FIELD OF INVENTION

[001] The present invention is directed to; inter alia, pharmaceutical compositions and methods for treating mood and stress related disorders.

BACKGROUND OF THE INVENTION

[002] Despite impressive progress in understanding the molecular, cellular and circuit-level correlates of major depression, the biological mechanisms that causally underlie this disease are still unclear, hindering the development of effective preventive and therapeutic procedures. One possible reason for this situation is that almost all research in this area focuses on the involvement of abnormalities in neuronal functioning, whereas the involvement of non-neuronal brain cells was not thoroughly examined. Over the last decade it has been suggested that glia cells, particularly astrocytes, may be also involved in the pathophysiology of depression, however, almost no research has focused on the role of microglia in this disease.

[003] Microglia, which comprise about 15% of brain cells, serve as the representatives of the immune system in the brain, and therefore their activation plays a major role in brain infection, injury, and neurodegenerative diseases. In addition, they are highly motile in their resting/quiescent state, and actively participate in synaptic changes, micro-damage repair, and neurogenesis.

[004] Microglial activation and brain inflammatory cytokines have been implicated in the responsiveness to stress, which both in humans and in animal models has been implicated as a major trigger for depression. Specifically, exposure to acute stressful conditions can directly induce hippocampal microglial activation, and the microglial inhibitor minocycline was reported to block stress-induced hypothalamic IL-1 secretion. Furthermore, exposure to repeated restraint or social disruption stress for 4- 14 days induced microglia proliferation and activation in several stress-responsive brain regions. In addition, the depressive and neurogenesis suppressive effects of chronic stress in rodents were found to be mediated by the microglial-derived cytokine interleukin-1. Together, these findings suggest that stress-induced microglial activation is critically involved in the development of depression and neurogenesis suppression. As such, the main hypothesis for treating stress and mood disorders proposes inhibiting microglial activation (e.g., Steiner et al., 2011, Journal of Neuroinflammation, 8:94).

[005] Nevertheless, the art does not describe or suggest that following the initial phase of microglial proliferation and activation there is a subsequent phase of microglia apoptosis, dystrophy and decline, which contradictory to the art, should be treated by microglia stimulation.

[006] There is an unmet need for improved compositions and methods for treating and/or ameliorating mood and stress related disorders.

SUMMARY OF THE INVENTION

[007] The present invention relates to methods and compositions for treating a mood disorder or stress related disorder in a subject in need thereof. In some embodiments, there is provided use or administration of least one microglia activator or a combination of microglia activators for treatment of mood or stress related disorders.

[008] According to one aspect the present invention provides a method for treating or attenuating a mood or stress-related disorder in a subject having normal or low inflammatory state, the method comprising administering to the subject a pharmaceutical composition comprising a therapeutically effective amount of at least one microglia stimulator selected from macrophage colony-stimulating factor (M- CSF), granulocyte-macrophage colony- stimulating factor (GM-CSF) and IL-34 or an active variant, fragment or derivative thereof, and at least one pharmaceutically acceptable carrier or diluent.

[009] In another embodiment, said inflammatory state is detected by determining the level of activated microglia. In another embodiment, said inflammatory state is detected by determining the level of dystrophic microglia. In another embodiment, said low inflammatory state is an increase in dystrophic microglia.

[010] In another embodiment, said inflammatory state (e.g., levels of activated or dystrophic microglia) is assessed by positron emission tomography (PET) imaging. In some embodiments, said inflammatory state is detected by determining the level of at least one inflammatory marker. In another embodiment, said inflammatory marker is C-reactive protein (CRP). In another embodiment, there is provided treatment of a subject suffering from a mood or stress related disorder having plasma CRP levels lower than 3 mg/L, lower than 2.5 mg/L or lower than 2 mg/L.

[Oi l] In another embodiment, said low inflammatory state is assessed by comparing the inflammatory state of said subject to a pre-determined inflammatory level. In another embodiment, said pre-determined inflammatory level is a pre-determined control level. In another embodiment, said pre-determined inflammatory level is an inflammatory level previously detected in said subject.

[012] In another embodiment, said stress-related disorder is selected from posttraumatic stress disorder, acute stress disorder, adjustment disorder, bereavement related disorder, general anxiety disorder, social anxiety disorder and anxiety disorder due to a medical condition. In another embodiment, said mood disorder is selected from major depressive disorder, unipolar major depressive episode, dysthymic disorder, treatment-resistant depression, bipolar depression, adjustment disorder with depressed mood, cyclothymic disorder, atypical depression, seasonal affective disorder, melancholic depression, psychotic depression, post-schizophrenic depression, melancholic depression, depression due to a general medical condition, post-viral fatigue syndrome and chronic fatigue syndrome.

[013] According to another aspect, the present invention provides a pharmaceutical composition for use in treating a mood or stress-related disorder in a subject having normal or low inflammatory state, the pharmaceutical composition comprising a therapeutically effective amount of at least one microglia stimulator selected from M- CSF, GM-CSF and IL-34 or an active variant, fragment or derivative thereof, and at least one pharmaceutically acceptable carrier or diluent.

[014] According to another aspect, the present invention provides use of a pharmaceutical composition comprising a therapeutically effective amount of at least one microglia stimulator selected from M-CSF, GM-CSF and IL-34 or an active variant, fragment or derivative thereof, and at least one pharmaceutically acceptable carrier or diluents, for the preparation of a medicament for treating a mood or stress- related disorder in a subject having normal or low inflammatory state.

[015] According to another aspect the present invention provides a method for the selection of an agent for treatment of a mood or stress related disorder in a subject, the method comprising: (a) contacting microglia cell culture with a putative agent; (b) assessing the effect of the agent on microglia activation or proliferation; and (c) selecting the agent(s) that increases microglia activation or proliferation; thereby selecting an agent for treatment of a mood or stress related disorder in a subject.

[016] In another embodiment, the method further comprises at least one step selected from the group consisting of: validating the effects of the selected agent of (c) on microglia activation and/or proliferation in depression -related areas within an animal model's (e.g., a rodent) brain; validating the effect of the selected agent of (c) in a behavioral model of the animal model; and selecting the agent(s) that reduce the stress induced depressive-like behavioral changes. In another embodiment, said behavioral model comprises a chronic unpredictable stress (CUS) exposure model. In another embodiment, said behavioral model is a chronic mild stress (CMS) exposure model.

[017] Further embodiments and the full scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[018] Figures la-j. Effects of CUS exposure on microglia number, morphology and functioning, (la) Effects of CUS exposure on the number of microglia in the dentate gyrus (DG) (t ₆ =2.53, P < 0.05; n=4/group). (lb) Representative picture of CX3CR1- GFP labeled microglia in the DG of control and (lc) CUS-exposed mice, (d) Effects of CUS exposure on the number of microglia in the medial prefrontal cortex (PFC) (p>0.1), and (le) the somatosensory cortex (p>0.1). (If) Representative pictures of a ramified microglial cell in the DG of a control mouse and (lg) a dystrophic microglial cell in the DG of CUS-exposed mouse, (lh) Effects of CUS exposure on the length of microglial processes (t ₆ =2.66, P < 0.05) and (li) the area of their soma (t ₆ =2.01, P < 0.05). (lj) Effects of CMS in rats on the mRNA expression levels of Iba-1, CDl lb and CD200 (t ₉ = 2.21, 2.34, and 4.13, respectively, p<0.05), as well as MHC-Π and CD200 receptor (CD200R). *p<0.05 compared with control untreated animals. [019] Figures 2a-m. Effects of short-term US exposure on microglial status. (2a) Microglia number in the DG of mice exposed to no stress (Control), 2 days or 4 days of US (F ₂ ,i7=28.30, p<0.01; n=5/group). (2b) Representative pictures of CX3CR1-GFP labeled ramified microglia in a control mouse and (2c) activated microglia in a mouse exposed to 2 days of US. (2d) Effects of 2 or 4 days US exposure on microglial soma size and (2e) processes length (F _2i n=7.06 and 14.27, respectively, P<0.05). (2f) At 24 hr post US initiation in rats, hippocampal mRNA expression of Iba-1 and MHC-II was increased (½=-1.18 and -3.49, P<0.05 and P<0.01, respectively; n=6/group), whereas CD200R expression was decreased (t ₁₀ =3.34, P<0.01). (2g) Effects of 2 days US exposure on the number of proliferating DG microglia (double-labeled for GFP and BrdU) (t ₆ =-4.997, p<0.01; n=4/group). (2h) Representative pictures of proliferating DG microglia in control and (2i) 2 days US-exposed mice (green=GFP; red=BrdU; the inset depicts the location of these cells within the DG (Blue=Dapi)). (2j) Effects of 3 days US exposure on the number of activated/apoptotic microglia (doubled -labeled for GFP and caspase-3) (t ₆ =-5.48, p<0.0001; n=4/group). (2k) Representative pictures of BrdU (green) and caspase-3 (red) doubled-labeled microglia in the DG of a stressed mouse. (21) TUNEL staining (red) in GFP-labeled DG microglia (green) from a mouse exposed to 4 days US. (2m) Higher magnification of an apoptotic microglia. *p<0.05 compared with the control group.

[020] Figures 3a-r. Effects of pharmacological and genetic modulation of microglial status on stress-induced depressive behavior. (3a) Effect of minocycline on the number of DG microglia following 2 days of US exposure (n=4-5/group) P<0.05, for stress by treatment interaction). (3b) Effects of minocycline on the number of caspase-3 and TUNEL-stained microglia in the DG following 3-4 days of US exposure (n=4-5/group) (t ₆ =4.09 and 1.93, respectively, p<0.05). (3c) Effect of minocycline on the number of DG microglia following 5 weeks of CUS exposure (n=5-6/group) (Fi _, 17=9.47, P<0.05, for stress by treatment interactions). (3d) Effects of minocycline on CUS-induced reductions in sucrose preference and (3e) social exploration (Fi,3i=25.389 and 3.54, P<0.0001 and P=0.07, respectively, for stress by treatment interactions). (3f) Effect of minocycline on CUS-induced reduction in neurogenesis (F _U7 =16.85 and 10.32, P<0.001 and P=0.059, for the stress and treatment effects, respectively, with no significant stress by treatment interaction). (3g) Effect of imipramine on the number of DG microglia following 2 days of US exposure (n=6- 9/group) (Fi 2 ₄ =7.91, P<0.01, for stress by treatment interaction). (3h) Effects of imipramine treatment throughout the CUS exposure period on CUS -induced suppression of DG microglia number, (3i) their processes length (F _3i i2=9.211 and 15.50, respectively, p<0.05, for the group by treatment interactions; n=6/group), and (3j) their soma size (for which the interaction effect did not reach statistical significance). (3k) Effects of imipramine on CUS-induced reductions in sucrose preference and (31) social exploration (F(l,20)=4.36 and 56.82, respectively, p<0.05, for group by treatment interactions; n=6/group). (3m) Effects of US on mRNA expression of IL-l-related genes in rats. At 24 hr post US initiation, hippocampal mRNA expression of IL-lra was decreased, whereas IL-IRI expression was increased (tio=5.12 and 3.25, p<0.01) and the expression of IL-1 and ICE was not altered; n=6/group). (3n) Effects of 2 days US exposure on DG microglia number in wild type (WT) vs. IL-lraTG mice (Fi _i34 =6.04, P<0.05 for stress by strain interaction; n=8- 10/group). (3o) Effects of CMS in rats on mRNA expression of IL-l-related genes. CMS exposure reduced ICE expression (t9 =2.21, P < 0.05), but it had no effects on other IL-l-related molecules (n=5-6/group). (3p) Effects of CUS on microglia number in WT and IL-lraTG mice (F _U9 =5.11, p<0.05, n=5-6/group). (3q) Effects of CUS on sucrose preference and (3r) social exploration in WT and IL-lraTG mice (Fi 28=3.87 and 6.04, respectively, P<0.05, for stress by strain interactions; n=7-8/group). *p<0.05 compared with water drinking or WT control mice.

[021] Figures 4a-p. Anti-depressive effects of microglial stimulation in CUS- exposed ("depressed-like") mice. (4a) Effects of LPS in Control and CUS -exposed mice on floating time and (4b) latency to first float in the Porsolt forced swim test (Fi _i48 =18.73 and 13.21, P<0.001 and P<0.01, respectively, for stress by treatment interactions; n=8-12/group). (4c) Effects of LPS on DG microglia number (F _3i3 i=9.80, P<0.05, for the overall effect of LPS), as well as (4d) the number of BrdU-labeled DG cells (Fi 2o=4.93, P<0.05, for stress by treatment interaction). (4e) Effects of M-CSF in Control and CUS-exposed mice on floating time (Fi ₃ i=5.60, P<0.05, for stress by treatment interaction) and (4f) latency to first float (P>0.1) in the Porsolt test (n=9/group). (4g) Effects of M-CSF on sucrose preference and (4h) social exploration (Fi,3i=7.75 and 5.52, P<0.01 and P<0.05, respectively, for stress by treatment by time interactions). (4i) Effects of M-CSF on DG microglia number (Fi 22=8.61, P<0.05, for the overall effect of M-CSF), as well as (4j) the number of GFP labeled microglia co- labeled for BrdU, and (4k) the number of non-microglia proliferating (BrdU-labeled) cells (Fi 2 ₄ =16.06 and 8.82, respectively, P<0.01, for stress by treatment interactions). (41) Effects of various treatments, administered to 5-weeks CUS-exposed mice over an additional two weeks CUS exposure period, on DG microglia number (F ₄₄₂ =8.61, P<0.001) (n=8-13/group), as well as on (4m) total floating time (F ₄ , ₄₉ =8.61, P<0.05) and (4n) the latency to first float in the Porsolt forced swim test (p>0.1). (4o) Effects of various treatments on sucrose preference ((F _{4 47} =6.76, P<0.001, for group by measurement (before vs. after treatment) interaction), and (4p) social exploration (F _4i48 =4.84, P<0.05 for treatment (before vs. after treatment) effect). *p<0.05 compared with Control non-stressed mice. **p<0.05 compared with CUS-exposed saline- injected mice and with LPS -injected control mice. *** p<0.05 compared with CUS- exposed saline-injected mice. +p<0.05 compared with both Control non-stressed mice and with CUS-exposed mice treated with GM-CSF. ++p<0.05 compared with the corresponding Control non-stressed mice either before or after drug treatment. #p<0.05 compared with the corresponding water- or minocycline-drinking CUS-exposed mice after drug treatment. †p<0.05 compared with the corresponding "Before drug" condition.

[022] Figure 5. A model representing the dynamic alterations in microglia status as a function of repeated stress exposure. An initial period of stress-induced IL-1 -dependent microglial proliferation and activation leads to subsequent microglial apoptosis, decline in numbers, and assumption of dystrophic morphology, which are causally related to the development of depressive symptomatology and suppressed neurogenesis.

[023] Figures 6a-k: Neurobehavioral effects of CUS exposure. (6a) Effects of CUS exposure on sucrose preference and (6b) social exploration (F^ ₂ 6=9.541and 18.00, p<0.05 and P<0.005, respectively; n=14/group). (6c) Effects of CUS on serum corticosterone levels (tio=2.099 p=0.05). (6d) Representative pictures of the DG in a control mouse (stained with Dapi), as well as (6e) GFP-labeled and (6f) Iba-l-labeled microglia in this DG area. (6g) Representative pictures of the DG in a CUS-exposed mouse (stained with Dapi), as well as (6e) GFP-labeled and (6f) Iba-l-labeled microglia in this DG area. (6j) Representative picture of a normal GFP-labeled microglia cell, as compared with (6k) Iba-1 labeling of the same cell. *p<0.05 compared with all other groups. [024] Figures 7a-f: Effects of CMS in rats on depressive-like symptoms and microglia number. Effects of CMS exposure on sucrose preference (7a; F _{5i l l} 5= 3.62, p<0.005) and (7b) social exploration (F ₃ =28.67, p<0.0001). (7c) Effects of CMS on the number of microglia/section of the DG (tg=l .93, p<0.05; n=5-6/group) and (7d) the prelimbic cortex (p>0.1). (7e) Representative pictures of CD l ib staining of microglia in control and (7f) CMS-exposed rats. *p<0.05 compared with the control group.

[025] Figures 8a-e: Effects of CMS in rats on depressive-like symptoms and microglia number. Effects of CMS exposure on sucrose preference (8a; F _{5i l l} 5= 3.62, p<0.005) and (8b) social exploration (F ₃ =28.67, p<0.0001). (8c) Effects of CMS on the number of microglia/section of the DG (tg=l .93, p<0.05; n=5-6/group) and (8d) the prelimbic cortex (p>0.1). (8e) Representative pictures of CD l ib staining of microglia in control and (8e) CMS-exposed rats. *p<0.05 compared with the control group.

[026] Figures 9a-d: Effects of LPS on the behavioral effects of CUS. (9a) Effects of acute LPS administration on locomotor activity (distance moved) in the open filed test and (9b) the time spent in the center of the open field, in control and CUS-exposed mice (Ρ _1ι54 =5.018 and 6.505, respectively, P < 0.05, for stress by treatment interactions; n=15-16/group). (9c) Effects of acute LPS administration on sucrose preference and (9d) social exploration in control and CUS-exposed mice before drug (saline/LPS) injection (i.e., at the completion of the 5-weeks CUS exposure) and after drug treatment (Fi, ₂₉ =25.625, P < 0.0005 and Fi, ₂₈ =15.146, P < 0.001, respectively, for the overall stress effect, but no significance for the treatment or stress by treatment interactions). *p<0.05 compared with saline-injected control mice.

[027] Figures lOa-b. Anxiolytic effects of LPS in CUS-exposed ("depressed-like") mice. (10a) Effects of acute LPS administration on locomotor activity (distance moved) in the open field test and (10b) the time spent in the center of the open field, in control and CUS-exposed mice (Ρ _1ι54 =5.018 and 6.505, respectively, P < 0.05, for stress by treatment interactions; n=15-16/group). *p<0.05 compared with saline- injected control mice.

DETAILED DESCRIPTION OF THE INVENTION

[028] The present invention relates to methods and compositions for treating a mood disorder or stress related disorder in a subject in need thereof. [029] The present invention is based, in part, on the surprising finding that a specific patient population suffering from mood and depressive disorders can be treated by microglial activation. Without wishing to be bound by any mechanism of action, the invention demonstrates for the first time that chronic stress (CS) produces dynamic, bidirectional alterations in microglia status, including a known initial phase of proliferation and activation followed by a surprising microglia apoptosis, dystrophy and decline (Fig. 5). As exemplified hereinbelow, stimulation of microglia in CUS- exposed mice exhibiting suppressed microglia reversed the depressive-like neuro- behavioral symptomatology.

[030] According to some embodiments, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a pharmaceutical composition comprising a therapeutically effective amount of at least one microglia stimulator or an active variant, fragment or derivative thereof, and at least one pharmaceutically acceptable carrier or diluents. In some embodiments, said at least one microglia stimulator is selected from the group consisting of: CpG oligonucleotides, ATP, a-Crystallin (small heat shock protein), CX3CR1 antagonist, albumin, Monophosphoryl Lipid A (MPLA) and LPA, or an active variant, fragment or derivative thereof.

[031] In one embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of CpG oligonucleotides.

[032] In another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of MPLA.

[033] In another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of LPA.

[034] In another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of ATP. [035] In another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of a-Crystallin.

[036] In another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of an CX3CR1 antagonist, including but not limited to antibody to the CX3CR1 receptor.

[037] In another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of albumin.

[038] In some embodiments, said subject is a subject having normal or low inflammatory state and said at least one microglia stimulator is a colony stimulating factor 1 receptor (CSF1R; also known as FMS and CD115) ligand (i.e., agonist). In another embodiment, said at least one microglia stimulator is selected from the group consisting of: macrophage colony- stimulating factor (M-CSF), granulocyte- macrophage colony-stimulating factor (GM-CSF) and IL-34 or an active variant, fragment or derivative thereof.

[039] According to another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising the steps of:

(a) identifying, at least once, the inflammatory state of the subject; and

(b) administering to a subject having a low or normal inflammatory state at least one microglia stimulator (e.g., M-CSF, GM-CSF and IL-34) or an active variant, fragment or derivative thereof.

[040] According to another embodiment, there is provided a method for treating or attenuating a mood or stress-related disorder in a subject in need thereof, the method comprising the steps of:

(a) identifying, at least once, the inflammatory state of the subject;

(b) administering to a subject having an increased inflammatory state at least one microglia suppressive agent (e.g., minocycline, IL-1 signaling blockers); and (c) administering to a subject having a low or normal inflammatory state at least one microglia stimulator or an active variant, fragment or derivative thereof.

[041] In another embodiment, said inflammatory state is detected by determining the level of activated microglia. In another embodiment, said inflammatory state is detected by determining the level of dystrophic microglia. In another embodiment, said low inflammatory state is an increase in dystrophic microglia.

[042] In some embodiments, said normal or low inflammatory state is detected by determining the level of at least one inflammatory marker. In another embodiment, said inflammatory marker is CRP. CRP is a sensitive, nonspecific, acute -phase protein, produced in response to most forms of tissue injury, infection, and inflammation. CRP is produced by Kupffer cells in the liver, which are regulated by cytokines, such as IL- 1, IL-6 and TNF . Based on its stability, assay precision, accuracy, and availability; and the availability of standards for proper assay calibration, the high sensitivity CRP assay was recommended as the preferred inflammatory marker for coronary vascular disease. In normal humans, with no overt inflammatory condition, 95% of the population has CRP values <10 mg/L. In another study more than 50% of the normal population was found to have CRP levels lower than 2 mg/L (Koenig et al., 1999).

[043] Several additional inflammatory markers can be also utilized for detection of a low inflammatory state, including IL-6 and TNF . In some embodiments, there is provided treatment of a subject suffering from a mood or stress related disorder having IL-6 or TNF levels lower than the levels of said cytokines in a control population (i.e., not having an inflammatory disease or disorder), typically less than 2.0 pg/ml for IL-6 and 3.8 pg/ml for TNF . The Erythrocyte Sedimentation Rate (ESR) can also be used to define the inflammatory state. In some embodiments, there is provided treatment of a subject suffering from a mood or stress related disorder having less than 6.3 mm/h for ESR.

[044] In another embodiment, said inflammatory state (e.g., levels of activated or dystrophic microglia) is assessed by positron emission tomography (PET) imaging. As known to one skilled in the art, microglia express the 18kDa translocator protein (TSPO), which can be quantified by several PET ligands (Owen and Matthews, Int Rev Neurobiol. 2011;101: 19-39). The most common ligand is [(11)C]PK11195 (also termed peripheral benzodiazepine receptor), but newer ligand, such as [18F]-FEPPA, [11C]PBR28 and [18F]DPA are also available.

[045] In some embodiments, the methods of the invention comprise assessing the inflammatory status of a subject at least twice, such as before and after treatment. In some embodiments, a subject is treated with a microglia activator of the invention when the microglia levels or activation rate are determined to be low or decreasing.

[046] In another embodiment, said low inflammatory state is assessed by comparing the inflammatory state of said subject to a pre-determined inflammatory level. In another embodiment, said pre-determined inflammatory level is a pre-determined control level. In another embodiment, said pre-determined inflammatory level is an inflammatory level previously detected in said subject.

Microglia activator

[047] Microglial activation refers to the fact that when infection, injury or disease occur in the brain and affect nerve cells, microglia in the central nervous system become "active," causing inflammation in the brain, similar to the manner in which white blood cells act in the rest of the body. Under some conditions, microglia act like the monocyte phagocytic system. Activated microglia can generate large quantities of superoxied anions, with hydroxyl radicals, singlet oxygen species and hydrogen peroxide being a downstream product, any of which can be assayed in the preparations utilized in such methods of the invention.

[048] Reactive microglia may be characterized by at least one of the following characteristics: 1) their cell bodies becoming larger, their processes becoming shorter and thicker, 2) an increase in the staining for several molecular activation markers, including Iba-1, Cdl lb and ED-1, 3) proliferation and clustering , 4) production and secretion of inflammatory mediators, including pro-inflammatory (e.g., interleukin (IL)-l, IL-6 and tumor necrosis factor- ) and anti-inflammatory (e.g., IL-10, IL-lra) cytokines, as well as additional inflammatory mediators (e.g., prostaglandins), 5) production and secretion of various neuroprotective factors, including brain- derived neurotrophic factor (BDNF) and insulin growth factor-1 (IGF-1), 6) production and secretion of chemoattractive factors (chemokines), which recruit microglia from within the brain to specific brain locations and facilitate the infiltration of peripheral immune cells, for example, white blood cells, as compared to that found in the non-reactive state. In some embodiments microglial activation is determined in at least one brain region or area, such as in the hippocampal dentate gyrus (DG), in the prelimbic cortex or in any depression-related area.

[049] The term "microglia activator" or "microglia stimulator" refers to a compound that may be nucleic acid based molecule, amino acid based molecule or a small organic molecule that causes activation of microglia as will be defined below. The activator may be an isolated full molecule, a fragment or a variant of the molecule as long as it causes microglia activation. The activator may cause the effect of microglia activation including but not limited to by acting directly on the microglia or by causing production, expression, secretion, of another agent effecting microglia activation.

[050] In some embodiments, the microglia activator increases at least one of the following, all being indicative of microglia activation: increase in hippocampal microglia number as well as increase in number of proliferating microglia (as determined for example by microglia labeled with BrdU); reversal or decrease in dystrophic changes in microglia and increase their of cell body size and processes length; an increase of the expression of activation markers (including Iba-1, CD l ib, and the inflammation -related enzyme caspase-1), and an increase in at least one neurotrophic factor such as BDNF and IGF-1.

[051] Non-limiting examples of microglia activators to be used in the method and composition of the invention alone or in combination include but are not limited to: M- CSF, GM-CSF, IL-34 and other Fms-tyrosine-kinase-receptor ligands; interleukin (IL)- 1 , IL-4; IL-6; IL-12/IL-23 p40 subunit; IL-27; Tumor necrosis factor-alpha (TNF- ), CD40 ligand (CD154), Interferon gamma (IFN ), Monophosphoryl lipid A (MPL), Protollin, Amphotericin B (AmB) (e.g., Fungizone), polyinosinic-polycytidylic acid (poly (I:C), imiquimod, loxoribine, R-848, CpG oligonucleotides, 12-myristate 13- acetate (PMA), ATP, CX3CR1 antagonist, a-Crystallin, albumin, Lysophosphatidic acid (LPA), Endotheline, Platelet-activating factor (PAF), aluminum hydroxide (alum), MF59, and Adjuvant System 03 (AS03)

[052] The term "macrophage colony stimulating factor receptor agonists", as used herein, relates to biologically active, recombinant, isolated peptides and proteins, including their biologically active fragments, peptidomimetics and small molecules that are capable of stimulating the Macrophage colony stimulating factor receptor, M- CSFR or c-fms.

[053] Macrophage colony-stimulating factor (M-CSF) is 70- to 90-kD membrane bound disulfide-linked homodimer glycoprotein that stimulates proliferation and supports survival and differentiation of cells of the mononuclear phagocyte series. Three related cDNA clones of human M-CSF have been isolated, representing long (beta) intermediate (gamma) and short (alpha) splicing variants from the single M-CSF gene. In some embodiments, M-CSF has an amino acid sequence selected from SEQ ID NO: 1-3.

[054] The M-CSF administered as part of the present invention may be a monomer or dimer. The dimer can be a homodimer composed of polypeptides having the above recited M-CSF reference amino acid sequences or a heterodimer composed of any two polypeptides having the above recited M-CSF reference amino acid sequences.

[055] Granulocyte-macrophage colony stimulating factor (GM-CSF) is a well-known growth factor. The GM-CSF that can be employed in the inventive methods described herein are those full length coding sequences, protein sequences, and the various functional variants, muteins, and mimetics that are known and available. The structure of both the coding DNA and protein are known as well as methods for recombinantly producing mammalian pluripotent granulocyte-macrophage colony- stimulating factor. Examples for different suitable forms and derivatives are sargramostim (Leukine®, Prokine®), Leucotropin® or molgramostim (Leucomax®). In some embodiments, GM-CSF comprises the amino acid sequence as set forth in SEQ ID NO: 4.

[056] IL-34 is a cytokine that promotes the proliferation, survival and differentiation of monocytes and macrophages. IL-34 promotes the release of pro-inflammatory chemokines, and thereby plays an important role in innate immunity and in inflammatory processes. Signaling of IL-34 is via CSF1R and its downstream effectors stimulate phosphorylation of MAPK1/ERK2 AND MAPK3/ERK1. In some embodiments, IL-34 has an amino acid sequence as set forth in SEQ ID NO: 5.

[057] In some embodiments, Interleukin-1 has an amino acid sequence as set in NP_000567. In some embodiments, IL-4 has an amino acid sequence as set in NP_000580. In some embodiments, IL-6 has an amino acid sequence as set in NP_000591. In some embodiments, IL-12 has an amino acid sequence as set in UniProt accession no. P29459 or IL-23 p40 subunit (e.g., NP_000873). In some embodiments, IL-27 has an amino acid sequence as set in UniProt accession no. Q6P676.

[058] In some embodiments, TNF- has an amino acid sequence as set in NP_000585. In some embodiments, IFN has an amino acid sequence as set in NP_000610. In some embodiments, CD40 ligand (CD154) has an amino acid sequence as set in NP_000065.

[059] In some embodiments, said microglia activator is at least one toll-like receptor agonist. Human microglia express all TLRs (1-9), and all agonists of these receptors can activate microglia (Olson and Miller, J Immunol. 2004 Sep 15;173(6):3916-24). Activators of TLR3, TLR4, TLR 7/8 and TLR 9 have been found as strong microglial activators. As demonstrated hereinbelow, TLR4 agonist LPS produces anti-depressive effects, indicating that at least one of TLR1-9 agonists can be used as microglia- stimulating anti-depressants agent of the invention.

[060] In some embodiments, said microglia activator is TLR3 agonists. Microglia express TLR3 and upon activation of this receptor exhibit marked activation, including cytokine production. In particular embodiments, said microglia activator is polyinosinic-polycytidylic acid (poly (I:C) ;CAS no. 24939-03-5), a synthetic double stranded RNA analog, found to activate microglia (i.e., induce an activated morphology and the production of inflammatory cytokines) through TLR3. Poly (I:C) may be particularly advantageous for the present invention since it can enter the brain, induce microglial activation and behavioral effects.

[061] In some embodiments, said microglia activator is at least one CpG oligonucleotide (of classes A, B and C). CpG oligonucleotide are known in the art as short single-stranded synthetic DNA molecules that contain a cytosine triphosphate deoxynucleotide ("C)" followed by a guaninetriphosphate deoxynucleotide ("G"). Bacterial DNA and synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG motifs (CpG ODN) have been shown to stimulate the cells of the innate immune system through TLR-9. CpG ODN are currently used as vaccine adjuvants, including Vaxlmmune™ and PF-3512676. CpG ODN and other TLR9 agonists can activate microglia cells. [062] In some embodiments, said microglia activator is selected from TLR7 and TLR8 agonists, e.g., guanosine-based imidazoquinoline compounds including but not limited to imiquimod and loxoribine.

[063] In some embodiments, said microglia activator is an adjuvant selected from MPL (e.g., CAS no. 1246298-63-4); protollin; Amphotericin B (AmB; e.g., CAS no. 1397-89-3) (Fungizone™); aluminum hydroxide (alum; e.g., CAS no. 21645-51-2 ); MF59 or Adjuvant System 03 (AS03).

[064] In some embodiments, said microglia activator is imiquimod (e.g., CAS no. 99011-02-6). In some embodiments, said microglia activator is loxoribine (e.g., CAS no. 121288-39-9). In some embodiments, said microglia activator is resiquimod (R- 848; e.g., CAS no. 144875-48-9).

[065] In some embodiments, said microglia activator is selected from Monophosphoryl lipid A (MPL; a chemically detoxified lipid A moiety derived from Salmonella Minnesota R595 LPS) or Lysophosphatidic acid (LPA).

[066] In some embodiments, said microglia activator is a CX3C receptor type 1 (CX3CR1) antagonist. In some embodiments, said microglia activator is an antibody to the CX3CR1.

[067] In some embodiments, said microglia activator is selected from 12-myristate 13-acetate (PMA) e.g., CAS no. 16561-29-8, ATP (e.g., CAS no. 56-56-5), a- Crystallin, albumin, Endotheline (e.g., UniProt accession no. P05305, P20800 or P14138) and platelet-activating factor (PAF; e.g., CAS no. 74389-68-7).

[068] In another embodiment, the method further comprises administering to the subject a combination of pro-inflammatory cytokines (including, but not limited to, IL- 1 , IL-6, IL-4, IL-6, IL -12/ IL -23 p40 subunit, IL-27, TNF- , IFN- ), as well as cytokine inducers and other microglia stimulators including, but not limited to CpG oligonucleotides (of classes A, B and C), CD40 ligand (CD154), 12-myristate 13- acetate (PMA), ATP, a-Crystallin (small heat shock protein), albumin and Lysophosphatidic acid (LPA).

[069] Included within the scope of the invention are polypeptides or polypeptide fragments being at least 70%, 75%, 80%, 85%, 90%, or 95% identical to the microglia activator described herein or fragments thereof. [070] As used herein, the term "polypeptide" is intended to encompass a singular "polypeptide" as well as plural "polypeptides," and refers to a molecule composed of monomers (amino acids) linearly linked by amide bonds (also known as peptide bonds). The term "polypeptide" refers to any chain or chains of two or more amino acids, and does not refer to a specific length of the product. Thus, peptides, dipeptides, tripeptides, oligopeptides, "protein," "amino acid chain," or any other term used to refer to a chain or chains of two or more amino acids, are included within the definition of "polypeptide," and the term "polypeptide" may be used instead of, or interchangeably with any of these terms. The term "polypeptide" is also intended to refer to the products of post-expression modifications of the polypeptide, including without limitation glycosylation, acetylation, phosphorylation, amidation, derivatization by known protecting/blocking groups, proteolytic cleavage, or modification by non-naturally occurring amino acids. A polypeptide may be derived from a natural biological source or produced by recombinant technology, but is not necessarily translated from a designated nucleic acid sequence. It may be generated in any manner, including by chemical synthesis.

[071] A polypeptide of the invention may be of a size of about 3 or more, 5 or more, 10 or more, 20 or more, 25 or more, 50 or more, 75 or more, 100 or more, 200 or more, 500 or more, 1,000 or more, or 2,000 or more amino acids. Polypeptides may have a defined three-dimensional structure, although they do not necessarily have such structure. Polypeptides with a defined three-dimensional structure are referred to as folded, and polypeptides which do not possess a defined three-dimensional structure, but rather can adopt a large number of different conformations, and are referred to as unfolded.

[072] By an "isolated" polypeptide or a fragment, variant, or derivative thereof is intended a polypeptide that is not in its natural milieu. No particular level of purification is required. For example, an isolated polypeptide can be removed from its native or natural environment. Recombinantly produced polypeptides and proteins expressed in host cells are considered isolated for purpose of the invention, as are native or recombinant polypeptides which have been identified and separated, fractionated, or partially or substantially purified by any suitable technique.

[073] In the present invention, a "polypeptide fragment" refers to a short amino acid sequence of a larger polypeptide. Protein fragments may be "free-standing," or comprised within a larger polypeptide of which the fragment forms a part of region. Representative examples of polypeptide fragments of the invention, include, for example, fragments comprising about 5 amino acids, about 10 amino acids, about 15 amino acids, about 20 amino acids, about 30 amino acids, about 40 amino acids, about 50 amino acids, about 60 amino acids, about 70 amino acids, about 80 amino acids, about 90 amino acids, about 100, about 200, and about 500 amino acids or more in length.

[074] The terms "fragment," "variant," and "derivative" when referring to a polypeptide of the present invention include any polypeptide which retains at least some biological activity. Polypeptides as described herein may include fragment, variant, or derivative molecules without limitation, so long as the polypeptide still serves its function. Microglia activators (e.g., M-CSF) polypeptides and polypeptide fragments of the present invention may include proteolytic fragments, deletion fragments and in particular, fragments which more easily reach the site of action when delivered to an animal. Polypeptide fragments further include any portion of the polypeptide which comprises an antigenic or immunogenic epitope of the native polypeptide, including linear as well as three-dimensional epitopes. Polypeptides and polypeptide fragments of the present invention may comprise variant regions, including fragments as described above, and also polypeptides with altered amino acid sequences due to amino acid substitutions, deletions, or insertions.

[075] Non-naturally occurring variants may be produced using art-known mutagenesis techniques. Polypeptides and polypeptide fragments of the invention may comprise conservative or non-conservative amino acid substitutions, deletions or additions and may also include derivative molecules. As used herein a "derivative" of a polypeptide or a polypeptide fragment refers to a subject polypeptide having one or more residues chemically derivatized by reaction of a functional side group. Also included as "derivatives" are those peptides which contain one or more naturally occurring amino acid derivatives of the twenty standard amino acids. For example, 4- hydroxyproline may be substituted for proline; 5-hydroxylysine may be substituted for lysine; 3-methylhistidine may be substituted for histidine; homoserine may be substituted for serine; and ornithine may be substituted for lysine.

[076] The term "polynucleotide" is intended to encompass a singular nucleic acid as well as plural nucleic acids, and refers to an isolated nucleic acid molecule or construct, e.g., messenger RNA (mRNA) or plasmid DNA (pDNA). A polynucleotide can contain the nucleotide sequence of the full length cDNA sequence, including the untranslated 5 and 3 sequences, the coding sequences. A polynucleotide may comprise a conventional phosphodiester bond or a non-conventional bond (e.g., an amide bond, such as found in peptide nucleic acids (PNA)). The polynucleotide can be composed of any polyribonucleotide or polydeoxyribonucleotide, which may be unmodified RNA or DNA or modified RNA or DNA. For example, polynucleotides can be composed of single- and double-stranded DNA, DNA that is a mixture of single- and double- stranded regions, single- and double-stranded RNA, and RNA that is mixture of single- and double-stranded regions, hybrid molecules comprising DNA and RNA that may be single- stranded or, more typically, double- stranded or a mixture of single- and double- stranded regions. In addition, the polynucleotides can be composed of triple- stranded regions comprising RNA or DNA or both RNA and DNA. polynucleotides may also contain one or more modified bases or DNA or RNA backbones modified for stability or for other reasons. "Modified" bases include, for example, tritylated bases and unusual bases such as inosine. A variety of modifications can be made to DNA and RNA; thus, "polynucleotide" embraces chemically, enzymatically, or metabolically modified forms.

[077] The term "nucleic acid" refers to any one or more nucleic acid segments, e.g., DNA or RNA fragments, present in a polynucleotide. By "isolated" nucleic acid or polynucleotide is intended a nucleic acid molecule, DNA or RNA, which has been removed from its native environment. For example, M-CSF contained in a vector is considered isolated for the purposes of the present invention. Further examples of an isolated polynucleotide include recombinant polynucleotides maintained in heterologous host cells or purified (partially or substantially) polynucleotides in solution. Isolated RNA molecules include in vivo or in vitro RNA transcripts of polynucleotides of the present invention. Isolated polynucleotides or nucleic acids according to the present invention further include such molecules produced synthetically. In addition, a polynucleotide or nucleic acid may be or may include a regulatory element such as a promoter, ribosome binding site, or a transcription terminator.

Pharmaceutical use [078] According to some embodiments, the present invention provides a pharmaceutical composition for use in treating a mood or stress-related disorder in a subject having normal or low inflammatory state, the pharmaceutical composition comprising a therapeutically effective amount of at least one microglia stimulator selected from M-CSF, GM-CSF and IL-34 or an active variant, fragment or derivative thereof, and at least one pharmaceutically acceptable carrier or diluent.

[079] The term "pharmaceutical composition", as used herein, refers to at least one microglia stimulator with chemical components such as diluents or carriers that do not cause unacceptable adverse side effects and that do not prevent microglial stimulation.

[080] As used herein, a "therapeutically effective amount" or "an amount effective" refers to an amount effective, at dosages and for periods of time necessary, to achieve the desired therapeutic result. A therapeutic result may be, e.g., lessening of symptoms, prolonged survival, improved mobility, improved social and vocational functioning, and the like. A therapeutic result need not be a "cure." A therapeutic result may also be prophylactic. Typically, since a prophylactic dose is used in subjects prior to or at an earlier stage of disease, the prophylactic ally effective amount will be less than the therapeutically effective amount.

[081] The amount of the peptides of the present invention, which will be effective in the treatment of a particular disorder or condition will depend on the nature of the disorder or condition and on the particular peptide and can be determined by standard clinical techniques known to a person skilled in the art. In addition, in vitro assays may optionally be employed to help identify optimal dosage ranges. The precise dose to be employed in the formulation will also depend on the route of administration, and the nature of the disease or disorder, and should be decided according to the judgment of the practitioner and each patient's circumstances. Effective doses can be extrapolated from dose-response curves derived from in- vitro or in-vivo animal model test bioassays or systems.

[082] In some embodiments, the microglia activator, for example, M-CSF is administered once per day, continuously or intermittently, such as until there is an improved in said mood or depressive symptomatology.

[083] In some embodiments, the therapeutically effective amount of the microglia activator, for example, M-CSF is from between 0.1 and 100 g/kg body weight per day. In some embodiments, the therapeutically effective amount is from between about 1 and 100 g/kg body weight per day. In some embodiments, the therapeutically effective amount is from between 1 and 75 g/kg body weight per day. In some embodiments, the therapeutically effective amount is from between 1 and 50 body weight per day. In some embodiments, the therapeutically effective amount is from between 1 and 40 g/kg body weight per day. In some embodiments, the therapeutically effective amount is from between 1 and about 30 g/kg body weight per day. In some embodiments, the therapeutically effective amount is from between 1 and 25 g/kg body weight per day.

[084] The term "treating" means an intervention performed with the intention of reversing or preventing the development or altering the pathology of, and thereby alleviating a disorder, disease or condition, including one or more symptoms of such disorder, disease, or condition. Preventing refers to prophylactic or preventative measures. The related term "treatment," as used herein, refers to the act of treating a disorder, symptom, disease or condition, as the term "treating" is defined above.

[085] By "subject" or "individual" or "animal" or "patient" or "mammal," is meant any subject, particularly a mammalian subject, for whom diagnosis, prognosis, or therapy is desired. Mammalian subjects include, but are not limited to, humans, domestic animals, farm animals, zoo animals, sport animals, pet animals such as dogs, cats, guinea pigs, rabbits, rats, mice, horses, cattle, cows; primates such as apes, monkeys, orangutans, and chimpanzees; canids such as dogs and wolves; felids such as cats, lions, and tigers; equids such as horses, donkeys, and zebras; food animals such as cows, pigs, and sheep; ungulates such as deer and giraffes; rodents such as mice, rats, hamsters and guinea pigs; and so on. In certain embodiments, the mammal is a human subject.

[086] The term "mood disorder" includes but is not limited to, depression of any type, including but not limited to unipolar major depressive episode, major depressive disorder, dysthymic disorder, treatment-resistant depression, bipolar depression, adjustment disorder with depressed mood, cyclothymic disorder, melancholic depression, psychotic depression, post-schizophrenic depression, depression due to a general medical condition, as well as to post-viral fatigue syndrome, and chronic fatigue syndrome. [087] In some embodiments, the invention includes treatment of a subject afflicted by schizophrenia, and particularly a schizophrenic subject characterized by low number and activity of microglia. In some embodiments, said subject is a schizophrenic subject afflicted by depression. In some embodiments said subject shows schizophrenic symptoms including but not limited to anhedonia and/or social problems/withdrawal. In some embodiments, the invention includes treatment of subtypes of schizophrenia, including but not limited to paranoidic schizophrenia, disorganized schizophrenia, catatonic schizophrenia, undifferentiated schizophrenia, residual schizophrenia and simple schizophrenia.

[088] The term "stress related disorder" includes but is not limited to stress-related disorders, including but not limited to Posttraumatic Stress Disorder, Acute Stress Disorder, Adjustment Disorder, Bereavement Related Disorder, Other Specified Trauma- or Stressor- Related Disorder and Unspecified Trauma, Generalised Anxiety Disorder, Anxiety Disorder due to general medical condition, and Anxiety disorder not otherwise specified.

[089] Preferably the treatment is for improving at least one parameter related to depression and/or stress responsiveness, including depressed mood, anhedonia, decrease in appetite and significant weight loss, insomnia or hypersomnia, psychomotor retardation, fatigue or loss of energy, diminished ability to think or concentrate or indecisiveness, helplessness, hopefulness, recurrent thoughts of death, a suicide attempt or a specific plan for committing suicide, excessive anxiety, uncontrollable worry, restlessness or feeling keyed up or on edge, being easily fatigued, difficulty concentrating or mind going blank, irritability, sleep disturbance (difficulty falling or staying asleep, or restless unsatisfying sleep), sense of numbing, detachment, or absence of emotional responsiveness, a reduction in awareness of his or her surroundings, depersonalization, derealization, anxiety or increased arousal (e.g., difficulty sleeping, irritability, poor concentration, hypervigilance, exaggerated startle response, motor restlessness), avoidance of places and situations, distress or impairment in social, occupational, or other important areas of functioning.

[090] The pharmaceutical compositions of the invention can be formulated in the form of a pharmaceutically acceptable salt of the peptides of the present invention or their analogs, or derivatives thereof. Pharmaceutically acceptable salts include those salts formed with free amino groups such as salts derived from non-toxic inorganic or organic acids such as hydrochloric, phosphoric, acetic, oxalic, tartaric acids, and the like, and those salts formed with free carboxyl groups such as salts derived from nontoxic inorganic or organic bases such as sodium, potassium, ammonium, calcium, ferric hydroxides, isopropylamine, triethylamine, 2-ethylamino ethanol, histidine, procaine, and the like.

[091] The term "pharmaceutically acceptable" means suitable for administration to a subject, e.g., a human. For example, the term "pharmaceutically acceptable" can mean approved by a regulatory agency of the Federal or a state government or listed in the U. S. Pharmacopeia or other generally recognized pharmacopeia for use in animals, and more particularly in humans. The term "carrier" refers to a diluent, adjuvant, excipient, or vehicle with which the therapeutic compound is administered. Such pharmaceutical carriers can be sterile liquids, such as water and oils, including those of petroleum, animal, vegetable or synthetic origin, such as peanut oil, soybean oil, mineral oil, sesame oil and the like, polyethylene glycols, glycerine, propylene glycol or other synthetic solvents. Water is a preferred carrier when the pharmaceutical composition is administered intravenously. Saline solutions and aqueous dextrose and glycerol solutions can also be employed as liquid carriers, particularly for injectable solutions. Suitable pharmaceutical excipients include starch, glucose, lactose, sucrose, gelatin, malt, rice, flour, chalk, silica gel, sodium stearate, glycerol monostearate, talc, sodium chloride, dried skim milk, glycerol, propylene glycol, water, ethanol and the like. The composition, if desired, can also contain minor amounts of wetting or emulsifying agents, or pH buffering agents such as acetates, citrates or phosphates. Antibacterial agents such as benzyl alcohol or methyl parabens; antioxidants such as ascorbic acid or sodium bisulfite; and agents for the adjustment of tonicity such as sodium chloride or dextrose are also envisioned.

[092] The compositions can take the form of solutions, suspensions, emulsions, colloidal dispersions, emulsions (oil-in-water or water-in-oil), sprays, aerosol, ointment, tablets, pills, capsules, powders, gels, creams, ointments, foams, pastes, sustained-release formulations and the like. In particular embodiments, the pharmaceutical compositions of the present invention are formulated for aerosol administration for inhalation by a subject in need thereof.

[093] In some embodiments, the composition of the invention is administered by intranasal or intraoral administration, using appropriate solutions, such as nasal solutions or sprays, aerosols or inhalants. Nasal solutions are usually aqueous solutions designed to be administered to the nasal passages in drops or sprays. Typically, nasal solutions are prepared so that they are similar in many respects to nasal secretions. Thus, the aqueous nasal solutions usually are isotonic and slightly buffered to maintain a pH of 5.5 to 6.5. In addition, antimicrobial preservatives, similar to those used in ophthalmic preparations, and appropriate drug stabilizers, if required, may be included in the formulation. Various commercial nasal and oral preparations for inhalation, aerosols and sprays are known and include, for example, antibiotics and antihistamines and are used for asthma prophylaxis.

[094] For intranasal or intraoral administration the composition of the invention is provided in a solution suitable for expelling the pharmaceutical dose in the form of a spray, wherein a therapeutic quantity of the pharmaceutical composition is contained within a reservoir of an apparatus for nasal or intraoral administration. The apparatus may comprise a pump spray device in which the means for expelling a dose comprises a metering pump. Alternatively, the apparatus comprises a pressurized spray device, in which the means for expelling a dose comprises a metering valve and the pharmaceutical composition further comprises a conventional propellant. Suitable propellants include one or mixture of chlorofluorocarbons, such as dichlorodifiuoromethane, trichlorofiuoromethane, dichloro-tetrafiuoroethane, hydro fiuorocarbons, such as 1 ,1 ,1 ,2-tetrafiuoroethane (HFC-134a) and 1 ,1,1 ,2,3,3,3 -heptafluoropropane (HFC-227) or carbon dioxide. Suitable pressurized spray devices are well known in the art and include those disclosed in, inter alia, WO 92/11190, U.S. 4,819,834, U.S. 4,407,481 and WO 97/09034, when adapted for producing a nasal spray, rather than an aerosol for inhalation, or a sublingual spray. The contents of the aforementioned publications are incorporated by reference herein in their entirety. Suitable nasal pump spray devices include the VP50, VP70 and VP100 models available from Valois S.A. in Marly Le Roi, France and the 50, 70 and 100 ul nasal pump sprays available from Pfeiffer GmbH in Radolfzell, Germany, although other models and sizes can be employed. In the aforementioned embodiments, a pharmaceutical dose or dose unit in accordance with the invention can be present within the metering chamber of the metering pump or valve.

[095] The compositions can be formulated as a suppository, with traditional binders and carriers such as triglycerides, microcrystalline cellulose, gum tragacanth or gelatin. Oral formulation can include standard carriers such as pharmaceutical grades of mannitol, lactose, starch, magnesium stearate, sodium saccharine, cellulose, magnesium carbonate, etc. Examples of suitable pharmaceutical carriers are described in: Remington's Pharmaceutical Sciences" by E.W. Martin, the contents of which are hereby incorporated by reference herein. Such compositions will contain a therapeutically effective amount of a peptide of the invention, preferably in a substantially purified form, together with a suitable amount of carrier so as to provide the form for proper administration to the subject.

[096] Microglial activators of the invention, polynucleotides encoding them, and compositions comprising them, can be delivered to a cell either through direct contact with the cell or via a carrier means. Carrier means for delivering microglial activators and compositions to cells are known in the art and include, for example, encapsulating the composition in a liposome moiety. Another means for delivery comprises attaching the microglial activator to a protein or nucleic acid that is targeted for delivery to the target cell. U.S. Pat. No. 6,960,648 and Published U.S. Patent Application Nos. 20030032594 and 20020120100 disclose amino acid sequences that can be coupled to another composition and that allows the composition to be translocated across biological membranes.

[097] The route of administration of the pharmaceutical composition will depend on the disease or condition to be treated. Suitable routes of administration include, but are not limited to, parenteral injections, e.g., intradermal, intravenous, intramuscular, intralesional, subcutaneous, intrathecal, and any other mode of injection as known in the art. Although the bioavailability of peptides administered by other routes can be lower than when administered via parenteral injection, by using appropriate formulations it is envisaged that it will be possible to administer the compositions of the invention via transdermal, oral, rectal, vaginal, topical, nasal, inhalation and ocular modes of treatment. In addition, it may be desirable to introduce the pharmaceutical compositions of the invention by any suitable route, including intraventricular and intrathecal injection; intraventricular injection may be facilitated by an intraventricular catheter, for example, attached to a reservoir. Pulmonary administration can also be employed, e.g., by use of an inhaler or nebulizer.

[098] In some embodiments, the route of administration is improved by encapsulating the pharmaceutical agent in nanoparticles, such as to protect the encapsulated drug from biological and/or chemical degradation, and/or to facilitate transport to the brain thereby targeting microglia.

[099] In another embodiment, the method further comprises administering to the subject vitamin E or a derivative thereof. As used herein, the term "vitamin E" includes alpha, beta, gamma, and delta-tocopherols and their derivatives, conjugates and metabolites. The vitamin E may also be a combination of alpha, beta, gamma, and delta-tocopherols. The alpha-form occurs naturally as the d-isomer known as d-.alpha.- tocopherol (d-2,5,7,8-tetramethyl-2-(4',8',12'-trimethyltridecyl)-6-chr omanol). Other forms of vitamin E which can be used include: d-.alpha.-tocopheryl acetate, d-.alpha.- tocopheryl succinate, d-.alpha.-tocopheryl nicotinate and d-.alpha.-tocopheryl linoleate.

[0100] In another embodiment, the method further comprises administering to the subject at least one of the following anti-depressant drugs, including fluoxetine, sertraline, venlafaxine, citalopram, parocetine, trazodone, amitriptyline, nortriptyline, imipramine, dothiepin, lofepramine, doxepin, protriptyline, tranylcypromine, moclobemide, bupropion, nefazodone, mirtazapine, Zolpidem, alprazolam, temazepam, diazepam, or buspirone.

Screening methods

[0101] The present invention provides methods for treating or attenuating mood or depressive disorders, in some embodiments in subjects having low inflammatory state, using compounds that stimulate c-fms (macrophage colony stimulating factor receptor agonists or ligands). Macrophage colony stimulating factor receptor agonists stimulate the macrophage colony stimulating factor receptor, M-CSFR or also called c-fms, and may be biologically active, recombinant, isolated peptides and proteins, including their biologically active fragments, peptidomimetics or small molecules. In certain embodiments, such compounds are orally active and can cross the blood brain barrier.

[0102] Macrophage colony stimulating factor receptor agonists can be identified experimentally using a variety of in vitro and/or in vivo models. Isolated macrophage colony stimulating factor receptor agonists can be screened for binding to various sites of the purified macrophage colony stimulating factor receptor proteins. Compounds can be evaluated as potential factors for treatment of mood or depressive disorders using animal models (e.g., monkey, rat, or mouse models). Said candidate compounds may also be identified by known pharmacology, structure analysis, or rational drug design using computer based modeling.

[0103] Candidate compounds and agents can be obtained from a wide variety of sources, including libraries of synthetic or natural compounds. For example, libraries of natural compounds in the form of bacterial, fungal, plant and animal extracts can be tested. Known pharmacological agents may be subjected to directed or random chemical modifications, e.g., alkylation, esterification, amidification, etc. to produce a library of structural analogs. Alternatively, a library of randomly or directed synthesized organic compounds or biomolecules (e.g., oligonucleotides and oligopeptides) can be used as a source of agents. Preparation and screening of combinatorial libraries are well known to those of skill in the art. See, e.g., U.S. Pat. No. 5,010,175, PCT Publication No. WO 93/20242, PCT Publication No. WO 92/00091, Chen et al., J. Amer. Chem. Soc. 116:2661 (1994), U.S. Pat. No. 5,539,083.

[0104] Candidate compounds may encompass numerous chemical classes, though typically they are organic molecules, preferably small organic compounds having a molecular weight of more than 50 and less than about 2,500 daltons. They may comprise functional groups necessary for structural interaction with proteins (e.g., hydrogen bonding), and typically include at least an amine, carbonyl, hydroxyl, or carboxyl group. They often comprise cyclical carbon or heterocyclic structures and/or aromatic or polyaromatic structures substituted with one or more functional groups. They may be found among biomolecules including peptides, saccharides, fatty acids, steroids, purines, and pyrimidines, and structural analogs thereof.

[0105] In some embodiments, the present invention provides a method for the selection of a candidate compound or agent from a plurality of compounds, for treating a mood disorder or stress related disorder in a subject, the method comprising:

(a) contacting or treating microglia cell culture with a candidate compound;

(b) assessing the effects of the candidate compound on microglia number and/or proliferation status, and optionally on hippocampal neurogenesis status;

(c) selecting those candidates that increase microglia number and activation and preferably those that enhance hippocampal neurogenesis as well; (d) validating the selected compounds of (c) in a behavioral model of mice, preferably mice that were exposed to chronic unpredictable stress (CUS); and

(e) selecting those compounds that were found to reduce the stress induced depressive-like behavioral changes.

[0106] In some embodiments, step (b) is performed using purified microglia cultures, derived from newborn CX3CR1-GFP (heterozygous or homozygous) mice or wild type (WT) mice to various concentrations of the candidate compound, or by administration of various doses of the candidate compound to CX3CR1-GFP or WT mice and assessing their effects on the number of hippocampal microglia.

[0107] Microglia status may be assessed by at least one of the following assays or a combination thereof: 1) Counting the number of hippocampal microglia, as well as proliferating microglia (i.e., microglia labeled with BrdU). It is expected that a suitable drug candidate will induce microglia proliferation and will reverse the CUS -induced reduction in hippocampal microglia number. 2) Assessing morphological changes in microglia, expecting that a suitable drug candidate will reverse the dystrophic changes in microglia and will increase their cell body size and processes length. 3) Assessing the expression of activation markers (including Iba-1, CD l ib, and the inflammasome- related enzyme caspase-1), expecting that a suitable drug candidate will reverse the CUS -induced decrease in these markers. 4) Assessing the levels of neurotrophins, including BDNF and IGF-1, expecting that a suitable drug candidate will reverse the CUS -induced decrease in these markers. Typically, histological measures are compared with the corresponding results in saline-treated mice.

[0108] Neurogenesis may be assessed by daily administration of BrdU along with the putative anti-depressant drug. Following termination of the behavioral tests, the number of BrdU- and doublecortin-labeled cells in the hippocampus may be assessed using immunohistochemical methods well known in the art. It is expected that a suitable drug candidate will reverse the CUS -induced reduction in neurogenesis.

[0109] The behavioral assessment of step (d) may be measured in CUS-exposed (and control) subjects immediately before as well as after the compound administration. [0110] Typically the assessment may be carried out by exposing the mice for a period of 5 weeks (along with a control group of CUS -exposed mice administered with saline).

[0111] The assessment of the depressive behavior may be done for example by using at least one of the following tests:

1) Porsolt forced swim test (duration of floating and time to first float)

2) Anhedonia in the sucrose preference test

3) Level of social exploration/interest

[0112] Selected compounds are those that show anti-depressive properties in this assessment set of tests (i.e., lower floating time, increased time to first float, higher sucrose preference and social exploration, as compared with their pre-drug condition and with respect to saline-injected controls).

[0113] In particular embodiments, a compound that will produce anti-depressive effects in the behavioral tests, reverse the CUS -induced reductions in hippocampal neurogenesis, increase the number of hippocampal microglia and will induce activated microglial morphology and elevated expression of activation markers will be considered a suitable candidate for clinical studies as "microglia-stimulating antidepressant (MSAD)" drug.

[0114] It is to be noted that the term "a" or "an" entity, refers to one or more of that entity; for example, "a polypeptide," is understood to represent one or more polypeptides. As such, the terms "a" (or "an"), "one or more," and "at least one" can be used interchangeably herein.

[0115] Throughout this specification and claims, the word "comprise," or variations such as "comprises" or "comprising," indicate the inclusion of any recited integer or group of integers but not the exclusion of any other integer or group of integers.

[0116] As used herein, the term "consists of," or variations such as "consist of or "consisting of," as used throughout the specification and claims, indicate the inclusion of any recited integer or group of integers, but that no additional integer or group of integers may be added to the specified method, structure or composition.

[0117] As used herein, the term "consists essentially of," or variations such as "consist essentially of or "consisting essentially of," as used throughout the specification and claims, indicate the inclusion of any recited integer or group of integers, and the optional inclusion of any recited integer or group of integers that do not materially change the basic or novel properties of the specified method, structure or composition.

[0118] Additional objects, advantages, and novel features of the present invention will become apparent to one ordinarily skilled in the art upon examination of the following examples, which are not intended to be limiting. Additionally, each of the various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below finds experimental support in the following examples.

EXAMPLES

[0119] Materials and methods

[0120] Subjects were 2-4 month old male mice or rats. Animals were housed 2 (mice) or 3-4 (rats) per cage in temperature (23+3°C) controlled rooms with standard rodent chow and water available ad libitum (unless mentioned otherwise). The behavioral experiments were conducted during the first half of the dark phase of a reversed 12 hr light/dark cycle. All experiments were approved by the Hebrew University or the University of Colorado Committees for Animal Care and Use. Most experiments were conducted in heterozygous mice expressing transgenic enhanced green fluorescent protein (EGFP) in conjunction with the CX3C (fractalkine) receptor type 1 (CX3CR1), which in the brain is expressed exclusively by resident microglia along with their C57BL WT littermates. In experiments on the IL-1 system we used mice with transgenic overexpression of IL-1 receptor antagonist (IL-lraTG) under the control of the glial fibrillary acidic protein (GFAP) promoter and their C57BL/6 x CBA WT controls. These mice were shown to secrete hlL-lra only within their central nervous system and to be completely unresponsive to exogenous or endogenous IL-1 . In studies on rats we used adult male Sprague-Dawley rats (weighing 260-300 g at beginning of the experiment) (Harlan Laboratories).

[0121] Stress exposure paradigms To induce chronic stress in mice the Chronic Unpredictable Stress (CUS) paradigm was employed, consisting of daily exposure to two of the following stressors in a random order over a 5 weeks period: cage shaking for 1 hr, lights on during the entire night (12 h), placement in 4°C cold room for 1 h, mild restraint (in small cages) for 2 h, 45° cage tilt for 14 h, lights off for 3 h during the daylight phase, wet cage for 14 h, flashing (stroboscopic) light for 6 h, noise in the room for 3 h and water deprivation for 12 h during the dark period. Shorter periods of unpredictable stress included various combinations of 2-3 of the above-mentioned stressors over a period of 2-4 days. To induce chronic stress in rats the Chronic Mild Stress (CMS) paradigm was used over a 5 weeks period, consisting of daily exposure to two of the following stressors in a random order: food deprivation or water deprivation for 12 hr during the dark phase of the circadian cycle, lights on during the entire night (12 h), soaked cage for 14 hr, 45° cage tilt for 3 hr, flashing light for 1 hr, radio noise in the room for 4 hr. Shorter periods of mild stress exposure included various combinations of 2-3 of the above-mentioned stressors over a period of 24 or 48 hr.

[0122] Behavioral measurements

[0123] Sucrose preference: Following baseline adaptation to sucrose for several days, animals were presented in the beginning of the dark circadian phase with two graduated drinking tubes, one containing tap water and the other 2% sucrose solution for 3 hr. Sucrose preference was calculated as the percent of sucrose consumption out of the total drinking volume.

[0124] Social exploration: Each subject was placed in an observation cage and allowed to habituate to the cage for 15 min, following which a male juvenile was placed in the cage. Social exploration, defined as the time of near contact between the nose of the subject and the juvenile conspecific, was then recorded for three minutes, using computerized in-house software.

[0125] The open field test: In the beginning of this test, each mouse was placed in the corner of a white 80X80 cm plastic arena with 50 cm-high walls. The time spent and distance traveled in the center of the box (60 cm area) vs. the outside zone (the rest of the box area) was measured over a three min period. Behavior was analyzed using Etho Vision XT video tracking system and software (Noldus, The Netherlands).

[0126] The Porsolt forced swim test: Mice were placed in 20 cm diameter glass cylinder filled to 10 cm with 21°C water and their behavior was analyzed for five min. The time spent in immobility, defined as the absence of all movement except motions required to maintain the animal's head above the water, and the latency to first immobility episode were recorded using an in-house computerized software.

[0127] Pharmacological treatments

The tricyclic antidepressant drug imipramine (Sigma, Israel) was administered via the drinking water at a dose of 20 mg/kg/day. LPS (from Escherichia coli, serotype 0111: B4, Sigma, Israel) was injected i.p. at a dose of 100 g/kg. M-CSF (Prospect, Israel) was administered i.p. at a dose of 100 g/kg.

[0128] Molecular and biochemical measurement

[0129] Real-Time RT-PCR: Total RNA was isolated from whole hippocampus utilizing a standard method of phenokchloroform extraction. For detailed descriptions of RNA isolation, cDNA synthesis, and PCR amplification protocols refer to prior publication. cDNA sequences were obtained from Genbank at the National Center for Biotechnology Information (NCBI; www.ncbi.nlm.nih.gov). Primer sequences were designed using the Operon Oligo Analysis Tool

(http://www.operon.com/technical/toolkit.aspx) and tested for sequence specificity using the Basic Local Alignment Search Tool at NCBI. Primers were obtained from Invitrogen. Primer specificity was verified by melt curve analyses. All primers were designed to span exon/exon boundaries and thus exclude amplification of genomic DNA. Primer sequences were as follows:

Iba-1, F: GGCAATGGAGATATCGATAT (SEQ ID NO: 6), R: AGAATCATTCTCAAGATGGC (SEQ ID NO: 7);

CDl lb, F: CTGGTACATCGAGACTTCTC (SEQ ID NO: 8), R: TTGGTCTCTGTCTGAGCCTT (SEQ ID NO: 9);

MHCII, F: AGCACTGGGAGTTTGAAGAG (SEQ ID NO: 10), R: AAGCCATCACCTCCTGGTAT (SEQ ID NO: 11);

CD200, F: TGTTCCGCTGATTGTTGGC (SEQ ID NO: 12), R: ATGGACACATTACGGTTGCC (SEQ ID NO: 13):

CD200R, F: TAGAGGGGGTGACCAATTAT (SEQ ID NO: 14) R: TACATTTTCTGCAGCCACTG (SEQ ID NO: 15); IL-lb, F: CCTTGTGCAAGTGTCTGAAG (SEQ ID NO: 16), R: GGGCTTGGAAGCAATCCTTA (SEQ ID NO: 17);

ICE, F: ATGCCGTGGAGAGAAACAAG (SEQ ID NO: 18), R: CCAGGACACATTATCTGGTG (SEQ ID NO: 19);

IL-1RA, F: GTCTGGAGATGACACCAAG (SEQ ID NO: 20), R: TCGGAGCGGATGAAGGTAA (SEQ ID NO: 21);

IL-1R1, F: ACCCAGTTCCTGACTTCAAG (SEQ ID NO: 22), R: AGTCCCTGTACCAAAGCACT (SEQ ID NO: 23).

[0130] Primers were obtained from Invitrogen and their specificity was verified by melt curve analyses. All primers were designed to span exon/exon boundaries and thus exclude amplification of genomic DNA. PCR amplification of cDNA was performed using the Quantitect SYBR Green PCR Kit (Qiagen, Valencia, CA). Formation of PCR product was monitored in real time using the MyiQ Single-Color Real-Time PCR Detection System (BioRad, Hercules, CA). Relative gene expression was determined by taking the expression ratio of the gene of interest to β-Actin.

[0131] Corticosterone ELISA: To measure corticosterone levels, mice were killed by decapitation and trunk blood was collected. Samples were centrifuged for 15 min at 4°C and the serum was removed. Levels of corticosterone were determined using ELISA Kit (Assaypro, USA).

[0132] Histological measurements

[0133] Immunohistochemistry: Animals were perfused trans-cardially with cold PBS followed by 4% paraformaldehyde in 0.1 M PBS, and the brains were quickly removed and placed in 4% paraformaldehyde. After 24 h the brains were placed in 30% sucrose solution in PBS and then frozen in OCT.

[0134] BrdU immunofluorescent staining was performed on 8 μιη frozen brain sections, fixated in 50% formamide/2Xssc (0.3M NaCl and 0.03M sodium citrate) for

2 hours at 65°C. After one wash with 2XSCC, sections were incubated in 2N HCl for 30 minutes at 37°C. Sections were then rinsed in 0.1M boric acid at pH 8.5 and washed

3 times in PBS, following which they were incubated in 3% normal goat serum in 1% BSA/0.1% Triton for 30 min at room temperature and then incubated with rat monoclonal anti-BrdU antibody (1:200, AbD serotec) for 48 hours at 4°C. Sections were then incubated with a secondary antibody (1:200 goat anti rat IgG, conjugated to Alexa 555, Invitrogene) for 2 hours at room temperature and counter-stained with DAPI (Sigma, Israel).

[0135] In some experiments microglia were visualized using an antibody to the microglia activation marker ionized calcium-binding adapter molecule- 1 (Iba-1). Brain sections were first fixed in 2% paraformaldehyde, and after 3 PBS washes they were incubated over-night in 3% normal goat serum in 5% BSA with the primary antibody (rabbit anti Iba-1 1:220, Wako, Japan) at 4°C. Sections were then incubated with the secondary antibody (goat anti rabbit, 1:200; Invitrogen) for 1 hour at RT and counter- stained with DAPI (Sigma, Israel).

[0136] In rats, microglia were visualized with an antibody that recognizes the cluster of differentiation molecule 11B (CD l ib), a microglia surface marker in brain. CD l ib staining was conducted using the avidin-biotin-horseradish peroxidase (ABC) reaction. Briefly, sections were washed 3 times in PBS followed by incubation in a 0.3% hydrogen peroxide solution. Sections were then incubated in mouse monoclonal antisera against CDl lb (1: 100; BD Pharmingen, United States) overnight at 4°C. The following day tissue was incubated in biotinylated goat -mouse IgG secondary antibody (1:200; Jackson ImmunoResearch, United States) at RT. Slices were next incubated in ABC (Vectastain Elite ABC kit, Vector Labs, United States) in PBS for 1 hour at RT. Next, sections were washed in phosphate buffer (without saline), and then exposed to a solution containing 3, 3'-diaminobenzidine (DAB), cobalt chloride, nickel ammonium sulfate, ammonium chloride, and glucose oxidase in phosphate buffer. The peroxidase reaction was initiated by the addition of a glucose solution and reacted with the tissue for approximately 10 minutes. The reaction was terminated by washing sections in PBS. Stained tissue sections were dehydrated in a series of alcohols and defatted with Histoclear. Slides were then cover-slipped with Permount.

[0137] Caspase 3 immunofluorescent staining was performed on 50 μιη frozen floating brain sections. The sections were fixed in methanol, washed 3 times with PBS and incubated over-night in 3% normal goat serum in 5% BSA with the primary antibody (rabbit anti Caspase-3 1:200, Cell signaling Technologies, Beverly, MA, USA) at 4°. Sections were then incubated with the secondary antibody (goat anti rabbit cy3, 1:200; Jeckson) for 1 hour at RT and counter- stained with DAPI (Sigma, Israel). [0138] Apoptosis was detected and quantified using a commercially available fluorescent terminal deoxynucleotidyl transferase nick-end labeling (TUNEL) kit, according to the manufacturer's protocol (Roche Diagnostics Corporation, Indianapolis, IN, USA). For positive control, sections were incubated with recombinant DNase I (Roche Diagnostics Corporation, Indianapolis, IN, USA).

[0139] Image analysis: Images were captured using Nikon Eclipse fluorescent microscope and CI Nikon confocal microscope and camera. The number of GFP- labeled microglia (and in some studies the number of BrdU-labeled microglia and other cells), as well and the microglial soma area were automatically measured at X10 magnification in a defined area exclusively containing the entire Dentate Gyrus or prelimbic cortex, for each slide, using Nikon Imaging Elements Software (NIS- Elements). Microglia processes length was measured by capturing images at 40X magnification and by manual tracing of the processes of 4 randomly selected cells/section their processes using the NIS Elements software.

[0140] Statistical analysis

[0141] All data are presented as mean +SEM. All statistical comparisons were computed using SPSS 19.0 software and consisted of t-tests, one-way, two-way or three-way analyses of variance (ANOVAs) followed by Tukey post hoc analyses, when appropriate.

EXAMPLE 1

Effects of Chronic stress on microglia number, morphology and activation markers expression

[0142] To examine the alterations in microglial status associated with stress-induced depression-like behavior, a group of mice was exposed to 5 weeks of chronic unpredictable stress (CUS), an established paradigm for producing depressive-like behavior, and assessed their depressive symptomatology as well as their microglia number and morphology, as compared with a group of control, non-stressed mice. CUS-exposed mice displayed significant decreases in sucrose preference and social exploration, along with increased corticosterone levels (Fig. 6a-c). Surprisingly, CUS- exposed mice exhibited decreased numbers of microglia in the hippocampal dentate gyrus (DG) (Fig. la-c) and to a lesser extent also in the medial prefrontal cortex (PFC) (Fig. Id), but not in the somatosensory cortex (Fig. le). Some microglia within the DG assumed a dystrophic morphology (Figs, lf-g), reflected by overall significant reductions in the length of microglial processes (Fig. lh) and in soma area (Fig. li). Similar effects were obtained when microglia were stained with Iba-1, verifying that these numerical and morphological alterations did not result from down-regulation of CX3CR1 (Figs. 6d-k). No apoptotic cells were found in either CUS -exposed or control mice following staining with either caspase-3 or TUNEL, suggesting that the microglial decline occurred at an earlier stage during the exposure to CUS.

[0143] Corroborating the generality of these findings, exposure of rats to chronic mild stress (CMS) resulted in significant decreases in sucrose preference (Fig. 7), and social exploration (Fig. 7b), along with decreased microglia number in the DG (Fig. 7c) and to a lesser (non- significant) extent in the medial prefrontal cortex (Fig. 7d). In addition, CMS exposure significantly reduced the mRNA expression levels of the microglial markers CD l ib and Iba-1, as well as the neuronal marker CD200, which normally keeps microglia in relative quiescence (Fig. lj). Furthermore, in the CMS-exposed, but not in control rats, high negative correlations were obtained between the changes in sucrose preference from baseline to 5 weeks post CMS initiation and the expression of CDl lb (r=-0.771 vs. 0.222, respectively, p<0.05) and Iba-1 (r=-0.437 vs. 0.191, respectively, not significant). That is, lower microglial marker expression was associated with greater anhedonia.

EXAMPLE 2

Effects of short-term exposure to US on microglia number, morphology and activation marker expression

[0144] In contrast with the findings presented above, previous studies on the effects of stress on microglial status reported activation, rather than suppression of these cells. Because these studies employed shorter periods of stress exposure than the 5 -weeks period used in the CUS model, we examined the microglial alterations induced by 1-4 days of US exposure. At day 2 following US initiation, the number of DG microglia was significantly increased (Fig. 2a), and the cells assumed an activated morphology (Figs. 2b, 2c), characterized by increased soma area (Fig. 2d) and reduced processes length (Fig. 2e). At day 4 following US initiation, microglia number and soma size reverted whereas the processes length remained shorter (Figs. 2a,d,e). At 24 hr following the initiation of US exposure, the mRNA expression of the microglia activation markers Iba-1 and MHC-II was significantly increased, whereas the expression of the microglial CD200 receptor (CD200R), which serves to keep microglia in relative quiescence, was significantly decreased (Fig. 2f). To determine the mechanisms underlying the initial increase and subsequent decrease in microglia number we investigated the effects of US on microglial proliferation and consequent apoptosis. Following 2 days of US exposure, but not at later time points, microglia exhibited substantial proliferation (Fig. 2g-i), whereas following 3 days of US exposure microglia exhibited a markedly increased labeling with caspase-3 (Fig. 2j-k, Fig. S4), which is critically important for microglia activation, but is also considered the "executioner of apoptosis", whose expression usually reflects a commitment to cell death. Indeed, at 4 (but not at 2) days post US -initiation, the stressed mice displayed a substantial number of TUNEL-stained apoptotic microglia (Fig. 21-m), whereas not even a single TUNEL-stained microglia cell was encountered in control (non-stressed) mice.

EXAMPLE 3

Effects of minocycline, imipramine and IL-1 signaling blockade on stress-induced dynamic microglial alteration and development of depressive symptomatology

[0145] To examine the hypothesis that the initial stress-induced microglial activation is involved in the subsequent microglia decline and development of depressive behaviors, we examined the effects of pharmacological blockade of microglial activation using minocycline and genetic blockade of the microglia-stimulating cytokine interleukin (IL)-l, using mice with transgenic over-expression of brain IL-1 receptor antagonist (IL-lraTG mice). Because antidepressant drugs were previously shown to exert antiinflammatory effects, including suppression of microglial activation, we also examined the microglial modulating effects of the tricyclic antidepressant imipramine. Minocycline administration (40 mg/kg/day via the drinking water, beginning 2 days before US initiation) completely reversed the increase in hippocampal microglia following 2 days of US exposure (Fig. 3a). Moreover, less caspase-3-labeled and TUNEL-stained apoptotic microglia were observed in US-exposed mice treated with minocycline (as compared with water only) at 3-4 days following the initiation of US exposure (Fig. 3b). Minocycline administration beginning two days before and continuing throughout the 5-weeks CUS exposure period blocked the decline in hippocampal microglia number (Fig. 3c) and completely prevented the suppressive effects of CUS on sucrose preference (Fig. 3d), with a similar trend for social exploration (Fig. 3e) and neurogenesis (Fig. 3f).

[0146] Imipramine administration (20 mg/kg/day via the drinking water, beginning 2 days before US initiation) completely reversed the increase in hippocampal microglia following 2 days of US exposure (Fig. 3g). Imipramine administration beginning two days before and continuing throughout the 5-weeks CUS exposure period blocked the CUS-induced decline in hippocampal microglia number (Fig. 3h) as well as their processes length (Fig. 3i) and soma size (Fig. 3j). The imipramine treatment also prevented the CUS-induced reductions in sucrose preference (Fig. 3k) and social exploration (Fig. 31).

[0147] Following 24 hr of US exposure, the mRNA expression of IL-1 and IL-1 converting enzyme (ICE; the inflammasome-associated enzyme required for processing and activating IL-1 ) did not change, however the expression of IL-lra was decreased and the expression of IL-1 receptor type I (IL-1RI) was increased (Fig. 3m), suggesting an overall increase in IL-1 signaling. This increase is functionally important because after 2 days of US exposure, WT mice displayed an increase in the number of DG microglia, whereas IL-lraTG mice showed no change in microglia number (Fig. 3n). Furthermore, acute intra-hippocampal administration of IL-1 in WT mice mimicked the effects of short-term US on microglia number and morphology (Fig. S3). In contrast with the effects of short-term US, exposure to CMS in rats significantly decreased the mRNA expression of ICE, but had no effect on the expression of IL-1 , IL-lra and IL-1R, representing an overall decreased IL-1 signaling (Fig. 3o). The impaired IL-1 signaling in IL-lraTG mice was associated with a complete blockade of the CUS-induced decrease in hippocampal microglia number (Fig. 3p), as well as the suppression of sucrose preference (Fig. 3q) and social exploration (Fig. 3r).

EXAMPLE 4

Effects of microglia stimulation in chronically stressed ("depressed-like") mice on depressive-like symptoms and neurogenesis

[0148] To provide direct causal evidence for the involvement of microglia depletion in CUS-induced depressive-like symptoms and suppressed neurogenesis, we examined the possible anti-depressive effects of microglia stimulation in CUS -exposed mice, using two of the most well characterized microglia activators - LPS (endotoxin) and M-CSF. In the first experiment, following verification that CUS exposure produced the expected decreases in sucrose consumption and social exploration, the control and CUS-exposed mice were acutely administrated with either LPS (100 g/kg, i.p.) or saline. Four to five hr later, their behavior was assessed in the Porsolt forced swim test (an established test for depressive-like behavior in rodents), as well as the open field, social exploration and sucrose preference tests (the latter was conducted 24 hr post- injections). Whereas in control (non-stressed) mice LPS increased floating time (Fig. 4a) and decreased the latency to first float (Fig. 4b), in CUS-exposed mice it produced the opposite effects, completely reversing the depression/despair-like behaviors. Similarly, LPS administration reduced locomotor activity (Fig. S4a) in the open field test and the time spent in the center of the field (considered a measure of anxiety) (Fig. S4b) in control mice, whereas in CUS-exposed mice LPS increased locomotor activity and the time spent in the center of the filed, reversing the suppressive effects of CUS on these parameters. LPS administration did not significantly reverse the CUS -induced decreases in sucrose preference (Fig. S4c) and social exploration (Fig. S4d). LPS administration produced an overall increase in DG microglia number, completely normalizing the CUS-induced reduction in microglia number (Fig. 4c). Finally, whereas in control mice LPS produced a small decrease in the generation of BrdU- labeled (new) DG cells, in CUS-exposed mice it caused a dramatic increase in cell generation, completely reversing the CUS-induced suppression of this process (Fig. 4d).

[0149] In the second experiment, following verification that CUS exposure produced the expected decreases in sucrose consumption and social exploration, the control and CUS-exposed mice were injected daily (i.p.) with either M-CSF (100 g/kg) or saline for 5 days. Following the fifth injection, saline-treated CUS-exposed mice displayed elevated floating time in the Porsolt forced swim test and this effect was reversed by M-CSF (Fig. 4e). A similar pattern of results was obtained with respect to the latency to first float, but this finding did not reach statistical significance (Fig. 4f). Treatment with M-CSF, but not saline also completely reversed the suppressive effects of CUS on sucrose preference (Fig. 4g) and social exploration (Fig. 4h). M-CSF administration produced an overall increase in DG microglia number, completely normalizing microglia number in CUS-exposed mice (Fig. 4i), as well as an overall increase in microglia proliferation (Fig. 4j). Furthermore, CUS-exposed mice injected with M- CSF also displayed a marked increase in the generation of new (BrdU-labeled) DG cells that are not microglia (Fig. 4k).

[0150] In a third experiment, we tested the effects of imipramine and minocycline (which, as shown above, produce anti-depressive effects when administered throughout the CUS exposure period) in CUS-exposed mice (i.e., when the mice already exhibited depressive-like symptoms). GM-CSF, which is known to induce microglial activation, was used as a positive control. Following verification that CUS exposure produced the expected decreases in sucrose consumption and social exploration, animals were treated with the drugs for 2 weeks (imipramine and minocycline were administered continuously via the drinking water and GM-CSF was injected i.p. 3 time/week). The various treatments produced differential effects on DG microglia number (Fig. 41), reflected by a significant group difference. Specifically, whereas both water and minocycline-drinking CUS-exposed mice displayed significant reductions in DG microglia numbers (as compared with control non-stressed mice), imipramine-treated mice displayed no differences from any of the groups whereas GM-CSF-treated CUS- exposed mice displayed significantly greater number of microglia as compared with the control CUS-exposed mice (Fig. 41). Water- and minocycline- drinking CUS- exposed mice displayed significantly elevated floating time in the Porsolt forced swim test, whereas imipramine and GM-CSF-treated mice showed no difference from non- stressed controls (Fig. 4m). A similar pattern of results was obtained with respect to the latency to first float (Fig. 4n). Treatment with GM-CSF, but not minocycline or imipramine, completely reversed the suppressive effects of CUS on sucrose preference (Fig. 4o). In the social exploration test, all the CUS-exposed groups displayed reduced exploration time before the treatment, and further reductions were evident during the 2- weeks drug administration period in the water-, minocycline- and imipramine-treated animals. In contrast, treatment with GM-CSF induced an elevation in the social exploration time (Fig. 4p).

[0151] Discussion

[0152] The results indicate that chronic stress produces dynamic, bi-directional alterations in microglia status, including an initial phase of proliferation and activation followed by apoptosis, dystrophy and decline. The effects of chronic stress were at least partly selective to the hippocampus, suggesting that microglia decline contributes, probably along with reductions in neurogenesis, neuronal elements and astrocyte numbers, to the known decreases in the volume of this structure in depressed humans. The results further demonstrate that stimulation of microglia in CUS -exposed mice reversed their depressive-like neuro-behavioral symptomatology, providing the first direct causal evidence that disturbances in microglial functioning has an etiological role in psychopathology, in general, and in major depression, in particular (Fig. 5).

[0153] The model suggests that depression cannot be treated unitarily. Indeed, effective anti-depressive procedures should take the inflammatory/microglial status into account, and depending on whether these cells are activated or suppressed, either microglia suppressive agents (e.g., minocycline, IL-1 signaling blockers) or microglia activating drugs (e.g., M-CSF, GM-CSF) should be employed.

[0154] The finding that a relatively short-term exposure to unpredictable stress (US) results in microglia proliferation and activation is consistent with previous studies, demonstrating that exposure to acute tail-shock stress results in hippocampal microglia activation and repeated daily restraint for 4-21 days produces increased microglial number and activated morphology in hippocampus and prefrontal cortex. The results further suggest that the US -induced microglia proliferation and activation depend on signaling via the IL-1 receptor, as they did not occur in mice with transgenic over- expression of IL-lra. Furthermore, the results suggest that the initial US-induced microglia activation was responsible for the subsequent microglial apoptosis and decline, as well as the development of depressive symptomatology and suppressed neurogenesis, because all of these effects were also prevented by minocycline or IL-lra over-expression, whereas minocycline administration to mice with established CUS- induced depressive-like behavior had no beneficial effect. Similarly to the effects of minocycline, the anti-depressive effects of imipramine also seem to depend at least partly on suppression of the initial microglial activation and the subsequent microglial decline, consistently with the known acute anti-inflammatory effects of antidepressant drugs, because imipramine administration to mice with established CUS -induced depressive-like behavior produced minimal beneficial effects.

[0155] The finding that stress-induced proliferation and activation was followed by microglial apoptosis corroborates the results of a previous study demonstrating the same phenomenon using flow cytometric methodology, although no apoptosis was found in other studies on the effects of repeated restraint stress on microglial status. Interestingly, the apoptosis-related molecule caspase-3, which in the present study was found to be elevated three days post-US initiation, has recently been shown to be critically involved in regulating microglia activation (consistently with other indices of microglia activation at this time, see Fig. 2). Caspase-3 elevation was shown to lead to microglial apoptosis in some studies, but not in others. Similar inconsistent findings also characterize the effects of various chronic stress procedures on inflammatory cytokines, with some studies reporting elevated brain cytokine levels, other studies reporting deceased levels and yet additional studies reporting no effects of chronic stressors on inflammatory cytokines levels. Together with the results of the present study, these findings suggest that various stress procedures produce either activation or decline of microglia number and functioning, as well as dynamic transitions between activation and decline, possibly depending on various parameters of the stress paradigm (e.g., type, severity, duration/chronicity) and other variables such as the subjects' genetics and environmental conditions. Based on the finding that the microglia decline occurred at a later phase of stress exposure than did microglia activation, it may be suggested that in humans microglia decline accounts for depression associated with chronic stressors such as poverty, medical disabilities, or lasting marital discord, which are indeed associated more strongly with depressive symptomatology than are acute stressors such as specific negative life events.

[0156] Over the last decade it has become evident that microglia are not merely idle cells, waiting to be activated by injury or infection, but rather are active and motile in their "quiescent" state, interacting with neurons and other cellular components, and participating in physiological processes such as synaptic formation and pruning. These findings are consistent with the emerging notion that immune-like processes play an important modulatory role in brain and behavioral functioning. However, despite the evolving literature on the physiological role of "quiescent" microglia, there is almost no data on their direct contribution to the regulation of behavioral/psychological processes.

[0157] The findings of the present study provide one of the first direct demonstrations that suppression of microglia activation status and reduction in their number can produce detrimental neuro -behavioral effects, in contrast with previous neurological and psychiatric literature, which implicated only microglial activation in neuro- and psychopathology. [0158] Administration of LPS to CUS-exposed mice, which increased hippocampal microglia number to normal levels, partially reversed the depressive-like behavioral symptoms and suppressed neurogenesis. Interestingly, in accordance with previous studies LPS produced the opposite effects, i.e., depressive like behavior and suppressed neurogenesis in non-stressed mice, emphasizing the importance of studying the properties of anti-depressive procedures using a chronic model of depression. The fact that LPS only partially reversed the CUS-induced depressive-like effects, despite completely reversing the effects of CUS on microglia and neurogenesis, may be related to the acute nature of this experiment, considering that LPS was injected only once and behavioral measurements were taken several hours before the animals were sacrificed (allowing more time for LPS to produce its effects on microglia and neurogenesis). In any case, the partial anti-depressive effects of LPS in CUS-exposed mice corroborate and explain the findings of a previous clinical study demonstrating a beneficial, anti- depressive effect of LPS in severely depressed patients. M-CSF can be more translationally relevant than LPS because it did not cause depressive symptoms in non- stressed mice, and it completely reversed the depressive symptoms after a few days of treatment. GM-CSF (also termed sargramostim) may be also translationally relevant because it has been marketed under the trade names Leukine® (Genzyme) and Leucomax® (Novartis) for more than two decades, mainly for replenishment of white blood cells following chemotherapy, acceleration of white blood cell recovery following autologous bone marrow transplantation. Thus, M-CSF, its derivatives and related molecules (e.g., IL-34), as well as GM-CSF and other microglia stimulators can serve as fast-acting anti-depressants, specifically in patients with major depression and stress-related disorders who exhibit normal or low levels of inflammatory markers.

[0159] Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.