FIELD OF INVENTION

The present invention relates to methods for assessing cell aging and for screening compounds slowing down or accelerating cell aging (ie. anti-aging or pro- senescence drugs respectively). In particular, the present invention relates to a cell surface nutrient transporter, in particular XPR1, as a biomarker of cell aging.

BACKGROUND OF INVENTION Regenerative medicine refers to the process of creating living, functional cells or tissues to repair and replace cells, tissue or organ function lost due to age, disease, damage or congenital defects. It may include replacing the damaged elements, but more usually corresponds to the administration of cells which will proliferate and/or differentiate in vivo in order to repair and/or replace the damaged tissue or organ. Regenerative medicine thus allows repair of previously irreparable tissues or organs, while overcoming the problem of the lack of organs available for transplantation.

Stem cells are defined by their self-renewal property (i.e. their ability to go through numerous cycles of cell division (mitosis) while maintaining an undifferentiated state) as well are their ability to generate differentiated cell types. Stem cells may be classified according to their potency, i.e. their differentiation potential. Totipotent stem cells may differentiate into all cell types and thereby construct a complete viable organism. They result from the fusion of an egg and sperm cell. Cells resulting from the differentiation of totipotent cells are pluripotent cells: these may differentiate in any cell types of the three germ layers. Multipotent stem cells may differentiate into a limited number of cell types, generally of the progeny of the tissue of their location. Oligopotent stem cells may only differentiate into a few cell types, such as, for example, lymphoid or myeloid stem cells. Finally, unipotent stem cells may generate only their own cell type.

On the contrary to embryonic stem cells, adult stem cells are found in differentiated tissues throughout the body after development. Sources of adult stem cells include, for example, bone marrow, blood, cornea and retina, brain, skeletal muscle, dental pulp, liver, skin, gastrointestinal tract or pancreas.

Among stem cells, mesenchymal stem cells (MSC) are multipotent stem cells that can readily differentiate into lineages including osteoblasts, myocytes, chondrocytes and adipocytes. The clinical potential of adult MSC has been recently fully documented, and these cells are currently used in hundreds of clinical trials over the world. For therapy, two main sources of MSC may be used. First, cells may be autologous to the subject to be treated: cells are harvested from said subject, grown in specific culture condition to selectively induce differentiation and further re-administered to the subject. Second, allogenic MSC may be used, in order to reduce the time and cost linked to the preparation of cells to be administered to the subject. Therefore, banks of allogenic cells have to be built and amplified in culture for use in therapy.

However, even if adult stem cells present self-renewal ability and potency, these properties are not endless. Indeed, as culture passages accumulate, the doubling time of cells tends to increase, until reaching a non-dividing state, usually called senescence. Moreover, the differentiation potential of stem cells decreases with passages. In addition, significant decreased MSC expansion and differentiation potential were observed with increasing donor age.

There is thus a need for systems for assessing the proliferative capacity and differentiation potential of MSC, i.e. for assessing cell aging of cells. Such system may thus allow checking the therapeutic potential of cells before administration to patients.

The US patent US 8,574,852 describes a method for evaluating cell aging by measuring the expression level of cofilin. Cofilin is an intracellular protein that can bind to actin filaments and promote their dynamics for motility, development, polarity or cytokinesis. Inventors demonstrated a correlation between expression level of cofilin in a target cell and the cellular age of said target cell.

Moreover, the European patent application EP 2 533 042 describes the detection of PW1 for monitoring cell aging. However, the assays of the prior art for assessing cell aging usually involve detection of intracellular proteins, which are thus not directly detectable. Indeed, permeabilization of the cell is required, thus increasing the complexity of the detection method. There is thus a need for a marker of cell aging, which is expressed on cell surface and whose expression is dependent on the age of said cell. Moreover, there is still a need for an early biomarker of cell aging, i.e. a biomarker whose expression, or change in expression, may be detected early in cell aging, in particular before the occurrence of any sign of senescence.

In the present invention, the Inventors demonstrated that the expression level of the XPR1 cell surface nutrient transporter correlates with cell aging. The present invention thus relates to the use of XPR1 as a biomarker (in particular as an early biomarker) of cell aging, and to the use of XPR1 in screening assays aiming at identifying compounds reversing, slowing-down or accelerating cell aging (ie. anti-aging or pro-senescence drugs respectively).

SUMMARY

The present invention thus relates to an in vitro method for assessing, evaluating, monitoring and/or predicting cell aging of a cell, wherein said method comprises measuring the expression level of at least one cell surface nutrient transporter on said cell. In one embodiment, the method of the invention further comprises comparing the measured expression level with a reference expression level.

In one embodiment, said at least one cell surface nutrient transporter is XPR1 and/or GLUT1, preferably XPR1. In one embodiment, said cell is an animal cell, preferably a human or equine cell. In another embodiment, said cell is a stem cell, preferably an adult stem cell, more preferably a MSC, provided that said cell is not a human embryonic stem cell.

In one embodiment, said expression level is assessed at the RNA level, preferably by RT-PCR, RT-qPCR, Northern Blot and/or hybridization techniques. In another embodiment, said expression level is assessed at the protein level, preferably the measurement of the expression level of at least one cell surface nutrient transporter corresponds to the detection and quantification of said at least one cell surface nutrient transporter on the cell surface. In one embodiment, said detection and quantification of at least one cell surface nutrient transporter on the cell surface corresponds to detecting and/or quantifying binding of a ligand to a cell surface nutrient transporter, preferably, said ligand is an antibody or is a receptor binding domain ligand (RBD) comprising a part or the totality of a receptor binding domain (RBD) derived from the soluble part of a glycoprotein of an enveloped virus. In one embodiment, said RBD is Xeno.RBD, and comprises or consists of the amino acid sequence SEQ ID NO: 1 or fragments thereof.

In one embodiment, said method is for assessing the quality of a batch of cells, preferably wherein said batch of cells is to be used in regenerative medicine or for in vitro screening assay.

The present invention also relates to the use of a cell surface nutrient transporter, preferably XPR1 and/or GLUT1, as a biomarker of cell aging or senescence, and/or as a biomarker of the accumulation of cell divisions, preferably as a biomarker of the proliferation capacity and/or of the differentiation capacity of a cell, or as a biomarker of sternness of a cell, or as a biomarker of the quality of a cell batch, in particular of a cell batch to be used in regenerative medicine or for in vitro screening assay. Another object of the invention is a screening method for identifying compounds impacting cell aging, wherein said screening method comprises determining or measuring the expression level of at least one cell surface nutrient transporter. In one embodiment, said at least one cell surface nutrient transporter is XPR1 and/or GLUT1, preferably XPR1. The present invention further relates to a kit for implementing the in vitro method of the invention or the screening method according of the invention, wherein said kit comprises means for determining or measuring the expression level of at least one cell surface nutrient transporter. In one embodiment, said means for determining or measuring the expression level of at least one cell surface nutrient transporter is a couple of PCR primer specific for said at least one cell surface nutrient transporter, an antibody specific for said at least one cell surface nutrient transporter and/or a RBD specific for said at least one cell surface nutrient transporter.

DEFINITIONS

In the present invention, the following terms have the following meanings:

"Cell aging" refers to the progressive modification of the phenotype of a cell due to the successive cell divisions and/or to environmental conditions (such as, for example, culture medium composition, oxygen concentration and the like). In particular, as used herein, the term "cell aging" refers to the significant alteration of the phenotype of the cell, reflected by the loss of cell capacities. For MSC, in particular, cell aging may refer to the loss of the differentiation capacity (such as, for example, the progressive loss of the capacity to differentiate into the adipogenic, osteogenic, myogenic and/or chondrogenic lineage(s)), and/or to the loss of proliferation capacity (characterized ultimately by the complete incapacity of the cell to actively divide). In one embodiment, the term "cell aging" refers to the loss of the differentiation capacity of a MSC, i.e. a MSC overcoming cell aging has lost the capacity to differentiate into one (or at least one) lineage selected from the adipogenic, osteogenic, myogenic and chondrogenic lineage. In one embodiment, the MSC overcoming cell aging has lost the capacity to differentiate into two lineages selected from the adipogenic, osteogenic, myogenic and chondrogenic lineage. In another embodiment, the MSC overcoming cell aging has lost the capacity to differentiate into three lineages selected from the adipogenic, osteogenic, myogenic and chondrogenic lineage (preferably into the adipogenic, the osteogenic and the chondrogenic lineages). In another embodiment, the MSC overcoming cell aging has lost the capacity to differentiate into the adipogenic, osteogenic, myogenic and chondrogenic lineages. In one embodiment, cell aging results in senescence, and an "aged cell" is a cell entered in senescence, i.e. presenting signs of senescence (such as, for example, late signs of senescence listed thereafter). In one embodiment, the term "cell aging" refers to the loss of the proliferation capacity of a MSC, preferably, the number of cell division before the MSC enters in senescence is at most 20, more preferably is at most 15, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 cell division(s).

"Senescence" or "replicative senescence" as used herein refers to cellular senescence, wherein cellular senescence refers to a cellular state characterized by morphological changes (such as, for example, enlarged and irregular cell shapes) and ultimately a stop of proliferation. Senescence is a complex phenomenon, whose causes are probably multiple and still largely unknown. In particular, senescence may imply shortening of telomeres or modified telomere structure, as well as accumulation of DNA damages. Examples of late signs of senescence include, but are not limited to, increased doubling time; morphological changes that may be easily detected by microscopy (such as, for example, larger cells, larger nucleus, irregular shape, flat shape, granular cytoplasm and the like); loss of the potential to differentiate along the adipogenic, osteogenic, myogenic and/or chondrogenic lineages, increased expression of lysosomal β-galactosidase or of pH6 β- galactosidase (SA-P-gal); modification of the transcriptome of the cell, including, without limitation, up-regulation of the expression of at least one gene selected from the list comprising human glycoprotein NMB, regeneration- associated muscle protease homolog (RAMP), p53 apoptosis effector related to PMP-22 (PERP), lymphocytes antigen 96 (LY96), signal transducer and activator of transcription 1 (STAT1), prion protein (PRNP), cyclin-dependent kinase inhibitor 2A and plasminogen activator inhibitor type 1, or down-regulation of the expression of at least one gene selected from the list comprising hyaluronic acid synthetase 1 (HAS1), inhibitor of DNA binding 1 (ID1) and osteoprotegrin ligand (TNFSF11); or up-regulation of micro-RNA, such as, for example, micro-RNAs selected from the group comprising has-mir-371, has-mir-369-5P, has-mir-29c, has-mir-499 and has- let-7f.

"Sternness" is a term referring to the common properties of stem cells, such as, for example, their self -renewal property (i.e. their ability to go through numerous cycles of cell division (mitosis) while maintaining an undifferentiated state) as well are their ability to generate differentiated cell types.

"Proliferation capacity" or "proliferation potential" (that may be used interchangeably): refers to the capacity of a cell to actively divide.

"Differentiation capacity" or "differentiation potential" (that may be used interchangeably): refers to the capacity of a cell to differentiate in at least one lineage. In one embodiment, the cell is MSC and the differentiation capacity of potential refers to the capacity of said MSC to differentiate into at least one of the osteogenic, myogenic, chondrogenic or adipogenic lineages; to differentiate into the three following lineages: osteogenic, chondrogenic and adipogenic lineages; or to differentiate into all of these 4 lineages.

"XPRl": refers to a phosphate exporter expressed by metazoans, in particular by humans, used as receptor by xenotropic murine leukemia virus (MLV), polytropic MLV and xenotropic murine leukemia virus-related virus (XMRV) (Giovannini et al, Cell Reports 3, 1866-1873, 2013). In one embodiment, XPRl is human XPRl (accession number AAH41142, SEQ ID NO: 21) encoded by SEQ ID NO: 22

(accession number BC041142.1). In one embodiment XPRl comprises or consists of an amino acid sequence presenting a sequence identity of at least 70% with SEQ ID NO: 21, preferably a sequence identity of at least 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99% or more with SEQ ID NO: 21. In one embodiment XPRl is encoded by a nucleotide sequence presenting a sequence identity of at least 70% with SEQ ID NO: 22, preferably a sequence identity of at least 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99% or more with SEQ ID NO: 22. In one embodiment, XPRl comprises or consists of a fragment of SEQ ID NO: 21, preferably a fragment of at least about 100 amino acids, more preferably of at least about 150, 200, 250, 300, 350, 400, 450, 500, 550 or 600 amino acids. As used herein; the term "identity", when used in a relationship between the sequences of two or more polypeptides or of two or more DNA sequences, refers to the degree of sequence relatedness between polypeptides or DNA sequences (respectively), as determined by the number of matches between strings of two or more amino acid residues or of two or more nucleotides respectively. "Identity" measures the percent of identical matches between the smaller of two or more sequences with gap alignments (if any) addressed by a particular mathematical model or computer program (i.e., "algorithms"). Identity of related polypeptides or DNA sequences can be readily calculated by known methods. Such methods include, but are not limited to, those described in Computational Molecular Biology, Lesk, A. M., ed., Oxford University Press, New York, 1988; Biocomputing: Informatics and Genome Projects, Smith, D. W., ed., Academic Press, New York, 1993; Computer Analysis of Sequence Data, Part 1, Griffin, A. M., and Griffin, H. G., eds., Humana Press, New Jersey, 1994; Sequence Analysis in Molecular Biology, von Heinje, G., Academic Press, 1987; Sequence Analysis Primer, Gribskov, M. and Devereux, J., eds., M. Stockton Press, New York, 1991; and Carillo et al., SIAM J. Applied Math. 48, 1073 (1988). Preferred methods for determining identity are designed to give the largest match between the sequences tested. Methods of determining identity are described in publicly available computer programs. Preferred computer program methods for determining identity between two sequences include the GCG program package, including GAP (Devereux et al., Nucl. Acid. Res. \2, 387 (1984); Genetics Computer Group, University of Wisconsin, Madison, Wis.), BLASTP, BLASTN, and FASTA (Altschul et al., J. Mol. Biol. 215, 403-410 (1990)). The BLASTX program is publicly available from the National Center for Biotechnology Information (NCBI) and other sources (BLAST Manual, Altschul et al. NCB/NLM/NIH Bethesda, Md. 20894; Altschul et al., supra). The well-known Smith Waterman algorithm may also be used to determine identity.

"GLUT1": refers to a glucose importer expressed by metazoans, in particular by humans, used as receptor by Human T Leukemia viruses (HTLV) in particular. In one embodiment, GLUT1 is human GLUT1 (accession number NP_006507.2, SEQ ID NO: 23) encoded by SEQ ID NO: 24 (accession number NM_006516.2). In one embodiment GLUT1 comprises or consists of an amino acid sequence presenting a sequence identity of at least 70% with SEQ ID NO: 23, preferably a sequence identity of at least 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99% or more with SEQ ID NO: 23. In one embodiment GLUT1 is encoded by a nucleotide sequence presenting a sequence identity of at least 70% with SEQ ID NO: 24, preferably a sequence identity of at least 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99% or more with SEQ ID NO: 24. In one embodiment, GLUT1 comprises or consists of a fragment of SEQ ID NO: 23, preferably a fragment of at least about 100 amino acids, more preferably of at least about 150, 200, 250, 300, 350, 400 or 450 amino acids.

"Ligand" refers to any substance that forms a complex with a cell surface nutrient transporter. Typical ligands include, but are not limited to, polypeptides and proteins. As used herein, a polypeptide refers to a linear polymer of amino acids (preferably at least 50 amino acids) linked together by peptide bonds. A protein specifically refers to a functional entity formed of one or more polypeptides, and optionally of non-polypeptides cof actors.

"About" preceding a figure means plus or less 10% of the value of said figure.

DETAILED DESCRIPTION

The present invention relates to a method, preferably an in vitro method for assessing, evaluating, monitoring and/or predicting cell aging of a cell, wherein said method comprises determining or measuring the expression level of at least one cell surface nutrient transporter on said cell. In one embodiment, said method further comprises comparing the measured expression level with a reference expression level.

With time, the doubling time of cells tends to increase, and the differentiation potential tends to decrease. Moreover, significant decreased MSC expansion and differentiation potential have been observed with increasing donor age. Consequently, in one embodiment, the method of the invention is for measuring, assessing or determining the proliferation potential of a cell. In a particular embodiment, the method of the invention is for determining the number of culture passages or of cell divisions before the occurrence of senescence, or before the occurrence of first signs of senescence, in particular before the absence of cell division, or before the occurrence of late signs of senescence.

Examples of late signs of senescence include, but are not limited to, absence of cell division or increased doubling time (for example an increase of at least about 20%, preferably of at least about 30, 40, or 50% of the doubling time of the cell); morphological changes that may be easily detected by microscopy (such as, for example, larger cells, larger nuclei, irregular shape, flat shape, granular cytoplasm and the like); increased levels of lysosomal beta-galactosidase (which may be measured, for example, using a senescence-associated beta-galactosidase staining kit, such as the one provided by Cell Signaling, USA); increased expression of lysosomal β-galactosidase or of pH6 β-galactosidase (SA-P-gal) which may be detected by methods known by the skilled artisan, such as, for example, RT-qPCR (detection of expression at the mRNA level) or Western Blot (detection of expression at the protein level); modification of the transcriptome of the cell that may be easily assessed (such as, for example, by RT- PCR), including, without limitation, up-regulation of the expression of at least one gene selected from the list comprising human glycoprotein NMB, regeneration-associated muscle protease homolog (RAMP), p53 apoptosis effector related to PMP-22 (PERP), lymphocytes antigen 96 (LY96), signal transducer and activator of transcription 1 (STAT1), prion protein (PRNP), cyclin-dependent kinase inhibitor 2A and plasminogen activator inhibitor type 1, or down-regulation of the expression of at least one gene selected from the list comprising hyaluronic acid synthetase 1 (HASl), inhibitor of DNA binding 1 (ID1) and osteoprotegrin ligand (TNFSF11); up-regulation of micro- RNA that may be easily assessed (such as, for example, by RT-PCR), such as, for example, micro-RNAs selected from the group comprising has-mir-371, has-mir-369- 5P, has-mir-29c, has-mir-499 and has-let-7f; and loss of the potential to differentiate into specific lineages, such as, for example, the adipogenic, osteogenic, myogenic and/or chondrogenic lineages. Methods for assessing the potential of a MSC to differentiate along the adipogenic, osteogenic and/or chondrogenic lineages are well known of the skilled artisan.

A non-limiting example of a method for assessing the potential of a MSC to differentiate along the adipogenic pathway is the following: cells were plated at 2.10 ⁴ cells/cm ² and cultured in DMEM with 10% FCS, 0.5 mM isobutylmethylxanthine (IBMX), ΙμΜ dexamethasone, 10 μΜ insulin, 200 μΜ indomethacine, and Oil Red-0 staining was performed after 21 days and analyzed semi-quantitatively at λ595 nm using a plate reader.

A non-limiting example of a method for assessing the potential of a MSC to differentiate along the osteogenic pathway is the following: cells are plated at 2.10 ⁴ cells/cm ² and cultured for 3 weeks in DMEM with 10% FCS (Invitrogen), 10 mM β-glycerophosphate, 10 ^"7 M dexamethasone, and 0.2 mM ascorbic acid and with medium changes every 3 to 4 days. After 21 days, cells are analyzed by Alcaline phosphatase von Kossa or Alizarin red staining. Alizarin red staining is semi- quantitatively analyzed at λ595 nm using a plate reader.

A non-limiting example of a method for assessing the potential of a MSC to differentiate along the chondrogenic pathway is the following: a pellet of 2.2.10 ⁵ cells is cultured in a differentiation medium (such as, for example, OriCell Mesenchymal Stem Cell Chondrogenic Differentiation Medium (Cyagen) or Mesenchymal Stem Cell Chondrogenic Differentiation Medium (Promocell)) for 3 weeks with subsequent assessment of acid mucopolysaccharides by 1% Alcian blue (Chroma, Kongen, Germany) for 10-30 minutes.

Moreover, in one embodiment, the method of the invention is for measuring, assessing or determining the differentiation potential of a cell. In a particular embodiment, the method of the invention is for determining the number of culture passages or of cell divisions that the cell may undergo before the disappearance of the differentiation potential of said cell. For example, the method of the invention may be for determining the number of culture passages or of cell divisions before the loss of the capacity to differentiate into one particular lineage (adipogenic, osteogenic, myogenic or chondrogenic lineages), into the three following lineages: adipogenic, osteogenic and chondrogenic lineages, or the complete loss of the capacity of differentiation of the cell (i.e. the loss of the capacity to differentiate into the adipogenic, osteogenic, myogenic and chondrogenic lineages). In one embodiment, the cell is an animal cell, such as, for example, a mammal cell, such as a rodent cell, a feline cell, a canine cell, an equine cell or a primate cell, preferably a human cell.

In one embodiment, the cell is a stem cell, preferably an adult stem cell, or a derivative thereof. Examples of adult stem cells include, but are not limited to, hematopoietic stem cells, mammary stem cells, intestinal stem cells, mesenchymal stem cells (MSC), endothelial stem cells, neural stem cells, olfactory adult stem cells, neural crest stem cells, testicular stem cells and muscle stem cells. In one embodiment, adult stem cells originate from bone marrow, mammary gland, intestine (in particular from the crypts of Lieberkuhn), placenta, adipose tissue, lung, blood, Wharton's jelly from umbilical cord, teeth, brain (such as, for example, from the subventricular zone, dentate gyrus or neocortex), nose (as olfactory adult stem cells may efficiently be harvested from olfactory mucosa cells from the lining of the nose), hair follicles, gastrointestinal tract, sciatic nerve, cardiac outflow tract, spinal and sympathetic ganglia or testicles.

In one embodiment, the stem cell is a multipotent stem cell, such as, for example, a MSC or a derivative thereof, such as, for example, osteocytes, chondrocytes, myocytes (e.g. cardiomyocytes) or adipocytes. In one embodiment, the MSC originate from bone- marrow, umbilical cord blood, Wharton's jelly (such as, for example, Wharton's jelly found within the umbilical cord), placenta, lung, adipose tissue, adult muscle, corneal stroma, teeth (such as, for example, from dental pulp of deciduous baby tooth), amniotic fluid, peripheral blood or the like. In one embodiment, the cell is hESC-SAOOl derived MSC. In another embodiment, the cell is human primary MSC isolated and/or derived from bone marrow. In another embodiment, the cell is equine MSC.

In another embodiment, the stem cell is a unipotent stem cell or a derivative thereof, such as, for example, a keratinocyte. In one embodiment, the cell is derived from a pluripotent stem cell such as an embryonic stem cell or an iPSC, wherein iPSC stands for induced pluripotent stem cell. iPSC is a type of pluripotent stem cells that can be generated directly from adult cells either by exposure to certain chemicals (such as, for example, valproic acid, BIX- 01294, DZNep, SB431412, PD0325901, thiazovivin and mixtures thereof) and/or through transfection or transduction (for example of the set of genes Oct4 (Pou5fl), Sox2, cMyc, and Klf4).

According to an embodiment, the cell is not a human embryonic stem cell, and/or the recovering of the cell does not require the destruction of a human embryo. In one embodiment, the cell is not a neuronal cell. In another embodiment, the cell is not a glial cell.

In one embodiment, the cells are cells recovered from a patient having a progeroid syndrome, preferably progeria cells. Progeroid syndromes are a group of rare genetic disorders that resemble to premature aging, a definition that can apply to a broad range of diseases. Familial Alzheimer's disease and familial Parkinson's disease are two well- known accelerated-aging diseases that are more frequent in older individuals while progeria (also referred as Hutchinson-Gilford Progeria Syndrome (HGPS)) is a very rare progressive disorder of childhood. It is characterized by features of premature aging (progeria), failure to thrive usually beginning in the first year of life resulting in short stature and low weight, deterioration of the layer of fat beneath the skin (subcutaneous adipose tissue), and characteristic craniofacial abnormalities, including frontal bossing, underdeveloped jaw (micrognathia), unusually prominent eyes and/or a small, "beaklike" nose.

In one embodiment, the cells are cells in culture, preferably are cell lines and/or are derived from primary cells, i.e. cells isolated straight from the tissue. In one embodiment, the cell is recovered from a sample from an individual, obtained for example by biopsy. Preferably, the step of recovering a sample from an individual is not part of the method of the present invention. According to an embodiment, the method of the invention is thus for assessing the quality of a cell line or of primary cells in culture, and may comprise assessing the proliferation potential and/or the differentiation potential of the cells at a specific time. According to this embodiment, the method of the invention may thus correspond to a quality control method, aiming at checking the quality of a cell bank or batch. Said method may be useful, for example, for checking the proliferation and differentiation potentials of stem cells to be used in regenerative therapy, or for checking the proliferation and differentiation potentials of cells to be used in in vitro screening assays.

As used herein, the term "cell surface nutrient transporter" refers to a nutrient transporter anchored in the plasma membrane of a cell. Mammalian cells take up necessary nutrients via "nutrient transporters" on the cell surface and expel catabolites and other components. Nutrients and metabolites or catabolites are, for example, carbohydrates, amino acids, inorganic phosphate, nucleosides, lipids, vitamins, heme, ions etc. Nutrient transporters may be divided based on passive or active mechanisms of function. Passive (or facilitated) transporters allow diffusion of solutes across membranes down their electrochemical gradient. Active transporters create solute gradients across membranes, utilizing diverse energy-coupling mechanisms, such as, for example, ATP synthesis or hydrolysis. In one embodiment, the cell surface nutrient transporter belongs to the SLC series, wherein SLC stands for Solute Linked Carriers.

Examples of cell surface nutrient transporters include, but are not limited to, transporters of glucose, such as, for example, glucose importers (such as, for example, GLUT1); transporters of inorganic phosphate, such as, for example, inorganic phosphate importers (such as, for example, PiTl or PiT2) or inorganic phosphate exporters (such as, for example, XPR1); transporters of amino acids, such as, for example, transporters of neutral amino acids (such as, for example, neutral amino acids importers (such as, for example, ASCT1 or ASCT2)), or transporters of cationic amino acids (such as, for example, CAT1); transporters of heme (such as, for example, FLVCR1); transporters of inositol, such as, for example, transporters of myo-inositol (such as, for example, SMIT1); and transporters of riboflavin, such as, for example, importers of riboflavin (such as, for example, RFT1, RFT3, PARI or PAR2).

In one embodiment, the cell surface nutrient transporter is a transporter of inorganic phosphate, such as, for example, an inorganic phosphate exporter (such as, for example, XPR1) or a transporter of glucose, such as, for example, a glucose importer (such as, for example, GLUT1).

In one embodiment, the method of the invention comprises measuring the expression level of XPR1 and/or of GLUT1. Preferably, the method of the invention comprises measuring the expression level of XPR1. In one embodiment, the method of the invention comprises measuring the expression level of XPR1 and of GLUT1.

In another embodiment, the at least one cell surface nutrient transporter does not consists in GLUT1, GLUT3 and/or GLUT4. In another embodiment, the at least one cell surface nutrient transporter does not consists in (a) glucose transporters.

As used herein, the term "expression" may refer alternatively to the transcription of a cell surface nutrient transporter (i.e. expression of the RNA) or to the translation (i.e. expression of the protein) of a cell surface nutrient transporter, or to the presence of the cell surface nutrient transporter at the surface of the cell.

Methods for determining the expression level are well-known from the skilled artisan, and include, without limitation, determining the transcriptome (in an embodiment wherein expression relates to transcription of a nutrient transporter) or proteome (in an embodiment wherein expression relates to translation of a nutrient transporter) of a cell.

In one embodiment of the invention, the expression of the cell surface nutrient transporter is assessed at the RNA level. Methods for assessing the transcription level of a transporter are well known in the prior art. Examples of such methods include, but are not limited to, RT-PCR, RT-qPCR, Northern Blot, hybridization techniques such as, for example, use of microarrays, and combination thereof including but not limited to, hybridization of amplicons obtained by RT-PCR, sequencing such as, for example, next-generation DNA sequencing (NGS) or RNA-seq (also known as "Whole Transcriptome Shotgun Sequencing") and the like.

Examples of PCR or qPCR primers that may be used for assessing the expression of XPR1 include, but are not limited to, the following couple of primers: Forward primer: 5 ' - AGAGCTTGGGAG ACAAAGC A-3 ' (SEQ ID NO: 25) - Reverse primer: 5'- GTGGACACAACATTCGCAAC-3 ' (SEQ ID NO: 26).

Examples of PCR or qPCR primers that may be used for assessing the expression of GLUT1 include, but are not limited to, the following couple of primers: Forward primer: 5 ' -TCACTGTGCTCCTGGTTCTG-3 ' (SEQ ID NO: 27) - Reverse primer: 5'- CCTCGGGTGTCTTGTCACTT-3 ' (SEQ ID NO: 28).

In one embodiment of the invention, the expression of the cell surface nutrient transporter is assessed at the protein level. Methods for determining a protein level in a sample are well-known in the art. Examples of such methods include, but are not limited to, immunohistochemistry, Multiplex methods (Luminex), western blot, enzyme-linked immunosorbent assay (ELISA), sandwich ELISA, fluorescent-linked immunosorbent assay (FLISA), enzyme immunoassay (EIA), radioimmunoassay (RIA), flow cytometry (FACS) and the like.

In one embodiment of the invention, determining the expression level of a cell surface nutrient transporter specifically corresponds to the detection and quantification of said nutrient transporter present on the cell surface. Methods for analyzing the presence of a protein on the cell surface are well-known to the skilled artisan and include, without limitation, FACS analysis, immunohistochemistry, western blot associated with cell fractionation, enzyme-linked immunosorbent assay (ELISA), sandwich ELISA, fluorescent-linked immunosorbent assay (FLISA), enzyme immunoassay (EIA), radioimmunoassay (RIA) or image analysis, for example high content analysis and the like.

In one embodiment, determining the expression level of at least one cell surface nutrient transporter corresponds to detecting and/or quantifying binding of a ligand to a cell surface nutrient transporter. Preferably, said ligand is a receptor binding domain ligand and the method of the invention comprises detecting and/or quantifying a complex formed between said receptor binding domain ligand and a cell surface nutrient transporter. In another embodiment, said ligand is an antibody specific of said cell surface nutrient transporter, and the method of the invention comprises detecting and/or quantifying a complex formed between said antibody and said cell surface nutrient transporter.

The expression "detecting and/or quantifying binding of a ligand, such as, for example, a receptor binding domain ligand, to a cell surface nutrient transporter" means that when a cell surface nutrient transporter is present a complex is formed between the nutrient transporter and the ligand. That complex can be detected if the ligand has been for example, but not limited to, covalently coupled with a detectable molecule such as an antibody constant fragment (Fc) or a fluorescent compound (e.g. Cyanine dye, Alexa dye, Quantum dye, etc). The complex can also be detected if the ligand has been tagged with different means well known to the person skilled in the art. For example, but without limitation, a tag used in the invention can be a tag selected from the group comprising or consisting of Hemaglutinin Tag, Poly Arginine Tag, Poly Histidine Tag, Myc Tag, Strep Tag, S-Tag, HAT Tag, 3x Flag Tag, Calmodulin-binding peptide Tag, SBP Tag, Chitin binding domain Tag, GST Tag, Maltose-Binding protein Tag, Fluorescent Protein Tag, T7 Tag, V5 Tag and Xpress Tag. The use of the ligand therefore allows on the one hand the identification and detection of the cell surface nutrient transporter depending on the ligand used, and on the other hand the quantification of the complex formed.

In one embodiment, detecting or quantifying binding is conducted by flow cytometry, immunofluorescence or image analysis, for example high content analysis. In a further aspect of the invention, the ligand is a receptor binding domain ligand, wherein said receptor binding domain ligand comprises a part or the totality of a receptor binding domain (RBD) derived from the soluble part of a glycoprotein of an enveloped virus that interacts with a cell surface nutrient transporter. Preferably, the ligand is soluble, i.e. it does not comprise a transmembrane domain, and is therefore not anchored to a membrane. The expression "derived from the soluble part of the glycoprotein of an enveloped virus" means that the ligand is a fragment or a part of a glycoprotein contained in the envelope of a virus and can be obtained, for example, by cloning.

The term "glycoprotein" is to be understood as meaning an envelope glycoprotein, a coat glycoprotein or a fusion glycoprotein", wherein the term "glycoprotein" refers to a protein containing oligosaccharide chains covalently attached to polypeptide side- chains.

The expression "that interacts with a cell surface nutrient transporter" means that the glycoprotein is liable to recognize a receptor present on the surface of the cell. In one embodiment, a ligand that interacts with a cell surface nutrient transporter will thus form a complex with said cell surface nutrient transporter, which complex may be detected by a method as hereinabove described.

The receptor binding domain ligand containing part or the totality of the RBD can be fused to an antibody constant fragment (such as, for example, Fc fragment from rabbit or from mouse), and/or chemically modified to add a fluorochrome, or a fluorescent compound (e.g. Cyanine dye, Alexa dye, Quantum dye, etc).

RBDs are found, in particular, in glycoproteins of the envelope of viruses, therefore, the receptor binding domain ligand contains the total RBD or a fragment or part of the RBD. In one embodiment, said virus is selected from the group comprising retroviruses, such as, for example, (i) gammaretroviruses such as for example, murine (MLV), feline (FeLV) or gibbon ape leukaemia virus (GaLV); and (ii) deltaretroviruses such as, for example, primate T cell leukaemia virus (such as, for example, human T cell leukaemia virus (HTLV) and simian T cell leukaemia virus (STLV)) or bovine leukaemia virus (BLV).

The gamma and deltaretroviruses encode an Env glycoprotein present in mature retrovirus virions. The Env protein is synthesized in the form of a propeptide, which is clived in Golgi apparatus by furine peptidase, resulting in two polypeptides: the transmembrane (TM) and the cell surface (SU) components. The SU domain contains two major subdomains: a domain of interaction with the TM domain and the RBD, the further being liable to interact with host cell membrane receptors.

In one embodiment, the soluble receptor binding domain ligand is isolated from the glycoprotein of Xenotropic Murine Leukaemia Virus, and is herein referred as Xeno.RBD.

In one embodiment, said Xeno.RBD comprises or consists of the amino acid sequence SEQ ID NO: 1 or fragments thereof.

In one embodiment, said fragment comprises or consists of amino acids 36 to 316 of SEQ ID NO: 1.

In one embodiment, said fragment comprises or consists of amino acids 1 to 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314 or 315 of SEQ ID NO: 1.

In another embodiment, said fragment comprises or consists of amino acids 36 to 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314 or 315 of SEQ ID NO: 1.

In another embodiment, said fragment comprises or consists of SEQ ID NO: 2, encoded by the DNA sequence SEQ ID NO: 3.

In another embodiment, said fragment comprises or consists of amino acids 36 to 296 of SEQ ID NO: 2.

In one embodiment, said Xeno.RBD comprises or consists of the amino acid sequence SEQ ID NO: 39 or fragments thereof.

In one embodiment, said fragment comprises or consists of amino acids 36 to 316 of SEQ ID NO: 39. In one embodiment, said fragment comprises or consists of amino acids 1 to 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314 or 315 of SEQ ID NO: 39.

In another embodiment, said fragment comprises or consists of amino acids 36 to 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314 or 315 of SEQ ID NO: 39.

In another embodiment, said fragment comprises or consists of SEQ ID NO: 40, encoded by the DNA sequence SEQ ID NO: 41.

In another embodiment, said fragment comprises or consists of amino acids 36 to 296 of SEQ ID NO: 40.

In one embodiment, the soluble receptor binding domain ligand is isolated from the glycoprotein of Xenotropic MRV, and is herein referred as XMRV.RBD. In one embodiment, said XMRV.RBD comprises or consists of the amino acid sequence SEQ ID NO: 29 or fragments thereof. In one embodiment, said fragment comprises or consists of amino acids 1 to 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282 or 283 of SEQ ID NO: 29. In one embodiment, said fragment comprises or consists of amino acids 33 to 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282 or 283 of SEQ ID NO: 29. In another embodiment, said fragment comprises or consists in SEQ ID NO: 35 (corresponding to amino acids 1 to 233 of SEQ ID NO: 29). In another embodiment, said fragment comprises or consists in amino acids 33 to 233 of SEQ ID NO: 29.

In one embodiment, the soluble receptor binding domain ligand is isolated from the glycoprotein of Polytropic MLV, and is herein referred as PMLV.RBD. In one embodiment, said PMLV.RBD comprises or consists of the amino acid sequence SEQ ID NO: 30 or fragments thereof.

In one embodiment, said fragment comprises or consists of amino acids 1 to 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278 or 279 of SEQ ID NO: 30.

In one embodiment, said fragment comprises or consists of amino acids 33 to 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278 or 279 of SEQ ID NO: 30.

In another embodiment, said fragment comprises or consists in SEQ ID NO: 36 (corresponding to amino acids 1 to 229 of SEQ ID NO: 30).

In another embodiment, said fragment comprises or consists of amino acids 33 to 229 of SEQ ID NO: 30.

In another embodiment, said PMLV.RBD comprises or consists of the amino acid sequence SEQ ID NO: 31 or fragments thereof.

In one embodiment, said fragment comprises or consists of amino acids 1 to 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269 or 270 of SEQ ID NO: 31. In one embodiment, said fragment comprises or consists of amino acids 33 to 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269 or 270 of SEQ ID NO: 31. In another embodiment, said fragment comprises or consists in SEQ ID NO: 37 (corresponding to amino acids 1 to 229 of SEQ ID NO: 31).

In another embodiment, said fragment comprises or consists of amino acids 33 to 229 of SEQ ID NO: 31.

In another embodiment, said PMLV.RBD comprises or consists of the amino acid sequence SEQ ID NO: 32 or fragments thereof.

In one embodiment, said fragment comprises or consists of amino acids 1 to 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278 or 279 of SEQ ID NO: 32.

In one embodiment, said fragment comprises or consists of amino acids 33 to 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278 or 279 of SEQ ID NO: 32.

In another embodiment, said fragment comprises or consists in SEQ ID NO: 38 (corresponding to amino acids 1 to 229 of SEQ ID NO: 32).

In another embodiment, said fragment comprises or consists of amino acids 33 to 229 of SEQ ID NO: 32. In one embodiment, the soluble receptor binding domain ligand is isolated from the glycoprotein of Human T Leukaemia Virus-2, and is herein referred as HTLV2.RBD. In one embodiment, said HTLV2.RBD comprises or consists of the amino acid sequence SEQ ID NO: 4 or fragments thereof.

In one embodiment, said fragment comprises or consists of amino acids 19 to 224 of SEQ ID NO: 4, or comprises or consists of amino acids 20 to 224 of SEQ ID NO: 4 or comprises or consists of amino acids 21 to 224 of SEQ ID NO: 4.

In one embodiment, said fragment comprises or consists of amino acids 1 to 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222 or 223 of SEQ ID NO: 4. In another embodiment, said fragment comprises or consists of amino acids 19, 20 or 21 to 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222 or 223 of SEQ ID NO: 4.

In another embodiment, said fragment comprises or consists of SEQ ID NO: 5, encoded by the DNA sequence SEQ ID NO: 6.

In another embodiment, said fragment comprises or consists of amino acids 19, 20 or 21 to 178 of SEQ ID NO: 5.

In one embodiment, the soluble receptor binding domain ligand is isolated from the glycoprotein of Human T Leukaemia Virus-1, and is herein referred as HTLV1.RBD. In one embodiment, said HTLV1.RBD comprises or consists of the amino acid sequence SEQ ID NO: 7 or fragments thereof.

In one embodiment, said fragments comprise or consist of amino acids 1 to 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207 or 208 of SEQ ID NO: 7. In one embodiment, said fragments comprise or consist of amino acids 21 to 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207 or 208 of SEQ ID NO: 7. In another embodiment, said fragments comprise or consist in SEQ ID NO: 33 (corresponding to amino acids 1 to 182 of SEQ ID NO: 7).

In one embodiment, said fragments comprise or consist of amino acids 21 to 182 of SEQ ID NO: 7. In one embodiment, the soluble receptor binding domain ligand is isolated from the glycoprotein of Human T Leukaemia Virus-4, and is herein referred as HTLV4.RBD. In one embodiment, said HTLV4.RBD comprises or consists of the amino acid sequence SEQ ID NO: 8 or fragments thereof.

In one embodiment, said fragments comprise or consist of amino acids 1 to 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203 or 204 of SEQ ID NO: 8.

In one embodiment, said fragments comprise or consist of amino acids 21 to 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203 or 204 of SEQ ID NO: 8. In another embodiment, said fragments comprise or consist in SEQ ID NO: 34 (corresponding to amino acids 1 to 178 of SEQ ID NO: 8).

In another embodiment, said fragments comprise or consist in amino acids 21 to 178 of SEQ ID NO: 8.

In another embodiment, said HTLV4.RBD comprises or consists of the amino acid sequence SEQ ID NO: 42 or fragments thereof.

In one embodiment, said fragments comprise or consist of amino acids 1 to 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340,

341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357,

358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374,

375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391,

392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408,

409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425,

426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442,

443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459,

460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476,

477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493,

494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510,

511, 512, 513, 514, 515, 516, 517, 518, 519, 520, 521, 522, 523, 524, 525, 526, 527,

528, 529, 530, 531, 532, 533, 534, 535, 536, 537, 538, 539, 540, 541, 542, 543, 544,

545, 546, 547, 548, 549, 550, 551, 552, 553, 554, 555, 556, 557, 558, 559, 560, 561,

562, 563, 564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, 577, 578,

579, 580, 581, 582, 583, 584, 585, 586, 587, 588, 589, 590, 591, 592, 593, 594, 595,

596, 597, 598, 599, 600, 601, 602, 603, 604, 605, 606, 607, 608, 609, 610, 611, 612,

613, 614, 615, 616, 617, 618, 619, 620, 621, 622, 623, 624, 625, 626, 627, 628, 629,

630, 631, 632, 633, 634, 635, 636, 637, 638, 639, 640, 641, 642, 643, 644, 645, 646,

647, 648, 649, 650, 651, 652, 653, 654, 655, 656, 657, 658, 659, 660, 661, 662, 663,

664, 665, 666, 667, 668, 669, 670, 671, 672, 673, 674, 675, 676, 677, 678, 679, 680,

681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 693, 694, 695, 696, 697,

698, 699, 700, 710, 711, 712, 713, 714, 715, 716, 717, 718, 719, 720, 721, 722, 723,

724, 725, 726, 727, 728, 729, 730, 731, 732, 733, or 734 of SEQ ID NO: 42. In one embodiment, said fragments comprise or consist of amino acids 24 to 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357,

358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374,

375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391,

392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408,

409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425,

426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442,

443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459,

460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476,

477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493,

494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510,

511, 512, 513, 514, 515, 516, 517, 518, 519, 520, 521, 522, 523, 524, 525, 526, 527,

528, 529, 530, 531, 532, 533, 534, 535, 536, 537, 538, 539, 540, 541, 542, 543, 544,

545, 546, 547, 548, 549, 550, 551, 552, 553, 554, 555, 556, 557, 558, 559, 560, 561,

562, 563, 564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, 577, 578,

579, 580, 581, 582, 583, 584, 585, 586, 587, 588, 589, 590, 591, 592, 593, 594, 595,

596, 597, 598, 599, 600, 601, 602, 603, 604, 605, 606, 607, 608, 609, 610, 611, 612,

613, 614, 615, 616, 617, 618, 619, 620, 621, 622, 623, 624, 625, 626, 627, 628, 629,

630, 631, 632, 633, 634, 635, 636, 637, 638, 639, 640, 641, 642, 643, 644, 645, 646,

647, 648, 649, 650, 651, 652, 653, 654, 655, 656, 657, 658, 659, 660, 661, 662, 663,

664, 665, 666, 667, 668, 669, 670, 671, 672, 673, 674, 675, 676, 677, 678, 679, 680,

681, 682, 683, 684, 685, 686, 687, 688, 689, 690, 691, 692, 693, 694, 695, 696, 697,

698, 699, 700, 710, 711, 712, 713, 714, 715, 716, 717, 718, 719, 720, 721, 722, 723,

724, 725, 726, 727, 728, 729, 730, 731, 732, 733, or 734 of SEQ ID NO: 42.

In another embodiment, said fragments comprise or consist of amino acids 22 to 237 of SEQ ID NO: 42, or comprise or consist of amino acids 23 to 237 of SEQ ID NO: 42, or comprise or consist of amino acids 24 to 237 of SEQ ID NO: 42.

In another embodiment, said fragments comprise or consist of amino acids 1 to 236 of SEQ ID NO: 42. In another embodiment, said fragments comprise or consist of amino acids 24 to 236 of SEQ ID NO: 42. In another embodiment, said fragments comprise or consist of SEQ ID NO: 42, encoded by the DNA sequence SEQ ID NO: 45.

In one embodiment, the soluble receptor binding domain ligand is isolated from the glycoprotein of Human T Leukemia Virus-3, and is herein referred as HTLV3.RBD. In one embodiment, said HTLV3.RBD comprises or consists of the amino acid sequence SEQ ID NO: 43 or fragments thereof.

In one embodiment, said fragments comprises or consists of amino acids 1 to 181, 182,

489 490, 491, or 492 of SEQ ID NO: 43 or fragments thereof.

In one embodiment, said fragments comprise or consist of amino acids 23 to 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, or 492 of SEQ ID NO: 43 or fragments thereof.

In another embodiment, said fragments comprise or consist of amino acids 1 to 180 of SEQ ID NO: 43. In another embodiment, said fragments comprise or consist of amino acids 23 to 180 of SEQ ID NO: 43. In another embodiment, said fragments comprise or consist of SEQ ID NO: 43, encoded by the DNA sequence SEQ ID NO: 44.

According to a preferred embodiment, receptor binding domain ligands are selected from the group comprising the sequences SEQ ID NO: 1, 2, 4, 5, 7, 8, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42 and 43, fragments and variants thereof, more preferably selected from the group comprising the sequences SEQ ID NO: 2, 5, 33, 34, 35, 36, 37, 38 and 40, fragments and variants thereof. According to another embodiment, receptor binding domain ligands are encoded by a DNA sequence selected from the group comprising the sequences SEQ ID NO: 3, 6, 41, 44 and 45. In one embodiment, the receptor binding domain ligand comprises or consists of a sequence presenting a sequence identity of at least 70% with one of the sequences SEQ ID NO: 1, 2, 4, 5, 7, 8, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42 and 43, preferably a sequence identity of at least about 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99% or more with one of the sequences SEQ ID NO: 1, 2, 4, 5, 7, 8, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42 and 43.

In another embodiment, the receptor binding domain ligand is encoded by a DNA sequence presenting a sequence identity of at least 70% with one of the sequences SEQ ID NO: 3, 6, 41, 44 and 45, preferably a sequence identity of at least about 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99% or more with one of the sequences SEQ ID NO: 3, 6, 41, 44 and 45.

In one embodiment, the receptor binding domain ligand is a variant of one of the polypeptide having the sequences SEQ ID NO: 1, 2, 4, 5, 7, 8, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 42 and 43. A polypeptide "variant" as the term is used herein, is a polypeptide that typically differs from a polypeptide specifically disclosed herein in one or more substitutions, deletions, additions and/or insertions. Such variants may be naturally occurring or may be synthetically generated, for example, by modifying one or more of the above polypeptide sequences and evaluating one or more biological activities of the polypeptide as described herein and/or using any of a number of techniques well known in the art. Modifications may be made in the structure of polypeptides and still obtain a functional molecule that encodes a variant or derivative polypeptide with desirable characteristics.

When it is desired to alter the amino acid sequence of a polypeptide to create an equivalent, or even an improved, variant or portion of a ligand of the invention, one skilled in the art will typically change one or more of the codons of the encoding DNA sequence. For example, certain amino acids may be substituted by other amino acids in a protein structure without appreciable loss of its ability to bind cell surface nutrient transporters. Since it is the binding capacity and nature of a protein that defines that protein's biological functional activity, certain amino acid sequence substitutions can be made in a protein sequence, and, of course, its underlying DNA coding sequence, and nevertheless obtain a protein with similar properties. It is thus contemplated that various changes may be made in the peptide sequences, or corresponding DNA sequences that encode said peptides without appreciable loss of their biological utility or activity. In many instances, a polypeptide variant will contain one or more conservative substitutions. A "conservative substitution" is one in which an amino acid is substituted by another amino acid that has similar properties, such that one skilled in the art of peptide chemistry would expect the secondary structure and hydropathic nature of the polypeptide to be substantially unchanged. As outlined above, amino acid substitutions are generally therefore based on the relative similarity of the amino acid side-chain substituents, for example, their hydrophobicity, hydrophilicity, charge, size, and the like. Exemplary substitutions that take various of the foregoing characteristics into consideration are well known to those of skill in the art and include: arginine and lysine; glutamate and aspartate; serine and threonine; glutamine and asparagine; and valine, leucine and isoleucine. Amino acid substitutions may further be made on the basis of similarity in polarity, charge, solubility, hydrophobicity, hydrophilicity and/or the amphipathic nature of the residues. For example, negatively charged amino acids include aspartic acid and glutamic acid; positively charged amino acids include lysine and arginine; and amino acids with uncharged polar head groups having similar hydrophilicity values include leucine, isoleucine and valine; glycine and alanine; asparagine and glutamine; and serine, threonine, phenylalanine and tyrosine. Other groups of amino acids that may represent conservative changes include: (1) ala, pro, gly, glu, asp, gin, asn, ser, thr; (2) cys, ser, tyr, thr; (3) val, ile, leu, met, ala, phe; (4) lys, arg, his; and (5) phe, tyr, trp, his. A variant may also, or alternatively, contain nonconservative changes. In a preferred embodiment, variant polypeptides differ from a native sequence by substitution, deletion or addition of 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 amino acids. Variants may also (or alternatively) be modified by, for example, the deletion or addition of amino acids that have minimal influence on the immunogenicity, secondary structure and hydropathic nature of the polypeptide. In one embodiment, the receptor binding domain ligand is a fusion protein comprising a part or the totality of a receptor binding domain fused to a detection tag, such as, for example, a Fc fragment or a GFP. Examples of Fc fragments include, but are not limited to, rabbit Fc fragment (amino acid sequence SEQ ID NO: 9, encoded by SEQ ID NO: 10), and mouse Fc fragment (amino acid sequence SEQ ID NO: 11, encoded by SEQ ID NO: 12).

In one embodiment, the receptor binding domain ligand is selected from the group comprising HTLV2.RBD fused to a mouse Fc fragment (encoded by the DNA sequence SEQ ID NO: 13), and Xeno.RBD fused to a rabbit Fc fragment (encoded by the DNA sequence SEQ ID NO: 14).

In one embodiment, the receptor binding domain ligand is obtained by a cloning method, such as, for example, using any production system known in the art, such as, for example, E. coli, yeast, baculovirus-insect cell, or mammalian cells such as HEK or CHO, expression system. In one embodiment, the sequence of the receptor binding domain ligand is fused in N-terminal to a peptide signal sequence allowing the secretion of said receptor binding domain ligand. Examples of peptide signal sequences include, but are not limited to, human IL-2 peptide signal (SEQ ID NO: 15), human albumin peptide signal (SEQ ID NO: 16), human chymotrypsinogen peptide signal (SEQ ID NO: 17), human trypsinogen-2 peptide signal (SEQ ID NO: 18), gaussia luciferase peptide signal (SEQ ID NO: 19), and mouse IgM peptide signal (SEQ ID NO: 20).

In one embodiment, the receptor binding domain ligand comprises a part or the totality of Xeno.RBD, XMRV.RBD or PMLV.RBD and binds to the XPR1 nutrient transporter. In one embodiment, the receptor binding domain ligand comprises a part or the totality of HTLV2.RBD, HTLV1.RBD, HTLV3.RBD or HTLV4.RBD and binds to the GLUT1 nutrient transporter.

As used herein, the term "reference" broadly encompasses any suitable reference expression level which may be used as a basis for comparison with respect to the measured expression level. In one embodiment, the standard reference is a personalized reference, determined earlier in the same culture of cells as the one used for determining the expression level. In a first embodiment of the invention the reference expression level is the expression level measured in a culture of cells before, preferably 1, 2, 3, 4, 5, 6, 8, 9, 10 days or more before, or 1, 2, 3, 4, 5, 6, 7, 8 weeks before, or 1, 2, 3, 4, 5 months or more before, or 1, 2, 3, 4, 5 years or more before the day of performing the assay of the invention. In a second embodiment, the cells were frozen and subsequently thawed, and the reference expression level is the expression level measured before freezing.

According to these embodiments, a difference between the measured expression level and the personalized reference expression level is indicative of cell aging or of senescence.

In one embodiment of the invention, the reference expression level is the expression level measured in a "young" batch of cells. In one embodiment, a young batch of cells presents complete differentiation properties (such as, for example, in the case of MSC, young cells are able to differentiate along the osteogenic, chondrogenic and adipogenic lineages, and optionally into the myogenic lineage). In another embodiment, a young batch of cells presents complete proliferation capacities, with normal doubling time (normal doubling time depends on the type of cells). In another embodiment, a young batch of cells present no signs (in particular no late signs) of senescence (examples of late signs of senescence are listed hereinabove). In one embodiment, a young batch of cells may further divide at least 10 times, preferably at least 15, 20, 25, 30, 35, 40, 45, or 50 times before the occurrence of signs of senescence (in particular of last signs of senescence as listed in the present invention). In one embodiment, a young batch of cells has been passaged 10 times or less, preferably 9, 8, 7, 6, 5, 4, 3, 2, 1 time(s). According to this embodiment, a difference between the measured expression level and the reference expression level is indicative of cell aging or of senescence.

In one embodiment of the invention, the reference expression level is the expression level measured in an "old" batch of cells. In one embodiment, an old batch of cells presents significantly altered differentiation capacities, such as, for example, the cell is not capable anymore to differentiate into at least one specific cell lineage (such as, for example, in the case of MSC, differentiation in at least one of the osteogenic, chondrogenic, adipogenic or myogenic lineages is not possible anymore); or is not capable to differentiate into the three following lineages: osteogenic, chondrogenic and adipogenic lineages; or is not capable to differentiate anymore. In another embodiment, an old batch of cells is a batch of cells showing absence of cell division or increased doubling time (for example, double time increased by at least 20%, preferably at least 30, 40, or 50% as compared to the normal doubling time, which depends on the type of cells). In another embodiment, an old batch of cells is a batch of cells showing signs (in particular late signs) of senescence (for a list, see hereinabove). In another embodiment, an old batch of cells has been passaged 10 times or more, preferably 20, 25, 30, 35, 40, 45, 50 times or more. According to this embodiment, the absence of difference between the measured expression level and the reference expression level is indicative of cell aging or of senescence.

In one embodiment, the reference is constructed using algorithms and other methods of statistical and hierarchical classification.

In another aspect, the reference expression level is stored in a database to provide a stored expression level and the stored expression level is used to determine the difference in the expression level. The database may, for example, be stored on a computer or a server. In one embodiment, the expression level is normalized, i.e. the expression level corresponds to a ratio between the expression of the cell surface nutrient transporter and the expression of another gene or protein.

In one embodiment, the cell surface nutrient transporter is XPRl and the other gene or protein used for normalization is GLUT1. In another embodiment, the cell surface nutrient transporter is GLUT1 and the other gene or protein used for normalization is XPRl. In another embodiment, the cell surface nutrient transporter is XPR1 or GLUT1, preferably is XPR1, and the other gene or protein used for normalization is selected from the group comprising GAPDH, CD29, CD44, CD73, CD105, and CD166.

In the present invention, two numeric values, in particular two expression levels, are considered as different if the first numeric value is higher (such as, for example, the first numeric value is about 20% higher than the second one, preferably is about 30, 40, 50, 60, 70, 80, 90% or more higher than the second one) or lower than the second one (such as, for example, the second numeric value is about 20% lower than the second one, preferably is about 30, 40, 50, 60, 70, 80, 90% or more lower than the second one). Another object of the present invention is a kit for implementing the method of the invention, wherein said kit comprises means for measuring the expression level of at least one cell surface nutrient transporter, preferably XPR1 and/or GLUT1, more preferably XPR1 and GLUTl.

In one embodiment, the expression level of at least one cell surface nutrient transporter is assessed at the RNA level, and the kit of the invention may comprise means for total RNA extraction, means for reverse transcription of total RNA, and means for quantifying the expression of RNA of at least one cell surface nutrient transporter, preferably XPR1 and/or GLUT1. In one embodiment, the means for quantifying the expression of RNA of at least one cell surface nutrient transporter, preferably XPR1 and/or GLUT1 are PCR or qPCR primers specific for said cell surface nutrient transporter, preferably XPR1 and/or GLUT1. Examples of PCT or qPCR primers specific for XPR1 include, but are not limited to, the following couple of primers: Forward primer: 5 ' - AGAGCTTGGGAGACAAAGCA-3 ' (SEQ ID NO: 25) - Reverse primer: 5 '-GTGGACACAACATTCGCAAC-3' (SEQ ID NO: 26). Examples of PCT or qPCR primers specific for GLUT1 include, but are not limited to, the following couple of primers: Forward primer: 5 ' -TCACTGTGCTCCTGGTTCTG-3 ' (SEQ ID NO: 27) - Reverse primer: 5 ' -CCTCGGGTGTCTTGTC ACTT-3 ' (SEQ ID NO: 28). In one embodiment, the kit also comprises reagents for carrying out a quantitative PCR (such as, for example, buffers, enzyme, and the like). In one embodiment, the kit of the invention may also comprise means for detecting the expression level of at least one normalization gene at the RNA level.

In another embodiment, the expression level of at least one cell surface nutrient transporter is assessed at the protein level, and the kit of the invention may comprise means for detecting the at least one cell surface nutrient transporter, preferably XPRl and/or GLUTl. In one embodiment, said means for detecting the at least one cell surface nutrient transporter is an antibody specific of said at least one cell surface nutrient transporter, preferably XPRl and/or GLUTl. In another embodiment, said means for detecting the at least one cell surface nutrient transporter is a RBD as defined in the present invention and specific of the at least one cell surface nutrient transporter. In one embodiment, the kit of the invention may also comprise means for detecting the expression level of at least one normalization protein.

The present invention also relates to a cell surface nutrient transporter, preferably XPRl and/or GLUTl, as a biomarker of cell aging (preferably, XPRl and/or GLUTl is an early biomarker of cell aging) or senescence, and/or as a biomarker of the accumulation of cell divisions.

The present invention also relates to a cell surface nutrient transporter, preferably XPRl and/or GLUTl, as a biomarker of the proliferation capacity and/or of the differentiation capacity of a cell. In one embodiment, the cell surface nutrient transporter, preferably XPRl and/or GLUTl, is a biomarker of sternness.

The present invention also relates to a cell surface nutrient transporter, preferably XPRl and/or GLUTl, as a biomarker of the quality of a cell batch, in particular of a cell batch to be used in regenerative medicine or a cell batch to be used in in vitro screening assays. The present invention also relates to an in vitro method of screening compounds impacting cell aging, such as, for example, compounds slowing down (anti-aging effect) or accelerating (pro-aging or pro-senescence effect) cell aging, wherein said screening method comprises determining or measuring the expression level of at least one cell surface nutrient transporter, preferably XPR1 and/or GLUT1, more preferably XPR1.

In one embodiment, the in vitro method is for screening compounds impacting progeroid syndromes, preferably, progeria. Progeroid syndromes and progeria are models of accelerated aging. In one embodiment, the in vitro method is for screening compounds that may be used for treating progeroid syndromes, preferably, for treating progeria, through anti-aging activity.

In one embodiment, the in vitro method is for screening compounds impacting cancer cells. Cancer cells are characterized by their ability to avoid senescence. In one embodiment, the in vitro method is for screening compounds that may be used for treating cancer cells, through pro-senescence (or pro-aging) activity.

In one embodiment, the method of the invention further comprises comparing the measured expression level with a reference expression level.

In one embodiment, the reference expression level is an index value or is derived from one or more risk prediction algorithms or computed indices for the anti- or pro-aging effect of the tested compound. A reference can be relative to a number or value derived from cell population studies, preferably based on cells which are the same as the ones used for testing the anti- or pro-aging effect of said compound, and which are cultured in the same culture medium with the same culture conditions. In another embodiment of the invention, the reference expression level is derived from the measurement of the expression level in a control cell sample exposed to a compound known not to present a pro- or anti-aging effect. According to this embodiment, a difference between the measured expression level and the reference expression level is indicative of the pro-or anti-aging effect of the tested compound. In another embodiment of the invention, the reference expression level is derived from the measurement of the expression level in a control cell sample exposed to a compound known to present a pro-or anti-aging effect. According to this embodiment, the absence of difference between the measured expression level and the reference expression level is indicative of the pro-or anti-aging effect of the tested compound.

In one embodiment of the invention, the reference expression level is derived from the measurement of the expression level in a control cell culture in the absence of the tested compound. According to this embodiment, a difference between the measured expression level and the reference expression level is indicative of the pro-or anti-aging effect of the tested compound.

In one embodiment, a culture of cells is provided, and separated in two different culture batches, wherein the first culture batch is exposed to the compound to be tested (for measuring the expression level) and the second culture batch is not exposed to the compound to be tested (for measuring the reference expression level).

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a combination of graphs showing the evolution of the ratio XPRl / GLUT1 with cell culture passages (P: passage). (A) Evolution of the doubling time of hESC- SA001 derived MSC with the number of cumulated divisions and culture passages. (B) Evolution of XPRl (left panel) and GLUT1 (right panel) cell surface expression with the number of cumulated divisions and culture passages. (C) Evolution of the XPRl / GLUT1 ratio with the number of cumulated divisions and culture passages. (D) Evolution of the XPRl mRNA relative expression level (normalized to GAPDH) with the number of cumulated divisions and culture passages.

Figure 2 is a histogram showing the expression of XPRl according to the number of culture passages (P) in an equine stem cell line. MESF: Molecules of Equivalent Soluble Fluorochrome. Figure 3 is a histogram showing the expression of XPRl according to the number of culture passages (P) in a bone marrow derived hMSC cell line. MESF: Molecules of Equivalent Soluble Fluorochrome. * p< 0,05; *** p<0,001 (Student's t Test). Figure 4 represents the expression of XPR1 and GLUT1 (A), the ratio XPR1/ GLUT1 (B) and cell proliferation (C) in a slow-down model of proliferation induced by a reduction in fetal calf serum.

Figure 5 represents the ratio XPR1/GLUT1 in a senescence accelerated-model of proliferation (mitomycin) and in an anti-aging model (rapamycin).

EXAMPLES

The present invention is further illustrated by the following examples. Example 1: Signature for monitoring the proliferation potential of human MSC Material and method

MSCs were detached using TrypLE Express (Life Technologies) for 5 minutes at 37°C and transferred into a 96-well V-shape microplate. 3.10 ⁴ cells were used for each binding. RBD were premixed pairwise (Glutl.RBD.mouseFc and Xeno.RBD.rabbitFc) in culture medium containing 0.1% sodium azide and ImM EDTA. RBD were added to MPCs and incubated at 37°C for 20 min. Cells were washed once with PBS/2% FCS and then incubated with Alexa Fluor 647 goat anti-rabbit IgG (Invitrogen, 1:400) and R- PE goat anti-mouse IgGl (Invitrogen, 1:200) antibodies in binding buffer (PBS/2% FCS/0.1% sodium azide/lmM EDTA), containing 0.3 μg/mL DAPI to restrict the analysis to live cells. After 30 min of incubation at 4°C, cells were washed and resuspended in binding buffer before flow cytometry analysis. Fluorescent signals were acquired on a FACSVerse flow cytometer (BD Biosciences) with 405, 488 and 640 nm excitation, and data analysis was performed using Flowjo software (Tree Star Inc.). Dead cells were excluded from the analysis. "Fluorescence minus one" (FMO) controls were used to establish background levels in RBD channels (R-PE and AF647). Signals were converted into molecules of equivalent soluble fluorochrome (MESF) values using calibration beads (R-PE and AF647 MESF Quantum Beads; Bangs Laboratories) according to the manufacturer's instructions. Results

As shown in Figure 1, the expression level of XPR1 increases with cumulated divisions of human MSC derived from embryonic stem cells (SA001 cells). Said increased expression is observed both on cell surface expression (Figure IB) and on mRNA expression (Figure ID). On the contrary, the expression level of GLUT1 decreases with cumulated divisions (Figure IB).

Moreover, as shown in Figure 1C, the ratio XPR1 / GLUT1 increases with cumulated divisions of human MSC derived from embryonic stem cells (SA001 cells).

These results confirm that XPR1, GLUT1 and the ratio XPR1 / GLUT1 may be used as biomarkers of cell aging.

Example 2: XPR1 expression in equine stem cells

Material and method

MSCs were detached using TrypLE Express (Life Technologies) for 5 minutes at 37°C and transferred into a 96-well V-shape microplate. 3.10 ⁴ cells were used for each binding. Xeno.RBD.rabbitFc, diluted in culture medium containing 0.1% sodium azide and ImM EDTA, was added to MPCs and incubated at 37°C for 20 min. Cells were washed once with PBS/2% FCS and then incubated with Alexa Fluor 647 goat anti- rabbit IgG (Invitrogen, 1:400) antibody in binding buffer (PBS/2% FCS/0.1% sodium azide/lmM EDTA), containing 0.3 μg/mL DAPI to restrict the analysis to live cells. After 30 min of incubation at 4°C, cells were washed and resuspended in binding buffer before flow cytometry analysis. Fluorescent signals were acquired on a FACS Verse flow cytometer (BD Biosciences) with 405, 488 and 640 nm excitation, and data analysis was performed using Flowjo software (Tree Star Inc.). Dead cells were excluded from the analysis. "Fluorescence minus one" (FMO) controls were used to establish background level in RBD channel (AF647). Signals were converted into molecules of equivalent soluble fluorochrome (MESF) values using calibration beads (AF647 MESF Quantum Beads; Bangs Laboratories) according to the manufacturer's instructions. Results

As shown in Figure 2, the expression level of XPRl increases with culture passages in equine MSC. This result thus demonstrates that XPRl is also a potential biomarker of cell aging in equine stem cells. Example 3: XPRl expression and cell aging of bone marrow - derived hMSC

Material and method

Human bone marrow-derived MSCs from 2 donors at different passages (P4, P6 and P8) were thawed and plated at a cell density of 60 cells/cm ² . Cells were expanded in complete medium containing a-MEM, L-glutamine, P/S and 16.5% lot-selected fetal calf serum. When cultures reached 80% confluence, cells were detached by trypsin (0.25% Trypsin/EDTA, Life Technologies) for 5 minutes at 37°C and the doubling time (h) was determined by counting each cell batch at each passage.

3.10 ⁴ cells of each batch at each passage were transferred into a 96-well V-shape microplate for each binding. Xeno.RBD.rabbitFc was prepared in culture medium containing 0.1% sodium azide and 1 mM EDTA. Xeno.RBD.rFc was added to MSCs and incubated at 37°C for 20 minutes. Cells were washed once with buffer B (PBS/2% FCS/0.1% sodium azide/ 1 mM EDTA) and then incubated with Alexa Fluor 647 goat anti-rabbit IgG (Life Technologies, 1/1 000) in buffer B containing 1 μg/mL DAPI to restrict analysis to live cells. After 30 minutes of incubation at 4°C in the dark, the cells were washed twice and resuspended in buffer B before flow cytometry analysis. Fluorescent signals were acquired on a FACSVerse flow cytometer (BD Biosciences) with 405, 488 and 640 nm excitation, and data analysis was performed using Flowjo software (Tree Star Inc.).

Dead cells were excluded from the analysis. Cells only labelled with the AF647 goat anti-rabbit secondary antibody were used to establish background level. Signals were converted into molecules and equivalent soluble fluorochrome (MESF) values using calibration beads (AF647 MESF Quantum Beads, Bangs Laboratories) according to the manufacturer's instructions. Data represent the mean +/- SD of a triplicate of labelling of XPR1 and statistical significance was determined using a Student's t-test. p<0.05 was considered to be significant.

Results As shown in Figure 3, XPR1 expression increases with the number of cell divisions (reflected by the number of passages), and with the doubling time for each donor. Increase of population doubling time indicates that cells are entering into replicative senescence.

These results confirm that XPR1 increase may be used as a biomarker of cell aging in hMSC derived from bone-marrow.

Example 4: XPR1 in a slow-down model of proliferation induced by a reduction of FCS in ES-derived hMSC.

Material and method hMSC derived from Embryonic Stem (ES) cells were cultured in decreasing concentrations of FCS (20, 5 and 1%) that did induce a slow-down in proliferation. When cultures reached 70-80% confluence, cells were detached using TrypLE Express (Life technologies) for 5 minutes at 37°C, and the doubling time (h) was determined by counting before transfer into a 96-well V-shape microplate. 3.10 ⁴ cells were used for each binding. RBDs were premixed pairwise (Glutl.RBD.mouseFc and Xeno.RBD.rabbitFc) in culture medium containing 0.1% sodium azide and 1 mM EDTA. RBDs were added to hMSCs and incubated at 37°C for 20 minutes. Cells were washed once with buffer B (PBS/2% FCS/0.1% sodium azide/1 mM EDTA) and then incubated with Alexa Fluor 647 goat anti-rabbit IgG (Life Technologies, 1/1 000) and R-PE goat anti-mouse IgGl (Life technologies, 1/100) in buffer B. After 30 minutes of incubation at 4°C in the dark, the cells were washed twice and resuspended in buffer B before flow cytometry analysis. Fluorescent signals were acquired on a FACS Verse flow cytometer (BD Biosciences) with 405, 488 and 640 nm excitation, and data analysis was performed using Flowjo software (Tree Star Inc.). "Fluorescence minus One" controls were used to establish background level (or noise) in RBD channel (R-PE and AF647) and signal/noise represents the expression level of XPR1 and Glutl. Data represent the mean +/- SD (3 replicates).

Results As shown in figure 4A the expression of XPR1 increases with lowering concentrations of FCS. At the opposite, the expression of Glutl decreases in ES-derived hMSC. In addition, the ratio XPRl/Glutl increases with lower concentrations of FCS (figure 4B). The doubling times (h) increases with lower concentrations of FCS, indicating a slowdown of cell proliferation (figure 4C) Altogether, these data confirm that XPR1, Glutl and the ratio XPRl/Glutl may be used as biomarkers of cell proliferation.

Example 5: XPR1 in a senescence accelerated- model induced by mitomycin C in ES- derived hMSC and in an anti-aging model induced by rapamycin in ES-derived hMSC.

Material and method Mitomycin treatment - 80% confluent hMSCs derived from ES cells were treated for 3 hours with 1 or 10 μg/mL of mitomycin C (MMC, Sigma) or none (control cells). After the pulse of MMC, the cells were washed twice with PBS, detached with TrypLE Express for 5 minutes a 37°C and re-plated in flasks for 4 additional days at the end of which the cells were detached for RBD labelling and the doubling time (h) was determined by counting.

3.10 ⁴ cells were used for each binding and were transferred into a 96- well V- shape microplate. RBDs were premixed pairwise (Glutl. RBD. mouseFc and Xeno.RBD.rabbitFc) in culture medium containing 0.1% sodium azide and 1 mM EDTA. RBDs were added to hMSCs and incubated at 37°C for 20 minutes. Cells were washed once with buffer B (PBS/2% FCS/0.1% sodium azide/1 mM EDTA) and then incubated with Alexa Fluor 647 goat anti-rabbit IgG (Life Technologies, 1/1 000) and R-PE goat anti-mouse IgGl (Life technologies, 1/100) in buffer B. After 30 minutes of incubation at 4°C in the dark, the cells were washed twice and resuspended in buffer B before flow cytometry analysis. Fluorescent signals were acquired on a FACS Verse flow cytometer (BD Biosciences) with 405, 488 and 640 nm excitation, and data analysis was performed using Flowjo software (Tree Star Inc.).

"Fluorescence minus One" controls were used to establish background level in RBD channels (R-PE and AF647). Signal/noise of 1 and 10 μg/mL mitomycin c conditions were normalized to control condition.

Rapamycin treatment - Rapamycin is described in the litterature for preventing in vitro cellular senescence in human cells and to slow aging in mice (Wilkinson, J.E., et al. (2012). Rapamycin slows aging in mice. Aging Cell 11, 675-682).

hMSC derived from ES cells were treated by 0.1 or 10 nM rapamycin (Sigma) 1 day after seeding at 5 000 cells/cm ² . 0.03% DMSO was used as control. Three days after the treatment, the cells were detached using TrypLE Express (Life technologies) for 5 minutes at 37°C and transferred into a 96-well V-shape microplate. 3.10 ⁴ cells were used for each binding. RBDs were premixed pairwise (Glutl.RBD.mouseFc and Xeno.RBD.rabbitFc) in culture medium containing 0.1% sodium azide and 1 mM EDTA. RBDs were added to MSCs and incubated at 37°C for 20 minutes. Cells were washed once with buffer B (PBS/2% FCS/0.1% sodium azide/1 mM EDTA) and then incubated with Alexa Fluor 647 goat anti-rabbit IgG (Life Technologies, 1/1 000) and R-PE goat anti-mouse IgGl (Life technologies, 1/100) in buffer B. After 30 minutes of incubation at 4°C in the dark, the cells were washed twice and resuspended in buffer B before flow cytometry analysis. Fluorescent signals were acquired on a FACS Verse flow cytometer (BD Biosciences) with 405, 488 and 640 nm excitation, and data analysis was performed using Flowjo software (Tree Star Inc.).

"Fluorescence minus One" controls were used to establish background level in RBD channels (R-PE and AF647). Signal/noise of 0.1 and 10 nM rapamycin conditions were normalized to control condition.

Result

Data represent XPRl/Glutl ratio on hMSC treated by 1 or 10 μg/mL of MMC or 0.1 or 10 nM of rapamycin normalized to the control condition (Figure 5). The pro- senescence drug Mitomycin C increases the XPRl/Glutl ratio in a dose- dependant manner which confirms that XPRl/Glutl ratio is a potential biomarker for accelerated cellular senescence, also called replicative senescence (mitomycin-treated cells are well known to stop cell proliferation). The anti-aging drug Rapamycin decreases the XPRl/Glutl ratio in a dose-dependant manner.

These data thus strongly support that the ratio XPRl/Glutl may be a biomarker for screening and identification of drugs impacting aging, such as, for example, anti-aging drugs or pro- senescence drugs.