Isolation, genomic and proteomic characterization of fibroblastic and epithelial limbal stem cells and evaluation of their multilineage differentiation capability

BACKGROUND: The limbus of the eye, located at the junction of the cornea and conjunctiva of the ocular surface, represents a unique stem cell niche in human body. A critical advantage of limbal cells is that they are easily accessible with a well established and minimally invasive procedure. Several groups have reported the gene expression profile of limbal and corneal epithelial cells that has significantly contributed to the understanding of several cellular pathways and intrinsic factors that underpin the phenotypic difference between the two cell types [1-3]. However the gene expression profile of the human limbal epithelial and limbal stromal cultured cells obtained from the native limbal tissue has not been addressed until now. The lack of specific molecular markers for the identification of the multipotent limbal subpopulation limited the investigation of their differentiation capability. SSEA4 (stage specific embryonic antigen) is recognized marker of pluripotent stem cells, such as embryonic stem cells, and not of multipotent stem cells. Actually, the researcher are agreed that SSEA4 expression is lost from stem cells as they start to differentiate. It is generally agreed that the LESC are characterized by special location in the limbus, clonality, cytokeratin profile, transformation-related protein 63 (p63) delta isomers, and ATP-binding cassette sub-family G member 2 (ABCG2) expression [4-5]. Fibroblast (f)-LSCs represent a multipotent stem cell population characterized as CD105+, CD73+, CD90+, CD34+ and CD45as well as by their selfrenewal and multi-lineage differentiation capability [6-7]. AIM: To identify the better protocol of isolation and expansion for LESC and f-LSCs to obtain two purified and distinct subpopulations for molecular and proteomic characterization to evaluate their multilineage differentiation capability. MATERIAL AND METHODS: Cytofluorimetric analisys; sphere formation assay; qRT PCR and proteomic assays; multilineage differentiation assays were performed. RESULTS: Both f-LSC and LESC populations expressed SSEA4 (principal staminal marker) 98.63 ± 3.5%; vs 78.32 ± 2.8%, respectively (at 2PD) and ABCG2 (limbal specific marker). The f-LSC genomic expression profile in comparison to LESC didn’t show significant differences and both cell populations kept a stem genomic expression profiles since 24 PD. f-LSC kept an high SSEA4 positive expression (98.63 ± 3.5%; 90.36 ± 1.4%; 2PD-14PD) whereas LESC showed a CK15+/ΔNp63+ pattern and a reduced expression of SSEA4 (78.32 ± 2.8% vs 13.90± 4.6 %; 2PD-14PD). f-LSC, but not LESC, were differentiated into pancreatic β-cell phenotype (insulin-producing beta-like cells). Furthermore, both f-LSC and LESC achieved terminal osteoblastic and adipose differentiation. By contrast, the specific for committed LESC towards definitive epithelial phenotype cytokeratin profile, reduced LESC multilineage differentiation skill. CONCLUSIONS: The identification of genetic profiles and novel molecular markers characteristic of specific cellular phenotypes is likely to assist in better defining the stages of progression from unspecialized cells to cell types of interest. Yet, translation of these promising basic science discoveries into successful cell replacement therapy in human subjects is likely to require sophisticated methods for assessing genetic and phenotypic stability.