Adult Corneal Limbal Epithelium: A Model for Studying Neural Potential of Non-Neural Stem Cells/Progenitors

Abstract

Recent studies suggest that tissue-specific stem cells possess much wider potential for differentiation than previously thought and can, in some instances, even cross germ layer boundaries. However, information is lacking regarding the efficiency and the fidelity of their differentiation along heterologous lineages. To address these issues of transdifferentiation, we have analyzed the heterologous potential of stem cells within the same germ layer. We report the neural potential of cells isolated from the limbal epithelium of the adult cornea. Limbal epithelium, which, like the neuroepithelium, is ectodermally derived, participates in the regeneration of cornea throughout life. We have observed that limbal epithelial cells, when removed from their niche and cultured in the presence of mitogens, begin to express neural progenitor markers. Based on the self-renewal property, it is likely that the nestin-positive progenitors are derived from limbal stem cells rather than transit-amplifying (TA) cells that have limited proliferating potential. In differentiation conditions, a subset of these cells acquire neural morphology and express transcripts and proteins specific to neurons and glia, suggesting their differentiation along neural lineage. The acquisition of neural properties is regulated by BMP signaling. Neural differentiation of these cells is also observed upon heterotopic transplantation. Investigation of functional differentiation of cells by electrophysiological analysis reveals properties consistent with the presence of glia that are influenced by extracellular cues. However, similar analyses coupled with Ca(2+) imaging suggest an incomplete differentiation of limbal epithelial-derived neural progenitors into neurons in the condition studied. Our study, therefore, draws attention toward the necessity for rigorous characterization of transdifferentiation and offers a model for characterizing neural potential of heterologous stem cells/progenitors.

Corneal Epithelial Cultures Generated from Organ-Cultured Limbal Tissue: Factors Influencing Epithelial Cell Growth

“…In their study, nestin expression is considered as a marker of limbal cell neural potential. 53 Less specifically, knowing that nestin expression has been described in progenitor cells in other human and animal tissues, 54−57 we consider its wide expression in our limbal cultures as a marker of poorly differentiated cells. This seems confirmed by vimentin expression.…”

Limbal stem cells: Central concepts of corneal epithelial homeostasis

“…Several studies however, have implied a high multipotent differentiation potential when appropriate combinations of cellular signalling molecules are encountered: Rabbit limbal epithelial cell sheets transformed into fibroblasts when transplanted onto limbal stroma [65] ; during the culture of human limbal explants, the limbal epithelial cells which invaded into the stroma underwent epithelialmesenchymal transition [66] ; mouse limbal epithelial cells expressed opsin when transplanted onto mice retina, indicating their potential to differentiate into rod photoreceptors [67] ; and the potential to transdifferentiate to neuronal cells was demonstrated by Zhao et al [68] . In their study, rat limbal cell isolates maintained in growth factordriven culture system expressed neuronal progenitors, β-tubulin, nestin and neurofilament.…”

Corneal epithelial stem cells in health and disease

“…LESC are derived from neural ectoderm (18) and it has been proposed that human corneal stem cells may have the potential to differentiate into neuronal cells. The cornea is a highly innervated tissue, and it has been found that a subpopulation of human corneal stem cells exhibit functional neuronal properties in vitro (18,19). It is therefore possible that human corneal stem cells could be directed to differentiate into neurons and used to treat neurodegenerative diseases.…”