A comparative study of two distinct populations of stem cells in the adult human retina : investigation of the role of Sox2 in the maintenance of progenicity

Field: Neuroscience (Life Sciences)

This thesis investigated differences and similarities between the stem cell population of the adult human retina and the progenitor cell population of the ciliary body. Stem cells from both the ciliary body and neural retina expressed the progenitor markers Sox2, Chx10 and Notch1 in situ. In contrast, Nestin expression was only observed in cells of the neural retina, particularly in the marginal region adjacent to the ora serrata. Nestin expression decreased towards the posterior retina where it co-stained with markers of Muller glia, including CRALBP and Vimentin. Nestin-positive cells in the retinal margin appeared as bundles of spindle cells lacking lamination and exhibiting glial morphology, resembling those seen in the ciliary marginal zone (CMZ) of fish and amphibians. Cells in this CM-like zone were able to re-enter the cell cycle upon retinal explant culture with EGF. \nDifferences in growth and differentiation in vitro were also observed between ciliary epithelium and Muller stem cells. Following dissociation from the retina, Muller cells could only be cultured as adherent cells whist ciliary epithelium could be cultured as both adherent cells and floating neurospheres. Ciliary epithelial cells dissociated from spheres could also be expanded as monolayers. Both cell types expressed SOX2, PAX6, CHX10 and NOTCH1 in culture, but as seen in situ, only Muller stem cells expressed Nestin in vitro. Upon isolation, both cell types retained the morphological features associated with their anatomical origin. In culture, ciliary epithelial cells displayed epithelial morphology and stained strongly for epithelial markers, whilst Muller cells maintained their glial morphology and stained for glial markers. These results strongly suggest that neural progenitor/stem cells reported in the ciliary body constitute a different population from those cells isolated from the neural retina. Transplantation of Muller stem cells onto explants of human retina resulted in limited migration. However, treatment of explants with Chondroitinase ABC (a matrix degrading enzyme) enhanced the migration of transplanted cells. These observations suggested that modification of the extracellular matrix may improve the migration and integration of transplanted cells. \nInvestigations on the expression of the transcription factors SOX2 and CHX10 in Muller stem cells showed that over 90% of these cells express high levels of SOX2 and that this expression was maintained under several culture conditions. In contrast, Chx10 expression was modified by growth factors and extracellular matrix proteins. Due to this high expression of Sox2, the role of this factor in the maintenance of progenicity in Muller stem cells, and its potential role in neural differentiation was examined. Conditions that promoted downregulation of Sox2 caused upregulation of photoreceptor mRNA, including Nrl, rhodopsin and S-opsin. The effects of Sox2 downregulation were further investigated by transfecting Muller stem cells with Sox2 shRNA. Transfected cells aquired a neural morphology and showed upregulation of mRNA and proteins expressed by terminally differentiated neurons. However, TUNEL and Caspase assays revealed that downregulation of Sox2 resulted in the induction of Muller stem cell apoptosis, indicating that expression of this transcription factor is necessary for survival of these cells.