Role of glast-positive glial cells during photoreceptor development

Purpose: Taking advantage of two transgenic lines, glast.DsRed and crx.gfp, that express fluorescent proteins in glial and photoreceptor cells respectively, we investigate the role of glast-positive glial cells (GPCs) in the survival/differentiation/proliferation of age-matched photoreceptor cells. Methods: Primary retinal cells were isolated from newborn transgenic mouse retina (glast.dsRed::crx.gfp) at postnatal day (P0/P1) and propagated in defined medium containing epidermal growth factor (EGF) and fibroblast growth factor 2 (bFGF). By flow-sorting another population of pure GPCs was isolated. Both populations were expanded and analyzed for the presence of specific retinal cell markers. Notably, the primary cell culture collected from the transgenic line glast.dsRed::crx.gfp showed a conspicuous presence of immature photoreceptors growing on top of GPCs. In order to reveal the role of such cells in the survival/differentiation/proliferation of photoreceptors we set up in vitro cultures of retina-derived cells that allowed long-term time-lapse recordings charting every cell division, death and differentiation event. To assess the regenerative potential of GPCs we challenged them with compounds mimicking retinal degeneration (NMU, NMDA, Zaprinast). Mass spectrometry (MS), immunostainings and other molecular approaches were performed to reveal adhesion molecules involved in the relationship between glial cells and photoreceptors. Results: Both primary cell lines were highly homogenous, with an elongated morphology and the majority expressed Müller glia markers (MG) such as glast, blbp, glt-1, vimentin, glutamine synthetase (GS), GFAP, cd44, mash1 and markers of reactive Müller glia such as nestin, pax6. Conversely, none of them were found positive for retinal neuron markers like tuj1, otx2, recoverin. Primary cultures of GPCs show the incapability of glial cells to give rise to photoreceptors in both wild type or degenerative environment. Furthermore, primary cultures of pure GPCs challenged with different compounds did not highlight the production of new glial cell-derived photoreceptors. Adhesion molecules involved in the contact between photoreceptors and glial cells are still under investigation. Conclusions: Primary glia cells do not give rise to photoreceptor cells in wt and degenerative conditions at least in vitro. The roles of glial cells seem to be more linked to the maintenance/proliferation of photoreceptor cells.