GLUTAMATE-INDUCED EPIGENETIC AND MORPHOLOGICAL CHANGES ALLOW RAT MOLLER CELL DEDIFFERENTIATION BUT NOT FURTHER ACQUISITION OF A PHOTORECEPTOR PHENOTYPE

Muller cells are not only the main glial cell type in the retina but also latent progenitor/stem cells, which in pathological conditions can transdifferentiate to a neuronal phenotype and regenerate the neurons lost in a mature retina. Several signal transduction pathways can induce the dedifferentiation of mature Muller cells to a progenitor-like state, including that stimulated by glutamate. However, the precise molecular mechanisms by which terminally differentiated cells are initially primed to acquire multipotency remain unclear. In the present study, we have characterized early genetic and epigenetic events that occur immediately after glutamate-induced dedifferentiation of fully differentiated Muller cells is initiated. Using Muller cell-enriched cultures from postnatal rats, we demonstrate that glutamate triggers a rapid dedifferentiation response characterized by changes in cell morphology coupled to the induction of progenitor cell marker gene expression (e.g., nestin, lin28 and sox2) within 1 h. Dedifferentiation involved the activation of N-methyl-D-aspartate and type II metabotropic glutamate receptors, as well as global DNA demethylation (evident through the decrease in methyl-CpG-binding protein 2 immunoreactivity) and an increase in gadd45-beta gene expression; although, early progenitor gene expression was only partially inhibited by pharmacological impairment of DNA methylation. Importantly, the expression of Muller glia identity genes (i.e., glutamine synthetase; cellular retinaldehyde binding protein, CRALBP) is retained through the process. Dedifferentiated Muller cells held an early neuronal differentiation potential similar to that observed in retinal progenitor-enriched cultures but, contrary to the latter, dedifferentiated Muller cells failed to further differentiate into mature photoreceptor lineages. We speculate that, in spite of the initial triggering of the dedifferentiation pathways, these cells may exhibit a certain degree of epigenetic memory that precludes them from further differentiation. (C) 2013 IBRO. Published by Elsevier Ltd. All rights reserved.