Epigenetic Regulation of Retinal Regeneration

SUMMARY: In fish and amphibians the retina is able to fully regenerate from a variety of insults but this ability is progressively lost in higher vertebrates, with modest regeneration in birds, and essentially no regeneration in mammals. Müller glia (MG) are responsible for retinal regeneration and due to the remarkable regenerative capacity of zebrafish, many studies have used this model to uncover the molecular underpinnings of retinal regeneration. In zebrafish, retinal damage first stimulates MG to adopt a gliotic state, but this is transient and they subsequently reprogram into a stem cell-like state, re-enter the cell cycle and generate multipotent neurogenic progenitors that then give rise to new neurons. In mammals, including humans, the initial MG gliotic response to damage is similar to that in zebrafish, but MG fail to reprogram or generate new neurons. Instead, gliotic responses persist and MG proliferate and eventually form a glial scar. Recent studies have demonstrated that ectopic expression of a key pro-regenerative gene identified in zebrafish, ascl1, in MG of the adult mouse retina, along with inhibition of histone deactylase (Hdac) activity, stimulates a MG regenerative response. Regeneration correlated with increased chromatin accessibility at critical pro-regenerative loci and when combined with other published studies, suggest that the epigenetic landscape regulating the expression of pro- regenerative genes during the injury and regenerative responses is a key regulator of the ability of MG to reprogram and stimulate regeneration. Our knowledge of the epigenetic regulation of retinal regeneration is limited and this is a critical knowledge gap in the field. Indeed, given that regenerative responses can be stimulated from normally non-regenerative MG in the mammalian eye, understanding the epigenetic regulation of retinal regeneration could be transformational in supporting the development of new therapeutics aimed at restoring neurons lost to retinal degenerative diseases and retinal injuries. We have focused on the epigenetic regulation of retinal regeneration, using the zebrafish MG reprogramming and regeneration model. We have generated strong preliminary data supporting a critical role for Brd proteins during MG-dependent retinal regeneration and more specifically, a role for Brd4 during MG reprogramming. We have identified genes regulated by Brd activity and developed and validated novel transgenic tools for manipulating Brd activity and Brd-dependent gene function in MG with spatial and temporal precision. We hypothesize that Brd-mediated regulation of gene expression in MG is a key regulator of MG reprogramming and MG-dependent retinal regeneration. Experiments in this proposal test this hypothesis by determining the roles of specific brd proteins during MG-dependent retinal regeneration and identifying the gene regulatory networks in which they function. When completed, the results of this study will be of high impact in identifying and validating Brd-dependent genes that could be useful on their own in stimulating reprogramming or regenerative responses from mammalian MG or, in combination with other factors, enhance reprogramming and regenerative responses from mammalian MG.