DNA demethylation is a driver for chick retina regeneration
BackgroundA promising avenue toward human retina regeneration lies in identifying the factors that promote cellular reprogramming to retinal neurons in organisms able to undergo retina regeneration. The embryonic chick can regenerate a complete neural retina, after retinectomy, via retinal pigment epithelium (RPE) reprogramming in the presence of FGF2. Cellular reprogramming resets the epigenetic landscape to drive shifts in transcriptional programs and cell identity. Here, we systematically analyzed the reprogramming competent chick RPE prior to injury, and during different stages of reprogramming. We examined the dynamic changes in the levels and distribution of histone marks and DNA modifications, as well as conducted a comprehensive analysis of the DNA methylome during this process.\n\nResultsIn addition to changes in the expression of genes associated with epigenetic modifications during RPE reprogramming, we observed dynamic changes in histone marks and intermediates of the process of DNA demethylation. At early times after injury, H3K27me3 and 5mC repression marks decreased while 5caC and the H3K4me3 activation mark increased, suggesting genome-wide changes in the bivalent chromatin, impaired DNA methylation, and active DNA demethylation in the chromatin reconfiguration of reprogramming RPE. Comprehensive analysis of the methylome by whole-genome bisulfite sequencing (WGBS) confirmed extensive rearrangements of DNA methylation patterns including differentially methylated regions (DMRs) found at promoters of genes associated with chromatin organization and fibroblast growth factor production. In contrast, genes associated with early RPE reprogramming are hypomethylated in the intact RPE and remain hypomethylated during the process. During the generation of a neuroepithelium (NE) at later stages of reprogramming, decreased levels of H3K27me3, 5mC, and 5hmC coincide with elevated levels of H3K27Ac and 5caC, indicating an active demethylation process and genome-wide changes in the active regulatory landscape. Finally, we identify Tet methylcytosine dioxygenase 3 (TET3) as an important factor for DNA demethylation and retina regeneration in the embryonic chick, capable of reprogramming RPE in the absence of exogenous FGF2.\n\nConclusionOur results demonstrated that injury signals early in RPE reprogramming trigger genome-wide dynamic changes in chromatin, including bivalent chromatin and DNA methylation. In the presence of FGF2 these dynamic modifications are further sustained in the commitment to form a new retina. We identify DNA demethylation as a key process driving the process of RPE reprogramming and identified TET3 as a factor able to reprogram RPE in absence of FGF2. Our findings reveal active DNA demethylation as an important process that may be applied to remove the epigenetic barriers in order to regenerate retina in mammals.