Chromatin access and epigenetic control over gene expression play important roles in regulating developmental processes. However, little is known about how chromatin access and epigenetic gene silencing influence mature glial cells and retinal regeneration. Herein, we investigate the expression and functions of S‐adenosylhomocysteine hydrolase (SAHH; AHCY) and histone methyltransferases (HMTs) during the formation of Müller glia (MG)‐derived progenitor cells (MGPCs) in the chick and mouse retinas. In chick, AHCY, AHCYL1 and AHCYL2, and many different HMTs are dynamically expressed by MG and MGPCs in damaged retinas. Inhibition of SAHH reduced levels of H3K27me3 and potently blocks the formation of proliferating MGPCs. By using a combination of single cell RNA‐seq and single cell ATAC‐seq, we find significant changes in gene expression and chromatin access in MG with SAHH inhibition and NMDA‐treatment; many of these genes are associated with glial and neuronal differentiation. A strong correlation across gene expression, chromatin access, and transcription factor motif access in MG was observed for transcription factors known to convey glial identity and promote retinal development. By comparison, in the mouse retina, inhibition of SAHH has no influence on the differentiation of neuron‐like cells from Ascl1‐overexpressing MG. We conclude that in the chick the activity of SAHH and HMTs are required for the reprogramming of MG into MGPCs by regulating chromatin access to transcription factors associated with glial differentiation and retinal development.

EZH2 (Enhancer of Zeste 2), a key component of the Polycomb Repressor Complex 2 (PRC2), is required for the differentiation of Muller glia (MG) during retinal development. However, nothing is known about the roles of EZH2 and PRC2 in mature or damaged retinas. Herein, we investigate the expression of PRC2-related factors and roles during the formation of Muller glia-derived progenitor cells (MGPCs) in the chick and mouse retinas. In chick, EZH2, PRC2-related factors, and EZH2-activators, JARID2 and AEBP1, are dynamically expressed by MG and MGPCs in damaged retinas. Inhibition of EZH2, EED or JARID2 blocks the formation of MGPCs. Similarly, inhibition of EZH2 suppresses the proliferation of progenitors in the circumferential marginal zone. By using a combination of single cell RNA-seq and single cell ATAC-seq, we find significant changes in gene expression and chromatin access in MG with EZH2 inhibition and NMDA-treatment, and many of these genes are associated with glial and neuronal differentiation. Correlation across gene expression, chromatin access and transcription factor motif access in MG was observed for transcription factors known to covey glial identity and promote retinal development. In the mouse retina, components of the PRC2 complex are downregulated in MG in damaged retinas and inhibition of EZH2 has no effect upon the formation of neuron-like cells from Ascl1-overexpressing MG. We conclude that in the chick, but not mouse, the activity of EZH2 is required for the reprogramming of MG into MGPCs by regulating chromatin access to transcription factors associated with glial differentiation and retinal development.

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