Di- and trimethylation of lysine 27 on histone H3 (H3K27me2/3) is an important gene repression mechanism. H3K27me2/3-specific demethylase, Jmjd3, was expressed in the inner nuclear layer during late retinal development. In contrast, H3K27 methyltransferase, Ezh2, was highly expressed in the embryonic retina but its expression decreased rapidly after birth. Jmjd3 loss of function in the developing retina resulted in failed differentiation of PKC-positive bipolar cell subsets (rod-ON-BP) and reduced transcription factor Bhlhb4 expression, which is critical for the differentiation of rod-ON-BP cells. Overexpression of Bhlhb4, but not of other BP cell-related genes, such as transcription factors Neurod and Chx10, in Jmjd3-knockdown retina rescued loss of PKC-positive BP cells. Populations of other retinal cell subsets were not significantly affected. In addition, proliferation activity and apoptotic cell number during retinal development were not affected by the loss of Jmjd3. Levels of histone H3 trimethyl Lys27 (H3K27me3) in the Bhlhb4 locus were lower in Islet-1-positive BP cells and amacrine cells than in the Islet-1-negative cell fraction. The Islet-1-negative cell fraction consisted mainly of photoreceptors, suggestive of lineage-specific demethylation of H3K27me3 in the Bhlhb4 locus. We propose that lineage-specific H3K27me3 demethylation of critical gene loci by spatiotemporal-specific Jmjd3 expression is required for appropriate maturation of retinal cells.
Keywords: histone methylation; interneuron; progenitor.