Roles of the histone methyltransferase complex PRC2 during retinal development in xenopus

Much has been done to define and characterize the mechanisms that control the fate of multipotent retinal progenitors during eye development, but our understanding of this process is still nascent. The histone methyltransferase complex PRC2 is a key regulator of differentiation during the development of organs such as skin and cortex, but its roles in vertebrate retinogenesis have not been explored. My work focused on investigating the possible involvement of PRC2 in the progression of retinal progenitors from proliferation to differentiation during eye development in Xenopus laevis embryos. In the first chapter, I report the cloning of Xenopus Suz12, and determine its expression pattern during development. Xsuz12 is provided maternally and its expression persists as development progresses, particularly in the developing central nervous system. Comparative analysis of the PRC2 core subunits Xez, Xeed, Xsuz12 and Xrbbp4 suggests that their expression largely overlaps in the nervous system. In the second chapter, I characterize in detail the retinal expression of the PRC2 core subunits, and explore its potential roles during development using a loss of function approach. I show that the transcripts of the PRC2 core subunits are coincidently expressed in retinal progenitors and are downregulated upon retinal differentiation. Surprisingly, I found that the levels of the H3K27me3 mark that is catalyzed by PRC2 greatly increase in terminally differentiated cells. Loss of PRC2 function led to a marked decrease in H3K27me3 in retinal cell types. Blocking the translation of the core subunits Xez or Xsuz12 caused a reduction in eye size, inhibition of differentiation genes and a bias toward generation of late born cell types. ChIP-seq analysis on whole embryos revealed that H3K27me3 transiently and selectively decorates a subset of genes expressed in the eye, some of which are known negative regulators of retinal differentiation. In the third chapter, I characterize the expression pattern of the newly identified binding partner of PRC2, Xjarid2, in the developing nervous system of Xenopus and found that it is particularly expressed in differentiated cells. Preliminary loss of function analysis suggests that Xjarid2 is required for neural differentiation, in agreement with data on PRC2 core subunits. Taken together, my data indicate that the PRC2 complex is an important regulator of retinal neurogenesis in Xenopus and highlights the contribution of histone methylation to the regulation of retinal proliferation and differentiation.