Normal formation of a vertebrate body plan and loss of tissue maintenance in the absence of ezh2.

Bilge San,Naomi D Chrispijn,Leonie M Kamminga,Marco Aben,Jeroen Bakkers,René F Ketting,Nadine Wittkopp,Simon J Van Heeringen,Anne K Lagendijk

doi:10.1038/srep24658

Abstract

Polycomb group (PcG) proteins are transcriptional repressors of numerous genes, many of which regulate cell cycle progression or developmental processes. We used zebrafish to study Enhancer of zeste homolog 2 (Ezh2), the PcG protein responsible for placing the transcriptional repressive H3K27me3 mark. We identified a nonsense mutant of ezh2 and generated maternal zygotic (MZ) ezh2 mutant embryos. In contrast to knockout mice for PcG proteins, MZezh2 mutant embryos gastrulate seemingly normal, but die around 2 days post fertilization displaying pleiotropic phenotypes. Expression analyses indicated that genes important for early development are not turned off properly, revealing a regulatory role for Ezh2 during zygotic gene expression. In addition, we suggest that Ezh2 regulates maternal mRNA loading of zygotes. Analyses of tissues arising later in development, such as heart, liver, and pancreas, indicated that Ezh2 is required for maintenance of differentiated cell fates. Our data imply that the primary role of Ezh2 is to maintain tissues after tissue specification. Furthermore, our work indicates that Ezh2 is essential to sustain tissue integrity and to set up proper maternal mRNA contribution, and presents a novel and powerful tool to study how PcG proteins contribute to early vertebrate development.

Highlights

PcG proteins are basically found in two complexes, Polycomb Repressive Complex 1 (PRC1) and PRC2
Before zygotic genome activation (ZGA), which starts around mid-blastula transition (MBT, 3.3 hours post fertilization) and is accompanied by degradation of maternal transcripts[22,23,24,25], levels of H3K4me[3] and H3K27me[3] are low
Ezh[2] has a WD repeat domain at the N-terminus, which is implicated in binding Eed and Suz[12] (Fig. 1a)

Summary

Introduction

PcG proteins are basically found in two complexes, Polycomb Repressive Complex 1 (PRC1) and PRC2. Most PcG mouse mutants display pre-gastrulation embryonic lethality[5,18,19] In mice, both homologs of RING1, Ring[1] and Rnf[2], are essential for development of primordial germ cells. It is clear from published work that PcG proteins are involved in conserved processes that are essential for organismal functioning, many critical questions remain unanswered It is not known what their role is during early development of a vertebrate system, a question that can be well addressed in zebrafish. A hint for this comes from rnf[2] mutant zebrafish embryos that die around 4–5 days post fertilization (dpf), a time at which organogenesis is normally completed, displaying defects in pectoral fin development[28]

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