The epigenetic modifier Usp22 regulates the transition of embryonic stem cells from self-renewal to differentiation via transcriptional repression of SOX2
Pluripotent embryonic stem cells (ESCs) undergo self-renewal until stimulated to differentiate down specific lineage pathways. Many of the transcriptional networks that drive the movement from a self-renewing ESC to a differentiating cell have been identified. However, fundamental questions remain unanswered about the epigenetic programs that control these changes in gene expression. Here we report that the histone ubiquitin hydrolase, Usp22, is a critical epigenetic modifier that controls this transition from self-renewal to differentiation. This study reveals that Usp22 is induced as ESCs differentiate. More importantly, both human and mouse ESCs require USP22 for proper differentiation. Conversely, ectopic expression of USP22 is sufficient to trigger the spontaneous differentiation of ESCs, even in the absence of other differentiation signals. Gene expression profiling in ESCs demonstrated broad defects in the transcription of genes linked to all three germ layers when USP22 levels were depleted. Remarkably, this analysis revealed that USP22 is also required for the repression of a limited number of pluripotency genes, including SOX2. These findings suggested that the loss of USP22 might result in both defective differentiation and lineage-specific gene expression due to the failure to repress pluripotency factors. As the core pluripotency factor most affected by USP22 depletion, SOX2\u27s role in this phenotype was explored. Epistasis experiments suggest the possibility that SOX2 de-repression may be responsible for the effects of USP22 depletion, as blocking the increase in SOX2 reversed the USP22 phenotype. Mechanistically, USP22 was found to directly occupy the SOX2 locus where it controls the relative level of histone H2B ubiquitylation. USP22-mediated changes in H2B ubiquitylation at SOX2 likely explain its effects on SOX2 transcription and pluripotency, since our studies show that RNF20, the E3 ligase responsible for H2B ubiquitylation, is essential for SOX2 expression in ESCs. Our study reveals an epigenetic mechanism that represses the core pluripotency transcriptional network in ESCs, allowing ESCs to transition from a state of self-renewal into lineage-specific differentiation programs.
