Histone H2A mono-ubiquitination is a crucial step to mediate PRC1-dependent repression of developmental genes to maintain ES cell identity.

Two distinct Polycomb complexes, PRC1 and PRC2, collaborate to maintain epigenetic repression of key developmental loci in embryonic stem cells (ESCs). PRC1 and PRC2 have histone modifying activities, catalyzing mono-ubiquitination of histone H2A (H2AK119u1) and trimethylation of H3 lysine 27 (H3K27me3), respectively. Compared to H3K27me3, localization and the role of H2AK119u1 are not fully understood in ESCs. Here we present genome-wide H2AK119u1 maps in ESCs and identify a group of genes at which H2AK119u1 is deposited in a Ring1-dependent manner. These genes are a distinctive subset of genes with H3K27me3 enrichment and are the central targets of Polycomb silencing that are required to maintain ESC identity. We further show that the H2A ubiquitination activity of PRC1 is dispensable for its target binding and its activity to compact chromatin at Hox loci, but is indispensable for efficient repression of target genes and thereby ESC maintenance. These data demonstrate that multiple effector mechanisms including H2A ubiquitination and chromatin compaction combine to mediate PRC1-dependent repression of genes that are crucial for the maintenance of ESC identity. Utilization of these diverse effector mechanisms might provide a means to maintain a repressive state that is robust yet highly responsive to developmental cues during ES cell self-renewal and differentiation.

A FLCN-TFE3 Feedback Loop Prevents Excessive Glycogenesis and Phagocyte Activation by Regulating Lysosome Activity.

The tumor suppressor folliculin (FLCN) suppresses nuclear translocation of TFE3, a master transcription factor for lysosomal biogenesis, via regulation of amino-acid-sensing Rag GTPases. However, the importance of this lysosomal regulation in mammalian physiology remains unclear. Following hematopoietic-lineage-specific Flcn deletion in mice, we found expansion of vacuolated phagocytes that accumulate glycogen in their cytoplasm, phenotypes reminiscent of lysosomal storage disorder (LSD). We report that TFE3 acts in a feedback loop to transcriptionally activate FLCN expression, and FLCN loss disrupts this loop, augmenting TFE3 activity. Tfe3 deletion in Flcn knockout mice reduces the number of phagocytes and ameliorates LSD-like phenotypes. We further reveal that TFE3 stimulates glycogenesis by promoting the expression of glycogenesis genes, including Gys1 and Gyg, upon loss of Flcn. Taken together, we propose that the FLCN-TFE3 feedback loop acts as a rheostat to control lysosome activity and prevents excessive glycogenesis and LSD-like phagocyte activation.

PCGF6-PRC1 suppresses premature differentiation of mouse embryonic stem cells by regulating germ cell-related genes.

The ring finger protein PCGF6 (polycomb group ring finger 6) interacts with RING1A/B and E2F6 associated factors to form a non-canonical PRC1 (polycomb repressive complex 1) known as PCGF6-PRC1. Here, we demonstrate that PCGF6-PRC1 plays a role in repressing a subset of PRC1 target genes by recruiting RING1B and mediating downstream mono-ubiquitination of histone H2A. PCGF6-PRC1 bound loci are highly enriched for promoters of germ cell-related genes in mouse embryonic stem cells (ESCs). Conditional ablation of Pcgf6 in ESCs leads to robust de-repression of such germ cell-related genes, in turn affecting cell growth and viability. We also find a role for PCGF6 in pre- and peri-implantation mouse embryonic development. We further show that a heterodimer of the transcription factors MAX and MGA recruits PCGF6 to target loci. PCGF6 thus links sequence specific target recognition by the MAX/MGA complex to PRC1-dependent transcriptional silencing of germ cell-specific genes in pluripotent stem cells.

Polycomb group proteins Ring1A/B are functionally linked to the core transcriptional regulatory circuitry to maintain ES cell identity.

The Polycomb group (PcG) proteins mediate heritable silencing of developmental regulators in metazoans, participating in one of two distinct multimeric protein complexes, the Polycomb repressive complexes 1 (PRC1) and 2 (PRC2). Although PRC2 has been shown to share target genes with the core transcription network, including Oct3/4, to maintain embryonic stem (ES) cells, it is still unclear whether PcG proteins and the core transcription network are functionally linked. Here, we identify an essential role for the core PRC1 components Ring1A/B in repressing developmental regulators in mouse ES cells and, thereby, in maintaining ES cell identity. A significant proportion of the PRC1 target genes are also repressed by Oct3/4. We demonstrate that engagement of PRC1 at target genes is Oct3/4-dependent, whereas engagement of Oct3/4 is PRC1-independent. Moreover, upon differentiation induced by Gata6 expression, most of the Ring1A/B target genes are derepressed and the binding of Ring1A/B to their target loci is also decreased. Collectively, these results indicate that Ring1A/B-mediated Polycomb silencing functions downstream of the core transcriptional regulatory circuitry to maintain ES cell identity.