Trimethylation of histone H3 lysine 27 (H3K27me3) by Polycomb repressive complex 2 (PRC2) is essential for transcriptional silencing of Polycomb target genes, whereas acetylation of H3K27 (H3K27ac) has recently been shown to be associated with many active mammalian genes. The Trithorax protein (TRX),which associates with the histone acetyltransferase CBP, is required for maintenance of transcriptionally active states and antagonizes Polycomb silencing, although the mechanism underlying this antagonism is unknown. Here we show that H3K27 is specifically acetylated by Drosophila CBP and its deacetylation involves RPD3. H3K27ac is present at high levels in early embryos and declines after 4 hours as H3K27me3 increases. Knockdown of E(Z)decreases H3K27me3 and increases H3K27ac in bulk histones and at the promoter of the repressed Polycomb target gene abd-A, suggesting that these indeed constitute alternative modifications at some H3K27 sites. Moderate overexpression of CBP in vivo causes a global increase in H3K27ac and a decrease in H3K27me3, and strongly enhances Polycomb mutant phenotypes. We also show that TRX is required for H3K27 acetylation. TRX overexpression also causes an increase in H3K27ac and a concomitant decrease in H3K27me3 and leads to defects in Polycomb silencing. Chromatin immunoprecipitation coupled with DNA microarray (ChIP-chip) analysis reveals that H3K27ac and H3K27me3 are mutually exclusive and that H3K27ac and H3K4me3 signals coincide at most sites. We propose that TRX-dependent acetylation of H3K27 by CBP prevents H3K27me3 at Polycomb target genes and constitutes a key part of the molecular mechanism by which TRX antagonizes or prevents Polycomb silencing.

Recent studies have identified a genetic variant rs641738 near two genes encoding membrane bound O-acyltransferase domain-containing 7 (MBOAT7) and transmembrane channel-like 4 (TMC4) that associate with increased risk of non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), alcohol-related cirrhosis, and liver fibrosis in those infected with viral hepatitis (Buch et al., 2015; Mancina et al., 2016; Luukkonen et al., 2016; Thabet et al., 2016; Viitasalo et al., 2016; Krawczyk et al., 2017; Thabet et al., 2017). Based on hepatic expression quantitative trait loci analysis, it has been suggested that MBOAT7 loss of function promotes liver disease progression (Buch et al., 2015; Mancina et al., 2016; Luukkonen et al., 2016; Thabet et al., 2016; Viitasalo et al., 2016; Krawczyk et al., 2017; Thabet et al., 2017), but this has never been formally tested. Here we show that Mboat7 loss, but not Tmc4, in mice is sufficient to promote the progression of NAFLD in the setting of high fat diet. Mboat7 loss of function is associated with accumulation of its substrate lysophosphatidylinositol (LPI) lipids, and direct administration of LPI promotes hepatic inflammatory and fibrotic transcriptional changes in an Mboat7-dependent manner. These studies reveal a novel role for MBOAT7-driven acylation of LPI lipids in suppressing the progression of NAFLD.

Polycomb Group (PcG) and Trithorax Group (TrxG) proteins are key epigenetic regulators of global transcription programs. Their antagonistic chromatin-modifying activities modulate the expression of many genes and affect many biological processes. Here we report that heterozygous mutations in two core subunits of Polycomb Repressive Complex 2 (PRC2), the histone H3 lysine 27 (H3K27)-specific methyltransferase E(Z) and its partner, the H3 binding protein ESC, increase longevity and reduce adult levels of trimethylated H3K27 (H3K27me3). Mutations in trithorax (trx), a well known antagonist of Polycomb silencing, elevate the H3K27me3 level of E(z) mutants and suppress their increased longevity. Like many long-lived mutants, E(z) and esc mutants exhibit increased resistance to oxidative stress and starvation, and these phenotypes are also suppressed by trx mutations. This suppression strongly suggests that both the longevity and stress resistance phenotypes of PRC2 mutants are specifically due to their reduced levels of H3K27me3 and the consequent perturbation of Polycomb silencing. Consistent with this, long-lived E(z) mutants exhibit derepression of Abd-B, a well-characterized direct target of Polycomb silencing, and Odc1, a putative direct target implicated in stress resistance. These findings establish a role for PRC2 and TRX in the modulation of organismal longevity and stress resistance and indicate that moderate perturbation of Polycomb silencing can increase longevity.aging | epigenetics | histone methyltransferase | Polycomb silencing

The body wall muscles in the Drosophila larva arise from interactions between Duf/Kirre and Irregular chiasm C-roughest (IrreC-rst)-expressing founder myoblasts and sticks-and-stones (SNS)-expressing fusion competent myoblasts in the embryo. Herein, we demonstrate that SNS mediates heterotypic adhesion of S2 cells with Duf/Kirre and IrreC-rst-expressing S2 cells, and colocalizes with these proteins at points of cell contact. These properties are independent of their transmembrane and cytoplasmic domains, and are observed quite readily with GPI-anchored forms of the ectodomains. Heterotypic interactions between Duf/Kirre and SNS-expressing S2 cells occur more rapidly and to a greater extent than homotypic interactions with other Duf/Kirre-expressing cells. In addition, Duf/Kirre and SNS are present in an immunoprecipitable complex from S2 cells. In the embryo, Duf/Kirre and SNS are present at points of contact between founder and fusion competent cells. Moreover, SNS clustering on the cell surface is dependent on Duf/Kirre and/or IrreC-rst. Finally, although the cytoplasmic and transmembrane domains of SNS are expendable for interactions in culture, they are essential for fusion of embryonic myoblasts.

Trithorax group (TrxG) proteins antagonize Polycomb silencing and are required for maintenance of transcriptionally active states. We previously showed that the Drosophila melanogaster acetyltransferase CREB-binding protein (CBP) acetylates histone H3 lysine 27 (H3K27ac), thereby directly blocking its trimethylation (H3K27me3) by Polycomb repressive complex 2 (PRC2) in Polycomb target genes. Here, we show that H3K27ac levels also depend on other TrxG proteins, including the histone H3K27-specific demethylase UTX and the chromatin-remodeling ATPase Brahma (BRM). We show that UTX and BRM are physically associated with CBP in vivo and that UTX, BRM, and CBP colocalize genome-wide on Polycomb response elements (PREs) and on many active Polycomb target genes marked by H3K27ac. UTX and BRM bind directly to conserved zinc fingers of CBP, suggesting that their individual activities are functionally coupled in vivo . The bromodomain-containing C terminus of BRM binds to the CBP PHD finger, enhances PHD binding to histone H3, and enhances in vitro acetylation of H3K27 by recombinant CBP. brm mutations and knockdown of UTX by RNA interference (RNAi) reduce H3K27ac levels and increase H3K27me3 levels. We propose that direct binding of UTX and BRM to CBP and their modulation of H3K27ac play an important role in antagonizing Polycomb silencing.