Polycomb group (PcG) proteins are transcriptional repressors that control processes ranging from the maintenance of cell fate decisions and stem cell pluripotency in animals to the control of flowering time in plants1–6. In Drosophila, genetic studies identified more than 15 different PcG proteins that are required to repress homeotic (HOX) and other developmental regulator genes in cells where they must stay inactive1,7,8. Biochemical analyses established that these PcG proteins exist in distinct multiprotein complexes that bind to and modify chromatin of target genes1–4. Among those, Polycomb repressive complex 1 (PRC1) and the related dRing-associated factors (dRAF) complex contain an E3 ligase activity for monoubiquitination of histone H2A (refs 1–4). Here we show that the uncharacterized Drosophila PcG gene calypso encodes the ubiquitin carboxy-terminal hydrolase BAP1. Biochemically purified Calypso exists in a complex with the PcG protein ASX, and this complex, named Polycomb repressive deubiquitinase (PR-DUB), is bound at PcG target genes in Drosophila. Reconstituted recombinant Drosophila and human PR-DUB complexes remove monoubiquitin from H2A but not from H2B in nucleosomes. Drosophila mutants lacking PR-DUB show a strong increase in the levels of monoubiquitinated H2A. A mutation that disrupts the catalytic activity of Calypso, or absence of the ASX subunit abolishes H2A deubiquitination in vitro and HOX gene repression in vivo. Polycomb gene silencing may thus entail a dynamic balance between H2A ubiquitination by PRC1 and dRAF, and H2A deubiquitination by PR-DUB.

Summary Long noncoding RNAs (lncRNAs) are often expressed in a development-specific manner, yet little is known about their roles in lineage commitment. Here, we identified Braveheart (Bvht), a heart-associated lncRNA in mouse. Using multiple embryonic stem cell (ESC) differentiation strategies, we show that Bvht is required for progression of nascent mesoderm towards a cardiac fate. We find that Bvht is necessary for activation of a core cardiovascular gene network and functions upstream of MesP1 (mesoderm posterior 1), a master regulator of a common multipotent cardiovascular progenitor. We also show that Bvht interacts with SUZ12, a component of Polycomb Repressive Complex 2 (PRC2), during cardiomyocyte differentiation suggesting that Bvht mediates epigenetic regulation of cardiac commitment. Finally, we demonstrate a role for Bvht in maintaining cardiac fate in neonatal cardiomyocytes. Together, our work provides evidence for a long noncoding RNA with critical roles in the establishment of the cardiovascular lineage during mammalian development.

Cardiogenesis in mammals requires exquisite control of gene expression and faulty regulation of transcriptional programs underpins congenital heart disease (CHD), the most common defect among live births. Similarly, many adult cardiac diseases involve transcriptional changes and sometimes have a developmental basis. Long non-coding RNAs (lncRNAs) are a novel class of transcripts that regulate cellular processes by controlling gene expression; however, detailed insights into their biological and mechanistic functions are only beginning to emerge. Here, we discuss recent findings suggesting that lncRNAs are important factors in regulation of mammalian cardiogenesis and in the pathogenesis of CHD as well as adult cardiac disease. We also outline potential methodological and conceptual considerations for future studies of lncRNAs in the heart and other contexts.

γ‐Secretase is involved in the production of amyloid β‐peptide, which is the principal component of amyloid plaques in the brains of patients with Alzheimer disease. γ‐Secretase is a complex composed of presenilin (PS), nicastrin, anterior pharynx‐defective phenotype 1 (Aph1) and PS enhancer 2 (Pen2). We previously proposed a mechanism of complex assembly by which unassembled subunits are retained in the endoplasmic reticulum (ER) and only the fully assembled complex is exported from the ER. We have now identified Retention in endoplasmic reticulum 1 (Rer1) as a protein that is involved in the retention/retrieval of unassembled Pen2 to the ER. Direct binding of unassembled Pen2 to Rer1 is mediated by the first transmembrane domain of Pen2, and a conserved asparagine in this domain is required. Downregulation of Rer1 leads to increased surface localization of Pen2, whereas overexpression of Rer1 stabilizes unassembled Pen2. To our knowledge, Rer1 is the first identified interaction partner of mammalian transmembrane‐based retention/retrieval signals.

Polycomb group (PcG) proteins exist in multiprotein complexes that modify chromatin to repress transcription. Drosophila PcG proteins Sex combs extra (Sce; dRing) and Posterior sex combs (Psc) are core subunits of PRC1-type complexes. The Sce:Psc module acts as an E3 ligase for monoubiquitylation of histone H2A, an activity thought to be crucial for repression by PRC1-type complexes. Here, we created an Sce knockout allele and show that depletion of Sce results in loss of H2A monoubiquitylation in developing Drosophila. Genome-wide profiling identified a set of target genes co-bound by Sce and all other PRC1 subunits. Analyses in mutants lacking individual PRC1 subunits reveals that these target genes comprise two distinct classes. Class I genes are misexpressed in mutants lacking any of the PRC1 subunits. Class II genes are only misexpressed in animals lacking the Psc-Su(z)2 and Polyhomeotic (Ph) subunits but remain stably repressed in the absence of the Sce and Polycomb (Pc) subunits. Repression of class II target genes therefore does not require Sce and H2A monoubiquitylation but might rely on the ability of Psc-Su(z)2 and Ph to inhibit nucleosome remodeling or to compact chromatin. Similarly, Sce does not provide tumor suppressor activity in larval tissues under conditions in which Psc-Su(z)2, Ph and Pc show such activity. Sce and H2A monoubiquitylation are therefore only crucial for repression of a subset of genes and processes regulated by PRC1-type complexes. Sce synergizes with the Polycomb repressive deubiquitinase (PR-DUB) complex to repress transcription at class I genes, suggesting that H2A monoubiquitylation must be appropriately balanced for their transcriptional repression.

No abstract

γ-Secretase is critically involved in the Notch pathway and in Alzheimer's disease. The four subunits of γ-secretase assemble in the endoplasmic reticulum (ER) and unassembled subunits are retained/retrieved to the ER by specific signals. We here describe a novel ERretention/retrieval signal in the transmembrane domain (TMD) 4 of presenilin 1, a subunit of γ-secretase. TMD4 also is essential for complex formation, conferring a dual role for this domain. Likewise, TMD1 of Pen2 is bifunctional as well. It carries an ER-retention/retrieval signal and is important for complex assembly by binding to TMD4. The two TMDs directly interact with each other and mask their respective ER-retention/retrieval signals, allowing surface transport of reporter proteins. Our data suggest a model how assembly of Pen2 into the nascent γ-secretase complex could mask TMD-based ER-retention/retrieval signals to allow plasma membrane transport of fully assembled γ-secretase.

The evolution of brain complexity correlates with an increased expression of long, noncoding (lnc) RNAs in neural tissues. Although prominent examples illustrate the potential of lncRNAs to scaffold and target epigenetic regulators to chromatin loci, only few cases have been described to function during brain development. We present a first functional characterization of the lncRNA LINC01322, which we term RUS for “RNA upstream of Slitrk3.” The RUS gene is well conserved in mammals by sequence and synteny next to the neurodevelopmental gene Slitrk3. RUS is exclusively expressed in neural cells and its expression increases during neuronal differentiation of mouse embryonic cortical neural stem cells. Depletion of RUS locks neuronal precursors in an intermediate state towards neuronal differentiation resulting in arrested cell cycle and increased apoptosis. RUS associates with chromatin in the vicinity of genes involved in neurogenesis, most of which change their expression upon RUS depletion. The identification of a range of epigenetic regulators as specific RUS interactors suggests that the lncRNA may mediate gene activation and repression in a highly context-dependent manner.