[Show abstract][Hide abstract] ABSTRACT: Cells rely on autophagy to clear misfolded proteins and damaged organelles to maintain cellular homeostasis. In this study we use the new autophagy inhibitor PIK-III to screen for autophagy substrates. PIK-III is a selective inhibitor of VPS34 that binds a unique hydrophobic pocket not present in related kinases such as PI(3)Kα. PIK-III acutely inhibits autophagy and de novo lipidation of LC3, and leads to the stabilization of autophagy substrates. By performing ubiquitin-affinity proteomics on PIK-III-treated cells we identified substrates including NCOA4, which accumulates in ATG7 -deficient cells and co-localizes with autolysosomes. NCOA4 directly binds ferritin heavy chain-1 (FTH1) to target the iron-binding ferritin complex with a relative molecular mass of 450,000 to autolysosomes following starvation or iron depletion. Interestingly, Ncoa4 −/− mice exhibit a profound accumulation of iron in splenic macrophages, which are critical for the reutilization of iron from engulfed red blood cells. Taken together, the results of this study provide a new mechanism for selective autophagy of ferritin and reveal a previously unappreciated role for autophagy and NCOA4 in the control of iron homeostasis in vivo. Cells rely on the autophagy pathway to survive diverse cellular insults such as nutrient depletion, accumulation of protein aggregates, damaged mitochondria or intracellular bacteria
[Show abstract][Hide abstract] ABSTRACT: Autophagy is a vesicular trafficking pathway that regulates degradation of aggregated proteins and damaged organelles. Initiation of autophagy requires several multiprotein signaling complexes such as the ULK1 kinase complex and the Vps34 lipid kinase complex, which generates phosphatidylinositol 3-phosphate (PtdIns (3)P) on the forming autophagosomal membrane. Alterations in autophagy have been reported for various diseases including myopathies. Here, we show that skeletal muscle autophagy is compromised in mice deficient for the X-linked myotubular myopathy (XLMTM)-associated PtdIns (3)P phosphatase myotubularin (MTM1). Mtm1-deficient muscle display several cellular abnormalities including a profound increase in ubiquitin aggregates and abnormal mitochondria. Further, we show that Mtm1 deficiency is accompanied by activation of mTORC1 signaling which persists even following starvation. In vivo pharmacological inhibition of mTOR is sufficient to normalize aberrant autophagy and improve muscle phenotypes in Mtm1 null mice. These results suggest that aberrant mTORC1 signaling and impaired autophagy are a consequence of the loss of Mtm1 and may play a primary role in disease pathogenesis.
[Show abstract][Hide abstract] ABSTRACT: R-spondin proteins strongly potentiate Wnt signalling and function as stem-cell growth factors. Despite the biological and therapeutic significance, the molecular mechanism of R-spondin action remains unclear. Here we show that the cell-surface transmembrane E3 ubiquitin ligase zinc and ring finger 3 (ZNRF3) and its homologue ring finger 43 (RNF43) are negative feedback regulators of Wnt signalling. ZNRF3 is associated with the Wnt receptor complex, and inhibits Wnt signalling by promoting the turnover of frizzled and LRP6. Inhibition of ZNRF3 enhances Wnt/β-catenin signalling and disrupts Wnt/planar cell polarity signalling in vivo. Notably, R-spondin mimics ZNRF3 inhibition by increasing the membrane level of Wnt receptors. Mechanistically, R-spondin interacts with the extracellular domain of ZNRF3 and induces the association between ZNRF3 and LGR4, which results in membrane clearance of ZNRF3. These data suggest that R-spondin enhances Wnt signalling by inhibiting ZNRF3. Our study provides new mechanistic insights into the regulation of Wnt receptor turnover, and reveals ZNRF3 as a tractable target for therapeutic exploration.
[Show abstract][Hide abstract] ABSTRACT: Trimethylation on H3K27 (H3K27me3) mediated by Polycomb repressive complex 2 (PRC2) has been linked to embryonic stem cell (ESC) identity and pluripotency. EZH2, the catalytic subunit of PRC2, has been reported as the sole histone methyltransferase that methylates H3K27 and mediates transcriptional silencing. Analysis of Ezh2(-/-) ESCs suggests existence of an additional enzyme(s) catalyzing H3K27 methylation. We have identified EZH1, a homolog of EZH2 that is physically present in a noncanonical PRC2 complex, as an H3K27 methyltransferase in vivo and in vitro. EZH1 colocalizes with the H3K27me3 mark on chromatin and preferentially preserves this mark on development-related genes in Ezh2(-/-) ESCs. Depletion of Ezh1 in cells lacking Ezh2 abolishes residual methylation on H3K27 and derepresses H3K27me3 target genes, demonstrating a role of EZH1 in safeguarding ESC identity. Ezh1 partially complements Ezh2 in executing pluripotency during ESC differentiation, suggesting that cell-fate transitions require epigenetic specificity.
[Show abstract][Hide abstract] ABSTRACT: Succinate acts as an extracellular mediator signaling through the G protein-coupled receptor GPR91. Here we show that dendritic cells had high expression of GPR91. In these cells, succinate triggered intracellular calcium mobilization, induced migratory responses and acted in synergy with Toll-like receptor ligands for the production of proinflammatory cytokines. Succinate also enhanced antigen-specific activation of human and mouse helper T cells. GPR91-deficient mice had less migration of Langerhans cells to draining lymph nodes and impaired tetanus toxoid-specific recall T cell responses. Furthermore, GPR91-deficient allografts elicited weaker transplant rejection than did the corresponding grafts from wild-type mice. Our results suggest that the succinate receptor GPR91 is involved in sensing immunological danger, which establishes a link between immunity and a metabolite of cellular respiration.
[Show abstract][Hide abstract] ABSTRACT: In eukaryotes, Suv39h H3K9 trimethyltransferases are required for pericentric heterochromatin formation and function. In early mouse preimplantation embryos, however, paternal pericentric heterochromatin lacks Suv39h-mediated H3K9me3 and downstream marks. Here we demonstrate Ezh2-independent targeting of maternally provided polycomb repressive complex 1 (PRC1) components to paternal heterochromatin. In Suv39h2 maternally deficient zygotes, PRC1 also associates with maternal heterochromatin lacking H3K9me3, thereby revealing hierarchy between repressive pathways. In Rnf2 maternally deficient zygotes, the PRC1 complex is disrupted, and levels of pericentric major satellite transcripts are increased at the paternal but not the maternal genome. We conclude that in early embryos, Suv39h-mediated H3K9me3 constitutes the dominant maternal transgenerational signal for pericentric heterochromatin formation. In absence of this signal, PRC1 functions as the default repressive back-up mechanism. Parental epigenetic asymmetry, also observed along cleavage chromosomes, is resolved by the end of the 8-cell stage--concurrent with blastomere polarization--marking the end of the maternal-to-embryonic transition.
[Show abstract][Hide abstract] ABSTRACT: Germ-line mutations in bone morphogenic protein type II receptor (Bmpr2) confer susceptibility to pulmonary arterial hypertension (PAH), which is characterized by obstructive vascular lesions in small arteries. The molecular and cellular mechanisms that account for the etiology of this disorder remain elusive, as does the role of Bmpr2 in postnatal tissue homeostasis. Here we show that in adult mice, stably silencing Bmpr2 expression by RNA interference does not increase pulmonary arterial resistance but results in severe mucosal hemorrhage, incomplete mural cell coverage on vessel walls, and gastrointestinal hyperplasia. We present evidence that BMP receptor signaling regulates vascular remodeling during angiogenesis by maintaining the expression of endothelial guidance molecules that promote vessel patterning and maturation and by counteracting growth factor-induced AKT activation. Attenuation of this function may cause vascular dysmorphogenesis and predisposition to angioproliferative diseases. Our findings provide a mechanistic link between PAH and other diseases associated with the BMP/TGF-beta pathways, such as hereditary hemorrhagic telangiectasia and juvenile polyposis syndrome.
[Show abstract][Hide abstract] ABSTRACT: Conventional approaches to produce transgenic mice recurrently yield unpredictable patterns and levels of transgene expression, a situation calling for the development of new techniques to overcome these drawbacks in the context of overexpression studies. Here we present an efficient method for rapid and reproducible transgenesis using the recombinase mediated cassette exchange (RMCE) (Bouhassira et al.: Blood 90:3332-3344, 1997) procedure. A lox511-EGFP-TK/neo-loxP cassette was placed under the control of the endogenous mouse beta-actin promoter. Heterozygous mice revealed strong and ubiquitous EGFP expression throughout embryogenesis and adulthood. Reproducibly, the same expression pattern was obtained with RMCE when it was used to replace the EGFP-harboring cassette by ECFP or placental alkaline phosphatase (PLAP) reporter genes (DePrimo et al.: Transgenic Res 5:459-466, 1996). Furthermore, the RMCE procedure proved efficient as well in embryonic stem (ES) cells as directly in zygotes. Our results demonstrate ubiquitous expression of floxed transgenes in the endogenous beta-actin locus and they support the general use of the beta-actin locus for targeted transgenesis.