Structure of C3b in complex with CRIg gives insights into regulation of complement activation

The complement system is a key part of the innate immune system, and is required for clearance of pathogens from the bloodstream. After exposure to pathogens, the third component of the complement system, C3, is cleaved to C3b which, after recruitment of factor B, initiates formation of the alternative pathway convertases. CRIg, a complement receptor expressed on macrophages, binds to C3b and iC3b mediating phagocytosis of the particles, but it is unknown how CRIg selectively recognizes proteolytic C3-fragments and whether binding of CRIg to C3b inhibits convertase activation. Here we present the crystal structure of C3b in complex with CRIg and, using CRIg mutants, provide evidence that CRIg acts as an inhibitor of the alternative pathway of complement. The structure shows that activation of C3 induces major structural rearrangements, including a dramatic movement (>80 A) of the thioester-bond-containing domain through which C3b attaches to pathogen surfaces. We show that CRIg is not only a phagocytic receptor, but also a potent inhibitor of the alternative pathway convertases. The structure provides insights into the complex macromolecular structural rearrangements that occur during complement activation and inhibition. Moreover, our structure-function studies relating the structural basis of complement activation and the means by which CRIg inhibits the convertases provide important clues to the development of therapeutics that target complement.     

Regulation of circadian behaviour and metabolism by synthetic REV-ERB agonists

Synchronizing rhythms of behaviour and metabolic processes is important for cardiovascular health and preventing metabolic diseases. The nuclear receptors REV-ERB-α and REV-ERB-β have an integral role in regulating the expression of core clock proteins driving rhythms in activity and metabolism. Here we describe the identification of potent synthetic REV-ERB agonists with in vivo activity. Administration of synthetic REV-ERB ligands alters circadian behaviour and the circadian pattern of core clock gene expression in the hypothalami of mice. The circadian pattern of expression of an array of metabolic genes in the liver, skeletal muscle and adipose tissue was also altered, resulting in increased energy expenditure. Treatment of diet-induced obese mice with a REV-ERB agonist decreased obesity by reducing fat mass and markedly improving dyslipidaemia and hyperglycaemia. These results indicate that synthetic REV-ERB ligands that pharmacologically target the circadian rhythm may be beneficial in the treatment of sleep disorders as well as metabolic diseases.     

The Drosophila melanogaster Genetic Reference Panel

A major challenge of biology is understanding the relationship between molecular genetic variation and variation in quantitative traits, including fitness. This relationship determines our ability to predict phenotypes from genotypes and to understand how evolutionary forces shape variation within and between species. Previous efforts to dissect the genotype-phenotype map were based on incomplete genotypic information. Here, we describe the Drosophila melanogaster Genetic Reference Panel (DGRP), a community resource for analysis of population genomics and quantitative traits. The DGRP consists of fully sequenced inbred lines derived from a natural population. Population genomic analyses reveal reduced polymorphism in centromeric autosomal regions and the X chromosome, evidence for positive and negative selection, and rapid evolution of the X chromosome. Many variants in novel genes, most at low frequency, are associated with quantitative traits and explain a large fraction of the phenotypic variance. The DGRP facilitates genotype-phenotype mapping using the power of Drosophila genetics.     

The MLH1 D132H variant is associated with susceptibility to sporadic colorectal cancer

Most susceptibility to colorectal cancer (CRC) is not accounted for by known risk factors. Because MLH1, MSH2 and MSH6 mutations underlie high-penetrance CRC susceptibility in hereditary nonpolyposis colon cancer (HNPCC), we hypothesized that attenuated alleles might also underlie susceptibility to sporadic CRC. We looked for gene variants associated with HNPCC in Israeli probands with familial CRC unstratified with respect to the microsatellite instability (MSI) phenotype. Association studies identified a new MLH1 variant (415G-->C, resulting in the amino acid substitution D132H) in approximately 1.3% of Israeli individuals with CRC self-described as Jewish, Christian and Muslim. MLH1 415C confers clinically significant susceptibility to CRC. In contrast to classic HNPCC, CRCs associated with MLH1 415C usually do not have the MSI defect, which is important for clinical mutation screening. Structural and functional analyses showed that the normal ATPase function of MLH1 is attenuated, but not eliminated, by the MLH1 415G-->C mutation. The new MLH1 variant confers a high risk of CRC and identifies a previously unrecognized mechanism in microsatellite-stable tumors. These studies suggest that variants of mismatch repair proteins with attenuated function may account for a higher proportion of susceptibility to sporadic microsatellite-stable CRC than previously assumed.