53BP1 facilitates long-range DNA end-joining during V(D)J recombination

Variable, diversity and joining (V(D)J) recombination and class-switch recombination use overlapping but distinct non-homologous end joining pathways to repair DNA double-strand-break intermediates. 53BP1 is a DNA-damage-response protein that is rapidly recruited to sites of chromosomal double-strand breaks, where it seems to function in a subset of ataxia telangiectasia mutated (ATM) kinase-, H2A histone family member X (H2AX, also known as H2AFX)- and mediator of DNA damage checkpoint 1 (MDC1)-dependent events. A 53BP1-dependent end-joining pathway has been described that is dispensable for V(D)J recombination but essential for class-switch recombination. Here we report a previously unrecognized defect in the joining phase of V(D)J recombination in 53BP1-deficient lymphocytes that is distinct from that found in classical non-homologous-end-joining-, H2ax-, Mdc1- and Atm-deficient mice. Absence of 53BP1 leads to impairment of distal V-DJ joining with extensive degradation of unrepaired coding ends and episomal signal joint reintegration at V(D)J junctions. This results in apoptosis, loss of T-cell receptor alpha locus integrity and lymphopenia. Further impairment of the apoptotic checkpoint causes propagation of lymphocytes that have antigen receptor breaks. These data suggest a more general role for 53BP1 in maintaining genomic stability during long-range joining of DNA breaks.     

Functional genomic screen reveals genes involved in lipid-droplet formation and utilization

Eukaryotic cells store neutral lipids in cytoplasmic lipid droplets enclosed in a monolayer of phospholipids and associated proteins. These dynamic organelles serve as the principal reservoirs for storing cellular energy and for the building blocks for membrane lipids. Excessive lipid accumulation in cells is a central feature of obesity, diabetes and atherosclerosis, yet remarkably little is known about lipid-droplet cell biology. Here we show, by means of a genome-wide RNA interference (RNAi) screen in Drosophila S2 cells that about 1.5% of all genes function in lipid-droplet formation and regulation. The phenotypes of the gene knockdowns sorted into five distinct phenotypic classes. Genes encoding enzymes of phospholipid biosynthesis proved to be determinants of lipid-droplet size and number, suggesting that the phospholipid composition of the monolayer profoundly affects droplet morphology and lipid utilization. A subset of the Arf1-COPI vesicular transport proteins also regulated droplet morphology and lipid utilization, thereby identifying a previously unrecognized function for this machinery. These phenotypes are conserved in mammalian cells, suggesting that insights from these studies are likely to be central to our understanding of human diseases involving excessive lipid storage.     

CLEC5A is critical for dengue-virus-induced lethal disease

Dengue haemorrhagic fever and dengue shock syndrome, the most severe responses to dengue virus (DV) infection, are characterized by plasma leakage (due to increased vascular permeability) and low platelet counts. CLEC5A (C-type lectin domain family 5, member A; also known as myeloid DAP12-associating lectin (MDL-1)) contains a C-type lectin-like fold similar to the natural-killer T-cell C-type lectin domains and associates with a 12-kDa DNAX-activating protein (DAP12) on myeloid cells. Here we show that CLEC5A interacts with the dengue virion directly and thereby brings about DAP12 phosphorylation. The CLEC5A-DV interaction does not result in viral entry but stimulates the release of proinflammatory cytokines. Blockade of CLEC5A-DV interaction suppresses the secretion of proinflammatory cytokines without affecting the release of interferon-alpha, supporting the notion that CLEC5A acts as a signalling receptor for proinflammatory cytokine release. Moreover, anti-CLEC5A monoclonal antibodies inhibit DV-induced plasma leakage, as well as subcutaneous and vital-organ haemorrhaging, and reduce the mortality of DV infection by about 50% in STAT1-deficient mice. Our observation that blockade of CLEC5A-mediated signalling attenuates the production of proinflammatory cytokines by macrophages infected with DV (either alone or complexed with an enhancing antibody) offers a promising strategy for alleviating tissue damage and increasing the survival of patients suffering from dengue haemorrhagic fever and dengue shock syndrome, and possibly even other virus-induced inflammatory diseases.     

Positive correlation between recombination rate and nucleotide diversity is shown under domestication selection in the chicken genome

Positive correlation between recombination rate and nucleoUde diversity has been observed in a wide variety of eukaryotes on megabase scale. On the basis of genome-wide chicken genetic variation map generated by comparing three domestic breeds with wild ancestor and the positions of markers on the genetic linkage map, we found that SNPs rates were similar for all chromosomes while the recombination rates increased in micro chromosomes. In other words no correlation exists in chromosome size. Nevertheless, when we scanned the genome by calculating the values of each characteristic within non-overlapping windows, instead of single value for each chromosomes, the nucleoUde diversity was found to be significantly correlated with the recombination rate （r=0.27, P〈0.0005）. Furthermore, the significant association not only existed between these two features, but also existed between all 6 pairwise combinations of nucleoUde diversity, recombination rate, GC content and average gene length. This co-variation is very meaningful for the studies of sequence evolution.     

An optical supernova associated with the X-ray flash XRF 060218

Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae that are more luminous than average and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB-supernovae were thought to be rare events. Whether X-ray flashes--analogues of GRBs, but with lower luminosities and fewer gamma-rays--can also be associated with supernovae, and whether they are intrinsically 'weak' events or typical GRBs viewed off the axis of the burst, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB-supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output and the supernova radio flux. Our data, combined with radio and X-ray observations, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB-supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB-supernovae.     

Boc is a receptor for sonic hedgehog in the guidance of commissural axons

In the spinal cord, sonic hedgehog (Shh) is secreted by the floor plate to control the generation of distinct classes of ventral neurons along the dorsoventral axis. Genetic and in vitro studies have shown that Shh also later acts as a midline-derived chemoattractant for commissural axons. However, the receptor(s) responsible for Shh attraction remain unknown. Here we show that two Robo-related proteins, Boc and Cdon, bind specifically to Shh and are therefore candidate receptors for the action of Shh as an axon guidance ligand. Boc is expressed by commissural neurons, and targeted disruption of Boc in mouse results in the misguidance of commissural axons towards the floor plate. RNA-interference-mediated knockdown of Boc impairs the ability of rat commissural axons to turn towards an ectopic source of Shh in vitro. Taken together, these data suggest that Boc is essential as a receptor for Shh in commissural axon guidance.