A secreted factor NimrodB4 promotes the elimination of apoptotic corpses by phagocytes in Drosophila.

Programmed cell death plays a fundamental role in development and tissue homeostasis. Professional and non-professional phagocytes achieve the proper recognition, uptake, and degradation of apoptotic cells, a process called efferocytosis. Failure in efferocytosis leads to autoimmune and neurodegenerative diseases. In Drosophila, two transmembrane proteins of the Nimrod family, Draper and SIMU, mediate the recognition and internalization of apoptotic corpses. Beyond this early step, little is known about how apoptotic cell degradation is regulated. Here, we study the function of a secreted member of the Nimrod family, NimB4, and reveal its crucial role in the clearance of apoptotic cells. We show that NimB4 is expressed by macrophages and glial cells, the two main types of phagocytes in Drosophila. Similar to draper mutants, NimB4 mutants accumulate apoptotic corpses during embryogenesis and in the larval brain. Our study points to the role of NimB4 in phagosome maturation, more specifically in the fusion between the phagosome and lysosomes. We propose that similar to bridging molecules, NimB4 binds to apoptotic corpses to engage a phagosome maturation program dedicated to efferocytosis.

The DEAD-box RNA helicase CshA is required for fatty acid homeostasis in Staphylococcus aureus.

Staphylococcus aureus is an opportunistic pathogen that can grow in a wide array of conditions: on abiotic surfaces, on the skin, in the nose, in planktonic or biofilm forms and can cause many type of infections. Consequently, S. aureus must be able to adapt rapidly to these changing growth conditions, an ability largely driven at the posttranscriptional level. RNA helicases of the DEAD-box family play an important part in this process. In particular, CshA, which is part of the degradosome, is required for the rapid turnover of certain mRNAs and its deletion results in cold-sensitivity. To understand the molecular basis of this phenotype, we conducted a large genetic screen isolating 82 independent suppressors of cold growth. Full genome sequencing revealed the fatty acid synthesis pathway affected in many suppressor strains. Consistent with that result, sublethal doses of triclosan, a FASII inhibitor, can partially restore growth of a cshA mutant in the cold. Overexpression of the genes involved in branched-chain fatty acid synthesis was also able to suppress the cold-sensitivity. Using gas chromatography analysis of fatty acids, we observed an imbalance of straight and branched-chain fatty acids in the cshA mutant, compared to the wild-type. This imbalance is compensated in the suppressor strains. Thus, we reveal for the first time that the cold sensitive growth phenotype of a DEAD-box mutant can be explained, at least partially, by an improper membrane composition. The defect correlates with an accumulation of the pyruvate dehydrogenase complex mRNA, which is inefficiently degraded in absence of CshA. We propose that the resulting accumulation of acetyl-CoA fuels straight-chained fatty acid production at the expense of the branched ones. Strikingly, addition of acetate into the medium mimics the cshA deletion phenotype, resulting in cold sensitivity suppressed by the mutations found in our genetic screen or by sublethal doses of triclosan.

Secretion Relieves Translational Co-repression by a Specialized Flagellin Paralog.

How cellular checkpoints couple the orderly assembly of macromolecular machines with cell-cycle progression is poorly understood. The alpha-proteobacterium Caulobacter crescentus assembles a single polar flagellum during each cell cycle. We discovered that the expression of multiple flagellin transcripts is licensed by a translational checkpoint responsive to a dual input signal: a secretion-competent hook-basal-body (HBB) structure and a surge in the FlaF secretion chaperone during cytokinesis, instructed by the cell-cycle program. We find that the unorthodox FljJ flagellin, one of the six flagellin paralogs, acts as a checkpoint linchpin, binding both FlaF and the FlbT translational regulator. FljJ recruits FlbT to inhibit translation at the 5' untranslated region in other flagellin transcripts before HBB assembly. Once FlaF is synthesized and stabilized, it directs FljJ secretion through the HBB, thereby separating FlbT from its co-activator and relieving translational inhibition. The FlbT/FlaF pair is wide spread and its functional properties are conserved in alpha-proteobacteria, including pathogens.

The adipokine NimrodB5 regulates peripheral hematopoiesis in Drosophila.

In animals, growth is regulated by the complex interplay between paracrine and endocrine signals. When food is scarce, tissues compete for nutrients, leading to critical resource allocation and prioritization. Little is known about how the immune system maturation is coordinated with the growth of other tissues. Here, we describe a signaling mechanism that regulates the number of hemocytes (blood cells) according to the nutritional state of the Drosophila larva. Specifically, we found that a secreted protein, NimB5, is produced in the fat body upon nutrient scarcity downstream of metabolic sensors and ecdysone signaling. NimB5 is then secreted and binds to hemocytes to down-regulate their proliferation and adhesion. Blocking this signaling loop results in conditional lethality when larvae are raised on a poor diet, due to excessive hemocyte numbers and insufficient energy storage. Similar regulatory mechanisms shaping the immune system in response to nutrient availability are likely to be widespread in animals.