Rab8a as a mitochondrial receptor for lipid droplets in skeletal muscle.

Dynamic interaction between lipid droplets (LDs) and mitochondria controls the mobilization of long-chain fatty acids (LCFAs) from LDs for mitochondrial ß-oxidation in skeletal muscle in response to energy stress. However, little is known about the composition and regulation of the tethering complex mediating LD-mitochondrion interaction. Here, we identify Rab8a as a mitochondrial receptor for LDs forming the tethering complex with the LD-associated PLIN5 in skeletal muscle. In rat L6 skeletal muscle cells, the energy sensor AMPK increases the GTP-bound active Rab8a that promotes LD-mitochondrion interaction through binding to PLIN5 upon starvation. The assembly of the Rab8a-PLIN5 tethering complex also recruits the adipose triglyceride lipase (ATGL), which couples LCFA mobilization from LDs with its transfer into mitochondria for ß-oxidation. Rab8a deficiency impairs fatty acid utilization and decreases endurance during exercise in a mouse model. These findings may help to elucidate the regulatory mechanisms underlying the beneficial effects of exercise on lipid homeostasis control.

TRIM24 is an insulin-responsive regulator of P-bodies.

Insulin is a potent inducer of mRNA transcription and translation, contributing to metabolic regulation. Insulin has also been suggested to regulate mRNA stability through the processing body (P-body) molecular machinery. However, whether and how insulin regulates mRNA stability via P-bodies is not clear. Here we show that the E3-ligase TRIM24 is a critical factor linking insulin signalling to P-bodies. Upon insulin stimulation, protein kinase B (PKB, also known as Akt) phosphorylates TRIM24 and stimulates its shuttling from the nucleus into the cytoplasm. TRIM24 interacts with several critical components of P-bodies in the cytoplasm, promoting their polyubiquitylation, which consequently stabilises Pparγ mRNA. Inactivation of TRIM24 E3-ligase activity or prevention of its phosphorylation via knockin mutations in mice promotes hepatic Pparγ degradation via P-bodies. Consequently, both knockin mutations alleviate hepatosteatosis in mice fed on a high-fat diet. Our results demonstrate the critical role of TRIM24 in linking insulin signalling to P-bodies and have therapeutic implications for the treatment of hepatosteatosis.

Spatiotemporal regulation of insulin signaling by liquid-liquid phase separation.

Insulin signals through its receptor to recruit insulin receptor substrates (IRS) and phosphatidylinositol 3-kinase (PI3K) to the plasma membrane for production of phosphatidylinositol-3,4,5-trisphosphate (PIP3) from phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2], which consequently activates protein kinase B (PKB). How insulin signals transduce from the plasma membrane into the cytoplasm is not clearly understood. Here we show that liquid-liquid phase separation (LLPS) plays a critical role in spatiotemporal control of insulin signaling through regulating multiple components including IRS1. Both protein concentration and insulin stimulation can drive the formation of intracellular IRS1 condensates through LLPS. Components including PI(4,5)P2, p85-PI3K and PDK1 are constitutively present in IRS1 condensates whereas production of PIP3 and recruitment of PKB in them are induced by insulin. Thus, IRS1 condensates function as intracellular signal hubs to mediate insulin signaling, whose formation is impaired in insulin resistant cells. Collectively, these data reveal an important function of LLPS in spatiotemporal control of insulin signaling.

Tissue-Specific Splicing and Dietary Interaction of a Mutant As160 Allele Determine Muscle Metabolic Fitness in Rodents.

Ethnic groups are physiologically and genetically adapted to their diets. Inuit bear a frequent AS160R684X mutation that causes type 2 diabetes. Whether this mutation evolutionarily confers adaptation in Inuit and how it causes metabolic disorders upon dietary changes are unknown due to limitations in human studies. Here, we develop a genetically modified rat model bearing an orthologous AS160R693X mutation, which mimics human patients exhibiting postprandial hyperglycemia and hyperinsulinemia. Importantly, a sugar-rich diet aggravates metabolic abnormalities in AS160R693X rats. The AS160R693X mutation diminishes a dominant long-variant AS160 without affecting a minor short-variant AS160 in skeletal muscle, which suppresses muscle glucose utilization but induces fatty acid oxidation. This fuel switch suggests a possible adaptation in Inuit who traditionally had lipid-rich hypoglycemic diets. Finally, induction of the short-variant AS160 restores glucose utilization in rat myocytes and a mouse model. Our findings have implications for development of precision treatments for patients bearing the AS160R684X mutation.

Receptor-interacting serine/threonine kinase 1- and 3-dependent inflammation induced in lungs of chicken infected with Pasteurella multocida.

Fowl cholera is a serious, highly contagious disease caused by the bacterium Pasteurella multocida (P. multocida) in a range of avian species and is characterized by an acute form of septicaemia. The pathogenic mechanism of chicken lung injury caused by the bacterium is unclear. Therefore, P. multocida Q (a reference standard strain isolated from chicken) and 1G1 (a clinic isolated strain from duck) were selected to infect chickens, establishing fowl cholera-induced laying hen models. Several important proteins involved in the process of lung injury were identified and quantified using immunohistochemistry and WB. The results showed that chicken lungs infected with bacteria for 24 h showed congestion and edema. The inflammatory factors HMGB1 and IL-6, intercellular matrix MMP, the cell apoptosis-associated caspase-3 and necrotic apoptosis signal molecules RIPK1 and RIPK3 were widely expressed in the lungs of group Q and were significantly different compared with those of 1G1 group and uninfected group (P < 0.05). The results indicated that RIPK1 and RIPK3 are involved in the injury process of chicken lungs after infection with P. multocida, and the mechanisms of lung injury induced by different strains are different.