Objective Skeletal muscle glucose disposal following a meal is mediated through insulin-stimulated movement of the GLUT4-containing vesicles to the cell surface. The highly conserved scaffold-protein β-catenin is an emerging regulator of vesicle trafficking in other tissues. Here, we investigated the involvement of β-catenin in skeletal muscle insulin-stimulated glucose transport. Methods Glucose homeostasis and transport was investigated in inducible muscle specific β-catenin knockout (BCAT-mKO) mice. The effect of β-catenin deletion and mutation of β-catenin serine 552 on signal transduction, glucose uptake and protein–protein interactions were determined in L6-G4-myc cells, and β-catenin insulin-responsive binding partners were identified via immunoprecipitation coupled to label-free proteomics. Results Skeletal muscle specific deletion of β-catenin impaired whole-body insulin sensitivity and insulin-stimulated glucose uptake into muscle independent of canonical Wnt signalling. In response to insulin, β-catenin was phosphorylated at serine 552 in an Akt-dependent manner, and in L6-G4-myc cells, mutation of β-catenin S552 impaired insulin-induced actin-polymerisation, resulting in attenuated insulin-induced glucose transport and GLUT4 translocation. β-catenin was found to interact with M-cadherin in an insulin-dependent β-catenin S552 -phosphorylation dependent manner, and loss of M-cadherin in L6-G4-myc cells attenuated insulin-induced actin-polymerisation and glucose transport. Conclusions Our data suggest that β-catenin is a novel mediator of glucose transport in skeletal muscle and may contribute to insulin-induced actin-cytoskeleton remodelling to support GLUT4 translocation.

Bumblebees (Bombus terrestris) fly at low ambient temperatures where other insects cannot, and to do so they must pre-warm their flight muscles. While some have proposed mechanisms, none fully explain how pre-flight thermogenesis occurs. Here, we present a novel hypothesis based on the less studied mitochondrial glycerol 3-phosphate dehydrogenase pathway (mGPDH). Using calorimetry, and high resolution respirometry coupled with fluorimetry, we report substrate oxidation by mGPDH in permeabilised flight muscles operates, in vitro, at a high flux, even in the absence of ADP. This may be facilitated by an endogenous, mGPDH-mediated uncoupling of mitochondria. This uncoupling increases ETS activity, which results in increased heat release. Furthermore, passive regulation of this mechanism is achieved via dampened temperature sensitivity of mGPDH relative to other respiratory pathways, and subsequent consumption of its substrate, glycerol 3-phosphate (G3P), at low temperatures. Mitochondrial GPDH may therefore facilitate pre-flight thermogenesis through poor mitochondrial coupling. We calculate this can occur at a sufficient rate to warm flight muscles until shivering commences, and until flight muscle function is adequate for bumblebees to fly in the cold.

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Key points Loss of β‐catenin impairs in vivo and isolated muscle exercise/contraction‐stimulated glucose uptake. β‐Catenin is required for exercise‐induced skeletal muscle actin cytoskeleton remodelling. β‐Catenin675 phosphorylation during exercise may be intensity dependent. Abstract The conserved structural protein β‐catenin is an emerging regulator of vesicle trafficking in multiple tissues and supports insulin‐stimulated glucose transporter 4 (GLUT4) translocation in skeletal muscle by facilitating cortical actin remodelling. Actin remodelling may be a convergence point between insulin and exercise/contraction‐stimulated glucose uptake. Here we investigated whether β‐catenin is involved in regulating exercise/contraction‐stimulated glucose uptake. We report that the muscle‐specific deletion of β‐catenin induced in adult mice (BCAT‐mKO) impairs both exercise‐ and contraction (isolated muscle)‐induced glucose uptake without affecting running performance or canonical exercise signalling pathways. Furthermore, high intensity exercise in mice and contraction of myotubes and isolated muscles led to the phosphorylation of β‐cateninS675, and this was impaired by Rac1 inhibition. Moderate intensity exercise in control and Rac1 muscle‐specific knockout mice did not induce muscle β‐cateninS675 phosphorylation, suggesting exercise intensity‐dependent regulation of β‐cateninS675. Introduction of a non‐phosphorylatable S675A mutant of β‐catenin into myoblasts impaired GLUT4 translocation and actin remodelling stimulated by carbachol, a Rac1 and RhoA activator. Exercise‐induced increases in cross‐sectional phalloidin staining (F‐actin marker) of gastrocnemius muscle was impaired in muscle from BCAT‐mKO mice. Collectively our findings suggest that β‐catenin is required for optimal glucose transport in muscle during exercise/contraction, potentially via facilitating actin cytoskeleton remodelling.

Excessive insulin signaling through the insulin receptor (IR) may play a role in the pathogenesis of diet-induced metabolic disease, including obesity and type 2 diabetes. Here we investigate whether heterozygous impairment of insulin receptor (IR) expression limited to peripheral, i.e. non-CNS, tissues of adult mice impacts the development of high-fat dietinduced metabolic deterioration. While exhibiting some features of insulin resistance, PerI-RKO +/− mice display a hepatic energy deficit accompanied by induction of energy-sensing AMPK, mitochondrial biogenesis, PPARα, unexpectedly leading to protection from, and reversal of hepatic lipid accumulation (steatosis hepatis, NAFLD). Consistently, and unlike in control mice, the PPARα activator fenofibrate fails to further affect hepatic lipid accumulation in PerIRKO +/− mice. Taken together, and opposing previously established diabetogenic features of insulin resistance, incomplete impairment of insulin signaling may mimic central aspects of calorie restriction to limit hepatic lipid accumulation during conditions of metabolic stress.

Systems genetics has begun to tackle the complexity of insulin resistance by capitalising on computational advances to study high-diversity populations. “Diversity Outbred in Australia (DOz)” is a population of genetically unique mice with profound metabolic heterogeneity. We leveraged this variance to explore skeletal muscle’s contribution to whole-body insulin action through metabolic phenotyping and skeletal muscle proteomics of 215 DOz mice. Linear modelling identified 553 proteins that associated with whole-body insulin sensitivity (Matsuda Index) including regulators of endocytosis and muscle proteostasis. To enrich for causality, we refined this network by focussing on negatively associated, genetically regulated proteins, resulting in a 76-protein fingerprint of insulin resistance. We sought to perturb this network and restore insulin action with small molecules by integrating the Broad Institute Connectivity Map platform andin vitroassays of insulin action using the Prestwick chemical library. These complimentary approaches identified the antibiotic thiostrepton as an insulin resistance reversal agent. Subsequent validation inex vivoinsulin resistant mouse muscle, and palmitate induced insulin resistant myotubes demonstrated potent insulin action restoration, potentially via up-regulation of glycolysis. This work demonstrates the value of a drug-centric framework to validate systems level analysis by identifying potential therapeutics for insulin resistance.

Genetic inhibition of the p110α isoform of phosphatidylinositol-3-kinase (PI3K) can increase murine lifespan, enhance mitochondrial function and alter tissue specific oxidative balance. Here, we investigated whether pharmacological inhibition of the p110α isoform of PI3K induces similar enhancement of mitochondrial function in middle-aged mice. Eight-month old male and female mice were fed a diet containing 0.3 g/kg of the p110α-selective inhibitor BYL-719 (BYL) or a vehicle diet (VEH) for six weeks. Mice consuming BYL-719 had higher fed blood glucose and insulin, and tended towards decreased body weight. After 72 h, gene expression of the mitochondrial biogenesis mediators Pgc1α, Tfam and Nrf1 was greater in liver of BYL-719 males only, but unchanged in skeletal muscle of either sex. Six weeks of BYL-719 treatment did not affect mitochondrial content or function in the liver or skeletal muscle of either sex. In livers of males only, the expression of the antioxidant genes Nfe2l2, Cat, Sod1 and Sod2 increased within 72 h of BYL-719 treatment, and remained higher after six weeks. This was associated with an increase in hepatic GSH content and catalase protein expression, and lower H2O2 levels. Our results suggest that pharmacological inhibition of p110α in adult mice does not affect liver or skeletal muscle mitochondrial function, but does show sex- and tissue-specific effects on upregulation of antioxidant response. Given BYL-719 was well-tolerated and improved oxidative balance, it may have potential to influence aging-associated metabolic decline in the long-term.