β-Catenin is required for optimal exercise- and contraction-stimulated skeletal muscle glucose uptake

The genetic and dietary landscape of the muscle insulin signalling network

Metabolic disease is caused by a combination of genetic and environmental factors, yet few studies have examined how these factors influence signal transduction, a key mediator of metabolism. Using mass spectrometry-based phosphoproteomics, we quantified 23,126 phosphosites in skeletal muscle of five genetically distinct mouse strains in two dietary environments, with and without acute in vivo insulin stimulation. Almost half of the insulin-regulated phosphoproteome was modified by genetic background on an ordinary diet, and high-fat high-sugar feeding affected insulin signalling in a strain-dependent manner. Our data revealed coregulated subnetworks within the insulin signalling pathway, expanding our understanding of the pathway's organisation. Furthermore, associating diverse signalling responses with insulin-stimulated glucose uptake uncovered regulators of muscle insulin responsiveness, including the regulatory phosphosite S469 on Pfkfb2, a key activator of glycolysis. Finally, we confirmed the role of glycolysis in modulating insulin action in insulin resistance. Our results underscore the significance of genetics in shaping global signalling responses and their adaptability to environmental changes, emphasising the utility of studying biological diversity with phosphoproteomics to discover key regulatory mechanisms of complex traits.

A grooved conduit combined with decellularized tissues for peripheral nerve regeneration

Peripheral nerve injury (PNI) is a common and severe clinical disease worldwide, which leads to a poor prognosis because of the complicated treatments and high morbidity. Autologous nerve grafting as the gold standard still cannot meet the needs of clinical nerve transplantation because of its low availability and limited size. The development of artificial nerve conduits was led to a novel direction for PNI treatment, while most of the currently developed artificial nerve conduits was lack biochemical cues to promote nerve regeneration. In this study, we designed a novel composite neural conduit by inserting decellularized the rat sciatic nerve or kidney in a poly (lactic-co-glycolic acid) (PLGA) grooved conduit. The nerve regeneration effect of all samples was analyzed using rat sciatic nerve defect model, where decellularized tissues and grooved PLGA conduit alone were used as controls. The degree of nerve regeneration was evaluated using the motor function, gastrocnemius recovery, and morphological and histological assessments suggested that the combination of a grooved conduit with decellularized tissues significantly promoted nerve regeneration compared with decellularized tissues and PLGA conduit alone. It is worth to note that the grooved conduits containing decellularized nerves have a promotive effect similar to that of autologous nerve grafting, suggesting that it could be an artificial nerve conduit used for clinical practice in the future.

Alternative glucose uptake mediated by β-catenin/RSK1 axis under stress stimuli in mammalian cells

Cells adapt to stress conditions by increasing glucose uptake as cytoprotective strategy. The efficiency of glucose uptake is determined by the translocation of glucose transporters (GLUTs) from cytosolic vesicles to cellular membranes in many tissues and cells. GLUT translocation is tightly controlled by the activation of Tre-2/BUB2/CDC16 1 domain family 4 (TBC1D4) via its phosphorylation. The mechanisms of glucose uptake under stress conditions remain to be clarified. In this study, we surprisingly found that glucose uptake is apparently increased for the early response to three stress stimuli, glucose starvation and the exposure to lipopolysaccharide (LPS) or deoxynivalenol (DON). The stress-induced glucose uptake was mainly controlled by the increment of β-catenin level and the activation of RSK1. Mechanistically, β-catenin directly interacted with RSK1 and TBC1D4, acting as the scaffold protein to recruit activated RSK1 to promote the phosphorylation of TBC1D4. In addition, β-catenin was further stabilized due to the inhibition of GSK3β kinase activity which is caused by activated RSK1 phosphorylating GSK3β at Ser9. In general, this triple protein complex consisting of β-catenin, phosphorylated RSK1, and TBC1D4 were increased in the early response to these stress signals, and consequently, further promoted the phosphorylation of TBC1D4 to facilitate the translocation of GLUT4 to the cell membrane. Our study revealed that the β-catenin/RSK1 axis contributed to the increment of glucose uptake for cellular adaption to these stress conditions, shedding new insights into cellular energy utilization under stress.

WNT/β-catenin pathway is a key regulator of cardiac function and energetic metabolism

The WNT/β-catenin pathway is a master regulator of cardiac development and growth, and its activity is low in healthy adult hearts. However, even this low activity is essential for maintaining normal heart function. Acute activation of the WNT/β-catenin signaling cascade is considered to be cardioprotective after infarction through the upregulation of prosurvival genes and reprogramming of metabolism. Chronically high WNT/β-catenin pathway activity causes profibrotic and hypertrophic effects in the adult heart. New data suggest more complex functions of β-catenin in metabolic maturation of the perinatal heart, establishing an adult pattern of glucose and fatty acid utilization. Additionally, low basal activity of the WNT/β-catenin cascade maintains oxidative metabolism in the adult heart, and this pathway is reactivated by physiological or pathological stimuli to meet the higher energy needs of the heart. This review summarizes the current state of knowledge of the organization of canonical WNT signaling and its function in cardiogenesis, heart maturation, adult heart function, and remodeling. We also discuss the role of the WNT/β-catenin pathway in cardiac glucose, lipid metabolism, and mitochondrial physiology.

A role for β-catenin in diet-induced skeletal muscle insulin resistance

A central characteristic of insulin resistance is the impaired ability for insulin to stimulate glucose uptake into skeletal muscle. While insulin resistance can occur distal to the canonical insulin receptor-PI3k-Akt signaling pathway, the signaling intermediates involved in the dysfunction are yet to be fully elucidated. β-catenin is an emerging distal regulator of skeletal muscle and adipocyte insulin-stimulated GLUT4 trafficking. Here, we investigate its role in skeletal muscle insulin resistance. Short-term (5-week) high-fat diet (HFD) decreased skeletal muscle β-catenin protein expression 27% (p = 0.03), and perturbed insulin-stimulated β-cateninS552 phosphorylation 21% (p = 0.009) without affecting insulin-stimulated Akt phosphorylation relative to chow-fed controls. Under chow conditions, mice with muscle-specific β-catenin deletion had impaired insulin responsiveness, whereas under HFD, both mice exhibited similar levels of insulin resistance (interaction effect of genotype × diet p < 0.05). Treatment of L6-GLUT4-myc myocytes with palmitate lower β-catenin protein expression by 75% (p = 0.02), and attenuated insulin-stimulated β-catenin phosphorylationS552 and actin remodeling (interaction effect of insulin × palmitate p < 0.05). Finally, β-cateninS552 phosphorylation was 45% lower in muscle biopsies from men with type 2 diabetes while total β-catenin expression was unchanged. These findings suggest that β-catenin dysfunction is associated with the development of insulin resistance.

Kalirin mediates Rac1 activation downstream of calcium/calmodulin-dependent protein kinase II to stimulate glucose uptake during muscle contraction

In this study, we investigated the role of calcium/calmodulin-dependent protein kinase II (CaMKII) in contraction-stimulated glucose uptake in skeletal muscle. C2C12 myotubes were contracted by electrical pulse stimulation (EPS), and treadmill running was used to exercise mice. The activities of CaMKII, the small G protein Rac1, and the Rac1 effector kinase PAK1 were elevated in muscle by running exercise or EPS, while they were lowered by the CaMKII inhibitor KN-93 and/or small interfering RNA (siRNA)-mediated knockdown. EPS induced the mRNA and protein expression of the Rac1-GEF Kalirin in a CaMKII-dependent manner. EPS-induced Rac1 activation was lowered by the Kalirin inhibitor ITX3 or siRNA-mediated Kalirin knockdown. KN-93, ITX3, and siRNA-mediated Kalirin knockdown reduced EPS-induced glucose uptake. These findings define a CaMKII-Kalirin-Rac1 signaling pathway that contributes to contraction-stimulated glucose uptake in skeletal muscle myotubes and tissue.

β-cells retain a pool of insulin-containing secretory vesicles regulated by adherens junctions and the cadherin binding protein p120 catenin

The β-cells of the islets of Langerhans are the sole producers of insulin in the human body. In response to rising glucose levels, insulin-containing vesicles inside β-cells fuse with the plasma membrane and release their cargo. However, the mechanisms regulating this process are only partly understood. Previous evidence indicated reductions in α-catenin elevate insulin release, while reductions in β-catenin decrease insulin release. α- and β-catenin contribute to cellular regulation in a range of ways but one is as members of the adherens junction complex and these contribute to the development of cell polarity in b-cells. Therefore, we investigated the effects of adherens junctions on insulin release. We show in INS-1E β-cells knockdown of either E- or N-cadherin had only small effects on insulin secretion, but simultaneous knockout of both cadherins resulted in a significant increase in basal insulin release to the same level as glucose-stimulated release. This double knockdown also significantly attenuated levels of p120 catenin, a cadherin binding partner involved in regulating cadherin turnover. Conversely, reducing p120 catenin levels with siRNA destabilized both E- and N-cadherin, and this was also associated with an increase in levels of insulin secreted from INS-1E cells. Furthermore, there were also changes in these cells consistent with higher insulin release, namely reductions in levels of F-actin and increased intracellular free Ca²⁺ levels in response to KCl-induced membrane depolarization. Taken together, these data provide evidence that adherens junctions play important roles in retaining a pool of insulin secretory vesicles within the cell and establish a role for p120 catenin in regulating this process.

Impaired expression of BCAT1 relates to muscle atrophy of mouse model of sarcopenia

Background

The underlying mechanism of muscle atrophy in sarcopenia is still not fully understood; branched chain aminotransferase 1(BCAT1) isocitrate dehydrogenase-1 encodes an evolutionarily conserved cytoplasmic aminotransferase for glutamate and branched-chain amino acids (BCAAs), thus constituting a regulatory component of cytoplasmic amino and keto acid metabolism. In human gliomas carrying wild-type isocitrate dehydrogenase-1, BCAT1 promotes cell proliferation through amino acid catabolism. Hence, the goals of this study were to unravel the potential role of BCAT1 expression in muscle atrophy and to explore the mechanisms underlying this process.

Methods

We first measured Bcat1 expression by RT-qPCR and western blotting in murine and cellular models of muscle atrophy. To understand how the Bcat1-driven changes sustained muscle cell growth, we analyzed reactive oxygen species (ROS) levels and activation of the mTORC1/S6K1 pathway in muscle cells. Furthermore, we performed Cell Counting Kit-8(CCK8) assays and fluorescence staining to evaluate growth rate of cells and ROS levels. Finally, we verified that depletion of Bcat1 impairs the growth rate of muscle cells and increases ROS levels, indicating that muscle atrophy resulted from the downregulation of the mTORC1/S6K1 pathway. Data were analyzed by two-tailed unpaired Student’s t-test or Mann-Whitney U test for two groups to determine statistical significance. Statistical analyses were performed using GraphPad Prism version 6.0 and SPSS 16.0 software.

Results

Bcat1 expression level in skeletal muscles was lower in murine and cellular models of sarcopenia than in the control groups. Bcat1 knockdown not only suppressed the growth of muscle cells but also increased the production of ROS. Impaired cell growth and increased ROS production was rescued by co-introduction of an shRNA-resistant Bcat1 cDNA or addition of the mTORC1 stimulator MYH1485. Muscle cells with Bcat1 knockdown featured lower mTORC1 and S6K1 phosphorylation (pS6K1) than NT muscle cells. Addition of either shRNA-resistant Bcat1 cDNA or MYH1485 rescued the suppression of cell growth, increase in ROS production, and decrease in pS6K1.

Conclusions

The branched chain amino acids catabolic enzyme BCAT1 is essential for the growth of muscle cells. BCAT1 expression contributes to sustained growth of muscle cells by activating mTOR signaling and reducing ROS production.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05332-7.

Interactions between insulin and exercise

The interaction between insulin and exercise is an example of balancing and modifying the effects of two opposing metabolic regulatory forces under varying conditions. While insulin is secreted after food intake and is the primary hormone increasing glucose storage as glycogen and fatty acid storage as triglycerides, exercise is a condition where fuel stores need to be mobilized and oxidized. Thus, during physical activity the fuel storage effects of insulin need to be suppressed. This is done primarily by inhibiting insulin secretion during exercise as well as activating local and systemic fuel mobilizing processes. In contrast, following exercise there is a need for refilling the fuel depots mobilized during exercise, particularly the glycogen stores in muscle. This process is facilitated by an increase in insulin sensitivity of the muscles previously engaged in physical activity which directs glucose to glycogen resynthesis. In physically trained individuals, insulin sensitivity is also higher than in untrained individuals due to adaptations in the vasculature, skeletal muscle and adipose tissue. In this paper, we review the interactions between insulin and exercise during and after exercise, as well as the effects of regular exercise training on insulin action.