Increased plasma creatine concentrations are associated with the risk of type 2 diabetes, but whether this alteration is associated with or causal for impairments in metabolism remains unexplored. Because skeletal muscle is the main disposal site of both creatine and glucose, we investigated the role of intramuscular creatine metabolism in the pathophysiology of insulin resistance in type 2 diabetes. In men with type 2 diabetes, plasma creatine concentrations were increased, and intramuscular phosphocreatine content was reduced. These alterations were coupled to reduced expression of sarcomeric mitochondrial creatine kinase 2 ( CKMT2 ). In C57BL/6 mice fed a high-fat diet, neither supplementation with creatine for 2 weeks nor treatment with the creatine analog β-GPA for 1 week induced changes in glucose tolerance, suggesting that increased circulating creatine was associated with insulin resistance rather than causing it. In C2C12 myotubes, silencing Ckmt2 using small interfering RNA reduced mitochondrial respiration, membrane potential, and glucose oxidation. Electroporation-mediated overexpression of Ckmt2 in skeletal muscle of high-fat diet–fed male mice increased mitochondrial respiration, independent of creatine availability. Given that overexpression of Ckmt2 improved mitochondrial function, we explored whether exercise regulates CKMT2 expression. Analysis of public data revealed that CKMT2 content was up-regulated by exercise training in both humans and mice. We reveal a previously underappreciated role of CKMT2 in mitochondrial homeostasis beyond its function for creatine phosphorylation, independent of insulin action. Collectively, our data provide functional evidence for how CKMT2 mediates mitochondrial dysfunction associated with type 2 diabetes.

Background: The maintenance of skeletal muscle plasticity upon changes in the environment, nutrient supply, and exercise depends on regulatory mechanisms that couple structural and metabolic adaptations. The mechanisms that interconnect both processes at the transcriptional level remain underexplored. Nr2f6, a nuclear receptor, regulates metabolism and cell differentiation in peripheral tissues. However, its role in the skeletal muscle is still elusive. Here, we aimed to investigate the effects of Nr2f6 modulation on muscle biology in vivo and in vitro.Methods: Global RNA‐seq was performed in Nr2f6 knockdown C2C12 myocytes (N = 4–5). Molecular and metabolic assays and proliferation experiments were performed using stable Nr2f6 knockdown and Nr2f6 overexpression C2C12 cell lines (N = 3–6). Nr2f6 content was evaluated in lipid overload models in vitro and in vivo (N = 3–6). In vivo experiments included Nr2f6 overexpression in mouse tibialis anterior muscle, followed by gene array transcriptomics and molecular assays (N = 4), ex vivo contractility experiments (N = 5), and histological analysis (N = 7). The conservation of Nr2f6 depletion effects was confirmed in primary skeletal muscle cells of humans and mice.Results: Nr2f6 knockdown upregulated genes associated with muscle differentiation, metabolism, and contraction, while cell cycle‐related genes were downregulated. In human skeletal muscle cells, Nr2f6 knockdown significantly increased the expression of myosin heavy chain genes (two‐fold to three‐fold) and siRNA‐mediated depletion of Nr2f6 increased maximal C2C12 myocyte's lipid oxidative capacity by 75% and protected against lipid‐induced cell death. Nr2f6 content decreased by 40% in lipid‐overloaded myotubes and by 50% in the skeletal muscle of mice fed a high‐fat diet. Nr2f6 overexpression in mice resulted in an atrophic and hypoplastic state, characterized by a significant reduction in muscle mass (15%) and myofibre content (18%), followed by an impairment (50%) in force production. These functional phenotypes were accompanied by the establishment of an inflammation‐like molecular signature and a decrease in the expression of genes involved in muscle contractility and oxidative metabolism, which was associated with the repression of the uncoupling protein 3 (20%) and PGC‐1α (30%) promoters activity following Nr2f6 overexpression in vitro. Additionally, Nr2f6 regulated core components of the cell division machinery, effectively decoupling muscle cell proliferation from differentiation.Conclusions: Our findings reveal a novel role for Nr2f6 as a molecular transducer that plays a crucial role in maintaining the balance between skeletal muscle contractile function and oxidative capacity. These results have significant implications for the development of potential therapeutic strategies for metabolic diseases and myopathies.

Obesity is a major public health problem and is a major contributor to the development of insulin resistance. In previous studies we observed that single-wavelength red or infrared photobiomodulation (PBM) improved insulin signaling in adipocytes and skeletal muscle of mice fed a high-fat diet, but information about the combination of different wavelengths, as well as the effect of different light doses (J/cm²) is lacking. Therefore, the aim of this study was to investigate the effects of different doses of dual-wavelength PBM on insulin signaling in muscle cell, and explore potential mechanisms involved. Mouse myoblasts (C2C12) were differentiated into myotubes and cultured in palmitic acid, sodium oleate and l-carnitine (PAL) to induce insulin resistance high or in glucose medium (CTRL). Then, they received SHAM treatment (lights off, 0 J/cm²) or PBM (660 + 850 nm; 2, 4 or 8 J/cm²). PAL induced insulin resistance (assessed by Akt phosphorylation at ser473), attenuated maximal citrate synthase activity, and increased the phosphorylation of c-Jun NH(2) terminal kinase (JNK) (T183/Y185). PBM at doses of 4 or 8 J/cm² reversed these PAL-induced responses. Furthermore, at doses of 2, 4 or 8 J/cm², PBM reversed the increase in mitofusin-2 content induced by PAL. In conclusion, the combination of dual-wavelength red and infrared PBM at doses of 4 and 8 J/cm² improved intracellular insulin signaling in musculoskeletal cells, and this effect appears to involve the modulation of mitochondrial function and the attenuation of the activation of stress kinases.