Deletion of Tgf-β1 From CD206+ M2 Macrophages Ameliorates Obesity-Induced Suppression of Myogenesis and AMPK Phosphorylation in Skeletal Muscle.

BACKGROUNDS: Obesity and diabetes impair the ability of the muscle to regenerate, repair and remodel, resulting in a gradual decrease in muscle mass and function. However, the underlying mechanisms and effective therapeutic strategies remain poorly understood. M2 macrophages within skeletal muscle play an important role in tissue recovery following injury. This study aims to investigate the role of M2 macrophages derived transforming growth factor-beta 1 (Tgf-β1) in regulation of skeletal muscle function under diet-induced obese conditions.

METHODS

An CD206+ M2 macrophage-specific Tgf-β1 gene knockout (Tgf-β1 KO) mouse model was generated by crossing CD206-CreERT2 mice with Tgf-β1f/f mice, followed by tamoxifen administration to induce Tgf-β1 gene deletion. Mice were then placed on a high-fat diet (HFD) for 12 weeks to develop obesity-induced skeletal muscle dysfunction. The study used multiple physiological and molecular analyses, including exercise tolerance, hanging time, grip strength, glucose and insulin tolerance tests, western blotting, RT-qPCR and others.

RESULTS

The present findings demonstrated improved exercise performance, as evidenced by increased running distance twice (p = 0.0008) in Tgf-β1 KO mice. Deletion of CD206+ M2 macrophage-specific Tgf-β1 stimulates fibro-adipogenic progenitors (FAPs), inducing Follistatin (Fst) expression by 1.70-fold (p = 0.04) and follistatin-like protein 1 (Fstl1) by 2.60-fold (p = 0.01) in tibialis anterior (TA), thus enhancing myogenesis. The Tgf-β1 KO mice showed increased muscle fibre type I (Myh7 by 2.40-fold, p = 0.005), muscle fibre type IIa (Myh2 by 1.50-fold, p = 0.003), type IIx (Myh1 by 2.35-fold, p = 0.002) in soleus and type II (Myh4 by 1.76-fold, p = 0.02) in TA. In Tgf-β1 KO mice, insulin-stimulated Akt phosphorylation was significantly increased in adipose tissue by 2.14-fold (p = 0.0003) and in the liver by 1.62-fold (p = 0.01). Besides, the Tgf-β1 KO mice showed increased circulating adiponectin by 1.23 fold (p = 0.003), thereby activating the AMPK/SIRT1/PGC-1α pathway with the phosphorylation level of AMPKα increased by 1.5-fold (p = 0.02), and PGC1α protein level increased by 2.2-fold (p = 0.02) via increased AdipoR1 mRNA expression by 1.8-fold in TA, p = 0.0001 and by 1.8-fold in soleus, p = 0.01 in skeletal muscle, leading to improved mitochondrial function in skeletal muscle.

CONCLUSIONS

The CD206+ M2 macrophage-specific Tgf-β1 deletion ameliorates obesity-induced muscle dysfunction, potentially via two distinct mechanisms. Firstly, it enhances myogenesis by promoting FAP-mediated expression of Fst and Fstl1, thereby augmenting myogenesis-related gene expression in skeletal muscle. Secondly, it also induces adipocytes to produce and secrete adiponectin into the bloodstream, thereby enhancing mitochondrial function via the AMPK/SIRT1/PGC-1α pathway.