Myostatin deletion reverses myosteatosis and improves angiogenesis in obese mice

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): T32 Training Grant: Multi-Disciplinary Training Program in the Mechanisms of Cardiometabolic Disease Objective To test the hypothesis that increased muscle mass achieved through deletion of myostatin improves ischemic angiogenesis in obese mice. Methods db/db mice, a well-described model of obesity, were crossed with mice lacking myostatin (MSTN KO), a myokine that negatively regulates muscle differentiation and growth, to generate lean and obese mice with and without elevated muscle mass (MSTN KO). We utilized confocal and electron microscopy (EM) and nuclear magnetic resonance (NMR) spectroscopy to assess morphology and histology in the skeletal muscle (SKM) of db +/-_MSTN +/- mice. Muscle lipid contents were assessed via mass spectrometry. Femoral artery ligation was used to stimulate in vivo angiogenesis in response to ischemia. Gene expression was assessed by RNA Seq with mRNA and protein expression follow-up in isolated gastrocnemius skeletal muscle via RT-qPCR and western blotting. Results As previously described, myostatin deletion results in significantly increased muscle mass without altering whole-body mass, fat percentage, or activity levels in obese db/db mice. Blood glucose is markedly improved but plasma lipids remain elevated. In parallel with these improvements, we observed that obesity inhibits and MSTN deletion restores vascularization following hind limb ischemia. Despite persistent plasma lipemia, ectopic lipid deposition in skeletal muscle was largely ameliorated in obese mice lacking myostatin, a reduction largely explained by a decrease in tissue triglyceride levels. EM and confocal microscopy revealed accumulation of lipid in obese mouse muscle cells that was lacking in obese mice with MSTN KO. The resolved deposition occurred predominantly inside muscle cells with fat cell infiltration in external spaces largely unaffected. RNA Seq analysis of skeletal muscle revealed marked upregulation of lipogenesis pathways, notably the expression of stearoyl-CoA desaturase-1 (SCD1), the rate limiting enzyme for the oleate synthesis and subsequent triglyceride production. SCD1 was elevated in muscle from normal obese mice but not those lacking myostatin at both the protein and RNA level. Conclusion In summary, increased muscle mass in obese mice provides potent protection to vascular and metabolic health. A potent correlate of this improvement is the resolution of myosteatosis in obese, hypermuscular mice. These data suggest that resolution of myosteatosis independently of muscle mass may afford similar protection, possibly by targeted deletion or blockade of SCD1.