Role of Gs-alpha in regulation of skeletal muscle metabolism

我们通过将肌肉重链启动子-cre重组酶转基因小鼠与在Gs-alpha外显子1周围具有loxP重组位点的floxed Gs-alpha小鼠重复交配，产生骨骼肌中具有Gs-alpha缺陷的小鼠（MGsKO小鼠）。MGsKO小鼠存活正常，没有明显的物理表型。MGsKO小鼠具有正常的生长和体重和组成，并且食物摄入和能量消耗没有变化。这些结果表明，肌肉中Gs-alpha的损失似乎不会影响常规卡路里饮食的全身能量代谢。对高脂肪饮食的研究也没有显示代谢差异的证据。葡萄糖代谢的研究（基线血清化学、葡萄糖和胰岛素耐受性测试、高胰岛素血正葡萄糖钳夹研究和离体肌肉中的葡萄糖摄取）显示，MGsKO小鼠是葡萄糖不耐受的，尽管事实上小鼠具有正常的胰岛素分泌和对施用的胰岛素的低血糖应答。事实上，在离体肌肉中的研究显示，在存在最大胰岛素刺激的情况下，基础葡萄糖摄取增加，并且葡萄糖摄取没有变化，尽管在MGsKO小鼠的肌肉中，胰岛素相对于基线的增加较低。此外，响应于AMP类似物AICAR的肌肉葡萄糖摄取不受影响，表明AMP激酶及其下游途径保持完整。MGsKO患者的骨骼肌显示出显著萎缩，纤维横截面积减少。此外，根据肌球蛋白重链亚型和动力学特性，从1型（白色，快缩）纤维转换为2型（红色，慢缩）纤维，即使肌肉的线粒体含量和氧化能力降低，PGC-1 α（线粒体氧化的已知转录诱导物）表达降低，也会转换为1型纤维。因此，在MGsKO小鼠中，纤维类型转换与代谢特性的预期变化之间存在分离。这些小鼠也被证明可用于检查心肌中肌钙蛋白加工作为β-肾上腺素能信号传导减少的补偿机制的作用。最近的研究表明，这种向2型肌肉的转变是渐进的，可能是肌肉质量减少的一种补偿机制，可能与衰老和肌肉萎缩性疾病中的情况相似。

We generated mice with Gs-alpha deficiency in skeletal muscle (MGsKO mice) by repeated matings of muscle heavy chain promoter -cre recombinase transgenic mice with floxed Gs-alpha mice which have loxP recombination sites surrounding Gs-alpha exon 1. MGsKO mice have normal survival and no obvious physical phenotype. MGsKO mice had normal growth and body weight and composition and no changes in food intake and energy expenditure. These results show that loss of Gs-alpha in muscle does not appear to affect whole body energy metabolism on a regular calorie diet. Studies on high fat diet also show no evidence of differences in metabolism. Studies in glucose metabolism (baseline serum chemistries, glucose and insulin tolerance tests, hyperinsulinemic euglycemic clamp studies, and glucose uptake in isolated muscles) show that MGsKO mice are glucose intolerant despite the fact that the mice have normal insulin secretion and hypoglycemic response to administered insulin. In fact studies in isolated muscles show basal glucose uptake to be increased and with no change in glucose uptake in the presence of maximal insulin stimulation, although the increase from baseline with insulin was lower in muscles from MGsKO mice. In addition, muscle glucose uptake in response to the AMP analog AICAR was unaffected, indicating that AMP kinase and its downstream pathways remain intact. Skeletal muscles in MGsKO show significant atrophy with reduced fiber cross-sectional area. In addition, there is a switch in from type 1 (white, fast-twitch) to type 2 (red, slow-twitch) fibers based upon myosin heavy chain subtypes and kinetic properties even though the muscles have reduced mitochondrial content and oxidative capacity and reduced expression of PGC-1alpha, a known transcriptional inducer of mitochondrial oxifation and switch to type 1 fibers. Therefore in MGsKO mice there is a dissociation between the fiber type switch and the expected changes in metabolic properties. These mice have also proven useful to examine the role of troponin processing in cardiac muscle as a compensatory mechanism to reduced beta-adrenergic signaling. More recent studies have suggested that this switch to type 2 muscle is progressive and may be a compensatory mechanism for reduced muscle mass, and may mimic the situation seen in aging and muscle wasting disorders.