Redefining the Role of FKBP12 in Skeletal Muscle

摘要 骨骼肌（SkM）特异性降低小分子12 kDa FK 506结合蛋白FKBP 12的小鼠， (FKD小鼠）表现出改善的耐力、胰岛素介导的葡萄糖清除率和骨矿物质密度， 以及减少身体脂肪和抵抗高脂肪饮食的体重增加。低剂量雷帕霉素和SLF 从其结合配偶体置换FKBP 12的合成配体（FKBP 12的合成配体）模拟FKBP 12的作用 SkM缺乏，表明这些药物有潜力作为干预措施，以改善肌肉功能， 新陈代谢. FKBP 12在SkM中的主要靶点是肌浆网（SR）Ca 2+释放通道， RyR 1，它控制兴奋-收缩偶联（ECC）过程中细胞内Ca 2+的释放。 从RyR 1中部分去除FKBP 12（遗传或通过低剂量雷帕霉素或SLF治疗） 增加肌浆Ca 2+瞬变和Ca 2+流入肌纤维的幅度， 重复刺激。两者都能增强SR的Ca ~（2+）释放和增加Ca ~（2+）内流，这与部分 FKBP 12从RyR 1中的去除被钙调蛋白依赖性蛋白激酶II（CaMKII）抑制剂阻断 钙库操纵的钙内流（SOCE）。然而，增加Ca 2+释放和 流入导致改善肌肉功能和代谢仍然未知。我们假设存在一个 可调反馈回路，在功能上耦合ECC，SOCE和线粒体Ca 2+摄取，以调节 肌肉功能和新陈代谢。本申请的具体目的是：A1.定义FKBP 12的角色 和RyR 1磷酸化在调节重复刺激期间Ca 2+瞬变的幅度中的作用， 改善SkM性能和新陈代谢。2.定义增强Ca 2+储存再填充的反馈回路 和ATP产生来维持FKBP 12缺陷小鼠中改善的肌肉性能和代谢。3. 评价SLF改善肌肉功能和代谢的治疗潜力。我们的长期目标是 开发干预措施，以改善因年龄而无法进行剧烈运动的人的肌肉功能， 肌肉疾病、肥胖和/或患有II型糖尿病。

Abstract Mice with a skeletal muscle (SkM)-specific decrease in the small 12 kDa FK506 binding protein, FKBP12, (FKD mice) display improved endurance, insulin-mediated glucose clearance, and bone mineral density, as well as decreased body fat and resistance to weight gain on a high fat diet. Low doses of rapamycin and SLF (synthetic ligand for FKBP12) that displace FKBP12 from its binding partners mimic the effects of FKBP12 deficiency in SkM, suggesting these drugs have potential as interventions to improve muscle function and metabolism. The primary target of FKBP12 in SkM is the sarcoplasmic reticulum (SR) Ca2+ release channel, RyR1, which controls the release of Ca2+ from intracellular stores during excitation-contraction coupling (ECC). Partial removal of FKBP12 from RyR1 (genetically or by treatment with low doses of rapamycin or SLF) increases both the amplitude of the myoplasmic Ca2+ transient and Ca2+ influx into the muscle fiber during repetitive stimulation. Both enhanced SR Ca2+ release and increased Ca2+ influx associated with partial removal of FKBP12 from RyR1 are blocked by inhibitors of calmodulin-dependent protein kinase II (CaMKII) and store-operated Ca2+ entry (SOCE). However, the mechanisms by which increases in Ca2+ release and influx result in improved muscle function and metabolism remain unknown. We hypothesize the existence of a tunable feedback loop that functionally couples ECC, SOCE, and mitochondrial Ca2+ uptake to modulate muscle function and metabolism. The specific aims of this application are to: A1. Define the roles of FKBP12 and RyR1 phosphorylation in regulating the amplitude of the Ca2+ transient during repetitive stimulation and improving SkM performance and metabolism. 2. Define the feedback loop that enhances Ca2+ store refilling and ATP production to sustain the improved muscle performance and metabolism in FKBP12 deficient mice. 3. Evaluate the therapeutic potential of SLF to improve muscle function and metabolism. Our long-term goal is to develop interventions to improve muscle function in people who cannot perform strenuous exercise due to age, muscle disease, obesity and/or have type II diabetes.