Effect of acute exercise on activation of the calcineurin signaling pathway involved in development of slow muscle fibers

为了研究钙调神经磷酸酶在慢肌纤维发育中的作用，我们首先比较了钙调神经磷酸酶信号通路中基因在慢肌纤维和快肌纤维中的mRNA表达。结果表明，慢肌纤维中肌细胞特异性钙调磷酸酶抑制蛋白1（MCIP 1）的mRNA表达至少是快肌纤维的10倍。骨骼肌失神经支配后，快、慢肌纤维的分化表型消失，MCIP 1 mRNA表达的差异消失，急性运动后4 h内，骨骼肌MCIP 1 mRNA表达被短暂诱导。在MCIP 1启动子序列中，存在一系列可能与钙调磷酸酶活性相关的NFAT结合位点。事实上，钙调磷酸酶直接调节MCIP 1启动子通过这些NFAT结合位点。因此，MCIP 1 mRNA的表达水平可能直接反映了骨骼肌中钙调磷酸酶的活性，否则很难在体内进行监测。接下来，我们研究了大鼠在不同速度和时间的跑台运动后，急性运动强度与骨骼肌中MCIP 1 mRNA诱导之间的关系。MCIP 1 mRNA的表达与急性运动强度成正比，且在极轻度运动（12 m/min，10 min）时可诱导MCIP 1 mRNA的表达。即使在8 ~ 16 m/min的速度变化120 m时，大鼠骨骼肌中的MCIP 1 mRNA也同样被诱导。这些结果表明，MCIP 1 mRNA的表达水平是骨骼肌中钙调神经磷酸酶活性的良好指标，即使是非常温和的运动也足以激活钙调神经磷酸酶信号通路，这对慢肌纤维的发育和维持至关重要。

To investigate the role of the calcineurin in development of slow muscle fibers, we first compared the mRNA expression of genes in the calcineurin signaling pathway in slow muscle fibers with that in fast muscle fibers. It revealed that the mRNA expression of myocyte specific calcineurin inhibitory protein 1 (MCIP1) was at least 10 times higher in slow muscle fibers than in fast muscle fibers. The difference in the MCIP1 mRNA expression was lost, when slow and fast muscle fibers lost their differentiated phenotypes after denervation of skeletal muscles.The expression of MCIP1 mRNA was transiently induced in skeletal muscles within 4 hours after acute exercise. In the MCIP1 promoter sequence, there is an array of tentative NFAT binding sites, which might respond to the calcineurin activity. Indeed, calcineurin directly regulated the MCIP1 promoter via these NFAT binding sites. Therefore, the expression levels of MCIP1 mRNA might directly reflect the calcineurin activity in skeletal muscle, which is otherwise very difficult to monitor in vivo.We next examined the relationship between the intensity of acute exercise and the induction of MCIP1 mRNA in rat skeletal muscles, after treadmill running at various speed and time. The expression of MCIP1 mRNA was proportionally correlated with the intensity of acute exercise, and it was induced by very mild exercise (12 m/min for 10 min). Even when varying speed from 8〜16 m/min for 120 m, the MCIP1 mRNA was equally induced in rat skeletal muscles. These results suggest that the expression level of MCIP1 mRNA is a good indicator of the calcineurin activity in skeletal muscle, and even very mild exercise is enough to activate the calcineurin signaling pathway, which is important for development and maintenance of slow muscle fibers.