Ravel-Chapuis Aymeric, Bélanger Guy, Côté Jocelyn, Michel Robin N, Jasmin Bernard J
Department of Cellular and Molecular Medicine and Center for Neuromuscular Disease, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada.
Department of Exercise Science, Faculty of Arts and Science, Concordia University, Montreal, QC, Canada.
Hum Mol Genet. 2017 Jun 15;26(12):2192-2206. doi: 10.1093/hmg/ddx109.
Myotonic Dystrophy type 1 (DM1) is caused by an expansion of CUG repeats in DMPK mRNAs. This mutation affects alternative splicing through misregulation of RNA-binding proteins. Amongst pre-mRNAs that are mis-spliced, several code for proteins involved in calcium homeostasis suggesting that calcium-handling and signaling are perturbed in DM1. Here, we analyzed expression of such proteins in DM1 mouse muscle. We found that the levels of several sarcoplasmic reticulum proteins (SERCA1, sarcolipin and calsequestrin) are altered, likely contributing to an imbalance in calcium homeostasis. We also observed that calcineurin (CnA) signaling is hyperactivated in DM1 muscle. Indeed, CnA expression and phosphatase activity are both markedly increased in DM1 muscle. Coherent with this, we found that activators of the CnA pathway (MLP, FHL1) are also elevated. Consequently, NFATc1 expression is increased in DM1 muscle and becomes relocalized to myonuclei, together with an up-regulation of its transcriptional targets (RCAN1.4 and myoglobin). Accordingly, DM1 mouse muscles display an increase in oxidative metabolism and fiber hypertrophy. To determine the functional consequences of this CnA hyperactivation, we administered cyclosporine A, an inhibitor of CnA, to DM1 mice. Muscles of treated DM1 mice showed an increase in CUGBP1 levels, and an exacerbation of key alternative splicing events associated with DM1. Finally, inhibition of CnA in cultured human DM1 myoblasts also resulted in a splicing exacerbation of the insulin receptor. Together, these findings show for the first time that calcium-CnA signaling is hyperactivated in DM1 muscle and that such hyperactivation represents a beneficial compensatory adaptation to the disease.
1型强直性肌营养不良（DM1）由DMPK mRNA中CUG重复序列的扩增引起。这种突变通过RNA结合蛋白的调控异常影响可变剪接。在错误剪接的前体mRNA中，有几种编码参与钙稳态的蛋白质，这表明DM1中钙处理和信号传导受到干扰。在此，我们分析了这些蛋白质在DM1小鼠肌肉中的表达。我们发现几种肌浆网蛋白（SERCA1、肌脂蛋白和肌钙蛋白）的水平发生了改变，这可能导致钙稳态失衡。我们还观察到DM1肌肉中钙调神经磷酸酶（CnA）信号过度激活。事实上，DM1肌肉中CnA的表达和磷酸酶活性均显著增加。与此一致，我们发现CnA途径的激活剂（MLP、FHL1）也升高。因此，DM1肌肉中NFATc1的表达增加，并重新定位于肌细胞核，同时其转录靶点（RCAN1.4和肌红蛋白）上调。相应地，DM1小鼠肌肉的氧化代谢增加，纤维肥大。为了确定这种CnA过度激活的功能后果，我们给DM1小鼠施用了CnA抑制剂环孢素A。接受治疗的DM1小鼠的肌肉中CUGBP1水平升高，与DM1相关的关键可变剪接事件加剧。最后，在培养的人DM1成肌细胞中抑制CnA也导致胰岛素受体的剪接加剧。总之，这些发现首次表明DM1肌肉中钙-CnA信号过度激活，并且这种过度激活代表了对该疾病的有益代偿性适应。
