Stupka Nicole, Plant David R, Schertzer Jonathan D, Emerson Tennent M, Bassel-Duby Rhonda, Olson Eric N, Lynch Gordon S
Department of Physiology, the University of Melbourne, Victoria 3010, Australia.
J Physiol. 2006 Sep 1;575(Pt 2):645-56. doi: 10.1113/jphysiol.2006.108472. Epub 2006 Jun 22.
Utrophin expression is regulated by calcineurin and up-regulating utrophin can decrease the susceptibility of dystrophic skeletal muscle to contraction-induced injury. We overexpressed the constitutively active calcineurin-A alpha in skeletal muscle of mdx dystrophic mice (mdx CnA*) and examined the tibialis anterior muscle to determine whether the presence of activated calcineurin promotes resistance to muscle damage after lengthening contractions. Two stretches (10 s apart) of 40% strain relative to muscle fibre length were initiated from the plateau of a maximal isometric tetanic contraction. Muscle damage was assessed 1, 5 and 15 min later by the deficit in maximum isometric force and by quantifying the proportion of muscle fibres staining positive for intracytoplasmic albumin. The force deficit at all time points after the lengthening contractions was approximately 80% in mdx muscles and 30% in mdx CnA* muscles. The proportion of albumin-positive fibres was significantly less in control and injured muscles from mdx CnA* mice than from mdx mice. Compared with mdx mice, mean fibre cross-sectional area was 50% less in muscles from mdx CnA* mice. Furthermore, muscles from mdx CnA* mice exhibited a higher proportion of fibres expressing the slow(er) myosin heavy chain (MyHC) I and IIa isoforms, prolonged contraction and relaxation times, lower absolute and normalized maximum forces, and a clear leftward shift of the frequency-force relationship with greater force production at lower stimulation frequencies. These are structural and functional markers of a slower muscle phenotype. Taken together, our findings show that muscles from mdx CnA* mice have a smaller mean fibre cross-sectional area, a greater sarcolemmal to cytoplasmic volume ratio, and an increase in utrophin expression, promoting an attenuated susceptibility to contraction-induced injury. We conclude that increased calcineurin activity may confer functional benefits to dystrophic skeletal muscles.
肌养蛋白的表达受钙调神经磷酸酶调控，上调肌养蛋白可降低营养不良性骨骼肌对收缩诱导损伤的易感性。我们在mdx营养不良小鼠（mdx CnA*）的骨骼肌中过表达组成型活性钙调神经磷酸酶-Aα，并检查胫前肌，以确定活化的钙调神经磷酸酶是否能增强延长收缩后肌肉对损伤的抵抗力。从最大等长强直收缩的平台期开始，进行两次相对于肌纤维长度40%应变的拉伸（间隔10秒）。在拉伸后1、5和15分钟，通过最大等长力的下降以及量化胞浆白蛋白染色阳性的肌纤维比例来评估肌肉损伤。延长收缩后所有时间点，mdx肌肉的力下降约80%，而mdx CnA肌肉为30%。mdx CnA小鼠对照和损伤肌肉中白蛋白阳性纤维的比例显著低于mdx小鼠。与mdx小鼠相比，mdx CnA小鼠肌肉的平均纤维横截面积小50%。此外，mdx CnA小鼠的肌肉中表达较慢肌球蛋白重链（MyHC）I和IIa亚型的纤维比例更高，收缩和舒张时间延长，绝对和标准化最大力更低，频率-力关系明显左移，在较低刺激频率下产生更大力量。这些是较慢肌肉表型的结构和功能标志物。综上所述，我们的研究结果表明，mdx CnA*小鼠的肌肉平均纤维横截面积较小，肌膜与细胞质体积比更大，肌养蛋白表达增加，从而降低了对收缩诱导损伤的易感性。我们得出结论，钙调神经磷酸酶活性增加可能为营养不良性骨骼肌带来功能益处。
