Department of Physiology, School of Medical Sciences, University of New South Wales, Sydney, NSW 2052, Australia.
Exp Physiol. 2010 May;95(5):641-56. doi: 10.1113/expphysiol.2009.052019. Epub 2010 Feb 5.
In the dystrophinopathies, skeletal muscle fibres undergo cycles of degeneration and regeneration, with regenerated fibres displaying a branched morphology. This study tests the hypothesis that regenerated branched fibres are mechanically weakened by the presence of branches and are damaged by contractions which do not affect unbranched dystrophin-negative fibres. Experiments were carried out on single fast-twitch fibres and whole muscle from the dystrophin-negative mdx mouse. Fura-2 was ionophoresed into fibres to measure intracellular Ca(2+) concentration (Ca(2+)). Single branched mdx fibres have abnormal Ca(2+) kinetics, with the Ca(2+) transient at the peak of the twitch depressed, are damaged by fatiguing activation, resulting in a breakdown of Ca(2+) homeostasis, and break at branch points when submaximally activated in skinned fibre experiments. When old intact isolated mdx muscles, with >90% branched fibres, are eccentrically activated with a moderate eccentric protocol there is a 40 +/- 8% reduction in maximal force. Isolated single fibres from these muscles show areas of damage at fibre branch points. This same eccentric protocol causes no force loss in either littermate control muscles or mdx muscles with <10% branched fibres. I present a two-stage hypothesis for muscle damage in the dystrophinopathies, as follows: stage 1, the absence of dystrophin disrupts ion channel function, causing an activation of necrotizing Ca(2+)-activated proteases, which results in regenerated branched fibres; and stage 2, branched fibres are mechanically damaged during contraction. These results may have implications when considering therapies for boys with Duchenne muscular dystrophy. In particular, any therapy aimed at rescuing the defective gene will presumably have to do so before the number of branched fibres has increased to a level where the muscle is mechanically compromised.
在肌营养不良症中，骨骼肌纤维经历退化和再生的循环，再生的纤维呈现分支形态。本研究检验了以下假设：即分支再生纤维由于分支的存在而机械弱化，并在不影响未分支的抗肌萎缩蛋白阴性纤维的收缩中受损。实验在抗肌萎缩蛋白阴性 mdx 小鼠的单个快速抽搐纤维和整块肌肉上进行。将 Fura-2 离子导入纤维以测量细胞内 Ca(2+)浓度(Ca(2+))。单个分支的 mdx 纤维具有异常的 Ca(2+)动力学，抽搐峰值时的 Ca(2+)瞬变被抑制，在疲劳激活下受损，导致 Ca(2+)稳态破坏，并在去神经纤维实验中在分支点断裂。当用适度的离心方案激活陈旧的完整的分离的 mdx 肌肉时，超过 90%的分支纤维，最大力减少 40 +/- 8%。从这些肌肉中分离的单个纤维显示在纤维分支点处有损伤区域。同一离心方案在同窝对照肌肉或分支纤维<10%的 mdx 肌肉中不会引起力损失。我提出了一个肌营养不良症中肌肉损伤的两阶段假说，如下所示：第 1 阶段，抗肌萎缩蛋白的缺失破坏了离子通道功能，导致激活了坏死的 Ca(2+)-激活蛋白酶，从而导致再生的分支纤维；第 2 阶段，在收缩过程中分支纤维受到机械损伤。当考虑治疗杜氏肌营养不良症的男孩时，这些结果可能具有重要意义。特别是，任何旨在拯救有缺陷基因的治疗方法，很可能必须在分支纤维数量增加到使肌肉受到机械损伤的水平之前进行。
