Physiology and Bosch Institute, University of Sydney, Sydney, New South Wales, Australia.
Department of Physiology, The University of Melbourne, Melbourne, Victoria, Australia.
J Physiol. 2019 Sep;597(18):4831-4850. doi: 10.1113/JP277839. Epub 2019 Aug 18.
Adeno-associated viral vector was used to elevate the expression of muscle specific kinase (MuSK) and rapsyn (a cytoplasmic MuSK effector protein) in the tibialis anterior muscle of wild-type and dystrophic (mdx) mice. In mdx mice, enhanced expression of either MuSK or rapsyn ameliorated the acute loss of muscle force associated with strain injury. Increases in sarcolemmal immunolabelling for utrophin and β-dystroglycan suggest a mechanism for the protective effect of MuSK in mdx muscles. MuSK also caused subtle changes to the structure and function of the neuromuscular junction, suggesting novel roles for MuSK in muscle physiology and pathophysiology.
Muscle specific kinase (MuSK) has a well-defined role in stabilizing the developing mammalian neuromuscular junction, but MuSK might also be protective in some neuromuscular diseases. In the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy, limb muscles are especially fragile. We injected the tibialis anterior muscle of 8-week-old mdx and wild-type (C57BL10) mice with adeno-associated viral vectors encoding either MuSK or rapsyn (a cytoplasmic MuSK effector protein) fused to green fluorescent protein (MuSK-GFP and rapsyn-GFP, respectively). Contralateral muscles injected with empty vector served as controls. One month later mice were anaesthetized with isoflurane and isometric force-producing capacity was recorded from the distal tendon. MuSK-GFP caused an unexpected decay in nerve-evoked tetanic force, both in wild-type and mdx muscles, without affecting contraction elicited by direct electrical stimulation of the muscle. Muscle fragility was probed by challenging muscles with a strain injury protocol consisting of a series of four strain-producing eccentric contractions in vivo. When applied to muscles of mdx mice, eccentric contraction produced an acute 27% reduction in directly evoked muscle force output, affirming the susceptibility of mdx muscles to strain injury. mdx muscles overexpressing MuSK-GFP or rapsyn-GFP exhibited significantly milder force deficits after the eccentric contraction challenge (15% and 14%, respectively). The protective effect of MuSK-GFP in muscles of mdx mice was associated with increased immunolabelling for utrophin and β-dystroglycan in the sarcolemma. Elevating the expression of MuSK or rapsyn revealed several distinct synaptic and extrasynaptic effects, suggesting novel roles for MuSK signalling in muscle physiology and pathophysiology.
腺相关病毒载体被用于提高野生型和营养不良型（mdx）小鼠的比目鱼肌中肌肉特异性激酶（MuSK）和rapsyn（细胞质 MuSK 效应蛋白）的表达。在 mdx 小鼠中，MuSK 或 rapsyn 的表达增强减轻了与应变损伤相关的肌肉力量的急性丧失。肌膜免疫标记物 utrophin 和 β-肌营养不良蛋白的增加表明 MuSK 在 mdx 肌肉中的保护作用的一种机制。MuSK 还对神经肌肉接头的结构和功能产生细微变化，表明 MuSK 在肌肉生理学和病理生理学中具有新的作用。
肌肉特异性激酶（MuSK）在稳定发育中的哺乳动物神经肌肉接头中具有明确的作用，但 MuSK 在某些神经肌肉疾病中也可能具有保护作用。在营养不良型肌营养不良症的肌营养不良蛋白缺陷型 mdx 小鼠模型中，四肢肌肉特别脆弱。我们用腺相关病毒载体将编码 MuSK 或 rapsyn（细胞质 MuSK 效应蛋白）融合绿色荧光蛋白（MuSK-GFP 和 rapsyn-GFP）的病毒载体注射到 8 周龄 mdx 和野生型（C57BL10）小鼠的比目鱼肌中。用空载体注射对侧肌肉作为对照。一个月后，用异氟烷麻醉小鼠，并从远端肌腱记录等长产生力的能力。MuSK-GFP 引起了意想不到的神经诱发强直收缩力的衰减，无论是在野生型还是 mdx 肌肉中，而不影响肌肉直接电刺激引起的收缩。通过一种由体内四次应变产生的偏心收缩组成的应变损伤方案来探测肌肉脆弱性。当应用于 mdx 小鼠的肌肉时，偏心收缩导致直接诱发的肌肉力输出急性减少 27%，证实了 mdx 肌肉对应变损伤的易感性。过表达 MuSK-GFP 或 rapsyn-GFP 的 mdx 肌肉在偏心收缩后表现出明显较轻的力量缺陷（分别为 15%和 14%）。MuSK-GFP 在 mdx 小鼠肌肉中的保护作用与肌膜中 utrophin 和 β-肌营养不良蛋白的免疫标记增加有关。提高 MuSK 或 rapsyn 的表达揭示了几个不同的突触和 extrasynaptic 效应，表明 MuSK 信号在肌肉生理学和病理生理学中的新作用。
