Fast-to-Slow Transition of Skeletal Muscle Contractile Function and Corresponding Changes in Myosin Heavy and Light Chain Formation in the R6/2 Mouse Model of Huntington’s Disease

Huntington´s disease (HD) is a hereditary neurodegenerative disease resulting from an expanded polyglutamine sequence (poly-Q) in the protein huntingtin (HTT). Various studies report atrophy and metabolic pathology of skeletal muscle in HD and suggest as part of the process a fast-to-slow fiber type transition that may be caused by the pathological changes in central motor control or/and by mutant HTT in the muscle tissue itself. To investigate muscle pathology in HD, we used R6/2 mice, a common animal model for a rapidly progressing variant of the disease expressing exon 1 of the mutant human gene. We investigated alterations in the extensor digitorum longus (EDL), a typical fast-twitch muscle, and the soleus (SOL), a slow-twitch muscle. We focussed on mechanographic measurements of excised muscles using single and repetitive electrical stimulation and on the expression of the various myosin isoforms (heavy and light chains) using dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of whole muscle and single fiber preparations. In EDL of R6/2, the functional tests showed a left shift of the force-frequency relation and decrease in specific force. Moreover, the estimated relative contribution of the fastest myosin isoform MyHC IIb decreased, whereas the contribution of the slower MyHC IIx isoform increased. An additional change occurred in the alkali MyLC forms showing a decrease in 3f and an increase in 1f level. In SOL, a shift from fast MyHC IIa to the slow isoform I was detectable in male R6/2 mice only, and there was no evidence of isoform interconversion in the MyLC pattern. These alterations point to a partial remodeling of the contractile apparatus of R6/2 mice towards a slower contractile phenotype, predominantly in fast glycolytic fibers.亨廷顿舞蹈病（Huntington´s disease, HD）是一种遗传性神经退行性疾病，由亨廷顿蛋白（huntingtin, HTT）中异常扩增的多聚谷氨酰胺序列（poly-Q）所引发。多项研究报道了HD患者的骨骼肌萎缩与代谢病理改变，并提示疾病进程中存在肌纤维类型从快肌向慢肌的转变，该转变可能由中枢运动控制的病理变化或/和肌肉组织自身的突变型HTT所导致。为探究HD的肌肉病理特征，本研究采用了R6/2小鼠——一种表达突变型人类基因外显子1的快速进展型HD常用动物模型。我们分别对伸趾长肌（extensor digitorum longus, EDL，典型快缩肌）和比目鱼肌（soleus, SOL，慢缩肌）的变化进行了检测。研究重点包括：采用单次及重复电刺激对离体肌肉进行肌动描记测量，以及通过十二烷基硫酸钠-聚丙烯酰胺凝胶电泳（SDS-PAGE）对全肌肉及单纤维样本进行分析，以此检测各类肌球蛋白同工型（重链与轻链）的表达情况。在R6/2小鼠的伸趾长肌中，功能测试显示肌力-频率曲线左移，比肌力下降。此外，最快的肌球蛋白重链同工型MyHC IIb的估计相对占比降低，而较慢的MyHC IIx同工型的占比升高。碱溶性肌球蛋白轻链（alkali MyLC）的形式也出现了相应变化：3f亚型水平下降，1f亚型水平上升。在比目鱼肌中，仅在雄性R6/2小鼠中可检测到肌球蛋白重链从快肌型MyHC IIa向慢肌型I的转变，且未观察到肌球蛋白轻链谱型出现同工型互变。上述改变提示，R6/2小鼠的肌肉收缩装置发生了部分重塑，主要在快糖酵解肌纤维中向更慢的收缩表型转变。