Department of Neurobiology, Physiology, and Behavior, University of California Davis.
Department of Physical Medicine and Rehabilitation, University of California Davis.
J Physiol. 2021 Feb;599(3):943-962. doi: 10.1113/JP280656. Epub 2020 Dec 18.
The amount of fibrotic material in dystrophic mouse muscles relates to contractile function, but not passive function. Collagen fibres in skeletal muscle are associated with increased passive muscle stiffness in fibrotic muscles. The alignment of collagen is independently associated with passive stiffness in dystrophic skeletal muscles. These outcomes demonstrate that collagen architecture rather than collagen content should be a target of anti-fibrotic therapies to treat muscle stiffness.
Fibrosis is prominent in many skeletal muscle pathologies including dystrophies, neurological disorders, cachexia, chronic kidney disease, sarcopenia and metabolic disorders. Fibrosis in muscle is associated with decreased contractile forces and increased passive stiffness that limits joint mobility leading to contractures. However, the assumption that more fibrotic material is directly related to decreased function has not held true. Here we utilize novel measurement of extracellular matrix (ECM) and collagen architecture to relate ECM form to muscle function. We used mdx mice, a model for Duchenne muscular dystrophy that becomes fibrotic, and wildtype mice. In this model, extensor digitorum longus (EDL) muscle was significantly stiffer, but with similar total collagen, while the soleus muscle did not change stiffness, but increased collagen. The stiffness of the EDL was associated with increased collagen crosslinking as determined by collagen solubility. Measurement of ECM alignment using polarized light microscopy showed a robust relationship between stiffness and alignment for wildtype muscle that broke down in mdx muscles. Direct visualization of large collagen fibres with second harmonic generation imaging revealed their relative abundance in stiff muscles. Collagen fibre alignment was linked to stiffness across all muscles investigated and the most significant factor in a multiple linear regression-based model of muscle stiffness from ECM parameters. This work establishes novel characteristics of skeletal muscle ECM architecture and provides evidence for a mechanical function of collagen fibres in muscle. This finding suggests that anti-fibrotic strategies to enhance muscle function and excessive stiffness should target large collagen fibres and their alignment rather than total collagen.
营养不良小鼠肌肉中的纤维组织含量与收缩功能有关，但与被动功能无关。在纤维化肌肉中，骨骼肌中的胶原纤维与被动肌肉僵硬增加有关。胶原的排列与营养不良骨骼肌的被动僵硬独立相关。这些结果表明，胶原结构而不是胶原含量应该成为抗纤维化治疗的目标，以治疗肌肉僵硬。
纤维化在许多骨骼肌疾病中都很明显，包括营养不良、神经紊乱、恶病质、慢性肾病、肌肉减少症和代谢紊乱。肌肉中的纤维化与收缩力降低和被动僵硬增加有关，这会限制关节活动度，导致挛缩。然而，纤维组织越多与功能下降直接相关的假设并不成立。在这里，我们利用细胞外基质 (ECM) 和胶原结构的新测量方法，将 ECM 形态与肌肉功能联系起来。我们使用 mdx 小鼠，这是一种营养不良的模型，会发生纤维化，以及野生型小鼠。在这个模型中，伸趾长肌 (EDL) 肌肉明显更硬，但总胶原相似，而比目鱼肌肌肉没有改变僵硬度，但增加了胶原。EDL 肌肉的僵硬度与胶原交联的增加有关，这是通过胶原溶解度来确定的。偏振光显微镜测量 ECM 排列显示，野生型肌肉的僵硬度与排列之间存在很强的关系，但在 mdx 肌肉中这种关系破裂。使用二次谐波产生成像直接观察大的胶原纤维显示出它们在僵硬肌肉中的相对丰度。胶原纤维排列与所有研究肌肉的僵硬度相关，并且在基于 ECM 参数的肌肉僵硬多线性回归模型中是最重要的因素。这项工作确立了骨骼肌 ECM 结构的新特征，并为肌肉中胶原纤维的机械功能提供了证据。这一发现表明，为了增强肌肉功能和过度僵硬，抗纤维化策略应该针对大的胶原纤维及其排列，而不是总胶原。
