Novel Mechanisms of Extracellular Matrix Regulation During Development

骨骼肌的特化和形态发生对正常胚胎发育至关重要。的 先天性肌营养不良症导致衰弱性疾病，这一事实突出了肌肉的关键作用 在儿童中，几乎所有受影响的患者都与过早死亡有关。许多先天性肌肉 营养不良，如Duchenne、Ulrich和Merosin缺陷型肌营养不良，是由突变引起的 在粘附复合物中，锚肌细胞到它们周围的细胞外基质（肌基质）。在 除了肌肉发育所需外，肌基质在肌肉中起着重要作用， 因为它承担了大部分的被动负荷。尽管肌瘤在子宫内膜增生中 肌肉发育和生理学，肌基质发育的机制尚不清楚。在 特别是，介导肌基质组成动态变化的机制还不清楚。 纤维连接蛋白（Fn）是肌肉发育和再生所必需的肌基质蛋白。Fn是 在肌肉发育和再生过程中瞬时上调。随后的Fn下调 水平是重要的，因为过量的Fn沉积导致纤维化。纤维化是一种 细胞外基质蛋白在骨骼肌的情况下，收缩性肌肉组织被替换为 富含纤维素的纤维化物质减少肌肉功能。因此，必须严格控制Fn水平。的 肌肉发育过程中介导Fn聚合的机制开始被阐明，但 关于Fn是如何下调的还不清楚。我们的中心假设是， 沿着基质金属蛋白酶11（Mmp 11）和基质金属蛋白酶组织抑制剂2a（Timp 2a）， 包括控制Fn水平、肌纤维类型特化和肌肉 形态发生这一假设是基于我们的初步数据，表明层粘连蛋白聚合 介导Mmp 11定位，Mmp 11是Fn下调的必要和充分条件， 斑马鱼发育初步数据还表明，Timp 2a是Fn调节所必需的。理解 Fn在肌肉发育和再生过程中的调节是重要的，因为瞬时Fn是必需的; 但过多或持续的Fn是有害的。这些实验是创新的，因为它们是第一个 解决了在发育过程中Fn下调的潜在机制，首次解决了Mmp 11功能 在肌肉发育过程中，第一个分析层粘连蛋白和Fn之间的串扰。这一贡献 研究将是确定新的机制，调节Fn水平和Fn的作用，在肌纤维类型 规范.这项研究的完成将大大推进我们的长期目标，即了解 肌肉细胞和它们的肌基质之间的信号传导如何介导肌肉发育，稳态， 再生

The specification and morphogenesis of skeletal muscle is critical for normal embryonic development. The critical role of muscle is highlighted by the fact that congenital muscular dystrophies cause debilitating disease in children and are associated with premature death in nearly all patients affected. Many congenital muscular dystrophies, such as Duchenne, Ulrich, and Merosin-deficient muscular dystrophies, are caused by mutations in adhesion complexes that anchor muscle cells to their surrounding extracellular matrix (the myomatrix). In addition to being required for muscle development, the myomatrix plays a fundamental role in muscle homeostasis because it bears much of the passive load. Despite the critical role that the myomatrix plays in muscle development and physiology, the mechanisms underlying myomatrix development are not known. In particular, mechanisms that mediate the dynamic changes in myomatrix composition are not well understood. Fibronectin (Fn) is a myomatrix protein that is necessary for muscle development and regeneration. Fn is transiently upregulated during muscle development and regeneration. The subsequent downregulation of Fn levels is important because excess Fn deposition leads to fibrosis. Fibrosis is the aberrant deposition of extracellular matrix proteins. In the context of skeletal muscle, the replacement of contractile muscle tissue with Fn-rich fibrotic material diminishes muscle function. Therefore, Fn levels have to be tightly controlled. The mechanisms that mediate Fn polymerization during muscle development are beginning to be elucidated, but nothing is known about how Fn is downregulated. Our central hypothesis is that the myomatrix protein laminin, along with Matrix Metalloproteinase 11 (Mmp11) and Tissue Inhibitor of Matrix Metalloproteinase 2a (Timp2a), comprise a regulatory network that controls Fn levels, muscle fiber type specification, and muscle morphogenesis. This hypothesis is based on our preliminary data showing that laminin polymerization mediates Mmp11 localization and that Mmp11 is necessary and sufficient for Fn downregulation during zebrafish development. Preliminary data also suggest that Timp2a is required for Fn regulation. Understanding Fn regulation during muscle development and regeneration is significant because transient Fn is necessary; but too much or sustained Fn is deleterious. These experiments are innovative because they are the first to address mechanisms underlying Fn downregulation during development, the first to address Mmp11 function during muscle development, and the first to analyze crosstalk between laminin and Fn. The contribution of this research will be identification of novel mechanisms regulating Fn levels and roles for Fn in muscle fiber type specification. Completion of this research will significantly advance our long-term goal, which is to understand how signaling between muscle cells and their myomatrix mediates muscle development, homeostasis, and regeneration.