MYD88 Signaling in Mammalian Myoblast Fusion

 描述(由PI提供)：成肌细胞融合是一个关键事件，不仅对于胚胎发育过程中骨骼肌的发育是必要的，而且对于成年肌纤维损伤后的再生和负荷诱导的骨骼肌肥大也是必需的。在现有或引入的肌源性细胞中增强肌细胞融合的自然过程在治疗退行性肌肉疾病方面具有巨大的治疗潜力。然而，脊椎动物成肌细胞融合的机制仍然知之甚少。越来越多的证据表明，在哺乳动物系统中，成肌细胞的融合涉及到特定的细胞信号通路的激活，如MAPK、非典范的NF-κB、典范的Wnt信号。然而，导致它们激活的上游信号机制以及这些通路的激活如何在肌形成过程中促进成肌细胞融合仍然知之甚少。我们的初步研究已经证实，髓系分化初级反应基因88(MyD88)在体外和体内都是成肌细胞融合的关键调控因子。我们的结果表明，MyD88的水平在肌肉发生过程中升高。MyD88的基因消融损害成肌细胞的融合，而MyD88的过表达促进了培养成肌细胞肌管的形成。此外，我们已经获得了MyD88在肌肉发生过程中调节特定的ProFusion信号通路激活的初步证据。基于我们的初步研究，我们建议建立MyD88在体外和体内介导骨骼肌形成的作用，并描述MyD88介导的信号和分子机制。我们的工作假设是：(I)MyD88通过增强特定融合分子的基因表达在多种条件下促进成肌细胞融合；(Ii)MyD88协调规范的Wnt和非规范的NF-κB信号的激活，以促进肌肉发生过程中的成肌细胞融合；(Iii)转录和转录后机制调节MyD88在成肌细胞分化过程中的水平。我们将通过解决以下三个具体目标来检验这些假设。目的I：确定MyD88在成肌细胞融合中的作用并研究其分子机制；目的II：研究MyD88促进成肌细胞融合的信号网络；目的III：研究MyD88在肌细胞发生过程中水平升高的机制。该项目的成功完成将为成肌细胞融合的机制提供关键的见解，并将导致确定用于治疗肌肉疾病的新的药物靶点。

 DESCRIPTION (provided by PI): Myoblast fusion is a critical event that is required not only for the development of skeletal muscle during embryogenesis but also for the regeneration of adult myofibers upon injury and for load-induced skeletal muscle hypertrophy. Augmenting the natural process of muscle cell fusion in existing or introduced myogenic cells has enormous therapeutic potential to treat degenerative muscle diseases. However, the mechanisms of myoblast fusion in vertebrates remain less understood. Accumulating evidence suggests that myoblast fusion in mammalian system involves the activation of specific cell signaling pathways such as MAPK, non-canonical NF-κB, canonical Wnt signaling. However, the upstream signaling mechanisms leading to their activation and how the activation of these pathways promotes myoblast fusion during myogenesis remain poorly understood. Our preliminary studies have identified that myeloid differentiation primary response gene 88 (Myd88) is a critical regulator of myoblast fusion both in vitro and in vivo. Our results show that the levels of Myd88 are increased during myogenesis. Genetic ablation of Myd88 impairs myoblast fusion whereas overexpression of Myd88 enhances the formation of myotubes in cultured myoblasts. Furthermore, we have obtained initial evidence that Myd88 regulates the activation of specific profusion signaling pathways during myogenesis. Based on our preliminary studies, we have proposed to establish the role and delineate the signaling and molecular mechanisms by which Myd88 mediates skeletal muscle formation in vitro and in vivo. Our working hypotheses are: (I) Myd88 promotes myoblast fusion in multiple conditions through augmenting the gene expression of specific profusion molecules; (II) Myd88 coordinates the activation of canonical Wnt and non-canonical NF-κB signaling to promote myoblast fusion during myogenesis; (III) Both transcriptional and post-transcriptional mechanisms regulate the levels of Myd88 in differentiating myoblasts. We will test these hypotheses by addressing the following three specific aims. Aim I: Establish the role and investigate molecular mechanisms by which Myd88 mediates myoblast fusion; Aim II: Investigate the signaling networks through which Myd88 promotes myoblast fusion; and Aim III: Investigate the mechanisms by which levels of Myd88 are increased during myogenesis. Successful completion of this project will provide critical insights into the mechanisms of myoblast fusion and will lead to the identification of novel drug targets for treatment of muscle disorders.