Doherty Katherine R, Cave Andrew, Davis Dawn Belt, Delmonte Anthony J, Posey Avery, Earley Judy U, Hadhazy Michele, McNally Elizabeth M
Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, IL 60637, USA.
Development. 2005 Dec;132(24):5565-75. doi: 10.1242/dev.02155. Epub 2005 Nov 9.
Muscle growth occurs during embryonic development and continues in adult life as regeneration. During embryonic muscle growth and regeneration in mature muscle, singly nucleated myoblasts fuse to each other to form myotubes. In muscle growth, singly nucleated myoblasts can also fuse to existing large, syncytial myofibers as a mechanism of increasing muscle mass without increasing myofiber number. Myoblast fusion requires the alignment and fusion of two apposed lipid bilayers. The repair of muscle plasma membrane disruptions also relies on the fusion of two apposed lipid bilayers. The protein dysferlin, the product of the Limb Girdle Muscular Dystrophy type 2 locus, has been shown to be necessary for efficient, calcium-sensitive, membrane resealing. We now show that the related protein myoferlin is highly expressed in myoblasts undergoing fusion, and is expressed at the site of myoblasts fusing to myotubes. Like dysferlin, we found that myoferlin binds phospholipids in a calcium-sensitive manner that requires the first C2A domain. We generated mice with a null allele of myoferlin. Myoferlin null myoblasts undergo initial fusion events, but they form large myotubes less efficiently in vitro, consistent with a defect in a later stage of myogenesis. In vivo, myoferlin null mice have smaller muscles than controls do, and myoferlin null muscle lacks large diameter myofibers. Additionally, myoferlin null muscle does not regenerate as well as wild-type muscle does, and instead displays a dystrophic phenotype. These data support a role for myoferlin in the maturation of myotubes and the formation of large myotubes that arise from the fusion of myoblasts to multinucleate myotubes.
肌肉生长在胚胎发育期间发生，并在成年期以再生的形式持续进行。在胚胎期肌肉生长以及成熟肌肉的再生过程中，单核成肌细胞相互融合形成肌管。在肌肉生长过程中，单核成肌细胞也可与现有的大型多核肌纤维融合，作为增加肌肉质量而不增加肌纤维数量的一种机制。成肌细胞融合需要两个相对的脂质双层的对齐和融合。肌肉质膜破坏的修复也依赖于两个相对脂质双层的融合。肢带型肌营养不良2型基因座的产物——dysferlin蛋白，已被证明是高效、钙敏感的膜重新封闭所必需的。我们现在表明，相关蛋白肌铁蛋白在正在融合的成肌细胞中高度表达，并在成肌细胞与肌管融合的部位表达。与dysferlin一样，我们发现肌铁蛋白以钙敏感的方式结合磷脂，这需要第一个C2A结构域。我们构建了肌铁蛋白无效等位基因的小鼠。肌铁蛋白缺失的成肌细胞经历初始融合事件，但它们在体外形成大型肌管的效率较低，这与肌生成后期的缺陷一致。在体内，肌铁蛋白缺失的小鼠肌肉比对照组小，且肌铁蛋白缺失的肌肉缺乏大直径肌纤维。此外，肌铁蛋白缺失的肌肉再生能力不如野生型肌肉，而是表现出营养不良的表型。这些数据支持肌铁蛋白在肌管成熟以及由成肌细胞融合形成多核肌管所产生的大型肌管形成中发挥作用。
