Volonte Daniela, Peoples Aaron J, Galbiati Ferruccio
Department of Pharmacology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, USA.
Mol Biol Cell. 2003 Oct;14(10):4075-88. doi: 10.1091/mbc.e03-03-0161. Epub 2003 Aug 7.
Caveolae are vesicular invaginations of the plasma membrane. Caveolin-3 is the principal structural component of caveolae in skeletal muscle cells in vivo. We have recently generated caveolin-3 transgenic mice and demonstrated that overexpression of wild-type caveolin-3 in skeletal muscle fibers is sufficient to induce a Duchenne-like muscular dystrophy phenotype. In addition, we have shown that caveolin-3 null mice display mild muscle fiber degeneration and T-tubule system abnormalities. These data are consistent with the mild phenotype observed in Limb-girdle muscular dystrophy-1C (LGMD-1C) in humans, characterized by a approximately 95% reduction of caveolin-3 expression. Thus, caveolin-3 transgenic and null mice represent valid mouse models to study Duchenne muscular dystrophy (DMD) and LGMD-1C, respectively, in humans. Here, we derived conditionally immortalized precursor skeletal muscle cells from caveolin-3 transgenic and null mice. We show that overexpression of caveolin-3 inhibits myoblast fusion to multinucleated myotubes and lack of caveolin-3 enhances the fusion process. M-cadherin and microtubules have been proposed to mediate the fusion of myoblasts to myotubes. Interestingly, we show that M-cadherin is downregulated in caveolin-3 transgenic cells and upregulated in caveolin-3 null cells. For the first time, variations of M-cadherin expression have been linked to a muscular dystrophy phenotype. In addition, we demonstrate that microtubules are disorganized in caveolin-3 null myotubes, indicating the importance of the cytoskeleton network in mediating the phenotype observed in these cells. Taken together, these results propose caveolin-3 as a key player in myoblast fusion and suggest that defects of the fusion process may represent additional molecular mechanisms underlying the pathogenesis of DMD and LGMD-1C in humans.
小窝是质膜的囊泡状内陷结构。小窝蛋白-3是体内骨骼肌细胞中小窝的主要结构成分。我们最近培育出了小窝蛋白-3转基因小鼠，并证明在骨骼肌纤维中过表达野生型小窝蛋白-3足以诱发杜兴氏样肌营养不良表型。此外，我们还表明小窝蛋白-3基因敲除小鼠表现出轻度的肌纤维退化和T小管系统异常。这些数据与人类肢带型肌营养不良1C（LGMD-1C）中观察到的轻度表型一致，其特征是小窝蛋白-3表达降低约95%。因此，小窝蛋白-3转基因和基因敲除小鼠分别代表了研究人类杜兴氏肌营养不良（DMD）和LGMD-1C的有效小鼠模型。在此，我们从小窝蛋白-3转基因和基因敲除小鼠中获得了条件永生化的前体骨骼肌细胞。我们发现，小窝蛋白-3的过表达抑制成肌细胞融合形成多核肌管，而缺乏小窝蛋白-3则增强融合过程。有人提出M-钙黏蛋白和微管介导成肌细胞与肌管的融合。有趣的是，我们发现M-钙黏蛋白在小窝蛋白-3转基因细胞中表达下调，而在小窝蛋白-3基因敲除细胞中表达上调。首次将M-钙黏蛋白表达的变化与肌营养不良表型联系起来。此外，我们证明在小窝蛋白-3基因敲除的肌管中微管排列紊乱，表明细胞骨架网络在介导这些细胞中观察到的表型方面具有重要作用。综上所述，这些结果表明小窝蛋白-3是成肌细胞融合中的关键因子，并提示融合过程的缺陷可能是人类DMD和LGMD-1C发病机制的额外分子机制。
