Takekura H, Nishi M, Noda T, Takeshima H, Franzini-Armstrong C
Department of Physiology, National Institute of Fitness and Sports, Kagoshima, Japan.
Proc Natl Acad Sci U S A. 1995 Apr 11;92(8):3381-5. doi: 10.1073/pnas.92.8.3381.
Junctions that mediate excitation-contraction (e-c) coupling are formed between the sarcoplasmic reticulum (SR) and either the surface membrane or the transverse (T) tubules in normal skeletal muscle. Two structural components of the junctions, the feet of the SR and the tetrads of T tubules, have been identified respectively as ryanodine receptors (RyRs, or SR calcium-release channels), and as groups of four dihydropyridine receptors (DHPRs, or voltage sensors of e-c coupling). A targeted mutation (skrrm1) of the gene for skeletal muscle RyRs in mice results in the absence of e-c coupling in homozygous offspring of transgenic parents. The mutant gene is expected to produce no functional RyRs, and we have named the mutant mice "dyspedic" because they lack feet--the cytoplasmic domain of RyRs anchored in the SR membrane. We have examined the development of junctions in skeletal muscle fibers from normal and dyspedic embryos. Surprisingly, despite the absence of RyRs, junctions are formed in dyspedic myotubes, but the junctional gap between the SR and T tubule is narrow, presumably because the feet are missing. Tetrads are also absent from these junctions. The results confirm the identity of RyRs and feet and a major role for RyRs and tetrads in e-c coupling. Since junctions form in the absence of feet and tetrads, coupling of SR to surface membrane and T tubules appears to be mediated by additional proteins, distinct from either RyRs or DHPRs.
在正常骨骼肌中，介导兴奋 - 收缩（e - c）偶联的连接结构形成于肌浆网（SR）与表面膜或横管（T管）之间。这些连接结构的两个组成部分，即SR的脚和T管的四联体，已分别被鉴定为兰尼碱受体（RyRs，即SR钙释放通道）和由四个二氢吡啶受体（DHPRs，即e - c偶联的电压传感器）组成的基团。小鼠骨骼肌RyRs基因的靶向突变（skrrm1）导致转基因亲本的纯合后代中不存在e - c偶联。预计该突变基因不会产生有功能的RyRs，我们将突变小鼠命名为“dyspedic”，因为它们缺少脚——即锚定在SR膜上的RyRs的胞质结构域。我们研究了正常和dyspedic胚胎骨骼肌纤维中连接结构的发育情况。令人惊讶的是，尽管缺少RyRs，但dyspedic肌管中仍形成了连接结构，不过SR与T管之间的连接间隙很窄，推测是因为缺少脚。这些连接结构中也没有四联体。结果证实了RyRs与脚的一致性，以及RyRs和四联体在e - c偶联中的主要作用。由于在缺少脚和四联体的情况下仍能形成连接结构，SR与表面膜和T管的偶联似乎是由不同于RyRs或DHPRs的其他蛋白质介导的。
