Department of Pathology, Anatomy and Cell Biology, Thomas Jefferson University, Philadelphia, PA 19107, USA.
Cell Calcium. 2012 Nov;52(5):355-65. doi: 10.1016/j.ceca.2012.05.012. Epub 2012 Jul 10.
Emerging evidence indicates that mitochondria are locally coupled to endoplasmic reticulum (ER) Ca2+ release in myoblasts and to sarcoplasmic reticulum (SR) Ca2+ release in differentiated muscle fibers in order to regulate cytoplasmic calcium dynamics and match metabolism with cell activity. However, the mechanism of the developmental transition from ER to SR coupling remains unclear. We have studied mitochondrial sensing of IP3 receptor (IP3R)- and ryanodine receptor (RyR)-mediated Ca2+ signals in H9c2 myoblasts and differentiating myotubes, as well as the attendant changes in mitochondrial morphology. Mitochondria in myoblasts were largely elongated, luminally connected and relatively few in number, whereas the myotubes were densely packed with globular mitochondria that displayed limited luminal continuity. Vasopressin, an IP3-linked agonist, evoked a large cytoplasmic Ca2+ ([Ca2+]c) increase in myoblasts, whereas it elicited a smaller response in myotubes. Conversely, RyR-mediated Ca2+ release induced by caffeine, was not observed in myoblasts, but triggered a large [Ca2+]c signal in myotubes. Both the IP3R and the RyR-mediated [Ca2+]c rise was closely associated with a mitochondrial matrix Ca2+ ([Ca2+]m) signal. Every myotube that showed a [Ca2+]c spike also displayed a [Ca2+]m response. Addition of IP3 to permeabilized myoblasts and caffeine to permeabilized myotubes also resulted in a rapid [Ca2+]m rise, indicating that Ca2+ was delivered via local coupling of the ER/SR and mitochondria. Thus, as RyRs are expressed during muscle differentiation, the local connection between RyR and mitochondrial Ca2+ uptake sites also appears. When RyR1 was exogenously introduced to myoblasts by overexpression, the [Ca2+]m signal appeared together with the [Ca2+]c signal, however the mitochondrial morphology remained unchanged. Thus, RyR expression alone is sufficient to induce the steps essential for their alignment with mitochondrial Ca2+ uptake sites, whereas the mitochondrial proliferation and reshaping utilize either downstream or alternative pathways.
新出现的证据表明，在线粒体中，内质网（ER）钙释放与肌浆网（SR）钙释放局部偶联，以调节细胞质钙动力学并使代谢与细胞活动相匹配。然而，从 ER 到 SR 偶联的发育转变机制尚不清楚。我们研究了 H9c2 成肌细胞和分化的肌管中线粒体对 IP3 受体（IP3R）和 Ryanodine 受体（RyR）介导的 Ca2+信号的感应，以及伴随的线粒体形态变化。成肌细胞中的线粒体大多呈长形，内腔连接且数量较少，而肌管中则充满了球形线粒体，内腔连续性有限。血管加压素是一种与 IP3 相关的激动剂，可在成肌细胞中引发较大的细胞质 Ca2+（[Ca2+]c）增加，而在肌管中则引发较小的反应。相反，咖啡因诱导的 RyR 介导的 Ca2+释放在成肌细胞中观察不到，但在肌管中引发了较大的 [Ca2+]c 信号。IP3R 和 RyR 介导的 [Ca2+]c 上升都与线粒体基质 Ca2+（[Ca2+]m）信号密切相关。每个显示 [Ca2+]c 峰的肌管也显示 [Ca2+]m 反应。向通透的成肌细胞中添加 IP3 和向通透的肌管中添加咖啡因也导致快速的 [Ca2+]m 上升，表明 Ca2+是通过 ER/SR 和线粒体的局部偶联传递的。因此，随着 RyR 在肌肉分化过程中的表达，RyR 和线粒体 Ca2+摄取位点之间的局部连接也出现了。当通过过表达将 RyR1 外源性引入成肌细胞时，[Ca2+]m 信号与 [Ca2+]c 信号一起出现，然而线粒体形态保持不变。因此，RyR 的表达本身足以诱导与线粒体 Ca2+摄取位点对齐的必要步骤，而线粒体的增殖和重塑则利用下游或替代途径。
