Retrograde Ca2+ signaling in C2C12 skeletal myocytes in response to mitochondrial genetic and metabolic stress: a novel mode of inter-organelle crosstalk

We have investigated the mechanism of mitochondrial-nuclear crosstalk durin g cellular stress in mouse C2C12 myocytes, For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatme nt or myocytes treated with known mitochondrial metabolic inhibitors, inclu ding carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomyci n and azide. Both genetic and metabolic stresses similarly affected mitocho ndrial membrane potential (Delta psi(m),) and electron transport-coupled AT P synthesis, which was also accompanied by an elevated steady-state cytosol ic Ca2+ level ([Ca2+](i)). The mitochondrial stress resulted in: (i) an enh anced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1) , hence potentiating the Ca2+ release in response to its modulator, caffein e; (ii) enhanced levels of Ca2+-responsive factors calineurin, calcineurin- dependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transc ription factor 2); (iii) reduced levels of transcription factor, NF-KB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene , These cellular changes, including the steady-state [Ca2+](i) were normali zed in genetically reverted cells which contain near-normal mtDNA levels, W e propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca2+](i) changes, which are likely to be due to reduced ATP and Ca2+ efflux. Our results indicate that the mitochondrial stress signal affe cts a variety of cellular processes, in addition to mitochondrial membrane biogenesis.