RESUMO
Mitochondria are crucial organelles in the production of energy and in the control of signalling cascades. A machinery of pro-fusion and fission proteins regulates their morphology and subcellular localization. In muscle this results in an orderly pattern of intermyofibrillar and subsarcolemmal mitochondria. Muscular atrophy is a genetically controlled process involving the activation of the autophagy-lysosome and the ubiquitin-proteasome systems. Whether and how the mitochondria are involved in muscular atrophy is unknown. Here, we show that the mitochondria are removed through autophagy system and that changes in mitochondrial network occur in atrophying muscles. Expression of the fission machinery is per se sufficient to cause muscle wasting in adult animals, by triggering organelle dysfunction and AMPK activation. Conversely, inhibition of the mitochondrial fission inhibits muscle loss during fasting and after FoxO3 overexpression. Mitochondrial-dependent muscle atrophy requires AMPK activation as inhibition of AMPK restores muscle size in myofibres with altered mitochondria. Thus, disruption of the mitochondrial network is an essential amplificatory loop of the muscular atrophy programme.
Assuntos
Mitocôndrias/metabolismo , Atrofia Muscular/patologia , Proteínas Quinases Ativadas por AMP/metabolismo , Animais , Autofagia , Linhagem Celular , Humanos , Camundongos , Microscopia de Fluorescência/métodos , Modelos Biológicos , Músculo Esquelético/patologia , Interferência de RNA , Transdução de Sinais , Fatores de Tempo , TransfecçãoRESUMO
Autophagy allows cell survival during starvation through the bulk degradation of proteins and organelles by lysosomal enzymes. However, the mechanisms responsible for the induction and regulation of the autophagy program are poorly understood. Here we show that the FoxO3 transcription factor, which plays a critical role in muscle atrophy, is necessary and sufficient for the induction of autophagy in skeletal muscle in vivo. Akt/PKB activation blocks FoxO3 activation and autophagy, and this effect is not prevented by rapamycin. FoxO3 controls the transcription of autophagy-related genes, including LC3 and Bnip3, and Bnip3 appears to mediate the effect of FoxO3 on autophagy. This effect is not prevented by proteasome inhibitors. Thus, FoxO3 controls the two major systems of protein breakdown in skeletal muscle, the ubiquitin-proteasomal and autophagic/lysosomal pathways, independently. These findings point to FoxO3 and Bnip3 as potential therapeutic targets in muscle wasting disorders and other degenerative and neoplastic diseases in which autophagy is involved.
Assuntos
Autofagia/fisiologia , Fatores de Transcrição Forkhead/metabolismo , Músculo Esquelético/citologia , Músculo Esquelético/metabolismo , Animais , Autofagia/genética , Proteína Forkhead Box O3 , Fatores de Transcrição Forkhead/antagonistas & inibidores , Fatores de Transcrição Forkhead/genética , Regulação da Expressão Gênica , Lisossomos/metabolismo , Proteínas de Membrana/genética , Proteínas de Membrana/metabolismo , Camundongos , Camundongos Transgênicos , Proteínas Associadas aos Microtúbulos/genética , Proteínas Associadas aos Microtúbulos/metabolismo , Proteínas Mitocondriais/genética , Proteínas Mitocondriais/metabolismo , Modelos Biológicos , Atrofia Muscular/genética , Atrofia Muscular/metabolismo , Atrofia Muscular/patologia , Complexo de Endopeptidases do Proteassoma/metabolismo , Proteínas Quinases/genética , Proteínas Quinases/metabolismo , Proteínas Proto-Oncogênicas c-akt/genética , Proteínas Proto-Oncogênicas c-akt/metabolismo , Interferência de RNA , Serina-Treonina Quinases TOR , Ubiquitina/metabolismoRESUMO
The presence and role of functional inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) in adult skeletal muscle are controversial. The current consensus is that, in adult striated muscle, the relative amount of IP(3)Rs is too low and the kinetics of Ca(2+) release from IP(3)R is too slow compared with ryanodine receptors to contribute to the Ca(2+) transient during excitation-contraction coupling. However, it has been suggested that IP(3)-dependent Ca(2+) release may be involved in signaling cascades leading to regulation of muscle gene expression. We have reinvestigated IP(3)-dependent Ca(2+) release in isolated flexor digitorum brevis (FDB) muscle fibers from adult mice. Although Ca(2+) transients were readily induced in cultured C2C12 muscle cells by (a) UTP stimulation, (b) direct injection of IP(3), or (c) photolysis of membrane-permeant caged IP(3), no statistically significant change in calcium signal was detected in adult FDB fibers. We conclude that the IP(3)-IP(3)R system does not appear to affect global calcium levels in adult mouse skeletal muscle.