Mechanism of Mitochondria-induced Progressive Muscle Wasting

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Muscle wasting (or atrophy) is defined by reduced myofiber size, number and strength. It occurs in aging, muscle disuse, denervation, cancer, AIDS, diabetes and cardiac failure, which increases frailty, morbidity and mortality. Under physiological conditions, myofiber size is maintained by a balance between protein synthesis and degradation. How proteostasis is unbalanced under various atrophying conditions is poorly understood. Mitochondrial dysfunction has been proposed to contribute to progressive muscle atrophy. Most studies have been focused on the role of mitochondria in energy production, oxidative stress and apoptosis. However, recent studies showed that considerable levels of bioenergetic deficiency and oxidative stress are not sufficient to cause myofiber shrinkage and progressive muscle wasting. If mitochondrial stress causes muscle wasting, it would have to involve a novel mechanism. We recently generated a transgenic mouse line that moderately overexpresses Ant1, the muscle/heart isoform of adenine nucleotide translocase involved in ATP/ADP exchange across the inner membrane of mitochondria (IMM), to model one of the most common muscle diseases known as Facioscapulohumeral Muscular Dystrophy (FSHD). We found that the ANT1-transgenic mice have reduced myofiber size and progressively lose muscle mass. Our preliminary studies support the idea that, in addition to moderate bioenergetic defect, ANT1-overexpression causes proteostatic stress in the cytosol. We hypothesize that Ant1 overloading may disturb protein import and cause mitochondrial Precursor Overaccumulation Stress (mPOS), a novel mitochondria-induced stress characterized by the overaccumulation of unimported proteins in the cytosol. We also propose that proteostatic adaptation to mPOS may chronically reduce protein synthesis and increase protein degradation, which ultimately leads to muscle wasting. In this application, we will test these hypotheses by proposing the following specific aims. (1) We will test whether moderate ANT1 overexpression is sufficient to induce proteostatic stress in the cytosol. (2) We will identify cytosolic pathways that are important for the triage of unimported mitochondrial proteins. (3) We will test the hypothesis that Ant1-induced muscle atrophy results from reduced protein synthesis and/or increased proteasomal and autophagy activities, triggered as stress responses to mPOS. Success of the project may lead to the discovery of a novel mechanism by which mitochondria affect muscle mass homeostasis. The results could have direct implications for the understanding of several diseases that involve ANT1 overexpression, including FSHD, dilated cardiomyopathy and Rett syndrome. Finally, the validation of the mPOS model in mice could have broad implications for the understanding of other mitochondrial disorders that affect protein import.