Effect of Chronic Muscle Denervation on Mitochondrial Biogenesis and Apoptosis

Abstract

1825 Chronic muscle disuse, such as that produced by denervation, reduces mitochondrial content and produces muscle atrophy. These changes in muscle phenotype could be provoked by alterations in important regulators of mitochondrial biogenesis, such as the nuclear coactivator PGC-1α, and by increases in muscle fiber cell death via apoptosis. However, the role of PGC-1α following denervation has not been defined, and limited research has assessed apoptotic mechanisms as contributing factors to denervation-induced muscle atrophy. PURPOSE: To assess time-dependent alterations in key proteins involved in mitochondrial biogenesis (PGC-1α, cytochrome c, Tfam, cytochrome oxidase (COX) activity) and apoptosis (HSP70, Bcl-2, BAX, AIF) following chronic muscle denervation. METHODS: Male Sprague-Dawley rats (n = 20) underwent unilateral denervation (DEN) of the sciatic nerve for 5, 21, or 42 days (n = 3–4 animals/day). Denervated and contralateral (control) red gastrocnemius muscles were used for biochemical analyses. RESULTS: Denervation decreased PGC-1α expression by 45% following 5 days of DEN, which was further decreased by 51% and 65% of control at 21 and 42 days, respectively. COX activity declined with time and matched PGC-1α expression, with decreases of 35%, 55%, and 70% at 5, 21, and 42 days of DEN. These changes occurred coincident with 45% decreases in both Tfam and cytochrome c expression at 42 days of DEN. In contrast, proapoptotic BAX expression was elevated by 88% and 115% after 21 and 42 days of DEN, while coincident decreases of 79% and 89% occurred for the expression of the antiapoptotic protein Bcl-2. These changes dramatically increased the BAX:Bcl-2 ratio in denervated muscle. An increase in this ratio is indicative of a greater susceptibility to apoptosis. Additionally, 42 days of DEN resulted in increased (2.0-fold) expression of pro-apoptotic AIF, as well as increased levels (2.9-fold) of anti-apoptotic HSP70. CONCLUSIONS: These results indicate PGC-1α could be an important mediator of the decrements in mitochondrial content following periods of chronic muscle disuse. In addition, the profound shift in BAX:Bcl-2 ratio suggests that denervated muscle has a greater susceptibility to apoptosis and that this likely contributes to denervationinduced atrophy of skeletal muscle. Support: NSERC, Canada.