| Reactive oxygen species (ROS) and oxidative stress have been implicated in the etiology of age related loss of muscle mass and function, also known as sarcopenia. However, direct casual evidence of ROS mediating muscle atrophy has not been thoroughly tested. To investigate the role of chronic oxidative stress in vivo, we used a mouse model that lacks a major antioxidant enzyme CuZnSOD (Sod1-/-). The Sod1-/- mice are characterized as having very high levels of oxidative stress and elevated levels of oxidative damage to lipid, protein, and DNA in number of tissues, including skeletal muscle. Moreover, Sod1-/- mice show a dramatic age related loss of skeletal muscle mass that is accelerated compared to wild-type mice.;Using this animal model, we first investigated the role of in vivo oxidative stress on mitochondrial function. EM analysis revealed that in aged Sod1-/- muscle, a significant increase in both interfibrillar and subsarcolemmal mitochondria number was observed. However, despite having more mitochondria, Sod1-/- muscle show a significant decline in mitochondrial function with higher production of ROS. These changes became more apparent as animals aged. To further investigate the mechanism of mitochondrial dysfunction in response to oxidative stress, we examined the mitochondrial apoptotic pathway. Sod1-/- mitochondria show more rapid induction of mitochondrial PT with higher release of the pro-apoptotic proteins, cytochrome c and AIF. These data were supported by the observation of the upregulation of caspase-3 activity and increased levels of apoptotic nuclei measured in a cell free system.;Continuing our study of oxidative stress induced muscle atrophy, we next investigated whether neuromuscular innervations is altered in Sod1 -/- mice. Morphological analyses showed that postsynaptic endplates are severely disrupted in Sod1-/- muscle by 20 months age. Synaptic cleft are reduced and AChR are significantly fragmented. In an agreement with these observations, we found a significant decrease in AChR protein level with subsequent increase in denervated NMJs. At 20 months of age ∼ 80% of the NMJ were found to be denervated in Sod1-/-. Furthermore, subsarcolemmal mitochondria, which function to support neuromuscular transmission, also exhibit a dramatic decline in ATP generation with concomitant increase in ROS generation. Collectively, these data suggest that mitochondrial function and tight regulation of oxidative stress play a critical role in maintaining neuromuscular innervations.;The maintenance of skeletal muscle mass depends on the overall balance between the rates of protein synthesis and breakdown. Thus, age related muscle atrophy may result from the decreased protein synthesis, increased proteolysis, or simultaneous changes in both processes governed by complex multifactorial mechanisms. Growing evidence implicates oxidative stress and ROS as an important regulator of proteolysis. Thus we investigated whether Sod1 -/- mice have increased oxidative protein damage and accelerated proteolysis with age. In Sod1-/- muscle, a significant increase in protein carbonyl groups were observed suggesting increased oxidative protein damage. In addition, as muscle atrophy increases with age, an upregulation of cysteine protease, calpain and caspase-3 activities were detected. These proteases are known to play a role in initiating the breakdown of sarcomeres. Furthermore, an enhanced muscle atrophy was also coupled with the activation of a major proteolytic system, the ubiquitin proteasome pathway. A significant increase in ubiquitin conjugated products as well as elevated activities of 26S core peptidase activity were found in Sod1 -/- muscle.;In summary, our results clearly demonstrate that regulation of oxidant production and maintenance of mitochondria function are two critical components in preserving skeletal muscle mass and function with age. |
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