Aging is accompanied by loss of muscle mass and force, known as
sarcopenia.
Muscle atrophy, weakness, and neuromuscular junction (NMJ) degeneration reminiscent of normal muscle aging are observed early in adulthood for mice deficient in
Cu, Zn-superoxide dismutase (SOD, Sod1-/-). Muscles of Sod1-/- mice also display impaired mitochondrial
ATP production and increased mitochondrial
reactive oxygen species (ROS) generation implicating oxidative stress in
sarcopenia. Restoration of CuZnSOD specifically in neurons of Sod1-/- mice (SynTgSod1-/-) prevents
muscle atrophy and loss of force, but whether muscle mitochondrial function is preserved is not known. To establish links among CuZnSOD expression, mitochondrial function, and
sarcopenia, we examined contractile properties, mitochondrial function and ROS production, intracellular
calcium transients (ICT), and NMJ morphology in lumbrical muscles of 7-9 month wild type (WT), Sod1-/-, and SynTgSod1-/- mice. Compared with WT values, mitochondrial ROS production was increased 2.9-fold under basal conditions and 2.2-fold with addition of
glutamate and
malate in Sod1-/- muscle fibers while oxygen consumption was not significantly altered. In addition,
NADH recovery was blunted following contraction and the peak of the ICT was decreased by 25%. Mitochondrial function, ROS generation and
calcium handling were restored to WT values in SynTgSod1-/- mice, despite continued lack of CuZnSOD in muscle. NMJ
denervation and fragmentation were also fully rescued in SynTgSod1-/- mice suggesting that muscle mitochondrial and
calcium handling defects in Sod1-/- mice are secondary to neuronal oxidative stress and its effects on the NMJ rather than the lack of muscle CuZnSOD. We conclude that intact neuronal function and innervation are key to maintaining excitation-contraction coupling and muscle mitochondrial function.