MnSod deficiency causes significant mitochondrial abnormalities and contractile dysfunction in skeletal muscle, but does not decrease muscle mass

Loss of innervation and consequent disruption of the neuromuscular junction (NMJ) in skeletal muscle results in mitochondrial dysfunction and increased ROS that are associated with muscle atrophy and weakness. To directly test the role of mitochondrial dysfunction and ROS in this series of events independent of loss of innervation, we generated a muscle specific knockout mice in which MnSOD, the primary superoxide anion scavenger in the mitochondrial matrix, was deleted using a Cre-Lox approach with Cre driven by a constitutively active human skeletal actin promoter (HSA-Cre). We measured mitochondrial ROS production and respiration using permeabilized fibers from red gastrocnemius muscle in wild type and MnSod deficient (m Sod2 KO) mice at 6-10 months of age. In m Sod2 KO mice, ROS generation was elevated by ~5 fold, and significant defects in mitochondrial respiration were observed-most pronounced in complex II-mediated respiration. Mitochondrial calcium retention capacity was also decreased by 70% in m Sod2 KO mice. To determine the effect of reduced MnSOD and increased superoxide on muscle function, we measured in situ contractile function in gastrocnemius muscle. Maximum isometric specific force was decreased by ~30% in mutant mice with direct stimulation on muscle, but the reduction in force was greater (~50%) after stimulation of the sciatic nerve. Surprisingly, gastrocnemius mass was increased in m Sod2 KO mice potentially due to compensatory mechanism by fiber branching. Our findings suggest that increased mitochondrial ROS generation and subsequent oxidative stress play an important role in regulating contractile function of skeletal muscle but did not cause muscle atrophy. This result supports the concept that mitochondrial ROS and dysfunction are not critical mediators of denervation-induced atrophy.