Muscle specific MnSOD deficiency leads to complex II‐specific inactivity of ETC and contractile dysfunction, but increases muscle mass

Abstract

Multiple pathological conditions exhibit excess mitochondrial ROS and muscle weakness in parallel, including aging, denervation and cancer cachexia. To directly test the role of mitochondrial ROS in skeletal muscle weakness and atrophy we generated a skeletal muscle specific knockout mouse model in which MnSOD (Sod2), the primary superoxide anion scavenger in the mitochondrial matrix, was deleted using a Cre‐Lox approach driven by a constitutively active human skeletal actin promoter (HSA‐Cre). Female wild type and MnSOD deficient (mSod2 KO) mice were studied at 6–8 months of age. In mSod2 KO mice, the observed rate of superoxide generation from isolated skeletal muscle mitochondria was significantly elevated, and hydrogen peroxide generation using permeabilized fibers increased by ~5 fold. ATP generation rate and electron transport chain (ETC) activity were severely reduced in response to Complex II linked substrate, suggesting superoxide‐induced complex II inactivity. Reduced ATP generation rate and ETC activity were consistent with severe exercise intolerance in mSod2 KO mice. To determine the effects of mitochondrial ROS on skeletal muscle weakness, we first tested in situ contractile function using gastrocnemius. Maximum isometric specific force was decreased by ~30 % in mSod2 KO mice with direct stimulation on muscle, but the reduction in force was further diminished (~50%) after stimulation of the sciatic nerve suggesting neuromuscular disruption. Morphological assessment exhibited increases in NMJ size and fragmentation in mSod2 KO mice supporting that mitochondrial dysfunction can feed back to initiate NMJ disruption. Surprisingly, gastrocnemius muscle mass was increased in mSod2 KO mice, which was associated with increased numbers of central nuclei and hyperplasia via fiber branching. In summary, our data suggest that excess mitochondrial superoxide caused a selective loss of complex II activity and mitochondrial defects, leading to severe exercise intolerance. Increased cytoplasmic hydrogen peroxide induced necrosis‐mediated regeneration, which increased muscle mass by fiber branching.Support or Funding InformationNIH/NIA to Dr. Van Remmen (R01 AG050676) and a VA Merit grant to HVRThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.