Using MRI to measure in vivo free radical production and perfusion dynamics in a mouse model of elevated oxidative stress and neurogenic atrophy.

Mitochondrial dysfunction, reactive oxygen species (ROS) and oxidative damage have been implicated to play a causative role in age-related skeletal muscle atrophy and weakness (i.e. sarcopenia). Mice lacking the superoxide scavenger CuZnSOD (Sod1^-/-) exhibit high levels of oxygen-derived radicals and oxidative damage, associated with neuronal and muscular phenotypes consistent with sarcopenia. We used magnetic resonance imaging (MRI) technology combined with immunospin-trapping (IST) to measure in vivo free radical levels in skeletal muscle from wildtype, Sod1^-/- and SynTgSod1^-/- mice, a mouse model generated using targeted expression of the human Sod1 transgene specifically in neuronal tissues to determine the impact of motor neuron degeneration in muscle atrophy. By combining the spin trap DMPO (5,5-dimethyl-1-pyrroline N-oxide) and molecular MRI (mMRI), we monitored the level of free radicals in mouse hindlimb muscle. The level of membrane-bound macromolecular radicals in the quadriceps muscle was elevated by ~3-fold in Sod1^-/- mice, but normalized to wildtype levels in SynTgSod1^-/- rescue mice. Skeletal muscle mass was reduced by ~25-30% in Sod1^-/- mice, but fully reversed in muscle from SynTgSod1^-/- mice. Using perfusion MRI we also measured the dynamics of blood flow within mouse hindlimb. Relative muscle blood flow in Sod1^-/- is decreased to ~50% of wildtype and remained low in the SynTgSod1^-/- mice. Our findings are significant in that we have shown for the first time that in vivo free radical production in skeletal muscle is directly correlated to muscle atrophy in an experimental model of oxidative stress. Neuron-specific expression of CuZnSOD reverses the in vivo free radical production in skeletal muscle in the Sod1^-/- mouse model and prevents muscle atrophy. These results further support the feasibility of using in vivo assessments of redox status in the progression of a pathological process such as sarcopenia. This approach can also be valuable for evaluating responses to pharmacologic interventions.