Progressive impairment of CaV1.1 function in the skeletal muscle of mice expressing a mutant type 1 Cu/Zn superoxide dismutase (G93A) linked to amyotrophic lateral sclerosis

Background: Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder that is typically fatal within 3-5 years of diagnosis. While motoneuron death is the defining characteristic of ALS, the events that underlie its pathology are not restricted to the nervous system. In this regard, ALS muscle atrophies and weakens significantly before presentation of neurological symptoms. Since the skeletal muscle L-type Ca2+ channel (Ca(V)1.1) is a key regulator of both mass and force, we investigated whether Ca(V)1.1 function is impaired in the muscle of two distinct mouse models carrying an ALS-linked mutation. Methods: We recorded L-type currents, charge movements, and myoplasmic Ca2+ transients from dissociated flexor digitorum brevis (FDB) fibers to assess Ca(V)1.1 function in two mouse models expressing a type 1 Cu/Zn superoxide dismutase mutant (SOD1(G93A)). Results: In FDB fibers obtained from "symptomatic" global SOD1(G93A) mice, we observed a substantial reduction of SR Ca2+ release in response to depolarization relative to fibers harvested from age-matched control mice. L-type current and charge movement were both reduced by similar to 40 % in symptomatic SOD1(G93A) fibers when compared to control fibers. Ca2+ transients were not significantly reduced in similar experiments performed with FDB fibers obtained from "early-symptomatic" SOD1(G93A) mice, but L-type current and charge movement were decreased (similar to 30 and similar to 20 %, respectively). Reductions in SR Ca2+ release (similar to 35 %), L-type current (similar to 20 %), and charge movement (similar to 15 %) were also observed in fibers obtained from another model where SOD1(G93A) expression was restricted to skeletal muscle. Conclusions: We report reductions in EC coupling, L-type current density, and charge movement in FDB fibers obtained from symptomatic global SOD1(G93A) mice. Experiments performed with FDB fibers obtained from early-symptomatic SOD1(G93A) and skeletal muscle autonomous MLC/SOD1(G93A) mice support the idea that events occurring locally in the skeletal muscle contribute to the impairment of Ca(V)1.1 function in ALS muscle independently of innervation status.