SOD1 A4V mutation increases Nav1.3 channel excitability on xenopus laevis oocyte

Amyotrophic lateral sclerosis (ALS) is a lethal, paralytic disease caused by degeneration of motor neurons in the spinal cord, brain stem and motor cortex. Mutations in the gene encoding copper/zinc superoxide dismutase (SOD1) are present in 20 % of familial ALS and 2 % of all ALS cases. The most common SOD1 gene mutation in North America is a missense mutation substituting valine for alanine (A4V). In this study, sodium channel currents in oocytes expressing either wild type or mutant (A4V) SOD1 protein were analyzed. In this study elicited on Xenopus Laevis oocyte, it is demonstrated that the A4V mutation confers a propensity to hyperexcitability on a voltage dependent sodium channel (Nav) mediated by heightened total Na+ conductance and a hyperpolarizing shift in the voltage dependence of Nav 1.3 activation. To estimate the impact of these channel effects on excitability in an intact neuron, these changes were simulated in the program NEURON; this shows that the changes induced by mutant SOD1 increase the spontaneous firing frequency of the simulated neuron. These findings are consistent with the view that excessive excitability of neurons is one component in the pathogenesis of this disease.