Forty Years of Sodium Channels: Structure, Function, Pharmacology, and Epilepsy

Voltage-gated sodium channels initiate action potentials in brain neurons. In the 1970s, much was known about the function of sodium channels from measurements of ionic currents using the voltage clamp method, but there was no information about the sodium channel molecules themselves. As a postdoctoral fellow and staff scientist at the National Institutes of Health, I developed neurotoxins as molecular probes of sodium channels in cultured neuroblastoma cells. During those years, Bruce Ransom and I crossed paths as members of the laboratories of Marshall Nirenberg and Philip Nelson and shared insights about sodium channels in neuroblastoma cells from my work and electrical excitability and synaptic transmission in cultured spinal cord neurons from Bruce's pioneering electrophysiological studies. When I established my laboratory at the University of Washington in 1977, my colleagues and I used those neurotoxins to identify the protein subunits of sodium channels, purify them, and reconstitute their ion conductance activity in pure form. Subsequent studies identified the molecular basis for the main functions of sodium channels-voltage-dependent activation, rapid and selective ion conductance, and fast inactivation. Bruce Ransom and I re-connected in the 1990s, as ski buddies at the Winter Conference on Brain Research and as faculty colleagues at the University of Washington when Bruce became our founding Chair of Neurology and provided visionary leadership of that department. In the past decade my work on sodium channels has evolved into structural biology. Molecular modeling and X-ray crystallographic studies have given new views of sodium channel function at atomic resolution. Sodium channels are also the molecular targets for genetic diseases, including Dravet Syndrome, an intractable pediatric epilepsy disorder with major co-morbidities of cognitive deficit, autistic-like behaviors, and premature death that is caused by loss-of-function mutations in the brain sodium channel Na_V1.1. Our work on a mouse genetic model of this disease has shown that its multi-faceted pathophysiology and co-morbidities derive from selective loss of electrical excitability and action potential firing in GABAergic inhibitory neurons, which disinhibits neural circuits throughout the brain and leads directly to the epilepsy, premature death and complex co-morbidities of this disease. It has been rewarding for me to use our developing knowledge of sodium channels to help understand the pathophysiology and to suggest potential therapeutic approaches for this devastating childhood disease.