Genetic modifications of voltage-sensitive sodium channels in Drosophila: Gene dosage studies of the seizure locus

Abstract

We have identified a genetic locus in Drosophila melanogaster whose product appears to have a structural role in the formation of functional voltage-sensitive sodium channels. This locus, designated seizure, is defined by two temperature-sensitive alleles (sei(ts-1) and sei(ts-2)), each of which causes convulsive seizures followed by a rapid but reversible paralysis of adults at restrictive temperatures above 38°C. Previous work had shown that sei(ts-2) extracts display an altered pH dependence and an abnormally high K(d) for [3H]saxitoxin binding at high temperatures, suggesting that sodium channels in sei(ts-2) mutants have an altered structure (Jackson F.R., S.D. Wilson, G.R. Strichartz, and L.M. Hall (1984) Nature 308: 189-191). These binding studies have now been extended to extracts of sei(ts-1) which have a K(d) not significantly different from wild-type at all assay temperatures. However, sei(ts-1) extracts show a reduced number of saxitoxin binding sites (B(max)) relative to wild-type. This reduction is only 5 to 18% at 0°C but is 17 to 37% at 39° C, suggesting that under certain conditions sodium channels in the sei(ts-1) mutant are more labile than those of wild-type. Cytogenetic studies demonstrate that the seizure locus maps within region 60A to 60B8-10 on the second chromosome. Gene dosage analysis of ~99.7% of the genome, including this second chromosome region, failed to detect a wild-type locus whose dose affected saxitoxin-binding activity. Nevertheless, the mutant sei(ts-2) allele has codominant and dose-dependent effects on paralytic behavior and saxitoxin-binding activity. In addition, genetic interactions between sei(ts-2) and nap(ts) (a nonallelic mutation that reduces number of sodium channels) suggest that sei(ts) mutants have enhanced neuronal membrane excitability at the restrictive temperature. These genetic studies, in concert with the biochemical evidence for a structural alteration of sodium channels in sei(ts) extracts, suggest that the seizure locus encodes a component of the voltage-sensitive sodium channel.