Worm migraines: Characterization of a Gain-of-Function Mutation in the Voltage-Gated Calcium Channel, UNC-2/CaV2

Abstract

The fusion of synaptic vesicles with the cell membrane is initiated by the activation of presynaptic voltage-gated calcium channels (CaV2). C. elegans has a single predicted presynaptic CaV2a subunit, encoded by the unc-2 gene. We have identified an unc-2 gain-of-function (gf) mutant, which displays an increased reversal frequency, as well as hyperactive locomotion and egg-laying. In contrast, unc-2 loss-of-function mutants are uncoordinated and sluggish. Transgenic animals that express the unc-2(gf) in the backward locomotion interneurons exhibit an increased reversal frequency, while expression in the hermaphrodite specific neurons (HSN) produces hyperactive egg-laying. This suggests that the unc-2(gf) transgene can be used to chronically hyper-activate selective neurons within a neural circuit. unc-2(gf) mutants are hypersensitive to the acetylcholine esterase inhibitor, aldicarb and show an increased frequency of endogenous synaptic release events, indicating elevated levels of neurotransmitter release. Whole cell recordings from HEK cells expressing the human CaV2.1a with the corresponding unc-2(gf) mutation revealed activation at a lower membrane potential and a higher current density than the wild type. Our findings are similar to those reported for mutations in the human CaV2.1a, CACNA1A, that cause Familial Hemiplegic Migraine (FHM1). Interestingly, transgenic animals that express unc-2 carrying corresponding FHM1 mutations display a hyperactive phenotype similar to that of the unc-2(gf) mutants. Therefore, the unc-2(gf) mutants provide an invertebrate model to study mechanisms underlying FHM1. To understand mechanisms of CaV2 function in vivo, we performed a screen for mutants that suppress the hyperactive phenotype of unc-2(gf) mutants. Genes identified from this suppressor screen may ultimately provide novel targets for the treatments of calcium channelopathies.