Regulation of Eag by Ca2+/calmodulin controls presynaptic excitability in Drosophila

Abstract

Drosophila ether-à-go-go ( eag) is the founding member of a large family of voltage-gated K⁺ channels, the KCNH family, which includes Kv10, 11, and 12. Concurrent binding of calcium/calmodulin (Ca²⁺/CaM) to NH₂- and COOH-terminal sites inhibits mammalian EAG1 channels at submicromolar Ca²⁺ concentrations, likely by causing pore constriction. Although the Drosophila EAG channel was believed to be Ca²⁺-insensitive (Schönherr R, Löber K, Heinemann SH. EMBO J 19: 3263-3271, 2000.), both the NH2- and COOH-terminal sites are conserved. In this study we show that Drosophila EAG is inhibited by high Ca²⁺ concentrations that are only present at plasma membrane Ca²⁺ channel microdomains. To test the role of this regulation in vivo, we engineered mutations that block CaM-binding to the major COOH-terminal site of the endogenous eag locus, disrupting Ca²⁺-dependent inhibition. eag CaMBD mutants have reduced evoked release from larval motor neuron presynaptic terminals and show decreased Ca²⁺ influx in stimulated adult projection neuron presynaptic terminals, consistent with an increase in K⁺ conductance. These results are predicted by a conductance-based multicompartment model of the presynaptic terminal in which some fraction of EAG is localized to the Ca²⁺ channel microdomains that control neurotransmitter release. The reduction of release in the larval neuromuscular junction drives a compensatory increase in motor neuron somatic excitability. This misregulation of synaptic and somatic excitability has consequences for systems-level processes and leads to defects in associative memory formation in adults. NEW & NOTEWORTHY Regulation of excitability is critical to tuning the nervous system for complex behaviors. We demonstrate in this article that the EAG family of voltage-gated K⁺ channels exhibit conserved gating by Ca²⁺/CaM. Disruption of this inhibition in Drosophila results in decreased evoked neurotransmitter release due to truncated Ca²⁺ influx in presynaptic terminals. In adults, disrupted Ca²⁺ dynamics cripples memory formation. These data demonstrate that the biophysical details of channels have important implications for cell function and behavior.

Keywords: KCNH channel; Kv10 channel; calcium/CaM; genetics; potassium channel.