Regulation of Eag by Ca/calmodulin Controls Presynaptic Excitability in Drosophila

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 (Ca/CaM) to NH- and COOH-terminal sites inhibits mammalian EAG1 channels at submicromolar Ca concentrations, likely by causing pore constriction. Although the Drosophila EAG channel was believed to be Ca-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 Ca concentrations that are only present at plasma membrane Ca 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 Ca-dependent inhibition. eag CaMBD mutants have reduced evoked release from larval motor neuron presynaptic terminals and show decreased Ca 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 Ca 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 Ca/CaM. Disruption of this inhibition in Drosophila results in decreased evoked neurotransmitter release due to truncated Ca influx in presynaptic terminals. In adults, disrupted Ca dynamics cripples memory formation. These data demonstrate that the biophysical details of channels have important implications for cell function and behavior.