Intracellular regions of the Eag potassium channel play a critical role in generation of voltage-dependent currents

Regulation of Eag by Ca2+/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 (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.

Cytoplasmic domains and voltage-dependent potassium channel gating

The basic architecture of the voltage-dependent K⁺ channels (Kv channels) corresponds to a transmembrane protein core in which the permeation pore, the voltage-sensing components and the gating machinery (cytoplasmic facing gate and sensor–gate coupler) reside. Usually, large protein tails are attached to this core, hanging toward the inside of the cell. These cytoplasmic regions are essential for normal channel function and, due to their accessibility to the cytoplasmic environment, constitute obvious targets for cell-physiological control of channel behavior. Here we review the present knowledge about the molecular organization of these intracellular channel regions and their role in both setting and controlling Kv voltage-dependent gating properties. This includes the influence that they exert on Kv rapid/N-type inactivation and on activation/deactivation gating of Shaker-like and eag-type Kv channels. Some illustrative examples about the relevance of these cytoplasmic domains determining the possibilities for modulation of Kv channel gating by cellular components are also considered.

Metabotropic glutamate receptor 5 regulates excitability and Kv4.2-containing K⁺ channels primarily in excitatory neurons of the spinal dorsal horn

Metabotropic glutamate (mGlu) receptors play important roles in the modulation of nociception. Previous studies demonstrated that mGlu5 modulates nociceptive plasticity via activation of ERK signaling. We have reported recently that the Kv4.2 K(+) channel subunit underlies A-type currents in spinal cord dorsal horn neurons and that this channel is modulated by mGlu5-ERK signaling. In the present study, we tested the hypothesis that modulation of Kv4.2 by mGlu5 occurs in excitatory spinal dorsal horn neurons. With the use of a transgenic mouse strain expressing enhanced green fluorescent protein (GFP) under control of the promoter for the γ-amino butyric acid (GABA)-synthesizing enzyme, glutamic acid decarboxylase 67 (GAD67), we found that these GABAergic neurons express less Kv4.2-mediated A-type current than non-GAD67-GFP neurons. Furthermore, the mGlu1/5 agonist, (R,S)-3,5-dihydroxyphenylglycine, had no modulatory effects on A-type currents or neuronal excitability in this subgroup of GABAergic neurons but robustly modulated A-type currents and neuronal excitability in non-GFP-expressing neurons. Immunofluorescence studies revealed that Kv4.2 was highly colocalized with markers of excitatory neurons, such as vesicular glutamate transporter 1/2, PKCγ, and neurokinin 1, in cultured dorsal horn neurons. These results indicate that mGlu5-Kv4.2 signaling is associated with excitatory dorsal horn neurons and suggest that the pronociceptive effects of mGlu5 activation in the spinal cord likely involve enhanced excitability of excitatory neurons.