BK and CaV1.3 Channels Organize in Clusters that Control Excitability in Neurons

Abstract

BK channels are calcium-dependent potassium channels that couple to voltage-gated calcium channels. Because of their calcium dependence, activation curves of BK channels reflect activation curves of their coupling partners. CaV1.3 channels are low-voltage activated voltage-gated calcium channels. We hypothesized that coupling of BK and CaV1.3 channels allows BK channels to activate with small depolarizations. This hypothesis assumes that BK and CaV1.3 channels are organized in nanodomains, with < 20 nm separation that achieve high concentrations of calcium next to BK channels. We studied the electrophysiological properties of BK and CaV1.3 channels in an expression system and assessed the distance between them using single-molecule localization (GSD) microscopy. We found that BK channels indeed start activating at −40 mV when coexpressed with CaV1.3 channels. Imaging showed that only 3% of CaV1.3 channels are at < 20 nm from BK channels. We did not find evidence to support a 1:1 ratio of BK and CaV1.3 clusters in nanodomains. Instead we observed a pattern of 4-6 CaV1.3 channels surrounding BK channels within 80 nm, as if BK and CaV1.3 channels were organized in a multichannel complex. Similar distributions are observed in neurons. Our results suggest an alternative hypothesis, domains of multiple channels, to explain coupling of BK and CaV1.3 channels. For comparison, we coexpressed BK channels with high-voltage activated CaV2.2 channels. Now, BK channels activated at higher-voltages. Electrophysiological and GSD experiments revealed that, unlike CaV1.3 channels, CaV2.2 channels localize further away from BK channels and with no special spatial distribution. Supported by NIH grants R37NS008174 (BH), R01HL085686, and R01HL085870 (LFS).