C-terminal modulation of hyperpolarization-activated HCN channels bycAMP and protein-protein interactions

Modulation of the I_h current produced by HCN pacemaker channels controls rhythmic activity in heart and brain. Ion channel regulation occurs through intrinsic mechanisms, which require no other protein molecules besides the channel, and extrinsic mechanisms, which involve interactions between the channel and other proteins. We have found that the same region of the HCN channel—the cytoplasmic C-terminus—can mediate both types of regulation.; The HCN channels are members of the voltage-gated potassium channel superfamily. All four HCN isoforms have a cyclic nucleotide-binding domain in the channel C-terminus, allowing CAMP to directly facilitate channel activation; however, both the basal voltage-dependence of activation and the extent of CAMP modulation vary among channel isoforms. To investigate these differences, we constructed C-terminal truncation mutants from two channel isoforms. Deletion of the binding domain facilitated opening of both isoforms. Moreover, the degree of facilitation matched that of cAMP modulation for each full length isoform. We developed a model in which the C-terminus inhibits channel gating: differences among HCN isoforms in both basal voltage-dependence and response to cAMP result largely from different degrees of C-terminal inhibition.; Depending on neuron type, HCN1 can be found tightly localized to distal dendrites or axon terminals. To explore potential mechanisms underlying this extrinsic channel regulation, we performed a yeast two-hybrid screen using the HCN1 C-terminus as a bait. HIP1, a brain specific cytoplasmic protein, bound all four HCN channel isoforms, showed an overlapping expression pattern with HCN1, and interacted with HCN1 in vivo. HIP1 caused the internalization of HCN channel protein, profoundly decreasing the magnitude of both exogenous and native HCN currents.; These two mechanisms of HCN channel modulation provide insight into the complex channel physiology. One mechanism provides for dynamic adjustments of channel gating; the other involves a slower regulation of channel trafficking. Together, they offer precise temporal and spatial control over channel function, allowing the channels to serve specialized roles that range from modification of synaptic integration to regulation of neurotransmitter release.