From the Cover: Structural basis for the mutual antagonism of cAMP and TRIP8b in regulating HCN channel function

cAMP signaling in the brain mediates several higher order neural processes. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels directly bind cAMP through their cytoplasmic cyclic nucleotide binding domain (CNBD), thus playing a unique role in brain function. Neuronal HCN channels are also regulated by tetratricopeptide repeat-containing Rab8b interacting protein (TRIP8b), an auxiliary subunit that antagonizes the effects of cAMP by interacting with the channel CNBD. To unravel the molecular mechanisms underlying the dual regulation of HCN channel activity by cAMP/TRIP8b, we determined the NMR solution structure of the HCN2 channel CNBD in the cAMP-free form and mapped on it the TRIP8b interaction site. We reconstruct here the full conformational changes induced by cAMP binding to the HCN channel CNBD. Our results show that TRIP8b does not compete with cAMP for the same binding region; rather, it exerts its inhibitory action through an allosteric mechanism, preventing the cAMP-induced conformational changes in the HCN channel CNBD.Hyperpolarization-activated cyclic nucleotide-gated (HCN1–4) channels are the molecular determinants of the h-current (I_h), which regulates critical neuronal properties, including membrane resting potential, dendritic excitability, and intrinsic rhythmicity (1). HCN channels are dually regulated by voltage and by binding of cAMP to the cyclic nucleotide binding domain (CNBD), which is found on the cytoplasmic C-terminal tail of the channel. The CNBD exerts a tonic inhibition on the channel pore, with the opening transition of the channel being allosterically coupled to the conformational changes in the CNBD induced by cAMP binding (2). Thus, the closed-to-open transition of the channel is thought to reflect the transition from the cAMP-free conformation to the cAMP-bound conformation of the CNBD, which stabilize, respectively, the closed and open states of the channel (2, 3). The C-linker, an α-helical folded domain that connects the CNBD to the pore region, conveys the regulation of channel gating from the CNBD to the pore (4–6). As a result of this allosteric mechanism, the binding of cAMP shifts the voltage dependence of the HCN channel opening to more positive potentials and increases maximal I_h at extreme negative voltages, where voltage gating is complete.In addition to cAMP, HCN channels in the brain are regulated by auxiliary proteins, such as TRIP8b, a cytosolic β-subunit of neuronal HCN channels, which inhibits channel activation by antagonizing the effects of cAMP (7–9). We have previously shown that TRIP8b_core, an 80-aa sequence located in the TRIP8b protein core that directly interacts with the C-linker/CNBD region of HCN channels, is necessary and sufficient to prevent all of the effects of cAMP on the channel (10, 11). TRIP8b_core decreases both the sensitivity of the channel to cAMP [half maximal concentration (k_1/2)] and the efficacy of cAMP in inducing channel opening [half activation voltage (V_1/2)]; conversely, cAMP binding inhibits these actions of TRIP8b. These mutually antagonistic effects are well described by a cyclic allosteric model in which TRIP8b binding reduces the affinity of the channel for cAMP, with the affinity of the open state for cAMP being reduced to a greater extent than the cAMP affinity of the closed state (11).A second important action of TRIP8b is to reduce maximal current through HCN channels in the absence of cAMP (11). As a consequence, application of cAMP produces a larger increase in maximal I_h in the presence of TRIP8b than in its absence. The observation that TRIP8b exerts opposing influences on the two major actions of cAMP on HCN channel function, namely, reduces the effect of cAMP to shift the voltage dependence of channel gating but enhances the effect of cAMP to increase maximal current, has important implications for the ability of cAMP to modulate neuronal excitability in vivo. Thus, the relative extent by which neuromodulatory transmitters alter maximal I_h or shift the voltage dependence of HCN channel gating can vary widely among distinct classes of neurons (12–14). The differential expression of TRIP8b may provide a mechanistic explanation for this finding, because in neurons with high levels of TRIP8b expression, cAMP will exert a larger action to enhance maximal current, and a smaller action to alter the voltage dependence of channel gating, compared with neurons in which TRIP8b expression is low. Such fine-tuning broadens the range of physiological actions that cAMP can exert to modulate neuronal firing.In the present study, we address the structural basis for the mutually antagonistic effects of cAMP and TRIP8b on HCN channel function. Although our previous biochemical and electrophysiological data strongly support the hypothesis that TRIP8b and cAMP binding sites do not overlap, direct structural information on the TRIP8b–CNBD complex is required to validate the allosteric antagonism model of interaction between the two ligands. A plausible hypothesis for the antagonistic effect of TRIP8b and cAMP is that each of the two ligands stabilizes the CNBD in a conformation that decreases the affinity for the other. To test this hypothesis, we first generated the 3D structure of the cAMP-free HCN2 channel CNBD using solution NMR spectroscopy and then characterized its interaction with the TRIP8b_core fragment. By comparing the cAMP-free with the available cAMP-bound HCN2 channel CNBD structure (15, 16), we reconstruct the full conformational changes induced by cAMP binding, revealing critical transitions occurring in the P- and C-helices of the CNBD, and further highlighting the role of the N-terminal helical bundle in transducing the movements of the CNBD to the channel pore. We next identify, through NMR titration, site-directed mutagenesis, and biochemical interaction assays, the binding site of TRIP8b_core on the cAMP-free form of the HCN2 channel CNBD. Our results demonstrate that cAMP and TRIP8b do not directly compete for the same binding region and support a model of mutual allosteric inhibition between cAMP and TRIP8b. Finally, our results clarify the mechanism by which TRIP8b antagonizes the effect of cAMP on channel gating: TRIP8b directly interacts with two mobile elements that drive the ligand-induced conformational changes in the CNBD. TRIP8b binding to the CNBD therefore prevents the cAMP-induced transition and stabilizes the channel in the cAMP-free conformation.