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Molecular mechanism of cAMP modulation of HCN pacemaker channels

Abstract

Hyperpolarization-activated cation channels of the HCN gene family1,2,3,4,5,6 contribute to spontaneous rhythmic activity in both heart7 and brain5,6,8. All four family members contain both a core transmembrane segment domain, homologous to the S1–S6 regions of voltage-gated K+ channels, and a carboxy-terminal 120 amino-acid cyclic nucleotide-binding domain (CNBD) motif. Homologous CNBDs are responsible for the direct activation of cyclic nucleotide-gated channels and for modulation of the HERG voltage-gated K+ channel—important for visual and olfactory signalling9 and for cardiac repolarization10, respectively. The direct binding of cyclic AMP to the cytoplasmic site on HCN channels permits the channels to open more rapidly and completely after repolarization of the action potential1,2,11, thereby accelerating rhythmogenesis6,7,8. However, the mechanism by which cAMP binding modulates HCN channel gating and the basis for functional differences between HCN isoforms remain unknown. Here we demonstrate by constructing truncation mutants that the CNBD inhibits activation of the core transmembrane domain. cAMP binding relieves this inhibition. Differences in activation gating and extent of cAMP modulation between the HCN1 and HCN2 isoforms result largely from differences in the efficacy of CNBD inhibition.

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Figure 1: HCN1 and HCN2 channels differ in steady-state voltage dependence, kinetics of activation, and response to cAMP.
Figure 2: Effect of C-terminal truncation on steady-state voltage dependence in the presence and absence of cAMP.
Figure 4: Model for cAMP modulation of HCN channel gating.
Figure 3: Deletion of CNBD mimics cAMP-induced increase in activation rate.

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Acknowledgements

We thank J. Riley, H. Yao and E. Odell for their technical assistance. This work was partly supported by grants from the Whitehall Foundation (G.R.T.), NIH Medical Scientist Training Program (B.J.W.) and NINDS (S.A.S).

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Correspondence to Gareth R. Tibbs.

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Wainger, B., DeGennaro, M., Santoro, B. et al. Molecular mechanism of cAMP modulation of HCN pacemaker channels. Nature 411, 805–810 (2001). https://doi.org/10.1038/35081088

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