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HCN channel
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are intermembrane proteins that serve as nonselective ligand-gated cation channels in the plasma membranes of heart and brain cells.〔Luthi A, McCormick DA. 1998. Neuron. H-current: properties of a neuronal and network pacemaker. Vol. 21. pp 9-12.〕 HCN channels are sometimes referred to as “pacemaker channels” because they help to generate rhythmic activity within groups of heart and brain cells. HCN channels are encoded by four genes (HCN1, 2, 3, 4) and are widely expressed throughout the heart and the central nervous system.〔Kaupp UB, Seifert R. Molecular diversity of pacemaker ion channels (2001)Annu Rev Physiol. 63:235-57. Review.〕〔Notomi T, Shigemoto R (2004) Immunohistochemical localization of Ih channel subunits, HCN1-4, in the rat brain. J Comp Neurol 471: 241-276.
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The current through HCN channels, designated If or Ih, plays a key role in the control of cardiac and neuronal rhythmicity ("pacemaker current"). Expression of single isoforms in heterologous systems such as human embryonic kidney (HEK) cells, Chinese hamster ovary (CHO) cells, and ''Xenopus'' oocytes yields homotetrameric channels able to generate ion currents with properties similar to those of the native If/Ih current, but with quantitative differences in the voltage-dependence, activation/deactivation kinetics and sensitivity to the nucleotide cyclic AMP (cAMP): HCN1 channels show the more positive threshold for activation, the fastest activation kinetics, and the lowest sensitivity to cAMP, while HCN4 channels are slowly gating and strongly sensitive to cAMP. HCN2 and HCN3 have intermediate properties.〔Wahl-Schott C, Biel M. (2009) HCN channels: structure, cellular regulation and physiological function. Cell Mol Life Sci. 2009 Feb;66(3):470-94. Review〕〔Baruscotti, M., Bucchi, A., DiFrancesco, D. (2005). Physiology and pharmacology of the cardiac pacemaker ("funny") current. Pharmacology & Therapeutics, 107, 59-79〕〔Santoro B, Tibbs GR. (1999) The HCN gene family: molecular basis of the hyperpolarization-activated pacemaker channels. Ann N Y Acad Sci. Apr 30;868:741-64. Review〕
== Structure ==
Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels belong to the superfamily of voltage-gated K+ (Kv) and cyclic nucleotide-gated (CNG) channels. HCN channels are thought to consist of four either identical or non-identical subunits that are integrally embedded in the cell membrane to create an ion-conducting pore.〔(Swiss-Prot entry )〕 Each subunit comprises six membrane-spanning (S1–6) domains which include a putative voltage sensor (S4) and a pore region between S5 and S6 carrying the GYG triplet signature of K+-permeable channels, and a cyclic nucleotide-binding domain (CNBD) in the C-terminus. HCN isoforms are highly conserved in their core transmembrane regions and cyclic nucleotide binding domain (80–90% identical), but diverge in their amino- and carboxy-terminal cytoplasmic regions. The ribbon model to the right depicts HCN1.〔
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