{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nHyperpolarization‐activated cyclic nucleotide–gated (HCN) channels are essential determinants of rhythmic electrical activity in both the heart and the brain. They underlie the pacemaker current (Iₕ) by generating slowly activating inward cation currents upon hyperpolarization, and their gating is powerfully modulated by cyclic nucleotides (cAMP, cGMP). Through these actions, HCN channels set the resting membrane potential, regulate spontaneous firing rates, and fine‐tune synaptic integration. Such functions are critical not only for maintaining a steady heartbeat but also for controlling neuronal excitability, as evidenced by studies in cardiomyocytes and diverse neural populations."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "12"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nDetailed structural and biophysical investigations have elucidated the molecular mechanisms that orchestrate HCN channel gating and modulation. X‐ray crystallography together with mutagenesis and electrophysiological studies have highlighted the crucial roles of the cyclic nucleotide–binding domain (CNBD) and the adjoining C‐linker in mediating ligand specificity and allosteric changes. Likewise, interactions within the S4–S5 linker couple voltage‐sensor movements to opening of the channel’s activation gate. Moreover, modifications such as N-linked glycosylation and binding of membrane phospholipids (e.g. PI[4,5]P₂) further modulate channel activity, while a conserved amino-terminal domain in HCN2 has been shown to promote subunit co-assembly and proper trafficking—a function that can be conceptually linked to roles attributed to domains such as NHSL1 in organizing protein complexes."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "13", "end_ref": "25"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nAlterations in HCN channel function—as shown by genetic deletion, pharmacological blockade, or modulation by accessory proteins—are implicated in a variety of pathophysiological conditions including cardiac dysrhythmia, absence epilepsy, neuropathic pain, and mood disorders. In several models, disruption of HCN2 function perturbs normal rhythmic firing in brain regions (such as thalamocortical circuits and ventral tegmental area dopamine neurons) and compromises cardiac pacemaking, while interactions with cytoskeletal or scaffolding proteins (like filamin A or tyrosine phosphatases) further fine-tune their localization and kinetics. Collectively, these findings underscore the therapeutic potential of targeting HCN channels to correct aberrant excitability."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "26", "end_ref": "37"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Catherine Proenza, Damiano Angoli, Eugene Agranovich, et al. 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"}, {"type": "b", "children": [{"type": "t", "text": "A conserved domain in the NH2 terminus important for assembly and functional expression of pacemaker channels."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2002)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.M208477200"}], "href": "https://doi.org/10.1074/jbc.M208477200"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12193608"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12193608"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Andreas Ludwig, Thomas Budde, Juliane Stieber, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Absence epilepsy and sinus dysrhythmia in mice lacking the pacemaker channel HCN2."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "EMBO J (2003)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1093/emboj/cdg032"}], "href": "https://doi.org/10.1093/emboj/cdg032"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12514127"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12514127"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Ezana M Azene, Tian Xue, Ronald A Li "}, {"type": "b", "children": [{"type": "t", "text": "Molecular basis of the effect of potassium on heterologously expressed pacemaker (HCN) channels."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Physiol (2003)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1113/jphysiol.2003.039768"}], "href": "https://doi.org/10.1113/jphysiol.2003.039768"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12562911"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12562911"}]}, {"type": "r", "ref": 5, "children": [{"type": "t", "text": "William N Zagotta, Nelson B Olivier, Kevin D Black, et al. 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