1.A.1.5.32
HCN1 is a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel of 890 aas and 6 TMSs that opens due to inward movement of the positive charges in the fourth transmembrane domain (S4).  These channels open after only two S4s have moved, and S4 motion is rate limiting during voltage activation of spHCN channels (Bruening-Wright et al. 2007). HCN1 exhibits weak selectivity for potassium over sodium ions.  It's structure (3.5 Å resolution) is known (Lee and MacKinnon 2017).  It contributes to the native pacemaker currents in heart and neurons. It may also mediate responses to sour stimuli. It is inhibited by Cs⁺, zatebradine, capsazepine and ZD7288 (Gill et al. 2004). HCN1 mutational variants include epileptic encephalopathy and common generalized epilepsy. HCN1 has a pivotal function in brain development and control of neuronal excitability (Marini et al. 2018). The interaction with filamin A seems to contribute to localizing HCN1 channels to specific neuronal areas and to modulating channel activity (Gravante et al. 2004). The HCN domain is required for HCN channel cell-surface expression, and it couples voltage- and cAMP-dependent gating mechanisms (Wang et al. 2020). Changes in the local S4 environment provide a voltage-sensing mechanism for mammalian hyperpolarization-activated HCN channels (Bell et al. 2004). Cation leak is an important pathogenic mechanism in HCN1-mediated developmental and epileptic encephalopathy (DEE), and seizures are exacerbated by sodium channel blockers in patients with HCN1 variants that cause cation leak (McKenzie et al. 2023). HCN1 epilepsy is progressing from genetics and mechanisms to precision therapies (Bleakley and Reid 2023).  Opioid-Induced Hyperalgesia and Tolerance Are Driven by HCN Ion Channels (Han et al. 2024).