Biophysics of leaky HCN ion channels

The hyperpolarisation-activated, cyclic nucleotide-gated (HCN) channel, opens and conducts positively charged ions when the transmembrane voltage is negative on the inside. Several variants in the HCN1 subtype channel have been reported in patients with severe epilepsy. Functional analyses of these variants revealed a converging functional impact of ‘leaky’ channels, which remain open and conduct ions at membrane voltages in which the channels are meant to be closed. This project will gain an understanding of how these variants cause a ‘leaky’ phenotype at the molecular level to result in severe epilepsy.

HCN channels play a role in regulating pacemaker current in cardiac muscle cells and CNS neurons. Pathogenic variants in HCN channels have been associated with various neurological conditions including neuropathic pain and epilepsy. There are four HCN isoforms in the mammals (HCN1-4), and each channel is a tetramer, comprising of four subunits. Each subunit has six transmembrane segments with distinct functions. Emerging studies are beginning to provide insight into how these transmembrane segments interact within and between subunits to facilitate channel opening, although the actual mechanism remains poorly understood.

Understanding how HCN channels open and close and elucidating HCN structure-function relationships is essential for multiple applications. These include knowledge transfer for targeted compound development and mechanistic understanding of how mutations affect biophysical properties of the channel to lead to disease phenotype.

Hung A, Forster IC, Mckenzie CE, Berecki G, Petrou S, Kathirvel A, Soh MS, Reid CA, (2021), ‘Biophysical analysis of an HCN1 epilepsy variant suggests a critical role for S5 helix Met-305 in voltage sensor to pore domain coupling’, Progress in Biophysics and Molecular Biology, 166:156-172, doi: 10.1016/j.pbiomolbio.2021.07.005. Epub 2021 Jul 21. Erratum in: Prog Biophys Mol Biol. 2022 Aug;172:90.

Bleakley LE, McKenzie CE, Soh MS, Forster IC, Pinares-Garcia P, Sedo A, Kathirvel A, Churilov L, Jancovski N, Maljevic S, Berkovic SF, Scheffer IE, Petrou S, Santoro B, Reid CA (2021), ‘Cation leak underlies neuronal excitability in an HCN1 developmental and epileptic encephalopathy’ Brain, 144(7):2060-2073,  doi: 10.1093/brain/awab145

Mckenzie CE, Ho CJ, Forster IC, Soh MS, Phillips AM, Chang YC, Scheffer IE, Reid CA, Tsai MH (2022), Impaired Color Recognition in HCN1 Epilepsy: A Single Case Report’, Frontiers in Neurology, 13:834252, doi: 10.3389/fneur.2022.834252