Loss of HCN channel mediated Ih current following seizures accounts for movement dysfunction

The Contribution of HCN Channelopathies in Different Epileptic Syndromes, Mechanisms, Modulators, and Potential Treatment Targets: A Systematic Review

Background

Hyperpolarization-activated cyclic nucleotide-gated (HCN) current reduces dendritic summation, suppresses dendritic calcium spikes, and enables inhibitory GABA-mediated postsynaptic potentials, thereby suppressing epilepsy. However, it is unclear whether increased HCN current can produce epilepsy. We hypothesized that gain-of-function (GOF) and loss-of-function (LOF) variants of HCN channel genes may cause epilepsy.

Objectives

This systematic review aims to summarize the role of HCN channelopathies in epilepsy, update genetic findings in patients, create genotype–phenotype correlations, and discuss animal models, GOF and LOF mechanisms, and potential treatment targets.

Methods

The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement, for all years until August 2021.

Results

We identified pathogenic variants of HCN1 (n = 24), HCN2 (n = 8), HCN3 (n = 2), and HCN4 (n = 6) that were associated with epilepsy in 74 cases (43 HCN1, 20 HCN2, 2 HCN3, and 9 HCN4). Epilepsy was associated with GOF and LOF variants, and the mechanisms were indeterminate. Less than half of the cases became seizure-free and some developed drug-resistant epilepsy. Of the 74 cases, 12 (16.2%) died, comprising HCN1 (n = 4), HCN2 (n = 2), HCN3 (n = 2), and HCN4 (n = 4). Of the deceased cases, 10 (83%) had a sudden unexpected death in epilepsy (SUDEP) and 2 (16.7%) due to cardiopulmonary failure. SUDEP affected more adults (n = 10) than children (n = 2). HCN1 variants p.M234R, p.C329S, p.V414M, p.M153I, and p.M305L, as well as HCN2 variants p.S632W and delPPP (p.719–721), were associated with different phenotypes. HCN1 p.L157V and HCN4 p.R550C were associated with genetic generalized epilepsy. There are several HCN animal models, pharmacological targets, and modulators, but precise drugs have not been developed. Currently, there are no HCN channel openers.

Conclusion

We recommend clinicians to include HCN genes in epilepsy gene panels. Researchers should explore the possible underlying mechanisms for GOF and LOF variants by identifying the specific neuronal subtypes and neuroanatomical locations of each identified pathogenic variant. Researchers should identify specific HCN channel openers and blockers with high binding affinity. Such information will give clarity to the involvement of HCN channelopathies in epilepsy and provide the opportunity to develop targeted treatments.

Nerve excitability differences in slow and fast motor axons of the rat: more than just Ih

The objective was to determine biophysical differences between fast and slow motor axons using threshold tracking and demonstrate confounds related to anesthetic. Nerve excitability of motor axons innervating the slow-twitch soleus (SOL) and fast-twitch tibialis anterior (TA) muscles was tested. The experiments were conducted with pentobarbital sodium (SP) anesthetic and compared with previous results that used ketamine-xylazine (KX). Nerve excitability indices measured with SP show definitive differences between TA and SOL motor axons that extend beyond previous reports. Nerve excitability indices sensitive to changes in I_h indicated an increase in SOL axons compared with TA axons [e.g., S3 t = 7.949 (df = 10), P < 0.001; hyperpolarizing threshold electrotonus (90-100 ms), t = 2.659 (df = 20); P = 0.01; hyperpolarizing I/V slope, t = 4.308 (df = 19); P < 0.001]. SOL axons also had a longer strength-duration time constant [t = 3.35 (df = 20); P = 0.003] and a longer and larger magnitude relative refractory period [RRP (ms) t = 3.53 (df = 12); P = 0.004; Refractoriness at 2 ms, t = 0.0055 (df = 9); P = 0.006]. Anesthetic choice affected many measures of peripheral nerve excitability with differences most apparent in tests of threshold electrotonus and recovery cycle. For example, recovery cycle with KX lacked a clear superexcitable and late subexcitable period. We conclude that KX had a confounding effect on nerve excitability results consistent with ischemic depolarization. Results using SP revealed the full extent of differences in nerve excitability measures between putative slow and fast motor axons of the rat. These results provide empirical evidence, beyond conduction velocity, that the biophysical properties of motor axons vary with the type of muscle fiber innervated. These differences suggest that fast axons may be predisposed to dysfunction during hyperpolarizing stresses, e.g., electrogenic sodium pumping following sustained impulse conduction.NEW & NOTEWORTHY Nerve excitability testing is a tool used to provide insight into the properties of ion channels in peripheral nerves. It is used clinically to assess pathophysiology of axons. Researchers customarily think of motor axons as homogeneous; however, we demonstrate there are clear differences between fast and slow axons in the rat. This is important for interpreting results with selective motor neuronopathy, like aging where fast axons are at high risk of degeneration.