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Abstract: Transient voltage-gated sodium currents are essential for the initiation and conduction of action potentials in neurons and cardiomyocytes. The amplitude and duration of sodium currents are tuned by intracellular fibroblast growth factor homologous factors (FHFs/iFGFs) that associate with the cytoplasmic tails of voltage-gated sodium channels (Na v s), and genetic ablation of Fhf genes disturbs neurological and cardiac functions. Among reported phenotypes, Fhf2null mice undergo lethal hyperthermia-induced cardiac conduction block attributable to the combined effects of FHF2 deficiency and elevated temperature on the cardiac sodium channel (Na v 1.5) inactivation rate. Fhf2null mice also display a lack of heat nociception, while retaining other somatosensory capabilities. Here, we use electrophysiological and computational methods to show that the heat nociception deficit can be explained by the combined effects of elevated temperature and FHF2 deficiency on the fast inactivation gating of Na v 1.7 and tetrodotoxin-resistant sodium channels expressed in dorsal root ganglion C fibers. Hence, neurological and cardiac heat-associated deficits in Fhf2null mice derive from shared impacts of FHF deficiency and temperature towards Na v inactivation gating kinetics in distinct tissues., (Copyright © 2022 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the International Association for the Study of Pain.)

Objective: Fibroblast growth factor homologous factors (FHFs) are brain and cardiac sodium channel-binding proteins that modulate channel density and inactivation gating. A recurrent de novo gain-of-function missense mutation in the FHF1(FGF12) gene (p.Arg52His) is associated with early infantile epileptic encephalopathy 47 (EIEE47; Online Mendelian Inheritance in Man database 617166). To determine whether the FHF1 missense mutation is sufficient to cause EIEE and to establish an animal model for EIEE47, we sought to engineer this mutation into mice., Methods: The Arg52His mutation was introduced into fertilized eggs by CRISPR (clustered regularly interspaced short palindromic repeats) editing to generate Fhf1 R52H /F+ mice. Spontaneous epileptiform events in Fhf1 R52H /+ mice were assessed by cortical electroencephalography (EEG) and video monitoring. Basal heart rhythm and seizure-induced arrhythmia were recorded by electrocardiography. Modulation of cardiac sodium channel inactivation by FHF1B R52H protein was assayed by voltage-clamp recordings of FHF-deficient mouse cardiomyocytes infected with adenoviruses expressing wild-type FHF1B or FHF1B R52H protein., Results: All Fhf1 R52H /+ mice experienced seizure or seizurelike episodes with lethal ending between 12 and 26 days of age. EEG recordings in 19-20-day-old mice confirmed sudden unexpected death in epilepsy (SUDEP) as severe tonic seizures immediately preceding loss of brain activity and death. Within 2-53 s after lethal seizure onset, heart rate abruptly declined from 572 ± 16 bpm to 108 ± 15 bpm, suggesting a parasympathetic surge accompanying seizures that may have contributed to SUDEP. Although ectopic overexpression of FHF1B R52H in cardiomyocytes induced a 15-mV depolarizing shift in voltage of steady-state sodium channel inactivation and slowed the rate of channel inactivation, heart rhythm was normal in Fhf1 R52H /+ mice prior to seizure., Significance: The Fhf1 missense mutation p.Arg52His induces epileptic encephalopathy with full penetrance in mice. Both Fhf1 (p.Arg52His) and Scn8a (p.Asn1768Asp) missense mutations enhance sodium channel Na v 1.6 currents and induce SUDEP with bradycardia in mice, suggesting an FHF1/Na v 1.6 functional axis underlying altered brain sodium channel gating in epileptic encephalopathy., (© 2021 International League Against Epilepsy.)

Fibroblast growth factor homologous factors (FHFs) are intracellular proteins which regulate voltage-gated sodium (Na v ) channels in the brain and other tissues. FHF dysfunction has been linked to neurological disorders including epilepsy. Here, we describe two sibling pairs and three unrelated males who presented in infancy with intractable focal seizures and severe developmental delay. Whole-exome sequencing identified hemi- and heterozygous variants in the N-terminal domain of the A isoform of FHF2 (FHF2A). The X-linked FHF2 gene (also known as FGF13) has alternative first exons which produce multiple protein isoforms that differ in their N-terminal sequence. The variants were located at highly conserved residues in the FHF2A inactivation particle that competes with the intrinsic fast inactivation mechanism of Na v channels. Functional characterization of mutant FHF2A co-expressed with wild-type Na v 1.6 (SCN8A) revealed that mutant FHF2A proteins lost the ability to induce rapid-onset, long-term blockade of the channel while retaining pro-excitatory properties. These gain-of-function effects are likely to increase neuronal excitability consistent with the epileptic potential of FHF2 variants. Our findings demonstrate that FHF2 variants are a cause of infantile-onset developmental and epileptic encephalopathy and underline the critical role of the FHF2A isoform in regulating Na v channel function., (Copyright © 2020 American Society of Human Genetics. Published by Elsevier Inc. All rights reserved.)

Fever is a highly conserved systemic response to infection dating back over 600 million years. Although conferring a survival benefit, fever can negatively impact the function of excitable tissues, such as the heart, producing cardiac arrhythmias. Here we show that mice lacking fibroblast growth factor homologous factor 2 (FHF2) have normal cardiac rhythm at baseline, but increasing core body temperature by as little as 3 °C causes coved-type ST elevations and progressive conduction failure that is fully reversible upon return to normothermia. FHF2-deficient cardiomyocytes generate action potentials upon current injection at 25 °C but are unexcitable at 40 °C. The absence of FHF2 accelerates the rate of closed-state and open-state sodium channel inactivation, which synergizes with temperature-dependent enhancement of inactivation rate to severely suppress cardiac sodium currents at elevated temperatures. Our experimental and computational results identify an essential role for FHF2 in dictating myocardial excitability and conduction that safeguards against temperature-sensitive conduction failure.

Neurons in vertebrate central nervous systems initiate and conduct sodium action potentials in distinct subcellular compartments that differ architecturally and electrically. Here, we report several unanticipated passive and active properties of the cerebellar granule cell's unmyelinated axon. Whereas spike initiation at the axon initial segment relies on sodium channel (Na v )-associated fibroblast growth factor homologous factor (FHF) proteins to delay Na v inactivation, distal axonal Na v s show little FHF association or FHF requirement for high-frequency transmission, velocity and waveforms of conducting action potentials. In addition, leak conductance density along the distal axon is estimated as <1% that of somatodendritic membrane. The faster inactivation rate of FHF-free Na v s together with very low axonal leak conductance serves to minimize ionic fluxes and energetic demand during repetitive spike conduction and at rest. The absence of FHFs from Na v s at nodes of Ranvier in the central nervous system suggests a similar mechanism of current flux minimization along myelinated axons.