SCN5A variant that blocks fibroblast growth factor homologous factor regulation causes human arrhythmia

The Extent of Edgetic Perturbations in the Human Interactome Caused by Population-Specific Mutations

Until recently, efforts in population genetics have been focused primarily on people of European ancestry. To attenuate this bias, global population studies, such as the 1000 Genomes Project, have revealed differences in genetic variation across ethnic groups. How many of these differences can be attributed to population-specific traits? To answer this question, the mutation data must be linked with functional outcomes. A new "edgotype" concept has been proposed, which emphasizes the interaction-specific, "edgetic", perturbations caused by mutations in the interacting proteins. In this work, we performed systematic in silico edgetic profiling of ~50,000 non-synonymous SNVs (nsSNVs) from the 1000 Genomes Project by leveraging our semi-supervised learning approach SNP-IN tool on a comprehensive set of over 10,000 protein interaction complexes. We interrogated the functional roles of the variants and their impact on the human interactome and compared the results with the pathogenic variants disrupting PPIs in the same interactome. Our results demonstrated that a considerable number of nsSNVs from healthy populations could rewire the interactome. We also showed that the proteins enriched with interaction-disrupting mutations were associated with diverse functions and had implications in a broad spectrum of diseases. Further analysis indicated that distinct gene edgetic profiles among major populations could shed light on the molecular mechanisms behind the population phenotypic variances. Finally, the network analysis revealed that the disease-associated modules surprisingly harbored a higher density of interaction-disrupting mutations from healthy populations. The variation in the cumulative network damage within these modules could potentially account for the observed disparities in disease susceptibility, which are distinctly specific to certain populations. Our work demonstrates the feasibility of a large-scale in silico edgetic study, and reveals insights into the orchestrated play of population-specific mutations in the human interactome.

Determinants of iFGF13-mediated regulation of myocardial voltage-gated sodium (NaV) channels in mouse

Posttranslational regulation of cardiac NaV1.5 channels is critical in modulating channel expression and function, yet their regulation by phosphorylation of accessory proteins has gone largely unexplored. Using phosphoproteomic analysis of NaV channel complexes from adult mouse left ventricles, we identified nine phosphorylation sites on intracellular fibroblast growth factor 13 (iFGF13). To explore the potential roles of these phosphosites in regulating cardiac NaV currents, we abolished expression of iFGF13 in neonatal and adult mouse ventricular myocytes and rescued it with wild-type (WT), phosphosilent, or phosphomimetic iFGF13-VY. While the increased rate of closed-state inactivation of NaV channels induced by Fgf13 knockout in adult cardiomyocytes was completely restored by adenoviral-mediated expression of WT iFGF13-VY, only partial rescue was observed in neonatal cardiomyocytes after knockdown. The knockdown of iFGF13 in neonatal ventricular myocytes also shifted the voltage dependence of channel activation toward hyperpolarized potentials, a shift that was not reversed by WT iFGF13-VY expression. Additionally, we found that iFGF13-VY is the predominant isoform in adult ventricular myocytes, whereas both iFGF13-VY and iFGF13-S are expressed comparably in neonatal ventricular myocytes. Similar to WT iFGF13-VY, each of the iFGF13-VY phosphomutants studied restored NaV channel inactivation properties in both models. Lastly, Fgf13 knockout also increased the late Na+ current in adult cardiomyocytes, and this effect was restored with expression of WT and phosphosilent iFGF13-VY. Together, our results demonstrate that iFGF13 is highly phosphorylated and displays differential isoform expression in neonatal and adult ventricular myocytes. While we found no roles for iFGF13 phosphorylation, our results demonstrate differential effects of iFGF13 on neonatal and adult mouse ventricular NaV channels.

Differential regulation of cardiac sodium channels by intracellular fibroblast growth factors

Voltage-gated sodium (NaV) channels are responsible for the initiation and propagation of action potentials. In the heart, the predominant NaV1.5 α subunit is composed of four homologous repeats (I-IV) and forms a macromolecular complex with multiple accessory proteins, including intracellular fibroblast growth factors (iFGF). In spite of high homology, each of the iFGFs, iFGF11-iFGF14, as well as the individual iFGF splice variants, differentially regulates NaV channel gating, and the mechanisms underlying these differential effects remain elusive. Much of the work exploring iFGF regulation of NaV1.5 has been performed in mouse and rat ventricular myocytes in which iFGF13VY is the predominant iFGF expressed, whereas investigation into NaV1.5 regulation by the human heart-dominant iFGF12B is lacking. In this study, we used a mouse model with cardiac-specific Fgf13 deletion to study the consequences of iFGF13VY and iFGF12B expression. We observed distinct effects on the voltage-dependences of activation and inactivation of the sodium currents (INa), as well as on the kinetics of peak INa decay. Results in native myocytes were recapitulated with human NaV1.5 heterologously expressed in Xenopus oocytes, and additional experiments using voltage-clamp fluorometry (VCF) revealed iFGF-specific effects on the activation of the NaV1.5 voltage sensor domain in repeat IV (VSD-IV). iFGF chimeras further unveiled roles for all three iFGF domains (i.e., the N-terminus, core, and C-terminus) on the regulation of VSD-IV, and a slower time domain of inactivation. We present here a novel mechanism of iFGF regulation that is specific to individual iFGF isoforms and that leads to distinct functional effects on NaV channel/current kinetics.

FHF2 phosphorylation and regulation of native myocardial Na V 1.5 channels

Phosphorylation of the cardiac Na V 1.5 channel pore-forming subunit is extensive and critical in modulating channel expression and function, yet the regulation of Na V 1.5 by phosphorylation of its accessory proteins remains elusive. Using a phosphoproteomic analysis of Na V channel complexes purified from mouse left ventricles, we identified nine phosphorylation sites on Fibroblast growth factor Homologous Factor 2 (FHF2). To determine the roles of phosphosites in regulating Na V 1.5, we developed two models from neonatal and adult mouse ventricular cardiomyocytes in which FHF2 expression is knockdown and rescued by WT, phosphosilent or phosphomimetic FHF2-VY. While the increased rates of closed-state and open-state inactivation of Na V channels induced by the FHF2 knockdown are completely restored by the FHF2-VY isoform in adult cardiomyocytes, sole a partial rescue is obtained in neonatal cardiomyocytes. The FHF2 knockdown also shifts the voltage-dependence of activation towards hyperpolarized potentials in neonatal cardiomyocytes, which is not rescued by FHF2-VY. Parallel investigations showed that the FHF2-VY isoform is predominant in adult cardiomyocytes, while expression of FHF2-VY and FHF2-A is comparable in neonatal cardiomyocytes. Similar to WT FHF2-VY, however, each FHF2-VY phosphomutant restores the Na V channel inactivation properties in both models, preventing identification of FHF2 phosphosite roles. FHF2 knockdown also increases the late Na + current in adult cardiomyocytes, which is restored similarly by WT and phosphosilent FHF2-VY. Together, our results demonstrate that ventricular FHF2 is highly phosphorylated, implicate differential roles for FHF2 in regulating neonatal and adult mouse ventricular Na V 1.5, and suggest that the regulation of Na V 1.5 by FHF2 phosphorylation is highly complex.

eTOC Summary

Lesage et al . identify the phosphorylation sites of FHF2 from mouse left ventricular Na V 1.5 channel complexes. While no roles for FHF2 phosphosites could be recognized yet, the findings demonstrate differential FHF2-dependent regulation of neonatal and adult mouse ventricular Na V 1.5 channels.

BIOLOGICAL ANTIARRHYTHMICS-SODIUM CHANNEL INTERACTING PROTEINS

Voltage gated Na channels (NaV) are essential for excitation of tissues. Mutations in NaVs cause a spectrum of human disease from autism and epilepsy to cardiac arrhythmias to skeletal myotonias. The carboxyl termini (CT) of NaV channels are hotspots for disease-causing mutations and are richly invested with protein interaction sites. We have focused on the regulation of NaV by two proteins that bind in this region: calmodulin (CaM) and non-secreted fibroblast growth factors (iFGF or FHF). CaM regulates NaV gating, mediating Ca2+-dependent inactivation (CDI) in a channel isoform-specific manner, while Ca2+-free CaM (apo-CaM) binding broadly regulates NaV opening and suppresses the arrhythmogenic late Na current (INa-L). FHFs inhibit CDI, in NaV isoforms that exhibit this property, and potently suppress INa-L, the latter requiring the amino terminus of the FHF. A peptide comprised of the first 39 amino acids of FHF1A is sufficient to inhibit INa-L, constituting a credible specific antiarrhythmic.

Inhibition of late sodium current via PI3K/Akt signaling prevents cellular remodeling in tachypacing-induced HL-1 atrial myocytes

An aberrant late sodium current (I_Na,Late) caused by a mutation in the cardiac sodium channel (Na_v1.5) has emerged as a contributor to electrical remodeling that causes susceptibility to atrial fibrillation (AF). Although downregulation of phosphoinositide 3-kinase (PI3K)/Akt signaling is associated with AF, the molecular mechanisms underlying the negative regulation of I_Na,Late in AF remain unclear, and potential therapeutic approaches are needed. In this work, we constructed a tachypacing-induced cellular model of AF by exposing HL-1 myocytes to rapid electrical stimulation (1.5 V/cm, 4 ms, 10 Hz) for 6 h. Then, we gathered data using confocal Ca²⁺ imaging, immunofluorescence, patch-clamp recordings, and immunoblots. The tachypacing cells displayed irregular Ca²⁺ release, delayed afterdepolarization, prolonged action potential duration, and reduced PI3K/Akt signaling compared with controls. Those detrimental effects were related to increased I_Na,Late and were significantly mediated by treatment with the I_Na,Late blocker ranolazine. Furthermore, decreased PI3K/Akt signaling via PI3K inhibition increased I_Na,Late and subsequent aberrant myocyte excitability, which were abolished by I_Na,Late inhibition, suggesting that PI3K/Akt signaling is responsible for regulating pathogenic I_Na,Late. These results indicate that PI3K/Akt signaling is critical for regulating I_Na,Late and electrical remodeling, supporting the use of PI3K/Akt-mediated I_Na,Late as a therapeutic target for AF.

FGF12 is a novel component of the nucleolar NOLC1/TCOF1 ribosome biogenesis complex

Among the FGF proteins, the least characterized superfamily is the group of fibroblast growth factor homologous factors (FHFs). To date, the main role of FHFs has been primarily seen in the modulation of voltage-gated ion channels, but a full picture of the function of FHFs inside the cell is far from complete. In the present study, we focused on identifying novel FGF12 binding partners to indicate its intracellular functions. Among the identified proteins, a significant number were nuclear proteins, especially RNA-binding proteins involved in translational processes, such as ribosomal processing and modification. We have demonstrated that FGF12 is localized to the nucleolus, where it interacts with NOLC1 and TCOF1, proteins involved in the assembly of functional ribosomes. Interactions with both NOLC1 and TCOF1 are unique to FGF12, as other FHF proteins only bind to TCOF1. The formation of nucleolar FGF12 complexes with NOLC1 and TCOF1 is phosphorylation-dependent and requires the C-terminal region of FGF12. Surprisingly, NOLC1 and TCOF1 are unable to interact with each other in the absence of FGF12. Taken together, our data link FHF proteins to nucleoli for the first time and suggest a novel and unexpected role for FGF12 in ribosome biogenesis. Video Abstract.

Fibroblast growth factor homologous factors serve as a molecular rheostat in tuning arrhythmogenic cardiac late sodium current

Voltage-gated sodium (Nav1.5) channels support the genesis and brisk spatial propagation of action potentials in the heart. Disruption of NaV1.5 inactivation results in a small persistent Na influx known as late Na current (I Na,L), which has emerged as a common pathogenic mechanism in both congenital and acquired cardiac arrhythmogenic syndromes. Here, using low-noise multi-channel recordings in heterologous systems, LQTS3 patient-derived iPSCs cardiomyocytes, and mouse ventricular myocytes, we demonstrate that the intracellular fibroblast growth factor homologous factors (FHF1-4) tune pathogenic I Na,L in an isoform-specific manner. This scheme suggests a complex orchestration of I Na,L in cardiomyocytes that may contribute to variable disease expressivity of NaV1.5 channelopathies. We further leverage these observations to engineer a peptide-inhibitor of I Na,L with a higher efficacy as compared to a well-established small-molecule inhibitor. Overall, these findings lend insights into molecular mechanisms underlying FHF regulation of I Na,L in pathophysiology and outline potential therapeutic avenues.

Ion Channel and Ubiquitin Differential Expression during Erythromycin-Induced Anhidrosis in Foals

Macrolide drugs are the treatment of choice for Rhodococcus equi infections, despite severe side-effects temporary anhidrosis as a. To better understand the molecular biology leading to macrolide induced anhidrosis, we performed skin biopsies and Quantitative Intradermal Terbutaline Sweat Tests (QITSTs) in six healthy pony-cross foals for three different timepoints during erythromycin administration-pre-treatment (baseline), during anhidrosis and post-recovery. RNA sequencing of biopsies followed by differential gene expression analysis compared both pre and post normal sweating timepoints to the erythromycin induced anhidrosis episode. After Bonferroni correction for multiple testing, 132 gene transcripts were significantly differentially expressed during the anhidrotic timepoint. Gene ontology analysis of the full differentially expressed gene set identified over-represented biological functions for ubiquitination and ion-channel function, both biologically relevant to sweat production. These same mechanisms were previously implicated in heritable equine idiopathic anhidrosis and sweat gland function and their involvement in macrolide-induced temporary anhidrosis warrants further investigation.

Ca2+-dependent modulation of voltage-gated myocyte sodium channels

Voltage-dependent Na⁺ channel activation underlies action potential generation fundamental to cellular excitability. In skeletal and cardiac muscle this triggers contraction via ryanodine-receptor (RyR)-mediated sarcoplasmic reticular (SR) Ca²⁺ release. We here review potential feedback actions of intracellular [Ca²⁺] ([Ca²⁺]_i) on Na⁺ channel activity, surveying their structural, genetic and cellular and functional implications, translating these to their possible clinical importance. In addition to phosphorylation sites, both Nav1.4 and Nav1.5 possess potentially regulatory binding sites for Ca²⁺ and/or the Ca^2+-sensor calmodulin in their inactivating III–IV linker and C-terminal domains (CTD), where mutations are associated with a range of skeletal and cardiac muscle diseases. We summarize in vitro cell-attached patch clamp studies reporting correspondingly diverse, direct and indirect, Ca²⁺ effects upon maximal Nav1.4 and Nav1.5 currents (I_max) and their half-maximal voltages (V_1/2) characterizing channel gating, in cellular expression systems and isolated myocytes. Interventions increasing cytoplasmic [Ca²⁺]_i down-regulated I_max leaving V_1/2 constant in native loose patch clamped, wild-type murine skeletal and cardiac myocytes. They correspondingly reduced action potential upstroke rates and conduction velocities, causing pro-arrhythmic effects in intact perfused hearts. Genetically modified murine RyR2-P2328S hearts modelling catecholaminergic polymorphic ventricular tachycardia (CPVT), recapitulated clinical ventricular and atrial pro-arrhythmic phenotypes following catecholaminergic challenge. These accompanied reductions in action potential conduction velocities. The latter were reversed by flecainide at RyR-blocking concentrations specifically in RyR2-P2328S as opposed to wild-type hearts, suggesting a basis for its recent therapeutic application in CPVT. We finally explore the relevance of these mechanisms in further genetic paradigms for commoner metabolic and structural cardiac disease.

Differential Modulation of the Voltage-Gated Na+ Channel 1.6 by Peptides Derived From Fibroblast Growth Factor 14

The voltage-gated Na⁺ (Nav) channel is a primary molecular determinant of the initiation and propagation of the action potential. Despite the central role of the pore-forming α subunit in conferring this functionality, protein:protein interactions (PPI) between the α subunit and auxiliary proteins are necessary for the full physiological activity of Nav channels. In the central nervous system (CNS), one such PPI occurs between the C-terminal domain of the Nav1.6 channel and fibroblast growth factor 14 (FGF14). Given the primacy of this PPI in regulating the excitability of neurons in clinically relevant brain regions, peptides targeting the FGF14:Nav1.6 PPI interface could be of pre-clinical value. In this work, we pharmacologically evaluated peptides derived from FGF14 that correspond to residues that are at FGF14's PPI interface with the CTD of Nav1.6. These peptides, Pro-Leu-Glu-Val (PLEV) and Glu-Tyr-Tyr-Val (EYYV), which correspond to residues of the β12 sheet and β8-β9 loop of FGF14, respectively, were shown to inhibit FGF14:Nav1.6 complex assembly. In functional studies using whole-cell patch-clamp electrophysiology, PLEV and EYYV were shown to confer differential modulation of Nav1.6-mediated currents through mechanisms dependent upon the presence of FGF14. Crucially, these FGF14-dependent effects of PLEV and EYYV on Nav1.6-mediated currents were further shown to be dependent on the N-terminal domain of FGF14. Overall, these data suggest that the PLEV and EYYV peptides represent scaffolds to interrogate the Nav1.6 channel macromolecular complex in an effort to develop targeted pharmacological modulators.

Heritable arrhythmia syndromes associated with abnormal cardiac sodium channel function: ionic and non-ionic mechanisms

The cardiac sodium channel NaV1.5, encoded by the SCN5A gene, is responsible for the fast upstroke of the action potential. Mutations in SCN5A may cause sodium channel dysfunction by decreasing peak sodium current, which slows conduction and facilitates reentry-based arrhythmias, and by enhancing late sodium current, which prolongs the action potential and sets the stage for early afterdepolarization and arrhythmias. Yet, some NaV1.5-related disorders, in particular structural abnormalities, cannot be directly or solely explained on the basis of defective NaV1.5 expression or biophysics. An emerging concept that may explain the large disease spectrum associated with SCN5A mutations centres around the multifunctionality of the NaV1.5 complex. In this alternative view, alterations in NaV1.5 affect processes that are independent of its canonical ion-conducting role. We here propose a novel classification of NaV1.5 (dys)function, categorized into (i) direct ionic effects of sodium influx through NaV1.5 on membrane potential and consequent action potential generation, (ii) indirect ionic effects of sodium influx on intracellular homeostasis and signalling, and (iii) non-ionic effects of NaV1.5, independent of sodium influx, through interactions with macromolecular complexes within the different microdomains of the cardiomyocyte. These indirect ionic and non-ionic processes may, acting alone or in concert, contribute significantly to arrhythmogenesis. Hence, further exploration of these multifunctional effects of NaV1.5 is essential for the development of novel preventive and therapeutic strategies.

Nav1.3 and FGF14 are primary determinants of the TTX-sensitive sodium current in mouse adrenal chromaffin cells

Adrenal chromaffin cells (CCs) in rodents express rapidly inactivating, tetrodotoxin (TTX)-sensitive sodium channels. The resulting current has generally been attributed to Nav1.7, although a possible role for Nav1.3 has also been suggested. Nav channels in rat CCs rapidly inactivate via two independent pathways which differ in their time course of recovery. One subpopulation recovers with time constants similar to traditional fast inactivation and the other ∼10-fold slower, but both pathways can act within a single homogenous population of channels. Here, we use Nav1.3 KO mice to probe the properties and molecular components of Nav current in CCs. We find that the absence of Nav1.3 abolishes all Nav current in about half of CCs examined, while a small, fast inactivating Nav current is still observed in the rest. To probe possible molecular components underlying slow recovery from inactivation, we used mice null for fibroblast growth factor homology factor 14 (FGF14). In these cells, the slow component of recovery from fast inactivation is completely absent in most CCs, with no change in the time constant of fast recovery. The use dependence of Nav current reduction during trains of stimuli in WT cells is completely abolished in FGF14 KO mice, directly demonstrating a role for slow recovery from inactivation in determining Nav current availability. Our results indicate that FGF14-mediated inactivation is the major determinant defining use-dependent changes in Nav availability in CCs. These results establish that Nav1.3, like other Nav isoforms, can also partner with FGF subunits, strongly regulating Nav channel function.

Association Between Obesity-Mediated Atrial Fibrillation and Therapy With Sodium Channel Blocker Antiarrhythmic Drugs

Importance

The association between obesity, an established risk factor for atrial fibrillation (AF), and response to antiarrhythmic drugs (AADs) remains unclear.

Objective

To test the hypothesis that obesity differentially mediates response to AADs in patients with symptomatic AF and in mice with diet-induced obesity (DIO) and pacing induced AF.

Design, Setting, and Participants

An observational cohort study was conducted including 311 patients enrolled in a clinical-genetic registry. Mice fed a high-fat diet for 10 weeks were also evaluated. The study was conducted from January 1, 2018, to June 2, 2019.

Main Outcomes and Measures

Symptomatic response was defined as continuation of the same AAD for at least 3 months. Nonresponse was defined as discontinuation of the AAD within 3 months of initiation because of poor symptomatic control of AF necessitating alternative rhythm control therapy. Outcome measures in DIO mice were pacing-induced AF and suppression of AF after 2 weeks of treatment with flecainide acetate or sotalol hydrochloride.

Results

A total of 311 patients (mean [SD] age, 65 [12] years; 120 women [38.6%]) met the entry criteria and were treated with a class I or III AAD for symptomatic AF. Nonresponse to class I AADs in patients with obesity was less than in those without obesity (30% [obese] vs 6% [nonobese]; difference, 0.24; 95% CI, 0.11-0.37; P = .001). Both groups had similar symptomatic response to a potassium channel blocker AAD. On multivariate analysis, obesity, AAD class (class I vs III AAD [obese] odds ratio [OR], 4.54; 95% Wald CI, 1.84-11.20; P = .001), female vs male sex (OR, 2.31; 95% Wald CI, 1.07-4.99; P = .03), and hyperthyroidism (OR, 4.95; 95% Wald CI, 1.23-20.00; P = .02) were significant indicators of the probability of failure to respond to AADs. Pacing induced AF in 100% of DIO mice vs 30% (P < .001) in controls. Furthermore, DIO mice showed a greater reduction in AF burden when treated with sotalol compared with flecainide (85% vs 25%; P < .01).

Conclusions and Relevance

Results suggest that obesity differentially mediates response to AADs in patients and in mice with AF, possibly reducing the therapeutic effectiveness of sodium channel blockers. These findings may have implications for the management of AF in patients with obesity.

Characterization of an N-terminal Nav1.5 channel variant - a potential risk factor for arrhythmias and sudden death?

Background

Alterations in the SCN5A gene encoding the cardiac sodium channel Na_v1.5 have been linked to a number of arrhythmia syndromes and diseases including long-QT syndrome (LQTS), Brugada syndrome (BrS) and dilative cardiomyopathy (DCM), which may predispose to fatal arrhythmias and sudden death. We identified the heterozygous variant c.316A > G, p.(Ser106Gly) in a 35-year-old patient with survived cardiac arrest. In the present study, we aimed to investigate the functional impact of the variant to clarify the medical relevance.

Methods

Mutant as well as wild type GFP tagged Na_v1.5 channels were expressed in HEK293 cells. We performed functional characterization experiments using patch-clamp technique.

Results

Electrophysiological measurements indicated, that the detected missense variant alters Nav1.5 channel functionality leading to a gain-of-function effect. Cells expressing S106G channels show an increase in Na_v1.5 current over the entire voltage window.

Conclusion

The results support the assumption that the detected sequence aberration alters Na_v1.5 channel function and may predispose to cardiac arrhythmias and sudden cardiac death.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12881-020-01170-3.

Design, Synthesis, and Pharmacological Evaluation of Analogues Derived from the PLEV Tetrapeptide as Protein-Protein Interaction Modulators of Voltage-Gated Sodium Channel 1.6

The voltage-gated Na⁺ (Na_v) channel is the molecular determinant of excitability. Disruption of protein-protein interactions (PPIs) between Na_v1.6 and fibroblast growth factor 14 (FGF14) leads to impaired excitability of neurons in clinically relevant brain areas associated with channelopathies. Here, we designed, synthesized, and pharmacologically characterized new peptidomimetics based on a PLEV tetrapeptide scaffold derived from the FGF14:Na_v1.6 PPI interface. Addition of an N-terminal 1-adamantanecarbonyl pharmacophore significantly improved peptidomimetic inhibitory potency. Surface plasmon resonance studies revealed that while this moiety was sufficient to confer binding to FGF14, altering the C-terminal moiety from methoxy (21a) to π bond-containing (23a and 23b) or cycloalkane substituents (23e) abrogated the binding to Na_v1.6. Whole-cell patch-clamp electrophysiology subsequently revealed that 21a had functionally relevant interactions with both the C-terminal tail of Na_v1.6 and FGF14. Collectively, these findings support that 21a (PW0564) may serve as a promising lead to develop target-selective neurotherapeutics by modulating protein-channel interactions.

Fibroblast growth factor homologous factors tune arrhythmogenic late NaV1.5 current in calmodulin binding-deficient channels

The Ca2+-binding protein calmodulin has emerged as a pivotal player in tuning Na+ channel function, although its impact in vivo remains to be resolved. Here, we identify the role of calmodulin and the NaV1.5 interactome in regulating late Na+ current in cardiomyocytes. We created transgenic mice with cardiac-specific expression of human NaV1.5 channels with alanine substitutions for the IQ motif (IQ/AA). The mutations rendered the channels incapable of binding calmodulin to the C-terminus. The IQ/AA transgenic mice exhibited normal ventricular repolarization without arrhythmias and an absence of increased late Na+ current. In comparison, transgenic mice expressing a lidocaine-resistant (F1759A) human NaV1.5 demonstrated increased late Na+ current and prolonged repolarization in cardiomyocytes, with spontaneous arrhythmias. To determine regulatory factors that prevent late Na+ current for the IQ/AA mutant channel, we considered fibroblast growth factor homologous factors (FHFs), which are within the NaV1.5 proteomic subdomain shown by proximity labeling in transgenic mice expressing NaV1.5 conjugated to ascorbate peroxidase. We found that FGF13 diminished late current of the IQ/AA but not F1759A mutant cardiomyocytes, suggesting that endogenous FHFs may serve to prevent late Na+ current in mouse cardiomyocytes. Leveraging endogenous mechanisms may furnish an alternative avenue for developing novel pharmacology that selectively blunts late Na+ current.

Ionic Mechanisms of Impulse Propagation Failure in the FHF2-Deficient Heart

RATIONALE

FHFs (fibroblast growth factor homologous factors) are key regulators of sodium channel (Na_V) inactivation. Mutations in these critical proteins have been implicated in human diseases including Brugada syndrome, idiopathic ventricular arrhythmias, and epileptic encephalopathy. The underlying ionic mechanisms by which reduced Na_v availability in Fhf2 knockout (Fhf2^KO) mice predisposes to abnormal excitability at the tissue level are not well defined.

OBJECTIVE

Using animal models and theoretical multicellular linear strands, we examined how FHF2 orchestrates the interdependency of sodium, calcium, and gap junctional conductances to safeguard cardiac conduction.

METHODS AND RESULTS

Fhf2^KO mice were challenged by reducing calcium conductance (gCa_V) using verapamil or by reducing gap junctional conductance (Gj) using carbenoxolone or by backcrossing into a cardiomyocyte-specific Cx43 (connexin 43) heterozygous background. All conditions produced conduction block in Fhf2^KO mice, with Fhf2 wild-type (Fhf2^WT) mice showing normal impulse propagation. To explore the ionic mechanisms of block in Fhf2^KO hearts, multicellular linear strand models incorporating FHF2-deficient Na_v inactivation properties were constructed and faithfully recapitulated conduction abnormalities seen in mutant hearts. The mechanisms of conduction block in mutant strands with reduced gCa_V or diminished Gj are very different. Enhanced Na_v inactivation due to FHF2 deficiency shifts dependence onto calcium current (I_Ca) to sustain electrotonic driving force, axial current flow, and action potential (AP) generation from cell-to-cell. In the setting of diminished Gj, slower charging time from upstream cells conspires with accelerated Na_v inactivation in mutant strands to prevent sufficient downstream cell charging for AP propagation.

CONCLUSIONS

FHF2-dependent effects on Na_v inactivation ensure adequate sodium current (I_Na) reserve to safeguard against numerous threats to reliable cardiac impulse propagation.

Phosphoproteomic analysis identifies phospho-Threonine-17 site of phospholamban important in low molecular weight isoform of fibroblast growth factor 2-induced protection against post-ischemic cardiac dysfunction

RATIONALE

Among its many biological roles, fibroblast growth factor 2 (FGF2) protects the heart from dysfunction and damage associated with an ischemic attack. Our laboratory demonstrated that its protection against myocardial dysfunction occurs by the low molecular weight (LMW) isoform of FGF2, while the high molecular weight (HMW) isoforms are associated with a worsening in post-ischemic recovery of cardiac function. LMW FGF2-mediated cardioprotection is facilitated by activation of multiple kinases, including PKCalpha, PKCepsilon, and ERK, and inhibition of p38 and JNK.

OBJECTIVE

Yet, the substrates of those kinases associated with LMW FGF2-induced cardioprotection against myocardial dysfunction remain to be elucidated.

METHODS AND RESULTS

To identify substrates in LMW FGF2 improvement of post-ischemic cardiac function, mouse hearts expressing only LMW FGF2 were subjected to ischemia-reperfusion (I/R) injury and analyzed by a mass spectrometry (MS)-based quantitative phosphoproteomic strategy. MS analysis identified 50 phosphorylation sites from 7 sarcoendoplasmic reticulum (SR) proteins that were significantly altered in I/R-treated hearts only expressing LMW FGF2 compared to those hearts lacking FGF2. One of those phosphorylated SR proteins identified was phospholamban (PLB), which exhibited rapid, increased phosphorylation at Threonine-17 (Thr17) after I/R in hearts expressing only LMW FGF2; this was further validated using Selected Reaction Monitoring-based MS workflow. To demonstrate a mechanistic role of phospho-Thr17 PLB in LMW FGF2-mediated cardioprotection, hearts only expressing LMW FGF2 and those expressing only LMW FGF2 with a mutant PLB lacking phosphorylatable Thr17 (Thr17Ala PLB) were subjected to I/R. Hearts only expressing LMW FGF2 showed significantly improved recovery of cardiac function following I/R (p < 0.05), and this functional improvement was significantly abrogated in hearts expressing LMW FGF2 and Thr17Ala PLB (p < 0.05).

CONCLUSION

The findings indicate that LMW FGF2 modulates intracellular calcium handling/cycling via regulatory changes in SR proteins essential for recovery from I/R injury, and thereby protects the heart from post-ischemic cardiac dysfunction.

Ion Channel and Structural Remodeling in Obesity-Mediated Atrial Fibrillation

BACKGROUND

Epidemiological studies have established obesity as an independent risk factor for atrial fibrillation (AF), but the underlying pathophysiological mechanisms remain unclear. Reduced cardiac sodium channel expression is a known causal mechanism in AF. We hypothesized that obesity decreases Nav1.5 expression via enhanced oxidative stress, thus reducing INa, and enhancing susceptibility to AF.

METHODS

To elucidate the underlying electrophysiological mechanisms a diet-induced obese mouse model was used. Weight, blood pressure, glucose, F2-isoprostanes, NOX2 (NADPH oxidase 2), and PKC (protein kinase C) were measured in obese mice and compared with lean controls. Invasive electrophysiological, immunohistochemistry, Western blotting, and patch clamping of membrane potentials was performed to evaluate the molecular and electrophysiological phenotype of atrial myocytes.

RESULTS

Pacing-induced AF in 100% of diet-induced obese mice versus 25% in controls (P<0.01) with increased AF burden. Cardiac sodium channel expression, INa and atrial action potential duration were reduced and potassium channel expression (Kv1.5) and current (IKur) and F2-isoprostanes, NOX2, and PKC-α/δ expression and atrial fibrosis were significantly increased in diet-induced obese mice as compared with controls. A mitochondrial antioxidant reduced AF burden, restored INa, ICa,L, IKur, action potential duration, and reversed atrial fibrosis in diet-induced obese mice as compared with controls.

CONCLUSIONS

Inducible AF in obese mice is mediated, in part, by a combined effect of sodium, potassium, and calcium channel remodeling and atrial fibrosis. Mitochondrial antioxidant therapy abrogated the ion channel and structural remodeling and reversed the obesity-induced AF burden. Our findings have important implications for the management of obesity-mediated AF in patients. Graphic Abstract: A graphic abstract is available for this article.

Structures Illuminate Cardiac Ion Channel Functions in Health and in Long QT Syndrome

The cardiac action potential is critical to the production of a synchronized heartbeat. This electrical impulse is governed by the intricate activity of cardiac ion channels, among them the cardiac voltage-gated potassium (Kv) channels KCNQ1 and hERG as well as the voltage-gated sodium (Nav) channel encoded by SCN5A. Each channel performs a highly distinct function, despite sharing a common topology and structural components. These three channels are also the primary proteins mutated in congenital long QT syndrome (LQTS), a genetic condition that predisposes to cardiac arrhythmia and sudden cardiac death due to impaired repolarization of the action potential and has a particular proclivity for reentrant ventricular arrhythmias. Recent cryo-electron microscopy structures of human KCNQ1 and hERG, along with the rat homolog of SCN5A and other mammalian sodium channels, provide atomic-level insight into the structure and function of these proteins that advance our understanding of their distinct functions in the cardiac action potential, as well as the molecular basis of LQTS. In this review, the gating, regulation, LQTS mechanisms, and pharmacological properties of KCNQ1, hERG, and SCN5A are discussed in light of these recent structural findings.

An interaction between the III-IV linker and CTD in NaV1.5 confers regulation of inactivation by CaM and FHF

Voltage gated sodium channel (VGSC) activation drives the action potential upstroke in cardiac myocytes, skeletal muscles, and neurons. After opening, VGSCs rapidly enter a non-conducting, inactivated state. Impaired inactivation causes persistent inward current and underlies cardiac arrhythmias. VGSC auxiliary proteins calmodulin (CaM) and fibroblast growth factor homologous factors (FHFs) bind to the channel's C-terminal domain (CTD) and limit pathogenic persistent currents. The structural details and mechanisms mediating these effects are not clear. Building on recently published cryo-EM structures, we show that CaM and FHF limit persistent currents in the cardiac NaV1.5 VGSC by stabilizing an interaction between the channel's CTD and III-IV linker region. Perturbation of this intramolecular interaction increases persistent current and shifts the voltage dependence of steady-state inactivation. Interestingly, the NaV1.5 residues involved in the interaction are sites mutated in the arrhythmogenic long QT3 syndrome (LQT3). Along with electrophysiological investigations of this interaction, we present structural models that suggest how CaM and FHF stabilize the interaction and thereby limit the persistent current. The critical residues at the interaction site are conserved among VGSC isoforms, and subtle substitutions provide an explanation for differences in inactivation among the isoforms.

The cardiac CaMKII-Nav1.5 relationship: From physiology to pathology

The SCN5A gene encodes Nav1.5, which, as the cardiac voltage-gated Na+ channel's pore-forming α subunit, is crucial for the initiation and propagation of atrial and ventricular action potentials. The arrhythmogenic propensity of inherited SCN5A mutations implicates the Na+ channel in determining cardiomyocyte excitability under normal conditions. Cytosolic kinases have long been known to alter the kinetic profile of Nav1.5 inactivation via phosphorylation of specific residues. Recent substantiation of both the role of calmodulin-dependent kinase II (CaMKII) in modulating the properties of the Nav1.5 inactivation gate and the significant rise in oxidation-dependent autonomous CaMKII activity in structural heart disease has raised the possibility of a novel pathway for acquired arrhythmias - the CaMKII-Nav1.5 relationship. The aim of this review is to: (1) outline the relationship's translation from physiological adaptation to pathological vicious circle; and (2) discuss the relative merits of each of its components as pharmacological targets.

Fibroblast growth factor homologous factors are potential ion channel modifiers associated with cardiac arrhythmias

Stable electrical activity in cardiac myocytes is the basis of maintaining normal myocardial systolic and diastolic function. Cardiac ionic currents and their associated regulatory proteins are crucial to myocyte excitability and heart function. Fibroblast growth factor homologous factors (FHFs) are intracellular noncanonical fibroblast growth factors (FGFs) that are incapable of activating FGF receptors. The main functions of FHFs are to regulate ion channels and influence excitability, which are processes involved in sustaining normal cardiac function. In addition to their regulatory effect on ion channels, FHFs can be regulators of cardiac hypertrophic signaling and alter signaling pathways, including the protein kinase, NF<kappa>B, and p53 pathways, which are related to the pathological processes of heart diseases. This review emphasizes FHF-mediated regulation of cardiac excitability and the association of FHFs with cardiac arrhythmias and explores the idea that abnormal FHFs may be an unrecognized cause of cardiac disorders.

Comparing human iPSC-cardiomyocytes versus HEK293T cells unveils disease-causing effects of Brugada mutation A735V of NaV1.5 sodium channels

Loss-of-function mutations of the SCN5A gene encoding for the sodium channel α-subunit NaV1.5 result in the autosomal dominant hereditary disease Brugada Syndrome (BrS) with a high risk of sudden cardiac death in the adult. We here engineered human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) carrying the CRISPR/Cas9 introduced BrS-mutation p.A735V-NaV1.5 (g.2204C > T in exon 14 of SCN5A) as a novel model independent of patient´s genetic background. Recent studies raised concern regarding the use of hiPSC-CMs for studying adult-onset hereditary diseases due to cells' immature phenotype. To tackle this concern, long-term cultivation of hiPSC-CMs on a stiff matrix (27-42 days) was applied to promote maturation. Patch clamp recordings of A735V mutated hiPSC-CMs revealed a substantially reduced upstroke velocity and sodium current density, a prominent rightward shift of the steady state activation curve and decelerated recovery from inactivation as compared to isogenic hiPSC-CMs controls. These observations were substantiated by a comparative study on mutant A735V-NaV1.5 channels heterologously expressed in HEK293T cells. In contrast to mutated hiPSC-CMs, a leftward shift of sodium channel inactivation was not observed in HEK293T, emphasizing the importance of investigating mechanisms of BrS in independent systems. Overall, our approach supports hiPSC-CMs' relevance for investigating channelopathies in a dish.

Identification of peptidomimetics as novel chemical probes modulating fibroblast growth factor 14 (FGF14) and voltage-gated sodium channel 1.6 (Nav1.6) protein-protein interactions

The voltage-gated sodium (Nav) channel is the molecular determinant of action potential in neurons. Protein-protein interactions (PPI) between the intracellular Nav1.6 C-tail and its regulatory protein fibroblast growth factor 14 (FGF14) provide an ideal and largely untapped opportunity for development of neurochemical probes. Based on a previously identified peptide FLPK, mapped to the FGF14:FGF14 PPI interface, we have designed and synthesized a series of peptidomimetics with the intent of increasing clogP values and improving cell permeability relative to the parental lead peptide. In-cell screening using the split-luciferase complementation (LCA) assay identified ZL0177 (13) as the most potent inhibitor of the FGF14:Nav1.6 channel complex assembly with an apparent IC₅₀ of 11 μM. Whole-cell patch-clamp recordings demonstrated that ZL0177 significantly reduced Nav1.6-mediated transient current density and induced a depolarizing shift of the channel voltage-dependence of activation. Docking studies revealed strong interactions between ZL0177 and Nav1.6, mediated by hydrogen bonds, cation-π interactions and hydrophobic contacts. All together these results suggest that ZL0177 retains some key features of FGF14-dependent modulation of Nav1.6 currents. Overall, ZL0177 provides a chemical scaffold for developing Nav channel modulators as pharmacological probes with therapeutic potential of interest for a broad range of CNS and PNS disorders.

CardioVAI: An automatic implementation of ACMG-AMP variant interpretation guidelines in the diagnosis of cardiovascular diseases

Variant interpretation for the diagnosis of genetic diseases is a complex process. The American College of Medical Genetics and Genomics, with the Association for Molecular Pathology, have proposed a set of evidence-based guidelines to support variant pathogenicity assessment and reporting in Mendelian diseases. Cardiovascular disorders are a field of application of these guidelines, but practical implementation is challenging due to the genetic disease heterogeneity and the complexity of information sources that need to be integrated. Decision support systems able to automate variant interpretation in the light of specific disease domains are demanded. We implemented CardioVAI (Cardio Variant Interpreter), an automated system for guidelines based variant classification in cardiovascular-related genes. Different omics-resources were integrated to assess pathogenicity of every genomic variant in 72 cardiovascular diseases related genes. We validated our method on benchmark datasets of high-confident assessed variants, reaching pathogenicity and benignity concordance up to 83 and 97.08%, respectively. We compared CardioVAI to similar methods and analyzed the main differences in terms of guidelines implementation. We finally made available CardioVAI as a web resource (http://cardiovai.engenome.com/) that allows users to further specialize guidelines recommendations.

SCN5A Variants: Association With Cardiac Disorders

The SCN5A gene encodes the alpha subunit of the main cardiac sodium channel Na_v1.5. This channel predominates inward sodium current (INa) and plays a critical role in regulation of cardiac electrophysiological function. Since 1995, SCN5A variants have been found to be causatively associated with Brugada syndrome, long QT syndrome, cardiac conduction system dysfunction, dilated cardiomyopathy, etc. Previous genetic, electrophysiological, and molecular studies have identified the arrhythmic and cardiac structural characteristics induced by SCN5A variants. However, due to the variation of disease manifestations and genetic background, impact of environmental factors, as well as the presence of mixed phenotypes, the detailed and individualized physiological mechanisms in various SCN5A-related syndromes are not fully elucidated. This review summarizes the current knowledge of SCN5A genetic variations in different SCN5A-related cardiac disorders and the newly developed therapy strategies potentially useful to prevent and treat these disorders in clinical setting.

Dysfunctional Nav1.5 channels due to SCN5A mutations

The voltage-gated sodium channel 1.5 (Nav1.5), encoded by the SCN5A gene, is responsible for the rising phase of the action potential of cardiomyocytes. The sodium current mediated by Nav1.5 consists of peak and late components (I_Na-P and I_Na-L). Mutant Nav1.5 causes alterations in the peak and late sodium current and is associated with an increasingly wide range of congenital arrhythmias. More than 400 mutations have been identified in the SCN5A gene. Although the mechanisms of SCN5A mutations leading to a variety of arrhythmias can be classified according to the alteration of I_Na-P and I_Na-L as gain-of-function, loss-of-function and both, few researchers have summarized the mechanisms in this way before. In this review article, we aim to review the mechanisms underlying dysfunctional Nav1.5 due to SCN5A mutations and to provide some new insights into further approaches in the treatment of arrhythmias. Impact statement The field of ion channelopathy caused by dysfunctional Nav1.5 due to SCN5A mutations is rapidly evolving as novel technologies of electrophysiology are introduced and our understanding of the mechanisms of various arrhythmias develops. In this review, we focus on the dysfunctional Nav1.5 related to arrhythmias and the underlying mechanisms. We update SCN5A mutations in a precise way since 2013 and presents novel classifications of SCN5A mutations responsible for the dysfunction of the peak (I_Na-P) and late (I_Na-L) sodium channels based on their phenotypes, including loss-, gain-, and coexistence of gain- and loss-of function mutations in I_Na-P, I_Na-L, respectively. We hope this review will provide a new comprehensive way to better understand the electrophysiological mechanisms underlying arrhythmias from cell to bedside, promoting the management of various arrhythmias in practice.

Functional Modulation of Voltage-Gated Sodium Channels by a FGF14-Based Peptidomimetic

Protein-protein interactions (PPI) offer unexploited opportunities for CNS drug discovery and neurochemical probe development. Here, we present ZL181, a novel peptidomimetic targeting the PPI interface of the voltage-gated Na⁺ channel Nav1.6 and its regulatory protein fibroblast growth factor 14 (FGF14). ZL181 binds to FGF14 and inhibits its interaction with the Nav1.6 channel C-tail. In HEK-Nav1.6 expressing cells, ZL181 acts synergistically with FGF14 to suppress Nav1.6 current density and to slow kinetics of fast inactivation, but antagonizes FGF14 modulation of steady-state inactivation that is regulated by the N-terminal tail of the protein. In medium spiny neurons in the nucleus accumbens, ZL181 suppresses excitability by a mechanism that is dependent upon expression of FGF14 and is consistent with a state-dependent inhibition of FGF14. Overall, ZL181 and derivatives could lay the ground for developing allosteric modulators of Nav channels that are of interest for a broad range of CNS disorders.

Mechanisms and models of cardiac sodium channel inactivation

Shortly after cardiac Na+ channels activate and initiate the action potential, inactivation ensues within milliseconds, attenuating the peak Na+ current, INa, and allowing the cell membrane to repolarize. A very limited number of Na+ channels that do not inactivate carry a persistent INa, or late INa. While late INa is only a small fraction of peak magnitude, it significantly prolongs ventricular action potential duration, which predisposes patients to arrhythmia. Here, we review our current understanding of inactivation mechanisms, their regulation, and how they have been modeled computationally. Based on this body of work, we conclude that inactivation and its connection to late INa would be best modeled with a "feet-on-the-door" approach where multiple channel components participate in determining inactivation and late INa. This model reflects experimental findings showing that perturbation of many channel locations can destabilize inactivation and cause pathological late INa.

C-terminal phosphorylation of NaV1.5 impairs FGF13-dependent regulation of channel inactivation

Voltage-gated Na+ (NaV) channels are key regulators of myocardial excitability, and Ca2+/calmodulin-dependent protein kinase II (CaMKII)-dependent alterations in NaV1.5 channel inactivation are emerging as a critical determinant of arrhythmias in heart failure. However, the global native phosphorylation pattern of NaV1.5 subunits associated with these arrhythmogenic disorders and the associated channel regulatory defects remain unknown. Here, we undertook phosphoproteomic analyses to identify and quantify in situ the phosphorylation sites in the NaV1.5 proteins purified from adult WT and failing CaMKIIδc-overexpressing (CaMKIIδc-Tg) mouse ventricles. Of 19 native NaV1.5 phosphorylation sites identified, two C-terminal phosphoserines at positions 1938 and 1989 showed increased phosphorylation in the CaMKIIδc-Tg compared with the WT ventricles. We then tested the hypothesis that phosphorylation at these two sites impairs fibroblast growth factor 13 (FGF13)-dependent regulation of NaV1.5 channel inactivation. Whole-cell voltage-clamp analyses in HEK293 cells demonstrated that FGF13 increases NaV1.5 channel availability and decreases late Na+ current, two effects that were abrogated with NaV1.5 mutants mimicking phosphorylation at both sites. Additional co-immunoprecipitation experiments revealed that FGF13 potentiates the binding of calmodulin to NaV1.5 and that phosphomimetic mutations at both sites decrease the interaction of FGF13 and, consequently, of calmodulin with NaV1.5. Together, we have identified two novel native phosphorylation sites in the C terminus of NaV1.5 that impair FGF13-dependent regulation of channel inactivation and may contribute to CaMKIIδc-dependent arrhythmogenic disorders in failing hearts.

Gene Expression Networks in the Murine Pulmonary Myocardium Provide Insight into the Pathobiology of Atrial Fibrillation

The pulmonary myocardium is a muscular coat surrounding the pulmonary and caval veins. Although its definitive physiological function is unknown, it may have a pathological role as the source of ectopic beats initiating atrial fibrillation. How the pulmonary myocardium gains pacemaker function is not clearly defined, although recent evidence indicates that changed transcriptional gene expression networks are at fault. The gene expression profile of this distinct cell type in situ was examined to investigate underlying molecular events that might contribute to atrial fibrillation. Via systems genetics, a whole-lung transcriptome data set from the BXD recombinant inbred mouse resource was analyzed, uncovering a pulmonary cardiomyocyte gene network of 24 transcripts, coordinately regulated by chromosome 1 and 2 loci. Promoter enrichment analysis and interrogation of publicly available ChIP-seq data suggested that transcription of this gene network may be regulated by the concerted activity of NKX2-5, serum response factor, myocyte enhancer factor 2, and also, at a post-transcriptional level, by RNA binding protein motif 20. Gene ontology terms indicate that this gene network overlaps with molecular markers of the stressed heart. Therefore, we propose that perturbed regulation of this gene network might lead to altered calcium handling, myocyte growth, and contractile force contributing to the aberrant electrophysiological properties observed in atrial fibrillation. We reveal novel molecular interactions and pathways representing possible therapeutic targets for atrial fibrillation. In addition, we highlight the utility of recombinant inbred mouse resources in detecting and characterizing gene expression networks of relatively small populations of cells that have a pathological significance.

De Novo FGF12 (Fibroblast Growth Factor 12) Functional Variation Is Potentially Associated With Idiopathic Ventricular Tachycardia

Background

Idiopathic ventricular tachycardia (VT) is a type of cardiac arrhythmia occurring in structurally normal hearts. The heritability of idiopathic VT remains to be clarified, and numerous genetic factors responsible for development of idiopathic VT are as yet unclear. Variations in FGF12 (fibroblast growth factor 12), which is expressed in the human ventricle and modulates the cardiac Na⁺ channel Na_V1.5, may play an important role in the genetic pathogenesis of VT.

Methods and Results

We tested the hypothesis that genetic variations in FGF12 are associated with VT in 2 independent Chinese cohorts and resequenced all the exons and exon–intron boundaries and the 5′ and 3′ untranslated regions of FGF12 in 320 unrelated participants with idiopathic VT. For population‐based case–control association studies, we chose 3 single‐nucleotide polymorphisms—rs1460922, rs4687326, and rs2686464—which included all the exons of FGF12. The results showed that the single‐nucleotide polymorphism rs1460922 in FGF12 was significantly associated with VT after adjusting for covariates of sex and age in 2 independent Chinese populations: adjusted P=0.015 (odds ratio: 1.54 [95% CI, 1.09–2.19]) in the discovery sample, adjusted P=0.018 (odds ratio: 1.64 [95% CI, 1.09–2.48]) in the replication sample, and adjusted P=2.52E‐04 (odds ratio: 1.59 [95% CI, 1.24–2.03]) in the combined sample. After resequencing all amino acid coding regions and untranslated regions of FGF12, 5 rare variations were identified. The result of western blotting revealed that a de novo functional variation, p.P211Q (1.84% of 163 patients with right ventricular outflow tract VT), could downregulate FGF12 expression significantly.

Conclusions

In this study, we observed that rs1460922 of FGF12 was significantly associated with VT and identified that a de novo variation of FGF12 may be an important genetic risk factor for the pathogenesis of VT.

Inducible Fgf13 ablation enhances caveolae-mediated cardioprotection during cardiac pressure overload

The fibroblast growth factor (FGF) homologous factor FGF13, a noncanonical FGF, has been best characterized as a voltage-gated Na+ channel auxiliary subunit. Other cellular functions have been suggested, but not explored. In inducible, cardiac-specific Fgf13 knockout mice, we found-even in the context of the expected reduction in Na+ channel current-an unanticipated protection from the maladaptive hypertrophic response to pressure overload. To uncover the underlying mechanisms, we searched for components of the FGF13 interactome in cardiomyocytes and discovered the complete set of the cavin family of caveolar coat proteins. Detailed biochemical investigations showed that FGF13 acts as a negative regulator of caveolae abundance in cardiomyocytes by controlling the relative distribution of cavin 1 between the sarcolemma and cytosol. In cardiac-specific Fgf13 knockout mice, cavin 1 redistribution to the sarcolemma stabilized the caveolar structural protein caveolin 3. The consequent increase in caveolae density afforded protection against pressure overload-induced cardiac dysfunction by two mechanisms: (i) enhancing cardioprotective signaling pathways enriched in caveolae, and (ii) increasing the caveolar membrane reserve available to buffer membrane tension. Thus, our results uncover unexpected roles for a FGF homologous factor and establish FGF13 as a regulator of caveolae-mediated mechanoprotection and adaptive hypertrophic signaling.

Sodium channel biophysics, late sodium current and genetic arrhythmic syndromes

Arrhythmias arise from breakdown of orderly action potential (AP) activation, propagation and recovery driven by interactive opening and closing of successive voltage-gated ion channels, in which one or more Na⁺ current components play critical parts. Early peak, Na⁺ currents (I_Na) reflecting channel activation drive the AP upstroke central to cellular activation and its propagation. Sustained late Na⁺ currents (I_Na-L) include contributions from a component with a delayed inactivation timecourse influencing AP duration (APD) and refractoriness, potentially causing pro-arrhythmic phenotypes. The magnitude of I_Na-L can be analysed through overlaps or otherwise in the overall voltage dependences of the steady-state properties and kinetics of activation and inactivation of the Na⁺ conductance. This was useful in analysing repetitive firing associated with paramyotonia congenita in skeletal muscle. Similarly, genetic cardiac Na⁺ channel abnormalities increasing I_Na-L are implicated in triggering phenomena of automaticity, early and delayed afterdepolarisations and arrhythmic substrate. This review illustrates a wide range of situations that may accentuate I_Na-L. These include (1) overlaps between steady-state activation and inactivation increasing window current, (2) kinetic deficiencies in Na⁺ channel inactivation leading to bursting phenomena associated with repetitive channel openings and (3) non-equilibrium gating processes causing channel re-opening due to more rapid recoveries from inactivation. All these biophysical possibilities were identified in a selection of abnormal human SCN5A genotypes. The latter presented as a broad range of clinical arrhythmic phenotypes, for which effective therapeutic intervention would require specific identification and targeting of the diverse electrophysiological abnormalities underlying their increased I_Na-L.

Conditional knockout of Fgf13 in murine hearts increases arrhythmia susceptibility and reveals novel ion channel modulatory roles

The intracellular fibroblast growth factors (iFGF/FHFs) bind directly to cardiac voltage gated Na⁺ channels, and modulate their function. Mutations that affect iFGF/FHF-Na⁺ channel interaction are associated with arrhythmia syndromes. Although suspected to modulate other ionic currents, such as Ca²⁺ channels based on acute knockdown experiments in isolated cardiomyocytes, the in vivo consequences of iFGF/FHF gene ablation on cardiac electrical activity are still unknown. We generated inducible, cardiomyocyte-restricted Fgf13 knockout mice to determine the resultant effects of Fgf13 gene ablation. Patch clamp recordings from ventricular myocytes isolated from Fgf13 knockout mice showed a ~25% reduction in peak Na⁺ channel current density and a hyperpolarizing shift in steady-state inactivation. Electrocardiograms on Fgf13 knockout mice showed a prolonged QRS duration. The Na⁺ channel blocker flecainide further prolonged QRS duration and triggered ventricular tachyarrhythmias only in Fgf13 knockout mice, suggesting that arrhythmia vulnerability resulted, at least in part, from a loss of functioning Na⁺ channels. Consistent with these effects on Na⁺ channels, action potentials in Fgf13 knockout mice, compared to Cre control mice, exhibited slower upstrokes and reduced amplitude, but unexpectedly had longer durations. We investigated candidate sources of the prolonged action potential durations in myocytes from Fgf13 knockout mice and found a reduction of the transient outward K⁺ current (I_to). Fgf13 knockout did not alter whole-cell protein levels of Kv4.2 and Kv4.3, the I_to pore-forming subunits, but did decrease Kv4.2 and Kv4.3 at the sarcolemma. No changes were seen in the sustained outward K⁺ current or voltage-gated Ca²⁺ current, other candidate contributors to the increased action potential duration. These results implicate that FGF13 is a critical cardiac Na⁺ channel modulator and Fgf13 knockout mice have increased arrhythmia susceptibility in the setting of Na⁺ channel blockade. The unanticipated effect on I_to revealed new FGF13 properties and the unexpected lack of an effect on voltage-gated Ca²⁺ channels highlight potential compensatory changes in vivo not readily revealed with acute Fgf13 knockdown in cultured cardiomyocytes.

The influence of sodium on pathophysiology of multiple sclerosis

Multiple sclerosis (MS) is a chronic, inflammatory, autoimmune disease of the central nervous system, and is an important cause of disability in young adults. In genetically susceptible individuals, several environmental factors may play a partial role in the pathogenesis of MS. Some studies suggests that high-salt diet (>5 g/day) may contribute to the MS and other autoimmune disease development through the induction of pathogenic Th17 cells and pro-inflammatory cytokines in both humans and mice. However, the precise mechanisms of pro-inflammatory effect of sodium chloride intake are not yet explained. The purpose of this review was to discuss the present state of knowledge on the potential role of environmental and dietary factors, particularly sodium chloride on the development and course of MS.

Cardiac Na Channels: Structure to Function

Heart rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. Opening of the primary cardiac voltage-gated sodium (NaV1.5) channel initiates cellular depolarization and the propagation of an electrical action potential that promotes coordinated contraction of the heart. The regularity of these contractile waves is critically important since it drives the primary function of the heart: to act as a pump that delivers blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. Perturbations to NaV1.5 may alter the structure, and hence the function, of the ion channel and are associated downstream with a wide variety of cardiac conduction pathologies, such as arrhythmias.

Current view on regulation of voltage-gated sodium channels by calcium and auxiliary proteins

In cardiac and skeletal myocytes, and in most neurons, the opening of voltage-gated Na(+) channels (NaV channels) triggers action potentials, a process that is regulated via the interactions of the channels' intercellular C-termini with auxiliary proteins and/or Ca(2+) . The molecular and structural details for how Ca(2+) and/or auxiliary proteins modulate NaV channel function, however, have eluded a concise mechanistic explanation and details have been shrouded for the last decade behind controversy about whether Ca(2+) acts directly upon the NaV channel or through interacting proteins, such as the Ca(2+) binding protein calmodulin (CaM). Here, we review recent advances in defining the structure of NaV intracellular C-termini and associated proteins such as CaM or fibroblast growth factor homologous factors (FHFs) to reveal new insights into how Ca(2+) affects NaV function, and how altered Ca(2+) -dependent or FHF-mediated regulation of NaV channels is perturbed in various disease states through mutations that disrupt CaM or FHF interaction.

FGF13 modulates the gating properties of the cardiac sodium channel Nav1.5 in an isoform-specific manner

FGF13 (FHF2), the major fibroblast growth factor homologous factor (FHF) in rodent heart, directly binds to the C-terminus of the main cardiac sodium channel, Na_V1.5. Knockdown of FGF13 in cardiomyocytes induces slowed ventricular conduction by altering Na_V1.5 function. FGF13 has five splice variants, each of which possess the same core region and C terminus but differing in their respective N termini. Whether and how these alternatively spliced N termini impart isoform-specific regulation of Na_V1.5, however, has not been reported. Here, we exploited a heterologous expression to explore the specific modulatory effects of FGF13 splice variants FGF13S, FGF13U and FGF13YV on Na_V1.5 function. We found these three splice variants differentially modulated Na_V1.5 current density. Although steady-state activation was unaltered by any of the FGF13 isoforms (compared to control cells expressing Nav1.5 but not expressing FGF13), open-state fast inactivation and closed-state fast inactivation were markedly slowed, steady-state availability was significantly shifted toward the depolarizing direction, and the window current was increased by each of FGF13 isoforms. Most strikingly, FGF13S hastened the rate of Na_V1.5 entry into the slow inactivation state and induced a dramatic slowing of recovery from inactivation, which caused a large decrease in current after either low or high frequency stimulation. Overall, these data showed the diversity of the roles of the FGF13 N-termini in Na_V1.5 channel modulation and suggested the importance of isoform-specific regulation.

Gain-of-function FHF1 mutation causes early-onset epileptic encephalopathy with cerebellar atrophy

OBJECTIVE

Voltage-gated sodium channel (Nav)-encoding genes are among early-onset epileptic encephalopathies (EOEE) targets, suggesting that other genes encoding Nav-binding proteins, such as fibroblast growth factor homologous factors (FHFs), may also play roles in these disorders.

METHODS

To identify additional genes for EOEE, we performed whole-exome sequencing in a family quintet with 2 siblings with a lethal disease characterized by EOEE and cerebellar atrophy. The pathogenic nature and functional consequences of the identified sequence alteration were determined by electrophysiologic studies in vitro and in vivo.

RESULTS

A de novo heterozygous missense mutation was identified in the FHF1 gene (FHF1AR114H, FHF1BR52H) in the 2 affected siblings. The mutant FHF1 proteins had a strong gain-of-function phenotype in transfected Neuro2A cells, enhancing the depolarizing shifts in Nav1.6 voltage-dependent fast inactivation, predicting increased neuronal excitability. Surprisingly, the gain-of-function effect is predicted to result from weaker interaction of mutant FHF1 with the Nav cytoplasmic tail. Transgenic overexpression of mutant FHF1B in zebrafish larvae enhanced epileptiform discharges, demonstrating the epileptic potential of this FHF1 mutation in the affected children.

CONCLUSIONS

Our data demonstrate that gain-of-function FHF mutations can cause neurologic disorder, and expand the repertoire of genetic causes (FHF1) and mechanisms (altered Nav gating) underlying EOEE and cerebellar atrophy.

Dysfunction of the β2-spectrin-based pathway in human heart failure

β2-Spectrin is critical for integrating membrane and cytoskeletal domains in excitable and nonexcitable cells. The role of β2-spectrin for vertebrate function is illustrated by dysfunction of β2-spectrin-based pathways in disease. Recently, defects in β2-spectrin association with protein partner ankyrin-B were identified in congenital forms of human arrhythmia. However, the role of β2-spectrin in common forms of acquired heart failure and arrhythmia is unknown. We report that β2-spectrin protein levels are significantly altered in human cardiovascular disease as well as in large and small animal cardiovascular disease models. Specifically, β2-spectrin levels were decreased in atrial samples of patients with atrial fibrillation compared with tissue from patients in sinus rhythm. Furthermore, compared with left ventricular samples from nonfailing hearts, β2-spectrin levels were significantly decreased in left ventricle of ischemic- and nonischemic heart failure patients. Left ventricle samples of canine and murine heart failure models confirm reduced β2-spectrin protein levels. Mechanistically, we identify that β2-spectrin levels are tightly regulated by posttranslational mechanisms, namely Ca(2+)- and calpain-dependent proteases. Furthermore, consistent with this data, we observed Ca(2+)- and calpain-dependent loss of β2-spectrin downstream effector proteins, including ankyrin-B in heart. In summary, our findings illustrate that β2-spectrin and downstream molecules are regulated in multiple forms of cardiovascular disease via Ca(2+)- and calpain-dependent proteolysis.

Identification of Amino Acid Residues in Fibroblast Growth Factor 14 (FGF14) Required for Structure-Function Interactions with Voltage-gated Sodium Channel Nav1.6

The voltage-gated Na(+) (Nav) channel provides the basis for electrical excitability in the brain. This channel is regulated by a number of accessory proteins including fibroblast growth factor 14 (FGF14), a member of the intracellular FGF family. In addition to forming homodimers, FGF14 binds directly to the Nav1.6 channel C-tail, regulating channel gating and expression, properties that are required for intrinsic excitability in neurons. Seeking amino acid residues with unique roles at the protein-protein interaction interface (PPI) of FGF14·Nav1.6, we engineered model-guided mutations of FGF14 and validated their impact on the FGF14·Nav1.6 complex and the FGF14:FGF14 dimer formation using a luciferase assay. Divergence was found in the β-9 sheet of FGF14 where an alanine (Ala) mutation of Val-160 impaired binding to Nav1.6 but had no effect on FGF14:FGF14 dimer formation. Additional analysis revealed also a key role of residues Lys-74/Ile-76 at the N-terminal of FGF14 in the FGF14·Nav1.6 complex and FGF14:FGF14 dimer formation. Using whole-cell patch clamp electrophysiology, we demonstrated that either the FGF14(V160A) or the FGF14(K74A/I76A) mutation was sufficient to abolish the FGF14-dependent regulation of peak transient Na(+) currents and the voltage-dependent activation and steady-state inactivation of Nav1.6; but only V160A with a concomitant alanine mutation at Tyr-158 could impede FGF14-dependent modulation of the channel fast inactivation. Intrinsic fluorescence spectroscopy of purified proteins confirmed a stronger binding reduction of FGF14(V160A) to the Nav1.6 C-tail compared with FGF14(K74A/I76A) Altogether these studies indicate that the β-9 sheet and the N terminus of FGF14 are well positioned targets for drug development of PPI-based allosteric modulators of Nav channels.