{"gene":"SCN4A","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":2003,"finding":"The V1442E mutation in SCN4A (Nav1.4) markedly enhances fast inactivation close to the resting potential and causes enhanced use-dependent inactivation on high-frequency stimulation, resulting in failure to generate action potentials and a myasthenic phenotype; demonstrated by expression of genetically engineered channels in HEK cells.","method":"Whole-cell patch clamp in HEK293 cells expressing mutant Nav1.4 channels; mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro functional reconstitution with mutagenesis in heterologous system, mechanistic link established","pmids":["12766226"],"is_preprint":false},{"year":1996,"finding":"Fast and slow inactivation of Skm1 (Nav1.4) are mechanistically coupled: abolishing fast inactivation by IFM1303QQQ mutation in the DIII-IV inactivation loop allows slow inactivation to occur more quickly and completely, demonstrating that fast inactivation reduces the probability of subsequent slow inactivation; slow inactivation remains intact even when fast inactivation is removed.","method":"On-cell macropatch recording from Xenopus oocytes co-expressing Skm1 alpha and beta1 subunits; site-directed mutagenesis of IFM motif","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in oocytes with mutagenesis, clear mechanistic dissection of two inactivation processes","pmids":["8968581"],"is_preprint":false},{"year":2008,"finding":"Calmodulin (CaM) associates constitutively with the C-terminus of Nav1.4 via the IQ motif in a conformation-independent manner (shown by FRET in mammalian cells). Mutation of the IQ motif reduces surface expression of Nav1.4 and eliminates CaM modulation of gating. A single CaM fused to the C-terminus is sufficient to modulate Na+ current and shift the voltage dependence of inactivation, establishing CaM as an ancillary subunit that facilitates channel trafficking and regulates inactivation.","method":"FRET, mutagenesis of IQ motif, CaM-fusion constructs with variable glycine linkers, whole-cell patch clamp in mammalian cells","journal":"The Journal of general physiology","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal methods (FRET, mutagenesis, fusion constructs) in single study with rigorous controls","pmids":["18270170"],"is_preprint":false},{"year":2012,"finding":"Single histidine substitutions of S4 arginine residues in each voltage sensor domain (VSD) of Nav1.4 produce proton leak currents (gating pore currents) that report the resting-state position of each S4 segment. DI/R1H, DII/R1H, and DIII/R2H produce leak currents indicating accessible positions, while DIV S4 residues do not produce appreciable proton currents due to a longer hydrophobic septum that prevents water from forming a proton conduction pathway, revealing domain-specific resting state topologies.","method":"Site-directed mutagenesis with histidine substitutions, whole-cell patch clamp proton current measurements, molecular dynamics simulations of resting-state VSD structures","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — functional mutagenesis combined with structural MD modeling, multiple domains characterized","pmids":["23134726"],"is_preprint":false},{"year":2008,"finding":"Three HypoPP-associated missense mutations at R666 (R672 in human) in the domain II S4 segment of Nav1.4 create accessory ionic permeation pathways (gating pore currents) permeable to K+ and Na+, exhibiting inward rectification and saturation consistent with a barrier model with a single cation binding site near the external surface; R666H creates a proton-selective gating pore. These low-amplitude inward currents at resting potential are proposed to cause pathological depolarization during paralytic attacks.","method":"Whole-cell voltage clamp of mutant Nav1.4 channels expressed in heterologous system; barrier model fitting","journal":"The Journal of general physiology","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with multiple mutants, biophysical modeling, mechanistic link to disease pathogenesis established","pmids":["18824591"],"is_preprint":false},{"year":2011,"finding":"Knock-in mice carrying the NaV1.4-R669H (ortholog of human R672H) HypoPP mutation show transient loss of muscle excitability and weakness under low-K+ challenge. Affected fibers exhibit an anomalous inward current at hyperpolarized potentials (gating pore current) consistent with a leaky voltage sensor as the trigger for paralytic attacks, plus reduced action potential amplitude indicating concurrent loss-of-function. Recovery is sensitive to Na+/K+-ATPase inhibition by ouabain.","method":"Knock-in mouse model; ex vivo muscle fiber recordings; low-K+ and high-K+ challenge; pharmacological ouabain treatment","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — knock-in model with defined electrophysiological phenotype and pharmacological validation, replicated gating pore mechanism in vivo","pmids":["21881211"],"is_preprint":false},{"year":2015,"finding":"Recessive loss-of-function SCN4A mutations causing fully non-functional or reduced-activity Nav1.4 channels are sufficient to cause severe congenital myopathy and fetal hypokinesia. In vitro functional assessment in HEK293 cells demonstrated that each mutation either abolished or markedly reduced channel activity, showing that different degrees of loss-of-function attenuate the skeletal muscle action potential amplitude below the threshold needed for normal muscle contraction.","method":"Whole exome sequencing; whole-cell patch clamp in HEK293 cells expressing mutant Nav1.4; clinical correlation","journal":"Brain : a journal of neurology","confidence":"High","confidence_rationale":"Tier 1-2 — functional reconstitution in multiple mutations from multiple families with clear mechanistic outcome","pmids":["26700687"],"is_preprint":false},{"year":2016,"finding":"Heterozygous Nav1.4 null mice (exon 12 deletion) have half-normal sodium current density in skeletal muscle, no compensatory upregulation of fetal Nav1.5, and manifest latent myasthenia with a right-shifted force-stimulus relation without periodic paralysis. Homozygous null mice die by postnatal day 2, establishing that Nav1.4 is essential for postnatal muscle excitability.","method":"Knockout mouse model (exon 12 deletion); ex vivo muscle electrophysiology; Western blot for Nav1.5","journal":"Brain : a journal of neurology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined electrophysiological and functional phenotype, clear loss-of-function consequence","pmids":["27048647"],"is_preprint":false},{"year":2003,"finding":"Veratridine (VTD) binds within the inner vestibule of Nav1.4 at a site that overlaps extensively with the local anesthetic receptor (residues N434 in D1-S6, L1280 in D3-S6, F1579 in D4-S6). Triple cysteine substitution (N434C/L1280C/F1579C) abolishes VTD-induced persistent opening but converts VTD into a high-affinity channel blocker, indicating receptor-bound VTD stabilizes the open conformation and reduces unitary conductance from the inner vestibule.","method":"Site-directed mutagenesis; whole-cell voltage clamp of mutant Nav1.4 in HEK cells; pharmacological characterization with VTD, amitriptyline, bupivacaine","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis plus pharmacological reconstitution defines binding site mechanism","pmids":["12626674"],"is_preprint":false},{"year":2003,"finding":"Residue L1280 in D3-S6 of Nav1.4 directly interacts with bupivacaine enantiomers in the inactivated channel state. Substitutions at L1280 with aromatic groups increase inactivated-state block, while positively charged substitutions decrease it; several substitutions (L1280E, L1280N, L1280Q, L1280R) confer stereoselectivity for R(+)-bupivacaine, the opposite of the N434R effect in D1-S6, indicating L1280 and N434 face each other in the pore.","method":"Site-directed mutagenesis; whole-cell voltage clamp of mutant Nav1.4 in HEK293T cells; pharmacological analysis with bupivacaine enantiomers","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with functional and pharmacological readouts defines binding site geometry","pmids":["12761351"],"is_preprint":false},{"year":2001,"finding":"A phenylalanine residue at D3-S6 position F1278 of Nav1.4 is critical for pyrethroid (deltamethrin) action. The double mutant I687M/F1278I renders channels greatly resistant to deltamethrin while retaining batrachotoxin (BTX) sensitivity, and the adjacent residues S1276 and L1280 are critical for BTX action, placing the deltamethrin and BTX receptors on opposite surfaces of the D3-S6 helix.","method":"Site-directed mutagenesis; whole-cell voltage clamp in HEK cells; pharmacological characterization with deltamethrin and batrachotoxin","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with multiple toxins defines distinct receptor sites on S6 helix","pmids":["11502895"],"is_preprint":false},{"year":2006,"finding":"The beta1 subunit modulates Nav1.4 gating by increasing negative surface charge density near the voltage-sensing machinery. Co-expression of beta1 potentiates the shift of half-activation potentials induced by changes in extracellular Ca2+ concentration without affecting Ca2+ binding to the open or closed pore, attributable to the highly sialylated nature of the beta1 subunit.","method":"Whole-cell patch clamp of Nav1.4 ± beta1 subunit in HEK293 cells; extracellular Ca2+ titration experiments","journal":"Experimental brain research","confidence":"Medium","confidence_rationale":"Tier 2 — clean functional comparison with defined mechanistic interpretation, single lab","pmids":["16432696"],"is_preprint":false},{"year":2008,"finding":"Co-expression of the beta1 subunit impedes slow inactivation of Nav1.4 in HEK cells, right-shifting the voltage dependence and hastening recovery from slow inactivation. This effect depends on the extracellular Ig-like domain of beta1 but is independent of its intracellular C-terminal tail and is independent of the negative coupling between fast and slow inactivation.","method":"Whole-cell patch clamp of Nav1.4 ± beta1 in HEK cells; beta1 domain deletion mutants","journal":"Pflugers Archiv : European journal of physiology","confidence":"Medium","confidence_rationale":"Tier 2 — domain-mapping mutagenesis combined with functional recordings, single lab","pmids":["18941776"],"is_preprint":false},{"year":2011,"finding":"Polysialic acid (polySia) and sialic acid (Sia) attached to Nav1.4 differentially regulate channel gating: loss of Sia shifts voltage-dependent activation and steady-state inactivation to more depolarized potentials and affects recovery from fast inactivation; loss of polySia shifts these parameters in the opposite direction. Both modifications act through electrostatic mechanisms at the channel surface.","method":"Expression of Nav1.4 in CHO cell mutants deficient in sialylation or polysialylation; whole-cell patch clamp; immunoblot gel shift analysis","journal":"Pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — genetically defined cell lines with functional electrophysiology and biochemical confirmation, single lab","pmids":["21606664"],"is_preprint":false},{"year":2014,"finding":"Sialic acids attached to both N-glycans and O-glycans within the Nav1.4 D1S5-S6 linker modulate channel gating through electrostatic mechanisms, with similar relative contributions from each glycan type. Preventing either N-glycan or O-glycan sialylation shifts all voltage-dependent parameters to intermediate values between full and no sialylation.","method":"Expression of Nav1.4 in CHO cell lines with selective loss of N-glycan or O-glycan sialylation; whole-cell patch clamp; immunoblot gel shift","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 — genetically defined cell systems with multiple parameters measured, single lab","pmids":["25450184"],"is_preprint":false},{"year":2003,"finding":"The P1158S mutation in SCN4A (Nav1.4) causes hyperpolarizing shifts in voltage dependence of both activation and inactivation at cold temperature and slows inactivation rate, mechanistically explaining both cold-induced paralysis (at low potassium) and myotonia (at normal potassium) as demonstrated by patch clamp and computer simulation.","method":"Amphotericin B perforated patch clamp at 22°C and 32°C in tsA201 cells; computer simulation incorporating mutant gating parameters","journal":"Neurology","confidence":"Medium","confidence_rationale":"Tier 1-2 — functional mutagenesis at two temperatures plus modeling, single lab","pmids":["14557559"],"is_preprint":false},{"year":2003,"finding":"The II-III loop of Nav1.4 (residue Ser-906) is important for slow inactivation: substitutions at S906 with varying hydrophobicity systematically shift steady-state slow inactivation voltage dependence and alter recovery kinetics, with hydrophobic residues enhancing and polar residues destabilizing the slow-inactivated state.","method":"Site-directed mutagenesis; whole-cell patch clamp in heterologous expression system","journal":"Pflugers Archiv : European journal of physiology","confidence":"Medium","confidence_rationale":"Tier 1 — systematic mutagenesis with functional readout, single lab","pmids":["12898257"],"is_preprint":false},{"year":2013,"finding":"Ca2+ and calmodulin regulate Nav1.4 inactivation gating through the EF-hand-like (EFL) region of the C-terminus. The myotonia mutation F1705I in human Nav1.4 renders inactivation gating Ca2+-sensitive (unlike wild-type human Nav1.4), slows INa decay in a Ca2+-dependent manner, and shifts the V1/2 of inactivation with CaM overexpression, with species differences in Ca2+ sensitivity attributable to sequence divergence in the EFL region.","method":"Whole-cell patch clamp of wild-type and mutant human and rat Nav1.4 in heterologous cells; Ca2+ and CaM manipulation experiments; mutagenesis","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 1-2 — functional comparison across species and mutants with mechanistic dissection, single lab","pmids":["24324661"],"is_preprint":false},{"year":2020,"finding":"Mutations in the EF hand-like motif of the Nav1.4 C-terminus (E1702del, E1702K, T1700_E1703del) impair fast inactivation as demonstrated by whole-cell patch clamp, establishing that the C-terminal EF hand-like region is important for regulating fast inactivation and that its disruption causes nondystrophic myotonia.","method":"Whole-cell patch clamp recording of heterologously expressed mutant Nav1.4 channels","journal":"Muscle & nerve","confidence":"Medium","confidence_rationale":"Tier 1 — functional reconstitution of three mutations at a defined structural motif, single lab","pmids":["32129495"],"is_preprint":false},{"year":2008,"finding":"The novel I141V mutation in the first transmembrane segment (S1) of domain I of Nav1.4 causes a -12.9 mV hyperpolarizing shift in voltage dependence of activation (approximately twofold increase in window current) and a -8.7 mV shift of slow inactivation with accelerated entry, establishing gain-of-function in activation as the mechanism of myotonia and enhanced slow inactivation as a protective factor against depolarization-induced paralysis.","method":"Whole-cell patch clamp of mutant Nav1.4 in HEK293 cells","journal":"Neurology","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro functional reconstitution with mutagenesis and multiple gating parameters, single lab","pmids":["19015483"],"is_preprint":false},{"year":2015,"finding":"The recessive p.R1454W mutation in SCN4A causes enhanced fast and slow inactivation, slower recovery from inactivated states, and frequency-dependent regulation of Nav1.4, resulting in loss-of-function and a combined myasthenic syndrome/periodic paralysis phenotype by decreasing channel availability for action potential generation at the neuromuscular junction.","method":"Whole-cell patch clamp in mammalian cell background (heterologous expression); genetic testing","journal":"Neurology","confidence":"Medium","confidence_rationale":"Tier 1 — functional reconstitution with multiple inactivation parameters measured, single lab","pmids":["26659129"],"is_preprint":false},{"year":2008,"finding":"Adenoviral overexpression of skeletal muscle Nav1.4 (SkM1) in depolarized epicardial border zones of infarcted canine myocardium restores action potential upstroke velocity (Vmax) at depolarized membrane potentials and reduces inducible sustained ventricular tachyarrhythmia, because Nav1.4 activates at more negative potentials than cardiac SCN5A and thus remains functional in partially depolarized tissue.","method":"Adenoviral gene transfer; in vivo epicardial mapping and programmed stimulation; in vitro microelectrode studies; immunohistochemistry; computational modeling","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods in vivo and in vitro, with computational validation; strong mechanistic demonstration","pmids":["19103989"],"is_preprint":false},{"year":2010,"finding":"Adenoviral expression of Nav1.4 (SkM1) in murine ventricular myocardium increased action potential Vmax and conduction velocity under hyperkalemic and ischemic conditions, and reduced incidence and duration of reperfusion ventricular tachycardia, demonstrating that Nav1.4's more negative activation threshold preserves conduction when cardiac Nav1.5 is inactivated by depolarization.","method":"Adenoviral injection; in vivo coronary ligation/reperfusion model; ex vivo multisite microelectrode mapping; Western blot; simulated ischemia (elevated K+, acidosis)","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 — in vivo plus in vitro functional demonstration with pharmacological and molecular validation, replicated concept from prior study","pmids":["20823275"],"is_preprint":false},{"year":2013,"finding":"Co-injection of HCN2 and Nav1.4 (SkM1) adenoviral constructs into left bundle branches of AV-blocked dogs produces stable biological pacemaker activity at physiological rates. Nav1.4 contributes by hyperpolarizing the action potential threshold in isolated LV preparations, improving the efficiency of HCN2-driven spontaneous depolarizations and allowing stable autonomous pacing without electronic backup.","method":"Adenoviral gene transfer in vivo; in vivo Holter/ECG monitoring; in vitro isolated LV action potential recordings; AP threshold measurements","journal":"Journal of the American College of Cardiology","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo functional demonstration plus in vitro mechanistic validation of threshold shift, single study","pmids":["23395072"],"is_preprint":false},{"year":2014,"finding":"A novel SCN4A mutation (p.I588V) in a patient with myotonia and periodic paralysis causes metabolic abnormalities in a mouse knock-in model (draggen mice, equivalent mutation p.I582V), including AMPK activation associated with immobility attacks, revealing that Nav1.4 dysfunction has downstream systemic metabolic consequences beyond direct electrophysiological effects.","method":"N-ethyl-N-nitrosourea mutagenesis mouse model; in vivo metabolic and behavioral characterization; AMPK activity measurements","journal":"Brain : a journal of neurology","confidence":"Medium","confidence_rationale":"Tier 2 — mouse knock-in model with multiple systemic readouts, novel downstream pathway identified, single lab","pmids":["25348630"],"is_preprint":false},{"year":2020,"finding":"The V445M missense mutation in Nav1.4 causes a hyperpolarizing shift of activation and inactivation curves with increased window current, and when co-expressed with the Navβ4 peptide, generates larger resurgent Na+ currents with a delayed time to peak compared to wild-type, providing a mechanism for repetitive firing and recurrent myotonia.","method":"Whole-cell patch clamp in CHO cells expressing mutant Nav1.4 ± Navβ4 peptide; analysis of transient, persistent, and resurgent Na+ currents","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 1 — functional reconstitution with Navβ4 co-expression dissecting resurgent current mechanism, single lab","pmids":["32276507"],"is_preprint":false},{"year":2015,"finding":"The SCN4A pore mutation p.Gly1537Ser segregates with essential tremor and epilepsy susceptibility in a large Spanish family. Patch clamp analysis revealed that this mutation produces more rapid kinetics at near-threshold potentials, alters ion selectivity to allow K+ and NH4+ conductance through the pore, with the aberrant cation flux proposed to contribute to tremor and increased seizure susceptibility.","method":"Whole exome sequencing; site-directed mutagenesis; patch clamp electrophysiology; mouse and human brain expression analysis","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 1-2 — functional mutagenesis with ion selectivity characterization and genetic segregation, single lab","pmids":["26427606"],"is_preprint":false},{"year":2015,"finding":"The SCN4A p.P72L mutation (in the cytoplasmic N-terminus of Nav1.4, a region with no previously reported mutations) causes a -5 mV hyperpolarizing shift of the voltage dependence of activation, increasing cell excitability and potentially enhancing the myotonic phenotype of a DM2 patient carrying this variant.","method":"Whole-cell voltage-clamp analysis in tsA201 cells expressing mutant Nav1.4","journal":"Neuromuscular disorders : NMD","confidence":"Low","confidence_rationale":"Tier 3 — single functional measurement, single lab, case report context","pmids":["25660391"],"is_preprint":false},{"year":2003,"finding":"The paramyotonia congenita SCN4A mutation R1448C is expressed in normal human heart (SCN4A alpha-subunit mRNA detected by RT-PCR), and affected family members show repolarization abnormalities on ECG, establishing that SCN4A is expressed in cardiac tissue and may contribute to cardiac electrophysiology.","method":"RT-PCR of human heart tissue; clinical ECG analysis in mutation carriers","journal":"Neurology","confidence":"Low","confidence_rationale":"Tier 3 — expression demonstrated by RT-PCR in heart, functional consequence inferred from clinical observation, no direct cardiac electrophysiology","pmids":["12552059"],"is_preprint":false}],"current_model":"Nav1.4 (encoded by SCN4A) is the principal voltage-gated sodium channel of adult skeletal muscle that initiates and propagates the action potential required for muscle contraction; its alpha-subunit contains four homologous domains each with a voltage-sensor S4 segment and pore-lining S6 segments, and undergoes distinct fast inactivation (mediated by the DIII-IV IFM loop docking onto the inner vestibule) and slow inactivation (regulated by the II-III loop at Ser-906 and negatively coupled to fast inactivation via the beta1 subunit's extracellular Ig domain); calmodulin binds constitutively to the C-terminal IQ motif to modulate inactivation voltage-dependence and facilitate surface trafficking; the C-terminal EF hand-like region further regulates fast inactivation in a Ca2+-sensitive manner; local anesthetic and veratridine binding sites overlap in the inner vestibule at D1-S6 (N434), D3-S6 (L1280, F1278), and D4-S6 residues; gating is additionally tuned by sialic acids on N- and O-glycans at the D1S5-S6 linker via electrostatic surface charge; gain-of-function mutations cause myotonia or periodic paralysis by impairing inactivation or enhancing activation window currents, while loss-of-function mutations (including null alleles) reduce action potential amplitude causing myasthenic syndrome or congenital myopathy, and specific S4 arginine-to-histidine/glycine/cysteine substitutions create pathological gating pore currents that depolarize the resting membrane and trigger hypokalemic paralytic attacks."},"narrative":{"teleology":[{"year":1996,"claim":"Establishing that fast and slow inactivation are mechanistically coupled resolved whether these were independent processes: removing fast inactivation (IFM→QQQ) paradoxically accelerated slow inactivation, showing that fast inactivation normally reduces slow inactivation probability.","evidence":"On-cell macropatch recording from Xenopus oocytes co-expressing Nav1.4 α and β1 with IFM motif mutagenesis","pmids":["8968581"],"confidence":"High","gaps":["Structural basis for the coupling between the DIII–IV loop and the slow inactivation gate was not resolved","Whether coupling is direct or allosteric through the pore domain was not determined"]},{"year":2001,"claim":"Mapping neurotoxin and insecticide binding sites on the D3-S6 helix revealed that pyrethroid (deltamethrin) and batrachotoxin receptors occupy opposite faces of the same helix, establishing domain-specific pharmacological topology of the inner pore.","evidence":"Systematic site-directed mutagenesis of D3-S6 residues with voltage clamp in HEK cells using deltamethrin and batrachotoxin","pmids":["11502895"],"confidence":"High","gaps":["No structural visualization of toxin-bound conformations","Contributions of other S6 segments to pyrethroid sensitivity were not fully addressed"]},{"year":2003,"claim":"Multiple studies converged to define the inner vestibule pharmacology and disease-linked gating mechanisms: veratridine and local anesthetics share overlapping binding determinants at N434/L1280/F1579; L1280 directly contacts bupivacaine enantiomers; S906 in the II–III loop tunes slow inactivation; and the V1442E mutation demonstrated that enhanced inactivation causes myasthenic loss-of-function while P1158S showed temperature-dependent gain-of-function causing cold-induced paralysis.","evidence":"Site-directed mutagenesis with voltage clamp in HEK cells; pharmacological characterization with VTD, bupivacaine enantiomers; computer simulations of mutant gating at different temperatures","pmids":["12626674","12761351","12898257","12766226","14557559"],"confidence":"High","gaps":["No high-resolution structure of the Nav1.4 inner vestibule to confirm residue orientations","Slow inactivation structural gate identity beyond S906 contribution remained unknown","Temperature-dependence mechanism of P1158S at the atomic level was not resolved"]},{"year":2006,"claim":"The β1 subunit's role in gating was linked to sialylation-dependent surface charge: β1 potentiates extracellular Ca²⁺-dependent shifts in activation through its highly sialylated extracellular domain, establishing that glycosylation of auxiliary subunits tunes voltage sensing.","evidence":"Whole-cell patch clamp of Nav1.4 ± β1 in HEK293 cells with Ca²⁺ titration","pmids":["16432696"],"confidence":"Medium","gaps":["Identity of specific sialylated residues on β1 responsible for the effect was not determined","Whether β1 sialylation affects slow inactivation coupling was not tested"]},{"year":2008,"claim":"Three advances reshaped understanding of Nav1.4 regulation and disease: calmodulin was established as a constitutive ancillary subunit binding the IQ motif to modulate inactivation and trafficking; the β1 Ig domain was shown to independently impede slow inactivation; and HypoPP-associated R666 mutations were demonstrated to create gating pore currents—anomalous cation leak through the voltage sensor domain—as the molecular basis of hypokalemic paralysis.","evidence":"FRET and CaM-fusion constructs in mammalian cells; β1 domain-deletion mutagenesis with patch clamp; voltage clamp of R666 mutants in heterologous cells with barrier model fitting","pmids":["18270170","18941776","18824591","19015483"],"confidence":"High","gaps":["Whether CaM binding is Ca²⁺-dependent at physiological [Ca²⁺] in muscle fibers was unclear","In vivo demonstration of gating pore currents had not yet been achieved","Whether β1 Ig domain contacts the pore or acts allosterically on slow inactivation was not resolved"]},{"year":2011,"claim":"Two findings extended the gating pore and glycosylation mechanisms: knock-in mice carrying R669H validated that gating pore currents cause paralytic attacks under low-K⁺ challenge in vivo, with concurrent loss-of-function reducing AP amplitude; and polysialic acid versus sialic acid were shown to shift gating in opposite directions through electrostatic mechanisms.","evidence":"Knock-in mouse model with ex vivo muscle recordings and pharmacological challenge; Nav1.4 expression in CHO sialylation-deficient mutant lines with patch clamp","pmids":["21881211","21606664"],"confidence":"High","gaps":["Mechanism by which Na⁺/K⁺-ATPase modulates recovery from paralysis was not fully dissected","Whether polySia is present on native muscle Nav1.4 at physiological levels was not confirmed"]},{"year":2012,"claim":"Histidine-scanning of S4 arginines across all four voltage-sensor domains revealed domain-specific resting-state topologies: DI, DII, and DIII S4 segments are accessible to proton conduction at rest, while DIV has a longer hydrophobic septum preventing gating pore formation, explaining why disease-causing gating pore mutations cluster in DI–DIII.","evidence":"Arginine-to-histidine mutagenesis in all four domains with proton current measurements and molecular dynamics simulations","pmids":["23134726"],"confidence":"High","gaps":["No experimental structure of the resting-state VSD to directly validate the MD models","Whether the DIV septum length difference is conserved across Nav isoforms was not tested"]},{"year":2013,"claim":"The C-terminal EF-hand-like region was identified as a Ca²⁺-sensitive regulator of inactivation: the myotonia mutation F1705I rendered inactivation Ca²⁺-dependent (unlike wild-type human Nav1.4), with species differences in the EFL explaining divergent Ca²⁺ sensitivity between human and rat channels.","evidence":"Patch clamp of wild-type and mutant human/rat Nav1.4 with Ca²⁺ and CaM manipulation in heterologous cells","pmids":["24324661"],"confidence":"Medium","gaps":["Whether native muscle fibers show EFL-mediated Ca²⁺-dependent inactivation changes was not demonstrated","Structural basis for species-specific Ca²⁺ sensitivity in the EFL was not resolved"]},{"year":2014,"claim":"Both N-glycan and O-glycan sialylation within the D1S5–S6 linker were shown to contribute comparably to electrostatic gating modulation, and a knock-in mouse model revealed that Nav1.4 dysfunction activates AMPK and produces systemic metabolic consequences beyond excitability defects.","evidence":"Nav1.4 expression in CHO lines with selective glycosylation deficiencies; ENU-mutagenesis knock-in mouse with metabolic characterization","pmids":["25450184","25348630"],"confidence":"Medium","gaps":["Whether AMPK activation is a direct consequence of reduced muscle activity or an independent signaling pathway was not resolved","Specific O-glycosylation sites on D1S5–S6 linker were not mapped"]},{"year":2015,"claim":"The phenotypic spectrum of SCN4A was extended in both directions: recessive null/hypomorphic mutations were shown to cause severe congenital myopathy and fetal hypokinesia through complete loss of channel function, while Nav1.4 knockout mice confirmed that homozygous loss is lethal and heterozygous loss produces latent myasthenia without compensatory Nav1.5 upregulation; separately, a pore mutation (G1537S) causing altered ion selectivity segregated with essential tremor and epilepsy.","evidence":"Whole-cell patch clamp of multiple loss-of-function mutants in HEK293 cells; exon-12-deletion knockout mice with ex vivo electrophysiology; whole-exome sequencing with functional characterization of G1537S","pmids":["26700687","27048647","26427606"],"confidence":"High","gaps":["Whether partial Nav1.4 loss-of-function is compensated by other sodium channels in specific muscle types was not tested","Brain expression of Nav1.4 and its contribution to tremor/epilepsy require direct electrophysiological confirmation"]},{"year":2020,"claim":"Further mechanistic refinement showed that EF-hand-like motif deletions/substitutions impair fast inactivation causing nondystrophic myotonia, and that Nav1.4 mutations can generate resurgent sodium currents (in the presence of Navβ4 peptide) providing a mechanism for repetitive firing in myotonia.","evidence":"Patch clamp of EFL mutants in heterologous cells; V445M mutant co-expressed with Navβ4 peptide in CHO cells analyzing resurgent currents","pmids":["32129495","32276507"],"confidence":"Medium","gaps":["Whether Navβ4-mediated resurgent currents occur in native skeletal muscle fibers is not established","Structure of the EFL in the context of the full-length channel is not resolved"]},{"year":null,"claim":"Despite extensive biophysical characterization, a high-resolution structure of the full-length human Nav1.4 in multiple gating states with bound CaM and β subunits is lacking; the structural basis of slow inactivation, the mechanism coupling EFL Ca²⁺ sensing to the inactivation gate, and whether Nav1.4 has functional roles in non-skeletal muscle tissues (heart, brain) remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No cryo-EM structure of human Nav1.4 in the slow-inactivated state","Mechanism linking gating pore currents to downstream metabolic (AMPK) activation is unknown","Functional significance of SCN4A expression in cardiac tissue is not established by direct electrophysiology"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,1,4,5,6,7,8]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[3,4]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,5,6,7,21,22]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,1,5,6,7]},{"term_id":"R-HSA-397014","term_label":"Muscle contraction","supporting_discovery_ids":[5,6,7]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,4,5,6,15,20]}],"complexes":[],"partners":["SCN1B","CALM1","HCN2","SCN4B"],"other_free_text":[]},"mechanistic_narrative":"SCN4A encodes Nav1.4, the principal voltage-gated sodium channel α-subunit of adult skeletal muscle, responsible for initiating and propagating action potentials required for muscle contraction. The channel undergoes mechanistically coupled fast inactivation (mediated by the DIII–IV IFM motif) and slow inactivation (regulated by the II–III loop at Ser-906 and modulated by the β1 subunit's extracellular Ig domain), with calmodulin constitutively bound at the C-terminal IQ motif to regulate inactivation voltage-dependence and surface trafficking, and sialic acids on N- and O-glycans electrostatically tuning gating [PMID:8968581, PMID:18270170, PMID:12898257, PMID:18941776, PMID:25450184]. Gain-of-function mutations that enhance activation window currents or impair inactivation cause myotonia or periodic paralysis, while loss-of-function mutations—including null alleles that are lethal in homozygous knockout mice—reduce action potential amplitude causing congenital myasthenic syndrome or myopathy; specific S4 arginine substitutions create pathological gating pore currents that depolarize resting membrane potential and trigger hypokalemic paralytic attacks [PMID:19015483, PMID:26700687, PMID:27048647, PMID:18824591, PMID:21881211]. The inner vestibule formed by D1-S6 (N434), D3-S6 (L1280, F1278), and D4-S6 residues constitutes overlapping binding sites for local anesthetics, veratridine, and pyrethroids [PMID:12626674, PMID:12761351, PMID:11502895]."},"prefetch_data":{"uniprot":{"accession":"P35499","full_name":"Sodium channel protein type 4 subunit alpha","aliases":["SkM1","Sodium channel protein skeletal muscle subunit alpha","Sodium channel protein type IV subunit alpha","Voltage-gated sodium channel subunit alpha Nav1.4"],"length_aa":1836,"mass_kda":208.1,"function":"Pore-forming subunit of Nav1.4, a voltage-gated sodium (Nav) channel that directly mediates the depolarizing phase of action potentials in excitable membranes. Navs, also called VGSCs (voltage-gated sodium channels) or VDSCs (voltage-dependent sodium channels), operate by switching between closed and open conformations depending on the voltage difference across the membrane. In the open conformation they allow Na(+) ions to selectively pass through the pore, along their electrochemical gradient. The influx of Na+ ions provokes membrane depolarization, initiating the propagation of electrical signals throughout cells and tissues (PubMed:12766226, PubMed:15318338, PubMed:16890191, PubMed:17898326, PubMed:18690054, PubMed:19347921, PubMed:25707578, PubMed:26659129, PubMed:26700687, PubMed:29992740, PubMed:30190309). 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SYNDROME, CONGENITAL, 16; CMS16","url":"https://www.omim.org/entry/614198"},{"mim_id":"613345","title":"HYPOKALEMIC PERIODIC PARALYSIS, TYPE 2; HOKPP2","url":"https://www.omim.org/entry/613345"},{"mim_id":"608390","title":"MYOTONIA, POTASSIUM-AGGRAVATED","url":"https://www.omim.org/entry/608390"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Golgi apparatus","reliability":"Uncertain"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"skeletal muscle","ntpm":128.7},{"tissue":"tongue","ntpm":50.4}],"url":"https://www.proteinatlas.org/search/SCN4A"},"hgnc":{"alias_symbol":["Nav1.4","HYPP","SkM1"],"prev_symbol":["HYKPP"]},"alphafold":{"accession":"P35499","domains":[{"cath_id":"-","chopping":"598-674","consensus_level":"medium","plddt":83.1932,"start":598,"end":674},{"cath_id":"1.10.287.70","chopping":"700-809","consensus_level":"high","plddt":84.1468,"start":700,"end":809},{"cath_id":"1.20.120.350","chopping":"1031-1140","consensus_level":"high","plddt":84.7598,"start":1031,"end":1140},{"cath_id":"1.10.287.70","chopping":"1144-1187_1224-1311","consensus_level":"medium","plddt":84.5264,"start":1144,"end":1311},{"cath_id":"1.20.120.350","chopping":"1338-1470","consensus_level":"medium","plddt":82.084,"start":1338,"end":1470},{"cath_id":"1.10.238.10","chopping":"1611-1728","consensus_level":"high","plddt":75.7139,"start":1611,"end":1728}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P35499","model_url":"https://alphafold.ebi.ac.uk/files/AF-P35499-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P35499-F1-predicted_aligned_error_v6.png","plddt_mean":72.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SCN4A","jax_strain_url":"https://www.jax.org/strain/search?query=SCN4A"},"sequence":{"accession":"P35499","fasta_url":"https://rest.uniprot.org/uniprotkb/P35499.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P35499/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P35499"}},"corpus_meta":[{"pmid":"11353725","id":"PMC_11353725","title":"Hypokalaemic 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K-562.","date":"2017","source":"Oncotarget","url":"https://pubmed.ncbi.nlm.nih.gov/28388554","citation_count":14,"is_preprint":false},{"pmid":"26256659","id":"PMC_26256659","title":"Whole-Body Muscle MRI in Patients with Hyperkalemic Periodic Paralysis Carrying the SCN4A Mutation T704M: Evidence for Chronic Progressive Myopathy with Selective Muscle Involvement.","date":"2015","source":"Journal of clinical neurology (Seoul, Korea)","url":"https://pubmed.ncbi.nlm.nih.gov/26256659","citation_count":14,"is_preprint":false},{"pmid":"30415163","id":"PMC_30415163","title":"A novel all-trans retinoic acid derivative inhibits proliferation and induces apoptosis of myelodysplastic syndromes cell line SKM-1 cells via up-regulating p53.","date":"2018","source":"International immunopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/30415163","citation_count":13,"is_preprint":false},{"pmid":"27415035","id":"PMC_27415035","title":"Sequence CLCN1 and SCN4A in patients with Nondystrophic myotonias in Chinese populations: Genetic and pedigree analysis of 10 families and review of the literature.","date":"2016","source":"Channels (Austin, Tex.)","url":"https://pubmed.ncbi.nlm.nih.gov/27415035","citation_count":13,"is_preprint":false},{"pmid":"36090556","id":"PMC_36090556","title":"Case report: Novel SCN4A variant associated with a severe congenital myasthenic syndrome/myopathy phenotype.","date":"2022","source":"Frontiers in pediatrics","url":"https://pubmed.ncbi.nlm.nih.gov/36090556","citation_count":12,"is_preprint":false},{"pmid":"19077043","id":"PMC_19077043","title":"Tubular aggregates in paralysis periodica paramyotonica with T704M mutation of SCN4A.","date":"2008","source":"Neuropathology : official journal of the Japanese Society of Neuropathology","url":"https://pubmed.ncbi.nlm.nih.gov/19077043","citation_count":11,"is_preprint":false},{"pmid":"30172468","id":"PMC_30172468","title":"Lower-extremity magnetic resonance imaging in patients with hyperkalemic periodic paralysis carrying the SCN4A mutation T704M: 30-month follow-up of seven patients.","date":"2018","source":"Neuromuscular disorders : NMD","url":"https://pubmed.ncbi.nlm.nih.gov/30172468","citation_count":11,"is_preprint":false},{"pmid":"1333556","id":"PMC_1333556","title":"The monocytic cell line SKM-1 strongly expresses the myeloperoxidase gene.","date":"1992","source":"Leukemia","url":"https://pubmed.ncbi.nlm.nih.gov/1333556","citation_count":11,"is_preprint":false},{"pmid":"32276507","id":"PMC_32276507","title":"Changes of Resurgent Na+ Currents in the Nav1.4 Channel Resulting from an SCN4A Mutation Contributing to Sodium Channel Myotonia.","date":"2020","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/32276507","citation_count":10,"is_preprint":false},{"pmid":"24968412","id":"PMC_24968412","title":"Vincristine-induced expression of P-glycoprotein in MOLM-13 and SKM-1 acute myeloid leukemia cell lines is associated with coexpression of nestin transcript.","date":"2014","source":"General physiology and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/24968412","citation_count":10,"is_preprint":false},{"pmid":"16801039","id":"PMC_16801039","title":"Paralysis periodica paramyotonica caused by SCN4A Arg1448Cys mutation.","date":"2006","source":"Journal of the Formosan Medical Association = Taiwan yi zhi","url":"https://pubmed.ncbi.nlm.nih.gov/16801039","citation_count":10,"is_preprint":false},{"pmid":"31609695","id":"PMC_31609695","title":"A Novel De Novo Heterozygous SCN4a Mutation Causing Congenital Myopathy, Myotonia and Multiple Congenital Anomalies.","date":"2019","source":"Journal of neuromuscular diseases","url":"https://pubmed.ncbi.nlm.nih.gov/31609695","citation_count":9,"is_preprint":false},{"pmid":"32129495","id":"PMC_32129495","title":"EF hand-like motif mutations of Nav1.4 C-terminus cause myotonic syndrome by impairing fast inactivation.","date":"2020","source":"Muscle & 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transition.","date":"2019","source":"Glycoconjugate journal","url":"https://pubmed.ncbi.nlm.nih.gov/31863309","citation_count":9,"is_preprint":false},{"pmid":"22617007","id":"PMC_22617007","title":"A sodium channel myotonia due to a novel SCN4A mutation accompanied by acquired autoimmune myasthenia gravis.","date":"2012","source":"Neuroscience letters","url":"https://pubmed.ncbi.nlm.nih.gov/22617007","citation_count":9,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":47509,"output_tokens":7375,"usd":0.126576},"stage2":{"model":"claude-opus-4-6","input_tokens":11047,"output_tokens":3608,"usd":0.218152},"total_usd":0.344728,"stage1_batch_id":"msgbatch_011rkcQdVV1RaFq9XhcG6df6","stage2_batch_id":"msgbatch_01RzgrJafdF4EWHhj5mNi6dh","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"The V1442E mutation in SCN4A (Nav1.4) markedly enhances fast inactivation close to the resting potential and causes enhanced use-dependent inactivation on high-frequency stimulation, resulting in failure to generate action potentials and a myasthenic phenotype; demonstrated by expression of genetically engineered channels in HEK cells.\",\n      \"method\": \"Whole-cell patch clamp in HEK293 cells expressing mutant Nav1.4 channels; mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro functional reconstitution with mutagenesis in heterologous system, mechanistic link established\",\n      \"pmids\": [\"12766226\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Fast and slow inactivation of Skm1 (Nav1.4) are mechanistically coupled: abolishing fast inactivation by IFM1303QQQ mutation in the DIII-IV inactivation loop allows slow inactivation to occur more quickly and completely, demonstrating that fast inactivation reduces the probability of subsequent slow inactivation; slow inactivation remains intact even when fast inactivation is removed.\",\n      \"method\": \"On-cell macropatch recording from Xenopus oocytes co-expressing Skm1 alpha and beta1 subunits; site-directed mutagenesis of IFM motif\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in oocytes with mutagenesis, clear mechanistic dissection of two inactivation processes\",\n      \"pmids\": [\"8968581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Calmodulin (CaM) associates constitutively with the C-terminus of Nav1.4 via the IQ motif in a conformation-independent manner (shown by FRET in mammalian cells). Mutation of the IQ motif reduces surface expression of Nav1.4 and eliminates CaM modulation of gating. A single CaM fused to the C-terminus is sufficient to modulate Na+ current and shift the voltage dependence of inactivation, establishing CaM as an ancillary subunit that facilitates channel trafficking and regulates inactivation.\",\n      \"method\": \"FRET, mutagenesis of IQ motif, CaM-fusion constructs with variable glycine linkers, whole-cell patch clamp in mammalian cells\",\n      \"journal\": \"The Journal of general physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal methods (FRET, mutagenesis, fusion constructs) in single study with rigorous controls\",\n      \"pmids\": [\"18270170\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Single histidine substitutions of S4 arginine residues in each voltage sensor domain (VSD) of Nav1.4 produce proton leak currents (gating pore currents) that report the resting-state position of each S4 segment. DI/R1H, DII/R1H, and DIII/R2H produce leak currents indicating accessible positions, while DIV S4 residues do not produce appreciable proton currents due to a longer hydrophobic septum that prevents water from forming a proton conduction pathway, revealing domain-specific resting state topologies.\",\n      \"method\": \"Site-directed mutagenesis with histidine substitutions, whole-cell patch clamp proton current measurements, molecular dynamics simulations of resting-state VSD structures\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — functional mutagenesis combined with structural MD modeling, multiple domains characterized\",\n      \"pmids\": [\"23134726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Three HypoPP-associated missense mutations at R666 (R672 in human) in the domain II S4 segment of Nav1.4 create accessory ionic permeation pathways (gating pore currents) permeable to K+ and Na+, exhibiting inward rectification and saturation consistent with a barrier model with a single cation binding site near the external surface; R666H creates a proton-selective gating pore. These low-amplitude inward currents at resting potential are proposed to cause pathological depolarization during paralytic attacks.\",\n      \"method\": \"Whole-cell voltage clamp of mutant Nav1.4 channels expressed in heterologous system; barrier model fitting\",\n      \"journal\": \"The Journal of general physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with multiple mutants, biophysical modeling, mechanistic link to disease pathogenesis established\",\n      \"pmids\": [\"18824591\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Knock-in mice carrying the NaV1.4-R669H (ortholog of human R672H) HypoPP mutation show transient loss of muscle excitability and weakness under low-K+ challenge. Affected fibers exhibit an anomalous inward current at hyperpolarized potentials (gating pore current) consistent with a leaky voltage sensor as the trigger for paralytic attacks, plus reduced action potential amplitude indicating concurrent loss-of-function. Recovery is sensitive to Na+/K+-ATPase inhibition by ouabain.\",\n      \"method\": \"Knock-in mouse model; ex vivo muscle fiber recordings; low-K+ and high-K+ challenge; pharmacological ouabain treatment\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knock-in model with defined electrophysiological phenotype and pharmacological validation, replicated gating pore mechanism in vivo\",\n      \"pmids\": [\"21881211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Recessive loss-of-function SCN4A mutations causing fully non-functional or reduced-activity Nav1.4 channels are sufficient to cause severe congenital myopathy and fetal hypokinesia. In vitro functional assessment in HEK293 cells demonstrated that each mutation either abolished or markedly reduced channel activity, showing that different degrees of loss-of-function attenuate the skeletal muscle action potential amplitude below the threshold needed for normal muscle contraction.\",\n      \"method\": \"Whole exome sequencing; whole-cell patch clamp in HEK293 cells expressing mutant Nav1.4; clinical correlation\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — functional reconstitution in multiple mutations from multiple families with clear mechanistic outcome\",\n      \"pmids\": [\"26700687\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Heterozygous Nav1.4 null mice (exon 12 deletion) have half-normal sodium current density in skeletal muscle, no compensatory upregulation of fetal Nav1.5, and manifest latent myasthenia with a right-shifted force-stimulus relation without periodic paralysis. Homozygous null mice die by postnatal day 2, establishing that Nav1.4 is essential for postnatal muscle excitability.\",\n      \"method\": \"Knockout mouse model (exon 12 deletion); ex vivo muscle electrophysiology; Western blot for Nav1.5\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined electrophysiological and functional phenotype, clear loss-of-function consequence\",\n      \"pmids\": [\"27048647\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Veratridine (VTD) binds within the inner vestibule of Nav1.4 at a site that overlaps extensively with the local anesthetic receptor (residues N434 in D1-S6, L1280 in D3-S6, F1579 in D4-S6). Triple cysteine substitution (N434C/L1280C/F1579C) abolishes VTD-induced persistent opening but converts VTD into a high-affinity channel blocker, indicating receptor-bound VTD stabilizes the open conformation and reduces unitary conductance from the inner vestibule.\",\n      \"method\": \"Site-directed mutagenesis; whole-cell voltage clamp of mutant Nav1.4 in HEK cells; pharmacological characterization with VTD, amitriptyline, bupivacaine\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis plus pharmacological reconstitution defines binding site mechanism\",\n      \"pmids\": [\"12626674\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Residue L1280 in D3-S6 of Nav1.4 directly interacts with bupivacaine enantiomers in the inactivated channel state. Substitutions at L1280 with aromatic groups increase inactivated-state block, while positively charged substitutions decrease it; several substitutions (L1280E, L1280N, L1280Q, L1280R) confer stereoselectivity for R(+)-bupivacaine, the opposite of the N434R effect in D1-S6, indicating L1280 and N434 face each other in the pore.\",\n      \"method\": \"Site-directed mutagenesis; whole-cell voltage clamp of mutant Nav1.4 in HEK293T cells; pharmacological analysis with bupivacaine enantiomers\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with functional and pharmacological readouts defines binding site geometry\",\n      \"pmids\": [\"12761351\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A phenylalanine residue at D3-S6 position F1278 of Nav1.4 is critical for pyrethroid (deltamethrin) action. The double mutant I687M/F1278I renders channels greatly resistant to deltamethrin while retaining batrachotoxin (BTX) sensitivity, and the adjacent residues S1276 and L1280 are critical for BTX action, placing the deltamethrin and BTX receptors on opposite surfaces of the D3-S6 helix.\",\n      \"method\": \"Site-directed mutagenesis; whole-cell voltage clamp in HEK cells; pharmacological characterization with deltamethrin and batrachotoxin\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with multiple toxins defines distinct receptor sites on S6 helix\",\n      \"pmids\": [\"11502895\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The beta1 subunit modulates Nav1.4 gating by increasing negative surface charge density near the voltage-sensing machinery. Co-expression of beta1 potentiates the shift of half-activation potentials induced by changes in extracellular Ca2+ concentration without affecting Ca2+ binding to the open or closed pore, attributable to the highly sialylated nature of the beta1 subunit.\",\n      \"method\": \"Whole-cell patch clamp of Nav1.4 ± beta1 subunit in HEK293 cells; extracellular Ca2+ titration experiments\",\n      \"journal\": \"Experimental brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean functional comparison with defined mechanistic interpretation, single lab\",\n      \"pmids\": [\"16432696\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Co-expression of the beta1 subunit impedes slow inactivation of Nav1.4 in HEK cells, right-shifting the voltage dependence and hastening recovery from slow inactivation. This effect depends on the extracellular Ig-like domain of beta1 but is independent of its intracellular C-terminal tail and is independent of the negative coupling between fast and slow inactivation.\",\n      \"method\": \"Whole-cell patch clamp of Nav1.4 ± beta1 in HEK cells; beta1 domain deletion mutants\",\n      \"journal\": \"Pflugers Archiv : European journal of physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain-mapping mutagenesis combined with functional recordings, single lab\",\n      \"pmids\": [\"18941776\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Polysialic acid (polySia) and sialic acid (Sia) attached to Nav1.4 differentially regulate channel gating: loss of Sia shifts voltage-dependent activation and steady-state inactivation to more depolarized potentials and affects recovery from fast inactivation; loss of polySia shifts these parameters in the opposite direction. Both modifications act through electrostatic mechanisms at the channel surface.\",\n      \"method\": \"Expression of Nav1.4 in CHO cell mutants deficient in sialylation or polysialylation; whole-cell patch clamp; immunoblot gel shift analysis\",\n      \"journal\": \"Pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetically defined cell lines with functional electrophysiology and biochemical confirmation, single lab\",\n      \"pmids\": [\"21606664\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Sialic acids attached to both N-glycans and O-glycans within the Nav1.4 D1S5-S6 linker modulate channel gating through electrostatic mechanisms, with similar relative contributions from each glycan type. Preventing either N-glycan or O-glycan sialylation shifts all voltage-dependent parameters to intermediate values between full and no sialylation.\",\n      \"method\": \"Expression of Nav1.4 in CHO cell lines with selective loss of N-glycan or O-glycan sialylation; whole-cell patch clamp; immunoblot gel shift\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetically defined cell systems with multiple parameters measured, single lab\",\n      \"pmids\": [\"25450184\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The P1158S mutation in SCN4A (Nav1.4) causes hyperpolarizing shifts in voltage dependence of both activation and inactivation at cold temperature and slows inactivation rate, mechanistically explaining both cold-induced paralysis (at low potassium) and myotonia (at normal potassium) as demonstrated by patch clamp and computer simulation.\",\n      \"method\": \"Amphotericin B perforated patch clamp at 22°C and 32°C in tsA201 cells; computer simulation incorporating mutant gating parameters\",\n      \"journal\": \"Neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — functional mutagenesis at two temperatures plus modeling, single lab\",\n      \"pmids\": [\"14557559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The II-III loop of Nav1.4 (residue Ser-906) is important for slow inactivation: substitutions at S906 with varying hydrophobicity systematically shift steady-state slow inactivation voltage dependence and alter recovery kinetics, with hydrophobic residues enhancing and polar residues destabilizing the slow-inactivated state.\",\n      \"method\": \"Site-directed mutagenesis; whole-cell patch clamp in heterologous expression system\",\n      \"journal\": \"Pflugers Archiv : European journal of physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with functional readout, single lab\",\n      \"pmids\": [\"12898257\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Ca2+ and calmodulin regulate Nav1.4 inactivation gating through the EF-hand-like (EFL) region of the C-terminus. The myotonia mutation F1705I in human Nav1.4 renders inactivation gating Ca2+-sensitive (unlike wild-type human Nav1.4), slows INa decay in a Ca2+-dependent manner, and shifts the V1/2 of inactivation with CaM overexpression, with species differences in Ca2+ sensitivity attributable to sequence divergence in the EFL region.\",\n      \"method\": \"Whole-cell patch clamp of wild-type and mutant human and rat Nav1.4 in heterologous cells; Ca2+ and CaM manipulation experiments; mutagenesis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — functional comparison across species and mutants with mechanistic dissection, single lab\",\n      \"pmids\": [\"24324661\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Mutations in the EF hand-like motif of the Nav1.4 C-terminus (E1702del, E1702K, T1700_E1703del) impair fast inactivation as demonstrated by whole-cell patch clamp, establishing that the C-terminal EF hand-like region is important for regulating fast inactivation and that its disruption causes nondystrophic myotonia.\",\n      \"method\": \"Whole-cell patch clamp recording of heterologously expressed mutant Nav1.4 channels\",\n      \"journal\": \"Muscle & nerve\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — functional reconstitution of three mutations at a defined structural motif, single lab\",\n      \"pmids\": [\"32129495\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The novel I141V mutation in the first transmembrane segment (S1) of domain I of Nav1.4 causes a -12.9 mV hyperpolarizing shift in voltage dependence of activation (approximately twofold increase in window current) and a -8.7 mV shift of slow inactivation with accelerated entry, establishing gain-of-function in activation as the mechanism of myotonia and enhanced slow inactivation as a protective factor against depolarization-induced paralysis.\",\n      \"method\": \"Whole-cell patch clamp of mutant Nav1.4 in HEK293 cells\",\n      \"journal\": \"Neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro functional reconstitution with mutagenesis and multiple gating parameters, single lab\",\n      \"pmids\": [\"19015483\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The recessive p.R1454W mutation in SCN4A causes enhanced fast and slow inactivation, slower recovery from inactivated states, and frequency-dependent regulation of Nav1.4, resulting in loss-of-function and a combined myasthenic syndrome/periodic paralysis phenotype by decreasing channel availability for action potential generation at the neuromuscular junction.\",\n      \"method\": \"Whole-cell patch clamp in mammalian cell background (heterologous expression); genetic testing\",\n      \"journal\": \"Neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — functional reconstitution with multiple inactivation parameters measured, single lab\",\n      \"pmids\": [\"26659129\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Adenoviral overexpression of skeletal muscle Nav1.4 (SkM1) in depolarized epicardial border zones of infarcted canine myocardium restores action potential upstroke velocity (Vmax) at depolarized membrane potentials and reduces inducible sustained ventricular tachyarrhythmia, because Nav1.4 activates at more negative potentials than cardiac SCN5A and thus remains functional in partially depolarized tissue.\",\n      \"method\": \"Adenoviral gene transfer; in vivo epicardial mapping and programmed stimulation; in vitro microelectrode studies; immunohistochemistry; computational modeling\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in vivo and in vitro, with computational validation; strong mechanistic demonstration\",\n      \"pmids\": [\"19103989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Adenoviral expression of Nav1.4 (SkM1) in murine ventricular myocardium increased action potential Vmax and conduction velocity under hyperkalemic and ischemic conditions, and reduced incidence and duration of reperfusion ventricular tachycardia, demonstrating that Nav1.4's more negative activation threshold preserves conduction when cardiac Nav1.5 is inactivated by depolarization.\",\n      \"method\": \"Adenoviral injection; in vivo coronary ligation/reperfusion model; ex vivo multisite microelectrode mapping; Western blot; simulated ischemia (elevated K+, acidosis)\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo plus in vitro functional demonstration with pharmacological and molecular validation, replicated concept from prior study\",\n      \"pmids\": [\"20823275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Co-injection of HCN2 and Nav1.4 (SkM1) adenoviral constructs into left bundle branches of AV-blocked dogs produces stable biological pacemaker activity at physiological rates. Nav1.4 contributes by hyperpolarizing the action potential threshold in isolated LV preparations, improving the efficiency of HCN2-driven spontaneous depolarizations and allowing stable autonomous pacing without electronic backup.\",\n      \"method\": \"Adenoviral gene transfer in vivo; in vivo Holter/ECG monitoring; in vitro isolated LV action potential recordings; AP threshold measurements\",\n      \"journal\": \"Journal of the American College of Cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo functional demonstration plus in vitro mechanistic validation of threshold shift, single study\",\n      \"pmids\": [\"23395072\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A novel SCN4A mutation (p.I588V) in a patient with myotonia and periodic paralysis causes metabolic abnormalities in a mouse knock-in model (draggen mice, equivalent mutation p.I582V), including AMPK activation associated with immobility attacks, revealing that Nav1.4 dysfunction has downstream systemic metabolic consequences beyond direct electrophysiological effects.\",\n      \"method\": \"N-ethyl-N-nitrosourea mutagenesis mouse model; in vivo metabolic and behavioral characterization; AMPK activity measurements\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mouse knock-in model with multiple systemic readouts, novel downstream pathway identified, single lab\",\n      \"pmids\": [\"25348630\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The V445M missense mutation in Nav1.4 causes a hyperpolarizing shift of activation and inactivation curves with increased window current, and when co-expressed with the Navβ4 peptide, generates larger resurgent Na+ currents with a delayed time to peak compared to wild-type, providing a mechanism for repetitive firing and recurrent myotonia.\",\n      \"method\": \"Whole-cell patch clamp in CHO cells expressing mutant Nav1.4 ± Navβ4 peptide; analysis of transient, persistent, and resurgent Na+ currents\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — functional reconstitution with Navβ4 co-expression dissecting resurgent current mechanism, single lab\",\n      \"pmids\": [\"32276507\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The SCN4A pore mutation p.Gly1537Ser segregates with essential tremor and epilepsy susceptibility in a large Spanish family. Patch clamp analysis revealed that this mutation produces more rapid kinetics at near-threshold potentials, alters ion selectivity to allow K+ and NH4+ conductance through the pore, with the aberrant cation flux proposed to contribute to tremor and increased seizure susceptibility.\",\n      \"method\": \"Whole exome sequencing; site-directed mutagenesis; patch clamp electrophysiology; mouse and human brain expression analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — functional mutagenesis with ion selectivity characterization and genetic segregation, single lab\",\n      \"pmids\": [\"26427606\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The SCN4A p.P72L mutation (in the cytoplasmic N-terminus of Nav1.4, a region with no previously reported mutations) causes a -5 mV hyperpolarizing shift of the voltage dependence of activation, increasing cell excitability and potentially enhancing the myotonic phenotype of a DM2 patient carrying this variant.\",\n      \"method\": \"Whole-cell voltage-clamp analysis in tsA201 cells expressing mutant Nav1.4\",\n      \"journal\": \"Neuromuscular disorders : NMD\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single functional measurement, single lab, case report context\",\n      \"pmids\": [\"25660391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The paramyotonia congenita SCN4A mutation R1448C is expressed in normal human heart (SCN4A alpha-subunit mRNA detected by RT-PCR), and affected family members show repolarization abnormalities on ECG, establishing that SCN4A is expressed in cardiac tissue and may contribute to cardiac electrophysiology.\",\n      \"method\": \"RT-PCR of human heart tissue; clinical ECG analysis in mutation carriers\",\n      \"journal\": \"Neurology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — expression demonstrated by RT-PCR in heart, functional consequence inferred from clinical observation, no direct cardiac electrophysiology\",\n      \"pmids\": [\"12552059\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Nav1.4 (encoded by SCN4A) is the principal voltage-gated sodium channel of adult skeletal muscle that initiates and propagates the action potential required for muscle contraction; its alpha-subunit contains four homologous domains each with a voltage-sensor S4 segment and pore-lining S6 segments, and undergoes distinct fast inactivation (mediated by the DIII-IV IFM loop docking onto the inner vestibule) and slow inactivation (regulated by the II-III loop at Ser-906 and negatively coupled to fast inactivation via the beta1 subunit's extracellular Ig domain); calmodulin binds constitutively to the C-terminal IQ motif to modulate inactivation voltage-dependence and facilitate surface trafficking; the C-terminal EF hand-like region further regulates fast inactivation in a Ca2+-sensitive manner; local anesthetic and veratridine binding sites overlap in the inner vestibule at D1-S6 (N434), D3-S6 (L1280, F1278), and D4-S6 residues; gating is additionally tuned by sialic acids on N- and O-glycans at the D1S5-S6 linker via electrostatic surface charge; gain-of-function mutations cause myotonia or periodic paralysis by impairing inactivation or enhancing activation window currents, while loss-of-function mutations (including null alleles) reduce action potential amplitude causing myasthenic syndrome or congenital myopathy, and specific S4 arginine-to-histidine/glycine/cysteine substitutions create pathological gating pore currents that depolarize the resting membrane and trigger hypokalemic paralytic attacks.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SCN4A encodes Nav1.4, the principal voltage-gated sodium channel α-subunit of adult skeletal muscle, responsible for initiating and propagating action potentials required for muscle contraction. The channel undergoes mechanistically coupled fast inactivation (mediated by the DIII–IV IFM motif) and slow inactivation (regulated by the II–III loop at Ser-906 and modulated by the β1 subunit's extracellular Ig domain), with calmodulin constitutively bound at the C-terminal IQ motif to regulate inactivation voltage-dependence and surface trafficking, and sialic acids on N- and O-glycans electrostatically tuning gating [PMID:8968581, PMID:18270170, PMID:12898257, PMID:18941776, PMID:25450184]. Gain-of-function mutations that enhance activation window currents or impair inactivation cause myotonia or periodic paralysis, while loss-of-function mutations—including null alleles that are lethal in homozygous knockout mice—reduce action potential amplitude causing congenital myasthenic syndrome or myopathy; specific S4 arginine substitutions create pathological gating pore currents that depolarize resting membrane potential and trigger hypokalemic paralytic attacks [PMID:19015483, PMID:26700687, PMID:27048647, PMID:18824591, PMID:21881211]. The inner vestibule formed by D1-S6 (N434), D3-S6 (L1280, F1278), and D4-S6 residues constitutes overlapping binding sites for local anesthetics, veratridine, and pyrethroids [PMID:12626674, PMID:12761351, PMID:11502895].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing that fast and slow inactivation are mechanistically coupled resolved whether these were independent processes: removing fast inactivation (IFM→QQQ) paradoxically accelerated slow inactivation, showing that fast inactivation normally reduces slow inactivation probability.\",\n      \"evidence\": \"On-cell macropatch recording from Xenopus oocytes co-expressing Nav1.4 α and β1 with IFM motif mutagenesis\",\n      \"pmids\": [\"8968581\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for the coupling between the DIII–IV loop and the slow inactivation gate was not resolved\", \"Whether coupling is direct or allosteric through the pore domain was not determined\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Mapping neurotoxin and insecticide binding sites on the D3-S6 helix revealed that pyrethroid (deltamethrin) and batrachotoxin receptors occupy opposite faces of the same helix, establishing domain-specific pharmacological topology of the inner pore.\",\n      \"evidence\": \"Systematic site-directed mutagenesis of D3-S6 residues with voltage clamp in HEK cells using deltamethrin and batrachotoxin\",\n      \"pmids\": [\"11502895\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural visualization of toxin-bound conformations\", \"Contributions of other S6 segments to pyrethroid sensitivity were not fully addressed\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Multiple studies converged to define the inner vestibule pharmacology and disease-linked gating mechanisms: veratridine and local anesthetics share overlapping binding determinants at N434/L1280/F1579; L1280 directly contacts bupivacaine enantiomers; S906 in the II–III loop tunes slow inactivation; and the V1442E mutation demonstrated that enhanced inactivation causes myasthenic loss-of-function while P1158S showed temperature-dependent gain-of-function causing cold-induced paralysis.\",\n      \"evidence\": \"Site-directed mutagenesis with voltage clamp in HEK cells; pharmacological characterization with VTD, bupivacaine enantiomers; computer simulations of mutant gating at different temperatures\",\n      \"pmids\": [\"12626674\", \"12761351\", \"12898257\", \"12766226\", \"14557559\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of the Nav1.4 inner vestibule to confirm residue orientations\", \"Slow inactivation structural gate identity beyond S906 contribution remained unknown\", \"Temperature-dependence mechanism of P1158S at the atomic level was not resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"The β1 subunit's role in gating was linked to sialylation-dependent surface charge: β1 potentiates extracellular Ca²⁺-dependent shifts in activation through its highly sialylated extracellular domain, establishing that glycosylation of auxiliary subunits tunes voltage sensing.\",\n      \"evidence\": \"Whole-cell patch clamp of Nav1.4 ± β1 in HEK293 cells with Ca²⁺ titration\",\n      \"pmids\": [\"16432696\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of specific sialylated residues on β1 responsible for the effect was not determined\", \"Whether β1 sialylation affects slow inactivation coupling was not tested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Three advances reshaped understanding of Nav1.4 regulation and disease: calmodulin was established as a constitutive ancillary subunit binding the IQ motif to modulate inactivation and trafficking; the β1 Ig domain was shown to independently impede slow inactivation; and HypoPP-associated R666 mutations were demonstrated to create gating pore currents—anomalous cation leak through the voltage sensor domain—as the molecular basis of hypokalemic paralysis.\",\n      \"evidence\": \"FRET and CaM-fusion constructs in mammalian cells; β1 domain-deletion mutagenesis with patch clamp; voltage clamp of R666 mutants in heterologous cells with barrier model fitting\",\n      \"pmids\": [\"18270170\", \"18941776\", \"18824591\", \"19015483\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CaM binding is Ca²⁺-dependent at physiological [Ca²⁺] in muscle fibers was unclear\", \"In vivo demonstration of gating pore currents had not yet been achieved\", \"Whether β1 Ig domain contacts the pore or acts allosterically on slow inactivation was not resolved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Two findings extended the gating pore and glycosylation mechanisms: knock-in mice carrying R669H validated that gating pore currents cause paralytic attacks under low-K⁺ challenge in vivo, with concurrent loss-of-function reducing AP amplitude; and polysialic acid versus sialic acid were shown to shift gating in opposite directions through electrostatic mechanisms.\",\n      \"evidence\": \"Knock-in mouse model with ex vivo muscle recordings and pharmacological challenge; Nav1.4 expression in CHO sialylation-deficient mutant lines with patch clamp\",\n      \"pmids\": [\"21881211\", \"21606664\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which Na⁺/K⁺-ATPase modulates recovery from paralysis was not fully dissected\", \"Whether polySia is present on native muscle Nav1.4 at physiological levels was not confirmed\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Histidine-scanning of S4 arginines across all four voltage-sensor domains revealed domain-specific resting-state topologies: DI, DII, and DIII S4 segments are accessible to proton conduction at rest, while DIV has a longer hydrophobic septum preventing gating pore formation, explaining why disease-causing gating pore mutations cluster in DI–DIII.\",\n      \"evidence\": \"Arginine-to-histidine mutagenesis in all four domains with proton current measurements and molecular dynamics simulations\",\n      \"pmids\": [\"23134726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental structure of the resting-state VSD to directly validate the MD models\", \"Whether the DIV septum length difference is conserved across Nav isoforms was not tested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The C-terminal EF-hand-like region was identified as a Ca²⁺-sensitive regulator of inactivation: the myotonia mutation F1705I rendered inactivation Ca²⁺-dependent (unlike wild-type human Nav1.4), with species differences in the EFL explaining divergent Ca²⁺ sensitivity between human and rat channels.\",\n      \"evidence\": \"Patch clamp of wild-type and mutant human/rat Nav1.4 with Ca²⁺ and CaM manipulation in heterologous cells\",\n      \"pmids\": [\"24324661\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether native muscle fibers show EFL-mediated Ca²⁺-dependent inactivation changes was not demonstrated\", \"Structural basis for species-specific Ca²⁺ sensitivity in the EFL was not resolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Both N-glycan and O-glycan sialylation within the D1S5–S6 linker were shown to contribute comparably to electrostatic gating modulation, and a knock-in mouse model revealed that Nav1.4 dysfunction activates AMPK and produces systemic metabolic consequences beyond excitability defects.\",\n      \"evidence\": \"Nav1.4 expression in CHO lines with selective glycosylation deficiencies; ENU-mutagenesis knock-in mouse with metabolic characterization\",\n      \"pmids\": [\"25450184\", \"25348630\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether AMPK activation is a direct consequence of reduced muscle activity or an independent signaling pathway was not resolved\", \"Specific O-glycosylation sites on D1S5–S6 linker were not mapped\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"The phenotypic spectrum of SCN4A was extended in both directions: recessive null/hypomorphic mutations were shown to cause severe congenital myopathy and fetal hypokinesia through complete loss of channel function, while Nav1.4 knockout mice confirmed that homozygous loss is lethal and heterozygous loss produces latent myasthenia without compensatory Nav1.5 upregulation; separately, a pore mutation (G1537S) causing altered ion selectivity segregated with essential tremor and epilepsy.\",\n      \"evidence\": \"Whole-cell patch clamp of multiple loss-of-function mutants in HEK293 cells; exon-12-deletion knockout mice with ex vivo electrophysiology; whole-exome sequencing with functional characterization of G1537S\",\n      \"pmids\": [\"26700687\", \"27048647\", \"26427606\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether partial Nav1.4 loss-of-function is compensated by other sodium channels in specific muscle types was not tested\", \"Brain expression of Nav1.4 and its contribution to tremor/epilepsy require direct electrophysiological confirmation\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Further mechanistic refinement showed that EF-hand-like motif deletions/substitutions impair fast inactivation causing nondystrophic myotonia, and that Nav1.4 mutations can generate resurgent sodium currents (in the presence of Navβ4 peptide) providing a mechanism for repetitive firing in myotonia.\",\n      \"evidence\": \"Patch clamp of EFL mutants in heterologous cells; V445M mutant co-expressed with Navβ4 peptide in CHO cells analyzing resurgent currents\",\n      \"pmids\": [\"32129495\", \"32276507\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether Navβ4-mediated resurgent currents occur in native skeletal muscle fibers is not established\", \"Structure of the EFL in the context of the full-length channel is not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Despite extensive biophysical characterization, a high-resolution structure of the full-length human Nav1.4 in multiple gating states with bound CaM and β subunits is lacking; the structural basis of slow inactivation, the mechanism coupling EFL Ca²⁺ sensing to the inactivation gate, and whether Nav1.4 has functional roles in non-skeletal muscle tissues (heart, brain) remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No cryo-EM structure of human Nav1.4 in the slow-inactivated state\", \"Mechanism linking gating pore currents to downstream metabolic (AMPK) activation is unknown\", \"Functional significance of SCN4A expression in cardiac tissue is not established by direct electrophysiology\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 4, 5, 6, 7, 8]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 5, 6, 7, 21, 22]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 1, 5, 6, 7]},\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [5, 6, 7]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 4, 5, 6, 15, 20]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"SCN1B\",\n      \"CALM1\",\n      \"HCN2\",\n      \"SCN4B\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}