Sodium channel ␤1 subunits modulate ␣ subunit gating and cell surface expression and participate in cell adhesive interactions in vitro. ␤1 (Ϫ/Ϫ) mice appear ataxic and display spontaneous generalized seizures. In the optic nerve, the fastest components of the compound action potential are slowed and the number of mature nodes of Ranvier is reduced, but Na v 1.6, contactin, caspr 1, and K v 1 channels are all localized normally at nodes. At the ultrastructural level, the paranodal septate-like junctions immediately adjacent to the node are missing in a subset of axons, suggesting that ␤1 may participate in axo-glial communication at the periphery of the nodal gap. Sodium currents in dissociated hippocampal neurons are normal, but Na v 1.1 expression is reduced and Na v 1.3 expression is increased in a subset of pyramidal neurons in the CA2/CA3 region, suggesting a basis for the epileptic phenotype. Our results show that ␤1 subunits play important roles in the regulation of sodium channel density and localization, are involved in axo-glial communication at nodes of Ranvier, and are required for normal action potential conduction and control of excitability in vivo.

Dravet syndrome (also called severe myoclonic epilepsy of infancy) is one of the most severe forms of childhood epilepsy. Most patients have heterozygous mutations in SCN1A, encoding voltage-gated sodium channel Na v 1.1 ␣ subunits. Sodium channels are modulated by ␤1 subunits, encoded by SCN1B, a gene also linked to epilepsy. Here we report the first patient with Dravet syndrome associated with a recessive mutation in SCN1B (p.R125C). Biochemical characterization of p.R125C in a heterologous system demonstrated little to no cell surface expression despite normal total cellular expression. This occurred regardless of coexpression of Na v 1.1 ␣ subunits. Because the patient was homozygous for the mutation, these data suggest a functional SCN1B null phenotype. To understand the consequences of the lack of ␤1 cell surface expression in vivo, hippocampal slice recordings were performed in Scn1b Ϫ/Ϫ versus Scn1b ϩ/ϩ mice. Scn1b Ϫ/Ϫ CA3 neurons fired evoked action potentials with a significantly higher peak voltage and significantly greater amplitude compared with wild type. However, in contrast to the Scn1a ϩ/Ϫ model of Dravet syndrome, we found no measurable differences in sodium current density in acutely dissociated CA3 hippocampal neurons. Whereas Scn1b Ϫ/Ϫ mice seize spontaneously, the seizure susceptibility of Scn1b ϩ/Ϫ mice was similar to wild type, suggesting that, like the parents of this patient, one functional SCN1B allele is sufficient for normal control of electrical excitability. We conclude that SCN1B p.R125C is an autosomal recessive cause of Dravet syndrome through functional gene inactivation.

Voltage-gated sodium channels (VGSCs) are responsible for initiation and propagation of action potentials in excitable cells. VGSCs in mammalian brain are heterotrimeric complexes of α and β subunits. Originally called “auxiliary,” we now know that β subunit proteins are multifunctional signaling molecules that play roles in both excitable and non-excitable cell types, and with or without the pore-forming α subunit present. β subunits function in VGSC and potassium channel modulation, cell adhesion, and gene regulation, with particularly important roles in brain development. Mutations in the genes encoding β subunits are linked to a number of diseases, including epilepsy, sudden death syndromes like SUDEP and SIDS, and cardiac arrhythmia. While VGSC β subunit-specific drugs have not yet been developed, this protein family is an emerging therapeutic target.

OBJECTIVE Neuronal channelopathies cause brain disorders including epilepsy, migraine and ataxia. Despite the development of mouse models, pathophysiological mechanisms for these disorders remain uncertain. One particularly devastating channelopathy is Dravet Syndrome (DS), a severe childhood epilepsy typically caused by de novo dominant mutations in the SCN1A gene encoding the voltage-gated sodium channel Nav1.1. Heterologous expression of mutant channels suggests loss-of-function, raising the quandary of how loss of sodium channels underlying action potentials produces hyperexcitability. Mouse model studies suggest that decreased Nav1.1 function in interneurons causes disinhibition. We sought to determine how mutant SCN1A affects human neurons using the induced pluripotent stem cell (iPSC) method to generate patient-specific neurons. METHODS Forebrain-like pyramidal- and bipolar-shaped neurons are derived from two DS subjects and three human controls by iPSC reprogramming of fibroblasts. DS and control iPSC-derived neurons are compared using whole-cell patch clamp recordings. Sodium current density and intrinsic neuronal excitability are examined. RESULTS Neural progenitors from DS and human control iPSCs display a forebrain identity and differentiate into bipolar- and pyramidal-shaped neurons. DS patient-derived neurons show increased sodium currents in both bipolar- and pyramidal-shaped neurons. Consistent with increased sodium currents, both types of patient-derived neurons show spontaneous bursting and other evidence of hyperexcitability. Sodium channel transcripts are not elevated, consistent with a post-translational mechanism. INTERPRETATION These data demonstrate that epilepsy patient-specific iPSC-derived neurons are useful for modeling epileptic-like hyperactivity. Our findings reveal a previously unrecognized cell-autonomous epilepsy mechanism potentially underlying Dravet Syndrome, and offer a platform for screening new anti-epileptic therapies.

Scn1b null mice have a severe neurological and cardiac phenotype. Human mutations in SCN1B result in epilepsy and cardiac arrhythmia. SCN1B is expressed as two developmentally regulated splice variants, β1 and β1B, that are each expressed in brain and heart in rodents and humans. Here we studied the structure and function of β1B and investigated a novel human SCN1B epilepsy-related mutation (p.G257R) unique to β1B. We show that wild-type β1B is not a transmembrane protein, but a soluble protein expressed predominantly during embryonic development that promotes neurite outgrowth. Association of β1B with voltage-gated Na+ channels (VGSCs) Nav1.1 or Nav1.3 is not detectable by immunoprecipitation and β1B does not affect Nav1.3 cell surface expression as measured by [3H]-saxitoxin binding. However, β1B co-expression results in subtle alteration of Nav1.3 currents in transfected cells, suggesting that β1B may modulate Na+ current in brain. Similar to the previously characterized p.R125C mutation, p.G257R results in intracellular retention of β1B, generating a functional null allele. In contrast, two other SCN1B mutations associated with epilepsy, p.C121W and p.R85H, are expressed at the cell surface. We propose that β1B p.G257R may contribute to epilepsy through a mechanism that includes intracellular retention resulting in aberrant neuronal pathfinding.