Voltage-sensitive sodium channels are responsible for the initiation and propagation of the action potential and therefore are important for neuronal excitability. Complementary DNA clones encoding the beta 1 subunit of the rat brain sodium channel were isolated by a combination of polymerase chain reaction and library screening techniques. The deduced primary structure indicates that the beta 1 subunit is a 22,851-dalton protein that contains a single putative transmembrane domain and four potential extracellular N-linked glycosylation sites, consistent with biochemical data. Northern blot analysis reveals a 1,400-nucleotide messenger RNA in rat brain, heart, skeletal muscle, and spinal cord. Coexpression of beta 1 subunits with alpha subunits increases the size of the peak sodium current, accelerates its inactivation, and shifts the voltage dependence of inactivation to more negative membrane potentials. These results indicate that the beta 1 subunit is crucial in the assembly, expression, and functional modulation of the heterotrimeric complex of the rat brain sodium channel.

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.

Voltage-gated sodium channels are glycoprotein complexes responsible for initiation and propagation of action potentials in excitable cells such as central and peripheral neurons, cardiac and skeletal muscle myocytes, and neuroendocrine cells. Mammalian sodium channels are heterotrimers, composed of a central, pore-forming alpha subunit and two auxiliary beta subunits. The alpha subunits form a gene family with at least 10 members. Mutations in alpha subunit genes have been linked to paroxysmal disorders such as epilepsy, long QT syndrome, and hyperkalemic periodic paralysis in humans, and motor endplate disease and cerebellar ataxia in mice. Three genes encode sodium channel beta subunits with at least one alternative splice product. A mutation in the beta 1 subunit gene has been linked to generalized epilepsy with febrile seizures plus type 1 (GEFS + 1) in a human family with this disease. Sodium channel beta subunits are multifunctional. They modulate channel gating and regulate the level of channel expression at the plasma membrane. More recently, they have been shown to function as cell adhesion molecules in terms of interaction with extracellular matrix, regulation of cell migration, cellular aggregation, and interaction with the cytoskeleton. Structure-function studies have resulted in the preliminary assignment of functional domains in the beta 1 subunit. A sodium channel signaling complex is proposed that involves beta subunits as channel modulators as well as cell adhesion molecules, other cell adhesion molecules such as neurofascin and contactin, RPTP beta, and extracellular matrix molecules such as tenascin.

Dravet syndrome (DS) is an intractable developmental and epileptic encephalopathy caused largely by de novo variants in the SCN1A gene, resulting in haploinsufficiency of the voltage-gated sodium channel α subunit NaV1.1. Here, we used Targeted Augmentation of Nuclear Gene Output (TANGO) technology, which modulates naturally occurring, nonproductive splicing events to increase target gene and protein expression and ameliorate disease phenotype in a mouse model. We identified antisense oligonucleotides (ASOs) that specifically increase the expression of productive Scn1a transcript in human cell lines, as well as in mouse brain. We show that a single intracerebroventricular dose of a lead ASO at postnatal day 2 or 14 reduced the incidence of electrographic seizures and sudden unexpected death in epilepsy (SUDEP) in the F1:129S-Scn1a+/− × C57BL/6J mouse model of DS. Increased expression of productive Scn1a transcript and NaV1.1 protein was confirmed in brains of treated mice. Our results suggest that TANGO may provide a unique, gene-specific approach for the treatment of DS.

Background-Sodium channels isolated from mammalian brain are composed of ␣-, ␤ 1 -, and ␤ 2 -subunits. The composition of sodium channels in cardiac muscle, however, has not been defined, and disagreement exists over which ␤-subunits are expressed in the myocytes. Some investigators have demonstrated ␤ 1 expression in heart. Others have not detected any auxiliary subunits. On the basis of Northern blot analysis of total RNA, ␤ 2 expression has been thought to be exclusive to neurons and absent from cardiac muscle. Methods and Results-The goal of this study was to define the subunit composition of cardiac sodium channels in myocytes. We show that cardiac sodium channels are composed of ␣-, ␤ 1 -, and ␤ 2 -subunits. Nav1.5 and Nav1.1 are expressed in myocytes and are associated with ␤ 1 -and ␤ 2 -subunits. Immunocytochemical localization of Nav1.1, ␤ 1 , and ␤ 2 in adult heart sections showed that these subunits are expressed at the Z lines, as shown previously for Nav1.5. Coexpression of Nav1.5 with ␤ 2 in transfected cells resulted in no detectable changes in sodium current. Conclusions-Cardiac sodium channels are composed of ␣-(Nav1.1 or Nav1.5), ␤ 1 -, and ␤ 2 -subunits. Although ␤ 1 -subunits modulate cardiac sodium channel current, ␤ 2 -subunit function in heart may be limited to cell adhesion.

Contactin (also known as F3, F11) is a surface glycoprotein that has significant homology with the beta2 subunit of voltage-gated Na(+) channels. Contactin and Na(+) channels can be reciprocally coimmunoprecipitated from brain homogenates, indicating association within a complex. Cells cotransfected with Na(+) channel Na(v)1.2alpha and beta1 subunits and contactin have threefold to fourfold higher peak Na(+) currents than cells with Na(v)1.2alpha alone, Na(v)1.2/beta1, Na(v)1.2/contactin, or Na(v)1.2/beta1/beta2. These cells also have a correspondingly higher saxitoxin binding, suggesting an increased Na(+) channel surface membrane density. Coimmunoprecipitation of different subunits from cell lines shows that contactin interacts specifically with the beta1 subunit. In the PNS, immunocytochemical studies show a transient colocalization of contactin and Na(+) channels at new nodes of Ranvier forming during remyelination. In the CNS, there is a particularly high level of colocalization of Na(+) channels and contactin at nodes both during development and in the adult. Contactin may thus significantly influence the functional expression and distribution of Na(+) channels in neurons.

Computational modeling indicates that cardiac conduction may involve ephaptic coupling – intercellular communication involving electrochemical signaling across narrow extracellular clefts between cardiomyocytes. We hypothesized that β1(SCN1B) –mediated adhesion scaffolds trans-activating NaV1.5 (SCN5A) channels within narrow (<30 nm) perinexal clefts adjacent to gap junctions (GJs), facilitating ephaptic coupling. Super-resolution imaging indicated preferential β1 localization at the perinexus, where it co-locates with NaV1.5. Smart patch clamp (SPC) indicated greater sodium current density (INa) at perinexi, relative to non-junctional sites. A novel, rationally designed peptide, βadp1, potently and selectively inhibited β1-mediated adhesion, in electric cell-substrate impedance sensing studies. βadp1 significantly widened perinexi in guinea pig ventricles, and selectively reduced perinexal INa, but not whole cell INa, in myocyte monolayers. In optical mapping studies, βadp1 precipitated arrhythmogenic conduction slowing. In summary, β1-mediated adhesion at the perinexus facilitates action potential propagation between cardiomyocytes, and may represent a novel target for anti-arrhythmic therapies.

Generalized epilepsy with febrile seizures plus type 1 is an inherited human epileptic syndrome, associated with a cysteine-to-tryptophan (C121W) mutation in the extracellular immunoglobin domain of the auxiliary beta1 subunit of the voltage-gated sodium channel. The mutation disrupts beta1 function, but how this leads to epilepsy is not understood. In this study, we make several observations that may be relevant for understanding why this beta1 mutation results in seizures. First, using electrophysiological recordings from mammalian cell lines, coexpressing sodium channel alpha subunits and either wild-type beta1 or C121Wbeta1, we show that loss of beta1 functional modulation, caused by the C121W mutation, leads to increased sodium channel availability at hyperpolarized membrane potentials and reduced sodium channel rundown during high-frequency channel activity, compared with channels coexpressed with wild-type beta1. In contrast, neither wild-type beta1 nor C121Wbeta1 significantly affected sodium current time course or the voltage dependence of channel activation. We also show, using a Drosophila S2 cell adhesion assay, that the C121W mutation disrupts beta1-beta1 homophilic cell adhesion, suggesting that the mutation may alter the ability of beta1 to mediate protein-protein interactions critical for sodium channel localization. Finally, we demonstrate that neither functional modulation nor cell adhesion mediated by wild-type beta1 is occluded by coexpression of C121Wbeta1, arguing against the idea that the mutant beta1 acts as a dominant-negative subunit. Together, these data suggest that C121Wbeta1 causes subtle effects on channel function and subcellular distribution that bias neurons toward hyperexcitabity and epileptogenesis.