Voltage-gated sodium channels in brain neurons are complexes of a pore-forming alpha subunit with smaller beta 1 and beta 2 subunits. cDNA cloning and sequencing showed that the beta 2 subunit is a 186 residue glycoprotein with an extracellular NH2-terminal domain containing an immunoglobulin-like fold with similarity to the neural cell adhesion molecule (CAM) contactin, a single transmembrane segment, and a small intracellular domain. Coexpression of beta 2 with alpha subunits in Xenopus oocytes increases functional expression, modulates gating, and causes up to a 4-fold increase in the capacitance of the oocyte, which results from an increase in the surface area of the plasma membrane microvilli. beta 2 subunits are unique among the auxiliary subunits of ion channels in combining channel modulation with a CAM motif and the ability to expand the cell membrane surface area. They may be important regulators of sodium channel expression and localization in neurons.

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Voltage-gated sodium channels with "resurgent" kinetics are specialized for high-frequency firing. The alpha subunits interact with a blocking protein that binds open channels upon depolarization and unbinds upon repolarization, producing resurgent sodium current. By limiting classical inactivation, the cycle of block and unblock shortens refractory periods. To characterize the blocker in Purkinje neurons, we briefly exposed inside-out patches to substrate-specific proteases. Trypsin and chymotrypsin each removed resurgent current, consistent with established roles for positively charged and hydrophobic/aromatic groups in blocking sodium channels. In Purkinje cells, the only known sodium channel-associated subunit that has a cytoplasmic sequence with several positive charges and clustered hydrophobic/aromatic residues is beta4 (KKLITFILKKTREK; beta4(154-167)). After enzymatic removal of block, beta4(154-167) fully reconstituted resurgent current, whereas scrambled or point-mutated peptides were ineffective. In CA3 pyramidal neurons, which lack beta4 and endogenous block, beta4(154-167) generated resurgent current. Thus, beta4 may be the endogenous open-channel blocker responsible for resurgent kinetics.

Voltage-gated Na+ channels (VGSCs) in mammals contain a pore-forming α subunit and one or more β subunits. There are five mammalian β subunits in total: β1, β1B, β2, β3, and β4, encoded by four genes: SCN1B–SCN4B. With the exception of the SCN1B splice variant, β1B, the β subunits are type I topology transmembrane proteins. In contrast, β1B lacks a transmembrane domain and is a secreted protein. A growing body of work shows that VGSC β subunits are multifunctional. While they do not form the ion channel pore, β subunits alter gating, voltage-dependence, and kinetics of VGSCα subunits and thus regulate cellular excitability in vivo. In addition to their roles in channel modulation, β subunits are members of the immunoglobulin superfamily of cell adhesion molecules and regulate cell adhesion and migration. β subunits are also substrates for sequential proteolytic cleavage by secretases. An example of the multifunctional nature of β subunits is β1, encoded by SCN1B, that plays a critical role in neuronal migration and pathfinding during brain development, and whose function is dependent on Na+ current and γ-secretase activity. Functional deletion of SCN1B results in Dravet Syndrome, a severe and intractable pediatric epileptic encephalopathy. β subunits are emerging as key players in a wide variety of physiopathologies, including epilepsy, cardiac arrhythmia, multiple sclerosis, Huntington’s disease, neuropsychiatric disorders, neuropathic and inflammatory pain, and cancer. β subunits mediate multiple signaling pathways on different timescales, regulating electrical excitability, adhesion, migration, pathfinding, and transcription. Importantly, some β subunit functions may operate independently of α subunits. Thus, β subunits perform critical roles during development and disease. As such, they may prove useful in disease diagnosis and therapy.

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.

Sodium channels isolated from mammalian brain are composed of ␣, ␤1, and ␤2 subunits. The auxiliary ␤ subunits do not form the ion conducting pore, yet play important roles in channel modulation and plasma membrane expression. ␤1 and ␤2 are transmembrane proteins with one extracellular V-set immunoglobulin (Ig) protein domain. It has been shown recently that ␤1 and ␤2 interact with the extracellular matrix proteins tenascin-C and tenascin-R. In the present study we show that rat brain ␤1 and ␤2, but not ␣IIA, subunits interact in a trans-homophilic fashion, resulting in recruitment of the cytoskeletal protein ankyrin to sites of cell-cell contact in transfected Drosophila S2 cells. Whereas ␣IIA subunits expressed alone do not cause cellular aggregation, ␤ subunits co-expressed with ␣IIA retain the ability to adhere and recruit ankyrin. Truncated ␤ subunits lacking cytoplasmic domains interact homophilically to produce cell aggregation but do not recruit ankyrin. Thus, the cytoplasmic domains of ␤1 and ␤2 are required for cytoskeletal interactions. It is hypothesized that sodium channel ␤ subunits serve as a critical communication link between the extracellular and intracellular environments of the neuron and may play a role in sodium channel placement at nodes of Ranvier.Control of the cell surface density and localization of voltagegated sodium channels are critical aspects of neuronal function. This is especially important at nodes of Ranvier of myelinated axons where high densities of sodium channels are needed for rapid and reliable saltatory conduction (1, 2). Sodium channels from mammalian brain are heterotrimeric structures composed of a central pore forming ␣ subunit and two auxiliary subunits, ␤1 and ␤2 (3, 4). Though the ␤ subunits do not form the ion conducting pore, they play critical roles in channel gating, voltage dependence of activation and inactivation, and channel plasma membrane expression levels (5-8). ␤1 and ␤2 contain Ig-like extracellular domains and are members of the V-set of the Ig superfamily, which includes many cell adhesion molecules (CAMs) 1 (9). ␤1 and ␤2 have recently been shown to interact with the extracellular matrix proteins tenascin-C and tenascin-R (10, 11). Transfected cells expressing ␤1 or ␤2 subunits initially bind to and then are repelled from a tenascin-R substrate (11). Purified sodium channels or the bacterially expressed ␤2 subunit extracellular domain bind tenascin-C and tenascin-R in an enzyme-linked immunosorbent type biochemical assay (10). These results suggested that sodium channel ␤ subunits function as CAMs. CAMs of the L1 family also bind directly to cytoskeletal elements such as ankyrin (12,13,(15)(16)(17). Sodium channels have been shown to colocalize with ankyrin G and spectrin at axon initial segments and nodes of Ranvier, and there is some evidence to show that sodium channels bind ankyrin directly in vitro (18 -22). However, it is not known to which sodium channel subunit ankyrin binds. Because ␤1 and ␤2 are structurally and functionally homologous ...

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.

Key Words: plakophilin-2 Ⅲ intercalated disc Ⅲ arrhythmogenic right ventricular cardiomyopathy Ⅲ cardiac desmosomes A high-resolution image of the site of end-end contact between cardiomyocytes reveals an electron-dense organization called "the intercalated disc." Its classic definition involves 3 structures: desmosomes and adherens junctions, providing mechanical coupling; and gap junctions, allowing electric/metabolic synchronization between cells. Recent studies show that other molecules, not directly involved in intercellular coupling, also reside preferentially at the intercalated disc. Among them is Na V 1.5, the major ␣ subunit of the cardiac sodium channel. 1 Here, we ask whether Na v 1.5 and the desmosomal protein plakophilin-2 (PKP2) coexist in the same molecular complex and whether loss of PKP2 expression affects (1) the amplitude and kinetics of the sodium current and (2) action potential propagation in a monolayer of cardiomyocytes. Our data demonstrate a functional crosstalk between a protein defined in the context of intercellular junctions (PKP2) and another protein that is fundamental to the electrical behavior of the single myocyte.