Lori L. Isom

Sodium Channel β Subunits: Anything but Auxiliary

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 α subunit and two auxiliary β subunits. The α subunits form a gene family with at least 10 members. Mutations in α 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 β subunits with at least one alternative splice product. A mutation in the β1 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 β 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 β1 subunit. A sodium channel signaling complex is proposed that involves β subunits as channel modulators as well as cell adhesion molecules, other cell adhesion molecules such as neurofascin and contactin, RPTPβ, and extracellular matrix molecules such as tenascin.

Differential Control of Clustering of the Sodium Channels Nav1.2 and Nav1.6 at Developing CNS Nodes of Ranvier

Na(v)1.6 is the main sodium channel isoform at adult nodes of Ranvier. Here, we show that Na(v)1.2 and its beta2 subunit, but not Na(v)1.6 or beta1, are clustered in developing central nervous system nodes and that clustering of Na(v)1.2 and Na(v)1.6 is differentially controlled. Oligodendrocyte-conditioned medium is sufficient to induce clustering of Na(v)1.2 alpha and beta2 subunits along central nervous system axons in vitro. This clustering is regulated by electrical activity and requires an intact actin cytoskeleton and synthesis of a non-sodium channel protein. Neither soluble- or contact-mediated glial signals induce clustering of Na(v)1.6 or beta1 in a nonmyelinating culture system. These data reveal that the sequential clustering of Na(v)1.2 and Na(v)1.6 channels is differentially controlled and suggest that myelination induces Na(v)1.6 clustering.

Loss of Plakophilin-2 Expression Leads to Decreased Sodium Current and Slower Conduction Velocity in Cultured Cardiac Myocytes

Rationale: Plakophilin-2 (PKP2) is an essential component of the cardiac desmosome. Recent data show that it interacts with other molecules of the intercalated disc. Separate studies show preferential localization of the voltage-gated sodium channel (NaV1.5) to this region. Objective: To establish the association of PKP2 with sodium channels and its role on action potential propagation. Methods and Results: Biochemical, patch clamp, and optical mapping experiments demonstrate that PKP2 associates with NaV1.5, and that knockdown of PKP2 expression alters the properties of the sodium current, and the velocity of action potential propagation in cultured cardiomyocytes. Conclusions: These results emphasize the importance of intermolecular interactions between proteins relevant to mechanical junctions, and those involved in electric synchrony. Possible relevance to the pathogenesis of arrhythmogenic right ventricular cardiomyopathy is discussed.

Mice Lacking Sodium Channel β1 Subunits Display Defects in Neuronal Excitability, Sodium Channel Expression, and Nodal Architecture

Sodium channel β1 subunits modulate α subunit gating and cell surface expression and participate in cell adhesive interactions in vitro. β1(-/-) 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 Nav1.6, contactin, caspr 1, and Kv1 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 β1 may participate in axo-glial communication at the periphery of the nodal gap. Sodium currents in dissociated hippocampal neurons are normal, but Nav1.1 expression is reduced and Nav1.3 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 β1 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.

Tenascin-R is a functional modulator of sodium channel beta subunits.

Voltage-gated sodium channels isolated from mammalian brain are composed of α, β1, and β2 subunits. The α subunit forms the ion conducting pore of the channel, whereas the β1 and β2 subunits modulate channel function, as well as channel plasma membrane expression levels. β1 and β2 each contain a single, extracellular Ig-like domain with structural similarity to the neural cell adhesion molecule (CAM), myelin Po. β2 contains strong amino acid homology to the third Ig domain and to the juxtamembrane region of F3/contactin. Many CAMs of the Ig superfamily have been shown to interact with extracellular matrix molecules. We hypothesized that β2 may interact with tenascin-R (TN-R), an extracellular matrix molecule that is secreted by oligodendrocytes during myelination and that binds F3-contactin. We show here that cells expressing sodium channel β1 or β2 subunits are functionally modulated by TN-R. Transfected cells stably expressing β1 or β2 subunits initially recognized and then were repelled from TN-R substrates. The cysteine-rich amino-terminal domain of TN-R expressed as a recombinant peptide, termed EGF-L, appears to be responsible for the repellent effect on β subunit-expressing cells. The epidermal growth factor-like repeats and fibronectin-like repeats 6–8 are most effective in the initial adhesion of β subunit-expressing cells. Application of EGF-L to αIIAβ1β2 channels expressed in Xenopus oocytes potentiated expressed sodium currents without significantly altering current time course or the voltage dependence of current activation or inactivation. Thus, sodium channel β subunits appear to function as CAMs, and TN-R may be an important regulator of sodium channel localization and function in neurons.

A Functional Null Mutation of SCN1B in a Patient with Dravet Syndrome

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 Nav1.1 α subunits. Sodium channels are modulated by β1 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 Nav1.1 α 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 β1 cell surface expression in vivo, hippocampal slice recordings were performed in Scn1b−/− versus Scn1b+/+ mice. Scn1b−/− 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+/− model of Dravet syndrome, we found no measurable differences in sodium current density in acutely dissociated CA3 hippocampal neurons. Whereas Scn1b−/− mice seize spontaneously, the seizure susceptibility of Scn1b+/− 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.

Interactions Between Ankyrin-G, Plakophilin-2, and Connexin43 at the Cardiac Intercalated Disc

Rationale: The early description of the intercalated disc defined 3 structures, all of them involved in cell-cell communication: desmosomes, gap junctions, and adherens junctions. Current evidence demonstrates that molecules not involved in providing a physical continuum between cells also populate the intercalated disc. Key among them is the voltage-gated sodium channel complex. An important component of this complex is the cytoskeletal adaptor protein Ankyrin-G (AnkG). Objective: To test the hypothesis that AnkG partners with desmosome and gap junction molecules and exerts a functional effect on intercellular communication in the heart. Methods and Results: We used a combination of microscopy, immunochemistry, patch-clamp, and optical mapping to assess the interactions between AnkG, Plakophilin-2, and Connexin43. Coimmunoprecipitation studies from rat heart lysate demonstrated associations between the 3 molecules. With the use of siRNA technology, we demonstrated that loss of AnkG expression caused significant changes in subcellular distribution and/or abundance of PKP2 and Connexin43 as well as a decrease in intercellular adhesion strength and electric coupling. Regulation of AnkG and of Nav1.5 by Plakophilin-2 was also demonstrated. Finally, optical mapping experiments in AnkG-silenced cells demonstrated a shift in the minimal frequency at which rate-dependence activation block was observed. Conclusions: These experiments support the hypothesis that AnkG is a key functional component of the intercalated disc at the intersection of 3 complexes often considered independent: the voltage-gated sodium channel, gap junctions, and the cardiac desmosome. Possible implications to the pathophysiology of inherited arrhythmias (such as arrhythmogenic right ventricular cardiomyopathy) are discussed.

Reduced sodium channel density, altered voltage dependence of inactivation, and increased susceptibility to seizures in mice lacking sodium channel β2-subunits

Sodium channel β-subunits modulate channel gating, assembly, and cell surface expression in heterologous cell systems. We generated β2−/− mice to investigate the role of β2 in control of sodium channel density, localization, and function in neurons in vivo. Measurements of [3H]saxitoxin (STX) binding showed a significant reduction in the level of plasma membrane sodium channels in β2−/− neurons. The loss of β2 resulted in negative shifts in the voltage dependence of inactivation as well as significant decreases in sodium current density in acutely dissociated hippocampal neurons. The integral of the compound action potential in optic nerve was significantly reduced, and the threshold for action potential generation was increased, indicating a reduction in the level of functional plasma membrane sodium channels. In contrast, the conduction velocity, the number and size of axons in the optic nerve, and the specific localization of Nav1.6 channels in the nodes of Ranvier were unchanged. β2−/− mice displayed increased susceptibility to seizures, as indicated by reduced latency and threshold for pilocarpine-induced seizures, but seemed normal in other neurological tests. Our observations show that β2-subunits play an important role in the regulation of sodium channel density and function in neurons in vivo and are required for normal action potential generation and control of excitability.

Na+ Channel β Subunits: Overachievers of the Ion Channel Family

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.

A Novel Epilepsy Mutation in the Sodium Channel SCN1A Identifies a Cytoplasmic Domain for β Subunit Interaction

A mutation in the sodium channel SCN1A was identified in a small Italian family with dominantly inherited generalized epilepsy with febrile seizures plus (GEFS+). The mutation, D1866Y, alters an evolutionarily conserved aspartate residue in the C-terminal cytoplasmic domain of the sodium channel α subunit. The mutation decreased modulation of the α subunit by β1, which normally causes a negative shift in the voltage dependence of inactivation in oocytes. There was less of a shift with the mutant channel, resulting in a 10 mV difference between the wild-type and mutant channels in the presence of β1. This shift increased the magnitude of the window current, which resulted in more persistent current during a voltage ramp. Computational analysis suggests that neurons expressing the mutant channels will fire an action potential with a shorter onset delay in response to a threshold current injection, and that they will fire multiple action potentials with a shorter interspike interval at a higher input stimulus. These results suggest a causal relationship between a positive shift in the voltage dependence of sodium channel inactivation and spontaneous seizure activity. Direct interaction between the cytoplasmic C-terminal domain of the wild-typeα subunit with the β1or β3 subunit was first demonstrated by yeast two-hybrid analysis. The SCN1A peptide K1846-R1886 is sufficient for β subunit interaction. Coimmunoprecipitation from transfected mammalian cells confirmed the interaction between the C-terminal domains of the α and β1 subunits. The D1866Y mutation weakens this interaction, demonstrating a novel molecular mechanism leading to seizure susceptibility.