A single B1 subunit mapped to mouse chromosome 7 may be a common component of Na channel isoforms from brain, skeletal muscle and heart

scn1bb, a zebrafish ortholog of SCN1B expressed in excitable and nonexcitable cells, affects motor neuron axon morphology and touch sensitivity

Voltage-gated Na(+) channels initiate and propagate action potentials in excitable cells. Mammalian Na(+) channels are composed of one pore-forming alpha-subunit and two beta-subunits. SCN1B encodes the Na(+) channel beta1-subunit that modulates channel gating and voltage dependence, regulates channel cell surface expression, and functions as a cell adhesion molecule (CAM). We recently identified scn1ba, a zebrafish ortholog of SCN1B. Here we report that zebrafish express a second beta1-like paralog, scn1bb. In contrast to the restricted expression of scn1ba mRNA in excitable cells, we detected scn1bb transcripts and protein in several ectodermal derivatives including neurons, glia, the lateral line, peripheral sensory structures, and tissues derived from other germ layers such as the pronephros. As expected for beta1-subunits, elimination of Scn1bb protein in vivo by morpholino knock-down reduced Na(+) current amplitudes in Rohon-Beard neurons of zebrafish embryos, consistent with effects observed in heterologous systems. Further, after Scn1bb knock-down, zebrafish embryos displayed defects in Rohon-Beard mediated touch sensitivity, demonstrating the significance of Scn1bb modulation of Na(+) current to organismal behavior. In addition to effects associated with Na(+) current modulation, Scn1bb knockdown produced phenotypes consistent with CAM functions. In particular, morpholino knock-down led to abnormal development of ventrally projecting spinal neuron axons, defasciculation of the olfactory nerve, and increased hair cell number in the inner ear. We propose that, in addition to modulation of electrical excitability, Scn1bb plays critical developmental roles by functioning as a CAM in the zebrafish embryonic nervous system.

Sodium channel beta1 subunits promote neurite outgrowth in cerebellar granule neurons

Many immunoglobulin superfamily members are integral in development through regulation of processes such as growth cone guidance, cell migration, and neurite outgrowth. We demonstrate that homophilic interactions between voltage-gated sodium channel beta1 subunits promote neurite extension in cerebellar granule neurons. Neurons isolated from wild-type or beta1(-/-) mice were plated on top of parental, mock-, or beta1-transfected fibroblasts. Wild-type neurons consistently showed increased neurite length when grown on beta1-transfected monolayers, whereas beta1(-/-) neurons showed no increase compared with control conditions. beta1-mediated neurite extension was mimicked using a soluble beta1 extracellular domain and was blocked by antibodies directed against the beta1 extracellular domain. Immunohistochemical analysis suggests that the beta1 and beta4 subunits, but not beta2 and beta3, are expressed in cerebellar Bergmann glia as well as granule neurons. These results suggest a novel role for beta1 during neuronal development and are the first demonstration of a functional role for sodium channel beta subunit-mediated cell adhesive interactions.

Adenoviral-mediated expression of functional Na+ channel beta1 subunits tagged with a yellow fluorescent protein

Voltage-gated sodium (Na(+)) channels typically contain a pore-forming alpha subunit and one or two auxiliary beta subunits. Although initial characterization of known alpha and beta subunits has been facilitated by expression in heterologous cells, to understand fully the differences between individual subunits and the functional consequences of selective subunit expression, there is a need to acutely manipulate expression in cells that endogenously express Na(+) channels. To this end, we have constructed a recombinant adenovirus containing a cDNA for a mouse Na(+) channel beta1 subunit with a yellow fluorescent protein fused to its C-terminus (Ad-beta1-EYFP), and with fluorescence microscopy detected beta1-EYFP expression in primary cerebellar neurons and Chinese hamster ovary (CHO) cells upon transduction with this adenovirus, including expression in the plasma membrane. Consistent with this, patch clamp recordings confirmed that Na(+) currents in CHO cells expressing mouse Na(v)1.4 alpha subunits were appropriately modified by the viral-mediated expression of beta1-EYFP subunits. The results demonstrate that adenoviral-mediated gene delivery can be used effectively to express epitope-tagged Na(+) channel subunits with properties similar to wild-type subunits, and suggest that Ad-beta1-EYFP will be a useful reagent for investigating Na(+) channels in a variety of excitable cell types, including neurons.

Nav1.3 sodium channels: rapid repriming and slow closed-state inactivation display quantitative differences after expression in a mammalian cell line and in spinal sensory neurons

Although rat brain Nav1.3 voltage-gated sodium channels have been expressed and studied in Xenopus oocytes, these channels have not been studied after their expression in mammalian cells. We characterized the properties of the rat brain Nav1.3 sodium channels expressed in human embryonic kidney (HEK) 293 cells. Nav1.3 channels generated fast-activating and fast-inactivating currents. Recovery from inactivation was relatively rapid at negative potentials (<-80 mV) but was slow at more positive potentials. Development of closed-state inactivation was slow, and, as predicted on this basis, Nav1.3 channels generated large ramp currents in response to slow depolarizations. Coexpression of beta3 subunits had small but significant effects on the kinetic and voltage-dependent properties of Nav1.3 currents in HEK 293 cells, but coexpression of beta1 and beta2 subunits had little or no effect on Nav1.3 properties. Nav1.3 channels, mutated to be tetrodotoxin-resistant (TTX-R), were expressed in SNS-null dorsal root ganglion (DRG) neurons via biolistics and were compared with the same construct expressed in HEK 293 cells. The voltage dependence of steady-state inactivation was approximately 7 mV more depolarized in SNS-null DRG neurons, demonstrating the importance of background cell type in determining physiological properties. Moreover, consistent with the idea that cellular factors can modulate the properties of Nav1.3, the repriming kinetics were twofold faster in the neurons than in the HEK 293 cells. The rapid repriming of Nav1.3 suggests that it contributes to the acceleration of repriming of TTX-sensitive (TTX-S) sodium currents that are seen after peripheral axotomy of DRG neurons. The relatively rapid recovery from inactivation and the slow closed-state inactivation kinetics of Nav1.3 channels suggest that neurons expressing Nav1.3 may exhibit a reduced threshold and/or a relatively high frequency of firing.

Resurgence of sodium channel research

A variety of isoforms of mammalian voltage-gated sodium channels have been described. Ten genes encoding sodium channel alpha subunits have been identified, and nine of those isoforms have been functionally expressed in exogenous systems. The alpha subunit is associated with accessory beta subunits in some tissues, and three genes encoding different beta subunits have been identified. The alpha subunit isoforms have distinct patterns of development and localization in the nervous system, skeletal and cardiac muscle. In addition, many of the isoforms demonstrate subtle differences in their functional properties. However, there are no clear subfamilies of the channels, unlike the situation with potassium and calcium channels. The subtle differences in the functional properties of the sodium channel isoforms result in unique conductances in specific cell types, which have important physiological effects for the organism. Small alterations in the electrophysiological properties of the channel resulting from mutations in specific isoforms cause human diseases such as periodic paralysis, long QT syndrome, and epilepsy.

On the cause of mental retardation in Down syndrome: extrapolation from full and segmental trisomy 16 mouse models

Down syndrome (DS, trisomy 21, Ts21) is the most common known cause of mental retardation. In vivo structural brain imaging in young DS adults, and post-mortem studies, indicate a normal brain size after correction for height, and the absence of neuropathology. Functional imaging with positron emission tomography (PET) shows normal brain glucose metabolism, but fewer significant correlations between metabolic rates in different brain regions than in controls, suggesting reduced functional connections between brain circuit elements. Cultured neurons from Ts21 fetuses and from fetuses of an animal model for DS, the trisomy 16 (Ts16) mouse, do not differ from controls with regard to passive electrical membrane properties, including resting potential and membrane resistance. On the other hand, the trisomic neurons demonstrate abnormal active electrical and biochemical properties (duration of action potential and its rates of depolarization and repolarization, altered kinetics of active Na(+), Ca(2+) and K(+) currents, altered membrane densities of Na(+) and Ca(2+) channels). Another animal model, the adult segmental trisomy 16 mouse (Ts65Dn), demonstrates reduced long-term potentiation and increased long-term depression (models for learning and memory related to synaptic plasticity) in the CA1 region of the hippocampus. Evidence suggests that the abnormalities in the trisomy mouse models are related to defective signal transduction pathways involving the phosphoinositide cycle, protein kinase A and protein kinase C. The phenotypes of DS and its mouse models do not involve abnormal gene products due to mutations or deletions, but result from altered expression of genes on human chromosome 21 or mouse chromosome 16, respectively. To the extent that the defects in signal transduction and in active electrical properties, including synaptic plasticity, that are found in the Ts16 and Ts65Dn mouse models, are found in the brain of DS subjects, we postulate that mental retardation in DS results from such abnormalities. Changes in timing and synaptic interaction between neurons during development can lead to less than optimal functioning of neural circuitry and signaling then and in later life.

Cloning, localization, and functional expression of sodium channel beta1A subunits

Auxiliary beta1 subunits of voltage-gated sodium channels have been shown to be cell adhesion molecules of the Ig superfamily. Co-expression of alpha and beta1 subunits modulates channel gating as well as plasma membrane expression levels. We have cloned, sequenced, and expressed a splice variant of beta1, termed beta1A, that results from an apparent intron retention event. beta1 and beta1A are structurally homologous proteins with type I membrane topology; however, they contain little to no amino acid homology beyond the shared Ig loop region. beta1A mRNA expression is developmentally regulated in rat brain such that it is complementary to beta1. beta1A mRNA is expressed during embryonic development, and then its expression becomes undetectable after birth, concomitant with the onset of beta1 expression. In contrast, beta1A mRNA is expressed in adult adrenal gland and heart. Western blot analysis revealed beta1A protein expression in heart, skeletal muscle, and adrenal gland but not in adult brain or spinal cord. Immunocytochemical analysis of beta1A expression revealed selective expression in brain and spinal cord neurons, with high expression in heart and all dorsal root ganglia neurons. Co-expression of alphaIIA and beta1A subunits in Chinese hamster lung 1610 cells results in a 2.5-fold increase in sodium current density compared with cells expressing alphaIIA alone. This increase in current density reflected two effects of beta1A: 1) an increase in the proportion of cells expressing detectable sodium currents and 2) an increase in the level of functional sodium channels in expressing cells. [(3)H]Saxitoxin binding analysis revealed a 4-fold increase in B(max) with no change in K(D) in cells coexpressing alphaIIA and beta1A compared with cells expressing alphaIIA alone. beta1A-expressing cell lines also revealed subtle differences in sodium channel activation and inactivation. These effects of beta1A subunits on sodium channel function may be physiologically important events in the development of excitable cells.

Diversity of mammalian voltage-gated sodium channels

A variety of different isoforms of mammalian voltage-gated sodium channels have been identified. These channels can be classified into three different types. Eight type 1 isoforms have been identified in the CNS, PNS, skeletal muscle, and heart. All of these channels have been expressed in exogenous systems, and all of the genes have been mapped. Three type 2 isoforms have been identified in heart, uterus, and muscle. These channels diverge from the type 1 channels in critical regions, and have not been functionally expressed, so their significance is unknown. A single isoform identified in the PNS may represent a third class of channels, in that it diverges from both type 1 and 2 channels. The type 3 channel has not been functionally expressed.

Molecular determinants of Na+ channel function in the extracellular domain of the beta1 subunit

The rat brain voltage-gated Na+ channel is composed of three glycoprotein subunits: the pore-forming alpha subunit and two auxiliary subunits, beta1 and beta2, which contain immunoglobulin (Ig)-like folds in their extracellular domains. When expressed in Xenopus oocytes, beta1 modulates the gating properties of the channel-forming type IIA alpha subunit, resulting in an acceleration of inactivation. We have used a combination of deletion, alanine-scanning, site-directed, and chimeric mutagenesis strategies to examine the importance of different structural features of the beta1 subunit in the modulation of alphaIIA function, with an emphasis on the extracellular domain. Deletion analysis revealed that the extracellular domain is required for function, but the intracellular domain is not. The mutation of four putative sites of N-linked glycosylation showed that they are not required for beta1 function. Mutations of hydrophobic residues in the core beta sheets of the Ig fold disrupted beta1 function, whereas substitution of amino acid residues in connecting segments had no effect. Mutations of acidic residues in the A/A' strand of the Ig fold reduced the effectiveness of the beta1 subunit in modulating the rate of inactivation but did not significantly affect the association of the mutant beta1 subunit with the alphaIIA subunit or its effect on recovery from inactivation. Our data suggest that the Ig fold of the beta1 extracellular domain serves as a scaffold that presents the charged residues of the A/A' strands for interaction with the pore-forming alpha subunit.

Sequence of the voltage-gated sodium channel beta1-subunit in wild-type and in quivering mice

SCN1B, the human gene encoding the beta1-subunit of the voltage-gated sodium channel has previously been cloned and mapped to Chr 19q13.1. The sequence of the homologous mouse gene, Scn1b, has now been determined from cDNA. The mouse gene is highly conserved, encoding a predicted protein with 99%, 98% and 96% amino acid identity to the rat, rabbit, and human homologs, respectively. DNA sequence conservation is also striking in the 3' untranslated region which shows 67% and 98% to human and rat, respectively. Unlike the human and rat homologs, high expression of mRNA from the mouse gene is confined to adult skeletal muscle and brain, and is not observed in heart. As Scnlb maps to Chr 7, in close genetic proximity to the quivering gene (qv), the coding region of Scnlb was also cloned from a qvJ/qvJ homozygous mouse and assessed as a candidate for the site of this genetic defect. Comparison of qv and wild-type cDNAs showed no changes in the predicted amino acid sequence that could cause the qv phenotype. However, three silent polymorphisms in the DNA coding region indicate that Scn1b is close to qv, and is within a region of genetic identity with DBA/2J, the inbred background on which the qvJ allele arose.

Reduced expression of voltage-gated sodium channels in neurons cultured from trisomy 16 mouse hippocampus

Voltage-gated sodium channels are responsible for the initial depolarizing phase of the action potential. In hippocampal neurons cultured from trisomy 16 (Ts16) mice (a model for Down's syndrome), the maximum inward conductance mediated by these channels was reduced 47% relative to control diploid neurons. This reduced conductance was reflected in a 35% decrease in binding of radiolabeled saxitoxin, a sodium channel-specific ligand, indicating expression of fewer channels in these neurons. The mRNAs encoding the alpha and beta 1 subunits were, however, present at the same levels in Ts16 neurons and control diploid neurons. Thus, the altered regulation of voltage-gated sodium channels in Ts16 neurons is apparently a post-transcriptional event and possible mechanisms are discussed.

Channel activators reduce the expression of sodium channel alpha-subunit mRNA in developing neurons

The expression of rat brain sodium channel alpha-subunit (Na+I, Na+II and Na+III) and beta 1-subunit mRNAs was examined in rat fetal brain neurons in culture. A combined technique of reverse transcription and polymerase chain reaction (RT-PCR) was used. Two different PCR primer sets were designed to obtain simultaneous amplification of the three alpha-subunit mRNAs. All three molecules were detected in fetal neurons but the expression pattern (Na+III > Na+II > > Na+I) was different than that observed in adult tissue (Na+II > Na+I > Na+III). Expression of the beta 1-subunit mRNA was detected using a specific PCR primer set. Doublet bands were amplified, from fetal cells and adult brain mRNA. To get further insight into the molecular mechanism that underlie activity dependent plasticity of sodium channels, we studied the effect on the expression of sodium channel subunits mRNA of a 60 h incubation of cells in the presence of a scorpion neurotoxin that blocks channel inactivation. An overall decrease in the expression of all three alpha-subunit mRNAs was observed whereas the beta 1-subunit mRNA was unaffected by the same treatment. When cells were incubated with the scorpion neurotoxin together with tetrodotoxin, to block Na+ influx through channels, the decrease in mRNA expression was not observed. Finally, a 60 h continuous depolarization of cells induced by application of a high concentration KC1 solution did not mimic the effect of the scorpion toxin. These observations suggest that a persistent activation of the sodium channels is able to down-regulate mRNA expression for alpha-subunits but not for the beta 1-subunit.

Serine-1321-independent regulation of the mu 1 adult skeletal muscle Na+ channel by protein kinase C

The adult skeletal muscle Na+ channel mu1 possesses a highly conserved segment between subunit domains III and IV containing a consensus protein kinase C (PKC) phosphorylation site that, in the neuronal isoform, acts as a master control for "convergent" regulation by PKC and cAMP-dependent protein kinase. It lacks an approximately 200-aa segment between domains I and II though to modulate channel gating. We here demonstrate that mu1 is regulated by PKC (but not cAMP-dependent protein kinase) in a manner distinct from that observed for the neuronal isoforms, suggesting that under the same conditions muscle excitation could be uncoupled from motor neuron input. Maximal phosphorylation by PKC, in the presence of phosphatase inhibitors, reduced peak Na+ currents by approximately 90% by decreasing the maximal conductance, caused a -15 mV shift in the midpoint of steady-state inactivation, and caused a slight speeding of inactivation. Surprisingly, these effects were not affected by mutation of the conserved serine (serine-1321) in the interdomain III-IV loop. the pattern of current suppression and gating modification by PKC resembles the response of muscle Na+ channels to inhibitory factors present in the serum and cerebrospinal fluid of patients with Guillain-Barré syndrome, multiple sclerosis, and idiopathic demyelinating polyradiculoneuritis.

Structure and function of the beta 2 subunit of brain sodium channels, a transmembrane glycoprotein with a CAM motif

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.

Modulation of cardiac Na+ channel expression in Xenopus oocytes by beta 1 subunits

Voltage-gated Na+ channels consist of a large alpha subunit of 260 kDa associated with beta 1 and/or beta 2 subunits of 36 and 33 kDa, respectively. alpha subunits of rat cardiac Na+ channels (rH1) are functional when expressed alone in Xenopus oocytes or mammalian cells. beta 1 subunits are present in the heart, and localization of beta 1 subunit mRNA by in situ hybridization shows expression in the perinuclear cytoplasm of cardiac myocytes. Coexpression of beta 1 subunits with rH1 alpha subunits in Xenopus oocytes increases Na+ currents up to 6-fold in a concentration-dependent manner. However, no effects of beta 1 subunit coexpression on the kinetics or voltage dependence of the rH1 Na+ current were detected. Increased expression of Na+ currents is not observed when an equivalent mRNA encoding a nonfunctional mutant beta 1 subunit is coexpressed. Our results show that beta 1 subunits are expressed in cardiac muscle cells and that they interact with alpha subunits to increase the expression of cardiac Na+ channels in Xenopus oocytes, suggesting that beta 1 subunits are important determinants of the level of excitability of cardiac myocytes in vivo.

A simplified method for single-cell RT-PCR that can detect and distinguish genomic DNA and mRNA transcripts

Images