Regulation of Na+ channel beta 1 and beta 2 subunit mRNA levels in cultured rat astrocytes

Sodium channel β subunits: emerging targets in channelopathies

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in excitable cells. VGSCs in mammalian brain are heterotrimeric complexes of α and β subunits. Although β subunits were originally termed auxiliary, we now know that they are multifunctional signaling molecules that play roles in both excitable and nonexcitable 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. Although VGSC β subunit-specific drugs have not yet been developed, this protein family is an emerging therapeutic target.

The role of non-pore-forming β subunits in physiology and pathophysiology of voltage-gated sodium channels

Voltage-gated sodium channel β1 and β2 subunits were discovered as auxiliary proteins that co-purify with pore-forming α subunits in brain. The other family members, β1B, β3, and β4, were identified by homology and shown to modulate sodium current in heterologous systems. Work over the past 2 decades, however, has provided strong evidence that these proteins are not simply ancillary ion channel subunits, but are multifunctional signaling proteins in their own right, playing both conducting (channel modulatory) and nonconducting roles in cell signaling. Here, we discuss evidence that sodium channel β subunits not only regulate sodium channel function and localization but also modulate voltage-gated potassium channels. In their nonconducting roles, VGSC β subunits function as immunoglobulin superfamily cell adhesion molecules that modulate brain development by influencing cell proliferation and migration, axon outgrowth, axonal fasciculation, and neuronal pathfinding. Mutations in genes encoding β subunits are linked to paroxysmal diseases including epilepsy, cardiac arrhythmia, and sudden infant death syndrome. Finally, β subunits may be targets for the future development of novel therapeutics.

Electrophysiology and beyond: multiple roles of Na+ channel β subunits in development and disease

Voltage-gated Na+ channel (VGSC) β Subunits are not "auxiliary." These multi-functional molecules not only modulate Na+ current (I(Na)), but also function as cell adhesion molecules (CAMs)-playing roles in aggregation, migration, invasion, neurite outgrowth, and axonal fasciculation. β subunits are integral members of VGSC signaling complexes at nodes of Ranvier, axon initial segments, and cardiac intercalated disks, regulating action potential propagation through critical intermolecular and cell-cell communication events. At least in vitro, many β subunit cell adhesive functions occur both in the presence and absence of pore-forming VGSC α subunits, and in vivo β subunits are expressed in excitable as well as non-excitable cells, thus β subunits may play important functional roles on their own, in the absence of α subunits. VGSC β1 subunits are essential for life and appear to be especially important during brain development. Mutations in β subunit genes result in a variety of human neurological and cardiovascular diseases. Moreover, some cancer cells exhibit alterations in β subunit expression during metastasis. In short, these proteins, originally thought of as merely accessory to α subunits, are critical players in their own right in human health and disease. Here we discuss the role of VGSC β subunits in the nervous system.

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.

Floxed allele for conditional inactivation of the voltage-gated sodium channel β1 subunitScn1b

The voltage-gated sodium channel gene Scn1b encodes the auxiliary subunit beta1, which is widely distributed in neurons and glia of the central and peripheral nervous systems, cardiac myocytes, skeletal muscle myocytes, and neuroendocrine cells. We showed previously that the Scn1b null mutation results in a complex and severe phenotype that includes retarded growth, seizures, ataxia, and death by postnatal day 21. We generated a floxed allele of Scn1b by inserting loxP sites surrounding the second coding exon. Ubiquitous deletion of the floxed exon by Cre recombinase using CMV-Cre-transgenic mice produced the Scn1b(del) allele. The null phenotype of Scn1b(del) homozygotes is indistinguishable from that of Scn1b nulls and confirms the invivo inactivation of Scn1b. Conditional inactivation ofthe floxed allele will make it possible to circumvent the lethality that results from complete loss of this gene, such that the physiological role of Scn1b in specific cell types and/or specific developmental time points can be investigated.

Expression and regulation of voltage-gated sodium channel beta1 subunit protein in human gliosis-associated pathologies

Auxiliary beta1 subunits of voltage-gated sodium channels (NaChs) critically regulate channel activity and may also act as cell adhesion molecules (CAMs). In a recent study we have shown that the expression of beta1 NaCh protein is increased in reactive astrocytes in a rat epilepsy model of mesial temporal lobe epilepsy. The present study was undertaken to examine whether changes of NaCh beta1 subunit protein expression are also associated with structural changes occurring in human reactive astrocytes under different pathological conditions in vivo, as well as in response to changing environmental conditions in vitro. Strong beta1 astroglial immunoreactivity was present in human brain tissue from patients with astrogliosis. The over-expression of beta1 protein in reactive glia was observed in both epilepsy-associated brain pathologies (temporal lobe epilepsy, cortical dysplasia), as well as non-epileptic (cerebral infarction, multiple sclerosis, amyotrophic lateral sclerosis, meningo-encephalitis) disorders. The up-regulation of beta1 subunit protein in astrocytes can be reproduced in vitro. beta1 protein is highly expressed in human astrocytes cultured in the presence of trophic factors, under conditions in which they show morphology similar to the morphology of cells undergoing reactive gliosis. The growth factor-induced overexpression of beta1 protein was abrogated by PD98059, which inhibits the mitogen-activated protein kinase pathway. These findings demonstrate that the expression of NaCh beta1 subunit protein in astrocytes is plastic, and indicate a novel mechanism for modulation of glial function in gliosis-associated pathologies.

Structural requirements for interaction of sodium channel beta 1 subunits with ankyrin

Sodium channel beta subunits modulate channel kinetic properties and cell surface expression levels and function as cell adhesion molecules. beta 1 and beta 2 participate in homophilic cell adhesion resulting in ankyrin recruitment to cell contact sites. We hypothesized that a tyrosine residue in the cytoplasmic domain of beta 1 may be important for ankyrin recruitment and tested our hypothesis using beta 1 mutants replacing Tyr(181) with alanine (beta 1Y181A), phenylalanine (beta 1Y181F), or glutamate (beta 1Y181E), or a truncated construct deleting all residues beyond Tyr(181) (beta 1L182(STOP)). Ankyrin recruitment was observed in beta 1L182(STOP), showing that residues Ile(166)-Tyr(181) contain the major ankyrin recruiting activity of beta 1. Ankyrin recruitment was abolished in beta 1Y181E, suggesting that tyrosine phosphorylation of beta 1 may inhibit beta 1-ankyrin interactions. Ankyrin(G) and beta 1 associate in rat brain membranes and in transfected cells expressing beta 1 and ankyrin(G) in the absence of sodium channel alpha subunits. beta 1 subunits are recognized by anti-phosphotyrosine antibodies following treatment of these cell lines with fibroblast growth factor. beta 1 and ankryin(G) association is not detectable in cells following treatment with fibroblast growth factor. Ankyrin(G) and beta 1Y181E do not associate even in the absence of fibroblast growth factor treatment. beta 1 subunit-mediated cell adhesion and ankyrin recruitment may contribute to sodium channel placement at nodes of Ranvier. The phosphorylation state of beta 1Y181 may be a critical regulatory step in these developmental processes.

Heterogeneous increases of cytoplasmic calcium: distinct effects on down-regulation of cell surface sodium channels and sodium channel subunit mRNA levels

1. Long-term (> or = 12 h) treatment of cultured bovine adrenal chromaffin cells with A23187 (a Ca(2+) ionophore) or thapsigargin (TG) [an inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)] caused a time- and concentration-dependent reduction of cell surface [(3)H]-saxitoxin (STX) binding capacity, but did not change the K:(D:) value. In A23187- or TG-treated cells, veratridine-induced (22)Na(+) influx was reduced (with no change in veratridine EC(50) value) while it was enhanced by alpha-scorpion venom, beta-scorpion venom, or Ptychodiscus brevis toxin-3, like in nontreated cells. 2. The A23187- or TG-induced decrease of [(3)H]-STX binding was diminished by BAPTA-AM. EGTA also inhibited the decreasing effect of A23187. A23187 caused a rapid, monophasic and persistent increase in intracellular concentration of Ca(2+) ([Ca(2+)](i)) to a greater extent than that observed with TG. 2,5-Di-(t-butyl)-1,4-benzohydroquinone (DBHQ) (an inhibitor of SERCA) produced only a rapid monophasic increase in [Ca(2+)](i), without any effect on [(3)H]-STX binding. 3. Reduction in [(3)H]-STX binding capacity induced by A23187 or TG was attenuated by Gö6976 (an inhibitor of conventional protein kinase C) or calpastatin peptide (an inhibitor of calpain). When the internalization rate of cell surface Na(+) channels was measured in the presence of brefeldin A (an inhibitor of vesicular exit from the trans-Golgi network), A23187 or TG accelerated the reduction of [(3)H]-STX binding capacity. 4. Six hours treatment with A23187 lowered Na(+) channel alpha- and beta(1)-subunit mRNA levels, whereas TG had no effect. 5. These results suggest that elevation of [Ca(2+)](i) caused by A23187, TG or DBHQ exerted differential effects on down-regulation of cell surface functional Na(+) channels and Na(+) channel subunit mRNA levels.

Sodium channel beta1 and beta2 subunits parallel SNS/PN3 alpha-subunit changes in injured human sensory neurons

Voltage-gated sodium channels consist of a pore-containing alpha-subunit and one or more auxiliary beta-subunits, which may modulate channel function. We previously demonstrated that sodium channel SNS/PN3 alpha-subunits were decreased in human sensory cell bodies after spinal root avulsion injury, and accumulated at injured nerve terminals in pain states. Using specific antibodies for immunohistochemistry, we have now detected sodium channel beta1 and beta2 subunits in sensory cell bodies within control human postmortem sensory ganglia (78% of small/medium (< or = 50 microm) and 68% of large (> or = 50 microm) cells); their changes in cervical sensory ganglia after avulsion injury paralleled those described for SNS/PN3 alpha-subunits. Our results suggest that alpha- and beta-subunits share common regulatory mechanisms, but present distinct targets for novel analgesics.

Sodium channel beta 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 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.

I. Cellular and molecular biology of sodium channel beta-subunits: therapeutic implications for pain?

Voltage-gated sodium channel alpha-subunits have been shown to be key mediators of the pathophysiology of pain. The present review considers the role of sodium channel auxiliary beta-subunits in channel modulation, channel protein expression levels, and interactions with extracellular matrix and cytoskeletal signaling molecules. Although beta-subunits have not yet been directly implicated in pain mechanisms, their intimate association with and ability to regulate alpha-subunits predicts that they may be a viable target for therapeutic intervention in the future. It is proposed that multifunctional sodium channel beta-subunits provide a critical link between extracellular and intracellular signaling molecules and thus have the ability to fine tune channel activity and electrical excitability.

The sodium channel auxiliary subunits beta1 and beta2 are differentially expressed in the spinal cord of neuropathic rats

Neuropathic pain is thought to arise from ectopic discharges at the site of injury within the peripheral nervous system, and is manifest as a general increase in the level of neuronal excitability within primary afferent fibres and their synaptic contacts within the spinal cord. Voltage-activated Na+ channel blockers such as lamotrigine have been shown to be clinically effective in the treatment of neuropathic pain. Na+ channels are structurally diverse comprising a principal a subunit (of which there are variable isoforms) and two auxiliary subunits termed beta1 and beta2. Both beta subunits affect the rates of channel activation and inactivation, and can modify alpha subunit density within the plasma membrane. In addition, these subunits may interact with extracellular matrix molecules to affect growth and myelination of axons. Using in situ hybridization histochemistry we have shown that the expression of the beta1 and beta2 subunits within the dorsal horn of the spinal cord of neuropathic rats is differentially regulated by a chronic constrictive injury to the sciatic nerve. At days 12-15 post-neuropathy, beta1 messenger RNA levels had increased, whereas beta2 messenger RNA levels had decreased significantly within laminae I, II on the ipsilateral side of the cord relative to the contralateral side. Within laminae III-IV beta2 messenger RNA levels showed a small but significant decrease on the ipsilateral side relative to the contralateral side, whilst expression of beta1 messenger RNA remained unchanged. Thus, differential regulation of the individual beta subunit types may (through their distinct influences on Na+ channel function) contribute to altered excitability of central neurons after neuropathic injury.

Molecular and functional remodeling of electrogenic membrane of hypothalamic neurons in response to changes in their input

Neurons respond to stimuli by integrating generator and synaptic potentials and generating action potentials. However, whether the underlying electrogenic machinery within neurons itself changes, in response to alterations in input, is not known. To determine whether there are changes in Na+ channel expression and function within neurons in response to altered input, we exposed magnocellular neurosecretory cells (MNCs) in the rat supraoptic nucleus to different osmotic milieus by salt-loading and studied Na+ channel mRNA and protein, and Na+ currents, in these cells. In situ hybridization demonstrated significantly increased mRNA levels for alpha-II, Na6, beta1 and beta2 Na+ channel subunits, and immunohistochemistry/immunoblotting showed increased Na+ channel protein after salt-loading. Using patch-clamp recordings to examine the deployment of functional Na+ channels in the membranes of MNCs, we observed an increase in the amplitude of the transient Na+ current after salt-loading and an even greater increase in amplitude and density of the persistent Na+ current evoked at subthreshold potentials by slow ramp depolarizations. These results demonstrate that MNCs respond to salt-loading by selectively synthesizing additional, functional Na+ channel subtypes whose deployment in the membrane changes its electrogenic properties. Thus, neurons may respond to changes in their input not only by producing different patterns of electrical activity, but also by remodeling the electrogenic machinery that underlies this activity.

Expression of the mRNA for the beta 2 subunit of the voltage-dependent sodium channel in rat CNS

Expression of the voltage-dependent sodium channel has been analysed in adult rat central nervous system by Northern blotting and in situ hybridization. Northern blots showed that all the territories studied express beta 2 transcripts, albeit with widely varying levels (with cerebellum >> hippocampus > brain > brainstem > spinal cord). In situ hybridization confirmed that in these structures, all the neuronal cell bodies contain beta 2 mRNA; expression was particularly high in the granule cells of the cerebellum, in both pyramidal cell layer and dentate gyrus in the hippocampus, and in spinal cord motor neurons. Northern blots also showed that RNA extracted from optic nerve and cultured cortical astrocytes contained beta 2 mRNA, while it was totally absent from sciatic nerve. In situ hybridization evidenced the presence of a numerous population of beta 2-positive cells in cerebellum white matter, spinal cord white matter, and in corpus callosum, where frontal sections showed labelled cells arranged in the chain-like or row pattern typical of interfascicular oligodendrocytes. Combination of antiglial fibrillary acid protein (GFAP) immunofluorescent histochemistry with detection of beta 2 mRNA evidenced that expression of the transcripts was indeed restricted to GFAP-negative cells in white matter.

Multiple pathways regulate the expression of genes encoding sodium channel subunits in developing neurons

In primary cultures of fetal neurons, activation of sodium channels with either alpha-scorpion toxin or veratridine caused a rapid and persistent decrease of mRNAs encoding beta2 and different sodium channel alpha mRNAs. In contrast, beta1 subunit mRNA was up-regulated by sodium channel activation. This phenomenon was calcium-independent. The effects of activating toxins on mRNAs of different sodium channel subunits were mimicked by membrane depolarization. An important aspect of this study was the demonstration that cAMP also caused rapid reduction of alphaI, alphaII and alphaIII mRNA levels whereas beta1 subunit mRNA was up regulated and beta2 subunit mRNA was not affected. Sodium channel activation by veratridine was shown to increase cAMP immunoreactivity in cultured neurons, but alphaII mRNA down-regulation induced by activating toxins was not reversed by protein kinase A antagonists, indicating that this phenomenon is not protein kinase A dependent. The effects of cAMP and membrane depolarisation were antagonized by the PKA inhibitor H89. These results are indicative of the existence of multiple and independent regulatory pathways modulating the expression of sodium channel genes in the developing central nervous system.