Regulation of muscle sodium channel transcripts during development and in response to denervation

Epigenetic Changes Governing Scn5a Expression in Denervated Skeletal Muscle

The SCN5A gene encodes the α-subunit of the voltage-gated cardiac sodium channel (Na_V1.5), a key player in cardiac action potential depolarization. Genetic variants in protein-coding regions of the human SCN5A have been largely associated with inherited cardiac arrhythmias. Increasing evidence also suggests that aberrant expression of the SCN5A gene could increase susceptibility to arrhythmogenic diseases, but the mechanisms governing SCN5A expression are not yet well understood. To gain insights into the molecular basis of SCN5A gene regulation, we used rat gastrocnemius muscle four days following denervation, a process well known to stimulate Scn5a expression. Our results show that denervation of rat skeletal muscle induces the expression of the adult cardiac Scn5a isoform. RNA-seq experiments reveal that denervation leads to significant changes in the transcriptome, with Scn5a amongst the fifty top upregulated genes. Consistent with this increase in expression, ChIP-qPCR assays show enrichment of H3K27ac and H3K4me3 and binding of the transcription factor Gata4 near the Scn5a promoter region. Also, Gata4 mRNA levels are significantly induced upon denervation. Genome-wide analysis of H3K27ac by ChIP-seq suggest that a super enhancer recently described to regulate Scn5a in cardiac tissue is activated in response to denervation. Altogether, our experiments reveal that similar mechanisms regulate the expression of Scn5a in denervated muscle and cardiac tissue, suggesting a conserved pathway for SCN5A expression among striated muscles.

Allosteric regulators selectively prevent Ca2+-feedback of CaV and NaV channels

Calmodulin (CaM) serves as a pervasive regulatory subunit of Ca_V1, Ca_V2, and Na_V1 channels, exploiting a functionally conserved carboxy-tail element to afford dynamic Ca²⁺-feedback of cellular excitability in neurons and cardiomyocytes. Yet this modularity counters functional adaptability, as global changes in ambient CaM indiscriminately alter its targets. Here, we demonstrate that two structurally unrelated proteins, SH3 and cysteine-rich domain (stac) and fibroblast growth factor homologous factors (fhf) selectively diminish Ca²⁺/CaM-regulation of Ca_V1 and Na_V1 families, respectively. The two proteins operate on allosteric sites within upstream portions of respective channel carboxy-tails, distinct from the CaM-binding interface. Generalizing this mechanism, insertion of a short RxxK binding motif into Ca_V1.3 carboxy-tail confers synthetic switching of CaM regulation by Mona SH3 domain. Overall, our findings identify a general class of auxiliary proteins that modify Ca²⁺/CaM signaling to individual targets allowing spatial and temporal orchestration of feedback, and outline strategies for engineering Ca²⁺/CaM signaling to individual targets.

Voltage-gated sodium channels: (NaV )igating the field to determine their contribution to visceral nociception

Chronic visceral pain, altered motility and bladder dysfunction are common, yet poorly managed symptoms of functional and inflammatory disorders of the gastrointestinal and urinary tracts. Recently, numerous human channelopathies of the voltage-gated sodium (NaV ) channel family have been identified, which induce either painful neuropathies, an insensitivity to pain, or alterations in smooth muscle function. The identification of these disorders, in addition to the recent utilisation of genetically modified NaV mice and specific NaV channel modulators, has shed new light on how NaV channels contribute to the function of neuronal and non-neuronal tissues within the gastrointestinal tract and bladder. Here we review the current pre-clinical and clinical evidence to reveal how the nine NaV channel family members (NaV 1.1-NaV 1.9) contribute to abdominal visceral function in normal and disease states.

Defects in Neuromuscular Transmission May Underlie Motor Dysfunction in Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy (SBMA) in men is an androgen-dependent neuromuscular disease caused by expanded CAG repeats in the androgen receptor (AR). Whether muscle or motor neuron dysfunction or both underlies motor impairment in SBMA is unknown. Muscles of SBMA mice show significant contractile dysfunction, implicating them as a likely source of motor dysfunction, but whether disease also impairs neuromuscular transmission is an open question. Thus, we examined synaptic function in three well-studied SBMA mouse models-the AR97Q, knock-in (KI), and myogenic141 models-by recording in vitro miniature and evoked end-plate potentials (MEPPs and EPPs, respectively) intracellularly from adult muscle fibers. We found striking defects in neuromuscular transmission suggesting that toxic AR in SBMA impairs both presynaptic and postsynaptic mechanisms. Notably, SBMA causes neuromuscular synapses to become weak and muscles to become hyperexcitable in all three models. Presynaptic defects included deficits in quantal content, reduced size of the readily releasable pool, and impaired short-term facilitation. Postsynaptic defects included prolonged decay times for both MEPPs and EPPs, marked resistance to μ-conotoxin (a sodium channel blocker), and enhanced membrane excitability. Quantitative PCR revealed robust upregulation of mRNAs encoding neonatal isoforms of the AChR (γ-subunit) and the voltage-gated sodium channel (NaV1.5) in diseased adult muscles of all three models, consistent with the observed slowing of synaptic potentials and resistance to μ-conotoxin. These findings suggest that muscles of SBMA patients regress to an immature state that impairs neuromuscular function.

SIGNIFICANCE STATEMENT

We have discovered that SBMA is accompanied by marked defects in neuromuscular synaptic transmission involving both presynaptic and postsynaptic mechanisms. For three different mouse models, we find that diseased synapses are weak, having reduced quantal content due to reductions in the size of the readily releasable pool and/or probability of release. Synaptic potentials in diseased adult fibers are slowed, explained by an aberrant upregulation of the neonatal isoform of the acetylcholine receptor. Diseased fibers also show marked resistance to μ-conotoxin, explained by an aberrant upregulation in the neonatal isoform of the sodium channel, and are hyperexcitable, reminiscent of myotonic dystrophy, showing anode-break action potentials. This work identifies several new molecular targets for recovering function in SBMA.

Pharmacological characterisation of the highly NaV1.7 selective spider venom peptide Pn3a

Human genetic studies have implicated the voltage-gated sodium channel NaV1.7 as a therapeutic target for the treatment of pain. A novel peptide, μ-theraphotoxin-Pn3a, isolated from venom of the tarantula Pamphobeteus nigricolor, potently inhibits NaV1.7 (IC50 0.9 nM) with at least 40-1000-fold selectivity over all other NaV subtypes. Despite on-target activity in small-diameter dorsal root ganglia, spinal slices, and in a mouse model of pain induced by NaV1.7 activation, Pn3a alone displayed no analgesic activity in formalin-, carrageenan- or FCA-induced pain in rodents when administered systemically. A broad lack of analgesic activity was also found for the selective NaV1.7 inhibitors PF-04856264 and phlotoxin 1. However, when administered with subtherapeutic doses of opioids or the enkephalinase inhibitor thiorphan, these subtype-selective NaV1.7 inhibitors produced profound analgesia. Our results suggest that in these inflammatory models, acute administration of peripherally restricted NaV1.7 inhibitors can only produce analgesia when administered in combination with an opioid.

Crude oil exposures reveal roles for intracellular calcium cycling in haddock craniofacial and cardiac development

Recent studies have shown that crude oil exposure affects cardiac development in fish by disrupting excitation-contraction (EC) coupling. We previously found that eggs of Atlantic haddock (Melanogrammus aeglefinus) bind dispersed oil droplets, potentially leading to more profound toxic effects from uptake of polycyclic aromatic hydrocarbons (PAHs). Using lower concentrations of dispersed crude oil (0.7-7 μg/L ∑PAH), here we exposed a broader range of developmental stages over both short and prolonged durations. We quantified effects on cardiac function and morphogenesis, characterized novel craniofacial defects, and examined the expression of genes encoding potential targets underlying cardiac and craniofacial defects. Because of oil droplet binding, a 24-hr exposure was sufficient to create severe cardiac and craniofacial abnormalities. The specific nature of the craniofacial abnormalities suggests that crude oil may target common craniofacial and cardiac precursor cells either directly or indirectly by affecting ion channels and intracellular calcium in particular. Furthermore, down-regulation of genes encoding specific components of the EC coupling machinery suggests that crude oil disrupts excitation-transcription coupling or normal feedback regulation of ion channels blocked by PAHs. These data support a unifying hypothesis whereby depletion of intracellular calcium pools by crude oil-derived PAHs disrupts several pathways critical for organogenesis in fish.

Development of a μO-Conotoxin Analogue with Improved Lipid Membrane Interactions and Potency for the Analgesic Sodium Channel NaV1.8

The μO-conotoxins MrVIA, MrVIB, and MfVIA inhibit the voltage-gated sodium channel NaV1.8, a well described target for the treatment of pain; however, little is known about the residues or structural elements that define this activity. In this study, we determined the three-dimensional structure of MfVIA, examined its membrane binding properties, performed alanine-scanning mutagenesis, and identified residues important for its activity at human NaV1.8. A second round of mutations resulted in (E5K,E8K)MfVIA, a double mutant with greater positive surface charge and greater affinity for lipid membranes compared with MfVIA. This analogue had increased potency at NaV1.8 and was analgesic in the mouse formalin assay.

Analgesic Effects of GpTx-1, PF-04856264 and CNV1014802 in a Mouse Model of NaV1.7-Mediated Pain

Loss-of-function mutations of Na(V)1.7 lead to congenital insensitivity to pain, a rare condition resulting in individuals who are otherwise normal except for the inability to sense pain, making pharmacological inhibition of Na(V)1.7 a promising therapeutic strategy for the treatment of pain. We characterized a novel mouse model of Na(V)1.7-mediated pain based on intraplantar injection of the scorpion toxin OD1, which is suitable for rapid in vivo profiling of Na(V)1.7 inhibitors. Intraplantar injection of OD1 caused spontaneous pain behaviors, which were reversed by co-injection with Na(V)1.7 inhibitors and significantly reduced in Na(V)1.7(-/-) mice. To validate the use of the model for profiling Na(V)1.7 inhibitors, we determined the Na(V) selectivity and tested the efficacy of the reported Na(V)1.7 inhibitors GpTx-1, PF-04856264 and CNV1014802 (raxatrigine). GpTx-1 selectively inhibited Na(V)1.7 and was effective when co-administered with OD1, but lacked efficacy when delivered systemically. PF-04856264 state-dependently and selectively inhibited Na(V)1.7 and significantly reduced OD1-induced spontaneous pain when delivered locally and systemically. CNV1014802 state-dependently, but non-selectively, inhibited Na(V) channels and was only effective in the OD1 model when delivered systemically. Our novel model of Na(V)1.7-mediated pain based on intraplantar injection of OD1 is thus suitable for the rapid in vivo characterization of the analgesic efficacy of Na(V)1.7 inhibitors.

cAMP/PKA Pathways and S56 Phosphorylation Are Involved in AA/PGE2-Induced Increases in rNaV1.4 Current

Arachidonic acid (AA) and its metabolites are important second messengers for ion channel modulation. The effects of extracellular application of AA and its non-metabolized analogue on muscle rNa_V1.4 Na⁺ current has been studied, but little is known about the effects of intracellular application of AA on this channel isoform. Here, we report that intracellular application of AA significantly augmented the rNa_V1.4 current peak without modulating the steady-state activation and inactivation properties of the rNa_V1.4 channel. These results differed from the effects of extracellular application of AA on rNa_V1.4 current. The effects of intracellular AA were mimicked by prostaglandin E₂ but not eicosatetraynoic acid (ETYA), the non-metabolized analogue of AA, and were eliminated by treatment with cyclooxygenase inhibitors, flufenamic acid, or indomethacin. AA/PGE₂-induced activation of rNa_V1.4 channels was mimicked by a cAMP analogue (db-cAMP) and eliminated by a PKA inhibitor, PKAi. Furthermore, inhibition of EP2 and EP4 (PGE₂ receptors) with AH6809 and AH23848 reduced the intracellular AA/PGE₂-induced increase of rNa_V1.4 current. Two mutated channels, rNa_V1.4S56A and rNa_V1.4T21A, were designed to investigate the role of predicted phosphorylation sites in the AA/PGE₂–mediated regulation of rNa_V1.4 currents. In rNa_V1.4S56A, the effects of intracellular db-cAMP, AA, and PGE₂ were significantly reduced. The results of the present study suggest that intracellular AA augments rNa_V1.4 current by PGE₂/EP receptor-mediated activation of the cAMP/PKA pathway, and that the S56 residue on the channel protein is important for this process.

Conservation of Ca2+/calmodulin regulation across Na and Ca2+ channels

Voltage-gated Na and Ca2+ channels comprise distinct ion channel superfamilies, yet the carboxy tails of these channels exhibit high homology, hinting at a long-shared and purposeful module. For different Ca2+ channels, carboxyl-tail interactions with calmodulin do elaborate robust and similar forms of Ca2+ regulation. However, Na channels have only shown subtler Ca2+ modulation that differs among reports, challenging attempts at unified understanding. Here, by rapid Ca2+ photorelease onto Na channels, we reset this view of Na channel regulation. For cardiac-muscle channels (NaV1.5), reported effects from which most mechanistic proposals derive, we observe no Ca2+ modulation. Conversely, for skeletal-muscle channels (NaV1.4), we uncover fast Ca2+ regulation eerily similar to that of Ca2+ channels. Channelopathic myotonia mutations halve NaV1.4 Ca2+ regulation, and transplanting the NaV1.4 carboxy tail onto Ca2+ channels recapitulates Ca2+ regulation. Thus, we argue for the persistence and physiological relevance of an ancient Ca2+ regulatory module across Na and Ca2+ channels.

Molecular surface of JZTX-V (β-Theraphotoxin-Cj2a) interacting with voltage-gated sodium channel subtype NaV1.4

Voltage-gated sodium channels (VGSCs; Na_V1.1–Na_V1.9) have been proven to be critical in controlling the function of excitable cells, and human genetic evidence shows that aberrant function of these channels causes channelopathies, including epilepsy, arrhythmia, paralytic myotonia, and pain. The effects of peptide toxins, especially those isolated from spider venom, have shed light on the structure–function relationship of these channels. However, most of these toxins have not been analyzed in detail. In particular, the bioactive faces of these toxins have not been determined. Jingzhaotoxin (JZTX)-V (also known as β-theraphotoxin-Cj2a) is a 29-amino acid peptide toxin isolated from the venom of the spider Chilobrachys jingzhao. JZTX-V adopts an inhibitory cysteine knot (ICK) motif and has an inhibitory effect on voltage-gated sodium and potassium channels. Previous experiments have shown that JZTX-V has an inhibitory effect on TTX-S and TTX-R sodium currents on rat DRG cells with IC₅₀ values of 27.6 and 30.2 nM, respectively, and is able to shift the activation and inactivation curves to the depolarizing and the hyperpolarizing direction, respectively. Here, we show that JZTX-V has a much stronger inhibitory effect on Na_V1.4, the isoform of voltage-gated sodium channels predominantly expressed in skeletal muscle cells, with an IC₅₀ value of 5.12 nM, compared with IC₅₀ values of 61.7–2700 nM for other heterologously expressed Na_V1 subtypes. Furthermore, we investigated the bioactive surface of JZTX-V by alanine-scanning the effect of toxin on Na_V1.4 and demonstrate that the bioactive face of JZTX-V is composed of three hydrophobic (W5, M6, and W7) and two cationic (R20 and K22) residues. Our results establish that, consistent with previous assumptions, JZTX-V is a Janus-faced toxin which may be a useful tool for the further investigation of the structure and function of sodium channels.

Sigma-1 receptor agonists directly inhibit Nav1.2/1.4 channels

(+)-SKF 10047 (N-allyl-normetazocine) is a prototypic and specific sigma-1 receptor agonist that has been used extensively to study the function of sigma-1 receptors. (+)-SKF 10047 inhibits K(+), Na(+) and Ca2+ channels via sigma-1 receptor activation. We found that (+)-SKF 10047 inhibited Na(V)1.2 and Na(V)1.4 channels independently of sigma-1 receptor activation. (+)-SKF 10047 equally inhibited Na(V)1.2/1.4 channel currents in HEK293T cells with abundant sigma-1 receptor expression and in COS-7 cells, which barely express sigma-1 receptors. The sigma-1 receptor antagonists BD 1063,BD 1047 and NE-100 did not block the inhibitory effects of (+)-SKF-10047. Blocking of the PKA, PKC and G-protein pathways did not affect (+)-SKF 10047 inhibition of Na(V)1.2 channel currents. The sigma-1 receptor agonists Dextromethorphan (DM) and 1,3-di-o-tolyl-guanidine (DTG) also inhibited Na(V)1.2 currents through a sigma-1 receptor-independent pathway. The (+)-SKF 10047 inhibition of Na(V)1.2 currents was use- and frequency-dependent. Point mutations demonstrated the importance of Phe(1764) and Tyr(1771) in the IV-segment 6 domain of the Na(V)1.2 channel and Phe(1579) in the Na(V)1.4 channel for (+)-SKF 10047 inhibition. In conclusion, our results suggest that sigma-1 receptor agonists directly inhibit Na(V)1.2/1.4 channels and that these interactions should be given special attention for future sigma-1 receptor function studies.

Genetic architecture of a feeding adaptation: garter snake (Thamnophis) resistance to tetrodotoxin bearing prey

Detailing the genetic basis of adaptive variation in natural populations is a first step towards understanding the process of adaptive evolution, yet few ecologically relevant traits have been characterized at the genetic level in wild populations. Traits that mediate coevolutionary interactions between species are ideal for studying adaptation because of the intensity of selection and the well-characterized ecological context. We have previously described the ecological context, evolutionary history and partial genetic basis of tetrodotoxin (TTX) resistance in garter snakes (Thamnophis). Derived mutations in a voltage-gated sodium channel gene (Na(v)1.4) in three garter snake species are associated with resistance to TTX, the lethal neurotoxin found in their newt prey (Taricha). Here we evaluate the contribution of Na(v)1.4 alleles to TTX resistance in two of those species from central coastal California. We measured the phenotypes (TTX resistance) and genotypes (Na(v)1.4 and microsatellites) in a local sample of Thamnophis atratus and Thamnophis sirtalis. Allelic variation in Na(v)1.4 explains 23 per cent of the variation in TTX resistance in T. atratus while variation in a haphazard sample of the genome (neutral microsatellite markers) shows no association with the phenotype. Similarly, allelic variation in Na(v)1.4 correlates almost perfectly with TTX resistance in T. sirtalis, but neutral variation does not. These strong correlations suggest that Na(v)1.4 is a major effect locus. The simple genetic architecture of TTX resistance in garter snakes may significantly impact the dynamics of phenotypic coevolution. Fixation of a few alleles of major effect in some garter snake populations may have led to the evolution of extreme phenotypes and an 'escape' from the arms race with newts.

μ-conotoxin KIIIA derivatives with divergent affinities versus efficacies in blocking voltage-gated sodium channels

The possibility of independently manipulating the affinity and efficacy of pore-blocking ligands of sodium channels is of interest for the development of new drugs for the treatment of pain. The analgesic mu-conotoxin KIIIA (KIIIA), a 16-residue peptide with three disulfide bridges, is a pore blocker of voltage-gated sodium channels, including neuronal subtype Na(V)1.2 (K(d) = 5 nM). At saturating concentrations, KIIIA incompletely blocks the sodium current of Na(V)1.2, leaving a 5% residual current (rI(Na)). Lys7 is an important residue: the K7A mutation decreases both the efficacy (i.e., increases rI(Na) to 23%) and the affinity of the peptide (K(d) = 115 nM). In this report, various replacements of residue 7 were examined to determine whether affinity and efficacy were inexorably linked. Because of their facile chemical synthesis, KIIIA analogues that had as a core structure the disulfide-depleted KIIIA[C1A,C2U,C9A,C15U] (where U is selenocysteine) or ddKIIIA were used. Analogues ddKIIIA and ddKIIIA[K7X], where X represents one of nine different amino acids, were tested on voltage-clamped Xenopus oocytes expressing rat Na(V)1.2 or Na(V)1.4. Their affinities ranged from 0.01 to 36 muM and rI(Na) values from 2 to 42%, and these two variables appeared to be uncorrelated. Instead, rI(Na) varied inversely with side chain size, and remarkably charge and hydrophobicity appeared to be inconsequential. The ability to manipulate a mu-conopeptide's affinity and efficacy, as well as its capacity to interfere with subsequent tetrodotoxin binding, greatly expands its scope as a reagent for probing sodium channel structure and function and may also lead to the development of mu-conotoxins as safe analgesics.

Cell membrane expression of cardiac sodium channel Na(v)1.5 is modulated by alpha-actinin-2 interaction

Cardiac sodium channel Na(v)1.5 plays a critical role in heart excitability and conduction. The molecular mechanism that underlies the expression of Na(v)1.5 at the cell membrane is poorly understood. Previous studies demonstrated that cytoskeleton proteins can be involved in the regulation of cell surface expression and localization of several ion channels. We performed a yeast two-hybrid screen to identify Na(v)1.5-associated proteins that may be involved in channel function and expression. We identified alpha-actinin-2 as an interacting partner of the cytoplasmic loop connecting domains III and IV of Na(v)1.5 (Na(v)1.5/LIII-IV). Co-immunoprecipitation and His(6) pull-down assays confirmed the physical association between Na(v)1.5 and alpha-actinin-2 and showed that the spectrin-like repeat domain is essential for binding of alpha-actinin-2 to Na(v)1.5. Patch-clamp studies revealed that the interaction with alpha-actinin-2 increases sodium channel density without changing their gating properties. Consistent with these findings, coexpression of alpha-actinin-2 and Na(v)1.5 in tsA201 cells led to an increase in the level of expression of Na(v)1.5 at the cell membrane as determined by cell surface biotinylation. Lastly, immunostaining experiments showed that alpha-actinin-2 was colocalized with Na(v)1.5 along the Z-lines and in the plasma membrane. Our data suggest that alpha-actinin-2, which is known to regulate the functional expression of the potassium channels, may play a role in anchoring Na(v)1.5 to the membrane by connecting the channel to the actin cytoskeleton network.

Regulated and aberrant glycosylation modulate cardiac electrical signaling

Millions afflicted with Chagas disease and other disorders of aberrant glycosylation suffer symptoms consistent with altered electrical signaling such as arrhythmias, decreased neuronal conduction velocity, and hyporeflexia. Cardiac, neuronal, and muscle electrical signaling is controlled and modulated by changes in voltage-gated ion channel activity that occur through physiological and pathological processes such as development, epilepsy, and cardiomyopathy. Glycans attached to ion channels alter channel activity through isoform-specific mechanisms. Here we show that regulated and aberrant glycosylation modulate cardiac ion channel activity and electrical signaling through a cell-specific mechanism. Data show that nearly half of 239 glycosylation-associated genes (glycogenes) were significantly differentially expressed among neonatal and adult atrial and ventricular myocytes. The N-glycan structures produced among cardiomyocyte types were markedly variable. Thus, the cardiac glycome, defined as the complete set of glycan structures produced in the heart, is remodeled. One glycogene, ST8sia2, a polysialyltransferase, is expressed only in the neonatal atrium. Cardiomyocyte electrical signaling was compared in control and ST8sia2((-/-)) neonatal atrial and ventricular myocytes. Action potential waveforms and gating of less sialylated voltage-gated Na+ channels were altered consistently in ST8sia2((-/-)) atrial myocytes. ST8sia2 expression had no effect on ventricular myocyte excitability. Thus, the regulated (between atrium and ventricle) and aberrant (knockout in the neonatal atrium) expression of a single glycogene was sufficient to modulate cardiomyocyte excitability. A mechanism is described by which cardiac function is controlled and modulated through physiological and pathological processes that involve regulated and aberrant glycosylation.

Sodium current properties of primary skeletal myocytes and cardiomyocytes derived from different mouse strains

The mouse has become the preferred animal for genetic manipulations. Because of the diverse genetic backgrounds of various mouse strains, these can manifest strikingly different characteristics. Here, we studied the functional properties of currents through voltage-gated sodium channels in primary cultures of skeletal myocytes and cardiomyocytes derived from the three commonly used mouse strains BL6, 129/Sv, and FVB, by using the whole-cell patch-clamp technique. We found strain-specific sodium current function in skeletal myocytes, which could partly be explained by differences in sodium channel isoform expression. In addition, we found significant effects of cell source (neonatal or adult animal-derived) and variation of the differentiation time period. In contrast to skeletal myocytes, sodium current function in cardiomyocytes was similar in all strains. Our findings are relevant for the design and proper interpretation of electrophysiological studies, which use excitable cells in primary culture as a model system.

Repetitive firing triggers clustering of Kv2.1 potassium channels in Aplysia neurons

The Kv2.1 gene encodes a highly conserved delayed rectifier potassium channel that is widely expressed in neurons of the central nervous system. In the bag cell neurons of Aplysia, Kv2.1 channels contribute to the repolarization of action potentials during a prolonged afterdischarge that triggers a series of reproductive behaviors. Partial inactivation of Aplysia Kv2.1 during repetitive firing produces frequency-dependent broadening of action potentials during the afterdischarge. We have now found that, as in mammalian neurons, Kv2.1 channels in bag cell neurons are localized to ring-like clusters in the plasma membrane of the soma and proximal dendrites. Either elevation of cyclic AMP levels or direct electrical stimulation of afterdischarge rapidly enhanced formation of these clusters on the somata of these neurons. In contrast, injection of a 13-amino acid peptide corresponding to a region in the C terminus that is required for clustering of Kv2.1 channels produced disassociation of the clusters, resulting in a more uniform distribution over the somata. Voltage clamp recordings demonstrated that peptide-induced dissociation of the Kv2.1 clusters is associated with an increase in the amplitude of delayed rectifier current and a shift of activation toward more negative potentials. In current clamp recording, injection of the unclustering peptide reduced the width of action potentials and reduced frequency-dependent broadening of action potentials. Our results suggest that rapid redistribution of Kv2.1 channels occurs during physiological changes in neuronal excitability.

Size Matters: Erythromelalgia Mutation S241T in Nav1.7 Alters Channel Gating

The Nav1.7 sodium channel is preferentially expressed in most nociceptive dorsal root ganglion neurons and in sympathetic neurons. Inherited erythromelalgia (IEM, also known as erythermalgia), an autosomal dominant neuropathy characterized by burning pain in the extremities in response to mild warmth, has been linked to mutations in Nav1.7. Recently, a substitution of Ser-241 by threonine (S241T) in the domain I S4-S5 linker of Nav1.7 was identified in a family with IEM. To investigate the possible causative role of this mutation in the pathophysiology of IEM, we used whole-cell voltage-clamp analysis to study the effects of S241T on Nav1.7 gating in HEK293 cells. We found a hyperpolarizing shift of activation midpoint by 8.4 mV, an accelerated time to peak, slowing of deactivation, and an increase in the current in response to small, slow depolarizations. Additionally, S241T produced an enhancement of slow inactivation, shifting the midpoint by -12.3 mV. Because serine and threonine have similar biochemical properties, the S241T substitution suggested that the size of the side chain at this position affected channel gating. To test this hypothesis, we investigated the effect of S241A and S241L substitutions on the gating properties of Nav1.7. Although S241A did not alter the properties of the channel, S241L mimicked the effects of S241T. We conclude that the linker between S4 and S5 in domain I of Nav1.7 modulates gating of this channel, and that a larger side chain at position 241 interferes with its gating mechanisms.

Embryonic and larval expression of zebrafish voltage-gated sodium channel alpha-subunit genes

Whereas it is known that voltage-gated calcium channels play important roles during development, potential embryonic roles of voltage-gated sodium channels have received much less attention. Voltage-gated sodium channels consist of pore-forming alpha-subunits (Na(v)1) and auxiliary beta-subunits. Here, we report the embryonic and larval expression patterns for all eight members of the gene family (scna) coding for zebrafish Na(v)1 proteins. We find that each scna gene displays a distinct expression pattern that is temporally and spatially dynamic during embryonic and larval stages. Overall, our findings indicate that scna gene expression occurs sufficiently early during embryogenesis to play developmental roles for both muscle and nervous tissues.

The denervated muscle: facts and hypotheses. A historical review

Denervation changes in skeletal muscle (atrophy; alterations of myofibrillar expression, muscle membrane electrical properties, ACh sensitivity and excitation-contraction coupling process; fibrillation), and their possible causes are reviewed. All changes can be counteracted by muscle electrostimulation, while denervation-like effects can be caused by the complete conduction block in muscle nerve. These results do not support the hypothesis that the lack of neurotrophic, non-motor factors plays a role in denervation phenomena. Instead they support the view that the lack of neuromotor discharge is the only cause of the phenomena and that neuromotor activity is an essential factor in regulating muscle properties. However, some experimental results cannot apparently be explained by the lack of neuromotor impulses, and may still suggest that neurotrophic influences exist. A hypothesis is that neurotrophic factors, too feeble to maintain a role in completely differentiated, adult muscles, can concur with neuromotor activity in the differentiation of immature, developing muscles.

Evolutionary diversification of TTX-resistant sodium channels in a predator–prey interaction

Understanding the molecular genetic basis of adaptations provides incomparable insight into the genetic mechanisms by which evolutionary diversification takes place. Whether the evolution of common traits in different lineages proceeds by similar or unique mutations, and the degree to which phenotypic evolution is controlled by changes in gene regulation as opposed to gene function, are fundamental questions in evolutionary biology that require such an understanding of genetic mechanisms. Here we identify novel changes in the molecular structure of a sodium channel expressed in snake skeletal muscle, tsNa(V)1.4, that are responsible for differences in tetrodotoxin (TTX) resistance among garter snake populations coevolving with toxic newts. By the functional expression of tsNa(V)1.4, we show how differences in the amino-acid sequence of the channel affect TTX binding and impart different levels of resistance in four snake populations. These results indicate that the evolution of a physiological trait has occurred through a series of unique functional changes in a gene that is otherwise highly conserved among vertebrates.

Expression and distribution of a small-conductance calcium-activated potassium channel (SK3) protein in skeletal muscles from myotonic muscular dystrophy patients and congenital myotonic mice

The SK3 channel, a small-conductance calcium-activated potassium channel, is expressed in immature fibers of skeletal muscle and becomes down regulated after innervation. We have previously shown that the level of mRNA of the SK3 channel is increased in muscle from myotonic dystrophy. In this study, we have carried out an immunohistochemical study using a polyclonal anti-SK3 antibody. SK3 protein is partly expressed at the cell membrane of normal sized fibers in myotonic dystrophy. Although SK3 channels are also expressed in muscles from polymyositis and amyotrophic lateral sclerosis, the positive staining is observed only in regenerating or denervated fibers. No expression of SK3 protein in a myotonic mouse (ADR) suggests that the increase in the SK3 channel in myotonic dystrophy is not due to hyperexcitability. These data support the hypothesis of a differentiation defect in myotonic dystrophy.

SK channels are necessary but not sufficient for denervation-induced hyperexcitability

Skeletal muscle hyperexcitability is characteristically associated with denervation. Expression of SK3, a small conductance Ca(2+)-activated K(+) channel (SK channel) in skeletal muscle is induced by denervation, and direct application of apamin, a peptide blocker of SK channels, dramatically reduces hyperexcitability. To investigate the role of SK3 channels in denervation- induced hyperexcitability, SK3 expression was manipulated using a transgenic mouse that harbors a tetracycline-regulated SK3 gene. Electromyographic (EMG) recordings from anterior tibial (AT) muscle showed that denervated muscle from transgenic or wild-type animals had equivalent hyperexcitability that was blocked by apamin. In contrast, denervated skeletal muscle from SK3tTA mice lacking SK3 channels showed little or no hyperexcitability, similar to results from wild-type innervated skeletal muscle. However, innervated skeletal muscle from SK3tTA mice containing SK3 channels did not show hyperexcitability. The results demonstrate that SK3 channels are necessary but not sufficient for denervation-induced skeletal muscle hyperexcitability.

Sodium influx during action potential in innervated and denervated rat skeletal muscles

Resting Na(+) influx (J(i)(Na)) was measured in innervated and denervated (1-6 days) rat extensor digitorum longus muscle in the absence and presence of 2 micromol/L tetrodotoxin (TTX). The mean value of Na(+) permeability (P(Na)) in innervated muscles was 49.6 +/- 2.6 pm.s(-1). At the second day postdenervation, it decreased by about 45%. This was followed, between the second and fourth days, by a sharp rise, which by the sixth day reached a steady value approximately 2.5 times greater than that of innervated muscles. This, most likely, generated the 30% increase in internal [Na(+)] concentration ([Na(+)](I)) observed at this time. Tetrodotoxin reduced P(Na) of both innervated and denervated muscles by about 25%. In 6-day denervated muscles, virtually all the TTX effect on P(Na) represents the blockage of TTX-resistant Na(+) channels. Denervation produced a depolarization of about 20 mV by the sixth day. The extra J(i)(Na) per action potential (AP) decreased monotonically with time after denervation from 20.0 +/- 3.8 in innervated to 11.1 +/- 1.0 nmol.g(-1).AP(-1) in 6-day denervated muscles. The overshoot of the AP decreased from 15 +/- 1 in innervated to 7 +/- 1 mV in 6-day denervated muscles. Likewise, the maximum rate of rise (+dV/dt), an expression of the inward Na(+) current, fell from 305 +/- 14 in innervated to 188 +/- 18 V.s(-1) in 6-day denervated muscles. The estimated 6-day denervated/innervated ratio of peak Na(+) conductance (g(Na)) was 0.67. The changes in AP parameters promoted by denervation were substantially reduced when both innervated and denervated fibers were hyperpolarized to -90 mV. These results suggest that the depolarization, mainly due to the increase in P(Na) /P(K) ratio, increases Na(+) inactivation and consequently reduces peak g(Na), in spite of the absolute increment in resting TTX-sensitive P(Na). This, in addition to the moderate reduction in the inward driving force on Na(+), decreases the inward Na(+) current and the extra J(i)(Na) per AP.

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.

Primary structure and developmental expression of zebrafish sodium channel Na(v)1.6 during neurogenesis

A zebrafish sodium channel cDNA encoding a 1949-amino acid polypeptide, Na(v)1.6, was isolated. Two transcripts were detected in zebrafish adult brain but not in cardiac or skeletal muscle. The RNase protection analysis confirmed the neural specificity of zebrafish Na(v)1.6 24 hours postfertilization (hpf) Na(v)1.6 was expressed in the trigeminal ganglion, anterior and posterior lateral line ganglia, rhombomeres, and Rohon-Beard neurons. This preferential localization suggests that Na(v)1.6 plays an important role in tactile sensitivity. The abundance of zebrafish Na(v) 1.6 mRNA in the central and peripheral nervous systems increased markedly between 48 and 72 hpf, during the maturation of the nervous system.

The expression of ion channel mRNAs in skeletal muscles from patients with myotonic muscular dystrophy

We investigated gene expression patterns of ion channels including the apamin-sensitive small-conductance Ca(2+)-activated K(+) (SK3) channel, the adult isoform of the skeletal muscle Na(+) channel (SkM1), the fetal isoform of skeletal muscle Na(+) channel (H1), and the Cl(-) channel (ClC-1) by using the semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) for muscle samples from patients with adult onset myotonic dystrophy (DM), amyotrophic lateral sclerosis, and polymyositis. Patients with DM showed a significant increase in SK3 mRNA but not in mRNAs for other ion channels. The increased expression of SK3 gene in DM did not correlate with H1, the marker of muscle denervation, or the percentage of type 2C fiber, the marker of muscle regeneration.

Effects of channel cytoplasmic regions on the activation mechanisms of cardiac versus skeletal muscle Na(+) channels

Functional comparison of skeletal muscle (rSkM1) and cardiac (hH1) voltage-gated sodium channel isoforms expressed in Chinese hamster ovary cells showed rSkM1 half-activation (V(a)) and inactivation (V(i)) voltages 7 and 10 mV more depolarized than hH1 V(a) and V(i), respectively. Internal papain perfusion removed fast inactivation from each isoform and caused a 20-mV hyperpolarizing shift in hH1 V(a), with an insignificant change in rSkM1 V(a). Activation voltage of the inactivation-deficient hH1 mutant, hH1Q3, was nearly identical to wild-type hH1 V(a), both before and after papain treatment, with hH1Q3 V(a) also shifted by nearly 20 mV after internal papain perfusion. These data indicate that while papain removes both hH1 and rSkM1 inactivation, it has a second effect only on hH1 that causes a shift in activation voltage. Internal treatment with an antibody directed against the III-IV linker essentially mimicked papain treatment by removing some inactivation from each isoform and causing a 12-mV shift in hH1 V(a), while rSkM1 V(a) remained constant. This suggests that some channel segment within, near, or interacting with the III-IV linker is involved in establishing hH1 activation voltage. Together the data show that rSkM1 and hH1 activation mechanisms are different and are the first to suggest a role for a cytoplasmic structure in the voltage-dependent activation of cardiac sodium channels.

Skeletal muscle and small-conductance calcium-activated potassium channels

Skeletal muscle becomes hyperexcitable following denervation and when cultured in the absence of nerve cells. In these circumstances, the bee venom peptide toxin apamin, a blocker of small-conductance calcium-activated potassium (SK) channels, dramatically reduces the hyperexcitability. In this report, we show that SK3 channels are expressed in denervated skeletal muscle and in L6 cells. Action potentials evoked from normal innervated rat skeletal muscle did not exhibit an afterhyperpolarization, indicating a lack of SK channel activity; very low levels of apamin binding sites, SK3 protein, or SK3 mRNA were present. However, denervation resulted in apamin-sensitive afterhyperpolarizations and increased apamin binding sites, SK3 protein, and SK3 mRNA. Cultured rat L6 myoblasts and differentiated L6 myotubes contained similar levels of SK3 mRNA, although apamin-sensitive SK currents and apamin binding sites were detected only following myotube differentiation. Therefore, different molecular mechanisms govern SK3 expression levels in denervated muscle compared with muscle cells differentiated in culture.

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.

Neural influence on protein kinase C isoform expression in skeletal muscle

Protein kinase C (PKC) is a family of enzymes involved in synapse formation and signal transduction at the neuromuscular junction. Two PKC isoforms, classical PKC alpha and novel PKC theta, have been shown to be enriched in skeletal muscle or localized to the endplate. We examined the role of nerve in regulating the expression of these PKC isoforms in rat skeletal muscle by denervating diaphragm muscle and measuring PKC protein expression at various postoperative times. nPKC theta protein levels decreased 65% after denervation, whereas cPKC alpha levels increased 80% compared with control hemidiaphragms. These results suggest that innervation regulates PKC theta and alpha isoform expression in skeletal muscle. To explore further how nerve regulates PKC expression, we characterized PKC isoform expression in rat myotubes deprived of neural input. Myoblast expression of nPKC theta was low, and the increase in nPKC theta expression that occurred during differentiation into myotubes resulted in levels of nPKC theta significantly below adult skeletal muscle. cPKC alpha expression in myoblastic increased during differentiation to levels that exceeded expression in adult skeletal muscle. Coculturing myotubes within neuroblastoma X glioma hybrid clonal cell line (NG108-15) increased nPKC theta expression, but not cPKC alpha, suggesting that nPKC theta in skeletal muscle and myotubes is regulated by nerve contact or by a factor(s) provided by nerve. Treating myotubes with tetrodotoxin did not affect either basal- or NG108-15 cell-stimulated nPKC theta expression. Together these results suggest that expression of nPKC theta in skeletal muscle is regulated by a transynaptic interaction with nerve that specifically influences nPKC theta expression.

Expression and distribution of sodium channels in short- and long-term denervated rodent skeletal muscles

1. Loose-patch voltage-clamp recordings were made from rat and mouse skeletal muscle fibres denervated for up to 6 weeks. Innervated muscles possessed a Na+ current density of 107 +/- 3.3 mA cm-2 in endplate membrane, and 6.3 +/- 0.6 mA cm-2 in extrajunctional membrane. This high concentration of Na+ channels at the endplate was gradually reduced following denervation. After 6 weeks of denervation, the endplate Na+ channel concentration was reduced by 40-50%, and the density of Na+ channels in extrajunctional membrane was increased by about 30%. 2. The tetrodotoxin (TTX)-resistant form of the Na+ channel appeared after 3 days of denervation and comprised approximately 43% of the endplate Na+ channels 5-6 days after denervation. Subsequently, TTX-resistant Na+ channels were reduced in density to approximately 25% of the postjunctional Na+ channels and remained at this level up to 6 weeks after denervation. 3. RNase protection analysis showed that mRNA encoding the TTX-resistant Na+ channel was virtually absent in innervated muscle, rose > 50-fold after 3 days of denervation, then decreased by 95% 6 weeks after denervation. The density of TTX-resistant Na+ channels correlated qualitatively with changes in mRNA levels. 4. These results suggest that the density of Na+ channels at neuromuscular junctions is maintained by two mechanisms, one influenced by the nerve terminal and the other independent of innervation.

Tetrodotoxin-resistant sodium channels

1. Tetrodotoxin (TTX) has been widely used as a chemical tool for blocking Na+ channels. However, reports are accumulating that some Na+ channels are resistant to TTX in various tissues and in different animal species. Studying the sensitivity of Na+ channels to TTX may provide us with an insight into the evolution of Na+ channels. 2. Na+ channels present in TTX-carrying animals such as pufferfish and some types of shellfish, frogs, salamanders, octopuses, etc., are resistant to TTX. 3. Denervation converts TTX-sensitive Na+ channels to TTX-resistant ones in skeletal muscle cells, i.e., reverting-back phenomenon. Also, undifferentiated skeletal muscle cells contain TTX-resistant Na+ channels. Cardiac muscle cells and some types of smooth muscle cells are considerably insensitive to TTX. 4. TTX-resistant Na+ channels have been found in cell bodies of many peripheral nervous system (PNS) neurons in both immature and mature animals. However, TTX-resistant Na+ channels have been reported in only a few types of central nervous system (CNS). Axons of PNS and CNS neurons are sensitive to TTX. However, some glial cells have TTX-resistant Na+ channels. 5. Properties of TTX-sensitive and TTX-resistant Na+ channels are different. Like Ca2+ channels, TTX-resistant Na+ channels can be blocked by inorganic (Co2+, Mn2+, Ni2+, Cd2+, Zn2+, La3+) and organic (D-600) Ca2+ channel blockers. Usually, TTX-resistant Na+ channels show smaller single-channel conductance, slower kinetics, and a more positive current-voltage relation than TTX-sensitive ones. 6. Molecular aspects of the TTX-resistant Na+ channel have been described. The structure of the channel has been revealed, and changing its amino acid(s) alters the sensitivity of the Na+ channel to TTX. 7. TTX-sensitive Na+ channels seem to be used preferentially in differentiated cells and in higher animals instead of TTX-resistant Na+ channels for rapid and effective processing of information. 8. Possible evolution courses for Na+ and Ca2+ channels are discussed with regard to ontogenesis and phylogenesis.

Molecular cloning and functional analysis of the promoter of rat skeletal muscle voltage-sensitive sodium channel subtype 2 (rSkM2): evidence for muscle-specific nuclear protein binding to the core promoter

rSkM2 is a tetrodotoxin-resistant rat skeletal muscle voltage-sensitive sodium channel that is expressed in immature and denervated skeletal muscle and in adult heart. We have isolated a 3.7-kb gene segment that contains the first exon, multiple transcription initiation sites, the core promoter (nt -102 to +1), GC-rich elements (Sp1 recognition sites), three overlapping C-rich motifs (important for muscle-specific expression of some muscle genes), and multiple CANNTG (E-box) motifs (MyoD binding sites). A deletion analysis of the 5' upstream 2.8-kb segment, driving the rSkM2 core promoter, has localized a muscle-restrictive enhancer element (MRSE) at least 2 kb upstream from the core promoter. The core promoter is silenced by an additional cis element (-645/-506). The positive and negative cis-elements together drive transcription of the chloramphenicol acetyltransferase (CAT) reporter gene from the core promoter at about the same level as does the core promoter alone in a skeletal muscle differentiation stage-specific manner. Gel-shift assays have identified sequence- and cell-type-specific proteins that bind to a 16-bp region (-44/-29) containing C-rich motifs. Muscle-specific complexes formed from muscle cell nuclear extracts and a 16-bp element (-44/-29) are competed by unlabeled -44/-29 oligonucleotide but not by several mutant oligonucleotides that implicate nucleotides -40 to -38 and -34 to -32 in the binding of a nuclear protein (designated SkM2 transcription factor 1, SkM2-TF1). We conclude that rSkM2 gene expression depends on the interactions of positive and negative transcriptional regulators with tissue- and developmental stage-specific core promoter elements.

Sodium channels aggregate at former synaptic sites in innervated and denervated regenerating muscles

The role of innervation in the establishment and regulation of the synaptic density of voltage-activated Na channels (NaChs) was investigated at regenerating neuromuscular junctions. Rat muscles were induced to degenerate after injection of the Australian tiger snake toxin, notexin. The loose-patch voltage clamp technique was used to measure the density and distribution of NaChs on muscle fibers regenerating with or without innervation. In either case, new myofibers formed within the original basal lamina sheaths, and, NaChs became concentrated at regenerating endplates nearly as soon as they formed. The subsequent increase in synaptic NaCh density followed a time course similar to postnatal muscles. Neuromuscular endplates regenerating after denervation, with no nerve terminals present, had NaCh densities not significantly different from endplates regenerating in the presence of nerve terminals. The results show that the nerve terminal is not required for the development of an enriched NaCh density at regenerating neuromuscular synapses and implicate Schwann cells or basal lamina as the origin of the signal for NaCh aggregation. In contrast, the change in expression from the immature to the mature form of the NaCh isoform that normally accompanies development occurred only partially on muscles regenerating in the absence of innervation. This aspect of NaCh regulation is thus dependent upon innervation.

Nerve growth factor regulates the abundance and distribution of K+ channels in PC12 cells

We examined the effect of nerve growth factor (NGF) treatment on expression of a neuronal delayed rectifler K+ channel subtype, Kv2.1 (drk1), in PC12 cells. Anti-Kv2.1 antibodies recognized a single polypeptide population of M(r) = 132 kD in PC12 cell membranes, distinct from the more heterogeneous population found in adult rat brain. In response to NGF treatment, levels of Kv2.1 polypeptide in PC12 membranes increased fourfold. This increase in polypeptide levels could be seen within 12 h, and elevated levels were maintained for at least 6 d of continuous NGF treatment. RNase protection assays indicate that this increase in Kv2.1 protein occurs without an increase in steady state levels of Kv2.1 mRNA following NGF treatment. Immunofluorescent localization of the Kv2.1 polypeptide revealed plasma membrane-associated staining of cell bodies in both untreated and NGF-treated PC12 cells. In undifferentiated cells, intense staining is seen at sites of cell-cell and cell-substratum contact. In differentiated cells the most intense Kv2.1 staining is observed in neuritic growth cones. These studies show that in PC12 cells both the abundance and distribution of the Kv2.1 k+ channel are regulated by NGF, and suggest that PC12 cells provide a model for the selective expression of Kv2.1 in neuritic endings.

Neuronal growth factor regulation of two different sodium channel types through distinct signal transduction pathways

Neuronal growth factors regulate the expression of voltage-activated sodium current in differentiating sympathetic neurons and PC12 cells. We show that, in PC12 cells, the NGF- and FGF-induced sodium current results from increased expression of two distinct sodium channel types. Sodium current results from the rapid induction of a novel sodium channel transcript, also found in peripheral neurons, and from the long term induction of brain type II/IIA mRNA. Expression of the type II/IIA sodium channel requires activation of the cyclic AMP-dependent protein kinase (A-kinase), whereas induction of the peripheral neuron type sodium channel occurs through an A-kinase-independent signal transduction pathway. These findings suggest that the two sodium channel types act in concert to ensure the generation of action potentials during neuronal differentiation.

Expression of Kv2.1 delayed rectifier K+ channel isoforms in the developing rat brain

Mammalian central neurons regulate a diverse set of functions by changes in spatial and temporal expression patterns of a large family of K+ channel gene products. Here the temporal patterns of expression of Kv2.1 (drk1) K+ channel mRNAs and polypeptides have been studied in rat brain. Northern blots using Kv2.1-specific probes show that three size classes (4.4, 9.0, 11.5 kb) of Kv2.1 K+ channel transcripts are present in rat brain. These mRNAs show differential patterns of expression over embryonic and postnatal developmental, with the 4.4 kb transcript being the predominant embryonic Kv2.1 mRNA, and the 11.5 kb transcript being the predominant adult mRNA. Immunoblots using Kv2.1-specific site-directed antibodies show the existence of multiple Kv2.1 polypeptides, which differ in their mobility on SDS polyacrylamide gels, in their immunoreactivity to a carboxyl terminal-directed anti-Kv2.1 antibody, and in their developmental expression. Thus Kv2.1 polypeptide isoforms exhibit discrete temporal patterning during neuronal development, implying distinct roles for these channel proteins in vivo.

Periodic paralysis, myotonia congenita and sarcolemmal ion channels: a success of the candidate gene approach

The classification of periodic paralyses and myotonic syndromes has been a subject of debates for the last 40 yr. Recent advances in molecular biology have led geneticists to reconsider this old question, using a candidate gene approach. Two groups of disorders have now emerged: (1) muscle sodium channel-associated diseases which include hyperkalemic periodic paralysis and its clinical variants, as well as paramyotonia congenita; (2) muscle chloride channel-associated disorders which comprise both the dominant and recessive form of myotonia congenita. This review is focussed on these recent discoveries.

Developmental expression of voltage-sensitive K+ channels in mouse skeletal muscle and C2C12 cells

The developmental expression of voltage-sensitive K+ channels was analyzed by Northern blot in mouse skeletal muscle. Of nine Shaker-like genes studied, eight are expressed in this mammalian muscle. Their expression is differentially regulated during development. The mouse cell line C2C12 has been used to study expression of voltage-sensitive K+ channels during in vitro myotube differentiation. Different voltage-sensitive K+ channel messages are also expressed in these cells which display a pattern of expression depending upon the differentiation stage. The message for the very peculiar K+ channel of IsK type could only be detected by polymerase chain reaction on skeletal muscle mRNA.

muI Na+ channels expressed transiently in human embryonic kidney cells: biochemical and biophysical properties

We describe the transient expression of the rat skeletal muscle muI Na+ channel in human embryonic kidney (HEK 293) cells. Functional channels appear at a density of approximately 30 in a 10 microns 2 patch, comparable to those of native excitable cells. Unlike muI currents in oocytes, inactivation gating is predominantly (approximately 97%) fast, although clear evidence is provided for noninactivating gating modes, which have been linked to anomalous behavior in the inherited disorder hyperkalemic periodic paralysis. Sequence-specific antibodies detect a approximately 230 kd glycopeptide. The majority of molecules acquire only neutral oligosaccharides and are retained within the cell. Electrophoretic mobility on SDS gels suggests the molecules may acquire covalently attached lipid. The channel is readily phosphorylated by activation of the protein kinase A and protein kinase C second messenger pathways.

Primary structure of the adult human skeletal muscle voltage-dependent sodium channel

The gene encoding the principal voltage-dependent sodium channel expressed in adult human skeletal muscle (SCN4A) has recently been linked to the pathogenesis of human hyperkalemic periodic paralysis and paramyotonia congenita. We report the cloning and nucleotide sequence determination of the normal product of this gene. The 7,823 nucleotide complementary DNA, designated hSkM1, encodes a 1,836 amino acid protein that exhibits 92% identity with the tetrodotoxin-sensitive rat skeletal muscle sodium channel alpha subunit, but lower homology with either the human heart sodium channel or with other sodium channels from immature rat muscle or rat brain. Specific hSkM1 RNA transcripts are expressed in adult human skeletal muscle but not in heart, brain, or uterus. The SCN4A gene product, hSkM1, is the human homologue of rSkM1, the tetrodotoxin-sensitive sodium channel characteristic of adult rat skeletal muscle. This structural information should provide the necessary backdrop for identifying and evaluating mutations affecting the function of this channel in the periodic paralyses.