Functional association of the beta 1 subunit with human cardiac (hH1) and rat skeletal muscle (mu 1) sodium channel alpha subunits expressed in Xenopus oocytes

Beta-subunit-eliminated eHAP expression (BeHAPe) cells reveal subunit regulation of the cardiac voltage-gated sodium channel

Voltage-gated sodium (NaV) channels drive the upstroke of the action potential and are comprised of a pore-forming α-subunit and regulatory β-subunits. The β-subunits modulate the gating, trafficking, and pharmacology of the α-subunit. These functions are routinely assessed by ectopic expression in heterologous cells. However, currently available expression systems may not capture the full range of these effects since they contain endogenous β-subunits. To better reveal β-subunit functions, we engineered a human cell line devoid of endogenous NaV β-subunits and their immediate phylogenetic relatives. This new cell line, β-subunit-eliminated eHAP expression (BeHAPe) cells, were derived from haploid eHAP cells by engineering inactivating mutations in the β-subunits SCN1B, SCN2B, SCN3B, and SCN4B, and other subfamily members MPZ (myelin protein zero(P0)), MPZL1, MPZL2, MPZL3, and JAML. In diploid BeHAPe cells, the cardiac NaV α-subunit, NaV1.5, was highly sensitive to β-subunit modulation and revealed that each β-subunit and even MPZ imparted unique gating properties. Furthermore, combining β1 and β2 with NaV1.5 generated a sodium channel with hybrid properties, distinct from the effects of the individual subunits. Thus, this approach revealed an expanded ability of β-subunits to regulate NaV1.5 activity and can be used to improve the characterization of other α/β NaV complexes.

Cardiac sodium channel complexes and arrhythmia: structural and functional roles of the β1 and β3 subunits

In cardiac myocytes, the voltage-gated sodium channel NaV 1.5 opens in response to membrane depolarisation and initiates the action potential. The NaV 1.5 channel is typically associated with regulatory β-subunits that modify gating and trafficking behaviour. These β-subunits contain a single extracellular immunoglobulin (Ig) domain, a single transmembrane α-helix and an intracellular region. Here we focus on the role of the β1 and β3 subunits in regulating NaV 1.5. We catalogue β1 and β3 domain specific mutations that have been associated with inherited cardiac arrhythmia, including Brugada syndrome, long QT syndrome, atrial fibrillation and sudden death. We discuss how new structural insights into these proteins raises new questions about physiological function.

Closed-state inactivation of cardiac, skeletal, and neuronal sodium channels is isoform specific

Voltage-gated sodium (Nav) channels produce the upstroke of action potentials in excitable tissues throughout the body. The gating of these channels is determined by the asynchronous movements of four voltage-sensing domains (VSDs). Past studies on the skeletal muscle Nav1.4 channel have indicated that VSD-I, -II, and -III are sufficient for pore opening, whereas VSD-IV movement is sufficient for channel inactivation. Here, we studied the cardiac sodium channel, Nav1.5, using charge-neutralizing mutations and voltage-clamp fluorometry. Our results reveal that both VSD-III and -IV are necessary for Nav1.5 inactivation, and that steady-state inactivation can be modulated by all VSDs. We also demonstrate that channel activation is partially determined by VSD-IV movement. Kinetic modeling suggests that these observations can be explained from the cardiac channel's propensity to enter closed-state inactivation (CSI), which is significantly higher than that of other Nav channels. We show that skeletal muscle Nav1.4, cardiac Nav1.5, and neuronal Nav1.6 all have different propensities for CSI and postulate that these differences produce isoform-dependent roles for the four VSDs.

Structural Basis for Pore Blockade of the Human Cardiac Sodium Channel Nav 1.5 by the Antiarrhythmic Drug Quinidine*

Na_v 1.5, the primary voltage-gated Na⁺ (Na_v ) channel in heart, is a major target for class I antiarrhythmic agents. Here we present the cryo-EM structure of full-length human Na_v 1.5 bound to quinidine, a class Ia antiarrhythmic drug, at 3.3 Å resolution. Quinidine is positioned right beneath the selectivity filter in the pore domain and coordinated by residues from repeats I, III, and IV. Pore blockade by quinidine is achieved through both direct obstruction of the ion permeation path and induced rotation of an invariant Tyr residue that tightens the intracellular gate. Structural comparison with a truncated rat Na_v 1.5 in the presence of flecainide, a class Ic agent, reveals distinct binding poses for the two antiarrhythmics within the pore domain. Our work reported here, along with previous studies, reveals the molecular basis for the mechanism of action of class I antiarrhythmic drugs.

Action potentials in Xenopus oocytes triggered by blue light

Voltage-gated sodium (Na⁺) channels are responsible for the fast upstroke of the action potential of excitable cells. The different α subunits of Na⁺ channels respond to brief membrane depolarizations above a threshold level by undergoing conformational changes that result in the opening of the pore and a subsequent inward flux of Na⁺. Physiologically, these initial membrane depolarizations are caused by other ion channels that are activated by a variety of stimuli such as mechanical stretch, temperature changes, and various ligands. In the present study, we developed an optogenetic approach to activate Na⁺ channels and elicit action potentials in Xenopus laevis oocytes. All recordings were performed by the two-microelectrode technique. We first coupled channelrhodopsin-2 (ChR2), a light-sensitive ion channel of the green alga Chlamydomonas reinhardtii, to the auxiliary β1 subunit of voltage-gated Na⁺ channels. The resulting fusion construct, β1-ChR2, retained the ability to modulate Na⁺ channel kinetics and generate photosensitive inward currents. Stimulation of Xenopus oocytes coexpressing the skeletal muscle Na⁺ channel Na_v1.4 and β1-ChR2 with 25-ms lasting blue-light pulses resulted in rapid alterations of the membrane potential strongly resembling typical action potentials of excitable cells. Blocking Na_v1.4 with tetrodotoxin prevented the fast upstroke and the reversal of the membrane potential. Coexpression of the voltage-gated K⁺ channel K_v2.1 facilitated action potential repolarization considerably. Light-induced action potentials were also obtained by coexpressing β1-ChR2 with either the neuronal Na⁺ channel Na_v1.2 or the cardiac-specific isoform Na_v1.5. Potential applications of this novel optogenetic tool are discussed.

Cross-kingdom auxiliary subunit modulation of a voltage-gated sodium channel

Voltage-gated, sodium ion-selective channels (NaV) generate electrical signals contributing to the upstroke of the action potential in animals. NaVs are also found in bacteria and are members of a larger family of tetrameric voltage-gated channels that includes CaVs, KVs, and NaVs. Prokaryotic NaVs likely emerged from a homotetrameric Ca2+-selective voltage-gated progenerator, and later developed Na+ selectivity independently. The NaV signaling complex in eukaryotes contains auxiliary proteins, termed beta (β) subunits, which are potent modulators of the expression profiles and voltage-gated properties of the NaV pore, but it is unknown whether they can functionally interact with prokaryotic NaV channels. Herein, we report that the eukaryotic NaVβ1-subunit isoform interacts with and enhances the surface expression as well as the voltage-dependent gating properties of the bacterial NaV, NaChBac in Xenopus oocytes. A phylogenetic analysis of the β-subunit gene family proteins confirms that these proteins appeared roughly 420 million years ago and that they have no clear homologues in bacterial phyla. However, a comparison between eukaryotic and bacterial NaV structures highlighted the presence of a conserved fold, which could support interactions with the β-subunit. Our electrophysiological, biochemical, structural, and bioinformatics results suggests that the prerequisites for β-subunit regulation are an evolutionarily stable and intrinsic property of some voltage-gated channels.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Sodium channel haploinsufficiency and structural change in ventricular arrhythmogenesis

Normal cardiac excitation involves orderly conduction of electrical activation and recovery dependent upon surface membrane, voltage-gated, sodium (Na(+) ) channel α-subunits (Nav 1.5). We summarize experimental studies of physiological and clinical consequences of loss-of-function Na(+) channel mutations. Of these conditions, Brugada syndrome (BrS) and progressive cardiac conduction defect (PCCD) are associated with sudden, often fatal, ventricular tachycardia (VT) or fibrillation. Mouse Scn5a(+/-) hearts replicate important clinical phenotypes modelling these human conditions. The arrhythmic phenotype is associated not only with the primary biophysical change but also with additional, anatomical abnormalities, in turn dependent upon age and sex, each themselves exerting arrhythmic effects. Available evidence suggests a unified binary scheme for the development of arrhythmia in both BrS and PCCD. Previous biophysical studies suggested that Nav 1.5 deficiency produces a background electrophysiological defect compromising conduction, thereby producing an arrhythmic substrate unmasked by flecainide or ajmaline challenge. More recent reports further suggest a progressive decline in conduction velocity and increase in its dispersion particularly in ageing male Nav 1.5 haploinsufficient compared to WT hearts. This appears to involve a selective appearance of slow conduction at the expense of rapidly conducting pathways with changes in their frequency distributions. These changes were related to increased cardiac fibrosis. It is thus the combination of the structural and biophysical changes both accentuating arrhythmic substrate that may produce arrhythmic tendency. This binary scheme explains the combined requirement for separate, biophysical and structural changes, particularly occurring in ageing Nav 1.5 haploinsufficient males in producing clinical arrhythmia.

Post-translational modifications of voltage-gated sodium channels in chronic pain syndromes

In the peripheral sensory nervous system the neuronal expression of voltage-gated sodium channels (Navs) is very important for the transmission of nociceptive information since they give rise to the upstroke of the action potential (AP). Navs are composed of nine different isoforms with distinct biophysical properties. Studying the mutations associated with the increase or absence of pain sensitivity in humans, as well as other expression studies, have highlighted Nav1.7, Nav1.8, and Nav1.9 as being the most important contributors to the control of nociceptive neuronal electrogenesis. Modulating their expression and/or function can impact the shape of the AP and consequently modify nociceptive transmission, a process that is observed in persistent pain conditions. Post-translational modification (PTM) of Navs is a well-known process that modifies their expression and function. In chronic pain syndromes, the release of inflammatory molecules into the direct environment of dorsal root ganglia (DRG) sensory neurons leads to an abnormal activation of enzymes that induce Navs PTM. The addition of small molecules, i.e., peptides, phosphoryl groups, ubiquitin moieties and/or carbohydrates, can modify the function of Navs in two different ways: via direct physical interference with Nav gating, or via the control of Nav trafficking. Both mechanisms have a profound impact on neuronal excitability. In this review we will discuss the role of Protein Kinase A, B, and C, Mitogen Activated Protein Kinases and Ca++/Calmodulin-dependent Kinase II in peripheral chronic pain syndromes. We will also discuss more recent findings that the ubiquitination of Nav1.7 by Nedd4-2 and the effect of methylglyoxal on Nav1.8 are also implicated in the development of experimental neuropathic pain. We will address the potential roles of other PTMs in chronic pain and highlight the need for further investigation of PTMs of Navs in order to develop new pharmacological tools to alleviate pain.

The cardiac sodium channel gene SCN5A and its gene product NaV1.5: Role in physiology and pathophysiology

The gene SCN5A encodes the main cardiac sodium channel NaV1.5. This channel predominates the cardiac sodium current, INa, which underlies the fast upstroke of the cardiac action potential. As such, it plays a crucial role in cardiac electrophysiology. Over the last 60years a tremendous amount of knowledge regarding its function at the electrophysiological and molecular level has been acquired. Furthermore, genetic studies have shown that mutations in SCN5A are associated with multiple cardiac diseases (e.g. Brugada syndrome, Long QT syndrome, conduction disease and cardiomyopathy), while genetic variation in the general population has been associated with differences in cardiac conduction and risk of arrhythmia through genome wide association studies. In this review we aim to give an overview of the current knowledge (and the gaps therein) on SCN5A and NaV1.5.

Comparison of Gating Properties and Use-Dependent Block of Nav1.5 and Nav1.7 Channels by Anti-Arrhythmics Mexiletine and Lidocaine

Mexiletine and lidocaine are widely used class IB anti-arrhythmic drugs that are considered to act by blocking voltage-gated open sodium currents for treatment of ventricular arrhythmias and relief of pain. To gain mechanistic insights into action of anti-arrhythmics, we characterized biophysical properties of Na_v1.5 and Na_v1.7 channels stably expressed in HEK293 cells and compared their use-dependent block in response to mexiletine and lidocaine using whole-cell patch clamp recordings. While the voltage-dependent activation of Na_v1.5 or Na_v1.7 was not affected by mexiletine and lidocaine, the steady-state fast and slow inactivation of Na_v1.5 and Na_v1.7 were significantly shifted to hyperpolarized direction by either mexiletine or lidocaine in dose-dependent manner. Both mexiletine and lidocaine enhanced the slow component of closed-state inactivation, with mexiletine exerting stronger inhibition on either Na_v1.5 or Na_v1.7. The recovery from inactivation of Na_v1.5 or Na_v1.7 was significantly prolonged by mexiletine compared to lidocaine. Furthermore, mexiletine displayed a pronounced and prominent use-dependent inhibition of Na_v1.5 than lidocaine, but not Na_v1.7 channels. Taken together, our findings demonstrate differential responses to blockade by mexiletine and lidocaine that preferentially affect the gating of Na_v1.5, as compared to Na_v1.7; and mexiletine exhibits stronger use-dependent block of Na_v1.5. The differential gating properties of Na_v1.5 and Na_v1.7 in response to mexiletine and lidocaine may help explain the drug effectiveness and advance in new designs of safe and specific sodium channel blockers for treatment of cardiac arrhythmia or pain.

Voltage-gated sodium channels in the mammalian heart

Mammalian species express nine functional voltage-gated Na(+) channels. Three of them, the cardiac-specific isoform Nav1.5 and the neuronal isoforms Nav1.8 and Nav1.9, are relatively resistant to the neurotoxin tetrodotoxin (TTX; IC50 ≥ 1 μM). The other six isoforms are highly sensitive to TTX with IC50 values in the nanomolar range. These isoforms are expressed in the central nervous system (Nav1.1, Nav1.2, Nav1.3, Nav1.6), in the skeletal muscle (Nav1.4), and in the peripheral nervous system (Nav1.6, Nav1.7). The isoform Nav1.5, encoded by the SCN5A gene, is responsible for the upstroke of the action potential in the heart. Mutations in SCN5A are associated with a variety of life-threatening arrhythmias, like long QT syndrome type 3 (LQT3), Brugada syndrome (BrS) or cardiac conduction disease (CCD). Previous immunohistochemical and electrophysiological assays demonstrated the cardiac expression of neuronal and skeletal muscle Na(+) channels in the heart of various mammals, which led to far-reaching speculations on their function. However, when comparing the Na(+) channel mRNA patterns in the heart of various mammalian species, only minute quantities of transcripts for TTX-sensitive Na(+) channels were detectable in whole pig and human hearts, suggesting that these channels are not involved in cardiac excitation phenomena in higher mammals. This conclusion is strongly supported by the fact that mutations in TTX-sensitive Na(+) channels were associated with epilepsy or skeletal muscle diseases, rather than with a pathological cardiac phenotype. Moreover, previous data from TTX-intoxicated animals and from cases of human tetrodotoxication showed that low TTX dosages caused at most little alterations of both the cardiac output and the electrocardiogram. Recently, genome-wide association studies identified SCN10A, the gene encoding Nav1.8, as a determinant of cardiac conduction parameters, and mutations in SCN10A have been associated with BrS. These novel findings opened a fascinating new research area in the cardiac ion channel field, and the on-going debate on how SCN10A/Nav1.8 affects cardiac conduction is very exciting.

Diseases caused by mutations in Nav1.5 interacting proteins

Sodium current in the heart flows principally through the pore protein NaV1.5, which is part of a complex of interacting proteins that serve both to target and localize the complex in the membrane, and to modulate function by such post-translational modifications as phosphorylation and nitrosylation. Multiple mutations in seven different NaV1.5 interacting proteins have been associated with dysfunctional sodium current and inherited cardiac diseases, including long QT syndrome, Brugada syndrome, atrial fibrillation, and cardiomyopathy, as well as sudden infant death syndrome (SIDS). Mutations in as yet unidentified interacting proteins may account for cardiac disease for which a genetic basis has not yet been established. Characterizing the mechanisms by which these mutations cause disease may give insight into etiologies and treatments of more common acquired cardiac disease, such as ischemia and heart failure.

β1- and β3- voltage-gated sodium channel subunits modulate cell surface expression and glycosylation of Nav1.7 in HEK293 cells

Voltage-gated sodium channels (Navs) are glycoproteins composed of a pore-forming α-subunit and associated β-subunits that regulate Nav α-subunit plasma membrane density and biophysical properties. Glycosylation of the Nav α-subunit also directly affects Navs gating. β-subunits and glycosylation thus comodulate Nav α-subunit gating. We hypothesized that β-subunits could directly influence α-subunit glycosylation. Whole-cell patch clamp of HEK293 cells revealed that both β1- and β3-subunits coexpression shifted V ½ of steady-state activation and inactivation and increased Nav1.7-mediated I Na density. Biotinylation of cell surface proteins, combined with the use of deglycosydases, confirmed that Nav1.7 α-subunits exist in multiple glycosylated states. The α-subunit intracellular fraction was found in a core-glycosylated state, migrating at ~250 kDa. At the plasma membrane, in addition to the core-glycosylated form, a fully glycosylated form of Nav1.7 (~280 kDa) was observed. This higher band shifted to an intermediate band (~260 kDa) when β1-subunits were coexpressed, suggesting that the β1-subunit promotes an alternative glycosylated form of Nav1.7. Furthermore, the β1-subunit increased the expression of this alternative glycosylated form and the β3-subunit increased the expression of the core-glycosylated form of Nav1.7. This study describes a novel role for β1- and β3-subunits in the modulation of Nav1.7 α-subunit glycosylation and cell surface expression.

A naturally occurring amino acid substitution in the voltage-dependent sodium channel selectivity filter affects channel gating

The pore of sodium channels contains a selectivity filter made of 4 amino acids, D/E/K/A. In voltage sensitive sodium channel (Nav) channels from jellyfish to human the fourth amino acid is Ala. This Ala, when mutated to Asp, promotes slow inactivation. In some Nav channels of pufferfishes, the Ala is replaced with Gly. We studied the biophysical properties of an Ala-to-Gly substitution (A1529G) in rat Nav1.4 channel expressed in Xenopus oocytes alone or with a β1 subunit. The Ala-to-Gly substitution does not affect monovalent cation selectivity and positively shifts the voltage-dependent inactivation curve, although co-expression with a β1 subunit eliminates the difference between A1529G and WT. There is almost no difference in channel fast inactivation, but the β1 subunit accelerates WT current inactivation significantly more than it does the A1529G channels. The Ala-to-Gly substitution mainly influences the rate of recovery from slow inactivation. Again, the β1 subunit is less effective on speeding recovery of A1529G than the WT. We searched Nav channels in numerous databases and noted at least four other independent Ala-to-Gly substitutions in Nav channels in teleost fishes. Thus, the Ala-to-Gly substitution occurs more frequently than previously realized, possibly under selection for alterations of channel gating.

Channelopathies from mutations in the cardiac sodium channel protein complex

The cardiac sodium current underlies excitability in heart, and inherited abnormalities of the proteins regulating and conducting this current cause inherited arrhythmia syndromes. This review focuses on inherited mutations in non-pore forming proteins of sodium channel complexes that cause cardiac arrhythmia, and the deduced mechanisms by which they affect function and dysfunction of the cardiac sodium current. Defining the structure and function of these complexes and how they are regulated will contribute to understanding the possible roles for this complex in normal and abnormal physiology and homeostasis. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Arrhythmogenesis in Brugada syndrome: impact and constrains of current concepts

Brugada syndrome (BrS), an inherited arrhythmogenic disease first described in 1992, is characterized by ST segment elevations on the electrocardiogram in the right precordium and by a high occurrence of arrhythmias including the life-threatening ventricular tachycardia/fibrillation. Knowledge of the underlying mechanisms of formation of arrhythmogenic substrate in BrS is essential, namely for the risk stratification of BrS patients and their therapy which is still restrained almost exclusively to the implantation of cardioverter/defibrillator. In spite of many crucial findings in this field published within recent years, the final consistent view has not been established so far. Hence, BrS described 20 years ago remains an actual topic of both clinical and experimental studies. This review presents an overview of the current knowledge related to the pathogenesis of BrS arrhythmogenic substrate, namely of the genetic basis of BrS, functional consequences of mutations related to BrS, and arrhythmogenic mechanisms in BrS.

Cardiac sodium channel Nav1.5 mutations and cardiac arrhythmia

As a major cardiac voltage-gated sodium channel isoform in the heart, the Nav1.5 channel is essential for cardiac action potential initiation and subsequent propagation throughout the heart. Mutations of Nav1.5 have been linked to a variety of cardiac diseases such as long QT syndrome (LQTs), Brugada syndrome, cardiac conduction defect, atrial fibrillation, and dilated cardiomyopathy. The mutagenesis approach and heterologous expression systems are most frequently used to study the function of this channel. This review focuses primarily on recent findings of Nav1.5 mutations associated with type 3 long QT syndrome (LQT3) in particular. Understanding the functional changes of the Nav1.5 mutation may offer critical insight into the mechanism of long QT3 syndrome. In addition, this review provides the updated information on the current progress of using various experimental model systems to study primarily the long QT3 syndrome.

A novel rare variant in SCN1Bb linked to Brugada syndrome and SIDS by combined modulation of Na(v)1.5 and K(v)4.3 channel currents

BACKGROUND

Cardiac sodium channel β-subunit mutations have been associated with several inherited cardiac arrhythmia syndromes.

OBJECTIVE

To identify and characterize variations in SCN1Bb associated with Brugada syndrome (BrS) and sudden infant death syndrome (SIDS).

METHODS

All known exons and intron borders of the BrS-susceptibility genes were amplified and sequenced in both directions. Wild type (WT) and mutant genes were expressed in TSA201 cells and studied using co-immunoprecipitation and whole-cell patch-clamp techniques.

RESULTS

Patient 1 was a 44-year-old man with an ajmaline-induced type 1 ST-segment elevation in V1 and V2 supporting the diagnosis of BrS. Patient 2 was a 62-year-old woman displaying a coved-type BrS electrocardiogram who developed cardiac arrest during fever. Patient 3 was a 4-month-old female SIDS case. A R214Q variant was detected in exon 3A of SCN1Bb (Na(v)1B) in all three probands, but not in any other gene previously associated with BrS or SIDS. R214Q was identified in 4 of 807 ethnically-matched healthy controls (0.50%). Co-expression of SCN5A/WT + SCN1Bb/R214Q resulted in peak sodium channel current (I(Na)) 56.5% smaller compared to SCN5A/WT + SCN1Bb/WT (n = 11-12, P<0.05). Co-expression of KCND3/WT + SCN1Bb/R214Q induced a Kv4.3 current (transient outward potassium current, I(to)) 70.6% greater compared with KCND3/WT + SCN1Bb/WT (n = 10-11, P<0.01). Co-immunoprecipitation indicated structural association between Na(v)β1B and Na(v)1.5 and K(v)4.3.

CONCLUSION

Our results suggest that R214Q variation in SCN1Bb is a functional polymorphism that may serve as a modifier of the substrate responsible for BrS or SIDS phenotypes via a combined loss of function of sodium channel current and gain of function of transient outward potassium current.

Tubulin polymerization modifies cardiac sodium channel expression and gating

AIMS

Treatment with the anticancer drug taxol (TXL), which polymerizes the cytoskeleton protein tubulin, may evoke cardiac arrhythmias based on reduced human cardiac sodium channel (Na(v)1.5) function. Therefore, we investigated whether enhanced tubulin polymerization by TXL affects Na(v)1.5 function and expression and whether these effects are beta1-subunit-mediated.

METHODS AND RESULTS

Human embryonic kidney (HEK293) cells, transfected with SCN5A cDNA alone (Na(v)1.5) or together with SCN1B cDNA (Na(v)1.5 + beta1), and neonatal rat cardiomyocytes (NRCs) were incubated in the presence and in the absence of 100 microM TXL. Sodium current (I(Na)) characteristics were studied using patch-clamp techniques. Na(v)1.5 membrane expression was determined by immunocytochemistry and confocal microscopy. Pre-treatment with TXL reduced peak I(Na) amplitude nearly two-fold in both Na(v)1.5 and Na(v)1.5 + beta1, as well as in NRCs, compared with untreated cells. Accordingly, HEK293 cells and NRCs stained with anti-Na(v)1.5 antibody revealed a reduced membrane-labelling intensity in the TXL-treated groups. In addition, TXL accelerated I(Na) decay of Na(v)1.5 + beta1, whereas I(Na) decay of Na(v)1.5 remained unaltered. Finally, TXL reduced the fraction of channels that slow inactivated from 31% to 18%, and increased the time constant of slow inactivation by two-fold in Na(v)1.5. Conversely, slow inactivation properties of Na(v)1.5 + beta1 were unchanged by TXL.

CONCLUSION

Enhanced tubulin polymerization reduces sarcolemmal Na(v)1.5 expression and I(Na) amplitude in a beta1-subunit-independent fashion and causes I(Na) fast and slow inactivation impairment in a beta1-subunit-dependent way. These changes may underlie conduction-slowing-dependent cardiac arrhythmias under conditions of enhanced tubulin polymerization, e.g. TXL treatment and heart failure.

The importance of serine 161 in the sodium channel beta3 subunit for modulation of Na(V)1.2 gating

Voltage-gated sodium (Na) channels contribute to the regulation of cellular excitability due to their role in the generation and propagation of action potentials. They are composed of a pore-forming alpha subunit and are modulated by at least two of four distinct beta subunits (beta1-4). Recent studies have implicated a role for the intracellular domain of beta subunits in modulating Na channel gating and trafficking. In beta3, the intracellular domain contains a serine residue at position 161 that is replaced by an alanine in beta1. In this study, we have probed the functional importance of beta3S161 for modulating Na channel gating. Wild-type beta3 and point mutations beta3S161A or beta3S161E were individually co-expressed in HEK 293 cells stably expressing human Na(v)1.2. WTbeta3 expression increased Na current density, shifted steady-state inactivation in a depolarized direction, and accelerated the kinetics of recovery from inactivation of the Na current. Analogous effects were observed with beta3S161E co-expression. In contrast, beta3S161A abolished the shifts in steady-state inactivation and recovery from inactivation of the Na current, but did increase Na current density. Immunocytochemistry and Western blot experiments demonstrate membrane expression of WTbeta3, beta3S161E, and beta3S161A, suggesting that the differences in Na channel gating were not due to disruptions in beta subunit trafficking. These studies suggest that modification of beta3S161 may be important in modulating Na-channel gating.

A novel adhesion molecule in human breast cancer cells: voltage-gated Na+ channel beta1 subunit

Voltage-gated Na(+) channels (VGSCs), predominantly the 'neonatal' splice form of Na(v)1.5 (nNa(v)1.5), are upregulated in metastatic breast cancer (BCa) and potentiate metastatic cell behaviours. VGSCs comprise one pore-forming alpha subunit and one or more beta subunits. The latter modulate VGSC expression and gating, and can function as cell adhesion molecules of the immunoglobulin superfamily. The aims of this study were (1) to determine which beta subunits were expressed in weakly metastatic MCF-7 and strongly metastatic MDA-MB-231 human BCa cells, and (2) to investigate the possible role of beta subunits in adhesion and migration. In both cell lines, the beta subunit mRNA expression profile was SCN1B (encoding beta1)>>SCN4B (encoding beta4)>SCN2B (encoding beta2); SCN3B (encoding beta3) was not detected. MCF-7 cells had much higher levels of all beta subunit mRNAs than MDA-MB-231 cells, and beta1 mRNA was the most abundant. Similarly, beta1 protein was strongly expressed in MCF-7 and barely detectable in MDA-MB-231 cells. In MCF-7 cells transfected with siRNA targeting beta1, adhesion was reduced by 35%, while migration was increased by 121%. The increase in migration was reversed by tetrodotoxin (TTX). In addition, levels of nNa(v)1.5 mRNA and protein were increased following beta1 down-regulation. Stable expression of beta1 in MDA-MB-231 cells increased functional VGSC activity, process length and adhesion, and reduced lateral motility and proliferation. We conclude that beta1 is a novel cell adhesion molecule in BCa cells and can control VGSC (nNa(v)1.5) expression and, concomitantly, cellular migration.

A mutation in the beta 3 subunit of the cardiac sodium channel associated with Brugada ECG phenotype

BACKGROUND

Brugada syndrome, characterized by ST-segment elevation in the right precordial ECG leads and the development of life-threatening ventricular arrhythmias, has been associated with mutations in 6 different genes. We identify and characterize a mutation in a new gene.

METHODS AND RESULTS

A 64-year-old white male displayed a type 1 ST-segment elevation in V1 and V2 during procainamide challenge. Polymerase chain reaction-based direct sequencing was performed using a candidate gene approach. A missense mutation (L10P) was detected in exon 1 of SCN3B, the beta 3 subunit of the cardiac sodium channel, but not in any other gene known to be associated with Brugada syndrome or in 296 controls. Wild-type (WT) and mutant genes were expressed in TSA201 cells and studied using whole-cell patch-clamp techniques. Coexpression of SCN5A/WT+SCN1B/WT+SCN3B/L10P resulted in an 82.6% decrease in peak sodium current density, accelerated inactivation, slowed reactivation, and a -9.6-mV shift of half-inactivation voltage compared with SCN5A/WT+SCN1B/WT+SCN3B/WT. Confocal microscopy revealed that SCN5A/WT channels tagged with green fluorescent protein are localized to the cell surface when coexpressed with WT SCN1B and SCN3B but remain trapped in intracellular organelles when coexpressed with SCN1B/WT and SCN3B/L10P. Western blot analysis confirmed the presence of Na(V)beta 3 in human ventricular myocardium.

CONCLUSIONS

Our results provide support for the hypothesis that mutations in SCN3B can lead to loss of transport and functional expression of the hNa(v)1.5 protein, leading to reduction in sodium channel current and clinical manifestation of a Brugada phenotype.

Sodium channel β1 subunit mutations associated with Brugada syndrome and cardiac conduction disease in humans

Brugada syndrome is a genetic disease associated with sudden cardiac death that is characterized by ventricular fibrillation and right precordial ST segment elevation on ECG. Loss-of-function mutations in SCN5A, which encodes the predominant cardiac sodium channel alpha subunit NaV1.5, can cause Brugada syndrome and cardiac conduction disease. However, SCN5A mutations are not detected in the majority of patients with these syndromes, suggesting that other genes can cause or modify presentation of these disorders. Here, we investigated SCN1B, which encodes the function-modifying sodium channel beta1 subunit, in 282 probands with Brugada syndrome and in 44 patients with conduction disease, none of whom had SCN5A mutations. We identified 3 mutations segregating with arrhythmia in 3 kindreds. Two of these mutations were located in a newly described alternately processed transcript, beta1B. Both the canonical and alternately processed transcripts were expressed in the human heart and were expressed to a greater degree in Purkinje fibers than in heart muscle, consistent with the clinical presentation of conduction disease. Sodium current was lower when NaV1.5 was coexpressed with mutant beta1 or beta1B subunits than when it was coexpressed with WT subunits. These findings implicate SCN1B as a disease gene for human arrhythmia susceptibility.

Sodium channel Scn1b null mice exhibit prolonged QT and RR intervals

In neurons, voltage-gated sodium channel beta subunits regulate the expression levels, subcellular localization, and electrophysiological properties of sodium channel alpha subunits. However, the contribution of beta subunits to sodium channel function in heart is poorly understood. We examined the role of beta1 in cardiac excitability using Scn1b null mice. Compared to wildtype mice, electrocardiograms recorded from Scn1b null mice displayed longer RR intervals and extended QT(c) intervals, both before and after autonomic block. In acutely dissociated ventricular myocytes, loss of beta1 expression resulted in a approximately 1.6-fold increase in both peak and persistent sodium current while channel gating and kinetics were unaffected. Na(v)1.5 expression increased in null myocytes approximately 1.3-fold. Action potential recordings in acutely dissociated ventricular myocytes showed slowed repolarization, supporting the extended QT(c) interval. Immunostaining of individual myocytes or ventricular sections revealed no discernable alterations in the localization of sodium channel alpha or beta subunits, ankyrin(B), ankyrin(G), N-cadherin, or connexin-43. Together, these results suggest that beta1 is critical for normal cardiac excitability and loss of beta1 may be associated with a long QT phenotype.

Molecular identification and functional role of voltage-gated sodium channels in rat carotid body chemoreceptor cells. Regulation of expression by chronic hypoxia in vivo

We have assessed the expression, molecular identification and functional role of Na+ channels (Na(v)) in carotid bodies (CB) obtained from normoxic and chronically hypoxic adult rats. Veratridine evoked release of catecholamines (CA) from an in vitro preparation of intact CBs obtained from normoxic animals, the response being Ca2+ and Na+-dependent and sensitive to tetrodotoxin (TTX). TTX inhibited by 25-50% the CA release response evoked by graded hypoxia. Immunoblot assays demonstrated the presence of Na(v)alpha-subunit (c. 220 kDa) in crude homogenates from rat CBs, being evident an up-regulation (60%) of this protein in the CBs obtained from chronically hypoxic rats (10% O2; 7 days). This up-regulation was accompanied by an enhanced TTX-sensitive release response to veratridine, and by an enhanced ventilatory response to acute hypoxic stimuli. RT-PCR studies demonstrated the expression of mRNA for Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.6 and Na(v)1.7 isoforms. At least three isoforms, Na(v)1.1, Na(v)1.3 and Na(v)1.6 co-localized with tyrosine hydroxylase in all chemoreceptor cells. RT-PCR and immunocytochemistry indicated that Na(v)1.1 isoform was up-regulated by chronic hypoxia in chemoreceptor cells. We conclude that Na(v) up-regulation represents an adaptive mechanism to increase chemoreceptor sensitivity during acclimatization to sustained hypoxia as evidenced by enhanced ventilatory responses to acute hypoxic tests.

Molecular cloning and analysis of zebrafish voltage-gated sodium channel beta subunit genes: implications for the evolution of electrical signaling in vertebrates

Background

Action potential generation in excitable cells such as myocytes and neurons critically depends on voltage-gated sodium channels. In mammals, sodium channels exist as macromolecular complexes that include a pore-forming alpha subunit and 1 or more modulatory beta subunits. Although alpha subunit genes have been cloned from diverse metazoans including flies, jellyfish, and humans, beta subunits have not previously been identified in any non-mammalian species. To gain further insight into the evolution of electrical signaling in vertebrates, we investigated beta subunit genes in the teleost Danio rerio (zebrafish).

Results

We identified and cloned single zebrafish gene homologs for beta1-beta3 (zbeta1-zbeta3) and duplicate genes for beta4 (zbeta4.1, zbeta4.2). Sodium channel beta subunit loci are similarly organized in fish and mammalian genomes. Unlike their mammalian counterparts, zbeta1 and zbeta2 subunit genes display extensive alternative splicing. Zebrafish beta subunit genes and their splice variants are differentially-expressed in excitable tissues, indicating tissue-specific regulation of zbeta1-4 expression and splicing. Co-expression of the genes encoding zbeta1 and the zebrafish sodium channel alpha subunit Na_v1.5 in Chinese Hamster Ovary cells increased sodium current and altered channel gating, demonstrating functional interactions between zebrafish alpha and beta subunits. Analysis of the synteny and phylogeny of mammalian, teleost, amphibian, and avian beta subunit and related genes indicated that all extant vertebrate beta subunits are orthologous, that beta2/beta4 and beta1/beta3 share common ancestry, and that beta subunits are closely related to other proteins sharing the V-type immunoglobulin domain structure. Vertebrate sodium channel beta subunit genes were not identified in the genomes of invertebrate chordates and are unrelated to known subunits of the para sodium channel in Drosophila.

Conclusion

The identification of conserved orthologs to all 4 voltage-gated sodium channel beta subunit genes in zebrafish and the lack of evidence for beta subunit genes in invertebrate chordates together indicate that this gene family emerged early in vertebrate evolution, prior to the divergence of teleosts and tetrapods. The evolutionary history of sodium channel beta subunits suggests that these genes may have played a key role in the diversification and specialization of electrical signaling in early vertebrates.

The intracellular domain of the beta 2 subunit modulates the gating of cardiac Na v 1.5 channels

We have previously shown that the transmembrane segment plus either the extracellular or intracellular domain of the beta1 subunit are required to modify cardiac Na(v)1.5 channels. In this study, we coexpressed the intracellular domain of the beta2 subunit in a beta1/beta2 chimera with Na(v)1.5 channels in Xenopus oocytes and obtained an atypical recovery behavior of Na(v)1.5 channels not reported before for other Na(+) channels: Recovery times of up to 20 ms at -120 mV produced a similar fast recovery as observed for Na(v)1.5/beta1 channels, but the current amplitude decreased again at longer recovery times and reached a steady-state level after 1-2 s with current amplitudes of only 43 +/- 2% of the value at 20 ms. Current reduction was accompanied by slowed inactivation and by a shift of steady-state activation toward depolarized potentials by 9 mV. All effects were reversible and they were not seen when deleting the beta2 intracellular domain. These results describe the first functional effects of a beta2 subunit region on Na(v)1.5 channels and suggest a novel closed state in cardiac Na(+) channels accessible at hyperpolarized potentials.

Individual variation and hormonal modulation of a sodium channel β subunit in the electric organ correlate with variation in a social signal

The sodium channel beta1 subunit affects sodium channel gating and surface density, but little is known about the factors that regulate beta1 expression or its participation in the fine control of cellular excitability. In this study we examined whether graded expression of the beta1 subunit contributes to the gradient in sodium current inactivation, which is tightly controlled and directly related to a social behavior, the electric organ discharge (EOD), in a weakly electric fish Sternopygus macrurus. We found the mRNA and protein levels of beta1 in the electric organ both correlate with EOD frequency. We identified a novel mRNA splice form of this gene and found the splicing preference for this novel splice form also correlates with EOD frequency. Androgen implants lowered EOD frequency and decreased the beta1 mRNA level but did not affect splicing. Coexpression of each splice form in Xenopus oocytes with either the human muscle sodium channel gene, hNav1.4, or a Sternopygus ortholog, smNav1.4b, sped the rate of inactivation of the sodium current and shifted the steady-state inactivation toward less negative membrane potentials. The translational product of the novel mRNA splice form lacks a previously identified important tyrosine residue but still functions normally. The properties of the fish alpha and coexpressed beta1 subunits in the oocyte replicate those of the electric organ's endogenous sodium current. These data highlight the role of ion channel beta subunits in regulating cellular excitability.

Modulation of cardiac Na(+) and Ca(2+) currents by CaM and CaMKII

Sarcolemmal sodium (Na) and calcium (Ca) currents are fundamentally involved in shaping the cardiac action potential. Alterations in Na or Ca currents can change action potential characteristics and therefore might result in cardiac arrhythmias. Also, these ions contribute to excitation-contraction coupling and therefore are important in myocyte shortening and contractility of the heart. This review article summarizes how sarcolemmal Na and Ca channels are regulated with an emphasis on the novel role of Ca-dependent proteins Calmodulin (CaM) and especially Ca/CaM-dependent protein kinase II (CaMKII) to modulate sarcolemmal Na and Ca channels in the heart.

Compound heterozygous mutations P336L and I1660V in the human cardiac sodium channel associated with the Brugada syndrome

BACKGROUND

Loss-of-function mutations in SCN5A have been associated with the Brugada syndrome. We report the first Brugada syndrome family with compound heterozygous mutations in SCN5A. The proband inherited 1 mutation from each parent and transmitted 1 to each daughter.

METHODS AND RESULTS

The effects of the mutations on the function of the sodium channel were evaluated with heterologous expression in TSA201 cells, patch-clamp study, and confocal microscopy. Genetic analysis revealed that the proband carried 2 heterozygous missense mutations (P336L and I1660V) on separate alleles. He displayed a coved-type ST-segment elevation and a prolonged PR interval (280 ms). One daughter inherited P336L and exhibited a prolonged PR (210 ms). The other daughter inherited mutation I1660V and displayed a normal PR interval. Both daughters had a slightly elevated, upsloping ST-segment elevation. The parents had normal ECGs. Patch-clamp analysis showed that the P336L mutation reduced I(Na) by 85% relative to wild type. The I1660V mutation produced little measurable current, which was rescued by room temperature incubation for 48 hours. Sodium channel blockers also rescued the I1660V current, with mexiletine proving to be the most effective. Confocal immunofluorescence showed that I1660V channels conjugated to green fluorescent protein remained trapped in intracellular organelles.

CONCLUSIONS

Mutation P336L produced a reduction in cardiac I(Na), whereas I1660V abolished it. Only the proband carrying both mutations displayed the Brugada syndrome phenotype, whereas neither mutation alone produced the clinical phenotype. I1660V channels could be rescued pharmacologically and by incubation at room temperature. The present data highlight the role of compound heterozygosity in modulating the phenotypic expression and penetrance of Brugada syndrome.

Novel Brugada syndrome-causing mutation in ion-conducting pore of cardiac Na+ channel does not affect ion selectivity properties

AIM

Brugada syndrome is an inherited cardiac disease with an increased risk of sudden cardiac death. Thus far Brugada syndrome has been linked only to mutations in SCN5A, the gene encoding the alpha-subunit of cardiac Na+ channel. In this study, a novel SCN5A gene mutation (D1714G) is reported, which has been found in a 57-year-old male patient. Since the mutation is located in a segment of the ion-conducting pore of the cardiac Na+ channel, which putatively determines ion selectivity, it may affect ion selectivity properties.

METHODS

HEK-293 cells were transfected with wild-type (WT) or D1714G alpha-subunit and beta-subunit cDNA. Whole-cell configuration of the patch-clamp technique was used to study biophysical properties at room temperature (21 degrees C) and physiological temperature (36 degrees C). This study represents the first measurements of human Na+ channel kinetics at 36 degrees C. Ion selectivity, current density, and gating properties of WT and D1714G channel were studied.

RESULTS

D1714G channel yielded nearly 80% reduction of Na+ current density at 21 and 36 degrees C. At both temperatures, no significant changes were observed in V(1/2) values and slope factors for voltage-dependent activation and inactivation. At 36 degrees C, but not at 21 degrees C, D1714G channel exhibited more slow inactivation compared with WT channel. Ion selectivity properties were not affected by the mutation at both temperatures, as assessed by either current or permeability ratio.

CONCLUSION

This study shows no changes in ion selectivity properties of D1714G channel. However, the profoundly decreased current density associated with the D1714G mutation may explain the Brugada syndrome phenotype in our patient.

Modulation of Nav1.5 Channel Function by an Alternatively Spliced Sequence in the DII/DIII Linker Region

In the present study, we identified a novel splice variant of the human cardiac Na(+) channel Na(v)1.5 (Na(v)1.5d), in which a 40-amino acid sequence of the DII/DIII intracellular linker is missing due to a partial deletion of exon 17. Expression of Na(v)1.5d occurred in embryonic and adult hearts of either sex, indicating that the respective alternative splicing is neither age-dependent nor gender-specific. In contrast, Na(v)1.5d was not detected in the mouse heart, indicating that alternative splicing of Na(v)1.5 is species-dependent. In HEK293 cells, splice variant Na(v)1.5d generated voltage-dependent Na(+) currents that were markedly reduced compared with wild-type Na(v)1.5. Experiments with mexiletine and 8-bromo-cyclic AMP suggested that the trafficking of Na(v)1.5d channels was not impaired. However, single-channel recordings showed that the whole-cell current reduction was largely due to a significantly reduced open probability. Additionally, steady-state activation and inactivation were shifted to depolarized potentials by 15.9 and 5.1 mV, respectively. Systematic mutagenesis analysis of the spliced region provided evidence that a short amphiphilic region in the DII/DIII linker resembling an S4 voltage sensor of voltage-gated ion channels is an important determinant of Na(v)1.5 channel gating. Moreover, the present study identified novel short sequence motifs within this amphiphilic region that specifically affect the voltage dependence of steady-state activation and inactivation and current amplitude of human Na(v)1.5.

A mutation in the local anaesthetic binding site abolishes toluene effects in sodium channels

Toluene is a solvent of abuse that inhibits cardiac sodium channels in a manner that resembles the action of local anaesthetics. The purpose of this work was to analyze toluene effects on skeletal muscle sodium channels with and without beta1 subunit (Nav1.4+beta1 and Nav1.4-beta1, respectively) expressed in Xenopus laevis oocytes and to compare them with those produced in the F1579A mutant channel lacking a local anaesthetic binding site. Toluene inhibited Nav1.4 sodium currents (IC50=2.7 mM in Nav1.4+beta1 and 2.2 mM in Nav1.4-beta1 in a concentration dependent way. Toluene (3 mM) blocked sodium currents in Nav1.4 channels proportionally throughout the entire current-voltage relationship producing inactivation at more negative potentials. Minimal inhibition was produced by 3 mM toluene in F1579A mutant channels. Recovery from inactivation was slower both in Nav1.4 and F1579A channels in the presence of 3 mM toluene. The solvent blocked sodium currents in a use-dependent and frequency-dependent manner in Nav1.4 channels. A single mutation in the local anaesthetic binding site of Nav1.4 channels almost abolished toluene effects. These results suggest that this site is important for toluene action.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Expression pattern of neuronal and skeletal muscle voltage-gated Na+ channels in the developing mouse heart

In the mammalian heart, a variety of voltage-gated Na(+) channel transcripts and proteins have been detected. However, little quantitative information is available on the abundance of each transcript during development, or the contribution of TTX-sensitive Na(+) channels to the cardiac sodium current (I(Na)). Using competitive and real-time RT-PCR we investigated the transcription of six Na(+) channels (Na(v)1.1-Na(v)1.6) and the beta1 subunit during mouse heart development. Na(v)1.5 was predominantly expressed in the adult heart, whereas the splice variant Na(v)1.5a was the major Na(+) channel isoform in embryonic hearts. The TTX-resistant Na(+) channel transcripts (Na(v)1.5 and Na(v)1.5a) increased 1.7-fold during postnatal development. Transcripts encoding TTX-sensitive Na(+) channels (Na(v)1.1-Na(v)1.4) and the beta1 subunit gradually increased up to fourfold from postnatal day (P)1 to P126, while the Na(v)1.6 transcript level remained low and constant over the same period. In adults, TTX-sensitive channel mRNA accounted for 30-40% of the channel pool in whole-heart preparations (Na(v)1.3 > Na(v)1.4 > Na(v)1.2 >> Na(v)1.1 approximately Na(v)1.6), and 16% in mRNA from isolated cardiomyocytes (Na(v)1.4 > Na(v)1.3 > Na(v)1.2 > Na(v)1.1 > Na(v)1.6). Confocal immunofluorescence on ventricular myocytes suggested that Na(v)1.1 and Na(v)1.2 were localized at the intercalated disks and in the t tubules. Na(v)1.3 labelling predominantly produced a diffuse but strong intracellular signal. Na(v)1.6 fluorescence was detected only along the Z lines. Electrophysiological recordings showed that TTX-sensitive and TTX-resistant Na(+) channels, respectively, accounted for 8% and 92% of the I(Na) in adult ventricular cardiomyocytes. Our data suggest that neuronal and skeletal muscle Na(+) channels contribute to the action potential of cardiomyocytes in the adult mammalian heart.

Modulation of Na(v)1.5 by beta1-- and beta3-subunit co-expression in mammalian cells

Cardiac sodium channels (Na(v)1.5) comprise a pore-forming alpha-subunit and auxiliary beta-subunits that modulate channel function. In the heart, beta1 is expressed throughout the atria and ventricles, whilst beta3 is present only in the ventricles and Purkinje fibers. In view of this expression pattern, we determined the effects of beta3 and beta1 co-expression alone, and in combination, on Na(v)1.5 stably expressed in Chinese hamster ovary cells. The current/voltage relationship was shifted -5 mV with either beta1 or beta3 co-expression alone and -10 mV with co-expression of both beta1 and beta3. In addition, beta3 and beta1/beta3 co-expression accelerated macroscopic current decay. There were significant hyperpolarizing shifts in equilibrium gating relationships with co-expression of beta1 and beta3 alone and in combination. Co-expression of beta1/beta3 together resulted in a greater hyperpolarizing shift in channel availability, and an increase in the slopes of equilibrium gating relationships. Co-expression of beta3 and beta1/beta3, but not beta1, slowed recovery from inactivation at -90 mV. Development of inactivation at -70 and -50 mV was accelerated by beta-subunit co-expression alone and in combination. beta-Subunit co-expression also reduced the late Na current measured at 200 ms. In conclusion, beta-subunits modulate Na(v)1.5 gating with important differences between co-expression of beta1 and beta3 alone and beta1/beta3 together.

Role of auxiliary beta1-, beta2-, and beta3-subunits and their interaction with Na(v)1.8 voltage-gated sodium channel

The nociceptive C-fibers of the dorsal root ganglion express several sodium channel isoforms that associate with one or more regulatory beta-subunits (beta1-beta4). To determine the effects of individual and combinations of the beta-subunit isoforms, we co-expressed Nav1.8 in combination with these beta-subunits in Xenopus oocytes. Whole-cell inward sodium currents were recorded using the two-microelectrode voltage clamp method. Our studies revealed that the co-expression beta1 alone or in combination with other beta-subunits enhanced current amplitudes, accelerated current decay kinetics, and negatively shifted the steady-state curves. In contrast, beta2 alone and in combination with beta1 altered steady-state inactivation of Nav1.8 to more depolarized potentials. Co-expression of beta3 shifted steady-state inactivation to more depolarized potentials; however, combined beta1beta3 expression caused no shift in channel availability. The results in this study suggest that the functional behavior of Nav1.8 will vary depending on the type of beta-subunit that expressed under normal and disease states.

Distinct Subcellular Localization of Different Sodium Channel α and β Subunits in Single Ventricular Myocytes From Mouse Heart

Background— Voltage-gated sodium channels composed of pore-forming α and auxiliary β subunits are responsible for the rising phase of the action potential in cardiac muscle, but their localizations have not yet been clearly defined.

Methods and Results— Immunocytochemical studies show that the principal cardiac α subunit isoform Na v 1.5 and the β2 subunit are preferentially localized in intercalated disks, identified by immunostaining of connexin 43, the major protein of cardiac gap junctions. The brain α subunit isoforms Na v 1.1, Na v 1.3, and Na v 1.6 are preferentially localized with β1 and β3 subunits in the transverse tubules, identified by immunostaining of α-actinin, a cardiac z-line protein. The β1 subunit is also present in a small fraction of intercalated disks. The recently cloned β4 subunit, which closely resembles β2 in amino acid sequence, is also expressed in ventricular myocytes and is localized in intercalated disks as are β2 and Na v 1.5.

Conclusions— Our results suggest that the primary sodium channels present in ventricular myocytes are composed of Na v 1.5 plus β2 and/or β4 subunits in intercalated disks and Na v 1.1, Na v 1.3, and Na v 1.6 plus β1 and/or β3 subunits in the transverse tubules.

FRET between cardiac Na+ channel subunits measured with a confocal microscope and a streak camera

When and where proteins associate is a central question in many biomolecular studies. Förster resonance energy transfer (FRET) measurements can be used to address this question when the interacting proteins are labeled with appropriate donor and acceptor fluorophores. We describe an improved method to determine FRET efficiency that uses a mode-locked laser, a confocal microscope and a streak camera. We applied this method to study the association of alpha and beta(1) subunits of the human cardiac sodium channel. The subunits were tagged with the cyan and yellow variants of the green fluorescent protein (GFP) and expressed in human embryonic kidney (HEK293) cells. Pronounced FRET between the channel subunits in the endoplasmic reticulum (ER) suggested that the subunits associate before they reach the plasma membrane. The described method allows simultaneous measurement of donor and acceptor fluorescence decays and provides an intrinsically validated estimate of FRET efficiency.

A Ubiquitous Splice Variant and a Common Polymorphism Affect Heterologous Expression of Recombinant Human SCN5A Heart Sodium Channels

Amino acid sequence variations in SCN5A are known to affect function of wild-type channels and also those with coexisting mutations; therefore, it is important to know the exact sequence and function of channels most commonly present in human myocardium. SCN5A was analyzed in control panels of human alleles, demonstrating that the existing clones (hH1, hH1a, hH1b) each contained a rare variant and thus none represented the common sequence. Confirming prior work, the H558R polymorphism was present in ≈30% of subjects. Quantitative mRNA analysis from human hearts showed that a shorter 2015 amino acid splice variant lacking glutamine at position 1077 (Q1077del) made up 65% of the transcript in every heart examined. Age, sex, race, or structural heart disease did not affect this proportion of Q1077del. Estimated population frequencies for the four common variants were 25% SCN5A, 10% [H558R], 45% [Q1077del], and 20% [H558R;Q1077del], where the reference sequence SCN5A is GenBank AC137587. When expressed in HEK-293 cells, these common variants had a more positive mid-point of the voltage dependence of inactivation than the standard clone hH1. Also, channels containing Q1077 expressed smaller currents. When H558R was present with Q1077 ([H558R]), current expression was profoundly reduced despite normal trafficking to the cell surface. Thus, four variant sequences for SCN5A are commonly present in human myocardium and they exhibit functional differences among themselves and with the previous standard clone. These results have implications for the choice of background sequence for experiments with heterologous expression systems, and possibly implications for electrophysiological function in vivo.