Partial characterization of the rH1 sodium channel protein from rat heart using subtype-specific antibodies

Glycans and Carbohydrate-Binding/Transforming Proteins in Axon Physiology

The mature nervous system relies on the polarized morphology of neurons for a directed flow of information. These highly polarized cells use their somatodendritic domain to receive and integrate input signals while the axon is responsible for the propagation and transmission of the output signal. However, the axon must perform different functions throughout development before being fully functional for the transmission of information in the form of electrical signals. During the development of the nervous system, axons perform environmental sensing functions, which allow them to navigate through other regions until a final target is reached. Some axons must also establish a regulated contact with other cells before reaching maturity, such as with myelinating glial cells in the case of myelinated axons. Mature axons must then acquire the structural and functional characteristics that allow them to perform their role as part of the information processing and transmitting unit that is the neuron. Finally, in the event of an injury to the nervous system, damaged axons must try to reacquire some of their immature characteristics in a regeneration attempt, which is mostly successful in the PNS but fails in the CNS. Throughout all these steps, glycans perform functions of the outermost importance. Glycans expressed by the axon, as well as by their surrounding environment and contacting cells, encode key information, which is fine-tuned by glycan modifying enzymes and decoded by glycan binding proteins so that the development, guidance, myelination, and electrical transmission functions can be reliably performed. In this chapter, we will provide illustrative examples of how glycans and their binding/transforming proteins code and decode instructive information necessary for fundamental processes in axon physiology.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Initiation and entrainment of multicellular automaticity via diffusion limited extracellular domains

Electrically excitable cells often spontaneously and synchronously depolarize in vitro and in vivo preparations. It remains unclear how cells entrain and autorhythmically activate above the intrinsic mean activation frequency of isolated cells with or without pacemaking mechanisms. Recent studies suggest that cyclic ion accumulation and depletion in diffusion-limited extracellular volumes modulate electrophysiology by ephaptic mechanisms (nongap junction or synaptic coupling). This report explores how potassium accumulation and depletion in a restricted extracellular domain induces spontaneous action potentials in two different computational models of excitable cells without gap junctional coupling: Hodgkin-Huxley and Luo-Rudy. Importantly, neither model will spontaneously activate on its own without external stimuli. Simulations demonstrate that cells sharing a diffusion-limited extracellular compartment can become autorhythmic and entrained despite intercellular electrical heterogeneity. Autorhythmic frequency is modulated by the cleft volume and potassium fluxes through the cleft. Additionally, inexcitable cells can suppress or induce autorhythmic activity in an excitable cell via a shared cleft. Diffusion-limited shared clefts can also entrain repolarization. Critically, this model predicts a mechanism by which diffusion-limited shared clefts can initiate, entrain, and modulate multicellular automaticity in the absence of gap junctions.

Late Sodium Current Inhibitors as Potential Antiarrhythmic Agents

Based on recent findings, an increased late sodium current (INa,late) plays an important pathophysiological role in cardiac diseases, including rhythm disorders. The article first describes what is INa,late and how it functions under physiological circumstances. Next, it shows the wide range of cellular mechanisms that can contribute to an increased INa,late in heart diseases, and also discusses how the upregulated INa,late can play a role in the generation of cardiac arrhythmias. The last part of the article is about INa,late inhibiting drugs as potential antiarrhythmic agents, based on experimental and preclinical data as well as in the light of clinical trials.

Ankyrin-G mediates targeting of both Na+ and KATP channels to the rat cardiac intercalated disc

We investigated targeting mechanisms of Na⁺ and K_ATP channels to the intercalated disk (ICD) of cardiomyocytes. Patch clamp and surface biotinylation data show reciprocal downregulation of each other’s surface density. Mutagenesis of the Kir6.2 ankyrin binding site disrupts this functional coupling. Duplex patch clamping and Angle SICM recordings show that I_Na and I_KATP functionally co-localize at the rat ICD, but not at the lateral membrane. Quantitative STORM imaging show that Na⁺ and K_ATP channels are localized close to each other and to AnkG, but not to AnkB, at the ICD. Peptides corresponding to Nav1.5 and Kir6.2 ankyrin binding sites dysregulate targeting of both Na⁺ and K_ATP channels to the ICD, but not to lateral membranes. Finally, a clinically relevant gene variant that disrupts K_ATP channel trafficking also regulates Na⁺ channel surface expression. The functional coupling between these two channels need to be considered when assessing clinical variants and therapeutics.

AMPK-mediated degradation of Nav1.5 through autophagy

The voltage-gated cardiac sodium channel, Nav1.5, is the key component that controls cardiac excitative electrical impulse and propagation. However, the dynamic alterations of Nav1.5 during cardiac ischemia and reperfusion (I/R) are seldom reported. We found that the protein levels of rat cardiac Nav1.5 were significantly decreased in response to cardiac I/R injury. By simulating I/R injury in cells through activating AMPK by glucose deprivation, AMPK activator treatment, or hypoxia and reoxygenation (H/R), we found that Nav1.5 was down-regulated by AMPK-mediated autophagic degradation. Furthermore, AMPK was found to phosphorylate Nav1.5 at threonine (T) 101, which then regulates the interaction between Nav1.5 and the autophagic adaptor protein, microtubule-associated protein 1 light chain 3 (LC3), by exposing the LC3-interacting region adjacent to T101 in Nav1.5. This study highlights an instrumental role of AMPK in mediating the autophagic degradation of Nav1.5 during cardiac I/R injury.-Liu, X., Chen, Z., Han, Z., Liu, Y., Wu, X., Peng, Y., Di, W., Lan, R., Sun, B., Xu, B., Xu, W. AMPK-mediated degradation of Nav1.5 through autophagy.

Posttranslational Modification of Sodium Channels

Voltage-gated sodium channels (VGSCs) are critical determinants of excitability. The properties of VGSCs are thought to be tightly controlled. However, VGSCs are also subjected to extensive modifications. Multiple posttranslational modifications that covalently modify VGSCs in neurons and muscle have been identified. These include, but are not limited to, phosphorylation, ubiquitination, palmitoylation, nitrosylation, glycosylation, and SUMOylation. Posttranslational modifications of VGSCs can have profound impact on cellular excitability, contributing to normal and abnormal physiology. Despite four decades of research, the complexity of VGSC modulation is still being determined. While some modifications have similar effects on the various VGSC isoforms, others have isoform-specific interactions. In addition, while much has been learned about how individual modifications can impact VGSC function, there is still more to be learned about how different modifications can interact. Here we review what is known about VGSC posttranslational modifications with a focus on the breadth and complexity of the regulatory mechanisms that impact VGSC properties.

Identification of Glycosylation Sites Essential for Surface Expression of the CaVα2δ1 Subunit and Modulation of the Cardiac CaV1.2 Channel Activity

Alteration in the L-type current density is one aspect of the electrical remodeling observed in patients suffering from cardiac arrhythmias. Changes in channel function could result from variations in the protein biogenesis, stability, post-translational modification, and/or trafficking in any of the regulatory subunits forming cardiac L-type Ca(2+) channel complexes. CaVα2δ1 is potentially the most heavily N-glycosylated subunit in the cardiac L-type CaV1.2 channel complex. Here, we show that enzymatic removal of N-glycans produced a 50-kDa shift in the mobility of cardiac and recombinant CaVα2δ1 proteins. This change was also observed upon simultaneous mutation of the 16 Asn sites. Nonetheless, the mutation of only 6/16 sites was sufficient to significantly 1) reduce the steady-state cell surface fluorescence of CaVα2δ1 as characterized by two-color flow cytometry assays and confocal imaging; 2) decrease protein stability estimated from cycloheximide chase assays; and 3) prevent the CaVα2δ1-mediated increase in the peak current density and voltage-dependent gating of CaV1.2. Reversing the N348Q and N812Q mutations in the non-operational sextuplet Asn mutant protein partially restored CaVα2δ1 function. Single mutation N663Q and double mutations N348Q/N468Q, N348Q/N812Q, and N468Q/N812Q decreased protein stability/synthesis and nearly abolished steady-state cell surface density of CaVα2δ1 as well as the CaVα2δ1-induced up-regulation of L-type currents. These results demonstrate that Asn-663 and to a lesser extent Asn-348, Asn-468, and Asn-812 contribute to protein stability/synthesis of CaVα2δ1, and furthermore that N-glycosylation of CaVα2δ1 is essential to produce functional L-type Ca(2+) channels.

Regulation of the cardiac Na+ channel NaV1.5 by post-translational modifications

The cardiac voltage-gated Na(+) channel, Na(V)1.5, is responsible for the upstroke of the action potential in cardiomyocytes and for efficient propagation of the electrical impulse in the myocardium. Even subtle alterations of Na(V)1.5 function, as caused by mutations in its gene SCN5A, may lead to many different arrhythmic phenotypes in carrier patients. In addition, acquired malfunctions of Na(V)1.5 that are secondary to cardiac disorders such as heart failure and cardiomyopathies, may also play significant roles in arrhythmogenesis. While it is clear that the regulation of Na(V)1.5 protein expression and function tightly depends on genetic mechanisms, recent studies have demonstrated that Na(V)1.5 is the target of various post-translational modifications that are pivotal not only in physiological conditions, but also in disease. In this review, we examine the recent literature demonstrating glycosylation, phosphorylation by Protein Kinases A and C, Ca(2+)/Calmodulin-dependent protein Kinase II, Phosphatidylinositol 3-Kinase, Serum- and Glucocorticoid-inducible Kinases, Fyn and Adenosine Monophosphate-activated Protein Kinase, methylation, acetylation, redox modifications, and ubiquitylation of Na(V)1.5. Modern and sensitive mass spectrometry approaches, applied directly to channel proteins that were purified from native cardiac tissues, have enabled the determination of the precise location of post-translational modification sites, thus providing essential information for understanding the mechanistic details of these regulations. The current challenge is first, to understand the roles of these modifications on the expression and the function of Na(V)1.5, and second, to further identify other chemical modifications. It is postulated that the diversity of phenotypes observed with Na(V)1.5-dependent disorders may partially arise from the complex post-translational modifications of channel protein components.

Nav1.5 channels can reach the plasma membrane through distinct N-glycosylation states

BACKGROUND

Like many voltage-gated sodium channels, the cardiac isoform Nav1.5 is well known as a glycoprotein which necessarily undergoes N-glycosylation processing during its transit to the plasma membrane. In some cardiac disorders, especially the Brugada syndrome (BrS), mutations in Nav1.5 encoding gene lead to intracellular retention and consequently trafficking defect of these proteins. We used two BrS mutants as tools to clarify both Nav1.5 glycosylation states and associated secretory behaviors.

METHODS

Patch-clamp recordings and surface biotinylation assays of HEK293T cells expressing wild-type (WT) and/or mutant Nav1.5 proteins were performed to assess the impact of mutant co-expression on the membrane activity and localization of WT channels. Enzymatic deglycosylation assays and brefeldin A (BFA) treatments were also employed to further characterize recombinant and native Nav1.5 maturation.

RESULTS

The present data demonstrate that Nav1.5 channels mainly exist as two differentially glycosylated forms. We reveal that dominant negative effects induced by BrS mutants upon WT channel current result from the abnormal surface expression of the fully-glycosylated forms exclusively. Furthermore, we show that core-glycosylated channels can be found at the surface membrane of BFA-treated or untreated cells, but obviously without generating any sodium current.

CONCLUSIONS

Our findings provide evidence that native and recombinant Nav1.5 subunits are expressed as two distinct matured forms. Fully-glycosylated state of Nav1.5 seems to determine its functionality whereas core-glycosylated forms might be transported to the plasma membrane through an unconventional Golgi-independent secretory route.

GENERAL SIGNIFICANCE

This work highlights that N-linked glycosylation processing would be critical for Nav1.5 membrane trafficking and function.

The late sodium current in heart failure: pathophysiology and clinical relevance

Large and growing body of data suggest that an increased late sodium current (I_Na,late ) can have a significant pathophysiological role in heart failure and other heart diseases. The first goal of this article is to describe how I_Na,late functions under physiological circumstances. The second goal is to show the wide range of cellular mechanisms that can increase I_Na,late in cardiac disease, and also to describe how the up-regulated I_Na,late contributes to the pathophysiology of heart failure. The final section of the article discusses the possible use of I_Na,late -modifying drugs in heart failure, on the basis of experimental and preclinical data.

Ubiquitylation of voltage-gated sodium channels

Ion channel proteins are regulated by different types of posttranslational modifications. The focus of this review is the regulation of voltage-gated sodium channels (Navs) upon their ubiquitylation. The amiloride-sensitive epithelial sodium channel (ENaC) was the first ion channel shown to be regulated upon ubiquitylation. This modification results from the binding of ubiquitin ligase from the Nedd4 family to a protein-protein interaction domain, known as the PY motif, in the ENaC subunits. Many of the Navs have similar PY motifs, which have been demonstrated to be targets of Nedd4-dependent ubiquitylation, tagging them for internalization from the cell surface. The role of Nedd4-dependent regulation of the Nav membrane density in physiology and disease remains poorly understood. Two recent studies have provided evidence that Nedd4-2 is downregulated in dorsal root ganglion (DRG) neurons in both rat and mouse models of nerve injury-induced neuropathic pain. Using two different mouse models, one with a specific knockout of Nedd4-2 in sensory neurons and another where Nedd4-2 was overexpressed with the use of viral vectors, it was demonstrated that the neuropathy-linked neuronal hyperexcitability was the result of Nav1.7 and Nav1.8 overexpression due to Nedd4-2 downregulation. These studies provided the first in vivo evidence of the role of Nedd4-2-dependent regulation of Nav channels in a disease state. This ubiquitylation pathway may be involved in the development of symptoms and diseases linked to Nav-dependent hyperexcitability, such as pain, cardiac arrhythmias, epilepsy, migraine, and myotonias.

Modulation of voltage-gated ion channels by sialylation

Control and modulation of electrical signaling is vital to normal physiology, particularly in neurons, cardiac myocytes, and skeletal muscle. The orchestrated activities of variable sets of ion channels and transporters, including voltage-gated ion channels (VGICs), are responsible for initiation, conduction, and termination of the action potential (AP) in excitable cells. Slight changes in VGIC activity can lead to severe pathologies including arrhythmias, epilepsies, and paralyses, while normal excitability depends on the precise tuning of the AP waveform. VGICs are heavily posttranslationally modified, with upward of 30% of the mature channel mass consisting of N- and O-glycans. These glycans are terminated typically by negatively charged sialic acid residues that modulate voltage-dependent channel gating directly. The data indicate that sialic acids alter VGIC activity in isoform-specific manners, dependent in part, on the number/location of channel sialic acids attached to the pore-forming alpha and/or auxiliary subunits that often act through saturating electrostatic mechanisms. Additionally, cell-specific regulation of sialylation can affect VGIC gating distinctly. Thus, channel sialylation is likely regulated through two mechanisms that together contribute to a dynamic spectrum of possible gating motifs: a subunit-specific mechanism and regulated (aberrant) changes in the ability of the cell to glycosylate. Recent studies showed that neuronal and cardiac excitability is modulated through regulated changes in voltage-gated Na(+) channel sialylation, suggesting that both mechanisms of differential VGIC sialylation contribute to electrical signaling in the brain and heart. Together, the data provide insight into an important and novel paradigm involved in the control and modulation of electrical signaling.

Mining the virgin land of neurotoxicology: a novel paradigm of neurotoxic peptides action on glycosylated voltage-gated sodium channels

Voltage-gated sodium channels (VGSCs) are important membrane protein carrying on the molecular basis for action potentials (AP) in neuronal firings. Even though the structure-function studies were the most pursued spots, the posttranslation modification processes, such as glycosylation, phosphorylation, and alternative splicing associating with channel functions captured less eyesights. The accumulative research suggested an interaction between the sialic acids chains and ion-permeable pores, giving rise to subtle but significant impacts on channel gating. Sodium channel-specific neurotoxic toxins, a family of long-chain polypeptides originated from venomous animals, are found to potentially share the binding sites adjacent to glycosylated region on VGSCs. Thus, an interaction between toxin and glycosylated VGSC might hopefully join the campaign to approach the role of glycosylation in modulating VGSCs-involved neuronal network activity. This paper will cover the state-of-the-art advances of researches on glycosylation-mediated VGSCs function and the possible underlying mechanisms of interactions between toxin and glycosylated VGSCs, which may therefore, fulfill the knowledge in identifying the pharmacological targets and therapeutic values of VGSCs.

Biology of cardiac sodium channel Nav1.5 expression

Na(v)1.5, the pore forming α-subunit of the voltage-dependent cardiac Na(+) channel, is an integral membrane protein involved in the initiation and conduction of action potentials. Mutations in the gene-encoding Na(v)1.5, SCN5A, have been associated with a variety of arrhythmic disorders, including long QT, Brugada, and sick sinus syndromes as well as progressive cardiac conduction defect and atrial standstill. Moreover, alterations in the Na(v)1.5 expression level and/or sodium current density have been frequently noticed in acquired cardiac disorders, such as heart failure. The molecular mechanisms underlying these alterations are poorly understood, but are considered essential for conception of arrhythmogenesis and the development of therapeutic strategies for prevention or treatment of arrhythmias. The unravelling of such mechanisms requires critical molecular insight into the biology of Na(v)1.5 expression and function. Therefore, the aim of this review is to provide an up-to-date account of molecular determinants of normal Na(v)1.5 expression and function. The parts of the Na(v)1.5 life cycle that are discussed include (i) regulatory aspects of the SCN5A gene and transcript structure, (ii) the nature, molecular determinants, and functional consequences of Na(v)1.5 post-translational modifications, and (iii) the role of Na(v)1.5 interacting proteins in cellular trafficking. The reviewed studies have provided valuable information on how the Na(v)1.5 expression level, localization, and biophysical properties are regulated, but also revealed that our understanding of the underlying mechanisms is still limited.

Hereditary Inclusion Body Myopathy (HIBM2)

Hereditary inclusion body myopathy type 2 (HIBM2) is a myopathy characterized by progressive muscle weakness with early adult onset. The disease is the result of a recessive mutation in the Glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase gene (GNE), which results in reduced enzyme function and sialic acid levels. A majority of individuals with HIBM2 are from Iranian-Jewish or Japanese decent, but isolated cases have been identified world wide. This article reviews the diagnostic criteria for HIBM2. Current research with a highlight on the biology of the disease and the role of GNE in the sialic acid pathway are assessed. Finally, therapeutic investigations and animal models are discussed with a focus on future studies to better understand the pathology of Hereditary Inclusion Body Myopathy and move therapeutic agents towards clinical trials.

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.

The sodium channel Nav1.5a is the predominant isoform expressed in adult mouse dorsal root ganglia and exhibits distinct inactivation properties from the full-length Nav1.5 channel

Nav1.5 is the principal voltage-gated sodium channel expressed in heart, and is also expressed at lower abundance in embryonic dorsal root ganglia (DRG) with little or no expression reported postnatally. We report here the expression of Nav1.5 mRNA isoforms in adult mouse and rat DRG. The major isoform of mouse DRG is Nav1.5a, which encodes a protein with an IDII/III cytoplasmic loop reduced by 53 amino acids. Western blot analysis of adult mouse DRG membrane proteins confirmed the expression of Nav1.5 protein. The Na+ current produced by the Nav1.5a isoform has a voltage-dependent inactivation significantly shifted to more negative potentials (by approximately 5 mV) compared to the full-length Nav1.5 when expressed in the DRG neuroblastoma cell line ND7/23. These results imply that the alternatively spliced exon 18 of Nav1.5 plays a role in channel inactivation and that Nav1.5a is likely to make a significant contribution to adult DRG neuronal function.

Differential sialylation modulates voltage-gated Na+ channel gating throughout the developing myocardium

Voltage-gated sodium channel function from neonatal and adult rat cardiomyocytes was measured and compared. Channels from neonatal ventricles required an ∼10 mV greater depolarization for voltage-dependent gating events than did channels from neonatal atria and adult atria and ventricles. We questioned whether such gating shifts were due to developmental and/or chamber-dependent changes in channel-associated functional sialic acids. Thus, all gating characteristics for channels from neonatal atria and adult atria and ventricles shifted significantly to more depolarized potentials after removal of surface sialic acids. Desialylation of channels from neonatal ventricles did not affect channel gating. After removal of the complete surface N-glycosylation structures, gating of channels from neonatal atria and adult atria and ventricles shifted to depolarized potentials nearly identical to those measured for channels from neonatal ventricles. Gating of channels from neonatal ventricles were unaffected by such deglycosylation. Immunoblot gel shift analyses indicated that voltage-gated sodium channel α subunits from neonatal atria and adult atria and ventricles are more heavily sialylated than α subunits from neonatal ventricles. The data are consistent with approximately 15 more sialic acid residues attached to each α subunit from neonatal atria and adult atria and ventricles. The data indicate that differential sialylation of myocyte voltage-gated sodium channel α subunits is responsible for much of the developmental and chamber-specific remodeling of channel gating observed here. Further, cardiac excitability is likely impacted by these sialic acid–dependent gating effects, such as modulation of the rate of recovery from inactivation. A novel mechanism is described by which cardiac voltage-gated sodium channel gating and subsequently cardiac rhythms are modulated by changes in channel-associated sialic acids.

Caveolin-3 is adjacent to a group of extradyadic ryanodine receptors

Caveolae are present in almost all cells and concentrate a wide variety of signaling molecules, receptors, transporters, and ion pumps. We have investigated the distribution of the ryanodine receptor, the Na(+)/Ca(2+) exchanger, the predominant Na(+) channel isoform rH1, and the L-type calcium channel, Ca(v)1.2, relative to the muscle-specific caveolin isoform, caveolin-3, in adult rat ventricular myocytes. Three-dimensional immunofluorescence images were deconvolved and analyzed. Caveolin-3 colocalizes with all of these molecules at the surface of the cell, but there is no significant colocalization between caveolin-3 and either the Na(+)/Ca(2+) exchanger or the Na(+) channel in the cell interior. The distribution of the surface colocalization indicates that the caveolae that colocalize with each molecule form distinct populations. This organization indicates that there are multiple populations of caveolae separable by location and occupants. In the interior of the cell, caveolin-3 shows a marked colocalization with a population of ryanodine receptors that are separate from those within the dyad. Because of their location, the signaling molecules contained within these caveolae may have preferred access to the neighboring nondyadic ryanodine receptors.

The Sialic Acid Component of the β1 Subunit Modulates Voltage-gated Sodium Channel Function

Voltage-gated sodium channels (Nav) are responsible for initiation and propagation of nerve, skeletal muscle, and cardiac action potentials. Nav are composed of a pore-forming alpha subunit and often one to several modulating beta subunits. Previous work showed that terminal sialic acid residues attached to alpha subunits affect channel gating. Here we show that the fully sialylated beta1 subunit induces a uniform, hyperpolarizing shift in steady state and kinetic gating of the cardiac and two neuronal alpha subunit isoforms. Under conditions of reduced sialylation, the beta1-induced gating effect was eliminated. Consistent with this, mutation of beta1 N-glycosylation sites abolished all effects of beta1 on channel gating. Data also suggest an interaction between the cis effect of alpha sialic acids and the trans effect of beta1 sialic acids on channel gating. Thus, beta1 sialic acids had no effect gating on the of the heavily glycosylated skeletal muscle alpha subunit. However, when glycosylation of the skeletal muscle alpha subunit was reduced through chimeragenesis such that alpha sialic acids did not impact gating, beta1 sialic acids caused a significant hyperpolarizing shift in channel gating. Together, the data indicate that beta1 N-linked sialic acids can modulate Nav gating through an apparent saturating electrostatic mechanism. A model is proposed in which a spectrum of differentially sialylated Nav can directly modulate channel gating, thereby impacting cardiac, skeletal muscle, and neuronal excitability.

Isoform-specific effects of sialic acid on voltage-dependent Na+ channel gating: functional sialic acids are localized to the S5-S6 loop of domain I

The isoform specific role of sialic acid in human voltage-gated sodium channel gating was investigated through expression and chimeric analysis of two human isoforms, Na(v1.4) (hSkM1), and Na(v1.5) (hH1) in Chinese hamster ovary (CHO) cell lines. Immunoblot analyses indicate that both hSkM1 and hH1 are glycosylated and that hSkM1 is more glycosylated than hH1. Four sets of voltage-dependent parameters, the voltage of half-activation (V(a)), the voltage of half-inactivation (V(i)), the time constants for fast inactivation (tau(h)), and the time constants for recovery from inactivation (tau(rec)), were measured for hSkM1 and hH1 expressed in two CHO cell lines, Pro5 and Lec2, to determine the effect of changing sialylation on channel gating under conditions of full (Pro5) or reduced (Lec2) sialylation. For all parameters measured, hSkM1 gating showed a consistent 11-15 mV depolarizing shift under conditions of reduced sialylation, while hH1 showed no significant change in any gating parameter. Shifts in channel V(a) with changing external [Ca2+] indicated that sialylation of hSkM1, but not hH1, directly contributes to a negative surface potential. Functional analysis of two chimeras, hSkM1P1 and hH1P1, indicated that the responsible sialic acids are localized to the hSkM1 S5-S6 loop of domain I. When hSkM1 IS5-S6 was replaced by the analogous hH1 loop (hSkM1P1), changing sialylation had no significant effect on any voltage-dependent parameter. Conversely, when hSkM1 IS5-S6 was added to hH1 (hH1P1), all four parameters shifted by 6-7 mV in the depolarized direction under conditions of reduced sialylation. In summary, the gating of two human sodium channel isoforms show very different dependencies on sialic acid, with hSkM1 gating uniformly altered by sialic acid levels through an apparent electrostatic mechanism, while hH1 gating is unaffected by changing sialylation. Sialic acid-dependent gating can be removed or created by replacing or inserting hSkM1 IS5-S6, respectively, indicating that the functionally relevant sialic acid residues are localized to the first domain of the channel.

Differential contribution of sialic acid to the function of repolarizing K(+) currents in ventricular myocytes

We investigated the contribution of sialic acid residues to the K(+) currents involved in the repolarization of mouse ventricular myocytes. Ventricular K(+) currents had a rapidly inactivating component followed by slowly decaying and sustained components. This current was produced by the summation of three distinct currents: I(to), which contributed to the transient component; I(ss), which contributed to the sustained component; and I(K,slow), which contributed to both components. Incubation of ventricular myocytes with the sialidase neuraminidase reduced the amplitude of I(to) without altering I(K,slow) and I(ss). We found that the reduction in I(to) amplitude resulted from a depolarizing shift in the voltage of activation and a reduction in the conductance of I(to). Expression of Kv4.3 channels, a major contributor to I(to) in the ventricle, in a sialylation-deficient Chinese hamster ovary cell line (lec2) mimicked the effects of neuraminidase on the ventricular I(to). Furthermore, we showed that sialylated glycolipids have little effect on the voltage dependence of I(to). Finally, consistent with its actions on I(to), neuraminidase produced an increase in the duration of the action potential of ventricular myocytes and the frequency of early afterdepolarizations. We conclude that sialylation of the proteins forming Kv4 channels is important in determining the voltage dependence and conductance of I(to) and that incomplete glycosylation of these channels could lead to arrhythmias.

Distribution of proteins implicated in excitation-contraction coupling in rat ventricular myocytes

We have examined the distribution of ryanodine receptors, L-type Ca(2+) channels, calsequestrin, Na(+)/Ca(2+) exchangers, and voltage-gated Na(+) channels in adult rat ventricular myocytes. Enzymatically dissociated cells were fixed and dual-labeled with specific antibodies using standard immunocytochemistry protocols. Images were deconvolved to reverse the optical distortion produced by wide-field microscopes equipped with high numerical aperture objectives. Every image showed a well-ordered array of fluorescent spots, indicating that all of the proteins examined were distributed in discrete clusters throughout the cell. Mathematical analysis of the images revealed that dyads contained only ryanodine receptors, L-type Ca(2+) channels, and calsequestrin, and excluded Na(+)/Ca(2+) exchangers and voltage-gated Na(+) channels. The Na(+)/Ca(2+) exchanger and voltage-gated Na(+) channels were distributed largely within the t-tubules, on both transverse and axial elements, but were not co-localized. The t-tubule can therefore be subdivided into at least three structural domains; one of coupling (dyads), one containing the Na(+)/Ca(2+) exchanger, and one containing voltage-gated Na(+) channels. We conclude that if either the slip mode conductance of the Na(+) channel or the reverse mode of the Na(+)/Ca(2+) exchanger are to contribute to the contractile force, the fuzzy space must extend outside of the dyad.

Modulation of rat cardiac sodium channel by the stimulatory G protein alpha subunit

1. Modulation of cardiac sodium currents (INa) by the G protein stimulatory alpha subunit (Gsalpha) was studied using patch-clamp techniques on freshly dissociated rat ventricular myocytes. 2. Whole-cell recordings showed that stimulation of beta-adrenergic receptors with 10 microM isoprenaline (isoproterenol, ISO) enhanced INa by 68.4 +/- 9.6 % (mean +/- s.e.m.; n = 7, P < 0.05 vs. baseline). With the addition of 22 microgram ml-1 protein kinase A inhibitor (PKI) to the pipette solution, 10 microM ISO enhanced INa by 30.5 +/- 7.0 % (n = 7, P < 0.05 vs. baseline). With the pipette solution containing both PKI and 20 microgram ml-1 anti-Gsalpha IgG or 20 microgram ml-1 anti-Gsalpha IgG alone, 10 microM ISO produced no change in INa. 3. The effect of Gsalpha on INa was not due to changes in the steady-state activation or inactivation curves, the time course of current decay, the development of inactivation, or the recovery from inactivation. 4. Whole-cell INa was increased by 45.2 +/- 5.3% (n = 13, P < 0.05 vs. control) with pipette solution containing 1 microM Gsalpha27-42 peptide (amino acids 27-42 of rat brain Gsalpha) without altering the properties of Na+ channel kinetics. Furthermore, application of 1 nM Gsalpha27-42 to Na+ channels in inside-out macropatches increased the ensemble-averaged INa by 32.5 +/- 6.8 % (n = 8, P < 0.05 vs. baseline). The increase in INa was reversible upon Gsalpha27-42 peptide washout. Single channel experiments showed that the Gsalpha27-42 peptide did not alter the Na+ single channel current amplitude, the mean open time or the mean closed time, but increased the number of functional channels (N) in the patch. 5. Application of selected short amino acid segments (Gsalpha27-36, Gsalpha33-42 and Gsalpha30-39) of the 16 amino acid Gsalpha peptide (Gsalpha27-42 peptide) showed that only the C-terminal segment of this peptide (Gsalpha33-42) significantly increased INa in a dose-dependent fashion. These results show that cardiac INa is regulated by Gsalpha via a mechanism independent of PKA that results in an increase in the number of functional Na+ channels. In addition, a 10 residue domain (amino acids 33-42) near the N-terminus of Gsalpha is important in modulating cardiac Na+ channels.

cAMP-dependent phosphorylation of the tetrodotoxin-resistant voltage-dependent sodium channel SNS

1. Protein kinase A (PKA) modulation of tetrodotoxin-resistant (TTX-r) voltage-gated sodium channels may underly the hyperalgesic responses of mammalian sensory neurones. We have therefore examined PKA phosphorylation of the cloned alpha-subunit of the rat sensory neurone-specific TTX-r channel SNS. Phosphorylation of SNS was compared with that of a mutant channel, SNS(SA), in which all five PKA consensus sites (RXXS) within the intracellular I-II loop had been eliminated by site-directed mutagenesis (serine to alanine). 2. In vitro PKA phosphorylation and tryptic peptide mapping of SNS and mutant SNS(SA) I-II loops expressed as glutathione-S-transferase (GST) fusion proteins confirmed that the five mutated serines were the major PKA substrates within the SNS I-II loop. 3. SNS and SNS(SA) channels were transiently expressed in COS-7 cells and their electrophysiological properties compared. In wild-type SNS channels, forskolin and 8-bromo cAMP produced effects consistent with PKA phosphorylation. Mutant SNS(SA) currents, however, were not significantly affected by either agent. Thus, elimination of the I-II loop PKA consensus sites caused a marked reduction in PKA modulation of wild-type channels. 4. Under control conditions, the voltage dependence of activation of SNS(SA) current was shifted to depolarized potentials compared with SNS. This was associated with a slowing of SNS(SA) current inactivation at hyperpolarized potentials and suggested a tonic PKA phosphorylation of wild-type channels under basal conditions.5. We conclude that the major substrates involved in functional PKA modulation of the SNS channel are located within the intracellular I-II loop.

Structure and function of voltage-gated sodium channels

1. Sodium channels mediate fast depolarization and conduct electrical impulses throughout nerve, muscle and heart. This paper reviews the links between sodium channel structure and function. 2. Sodium channels have a modular architecture, with distinct regions for the pore and the gates. The separation is far from absolute, however, with extensive interaction among the various parts of the channel. 3. At a molecular level, sodium channels are not static: they move extensively in the course of gating and ion translocation. 4. Sodium channels bind local anaesthetics and various toxins. In some cases, the relevant sites have been partially identified. 5. Sodium channels are subject to regulation at the levels of transcription, subunit interaction and post-translational modification (notably glycosylation and phosphorylation).

Sodium channel distribution within the rabbit atrioventricular node as analysed by confocal microscopy

1. Paired 20 microns thick sections of fresh frozen tissue taken from the frontal plane of the rabbit atrioventricular (AV) nodal region were processed for histology and immunohistochemistry. Confocal microscopy was used to image the distribution of sodium channels using IgG (R12) developed against a highly conserved sequence in the interdomain 3-4 region of cloned sodium channels. 2. In ventricular and atrial cells, sodium channel immunofluorescence was localized to lateral membranes and T-tubules. In the open AV node, levels of sodium channel immunofluorescence in the transitional cell zone and in the lower nodal cell tract were comparable to that found in the atrial and ventricular myocardium. 3. In the enclosed AV node a gradation of sodium channel immunofluorescence is present such that peripherally located circumferential transitional cells display high levels of immunofluorescence, comparable to that of atrial and ventricular myocardium, while centrally located midnodal cells display decreased levels of or no immunofluorescence. 4. In order to correlate the distribution of sodium channels with the distribution of gap junctions, we used IgG directed against the carboxyl terminus of connexin43 (CT-360). Ventricular cell immunofluorescence was localized primarily to the intercalated disk region, while in the AV node, the pattern of distribution was found to be similar to that of sodium channels. 5. The reduced levels of and/or absence of immunofluorescence in the midnodal cell region indicates a paucity of sodium channel and connexin43 protein expression in this region of the AV node that would favour slow impulse conduction.

Modulation of the human cardiac sodium channel alpha-subunit by cAMP-dependent protein kinase and the responsible sequence domain

1. In order to investigate the modulation of human hH1 sodium channel alpha-subunits by cAMP-dependent protein kinase (PKA), the channel was expressed in oocytes of Xenopus laevis. 2. Cytosolic injection of cAMP, as well as of SP-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (SP-cAMPS, the S-diastereoisomeric configuration of the compound with respect to the phosphorus atom), resulted in a marked and significant increase in peak sodium current (INa,p). Cytosolic injections of RP-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (RP-cAMPS; a compound inhibitory to PKA) had no effect on peak current. 3. Kinetic parameters of steady-state activation, inactivation and recovery from inactivation were unchanged following stimulation of PKA activity, but a 42 +/- 5% (mean +/- S.E.M.) increase in maximal sodium conductance (delta gmax) could account for the observed increase in INa,p. 4. A set of chimerical sodium channels made from portions of the human cardiac hH1 alpha-subunit and the rat skeletal muscle SkM1 alpha-subunit (which is not affected by PKA stimulation) was generated. These were used to localize the structural determinant in the hH1 sequence responsible for PKA modulation of hH1. From our data we conclude that the effects of PKA on hH1 are conferred by the large cytosolic loop interconnecting transmembrane domains I and II, which is not conserved among sodium channel subtypes.

Molecular determinants of beta 1 subunit-induced gating modulation in voltage-dependent Na+ channels

Recombinant brain, skeletal muscle, and heart voltage-gated Na+ channel alpha subunits differ in their functional responses to an accessory beta 1 subunit when coexpressed in Xenopus oocytes. We exploited the distinct beta 1 subunit responses observed for the human heart (hH1) and human skeletal muscle (hSkM1) isoforms to identify determinants of this response. Chimeric alpha subunits were constructed by exchanging the S5-S6 interhelical loops of each domain between hH1 and hSkM1 and then examined for effects on inactivation induced by coexpressed beta 1 subunit in oocytes. Substitution of single S5-S6 loops in either domain 1 (D1/S5-S6) or domain 4 (D4/S5-S6) of hSkM1 by the corresponding segments of hH1 produced channels that exhibited an attenuated response to coexpressed beta 1 subunit. Substitutions of both D1/S5-S6 and D4/S5-S6 in hSkM1 by the corresponding loops from hH1 completely abolished the effects of the beta 1 subunit on inactivation. The reciprocal chimera in which both D1/S5-S6 and D4/S5-S6 from hSkM1 were transplanted into hH1 exhibited significant beta 1 responsiveness (accelerated inactivation). The region within D4/S5-S6 that conferred beta 1 responsiveness was determined to reside primarily within an extracellular loop between the putative pore-forming segment SS2 and D4/S6. Gating modulation was also demonstrated using a chimeric beta subunit consisting of the extracellular domains of beta 1 and the transmembrane and C-terminal domains of the rat brain beta 2 subunit. These results suggest that the D1/S5-S6 and D4/S5-S6 loops in the alpha subunit and the extracellular domain of the beta 1 subunit are important determinants of the beta 1 subunit-induced gating modulation in Na+ channels.

cAMP-dependent phosphorylation of two sites in the alpha subunit of the cardiac sodium channel

The voltage-sensitive Na+ channel is responsible for generating action potentials in the heart which are critical for coordinated cardiac muscle contraction. Cardiac Na+ channels are regulated by cAMP-dependent phosphorylation, but the sites of phosphorylation are not known. Using mammalian cells expressing the rat cardiac Na+ channel (rH1) alpha subunit and site-specific antibodies, we have shown that the alpha subunit of rat heart Na+ channel is phosphorylated selectively by cAMP-dependent protein kinase (PKA) in vitro and in intact cells. Analysis of the sites of phosphorylation by two-dimensional phosphopeptide mapping and site-directed mutagenesis of fusion proteins revealed that the cardiac alpha subunit is phosphorylated selectively in vitro by PKA on Ser526 and Ser529 in the intracellular loop connecting homologous domains I and II (LI-II). These two residues were phosphorylated in intact cells expressing the rH1 alpha subunit when PKA was activated. Our results define a different pattern of phosphorylation of LI-II of cardiac and brain Na+ channels and implicate phosphorylation of Ser526 and Ser529 in the differential regulation of cardiac and brain Na+ channels by PKA.

Cardiac sodium channels expressed in a peripheral neurotumor-derived cell line, RT4-B8

RT4-B is one of several cell lines derived from a multipotent stem cell line, RT4-AC, which originated from a rat peripheral neurotumor. Based on Northern blot and ribonuclease protection experiments, RT4-B8 cells have been proposed to express rat cardiac Na channel mRNA as the major isoform. We report here direct electrophysiological evidence that the expressed voltage-gated Na channels in the RT4-B8 cell line are of the cardiac phenotype with no evidence for subpopulations expressing other Na channel isoforms. Current activation half point (conductance) was -41 +/- 5 mV (n = 7) and the steady-state voltage-dependent availability half point was -89 +/- 1 mV. As expected for cardiac Na channels, the half concentration of block for tetrodotoxin block was 0.74 microM, for saxitoxin (STX) was 0.15 microM, and for the class 2B divalent cation Cd2+ was 67 microM. Block was well described by single-site dose-response relationships with no indication of a subpopulation with "neuronal" affinity. Single-channel conductance (140 mM Na+) was 10 pS and predicted the average number of channels open at peak Na current to be 3 channels/microns2. [3H]STX binding data were also consistent with a single population of low-affinity STX binding sites and predicted channel density to be 11 sites/microns2. No inwardly or outwardly rectifying K or Ca currents were detected electrophysiologically, although in some cells a small time-independent Cl current was detected. Reverse transcription-polymerase chain reaction of mRNA isolated from RT4-B8 cells demonstrated the presence of rat cardiac (rH1) and brain IIa alpha-subunit mRNA, as well as mRNA for the Na channel beta 1-subunit. Northern blot analysis confirmed the predominance of the rat cardiac Na mRNA compared with brain IIa. The beta 1-subunit mRNA levels were significantly lower than those detected in rat brain and heart mRNA but were comparable to the low level of beta 1-subunit mRNA detected in isolated rat ventricular myocytes.

Immunolocalization of the mNav2.3 Na+ channel in mouse heart: upregulation in myometrium during pregnancy

mNav2.3 is a putative voltage-dependent sodium channel (NaC) gene expressed in both mouse heart and uterus that shares only 45% amino acid identity with NaCs from gene subfamily 1. Immunofluorescence studies using polyclonal antibodies against two distinct epitopes revealed that mNav2.3 protein in heart colocalized with nerve-specific antibody binding. Similar mNav2.3-specific antibody staining was observed in virgin uterus. However, mNav2.3 expression in uterine nerve disappeared during late pregnancy, concurrent with an appearance in both the longitudinal and circular uterine smooth muscle, which reached a maximum at term and quickly declined within 2 days postpartum. mNav2.3 expression in term uterus often colocalized on the myocyte surface with connexin 43. The immunofluorescence results are supported by Western analysis in which the 217-kDa NaC increased during late pregnancy and declined 2 days postpartum. These data provide perhaps the most dramatic example of NaC regulation. The acute and transient upregulation in myometrium during gestation suggests the Nav2.3 channel plays a role in uterine function at term.

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

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

Expression of the sodium channel beta 1 subunit in rat skeletal muscle is selectively associated with the tetrodotoxin-sensitive alpha subunit isoform

Transcripts homologous to the rat brain sodium channel beta subunit (beta 1) are prominently expressed in both innervated and denervated adult skeletal muscle and in heart, but not in neonatal skeletal or cardiac muscle. Regulation of beta 1 mRNA expression closely parallels that of SkM1 alpha during development, after denervation in adult muscle, and in primary muscle culture, but does not follow SkM2 expression under any condition examined. In oocytes, beta 1 interacts functionally with SkM1 to modulate the abnormally slow inactivation kinetics observed with this alpha subunit expressed alone. We conclude that a common beta 1 subunit is expressed in skeletal muscle, heart, and brain and that in skeletal muscle, this subunit is specifically associated with the SkM1, rather than the SkM2, sodium channel isoform.

Structure, function and expression of voltage-dependent sodium channels

Voltage-dependent sodium channels control the transient inward current responsible for the action potential in most excitable cells. Members of this multigene family have been cloned, sequenced, and functionally expressed from various tissues and species, and common features of their structure have clearly emerged. Site-directed mutagenesis coupled with in vitro expression has provided additional insight into the relationship between structure and function. Subtle differences between sodium channel isoforms are also important, and aspects of the regulation of sodium channel gene expression and the modulation of channel function are becoming topics of increasing importance. Finally, sodium channel mutations have been directly linked to human disease, yielding insight into both disease pathophysiology and normal channel function. After a brief discussion of previous work, this review will focus on recent advances in each of these areas.