Selective localization of cardiac SCN5A sodium channels in limbic regions of rat brain

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Leaky Ca2+ release channel/ryanodine receptor 2 causes seizures and sudden cardiac death in mice

The Ca2+ release channel ryanodine receptor 2 (RyR2) is required for excitation-contraction coupling in the heart and is also present in the brain. Mutations in RyR2 have been linked to exercise-induced sudden cardiac death (catecholaminergic polymorphic ventricular tachycardia [CPVT]). CPVT-associated RyR2 mutations result in "leaky" RyR2 channels due to the decreased binding of the calstabin2 (FKBP12.6) subunit, which stabilizes the closed state of the channel. We found that mice heterozygous for the R2474S mutation in Ryr2 (Ryr2-R2474S mice) exhibited spontaneous generalized tonic-clonic seizures (which occurred in the absence of cardiac arrhythmias), exercise-induced ventricular arrhythmias, and sudden cardiac death. Treatment with a novel RyR2-specific compound (S107) that enhances the binding of calstabin2 to the mutant Ryr2-R2474S channel inhibited the channel leak and prevented cardiac arrhythmias and raised the seizure threshold. Thus, CPVT-associated mutant leaky Ryr2-R2474S channels in the brain can cause seizures in mice, independent of cardiac arrhythmias. Based on these data, we propose that CPVT is a combined neurocardiac disorder in which leaky RyR2 channels in the brain cause epilepsy, and the same leaky channels in the heart cause exercise-induced sudden cardiac death.

A mutation in telethonin alters Nav1.5 function

Excitable cells express a variety of ion channels that allow rapid exchange of ions with the extracellular space. Opening of Na(+) channels in excitable cells results in influx of Na(+) and cellular depolarization. The function of Na(v)1.5, an Na(+) channel expressed in the heart, brain, and gastrointestinal tract, is altered by interacting proteins. The pore-forming alpha-subunit of this channel is encoded by SCN5A. Genetic perturbations in SCN5A cause type 3 long QT syndrome and type 1 Brugada syndrome, two distinct heritable arrhythmia syndromes. Mutations in SCN5A are also associated with increased prevalence of gastrointestinal symptoms, suggesting that the Na(+) channel plays a role in normal gastrointestinal physiology and that alterations in its function may cause disease. We collected blood from patients with intestinal pseudo-obstruction (a disease associated with abnormal motility in the gut) and screened for mutations in SCN5A and ion channel-interacting proteins. A 42-year-old male patient was found to have a mutation in the gene TCAP, encoding for the small protein telethonin. Telethonin was found to be expressed in the human gastrointestinal smooth muscle, co-localized with Na(v)1.5, and co-immunoprecipitated with sodium channels. Expression of mutated telethonin, when co-expressed with SCN5A in HEK 293 cells, altered steady state activation kinetics of SCN5A, resulting in a doubling of the window current. These results suggest a new role for telethonin, namely that telethonin is a sodium channel-interacting protein. Also, mutations in telethonin can alter Na(v)1.5 kinetics and may play a role in intestinal pseudo-obstruction.

How do mutant Nav1.1 sodium channels cause epilepsy?

Voltage-gated sodium channels comprise pore-forming alpha subunits and auxiliary beta subunits. Nine different alpha subtypes, designated Nav1.1-Nav1.9 have been identified in excitable cells. Nav1.1, 1.2 and 1.6 are major subtypes in the adult mammalian brain. More than 200 mutations in the Nav1.1 alpha subtype have been linked to inherited epilepsy syndromes, ranging in severity from the comparatively mild disorder Generalized Epilepsy with Febrile Seizures Plus to the epileptic encephalopathy Severe Myoclonic Epilepsy of Infancy. Studies using heterologous expression and functional analysis of recombinant Nav1.1 channels suggest that epilepsy mutations in Nav1.1 may cause either gain-of-function or loss-of-function effects that are consistent with either increased or decreased neuronal excitability. How these diverse effects lead to epilepsy is poorly understood. This review summarizes the data on sodium channel mutations and epilepsy and builds a case for the hypothesis that most Nav1.1 mutations have their ultimate epileptogenic effects by reducing Nav1.1-mediated whole cell sodium currents in GABAergic neurons, resulting in widespread loss of brain inhibition, an ideal background for the genesis of epileptic seizures.

Distribution of the voltage-gated sodium channel Na(v)1.7 in the rat: expression in the autonomic and endocrine systems

It is generally accepted that the voltage-gated, tetrodotoxin-sensitive sodium channel, Na(V)1.7, is selectively expressed in peripheral ganglia. However, global deletion in mice of Na(V)1.7 leads to death shortly after birth (Nassar et al. [2004] Proc. Natl. Acad. Sci. U. S. A. 101:12706-12711), suggesting that this ion channel might be more widely expressed. To understand better the potential physiological function of this ion channel, we examined Na(V)1.7 expression in the rat by in situ hybridization and immunohistochemistry. As expected, highest mRNA expression levels are found in peripheral ganglia, and the protein is expressed within these ganglion cells and on the projections of these neurons in the central nervous system. Importantly, we found that Na(V)1.7 is present in discrete rat brain regions, and the unique distribution pattern implies a central involvement in endocrine and autonomic systems as well as analgesia. In addition, Na(V)1.7 expression was detected in the pituitary and adrenal glands. These results indicate that Na(V)1.7 is not only involved in the processing of sensory information but also participates in the regulation of autonomic and endocrine systems; more specifically, it could be implicated in such vital functions as fluid homeostasis and cardiovascular control.

Differential Expression of Intrinsic Membrane Currents in Defined Cell Types of the Anterolateral Bed Nucleus of the Stria Terminalis

The anterolateral group of the bed nucleus of the stria terminalis (BNSTALG) plays a critical role in a diverse array of behaviors, although little is known of the physiological properties of neurons in this region. Using whole cell patch-clamp recordings from rat BNSTALG slices in vitro, we describe three distinct physiological cell types. Type I neurons were characterized by the presence of a depolarizing sag in response to hyperpolarizing current injection that resembled activation of the hyperpolarization-activated cation current Ih and a regular firing pattern in response to depolarizing current injection. Type II neurons exhibited the same depolarizing sag in response to hyperpolarizing current injection, but burst-fired in response to depolarizing current injection, which was indicative of the activation of the low-threshold calcium current IT. Type III neurons did not exhibit a depolarizing sag in response to hyperpolarizing current injection, but instead exhibited a fast time-independent rectification that became more pronounced with increased amplitude of hyperpolarizing current injection, and was indicative of activation of the inwardly rectifying potassium current IK(IR). Type III neurons also exhibited a regular firing pattern in response to depolarizing current. Using voltage-clamp analysis we further characterized the primary active currents that shaped the physiological properties of these distinct cell types, including Ih, IT, IK(IR), the voltage-dependent potassium current IA, and the persistent sodium current INaP. The functional relevance of each cell type is discussed in relation to prior anatomical studies, as well as how these currents may interact to modulate neuronal activity within the BNSTALG.

Nav channel mechanosensitivity: activation and inactivation accelerate reversibly with stretch

Voltage-gated sodium channels (Nav) are modulated by many bilayer mechanical amphiphiles, but whether, like other voltage-gated channels (Kv, HCN, Cav), they respond to physical bilayer deformations is unknown. We expressed human heart Nav1.5 pore alpha-subunit in oocytes (where, unlike alphaNav1.4, alphaNav1.5 exhibits normal kinetics) and measured small macroscopic currents in cell-attached patches. Pipette pressure was used to reversibly stretch the membrane for comparison of I(Na)(t) before, during, and after stretch. At all voltages, and in a dose-dependent fashion, stretch accelerated the I(Na)(t) time course. The sign of membrane curvature was not relevant. Typical stretch stimuli reversibly accelerated both activation and inactivation by approximately 1.4-fold; normalization of peak I(Na)(t) followed by temporal scaling ( approximately 1.30- to 1.85-fold) resulted in full overlap of the stretch/no-stretch traces. Evidently the rate-limiting outward voltage sensor motion in the Nav1.5 activation path (as in Kv1) accelerated with stretch. Stretch-accelerated inactivation occurred even with activation saturated, so an independently stretch-modulated inactivation transition is also a possibility. Since Nav1.5 channel-stretch modulation was both reliable and reversible, and required stretch stimuli no more intense than what typically activates putative mechanotransducer channels (e.g., stretch-activated TRPC1-based currents), Nav channels join the ranks of putative mechanotransducers. It is noteworthy that at voltages near the activation threshold, moderate stretch increased the peak I(Na) amplitude approximately 1.5-fold. It will be important to determine whether stretch-modulated Nav current contributes to cardiac arrhythmias, to mechanosensory responses in interstitial cells of Cajal, to touch receptor responses, and to neuropathic (i.e., hypermechanosensitive) and/or normal pain reception.

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.

Lamotrigine in idiopathic epilepsy - increased risk of cardiac death?

OBJECTIVES

Lamotrigine (LTG) has recently been shown to inhibit the cardiac rapid delayed rectifier potassium ion current (Ikr). Ikr-blocking drugs may increase the risk of cardiac arrhythmia and sudden unexpected death. With this background, it may be of importance that in our outpatient clinic between August 1, 1995 and August 1, 2005 we registered four consecutive cases of sudden unexpected death in epilepsy (SUDEP) in non-hospitalized patients that were all being treated with LTG in monotherapy. Here we describe and discuss these cases, the relevant literature, and the reasons to question whether these events were as a result of coincidence alone.

METHODS

All the cases were collected consecutively at the outpatient clinic, Department of Neurology, Stavanger University Hospital, Norway. Clinical and pathological data were obtained and the relevant literature reviewed.

RESULTS

All were females with idiopathic epilepsy.

CONCLUSIONS

A systematic study is needed to reveal whether LTG may increase the risk of SUDEP in certain groups of patients.

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

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

Clinical aspects and physiopathology of Brugada syndrome: review of current concepts

Brugada syndrome (BS) is an inherited cardiac disorder characterized by typical electrocardiographic patterns of ST segment elevation in the precordial leads, right bundle branch block, fast polymorphic ventricular tachycardia in patients without any structural heart disease, and a high risk of sudden cardiac death. The incidence of BS is high in male vs. female (i.e., 8–10/1: male/female). The disorder is caused by mutations in the SCN5A gene encoding Nav1.5, the cardiac sodium channel, which is the only gene in which mutations were found to cause the disease. Mutations in SCN5A associated with the BS phenotype usually result in a loss of channel function by a reduction in Na+ currents. We review the clinical aspects, risk stratification, and therapeutic management of this important syndrome.

Gene duplications and evolution of vertebrate voltage-gated sodium channels

Voltage-gated sodium channels underlie action potential generation in excitable tissue. To establish the evolutionary mechanisms that shaped the vertebrate sodium channel alpha-subunit (SCNA) gene family and their encoded Nav1 proteins, we identified all SCNA genes in several teleost species. Molecular cloning revealed that teleosts have eight SCNA genes, compared to ten in another vertebrate lineage, mammals. Prior phylogenetic analyses have indicated that the genomes of both teleosts and tetrapods contain four monophyletic groups of SCNA genes, and that tandem duplications expanded the number of genes in two of the four mammalian groups. However, the number of genes in each group varies between teleosts and tetrapods, suggesting different evolutionary histories in the two vertebrate lineages. Our findings from phylogenetic analysis and chromosomal mapping of Danio rerio genes indicate that tandem duplications are an unlikely mechanism for generation of the extant teleost SCNA genes. Instead, analyses of other closely mapped genes in D. rerio as well as of SCNA genes from several teleost species all support the hypothesis that a whole-genome duplication was involved in expansion of the SCNA gene family in teleosts. Interestingly, despite their different evolutionary histories, mRNA analyses demonstrated a conservation of expression patterns for SCNA orthologues in teleosts and tetrapods, suggesting functional conservation.

Functional expression of "cardiac-type" Nav1.5 sodium channel in canine intracardiac ganglia

BACKGROUND

The autonomic nervous system has been implicated in several arrhythmogenic diseases, including long QT syndrome type 3 (LQT3) and Brugada syndrome. Scarce information on the cellular components of the intrinsic cardiac ganglia from higher mammals has limited our understanding of the role of the autonomic nervous system in such diseases.

OBJECTIVES

The purpose of this study was to isolate and characterize the electrophysiologic properties of canine intracardiac neurons.

METHODS

Action potentials (APs) and ionic currents were studied in enzymatically dissociated canine intracardiac neurons under current and voltage clamp conditions. Immunohistochemical and reverse transcription-polymerase chain reaction analysis was performed using freshly isolated intracardiac ganglia.

RESULTS

APs recorded from intracardiac neurons displayed a tetrodotoxin-resistant (TTX-R) component. TTX-R APs were abolished in the absence of sodium but persisted in the absence of external calcium. Immunohistochemical studies showed the presence of TTX-R sodium channels in these ganglia. Sodium currents were characterized by two components with different affinities for TTX: a tetrodotoxin-sensitive (TTX-S) component and a TTX-R component. TTX-S current inactivation was characteristic of neuronal sodium currents, whereas TTX-R current inactivation time constants were similar to those previously reported for Na(v)1.5 channels. TTX sensitivity (IC(50) = 1.17 microM) of the TTX-R component was in the range reported for Na(v)1.5 channels. Expression of Na(v)1.5 channels in intracardiac ganglia was confirmed by PCR analysis and sequencing.

CONCLUSION

Our results suggest that canine intracardiac neurons functionally express Na(v)1.5 channels. These findings open an exciting new door to our understanding of autonomically modulated arrhythmogenic diseases linked to mutations in Na(v)1.5 channels, including Brugada syndrome and LQT3.

Proximal persistent Na+ channels drive spike afterdepolarizations and associated bursting in adult CA1 pyramidal cells

In many principal brain neurons, the fast, all-or-none Na+ spike initiated at the proximal axon is followed by a slow, graded after depolarization (ADP). The spike ADP is critically important in determining the firing mode of many neurons; large ADPs cause neurons to fire bursts of spikes rather than solitary spikes. Nonetheless, not much is known about how and where spike ADPs are initiated. We addressed these questions in adult CA1 pyramidal cells, which manifest conspicuous somatic spike ADPs and an associated propensity for bursting, using sharp and patch microelectrode recordings in acutely isolated hippocampal slices and single neurons. Voltage-clamp commands mimicking spike waveforms evoked transient Na+ spike currents that declined quickly after the spike but were followed by substantial sustained Na+ spike after currents. Drugs that blocked the persistent Na+ current (INaP), markedly suppressed the sustained Na+ spike after currents, as well as spike ADPs and associated bursting. Ca2+ spike after currents were much smaller, and reducing them had no noticeable effect on the spike ADPs. Truncating the apical dendrites affected neither spike ADPs nor the firing modes of these neurons. Application of INaP blockers to truncated neurons, or their focal application to the somatic region of intact neurons, suppressed spike ADPs and associated bursting, whereas their focal application to distal dendrites did not. We conclude that the somatic spike ADPs are generated predominantly by persistent Na+ channels located at or near the soma. Through this action, proximal INaP critically determines the firing mode and spike output of adult CA1 pyramidal cells.

Sodium currents in medullary neurons isolated from the pre-Bötzinger complex region

The pre-Bötzinger complex (preBötC) in the ventrolateral medulla contains interneurons important for respiratory rhythm generation. Voltage-dependent sodium channels mediate transient current (I(NaT)), underlying action potentials, and persistent current (I(NaP)), contributing to repetitive firing, pacemaker properties, and the amplification of synaptic inputs. Voltage-clamp studies of the biophysical properties of these sodium currents were conducted on acutely dissociated preBötC region neurons. Reverse transcription-PCR demonstrated the presence of mRNA for Nav1.1, Nav1.2, and Nav1.6 alpha-subunits in individual neurons. A TTX-sensitive I(NaP) was evoked in all tested neurons by ramp depolarization from -80 to 0 mV. Including a constant in the Boltzmann equation for inactivation by estimating the steady-state fraction of Na+ channels available for inactivation allowed prediction of a window current that did not decay to 0 at voltages positive to -20 mV and closely matched the measured I(NaP). Riluzole (3 microM), a putative I(NaP) antagonist, reduced both I(NaP) and I(NaT) and produced a hyperpolarizing shift in the voltage dependence of steady-state inactivation. The latter decreased the predicted window current by an amount equivalent to the decrease in I(NaP). Riluzole also decreased the inactivation time constant at potentials in which the peak window/persistent currents are generated. Together, these findings imply that I(NaP) and I(NaT) arise from the same channels and that a simple modification of the Hodgkin-Huxley model can satisfactorily account for both currents. In the rostral ventral respiratory group (immediately caudal to preBötC), I(NaP) was also detected, but peak conductance, current density, and input resistance were smaller than in preBötC region cells.

Regulation of Na v channels in sensory neurons

Voltage-gated Na(+) channels have an essential role in the biophysical properties of nociceptive neurons. Factors that regulate Na(+) channel function are of interest from both pathophysiological and therapeutic perspectives. Increasing evidence indicates that changes in expression or inappropriate modulation of these channels leads to electrical instability of the cell membrane and the inappropriate spontaneous activity that is observed following nerve injury, and that this might contribute to neuropathic pain. The role of Na(v) channels in nociception depends on modulation by factors such as auxiliary beta-subunits, cytoskeletal proteins and the phosphorylation state of neurons. In this review we describe the modulation of Na(v) channels on sensory neurons by auxiliary beta-subunits, protein kinases and cytoskeletal proteins.

Tetrodotoxin-resistant Na+channels in human neuroblastoma cells are encoded by new variants of Nav1.5/SCN5A

Both tetrodotoxin-sensitive (TTX-S) and TTX-resistant (TTX-R) voltage-dependent Na+ channels are expressed in the human neuroblastoma cell line NB-1, but a gene encoding the TTX-R Na+ channel has not been identified. In this study, we have cloned cDNA encoding the alpha subunit of the TTX-R Na+ channel in NB-1 cells and designated it hNbR1. The longest open reading frame of hNbR1 (accession no. AB158469) encodes 2016 amino acid residues. Sequence analysis has indicated that hNbR1 is highly homologous with human cardiac Nav1.5/SCN5A with > 99% amino acid identity. The presence of a cysteine residue (Cys373) in the pore-loop region of domain I is consistent with the supposition that hNbR1 is resistant to TTX. Analysis of the genomic sequence of SCN5A revealed a new exon encoding S3 and S4 of domain I (exon 6A). In addition, an alternative splicing variant, lacking exon 18, that encodes 54 amino acids in the intracellular loop between domains II and III was found (hNbR1-2; accession no. AB158470). Na+ currents in human embryonic kidney cells (HEK293) transfected with hNbR1 or hNbR1-2 showed electrophysiological properties similar to those for TTX-R I(Na) in NB-1 cells. The IC50 for the TTX block was approximately 8 microM in both variants. These results suggest that SCN5A has a newly identified exon for alternative splicing and is more widely expressed than previously thought.

Low voltage-activated calcium and fast tetrodotoxin-resistant sodium currents define subtypes of cholinergic and noncholinergic neurons in rat basal forebrain

Neurons of the basal forebrain (BF) possess unique combinations of voltage-gated membrane currents. Here, we describe subtypes of rat basal forebrain neurons based on patch-clamp analysis of low-voltage activated (LVA) calcium and tetrodotoxin-resistant (TTX-R) sodium currents combined with single-cell RT-PCR analysis. Neurons were identified by mRNA expression of choline acetyltransferase (ChAT+, cholinergic) and glutamate decarboxylase (GAD67, GABAergic). Four cell types were encountered: ChAT+, GAD+, ChAT+/GAD+ and ChAT-/GAD- cells. Both ChAT+ and ChAT+/GAD+ cells (71/75) displayed LVA currents and most (34/39) expressed mRNA for LVA Ca(2+) channel subunits. Ca(v)3.2 was detected in 31/34 cholinergic neurons and Ca(v)3.1 was expressed in 6/34 cells. Three cells expressed both subunits. No single neurons showed Ca(v)3.3 mRNA expression, although BF tissue expression was observed. In young rats (2-4 mo), ChAT+/GAD+ cells displayed larger LVA current densities compared to ChAT+ neurons, while these latter neurons displayed an age-related increase in current densities. Most (29/38) noncholinergic neurons (GAD+ and ChAT-/GAD-) possessed fast TTX-R sodium currents resembling those mediated by Na(+) channel subunit Na(v)1.5. This subunit was expressed predominately in noncholinergic neurons. No cholinergic cells (0/75) displayed fast TTX-R currents. The TTX-R currents were faster and larger in GAD+ neurons compared to ChAT-/GAD- neurons. The properties of ChAT+/GAD+ neurons resemble those of ChAT+ neurons, rather than of GAD+ neurons. These results suggest novel features of subtypes of cholinergic and noncholinergic neurons within the BF that may provide new insights for understanding normal BF function.

Localization of voltage-gated ion channels in mammalian brain

The intrinsic electrical properties of neurons are shaped in large part by the action of voltage-gated ion channels. Molecular cloning studies have revealed a large family of ion channel genes, many of which are expressed in mammalian brain. Much recent effort has focused on determining the contribution of the protein products of these genes to neuronal function. This requires knowledge of the abundance and distribution of the constituent subunits of the channels in specific mammalian central neurons. Here we review progress made in recent studies aimed at localizing specific ion channel subunits using in situ hybridization and immunohistochemistry. We then discuss the implications of these results in terms of neuronal physiology and neuronal mechanisms underlying the observed patterns of expression.

Novel isoforms of the sodium channels Nav1.8 and Nav1.5 are produced by a conserved mechanism in mouse and rat

The voltage-gated sodium channel Na(v)1.8 is only expressed in subsets of neurons in dorsal root ganglia (DRG) and trigeminal and nodose ganglia. We have isolated mouse partial length Na(v)1.8 cDNA clones spanning the exon 17 sequence, which have 17 nucleotide substitutions and 12 predicted amino acid differences from the published sequence. The absence of a mutually exclusive alternative exon 17 was confirmed by sequencing 4.1 kilobases of genomic DNA spanning exons 16-18 of Scn10a. A novel cDNA isoform was identified, designated Na(v)1.8c, which results from alternative 3'-splice site selection at a CAG/CAG motif to exclude the codon for glutamine 1031 within the interdomain cytoplasmic loop IDII/III. The ratio of Na(v)1.8c (CAG-skipped) to Na(v)1.8 (CAG-inclusive) mRNA in mouse is approximately 2:1 in adult DRG, trigeminal ganglion, and neonatal DRG. A Na(v)1.8c isoform also occurs in rat DRG, but is less common. Of the two other tetrodotoxin-resistant channels, no analogous alternative splicing of mouse Na(v)1.9 was detected, whereas rare alternative splicing of Na(v)1.5 at a CAG/CAG motif resulted in the introduction of a CAG trinucleotide. This isoform, designated Na(v)1.5c, is conserved in rat and encodes an additional glutamine residue that disrupts a putative CK2 phosphorylation site. In summary, novel isoforms of Na(v)1.8 and Na(v)1.5 are each generated by alternative splicing at CAG/CAG motifs, which result in the absence or presence of predicted glutamine residues within the interdomain cytoplasmic loop IDII/III. Mutations of sodium channels within this cytoplasmic loop have previously been demonstrated to alter electrophysiological properties and cause cardiac arrhythmias and epilepsy.

The biology of epilepsy genes

Mutations in over 70 genes now define biological pathways leading to epilepsy, an episodic dysrhythmia of the cerebral cortex marked by abnormal network synchronization. Some of the inherited errors destabilize neuronal signaling by inflicting primary disorders of membrane excitability and synaptic transmission, whereas others do so indirectly by perturbing critical control points that balance the developmental assembly of inhibitory and excitatory circuits. The genetic diversity is now sufficient to discern short- and long-range functional convergence of epileptogenic molecular pathways, reducing the broad spectrum of primary molecular defects to a few common processes regulating cortical synchronization. Synaptic inhibition appears to be the most frequent target; however, each gene mutation retains unique phenotypic features. This review selects exemplary members of several gene families to illustrate principal categories of the disease and trace the biological pathways to epileptogenesis in the developing brain.

Modulation of the cardiac sodium channel Nav1.5 by fibroblast growth factor homologous factor 1B

We have previously shown that fibroblast growth factor homologous factor 1B (FHF1B), a cytosolic member of the fibroblast growth factor family, associates with the sensory neuron-specific channel Na(v)1.9 but not with the other sodium channels present in adult rat dorsal root ganglia neurons. We show in this study that FHF1B binds to the C terminus of the cardiac voltage-gated sodium channel Na(v)1.5 and modulates the properties of the channel. The N-terminal 41 amino acid residues of FHF1B are essential for binding to Na(v)1.5, and the conserved acidic rich domain (amino acids 1773-1832) in the C terminus of Na(v)1.5 is sufficient for association with this factor. Binding of the growth factor to recombinant wild type human Na(v)1.5 in human embryonic kidney 293 cells produces a significant hyperpolarizing shift in the voltage dependence of channel inactivation. An aspartic acid to glycine substitution at position 1790 of the channel, which underlies one of the LQT-3 phenotypes of cardiac arrythmias, abolishes the interaction of the Na(v)1.5 channel with FHF1B. This is the first report showing that interaction with a growth factor can modulate properties of a voltage-gated sodium channel.

Functional modulation of human brain Nav1.3 sodium channels, expressed in mammalian cells, by auxiliary beta 1, beta 2 and beta 3 subunits

Voltage-gated sodium channels consist of a pore-forming alpha subunit and two auxiliary beta subunits. Excitable cells express multiple alpha subtypes, designated Na(v)1.1-Na(v)1.9, and three beta subunits, designated beta1, beta2 and beta3. Understanding how the different alpha subtypes, in combination with the various beta subunits, determine sodium channel behavior is important for elucidating the molecular basis of sodium channel functional diversity. In this study, we used whole-cell electrophysiological recording to examine the properties of the human Na(v)1.3 alpha subtype, stably expressed in Chinese hamster ovary cells, and to investigate modulation of Na(v)1.3 function by beta1, beta2 and beta3 subunits. In the absence of beta subunits, human Na(v)1.3 formed channels that inactivated rapidly (tau(inactivation) approximately equals 0.5 ms at 0 mV) and almost completely by the end of 190-ms-long depolarizations. Using an intracellular solution with aspartate as the main anion, the midpoint for channel activation was approximately -12 mV. The midpoint for inactivation, determined using 100-ms conditioning pulses, was approximately -47 mV. The time constant for repriming of inactivated channels at -80 mV was approximately 6 ms. Coexpression of beta1 or beta3 did not affect inactivation time course or the voltage dependence of activation, but shifted the inactivation curve approximately 10 mV negative, and slowed the repriming rate ca. three-fold. beta2 did not affect channel properties, either by itself or in combination with beta1 or beta3. Na(v)1.3 expression is increased in damaged nociceptive peripheral afferents. This change in channel expression levels is correlated with the emergence of a rapidly inactivating and rapidly repriming sodium current, which has been proposed to contribute to the pathophysiology of neuropathic pain. The results of this study support the hypothesis that Na(v)1.3 may mediate this fast sodium current.

Localization of Nav1.5 sodium channel protein in the mouse brain

Na(v)1.5 or SCN5A is a member of the voltage-dependent family of sodium channels. The distribution of Na(v)1.5 protein was investigated in the mouse brain using immunohistochemistry. Immunostaining with a Na(v)1.5-specific antibody revealed that Na(v)1.5 protein was localized in certain distinct regions of brain including the cerebral cortex, thalamus, hypothalamus, basal ganglia, cerebellum and brain stem. Notably, we found that Na(v)1.5 protein co-localized with neurofilaments and clustered at a high density in the neuronal processes, mainly axons. These results suggest that Na(v)1.5 protein may play a role in the physiology of the central nervous system (generation and propagation of electrical signals by axons).

Voltage-gated sodium channels in epilepsy

Animal experiments, and particularly functional investigations on human chronically epileptic tissue as well as genetic studies in epilepsy patients and their families strongly suggest that some forms of epilepsy may share a pathogenetic mechanism: an alteration of voltage-gated sodium channels. This review summarizes recent data on changes of sodium channel expression, molecular structure and function associated with epilepsy, as well as on the interaction of new and established antiepileptic drugs with sodium currents. Although it remains to be determined precisely how and to what extent altered sodium-channel functions play a role in different epilepsy syndromes, future promising therapy approaches may include drugs modulating sodium currents, and particularly substances changing their inactivation characteristics.

Na(v)1.5 underlies the 'third TTX-R sodium current' in rat small DRG neurons

In addition to slow-inactivating and persistent TTX-R Na(+) currents produced by Na(v)1.8 and Na(v)1.9 Na(+) channels, respectively, a third TTX-R Na(+) current with fast activation and inactivation can be recorded in 80% of small neurons of dorsal root ganglia (DRG) from E15 rats, but in only 3% of adult small DRG neurons. The half-time for activation, the time constant for inactivation, and the midpoints of activation and inactivation of the third TTX-R Na(+) currents are significantly different from those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The estimated TTX K(i) (2.11+/-0.34 microM) of the third TTX-R Na(+) current is significantly lower than those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The Cd(2+) sensitivity of third TTX-R Na(+) current is closer to cardiac Na(+) currents. A concentration of 1 mM Cd(2+) is required to completely block this current, which is significantly lower than the 5 mM required to block Na(v)1.8 and Na(v)1.9 currents. The third TTX-R Na(+) channel is not co-expressed with Na(v)1.8 and Na(v)1.9 Na(+) channels in DRG neurons of E18 rats, at a time when all three currents show comparable densities. The physiological and pharmacological profiles of the third TTX-R Na(+) current are similar to those of the cardiac Na(+) channel Na(v)1.5 and RT-PCR and restriction enzyme polymorphism analysis, show a parallel pattern of expression of Na(v)1.5 in DRG during development. Taken together, these results demonstrate that Na(v)1.5 is expressed in a developmentally regulated manner in DRG neurons and suggest that Na(v)1.5 Na(+) channel produces the third TTX-R current.

SCN5A is expressed in human jejunal circular smooth muscle cells

Tetrodotoxin-resistant Na+currents are expressed in a variety of muscle cells including human jejunal circular smooth muscle (HJCSM) cells. The aim of this study was to determine the molecular identity of the pore-forming alpha-subunit of the HJCSM Na+ channel. Degenerate primers identified a cDNA fragment of 1.5 kb with 99% nucleotide homology with human cardiac SCN5A. The identified clone was also amplified from single smooth muscle cells by reverse transcriptase-polymerase chain reaction (RT-PCR). Northern blot analysis showed expression of full-length SCN5A. Laser capture microdissection was used to obtain highly purified populations of HJCSM cells. RT-PCR on the harvested cells showed that SCN5A was present in circular but not in longitudinal muscle. A similar result was obtained using a pan-Na+ channel antibody. The full-length sequence for SCN5A was obtained by combining standard polymerase chain reaction with 5' and 3' rapid amplification of cDNA end techniques. The intestinal SCN5A was nearly identical to the cardiac SCN5A. The data indicate that SCN5A is more widely distributed than previously thought and encodes the pore-forming alpha-subunit of the tetrodotoxin-resistant Na+ current in HJCSM cells.

Anticonvulsant profile and mechanism of action of propranolol and its two enantiomers

The anticonvulsant properties of the ss-adrenoceptor antagonist propranolol and its two enantiomers were examined in various screening tests in order to characterize the anticonvulsant profile as well as the possible molecular mechanism of action. These compounds dose-dependently raised the threshold for tonic electroshock seizures in mice and were effective in the traditional maximal electroshock test (ED (50)s 15- 20 mg kg (-1)i.p.). In combination with clinically used antiepileptics, the anticonvulsant effectiveness of the latter was significantly increased. In the pentylenetetrazol (85 mg kg (-1)s.c.) seizure threshold test, ( +/-)- and ( +)-propranolol were not effective in preventing clonic seizures. In unrestrained rats with chronically implanted electrodes in the dorsal hippocampus, propranolol and its ( +)-enantiomer equieffectively reduced the duration of electrically-evoked hippocampal afterdischarges (10 and 20 mg kg (-1)i.p.) and raised the focal stimulation threshold (20 mg kg (-1)i.p.). In amygdala-kindled rats, both drugs ( >or= 10 mg kg (-1)i.p.) reduced the seizure severity from stage 5 (generalized clonic-tonic) to stage 3 (unilateral forelimb) seizures. Furthermore, whole-cell patch-clamp experiments showed that ( +)- as well as ( -)-propranolol ( 10(-6)to 10(-4)M) depressed the fast inward sodium current in a concentration- and use-dependent manner in cultured rat cardiomyocytes and inhibited picrotoxin-induced burst firing activity of mouse spinal cord neurones in culture. In conclusion, propranolol and its two enantiomers have anticonvulsant effects in models for generalized tonic-clonic and complex partial seizures which may be accounted for by the sodium channel blocking and not by the ss-adrenoceptor blocking activity.

Selectivity and promiscuity of the first and second PDZ domains of PSD-95 and synapse-associated protein 102

PDZ domains typically interact with the very carboxyl terminus of their binding partners. Type 1 PDZ domains usually require valine, leucine, or isoleucine at the very COOH-terminal (P(0)) position, and serine or threonine 2 residues upstream at P(-2). We quantitatively defined the contributions of carboxyl-terminal residues to binding selectivity of the prototypic interactions of the PDZ domains of postsynaptic density protein 95 (PSD-95) and its homolog synapse-associated protein 90 (SAP102) with the NR2b subunit of the N-methyl-d-aspartate-type glutamate receptor. Our studies indicate that all of the last five residues of NR2b contribute to the binding selectivity. Prominent were a requirement for glutamate or glutamine at P(-3) and for valine at P(0) for high affinity binding and a preference for threonine over serine at P(-2), in the context of the last 11 residues of the NR2b COOH terminus. This analysis predicts a COOH-terminal (E/Q)(S/T)XV consensus sequence for the strongest binding to the first two PDZ domains of PSD-95 and SAP102. A search of the human genome sequences for proteins with a COOH-terminal (E/Q)(S/T)XV motif yielded 50 proteins, many of which have not been previously identified as PSD-95 or SAP102 binding partners. Two of these proteins, brain-specific angiogenesis inhibitor 1 and protein kinase Calpha, co-immunoprecipitated with PSD-95 and SAP102 from rat brain extracts.

Conditions sufficient for nonsynaptic epileptogenesis in the CA1 region of hippocampal slices

Nonsynaptic mechanisms exert a powerful influence on seizure threshold. It is well-established that nonsynaptic epileptiform activity can be induced in hippocampal slices by reducing extracellular Ca(2+) concentration. We show here that nonsynaptic epileptiform activity can be readily induced in vitro in normal (2 mM) Ca(2+) levels. Those conditions sufficient for nonsynaptic epileptogenesis in the CA1 region were determined by pharmacologically mimicking the effects of Ca(2+) reduction in normal Ca(2+) levels. Increasing neuronal excitability, by removing extracellular Mg(2+) and increasing extracellular K(+) (6-15 mM), induced epileptiform activity that was suppressed by postsynaptic receptor antagonists [D-(-)-2-amino-5-phosphonopentanoic acid, picrotoxin, and 6,7-dinitroquinoxaline-2,3-dione] and was therefore synaptic in nature. Similarly, epileptiform activity induced when neuronal excitability was increased in the presence of K(Ca) antagonists (verruculogen, charybdotoxin, norepinephrine, tetraethylammonium salt, and Ba(2+)) was found to be synaptic in nature. Decreases in osmolarity also failed to induce nonsynaptic epileptiform activity in the CA1 region. However, increasing neuronal excitability (by removing extracellular Mg(2+) and increasing extracellular K(+)) in the presence of Cd(2+), a nonselective Ca(2+) channel antagonist, or veratridine, a persistent sodium conductance enhancer, induced spontaneous nonsynaptic epileptiform activity in vitro. Both novel models were characterized using intracellular and ion-selective electrodes. The results of this study suggest that reducing extracellular Ca(2+) facilitates bursting by increasing neuronal excitability and inhibiting Ca(2+) influx, which might, in turn, enhance a persistent sodium conductance. Furthermore, these data show that nonsynaptic mechanisms can contribute to epileptiform activity in normal Ca(2+) levels.

Sodium current in human jejunal circular smooth muscle cells

BACKGROUND & AIMS

Sodium channels are key regulators of neuronal and muscle excitability. However, sodium channels have not been definitively identified in gastrointestinal smooth muscle. The aim of the present study was to determine if a Na(+) current is present in human jejunal circular smooth muscle cells.

METHODS

Currents were recorded from freshly dissociated cells using patch-clamp techniques. Complementary DNA (cDNA) libraries constructed from the dissociated cells were screened to determine if a message for alpha subunits of Na(+) channels was expressed. Smooth muscle cells were also collected using laser-capture microdissection and screened.

RESULTS

A tetrodotoxin-insensitive Na(+) channel was present in 80% of cells patch-clamped. Initial activation was at -65 mV with peak inward current at -30 mV. Steady-state inactivation and activation curves revealed a window current between -75 and -60 mV. The Na(+) current was blocked by lidocaine and internal and external QX314. A cDNA highly homologous to SCN5A, the alpha subunit of the cardiac Na(+) channel, was present in the cDNA libraries constructed from dissociated cells and from smooth muscle cells collected using laser-capture microdissection.

CONCLUSIONS

Human jejunal circular smooth muscle cells express a tetrodotoxin-insensitive Na(+) channel, probably SCN5A. Whether SCN5A plays a role in the pathophysiology of human gut dysmotilities remains to be determined.

Molecular genetic basis of sudden cardiac death

In this review, the up-to-date understanding of the molecular basis of disorders causing sudden death will be described. Two arrhythmic disorders causing sudden death have recently been well described at the molecular level, the long QT syndromes (LQTS) and Brugada syndrome, and in this article we will review the current scientific knowledge of each disease. A third disorder, hypertrophic cardiomyopathy (HCM), a myocardial disorder causing sudden death, has also been well studied. Finally, a disorder in which both myocardial abnormalities and rhythm abnormalities coexist, arrhythmogenic right ventricular dysplasia (ARVD) will also be described. The role of the pathologist in these studies will be highlighted.

D1/D5 dopamine receptor activation differentially modulates rapidly inactivating and persistent sodium currents in prefrontal cortex pyramidal neurons

Dopamine (DA) is a well established modulator of prefrontal cortex (PFC) function, yet the cellular mechanisms by which DA exerts its effects in this region are controversial. A major point of contention is the consequence of D(1) DA receptor activation. Several studies have argued that D(1) receptors enhance the excitability of PFC pyramidal neurons by augmenting voltage-dependent Na(+) currents, particularly persistent Na(+) currents. However, this conjecture is based on indirect evidence. To provide a direct test of this hypothesis, we combined voltage-clamp studies of acutely isolated layer V-VI prefrontal pyramidal neurons with single-cell RT-PCR profiling. Contrary to prediction, the activation of D(1) or D(5) DA receptors consistently suppressed rapidly inactivating Na(+) currents in identified corticostriatal pyramidal neurons. This modulation was attenuated by a D(1)/D(5) receptor antagonist, mimicked by a cAMP analog, and blocked by a protein kinase A (PKA) inhibitor. In the same cells the persistent component of the Na(+) current was unaffected by D(1)/D(5) receptor activation-suggesting that rapidly inactivating and persistent Na(+) currents arise in part from different channels. Single-cell RT-PCR profiling showed that pyramidal neurons coexpressed three alpha-subunit mRNAs (Nav1.1, 1.2, and 1.6) that code for the Na(+) channel pore. In neurons from Nav1.6 null mice the persistent Na(+) currents were significantly smaller than in wild-type neurons. Moreover, the residual persistent currents in these mutant neurons-which are attributable to Nav1.1/1.2 channels-were reduced significantly by PKA activation. These results argue that D(1)/D(5) DA receptor activation reduces the rapidly inactivating component of Na(+) current in PFC pyramidal neurons arising from Nav1.1/1.2 Na(+) channels but does not modulate effectively the persistent component of the Na(+) current that is attributable to Nav1.6 Na(+) channels.

Resurgence of sodium channel research

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

Genetics of brugada, long QT, and arrhythmogenic right ventricular dysplasia syndromes

This article outlines the up-to-date understanding of the molecular basis of primary ventricular arrhythmias. Two disorders have recently been well described at the molecular level, the long QT syndromes and Brugada syndrome, and this article reviews the current scientific knowledge of each disease. A third disorder, arrhythmogenic right ventricular dysplasia, which is on the cusp of understanding, will also be described.

Molecular biology and the prolonged QT syndromes

The prolonged QT syndromes are characterized by prolongation of the QT interval corrected for heart rate (QTc) on the surface electrocardiogram associated with T-wave abnormalities, relative bradycardia, and ventricular tachyarrhythmias, including polymorphic ventricular tachycardia and torsades de pointes. These patients tend to present with episodes of syncope, seizures, or sudden death typically triggered by exercise, emotion, noise, or, in some cases, sleep. These disorders of cardiac repolarization are commonly inherited, with the autosomal dominant form, Romano-Ward syndrome, most common. A rare autosomal recessive form associated with sensorineural deafness, Jervell and Lange-Nielsen syndrome, in which the cardiac disorder is autosomal dominant and deafness is a recessive trait, also occurs. The underlying genetic causes of these forms of prolonged QT interval syndromes are heterogeneous, with at least seven genes responsible for the clinical syndromes. All of the five genes identified to date encode ion channel proteins, suggesting this to be an ion channelopathy. In this review, the genetic basis of the prolonged QT interval syndromes will be discussed, genotype-phenotype correlations identified, and the approaches to genetic testing and treatments will be outlined.

Genotype and severity of long QT syndrome

OBJECTIVES

To describe the state of the art of our understanding of the long QT syndromes and to provide the genetic correlation of clinical severity of patients with this disorder.

DATE SOURCES

In this review, we outline data that were obtained from work in our laboratory, as well as information reported in the literature.

STUDY SELECTION

The information in this review spans the last decade; data were obtained from the studies that had the most impact, as well as from recent work at our laboratory.

DATA EXTRACTION

The data reported herein were extracted from the world literature on sudden death and the clinical aspects of long QT syndrome. The genes identified to date, mutations in these genes, and the biophysical perturbations in the mutated ion channels, as well as the severity of disease, are detailed.

DATA SYNTHESIS

The extracted data are described as a state-of-the-art review.

CONCLUSIONS

The long QT syndromes, genetically heterogeneous disorders due to mutations in genes encoding ion channels, are relatively common causes of syncope and sudden death. The affected genes, along with the genetic background of individuals, determine the clinical severity of disease. An understanding of the mechanisms responsible for long QT syndrome is expected to enable development of specific therapies.

The cardiac sodium channel mRNA is expressed in the developing and adult rat and human brain

Expression of the rat (RH-I/SkM2) and human (hH1/SCN5A) tetrodotoxin-resistant (TTX-R), voltage-sensitive sodium channels is thought to be specific to cardiac tissue. We detected RH-I/SkM2 mRNA in newborn rat brain using both RNase protection assay analysis and in situ hybridization and in adult rat brain using RNase protection assay analysis. This expression was observed primarily in developing limbic structures of the cerebrum and diencephalon, and in the medulla of the brain stem. Using RT-PCR analysis, we detected hH1/SCN5A mRNA in both fetal and adult human brain. Interestingly, mutations in the human cardiac sodium channel are known to lead to cardiac abnormalities, which result in arrhythmias and frequently in sudden cardiac death. If these mutant channels were also expressed in limbic regions of the brain, alterations in channel function could have drastic effects on the brain's signaling ability, possibly promoting seizure activity.

Coding and noncoding variation of the human calcium-channel beta4-subunit gene CACNB4 in patients with idiopathic generalized epilepsy and episodic ataxia

Inactivation of the beta4 subunit of the calcium channel in the mouse neurological mutant lethargic results in a complex neurological disorder that includes absence epilepsy and ataxia. To determine the role of the calcium-channel beta4-subunit gene CACNB4 on chromosome 2q22-23 in related human disorders, we screened for mutations in small pedigrees with familial epilepsy and ataxia. The premature-termination mutation R482X was identified in a patient with juvenile myoclonic epilepsy. The R482X protein lacks the 38 C-terminal amino acids containing part of an interaction domain for the alpha1 subunit. The missense mutation C104F was identified both in a German family with generalized epilepsy and praxis-induced seizures and in a French Canadian family with episodic ataxia. These coding mutations were not detected in 255 unaffected control individuals (510 chromosomes), and they may be considered candidate disease mutations. The results of functional tests of the truncated protein R482X in Xenopus laevis oocytes demonstrated a small decrease in the fast time constant for inactivation of the cotransfected alpha1 subunit. Further studies will be required to evaluate the in vivo consequences of these mutations. We also describe eight noncoding single-nucleotide substitutions, two of which are present at polymorphic frequency, and a previously unrecognized first intron of CACNB4 that interrupts exon 1 at codon 21.

Regulation of the cardiac voltage-gated Na+ channel (H1) by the ubiquitin-protein ligase Nedd4

The cardiac voltage-gated Na+ channel H1, involved in the generation of cardiac action potential, contains a C-terminal PY motif (xPPxY). Since PY motifs are known ligands to WW domains, we investigated their role for H1 regulation and the possible involvement of the WW domain containing ubiquitin-protein ligase Nedd4, taking advantage of the Xenopus oocyte system. Mutation of the PY motif leads to higher peak currents when compared to wild-type channel. Moreover, co-expression of Nedd4 reduced the peak currents, whereas an enzymatically inactive Nedd4 mutant increased them, likely by competing with endogenous Nedd4. The effect of Nedd4 was not observed in the PY motif mutated channel or in the skeletal muscle voltage-gated Na+ channel, which lacks a PY motif. We conclude that H1 may be regulated by Nedd4 depending on WW-PY interaction, and on an active ubiquitination site.

Identification of a novel human voltage-gated sodium channel alpha subunit gene, SCN12A

We have cloned a cDNA encoding a novel human voltage-gated sodium channel alpha subunit gene, SCN12A, from human brain. Two alternative splicing variants for SCN12A have been identified. The longest open reading frame of SCN12A encodes 1791 amino acid residues. The deduced amino acid sequence of SCN12A shows 37-73% similarity with various other mammalian sodium channels. The presence of a serine residue (S360) in the SS2 segment of domain I suggests that SCN12A is resistant to tetrodotoxin (TTX), as in the cases of rat Scn10a (rPN3/SNS) and rat Scn11a (NaN/SNS2). SCN12A is expressed predominantly in olfactory bulb, hippocampus, cerebellar cortex, spinal cord, spleen, small intestine, and placenta. Although expression level could not be determined, SCN12A is also expressed in dorsal root ganglia (DRG). Both neurons and glial cells express SCN12A. SCN12A maps to human chromosome 3p23-p21.3. These results suggest that SCN12A is a tetrodotoxin-resistant (TTX-R) sodium channel expressed in the central nervous system and nonneural tissues.

Molecular biology of arrhythmic syndromes

In this review, the up-to-date understanding of the molecular basis of primary ventricular arrhythmias will be outlined. Two disorders have recently been well described at the molecular level, the long QT syndromes and Brugada syndrome, and in this paper we review the current scientific knowledge of each disease.