Syntrophin gamma 2 regulates SCN5A gating by a PDZ domain-mediated interaction

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.

Mechanotransduction in gastrointestinal smooth muscle cells: role of mechanosensitive ion channels

Mechanosensation, the ability to properly sense mechanical stimuli and transduce them into physiologic responses, is an essential determinant of gastrointestinal (GI) function. Abnormalities in this process result in highly prevalent GI functional and motility disorders. In the GI tract, several cell types sense mechanical forces and transduce them into electrical signals, which elicit specific cellular responses. Some mechanosensitive cells like sensory neurons act as specialized mechanosensitive cells that detect forces and transduce signals into tissue-level physiological reactions. Nonspecialized mechanosensitive cells like smooth muscle cells (SMCs) adjust their function in response to forces. Mechanosensitive cells use various mechanoreceptors and mechanotransducers. Mechanoreceptors detect and convert force into electrical and biochemical signals, and mechanotransducers amplify and direct mechanoreceptor responses. Mechanoreceptors and mechanotransducers include ion channels, specialized cytoskeletal proteins, cell junction molecules, and G protein-coupled receptors. SMCs are particularly important due to their role as final effectors for motor function. Myogenic reflex-the ability of smooth muscle to contract in response to stretch rapidly-is a critical smooth muscle function. Such rapid mechanotransduction responses rely on mechano-gated and mechanosensitive ion channels, which alter their ion pores' opening in response to force, allowing fast electrical and Ca²⁺ responses. Although GI SMCs express a variety of such ion channels, their identities remain unknown. Recent advancements in electrophysiological, genetic, in vivo imaging, and multi-omic technologies broaden our understanding of how SMC mechano-gated and mechanosensitive ion channels regulate GI functions. This review discusses GI SMC mechanosensitivity's current developments with a particular emphasis on mechano-gated and mechanosensitive ion channels.

Phenotypic switch of smooth muscle cells in paediatric chronic intestinal pseudo-obstruction syndrome

Smooth Muscle Cells (SMC) are unique amongst all muscle cells in their capacity to modulate their phenotype. Indeed, SMCs do not terminally differentiate but instead harbour a remarkable capacity to dedifferentiate, switching between a quiescent contractile state and a highly proliferative and migratory phenotype, a quality often associated to SMC dysfunction. However, phenotypic plasticity remains poorly examined in the field of gastroenterology in particular in pathologies in which gut motor activity is impaired. Here, we assessed SMC status in biopsies of infants with chronic intestinal pseudo-obstruction (CIPO) syndrome, a life-threatening intestinal motility disorder. We showed that CIPO-SMCs harbour a decreased level of contractile markers. This phenotype is accompanied by an increase in Platelet-Derived Growth Factor Receptor-alpha (PDGFRA) expression. We showed that this modulation occurs without origin-related differences in CIPO circular and longitudinal-derived SMCs. As we characterized PDGFRA as a marker of digestive mesenchymal progenitors during embryogenesis, our results suggest a phenotypic switch of the CIPO-SMC towards an undifferentiated stage. The development of CIPO-SMC culture and the characterization of SMC phenotypic switch should enable us to design therapeutic approaches to promote SMC differentiation in CIPO.

microRNA overexpression in slow transit constipation leads to reduced NaV1.5 current and altered smooth muscle contractility

OBJECTIVE

This study was designed to evaluate the roles of microRNAs (miRNAs) in slow transit constipation (STC).

DESIGN

All human tissue samples were from the muscularis externa of the colon. Expression of 372 miRNAs was examined in a discovery cohort of four patients with STC versus three age/sex-matched controls by a quantitative PCR array. Upregulated miRNAs were examined by quantitative reverse transcription qPCR (RT-qPCR) in a validation cohort of seven patients with STC and age/sex-matched controls. The effect of a highly differentially expressed miRNA on a custom human smooth muscle cell line was examined in vitro by RT-qPCR, electrophysiology, traction force microscopy, and ex vivo by lentiviral transduction in rat muscularis externa organotypic cultures.

RESULTS

The expression of 13 miRNAs was increased in STC samples. Of those miRNAs, four were predicted to target SCN5A, the gene that encodes the Na⁺ channel Na_V1.5. The expression of SCN5A mRNA was decreased in STC samples. Let-7f significantly decreased Na⁺ current density in vitro in human smooth muscle cells. In rat muscularis externa organotypic cultures, overexpression of let-7f resulted in reduced frequency and amplitude of contraction.

CONCLUSIONS

A small group of miRNAs is upregulated in STC, and many of these miRNAs target the SCN5A-encoded Na⁺ channel Na_V1.5. Within this set, a novel Na_V1.5 regulator, let-7f, resulted in decreased Na_V1.5 expression, current density and reduced motility of GI smooth muscle. These results suggest Na_V1.5 and miRNAs as novel diagnostic and potential therapeutic targets in STC.

Systems Pharmacology Dissection of Multi-Scale Mechanisms of Action of Huo-Xiang-Zheng-Qi Formula for the Treatment of Gastrointestinal Diseases

Multi-components Traditional Chinese Medicine (TCM) treats various complex diseases (multi-etiologies and multi-symptoms) via herbs interactions to exert curative efficacy with less adverse effects. However, the ancient Chinese compatibility theory of herbs formula still remains ambiguous. Presently, this combination principle is dissected through a systems pharmacology study on the mechanism of action of a representative TCM formula, Huo-xiang-zheng-qi (HXZQ) prescription, on the treatment of functional dyspepsia (FD), a chronic or recurrent clinical disorder of digestive system, as typical gastrointestinal (GI) diseases which burden human physical and mental health heavily and widely. In approach, a systems pharmacology platform which incorporates the pharmacokinetic and pharmaco-dynamics evaluation, target fishing and network pharmacological analyses is employed. As a result, 132 chemicals and 48 proteins are identified as active compounds and FD-related targets, and the mechanism of HXZQ formula for the treatment of GI diseases is based on its three function modules of anti-inflammation, immune protection and gastrointestinal motility regulation mainly through four, i.e., PIK-AKT, JAK-STAT, Toll-like as well as Calcium signaling pathways. In addition, HXZQ formula conforms to the ancient compatibility rule of "Jun-Chen-Zuo-Shi" due to the different, while cooperative roles that herbs possess, specifically, the direct FD curative effects of GHX (serving as Jun drug), the anti-bacterial efficacy and major accompanying symptoms-reliving bioactivities of ZS and BZ (as Chen), the detoxication and ADME regulation capacities of GC (as Shi), as well as the minor symptoms-treating efficacy of the rest 7 herbs (as Zuo). This work not only provides an insight of the therapeutic mechanism of TCMs on treating GI diseases from a multi-scale perspective, but also may offer an efficient way for drug discovery and development from herbal medicine as complementary drugs.

Voltage-Dependent Sarcolemmal Ion Channel Abnormalities in the Dystrophin-Deficient Heart

Mutations in the gene encoding for the intracellular protein dystrophin cause severe forms of muscular dystrophy. These so-called dystrophinopathies are characterized by skeletal muscle weakness and degeneration. Dystrophin deficiency also gives rise to considerable complications in the heart, including cardiomyopathy development and arrhythmias. The current understanding of the pathomechanisms in the dystrophic heart is limited, but there is growing evidence that dysfunctional voltage-dependent ion channels in dystrophin-deficient cardiomyocytes play a significant role. Herein, we summarize the current knowledge about abnormalities in voltage-dependent sarcolemmal ion channel properties in the dystrophic heart, and discuss the potentially underlying mechanisms, as well as their pathophysiological relevance.

Functional consequences of genetic variation in sodium channel modifiers in early onset lone atrial fibrillation

Aim: We investigated the effect of variants in genes encoding sodium channel modifiers SNTA1 and GPD1L found in early onset atrial fibrillation (AF) patients. Patients & methods: Genetic screening in patients with early onset lone AF revealed three variants in GPD1L and SNTA1 in three AF patients. Functional analysis was performed by patch-clamp electrophysiology. Results: Co-expression of GPD1L or its p.A326E variant with NaV1.5 did not alter INa density or current kinetics. SNTA1 shifted the peak-current by -5 mV. The SNTA1-p.A257G variant significantly increased INa. SNTA1-p.P74L did not produce functional changes. Conclusion: Although genetic variation of sodium channel modifiers may contribute to development of AF at a molecular level, it is unlikely a monogenic cause of the disease.

Fluid flow modulates electrical activity in cardiac hERG potassium channels

Fluid movement within the heart generates substantial shear forces, but the effect of this mechanical stress on the electrical activity of the human heart has not been examined. The fast component of the delayed rectifier potassium currents responsible for repolarization of the cardiac action potential, Ikr, is encoded by the human ether-a-go-go related gene (hERG) channel. Here, we exposed hERG1a channel-expressing HEK293T cells to laminar shear stress (LSS) and observed that this mechanical stress increased the whole-cell current by 30-40%. LSS shifted the voltage dependence of steady-state activation of the hERG channel to the hyperpolarizing direction, accelerated the time course of activation and recovery from inactivation, slowed down deactivation, and shifted the steady-state inactivation to the positive direction, all of which favored the hERG open state. In contrast, the time course of inactivation was faster, favoring the closed state. Using specific inhibitors of focal adhesion kinase, a regulator of mechano-transduction via the integrin pathway, we also found that the LSS-induced modulation of the whole-cell current depended on the integrin pathway. The hERG1b channel variant, which lacks the Per-Arnt-Sim (PAS) domain, and long QT syndrome-associated variants having point mutations in the PAS domain were unaffected by LSS, suggesting that the PAS domain in hERG1a channel may be involved in sensing mechanical shear stress. We conclude that a mechano-electric feedback pathway modulates hERG channel activity through the integrin pathway, indicating that mechanical forces in the heart influence its electrical activity.

Distribution and function of voltage-gated sodium channels in the nervous system

Voltage-gated sodium channels (VGSCs) are the basic ion channels for neuronal excitability, which are crucial for the resting potential and the generation and propagation of action potentials in neurons. To date, at least nine distinct sodium channel isoforms have been detected in the nervous system. Recent studies have identified that voltage-gated sodium channels not only play an essential role in the normal electrophysiological activities of neurons but also have a close relationship with neurological diseases. In this study, the latest research findings regarding the structure, type, distribution, and function of VGSCs in the nervous system and their relationship to neurological diseases, such as epilepsy, neuropathic pain, brain tumors, neural trauma, and multiple sclerosis, are reviewed in detail.

Infant sudden death: Mutations responsible for impaired Nav1.5 channel trafficking and function

BACKGROUND

Two genetic variants in SCN5A, encoding the Nav1.5 Na⁺ channel α-subunit, were found in a 5-month-old girl who died suddenly in her sleep. The first variant is a missense mutation, resulting in an amino acid change (Q1832E), which has been described (but not characterized) in a patient with Brugada syndrome. The second is a nonsense mutation that produces a premature stop codon and a C-terminal truncation (R1944Δ).

METHODS AND RESULTS

To investigate their functional relevance with patch clamp experiments in transfected HEK-293 cells. The Q1832E mutation drastically reduced Nav1.5 current density. The R1944Δ C-terminal truncation had negligible effects on Nav1.5 current density. Neither of the mutations affected the voltage dependence of steady activation and inactivation or influenced the late Na⁺ current or the recovery from inactivation. Biochemical and immunofluorescent approaches demonstrated that the Q1832E mutation caused severe trafficking defects. Polymerase chain reaction cloning and sequencing the victim's genomic DNA allowed us to determine that the two variants were in trans. We investigated the functional consequences by coexpressing Nav1.5(Q1832E) and Nav1.5(R1944Δ), which led to a significantly reduced current amplitude relative to wild-type.

CONCLUSIONS

These sudden infant death syndrome (SIDS)-related variants caused a severely dysfunctional Nav1.5 channel, which was mainly due to trafficking defects caused by the Q1832E mutation. The decreased current density is likely to be a major contributing factor to arrhythmogenesis in Brugada syndrome and the sudden death of this SIDS victim.

Decreased inward rectifier potassium current IK1 in dystrophin-deficient ventricular cardiomyocytes

Kir2.x channels in ventricular cardiomyocytes (most prominently Kir2.1) account for the inward rectifier potassium current I_K1, which controls the resting membrane potential and the final phase of action potential repolarization. Recently it was hypothesized that the dystrophin-associated protein complex (DAPC) is important in the regulation of Kir2.x channels. To test this hypothesis, we investigated potential I_K1 abnormalities in dystrophin-deficient ventricular cardiomyocytes derived from the hearts of Duchenne muscular dystrophy mouse models. We found that I_K1 was substantially diminished in dystrophin-deficient cardiomyocytes when compared to wild type myocytes. This finding represents the first functional evidence for a significant role of the DAPC in the regulation of Kir2.x channels.

Mechanotransduction pulls the strings of matrix degradation at invadosome

Degradation of the extracellular matrix is a critical step of tumor cell invasion. Both protease-dependent and -independent mechanisms have been described as alternate processes in cancer cell motility. Interestingly, some effectors of protease-dependent degradation are focalized at invadosomes and are directly coupled with contractile and adhesive machineries composed of multiple mechanosensitive proteins. This review presents recent findings in protease-dependent mechanisms elucidating the ways the force affects extracellular matrix degradation by targeting protease expression and activity at invadosome. The aim is to highlight mechanosensing and mechanotransduction processes to direct the degradative activity at invadosomes, with the focus on membrane tension, proteases and mechanosensitive ion channels.

Physiological and Pathophysiological Insights of Nav1.4 and Nav1.5 Comparison

Mutations in Nav1.4 and Nav1.5 α-subunits have been associated with muscular and cardiac channelopathies, respectively. Despite intense research on the structure and function of these channels, a lot of information is still missing to delineate the various physiological and pathophysiological processes underlying their activity at the molecular level. Nav1.4 and Nav1.5 sequences are similar, suggesting structural and functional homologies between the two orthologous channels. This also suggests that any characteristics described for one channel subunit may shed light on the properties of the counterpart channel subunit. In this review article, after a brief clinical description of the muscular and cardiac channelopathies related to Nav1.4 and Nav1.5 mutations, respectively, we compare the knowledge accumulated in different aspects of the expression and function of Nav1.4 and Nav1.5 α-subunits: the regulation of the two encoding genes (SCN4A and SCN5A), the associated/regulatory proteins and at last, the functional effect of the same missense mutations detected in Nav1.4 and Nav1.5. First, it appears that more is known on Nav1.5 expression and accessory proteins. Because of the high homologies of Nav1.5 binding sites and equivalent Nav1.4 sites, Nav1.5-related results may guide future investigations on Nav1.4. Second, the analysis of the same missense mutations in Nav1.4 and Nav1.5 revealed intriguing similarities regarding their effects on membrane excitability and alteration in channel biophysics. We believe that such comparison may bring new cues to the physiopathology of cardiac and muscular diseases.

Late sodium current: A mechanism for angina, heart failure, and arrhythmia

The peak sodium current underlies excitability and conduction in heart muscle, but a late sodium current flowing after the peak contributes to maintaining and prolonging the action potential plateau, and also to intracellular sodium loading, which in turn increases intracellular calcium with consequent effects on arrhythmia and diastolic function. Late sodium current is pathologically increased in both genetic and acquired heart disease, making it an attractive target for therapy to treat arrhythmia, heart failure, and angina. This review provides an overview of the underlying bases for the clinical implications of late sodium current block.

Dynamitin affects cell-surface expression of voltage-gated sodium channel Nav1.5

The major cardiac voltage-gated sodium channel Nav1.5 associates with proteins that regulate its biosynthesis, localization, activity and degradation. Identification of partner proteins is crucial for a better understanding of the channel regulation. Using a yeast two-hybrid screen, we identified dynamitin as a Nav1.5-interacting protein. Dynamitin is part of the microtubule-binding multiprotein complex dynactin. When overexpressed it is a potent inhibitor of dynein/kinesin-mediated transport along the microtubules by disrupting the dynactin complex and dissociating cargoes from microtubules. The use of deletion constructs showed that the C-terminal domain of dynamitin is essential for binding to the first intracellular interdomain of Nav1.5. Co-immunoprecipitation assays confirmed the association between Nav1.5 and dynamitin in mouse heart extracts. Immunostaining experiments showed that dynamitin and Nav1.5 co-localize at intercalated discs of mouse cardiomyocytes. The whole-cell patch-clamp technique was applied to test the functional link between Nav1.5 and dynamitin. Dynamitin overexpression in HEK-293 (human embryonic kidney 293) cells expressing Nav1.5 resulted in a decrease in sodium current density in the membrane with no modification of the channel-gating properties. Biotinylation experiments produced similar information with a reduction in Nav1.5 at the cell surface when dynactin-dependent transport was inhibited. The present study strongly suggests that dynamitin is involved in the regulation of Nav1.5 cell-surface density.

Human macrophage SCN5A activates an innate immune signaling pathway for antiviral host defense

Pattern recognition receptors contain a binding domain for pathogen-associated molecular patterns coupled to a signaling domain that regulates transcription of host immune response genes. Here, a novel mechanism that links pathogen recognition to channel activation and downstream signaling is proposed. We demonstrate that an intracellular sodium channel variant, human macrophage SCN5A, initiates signaling and transcription through a calcium-dependent isoform of adenylate cyclase, ADCY8, and the transcription factor, ATF2. Pharmacological stimulation with a channel agonist or treatment with cytoplasmic poly(I:C), a mimic of viral dsRNA, activates this pathway to regulate expression of SP100-related genes and interferon β. Electrophysiological analysis reveals that the SCN5A variant mediates nonselective outward currents and a small, but detectable, inward current. Intracellular poly(I:C) markedly augments an inward voltage-sensitive sodium current and inhibits the outward nonselective current. These results suggest human macrophage SCN5A initiates signaling in an innate immune pathway relevant to antiviral host defense. It is postulated that SCN5A is a novel pathogen sensor and that this pathway represents a channel activation-dependent mechanism of transcriptional regulation.

The PDZ motif of the α1C subunit is not required for surface trafficking and adrenergic modulation of CaV1.2 channel in the heart

Voltage-gated Ca(2+) channels play a key role in initiating muscle excitation-contraction coupling, neurotransmitter release, gene expression, and hormone secretion. The association of CaV1.2 with a supramolecular complex impacts trafficking, localization, turnover, and, most importantly, multifaceted regulation of its function in the heart. Several studies hint at an important role for the C terminus of the α1C subunit as a hub for multidimensional regulation of CaV1.2 channel trafficking and function. Recent studies have demonstrated an important role for the four-residue PDZ binding motif at the C terminus of α1C in interacting with scaffold proteins containing PDZ domains, in the subcellular localization of CaV1.2 in neurons, and in the efficient signaling to cAMP-response element-binding protein in neurons. However, the role of the α1C PDZ ligand domain in the heart is not known. To determine whether the α1C PDZ motif is critical for CaV1.2 trafficking and function in cardiomyocytes, we generated transgenic mice with inducible expression of an N-terminal FLAG epitope-tagged dihydropyridine-resistant α1C with the PDZ motif deleted (ΔPDZ). These mice were crossed with α-myosin heavy chain reverse transcriptional transactivator transgenic mice, and the double-transgenic mice were fed doxycycline. The ΔPDZ channels expressed, trafficked to the membrane, and supported robust excitation-contraction coupling in the presence of nisoldipine, a dihydropyridine Ca(2+) channel blocker, providing functional evidence that they appropriately target to dyads. The ΔPDZ Ca(2+) channels were appropriately regulated by isoproterenol and forskolin. These data indicate that the α1C PDZ motif is not required for surface trafficking, localization to the dyad, or adrenergic stimulation of CaV1.2 in adult cardiomyocytes.

Diseases caused by mutations in Nav1.5 interacting proteins

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

Gastrointestinal motility and its enteric actors in mechanosensitivity: past and present

Coordinated contractions of the smooth muscle layers of the gastrointestinal (GI) tract are required to produce motor patterns that ensure normal GI motility. The crucial role of the enteric nervous system (ENS), the intrinsic ganglionated network located within the GI wall, has long been recognized in the generation of the main motor patterns. However, devising an appropriate motility requires the integration of informations emanating from the lumen of the GI tract. As already found more than half a century ago, the ability of the GI tract to respond to mechanical forces such as stretch is not restricted to neuronal mechanisms. Instead, mechanosensitivity is now recognized as a property of several non-neuronal cell types, the excitability of which is probably involved in shaping the motor patterns. This brief review gives an overview on how mechanosensitivity of different cell types in the GI tract has been established and, whenever available, on what ionic conductances are involved in mechanotransduction and their potential impact on normal GI motility.

Channelopathies from mutations in the cardiac sodium channel protein complex

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

Syntrophin proteins as Santa Claus: role(s) in cell signal transduction

Syntrophins are a family of cytoplasmic membrane-associated adaptor proteins, characterized by the presence of a unique domain organization comprised of a C-terminal syntrophin unique (SU) domain and an N-terminal pleckstrin homology (PH) domain that is split by insertion of a PDZ domain. Syntrophins have been recognized as an important component of many signaling events, and they seem to function more like the cell's own personal 'Santa Claus' that serves to 'gift' various signaling complexes with precise proteins that they 'wish for', and at the same time care enough for the spatial, temporal control of these signaling events, maintaining overall smooth functioning and general happiness of the cell. Syntrophins not only associate various ion channels and signaling proteins to the dystrophin-associated protein complex (DAPC), via a direct interaction with dystrophin protein but also serve as a link between the extracellular matrix and the intracellular downstream targets and cell cytoskeleton by interacting with F-actin. They play an important role in regulating the postsynaptic signal transduction, sarcolemmal localization of nNOS, EphA4 signaling at the neuromuscular junction, and G-protein mediated signaling. In our previous work, we reported a differential expression pattern of alpha-1-syntrophin (SNTA1) protein in esophageal and breast carcinomas. Implicated in several other pathologies, like cardiac dys-functioning, muscular dystrophies, diabetes, etc., these proteins provide a lot of scope for further studies. The present review focuses on the role of syntrophins in membrane targeting and regulation of cellular proteins, while highlighting their relevance in possible development and/or progression of pathologies including cancer which we have recently demonstrated.

Cardiac sodium channel NaV1.5 distribution in myocytes via interacting proteins: the multiple pool model

The cardiac sodium current (INa) is responsible for the rapid depolarization of cardiac cells, thus allowing for their contraction. It is also involved in regulating the duration of the cardiac action potential (AP) and propagation of the impulse throughout the myocardium. Cardiac INa is generated by the voltage-gated Na(+) channel, NaV1.5, a 2016-residue protein which forms the pore of the channel. Over the past years, hundreds of mutations in SCN5A, the human gene coding for NaV1.5, have been linked to many cardiac electrical disorders, including the congenital and acquired long QT syndrome, Brugada syndrome, conduction slowing, sick sinus syndrome, atrial fibrillation, and dilated cardiomyopathy. Similar to many membrane proteins, NaV1.5 has been found to be regulated by several interacting proteins. In some cases, these different proteins, which reside in distinct membrane compartments (i.e. lateral membrane vs. intercalated disks), have been shown to interact with the same regulatory domain of NaV1.5, thus suggesting that several pools of NaV1.5 channels may co-exist in cardiac cells. The aim of this review article is to summarize the recent works that demonstrate its interaction with regulatory proteins and illustrate the model that the sodium channel NaV1.5 resides in distinct and different pools in cardiac cells. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Ranolazine inhibits shear sensitivity of endogenous Na+ current and spontaneous action potentials in HL-1 cells

Na(V)1.5 is a mechanosensitive voltage-gated Na(+) channel encoded by the gene SCN5A, expressed in cardiac myocytes and required for phase 0 of the cardiac action potential (AP). In the cardiomyocyte, ranolazine inhibits depolarizing Na(+) current and delayed rectifier (I(Kr)) currents. Recently, ranolazine was also shown to be an inhibitor of Na(V)1.5 mechanosensitivity. Stretch also accelerates the firing frequency of the SA node, and fluid shear stress increases the beating rate of cultured cardiomyocytes in vitro. However, no cultured cell platform exists currently for examination of spontaneous electrical activity in response to mechanical stimulation. In the present study, flow of solution over atrial myocyte-derived HL-1 cultured cells was used to study shear stress mechanosensitivity of Na(+) current and spontaneous, endogenous rhythmic action potentials. In voltage-clamped HL-1 cells, bath flow increased peak Na(+) current by 14 ± 5%. In current-clamped cells, bath flow increased the frequency and decay rate of AP by 27 ± 12% and 18 ± 4%, respectively. Ranolazine blocked both responses to shear stress. This study suggests that cultured HL-1 cells are a viable in vitro model for detailed study of the effects of mechanical stimulation on spontaneous cardiac action potentials. Inhibition of the frequency and decay rate of action potentials in HL-1 cells are potential mechanisms behind the antiarrhythmic effect of ranolazine.

Membrane permeable local anesthetics modulate Na(V)1.5 mechanosensitivity

Voltage-gated sodium selective ion channel Na_V1.5 is expressed in the heart and the gastrointestinal tract, which are mechanically active organs. Na_V1.5 is mechanosensitive at stimuli that gate other mechanosensitive ion channels. Local anesthetic and antiarrhythmic drugs act upon Na_V1.5 to modulate activity by multiple mechanisms. This study examined whether Na_V1.5 mechanosensitivity is modulated by local anesthetics. Na_V1.5 channels wereexpressed in HEK-293 cells, and mechanosensitivity was tested in cell-attached and excised inside-out configurations. Using a novel protocol with paired voltage ladders and short pressure pulses, negative patch pressure (-30 mmHg) in both configurations produced a hyperpolarizing shift in the half-point of the voltage-dependence of activation (V_1/2a) and inactivation (V_1/2i) by about -10 mV. Lidocaine (50 µM) inhibited the pressure-induced shift of V_1/2a but not V_1/2i. Lidocaine inhibited the tonic increase in pressure-induced peak current in a use-dependence protocol, but it did not otherwise affect use-dependent block. The local anesthetic benzocaine, which does not show use-dependent block, also effectively blocked a pressure-induced shift in V_1/2a. Lidocaine inhibited mechanosensitivity in Na_V1.5 at the local anesthetic binding site mutated (F1760A). However, a membrane impermeable lidocaine analog QX-314 did not affect mechanosensitivity of F1760A Na_V1.5 when applied from either side of the membrane. These data suggest that the mechanism of lidocaine inhibition of the pressure-induced shift in the half-point of voltage-dependence of activation is separate from the mechanisms of use-dependent block. Modulation of Na_V1.5 mechanosensitivity by the membrane permeable local anesthetics may require hydrophobic access and may involve membrane-protein interactions.

Transcriptome profiling of the demosponge Amphimedon queenslandica reveals genome-wide events that accompany major life cycle transitions

Background

The biphasic life cycle with pelagic larva and benthic adult stages is widely observed in the animal kingdom, including the Porifera (sponges), which are the earliest branching metazoans. The demosponge, Amphimedon queenslandica, undergoes metamorphosis from a free-swimming larva into a sessile adult that bears no morphological resemblance to other animals. While the genome of A. queenslandica contains an extensive repertoire of genes very similar to that of complex bilaterians, it is as yet unclear how this is drawn upon to coordinate changing morphological features and ecological demands throughout the sponge life cycle.

Results

To identify genome-wide events that accompany the pelagobenthic transition in A. queenslandica, we compared global gene expression profiles at four key developmental stages by sequencing the poly(A) transcriptome using SOLiD technology. Large-scale changes in transcription were observed as sponge larvae settled on the benthos and began metamorphosis. Although previous systematics suggest that the only clear homology between Porifera and other animals is in the embryonic and larval stages, we observed extensive use of genes involved in metazoan-associated cellular processes throughout the sponge life cycle. Sponge-specific transcripts are not over-represented in the morphologically distinct adult; rather, many genes that encode typical metazoan features, such as cell adhesion and immunity, are upregulated. Our analysis further revealed gene families with candidate roles in competence, settlement, and metamorphosis in the sponge, including transcription factors, G-protein coupled receptors and other signaling molecules.

Conclusions

This first genome-wide study of the developmental transcriptome in an early branching metazoan highlights major transcriptional events that accompany the pelagobenthic transition and point to a network of regulatory mechanisms that coordinate changes in morphology with shifting environmental demands. Metazoan developmental and structural gene orthologs are well-integrated into the expression profiles at every stage of sponge development, including the adult. The utilization of genes involved in metazoan-associated processes throughout sponge development emphasizes the potential of the genome of the last common ancestor of animals to generate phenotypic complexity.

Ranolazine decreases mechanosensitivity of the voltage-gated sodium ion channel Na(v)1.5: a novel mechanism of drug action

BACKGROUND

Na(V)1.5 is a mechanosensitive voltage-gated sodium-selective ion channel responsible for the depolarizing current and maintenance of the action potential plateau in the heart. Ranolazine is a Na(V)1.5 antagonist with antianginal and antiarrhythmic properties.

METHODS AND RESULTS

Mechanosensitivity of Na(V)1.5 was tested in voltage-clamped whole cells and cell-attached patches by bath flow and patch pressure, respectively. In whole cells, bath flow increased peak inward current in both murine ventricular cardiac myocytes (24±8%) and human embryonic kidney 293 cells heterologously expressing Na(V)1.5 (18±3%). The flow-induced increases in peak current were blocked by ranolazine. In cell-attached patches from cardiac myocytes and Na(V)1.5-expressing human embryonic kidney 293 cells, negative pressure increased Na(V) peak currents by 27±18% and 18±4% and hyperpolarized voltage dependence of activation by -11 mV and -10 mV, respectively. In human embryonic kidney 293 cells, negative pressure also increased the window current (250%) and increased late open channel events (250%). Ranolazine decreased pressure-induced shift in the voltage dependence (IC(50) 54 μmol/L) and eliminated the pressure-induced increases in window current and late current event numbers. Block of Na(V)1.5 mechanosensitivity by ranolazine was not due to the known binding site on DIVS6 (F1760). The effect of ranolazine on mechanosensitivity of Na(V)1.5 was approximated by lidocaine. However, ionized ranolazine and charged lidocaine analog (QX-314) failed to block mechanosensitivity.

CONCLUSIONS

Ranolazine effectively inhibits mechanosensitivity of Na(V)1.5. The block of Na(V)1.5 mechanosensitivity by ranolazine does not utilize the established binding site and may require bilayer partitioning. Ranolazine block of Na(V)1.5 mechanosensitivity may be relevant in disorders of mechanoelectric dysfunction.

The Relationship Between Gastric Myoelectric Activity and SCN5A Mutation Suggesting Sodium Channelopathy in Patients With Brugada Syndrome and Functional Dyspepsia - A Pilot Study

BACKGROUND/AIMS

SCN5A encodes the cardiac-specific Na(V)1.5 sodium channel, and Brugada syndrome is a cardiac conduction disorder associated with sodium channel α-subunit (SCN5A) mutation. The SCN5A-encoded Na(V)1.5 channel is also found on gastrointestinal smooth muscle and interstitial cells of Cajal. We investigated the relationship between functional dyspepsia (FD) and SCN5A mutation to evaluate sodium channelopathy in FD.

METHODS

Patients with Brugada syndrome or FD were examined using upper endoscopy, electrogastrography (EGG), FD symptom questionnaire based on Rome III criteria and genetic testing for SCN5A mutation. Symptom scores of FD and EGG findings were analyzed according to SCN5A mutation.

RESULTS

A total of 17 patients (4 Brugada syndrome and 13 FD) participated in the study. An SCN5A mutation was noted in 75.0% of the patients with Brugada syndrome and in 1 (7.7%) of the patients with FD. Of 4 patients with SCN5A mutation, 2 (50%) had FD. Postprandial tachygastria and bradygastria were noted in 2 (50%) and 1 (25%) of the patients with SCN5A mutation, respectively. The EGG findings were not significantly different between positive and negative mutation in 17 patients.

CONCLUSIONS

Although we did not find statistically significant results, we suggest that it is meaningful to attempt to identify differences in symptoms and gastric myoelectric activity according to the presence of an SCN5A mutation by EGG analysis. The relationship between FD and sodium channelopathy should be elucidated in the future by a large-scale study.

Targeting ion channels for the treatment of gastrointestinal motility disorders

Gastrointestinal (GI) functional and motility disorders are highly prevalent and responsible for long-term morbidity and sometimes mortality in the affected patients. It is estimated that one in three persons has a GI functional or motility disorder. However, diagnosis and treatment of these widespread conditions remains challenging. This partly stems from the multisystem pathophysiology, including processing abnormalities in the central and peripheral (enteric) nervous systems and motor dysfunction in the GI wall. Interstitial cells of Cajal (ICCs) are central to the generation and propagation of the cyclical electrical activity and smooth muscle cells (SMCs) are responsible for electromechanical coupling. In these and other excitable cells voltage-sensitive ion channels (VSICs) are the main molecular units that generate and regulate electrical activity. Thus, VSICs are potential targets for intervention in GI motility disorders. Research in this area has flourished with advances in the experimental methods in molecular and structural biology and electrophysiology. However, our understanding of the molecular mechanisms responsible for the complex and variable electrical behavior of ICCs and SMCs remains incomplete. In this review, we focus on the slow waves and action potentials in ICCs and SMCs. We describe the constituent VSICs, which include voltage-gated sodium (Na(V)), calcium (Ca(V)), potassium (K(V), K(Ca)), chloride (Cl(-)) and nonselective ion channels (transient receptor potentials [TRPs]). VSICs have significant structural homology and common functional mechanisms. We outline the approaches and limitations and provide examples of targeting VSICs at the pores, voltage sensors and alternatively spliced sites. Rational drug design can come from an integrated view of the structure and mechanisms of gating and activation by voltage or mechanical stress.

Quantification of gastrointestinal sodium channelopathy

Na(v)1.5 sodium channels, encoded by SCN5A, have been identified in human gastrointestinal interstitial cells of Cajal (ICC) and smooth muscle cells (SMC). A recent study found a novel, rare missense R76C mutation of the sodium channel interacting protein telethonin in a patient with primary intestinal pseudo-obstruction. The presence of a mutation in a patient with a motility disorder, however, does not automatically imply a cause-effect relationship between the two. Patch clamp experiments on HEK-293 cells previously established that the R76C mutation altered Na(v)1.5 channel function. Here the process through which these data were quantified to create stationary Markov state models of wild-type and R76C channel function is described. The resulting channel descriptions were included in whole cell ICC and SMC computational models and simulations were performed to assess the cellular effects of the R76C mutation. The simulated ICC slow wave was decreased in duration and the resting membrane potential in the SMC was depolarized. Thus, the R76C mutation was sufficient to alter ICC and SMC cell electrophysiology. However, the cause-effect relationship between R76C and intestinal pseudo-obstruction remains an open question.

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.

Hydrogen sulfide is a partially redox-independent activator of the human jejunum Na+ channel, Nav1.5

Hydrogen sulfide (H(2)S) is produced endogenously by L-cysteine metabolism. H(2)S modulates several ion channels with an unclear mechanism of action. A possible mechanism is through reduction-oxidation reactions attributable to the redox potential of the sulfur moiety. The aims of this study were to determine the effects of the H(2)S donor NaHS on Na(V)1.5, a voltage-dependent sodium channel expressed in the gastrointestinal tract in human jejunum smooth muscle cells and interstitial cells of Cajal, and to elucidate whether H(2)S acts on Na(V)1.5 by redox reactions. Whole cell Na(+) currents were recorded in freshly dissociated human jejunum circular myocytes and Na(V)1.5-transfected human embryonic kidney-293 cells. RT-PCR amplified mRNA for H(2)S enzymes cystathionine β-synthase and cystathionine γ-lyase from the human jejunum. NaHS increased native Na(+) peak currents and shifted the half-point (V(1/2)) of steady-state activation and inactivation by +21 ± 2 mV and +15 ± 3 mV, respectively. Similar effects were seen on the heterologously expressed Na(V)1.5 α subunit with EC(50)s in the 10(-4) to 10(-3) M range. The reducing agent dithiothreitol (DTT) mimicked in part the effects of NaHS by increasing peak current and positively shifting steady-state activation. DTT together with NaHS had an additive effect on steady-state activation but not on peak current, suggesting that the latter may be altered via reduction. Pretreatment with the Hg(2+)-conjugated oxidizer thimerosal or the alkylating agent N-ethylmaleimide inhibited or decreased NaHS induction of Na(V)1.5 peak current. These studies show that H(2)S activates the gastrointestinal Na(+) channel, and the mechanism of action of H(2)S is partially redox independent.

Voltage-gated ion channel dysfunction precedes cardiomyopathy development in the dystrophic heart

Background

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is associated with severe cardiac complications including cardiomyopathy and cardiac arrhythmias. Recent research suggests that impaired voltage-gated ion channels in dystrophic cardiomyocytes accompany cardiac pathology. It is, however, unknown if the ion channel defects are primary effects of dystrophic gene mutations, or secondary effects of the developing cardiac pathology.

Methodology/Principal Findings

To address this question, we first investigated sodium channel impairments in cardiomyocytes derived from dystrophic neonatal mice prior to cardiomyopahty development, by using the whole cell patch clamp technique. Besides the most common model for DMD, the dystrophin-deficient mdx mouse, we also used mice additionally carrying an utrophin mutation. In neonatal cardiomyocytes, dystrophin-deficiency generated a 25% reduction in sodium current density. In addition, extra utrophin-deficiency significantly altered sodium channel gating parameters. Moreover, also calcium channel inactivation was considerably reduced in dystrophic neonatal cardiomyocytes, suggesting that ion channel abnormalities are universal primary effects of dystrophic gene mutations. To assess developmental changes, we also studied sodium channel impairments in cardiomyocytes derived from dystrophic adult mice, and compared them with the respective abnormalities in dystrophic neonatal cells. Here, we found a much stronger sodium current reduction in adult cardiomyocytes. The described sodium channel impairments slowed the upstroke of the action potential in adult cardiomyocytes, and only in dystrophic adult mice, the QRS interval of the electrocardiogram was prolonged.

Conclusions/Significance

Ion channel impairments precede pathology development in the dystrophic heart, and may thus be considered potential cardiomyopathy triggers.

Progressive Conduction Diseases

Progressive cardiac conduction disease (PCCD), a source of considerable morbidity, comprises a heterogeneous group of conditions resulting from genetic predisposition, environmental modifiers, and other physiologic determinants, including aging. The genetic factors include numerous mutations and variants within the cardiac sodium channel gene, SCN5A. The electrocardiographic phenotype has variable penetrance and is associated with appearances ranging from an isolated conduction disorder to an association with tachyarrhythmias and clinically significant cardiomyopathy. A heterozygotic Scn5a mouse model provides evidence that PCCD may lead to cardiac remodeling consistent with clinical observations in addition to slowing of intracardiac conduction. PCCD has also been associated with the altered expression of genes encoding other proteins involved in impulse propagation, including those responsible for Ca^2+- activated ion channels and cytoskeletal components, both in the presence or absence of structural abnormalities.

Mechanosensitivity of Nav1.5, a voltage-sensitive sodium channel

The voltage-sensitive sodium channel Na(v)1.5 (encoded by SCN5A) is expressed in electromechanical organs and is mechanosensitive. This study aimed to determine the mechanosensitive transitions of Na(v)1.5 at the molecular level. Na(v)1.5 was expressed in HEK 293 cells and mechanosensitivity was studied in cell-attached patches. Patch pressure up to -50 mmHg produced increases in current and large hyperpolarizing shifts of voltage dependence with graded shifts of half-activation and half-inactivation voltages (V(1/2)) by ∼0.7 mV mmHg(-1). Voltage dependence shifts affected channel kinetics by a single constant. This suggested that stretch accelerated only one of the activation transitions. Stretch accelerated voltage sensor movement, but not rate constants for gate opening and fast inactivation. Stretch also appeared to stabilize the inactivated states, since recovery from inactivation was slowed with stretch. Unitary conductance and maximum open probability were unaffected by stretch, but peak current was increased due to an increased number of active channels. Stretch effects were partially reversible, but recovery following a single stretch cycle required minutes. These data suggest that mechanical activation of Na(v)1.5 results in dose-dependent voltage dependence shifts of activation and inactivation due to mechanical modulation of the voltage sensors.

"CRASH"ing with the worm: insights into L1CAM functions and mechanisms

The L1 family of cell adhesion molecules (L1CAMs) in vertebrates has long been studied for its roles in nervous system development and function. Members of this family have been associated with distinct neurological disorders that include CRASH, autism, 3p syndrome, and schizophrenia. The conservation of L1CAMs in Drosophila and Caenorhabditis elegans allows the opportunity to take advantage of these simple model organisms and their accessible genetic manipulations to dissect L1CAM functions and mechanisms of action. This review summarizes the discoveries of L1CAMs made in C. elegans, showcasing this simple model organism as a powerful system to uncover L1CAM mechanisms and roles in healthy and diseased states.

The genetics of conduction disease

Conduction diseases (CD) include defects in impulse generation and conduction. Patients with CD may manifest a wide range of clinical presentations, from asymptomatic to potentially life-threatening arrhythmias. The pathophysiologic mechanisms underlying CD are diverse and may have implications for diagnosis, treatment, and prognosis. Known causes of functional CD include cardiac ion channelopathies or defects in modifying proteins, such as cytoskeletal proteins. Progress in molecular biology and genetics along with development of animal models has increased the understanding of the molecular mechanisms of these disorders. This article discusses the genetic basis for CD and its clinical implications.

A novel mechanism for the treatment of angina, arrhythmias, and diastolic dysfunction: inhibition of late I(Na) using ranolazine

Inhibition of the persistent or late Na current (INa) using ranolazine (Ranexa) represents a novel mechanism of action that was approved in the United States in 2006 and only recently in the European Union for use in patients with stable angina pectoris. In general, myocardial ischemia is associated with reduced adenosine triphosphate fluxes and decreased energy supply, resulting in severe disturbances of intracellular ion homeostasis in cardiac myocytes. In the recent years, increased late INa was suggested to contribute to this phenomenon by elevating intracellular Na concentration with subsequent rise in diastolic Ca levels by means of the sarcolemmal Na-Ca exchange system. Ranolazine, a specific inhibitor of late INa, reduces Na influx and hence ameliorates disturbed Na and Ca homeostasis. This is associated with a symptomatic improvement of angina in patients unlike other antianginal drugs without affecting heart rate or systemic blood pressure as shown in placebo-controlled studies. Therefore, ranolazine is a useful new option for patients with chronic stable angina not only as an add-on therapy. New clinical and experimental studies even point to potential antiarrhythmic effects, beneficial effects in diastolic heart failure, and under hyperglycemic conditions. In the present article, the relevant pathophysiological concepts for the role of late INa inhibition are reviewed and the most recent data from basic studies and clinical trials are summarized.

Regulation by scaffolding proteins of canonical transient receptor potential channels in striated muscle

Recent studies proposed a pivotal role of TRPC channels, in particular TRPC1, in the striated muscle tissue and in the development of calcium mishandling observed in dystrophin-deficient skeletal and cardiac muscle cells (Vandebrouck et al. in J Cell Biol 158:1089-1096, 2002; Williams and Allen in Am J Physiol Heart Circ Physiol 292:H846-H855, 2007; Stiber et al. in Mol Cell Biol 28:2637-2647, 2008). In skeletal muscle, TRPCs are proposed to function in a costameric macromolecular complex (Vandebrouck et al. in FASEB J 21:608-617, 2007; Gervasio et al. in J Cell Sci 121:2246-2255, 2008) in which scaffolding proteins and dystrophin are central components maintaining normal calcium entry (Stiber et al. in Mol Cell Biol 28:2637-2647, 2008; Sabourin et al. in J Biol Chem 284:36248-61, 2009). In this review, we shall summarize the roles played by scaffolding proteins in regulating the calcium entry through TRPC channels of skeletal muscle cells and the implications in muscle physiopathology. Interactions of TRPC1 with caveolin-3, Homer-1 and alpha-syntrophin will be addressed and these complexes will be compared with signalplex in other systems. The mechanosensitive function of scaffolding proteins will be discussed as well as interactions with TRPV2 channels regarding to calcium mishandling in Duchenne dystrophy.

Laser microdissection as a new tool to investigate site-specific gene expression in enteric ganglia of the human intestine

BACKGROUND

Myenteric ganglia are key-structures for the control of intestinal motility and their mRNA expression profiles might be altered under pathological conditions. A drawback of conventional RT-PCR from full-thickness specimens is that gene expression analysis is based on heterogeneously composed tissues. To overcome this problem, laser microdissection combined with real-time RT-PCR can be used to detect and quantify low levels of gene expression in isolated enteric ganglia.

METHODS

Fresh unfixed full-thickness specimens of sigmoid colon were obtained from patients (n = 8) with diseases unrelated to intestinal motility disorders. 10 microm cryo-sections were mounted on membrane-coated slides and ultra-rapidly stained with toluidine blue. Myenteric ganglia were isolated by laser microdissection and catapulting for mRNA isolation. Real-time RT-PCR was performed for selected growth factors, neurotransmitter receptors and specific cell type markers.

KEY RESULTS

Collection of 0.5 mm(2) of ganglionic tissue was sufficient to obtain positive RT-PCR results. Collection of 4 mm(2) resulted in ct-values allowing a reliable quantitative comparison of gene expression levels. mRNA analysis revealed that neurotrophic growth factor, neurotrophin-3, serotonin receptor 3A, PGP 9.5 and S100 beta are specifically expressed in myenteric ganglia of the human colon.

CONCLUSIONS & INFERENCES

Laser microdissection combined with real-time RT-PCR is a novel technique to reliably detect and quantify site-specific expression of low-abundance mRNAs (e.g. growth factors, neurotransmitter receptors) related to the human enteric nervous system. This technical approach expands the spectrum of available tools to characterize enteric neuropathologies underlying human gastrointestinal motility disorders at the molecular biological level.

Cytoskeletal basis of ion channel function in cardiac muscle

The heart is a force-generating organ that responds to self-generated electrical stimuli from specialized cardiomyocytes. This function is modulated by sympathetic and parasympathetic activity. In order to contract and accommodate the repetitive morphological changes induced by the cardiac cycle, cardiomyocytes depend on their highly evolved and specialized cytoskeletal apparatus. Defects in components of the cytoskeleton affect the ability of the cell to compensate at both functional and structural levels in the long term. In addition to structural remodeling, the myocardium becomes increasingly susceptible to altered electrical activity, leading to arrhythmogenesis. The development of arrhythmias secondary to structural remodeling defects has been noted, although the detailed molecular mechanisms are still elusive. Here, the author reviews the current knowledge of the molecular and functional relationships between the cytoskeleton and ion channels, and discusses the future impact of new data on molecular cardiology research and clinical practice.

alpha-1-syntrophin mutation and the long-QT syndrome: a disease of sodium channel disruption

BACKGROUND

Long-QT syndrome (LQTS) is an inherited disorder associated with sudden cardiac death. The cytoskeletal protein syntrophin-alpha(1) (SNTA1) is known to interact with the cardiac sodium channel (hNa(v)1.5), and we hypothesized that SNTA1 mutations might cause phenotypic LQTS in patients with genotypically normal hNa(v)1.5 by secondarily disturbing sodium channel function.

METHODS AND RESULTS

Mutational analysis of SNTA1 was performed on 39 LQTS patients (QTc> or =480 ms) with previously negative genetic screening for the known LQTS-causing genes. We identified a novel A257G-SNTA1 missense mutation, which affects a highly conserved residue, in 3 unrelated LQTS probands but not in 400 ethnic-matched control alleles. Only 1 of these probands had a preexisting family history of LQTS and sudden death with an additional intronic variant in KCNQ1. Electrophysiological analysis was performed using HEK-293 cells stably expressing hNa(v)1.5 and transiently transfected with either wild-type or mutant SNTA1 and, in neonatal rat cardiomyocytes, transiently transfected with either wild-type or mutant SNTA1. In both HEK-293 cells and neonatal rat cardiomyocytes, increased peak sodium currents were noted along with a 10-mV negative shift of the onset and peak of currents of the current-voltage relationships. In addition, A257G-SNTA1 shifted the steady-state activation (V(h)) leftward by 9.4 mV, whereas the voltage-dependent inactivation kinetics and the late sodium currents were similar to wild-type SNTA1.

CONCLUSION

SNTA1 is a new susceptibility gene for LQTS. A257G-SNTA1 can cause gain-of-function of Na(v)1.5 similar to the LQT3.

Regulation of TRPC1 and TRPC4 cation channels requires an alpha1-syntrophin-dependent complex in skeletal mouse myotubes

The dystrophin-associated protein complex (DAPC) is essential for skeletal muscle, and the lack of dystrophin in Duchenne muscular dystrophy results in a reduction of DAPC components such as syntrophins and in fiber necrosis. By anchoring various molecules, the syntrophins may confer a role in cell signaling to the DAPC. Calcium disorders and abnormally elevated cation influx in dystrophic muscle cells have suggested that the DAPC regulates some sarcolemmal cationic channels. We demonstrated previously that mini-dystrophin and alpha1-syntrophin restore normal cation entry in dystrophin-deficient myotubes and that sarcolemmal TRPC1 channels associate with dystrophin and the bound PDZ domain of alpha1-syntrophin. This study shows that small interfering RNA (siRNA) silencing of alpha1-syntrophin dysregulated cation influx in myotubes. Moreover, deletion of the PDZ-containing domain prevented restoration of normal cation entry by alpha1-syntrophin transfection in dystrophin-deficient myotubes. TRPC1 and TRPC4 channels are expressed at the sarcolemma of muscle cells; forced expression or siRNA silencing showed that cation influx regulated by alpha1-syntrophin is supported by TRPC1 and TRPC4. A molecular association was found between TRPC1 and TRPC4 channels and the alpha1-syntrophin-dystrophin complex. TRPC1 and TRPC4 channels may form sarcolemmal channels anchored to the DAPC, and alpha1-syntrophin is necessary to maintain the normal regulation of TRPC-supported cation entry in skeletal muscle. Cation channels with DAPC form a signaling complex that modulates cation entry and may be crucial for normal calcium homeostasis in skeletal muscles.

Sodium channel mutation in irritable bowel syndrome: evidence for an ion channelopathy

The SCN5A-encoded Na(v)1.5 Na(+) channel is expressed in interstitial cells of Cajal and smooth muscle in the circular layer of the human intestine. Patients with mutations in SCN5A are more likely to report gastrointestinal symptoms, especially abdominal pain. Twin and family studies of irritable bowel syndrome (IBS) suggest a genetic basis for IBS, but no genes have been identified to date. Therefore, our aims were to evaluate SCN5A as a candidate gene involved in the pathogenesis of IBS and to determine physiological consequences of identified mutations. Mutational analysis was performed on genomic DNA obtained from 49 subjects diagnosed with IBS who reported at least moderately severe abdominal pain. One patient hosted a loss-of-function missense mutation, G298S, that was not observed in >3,000 reference alleles derived from 1,500 healthy control subjects. Na(+) currents were recorded from the four common human SCN5A transcripts in transfected HEK-293 cells. Comparing Na(v)1.5 with G298S-SCN5A versus wild type in HEK cells, Na(+) current density was significantly less by 49-77%, and channel activation time was delayed in backgrounds that also contained the common H558R polymorphism. Single-channel measurements showed no change in Na(v)1.5 conductance. Mechanosensitivity was reduced in the H558/Q1077del transcript but not in the other three backgrounds. In conclusion, the G298S-SCN5A missense mutation caused a marked reduction of whole cell Na(+) current and loss of function of Na(v)1.5, suggesting SCN5A as a candidate gene in the pathophysiology of IBS.

Differential protein expression in heart in UT-B null mice with cardiac conduction defects

Cardiac conduction defects were found in transgenic mice deficient in urea transporter UT-B. To investigate the molecular mechanisms of the conduction defects caused by UT-B deletion, we studied the protein expression profiles of heart tissue (comprising most conduction system) in wild-type versus UT-B null mice at different ages. By two-dimensional electrophoresis-based comparative analysis, we found that more than dozen proteins were modulated (>two-fold) in the myocardium of UT-B null mice. Out of these modulated proteins, troponin T (TNNT2) presented significant changes in UT-B null mice at early stage prior to the development of P-R interval elongation, while the change of atrial natriuretic peptide (ANP) occurred only at late stage in UT-B null mice that had the AV block. These data indicate that UT-B deletion caused the dynamic expression regulation of TNNT2 and ANP, and these proteins may provide new clues to investigate the molecular events involved in cardiac conduction.

unc-44 Ankyrin and stn-2 gamma-syntrophin regulate sax-7 L1CAM function in maintaining neuronal positioning in Caenorhabditis elegans

The L1 family of single-pass transmembrane cell adhesion molecules (L1CAMs) is conserved from Caenorhabditis elegans and Drosophila to vertebrates and is required for axon guidance, neurite outgrowth, and maintenance of neuronal positions. The extracellular region of L1CAMs mediates cell adhesion via interactions with diverse cell-surface and extracellular matrix proteins. In contrast, less is known regarding the function of the intracellular domains in the L1CAM cytoplasmic tail. Previously, we identified a role of the C. elegans L1CAM homolog, SAX-7, in maintaining neuronal and axonal positioning. Here, we demonstrate that this function is dependent on three conserved motifs that reside in the SAX-7 cytoplasmic tail: (1) the FERM-binding motif, (2) the ankyrin-binding domain, and (3) the PDZ-binding motif. Furthermore, we provide molecular and genetic evidence that UNC-44 ankyrin and STN-2 gamma-syntrophin bind SAX-7 via the respective ankyrin-binding and PDZ-binding motifs to regulate SAX-7 function in maintaining neuronal positioning.

Alternative splicing regulates activation-induced cytidine deaminase (AID): implications for suppression of AID mutagenic activity in normal and malignant B cells

The mutagenic enzyme activation-induced cytidine deaminase (AID) is required for immunoglobulin class switch recombination (CSR) and somatic hypermutation (SHM) in germinal center (GC) B cells. Deregulated expression of AID is associated with various B-cell malignancies and, currently, it remains unclear how AID activity is extinguished to avoid illegitimate mutations. AID has also been shown to be alternatively spliced in malignant B cells, and there is limited evidence that this also occurs in normal blood B cells. The functional significance of these splice variants remains unknown. Here we show that normal GC human B cells and blood memory B cells similarly express AID splice variants and show for the first time that AID splicing variants are singly expressed in individual normal B cells as well as malignant B cells from chronic lymphocytic leukemia patients. We further demonstrate that the alternative AID splice variants display different activities ranging from inactivation of CSR to inactivation or heightened SHM activity. Our data therefore suggest that CSR and SHM are differentially switched off by varying the expression of splicing products of AID at the individual cell level. Most importantly, our findings suggest a novel tumor suppression mechanism by which unnecessary AID mutagenic activities are promptly contained for GC B cells.

Biochemical constitution of extracellular medium is critical for control of human breast cancer MDA-MB-231 cell motility

Although voltage-gated sodium channel (VGSC) activity, upregulated significantly in strongly metastatic human breast cancer cells, has been found to potentiate a variety of in vitro metastatic cell behaviors, the mechanism(s) regulating channel expression/activity is not clear. As a step toward identifying possible serum factors that might be responsible for this, we tested whether medium in which fetal bovine serum (FBS) was substituted with a commercial serum replacement agent (SR-2), comprising insulin and bovine serum albumin, would influence the VGSC-dependent in vitro metastatic cell behaviors. Human breast cancer MDA-MB-231 cells were used as a model. Measurements of lateral motility, transverse migration and adhesion showed consistently that the channel's involvement in metastatic cell behaviors depended on the extracellular biochemical conditions. In normal medium (5% FBS), tetrodotoxin (TTX), a highly specific blocker of VGSCs, suppressed these cellular behaviors, as reported before. In contrast, in SR-2 medium, TTX had opposite effects. However, blocking endogenous insulin/insulin-like growth factor receptor signaling with AG1024 eliminated or reversed the anomalous effects of TTX. Insulin added to serum-free medium increased migration, and TTX increased it further. In conclusion, (1) the biochemical constitution of the extracellular medium had a significant impact upon breast cancer cells' in vitro metastatic behaviors and (2) insulin, in particular, controlled the mode of the functional association between cells' VGSC activity and metastatic machinery.