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

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.

Biochemical characterization of MLC1 protein in astrocytes and its association with the dystrophin-glycoprotein complex

MLC1 gene mutations have been associated with megalencephalic leukoencephalopathy with subcortical cysts (MLC), a rare neurologic disorder in children. The MLC1 gene encodes a membrane protein (MLC1) with unknown function which is mainly expressed in astrocytes. Using a newly developed anti-human MLC1 polyclonal antibody, we have investigated the biochemical properties and localization of MLC1 in cultured astrocytes and brain tissue and searched for evidence of a relationship between MLC1 and proteins of the dystrophin-glycoprotein complex (DGC). Cultured astrocytes express two MLC1 components showing different solubilisation properties and subcellular distribution. Most importantly, we show that the membrane-associated component of MLC1 (60-64 kDa) localizes in astrocytic lipid rafts together with dystroglycan, syntrophin and caveolin-1, and co-fractionates with the DGC in whole rat brain tissue. In the human brain, MLC1 protein is expressed in astrocyte processes and ependymal cells, where it colocalizes with dystroglycan and syntrophin. These data indicate that the DGC may be involved in the organization and function of the MLC1 protein in astrocyte membranes.

Cardiac sodium channel mutation in atrial fibrillation

BACKGROUND

Mutations in the sodium channel SCN5A have been implicated in many cardiac disorders, including the long QT syndrome, Brugada syndrome, conduction system disease, and dilated cardiomyopathy with atrial arrhythmias.

OBJECTIVE

In view of the pleiotropic effects of SCN5A mutations, the purpose of this study was to examine a cohort of patients with familial atrial fibrillation (AF) for mutations in the SCN5A gene.

METHODS

Probands with AF were enrolled in the study between June 1, 2001 and February 10, 2004. Each patient underwent a standardized evaluation, which included an interview, physical examination, ECG, echocardiogram, and blood sample for genetic analysis. Direct sequencing of the coding region of SCN5A was used to screen for mutations in genomic DNA.

RESULTS

One hundred eighty-nine patients with AF were enrolled during the study period. From this cohort, a subset of 57 probands with a family history of AF in at least one first-degree relative was studied. Forty-seven subjects were men (82%); 45 had paroxysmal AF (79%). Echocardiography revealed ejection fraction 62% +/- 6.4 % and left atrial dimension 40 +/- 6.9 mm. A single mutation (N1986K) was observed in one family but was not present in more than 600 control chromosomes. Expression of the N1986K mutant in Xenopus oocytes revealed a hyperpolarizing shift in channel steady-state inactivation.

CONCLUSION

In a cohort with familial AF, a single SCN5A mutation causing the arrhythmia in one kindred was identified. These data extend the range of phenotypes observed with SCN5A mutations and suggest that variation in the SCN5A gene is not a major cause of familial AF.

Evolving concepts in the cellular control of gastrointestinal motility: neurogastroenterology and enteric sciences

The enteric nervous system is an independent nervous system with a complexity comparable with the central nervous system. This complex system is integrated into several other complex systems, such as interstitial cells of Cajal networks and immune cells. The result of these interactions is effective coordination of motility, secretion, and blood flow in the gastrointestinal tract. Loss of subsets of enteric nerves, of interstitial cells of Cajal, malfunction of smooth muscle, and alteration in immune cells have been identified as the basis of many motility disorders. The initial factors triggering these changes and how to intervene to prevent, halt, and reverse them needs to be understood.

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.

Mechanosensitive ion channels in interstitial cells of Cajal and smooth muscle of the gastrointestinal tract

Normal gastrointestinal (GI) motility is required to mix digestive enzymes and food and to move content along the GI tract. Underlying the complex motor patterns of the gut are electrical events that reflect ion flux across cell membranes. Smooth muscle electrical activity is directly influenced by GI interstitial cells of Cajal, whose rhythmic oscillations in membrane potential in part determine the excitability of GI smooth muscle and its response to neuronal input. Coordinated activity of the ion channels responsible for the conductances that underlie ion flux in both smooth muscle and interstitial cells is a requisite for normal motility. These conductances are regulated by many factors, including mechanical stress. Recent studies have revealed mechanosensitivity at the level of the ion channels, and the mechanosensor within the channel has been identified in many cases. This has led to better comprehension of the role of mechanosensitive conductances in normal physiology and will undoubtedly lead to understanding of the consequences of disturbances in these conductances.

Erbin enhances voltage-dependent facilitation of Ca(v)1.3 Ca2+ channels through relief of an autoinhibitory domain in the Ca(v)1.3 alpha1 subunit

Ca(v)1.3 (L-type) voltage-gated Ca2+ channels have emerged as key players controlling Ca2+ signals at excitatory synapses. Compared with the more widely expressed Ca(v)1.2 L-type channel, relatively little is known about the mechanisms that regulate Ca(v)1.3 channels. Here, we describe a new role for the PSD-95 (postsynaptic density-95)/Discs large/ZO-1 (zona occludens-1) (PDZ) domain-containing protein, erbin, in directly potentiating Ca(v)1.3. Erbin specifically forms a complex with Ca(v)1.3, but not Ca(v)1.2, in transfected cells. The significance of erbin/Ca(v)1.3 interactions is supported by colocalization in somatodendritic domains of cortical neurons in culture and coimmunoprecipitation from rat brain lysates. In electrophysiological recordings, erbin augments facilitation of Ca(v)1.3 currents by a conditioning prepulse, a process known as voltage-dependent facilitation (VDF). This effect requires a direct interaction of the erbin PDZ domain with a PDZ recognition site in the C-terminal domain (CT) of the long variant of the Ca(v)1.3 alpha1 subunit (alpha1 1.3). Compared with Ca(v)1.3, the Ca(v)1.3b splice variant, which lacks a large fraction of the alpha1 1.3 CT, shows robust VDF that is not further affected by erbin. When coexpressed as an independent entity with Ca(v)1.3b or Ca(v)1.3 plus erbin, the alpha1 1.3 CT strongly suppresses VDF, signifying an autoinhibitory function of this part of the channel. These modulatory effects of erbin, but not alpha1 1.3 CT, depend on the identity of the auxiliary Ca2+ channel beta subunit. Our findings reveal a novel mechanism by which PDZ interactions and alternative splicing of alpha1 1.3 may influence activity-dependent regulation of Ca(v)1.3 channels at the synapse.

Species dependent expression of intestinal smooth muscle mechanosensitive sodium channels

A mechanosensitive Na(+) current carried by Na(v)1.5 is present in human intestinal circular smooth muscle and contributes to regulation of intestinal motor function. Expression of this channel in different species is unknown. Our aim was to determine if Na(+) currents and message for the alpha subunit of the Na(+) channel (SCN5A) are found in circular smooth muscle cells of human, dog, pig, mouse and guinea pig jejunum. Currents were recorded using patch clamp techniques. Message for SCN5A was investigated using laser capture microdissection and reverse transcription polymerase chain reaction (RT-PCR). Na(+) currents were identified consistently in human and dog smooth muscle cells; however, Na(+) current was not found in pig (0/20) or guinea pig smooth muscle cells (0/21) and found only one mouse cell (1/21). SCN5A mRNA was found in circular muscle of human, dog, and mouse, but not in pig or guinea pig, and not in mouse longitudinal or mucosal layers. In summary, SCN5A message is expressed in, and Na(+) current recorded from, circular muscle layer of human and dog but not from pig and guinea pig. These data show that there are species differences in expression of the SCN5A-encoded Na(v)1.5 channel, suggesting species-specific differences in the electrophysiological response to mechanical and depolarizing stimuli.

Regulation of capacitative calcium entries by α1‐syntrophin: association of TRPC1 with dystrophin complex and the PDZ domain of α1‐syntrophin

Calcium mishandling in Duchenne dystrophic muscle suggested that dystrophin, a membrane-associated cytoskeleton protein, might regulate calcium signaling cascade such as calcium influx pathway. It was previously shown that abnormal calcium entries involve uncontrolled stretch-activated currents and store-operated Ca2+ currents supported by TRPC1 channels. Moreover, our recent work demonstrated that reintroduction of minidystrophin in dystrophic myotubes restores normal capacitative calcium entries (CCEs). However, until now, no molecular link between the dystrophin complex and calcium entry channels has been described. This study is the first to show by coimmunoprecipitation assays the molecular association of TRPC1 with dystrophin and alpha1-syntrophin in muscle cells. TRPC1 was also associated with alpha1-syntrophin in dystrophic muscle cells independently of dystrophin. Furthermore, glutathione S-transferase (GST) pull-down assays showed that TRPC1 binds to the alpha1-syntrophin PDZ domain. Transfected recombinant alpha1-syntrophin formed a complex with TRPC1 channels and restored normal CCEs in dystrophic muscle cells. We suggest that normal regulation of CCEs in skeletal muscle depends on the association between TRPC1 channels and alpha1-syntrophin that may anchor the store-operated channels to the dystrophin-associated protein complex (DAPC). The loss of this molecular association could participate in the calcium alterations observed in dystrophic muscle cells. This study provides a new model for the regulation of calcium influx by interaction with the scaffold of the DAPC in muscle cells.

Design of a selective chemical probe for class I PDZ domains

Covalent labeling has been widely used for structural and functional analyses of proteins. To target a wide range of PDZ domains, we designed a chemical scaffold mimicking the E/D-T/S-XV peptide, which is a PDZ domain that binds ligands in higher occurrence. A chemical probe (2) that contained this moiety alkylated diverse PDZ domains, including NHERF-1 PDZ2, and differentially visualized the cellular proteome.

Touch

Light touch, a sense of muscle position, and the responses to tissue-damaging levels of pressure all involve mechanosensitive sensory neurons that originate in the dorsal root or trigeminal ganglia. A variety of mechanisms of mechanotransduction are proposed. These ranges from direct activation of mechanically activated channels at the tips of sensory neurons to indirect effects of intracellular mediators, or chemical signals released from distended tissues, or specialized mechanosensory end organs. This chapter describes the properties of mechanosensitive channels present in sensory neurons and the potential molecular candidates that may underlie. Mechanically regulated electrical activity by touch and tissue damaging levels of pressure in sensory neurons seems to involve a variety of direct and indirect mechanisms and ion channels, and the involvement of specialized end organs in mechanotransduction complicates matters even more. Imaging studies are providing useful information about the events in the central nervous system associated with touch pain and allodynia (a pathological state where touch becomes painful this type of activity).

Modulation of host gene expression by the K15 protein of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) contains several open reading frames (ORFs) encoding proteins capable of initiating signal transduction pathways. Among them is the K15 ORF, which consists of eight exons encoding a protein with 12 predicted transmembrane domains and a cytoplasmic C terminus. When transiently expressed, the 8-exon K15 transcript gives rise to a protein with an apparent molecular mass of 45 kDa. K15 interacts with cellular proteins, TRAF (tumor necrosis factor receptor-associated factor) and Src kinases, and activates AP-1, NF-kappaB, and the mitogen-activated protein kinases (MAPKs) c-jun-N-terminal kinase and extracellular signal-regulated kinase. This signaling activity of K15 is related to phosphorylation of Y(481) of the K15 SH2-B motif Y(481)EEV. In this study we demonstrate the expression of an endogenous 45-kDa K15 protein in KSHV BAC36-infected epithelial cells. This endogenous K15 protein shows the same intracellular localization as transiently expressed K15, and expression kinetic studies suggest it to be a lytic gene. We have further determined the downstream target genes of K15 signaling using DNA oligonucleotide microarrays. We demonstrate that K15 is capable of inducing expression of multiple cytokines and chemokines, including interleukin-8 (IL-8), IL-6, CCL20, CCL2, CXCL3, and IL-1alpha/beta, as well as expression of Dscr1 and Cox-2. In epithelial cells, K15-induced upregulation of most genes was dependent on phosphorylation of Y(481), whereas in endothelial cells mutation of Y(481) did not result in a complete loss of Dscr1 and Cox-2 expression and NFAT-activity. Our study establishes K15 as one of the KSHV lytic genes that are inducing expression of multiple cytokines, which have been shown to play an important role in KSHV-associated pathogenesis.

Cardiac sodium channel Na(v)1.5 interacts with and is regulated by the protein tyrosine phosphatase PTPH1

In order to identify proteins interacting with the cardiac voltage-gated sodium channel Na(v)1.5, we used the last 66 amino acids of the C-terminus of the channel as bait to screen a human cardiac cDNA library. We identified the protein tyrosine phosphatase PTPH1 as an interacting protein. Pull-down experiments confirmed the interaction, and indicated that it depends on the PDZ-domain binding motif of Na(v)1.5. Co-expression experiments in HEK293 cells showed that PTPH1 shifts the Na(v)1.5 availability relationship toward hyperpolarized potentials, whereas an inactive PTPH1 or the tyrosine kinase Fyn does the opposite. The results of this study suggest that tyrosine phosphorylation destabilizes the inactivated state of Na(v)1.5.

New insights in the regulation of calcium transfers by muscle dystrophin-based cytoskeleton: implications in DMD

Calcium mishandling in Duchenne muscular dystrophy (DMD) suggested that dystrophin, a membrane-associated cytoskeleton protein, may regulate calcium-signalling cascades such as calcium entries. Calcium overload in human DMD myotubes is dependent on their contractile activity suggesting the involvement of channels being activated during contraction and/or calcium release. Forced expression of mini-dystrophin in dystrophin-deficient myotubes, reactivates appropriate sarcolemmal expression of dystrophin-associated proteins and restores normal calcium handling in the cytosol. Furthermore, the recombinant mini-dystrophin reduced the store-operated calcium influx across the sarcolemma, and the mitochondrial calcium uptake during this influx. A slow component of calcium release dependent on IP3R, as well as the production of IP3, were also reduced to normal levels by expression of mini-dystrophin. Our studies provide a new model for the convergent regulation of transmembrane calcium influx and IP3-dependent calcium release by the dystrophin-based cytoskeleton (DBC). We also suggest molecular association of such channels with DBC which may provide the scaffold for assembling a multiprotein-signalling complex that modulates the channel activity. This suggests that the loss of this molecular association could participate in the alteration of calcium homeostasis observed in DMD muscle cells.

Cardiac sodium channel Nav1.5 is regulated by a multiprotein complex composed of syntrophins and dystrophin

The cardiac sodium channel Na(v)1.5 plays a key role in cardiac excitability and conduction. The purpose of this study was to elucidate the role of the PDZ domain-binding motif formed by the last three residues (Ser-Ile-Val) of the Na(v)1.5 C-terminus. Pull-down experiments were performed using Na(v)1.5 C-terminus fusion proteins and human or mouse heart protein extracts, combined with mass spectrometry analysis. These experiments revealed that the C-terminus associates with dystrophin, and that this interaction was mediated by alpha- and beta-syntrophin proteins. Truncation of the PDZ domain-binding motif abolished the interaction. We used dystrophin-deficient mdx(5cv) mice to study the role of this protein complex in Na(v)1.5 function. Western blot experiments revealed a 50% decrease in the Na(v)1.5 protein levels in mdx(5cv) hearts, whereas Na(v)1.5 mRNA levels were unchanged. Patch-clamp experiments showed a 29% decrease of sodium current in isolated mdx(5cv) cardiomyocytes. Finally, ECG measurements of the mdx(5cv) mice exhibited a 19% reduction in the P wave amplitude, and an 18% increase of the QRS complex duration, compared with controls. These results indicate that the dystrophin protein complex is required for the proper expression and function of Na(v)1.5. In the absence of dystrophin, decreased sodium current may explain the alterations in cardiac conduction observed in patients with dystrophinopathies.

Gastrointestinal symptoms in families of patients with an SCN5A-encoded cardiac channelopathy: evidence of an intestinal channelopathy

OBJECTIVES

Recently, two ion channels associated with congenital long QT syndrome, the SCN5A-encoded Nav1.5 sodium channel and the KCNH2-encoded HERG potassium channel (IKr), have been found on gastrointestinal smooth muscle and interstitial cells of Cajal. The aim of this study was to determine if the cardiac channelopathy-associated mutations in SCN5A or KCNH2 are associated with GI symptom complexes.

METHODS

Mayo Clinic's Sudden Death Genomics Laboratory performed comprehensive mutational analysis on index patients referred for long QT syndrome genetic testing and their family members thus establishing a cohort of families for which the genotype status for SCN5A or KCNH2 is known. A valid GI symptom questionnaire was mailed to all family members (both genotype positive and genotype negative) in this cohort. The association between cardiac channel genotype and GI symptoms was assessed by logistic regression adjusted for age and sex.

RESULTS

Two hundred and nineteen (43% of 529) subjects returned the questionnaire. Fifty percent of the subjects with an SCN5A mutation reported abdominal pain compared to only 13% of controls (OR 5.7; 95% CI 1.3-24.4). Over 65% of subjects with an SCN5A mutation reported a GI symptom complex compared to 28% of controls (OR 5.2; 95% CI 1.5-18.3). No associations with KCNH2 genotype status were detected.

CONCLUSIONS

This study is the first to suggest an association between a well-defined cardiac channelopathy and GI symptoms. The role of sodium channelopathies in the pathogenesis of digestive diseases merits exploration.

Assembly of a perivascular astrocyte protein scaffold at the mammalian blood–brain barrier is dependent on α-syntrophin

alpha-Syntrophin, a member of the dystrophin-associated protein complex, is required for proper localization of the water channel aquaporin-4 at the blood-brain barrier. Mice lacking alpha-syntrophin have reduced levels of aquaporin-4 in perivascular astroglial endfeet. Consequently, they exhibit reduced edema and infarct volume in brain trauma models and reduced K+ clearance from the neuropil, leading to increased seizure susceptibility. We have used the alpha-syntrophin null mice to investigate whether alpha-syntrophin is required for proper localization of other components of the dystrophin complex at the blood-brain barrier. We find that alpha-syntrophin is required for the full recruitment of gamma2-syntrophin and alpha-dystrobrevin-2 to glial endfeet in adult cerebellum. In contrast, the localization of beta1- and beta2-syntrophin and alpha-dystrobrevin-1 at the blood-brain barrier is not dependent on the presence of alpha-syntrophin. The localization patterns of alpha-dystrobrevin-1 and -2 in wild type cerebellum are strikingly different; while alpha-dystrobrevin-1 is present in glial endfeet throughout the cerebellum, alpha-dystrobrevin-2 is restricted to glial endfeet in the granular layer alone. Finally, we show that the enrichment of dystrophin in glial endfeet depends on the presence of alpha-syntrophin. This finding is the first demonstration that dystrophin localization is dependent on syntrophin. Since the localization of gamma2-syntrophin, alpha-dystrobrevin-2, and dystrophin is contingent on alpha-syntrophin, we conclude that alpha-syntrophin is a central organizer of the astrocyte dystrophin complex, an important molecular scaffold for localization of aquaporin-4 at the blood-brain barrier.

Studying the mechanosensitivity of voltage-gated channels using oocyte patches

The mechanosensitivity of voltage-gated (VG) channels is of biophysical, physiological. and pathophysiological interest. Xenopus oocytes offer a critical advantage for investigating the electrophysiology of recombinant VG channels subjected to membrane stretch, namely, the ability to monitor macroscopic current from membrane patches. High-density channel expression in oocytes makes for macroscopic current in conventional-size, mechanically sturdy patches. With the patch configuration, precisely the same membrane that is voltage-clamped is the membrane subjected to on-off stretch stimuli. With patches, meaningful stretch dose responses are possible. Experimental design should facilitate within-patch comparisons wherever possible. The mechanoresponses of some VG channels depend critically on patch history. Methods for minimizing and coping with interference from endogenous voltage-dependent and stretch-activated endogenous channels are described.

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.

Differential effect of alpha-syntrophin knockout on aquaporin-4 and Kir4.1 expression in retinal macroglial cells in mice

Aquaporin-4 water channels and the inwardly rectifying potassium channels Kir4.1 are coexpressed in a highly polarized manner at the perivascular and subvitreal endfeet of retinal Müller cells and astrocytes. The present study was aimed at resolving the anchoring mechanisms responsible for the coexpression of these molecules. Both aquaporin-4 and Kir4.1 contain PDZ-domain binding motifs at their C-termini and it was recently shown that mice with targeted disruption of the dystrophin gene display altered distribution of aquaporin-4 and Kir4.1 in the retina. To test our hypothesis that alpha-syntrophin (a PDZ-domain containing protein of the dystrophin associated protein complex) is involved in aquaporin-4 and Kir4.1 anchoring in retinal cells, we studied the expression pattern of these molecules in alpha-syntrophin null mice. Judged by quantitative immunogold cytochemistry, deletion of the alpha-syntrophin gene causes a partial loss (by 70%) of aquaporin-4 labeling at astrocyte and Müller cell endfeet but no decrease in Kir4.1 labeling at these sites. These findings suggest that alpha-syntrophin is not involved in the anchoring of Kir4.1 and only partly responsible for the anchoring of aquaporin-4 in retinal endfeet membranes. Furthermore we show that wild type and alpha-syntrophin null mice exhibit strong beta1 syntrophin labeling at perivascular and subvitreal Müller cell endfeet, raising the possibility that beta1 syntrophin might be involved in the anchoring of Kir4.1 and the alpha-syntrophin independent pool of aquaporin-4.

Structure of the split PH domain and distinct lipid-binding properties of the PH-PDZ supramodule of alpha-syntrophin

Pleckstrin homology (PH) domains play diverse roles in cytoskeletal dynamics and signal transduction. Split PH domains represent a unique subclass of PH domains that have been implicated in interactions with complementary partial PH domains 'hidden' in many proteins. Whether partial PH domains exist as independent structural units alone and whether two halves of a split PH domain can fold together to form an intact PH domain are not known. Here, we solved the structure of the PH(N)-PDZ-PH(C) tandem of alpha-syntrophin. The split PH domain of alpha-syntrophin adopts a canonical PH domain fold. The isolated partial PH domains of alpha-syntrophin, although completely unfolded, remain soluble in solution. Mixing of the two isolated domains induces de novo folding and yields a stable PH domain. Our results demonstrate that two complementary partial PH domains are capable of binding to each other to form an intact PH domain. We further showed that the PH(N)-PDZ-PH(C) tandem forms a functionally distinct supramodule, in which the split PH domain and the PDZ domain function synergistically in binding to inositol phospholipids.

Structural abnormalities at neuromuscular synapses lacking multiple syntrophin isoforms

The syntrophins are modular adapter proteins that function by recruiting signaling molecules to the cytoskeleton via their direct association with proteins of the dystrophin protein family. We investigated the physiological function of beta2-syntrophin by generating a line of mice lacking this syntrophin isoform. The beta2-syntrophin null mice show no overt phenotype, or muscular dystrophy, and form structurally normal neuromuscular junctions (NMJs). To determine whether physiological consequences caused by the lack of beta2-syntrophin were masked by compensation from the alpha-syntrophin isoform, we crossed these mice with our previously described alpha-syntrophin null mice to produce mice lacking both isoforms. The alpha/beta2-syntrophin null mice have NMJs that are structurally more aberrant than those lacking only alpha-syntrophin. The NMJs of the alpha/beta2-syntrophin null mice have fewer junctional folds than either parent strain, and the remaining folds are abnormally shaped with few openings to the synaptic space. The levels of acetylcholine receptors are reduced to 23% of wild type in mice lacking both syntrophin isoforms. Furthermore, the alpha/beta2-syntrophin null mice ran significantly shorter distances on voluntary exercise wheels despite having normal neuromuscular junction transmission as determined by micro-electrode recording of endplate potentials. We conclude that both alpha-syntrophin and beta2-syntrophin play distinct roles in forming and maintaining NMJ structure and that each syntrophin can partially compensate for the loss of the other.

Dysfunction in ankyrin-based cellular pathways and human cardiac arrhythmia

Ankyrins are a family of multivalent membrane-adaptor proteins first identified in the erythrocyte over 25 years ago as a link between the anion exchanger and the spectrin-based cytoskeleton. Since their initial discovery, ankyrin function has been linked to protein targeting and membrane domain organization in a variety of cell types including erythrocytes, neurons, epithelial cells, and cardiomyocytes. Recent findings demonstrate that dysfunction in ankyrin-based cellular pathways in the heart leads to human ventricular arrhythmia and sudden cardiac death. This review will present an overview of the ankyrin family with special emphasis on the recently identified roles of ankyrin polypeptides in ion channel and transporter targeting in the heart.

Regulation of the Voltage-Gated Cardiac Sodium Channel Nav1.5 by Interacting Proteins

Na(v)1.5, the major cardiac voltage-gated Na(+) channel, plays a central role in the generation of the cardiac action potential and in the propagation of electrical impulses in the heart. Its importance for normal heart function has been recently exemplified by reports of numerous mutations found in the gene SCN5A--which encodes Na(v)1.5--in patients with various pathologic cardiac phenotypes, indicating that even subtle alterations of Na(v)1.5 cell biology and function may underlie human diseases. Similar to other ion channels, Na(v)1.5 is most likely part of dynamic multiprotein complexes located in the different cellular compartments. This review focuses on five intracellular proteins that have been recently reported to directly bind to and contribute to the regulation of Na(v)1.5: ankyrin proteins, fibroblast growth factor homologous factor 1B, calmodulin, Nedd4-like ubiquitin-protein ligases, and syntrophin proteins.

Membrane tension accelerates rate-limiting voltage-dependent activation and slow inactivation steps in a Shaker channel

A classical voltage-sensitive channel is tension sensitive—the kinetics of Shaker and S3–S4 linker deletion mutants change with membrane stretch (Tabarean, I.V., and C.E. Morris. 2002. Biophys. J. 82:2982–2994.). Does stretch distort the channel protein, producing novel channel states, or, more interestingly, are existing transitions inherently tension sensitive? We examined stretch and voltage dependence of mutant 5aa, whose ultra-simple activation (Gonzalez, C., E. Rosenman, F. Bezanilla, O. Alvarez, and R. Latorre. 2000. J. Gen. Physiol. 115:193–208.) and temporally matched activation and slow inactivation were ideal for these studies. We focused on macroscopic patch current parameters related to elementary channel transitions: maximum slope and delay of current rise, and time constant of current decline. Stretch altered the magnitude of these parameters, but not, or minimally, their voltage dependence. Maximum slope and delay versus voltage with and without stretch as well as current rising phases were well described by expressions derived for an irreversible four-step activation model, indicating there is no separate stretch-activated opening pathway. This model, with slow inactivation added, explains most of our data. From this we infer that the voltage-dependent activation path is inherently stretch sensitive. Simulated currents for schemes with additional activation steps were compared against datasets; this showed that generally, additional complexity was not called for. Because the voltage sensitivities of activation and inactivation differ, it was not possible to substitute depolarization for stretch so as to produce the same overall P_O time course. What we found, however, was that at a given voltage, stretch-accelerated current rise and decline almost identically—normalized current traces with and without stretch could be matched by a rescaling of time. Rate-limitation of the current falling phase by activation was ruled out. We hypothesize, therefore, that stretch-induced bilayer decompression facilitates an in-plane expansion of the protein in both activation and inactivation. Dynamic structural models of this class of channels will need to take into account the inherent mechanosensitivity of voltage-dependent gating.

Tyrosine-phosphorylated and Nonphosphorylated Sodium Channel β1 Subunits Are Differentially Localized in Cardiac Myocytes

Voltage-gated sodium channel alpha and beta subunits expressed in mammalian heart are differentially localized to t-tubules and intercalated disks. Sodium channel beta subunits are multifunctional molecules that participate in channel modulation and cell adhesion. Reversible, receptor-mediated changes in beta1 tyrosine phosphorylation modulate its ability to recruit and associate with ankyrin. The purpose of the present study was to test our hypothesis that tyrosine-phosphorylated beta1 (pYbeta1) and nonphosphorylated beta1 subunits may be differentially localized in heart and thus interact with different cytoskeletal and signaling proteins. We developed an antibody that specifically recognizes pYbeta1 and investigated the differential subcellular localization of beta1 and pYbeta1 in mouse ventricular myocytes. We found that pYbeta1 colocalized with connexin-43, N-cadherin, and Nav1.5 at intercalated disks but was not detected at the t-tubules. Anti-pYbeta1 immunoprecipitates N-cadherin from heart membranes and from cells transfected with beta1 and N-cadherin in the absence of other sodium channel subunits. pYbeta1 does not associate with ankyrinB in heart membranes. N-cadherin and connexin-43 associate with Nav1.5 in heart membranes as assessed by co-immunoprecipitation assays. We propose that sodium channel complexes at intercalated disks of ventricular myocytes are composed of Nav1.5 and pYbeta1 and that these complexes are in close association with both N-cadherin and connexin-43. beta1 phosphorylation appears to regulate its localization to differential subcellular domains.

Voltage-gated sodium channels and pain pathways

Acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. The peripheral input that drives pain perception thus depends on the presence of functional voltage-gated sodium channels. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. A combination of antisense and knock-out studies support a specialized role for these sodium channels in pain pathways, and pharmacological studies with conotoxins suggest that isotype-specific antagonists should be feasible. Taken together, these data suggest that isotype-specific sodium channel blockers could be useful analgesics.

Syntrophin is an actin-binding protein the cellular localization of which is regulated through cytoskeletal reorganization in skeletal muscle cells

We have characterized the interaction of syntrophin with F-actin. Subcellular fractionation of cardiac and skeletal muscle tissues showed that alpha-, beta1- and beta2-syntrophins were present in the soluble and the membrane fraction. Syntrophins are known to bind to the dystrophin-glycoprotein complex (DGC), but since the DGC is not present in the soluble fraction, it was concluded that some syntrophin did not associate with the DGC. Native syntrophins purified from the soluble fraction and recombinant syntrophins were both able to bind to F-actin, and binding occurred through several sites on syntrophin, including the second pleckstrin homology domain and the unique carboxyl-terminal domain. Syntrophin was also able to inhibit actin-activated myosin ATPase activity and actomyosin super-precipitation. alpha-Syntrophin co-localized with cortical F-actin fibers when expressed in Chinese hamster ovary cells, and deletion of the actin-binding region abolished co-localization. Most of exogenous or endogenous syntrophin also co-localized with stress fibers in endothelial and smooth muscle (A7r5) cells. However, syntrophins were mostly localized in the cytosol of serum-starved C2C12 or primary cultured skeletal muscle myotubes, and translocated to the membrane upon treatment with lysophosphatidic acid or the actin-stabilizing agent jasplakinolide. The actin-depolymerizing agent latrunculin-B abolished this syntrophin translocation. These findings suggest that syntrophin is an actin-binding protein the subcellular localization of which is regulated through cytoskeletal reorganization.

Sodium current in human intestinal interstitial cells of Cajal

Interstitial cells of Cajal (ICC) generate the electrical slow wave required for normal gastrointestinal motility. The ionic conductances expressed in human intestinal ICC are unknown. The aim of this study was to determine expression of a Na+ current in human intestinal ICC and to determine the effects of the Na+ current on the slow wave. Visually identified, freshly dissociated, single ICC were verified by the presence of c-kit mRNA by using single-cell RT-PCR. Standard whole cell currents were recorded from patch-clamped ICC held at -100 mV between pulse protocols. A Na+ current was identified in human intestinal ICC. The current activated at -55 mV and peaked at -30 mV. Extracellular N-methyl-d-glucamine abolished and QX-314 (500 microM) blocked the Na+ current, but nifedipine and Ni2+ did not. The Na+ current was activated by shear stress. Single-cell RT-PCR detected mRNA for the Na+ alpha-subunit SCN5A in single human intestinal ICC. Lidocaine (200 microm) and QX-314 (500 microM) decreased slow wave frequency, and stretch increased slow wave frequency. A mechanosensitive Na+ channel current is present in human intestinal ICC and appears to play a role in the control of intestinal motor function.

The stn-1 syntrophin gene of C.elegans is functionally related to dystrophin and dystrobrevin

Syntrophins are a family of PDZ domain-containing adaptor proteins required for receptor localization. Syntrophins are also associated with the dystrophin complex in muscles. We report here the molecular and functional characterization of the Caenorhabditis elegans gene stn-1 (F30A10.8), which encodes a syntrophin with homology to vertebrate alpha and beta-syntrophins. stn-1 is expressed in neurons and in muscles of C.elegans. stn-1 mutants resemble dystrophin (dys-1) and dystrobrevin (dyb-1) mutants: they are hyperactive, bend their heads when they move forward, tend to hypercontract, and are hypersensitive to the acetylcholinesterase inhibitor aldicarb. These phenotypes are suppressed when stn-1 is expressed under the control of a muscular promoter, indicating that they are caused by the absence of stn-1 in muscles. These results suggest that the role of syntrophin is linked to dystrophin function in C.elegans.