Mechanism of mitochondrial permeability transition pore induction and damage in the pancreas: inhibition prevents acute pancreatitis by protecting production of ATP.

OBJECTIVE: Acute pancreatitis is caused by toxins that induce acinar cell calcium overload, zymogen activation, cytokine release and cell death, yet is without specific drug therapy. Mitochondrial dysfunction has been implicated but the mechanism not established. DESIGN: We investigated the mechanism of induction and consequences of the mitochondrial permeability transition pore (MPTP) in the pancreas using cell biological methods including confocal microscopy, patch clamp technology and multiple clinically representative disease models. Effects of genetic and pharmacological inhibition of the MPTP were examined in isolated murine and human pancreatic acinar cells, and in hyperstimulation, bile acid, alcoholic and choline-deficient, ethionine-supplemented acute pancreatitis. RESULTS: MPTP opening was mediated by toxin-induced inositol trisphosphate and ryanodine receptor calcium channel release, and resulted in diminished ATP production, leading to impaired calcium clearance, defective autophagy, zymogen activation, cytokine production, phosphoglycerate mutase 5 activation and necrosis, which was prevented by intracellular ATP supplementation. When MPTP opening was inhibited genetically or pharmacologically, all biochemical, immunological and histopathological responses of acute pancreatitis in all four models were reduced or abolished. CONCLUSIONS: This work demonstrates the mechanism and consequences of MPTP opening to be fundamental to multiple forms of acute pancreatitis and validates the MPTP as a drug target for this disease.

Debio 0932, a new oral Hsp90 inhibitor, alleviates psoriasis in a xenograft transplantation model.

Debio 0932 is a novel oral heat shock protein 90 (Hsp90) inhibitor developed for anti-cancer therapy. Surprising-ly, during the first clinical trial, one psoriasis patient experienced complete remission of his skin manifestation. However, a possible therapeutic utility of Hsp90 in psoriasis has not previously been reported. The objective of the present study was to explore the ability of Debio 0932 to alleviate psoriasis in a preclinical model. A psoriasis xenograft transplantation model was employed where skin from 5 psoriasis patients was transplanted onto immunodeficient mice (8 xenografts per donor). Debio 0932 was administered perorally daily for 3 weeks and resulted in significant clinical alleviation of psoriasis by day 11 and reduced epidermal thickness evaluated post-treatment. Alleviation of psoriasis in the psoriasis xenograft transplantation model, which may be due to Hsp90's involvement in signalling pathways that are up-regulated in psoriasis, substantiates a potential role of Debio 0932 in psoriasis treatment.

The cyclophilin inhibitor alisporivir prevents hepatitis C virus-mediated mitochondrial dysfunction.

UNLABELLED: Alisporivir (Debio-025) is an analogue of cyclosporine A and represents the prototype of a new class of non-immunosuppressive cyclophilin inhibitors. In vitro and in vivo studies have shown that alisporivir inhibits hepatitis C virus (HCV) replication, and ongoing clinical trials are exploring its therapeutic potential in patients with chronic hepatitis C. Recent data suggest that the antiviral effect is mediated by inhibition of cyclophilin A, which is an essential host factor in the HCV life cycle. However, alisporivir also inhibits mitochondrial permeability transition by binding to cyclophilin D. Because HCV is known to affect mitochondrial function, we explored the effect of alisporivir on HCV protein-mediated mitochondrial dysfunction. Through the use of inducible cell lines, which allow to investigate the effects of HCV polyprotein expression independent from viral RNA replication and which recapitulate the major alterations of mitochondrial bioenergetics observed in infectious cell systems, we show that alisporivir prevents HCV protein-mediated decrease of cell respiration, collapse of mitochondrial membrane potential, overproduction of reactive oxygen species and mitochondrial calcium overload. Strikingly, some of the HCV-mediated mitochondrial dysfunctions could even be rescued by alisporivir. CONCLUSION: These observations provide new insights into the pathogenesis of HCV-related liver disease and reveal an additional mechanism of action of alisporivir that is likely beneficial in the treatment of chronic hepatitis C.

Phase I Study of Androgen Deprivation Therapy in Combination with Anti-PD-1 in Melanoma Patients Pretreated with Anti-PD-1.

PURPOSE: Androgen deprivation regenerates the thymus in adults, expanding of T-cell receptor V ß repertoire in blood and lymphoid organs and tumor-infiltrating lymphocytes in human prostate tumors. In melanoma murine models, androgen receptor promotes metastases and androgen blockade potentiates antitumor vaccine efficacy. This phase I study evaluated the safety, efficacy, and pharmocodynamics of androgen deprivation with the gonadotropin releasing hormone (GnRH) agonist triptorelin combined with nivolumab in male patients with melanoma resistant to anti-PD-1. PATIENTS AND METHODS: Adult male patients with advanced melanoma who progressed under anti-PD-1 containing regimens received triptorelin 3.75 mg every 4 weeks, nivolumab 3 mg/kg every 2 weeks, and bicalutamide 50 mg once daily during the first 28 days. Tumor response was first assessed after 3 months; adverse events (AE) were monitored throughout the study. T-cell receptor excision circles (TREC), a biomarker of thymus activity, were explored throughout the study. RESULTS: Of 14 patients, 4 were locally advanced and 10 had distant metastases. There were no grade 4 or 5 AEs. Five grade three AEs were reported in 4 patients. According to RECIST v1.1, best overall response was partial response (PR) in one patient with a pancreas metastasis, stable disease (SD) in 5 patients, and progressive disease in 8 patients. According to iRECIST, a second PR occurred after an initial pseudoprogression, TRECs increased in 2 patients, one with PR who also had an increase in TILs, and the second with SD. CONCLUSIONS: This combination was well tolerated. Disease control was obtained in 42.8% (RECIST) and 50% (iRECIST). The evidence for thymus rejuvenation was limited.

Exploratory window-of-opportunity trial to investigate the tumor pharmacokinetics/pharmacodynamics of the IAP antagonist Debio 1143 in patients with head and neck cancer.

Inhibitor of apoptosis proteins (IAPs) regulate apoptosis and modulate NF-κB signaling thereby driving expression of genes involved in immune/inflammatory responses. The orally available IAP antagonist Debio 1143 has potential to enhance tumor response to chemoradiotherapy and/or immunotherapy. Patients with pre-operative squamous cell carcinomas of the head and neck (SCCHN) received: Debio 1143 monotherapy (200 mg/day [D]1-15 +/- 2); Debio 1143 (200 mg/day D1-15 +/- 2) plus cisplatin (40 mg/m2 D 1 and 8); cisplatin alone (40 mg/m2 D 1 and 8; EudraCT: 2014-004655-31). Pharmacokinetic/pharmacodynamic effects were assessed in plasma and resected tumors. Primary end point; effect of Debio 1143 on cellular IAP-1 (cIAP-1). Levels of cIAP-1/-2, X-linked inhibitor of apoptosis protein (XIAP), tumor infiltrating lymphocytes (TILs), including CD8+ T cells, programmed cell death protein 1 (PD-1), PD-ligand 1 (PD-L1), and gene expression were also analyzed. Twenty-three of 26 patients completed treatment. In the Debio 1143 monotherapy cohort (n = 13), mean tumor concentrations of Debio 1143 were 18-fold (maximum 55.2-fold) greater than in plasma, exceeding the half-maximal inhibitory concentration for cIAPs and XIAP by 100 to 1000-fold, with significant engagement/degradation of cIAP-1 (p < 0.05). Overall, levels of CD8+ TILs, PD-1, and PD-L1 positive immune cells increased significantly (p < 0.05) following Debio 1143 treatment. Changes were observed in the expression of genes related to NF-κB signaling. Treatments were well-tolerated. Debio 1143 penetrated SCCHN tumors, engaged cIAP-1, and induced immune inflammatory changes in the tumor microenvironment. Based on the mode of action demonstrated here and in previous studies, these data support future combinations of Debio 1143 with immune-checkpoint agents.

Nedd4-2-dependent ubiquitylation and regulation of the cardiac potassium channel hERG1.

The voltage-gated cardiac potassium channel hERG1 (human ether-à-gogo-related gene 1) plays a key role in the repolarization phase of the cardiac action potential (AP). Mutations in its gene, KCNH2, can lead to defects in the biosynthesis and maturation of the channel, resulting in congenital long QT syndrome (LQTS). To identify the molecular mechanisms regulating the density of hERG1 channels at the plasma membrane, we investigated channel ubiquitylation by ubiquitin ligase Nedd4-2, a post-translational regulatory mechanism previously linked to other ion channels. We found that whole-cell hERG1 currents recorded in HEK293 cells were decreased upon neural precursor cell expressed developmentally down-regulated 4-2 (Nedd4-2) co-expression. The amount of hERG1 channels in total HEK293 lysates and at the cell surface, as assessed by Western blot and biotinylation assays, respectively, were concomitantly decreased. Nedd4-2 and hERG1 interact via a PY motif located in the C-terminus of hERG1. Finally, we determined that Nedd4-2 mediates ubiquitylation of hERG1 and that deletion of this motif affects Nedd4-2-dependent regulation. These results suggest that ubiquitylation of the hERG1 protein by Nedd4-2, and its subsequent down-regulation, could represent an important mechanism for modulation of the duration of the human cardiac action potential.

Kv4 potassium channels form a tripartite complex with the anchoring protein SAP97 and CaMKII in cardiac myocytes.

Membrane-associated guanylate kinase (MAGUK) proteins are major determinants of the organization of ion channels in the plasma membrane in various cell types. Here, we investigated the interaction between the MAGUK protein SAP97 and cardiac Kv4.2/3 channels, which account for a large part of the outward potassium current, I(to), in heart. We found that the Kv4.2 and Kv4.3 channels C termini interacted with SAP97 via a SAL amino acid sequence. SAP97 and Kv4.3 channels were colocalized in the sarcolemma of cardiomyocytes. In CHO cells, SAP97 clustered Kv4.3 channels in the plasma membrane and increased the current independently of the presence of KChIP and dipeptidyl peptidase-like protein-6. Suppression of SAP97 by using short hairpin RNA inhibited I(to) in cardiac myocytes, whereas its overexpression by using an adenovirus increased I(to). Kv4.3 channels without the SAL sequence were no longer regulated by Ca2+/calmodulin kinase (CaMK)II inhibitors. In cardiac myocytes, pull-down and coimmunoprecipitation assays showed that the Kv4 channel C terminus, SAP97, and CaMKII interact together, an interaction suppressed by SAP97 silencing and enhanced by SAP97 overexpression. In HEK293 cells, SAP97 silencing reproduced the effects of CaMKII inhibition on current kinetics and suppressed Kv4/CaMKII interactions. In conclusion, SAP97 is a major partner for surface expression and CaMKII-dependent regulation of cardiac Kv4 channels.

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.

Cardiac voltage-gated sodium channel Nav1.5 is regulated by Nedd4-2 mediated ubiquitination.

Na(v)1.5, the cardiac isoform of the voltage-gated Na+ channel, is critical to heart excitability and conduction. However, the mechanisms regulating its expression at the cell membrane are poorly understood. The Na(v)1.5 C-terminus contains a PY-motif (xPPxY) that is known to act as binding site for Nedd4/Nedd4-like ubiquitin-protein ligases. Because Nedd4-2 is well expressed in the heart, we investigated its role in the ubiquitination and regulation of Na(v)1.5. Yeast two-hybrid and GST-pulldown experiments revealed an interaction between Na(v)1.5 C-terminus and Nedd4-2, which was abrogated by mutating the essential tyrosine of the PY-motif. Ubiquitination of Na(v)1.5 was detected in both transfected HEK cells and heart extracts. Furthermore, Nedd4-2-dependent ubiquitination of Na(v)1.5 was observed. To test for a functional role of Nedd4-2, patch-clamp experiments were performed on HEK cells expressing wild-type and mutant forms of both Na(v)1.5 and Nedd4-2. Na(v)1.5 current density was decreased by 65% upon Nedd4-2 cotransfection, whereas the PY-motif mutant channels were not affected. In contrast, a catalytically inactive Nedd4-2 had no effect, indicating that ubiquitination mediates this downregulation. However, Nedd4-2 did not alter the whole-cell or the single channel biophysical properties of Na(v)1.5. Consistent with the functional findings, localization at the cell periphery of Na(v)1.5-YFP fusion proteins was reduced upon Nedd4-2 coexpression. The Nedd4-1 isoform did not regulate Na(v)1.5, suggesting that Nedd4-2 is a specific regulator of Na(v)1.5. These results demonstrate that Na(v)1.5 can be ubiquitinated in heart tissues and that the ubiquitin-protein ligase Nedd4-2 acts on Na(v)1.5 by decreasing the channel density at the cell surface.

Long-term amiodarone administration remodels expression of ion channel transcripts in the mouse heart.

BACKGROUND: The basis for the unique effectiveness of long-term amiodarone treatment on cardiac arrhythmias is incompletely understood. The present study investigated the pharmacogenomic profile of amiodarone on genes encoding ion-channel subunits. METHODS AND RESULTS: Adult male mice were treated for 6 weeks with vehicle or oral amiodarone at 30, 90, or 180 mg x kg(-1) x d(-1). Plasma and myocardial levels of amiodarone and N-desethylamiodarone increased dose-dependently, reaching therapeutic ranges observed in human. Plasma triiodothyronine levels decreased, whereas reverse triiodothyronine levels increased in amiodarone-treated animals. In ECG recordings, amiodarone dose-dependently prolonged the RR, PR, QRS, and corrected QT intervals. Specific microarrays containing probes for the complete ion-channel repertoire (IonChips) and real-time reverse transcription-polymerase chain reaction experiments demonstrated that amiodarone induced a dose-dependent remodeling in multiple ion-channel subunits. Genes encoding Na+ (SCN4A, SCN5A, SCN1B), connexin (GJA1), Ca2+ (CaCNA1C), and K+ channels (KCNA5, KCNB1, KCND2) were downregulated. In patch-clamp experiments, lower expression of K+ and Na+ channel genes was associated with decreased I(to,f), I(K,slow), and I(Na) currents. Inversely, other K+ channel alpha- and beta-subunits, such as KCNA4, KCNK1, KCNAB1, and KCNE3, were upregulated. CONCLUSIONS: Long-term amiodarone treatment induces a dose-dependent remodeling of ion-channel expression that is correlated with the cardiac electrophysiologic effects of the drug. This profile cannot be attributed solely to the amiodarone-induced cardiac hypothyroidism syndrome. Thus, in addition to the direct effect of the drug on membrane proteins, part of the therapeutic action of long-term amiodarone treatment is likely related to its effect on ion-channel transcripts.

Molecular and clinical determinants of drug-induced long QT syndrome: an iatrogenic channelopathy.

More than 70 drugs present on the Swiss market can cause drug-induced long QT syndrome (LQTS), which is associated with torsades de pointes (TdP) arrhythmias, potentially leading to sudden cardiac death. Basic and clinical investigations performed during the last decade have helped a better understanding of the mechanisms and risk factors of this serious public health problem. In their vast majority, QT interval prolonging drugs block the human ERG (hERG) channel involved in the repolarisation phase of the cardiac action potential, and thus lengthen the QT interval. Beside the well-known QT interval prolonging action of class IA, IC and III anti-arrhythmic drugs, many antibiotics, neurotropic, antifungal, and antimalarial drugs are also able to cause drug-induced LQTS. Reviewing the literature indicates that the risk of QT interval prolongation and TdP is increased in females, in patients with organic heart diseases and hypokalaemia. Furthermore in a few cases, genetic factors have also been reported. However thus far, no genetic test is available to detect at-risk patients, and in consequence, drug prescribers are still relying only on the clinical history and findings to perform an evaluation of the risk. Treatment of drug-induced LQTS and TdP includes identifying and withdrawing the culprit drug(s), infusing magnesium and, in resistant cases acceleration of the heart rate. In this review article we provide a list of QT interval prolonging drugs adapted to the pharmaceuticals found on the Swiss market that can be used as a check-list for drug prescribers and at-risk patients.

Proteasome inhibitor (MG132) rescues Nav1.5 protein content and the cardiac sodium current in dystrophin-deficient mdx (5cv) mice.

The cardiac voltage-gated sodium channel, Nav1.5, plays a central role in cardiac excitability and impulse propagation and associates with the dystrophin multiprotein complex at the lateral membrane of cardiomyocytes. It was previously shown that Nav1.5 protein content and the sodium current (l Na) were both decreased in cardiomyocytes of dystrophin-deficient mdx (5cv) mice. In this study, wild-type and mdx (5cv) mice were treated for 7 days with the proteasome inhibitor MG132 (10 µg/Kg/24 h) using implanted osmotic mini pumps. MG132 rescued both the total amount of Nav1.5 protein and l Na but, unlike in previous studies, de novo expression of dystrophin was not observed in skeletal or cardiac muscle. This study suggests that the reduced expression of Nav1.5 in dystrophin-deficient cells is dependent on proteasomal degradation.

Pharmacologic targeting or genetic deletion of mitochondrial cyclophilin D protects from NSAID-induced small intestinal ulceration in mice.

Small intestinal ulceration is a frequent and potentially serious condition associated with nonselective cyclooxygenase 1/2 inhibitors (nonsteroidal anti-inflammatory drugs, NSAIDs) including diclofenac (DCF). An initial topical effect involving mitochondria has been implicated in the pathogenesis, but the exact mechanisms of NSAID-induced enteropathy are unknown. We aimed at investigating whether DCF caused enterocyte demise via the mitochondrial permeability transition (mPT) and whether inhibition of critical mPT regulators might protect the mucosa from DCF injury. Cultured enterocytes (IEC-6) exposed to DCF readily underwent mPT-mediated cell death. We then targeted mitochondrial cyclophilin D (CypD), a key regulator of the mPT, in a mouse model of NSAID enteropathy. C57BL/6J mice were treated with an ulcerogenic dose of DCF (60 mg/kg, ip), followed (+ 1 h) by a non-cholestatic dose (10 mg/kg, ip) of the CypD inhibitor, cyclosporin A (CsA). CsA greatly reduced the extent of small intestinal ulceration. To avoid potential calcineurin-mediated effects, we used the non-immunosuppressive cyclosporin analog, D-MeAla(3)-EtVal(4)-cyclosporin (Debio 025). Debio 025 similarly protected the mucosa from DCF injury. To exclude drug-drug interactions, we exposed mice genetically deficient in mitochondrial CypD (peptidyl-prolyl cis-trans isomerase F [Ppif(-/-)]) to DCF. Ppif-null mice were largely protected from the ulcerogenic effects of DCF, whereas their wild-type littermates developed typical enteropathy. Enterocyte injury was preceded by upregulation of the proapoptotic transcription factor C/EBP homologous protein (Chop). Chop-null mice were refractory to DCF enteropathy, suggesting a critical role of endoplasmic reticulum stress induced by DCF. In conclusion, mitochondrial CypD plays a key role in NSAID-induced enteropathy, lending itself as a potentially new therapeutic target for cytoprotective intervention.

Variable Na(v)1.5 protein expression from the wild-type allele correlates with the penetrance of cardiac conduction disease in the Scn5a(+/-) mouse model.

BACKGROUND: Loss-of-function mutations in SCN5A, the gene encoding Na(v)1.5 Na+ channel, are associated with inherited cardiac conduction defects and Brugada syndrome, which both exhibit variable phenotypic penetrance of conduction defects. We investigated the mechanisms of this heterogeneity in a mouse model with heterozygous targeted disruption of Scn5a (Scn5a(+/-) mice) and compared our results to those obtained in patients with loss-of-function mutations in SCN5A. METHODOLOGY/PRINCIPAL FINDINGS: Based on ECG, 10-week-old Scn5a(+/-) mice were divided into 2 subgroups, one displaying severe ventricular conduction defects (QRS interval>18 ms) and one a mild phenotype (QRS< or = 18 ms; QRS in wild-type littermates: 10-18 ms). Phenotypic difference persisted with aging. At 10 weeks, the Na+ channel blocker ajmaline prolonged QRS interval similarly in both groups of Scn5a(+/-) mice. In contrast, in old mice (>53 weeks), ajmaline effect was larger in the severely affected subgroup. These data matched the clinical observations on patients with SCN5A loss-of-function mutations with either severe or mild conduction defects. Ventricular tachycardia developed in 5/10 old severely affected Scn5a(+/-) mice but not in mildly affected ones. Correspondingly, symptomatic SCN5A-mutated Brugada patients had more severe conduction defects than asymptomatic patients. Old severely affected Scn5a(+/-) mice but not mildly affected ones showed extensive cardiac fibrosis. Mildly affected Scn5a(+/-) mice had similar Na(v)1.5 mRNA but higher Na(v)1.5 protein expression, and moderately larger I(Na) current than severely affected Scn5a(+/-) mice. As a consequence, action potential upstroke velocity was more decreased in severely affected Scn5a(+/-) mice than in mildly affected ones. CONCLUSIONS: Scn5a(+/-) mice show similar phenotypic heterogeneity as SCN5A-mutated patients. In Scn5a(+/-) mice, phenotype severity correlates with wild-type Na(v)1.5 protein expression.

Aldosterone increases voltage-gated sodium current in ventricular myocytes.

The role of aldosterone in the pathogenesis of heart failure (HF) is still poorly understood. Recently, aldosterone has been shown to modulate the function of cardiac Ca(2+) and K(+) channels, thus playing a role in the electrical remodeling process. The goal of this work was to investigate the role of aldosterone on the cardiac Na(+) current (I(Na)). We analyzed the effects of aldosterone on I(Na) in isolated adult mouse ventricular myocytes, using the whole cell patch-clamp technique. After 24 h incubation with 1 microM aldosterone, the I(Na) density was significantly increased (+55%), without alteration of the biophysical properties and the cell membrane capacitance. Aldosterone (10 nM) increased the I(Na) by 23%. In 24-h coincubation experiments, with the use of actinomycin D, cycloheximide, or brefeldin A, the effect of aldosterone on I(Na) was abolished. Spironolactone (mineralocorticoid receptor antagonist, 10 microM) prevented the 1 microM aldosterone-dependent I(Na) increase, whereas RU-38486 (glucocorticoid receptor antagonist, 10 microM) did not. The action potential duration (APD) was longer in aldosterone-treated (APD(90): +53%) than in control myocytes. In addition, the L-type Ca(2+) current was also upregulated (+48%). We performed quantitative RT-PCR measurements and Western blots to quantify the mRNA and protein levels of Na(v)1.5 and Ca(v)1.2 (main channels mediating cardiac I(Na) and I(Ca)), but no significant difference was found. In conclusion, this study shows that aldosterone upregulates the cardiac I(Na) and suggest that this phenomenon may contribute to the HF-induced electrical remodeling process that may be reversed by spironolactone.

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.

Molecular determinants of voltage-gated sodium channel regulation by the Nedd4/Nedd4-like proteins.

The voltage-gated Na(+) channels (Na(v)) form a family composed of 10 genes. The COOH termini of Na(v) contain a cluster of amino acids that are nearly identical among 7 of the 10 members. This COOH-terminal sequence, PPSYDSV, is a PY motif known to bind to WW domains of E3 protein-ubiquitin ligases of the Nedd4 family. We recently reported that cardiac Na(v)1.5 is regulated by Nedd4-2. In this study, we further investigated the molecular determinants of regulation of Na(v) proteins. When expressed in HEK-293 cells and studied using whole cell voltage clamping, the neuronal Na(v)1.2 and Na(v)1.3 were also downregulated by Nedd4-2. Pull-down experiments using fusion proteins bearing the PY motif of Na(v)1.2, Na(v)1.3, and Na(v)1.5 indicated that mouse brain Nedd4-2 binds to the Na(v) PY motif. Using intrinsic tryptophan fluorescence imaging of WW domains, we found that Na(v)1.5 PY motif binds preferentially to the fourth WW domain of Nedd4-2 with a K(d) of approximately 55 muM. We tested the binding properties and the ability to ubiquitinate and downregulate Na(v)1.5 of three Nedd4-like E3s: Nedd4-1, Nedd4-2, and WWP2. Despite the fact that along with Nedd4-2, Nedd4-1 and WWP2 bind to Na(v)1.5 PY motif, only Nedd4-2 robustly ubiquitinated and downregulated Na(v)1.5. Interestingly, coexpression of WWP2 competed with the effect of Nedd4-2. Finally, using brefeldin A, we found that Nedd4-2 accelerated internalization of Na(v)1.5 stably expressed in HEK-293 cells. This study shows that Nedd4-dependent ubiquitination of Na(v) channels may represent a general mechanism regulating the excitability of neurons and myocytes via modulation of channel density at the plasma membrane.