Antiarrhythmic drug-induced internalization of the atrial-specific k+ channel kv1.5

Unravelling Novel SCN5A Mutations Linked to Brugada Syndrome: Functional, Structural, and Genetic Insights

Brugada Syndrome (BrS) is a rare inherited cardiac arrhythmia causing potentially fatal ventricular tachycardia or fibrillation, mainly occurring during rest or sleep in young individuals without heart structural issues. It increases the risk of sudden cardiac death, and its characteristic feature is an abnormal ST segment elevation on the ECG. While BrS has diverse genetic origins, a subset of cases can be conducted to mutations in the SCN5A gene, which encodes for the Nav1.5 sodium channel. Our study focused on three novel SCN5A mutations (p.A344S, p.N347K, and p.D349N) found in unrelated BrS families. Using patch clamp experiments, we found that these mutations disrupted sodium currents: p.A344S reduced current density, while p.N347K and p.D349N completely abolished it, leading to altered voltage dependence and inactivation kinetics when co-expressed with normal channels. We also explored the effects of mexiletine treatment, which can modulate ion channel function. Interestingly, the p.N347K and p.D349N mutations responded well to the treatment, rescuing the current density, while p.A344S showed a limited response. Structural analysis revealed these mutations were positioned in key regions of the channel, impacting its stability and function. This research deepens our understanding of BrS by uncovering the complex relationship between genetic mutations, ion channel behavior, and potential therapeutic interventions.

Insights of Endocytosis Signaling in Health and Disease

Endocytosis in mammalian cells is a fundamental cellular machinery that regulates vital physiological processes, such as the absorption of metabolites, release of neurotransmitters, uptake of hormone cellular defense, and delivery of biomolecules across the plasma membrane. A remarkable characteristic of the endocytic machinery is the sequential assembly of the complex proteins at the plasma membrane, followed by internalization and fusion of various biomolecules to different cellular compartments. In all eukaryotic cells, functional characterization of endocytic pathways is based on dynamics of the protein complex and signal transduction modules. To coordinate the assembly and functions of the numerous parts of the endocytic machinery, the endocytic proteins interact significantly within and between the modules. Clathrin-dependent and -independent endocytosis, caveolar pathway, and receptor mediated endocytosis have been attributed to a greater variety of physiological and pathophysiological roles such as, autophagy, metabolism, cell division, apoptosis, cellular defense, and intestinal permeabilization. Notably, any defect or alteration in the endocytic machinery results in the development of pathological consequences associated with human diseases such as cancer, cardiovascular diseases, neurological diseases, and inflammatory diseases. In this review, an in-depth endeavor has been made to illustrate the process of endocytosis, and associated mechanisms describing pathological manifestation associated with dysregulated endocytosis machinery.

The fates of internalized NaV1.7 channels in sensory neurons: Retrograde cotransport with other ion channels, axon-specific recycling, and degradation

Neuronal function relies on the maintenance of appropriate levels of various ion channels at the cell membrane, which is accomplished by balancing secretory, degradative, and recycling pathways. Neuronal function further depends on membrane specialization through polarized distribution of specific proteins to distinct neuronal compartments such as axons. Voltage-gated sodium channel NaV1.7, a threshold channel for firing action potentials in nociceptors, plays a major role in human pain, and its abundance in the plasma membrane is tightly regulated. We have recently characterized the anterograde axonal trafficking of NaV1.7 channels in Rab6A-positive vesicles, but the fate of internalized channels is not known. Membrane proteins that have undergone endocytosis can be directed into multiple pathways including those for degradation, recycling to the membrane, and transcytosis. Here, we demonstrate NaV1.7 endocytosis and dynein-dependent retrograde trafficking in Rab7-containing late endosomes together with other axonal membrane proteins using real-time imaging of live neurons. We show that some internalized NaV1.7 channels are delivered to lysosomes within the cell body, and that there is no evidence for NaV1.7 transcytosis. In addition, we show that NaV1.7 is recycled specifically to the axonal membrane as opposed to the soma membrane, suggesting a novel mechanism for the development of neuronal polarity. Together, these results shed light on the mechanisms by which neurons maintain excitable membranes and may inform efforts to target ion channel trafficking for the treatment of disorders of excitability.

Kv1.5 channels are regulated by PKC-mediated endocytic degradation

The voltage-gated potassium channel Kv1.5 plays important roles in the repolarization of atrial action potentials and regulation of the vascular tone. While the modulation of Kv1.5 function has been well studied, less is known about how the protein levels of Kv1.5 on the cell membrane are regulated. Here, through electrophysiological and biochemical analyses of Kv1.5 channels heterologously expressed in HEK293 cells and neonatal rat ventricular myocytes, as well as native Kv1.5 in human induced pluripotent stem cell (iPSC)-derived atrial cardiomyocytes, we found that activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate (PMA, 10 nM) diminished Kv1.5 current (I_Kv1.5) and protein levels of Kv1.5 in the plasma membrane. Mechanistically, PKC activation led to monoubiquitination and degradation of the mature Kv1.5 proteins. Overexpression of Vps24, a protein that sorts transmembrane proteins into lysosomes via the multivesicular body (MVB) pathway, accelerated, whereas the lysosome inhibitor bafilomycin A1 completely prevented PKC-mediated Kv1.5 degradation. Kv1.5, but not Kv1.1, Kv1.2, Kv1.3, or Kv1.4, was uniquely sensitive to PMA treatment. Sequence alignments suggested that residues within the N terminus of Kv1.5 are essential for PKC-mediated Kv1.5 reduction. Using N-terminal truncation as well as site-directed mutagenesis, we identified that Thr15 is the target site for PKC that mediates endocytic degradation of Kv1.5 channels. These findings indicate that alteration of protein levels in the plasma membrane represents an important regulatory mechanism of Kv1.5 channel function under PKC activation conditions.

KV1.5-KVβ1.3 Recycling Is PKC-Dependent

K_V1.5 channel function is modified by different regulatory subunits. K_Vβ1.3 subunits assemble with K_V1.5 channels and induce a fast and incomplete inactivation. Inhibition of PKC abolishes the K_Vβ1.3-induced fast inactivation, decreases the amplitude of the current K_V1.5–K_Vβ1.3 and modifies their pharmacology likely due to changes in the traffic of K_V1.5–K_Vβ1.3 channels in a PKC-dependent manner. In order to analyze this hypothesis, HEK293 cells were transfected with K_V1.5–K_Vβ1.3 channels, and currents were recorded by whole-cell configuration of the patch-clamp technique. The presence of K_V1.5 in the membrane was analyzed by biotinylation techniques, live cell imaging and confocal microscopy approaches. PKC inhibition resulted in a decrease of 33 ± 7% of channels in the cell surface due to reduced recycling to the plasma membrane, as was confirmed by confocal microscopy. Live cell imaging indicated that PKC inhibition almost abolished the recycling of the K_V1.5–K_Vβ1.3 channels, generating an accumulation of channels into the cytoplasm. All these results suggest that the trafficking regulation of K_V1.5–K_Vβ1.3 channels is dependent on phosphorylation by PKC and, therefore, they could represent a clinically relevant issue, mainly in those diseases that exhibit modifications in PKC activity.

Endocytosis: A Turnover Mechanism Controlling Ion Channel Function

Ion channels (IChs) are transmembrane proteins that selectively drive ions across membranes. The function of IChs partially relies on their abundance and proper location in the cell, fine-tuned by the delicate balance between secretory, endocytic, and degradative pathways. The disruption of this balance is associated with several diseases, such as Liddle's and long QT syndromes. Because of the vital role of these proteins in human health and disease, knowledge of ICh turnover is essential. Clathrin-dependent and -independent mechanisms have been the primary mechanisms identified with ICh endocytosis and degradation. Several molecular determinants recognized by the cellular internalization machinery have been discovered. Moreover, specific conditions can trigger the endocytosis of many IChs, such as the activation of certain receptors, hypokalemia, and some drugs. Ligand-dependent receptor activation primarily results in the posttranslational modification of IChs and the recruitment of important mediators, such as β-arrestins and ubiquitin ligases. However, endocytosis is not a final fate. Once internalized into endosomes, IChs are either sorted to lysosomes for degradation or recycled back to the plasma membrane. Rab proteins are crucial participants during these turnover steps. In this review, we describe the major ICh endocytic pathways, the signaling inputs triggering ICh internalization, and the key mediators of this essential cellular process.

Two New Neo-debromoaplysiatoxins-A Pair of Stereoisomers Exhibiting Potent Kv1.5 Ion Channel Inhibition Activities

A pair of stereoisomers possessing novel structures with 6/6/5 fused-ring systems, neo-debromoaplysiatoxin E (1) and neo-debromoaplysiatoxin F (2), were isolated from the marine cyanobacterium Lyngbya sp. Their structures were elucidated using various spectroscopic techniques including high resolution electrospray ionization mass spectroscopy (HRESIMS) and nuclear magnetic resonance (NMR). The absolute stereochemistry was determined by calculated electronic circular dichroism (ECD) and gauge-independent atomic orbital (GIAO) NMR shift calculation followed by DP4+ analysis. Significantly, this is the first report on aplysiatoxin derivatives with different absolute configurations at C9-C12 (1: 9S, 10R, 11S, 12S; 2: 9R, 10S, 11R, 12R). Compounds 1 and 2 exhibited potent blocking activities against Kv1.5 with IC50 values of 1.22 ± 0.22 μM and 2.85 ± 0.29 μM, respectively.

Long-term 4-AP treatment facilitates functional expression of human Kv1.5 channel

The human Kv1.5 channel (hKv1.5) produces the ultrarapid delayed rectifier potassium current (I_Kur), which is important for determining the repolarization of action potential in the cardiac atrium. However, the expression of I_Kur is reduced in patients with chronic atrial fibrillation. 4-Aminopyridine (4-AP) can specifically suppress I_Kur, suggesting that it modifies hKv1.5 as a chaperone molecule. Herein, the effects of long-term 4-AP treatment on hKv1.5 protein expression and function were investigated in HEK cells. 4-AP treatment (24 h) improved hKv1.5 protein levels, promoted hKv1.5 glycosylation, and facilitated the hKv1.5 current in a time-dependent manner. Long-term 4-AP treatment also markedly enhanced hKv1.5 localization in the cell membrane, endoplasmic reticulum, and Golgi. Importantly, the Ile508 residue located in the hKv1.5 channel pore was found to be important for 4-AP inhibitory activity. These results provide insight into developing hKv1.5 channel blocker that can functionally rescue I_Kur in patients with chronic atrial fibrillation.

The Pattern of mRNA Expression Is Changed in Sinoatrial Node from Goto-Kakizaki Type 2 Diabetic Rat Heart

BACKGROUND

In vivo experiments in Goto-Kakizaki (GK) type 2 diabetic rats have demonstrated reductions in heart rate from a young age. The expression of genes encoding more than 70 proteins that are associated with the generation and conduction of electrical activity in the GK sinoatrial node (SAN) have been evaluated to further clarify the molecular basis of the low heart rate.

MATERIALS AND METHODS

Heart rate and expression of genes were evaluated with an extracellular electrode and real-time RT-PCR, respectively. Rats aged 12-13 months were employed in these experiments.

RESULTS

Isolated spontaneous heart rate was reduced in GK heart (161 ± 12 bpm) compared to controls (229 ± 11 bpm). There were many differences in expression of mRNA, and some of these differences were of particular interest. Compared to control SAN, expression of some genes were downregulated in GK-SAN: gap junction, Gja1 (Cx43), Gja5 (Cx40), Gjc1 (Cx45), and Gjd3 (Cx31.9); cell membrane transport, Trpc1 (TRPC1) and Trpc6 (TRPC6); hyperpolarization-activated cyclic nucleotide-gated channels, Hcn1 (HCN1) and Hcn4 (HCN4); calcium channels, Cacna1d (Ca_v1.3), Cacna1g (Ca_v3.1), Cacna1h (Ca_v3.2), Cacna2d1 (Ca_vα2δ1), Cacna2d3 (Ca_vα2δ3), and Cacng4 (Cav _γ 4); and potassium channels, Kcna2 (K_v1.2), Kcna4 (K_v1.4), Kcna5 (K_v1.5), Kcnb1 (K_v2.1), Kcnd3 (K_v4.3), Kcnj2 (K_ir2.1), Kcnk1 (TWIK1), Kcnk5 (K_2P5.1), Kcnk6 (TWIK2), and Kcnn2 (SK2) whilst others were upregulated in GK-SAN: Ryr2 (RYR2) and Nppb (BNP).

CONCLUSIONS

This study provides new insight into the changing expression of genes in the sinoatrial node of diabetic heart.

Ion Channel Trafficking: Control of Ion Channel Density as a Target for Arrhythmias?

The shape of the cardiac action potential (AP) is determined by the contributions of numerous ion channels. Any dysfunction in the proper function or expression of these ion channels can result in a change in effective refractory period (ERP) and lead to arrhythmia. The processes underlying the correct targeting of ion channels to the plasma membrane are complex, and have not been fully characterized in cardiac myocytes. Emerging evidence highlights ion channel trafficking as a potential causative factor in certain acquired and inherited arrhythmias, and therapies which target trafficking as opposed to pore block are starting to receive attention. In this review we present the current evidence for the mechanisms which underlie precise control of cardiac ion channel trafficking and targeting.

Over-expression of microRNA-1 causes arrhythmia by disturbing intracellular trafficking system

Dysregulation of intracellular trafficking system plays a fundamental role in the progression of cardiovascular disease. Up-regulation of miR-1 contributes to arrhythmia, we sought to elucidate whether intracellular trafficking contributes to miR-1-driven arrhythmia. By performing microarray analyses of the transcriptome in the cardiomyocytes-specific over-expression of microRNA-1 (miR-1 Tg) mice and the WT mice, we found that these differentially expressed genes in miR-1 Tg mice were significantly enrichment with the trafficking-related biological processes, such as regulation of calcium ion transport. Also, the qRT-PCR and western blot results validated that Stx6, Braf, Ube3a, Mapk8ip3, Ap1s1, Ccz1 and Gja1, which are the trafficking-related genes, were significantly down-regulated in the miR-1 Tg mice. Moreover, we found that Stx6 was decreased in the heart of mice after myocardial infarction and in the hypoxic cardiomyocytes, and further confirmed that Stx6 is a target of miR-1. Meanwhile, knockdown of Stx6 in cardiomyocytes resulted in the impairments of PLM and L-type calcium channel, which leads to the increased resting ([Ca²⁺]_i). On the contrary, overexpression of Stx6 attenuated the impairments of miR-1 or hypoxia on PLM and L-type calcium channel. Thus, our studies reveals that trafficking-related gene Stx6 may regulate intracellular calcium and is involved in the occurrence of cardiac arrhythmia, which provides new insights in that miR-1 participates in arrhythmia by regulating the trafficking-related genes and pathway.

Genistein and tyrphostin AG556 decrease ultra-rapidly activating delayed rectifier K+ current of human atria by inhibiting EGF receptor tyrosine kinase

BACKGROUND AND PURPOSE

The ultra-rapidly activating delayed rectifier K⁺ current I_Kur (encoded by K_v 1.5 or KCNA5) plays an important role in human atrial repolarization. The present study investigates the regulation of this current by protein tyrosine kinases (PTKs).

EXPERIMENTAL APPROACH

Whole-cell patch voltage clamp technique and immunoprecipitation and Western blotting analysis were used to investigate whether the PTK inhibitors genistein, tyrphostin AG556 (AG556) and PP2 regulate human atrial I_Kur and hKv1.5 channels stably expressed in HEK 293 cells.

KEY RESULTS

Human atrial I_Kur was decreased by genistein (a broad-spectrum PTK inhibitor) and AG556 (a highly selective EGFR TK inhibitor) in a concentration-dependent manner. Inhibition of I_Kur induced by 30 μM genistein or 10 μM AG556 was significantly reversed by 1 mM orthovanadate (a protein tyrosine phosphatase inhibitor). Similar results were observed in HEK 293 cells stably expressing hK_v 1.5 channels. On the other hand, the Src family kinase inhibitor PP2 (1 μM) slightly enhanced I_Kur and hK_v 1.5 current, and the current increase was also reversed by orthovanadate. Immunoprecipitation and Western blotting analysis showed that genistein, AG556, and PP2 decreased tyrosine phosphorylation of hK_v 1.5 channels and that the decrease was countered by orthovanadate.

CONCLUSION AND IMPLICATIONS

The PTK inhibitors genistein and AG556 decrease human atrial I_Kur and cloned hK_v 1.5 channels by inhibiting EGFR TK, whereas the Src kinase inhibitor PP2 increases I_Kur and hK_v 1.5 current. These results imply that EGFR TK and the soluble Src kinases may have opposite effects on human atrial I_Kur .

Blockade of Kv1.5 channels by the antidepressant drug sertraline

Sertraline, a selective serotonin reuptake inhibitor (SSRI), has been reported to lead to cardiac toxicity even at therapeutic doses including sudden cardiac death and ventricular arrhythmia. And in a SSRI-independent manner, sertraline has been known to inhibit various voltage-dependent channels, which play an important role in regulation of cardiovascular system. In the present study, we investigated the action of sertraline on Kv1.5, which is one of cardiac ion channels. The eff ect of sertraline on the cloned neuronal rat Kv1.5 channels stably expressed in Chinese hamster ovary cells was investigated using the whole-cell patch-clamp technique. Sertraline reduced Kv1.5 whole-cell currents in a reversible concentration-dependent manner, with an IC 50 value and a Hill coefficient of 0.71 µM and 1.29, respectively. Sertraline accelerated the decay rate of inactivation of Kv1.5 currents without modifying the kinetics of current activation. The inhibition increased steeply between -20 and 0 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to +10 mV, inhibition displayed a weak voltage dependence, consistent with an electrical distance δ of 0.16. Sertraline slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of sertraline, were superimposed. Inhibition of Kv1.5 by sertraline was use-dependent. The present results suggest that sertraline acts on Kv1.5 currents as an open-channel blocker.

E3 ligase CHIP and Hsc70 regulate Kv1.5 protein expression and function in mammalian cells

Kv1.5 confers ultra-rapid delayed-rectifier potassium channel current (IKur) which contributes to repolarization of the atrial action potential. Kv1.5 proteins, degraded via the ubiquitin-proteasome pathway, decreased in some atrial fibrillation patients. Carboxyl-terminus heat shock cognate 70-interacting protein (CHIP), an E3 ubiquitin ligase, is known to ubiquitinate short-lived proteins. Here, we investigated the roles of CHIP in Kv1.5 degradation to provide insights into the mechanisms of Kv1.5 decreases and treatments targeting Kv1.5 for atrial fibrillation. Coexpression of CHIP with Kv1.5 in HEK293 cells increased Kv1.5 protein ubiquitination and decreased the protein level. Immunofluorescence revealed decreases of Kv1.5 proteins in the endoplasmic reticulum and on the cell membrane. A siRNA against CHIP suppressed Kv1.5 protein ubiquitination and increased its protein level. CHIP mutants, lacking either the N-terminal tetratricopeptide region domain or the C-terminal U-box domain, failed to exert these effects on Kv1.5 proteins. Immunoprecipitation showed that CHIP formed complexes with Kv1.5 proteins and heat shock cognate protein 70 (Hsc70). Effects of Hsc70 on Kv1.5 were similar to CHIP by altering interaction of CHIP with Kv1.5 protein. Coexpression of CHIP and Hsc70 with Kv1.5 additionally enhanced Kv1.5 ubiquitination. Kv1.5 currents were decreased by overexpression of CHIP or Hsc70 but were increased by knockdown of CHIP or Hsc70 in HEK 293 cells stably expressing Kv1.5. These effects of CHIP and Hsc70 were also observed on endogenous Kv1.5 in HL-1 mouse cardiomyocytes, decreasing IKur and prolonging action potential duration. These results indicate that CHIP decreases the Kv1.5 protein level and functional channel by facilitating its degradation in concert with chaperone Hsc70.

Polycomb-dependent repression of the potassium channel-encoding gene KCNA5 promotes cancer cell survival under conditions of stress

Relapse after clinical remission remains a leading cause of cancer-associated death. Although the mechanisms of tumor relapse are complex, the ability of cancer cells to survive physiological stress is a prerequisite for recurrence. Ewing sarcoma (ES) and neuroblastoma (NB) are aggressive cancers that frequently relapse after initial remission. In addition, both tumors overexpress the polycomb group (PcG) proteins BMI-1 and EZH2, which contribute to tumorigenicity. We have discovered that ES and NB resist hypoxic stress-induced death and that survival depends on PcG function. Epigenetic repression of developmental programs is the most well-established cancer-associated function of PcG proteins. However, we noted that voltage-gated potassium (Kv) channel genes are also targets of PcG regulation in stem cells. Given the role of potassium in regulating apoptosis, we reasoned that repression of Kv channel genes might have a role in cancer cell survival. Here we describe our novel finding that PcG-dependent repression of the Kv1.5 channel gene KCNA5 contributes to cancer cell survival under conditions of stress. We show that survival of cancer cells in stress is dependent upon suppression of Kv1.5 channel function. The KCNA5 promoter is marked in cancer cells with PcG-dependent chromatin repressive modifications that increase in hypoxia. Genetic and pharmacological inhibition of BMI-1 and EZH2, respectively, restore KCNA5 expression, which sensitizes cells to stress-induced death. In addition, ectopic expression of the Kv1.5 channel induces apoptotic cell death under conditions of hypoxia. These findings identify a novel role for PcG proteins in promoting cancer cell survival via repression of KCNA5.

Improved functional expression of human cardiac kv1.5 channels and trafficking-defective mutants by low temperature treatment

We herein investigated the effect of low temperature exposure on the expression, degradation, localization and activity of human Kv1.5 (hKv1.5). In hKv1.5-expressing CHO cells, the currents were significantly increased when cultured at a reduced temperature (28°C) compared to those observed at 37°C. Western blot analysis indicated that the protein levels (both immature and mature proteins) of hKv1.5 were significantly elevated under the hypothermic condition. Treatment with a proteasome inhibitor, MG132, significantly increased the immature, but not the mature, hKv1.5 protein at 37°C, however, there were no changes in either the immature or mature hKv1.5 proteins at low temperature following MG132 exposure. These observations suggest that the enhancement of the mature hKv1.5 protein at reduced temperature may not result from the inhibition of proteolysis. Moreover, the hKv1.5 fluorescence signal in the cells increased significantly on the cell surface at 28°C versus those cultured at 37°C. Importantly, the low temperature treatment markedly shifted the subcellular distribution of the mature hKv1.5, which showed considerable overlap with the trans-Golgi component. Experiments using tunicamycin, an inhibitor of N-glycosylation, indicated that the N-glycosylation of hKv1.5 is more effective at 28°C than at 37°C. Finally, the hypothermic treatment also rescued the protein expression and currents of trafficking-defective hKv1.5 mutants. These results indicate that low temperature exposure stabilizes the protein in the cellular organelles or on the plasma membrane, and modulates its maturation and trafficking, thus enhancing the currents of hKv1.5 and its trafficking defect mutants.

Role for myosin-V motor proteins in the selective delivery of Kv channel isoforms to the membrane surface of cardiac myocytes

RATIONALE

Kv1.5 (KCNA5) mediates the ultra-rapid delayed rectifier current that controls atrial action potential duration. Given its atrial-specific expression and alterations in human atrial fibrillation, Kv1.5 has emerged as a promising target for the treatment of atrial fibrillation. A necessary step in the development of novel agents that selectively modulate trafficking pathways is the identification of the cellular machinery controlling Kv1.5 surface density, of which little is yet known.

OBJECTIVE

To investigate the role of the unconventional myosin-V (MYO5A and MYO5B) motors in determining the cell surface density of Kv1.5.

METHODS AND RESULTS

Western blot analysis showed MYO5A and MYO5B expression in the heart, whereas disruption of endogenous motors selectively reduced IKur current in adult rat cardiomyocytes. Dominant negative constructs and short hairpin RNA silencing demonstrated a role for MYO5A and MYO5B in the surface trafficking of Kv1.5 and connexin-43 but not potassium voltage-gated channel, subfamily H (eag-related), member 2 (KCNH2). Live-cell imaging of Kv1.5-GFP and retrospective labeling of phalloidin demonstrated motility of Kv1.5 vesicles on actin tracts. MYO5A participated in anterograde trafficking, whereas MYO5B regulated postendocytic recycling. Overexpression of mutant motors revealed a selective role for Rab11 in coupling MYO5B to Kv1.5 recycling.

CONCLUSIONS

MYO5A and MYO5B control functionally distinct steps in the surface trafficking of Kv1.5. These isoform-specific trafficking pathways determine Kv1.5-encoded IKur in myocytes to regulate repolarizing current and, consequently, cardiac excitability. Therapeutic strategies that manipulate Kv1.5 selective trafficking pathways may prove useful in the treatment of arrhythmias.

Shear stress triggers insertion of voltage-gated potassium channels from intracellular compartments in atrial myocytes

Atrial myocytes are continuously exposed to mechanical forces including shear stress. However, in atrial myocytes, the effects of shear stress are poorly understood, particularly with respect to its effect on ion channel function. Here, we report that shear stress activated a large outward current from rat atrial myocytes, with a parallel decrease in action potential duration. The main ion channel underlying the increase in current was found to be Kv1.5, the recruitment of which could be directly observed by total internal reflection fluorescence microscopy, in response to shear stress. The effect was primarily attributable to recruitment of intracellular pools of Kv1.5 to the sarcolemma, as the response was prevented by the SNARE protein inhibitor N-ethylmaleimide and the calcium chelator BAPTA. The process required integrin signaling through focal adhesion kinase and relied on an intact microtubule system. Furthermore, in a rat model of chronic hemodynamic overload, myocytes showed an increase in basal current despite a decrease in Kv1.5 protein expression, with a reduced response to shear stress. Additionally, integrin beta1d expression and focal adhesion kinase activation were increased in this model. This data suggests that, under conditions of chronically increased mechanical stress, the integrin signaling pathway is overactivated, leading to increased functional Kv1.5 at the membrane and reducing the capacity of cells to further respond to mechanical challenge. Thus, pools of Kv1.5 may comprise an inducible reservoir that can facilitate the repolarization of the atrium under conditions of excessive mechanical stress.

Interaction of local anesthetics with the K (+) channel pore domain: KcsA as a model for drug-dependent tetramer stability

Local anesthetics and related drugs block ionic currents of Na (+) , K (+) and Ca ( 2+) conducted across the cell membrane by voltage-dependent ion channels. Many of these drugs bind in the permeation pathway, occlude the pore and stop ion movement. However channel-blocking drugs have also been associated with decreased membrane stability of certain tetrameric K (+) channels, similar to the destabilization of channel function observed at low extracellular K (+) concentration. Such drug-dependent stability may result from electrostatic repulsion of K (+) from the selectivity filter by a cationic drug molecule bound in the central cavity of the channel. In this study we used the pore domain of the KcsA K (+) channel protein to test this hypothesis experimentally with a biochemical assay of tetramer stability and theoretically by computational simulation of local anesthetic docking to the central cavity. We find that two common local anesthetics, lidocaine and tetracaine, promote thermal dissociation of the KcsA tetramer in a K (+) -dependent fashion. Docking simulations of these drugs with open, open-inactivated and closed crystal structures of KcsA yield many energetically favorable drug-channel complexes characterized by nonbonded attraction to pore-lining residues and electrostatic repulsion of K (+) . The results suggest that binding of cationic drugs to the inner cavity can reduce tetramer stability of K (+) channels.

Involvement of Kv1.5 protein in oxidative vascular endothelial cell injury

Endothelial injury related to oxidative stress is a key event in cardiovascular diseases, such as hypertension and atherosclerosis. The activation of the redox-sensitive Kv1.5 potassium channel mediates mitochondrial reactive oxygen species (ROS)-induced apoptosis in vascular smooth muscle cells and some cancer cells. Kv1.5 channel is therefore taken as a new potential therapeutic target for pulmonary hypertension and cancers. Although Kv1.5 is abundantly expressed in vascular endothelium, there is little knowledge of its role in endothelial injury related to oxidative stress. We found that DPO-1, a specific inhibitor of Kv1.5, attenuated H₂O₂-evoked endothelial cell apoptosis in an in vivo rat carotid arterial model. In human umbilical vein endothelial cells (HUVECs) and human pulmonary arterial endothelial cells (HPAECs), angiotensin II and oxLDL time- or concentration-dependently enhanced Kv1.5 protein expression in parallel with the production of intracellular ROS and endothelial cell injury. Moreover, siRNA-mediated knockdown of Kv1.5 attenuated, whereas adenovirus-mediated Kv1.5 cDNA overexpression enhanced oxLDL–induced cellular damage, NADPH oxidase and mitochondria-derived ROS production and restored the decrease in protein expression of mitochondria uncoupling protein 2 (UCP2). Collectively, these data suggest that Kv1.5 may play an important role in oxidative vascular endothelial injury.

Endocytic regulation of voltage-dependent potassium channels in the heart

Understanding the regulation of cardiac ion channels is critical for the prevention of arrhythmia caused by abnormal excitability. Ion channels can be regulated by a change in function (qualitative) and a change in number (quantitative). Functional changes have been extensively investigated for many ion channels including cardiac voltage-dependent potassium channels. By contrast, the regulation of ion channel numbers has not been widely examined, particularly with respect to acute modulation of ion channels. This article briefly summarizes stimulus-induced endocytic regulation of major voltage-dependent potassium channels in the heart. The stimuli known to cause their endocytosis include receptor activation, drugs, and low extracellular [K(+)], following which the potassium channels undergo either clathrin-mediated or caveolin-mediated endocytosis. Receptor-mediated endocytic regulation has been demonstrated for Kv1.2, Kv1.5, KCNQ1 (Kv7.1), and Kv4.3, while drug-induced endocytosis has been demonstrated for Kv1.5 and hERG. Low [K(+)](o)-induced endocytosis might be unique for hERG channels, whose electrophysiological characteristics are known to be under strong influence of [K(+)](o). Although the precise mechanisms have not been elucidated, it is obvious that major cardiac voltage-dependent potassium channels are modulated by endocytosis, which leads to changes in cardiac excitability.

Downregulation of Kv1.5 K channels by the AMP-activated protein kinase

BACKGROUND

The voltage gated K(+) channel Kv1.5 participates in the repolarization of a wide variety of cell types. Kv1.5 is downregulated during hypoxia, which is known to stimulate the energy-sensing AMP-activated serine/threonine protein kinase (AMPK). AMPK is a powerful regulator of nutrient transport and metabolism. Moreover, AMPK is known to downregulate several ion channels, an effect at least in part due to stimulation of the ubiquitin ligase Nedd4- 2. The present study explored whether AMPK regulates Kv1.5.

METHODS

cRNA encoding Kv1.5 was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (α1 β 1γ1), of constitutively active (γR70Q)AMPK (α1 β 1γ1(R70Q)), of inactive mutant (αK45R)AMPK (α1(K45R)β1γ1), or of Nedd4-2. Kv1.5 activity was determined by two-electrode voltage-clamp. Moreover, Kv1.5 protein abundance in the cell membrane was determined by chemiluminescence and immunostaining with subsequent confocal microscopy.

RESULTS

Coexpression of wild-type AMPK(WT) and constitutively active AMPK(γR70Q), but not of inactive AMPK(αK45R) significantly reduced Kv1.5-mediated currents. Coexpression of constitutively active AMPKγR70Q further reduced Kv1.5 K(+) channel protein abundance in the cell membrane. Co-expression of Nedd4-2 similarly downregulated Kv1.5-mediated currents.

CONCLUSION

AMPK is a potent regulator of Kv1.5. AMPK inhibits Kv1.5 presumably in part by activation of Nedd4- 2 with subsequent clearance of channel protein from the cell membrane.

Trafficking of intermediate (KCa3.1) and small (KCa2.x) conductance, Ca(2+)-activated K(+) channels: a novel target for medicinal chemistry efforts?

Ca(2+)-activated K(+) (KCa) channels play a pivotal role in the physiology of a wide variety of tissues and disease states, including vascular endothelia, secretory epithelia, certain cancers, red blood cells (RBC), neurons, and immune cells. Such widespread involvement has generated an intense interest in elucidating the function and regulation of these channels, with the goal of developing pharmacological strategies aimed at selective modulation of KCa channels in various disease states. Herein we give an overview of the molecular and functional properties of these channels and their therapeutic importance. We discuss the achievements made in designing pharmacological tools that control the function of KCa channels by modulating their gating properties. Moreover, this review discusses the recent advances in our understanding of KCa channel assembly and anterograde trafficking toward the plasma membrane, the micro-domains in which these channels are expressed within the cell, and finally the retrograde trafficking routes these channels take following endocytosis. As the regulation of intracellular trafficking by agonists as well as the protein-protein interactions that modify these events continue to be explored, we anticipate this will open new therapeutic avenues for the targeting of these channels based on the pharmacological modulation of KCa channel density at the plasma membrane.

Redox-sensitive sulfenic acid modification regulates surface expression of the cardiovascular voltage-gated potassium channel Kv1.5

RATIONALE

Kv1.5 (KCNA5) is expressed in the heart, where it underlies the I(Kur) current that controls atrial repolarization, and in the pulmonary vasculature, where it regulates vessel contractility in response to changes in oxygen tension. Atrial fibrillation and hypoxic pulmonary hypertension are characterized by downregulation of Kv1.5 protein expression, as well as with oxidative stress. Formation of sulfenic acid on cysteine residues of proteins is an important, dynamic mechanism for protein regulation under oxidative stress. Kv1.5 is widely reported to be redox-sensitive, and the channel possesses 6 potentially redox-sensitive intracellular cysteines. We therefore hypothesized that sulfenic acid modification of the channel itself may regulate Kv1.5 in response to oxidative stress.

OBJECTIVE

To investigate how oxidative stress, via redox-sensitive modification of the channel with sulfenic acid, regulates trafficking and expression of Kv1.5.

METHODS AND RESULTS

Labeling studies with the sulfenic acid-specific probe DAz and horseradish peroxidase-streptavidin Western blotting demonstrated a global increase in sulfenic acid-modified proteins in human patients with atrial fibrillation, as well as sulfenic acid modification to Kv1.5 in the heart. Further studies showed that Kv1.5 is modified with sulfenic acid on a single COOH-terminal cysteine (C581), and the level of sulfenic acid increases in response to oxidant exposure. Using live-cell immunofluorescence and whole-cell voltage-clamping, we found that modification of this cysteine is necessary and sufficient to reduce channel surface expression, promote its internalization, and block channel recycling back to the cell surface. Moreover, Western blotting demonstrated that sulfenic acid modification is a trigger for channel degradation under prolonged oxidative stress.

CONCLUSIONS

Sulfenic acid modification to proteins, which is elevated in diseased human heart, regulates Kv1.5 channel surface expression and stability under oxidative stress and diverts channel from a recycling pathway to degradation. This provides a molecular mechanism linking oxidative stress and downregulation of channel expression observed in cardiovascular diseases.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

Dynamic reciprocity of sodium and potassium channel expression in a macromolecular complex controls cardiac excitability and arrhythmia

The cardiac electrical impulse depends on an orchestrated interplay of transmembrane ionic currents in myocardial cells. Two critical ionic current mechanisms are the inwardly rectifying potassium current (I(K1)), which is important for maintenance of the cell resting membrane potential, and the sodium current (I(Na)), which provides a rapid depolarizing current during the upstroke of the action potential. By controlling the resting membrane potential, I(K1) modifies sodium channel availability and therefore, cell excitability, action potential duration, and velocity of impulse propagation. Additionally, I(K1)-I(Na) interactions are key determinants of electrical rotor frequency responsible for abnormal, often lethal, cardiac reentrant activity. Here, we have used a multidisciplinary approach based on molecular and biochemical techniques, acute gene transfer or silencing, and electrophysiology to show that I(K1)-I(Na) interactions involve a reciprocal modulation of expression of their respective channel proteins (Kir2.1 and Na(V)1.5) within a macromolecular complex. Thus, an increase in functional expression of one channel reciprocally modulates the other to enhance cardiac excitability. The modulation is model-independent; it is demonstrable in myocytes isolated from mouse and rat hearts and with transgenic and adenoviral-mediated overexpression/silencing. We also show that the post synaptic density, discs large, and zonula occludens-1 (PDZ) domain protein SAP97 is a component of this macromolecular complex. We show that the interplay between Na(v)1.5 and Kir2.1 has electrophysiological consequences on the myocardium and that SAP97 may affect the integrity of this complex or the nature of Na(v)1.5-Kir2.1 interactions. The reciprocal modulation between Na(v)1.5 and Kir2.1 and the respective ionic currents should be important in the ability of the heart to undergo self-sustaining cardiac rhythm disturbances.

Novel molecular targets for atrial fibrillation therapy

Atrial fibrillation is the most common type of cardiac arrhythmia, and is responsible for substantial morbidity and mortality in the general population. Current treatments have moderate efficacy and considerable risks, especially of pro-arrhythmia, highlighting the need for new therapeutic strategies. In recent years, substantial efforts have been invested in developing novel treatments that target the underlying molecular determinants of atrial fibrillation, and several new compounds are under development. This Review focuses on the mechanistic rationale for the development of new anti-atrial fibrillation drugs, on the molecular and structural motifs that they target and on the results obtained so far in experimental and clinical studies.

Delayed endosome-dependent CamKII and p38 kinase signaling in cardiomyocytes destabilizes Kv4.3 mRNA

The Kv4.3 transient outward current (I(to)) channel, which produces early repolarization in human cardiomyocytes, is downregulated with cardiac pathology. This is evident in cultured neonatal rat cardiomyocytes in which Angiotensin II (Ang II) acts via p38 mitogen-activated protein kinase (p38K) to increase apoptosis and induce Kv4.3 mRNA destabilization to downregulate the channel protein. However, it is not understood how p38K activation, which is activated transiently for minutes, induces downstream effects hours later. Here we show that there is a second phase of p38K activation. Inhibiting this delayed p38K activation eliminated Kv4.3 mRNA destabilization. Furthermore, inhibiting endosome generation left the transient activation of p38K intact, but blocked delayed p38K activation and the Kv4.3 effect. CamKII was also found to be required for delayed p38K activation and Kv4.3 mRNA destabilization. Finally, CamKII methionine oxidation and activation are biphasic, with the delayed phase requiring endosomes. Hence, in addition to participating in channel traffic, cardiomyocyte endosomes control channel mRNA expression by mediating delayed oxidative CamKII-p38K signaling.

Activation of voltage gated K⁺ channel Kv1.5 by β-catenin

Voltage-gated Kv1.5 channels are expressed in a wide variety of tissues including cardiac myocytes, smooth muscle and tumor cells. Kv1.5 channel activity is modified by N-cadherin, which in turn binds the multifunctional oncogenic protein β-catenin. The present experiments explored the effect of β-catenin on Kv1.5 channel activity. To this end, Kv1.5 was expressed in Xenopus oocytes with or without β-catenin and the voltage-gated Kv current determined by dual electrode voltage clamp. As a result, expression of β-catenin significantly increased the voltage-gated Kv current at positive potentials. The stimulating effect of β-catenin on Kv1.5 was not dependent on the stimulation of transcription since it was observed even in the presence of the transcription inhibitor actinomycin D. Specific antibody binding to surface Kv1.5 in Xenopus oocytes revealed that β-catenin enhances the membrane abundance of Kv1.5. Further experiments with brefeldin A showed that β-catenin fosters the insertion of Kv1.5 into rather than delaying the retrieval from the plasma membrane. According to electrophysiological recordings with mutant β-catenin, the effect on Kv1.5 requires the same protein domains that are required for association of β-catenin with cadherin. The experiments disclose a completely novel function of β-catenin, i.e. the regulation of Kv1.5 channel activity.

Novel pharmacological targets for the rhythm control management of atrial fibrillation

Atrial fibrillation (AF) is a growing clinical problem associated with increased morbidity and mortality. Development of safe and effective pharmacological treatments for AF is one of the greatest unmet medical needs facing our society. In spite of significant progress in non-pharmacological AF treatments (largely due to the use of catheter ablation techniques), anti-arrhythmic agents (AADs) remain first line therapy for rhythm control management of AF for most AF patients. When considering efficacy, safety and tolerability, currently available AADs for rhythm control of AF are less than optimal. Ion channel inhibition remains the principal strategy for termination of AF and prevention of its recurrence. Practical clinical experience indicates that multi-ion channel blockers are generally more optimal for rhythm control of AF compared to ion channel-selective blockers. Recent studies suggest that atrial-selective sodium channel block can lead to safe and effective suppression of AF and that concurrent inhibition of potassium ion channels may potentiate this effect. An important limitation of the ion channel block approach for AF treatment is that non-electrical factors (largely structural remodeling) may importantly determine the generation of AF, so that "upstream therapy", aimed at preventing or reversing structural remodeling, may be required for effective rhythm control management. This review focuses on novel pharmacological targets for the rhythm control management of AF.

Subunit-dependent axonal trafficking of distinct alpha heteromeric potassium channel complexes

Voltage-gated potassium (Kv) channels are critical for neuronal excitability and are targeted to specific subcellular compartments to carry out their unique functions. While it is widely believed that Kv channels exist as heteromeric complexes in neurons, direct tests of the hypothesis that specific heteromeric channel populations display divergent spatial and temporal dynamics are limited. Using a bimolecular fluorescence complementation approach, we monitored the assembly and localization of cell surface channel complexes in living cells. While PSD95-mediated clustering was subunit independent, selective visualization of heteromeric Kv complexes in rat hippocampal neurons revealed subunit-dependent localization that was not predicted by analyzing individual subunits. Assembly of Kv1.1 with Kv1.4 prevented axonal localization but not surface expression, while inclusion of Kv1.2 imparted clustering at presynaptic sites and decreased channel mobility within the axon. This mechanism by which specific Kv channel subunits can act in a dominant manner to impose unique trafficking properties to heteromeric complexes extended to Shab-related family of Kv channels. When coexpressed, Kv2.1 and Kv2.2 heteromultimers did not aggregate in somatodendritic clusters observed with expression of Kv2.1 alone. These studies demonstrate selective axonal trafficking and surface localization of distinct Kv channels based on their subunit composition.

Antidepressant-induced ubiquitination and degradation of the cardiac potassium channel hERG

The most common cause for adverse cardiac events by antidepressants is acquired long QT syndrome (acLQTS), which produces electrocardiographic abnormalities that have been associated with syncope, torsade de pointes arrhythmias, and sudden cardiac death. acLQTS is often caused by direct block of the cardiac potassium current I(Kr)/hERG, which is crucial for terminal repolarization in human heart. Importantly, desipramine belongs to a group of tricyclic antidepressant compounds that can simultaneously block hERG and inhibit its surface expression. Although up to 40% of all hERG blockers exert combined hERG block and trafficking inhibition, few of these compounds have been fully characterized at the cellular level. Here, we have studied in detail how desipramine inhibits hERG surface expression. We find a previously unrecognized combination of two entirely different mechanisms; desipramine increases hERG endocytosis and degradation as a consequence of drug-induced channel ubiquitination and simultaneously inhibits hERG forward trafficking from the endoplasmic reticulum. This unique combination of cellular effects in conjunction with acute channel block may explain why tricyclic antidepressants as a compound class are notorious for their association with arrhythmias and sudden cardiac death. Taken together, we describe the first example of drug-induced channel ubiquitination and degradation. Our data are directly relevant to the cardiac safety of not only tricyclic antidepressants but also other therapeutic compounds that exert multiple effects on hERG, as hERG trafficking and degradation phenotypes may go undetected in most preclinical safety assays designed to screen for acLQTS.

Auranofin protects against anthrax lethal toxin-induced activation of the Nlrp1b inflammasome

Anthrax lethal toxin (LT) is the major virulence factor for Bacillus anthracis. The lethal factor (LF) component of this bipartite toxin is a protease which, when transported into the cellular cytoplasm, cleaves mitogen-activated protein kinase kinase (MEK) family proteins and induces rapid toxicity in mouse macrophages through activation of the Nlrp1b inflammasome. A high-throughput screen was performed to identify synergistic LT-inhibitory drug combinations from within a library of approved drugs and molecular probes. From this screen we discovered that auranofin, an organogold compound with anti-inflammatory activity, strongly inhibited LT-mediated toxicity in mouse macrophages. Auranofin did not inhibit toxin transport into cells or MEK cleavage but inhibited both LT-mediated caspase-1 activation and caspase-1 catalytic activity. Thus, auranofin inhibited LT-mediated toxicity by preventing activation of the Nlrp1b inflammasome and the downstream actions that occur in response to the toxin. Idebenone, an analog of coenzyme Q, synergized with auranofin to increase its protective effect. We found that idebenone functions as an inhibitor of voltage-gated potassium channels and thus likely mediates synergy through inhibition of the potassium fluxes which have been shown to be required for Nlrp1b inflammasome activation.

Emerging concepts in the pharmacogenomics of arrhythmias: ion channel trafficking

Continuous, rhythmic beating of the heart requires exquisite control of expression, localization and function of cardiac ion channels - the foundations of the cardiac myocyte action potential. Disruption of any of these processes can alter the shape of the action potential, predisposing to cardiac arrhythmias. These arrhythmias can manifest in a variety of ways depending on both the channels involved and the type of disruption (i.e., gain or loss of function). As much as 1% of the population of developed countries is affected by cardiac arrhythmia each year, and a detailed understanding of the mechanism of each arrhythmia is crucial to developing and prescribing the proper therapies. Many of the antiarrhythmic drugs currently on the market were developed before the underlying cause of the arrhythmia was known, and as a result lack specificity, causing side effects. The majority of the available drugs target the conductance of cardiac ion channels, either by blocking or enhancing current through the channel. In recent years, however, it has become apparent that specific targeting of ion channel conductance may not be the most effective means for treatment. Here we review increasing evidence that suggests defects in ion channel trafficking play an important role in the etiology of arrhythmias, and small molecule approaches to correct trafficking defects will likely play an important role in the future of arrhythmia treatment.

Kv1.3: a potential pharmacological target for diabetes

K(+) channels, which are ubiquitous membrane proteins, play a central role in regulating the resting membrane potential and the shape and duration of the action potential in pancreatic beta-cells. There are at least three types of K(+) channels (K(ATP), K(Ca), and Kv2.1 channels) that are involved in glucose-stimulated insulin secretion in pancreatic beta-cells, and one type (Kv1.3) that is associated with the regulation of insulin sensitivity in peripheral target tissues. This article reviews the function of Kv1.3 channels that contribute to mediating insulin action in insulin-sensitive tissues. Pharmacological strategies for targeting Kv1.3 are then discussed with a focus on a rationale for the potential therapeutic use of Kv1.3 blocker in diabetic treatment.

Forward trafficking of ion channels: what the clinician needs to know

Each heartbeat requires precisely orchestrated action potential propagation through the myocardium, achieved by coordination of about a million ion channels on the surface of each cardiomyocyte. Specific ion channels must occur within discrete subdomains of the sarcolemma to exert their electrophysiological effects with highest efficiency (e.g., voltage-gated Ca(2+) channels at T-tubules and gap junctions at intercalated discs). Regulation of ion channel movement to their appropriate membrane subdomain is an exciting research frontier with opportunity for novel therapeutic manipulation of ion channels in the treatment of heart disease. Although much research has generally focused on internalization and subsequent degradation of ion channels, the field of forward trafficking of de novo ion channels from the cell interior to the sarcolemma has now emerged as a key regulatory step in cardiac electrophysiological function. In this brief review, we provide an overview of the current understanding of the cellular biology governing the forward trafficking of ion channels.

New developments in atrial antiarrhythmic drug therapy

Atrial fibrillation (AF) is a growing clinical problem associated with increased morbidity and mortality. Currently available antiarrhythmic drugs (AADs), although highly effective in acute cardioversion of paroxysmal AF, are generally only moderately successful in long-term maintenance of sinus rhythm. The use of AADs is often associated with an increased risk of ventricular proarrhythmia, extracardiac toxicity, and exacerbation of concomitant diseases such as heart failure. AF is commonly associated with intracardiac and extracardiac disease, which can modulate the efficacy and safety of AAD therapy. In light of the multifactorial intracardiac and extracardiac causes of AF generation, current development of anti-AF agents is focused on modulation of ion channel activity as well as on upstream therapies that reduce structural substrates. The available data indicate that multiple ion channel blockers exhibiting potent inhibition of peak I(Na) with relatively rapid unbinding kinetics, as well as inhibition of late I(Na) and I(Kr), may be preferable for the management of AF when considering both safety and efficacy.

Open channel block of Kv1.5 currents by citalopram

AIM

To examine whether selective serotonin reuptake inhibitor citalopram interacts with Kv1.5, one of the cardiovascular-specific Kv channel isoforms.

METHODS

The interaction between citalopram and Kv1.5 expressed in Chinese hamster ovary cells was studied using the whole-cell patch-clamp technique.

RESULTS

Citalopram reduced Kv1.5 whole-cell currents in a reversible concentration-dependent manner, with an IC(50) value and a Hill coefficient of 2.8+/-1.1 micromol/L and 0.8+/-0.3, respectively. Citalopram-induced inhibition of Kv1.5 is associated with time-dependent development of block without modifying the kinetics of current activation. The inhibition increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to 0 mV, inhibition displayed an additional voltage dependence, consistent with an electrical distance delta of 0.19. Citalopram slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of citalopram, were superimposed. Inhibition of Kv1.5 by citalopram was use-dependent.

CONCLUSION

The present results suggest that citalopram acts on Kv1.5 currents as an open-channel blocker, and much caution about arrhythmogenic risk is required when using citalopram in the treatment with depressed patients.

Multiple mechanisms of hERG liability: K+ current inhibition, disruption of protein trafficking, and apoptosis induced by amoxapine

The antidepressant amoxapine has been linked to cases of QT prolongation, acute heart failure, and sudden death. Inhibition of cardiac hERG (Kv11.1) potassium channels causes prolonged repolarization and is implicated in apoptosis. Apoptosis in association with amoxapine has not yet been reported. This study was designed to investigate amoxapine effects on hERG currents, hERG protein trafficking, and hERG-associated apoptosis in order to elucidate molecular mechanisms underlying cardiac side effects of the drug. hERG channels were expressed in Xenopus laevis oocytes and HEK 293 cells, and potassium currents were recorded using patch clamp and two-electrode voltage clamp electrophysiology. Protein trafficking was evaluated in HEK 293 cells by Western blot analysis, and cell viability was assessed in HEK cells by immunocytochemistry and colorimetric MTT assay. Amoxapine caused acute hERG blockade in oocytes (IC(50) = 21.6 microM) and in HEK 293 cells (IC(50) = 5.1 microM). Mutation of residues Y652 and F656 attenuated hERG blockade, suggesting drug binding to a receptor inside the channel pore. Channels were mainly blocked in open and inactivated states, and voltage dependence was observed with reduced inhibition at positive potentials. Amoxapine block was reverse frequency-dependent and caused accelerated and leftward-shifted inactivation. Furthermore, amoxapine application resulted in chronic reduction of hERG trafficking into the cell surface membrane (IC(50) = 15.3 microM). Finally, the antidepressant drug triggered apoptosis in cells expressing hERG channels. We provide evidence for triple mechanisms of hERG liability associated with amoxapine: (1) direct hERG current inhibition, (2) disruption of hERG protein trafficking, and (3) induction of apoptosis. Further experiments are required to validate a specific pro-apoptotic effect mediated through blockade of hERG channels.

Ion channel trafficking: a new therapeutic horizon for atrial fibrillation

Atrial fibrillation (AF) is a common cardiac arrhythmia with potentially life-threatening complications. Drug therapies for treatment of AF that seek long-term maintenance of normal sinus rhythm remain elusive due in large part to proarrhythmic ventricular actions. Kv1.5, which underlies the atrial specific I(Kur) current, is a major focus of research efforts seeking new therapeutic strategies and targets. Recent work has shown a novel effect of antiarrhythmic drugs where compounds that block Kv1.5 channel current also can alter ion channel trafficking. This work further suggests that the pleiotropic effects of antiarrhythmic drugs may be separable. Although this finding highlights the therapeutic potential for selective manipulation of ion channel surface density, it also reveals an uncertainty regarding the specificity of modulating trafficking pathways without risk of off-target effects. Future studies may show that specific alteration of Kv1.5 trafficking can overcome the proarrhythmic limitations of current pharmacotherapy and provide an effective method for long-term cardioversion in AF.

Inhibition of UV-B induced apoptosis in corneal epithelial cells by potassium channel modulators

The goal of this study was to determine whether prevention of K(+) loss can protect human corneal-limbal epithelial (HCLE) cells from UV-B induced apoptosis. Immunostaining for activated caspase-3 of HCLE cells exposed to 150-200 mJ/cm(2) UV-B demonstrated induction of apoptosis 6 h after exposure. The number of apoptotic cells was decreased by incubation in medium with 25 or 100 mM K(+). If this protection is due to a reduction of UV-induced K(+) loss then K(+) channel blockers should also protect HCLE cells from UV-B. Caspase-8 activity induced by exposure to UV-B at 150 mJ/cm(2) was significantly reduced when the cells were incubated in 0.3 microM BDS-I or 0.05-1 mM quinidine. Caspase-3 was also activated by UV-B and a reduction in activity was observed after incubation in 0.1-0.3 microM BDS-I and 0.1-1 mM quinidine. Induction of DNA fragmentation, as measured by the TUNEL assay, was decreased by treatment with 0.3 microM BDS-I and 0.01-0.05 mM quinidine. Patch-clamp recording showed activation of K(+) channels after exposure to UV-B and a decrease in outward K(+) current was observed following application of BDS-I. Quinidine did not block K(+) currents in HCLE cells, suggesting that the protective effect of quinidine occurs by a mechanism other than via K(+) channels. The effect of the K(+) channel blocker BDS-1 on HCLE cells exposed to UV-B confirms that preventing K(+) efflux protects corneal epithelial cells from apoptosis. This suggests the elevated [K(+)] in tears may protect the corneal epithelium from effects of ambient UV-B.