cAMP-dependent phosphorylation of two sites in the alpha subunit of the cardiac sodium channel

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Cardiac Conduction Velocity, Remodeling and Arrhythmogenesis

Cardiac electrophysiological disorders, in particular arrhythmias, are a key cause of morbidity and mortality throughout the world. There are two basic requirements for arrhythmogenesis: an underlying substrate and a trigger. Altered conduction velocity (CV) provides a key substrate for arrhythmogenesis, with slowed CV increasing the probability of re-entrant arrhythmias by reducing the length scale over which re-entry can occur. In this review, we examine methods to measure cardiac CV in vivo and ex vivo, discuss underlying determinants of CV, and address how pathological variations alter CV, potentially increasing arrhythmogenic risk. Finally, we will highlight future directions both for methodologies to measure CV and for possible treatments to restore normal CV.

Biophysical defects of an SCN5A V1667I mutation associated with epinephrine-induced marked QT prolongation

BACKGROUND

The epinephrine infusion test (EIT) typically induces marked QT prolongation in LQT1, but not LQT3, while the efficacy of β-blocker therapy is established in LQT1, but not LQT3. We encountered an LQT3 family, with an SCN5A V1667I mutation, that exhibited epinephrine-induced marked QT prolongation.

METHODS

Wild-type (WT) or V1667I-SCN5A was transiently expressed into tsA-201 cells, and whole-cell sodium currents (I_Na ) were recorded using patch-clamp techniques. To mimic the effects of epinephrine, I_Na was recorded after the application of protein kinase A (PKA) activator, 8-CPT-cAMP (200 μM), for 10 minutes.

RESULTS

The peak density of V1667I-I_Na was significantly larger than WT-I_Na (WT: 469 ± 48 pA/pF, n = 20; V1667I: 690 ± 62 pA/pF, n = 19, P < .01). The steady-state activation (SSA) and fast inactivation rate of V1667I-I_Na were comparable to WT-I_Na . V1667I-I_Na displayed a significant depolarizing shift in steady-state inactivation (SSI) in comparison to WT-I_Na (V_1/2 -WT: -88.1 ± 0.8 mV, n = 17; V1667I: -82.5 ± 1.1 mV, n = 17, P < .01), which increases window currents. Tetrodotoxin (30 μM)-sensitive persistent V1667I-I_Na was comparable to WT-I_Na . However, the ramp pulse protocol (RPP) displayed an increased hump in V1667I-I_Na in comparison to WT-I_Na . Although 8-CPT-cAMP shifted SSA to hyperpolarizing potentials in WT-I_Na and V1667I-I_Na to the same extent, it shifted SSI to hyperpolarizing potentials much less in V1667I-I_Na than in WT-I_Na (V_1/2 -WT: -92.7 ± 1.3 mV, n = 6; V1667I: -85.3 ± 1.6 mV, n = 6, P < .01). Concordantly, the RPP displayed an increased hump in V1667I-I_Na , but not in WT-I_Na .

CONCLUSIONS

We demonstrated an increase of V1667I-I_Na by PKA activation, which may provide a rationale for the efficacy of β-blocker therapy in some cases of LQT3.

"Heart Oddity": Intrinsically Reduced Excitability in the Right Ventricle Requires Compensation by Regionally Specific Stress Kinase Function

The traditional view of ventricular excitation and conduction is an all-or-nothing response mediated by a regenerative activation of the inward sodium channel, which gives rise to an essentially constant conduction velocity (CV). However, whereas there is no obvious biological need to tune-up ventricular conduction, the principal molecular components determining CV, such as sodium channels, inward-rectifier potassium channels, and gap junctional channels, are known targets of the “stress” protein kinases PKA and calcium/calmodulin dependent protein kinase II (CaMKII), and are thus regulatable by signal pathways converging on these kinases. In this mini-review we will expose deficiencies and controversies in our current understanding of how ventricular conduction is regulated by stress kinases, with a special focus on the chamber-specific dimension in this regulation. In particular, we will highlight an odd property of cardiac physiology: uniform CV in ventricles requires co-existence of mutually opposing gradients in cardiac excitability and stress kinase function. While the biological advantage of this peculiar feature remains obscure, it is important to recognize the clinical implications of this phenomenon pertinent to inherited or acquired conduction diseases and therapeutic interventions modulating activity of PKA or CaMKII.

Conduction in the right and left ventricle is differentially regulated by protein kinases and phosphatases: implications for arrhythmogenesis

The "stress" kinases cAMP-dependent protein kinase (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII), phosphorylate the Na+ channel Nav1.5 subunit to regulate its function. However, how the channel regulation translates to ventricular conduction is poorly understood. We hypothesized that the stress kinases positively and differentially regulate conduction in the right (RV) and the left (LV) ventricles. We applied the CaMKII blocker KN93 (2.75 μM), PKA blocker H89 (10 μM), and broad-acting phosphatase blocker calyculin (30 nM) in rabbit hearts paced at a cycle length (CL) of 150-8,000 ms. We used optical mapping to determine the distribution of local conduction delays (inverse of conduction velocity). Control hearts exhibited constant and uniform conduction at all tested CLs. Calyculin (15-min perfusion) accelerated conduction, with greater effect in the RV (by 15.3%) than in the LV (by 4.1%; P < 0.05). In contrast, both KN93 and H89 slowed down conduction in a chamber-, time-, and CL-dependent manner, with the strongest effect in the RV outflow tract (RVOT). Combined KN93 and H89 synergistically promoted conduction slowing in the RV (KN93: 24.7%; H89: 29.9%; and KN93 + H89: 114.2%; P = 0.0016) but not the LV. The progressive depression of RV conduction led to conduction block and reentrant arrhythmias. Protein expression levels of both the CaMKII-δ isoform and the PKA catalytic subunit were higher in the RVOT than in the apical LV (P < 0.05). Thus normal RV conduction requires a proper balance between kinase and phosphatase activity. Dysregulation of this balance due to pharmacological interventions or disease is potentially proarrhythmic. NEW & NOTEWORTHY We show that uniform ventricular conduction requires a precise physiological balance of the activities of calcium/calmodulin-dependent protein kinase II (CaMKII), PKA, and phosphatases, which involves region-specific expression of CaMKII and PKA. Inhibiting CaMKII and/or PKA activity elicits nonuniform conduction depression, with the right ventricle becoming vulnerable to the development of conduction disturbances and ventricular fibrillation/ventricular tachycardia.

Phosphorylation of cardiac voltage-gated sodium channel: Potential players with multiple dimensions

Cardiomyocytes are highly coordinated cells with multiple proteins organized in micro domains. Minor changes or interference in subcellular proteins can cause major disturbances in physiology. The cardiac sodium channel (Na_V1.5) is an important determinant of correct electrical activity in cardiomyocytes which are localized at intercalated discs, T‐tubules and lateral membranes in the form of a macromolecular complex with multiple interacting protein partners. The channel is tightly regulated by post‐translational modifications for smooth conduction and propagation of action potentials. Among regulatory mechanisms, phosphorylation is an enzymatic and reversible process which modulates Na_V1.5 channel function by attaching phosphate groups to serine, threonine or tyrosine residues. Phosphorylation of Na_V1.5 is implicated in both normal physiological and pathological processes and is carried out by multiple kinases. In this review, we discuss and summarize recent literature about the (a) structure of Na_V1.5 channel, (b) formation and subcellular localization of Na_V1.5 channel macromolecular complex, (c) post‐translational phosphorylation and regulation of Na_V1.5 channel, and (d) how these phosphorylation events of Na_V1.5 channel alter the biophysical properties and affect the channel during disease status. We expect, by reviewing these aspects will greatly improve our understanding of Na_V1.5 channel biology, physiology and pathology, which will also provide an insight into the mechanism of arrythmogenesis at molecular level.

AMPK-mediated degradation of Nav1.5 through autophagy

The voltage-gated cardiac sodium channel, Nav1.5, is the key component that controls cardiac excitative electrical impulse and propagation. However, the dynamic alterations of Nav1.5 during cardiac ischemia and reperfusion (I/R) are seldom reported. We found that the protein levels of rat cardiac Nav1.5 were significantly decreased in response to cardiac I/R injury. By simulating I/R injury in cells through activating AMPK by glucose deprivation, AMPK activator treatment, or hypoxia and reoxygenation (H/R), we found that Nav1.5 was down-regulated by AMPK-mediated autophagic degradation. Furthermore, AMPK was found to phosphorylate Nav1.5 at threonine (T) 101, which then regulates the interaction between Nav1.5 and the autophagic adaptor protein, microtubule-associated protein 1 light chain 3 (LC3), by exposing the LC3-interacting region adjacent to T101 in Nav1.5. This study highlights an instrumental role of AMPK in mediating the autophagic degradation of Nav1.5 during cardiac I/R injury.-Liu, X., Chen, Z., Han, Z., Liu, Y., Wu, X., Peng, Y., Di, W., Lan, R., Sun, B., Xu, B., Xu, W. AMPK-mediated degradation of Nav1.5 through autophagy.

β-adrenergic regulation of late Na+ current during cardiac action potential is mediated by both PKA and CaMKII

Late Na⁺ current (I_NaL) significantly contributes to shaping cardiac action potentials (APs) and increased I_NaL is associated with cardiac arrhythmias. β-adrenergic receptor (βAR) stimulation and its downstream signaling via protein kinase A (PKA) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) pathways are known to regulate I_NaL. However, it remains unclear how each of these pathways regulates I_NaL during the AP under physiological conditions. Here we performed AP-clamp experiments in rabbit ventricular myocytes to delineate the impact of each signaling pathway on I_NaL at different AP phases to understand the arrhythmogenic potential. During the physiological AP (2 Hz, 37 °C) we found that I_NaL had a basal level current independent of PKA, but partially dependent on CaMKII. βAR activation (10 nM isoproterenol, ISO) further enhanced I_NaL via both PKA and CaMKII pathways. However, PKA predominantly increased I_NaL early during the AP plateau, whereas CaMKII mainly increased I_NaL later in the plateau and during rapid repolarization. We also tested the role of key signaling pathways through exchange protein activated by cAMP (Epac), nitric oxide synthase (NOS) and reactive oxygen species (ROS). Direct Epac stimulation enhanced I_NaL similar to the βAR-induced CaMKII effect, while NOS inhibition prevented the βAR-induced CaMKII-dependent I_NaL enhancement. ROS generated by NADPH oxidase 2 (NOX2) also contributed to the ISO-induced I_NaL activation early in the AP. Taken together, our data reveal differential modulations of I_NaL by PKA and CaMKII signaling pathways at different AP phases. This nuanced and comprehensive view on the changes in I_NaL during AP deepens our understanding of the important role of I_NaL in reshaping the cardiac AP and arrhythmogenic potential under elevated sympathetic stimulation, which is relevant for designing therapeutic treatment of arrhythmias under pathological conditions.

Fusion of Ssm6a with a protein scaffold retains selectivity on NaV 1.7 and improves its therapeutic potential against chronic pain

Voltage-gated sodium channel Na_V 1.7 serves as an attractive target for chronic pain treatment. Several venom peptides were found to selectively inhibit Na_V 1.7 but with intrinsic problems. Among them, Ssm6a, a recently discovered centipede venom peptide, shows the greatest selectivity against Na_V 1.7, but dissociates from the target too fast and loses bioactivity in synthetic forms. As a disulfide-rich venom peptide, it is difficult to optimize Ssm6a by artificial mutagenesis and produce the peptide with common industrial manufacturing methods. Here, we developed a novel protein scaffold fusion strategy to address these concerns. Instead of directly mutating Ssm6a, we genetically fused Ssm6a with a protein scaffold engineered from human muscle fatty acid-binding protein. The resultant fusion protein, SP-TOX, maintained the selectivity and potency of Ssm6a upon Na_V 1.7 but dissociated from target at least 10 times more slowly. SP-TOX dramatically reduced inflammatory pain in a rat model through DRG-targeted delivery. Importantly, SP-TOX can be expressed cytosolically in Escherichia coli and purified in a cost-effective way. In summary, our study provided the first example of cytosolically expressed fusion protein with high potency and selectivity on Na_V 1.7. Our protein scaffold fusion approach may have its broad application in optimizing disulfide-rich venom peptides for therapeutic usage.

The control of cardiac ventricular excitability by autonomic pathways

Central to the genesis of ventricular cardiac arrhythmia are variations in determinants of excitability. These involve individual ionic channels and transporters in cardiac myocytes but also tissue factors such as variable conduction of the excitation wave, fibrosis and source-sink mismatch. It is also known that in certain diseases and particularly the channelopathies critical events occur with specific stressors. For example, in hereditary long QT syndrome due to mutations in KCNQ1 arrhythmic episodes are provoked by exercise and in particular swimming. Thus not only is the static substrate important but also how this is modified by dynamic signalling events associated with common physiological responses. In this review, we examine the regulation of ventricular excitability by signalling pathways from a cellular and tissue perspective in an effort to identify key processes, effectors and potential therapeutic approaches. We specifically focus on the autonomic nervous system and related signalling pathways.

Regulation of Cardiac Voltage-Gated Sodium Channel by Kinases: Roles of Protein Kinases A and C

In the heart, voltage-gated sodium (Na_v) channel (Na_v1.5) is defined by its pore-forming α-subunit and its auxiliary β-subunits, both of which are important for its critical contribution to the initiation and maintenance of the cardiac action potential (AP) that underlie normal heart rhythm. The physiological relevance of Na_v1.5 is further marked by the fact that inherited or congenital mutations in Na_v1.5 channel gene SCN5A lead to altered functional expression (including expression, trafficking, and current density), and are generally manifested in the form of distinct cardiac arrhythmic events, epilepsy, neuropathic pain, migraine, and neuromuscular disorders. However, despite significant advances in defining the pathophysiology of Na_v1.5, the molecular mechanisms that underlie its regulation and contribution to cardiac disorders are poorly understood. It is rapidly becoming evident that the functional expression (localization, trafficking and gating) of Na_v1.5 may be under modulation by post-translational modifications that are associated with phosphorylation. We review here the molecular basis of cardiac Na channel regulation by kinases (PKA and PKC) and the resulting functional consequences. Specifically, we discuss: (1) recent literature on the structural, molecular, and functional properties of cardiac Na_v1.5 channels; (2) how these properties may be altered by phosphorylation in disease states underlain by congenital mutations in Na_v1.5 channel and/or subunits such as long QT and Brugada syndromes. Our expectation is that understanding the roles of these distinct and complex phosphorylation processes on the functional expression of Na_v1.5 is likely to provide crucial mechanistic insights into Na channel associated arrhythmogenic events and will facilitate the development of novel therapeutic strategies.

Antiarrhythmic Action ofβ-Blockers: Potential Mechanisms

Sympathetic nervous system overactivity has been linked to ventricular tachyarrhythmias and sudden death. It has been hypothesized that the extent and nature of the arrhythmogenic effect of sympathetic stimulation depends on the underlying myocardial substrate, the mechanism of the arrhythmia, and the integrated effects of sympathetic stimulation in the particular individual circumstance. Multiple direct and indirect mechanisms of adrenergic action on the heart may benefit from the known antiarrhythmic actions of β-blocker therapy and other interventions that decrease sympathetic tone. The antiarrhythmic mechanism of β-blockade (and possibly α-blockade) will depend on the specific mechanism of the individual arrhythmia and will differ for those arrhythmias caused by tachycardia and ischemia, those caused by reentry and promoted by decreased conduction velocity and shortened refractoriness, and those caused by early or delayed afterdepolarizations, usually in the context of prolonged action potential duration. Antagonism of cardiac adrenergic activity by β-blockade in particular is the best-established drug therapy to prevent ventricular arrhythmias.

Cardiac Na Channels: Structure to Function

Heart rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. Opening of the primary cardiac voltage-gated sodium (NaV1.5) channel initiates cellular depolarization and the propagation of an electrical action potential that promotes coordinated contraction of the heart. The regularity of these contractile waves is critically important since it drives the primary function of the heart: to act as a pump that delivers blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. Perturbations to NaV1.5 may alter the structure, and hence the function, of the ion channel and are associated downstream with a wide variety of cardiac conduction pathologies, such as arrhythmias.

Conduction abnormalities and ventricular arrhythmogenesis: The roles of sodium channels and gap junctions

Ventricular arrhythmias arise from disruptions in the normal orderly sequence of electrical activation and recovery of the heart. They can be categorized into disorders affecting predominantly cellular depolarization or repolarization, or those involving action potential (AP) conduction. This article briefly discusses the factors causing conduction abnormalities in the form of unidirectional conduction block and reduced conduction velocity (CV). It then examines the roles that sodium channels and gap junctions play in AP conduction. Finally, it synthesizes experimental results to illustrate molecular mechanisms of how abnormalities in these proteins contribute to such conduction abnormalities and hence ventricular arrhythmogenesis, in acquired pathologies such as acute ischaemia and heart failure, as well as inherited arrhythmic syndromes.

CaMKII Phosphorylation of Na(V)1.5: Novel in Vitro Sites Identified by Mass Spectrometry and Reduced S516 Phosphorylation in Human Heart Failure

The cardiac voltage-gated sodium channel, Na(V)1.5, drives the upstroke of the cardiac action potential and is a critical determinant of myocyte excitability. Recently, calcium (Ca(2+))/calmodulin(CaM)-dependent protein kinase II (CaMKII) has emerged as a critical regulator of Na(V)1.5 function through phosphorylation of multiple residues including S516, T594, and S571, and these phosphorylation events may be important for the genesis of acquired arrhythmias, which occur in heart failure. However, phosphorylation of full-length human Na(V)1.5 has not been systematically analyzed and Na(V)1.5 phosphorylation in human heart failure is incompletely understood. In the present study, we used label-free mass spectrometry to assess phosphorylation of human Na(V)1.5 purified from HEK293 cells with full coverage of phosphorylatable sites and identified 23 sites that were phosphorylated by CaMKII in vitro. We confirmed phosphorylation of S516 and S571 by LC-MS/MS and found a decrease in S516 phosphorylation in human heart failure, using a novel phospho-specific antibody. This work furthers our understanding of the phosphorylation of Na(V)1.5 by CaMKII under normal and disease conditions, provides novel CaMKII target sites for functional validation, and provides the first phospho-proteomic map of full-length human Na(V)1.5.

Regulation of the cardiac Na+ channel NaV1.5 by post-translational modifications

The cardiac voltage-gated Na(+) channel, Na(V)1.5, is responsible for the upstroke of the action potential in cardiomyocytes and for efficient propagation of the electrical impulse in the myocardium. Even subtle alterations of Na(V)1.5 function, as caused by mutations in its gene SCN5A, may lead to many different arrhythmic phenotypes in carrier patients. In addition, acquired malfunctions of Na(V)1.5 that are secondary to cardiac disorders such as heart failure and cardiomyopathies, may also play significant roles in arrhythmogenesis. While it is clear that the regulation of Na(V)1.5 protein expression and function tightly depends on genetic mechanisms, recent studies have demonstrated that Na(V)1.5 is the target of various post-translational modifications that are pivotal not only in physiological conditions, but also in disease. In this review, we examine the recent literature demonstrating glycosylation, phosphorylation by Protein Kinases A and C, Ca(2+)/Calmodulin-dependent protein Kinase II, Phosphatidylinositol 3-Kinase, Serum- and Glucocorticoid-inducible Kinases, Fyn and Adenosine Monophosphate-activated Protein Kinase, methylation, acetylation, redox modifications, and ubiquitylation of Na(V)1.5. Modern and sensitive mass spectrometry approaches, applied directly to channel proteins that were purified from native cardiac tissues, have enabled the determination of the precise location of post-translational modification sites, thus providing essential information for understanding the mechanistic details of these regulations. The current challenge is first, to understand the roles of these modifications on the expression and the function of Na(V)1.5, and second, to further identify other chemical modifications. It is postulated that the diversity of phenotypes observed with Na(V)1.5-dependent disorders may partially arise from the complex post-translational modifications of channel protein components.

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

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

Anchored protein kinase A signalling in cardiac cellular electrophysiology

The cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is an elementary molecule involved in both acute and chronic modulation of cardiac function. Substantial research in recent years has highlighted the importance of A-kinase anchoring proteins (AKAP) therein as they act as the backbones of major macromolecular signalling complexes of the β-adrenergic/cAMP/PKA pathway. This review discusses the role of AKAP-associated protein complexes in acute and chronic cardiac modulation by dissecting their role in altering the activity of different ion channels, which underlie cardiac action potential (AP) generation. In addition, we review the involvement of different AKAP complexes in mechanisms of cardiac remodelling and arrhythmias.

Tubulin polymerization disrupts cardiac β-adrenergic regulation of late INa

AIMS

The anticancer drug paclitaxel (TXL) that polymerizes microtubules is associated with arrhythmias and sinus node dysfunction. TXL can alter membrane expression of Na channels (NaV1.5) and Na current (INa), but the mechanisms are unknown. Calcium/calmodulin-dependent protein kinase II (CaMKII) can be activated by β-adrenergic stimulation and regulates INa gating. We tested whether TXL interferes with isoproterenol (ISO)-induced activation of CaMKII and consequent INa regulation.

METHODS AND RESULTS

In wild-type mouse myocytes, the addition of ISO (1 µmol/L) resulted in increased CaMKII auto-phosphorylation (western blotting). This increase was completely abolished after pre-treatment with TXL (100 µmol/L, 1.5 h). The mechanism was further investigated in human embryonic kidney cells. TXL inhibited the ISO-induced β-arrestin translocation. Interestingly, both knockdown of β-arrestin2 expression using small interfering RNA and inhibition of exchange protein directly activated by cAMP (Epac) blocked the ISO-induced CaMKII auto-phosphorylation similar to TXL. The generation of cAMP, however, was unaltered (Epac1-camps). CaMKII-dependent Na channel function was measured using patch-clamp technique in isolated cardiomyoctes. ISO stimulation failed to induce CaMKII-dependent enhancement of late INa and Na channel inactivation (negative voltage shift in steady-state activation and enhanced intermediate inactivation) after pre-incubation with TXL. Consistent with this, TXL also inhibited ISO-induced CaMKII-specific Na channel phosphorylation (at serine 571 of NaV1.5).

CONCLUSION

Pre-incubation with TXL disrupts the ISO-dependent CaMKII activation and consequent Na channel regulation. This may be important for patients receiving TXL treatments, but also relevant for conditions of increased CaMKII expression and enhanced β-adrenergic stimulation like in heart failure.

Reactive oxygen species and excitation-contraction coupling in the context of cardiac pathology

Reactive oxygen species (ROS) are highly reactive oxygen-derived chemical compounds that are by-products of aerobic cellular metabolism as well as crucial second messengers in numerous signaling pathways. In excitation-contraction-coupling (ECC), which links electrical signaling and coordinated cardiac contraction, ROS have a severe impact on several key ion handling proteins such as ion channels and transporters, but also on regulating proteins such as protein kinases (e.g. CaMKII, PKA or PKC), thereby pivotally influencing the delicate balance of this finely tuned system. While essential as second messengers, ROS may be deleterious when excessively produced due to a disturbed balance in Na(+) and Ca(2+) handling, resulting in Na(+) and Ca(2+) overload, SR Ca(2+) loss and contractile dysfunction. This may, in the end, result in systolic and diastolic dysfunction and arrhythmias. This review aims to provide an overview of the single targets of ROS in ECC and to outline the role of ROS in major cardiac pathologies, such as heart failure and arrhythmogenesis. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System"

CaMKII-dependent regulation of cardiac Na(+) homeostasis

Na⁺ homeostasis is a key regulator of cardiac excitation and contraction. The cardiac voltage-gated Na⁺ channel, Na_V1.5, critically controls cell excitability, and altered channel gating has been implicated in both inherited and acquired arrhythmias. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), a serine/threonine kinase important in cardiac physiology and disease, phosphorylates Na_V1.5 at multiple sites within the first intracellular linker loop to regulate channel gating. Although CaMKII sites on the channel have been identified (S516, T594, S571), the relative role of each of these phospho-sites in channel gating properties remains unclear, whereby both loss-of-function (reduced availability) and gain-of-function (late Na⁺ current, I_NaL) effects have been reported. Our review highlights investigating the complex multi-site phospho-regulation of Na_V1.5 gating is crucial to understanding the genesis of acquired arrhythmias in heart failure (HF) and CaMKII activated conditions. In addition, the increased Na⁺ influx accompanying I_NaL may also indirectly contribute to arrhythmia by promoting Ca²⁺ overload. While the precise mechanisms of Na⁺ loading during HF remain unclear, and quantitative analyses of the contribution of I_NaL are lacking, disrupted Na⁺ homeostasis is a consistent feature of HF. Computational and experimental observations suggest that both increased diastolic Na⁺ influx and action potential prolongation due to systolic I_NaL contribute to disruption of Ca²⁺ handling in failing hearts. Furthermore, simulations reveal a synergistic interaction between perturbed Na⁺ fluxes and CaMKII, and confirm recent experimental findings of an arrhythmogenic feedback loop, whereby CaMKII activation is at once a cause and a consequence of Na⁺ loading.

PKA phosphorylation reshapes the pharmacological kinetics of BmK AS, a unique site-4 sodium channel-specific modulator

Although modulation of the activity of voltage-gated sodium channels (VGSCs) by protein kinase A (PKA) phosphorylation has been investigated in multiple preparations, the pharmacological sensitivity of VGSCs to scorpion toxins after PKA phosphorylation has rarely been approached. In this study, the effects of BmK AS, a sodium channel-specific modulator from Chinese scorpion Buthus martensi Karsch, on the voltage-dependent activation and inactivation of Nav1.2 were examined before and after PKA activation. After PKA phosphorylation, the pattern of dose-dependent modulation of BmK AS, on both Nav1.2α and Nav1.2 (α + β1) was reshaped. Meanwhile, the shifts in voltage-dependency of activation and inactivation induced by BmK AS were attenuated. The results suggested that PKA might play a role in different patterns how β-like toxins such as BmK AS modulate gating properties and peak currents of VGSCs.

Role of oxidants on calcium and sodium movement in healthy and diseased cardiac myocytes

In this review article we give an overview of current knowledge with respect to redox-sensitive alterations in Na(+) and Ca(2+) handling in the heart. In particular, we focus on redox-activated protein kinases including cAMP-dependent protein kinase A (PKA), protein kinase C (PKC), and Ca/calmodulin-dependent protein kinase II (CaMKII), as well as on redox-regulated downstream targets such as Na(+) and Ca(2+) transporters and channels. We highlight the pathological and physiological relevance of reactive oxygen species and some of its sources (such as NADPH oxidases, NOXes) for excitation-contraction coupling (ECC). A short outlook with respect to the clinical relevance of redox-dependent Na(+) and Ca(2+) imbalance will be given.

Dynamics of the late Na(+) current during cardiac action potential and its contribution to afterdepolarizations

The objective of this work is to examine the contribution of late Na(+) current (INa,L) to the cardiac action potential (AP) and arrhythmogenic activities. In spite of the rapidly growing interest toward this current, there is no publication available on experimental recording of the dynamic INa,L current as it flows during AP with Ca(2+) cycling. Also unknown is how the current profile changes when the Ca(2+)-calmodulin dependent protein kinase II (CaMKII) signaling is altered, and how the current contributes to the development of arrhythmias. In this study we use an innovative AP-clamp Sequential Dissection technique to directly record the INa,L current during the AP with Ca(2+) cycling in the guinea pig ventricular myocytes. First, we found that the magnitude of INa,L measured under AP-clamp is substantially larger than earlier studies indicated. CaMKII inhibition using KN-93 significantly reduced the current. Second, we recorded INa,L together with IKs, IKr, and IK1 in the same cell to understand how these currents counterbalance to shape the AP morphology. We found that the amplitude and the total charge carried by INa,L exceed that of IKs. Third, facilitation of INa,L by Anemone toxin II prolonged APD and induced Ca(2+) oscillations that led to early and delayed afterdepolarizations and triggered APs; these arrhythmogenic activities were eliminated by buffering Ca(2+) with BAPTA. In conclusion, INa,L contributes a significantly large inward current that prolongs APD and unbalances the Ca(2+) homeostasis to cause arrhythmogenic APs.

Determinants of myocardial conduction velocity: implications for arrhythmogenesis

Slowed myocardial conduction velocity (θ) is associated with an increased risk of re-entrant excitation, predisposing to cardiac arrhythmia. θ is determined by the ion channel and physical properties of cardiac myocytes and by their interconnections. Thus, θ is closely related to the maximum rate of action potential (AP) depolarization [(dV/dt)_max], as determined by the fast Na⁺ current (I_Na); the axial resistance (r_a) to local circuit current flow between cells; their membrane capacitances (c_m); and to the geometrical relationship between successive myocytes within cardiac tissue. These determinants are altered by a wide range of pathophysiological conditions. Firstly, I_Na is reduced by the impaired Na⁺ channel function that arises clinically during heart failure, ischemia, tachycardia, and following treatment with class I antiarrhythmic drugs. Such reductions also arise as a consequence of mutations in SCN5A such as those occurring in Lenègre disease, Brugada syndrome (BrS), sick sinus syndrome, and atrial fibrillation (AF). Secondly, r_a, may be increased due to gap junction decoupling following ischemia, ventricular hypertrophy, and heart failure, or as a result of mutations in CJA5 found in idiopathic AF and atrial standstill. Finally, either r_a or c_m could potentially be altered by fibrotic change through the resultant decoupling of myocyte–myocyte connections and coupling of myocytes with fibroblasts. Such changes are observed in myocardial infarction and cardiomyopathy or following mutations in MHC403 and SCN5A resulting in hypertrophic cardiomyopathy (HCM) or Lenègre disease, respectively. This review defines and quantifies the determinants of θ and summarizes experimental evidence that links changes in these determinants with reduced myocardial θ and arrhythmogenesis. It thereby identifies the diverse pathophysiological conditions in which abnormal θ may contribute to arrhythmia.

Post-translational modifications of the cardiac Na channel: contribution of CaMKII-dependent phosphorylation to acquired arrhythmias

The voltage-gated Na channel isoform 1.5 (NaV1.5) is the pore forming α-subunit of the voltage-gated cardiac Na channel, which is responsible for the initiation and propagation of cardiac action potentials. Mutations in the SCN5A gene encoding NaV1.5 have been linked to changes in the Na current leading to a variety of arrhythmogenic phenotypes, and alterations in the NaV1.5 expression level, Na current density, and/or gating have been observed in acquired cardiac disorders, including heart failure. The precise mechanisms underlying these abnormalities have not been fully elucidated. However, several recent studies have made it clear that NaV1.5 forms a macromolecular complex with a number of proteins that modulate its expression levels, localization, and gating and is the target of extensive post-translational modifications, which may also influence all these properties. We review here the molecular aspects of cardiac Na channel regulation and their functional consequences. In particular, we focus on the molecular and functional aspects of Na channel phosphorylation by the Ca/calmodulin-dependent protein kinase II, which is hyperactive in heart failure and has been causally linked to cardiac arrhythmia. Understanding the mechanisms of altered NaV1.5 expression and function is crucial for gaining insight into arrhythmogenesis and developing novel therapeutic strategies.

Redox regulation of sodium and calcium handling

SIGNIFICANCE

In heart failure (HF), contractile dysfunction and arrhythmias result from disturbed intracellular Ca handling. Activated stress kinases like cAMP-dependent protein kinase A (PKA), protein kinase C (PKC), and Ca/calmodulin-dependent protein kinase II (CaMKII), which are known to influence many Ca-regulatory proteins, are mechanistically involved.

RECENT ADVANCES

Beside classical activation pathways, it is becoming increasingly evident that reactive oxygen species (ROS) can directly oxidize these kinases, leading to alternative activation. Since HF is associated with increased ROS generation, ROS-activated serine/threonine kinases may play a crucial role in the disturbance of cellular Ca homeostasis. Many of the previously described ROS effects on ion channels and transporters are possibly mediated by these stress kinases. For instance, ROS have been shown to oxidize and activate CaMKII, thereby increasing Na influx through voltage-gated Na channels, which can lead to intracellular Na accumulation and action potential prolongation. Consequently, Ca entry via activated NCX is favored, which together with ROS-induced dysfunction of the sarcoplasmic reticulum can lead to dramatic intracellular Ca accumulation, diminished contractility, and arrhythmias.

CRITICAL ISSUES

While low amounts of ROS may regulate kinase activity, excessive uncontrolled ROS production may lead to direct redox modification of Ca handling proteins. Therefore, depending on the source and amount of ROS generated, ROS could have very different effects on Ca-handling proteins.

FUTURE DIRECTIONS

The discrimination between fine-tuned ROS signaling and unspecific ROS damage may be crucial for the understanding of heart failure development and important for the investigation of targeted treatment strategies.

Mass spectrometry-based identification of native cardiac Nav1.5 channel α subunit phosphorylation sites

Cardiac voltage-gated Na+ (Nav) channels are key determinants of action potential waveforms, refractoriness and propagation, and Nav1.5 is the main Nav pore-forming (α) subunit in the mammalian heart. Although direct phosphorylation of the Nav1.5 protein has been suggested to modulate various aspects of Nav channel physiology and pathophysiology, native Nav1.5 phosphorylation sites have not been identified. In the experiments here, a mass spectrometry (MS)-based proteomic approach was developed to identify native Nav1.5 phosphorylation sites directly. Using an anti-NavPAN antibody, Nav channel complexes were immunoprecipitated from adult mouse cardiac ventricles. The MS analyses revealed that this antibody immunoprecipitates several Nav α subunits in addition to Nav1.5, as well as several previously identified Nav channel associated/regulatory proteins. Label-free comparative and data-driven phosphoproteomic analyses of purified cardiac Nav1.5 protein identified 11 phosphorylation sites, 8 of which are novel. All the phosphorylation sites identified except one in the N-terminus are in the first intracellular linker loop, suggesting critical roles for this region in phosphorylation-dependent cardiac Nav channel regulation. Interestingly, commonly used prediction algorithms did not reliably predict these newly identified in situ phosphorylation sites. Taken together, the results presented provide the first in situ map of basal phosphorylation sites on the mouse cardiac Nav1.5 α subunit.

Biology of cardiac sodium channel Nav1.5 expression

Na(v)1.5, the pore forming α-subunit of the voltage-dependent cardiac Na(+) channel, is an integral membrane protein involved in the initiation and conduction of action potentials. Mutations in the gene-encoding Na(v)1.5, SCN5A, have been associated with a variety of arrhythmic disorders, including long QT, Brugada, and sick sinus syndromes as well as progressive cardiac conduction defect and atrial standstill. Moreover, alterations in the Na(v)1.5 expression level and/or sodium current density have been frequently noticed in acquired cardiac disorders, such as heart failure. The molecular mechanisms underlying these alterations are poorly understood, but are considered essential for conception of arrhythmogenesis and the development of therapeutic strategies for prevention or treatment of arrhythmias. The unravelling of such mechanisms requires critical molecular insight into the biology of Na(v)1.5 expression and function. Therefore, the aim of this review is to provide an up-to-date account of molecular determinants of normal Na(v)1.5 expression and function. The parts of the Na(v)1.5 life cycle that are discussed include (i) regulatory aspects of the SCN5A gene and transcript structure, (ii) the nature, molecular determinants, and functional consequences of Na(v)1.5 post-translational modifications, and (iii) the role of Na(v)1.5 interacting proteins in cellular trafficking. The reviewed studies have provided valuable information on how the Na(v)1.5 expression level, localization, and biophysical properties are regulated, but also revealed that our understanding of the underlying mechanisms is still limited.

Reactive oxygen species-activated Ca/calmodulin kinase IIδ is required for late I(Na) augmentation leading to cellular Na and Ca overload

RATIONALE

In heart failure Ca/calmodulin kinase (CaMK)II expression and reactive oxygen species (ROS) are increased. Both ROS and CaMKII can increase late I(Na) leading to intracellular Na accumulation and arrhythmias. It has been shown that ROS can activate CaMKII via oxidation.

OBJECTIVE

We tested whether CaMKIIδ is required for ROS-dependent late I(Na) regulation and whether ROS-induced Ca released from the sarcoplasmic reticulum (SR) is involved.

METHODS AND RESULTS

40 μmol/L H(2)O(2) significantly increased CaMKII oxidation and autophosphorylation in permeabilized rabbit cardiomyocytes. Without free [Ca](i) (5 mmol/L BAPTA/1 mmol/L Br(2)-BAPTA) or after SR depletion (caffeine 10 mmol/L, thapsigargin 5 μmol/L), the H(2)O(2)-dependent CaMKII oxidation and autophosphorylation was abolished. H(2)O(2) significantly increased SR Ca spark frequency (confocal microscopy) but reduced SR Ca load. In wild-type (WT) mouse myocytes, H(2)O(2) increased late I(Na) (whole cell patch-clamp). This increase was abolished in CaMKIIδ(-/-) myocytes. H(2)O(2)-induced [Na](i) and [Ca](i) accumulation (SBFI [sodium-binding benzofuran isophthalate] and Indo-1 epifluorescence) was significantly slowed in CaMKIIδ(-/-) myocytes (versus WT). CaMKIIδ(-/-) myocytes developed significantly less H(2)O(2)-induced arrhythmias and were more resistant to hypercontracture. Opposite results (increased late I(Na), [Na](i) and [Ca](i) accumulation) were obtained by overexpression of CaMKIIδ in rabbit myocytes (adenoviral gene transfer) reversible with CaMKII inhibition (10 μmol/L KN93 or 0.1 μmol/L AIP [autocamtide 2-related inhibitory peptide]).

CONCLUSIONS

Free [Ca](i) and a functional SR are required for ROS activation of CaMKII. ROS-activated CaMKIIδ enhances late I(Na), which may lead to cellular Na and Ca overload. This may be of relevance in hear failure, where enhanced ROS production meets increased CaMKII expression.

Regulation of sodium channel activity by phosphorylation

Voltage-gated sodium channels carry the major inward current responsible for action potential depolarization in excitable cells as well as providing additional inward current that modulates overall excitability. Both their expression and function is under tight control of protein phosphorylation by specific kinases and phosphatases and this control is particular to each type of sodium channel. This article examines the impact and mechanism of phosphorylation for isoforms where it has been studied in detail in an attempt to delineate common features as well as differences.

Estrogen and non-genomic upregulation of voltage-gated Na(+) channel activity in MDA-MB-231 human breast cancer cells: role in adhesion

External (but not internal) application of beta-estradiol (E2) increased the current amplitude of voltage-gated Na(+) channels (VGSCs) in MDA-MB-231 human breast cancer (BCa) cells. The G-protein activator GTP-gamma-S, by itself, also increased the VGSC current whilst the G-protein inhibitor GDP-beta-S decreased the effect of E2. Expression of GPR30 (a G-protein-coupled estrogen receptor) in MDA-MB-231 cells was confirmed by PCR, Western blot and immunocytochemistry. Importantly, G-1, a specific agonist for GPR30, also increased the VGSC current amplitude in a dose-dependent manner. Transfection and siRNA-silencing of GPR30 expression resulted in corresponding changes in GPR30 protein expression but only internally, and the response to E2 was not affected. The protein kinase A inhibitor, PKI, abolished the effect of E2, whilst forskolin, an adenylate cyclase activator, by itself, increased VGSC activity. On the other hand, pre-incubation of the MDA-MB-231 cells with brefeldin A (a trans-Golgi protein trafficking inhibitor) had no effect on the E2-induced increase in VGSC amplitude, indicating that such trafficking ('externalisation') of VGSC was not involved. Finally, acute application of E2 decreased cell adhesion whilst the specific VGSC blocker tetrodotoxin increased it. Co-application of E2 and tetrodotoxin inhibited the effect of E2 on cell adhesion, suggesting that the effect of E2 was mainly through VGSC activity. Pre-treatment of the cells with PKI abolished the effect of E2 on adhesion, consistent with the proposed role of PKA. Potential implications of the E2-induced non-genomic upregulation of VGSC activity for BCa progression are discussed.

Prostaglandin E2 promotes Na1.8 trafficking via its intracellular RRR motif through the protein kinase A pathway

Voltage-gated sodium channels (Na(v)) are essential for the initiation and propagation of action potentials in neurons. Na(v)1.8 activity is regulated by prostaglandin E(2) (PGE(2)). There is, however, no direct evidence showing the regulated trafficking of Na(v)1.8, and the molecular and cellular mechanism of PGE(2)-induced sodium channel trafficking is not clear. Here, we report that PGE(2) regulates the trafficking of Na(v)1.8 through the protein kinase A (PKA) signaling pathway, and an RRR motif in the first intracellular loop of Na(v)1.8 mediates this effect. In rat dorsal root ganglion (DRG) neurons, prolonged PGE(2) treatment enhanced Na(v)1.8 currents by increasing the channel density on the cell surface. Activation of PKA by forskolin had the same effect on DRG neurons and human embryonic kidney 293T cells expressing Na(v)1.8. Inhibition of PKA completely blocked the PGE(2)-promoted effect on Na(v)1.8. Mutation of five PKA phosphorylation sites or the RRR motif in the first intracellular loop of Na(v)1.8 abolished the PKA-promoted Na(v)1.8 surface expression. Furthermore, a membrane-tethered peptide containing the intracellular RRR motif disrupted the PGE(2)-induced promotion of the Na(v)1.8 current in DRG neurons. Our data indicate that PGE(2) promotes the surface expression of Na(v)1.8 via an intracellular RRR motif, and provide a novel mechanism for functional modulation of Na(v)1.8 by hyperalgesic agents.

Na+ channel regulation by Ca2+/calmodulin and Ca2+/calmodulin-dependent protein kinase II in guinea-pig ventricular myocytes

AIMS

Calmodulin (CaM) regulates Na+ channel gating through binding to an IQ-like motif in the C-terminus. Ca2+/CaM-dependent protein kinase II (CaMKII) regulates Ca2+ handling, and chronic overactivity of CaMKII is associated with left ventricular hypertrophy and dysfunction and lethal arrhythmias. However, the acute effects of Ca2+/CaM and CaMKII on cardiac Na+ channels are not fully understood.

METHODS AND RESULTS

Purified Na(V)1.5-glutathione-S-transferase fusion peptides were phosphorylated in vitro by CaMKII predominantly on the I-II linker. Whole-cell voltage-clamp was used to measure Na+ current (I(Na)) in isolated guinea-pig ventricular myocytes in the absence or presence of CaM or CaMKII in the pipette solution. CaMKII shifted the voltage dependence of Na+ channel availability by approximately +5 mV, hastened recovery from inactivation, decreased entry into intermediate or slow inactivation, and increased persistent (late) current, but did not change I(Na) decay. These CaMKII-induced changes of Na+ channel gating were completely abolished by a specific CaMKII inhibitor, autocamtide-2-related inhibitory peptide (AIP). Ca2+/CaM alone reproduced the CaMKII-induced changes of I(Na) availability and the fraction of channels undergoing slow inactivation, but did not alter recovery from inactivation or the magnitude of the late current. Furthermore, the CaM-induced changes were also completely abolished by AIP. On the other hand, cAMP-dependent protein kinase A inhibitors did not abolish the CaM/CaMKII-induced alterations of I(Na) function.

CONCLUSION

Ca2+/CaM and CaMKII have distinct effects on the inactivation phenotype of cardiac Na+ channels. The differences are consistent with CaM-independent effects of CaMKII on cardiac Na+ channel gating.

Beta-receptor activation increases sodium current in guinea pig heart

AIM

To study the influence of beta-receptor activation on sodium channel current and the physiological significance of increased sodium current with regard to the increased cardiac output caused by sympathetic excitation.

METHODS

Multiple experimental approaches, including ECG, action potential recording with conventional microelectrodes, whole-cell current measurements, single-channel recordings, and pumping-force measurements, were applied to guinea pig hearts and isolated ventricular myocytes.

RESULTS

Isoprenaline was found to dose-dependently shorten QRS waves, increase the amplitude and the V(max) of action potentials, augment the fast sodium current, and increase the occurrence frequencies and open time constants of the long-open and burst modes of the sodium channel. Increased levels of membrane-permeable cAMP have similar effects. In the presence of a calcium channel blocker, TTX reversed the increased pumping force produced by isoprenaline.

CONCLUSION

Beta-adrenergic modulation increases the inward sodium current and accelerates the conduction velocity within the ventricles by changing the sodium channel modes, which might both be conducive to the synchronous contraction of the heart and enhance its pumping function.

Regulation of cardiac excitation and contraction by p21 activated kinase-1

Cardiac excitation and contraction are regulated by a variety of signaling molecules. Central to the regulatory scheme are protein kinases and phosphatases that carry out reversible phosphorylation of different effectors. The process of beta-adrenergic stimulation mediated by cAMP dependent protein kinase (PKA) forms a well-known pathway considered as the most significant control mechanism in excitation and contraction as well as many other regulatory mechanisms in cardiac function. However, although dephosphorylation pathways are critical to these regulatory processes, signaling to phosphatases is relatively poorly understood. Emerging evidence indicates that regulation of phosphatases, which dampen the effect of beta-adrenergic stimulation, is also important. We review here functional studies of p21 activated kinase-1 (Pak1) and its potential role as an upstream signal for protein phosphatase PP2A in the heart. Pak1 is a serine/threonine protein kinase directly activated by the small GTPases Cdc42 and Rac1. Pak1 is highly expressed in different regions of the heart and modulates the activities of ion channels, sarcomeric proteins, and other phosphoproteins through up-regulation of PP2A activity. Coordination of Pak1 and PP2A activities is not only potentially involved in regulation of normal cardiac function, but is likely to be important in patho-physiological conditions.

Differential modulation of late sodium current by protein kinase A in R1623Q mutant of LQT3

AIMS

In the type 3 long QT syndrome (LQT3), shortening of the QT interval by overdrive pacing is used to prevent life-threatening arrhythmias. However, it is unclear whether accelerated heart rate induced by beta-adrenergic agents produces similar effects on the late sodium current (I(Na)) to those by overdrive pacing therapy. We analyzed the beta-adrenergic-like effects of protein kinase A and fluoride on I(Na) in R1623Q mutant channels.

MAIN METHODS

cDNA encoding either wild-type (WT) or R1623Q mutant of hNa(v)1.5 was stably transfected into HEK293 cells. I(Na) was recorded using a whole-cell patch-clamp technique at 23 degrees C.

KEY FINDINGS

In R1623Q channels, 2 mM pCPT-AMP and 120 mM fluoride significantly delayed macroscopic current decay and increased relative amplitude of the late I(Na) in a time-dependent manner. Modulations of peak I(Na) gating kinetics (activation, inactivation, recovery from inactivation) by fluoride were similar in WT and R1623Q channels. The effects of fluoride were almost completely abolished by concomitant dialysis with a protein kinase inhibitor. We also compared the effect of pacing with that of beta-adrenergic stimulation by analyzing the frequency-dependence of the late I(Na). Fluoride augmented frequency-dependent reduction of the late I(Na), which was due to preferential delay of recovery of late I(Na). However, the increase in late I(Na) by fluoride at steady-state was more potent than the frequency-dependent reduction of late I(Na).

SIGNIFICANCE

Different basic mechanisms participate in the QT interval shortening by pacing and beta-adrenergic stimulation in the LQT3.

Pharmacologic targets for atrial fibrillation

Despite advances in treatment, atrial fibrillation (AF) remains the most common arrhythmia in humans. Antiarrhythmic drug therapy continues to be a cornerstone of AF treatment, even in light of emerging non-pharmacologic therapies. Conventional antiarrhythmic drugs target cardiac ion channels and are often associated with modest AF suppression and the risk of ventricular proarrhythmia. Ongoing drug development has focused on targeting atrial-specific ion channels as well as novel non-ionic targets. Targeting non-ionic mechanisms may also provide new drugs directed towards the underlying mechanisms responsible for AF and possibly greater antiarrhythmic potency. Agents that act against these new targets may offer improved safety and efficacy in AF treatment.

Regulation of voltage-gated cardiac sodium current by epidermal growth factor receptor kinase in guinea pig ventricular myocytes

Voltage-gated cardiac fast sodium channel current (I(Na)) plays a critical role in the initiation and propagation of the myocardial action potential, and regulation of cardiac I(Na) by protein tyrosine kinases (PTKs) is not well documented, though it is known that ion channels are among the targets of PTKs. The present study was therefore designed to investigate whether/how cardiac I(Na) was modulated by PTKs in guinea pig ventricular myocytes using whole-cell patch clamp and immunoprecipitation and Western blotting approaches. It was found that cardiac I(Na) was enhanced by epidermal growth factor (EGF), and the effect was antagonized by the selective epidermal growth factor receptor (EGFR) kinase inhibitor tyrphostin AG556 while potentiated by orthovanadate (a protein tyrosine phosphatase (PTP) inhibitor). In addition, AG556 inhibited, while orthovanadate increased I(Na), and the inhibition of I(Na) by AG556 was antagonized by orthovanadate. Immunoprecipitation and Western blotting analysis demonstrated that tyrosine phosphorylation level of cardiac sodium channels was enhanced by EGF or orthovanadate, and reduced by AG556. The AG556-induced reduction of phosphorylation level was significantly reversed by orthovanadate. Our results demonstrate the novel information that EGFR kinase enhances, and PTPs reduce native cardiac I(Na) in guinea pig ventricular myocytes.

Reducing the Risk of Sudden Death in Heart Failure With β-Blockers

BACKGROUND

Heart failure (HF) is a serious cardiovascular syndrome that affects nearly 5 million people in the United States. A review of clinical data demonstrates that sudden cardiac death (SCD) accounts for approximately one-third of all HF deaths. This fatal outcome typically involves an unexpected electrical event leading to sustained cardiac arrhythmias resulting in cardiovascular collapse.

METHODS AND RESULTS

A systematic review of the literature was performed to serve as the basis for this review. Factors contributing directly to incidence of SCD in the HF population may include significant remodeling of the left ventricle (hypertrophy, dilation, and fibrosis) in addition to increased sympathetic activation. Using specific therapies to limit these mechanisms are beneficial in the HF patient by preventing SCD. Beta-blockers play a key role in the prevention of SCD for patients with HF by limiting the effects of circulating norepinephrine and by reducing left ventricular remodeling.

CONCLUSIONS

This review outlines the potential mechanisms and contributing factors of SCD in patients with HF and the impact of beta-blocker usage in the prevention of this fatal outcome for this growing patient population.

Effect of ethanol on cardiac single sodium channel gating

Alcohol in modest and higher doses has the potential to induce cardiac arrhythmias. The most famous alcohol-related arrhythmia is the "holiday heart syndrome". Furthermore, there is a clear association between excessive alcohol consumption and the risk of sudden cardiac death. However, the acute effects of ethanol on arrhythmia induction are not well understood. The effect of ethanol on single cardiac sodium channels has not been studied yet. To elucidate the effect of ethanol on human cardiac sodium channels we performed a patch clamp study in HEK-293 cells overexpressing the human cardiac sodium channel. We used HEK-293 cells overexpressing the human cardiac sodium channel (Na(1.5)). Single channel gating was investigated by the cell-attached patch clamp technique. Sodium channel currents were elicited by depolarizing pulses from -120 to -20mV for a duration of 150ms. Single channel availability, open probability and peak average current were assessed baseline and after addition of ethanol in increasing concentrations (0.50 per thousand (10.9mM), 1.00 per thousand (21.7mM), 2.00 per thousand (43.5mM) and 4.00 per thousand (87.0mM)). We found a concentration-dependent reduction of open probability which was statistically significant at 2.00 per thousand ethanol (66.5+/-14% of control). At higher concentrations (4.00 per thousand) also availability decreased to 66.5+/-11.0% of control. This resulted in a significant decrease of peak average current at 2.00 per thousand and at 4.00 per thousand ethanol (61.8+/-7.4 and 53.0+/-8.2% of control). For the first time the present study demonstrates acute inhibitory effects of ethanol on single cardiac sodium channel gating and provides one potential mechanism for the well known clinical observation that ethanol triggers supraventricular and ventricular arrhythmias.

Modulation of cardiac Na(+) and Ca(2+) currents by CaM and CaMKII

Sarcolemmal sodium (Na) and calcium (Ca) currents are fundamentally involved in shaping the cardiac action potential. Alterations in Na or Ca currents can change action potential characteristics and therefore might result in cardiac arrhythmias. Also, these ions contribute to excitation-contraction coupling and therefore are important in myocyte shortening and contractility of the heart. This review article summarizes how sarcolemmal Na and Ca channels are regulated with an emphasis on the novel role of Ca-dependent proteins Calmodulin (CaM) and especially Ca/CaM-dependent protein kinase II (CaMKII) to modulate sarcolemmal Na and Ca channels in the heart.

Autonomic Regulation of Voltage-Gated Cardiac Ion Channels

Altering voltage-gated ion channel currents, by changing channel number or voltage-dependent kinetics, regulates the propagation of action potentials along the plasma membrane of individual cells and from one cell to its neighbors. Functional increases in the number of cardiac sodium channels (Na(V)1.5) at the myocardial sarcolemma are accomplished by the regulation of caveolae by beta adrenergically stimulated G-proteins. We demonstrate that Na(V)1.5, Ca(V)1.2a, and K(V)1.5 channels specifically localize to isolated caveolar membranes, and to punctate regions of the sarcolemma labeled with caveolin-3. In addition, we show that Na(V)1.5, Ca(V)1.2a, and K(V)1.5 channel antibodies label the same subpopulation of isolated caveolae. Plasma membrane sheet assays demonstrate that Na(V)1.5, Ca(V)1.2a, and K(V)1.5 cluster with caveolin-3. This may have interesting implications for the way in which adrenergic pathways alter the cardiac action potential morphology and the velocity of the excitatory wave.

Mechanism of SNARE protein binding and regulation of Cav2 channels by phosphorylation of the synaptic protein interaction site

Ca(v)2.1 and Ca(v)2.2 channels conduct P/Q-type and N-type Ca(2+) currents that initiate neurotransmission and bind SNARE proteins through a synaptic protein interaction (synprint) site. PKC and CaMKII phosphorylate the synprint site and inhibit SNARE protein binding in vitro. Here we identify two separate microdomains that each bind syntaxin 1A and SNAP-25 in vitro and are regulated by PKC phosphorylation at serines 774 and 898 and CaMKII phosphorylation at serines 784 and 896. Activation of PKC resulted in its recruitment to and phosphorylation of Ca(V)2.2 channels, but PKC phosphorylation did not dissociate Ca(V)2.2 channel/syntaxin 1A complexes. Chimeric Ca(V)2.1a channels containing the synprint site of Ca(v)2.2 gain modulation by syntaxin 1A, which is blocked by PKC phosphorylation at the sites identified above. Our results support a bipartite model for the synprint site in which each SNARE-binding microdomain is controlled by a separate PKC and CaMKII phosphorylation site that regulates channel modulation by SNARE proteins.

Effects of ras and rap1 on electrical excitability of differentiated ng108-15 cells

Effects of two small G-proteins, Rap1 and Ras, on the sodium channel activity in NG108-15 cells were studied using sindbis virus-mediated gene transfer. When an activated Rap1A mutant (Rap1-12V, the activated mutant of Rap1 carrying glycine to valine substitution at codon 12) or a dominant-negative H-Ras mutant (Ras-17N, carrying serine to asparagine substitution at codon 17) was expressed in differentiated NG108-15 cells, the proportion of cells generating action potential decreased and the amplitudes of sodium current diminished. This effect was sensitive to an inhibitor of protein kinase A. The effects of a cyclic AMP (cAMP) analog (dibutyl cAMP) on sodium current in these cells were biphasic: inhibitory at lower concentrations (<100 microM) and enhancing at higher concentrations (200-500 microM). The inhibitory phase of cAMP effect was suppressed by an activated Ras mutant (Ras-12V) while the enhancing phase was suppressed by Rap1-12V. These data are consistent with the model that Rap1 and Ras function as counteracting regulators of voltage-gated sodium current through cAMP-dependent mechanisms.

Modulation of Nav1.7 and Nav1.8 peripheral nerve sodium channels by protein kinase A and protein kinase C

Voltage-gated Na+ channels (VGSC) are transmembrane proteins that are essential for the initiation and propagation of action potentials in neuronal excitability. Because neurons express a mixture of Na+ channel isoforms and protein kinase C (PKC) isozymes, the nature of which channel is being regulated by which PKC isozyme is not known. We showed that DRG VGSC Nav1.7 (TTX-sensitive) and Nav1.8 (TTX-resistant), expressed in Xenopus oocytes were differentially regulated by protein kinase A (PKA) and PKC isozymes using the two-electrode voltage-clamp method. PKA activation resulted in a dose-dependent potentiation of Nav1.8 currents and an attenuation of Nav1.7 currents. PKA-induced increases (Nav1.8) and decreases (Nav1.7) in peak currents were not associated with shifts in voltage-dependent activation or inactivation. The PKA-mediated increase in Nav1.8 current amplitude was prevented by chloroquine, suggesting that cell trafficking may contribute to the changes in Nav1.8 current amplitudes. A dose-dependent decrease in Nav1.7 and Nav1.8 currents was observed with the PKC activators phorbol 12-myristate, 13-acetate (PMA) and phorbol 12,13-dibutyrate. PMA induced shifts in the steady-state activation of Nav1.7 and Nav1.8 channels by 6.5 and 14 mV, respectively, in the depolarizing direction. The role of individual PKC isozymes in the regulation of Nav1.7 and Nav1.8 was determined using PKC-isozyme-specific peptide activators and inhibitors. The decrease in the Nav1.8 peak current induced by PMA was prevented by a specific epsilonPKC isozyme peptide antagonist, whereas the PMA effect on Nav1.7 was prevented by epsilonPKC and betaIIPKC peptide inhibitors. The data showed that Nav1.7 and Nav1.8 were differentially modulated by PKA and PKC. This is the first report demonstrating a functional role for epsilonPKC and betaIIPKC in the regulation of Nav1.7 and Nav1.8 Na+ channels. Identification of the particular PKC isozymes(s) that mediate the regulation of Na+ channels is essential for understanding the molecular mechanism involved in neuronal ion channel regulation in normal and pathological conditions.