Atrial fibrillation: Therapeutic potential of atrial K+ channel blockers

The SK4 channel allosteric blocker, BA6b9, reduces atrial fibrillation substrate in rats with reduced ejection fraction

Atrial fibrillation (AF), the most common cardiac arrhythmia, is strongly associated with several comorbidities including heart failure (HF). AF in general, and specifically in the context of HF, is progressive in nature and associated with poor clinical outcomes. Current therapies for AF are limited in number and efficacy and do not target the underlying causes of atrial remodeling such as inflammation or fibrosis. We previously identified the calcium-activated SK4 K+ channels, which are preferentially expressed in the atria relative to the ventricles in both rat and human hearts, as attractive druggable target for AF treatment. Here, we examined the ability of BA6b9, a novel allosteric inhibitor of SK4 channels that targets the specific calmodulin-PIP2 binding domain, to alter AF susceptibility and atrial remodeling in a systolic HF rat postmyocardial infarction (post-MI) model. Daily BA6b9 injection (20 mg/kg/day) for 3 weeks starting 1-week post-MI prolonged the atrial effective refractory period, reduced AF induction and duration, and dramatically prevented atrial structural remodeling. In the post-MI left atrium (LA), pronounced upregulation of the SK4 K+ channel was observed, with corresponding increases in collagen deposition, α-SMA levels, and NLRP3 inflammasome expression. Strikingly, BA6b9 treatment reversed these changes while also significantly reducing the lateralization of the atrial connexin Cx43 in the LA of post-MI rats. Our findings indicate that the blockade of SK4 K+ channels using BA6b9 not only favors rhythm control but also remarkably reduces atrial structural remodeling, a property that is highly desirable for novel AF therapies, particularly in patients with comorbid HF.

Exenatide reduces atrial fibrillation susceptibility by inhibiting hKv1.5 and hNav1.5 channels

Exenatide, a promising cardioprotective agent, protects against cardiac structural remodeling and diastolic dysfunction. Combined blockade of sodium and potassium channels is valuable for managing atrial fibrillation (AF). Here, we explored whether exenatide displayed anti-AF effects by inhibiting human Kv1.5 and Nav1.5 channels. We used the whole-cell patch-clamp technique to investigate the effects of exenatide on hKv1.5 and hNav1.5 channels expressed in human embryonic kidney 293 cells and studied the effects of exenatide on action potential (AP) and other cardiac ionic currents in rat atrial myocytes. Additionally, an electrical mapping system was used to explore the effects of exenatide on electrical properties and AF activity in isolated rat hearts. Finally, a rat AF model, established using acetylcholine and calcium chloride, was employed to evaluate the anti-AF potential of exenatide in rats. Exenatide reversibly suppressed IKv1.5 with IC50 of 3.08 μM, preferentially blocked the hKv1.5 channel in its closed state, and positively shifted the voltage-dependent activation curve. Exenatide also reversibly inhibited INav1.5 with IC50 of 3.30 μM, negatively shifted the voltage-dependent inactivation curve, and slowed its recovery from inactivation with significant use-dependency at 5 and 10 Hz. Furthermore, exenatide prolonged AP duration and suppressed the sustained K+ current (Iss) and transient outward K+ current (Ito), but without inhibition of L-type Ca2+ current (ICa,L) in rat atrial myocytes. Exenatide prevented AF incidence and duration in rat hearts and rats. These findings demonstrate that exenatide inhibits IKv1.5 and INav1.5in vitro and reduces AF susceptibility in isolated rat hearts and rats.

Research on atrial fibrillation mechanisms and prediction of therapeutic prospects: focus on the autonomic nervous system upstream pathways

Atrial fibrillation (AF) is the most common clinical arrhythmia disorder. It can easily lead to complications such as thromboembolism, palpitations, dizziness, angina, heart failure, and stroke. The disability and mortality rates associated with AF are extremely high, significantly affecting the quality of life and work of patients. With the deepening of research into the brain-heart connection, the link between AF and stroke has become increasingly evident. AF is now categorized as either Known Atrial Fibrillation (KAF) or Atrial Fibrillation Detected After Stroke (AFDAS), with stroke as the baseline. This article, through a literature review, briefly summarizes the current pathogenesis of KAF and AFDAS, as well as the status of their clinical pharmacological and non-pharmacological treatments. It has been found that the existing treatments for KAF and AFDAS have limited efficacy and are often associated with significant adverse reactions and a risk of recurrence. Moreover, most drugs and treatment methods tend to focus on a single mechanism pathway. For example, drugs targeting ion channels primarily modulate ion channels and have relatively limited impact on other pathways. This limitation underscores the need to break away from the "one disease, one target, one drug/measurement" dogma for the development of innovative treatments, promoting both drug and non-drug therapies and significantly improving the quality of clinical treatment. With the increasing refinement of the overall mechanisms of KAF and AFDAS, a deeper exploration of physiological pathology, and comprehensive research on the brain-heart relationship, it is imperative to shift from long-term symptom management to more precise and optimized treatment methods that are effective for almost all patients. We anticipate that drugs or non-drug therapies targeting the central nervous system and upstream pathways can guide the simultaneous treatment of multiple downstream pathways in AF, thereby becoming a new breakthrough in AF treatment research.

Multitargeting in cardioprotection: An example of biaromatic compounds

A multitarget drug design approach is actively developing in modern medicinal chemistry and pharmacology, especially with regard to multifactorial diseases such as cardiovascular diseases, cancer, and neurodegenerative diseases. A detailed study of many well-known drugs developed within the single-target approach also often reveals additional mechanisms of their real pharmacological action. One of the multitarget drug design approaches can be the identification of the basic pharmacophore models corresponding to a wide range of the required target ligands. Among such models in the group of cardioprotectors is the linked biaromatic system. This review develops the concept of a "basic pharmacophore" using the biaromatic pharmacophore of cardioprotectors as an example. It presents an analysis of possible biological targets for compounds corresponding to the biaromatic pharmacophore and an analysis of the spectrum of biological targets for the five most known and most studied cardioprotective drugs corresponding to this model, and their involvement in the biological effects of these drugs.

Tic-Tac: A Translational Approach in Mechanisms Associated with Irregular Heartbeat and Sinus Rhythm Restoration in Atrial Fibrillation Patients

Atrial fibrillation (AF) is a prevalent cardiac condition predominantly affecting older adults, characterized by irregular heartbeat rhythm. The condition often leads to significant disability and increased mortality rates. Traditionally, two therapeutic strategies have been employed for its treatment: heart rate control and rhythm control. Recent clinical studies have emphasized the critical role of early restoration of sinus rhythm in improving patient outcomes. The persistence of the irregular rhythm allows for the progression and structural remodeling of the atria, eventually leading to irreversible stages, as observed clinically when AF becomes permanent. Cardioversion to sinus rhythm alters this progression pattern through mechanisms that are still being studied. In this review, we provide an in-depth analysis of the pathophysiological mechanisms responsible for maintaining AF and how they are modified during sinus rhythm restoration using existing therapeutic strategies at different stages of clinical investigation. Moreover, we explore potential future therapeutic approaches, including the promising prospect of gene therapy.

TREK channels in Mechanotransduction: a Focus on the Cardiovascular System

Mechano-electric feedback is one of the most important subsystems operating in the cardiovascular system, but the underlying molecular mechanism remains rather unknown. Several proteins have been proposed to explain the molecular mechanism of mechano-transduction. Transient receptor potential (TRP) and Piezo channels appear to be the most important candidates to constitute the molecular mechanism behind of the inward current in response to a mechanical stimulus. However, the inhibitory/regulatory processes involving potassium channels that operate on the cardiac system are less well known. TWIK-Related potassium (TREK) channels have emerged as strong candidates due to their capacity for the regulation of the flow of potassium in response to mechanical stimuli. Current data strongly suggest that TREK channels play a role as mechano-transducers in different components of the cardiovascular system, not only at central (heart) but also at peripheral (vascular) level. In this context, this review summarizes and highlights the main existing evidence connecting this important subfamily of potassium channels with the cardiac mechano-transduction process, discussing molecular and biophysical aspects of such a connection.

Potassium channels, tumorigenesis and targeted drugs

Potassium channels play an important role in human physiological function. Recently, various molecular mechanisms have implicated abnormal functioning of potassium channels in the proliferation, migration, invasion, apoptosis, and cancer stem cell phenotype formation. Potassium channels also mediate the association of tumor cells with the tumor microenvironment. Meanwhile, potassium channels are important targets for cancer chemotherapy. A variety of drugs exert anti-cancer effects by modulating potassium channels in tumor cells. Therefore, there is a need to understand how potassium channels participate in tumor development and progression, which could reveal new, novel targets for cancer diagnosis and treatment. This review summarizes the roles of voltage-gated potassium channels, calcium-activated potassium channels, inwardly rectifying potassium channels, and two-pore domain potassium channels in tumorigenesis and the underlying mechanism of potassium channel-targeted drugs. Therefore, the study lays the foundation for rational and effective drug design and individualized clinical therapeutics.

Open channel block of Kv1.5 channels by HMQ1611

Kv1.5 channels conduct the ultra-rapid delayed rectifier potassium current (I_Kur). Pharmacological blockade of human Kv1.5 (hKv1.5) has been regarded as an effective treatment of re-entrant based atrial fibrillation, because Kv1.5 is highly expressed in human cardiac atria but scarcely in ventricles. The Kv1.5 blockade is also expected to be used in cancer therapeutics since Kv1.5 is overexpressed in some types of human tumors. Here, we investigated the blockade of hKv1.5 channels by HMQ1611, a symmetrical biphenyl derivative. hKv1.5 channels were heterologously expressed in Chinese hamster ovary cells. The effects of HMQ1611 on wild-type and 13 hKv1.5 mutant channels were examined using the whole-cell patch-clamp method, and molecular docking simulation was conducted to predict the docking position of HMQ1611 within Kv1.5 channels. We showed that HMQ1611 reversibly inhibited the hKv1.5 current in a concentration-dependent manner (IC₅₀ = 2.07 μM). HMQ1611 blockade of hKv1.5 current developed with time during depolarizing voltage-clamp steps, and this blockade was also voltage-dependent with a steep increase over the voltage range for channel openings. HMQ1611 inhibition was significantly reduced in the T479A, T480A, V505A, I508A, L510A, V512A, and V516A hKv1.5 mutant channels. Molecular docking analysis predicted that V505, V512, and T480 were involved in the blocking action of HMQ1611 on hKv1.5 channels. These results suggest that HMQ1611 inhibits hKv1.5 currents as an open channel blocker. Amino acid residues located at the base of the selectivity filter (T479 and T480) and in the S6 segment (V505, I508, L510, V512, and V516) of hKv1.5 appear to constitute potential binding sites for HMQ1611.

Allosteric inhibitors targeting the calmodulin-PIP2 interface of SK4 K+ channels for atrial fibrillation treatment

The Ca²⁺-activated SK4 K⁺ channel is gated by Ca²⁺-calmodulin (CaM) and is expressed in immune cells, brain, and heart. A cryoelectron microscopy (cryo-EM) structure of the human SK4 K⁺ channel recently revealed four CaM molecules per channel tetramer, where the apo CaM C-lobe and the holo CaM N-lobe interact with the proximal carboxyl terminus and the linker S4-S5, respectively, to gate the channel. Here, we show that phosphatidylinositol 4-5 bisphosphate (PIP2) potently activates SK4 channels by docking to the boundary of the CaM-binding domain. An allosteric blocker, BA6b9, was designed to act to the CaM-PIP2-binding domain, a previously untargeted region of SK4 channels, at the interface of the proximal carboxyl terminus and the linker S4-S5. Site-directed mutagenesis, molecular docking, and patch-clamp electrophysiology indicate that BA6b9 inhibits SK4 channels by interacting with two specific residues, Arg191 and His192 in the linker S4-S5, not conserved in SK1-SK3 subunits, thereby conferring selectivity and preventing the Ca²⁺-CaM N-lobe from properly interacting with the channel linker region. Immunohistochemistry of the SK4 channel protein in rat hearts showed a widespread expression in the sarcolemma of atrial myocytes, with a sarcomeric striated Z-band pattern, and a weaker occurrence in the ventricle but a marked incidence at the intercalated discs. BA6b9 significantly prolonged atrial and atrioventricular effective refractory periods in rat isolated hearts and reduced atrial fibrillation induction ex vivo. Our work suggests that inhibition of SK4 K⁺ channels by targeting drugs to the CaM-PIP2-binding domain provides a promising anti-arrhythmic therapy.

Regulation of cardiac ion channels by transcription factors: Looking for new opportunities of druggable targets for the treatment of arrhythmias

Cardiac electrical activity is governed by different ion channels that generate action potentials. Acquired or inherited abnormalities in the expression and/or function of ion channels usually result in electrophysiological changes that can cause cardiac arrhythmias. Transcription factors (TFs) control gene transcription by binding to specific DNA sequences adjacent to target genes. Linkage analysis, candidate-gene screening within families, and genome-wide association studies have linked rare and common genetic variants in the genes encoding TFs with genetically-determined cardiac arrhythmias. Besides its critical role in cardiac development, recent data demonstrated that they control cardiac electrical activity through the direct regulation of the expression and function of cardiac ion channels in adult hearts. This narrative review summarizes some studies showing functional data on regulation of the main human atrial and ventricular Na⁺, Ca²⁺, and K⁺ channels by cardiac TFs such as Pitx2c, Tbx20, Tbx5, Zfhx3, among others. The results have improved our understanding of the mechanisms regulating cardiac electrical activity and may open new avenues for therapeutic interventions in cardiac acquired or inherited arrhythmias through the identification of TFs as potential drug targets. Even though TFs have for a long time been considered as 'undruggable' targets, advances in structural biology have led to the identification of unique pockets in TFs amenable to be targeted with small-molecule drugs or peptides that are emerging as novel therapeutic drugs.

Dietary ω-3 fatty acids reduced atrial fibrillation vulnerability via attenuating myocardial endoplasmic reticulum stress and inflammation in a canine model of atrial fibrillation

BACKGROUND

Dietary consumption of ω-3 fatty acids is correlated with a reduced incidence of cardiovascular events. Here, we investigated the effect of dietary ω-3 fatty acids on atrial fibrillation (AF) vulnerability in a canine model of AF and explored the related mechanisms.

METHODS

Twenty four male beagle dogs (weight, 8-10 kg) were randomly divided into four groups: (a) sham-operated group (normal chow); (b) AF+FO [AF and normal chow supplemented with fish oil (FO): 0.6 g n-3 polyunsaturated fatty acids (ω-3 PUFA) /kg/day]; (c) AF group (normal chow); (d) sham-operated FO group (chow supplemented with FO: 0.6 g ω-3 PUFA/kg/day). AF was induced by rapid atrial pacing (RAP: 400 bpm for 4 weeks). Daily oral administration of FO was initiated 1 week before surgery and continued for 4 weeks post operation.

RESULTS

Atrial electric remodeling was significantly attenuated and AF vulnerability were significantly reduced in AF+FO group compared to AF group. Endoplasmic reticulum (ER) stress-related protein expression levels of glucose-regulated protein78, C/EBP homologous protein, cleaved-Caspase12, and phosphorylation of protein kinase R-like ER kinase as well as inflammatory cytokines interleukin-1β, interleukin-6, tumor necrosis factor-α in left atrium (LA) were significantly downregulated in AF+FO group than in AF group (all p<0.05). In addition, Masson staining revealed lower extent of LA interstitial fibrosis in AF+FO group than in AF group (p<0.01). Myocardial apoptosis was also significantly reduced in AF+FO group than in AF group (p<0.05).

CONCLUSIONS

Dietary ω-3 fatty acids could significantly reduce RAP-induced AF vulnerability, possibly via attenuating myocardial ER stress, inflammation, and apoptosis in this canine model of AF.

Carbon monoxide activation of delayed rectifier potassium currents of human cardiac fibroblasts through diverse pathways

To identify the effect and mechanism of carbon monoxide (CO) on delayed rectifier K⁺ currents (I_K) of human cardiac fibroblasts (HCFs), we used the wholecell mode patch-clamp technique. Application of CO delivered by carbon monoxidereleasing molecule-3 (CORM3) increased the amplitude of outward K⁺ currents, and diphenyl phosphine oxide-1 (a specific I_K blocker) inhibited the currents. CORM3- induced augmentation was blocked by pretreatment with nitric oxide synthase blockers (L-NG-monomethyl arginine citrate and L-NG-nitro arginine methyl ester). Pretreatment with KT5823 (a protein kinas G blocker), 1H-[1,-2,-4] oxadiazolo-[4,-3-a] quinoxalin-1-on (ODQ, a soluble guanylate cyclase blocker), KT5720 (a protein kinase A blocker), and SQ22536 (an adenylate cyclase blocker) blocked the CORM3 stimulating effect on I_K. In addition, pretreatment with SB239063 (a p38 mitogen-activated protein kinase [MAPK] blocker) and PD98059 (a p44/42 MAPK blocker) also blocked the CORM3's effect on the currents. When testing the involvement of S-nitrosylation, pretreatment of N-ethylmaleimide (a thiol-alkylating reagent) blocked CO-induced I_K activation and DL-dithiothreitol (a reducing agent) reversed this effect. Pretreatment with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)-21H,23H porphyrin manganese (III) pentachloride and manganese (III) tetrakis (4-benzoic acid) porphyrin chloride (superoxide dismutase mimetics), diphenyleneiodonium chloride (an NADPH oxidase blocker), or allopurinol (a xanthine oxidase blocker) also inhibited CO-induced I_K activation. These results suggest that CO enhances I_K in HCFs through the nitric oxide, phosphorylation by protein kinase G, protein kinase A, and MAPK, S-nitrosylation and reduction/oxidation (redox) signaling pathways.

Linked biaromatic compounds as cardioprotective agents

Cardiovascular diseases (CVDs) are widespread in the modern world, and their number is constantly growing. For a long time, CVDs have been the leading cause of morbidity and mortality worldwide. Drugs for the treatment of CVD have been developed almost since the beginning of the 20th century, and a large number of effective cardioprotective agents of various classes have been created. Nevertheless, the need for the design and development of new safe drugs for the treatment of CVD remains. Literature data indicate that a huge number of cardioprotective agents of various generations and mechanisms correspond to a single generalized pharmacophore model containing two aromatic nuclei linked by a linear linker. In this regard, we put forward a concept for the design of a new generation of cardioprotective agents with a multitarget mechanism of action within the indicated pharmacophore model. This review is devoted to a generalization of the currently known compounds with cardioprotective properties and corresponding to the pharmacophore model of biaromatic compounds linked by a linear linker. Particular attention is paid to the history of the creation of these drugs, approaches to their design, and analysis of the structure-action relationship within each class.

Ion Channel Expression and Electrophysiology of Singular Human (Primary and Induced Pluripotent Stem Cell-Derived) Cardiomyocytes

Subtype-specific human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are promising tools, e.g., to assess the potential of drugs to cause chronotropic effects (nodal hiPSC-CMs), atrial fibrillation (atrial hiPSC-CMs), or ventricular arrhythmias (ventricular hiPSC-CMs). We used single-cell patch-clamp reverse transcriptase-quantitative polymerase chain reaction to clarify the composition of the iCell cardiomyocyte population (Fujifilm Cellular Dynamics, Madison, WI, USA) and to compare it with atrial and ventricular Pluricytes (Ncardia, Charleroi, Belgium) and primary human atrial and ventricular cardiomyocytes. The comparison of beating and non-beating iCell cardiomyocytes did not support the presence of true nodal, atrial, and ventricular cells in this hiPSC-CM population. The comparison of atrial and ventricular Pluricytes with primary human cardiomyocytes showed trends, indicating the potential to derive more subtype-specific hiPSC-CM models using appropriate differentiation protocols. Nevertheless, the single-cell phenotypes of the majority of the hiPSC-CMs showed a combination of attributes which may be interpreted as a mixture of traits of adult cardiomyocyte subtypes: (i) nodal: spontaneous action potentials and high HCN4 expression and (ii) non-nodal: prominent INa-driven fast inward current and high expression of SCN5A. This may hamper the interpretation of the drug effects on parameters depending on a combination of ionic currents, such as beat rate. However, the proven expression of specific ion channels supports the evaluation of the drug effects on ionic currents in a more realistic cardiomyocyte environment than in recombinant non-cardiomyocyte systems.

Exosomes in atrial fibrillation: therapeutic potential and role as clinical biomarkers

Atrial fibrillation (AF), the most common cardiac arrhythmia, is a global epidemic. AF can cause heart failure and myocardial infarction and increase the risk of stroke, disability, and thromboembolic events. AF is becoming increasingly ubiquitous and is associated with increased morbidity and mortality at higher ages, resulting in an increasing threat to human health as well as substantial medical and social costs. Currently, treatment strategies for AF focus on controlling heart rate and rhythm with medications to restore and maintain sinus rhythm, but this approach has limitations. Catheter ablation is not entirely satisfactory and does not address the issues underlying AF. Research exploring the mechanisms causing AF is urgently needed for improved prevention, diagnosis, and treatment of AF. Exosomes are small vesicles (30-150 nm) released by cells that transmit information between cells. MicroRNAs in exosomes play an important role in the pathogenesis of AF and are established as a biomarker for AF. In this review, a summary of the role of exosomes in AF is presented. The role of exosomes and microRNAs in AF occurrence, their therapeutic potential, and their potential role as clinical biomarkers is considered. A better understanding of exosomes has the potential to improve the prognosis of AF patients worldwide, reducing the global medical burden of this disease.

Contribution of K2P Potassium Channels to Cardiac Physiology and Pathophysiology

Years before the first two-pore domain potassium channel (K2P) was cloned, certain ion channels had already been demonstrated to be present in the heart with characteristics and properties usually attributed to the TREK channels (a subfamily of K2P channels). K2P channels were later detected in cardiac tissue by RT-PCR, although the distribution of the different K2P subfamilies in the heart seems to depend on the species analyzed. In order to collect relevant information in this regard, we focus here on the TWIK, TASK and TREK cardiac channels, their putative roles in cardiac physiology and their implication in coronary pathologies. Most of the RNA expression data and electrophysiological recordings available to date support the presence of these different K2P subfamilies in distinct cardiac cells. Likewise, we show how these channels may be involved in certain pathologies, such as atrial fibrillation, long QT syndrome and Brugada syndrome.

Inhibition of Small-Conductance, Ca2+-Activated K+ Current by Ondansetron

Background: Small-conductance Ca²⁺-activated K⁺ channels (SK channels) have been proposed as antiarrhythmic targets for the treatment of atrial fibrillation. We previously demonstrated that the 5-HT₃ receptor antagonist ondansetron inhibits heterologously expressed, human SK2 (hSK2) currents as well as native cardiac SK currents in a physiological extra-/intracellular [K⁺] gradient at therapeutic (i.e., sub-micromolar) concentrations. A recent study, using symmetrical [K⁺] conditions, challenged this result. The goal of the present study was to revisit the inhibitory effect of ondansetron on hSK2-mediated currents in symmetrical [K⁺] conditions.

Experimental Approach: The whole-cell patch clamp technique was used to investigate the effects of ondansetron and apamin on hSK2-mediated currents expressed in HEK 293 cells. Currents were measured in symmetrical [K⁺] conditions in the presence of 100 nM [Ca²⁺]_o.

Results: Expression of hSK2 produced inwardly rectifying whole-cell currents in the presence of 400 nM free cytosolic Ca²⁺. Ondansetron inhibited whole-cell hSK2 currents with IC₅₀ values of 154 and 113 nM at −80 and 40 mV, respectively. Macroscopic current inhibited by ondansetron and current inhibited by apamin exhibited inwardly rectifying current-voltage relationships with similar reversal potentials (apamin, ∼5 mV and ondansetron, ∼2 mV). Ondansetron (1 μM) in the continuing presence of apamin (100 nM) had no effect on hSK2-mediated whole-cell currents. Wild-type HEK 293 cells did not express ondansetron- or apamin-sensitive currents.

Conclusion: Ondansetron in sub-micromolar concentrations inhibits hSK2 currents even under altered ionic conditions.

Targeting of Potassium Channels in Cardiac Arrhythmias

Cardiomyocytes are endowed with a complex repertoire of ion channels, responsible for the generation of action potentials (APs), travelling waves of electrical excitation, propagating throughout the heart and leading to cardiac contractions. Cardiac AP waveforms are shaped by a striking diversity of K⁺ channels. The pivotal role of K⁺ channels in cardiac health and disease is underscored by the dramatic impact that K⁺ channel dysfunction has on cardiac arrhythmias. The development of drugs targeted to specific K⁺ channels is expected to provide an optimized approach to antiarrhythmic therapy. Here, we review the functional roles of cardiac potassium channels under normal and diseased states. We survey current antiarrhythmic drugs (AADs) targeted to voltage-gated and Ca²⁺-activated K⁺ channels and highlight future research opportunities.

Atrial fibrillation and autonomic nervous system: A translational approach to guide therapeutic goals

The autonomic nervous system (ANS) is known to play an important role in the genesis and maintenance of atrial fibrillation (AF). Biomolecular and genetic mechanisms, anatomical knowledges with recent diagnostic techniques acquisitions, both invasive and non-invasive, have enabled greater therapeutic goals in patients affected by AF related to ANS imbalance. Catheter ablation of ganglionated plexi (GP) in the left and right atrium has been proposed in varied clinical conditions. Moreover interesting results arise from renal sympathetic denervation and vagal nerve stimulation. Despite all this, in the scenario of ANS modulation translational strategies we necessary must consider the treatment or correction of dynamic factors such as obesity, obstructive sleep apnea, lifestyle, food, and stress. Finally, new antiarrhythmic drugs, gene therapy and "ablatogenomic" could be represent exciting future therapeutic perspectives.

Atrial fibrillation risk in patients suffering from type I diabetes mellitus. A review of clinical and experimental evidence

Atrial fibrillation (AF) and diabetes mellitus (DM) are commonly encountered in clinical practice. Although, the long term macrovascular and microvascular sequela of DM are well validated, the association between the less prevalent type 1 DM (T1DM) and atrial arrhythmogenesis is poorly understood. In the present review we highlight the current experimental and clinical data addressing this complex interaction. Animal studies support that T1DM, characterized by insulin deficiency and glycemic variability, impairs phosphatidylinositol 3‑kinase (PI3K)/protein kinase B signaling pathway. This pathway holds a central role in atrial electrical and structural remodeling responsible for arrhythmia initiation and maintenance. The molecular ''footprint'' of T1DM in atrial myocytes seems to involve a state of increased oxidative stress, impaired glucose transportation, ionic channel dysregulation and eventually fibrosis. On the contrary only a few clinical studies have examined the role of T1DM as an independent risk factor for AF development, and are discussed here. Further research is needed to solidify the real magnitude of this association and to investigate the clinical implications of PI3K molecular signaling pathway in atrial fibrillation management.

Modulation of SK Channels: Insight Into Therapeutics of Atrial Fibrillation

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia in the world. Although much technological progress in the treatment of AF has been made, there is an urgent need for better treatment of AF due to its high rates of morbidity and mortality. The anti-arrhythmic drugs currently approved for marketing have significant limitations and side effects such as life-threatening ventricular arrhythmias and hypotension. The small conductance Ca²⁺-activated K⁺ channels (SK channels) are dependent on intracellular Ca²⁺ concentrations, which tightly integrate with membrane potential. Given the predominant expression in the atria of many species, including humans, they are now emerging as a therapeutic target for treating AF. This review aimed to illustrate the characteristics and function of SK channels. Moreover, it discussed the regulation of SK channels and their potential as a therapeutic target of AF.

Anxiety, Mental Stress, and Sudden Cardiac Arrest: Epidemiology, Possible Mechanisms and Future Research

Sudden cardiac arrest (SCA) is a leading cause of mortality and morbidity in affluent societies, which underscores the need to identify persons at risk. The etiology of SCA is however complex, with predisposing and precipitating factors interacting. Although anxiety and mental stress have been linked to SCA for decades, their precise role and impact remain unclear and the biological underpinnings are insufficiently understood. In this paper, we systematically reviewed various types of observational studies (total n = 20) examining the association between anxiety or mental stress and SCA. Multiple methodological considerations challenged the summarizing and interpretation of the findings. For anxiety, the overall picture suggests that it predisposes for SCA in physically healthy populations (unadjusted OR = 2.44; 95% CI: 1.06-5.59; n = 3). However, in populations at risk for SCA (n = 4), associations were heterogeneous but not significant. Anxiety may partly predispose to SCA by contributing to other risk factors such as cardiovascular disease and diabetes mellitus via mechanisms such as unhealthy lifestyle and metabolic abnormalities. Mental stress appears to precipitate SCA, presumably by more directly impacting on the cardiac ion channels that control the heart's electrical properties. This may lead to ventricular fibrillation, the arrhythmia that underlies SCA. To advance this field of research, experimental studies that unravel the underlying biological mechanisms are deemed important, and most easily designed for mental stress as a precipitating factor because of the short timeframe. These proof-of-concept studies should examine the whole pathway from the brain to the autonomic nervous system, and eventually to cardiac ion channels. Ultimately, such studies may facilitate the identification of persons at risk and the development of novel preventive strategies.

Expression of miRNAs in plasma exosomes derived from patients with atrial fibrillation

BACKGROUND

Studies have revealed the association between exosomes and cardiovascular diseases. However, the typical changes of plasma miRNAs in patients with atrial fibrillation (AF) are still controversial, the use of exosomal miRNAs to diagnose and predict the prognosis of AF has not been described.

HYPOTHESIS

We hypothesized that there were differences in the exosomal miRNAs between AF and normal sinus rhythm (SR) patients, which might be used as the novel biomarkers to reflect the progression of AF.

METHODS

miRNAs were isolated from the plasma of patients, and the target genes of differential miRNAs via enrichment analysis to discover potential pathogenesis related to AF. Combined with high-throughput sequencing results, real-time PCR was used to verify the relative expression of target miRNAs in patients.

RESULTS

This study confirmed that the expression of plasma-derived exosomal miRNAs between patients with AF and SR were different. Target gene enrichment analysis suggested that the target genes of 20 miRNAs, which were significantly upregulated were mainly enriched in biological processes such as gene expression process, inflammation response, enzyme modification, etc. Meanwhile, mitogen-activated protein kinase (MAPK), mammalian target of rapamycin (mTOR), and other pathways were highly enriched. The expressions of miR-92b-3p, miR-1306-5p, and miR-let-7b-3p had differences between patients with AF and SR.

CONCLUSION

These miRNAs and target genes were involved in the process of AF through affecting biological processes such as energy metabolism, lipid metabolism, inflammation, and enzyme activity. It suggested that the exosomal miRNAs might be used as the novel biomarkers to reflect the progression of AF.

Characterization of iCell cardiomyocytes using single-cell RNA-sequencing methods

INTRODUCTION

Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes are being evaluated for their use in pharmacological and toxicological testing, particularly for electrophysiological side effects. However, little is known about the composition of the commercially available iCell cardiomyocyte (Fuijifilm Cellular Dynamics) cultures and the transcriptomic phenotype of individual cells.

METHODS

We characterized iCell cardiomyocytes (assumed to be a mixture of nodal-, atrial-, and ventricular-like cardiomyocytes together with potential residual non-myocytes) using bulk RNA-sequencing, followed by investigation of cellular heterogeneity using two different single-cell RNA-sequencing platforms.

RESULTS

Bulk RNA-sequencing identified key cardiac markers (TNNT2, MYL7) as well as fibroblast associated genes (P4HB, VIM), and cardiac ion channels in the iCell cardiomyocyte culture. High-resolution single cell RNA-sequencing demonstrated that both, cardiac and fibroblast-related genes were co-expressed throughout the cell population. This approach resolved two cell clusters within iCell cardiomyocytes. Interestingly, these clusters could not be associated with known cardiac subtypes. However, transcripts of ion channels potentially useful as functional markers for cardiac subtypes were below the detection limits of the single-cell approaches used. Instead, one cluster (10.8% of the cells) is defined by co-expression of cardiac and cell cycle-related genes (e.g. TOP2A). Incorporation of bromodeoxyuridine further confirmed the capability of iCell cardiomyocytes to enter cell cycle.

DISCUSSION

The co-expression of cardiac related genes with cell cycle or fibroblast related genes may be interpreted either as aberrant or as an immature feature. However, this excludes the presence of a non-cardiomyocyte sub-population and indicates that some cardiomyocytes themselves enter cell cycle.

Inhibitory Effects of Dronedarone on Small Conductance Calcium Activated Potassium Channels in Patients with Chronic Atrial Fibrillation: Comparison to Amiodarone

BACKGROUND Dysfunction of small conductance calcium activated potassium (SK) channels plays a vital role in atrial arrhythmogenesis. Amiodarone and dronedarone are the most effective class III antiarrhythmic drugs. It is unclear whether the antiarrhythmic effect of amiodarone and dronedarone is related to SK channel inhibition. MATERIAL AND METHODS Tissue samples were obtained from the right atria of 46 patients with normal sinus rhythm and 39 patients with chronic atrial fibrillation. Isolated atrial myocytes were obtained by enzymatic dissociation. KCNN2 (SK2) channels were transiently expressed in human embryonic kidney (HEK)-293 cells. SK currents were recorded using whole-cell conventional patch clamp techniques. RESULTS Amiodarone and dronedarone showed a concentration-dependent inhibitory effect on SK currents (IKAS) in atrial myocytes from normal sinus rhythm patients and chronic atrial fibrillation patients. The suppressed efficacy of dronedarone and amiodarone on IKAS was greater in atrial myocytes from chronic atrial fibrillation patients than that from normal sinus rhythm patients. Furthermore, in patients with chronic atrial fibrillation, the IC₅₀ value was 2.42 µM with dronedarone and 8.03 µM with amiodarone. In HEK-293 cells with transiently transfected SK2 channels, both dronedarone and amiodarone had a dose-dependent inhibitory effect on IKAS. The IC₅₀ value was 1.7 µM with dronedarone and 7.2 µM with amiodarone in cells from patients with chronic atrial fibrillation. Compared to amiodarone, dronedarone is more efficacy to inhibit IKAS and could be a potential intervention for patients with chronic atrial fibrillation. CONCLUSIONS Dronedarone provides a great degree of IKAS inhibition in atrial myocytes from chronic atrial fibrillation than amiodarone. IKAS might be a potential target of amiodarone and dronedarone for the management of chronic atrial fibrillation.

Photosensitive nanocarriers for specific delivery of cargo into cells

Nanoencapsulation is a rapidly expanding technology to enclose cargo into inert material at the nanoscale size, which protects cargo from degradation, improves bioavailability and allows for controlled release. Encapsulation of drugs into functional nanocarriers enhances their specificity, targeting ability, efficiency, and effectiveness. Functionality may come from cell targeting biomolecules that direct nanocarriers to a specific cell or tissue. Delivery is usually mediated by diffusion and erosion mechanisms, but in some cases, this is not sufficient to reach the expected therapeutic effects. This work reports on the development of a new photoresponsive polymeric nanocarrier (PNc)-based nanobioconjugate (NBc) for specific photo-delivery of cargo into target cells. We readily synthesized the PNcs by modification of chitosan with ultraviolet (UV)-photosensitive azobenzene molecules, with Nile red and dofetilide as cargo models to prove the encapsulation/release concept. The PNcs were further functionalized with the cardiac targeting transmembrane peptide and efficiently internalized into cardiomyocytes, as a cell line model. Intracellular cargo-release was dramatically accelerated upon a very short UV-light irradiation time. Delivering cargo in a time-space controlled fashion by means of NBcs is a promising strategy to increase the intracellular cargo concentration, to decrease dose and cargo side effects, thereby improving the effectiveness of a therapeutic regime.

Losartan inhibits hyposmotic-induced increase of IKs current and shortening of action potential duration in guinea pig atrial myocytes

Objective:

The present study aims to investigate the effect of losartan, an selective angiotensin II type 1 receptor (AT₁R) blocker, on both the increase of I_Ks current and shortening of action potential duration (APD) induced by stretch of atrial myocytes, and to uncover the mechanism underlying the treatment of fibrillation (AF) by AT₁R blockers.

Methods:

Hyposmotic solution (Hypo-S) was applied in the guinea pig atrial myocytes to simulate cell stretch, then patch-clamp technique was applied to record the I_Ks and APD in atrial myocytes.

Results:

Hypo-S increased the I_Ks by 105.6%, while Hypo-S+1-20 µM of losartan only increased the I_Ks by 70.3-75.5% (p<0.05 vs. Hypo-S). Meanwhile, Hypo-S shortened APD₉₀ by 20.2%, while Hypo-S+1-20 µM of losartan shortened APD₉₀ by 13.03-14.56% (p<0.05 vs. Hypo-S).

Conclusion:

The above data indicate that the effect of losartan on the electrophysiological changes induced by stretch of atrial myocytes is associated with blocking of AT₁ receptor, and is beneficial for the treatment of AF that is often accompanied by the expansion of atrial myocytes and the increase of effective refractory period.

Atria-selective antiarrhythmic drugs in need of alliance partners

Atria-selective antiarrhythmic drugs in need of alliance partners. Guideline-based treatment of atrial fibrillation (AF) comprises prevention of thromboembolism and stroke, as well as antiarrhythmic therapy by drugs, electrical rhythm conversion, ablation and surgical procedures. Conventional antiarrhythmic drugs are burdened with unwanted side effects including a propensity of triggering life-threatening ventricular fibrillation. In order to solve this therapeutic dilemma, 'atria-selective' antiarrhythmic drugs have been developed for the treatment of supraventricular arrhythmias. These drugs are designed to aim at atrial targets, taking advantage of differences in atrial and ventricular ion channel expression and function. However it is not clear, whether such drugs are sufficiently antiarrhythmic or whether they are in need of an alliance partner for clinical efficacy. Atria-selective Na⁺ channel blockers display fast dissociation kinetics and high binding affinity to inactivated channels. Compounds targeting atria-selective K⁺ channels include blockers of ultra rapid delayed rectifier (Kv1.5) or acetylcholine-activated inward rectifier K⁺ channels (Kir3.x), inward rectifying K⁺ channels (Kir2.x), Ca²⁺-activated K⁺ channels of small conductance (SK), weakly rectifying two-pore domain K⁺ channels (K_2P), and transient receptor potential channels (TRP). Despite good antiarrhythmic data from in-vitro and animal model experiments, clinical efficacy of atria-selective antiarrhythmic drugs remains to be demonstrated. In the present review we will briefly summarize the novel compounds and their proposed antiarrhythmic action. In addition, we will discuss the evidence for putative improvement of antiarrhythmic efficacy and potency by addressing multiple pathophysiologically relevant targets as possible alliance partners.

Genetic ablation or pharmacological inhibition of Kv1.1 potassium channel subunits impairs atrial repolarization in mice

Voltage-gated Kv1.1 potassium channel α-subunits, encoded by the Kcna1 gene, have traditionally been regarded as neural-specific with no expression or function in the heart. However, recent data revealed that Kv1.1 subunits are expressed in atria where they may have an overlooked role in controlling repolarization and arrhythmia susceptibility independent of the nervous system. To explore this concept in more detail and to identify functional and molecular effects of Kv1.1 channel impairment in the heart, atrial cardiomyocyte patch-clamp electrophysiology and gene expression analyses were performed using Kcna1 knockout ( Kcna1-/-) mice. Specifically, we hypothesized that Kv1.1 subunits contribute to outward repolarizing K+ currents in mouse atria and that their absence prolongs cardiac action potentials. In voltage-clamp experiments, dendrotoxin-K (DTX-K), a Kv1.1-specific inhibitor, significantly reduced peak outward K+ currents in wild-type (WT) atrial cells but not Kcna1-/- cells, demonstrating an important contribution by Kv1.1-containing channels to mouse atrial repolarizing currents. In current-clamp recordings, Kcna1-/- atrial myocytes exhibited significant action potential prolongation which was exacerbated in right atria, effects that were partially recapitulated in WT cells by application of DTX-K. Quantitative RT-PCR measurements showed mRNA expression remodeling in Kcna1-/- atria for several ion channel genes that contribute to the atrial action potential including the Kcna5, Kcnh2, and Kcnj2 potassium channel genes and the Scn5a sodium channel gene. This study demonstrates a previously undescribed heart-intrinsic role for Kv1.1 subunits in mediating atrial repolarization, thereby adding a new member to the already diverse collection of known K+ channels in the heart.

Antiarrhythmic drugs for atrial fibrillation: Imminent impulses are emerging

Rhythm and rate strategies are considered equivalent for the management of atrial fibrillation (AF). Moreover, both strategies are intended for improving symptoms and quality of life. Despite the clinical availability of several antiarrhythmic drugs (AAD) the alternatives for the patient with comorbidities are significantly fewer because of the concern regarding many adverse effects, including proarrhythmias. The impetuous development of AF ablation gave rise to a false impression that AAD are a second line therapy. All these statements reflect, in fact, the weakness of the classical paradigm and classification regarding AAD and the gap between the current knowledge of AF mechanism and determinants and the "classical" AAD non-discriminatory action. A new paradigm in development of effective and safe AAD is based on modern knowledge of vulnerable parameters involved in the genesis and perpetuation of AF. New AAD will target specific triggers of AF and ion currents which are expressed preferentially in fibrillatory atrium. Such targets will include repolarizing currents and channels, as ultrarapid potassium current, two pore potassium current, the acetylcholine-gated potassium current, small-conductance calcium-dependent potassium channels, but, also, molecular targets involved in intracellular calcium kinetics, as Ca2+-calmodulin-dependent protein kinase, ryanodine receptors and non-coding miRNA. New mechanistic discoveries link AF to inflammation and modern anti-cytokine drugs. There is still a long way to win between basic research and clinical practice, but, without any doubt, antiarrhythmic drug therapy will remain and develop as a cornerstone therapy for AF not in conflict, but complementary and alternative to interventional therapy.

Small-conductance Ca2+-activated K+ channels: insights into their roles in cardiovascular disease

Life-threatening malignant arrhythmias in pathophysiological conditions can increase the mortality and morbidity of patients with cardiovascular diseases. Cardiac electrical activity depends on the coordinated propagation of excitatory stimuli and the generation of action potentials in cardiomyocytes. Action potential formation results from the opening and closing of ion channels. Recent studies have indicated that small-conductance calcium-activated potassium (SK) channels play a critical role in cardiac repolarization in pathophysiological but not normal physiological conditions. The aim of this review is to describe the role of SK channels in healthy and diseased hearts, to suggest cardiovascular pathophysiologic targets for intervention, and to discuss studies of agents that target SK channels for the treatment of cardiovascular diseases.

Voltage-gated and stretch-activated potassium channels in the human heart : Pathophysiological and clinical significance

Ion channels are essential for electrical signaling and contractility in cardiomyocytes. Detailed knowledge about the molecular function and regulation of cardiac ion channels is crucial for understanding cardiac physiology and pathophysiology especially in the field of arrhythmias. This review aims at providing a general overview on the identity, functional characteristics, and roles of voltage-gated as well as stretch-activated potassium selective channels in the heart. In particular, we will highlight potential therapeutic targets as well as the emerging fields of future investigations.

Multiple mechanisms mediating carbon monoxide inhibition of the voltage-gated K+ channel Kv1.5

The voltage-gated K+ channel has key roles in the vasculature and in atrial excitability and contributes to apoptosis in various tissues. In this study, we have explored its regulation by carbon monoxide (CO), a product of the cytoprotective heme oxygenase enzymes, and a recognized toxin. CO inhibited recombinant Kv1.5 expressed in HEK293 cells in a concentration-dependent manner that involved multiple signalling pathways. CO inhibition was partially reversed by superoxide dismutase mimetics and by suppression of mitochondrial reactive oxygen species. CO also elevated intracellular nitric oxide (NO) levels. Prevention of NO formation also partially reversed CO inhibition of Kv1.5, as did inhibition of soluble guanylyl cyclase. CO also elevated intracellular peroxynitrite levels, and a peroxynitrite scavenger markedly attenuated the ability of CO to inhibit Kv1.5. CO caused nitrosylation of Kv1.5, an effect that was also observed in C331A and C346A mutant forms of the channel, which had previously been suggested as nitrosylation sites within Kv1.5. Augmentation of Kv1.5 via exposure to hydrogen peroxide was fully reversed by CO. Native Kv1.5 recorded in HL-1 murine atrial cells was also inhibited by CO. Action potentials recorded in HL-1 cells were increased in amplitude and duration by CO, an effect mimicked and occluded by pharmacological inhibition of Kv1.5. Our data indicate that Kv1.5 is a target for modulation by CO via multiple mechanisms. This regulation has important implications for diverse cellular functions, including excitability, contractility and apoptosis.

Investigational antiarrhythmic agents: promising drugs in early clinical development

INTRODUCTION

Although there have been important technological advances for the treatment of cardiac arrhythmias (e.g., catheter ablation technology), antiarrhythmic drugs (AADs) remain the cornerstone therapy for the majority of patients with arrhythmias. Most of the currently available AADs were coincidental findings and did not result from a systematic development process based on known arrhythmogenic mechanisms and specific targets. During the last 20 years, our understanding of cardiac electrophysiology and fundamental arrhythmia mechanisms has increased significantly, resulting in the identification of new potential targets for mechanism-based antiarrhythmic therapy. Areas covered: Here, we review the state-of-the-art in arrhythmogenic mechanisms and AAD therapy. Thereafter, we focus on a number of antiarrhythmic targets that have received significant attention recently: atrial-specific K+-channels, the late Na+-current, the cardiac ryanodine-receptor channel type-2, and the small-conductance Ca2+-activated K+-channel. We highlight for each of these targets available antiarrhythmic agents and the evidence for their antiarrhythmic effect in animal models and early clinical development. Expert opinion: Targeting AADs to specific subgroups of well-phenotyped patients is likely necessary to detect improved outcomes that may be obscured in the population at large. In addition, specific combinations of selective AADs may have synergistic effects and may enable a mechanism-based tailored antiarrhythmic therapy.

Atrial-selective K+ channel blockers: potential antiarrhythmic drugs in atrial fibrillation?

In the wake of demographic change in Western countries, atrial fibrillation has reached an epidemiological scale, yet current strategies for drug treatment of the arrhythmia lack sufficient efficacy and safety. In search of novel medications, atrial-selective drugs that specifically target atrial over other cardiac functions have been developed. Here, I will address drugs acting on potassium (K⁺) channels that are either predominantly expressed in atria or possess electrophysiological properties distinct in atria from ventricles. These channels include the ultra-rapidly activating, delayed outward-rectifying Kv1.5 channel conducting I_Kur, the acetylcholine-activated inward-rectifying Kir3.1/Kir3.4 channel conducting I_K,ACh, the Ca²⁺-activated K⁺ channels of small conductance (SK) conducting I_SK, and the two-pore domain K⁺ (K2P) channels (tandem of P domains, weak inward-rectifying K⁺ channels (TWIK-1), TWIK-related acid-sensitive K⁺ channels (TASK-1 and TASK-3)) that are responsible for voltage-independent background currents I_TWIK-1, I_TASK-1, and I_TASK-3. Direct drug effects on these channels are described and their putative value in treatment of atrial fibrillation is discussed. Although many potential drug targets have emerged in the process of unravelling details of the pathophysiological mechanisms responsible for atrial fibrillation, we do not know whether novel antiarrhythmic drugs will be more successful when modulating many targets or a single specific one. The answer to this riddle can only be solved in a clinical context.

Concise Review: Criteria for Chamber-Specific Categorization of Human Cardiac Myocytes Derived from Pluripotent Stem Cells

Human pluripotent stem cell‐derived cardiomyocytes (PSC‐CMs) have great potential application in almost all areas of cardiovascular research. A current major goal of the field is to build on the past success of differentiation strategies to produce CMs with the properties of those originating from the different chambers of the adult human heart. With no anatomical origin or developmental pathway to draw on, the question of how to judge the success of such approaches and assess the chamber specificity of PSC‐CMs has become increasingly important; commonly used methods have substantial limitations and are based on limited evidence to form such an assessment. In this article, we discuss the need for chamber‐specific PSC‐CMs in a number of areas as well as current approaches used to assess these cells on their likeness to those from different chambers of the heart. Furthermore, describing in detail the structural and functional features that distinguish the different chamber‐specific human adult cardiac myocytes, we propose an evidence‐based tool to aid investigators in the phenotypic characterization of differentiated PSC‐CMs. Stem Cells2017;35:1881–1897