Tropical teleost fish Danio rerio is increasingly used as a model object for electrophysiological studies of human cardiac physiology and pathology. D. rerio is characterized by the similarity with humans in such functional parameters of the electrical activity of the heart as heart rate, action potential morphology, as well as in a set of ion currents depolarizing and repolarizing the cell membrane. D. rerio is easy to breed, easy to handle experimentally, and easy to genetically modify. This overview presents current data on the structural and functional organization of ion channels in D. rerio heart myocytes.

Introduction. The development of heart failure is closely associated with the appearance of life threatening arrhythmias, which are often a terminal event for these patients. An analysis of randomized clinical trials of inhibitors of sodium-glucose cotransporter type 2 indicates the clinically significant potential of these drugs as agents with antiarrhythmic properties. However, at the moment the full mechanism by which this effect can be realized is still not fully understood.Aim. To evaluate the effect of empagliflozin on the transmembrane calcium currents and the intracellular calcium transients on isolated ventricular cardiomyocytes of mice under conditions of normoglycemia.Materials and methods. In the experiment, ventricular cardiomyocytes were isolated from 12 outbred male mice. 2 groups were formed: group № 1 – control ventricular cardiomyocytes; group № 2 – ventricular cardiomyocytes after two hours incubation with 5 µmol/L empagliflozin solution. Transmembrane calcium currents were recorded and intracellular calcium transients were assessed.Results and discussion. Incubation of ventricular cardiomyocytes with empagliflozin significantly increased ICa current density and accelerated Ca2+ temporal dynamics. The amplitude of the Ca2+ wave and the rate of rise and decay were increased and the duration of the Ca2+ wave was shortened.Conclusion. The result of the experiment indicates that empagliflozin is able to modulate Ca2+-dependent mechanism of the excitation-contraction-coupling, enhancing and accelerating Ca2+ release into cytoplasm and reuptake. This presumably can optimize, namely reduce the time of systole and enhance it, which may be one of the important elements in the manifestation of empagliflozin antiarrhythmic properties.

BACKGROUND:Brugada syndrome (BrS) is a rare inherited cardiac arrhythmia with increased risk of sudden cardiac death. Mutations in gene SCN5A, which encodes the α-subunit of cardiac voltage-gated sodium channel NaV1.5, have been identified in over 20% of patients with BrS. However, only a small fraction of NaV1.5 variants, which are associated with BrS, are characterized in electrophysiological experiments.RESULTS:Here we explored variants V281A and L1582P, which were found in our patients with BrS, and variants F543L and K1419E, which are reportedly associated with BrS. Heterologous expression of the variants in CHO-K1 cells and the Western blot analysis demonstrated that each variant appeared at the cell surface. We further measured sodium current in the whole-cell voltage clamp configuration. Variant F543L produced robust sodium current with a hyperpolarizing shift in the voltage dependence of steady-state fast inactivation. Other variants did not produce detectable sodium currents, indicating a complete loss of function. In a recent cryoEM structure of the hNaV1.5 channel, residues V281, K1419, and L1582 are in close contacts with residues whose mutations are reportedly associated with BrS, indicating functional importance of respective contacts.CONCLUSIONS:Our results support the notion that loss of function of NaV1.5 or decrease of the channel activity is involved in the pathogenesis of BrS.

Empagliflozin, an inhibitor of sodium-glucose co-transporter 2 (iSGLT2), improves cardiovascular outcomes in patients with and without diabetes and possesses an antiarrhythmic activity. However, the mechanisms of these protective effects have not been fully elucidated. This study aimed to explore the impact of empagliflozin on ion channel activity and electrophysiological characteristics in the ventricular myocardium. The main cardiac ionic currents (INa, ICaL, ICaT, IKr, IKs) and action potentials (APs) were studied in zebrafish. Whole-cell currents were measured using the patch clamp method in the isolated ventricular cardiomyocytes. The conventional sharp glass microelectrode technique was applied for the recording of APs from the ventricular myocardium of the excised heart. Empagliflozin pretreatment compared to the control group enhanced potassium IKr step current density in the range of testing potentials from 0 to +30 mV, IKr tail current density in the range of testing potentials from +10 to +70 mV, and IKs current density in the range of testing potentials from −10 to +20 mV. Moreover, in the ventricular myocardium, empagliflozin pretreatment shortened AP duration APD as shown by reduced APD50 and APD90. Empagliflozin had no influence on sodium (INa) and L- and T-type calcium currents (ICaL and ICaT) in zebrafish ventricular cardiomyocytes. Thus, we conclude that empagliflozin increases the rapid and slow components of delayed rectifier K+ current (IKr and IKs). This mechanism could be favorable for cardiac protection.

The most important component of the filtration apparatus of the renal glomeruli is specialized visceral epithelial cells podocytes. The normal physiological function of podocytes is critically dependent on the proper regulation of the intracellular Ca2+ content; excessive Ca2+ influx in cells can lead to a disorder of cell morphology, podocytopathy, apoptosis and subsequent glomeruli damage. Podocytopathy is one of the primary characteristics of proteinuria and focal segmental glomerulosclerosis. One of the key proteins responsible for Ca2+ influx in podocytes is the TRPC6 channel. Since the first discovery of a mutation in a gene encoding TRPC6, the attention of the scientific community has been focused on studying the role of this ion channel in the onset and development of kidney diseases. Both an increase and a decrease in the functional activity of TRPC6 are associated with the manifestation of severe nephrotic syndromes leading to the end-stage of chronic kidney disease. The review contains materials related to the regulation of TRPC6 activity and the role of this channel in the pathogenesis of glomerular diseases.

Background: Mutations in desmosomal genes linked to arrhythmogenic cardiomyopathy are commonly associated with Wnt/beta-catenin signaling abnormalities and reduction of the sodium current density. Inhibitors of GSK3B were reported to restore sodium current and improve heart function in various arrhythmogenic cardiomyopathy models, but mechanisms underlying this effect remain unclear. We hypothesized that there is a crosstalk between desmosomal proteins, signaling pathways, and cardiac sodium channels. Methods and results: To reveal molecular mechanisms of arrhythmogenic cardiomyopathy, we established human iPSC-based model of this pathology. iPSC-derived cardiomyocytes from patient carrying two genetic variants in PKP2 gene demonstrated that PKP2 haploinsufficiency due to frameshift variant, in combination with the missense variant expressed from the second allele, was associated with decreased Wnt/beta-catenin activity and reduced sodium current. Different approaches were tested to restore impaired cardiomyocytes functions, including wild type PKP2 transduction, GSK3B inhibition and Wnt/beta-catenin signaling modulation. Inhibition of GSK3B led to the restoration of both Wnt/beta-catenin signaling activity and sodium current density in patient-specific cardiomyocytes while GSK3B activation led to the reduction of sodium current density. Moreover, we found that upon inhibition GSK3B sodium current was restored through Wnt/beta-catenin-independent mechanism. Conclusion: We propose that alterations in GSK3B-Wnt/beta-catenin signaling pathways lead to regulation of sodium current implying its role in molecular pathogenesis of arrhythmogenic cardiomyopathy.

Adipose tissue is an endocrine organ secreting signal peptides adipokines. Adipokines play a significant role in the regulation of the immune, endocrine, nervous, excretory systems. Adipokine imbalance leads to chronic inflammation, metabolic and cardiovascular diseases. Obesity as the primary reason of the adipokine imbalance is the leading risk factor of hypertension, atherosclerosis and coronary artery disease. Thus, experimental and clinical studies of adipokine biology are essential for the development of the novel therapeutic approaches for the management of cardiometabolic diseases. This review focuses on the role of adiponectin, leptin, chemerin, omentin and visfatin in physiological and pathophysiological processes in the cardiovascular system.

Little is known about the electrophysiological characteristics of telocytes found in the working myocardium and sinoatrial node (SAN). Telocyte expres -sion of HCN4 suggests the ability to generate pacemaker potentials. To prove the impulse conduction, the presence of voltage-gated sodium channels is required. It is assumed that telocytes are also located in the atrioventricular node (AVN).Aim. Morphological and electrophysiological study of AVN and SAN telocytes.Material and methods. Fragments of the right atrium, AVN of 7 hearts of recipients and 3 hearts of pigs were taken, respectively, during heart transplantation and after the experiment. Isolation of telocyte cultures, histological, immunohis tochemical tests with anti-CD117, anti-NaV1.5 (SCN5A), anti-CD34 antibodies, intravital confocal laser microscopy were carried out. The patch-clamp technique was used.Results. In AVN cultures, CD117+ cells with long processes were found. There was a potassium current with a density of 700 pA/pF during membrane depolarization up to +90 mW in humans and pigs using the patch-clamp technique. Calcium oscillations with a period of about 200 seconds in a pig with an increase in calcium concentration. In elongated cells located between cardiomyocytes and among the fibrous tissue of SAN perifocal zone, the co-expression of anti-NaV1.5 and anti-CD34 antibodies was revealed.Conclusion. In AVN, telocytes were found, in whose cultures potassium current and calcium oscillations were determined. SCN5A sodium channels were found in telocytes of the perifocal area of human SAN. This fact indicates the ability of cells to conduct an electrical impulse.

Small G-proteins of Rho family modulate the activity of several classes of ion channels, including K+ channels Kv1.2, Kir2.1, and ERG; Ca2+ channels; and epithelial Na+ channels. The present study was aimed to check the RhoA potential regulatory effects on Na+ current (INa) transferred by Na+ channel cardiac isoform NaV1.5 in heterologous expression system and in native rat cardiomyocytes. Whole-cell patch-clamp experiments showed that coexpression of NaV1.5 with the wild-type RhoA in CHO-K1 cell line caused 2.7-fold decrease of INa density with minimal influence on steady-state activation and inactivation. This effect was reproduced by the coexpression with a constitutively active RhoA, but not with a dominant negative RhoA. In isolated ventricular rat cardiomyocytes, a 5-h incubation with the RhoA activator narciclasine (5 × 10−6 M) reduced the maximal INa density by 38.8%. The RhoA-selective inhibitor rhosin (10−5 M) increased the maximal INa density by 25.3%. Experiments with sharp microelectrode recordings in isolated right ventricular wall preparations showed that 5 × 10−6 M narciclasine induced a significant reduction of action potential upstroke velocity after 2 h of incubation. Thus, RhoA might be considered as a potential negative regulator of sodium channels cardiac isoform NaV1.5.

The effect of small G-proteins of the Rho family on sodium current conducted by cardiac isoform Na V 1.5 of voltage-gated sodium channels was studied in heterologous expression system, CHO-K1 cell line transfected with a plasmid containing the Na V 1 . 5 gene. The influence of cotransfection with genes of wild-type, constitutively-active, and dominant-negative small G-proteins RhoA , Rac1 , and Cdc2 on the parameters of sodium current and its noninactivating component (I Na,late ) was estimated. Among three studied small G-proteins, only RhoA (wild-type and constitutively-active type) strongly affected sodium current reducing its peak amplitude, but not the value of I Na,late . Cotransfection with wild-type Rac1 resulted in a minor decrease in sodium current. Thus, small G-protein RhoA has potential capability for suppression of sodium current, although physiological relevance of this property has to be verified.

Mutations in gene SCN5A, which encodes cardiac voltage-gated sodium channel Nav1.5, are associated with multiple clinical phenotypes. Here we describe a novel A1294G genetic variant detected in a male patient with combined clinical phenotype including atrioventricular II block, Brugada-like ECG, septal fibrosis, right ventricular dilatation and decreased left ventricular contractility. Residue A1294 is located in the IIIS3–S4 extracellular loop, in proximity to several residues whose mutations are associated with sodium channelopathies. The wild-type channel Nav1.5 and mutant Nav1.5-A1294G were expressed in the CHO–K1 and HEK293T cells and whole-cell sodium currents were recorded using the patch-clamp method. The A1294G channels demonstrated a negative shift of steady-state inactivation, accelerated fast and slow inactivation and decelerated recovery from intermediate inactivation. Our study reveals biophysical mechanism of the Nav1.5-A1294G dysfunction, which may underlie the combined phenotypic manifestation observed in the patient.

Mutations in gene SCN5A, which encodes the α-subunit of cardiac voltage-gated sodium channel Nav1.5, are often associated with long QT syndrome type 3 (LQT3) and Brugada syndrome type 1 (BrS1). Recently large progress was achieved in an understanding of biophysical mechanisms of these arrhythmias. The LQT3 syndrome is associated with the gain-of-function due to accelerated activation, decelerated inactivation, appearance of sustained current, and accelerated recovery from inactivation. In contrast, The BrS1 syndrome is associated with loss-of-function due to defective channel trafficking, impaired activation; enhanced fast and slow inactivation, and decelerated recovery from inactivation. Mutations associated with inherited arrhythmias, can also disturb interactions of Nav1.5 with different proteins and/or ligands and cause abnormal reactions in response to pharmacological agents. Furthermore, mutations can affect post-translational modifications and sensitivity to pH and temperature. Here we review current knowledge on biophysical mechanisms of Brugada and long QT syndromes. We focus on limitations of studies that use heterologous expression systems and causes hampering our understanding of genotype-phenotype relations of SCN5A mutations.

Abstract Background Mutations in gene SCN5A, encoding cardiac potential-dependent sodium channel Nav1.5, are associated with various arrhythmogenic disorders among which the Brugada syndrome (BrS) and the Long QT syndrome (LQT) are the best characterized. BrS1 is associated with sodium channel dysfunction, which can be reflected by decreased current, impaired activation and enhanced inactivation. We found two novel mutations in our patients with BrS and explored their effect on fast and slow inactivation of cardiac sodium channel. Purpose The aim of this study was to investigate the effect of BrS (Y739D, L1582P) mutations on different inactivation processes in in vitro model. Methods Y739D and L1582P substitutions were introduced in SCN5A cDNA using site-directed mutagenesis. Sodium currents were recorded at room temperature in transfected HEK293-T cells using patch-clamp technique with holding potential −100 mV. In order to access the fast steady-state inactivation curve we used double-pulse protocol with 10 ms prepulses. To analyze voltage-dependence of slow inactivation we used two-pulse protocol with 10s prepulse, 20ms test pulse and 25ms interpulse at −100mV to allow recovery from fast inactivation. Electrophysiological measurements are presented as mean ±SEM. Results Y739D mutation affects highly conserved tyrosine 739 among voltage-gated sodium and calcium channels in the segment IIS2. Mutation L1582P located in the loop IVS4-S5, and leucine in this position is not conserved among voltage-gated channels superfamily. We have shown that Y739D leads to significant changes in both fast and slow inactivation, whereas L1582P enhanced slow inactivation only. Steady-state fast inactivation for Y739D was shifted on 8.9 mV towards more negative potentials compare with that for WT, while L1582P did not enhanced fast inactivation (V1/2 WT: −62.8±1.7 mV; Y739D: −71.7±2.3 mV; L1582P: −58.7±1.4 mV). Slow inactivation was increased for both substitutions (INa (+20mV)/INa (−100mV) WT: 0.45±0.03; Y739D: 0,34±0.09: L1582P: 0.38±0.04). Steady-state fast inactivation Conclusions Both mutations, observed in patients with Brugada syndrome, influence on the slow inactivation process. Enhanced fast inactivation was shown only for Y739D mutant. The more dramatic alterations in sodium channel biophysical characteristics are likely linked with mutated residue conservativity. Acknowledgement/Funding RSF #17-15-01292

Brugada syndrome is a rare hereditary arrhythmogenic disorder first described by Brugada brothers in 1992. Despite the large amount of clinical and experimental data, there is no complete understanding of genotype-phenotype relation in pathogenesis of the disease caused by missence mutations in SCN5A, which encodes the alpha-subunit of the major cardiac voltage-gated sodium channel Nav 1.5. The aim of this review is to summarize current knowledge on molecular, cellular and ionic mechanisms of the Brugada syndrome development. We focused on the clinical picture and physiological consequences of decreasing activity of Nav 1.5 and analyzed the impact of biophysical properties alterations on the pathological state. The mutation-specific influence of pharmacological agents and signalling proteins was described.

Muscular dystrophies caused by defects in various genes are often associated with impairment of calcium homeostasis. Studies of calcium currents are hampered because of the lack of a robust cellular model. Primary murine myotubes, formed upon satellite cell fusion, were examined for their utilization as a model of adult skeletal muscle. We enzymatically isolated satellite cells and induced them to differentiation to myotubes. Myotubes displayed morphological and physiological properties resembling adult muscle fibers. Desmin and myosin heavy chain immunoreactivity in the differentiated myotubes were similar to the mature muscle cross-striated pattern. The myotubes responded to electrical and chemical stimulations with sarcoplasmic reticulum calcium release. Presence of L-type calcium channels in the myotubes sarcolemma was confirmed via whole-cell patch-clamp technique. To assess the use of myotubes for studying functional mutation effects lentiviral transduction was applied. Satellite cells easily underwent transduction and were able to retain a positive expression of lentivirally encoded GFP up to and after the formation of myotubes, without changes in their physiological and morphological properties. Thus, we conclude that murine myotubes may serve as a fruitful cell model for investigating calcium homeostasis in muscular dystrophy and the effects of gene modifications can be assessed due to lentiviral transduction.

Relevant cell model is essential to study pathogenesis of muscle disorders. However, in the field of muscle research there is no ultimate cell line considered as a standard for studying muscular and neuromuscular diseases. Standard cell line claimed to be well differentiated in muscle lineage, be morphological and physiological similar to mature muscle cells and be easily genetically modified. Therefore, the goal of our study was to pick up available and fruitful cell model of muscle differentiation, that could be further applied for examination of muscular disorder pathogenesis in vitro. We characterized human mesenchymal stem cells (MSC), mature murine muscle fibers and primary murine satellite cells. It has been shown that MSC have very small capacity to myogenic differentiation; moreover, they were able to differentiate only in presence of C2C12 cells. Lentiviral transduction exhibited rather high toxic effect on primary myofibers, and positively transduced cells were not able to response to electrical stimulation, i. e. were functionally inactive. Satellite cells turned out to be the most fruitful cell model since they were easily transduced via lentiviruses and rapidly formed myotubes in differentiation media. Functional analysis of obtained myotubes has confirmed their ability to react to electrical and chemical stimulations; besides, potassium and calcium channels availability has been also demonstrated via patch-clump technique. Taken together, these results imply that satellite cells are the most promising cell line for further experiments aimed at exploring the molecular pathways of muscle pathologies.

The study of pathogenesis of muscle disorders needs an appropriate cell model. In the field of muscle research, there is no general cell line considered standard for studying muscular and neuromuscular diseases. Most cell lines are incapable of differentiation into a muscle lineage exhibiting morphological and physiological properties of mature muscle cells and can hardly be genetically modified. The goal of our study was to find an informative cell model of muscle differentiation suitable for examination of muscular pathogenesis in vitro. We assayed cultured human mesenchymal stem cells (MSCs), mature murine muscle fibers, and primary murine satellite cells. It was shown that MSCs had very low capacity for myogenic differentiation; they were able to differentiate only in the presence of C2C12 cells. Lentiviral transduction had a toxic effect in primary myofiber cultures, and positively transduced cells were unable to respond to electrical stimulation, i.e., were functionally inactive. Satellite cells proved to be the most appropriate cell model, since they were easily transduced with lentiviruses and rapidly formed myotubes in the differentiation medima. Functional analysis of induced myotubes showed their ability to react to electrical and chemical stimulation. The patch-clump technique showed the presence of potassium and calcium channels. Collectively, the results show that cultured satellite cells are the most promising cell line for further experiments to explore molecular pathways in muscle pathologies.