L-type Ca(2+) current in ventricular cardiomyocytes

Opposite effects of Gαi2 or Gαi3 deficiency on reduced basal density and attenuated β-adrenergic response of ventricular Ca2+ currents in myocytes of mice overexpressing the cardiac β1-adrenoceptor

Ca2+ currents (ICaL) carried by ventricular L-type Ca2+ channels (LTCC) are altered in failing hearts, and increased LTCC activity is discussed as a cause of cardiomyopathy. We have shown that lack of the inhibitory G-protein isoform Gαi3 improves cardiac outcome and survival in a murine heart-failure model of cardiac β1-adrenoceptor (β1-AR) overexpression (β1-tg), while lack of the Gαi2 isoform was detrimental in the same heart-failure model. Given the potential role of LTCC and their modulation by β-adrenergic signalling, we now analysed ventricular ICaL in β1-tg mice and in β1-tg mice lacking either Gαi2 or Gαi3. Using the patch-clamp technique, we recorded whole-cell ICaL in ventricular myocytes freshly isolated from adult mice. Compared to age-matched wild-type littermates, basal ICaL was reduced in myocytes from β1-tg mice both under basal conditions (- 8.1 ± 1.6 vs. - 5.5 ± 1.5 pA/pF) and upon β-adrenergic stimulation with 1 µM isoproterenol (- 14.3 ± 5.6 vs. - 7.4 ± 1.9 pA/pF). Lack of Gαi3 normalised basal ICaL to nearly wild-type levels (- 7.5 ± 1.6 pA/pF), while β-adrenergic response remained attenuated (- 9.5 ± 3.6 pA/pF). In contrast, the absence of Gαi2 did not restore basal ICaL (- 5.7 ± 1.8 pA/pF), but restored the β-adrenergic response of ICaL, with the difference from basal current even exceeding that in wild-type mice (- 12.2 ± 2.9 pA/pF).We propose that by restoring basal ICaL, Gαi3 deficiency might contribute to the restoration of contractility in β1-tg mice, while maintaining attenuation of the ICaL response upon β-adrenergic stimulation protects against deleterious effects mediated by enhanced β-AR signalling. In contrast, restored and even enhanced ICaL response to β-adrenergic stimulation might contribute to detrimental effects of Gαi2 deficiency observed in β1-tg mice previously.

Altered Protein Kinase A-Dependent Phosphorylation of Cav1.2 in Left Ventricular Myocardium from Cacna1c Haploinsufficient Rat Hearts

CACNA1C encodes the α1c subunit of the L-type Ca2+ channel, Cav1.2. Ventricular myocytes from haploinsufficient Cacna1c (Cacna1c+/-) rats exhibited reduced expression of Cav1.2 but an apparently normal sarcolemmal Ca2+ influx with an impaired response to sympathetic stress. We tested the hypothesis that the altered phosphorylation of Cav1.2 might underlie the sarcolemmal Ca2+ influx phenotype in Cacna1c+/- myocytes using immunoblotting of the left ventricular (LV) tissue from Cacna1c+/- versus wildtype (WT) hearts. Activation of cAMP-dependent protein kinase A (PKA) increases L-type Ca2+ current and phosphorylates Cav1.2 at serine-1928. Using an antibody directed against this phosphorylation site, we observed elevated phosphorylation of Cav1.2 at serine-1928 in LV myocardium from Cacna1c+/- rats under basal conditions (+110% versus WT). Sympathetic stress was simulated by isoprenaline (100 nM) in Langendorff-perfused hearts. Isoprenaline increased the phosphorylation of serine-1928 in Cacna1c+/- LV myocardium by ≈410%, but the increase was significantly smaller than in WT myocardium (≈650%). In conclusion, our study reveals altered PKA-dependent phosphorylation of Cav1.2 with elevated phosphorylation of serine-1928 under basal conditions and a diminished phosphorylation reserve during β-adrenergic stimulation. These alterations in the phosphorylation of Cav1.2 may explain the apparently normal sarcolemmal Ca2+ influx in Cacna1c+/- myocytes under basal conditions as well as the impaired response to sympathetic stimulation.

Spatiotemporal Cell Control via High-Precision Electronic Regulation of Microenvironmental pH

Accurate regulation of extracellular pH is crucial for controlling cell behaviors and functions. However, typical methods, which primarily rely on replacing cell culture media or using ionic diffusion, are slow, nondirectional, and lack spatiotemporal resolution. Here, we develop a microfabricated device that regulates microenvironmental pH within specific localized zones with high precision (uncertainty <0.1 pH units) and temporal resolution. The device uses a synchronization strategy that coordinates two processes: pulsatile modulation of pH through microelectrolysis and ultrasensitive graphene-electronic pH sensing, which operates in antiphase to the modulation. Using this device, we show real-time control of the dynamic behaviors of microscale clusters of bacteria (motility) and cardiomyocytes (calcium signaling and necrotic injury) in response to precisely regulated extracellular pH variations. Our device addresses the limitations of typical pH-altering techniques and holds significant potential to advance cell biology, physiology, tissue engineering, and regenerative medicine.

Exogenous Ketones in Cardiovascular Disease and Diabetes: From Bench to Bedside

Ketone bodies are molecules produced from fatty acids in the liver that act as energy carriers to peripheral tissues when glucose levels are low. Carbohydrate- and calorie-restricted diets, known to increase the levels of circulating ketone bodies, have attracted significant attention in recent years due to their potential health benefits in several diseases. Specifically, increasing ketones through dietary modulation has been reported to be beneficial for cardiovascular health and to improve glucose homeostasis and insulin resistance. Interestingly, although excessive production of ketones may lead to life-threatening ketoacidosis in diabetic patients, mounting evidence suggests that modest levels of ketones play adaptive and beneficial roles in pancreatic beta cells, although the exact mechanisms are still unknown. Of note, Sodium-Glucose Transporter 2 (SGLT2) inhibitors have been shown to increase the levels of beta-hydroxybutyrate (BHB), the most abundant ketone circulating in the human body, which may play a pivotal role in mediating some of their protective effects in cardiovascular health and diabetes. This systematic review provides a comprehensive overview of the scientific literature and presents an analysis of the effects of ketone bodies on cardiovascular pathophysiology and pancreatic beta cell function. The evidence from both preclinical and clinical studies indicates that exogenous ketones may have significant beneficial effects on both cardiomyocytes and pancreatic beta cells, making them intriguing candidates for potential cardioprotective therapies and to preserve beta cell function in patients with diabetes.

Cardiac Events and Survival in Patients With EGFR-Mutant Non-Small Cell Lung Cancer Treated With Osimertinib

Importance

Although it has been reported that osimertinib mesylate provides better survival benefits compared with first- or second-generation epidermal growth factor receptor tyrosine kinase inhibitors (EGFR TKIs), it remains unclear whether osimertinib is associated with more cancer therapy-related cardiac events (CTRCEs) compared with other EGFR TKIs, as does the extent of the association these adverse effects may have with overall survival. This issue is particularly critical due to the high prevalence of EGFR variants within Asian populations, including that of Taiwan.

Objective

To compare CTRCEs and their association with survival in patients treated with osimertinib vs other EGFR TKIs.

Design, Setting, and Participants

This cohort study was conducted at the National Cheng Kung University Hospital, a college hospital and tertiary academic referral center in Taiwan. The median follow-up duration was 23.2 (IQR, 15.2-31.5) months. A total of 401 patients with EGFR-mutant non-small cell lung cancer (NSCLC) beginning treatment with EGFR TKIs from September 1, 2019, to July 31, 2022, were retrospectively analyzed. CTRCEs included newly emerging arrhythmias, valvular heart diseases (moderate and more), myocardial infarction, and heart failure and were analyzed after adjusting for age, sex, smoking, alcohol consumption, body mass index, cardiovascular comorbidities, thoracic radiotherapy, and cardiovascular medications. Follow-up was completed January 31, 2024.

Exposure

Osimertinib.

Main Outcomes and Measures

The Cox proportional hazards model was used to estimate CTRCEs in patients treated with osimertinib or other EGFR TKIs. Considering that death can lower the incidence of CTRCEs, the competing risk method was used to calculate CTRCEs after adjusting for potential confounders. Multivariable Cox proportional hazard regression analysis for overall survival was used to explore whether CTRCEs were independently associated with overall survival.

Results

Among the 401 patients (253 [63.1%] female; mean [SD] age, 69.2 [11.3] years), 195 (48.6%) treated with osimertinib were matched with 206 (51.4%) treated with other EGFR TKIs. Occurrence of CTRCEs in patients receiving osimertinib was significantly higher compared with patients treated with other EGFR TKIs (29 [14.9%] vs 9 [4.4%]; hazard ratio [HR], 3.37; 95% CI, 1.56-7.26; P = .002). After adjustment for relevant cardiovascular risk factors, the HR of CTRCEs was significantly higher in the group treated with osimertinib (adjusted subdistribution HR, 4.00; 95% CI, 1.81-8.85; P < .001). In addition, CTRCEs were independently associated with overall survival (HR, 4.02; 95% CI, 2.44-6.63; P < .001).

Conclusions and Relevance

In this cohort study of patients with EGFR-mutant NSCLC, osimertinib was associated with a higher incidence of CTRCEs compared with other EGFR TKIs; CTRCEs were independently associated with overall survival. These findings highlight the need for ongoing cardiac monitoring in these patients, regardless of preexisting cardiac risk factors.

Role of the CaV1.2 distal carboxy terminus in the regulation of L-type current

L-type calcium channels are essential for the excitation-contraction coupling in cardiac muscle. The CaV1.2 channel is the most predominant isoform in the ventricle which consists of a multi-subunit membrane complex that includes the CaV1.2 pore-forming subunit and auxiliary subunits like CaVα2δ and CaVβ2b. The CaV1.2 channel's C-terminus undergoes proteolytic cleavage, and the distal C-terminal domain (DCtermD) associates with the channel core through two domains known as proximal and distal C-terminal regulatory domain (PCRD and DCRD, respectively). The interaction between the DCtermD and the remaining C-terminus reduces the channel activity and modifies voltage- and calcium-dependent inactivation mechanisms, leading to an autoinhibitory effect. In this study, we investigate how the interaction between DCRD and PCRD affects the inactivation processes and CaV1.2 activity. We expressed a 14-amino acid peptide miming the DCRD-PCRD interaction sequence in both heterologous systems and cardiomyocytes. Our results show that overexpression of this small peptide can displace the DCtermD and replicate the effects of the entire DCtermD on voltage-dependent inactivation and channel inhibition. However, the effect on calcium-dependent inactivation requires the full DCtermD and is prevented by overexpression of calmodulin. In conclusion, our results suggest that the interaction between DCRD and PCRD is sufficient to bring about the current inhibition and alter the voltage-dependent inactivation, possibly in an allosteric manner. Additionally, our data suggest that the DCtermD competitively modifies the calcium-dependent mechanism. The identified peptide sequence provides a valuable tool for further dissecting the molecular mechanisms that regulate L-type calcium channels' basal activity in cardiomyocytes.

β-adrenergic regulation of Ca2+ signaling in heart cells

β-adrenergic receptors (βARs) play significant roles in regulating Ca2+ signaling in cardiac myocytes, thus holding a key function in modulating heart performance. βARs regulate the influx of extracellular Ca2+ and the release and uptake of Ca2+ from the sarcoplasmic reticulum (SR) by activating key components such as L-type calcium channels (LTCCs), ryanodine receptors (RyRs) and phospholamban (PLN), mediated by the phosphorylation actions by protein kinase A (PKA). In cardiac myocytes, the presence of β2AR provides a protective mechanism against potential overstimulation of β1AR, which may aid in the restoration of cardiac dysfunctions. Understanding the Ca2+ regulatory signaling pathways of βARs in cardiac myocytes and the differences among various βAR subtypes are crucial in cardiology and hold great potential for developing treatments for heart diseases.

Diverse pathways in GPCR-mediated activation of Ca2+ mobilization in HEK293 cells

G protein-coupled receptors transduce extracellular stimuli into intracellular signaling. Ca2+ is a well-known second messenger that can be induced by G protein-coupled receptor activation through the primary canonical pathways involving Gαq- and Gβγ-mediated activation of phospholipase C-β (PLCβ). While some Gs-coupled receptors are shown to trigger Ca2+ mobilization, underlying mechanisms remain elusive. Here, we evaluated whether Gs-coupled receptors including the β2-adrenergic receptor (β2AR) and the prostaglandin EP2 and EP4 receptors (EP2R and EP4R) that are endogenously expressed in human embryonic kidney 293 (HEK293) cells utilize common pathways for mediating Ca2+ mobilization. For the β2AR, we found an essential role for Gq in agonist-promoted Ca2+ mobilization while genetic or pharmacological inhibition of Gs or Gi had minimal effect. β-agonist-promoted Ca2+ mobilization was effectively blocked by the Gq-selective inhibitor YM-254890 and was not observed in ΔGαq/11 or ΔPLCβ cells. Bioluminescence resonance energy transfer analysis also suggests agonist-dependent association of the β2AR with Gq. For the EP2R, which couples to Gs, agonist treatment induced Ca2+ mobilization in a pertussis toxin-sensitive but YM-254890-insensitive manner. In contrast, EP4R, which couples to Gs and Gi, exhibited Ca2+ mobilization that was sensitive to both pertussis toxin and YM-254890. Interestingly, both EP2R and EP4R were largely unable to induce Ca2+ mobilization in ΔGαs or ΔPLCβ cells, supporting a strong dependency on Gs signaling in HEK293 cells. Taken together, we identify differences in the signaling pathways that are used to mediate Ca2+ mobilization in HEK293 cells where the β2AR primarily uses Gq, EP2R uses Gs and Gi, and EP4R uses Gs, Gi, and Gq.

Rad protein: An essential player in L-type Ca2+ channel localization and modulation in cardiomyocytes

Rad is an emerging key Cav1.2 modulator. In the present issue of JGP, Elmore, Ahern et al. examine how the Rad C-terminus affects its subcellular distribution and Cav1.2 regulation.

Circadian regulation of Ca V 1.2 expression by RORα in the mouse heart

Background

In addition to show autonomous beating rhythmicity, the physiological functions of the heart present daily periodic oscillations. Notably the ventricular repolarization itself varies throughout the circadian cycle which was mainly related to the periodic expression of K + channels. However, the involvement of the L-type Ca 2+ channel (Ca V 1.2 encoded by Cacna1c gene) in these circadian variations remains elusive.

Methods

We used a transgenic mouse model (PCa-luc) that expresses the luciferase reporter under the control of the cardiac Cacna1c promoter and analyzed promoter activity by bioluminescent imaging, qPCR, immunoblot, Chromatin immunoprecipitation assay (ChIP) and Ca V 1.2 activity.

Results

Under normal 12:12h light-dark cycle, we observed in vivo a biphasic diurnal variation of promoter activities peaking at 9 and 19.5 Zeitgeber time (ZT). This was associated with a periodicity of Cacna1c mRNA levels preceding 24-h oscillations of Ca V 1.2 protein levels in ventricle (with a 1.5 h phase shift) but not in atrial heart tissues. The periodicity of promoter activities and Ca V 1.2 proteins, which correlated with biphasic oscillations of L-type Ca 2+ current conductance, persisted in isolated ventricular cardiomyocytes from PCa-Luc mice over the course of the 24-h cycle, suggesting an endogenous cardiac circadian regulation. Comparison of 24-h temporal patterns of clock gene expressions in ventricles and atrial tissues of the same mice revealed conserved circadian oscillations of the core clock genes except for the retinoid-related orphan receptor α gene (RORα), which remained constant throughout the course of a day in atrial tissues. In vitro we found that RORα is recruited to two specific regions on the Cacna1c promoter and that incubation with specific RORα inhibitor disrupted 24-h oscillations of ventricular promoter activities and Ca V 1.2 protein levels. Similar results were observed for pore forming subunits of the K + transient outward currents, K V 4.2 and K V 4.3.

Conclusions

These findings raise the possibility that the RORα-dependent rhythmic regulation of cardiac Ca V 1.2 and K V 4.2/4.3 throughout the daily cycle may play an important role in physiopathology of heart function.

Long QT syndrome-associated calmodulin variants disrupt the activity of the slowly activating delayed rectifier potassium channel

Calmodulin (CaM) is a highly conserved mediator of calcium (Ca2+ )-dependent signalling and modulates various cardiac ion channels. Genotyping has revealed several CaM mutations associated with long QT syndrome (LQTS). LQTS patients display prolonged ventricular recovery times (QT interval), increasing their risk of incurring life-threatening arrhythmic events. Loss-of-function mutations to Kv7.1 (which drives the slow delayed rectifier potassium current, IKs, a key ventricular repolarising current) are the largest contributor to congenital LQTS (>50% of cases). CaM modulates Kv7.1 to produce a Ca2+ -sensitive IKs, but little is known about the consequences of LQTS-associated CaM mutations on Kv7.1 function. Here, we present novel data characterising the biophysical and modulatory properties of three LQTS-associated CaM variants (D95V, N97I and D131H). We showed that mutations induced structural alterations in CaM and reduced affinity for Kv7.1, when compared with wild-type (WT). Using HEK293T cells expressing Kv7.1 channel subunits (KCNQ1/KCNE1) and patch-clamp electrophysiology, we demonstrated that LQTS-associated CaM variants reduced current density at systolic Ca2+ concentrations (1 μm), revealing a direct QT-prolonging modulatory effect. Our data highlight for the first time that LQTS-associated perturbations to CaM's structure impede complex formation with Kv7.1 and subsequently result in reduced IKs. This provides a novel mechanistic insight into how the perturbed structure-function relationship of CaM variants contributes to the LQTS phenotype. KEY POINTS: Calmodulin (CaM) is a ubiquitous, highly conserved calcium (Ca2+ ) sensor playing a key role in cardiac muscle contraction. Genotyping has revealed several CaM mutations associated with long QT syndrome (LQTS), a life-threatening cardiac arrhythmia syndrome. LQTS-associated CaM variants (D95V, N97I and D131H) induced structural alterations, altered binding to Kv7.1 and reduced IKs. Our data provide a novel mechanistic insight into how the perturbed structure-function relationship of CaM variants contributes to the LQTS phenotype.

In silico analysis of the contribution of cardiomyocyte-fibroblast electromechanical interaction to the arrhythmia

Although fibroblasts are about 5–10 times smaller than cardiomyocytes, their number in the ventricle is about twice that of cardiomyocytes. The high density of fibroblasts in myocardial tissue leads to a noticeable effect of their electromechanical interaction with cardiomyocytes on the electrical and mechanical functions of the latter. Our work focuses on the analysis of the mechanisms of spontaneous electrical and mechanical activity of the fibroblast-coupled cardiomyocyte during its calcium overload, which occurs in a variety of pathologies, including acute ischemia. For this study, we developed a mathematical model of the electromechanical interaction between cardiomyocyte and fibroblasts and used it to simulate the impact of overloading cardiomyocytes. In contrast to modeling only the electrical interaction between cardiomyocyte and fibroblasts, the following new features emerge in simulations with the model that accounts for both electrical and mechanical coupling and mechano-electrical feedback loops in the interacting cells. First, the activity of mechanosensitive ion channels in the coupled fibroblasts depolarizes their resting potential. Second, this additional depolarization increases the resting potential of the coupled myocyte, thus augmenting its susceptibility to triggered activity. The triggered activity associated with the cardiomyocyte calcium overload manifests itself in the model either as early afterdepolarizations or as extrasystoles, i.e., extra action potentials and extra contractions. Analysis of the model simulations showed that mechanics contribute significantly to the proarrhythmic effects in the cardiomyocyte overloaded with calcium and coupled with fibroblasts, and that mechano-electrical feedback loops in both the cardiomyocyte and fibroblasts play a key role in this phenomenon.

Role of Ca2+ in healthy and pathologic cardiac function: from normal excitation-contraction coupling to mutations that cause inherited arrhythmia

Calcium (Ca²⁺) ions are a key second messenger involved in the rhythmic excitation and contraction of cardiomyocytes throughout the heart. Proper function of Ca²⁺-handling proteins is required for healthy cardiac function, whereas disruption in any of these can cause cardiac arrhythmias. This comprehensive review provides a broad overview of the roles of Ca²⁺-handling proteins and their regulators in healthy cardiac function and the mechanisms by which mutations in these proteins contribute to inherited arrhythmias. Major Ca²⁺ channels and Ca²⁺-sensitive regulatory proteins involved in cardiac excitation-contraction coupling are discussed, with special emphasis on the function of the RyR2 macromolecular complex. Inherited arrhythmia disorders including catecholaminergic polymorphic ventricular tachycardia, long QT syndrome, Brugada syndrome, short QT syndrome, and arrhythmogenic right-ventricular cardiomyopathy are discussed with particular emphasis on subtypes caused by mutations in Ca²⁺-handling proteins.

Pathophysiology of Cav1.3 L-type calcium channels in the heart

Ca²⁺ plays a crucial role in excitation-contraction coupling in cardiac myocytes. Dysfunctional Ca²⁺ regulation alters the force of contraction and causes cardiac arrhythmias. Ca²⁺ entry into cardiomyocytes is mediated mainly through L-type Ca²⁺ channels, leading to the subsequent Ca²⁺ release from the sarcoplasmic reticulum. L-type Ca²⁺ channels are composed of the conventional Ca_v1.2, ubiquitously expressed in all heart chambers, and the developmentally regulated Ca_v1.3, exclusively expressed in the atria, sinoatrial node, and atrioventricular node in the adult heart. As such, Ca_v1.3 is implicated in the pathogenesis of sinoatrial and atrioventricular node dysfunction as well as atrial fibrillation. More recently, Ca_v1.3 de novo expression was suggested in heart failure. Here, we review the functional role, expression levels, and regulation of Ca_v1.3 in the heart, including in the context of cardiac diseases. We believe that the elucidation of the functional and molecular pathways regulating Ca_v1.3 in the heart will assist in developing novel targeted therapeutic interventions for the aforementioned arrhythmias.

Modulation of L-type calcium channels in Alzheimer's disease: A potential therapeutic target

Calcium plays a fundamental role in various signaling pathways and cellular processes in the human organism. In the nervous system, voltage-gated calcium channels such as L-type calcium channels (LTCCs) are critical elements in mediating neurotransmitter release, synaptic integration and plasticity. Dysfunction of LTCCs has been implicated in both aging and Alzheimer's Disease (AD), constituting a key component of calcium hypothesis of AD. As such, LTCCs are a promising drug target in AD. However, due to their structural and functional complexity, the mechanisms by which LTCCs contribute to AD are still unclear. In this review, we briefly summarize the structure, function, and modulation of LTCCs that are the backbone for understanding pathological processes involving LTCCs. We suggest targeting molecular pathways up-regulating LTCCs in AD may be a more promising approach, given the diverse physiological functions of LTCCs and the ineffectiveness of LTCC blockers in clinical studies.

Acetylcholine Reduces L-Type Calcium Current without Major Changes in Repolarization of Canine and Human Purkinje and Ventricular Tissue

Vagal nerve stimulation (VNS) holds a strong basis as a potentially effective treatment modality for chronic heart failure, which explains why a multicenter VNS study in heart failure with reduced ejection fraction is ongoing. However, more detailed information is required on the effect of acetylcholine (ACh) on repolarization in Purkinje and ventricular cardiac preparations to identify the advantages, risks, and underlying cellular mechanisms of VNS. Here, we studied the effect of ACh on the action potential (AP) of canine Purkinje fibers (PFs) and several human ventricular preparations. In addition, we characterized the effects of ACh on the L-type Ca²⁺ current (I_CaL) and AP of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and performed computer simulations to explain the observed effects. Using microelectrode recordings, we found a small but significant AP prolongation in canine PFs. In the human myocardium, ACh slightly prolonged the AP in the midmyocardium but resulted in minor AP shortening in subepicardial tissue. Perforated patch-clamp experiments on hiPSC-CMs demonstrated that 5 µM ACh caused an ≈15% decrease in I_CaL density without changes in gating properties. Using dynamic clamp, we found that under blocked K⁺ currents, 5 µM ACh resulted in an ≈23% decrease in AP duration at 90% of repolarization in hiPSC-CMs. Computer simulations using the O'Hara-Rudy human ventricular cell model revealed that the overall effect of ACh on AP duration is a tight interplay between the ACh-induced reduction in I_CaL and ACh-induced changes in K⁺ currents. In conclusion, ACh results in minor changes in AP repolarization and duration of canine PFs and human ventricular myocardium due to the concomitant inhibition of inward I_CaL and outward K⁺ currents, which limits changes in net repolarizing current and thus prevents major changes in AP repolarization.

Calcium Signaling Silencing in Atrial Fibrillation: Implications for Atrial Sodium Homeostasis

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia, affecting more than 33 million people worldwide. Despite important advances in therapy, AF's incidence remains high, and treatment often results in recurrence of the arrhythmia. A better understanding of the cellular and molecular changes that (1) trigger AF and (2) occur after the onset of AF will help to identify novel therapeutic targets. Over the past 20 years, a large body of research has shown that intracellular Ca2+ handling is dramatically altered in AF. While some of these changes are arrhythmogenic, other changes counteract cellular arrhythmogenic mechanisms (Calcium Signaling Silencing). The intracellular Na+ concentration ([Na+])i is a key regulator of intracellular Ca2+ handling in cardiac myocytes. Despite its importance in the regulation of intracellular Ca2+ handling, little is known about [Na+]i, its regulation, and how it might be changed in AF. Previous work suggests that there might be increases in the late component of the atrial Na+ current (INa,L) in AF, suggesting that [Na+]i levels might be high in AF. Indeed, a pharmacological blockade of INa,L has been suggested as a treatment for AF. Here, we review calcium signaling silencing and changes in intracellular Na+ homeostasis during AF. We summarize the proposed arrhythmogenic mechanisms associated with increases in INa,L during AF and discuss the evidence from clinical trials that have tested the pharmacological INa,L blocker ranolazine in the treatment of AF.

Mineralocorticoid receptor actions in cardiovascular development and disease

Mineralocorticoid receptors (MRs) are transcriptional regulators that mediate the diverse physiological and pathophysiological actions of corticosteroid hormones across many tissues. In the kidney aldosterone control of sodium/water resorption via DNA-binding actions of the MR is established. MRs also regulate tissues not involved in electrolyte homeostasis such as the heart, adipose tissue, brain, and inflammatory cells where the MRs can respond to both aldosterone and cortisol. The pathology of inappropriate MR activation in non-epithelial tissues are well-described, and steroidal antagonists of the MR have been clinically beneficial in the management of heart failure and blood pressure for decades. However, the role of cortisol-dependent MR activation in the physiological setting is less well defined. Like other steroid hormone receptors, the MR also regulates non-DNA-binding pathways including MAPK pathways and G protein coupled receptors to provide diversity to MR signaling. Whether nonDNA binding pathways are more relevant for MR activation in non-epithelial, versus epithelial, tissues remain unclear. This review will focus on molecular regulation of ligand-dependent MR activation and the physiology and pathophysiology of MR actions in the heart with a focus on the cardiomyocyte and provide a discussion of relevant genomic and non-genomic MR pathways and potential new transcriptional partners for the MR and their relevance for health and disease. Understanding MR actions in the heart will provide new insights into cell-selective mechanisms that underpin the therapeutic benefits of MRAs, and are a critical step towards developing next-generation tissue selective MR modulators with improved safety profiles.

L-Type Calcium Channel: Predicting Pathogenic/Likely Pathogenic Status for Variants of Uncertain Clinical Significance

(1) Background: Defects in gene CACNA1C, which encodes the pore-forming subunit of the human Cav1.2 channel (hCav1.2), are associated with cardiac disorders such as atrial fibrillation, long QT syndrome, conduction disorders, cardiomyopathies, and congenital heart defects. Clinical manifestations are known only for 12% of CACNA1C missense variants, which are listed in public databases. Bioinformatics approaches can be used to predict the pathogenic/likely pathogenic status for variants of uncertain clinical significance. Choosing a bioinformatics tool and pathogenicity threshold that are optimal for specific protein families increases the reliability of such predictions. (2) Methods and Results: We used databases ClinVar, Humsavar, gnomAD, and Ensembl to compose a dataset of pathogenic/likely pathogenic and benign variants of hCav1.2 and its 20 paralogues: voltage-gated sodium and calcium channels. We further tested the performance of sixteen in silico tools in predicting pathogenic variants. ClinPred demonstrated the best performance, followed by REVEL and MCap. In the subset of 309 uncharacterized variants of hCav1.2, ClinPred predicted the pathogenicity for 188 variants. Among these, 36 variants were also categorized as pathogenic/likely pathogenic in at least one paralogue of hCav1.2. (3) Conclusions: The bioinformatics tool ClinPred and the paralogue annotation method consensually predicted the pathogenic/likely pathogenic status for 36 uncharacterized variants of hCav1.2. An analogous approach can be used to classify missense variants of other calcium channels and novel variants of hCav1.2.

The zebrafish grime mutant uncovers an evolutionarily conserved role for Tmem161b in the control of cardiac rhythm

The establishment of cardiac function in the developing embryo is essential to ensure blood flow and, therefore, growth and survival of the animal. The molecular mechanisms controlling normal cardiac rhythm remain to be fully elucidated. From a forward genetic screen, we identified a unique mutant, grime, that displayed a specific cardiac arrhythmia phenotype. We show that loss-of-function mutations in tmem161b are responsible for the phenotype, identifying Tmem161b as a regulator of cardiac rhythm in zebrafish. To examine the evolutionary conservation of this function, we generated knockout mice for Tmem161b. Tmem161b knockout mice are neonatal lethal and cardiomyocytes exhibit arrhythmic calcium oscillations. Mechanistically, we find that Tmem161b is expressed at the cell membrane of excitable cells and live imaging shows it is required for action potential repolarization in the developing heart. Electrophysiology on isolated cardiomyocytes demonstrates that Tmem161b is essential to inhibit Ca²⁺ and K⁺ currents in cardiomyocytes. Importantly, Tmem161b haploinsufficiency leads to cardiac rhythm phenotypes, implicating it as a candidate gene in heritable cardiac arrhythmia. Overall, these data describe Tmem161b as a highly conserved regulator of cardiac rhythm that functions to modulate ion channel activity in zebrafish and mice.

Expanding the phenotype of CACNA1C mutation disorders

BACKGROUND

Pathogenic variants in the L-type Ca2+ channel gene CACNA1C cause a multi-system disorder that includes severe long QT syndrome (LQTS), congenital heart disease, dysmorphic facial features, syndactyly, abnormal immune function, and neuropsychiatric disorders, collectively known as Timothy syndrome. In 2015, a variant in CACNA1C (p.R518C) was reported to cause cardiac-only Timothy syndrome, a genetic disorder with a mixed phenotype of congenital heart disease, hypertrophic cardiomyopathy (HCM), and LQTS that lacked extra-cardiac features. We have identified a family harboring the p.R518C pathogenic variant with a wider spectrum of clinical manifestations.

METHODS

A four-generation family harboring the p.R518C pathogenic variant was reviewed in detail. The proband and his paternal great-uncle underwent comprehensive cardiac gene panel testing, and his remaining family members underwent cascade testing for the p.R518C pathogenic variant.

RESULTS

In addition to displaying cardinal features of CACNA1C disorders including LQTS, congenital heart disease, HCM, and sudden cardiac death, family members manifested atrial fibrillation and sick sinus syndrome.

CONCLUSION

Our report expands the cardiac phenotype of CACNA1C variants and reflects the variable expressivity of mutations in the L-type Ca2+ channel.

Diabetes-induced changes in cardiac voltage-gated ion channels

Diabetes mellitus affects the heart through various mechanisms such as microvascular defects, metabolic abnormalities, autonomic dysfunction and incompatible immune response. Furthermore, it can also cause functional and structural changes in the myocardium by a disease known as diabetic cardiomyopathy (DCM) in the absence of coronary artery disease. As DCM progresses it causes electrical remodeling of the heart, left ventricular dysfunction and heart failure. Electrophysiological changes in the diabetic heart contribute significantly to the incidence of arrhythmias and sudden cardiac death in diabetes mellitus patients. In recent studies, significant changes in repolarizing K+ currents, Na+ currents and L-type Ca2+ currents along with impaired Ca2+ homeostasis and defective contractile function have been identified in the diabetic heart. In addition, insulin levels and other trophic factors change significantly to maintain the ionic channel expression in diabetic patients. There are many diagnostic tools and management options for DCM, but it is difficult to detect its development and to effectively prevent its progress. In this review, diabetes-associated alterations in voltage-sensitive cardiac ion channels are comprehensively assessed to understand their potential role in the pathophysiology and pathogenesis of DCM.

M3, a 1,4-Dihydropyridine Derivative and Mixed L-/T-Type Calcium Channel Blocker, Attenuates Isoproterenol-Induced Toxicity in Male Wistar Rats

Recent evidence indicates that Ca²⁺ dysregulation is involved in the pathogenesis of isoproterenol (ISP)-induced biochemical toxicity and associated oxidative stress. In this study, we investigated the chemopreventive benefit of M3, a 1,4-dihydropyridine calcium channel blocker, against ISP-induced toxicity in male Wistar rats. Adult rats were divided into eight groups of six rats/group. Groups 1-5 received normal saline (control, 10 mL/kg/day, p.o.), ISP (85 mg/kg/day, s.c.), M3 lower dose (M3LD, 5 mg/kg, p.o.), M3 upper dose (M3UD, 20 mg/kg/day, p.o.), and Nifedipine (NFD, 20 mg/kg/day, p.o.), respectively. Others (groups 6-8) were pretreated with either M3LD, M3UD or NFD one hour before ISP administration. All rats were sacrificed 24 h after the last administration and changes in biochemical, hematological, and antioxidant parameters were assessed. Histologic examination of the heart, liver and kidney was also conducted. ISP elevated (p < 0.05) Ca²⁺, alanine aminotransferase, lactate dehydrogenase, triglycerides, and low-density lipoprotein levels when compared with control. Similarly, ISP increased levels of markers of renal function (p < 0.01), C-reactive protein (148.1%) and myocardial malondialdehyde (MDA, 88.7%) and tumor necrosis factor-alpha (109.2%). Platelet level was reduced (p < 0.05) in the ISP-intoxicated control rats. M3 exhibited antioxidant property, reduced levels of triglycerides, MDA and improved biochemical and hematological alterations associated with ISP toxicity. M3, however, was not effective in restoring histological changes that characterized ISP toxicity at the doses used. M3 offers chemopreventive benefits against ISP toxicity possibly through L-/T-type calcium channels blockade and modulatory actions on biochemical and antioxidant homeostasis.

Signal profiling of the β1AR reveals coupling to novel signalling pathways and distinct phenotypic responses mediated by β1AR and β2AR

A comprehensive understanding of signalling downstream of GPCRs requires a broad approach to capture novel signalling modalities in addition to established pathways. Here, using an array of sixteen validated BRET-based biosensors, we analyzed the ability of seven different β-adrenergic ligands to engage five distinct signalling pathways downstream of the β₁-adrenergic receptor (β₁AR). In addition to generating signalling signatures and capturing functional selectivity for the different ligands toward these pathways, we also revealed coupling to signalling pathways that have not previously been ascribed to the βAR. These include coupling to G_z and G₁₂ pathways. The signalling cascade linking the β₁AR to calcium mobilization was also characterized using a combination of BRET-based biosensors and CRISPR-engineered HEK 293 cells lacking the Gαs subunit or with pharmacological or genetically engineered pathway inhibitors. We show that both G_s and G₁₂ are required for the full calcium response. Our work highlights the power of combining signal profiling with genome editing approaches to capture the full complement of GPCR signalling activities in a given cell type and to probe their underlying mechanisms.

Discovery and characterization of a monogenetic insult, caveolin-3-V37L, that precipitated oligo-proteomic perturbations governing repolarization reserve

BACKGROUND

Caveolin-3 (Cav-3) is an essential scaffolding protein for caveolae formation in cardiomyocytes and targets multiple long QT syndrome (LQTS)-associated ion channels. Mutations in CAV3 have caused an LQT3-like accentuation in late sodium current, I_Na (Nav1.5). Here, we characterize a novel CAV3-V37L variant and determine whether it is the substrate for the patient's LQTS.

METHODS

The proband was a 39-year-old female with drug-induced, sudden cardiac arrest (SCA) with profound QT prolongation (QTc > 600 ms). Genetic testing revealed a rare CAV3-V37L variant of uncertain significance (VUS). Whole-cell patch clamp technique was used to measure I_Ks, I_Kr, I_Na, and I_{Ca, L} currents co-expressed with either CAV3-WT or CAV3-V37L in TSA201 cells and to measure the action potential duration (APD) in control human induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) overexpressed with CAV3-WT or CAV3-V37L.

RESULTS

CAV3-V37L did not affect Nav1.5 late current. Instead, CAV3-V37L resulted in 1) I_{Ca, L} with slower inactivation, a 1.5 fold increase in peak I_{Ca, L} current density and a 1.1 fold increase in I_{Ca, L} persistent current, 2) dramatically reduced I_Ks peak current density by 74.9%, 3) significantly reduced I_Kr peak current density by 31.1%, and 4) significantly prolonged the APD in hiPSC-CMs.

CONCLUSIONS

These functional validation assays enabled the promotion of CAV3-V37L from VUS status to a likely pathogenic variant. Although Nav1.5 was spared, this monogenetic insult precipitated an oligo-proteomic impact with a concomitant gain-of-function of I_{Ca, L} and loss-of-function of both I_Ks and I_Kr culminating in a marked prolongation of the cardiomyocyte's action potential duration.

Calcium Handling Defects and Cardiac Arrhythmia Syndromes

Calcium ions (Ca²⁺) play a major role in the cardiac excitation-contraction coupling. Intracellular Ca²⁺ concentration increases during systole and falls in diastole thereby determining cardiac contraction and relaxation. Normal cardiac function also requires perfect organization of the ion currents at the cellular level to drive action potentials and to maintain action potential propagation and electrical homogeneity at the tissue level. Any imbalance in Ca²⁺ homeostasis of a cardiac myocyte can lead to electrical disturbances. This review aims to discuss cardiac physiology and pathophysiology from the elementary membrane processes that can cause the electrical instability of the ventricular myocytes through intracellular Ca²⁺ handling maladies to inherited and acquired arrhythmias. Finally, the paper will discuss the current therapeutic approaches targeting cardiac arrhythmias.

Dexmedetomidine Exerted Anti-arrhythmic Effects in Rat With Ischemic Cardiomyopathy via Upregulation of Connexin 43 and Reduction of Fibrosis and Inflammation

Background

Persistent myocardial ischemia post-myocardial infarction can lead to fatal ventricular arrhythmias such as ventricular tachycardia and fibrillation, both of which carry high mortality rates. Dexmedetomidine (Dex) is a highly selective α2-agonist used in surgery for congenital cardiac disease because of its antiarrhythmic properties. Dex has previously been reported to prevent or terminate various arrhythmias. The purpose of the present study was to determine the anti-arrhythmic properties of Dex in the context of ischemic cardiomyopathy (ICM) after myocardial infarction.

Methods and Results

We randomly allocated 48 rats with ICM, created by persistent ligation of the left anterior descending artery for 4 weeks, into six groups: Sham (n = 8), Sham + BML (n = 8), ICM (n = 8), ICM + BML (n = 8), ICM + Dex (n = 8), and ICM + Dex + BML (n = 8). Treatments started after ICM was confirmed (the day after echocardiographic measurement) and continued for 4 weeks (inject intraperitoneally, daily). Dex inhibited the generation of collagens, cytokines, and other inflammatory mediators in rats with ICM via the suppression of NF-κB activation and increased the distribution of connexin 43 (Cx43) via phosphorylation of adenosine 5′-monophosphate-activated protein kinase (AMPK). Dex reduced the occurrence of spontaneous ventricular arrhythmias (ventricular premature beat or ventricular tachycardia), decreased the inducibility quotient of ventricular arrhythmias induced by PES, and partly improved cardiac contraction. The AMPK antagonist BML-275 dihydrochloride (BML) partly weakened the cardioprotective effect of Dex.

Conclusion

Dex conferred anti-arrhythmic effects in the context of ICM via upregulation of Cx43 and suppression of inflammation and fibrosis. The anti-arrhythmic and anti-inflammatory properties of Dex may be mediated by phosphorylation of AMPK and subsequent suppression of NF-κB activation.

Effects of nitric oxide on apoptosis and voltage-gated calcium channels in human cardiac myofibroblasts

We characterised the voltage-gated Ca²⁺ channels (VGCCs) in human cardiac fibroblasts (HCFs) and myofibroblasts (HCMFs) and investigated the effects of nitric oxide (NO) on apoptosis and on these channels. Western blotting and immunofluorescence analyses show that α-smooth muscle actin (a myofibroblast marker) was markedly expressed in passage (P) 12-15 but not in P4 HCF cells, whereas calponin (a fibroblast marker) was expressed only in P4 cells. Ca_V 1.2 (L-type) and Ca_V 3.3 (T-type) of VGCCs were highly expressed in P12-15 cells, but only weak Ca_V 2.3 (R-type) expression was identified in P4 cells using reverse transcription-polymerase chain reaction analysis. S-Nitroso-N-acetylpenicillamine (SNAP, an NO donor) decreased cell viability of HCMFs in a dose-dependent manner and induced apoptotic changes, and nifedipine (an L-type Ca²⁺ channel blocker) prevented apoptosis as shown with immunofluorescence staining and flow cytometry. Whole-cell mode patch-clamp recordings demonstrate the presence of L-type Ca²⁺ (I_{C a,L} ) and T-type Ca²⁺ (I_{C a,T} ) currents in HCMFs. SNAP inhibited I_{C a,L} of HCMFs, but pre-treatment with ODQ (a guanylate cyclase inhibitor) or KT5823 (a PKG inhibitor) prevented it. Pre-treating cells with KT5720 (a PKA inhibitor) or SQ22536 (an adenylate cyclase inhibitor) blocked SNAP-induced inhibition of I_{C a,L} . 8-Bromo-cyclic GMP or 8-bromo-cyclic AMP also inhibited I_{C a,L} . However, pre-treatment with N-ethylmaleimide (a thiol-alkylating reagent) did not block the SNAP effect, nor did DL-dithiothreitol (a reducing agent) reverse it. These data suggest that high concentrations of NO injure HCMFs and inhibit I_{C a,L} through the PKG and PKA signalling pathways but not through the S-nitrosylation pathway.

Engineered Cardiac Tissues Generated in the Biowire™ II: A Platform for Human-Based Drug Discovery

Recent advances in techniques to differentiate human induced pluripotent stem cells (hiPSCs) hold the promise of an unlimited supply of human derived cardiac cells from both healthy and disease populations. That promise has been tempered by the observation that hiPSC-derived cardiomyocytes (hiPSC-CMs) typically retain a fetal-like phenotype, raising concern about the translatability of the in vitro data obtained to drug safety, discovery and development studies. The Biowire™ II platform was used to generate 3D engineered cardiac tissues (ECTs) from hiPSC-CMs and cardiac fibroblasts. Long term electrical stimulation was employed to obtain ECTs that possess a phenotype like that of adult human myocardium including a lack of spontaneous beating, the presence of a positive force-frequency response from 1-4Hz and prominent post-rest potentiation. Pharmacology studies were performed in the ECTs to confirm the presence and functionality of pathways that modulate cardiac contractility in humans. Canonical responses were observed for compounds that act via the β-adrenergic/cAMP-mediated pathway, e.g. isoproterenol and milrinone; the L-type calcium channel, e.g. FPL64176 and nifedipine; and indirectly effect intracellular Ca2+ concentrations, e.g. digoxin. Expected positive inotropic responses were observed for compounds that modulate proteins of the cardiac sarcomere, e.g. omecamtiv mecarbil and levosimendan. ECTs generated in the BiowireTM II platform display adult-like properties and have canonical responses to cardiotherapeutic and cardiotoxic agents that affect contractility in humans via a variety of mechanisms. These data demonstrate that this human-based model can be used to assess the effects of novel compounds on contractility early in the drug discovery and development process.

Blockade of L-type Ca2+ channel attenuates doxorubicin-induced cardiomyopathy via suppression of CaMKII-NF-κB pathway

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and nuclear factor-kappa B (NF-κB) play crucial roles in pathogenesis of doxorubicin (DOX)-induced cardiomyopathy. Their activities are regulated by intracellular Ca²⁺. We hypothesized that blockade of L-type Ca²⁺ channel (LTCC) could attenuate DOX-induced cardiomyopathy by regulating CaMKII and NF-κB. DOX activated CaMKII and NF-κB through their phosphorylation and increased cleaved caspase 3 in cardiomyocytes. Pharmacological blockade or gene knockdown of LTCC by nifedipine or small interfering RNA, respectively, suppressed DOX-induced phosphorylation of CaMKII and NF-κB and apoptosis in cardiomyocytes, accompanied by decreasing intracellular Ca²⁺ concentration. Autocamtide 2-related inhibitory peptide (AIP), a selective CaMKII inhibitor, inhibited DOX-induced phosphorylation of NF-κB and cardiomyocyte apoptosis. Inhibition of NF-κB activity by ammonium pyrrolidinedithiocarbamate (PDTC) suppressed DOX-induced cardiomyocyte apoptosis. DOX-treatment (18 mg/kg via intravenous 3 injections over 1 week) increased phosphorylation of CaMKII and NF-κB in mouse hearts. Nifedipine (10 mg/kg/day) significantly suppressed DOX-induced phosphorylation of CaMKII and NF-κB and cardiomyocyte injury and apoptosis in mouse hearts. Moreover, it attenuated DOX-induced left ventricular dysfunction and dilatation. Our findings suggest that blockade of LTCC attenuates DOX-induced cardiomyocyte apoptosis via suppressing intracellular Ca²⁺ elevation and activation of CaMKII-NF-κB pathway. LTCC blockers might be potential therapeutic agents against DOX-induced cardiomyopathy.

Commotio cordis and L-type calcium channel mutation: Is there a link?

Commotio cordis is a rare phenomenon when ventricular fibrillation and sudden death occurs with a blunt, nonpenetrating blow to the chest. Individual susceptibility to commotio cordis has been demonstrated in swine models, and might be present in humans as well. We report a case of commotio cordis in an adolescent with a heterozygous mutation on the gene CACNA1C, encoding for an L-type calcium channel expressed in the heart. This genetic mutation has been previously associated with a phenotype of long-QT syndrome; however, this was not demonstrated in our patient despite extensive investigations. To the best of our knowledge, this is the first report of commotio cordis in which an ion-channel gene mutation involved in repolarization abnormalities has been documented. This finding might corroborate the hypothesis that a genetic predisposition plays a role in the individual susceptibility to this rare cause of cardiac arrest.

How Ca2+ influx is attenuated in the heart during a "fight or flight" response

Bazmi and Escobar highlight a recent investigation of the mechanisms that regulate Ca²⁺ influx during sympathetic stimulation.

Atomic Mechanisms of Timothy Syndrome-Associated Mutations in Calcium Channel Cav1.2

Timothy syndrome (TS) is a very rare multisystem disorder almost exclusively associated with mutations G402S and G406R in helix IS6 of Cav1.2. Recently, mutations R518C/H in helix IIS0 of the voltage sensing domain II (VSD-II) were described as a cause of cardiac-only TS. The three mutations are known to decelerate voltage-dependent inactivation (VDI). Here, we report a case of cardiac-only TS caused by mutation R518C. To explore possible impact of the three mutations on interdomain contacts, we modeled channel Cav1.2 using as templates Class Ia and Class II cryo-EM structures of presumably inactivated channel Cav1.1. In both models, R518 and several other residues in VSD-II donated H-bonds to the IS6-linked α1-interaction domain (AID). We further employed steered Monte Carlo energy minimizations to move helices S4–S5, S5, and S6 from the inactivated-state positions to those seen in the X-ray structures of the open and closed NavAb channel. In the open-state models, positions of AID and VSD-II were similar to those in Cav1.1. In the closed-state models, AID moved along the β subunit (Cavβ) toward the pore axis and shifted AID-bound VSD-II. In all the models R518 retained strong contacts with AID. Our calculations suggest that conformational changes in VSD-II upon its deactivation would shift AID along Cavβ toward the pore axis. The AID-linked IS6 would bend at flexible G402 and G406, facilitating the activation gate closure. Mutations R518C/H weakened the IIS0-AID contacts and would retard the AID shift. Mutations G406R and G402S stabilized the open state and would resist the pore closure. Several Cav1.2 mutations associated with long QT syndromes are consistent with this proposition. Our results provide a mechanistic rationale for the VDI deceleration caused by TS-associated mutations and suggest targets for further studies of calcium channelopathies.

Ups and downs of calcium in the heart

Contraction and relaxation of the heart result from cyclical changes of intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ). The entry of Ca²⁺ into the cell via the L-type Ca²⁺ current leads to the release of more from the sarcoplasmic reticulum (SR). Compared to other regulatory mechanisms such as phosphorylation, Ca²⁺ signalling is very rapid. However, since Ca²⁺ cannot be destroyed, Ca²⁺ signalling can only be controlled by pumping across membranes. In the steady state, on each beat, the amount of Ca²⁺ released from the SR must equal that taken back and influx and efflux across the sarcolemma must be equal. Any imbalance in these fluxes will result in a change of SR Ca²⁺ content and this provides a mechanism for regulation of SR Ca²⁺ content. These flux balance considerations also explain why simply potentiating Ca²⁺ release from the SR has no maintained effect on the amplitude of the Ca²⁺ transient. A low diastolic [Ca²⁺ ]_i is essential for cardiac relaxation, but the factors that control diastolic [Ca²⁺ ]_i are poorly understood. Recent work suggests that flux balance is also important here. In particular, decreasing SR function decreases the amplitude of the systolic Ca²⁺ transient and the resulting decrease of Ca²⁺ efflux results in an increase of diastolic [Ca²⁺ ]_i to maintain total efflux.

Calcium-dependent inactivation controls cardiac L-type Ca2+ currents under β-adrenergic stimulation

During a cardiac action potential, the activity of L-type Ca²⁺ channels (LTCCs) is modulated by voltage- and calcium-dependent inactivation processes. Morales et al. show that, in the context of β-adrenergic stimulation, calcium-dependent inactivation directs the regulation of LTCC activity, limiting calcium influx during the action potential.

The activity of L-type calcium channels is associated with the duration of the plateau phase of the cardiac action potential (AP) and it is controlled by voltage- and calcium-dependent inactivation (VDI and CDI, respectively). During β-adrenergic stimulation, an increase in the L-type current and parallel changes in VDI and CDI are observed during square pulses stimulation; however, how these modifications impact calcium currents during an AP remains controversial. Here, we examined the role of both inactivation processes on the L-type calcium current activity in newborn rat cardiomyocytes in control conditions and after stimulation with the β-adrenergic agonist isoproterenol. Our approach combines a self-AP clamp (sAP-Clamp) with the independent inhibition of VDI or CDI (by overexpressing Ca_Vβ_2a or calmodulin mutants, respectively) to directly record the L-type calcium current during the cardiac AP. We find that at room temperature (20–23°C) and in the absence of β-adrenergic stimulation, the L-type current recapitulates the AP kinetics. Furthermore, under our experimental setting, the activity of the sodium–calcium exchanger (NCX) does not affect the shape of the AP. We find that hindering either VDI or CDI prolongs the L-type current and the AP in parallel, suggesting that both inactivation processes modulate the L-type current during the AP. In the presence of isoproterenol, wild-type and VDI-inhibited cardiomyocytes display mismatched L-type calcium current with respect to their AP. In contrast, CDI-impaired cells maintain L-type current with kinetics similar to its AP, demonstrating that calcium-dependent inactivation governs L-type current kinetics during β-adrenergic stimulation.

Excitation of murine cardiac myocytes by nanosecond pulsed electric field

INTRODUCTION

Opening of voltage-gated sodium channels takes tens to hundreds of microseconds, and mechanisms of their opening by nanosecond pulsed electric field (nsPEF) stimuli remain elusive. This study was aimed at uncovering the mechanisms of how nsPEF elicits action potentials (APs) in cardiomyocytes.

METHODS AND RESULTS

Fluorescent imaging of optical APs (FluoVolt) and Ca²⁺ -transients (Fluo-4) was performed in enzymatically isolated murine ventricular cardiomyocytes stimulated by 200-nanosecond trapezoidal pulses. nsPEF stimulation evoked tetrodotoxin-sensitive APs accompanied or preceded by slow sustained depolarization (SSD) and, in most cells, by transient afterdepolarization waves. SSD threshold was lower than the AP threshold (1.26 ± 0.03 vs 1.34 ± 0.03 kV/cm, respectively, P < 0.001). Inhibition of l-type calcium and sodium-calcium exchanger currents reduced the SSD amplitude and increased the AP threshold ( P < 0.05). The threshold for Ca ²⁺ -transients (1.40 ± 0.04 kV/cm) was not significantly affected by a tetrodotoxin-verapamil cocktail, suggesting the activation of a Ca ²⁺ entry pathway independent from the opening of Na ⁺ or Ca ²⁺ voltage-gated channels. Removal of external Ca ²⁺ decreased the SSD amplitude ( P = 0.004) and blocked Ca ²⁺ -transients but not APs. The incidence of transient afterdepolarization waves was decreased by verapamil and by removal of external Ca ²⁺ ( P = 0.002).

CONCLUSIONS

The study established that nsPEF stimulation caused calcium entry into cardiac myocytes (including routes other than voltage-gated calcium channels) and SSD. Tetrodotoxin-sensitive APs were mediated by SSD, whose amplitude depended on the calcium entry. Plasma membrane electroporation was the most likely primary mechanism of SSD with additional contribution from l-type calcium and sodium-calcium exchanger currents.

The Role of 17β-Estradiol and Estrogen Receptors in Regulation of Ca2+ Channels and Mitochondrial Function in Cardiomyocytes

Numerous epidemiological, clinical, and animal studies showed that cardiac function and manifestation of cardiovascular diseases (CVDs) are different between males and females. The underlying reasons for these sex differences are definitely multifactorial, but major evidence points to a causal role of the sex steroid hormone 17β-estradiol (E2) and its receptors (ER) in the physiology and pathophysiology of the heart. Interestingly, it has been shown that cardiac calcium (Ca²⁺) ion channels and mitochondrial function are regulated in a sex-specific manner. Accurate mitochondrial function and Ca²⁺ signaling are of utmost importance for adequate heart function and crucial to maintaining the cardiovascular health. Due to the highly sensitive nature of these processes in the heart, this review article highlights the current knowledge regarding sex dimorphisms in the heart implicating the importance of E2 and ERs in the regulation of cardiac mitochondrial function and Ca²⁺ ion channels, thus the contractility. In particular, we provide an overview of in-vitro and in-vivo studies using either E2 deficiency; ER deficiency or selective ER activation, which suggest that E2 and ERs are strongly involved in these processes. In this context, this review also discusses the divergent E2-responses resulting from the activation of different ER subtypes in these processes. Detailed understanding of the E2 and ER-mediated molecular and cellular mechanisms in the heart under physiological and pathological conditions may help to design more specifically targeted drugs for the management of CVDs in men and women.

A multicentre study of patients with Timothy syndrome

Aims

Timothy syndrome (TS) is an extremely rare multisystem disorder characterized by marked QT prolongation, syndactyly, seizures, behavioural abnormalities, immunodeficiency, and hypoglycaemia. The aim of this study was to categorize the phenotypes and examine the outcomes of patients with TS.

Methods and results

All patients diagnosed with TS in the United Kingdom over a 24-year period were reviewed. Fifteen centres in the British Congenital Arrhythmia Group network were contacted to partake in the study. Six patients with TS were identified over a 24-year period (4 boys and 2 girls). Five out of the six patients were confirmed to have a CACNA1C mutation (p.Gly406Arg) and the other patient was diagnosed clinically. Early presentation with heart block, due to QT prolongation was frequently seen. Four are still alive, two of these have a pacemaker and two have undergone defibrillator implantation. Five out of six patients have had a documented cardiac arrest with three occurring under general anaesthesia. Two patients suffered a cardiac arrest while in hospital and resuscitation was unsuccessful, despite immediate access to a defibrillator. Surviving patients seem to have mild developmental delay and learning difficulties.

Conclusion

Timothy syndrome is a rare disorder with a high attrition rate if undiagnosed. Perioperative cardiac arrests are common and not always amenable to resuscitation. Longer-term survival is possible, however, patients invariably require pacemaker or defibrillator implantation.

An African loss-of-function CACNA1C variant p.T1787M associated with a risk of ventricular fibrillation

Calcium regulation plays a central role in cardiac function. Several variants in the calcium channel Ca_v1.2 have been implicated in arrhythmic syndromes. We screened patients with Brugada syndrome, short QT syndrome, early repolarisation syndrome, and idiopathic ventricular fibrillation to determine the frequency and pathogenicity of Ca_v1.2 variants. Ca_v1.2 related genes, CACNA1C, CACNB2 and CACNA2D1, were screened in 65 probands. Missense variants were introduced in the Ca_v1.2 alpha subunit plasmid by mutagenesis to assess their pathogenicity using patch clamp approaches. Six missense variants were identified in CACNA1C in five individuals. Five of them, A1648T, A1689T, G1795R, R1973Q, C1992F, showed no major alterations of the channel function. The sixth C-terminal variant, Ca_vα_1c-T1787M, present mostly in the African population, was identified in two patients with resuscitated cardiac arrest. The first patient originated from Cameroon and the second was an inhabitant of La Reunion Island with idiopathic ventricular fibrillation originating from Purkinje tissues. Patch-clamp analysis revealed that Ca_vα_1c-T1787M reduces the calcium and barium currents by increasing the auto-inhibition mediated by the C-terminal part and increases the voltage-dependent inhibition. We identified a loss-of-function variant, Ca_vα_1c-T1787M, present in 0.8% of the African population, as a new risk factor for ventricular arrhythmia.

A pore-localizing CACNA1C-E1115K missense mutation, identified in a patient with idiopathic QT prolongation, bradycardia, and autism spectrum disorder, converts the L-type calcium channel into a hybrid nonselective monovalent cation channel

BACKGROUND

Gain-of-function variants in the CACNA1C-encoded L-type calcium channel (LTCC, Ca_v1.2) cause type 8 long QT syndrome (LQT8). The pore region contains highly conserved glutamic acid (E) residues that collectively form the LTCC's selectivity filter. Here, we identified and characterized a pore-localizing missense variant, E1115K, that yielded a novel perturbation in the LTCC.

OBJECTIVE

The purpose of this study was to determine whether CACNA1C-E1115K alters the LTCC's selectivity and is the substrate for the patient's LQTS.

METHODS

The proband was a 14-year-old male with idiopathic QT prolongation and bradycardia. Genetic testing revealed a missense variant, CACNA1C-E1115K. The whole-cell patch clamp technique was used to measure CACNA1C-WT and -E1115K currents when heterologously expressed in TSA201 cells.

RESULTS

The CACNA1C-E1115K channel exhibited no inward calcium current. Instead, robust cardiac transient outward potassium current (I_to)-like outward currents that were blocked significantly by nifedipine were measured when 2 mM/0.1 mM extracellular/intracellular CaCl2 or 4 mM/141 mM extracellular/intracellular KCl was applied. Furthermore, when 140 mM extracellular NaCl was applied, the CACNA1C-E1115K channel revealed both robust inward persistent Na⁺ currents with slower inactivation and outward currents, which were also nifedipine sensitive. In contrast, CACNA1C-WT revealed only a small inward persistent Na⁺ current without a robust outward current.

CONCLUSION

This CACNA1C-E1115K variant destroyed the LTCC's calcium selectivity and instead converted the mutant channel into a channel with a marked increase in sodium-mediated inward currents and potassium-mediated outward currents. Despite the anticipated 50% reduction in LTCC, the creation of a new population of channels with accentuated inward and outward currents represents the likely pathogenic substrates for the patient's LQTS and arrhythmia phenotype.

Molecular mechanisms and physiological relevance of RGK proteins in the heart

The primary route of Ca²⁺ entry into cardiac myocytes is via 1,4-dihydropyridine-sensitive, voltage-gated L-type Ca²⁺ channels. Ca²⁺ influx through these channels influences duration of action potential and engages excitation-contraction (EC) coupling in both the atria and the myocardium. Members of the RGK (Rad, Rem, Rem2 and Gem/Kir) family of small GTP-binding proteins are potent, endogenously expressed inhibitors of cardiac L-type channels. Although much work has focused on the molecular mechanisms by which RGK proteins inhibit the Ca_V 1.2 and Ca_V 1.3 L-type channel isoforms that expressed in the heart, their impact on greater cardiac function is only beginning to come into focus. In this review, we summarize recent findings regarding the influence of RGK proteins on normal cardiac physiology and the pathological consequences of aberrant RGK activity.

Genetic Testing in the Evaluation of Unexplained Cardiac Arrest: From the CASPER (Cardiac Arrest Survivors With Preserved Ejection Fraction Registry)

BACKGROUND

Unexplained cardiac arrest may be because of an inherited arrhythmia syndrome. The role of genetic testing in cardiac arrest survivors without a definite clinical phenotype is unclear.

METHODS AND RESULTS

The CASPER (Cardiac Arrest Survivors with Preserved Ejection Fraction Registry) is a large registry of cardiac arrest survivors where initial assessment reveals normal coronary arteries, left ventricular function, and resting ECG. Of 375 cardiac arrest survivors in CASPER from 2006 to 2015, 174 underwent genetic testing. Patients were classified as phenotype-positive (n=72) or phenotype-negative (n=102). Genetic testing was performed at treating physicians' discretion in line with contemporary guidelines and availability. All genetic variants identified from original laboratory reports were reassessed by the investigators in line with modern criteria. Pathogenic variants were identified in 29 (17%) patients (60% channelopathy-associated and 40% cardiomyopathy-associated genes) and 70 variants of unknown significance were identified in 32 (18%) patients. Prior syncope (odds ratio, 4.0; 95% confidence interval, 1.6-9.7) and a family history of sudden death (odds ratio, 3.2; 95% confidence interval, 1.1-9.4) were independently associated with the presence of a pathogenic variant. In phenotype-negative patients, broad multiphenotype genetic testing led to higher yields (21% versus 8%; P=0.04) but was associated with more variants of unknown significance (55% versus 5%; P<0.01).

CONCLUSIONS

Genetic testing identifies a pathogenic variant in a significant proportion of unexplained cardiac arrest survivors. Prior syncope and family history of sudden death are predictors of a positive genetic test. Both arrhythmia and cardiomyopathy genes are implicated. Broad, multiphenotype testing revealed the highest frequency of pathogenic variants in phenotype-negative patients.

CLINICAL TRIAL REGISTRATION

https://www.clinicaltrials.gov. Unique Identifier: NCT00292032.

Voltage dependence of the Ca2+ transient in endocardial and epicardial myocytes from the left ventricle of Goto-Kakizaki type 2 diabetic rats

Diabetes mellitus is a major global health disorder and, currently, over 450 million people have diabetes with 90% suffering from type 2 diabetes. Left untreated, diabetes may lead to cardiovascular diseases which are a leading cause of death in diabetic patients. Calcium is the trigger and regulator of cardiac muscle contraction and derangement in cellular Ca²⁺ homeostasis, which can result in heart failure and sudden cardiac death. It is of paramount importance to investigate the regional involvement of Ca²⁺ in diabetes-induced cardiomyopathy. Therefore, the aim of this study was to investigate the voltage dependence of the Ca²⁺ transients in endocardial (ENDO) and epicardial (EPI) myocytes from the left ventricle of the Goto-Kakizaki (GK) rats, an experimental model of type 2 diabetes mellitus. Simultaneous measurement of L-type Ca²⁺ currents and Ca²⁺ transients was performed by whole-cell patch clamp techniques. GK rats displayed significantly increased heart weight, heart weight/body weight ratio, and non-fasting and fasting blood glucose compared to controls (CON). Although the voltage dependence of L-type Ca²⁺ current was unaltered, the voltage dependence of the Ca²⁺ transients was reduced to similar extents in EPI-GK and ENDO-GK compared to EPI-CON and ENDO-CON myocytes. TPK L-type Ca²⁺ current and Ca²⁺ transient were unaltered. THALF decay of L-type Ca²⁺ current was unaltered; however, THALF decay of the Ca²⁺ transient was shortened in ENDO and EPI myocytes from GK compared to CON rat hearts. In conclusion, the amplitude of L-type Ca²⁺ current was unaltered; however, the voltage dependence of the Ca²⁺ transient was reduced to similar extents in EPI and ENDO myocytes from GK rats compared to their respective controls, suggesting the possibility of dysfunctional sarcoplasmic reticulum Ca²⁺ transport in the GK diabetic rat hearts.

The Venom of Ornithoctonus huwena affect the electrophysiological stability of neonatal rat ventricular myocytes by inhibiting sodium, potassium and calcium current

Spider venoms are known to contain various toxins that are used as an effective means to capture their prey or to defend themselves against predators. An investigation of the properties of Ornithoctonus huwena (O.huwena) crude venom found that the venom can block neuromuscular transmission of isolated mouse phrenic nerve-diaphragm and sciatic nerve-sartorius preparations. However, little is known about its electrophysiological effects on cardiac myocytes. In this study, electrophysiological activities of ventricular myocytes were detected by 100 μg/mL venom of O.huwena, and whole cell patch-clamp technique was used to study the acute effects of the venom on action potential (AP), sodium current (I_Na), potassium currents (I_Kr, I_Ks, I_to1 and I_K1) and L-type calcium current (I_CaL). The results indicated that the venom prolongs APD₉₀ in a frequency-dependent manner in isolated neonatal rat ventricular myocytes. 100 μg/mL venom inhibited 72.3 ± 3.6% I_Na current, 58.3 ± 4.2% summit current and 54 ± 6.1% the end current of I_Kr, and 65 ± 3.3% I_CaL current, yet, didn't have obvious effect on I_Ks, I_to1 and I_K1 currents. In conclusion, the O.huwena venom represented a multifaceted pharmacological profile. It contains abundant of cardiac channel antagonists and might be valuable tools for investigation of both channels and anti- arrhythmic therapy development.

Cardiac voltage gated calcium channels and their regulation by β-adrenergic signaling

Voltage-gated calcium channels (VGCCs) are the predominant source of calcium influx in the heart leading to calcium-induced calcium release and ultimately excitation-contraction coupling. In the heart, VGCCs are modulated by the β-adrenergic signaling. Signaling through β-adrenergic receptors (βARs) and modulation of VGCCs by β-adrenergic signaling in the heart are critical signaling and changes to these have been significantly implicated in heart failure. However, data related to calcium channel dysfunction in heart failure is divergent and contradictory ranging from reduced function to no change in the calcium current. Many recent studies have highlighted the importance of functional and spatial microdomains in the heart and that may be the key to answer several puzzling questions. In this review, we have briefly discussed the types of VGCCs found in heart tissues, their structure, and significance in the normal and pathological condition of the heart. More importantly, we have reviewed the modulation of VGCCs by βARs in normal and pathological conditions incorporating functional and structural aspects. There are different types of βARs, each having their own significance in the functioning of the heart. Finally, we emphasize the importance of location of proteins as it relates to their function and modulation by co-signaling molecules. Its implication on the studies of heart failure is speculated.