Calcium and Excitation-Contraction Coupling in the Heart

Polymorphism rs7214723 in CAMKK1: a new genetic variant associated with cardiovascular diseases

Cardiovascular diseases (CVDs) are the leading cause of deaths worldwide. CVDs have a complex etiology due to the several factors underlying its development including environment, lifestyle, and genetics. Given the role of calcium signal transduction in several CVDs, we investigated via PCR-restriction fragment length polymorphism (RFLP) the single nucleotide polymorphism (SNP) rs7214723 within the calcium/calmodulin-dependent kinase kinase 1 (CAMKK1) gene coding for the Ca²⁺/calmodulin-dependent protein kinase kinase I. The variant rs7214723 causes E375G substitution within the kinase domain of CAMKK1. A cross-sectional study was conducted on 300 cardiac patients. RFLP-PCR technique was applied, and statistical analysis was performed to evaluate genotypic and allelic frequencies and to identify an association between SNP and risk of developing specific CVD. Genotype and allele frequencies for rs7214723 were statistically different between cardiopathic and several European reference populations. A logistic regression analysis adjusted for gender, age, diabetes, hypertension, BMI and previous history of malignancy was applied on cardiopathic genotypic data and no association was found between rs7214723 polymorphism and risk of developing specific coronary artery disease (CAD) and aortic stenosis (AS). These results suggest the potential role of rs7214723 in CVD susceptibility as a possible genetic biomarker.

Ca2+ mishandling in heart failure: Potential targets

Ca²⁺ mishandling is a common feature in several cardiovascular diseases such as heart failure (HF). In many cases, impairment of key players in intracellular Ca²⁺ homeostasis has been identified as the underlying mechanism of cardiac dysfunction and cardiac arrhythmias associated with HF. In this review, we summarize primary novel findings related to Ca²⁺ mishandling in HF progression. HF research has increasingly focused on the identification of new targets and the contribution of their role in Ca²⁺ handling to the progression of the disease. Recent research studies have identified potential targets in three major emerging areas implicated in regulation of Ca²⁺ handling: the innate immune system, bone metabolism factors and post-translational modification of key proteins involved in regulation of Ca²⁺ handling. Here, we describe their possible contributions to the progression of HF.

Non-Coding RNAs in the Cardiac Action Potential and Their Impact on Arrhythmogenic Cardiac Diseases

Cardiac arrhythmias are prevalent among humans across all age ranges, affecting millions of people worldwide. While cardiac arrhythmias vary widely in their clinical presentation, they possess shared complex electrophysiologic properties at cellular level that have not been fully studied. Over the last decade, our current understanding of the functional roles of non-coding RNAs have progressively increased. microRNAs represent the most studied type of small ncRNAs and it has been demonstrated that miRNAs play essential roles in multiple biological contexts, including normal development and diseases. In this review, we provide a comprehensive analysis of the functional contribution of non-coding RNAs, primarily microRNAs, to the normal configuration of the cardiac action potential, as well as their association to distinct types of arrhythmogenic cardiac diseases.

The Diabetic Cardiomyopathy: The Contributing Pathophysiological Mechanisms

Individuals with diabetes mellitus (DM) disclose a higher incidence and a poorer prognosis of heart failure (HF) than non-diabetic people, even in the absence of other HF risk factors. The adverse impact of diabetes on HF likely reflects an underlying “diabetic cardiomyopathy” (DM–CMP), which may by exacerbated by left ventricular hypertrophy and coronary artery disease (CAD). The pathogenesis of DM-CMP has been a hot topic of research since its first description and is still under active investigation, as a complex interplay among multiple mechanisms may play a role at systemic, myocardial, and cellular/molecular levels. Among these, metabolic abnormalities such as lipotoxicity and glucotoxicity, mitochondrial damage and dysfunction, oxidative stress, abnormal calcium signaling, inflammation, epigenetic factors, and others. These disturbances predispose the diabetic heart to extracellular remodeling and hypertrophy, thus leading to left ventricular diastolic and systolic dysfunction. This Review aims to outline the major pathophysiological changes and the underlying mechanisms leading to myocardial remodeling and cardiac functional derangement in DM-CMP.

Cellular mechanisms and recommended drug-based therapeutic options in diabetic cardiomyopathy

Diabetes mellitus (DM) is associated with a specific cardiac phenotype characterized by structural and functional alterations. This so-called diabetic cardiomyopathy (DM CM) is clinically relevant as patients with DM show high incidence of heart failure. Mechanistically, several parameters interact on the cardiomyocyte level leading to increased inflammation, apoptosis, reactive oxygen species and altered calcium signaling. This in turn provokes functional myocardial changes that might inter alia play into the worsened clinical outcome in DM patients. Therefore, efficient therapeutic options are urgently needed. This review focuses on mechanistic effects of currently recommended antidiabetic treatment and heart failure therapy for DM CM.

Human BIN1 isoforms grow, maintain and regenerate excitation-contraction couplons in adult rat and human stem cell-derived cardiomyocytes

AIMS

In ventricular myocytes, Transverse-tubules (T-tubules) are instrumental for excitation-contraction (EC) coupling and their disarray is a hallmark of cardiac diseases. BIN1 is a key contributor to their biogenesis. Our study set out to investigate the role of human BIN1 splice variants in the maintenance and regeneration of EC-coupling in rat adult ventricular myocytes and human induced pluripotent stem cell-derived cardiac myocytes (hiPS-CMs).

METHODS AND RESULTS

In heart samples from healthy human donors expression patterns of 5 BIN1 splice variants were identified. Following viral transduction of human BIN1 splice variants in cellular models of T-tubular disarray we employed high-speed confocal calcium imaging and Ca-CLEAN analysis to identify functional EC-coupling sites and T-tubular architecture. Adult rat ventricular myocytes were used to investigate the regeneration after loss and maintenance of EC-coupling while we studied the enhancement of EC-coupling in hiPS-CMs. All five human BIN1 splice variants induced de novo generation of T-tubules in both cell types. Isoforms with the phosphoinositide binding motif (PI) were most potent in maintenance and regeneration of T-tubules and functional EC-coupling in adult rat myocytes. In hiPSC-CMs, BIN1 variants with PI motiv induced de-novo generation of T-tubules, functional EC-coupling sites and enhanced calcium handling.

CONCLUSION(S)

BIN1 is essential for the maintenance, regeneration, and de-novo generation of functional T-tubules, especially isoforms with PI motifs. These T-tubules trigger the development of functional EC couplons resulting in enhanced calcium handling.

TRANSLATIONAL PERSPECTIVE

Cardiomyopathy and heart failure are among the most frequent causes of death in modern societies. Gene therapies and hiPSC technology are becoming increasingly promising, both for treatment and therapy development. On the cellular level, one of the common denominators of cardiac diseases is the concurrent loss of T-tubules essential for efficient EC-coupling. While initial approaches in animal models employing gene therapy with BIN1 have depicted encouraging improvements the expression pattern of BIN1 isoforms in the human heart is still elusive. The present study identifies a unique set of five distinct BIN1 isoforms in healthy human hearts and demonstrates their potency in both, T-tubule maintenance and re-generation after loss resulting in efficient EC-coupling. Noteworthy, PI-motif containing isoforms were potent trigger of de-novo generation of T-tubules and establishment of efficient EC-coupling in hiPSC-CMs. Therefore, the expression of BIN1 might be novel and promising for pharmaceutical treatment and gene therapy.

From Genetic Mutations to Molecular Basis of Heart Failure Treatment: An Overview of the Mechanism and Implication of the Novel Modulators for Cardiac Myosin

Heart failure (HF) is a syndrome encompassing several important etiologies that lead to the imbalance between oxygen demand and supply. Despite the usage of guideline-directed medical therapy for HF has shown better outcomes, novel therapeutic strategies are desirable, especially for patients with preserved or mildly reduced left ventricular ejection fraction. In this regard, understanding the molecular basis for cardiomyopathies is expected to fill in the knowledge gap and generate new therapies to improve prognosis for HF. This review discusses an evolutionary mechanism designed to regulate cardiac contraction and relaxation through the most often genetically determined cardiomyopathies associated with HF. In addition, both the myosin inhibitor and myosin activator are promising new treatments for cardiomyopathies. A comprehensive review from genetic mutations to the molecular basis of direct sarcomere modulators will help shed light on future studies for a better characterization of HF etiologies and potential therapeutic targets.

PAM: diverse roles in neuroendocrine cells, cardiomyocytes, and green algae

Our understanding of the ways in which peptides are used for communication in the nervous and endocrine systems began with the identification of oxytocin, vasopressin, and insulin, each of which is stored in electron-dense granules, ready for release in response to an appropriate stimulus. For each of these peptides, entry of its newly synthesized precursor into the ER lumen is followed by transport through the secretory pathway, exposing the precursor to a sequence of environments and enzymes that produce the bioactive products stored in mature granules. A final step in the biosynthesis of many peptides is C-terminal amidation by peptidylglycine α-amidating monooxygenase (PAM), an ascorbate- and copper-dependent membrane enzyme that enters secretory granules along with its soluble substrates. Biochemical and cell biological studies elucidated the highly conserved mechanism for amidated peptide production and raised many questions about PAM trafficking and the effects of PAM on cytoskeletal organization and gene expression. Phylogenetic studies and the discovery of active PAM in the ciliary membranes of Chlamydomonas reinhardtii, a green alga lacking secretory granules, suggested that a PAM-like enzyme was present in the last eukaryotic common ancestor. While the catalytic features of human and C. reinhardtii PAM are strikingly similar, the trafficking of PAM in C. reinhardtii and neuroendocrine cells and secretion of its amidated products differ. A comparison of PAM function in neuroendocrine cells, atrial myocytes, and C. reinhardtii reveals multiple ways in which altered trafficking allows PAM to accomplish different tasks in different species and cell types.

The Mystery of Diabetic Cardiomyopathy: From Early Concepts and Underlying Mechanisms to Novel Therapeutic Possibilities

Diabetic patients are predisposed to diabetic cardiomyopathy, a specific form of cardiomyopathy which is characterized by the development of myocardial fibrosis, cardiomyocyte hypertrophy, and apoptosis that develops independently of concomitant macrovascular and microvascular diabetic complications. Its pathophysiology is multifactorial and poorly understood and no specific therapeutic guideline has yet been established. Diabetic cardiomyopathy is a challenging diagnosis, made after excluding other potential entities, treated with different pharmacotherapeutic agents targeting various pathophysiological pathways that need yet to be unraveled. It has great clinical importance as diabetes is a disease with pandemic proportions. This review focuses on the potential mechanisms contributing to this entity, diagnostic options, as well as on potential therapeutic interventions taking in consideration their clinical feasibility and limitations in everyday practice. Besides conventional therapies, we discuss novel therapeutic possibilities that have not yet been translated into clinical practice.

Improving the diagnosis of heart failure in patients with atrial fibrillation

OBJECTIVE

To improve the echocardiographic assessment of heart failure in patients with atrial fibrillation (AF) by comparing conventional averaging of consecutive beats with an index-beat approach, whereby measurements are taken after two cycles with similar R-R interval.

METHODS

Transthoracic echocardiography was performed using a standardised and blinded protocol in patients enrolled in the RATE-AF (RAte control Therapy Evaluation in permanent Atrial Fibrillation) randomised trial. We compared reproducibility of the index-beat and conventional consecutive-beat methods to calculate left ventricular ejection fraction (LVEF), global longitudinal strain (GLS) and E/e' (mitral E wave max/average diastolic tissue Doppler velocity), and assessed intraoperator/interoperator variability, time efficiency and validity against natriuretic peptides.

RESULTS

160 patients were included, 46% of whom were women, with a median age of 75 years (IQR 69-82) and a median heart rate of 100 beats per minute (IQR 86-112). The index-beat had the lowest within-beat coefficient of variation for LVEF (32%, vs 51% for 5 consecutive beats and 53% for 10 consecutive beats), GLS (26%, vs 43% and 42%) and E/e' (25%, vs 41% and 41%). Intraoperator (n=50) and interoperator (n=18) reproducibility were both superior for index-beats and this method was quicker to perform (p<0.001): 35.4 s to measure E/e' (95% CI 33.1 to 37.8) compared with 44.7 s for 5-beat (95% CI 41.8 to 47.5) and 98.1 s for 10-beat (95% CI 91.7 to 104.4) analyses. Using a single index-beat did not compromise the association of LVEF, GLS or E/e' with natriuretic peptide levels.

CONCLUSIONS

Compared with averaging of multiple beats in patients with AF, the index-beat approach improves reproducibility and saves time without a negative impact on validity, potentially improving the diagnosis and classification of heart failure in patients with AF.

Cardiac function dependence on carbon monoxide

Nitric oxide, studied to evaluate its role in cardiovascular physiology, has cardioprotective and therapeutic effects in cellular signaling, mitochondrial function, and in regulating inflammatory processes. Heme oxygenase (major role in catabolism of heme into biliverdin, carbon monoxide (CO), and iron) has similar effects as well. CO has been suggested as the molecule that is responsible for many of the above mentioned cytoprotective and therapeutic pathways as CO is a signaling molecule in the control of physiological functions. This is counterintuitive as toxic effects are related to its binding to hemoglobin. However, CO is normally produced in the body. Experimental evidence indicates that this toxic gas, CO, exerts cytoprotective properties related to cellular stress including the heart and is being assessed for its cytoprotective and cytotherapeutic properties. While survival of adult cardiomyocytes depends on oxidative phosphorylation (survival and resulting cardiac function is impaired by mitochondrial damage), mitochondrial biogenesis is modified by the heme oxygenase-1/CO system and can result in promotion of mitochondrial biogenesis by associating mitochondrial redox status to the redox-active transcription factors. It has been suggested that the heme oxygenase-1/CO system is important in differentiation of embryonic stem cells and maturation of cardiomyocytes which is thought to mitigate progression of degenerative cardiovascular diseases. Effects on other cardiac cells are being studied. Acute exposure to air pollution (and, therefore, CO) is associated with cardiovascular mortality, myocardial infarction, and heart failure, but changes in the endogenous heme oxygenase-1 system (and, thereby, CO) positively affect cardiovascular health. We will review the effect of CO on heart health and function in this article.

Sex differences in echocardiographic predictors of bradycardia detected by implantable loop recorder in patients with syncope and palpitations

OBJECTIVE

Our aim was to investigate whether echocardiography may aid in identifying patients, specifically men, at risk of bradycardia as detected by implantable loop recorders (ILR) in patients evaluated for syncope and palpitations.

METHODS

We included ambulatory patients undergoing ILR implantation for syncope (84%), presyncope (9%), and palpitations (8%). Echocardiographic examination was performed prior to implantation (2.9 months [IQR 1.0-6.0 months]). Echocardiograms were analyzed for conventional and speckle tracking parameters. We examined time to first event of bradycardia, defined as (a) heart rate <30 beats/min and (b) ≥4 beats, including sinus arrest, asystole, sinoatrial block, and second- and third-degree atrioventricular nodal block. We applied Cox proportional hazards models.

RESULTS

A total of 285 patients we enrolled, and during a median time of 2.7 years [IQR 1.0, 3.3 years] of continuous heart rhythm monitoring, 84 (29%) had bradycardia detected by ILR. Patients with bradycardia were older (61 ± 19 years vs 55 ± 18 years, P = .01) and more frequently men (62% vs 44%, P = .01). Sex modified the association between echocardiographic parameters and bradycardia (P interaction <0.05 for all), such that left ventricular LV mass index (HR: 1.02 per 1g/m² increase [1.01-1.04], P < .001), LV ejection fraction (HR: 1.04 per 1% decrease [1.01-1.08], P = .02), and global longitudinal strain (HR: 1.09 per 1% decrease [1.01-1.19], P = .03) were associated with bradycardia in men but not women (P > .05 for all in female). After adjusting for baseline clinical characteristics, medical therapy, and loop indication, the abovementioned parameters remained significantly associated with incident bradycardia in men.

CONCLUSION

Echocardiographic parameters of LV structure and function may potentially be more useful for predicting bradycardia in men than women, among patients undergoing ILR implantation for syncope, presyncope, and palpations.

Mitochondrial Ca2+ Signaling in Health, Disease and Therapy

The divalent cation calcium (Ca²⁺) is considered one of the main second messengers inside cells and acts as the most prominent signal in a plethora of biological processes. Its homeostasis is guaranteed by an intricate and complex system of channels, pumps, and exchangers. In this context, by regulating cellular Ca²⁺ levels, mitochondria control both the uptake and release of Ca²⁺. Therefore, at the mitochondrial level, Ca²⁺ plays a dual role, participating in both vital physiological processes (ATP production and regulation of mitochondrial metabolism) and pathophysiological processes (cell death, cancer progression and metastasis). Hence, it is not surprising that alterations in mitochondrial Ca²⁺ (mCa²⁺) pathways or mutations in Ca²⁺ transporters affect the activities and functions of the entire cell. Indeed, it is widely recognized that dysregulation of mCa²⁺ signaling leads to various pathological scenarios, including cancer, neurological defects and cardiovascular diseases (CVDs). This review summarizes the current knowledge on the regulation of mCa²⁺ homeostasis, the related mechanisms and the significance of this regulation in physiology and human diseases. We also highlight strategies aimed at remedying mCa²⁺ dysregulation as promising therapeutical approaches.

Ryanodine receptor and immune-related molecules in diabetic cardiomyopathy

Hyperglycaemia is a major aetiological factor in the development of diabetic cardiomyopathy. Excessive hyperglycaemia increases the levels of reactive carbonyl species (RCS), reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the heart and causes derangements in calcium homeostasis, inflammation and immune‐system disorders. Ryanodine receptor 2 (RyR2) plays a key role in excitation–contraction coupling during heart contractions, including rhythmic contraction and relaxation of the heart. Cardiac inflammation has been indicated in part though interleukin 1 (IL‐1) signals, supporting a role for B and T lymphocytes in diabetic cardiomyopathy. Some of the post‐translational modifications of the ryanodine receptor (RyR) by RCS, ROS and RNS stress are known to affect its gating and Ca²⁺ sensitivity, which contributes to RyR dysregulation in diabetic cardiomyopathy. RyRs and immune‐related molecules are important signalling species in many physiological and pathophysiological processes in various heart and cardiovascular diseases. However, little is known regarding the mechanistic relationship between RyRs and immune‐related molecules in diabetes, as well as the mechanisms mediating complex communication among cardiomyocytes, fibroblasts and immune cells. This review highlights new findings on the complex cellular communications in the pathogenesis and progression of diabetic cardiomyopathy. We discuss potential therapeutic applications targeting RyRs and immune‐related molecules in diabetic complications.

Mechanisms of flecainide induced negative inotropy: An in silico study

It is imperative to develop better approaches to predict how antiarrhythmic drugs with multiple interactions and targets may alter the overall electrical and/or mechanical function of the heart. Safety Pharmacology studies have provided new insights into the multi-target effects of many different classes of drugs and have been aided by the addition of robust new in vitro and in silico technology. The primary focus of Safety Pharmacology studies has been to determine the risk profile of drugs and drug candidates by assessing their effects on repolarization of the cardiac action potential. However, for decades experimental and clinical studies have described substantial and potentially detrimental effects of Na⁺ channel blockers in addition to their well-known conduction slowing effects. One such side effect, associated with administration of some Na⁺ channel blocking drugs is negative inotropy. This reduces the pumping function of the heart, thereby resulting in hypotension. Flecainide is a well-known example of a Na⁺ channel blocking drug, that exhibits strong rate-dependent block of I_Na and may cause negative cardiac inotropy. While the phenomenon of Na⁺ channel suppression and resulting negative inotropy is well described, the mechanism(s) underlying this effect are not. Here, we set out to use a modeling and simulation approach to reveal plausible mechanisms that could explain the negative inotropic effect of flecainide. We utilized the Grandi-Bers model [1] of the cardiac ventricular myocyte because of its robust descriptions of ion homeostasis in order to characterize and resolve the relative effects of QRS widening, flecainide off-target effects and changes in intracellular Ca²⁺ and Na⁺ homeostasis. The results of our investigations and predictions reconcile multiple data sets and illustrate how multiple mechanisms may play a contributing role in the flecainide induced negative cardiac inotropic effect.

Mitochondrial Bioenergetics and Dynamism in the Failing Heart

The heart is responsible for pumping blood, nutrients, and oxygen from its cavities to the whole body through rhythmic and vigorous contractions. Heart function relies on a delicate balance between continuous energy consumption and generation that changes from birth to adulthood and depends on a very efficient oxidative metabolism and the ability to adapt to different conditions. In recent years, mitochondrial dysfunctions were recognized as the hallmark of the onset and development of manifold heart diseases (HDs), including heart failure (HF). HF is a severe condition for which there is currently no cure. In this condition, the failing heart is characterized by a disequilibrium in mitochondrial bioenergetics, which compromises the basal functions and includes the loss of oxygen and substrate availability, an altered metabolism, and inefficient energy production and utilization. This review concisely summarizes the bioenergetics and some other mitochondrial features in the heart with a focus on the features that become impaired in the failing heart.

Endolysosomal Ca2+ signaling in cardiovascular health and disease

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) regulates a plethora of functions in the cardiovascular (CV) system, including contraction in cardiomyocytes and vascular smooth muscle cells (VSMCs), and angiogenesis in vascular endothelial cells and endothelial colony forming cells. The sarco/endoplasmic reticulum (SR/ER) represents the largest endogenous Ca²⁺ store, which releases Ca²⁺ through ryanodine receptors (RyRs) and/or inositol-1,4,5-trisphosphate receptors (InsP₃Rs) upon extracellular stimulation. The acidic vesicles of the endolysosomal (EL) compartment represent an additional endogenous Ca²⁺ store, which is targeted by several second messengers, including nicotinic acid adenine dinucleotide phosphate (NAADP) and phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂], and may release intraluminal Ca²⁺ through multiple Ca²⁺ permeable channels, including two-pore channels 1 and 2 (TPC1-2) and Transient Receptor Potential Mucolipin 1 (TRPML1). Herein, we discuss the emerging, pathophysiological role of EL Ca²⁺ signaling in the CV system. We describe the role of cardiac TPCs in β-adrenoceptor stimulation, arrhythmia, hypertrophy, and ischemia-reperfusion injury. We then illustrate the role of EL Ca²⁺ signaling in VSMCs, where TPCs promote vasoconstriction and contribute to pulmonary artery hypertension and atherosclerosis, whereas TRPML1 sustains vasodilation and is also involved in atherosclerosis. Subsequently, we describe the mechanisms whereby endothelial TPCs promote vasodilation, contribute to neurovascular coupling in the brain and stimulate angiogenesis and vasculogenesis. Finally, we discuss about the possibility to target TPCs, which are likely to mediate CV cell infection by the Severe Acute Respiratory Disease-Coronavirus-2, with Food and Drug Administration-approved drugs to alleviate the detrimental effects of Coronavirus Disease-19 on the CV system.

Mitochondrial ROS and mitochondria-targeted antioxidants in the aged heart

Excessive mitochondrial ROS production has been causally linked to the pathophysiology of aging in the heart and other organs, and plays a deleterious role in several age-related cardiac pathologies, including myocardial ischemia-reperfusion injury and heart failure, the two worldwide leading causes of death and disability in the elderly. However, ROS generation is also a fundamental mitochondrial function that orchestrates several signaling pathways, some of them exerting cardioprotective effects. In cardiac myocytes, mitochondria are particularly abundant and are specialized in subcellular populations, in part determined by their relationships with other organelles and their cyclic calcium handling activity necessary for adequate myocardial contraction/relaxation and redox balance. Depending on their subcellular location, mitochondria can themselves be differentially targeted by ROS and display distinct age-dependent functional decline. Thus, precise mitochondria-targeted therapies aimed at counteracting unregulated ROS production are expected to have therapeutic benefits in certain aging-related heart conditions. However, for an adequate design of such therapies, it is necessary to unravel the complex and dynamic interactions between mitochondria and other cellular processes.

Mesenchymal stem cell-derived extracellular vesicles in the failing heart: past, present, and future

Cardiovascular disease (CVD) is the leading cause of death globally. Current treatment options include lifestyle changes, medication, and surgical intervention. However, many patients are unsuitable candidates for surgeries due to comorbidities, diffuse coronary artery disease, or advanced stages of heart failure. The search for new treatment options has recently transitioned from cell-based therapies to stem-cell-derived extracellular vesicles (EVs). A number of challenges remain in the EV field, including the effect of comorbidities, characterization, and delivery. However, recent revolutionary developments and insight into the potential of personalizing EV contents by bioengineering methods to alter specific signaling pathways in the ischemic myocardium hold promise. Here, we discuss the past limitations of cell-based therapies and recent EV studies involving in vivo, in vitro, and omics, and future challenges and opportunities in EV-based treatments in CVD.

A Heterozygous Mutation in Cardiac Troponin T Promotes Ca2+ Dysregulation and Adult Cardiomyopathy in Zebrafish

Cardiomyopathies are a group of heterogeneous diseases that affect the muscles of the heart, leading to early morbidity and mortality in young and adults. Genetic forms of cardiomyopathy are caused predominantly by mutations in structural components of the cardiomyocyte sarcomeres, the contractile units of the heart, which includes cardiac Troponin T (TnT). Here, we generated mutations with CRISPR/Cas9 technology in the zebrafish tnnt2a gene, encoding cardiac TnT, at a mutational “hotspot” site to establish a zebrafish model for genetic cardiomyopathies. We found that a heterozygous tnnt2a mutation deleting Arginine at position 94 and Lysine at position 95 of TnT causes progressive cardiac structural changes resulting in heart failure. The cardiac remodeling is presented by an enlarged atrium, decreased ventricle size, increased myocardial stress as well as increased fibrosis. As early as five days post fertilization, larvae carrying the TnT RK94del mutation display diastolic dysfunction and impaired calcium dynamics related to increased Ca²⁺ sensitivity. In conclusion, adult zebrafish with a heterozygous TnT-RK94del mutation develop cardiomyopathy as seen in patients with TnT mutations and therefore represent a promising model to study disease mechanisms and to screen for putative therapeutic compounds.

Abnormal Calcium Handling in Duchenne Muscular Dystrophy: Mechanisms and Potential Therapies

Duchenne muscular dystrophy (DMD) is an X-linked muscle-wasting disease caused by the loss of dystrophin. DMD is associated with muscle degeneration, necrosis, inflammation, fatty replacement, and fibrosis, resulting in muscle weakness, respiratory and cardiac failure, and premature death. There is no curative treatment. Investigations on disease-causing mechanisms offer an opportunity to identify new therapeutic targets to treat DMD. An abnormal elevation of the intracellular calcium (Cai2+) concentration in the dystrophin-deficient muscle is a major secondary event, which contributes to disease progression in DMD. Emerging studies have suggested that targeting Ca²⁺-handling proteins and/or mechanisms could be a promising therapeutic strategy for DMD. Here, we provide an updated overview of the mechanistic roles the sarcolemma, sarcoplasmic/endoplasmic reticulum, and mitochondria play in the abnormal and sustained elevation of Cai2+ levels and their involvement in DMD pathogenesis. We also discuss current approaches aimed at restoring Ca²⁺ homeostasis as potential therapies for DMD.

Admission Serum Ionized and Total Calcium as New Predictors of Mortality in Patients with Cardiogenic Shock

Background

Although serum calcium has been proven to be a predictor of mortality in a wide range of diseases, its prognostic value in critically ill patients with cardiogenic shock (CS) remains unknown. This retrospective observational study is aimed at investigating the association of admission calcium with mortality among CS patients.

Methods

Critically ill patients diagnosed with CS in the Medical Information Mart for Intensive Care-III (MIMIC-III) database were included in our study. The study endpoints included 30-day, 90-day, and 365-day all-cause mortalities. First, admission serum ionized calcium (iCa) and total calcium (tCa) levels were analyzed as continuous variables using restricted cubic spline Cox regression models to evaluate the possible nonlinear relationship between serum calcium and mortality. Second, patients with CS were assigned to four groups according to the quartiles (Q1-Q4) of serum iCa and tCa levels, respectively. In addition, multivariable Cox regression analyses were used to assess the independent association of the quartiles of iCa and tCa with clinical outcomes.

Results

A total of 921 patients hospitalized with CS were enrolled in this study. A nonlinear relationship between serum calcium levels and 30-day mortality was observed (all P values for nonlinear trend < 0.001). Furthermore, multivariable Cox analysis showed that compared with the reference quartile (Q3: 1.11 ≤ iCa < 1.17 mmol/L), the lowest serum iCa level quartile (Q1: iCa < 1.04 mmol/L) was independently associated with an increased risk of 30-day mortality (Q1 vs. Q3: HR 1.35, 95% CI 1.00-1.83, P = 0.049), 90-day mortality (Q1 vs. Q3: HR 1.36, 95% CI 1.03-1.80, P = 0.030), and 365-day mortality (Q1 vs. Q3: HR 1.28, 95% CI 1.01-1.67, P = 0.046) in patients with CS.

Conclusions

Lower serum iCa levels on admission were potential predictors of an increased risk of mortality in critically ill patients with CS.

Chenodeoxycholic and deoxycholic acids induced positive inotropic and negative chronotropic effects on rat heart

Bile acids are endogenous amphiphilic steroids from the metabolites of cholesterol. Studies showed that they might contribute to the pathogenesis of cardiopathy in cholestatic liver diseases. Chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA) is associated with colon cancer, gallstones, and gastrointestinal disorders. However, little information is available regarding their cardiac effects. Here, we reported that CDCA (100 μM) and DCA (100 μM) significantly increased the left ventricular developed pressure of the isolated rat hearts to 122.3 ± 5.6% and 145.1 ± 13.7%, and the maximal rate of the pressure development rising and descending (± dP/dtmax) to 103.4 ± 17.6% and 124.4 ± 37.7% of the basal levels, respectively. They decreased the heart rate and prolonged the RR, QRS, and QT intervals of Langendorff-perfused hearts in a concentration-dependent manner. Moreover, CDCA and DCA increased the developed tension of left ventricular muscle and the cytosolic Ca2+ concentrations in left ventricular myocytes; these functions positively coordinated with their inotropic effects on hearts. Additionally, CDCA (150 μM) and DCA (100 μM) decreased the sinoatrial node beating rate to 80.6 ± 3.0% and 79.7 ± 0.9% of the basal rate (334.2 ± 10.7 bpm), respectively. These results were consistent with their chronotropic effects. In conclusion, CDCA and DCA induced positive inotropic effects by elevating the Ca2+ in left ventricular myocytes. They exerted negative chronotropic effects by lowering the pace of the sinoatrial node in rat heart. These results indicated that the potential role of bile acids in cardiopathy related to cholestasis.

Calcium signaling in endocardial and epicardial ventricular myocytes from streptozotocin-induced diabetic rats

AIMS/INTRODUCTION

Abnormalities in Ca2+ signaling have a key role in hemodynamic dysfunction in diabetic heart. The purpose of this study was to explore the effects of streptozotocin (STZ)-induced diabetes on Ca2+ signaling in epicardial (EPI) and endocardial (ENDO) cells of the left ventricle after 5-6 months of STZ injection.

MATERIALS AND METHODS

Whole-cell patch clamp was used to measure the L-type Ca2+ channel (LTCC) and Na+ /Ca2+ exchanger currents. Fluorescence photometry techniques were used to measure intracellular free Ca2+ concentration.

RESULTS

Although the LTCC current was not significantly altered, the amplitude of Ca2+ transients increased significantly in EPI-STZ and ENDO-STZ compared with controls. Time to peak LTCC current, time to peak Ca2+ transient, time to half decay of LTCC current and time to half decay of Ca2+ transients were not significantly changed in EPI-STZ and ENDO-STZ myocytes compared with controls. The Na+ /Ca2+ exchanger current was significantly smaller in EPI-STZ and in ENDO-STZ compared with controls.

CONCLUSIONS

STZ-induced diabetes resulted in an increase in amplitude of Ca2+ transients in EPI and ENDO myocytes that was independent of the LTCC current. Such an effect can be attributed, at least in part, to the dysfunction of the Na+ /Ca2+ exchanger. Additional studies are warranted to improve our understanding of the regional impact of diabetes on Ca2+ signaling, which will facilitate the discovery of new targeted treatments for diabetic cardiomyopathy.

Cx43 hemichannel microdomain signaling at the intercalated disc enhances cardiac excitability

Cx43, a major cardiac connexin, forms precursor hemichannels that accrue at the intercalated disc to assemble as gap junctions. While gap junctions are crucial for electrical conduction in the heart, little is known about the potential roles of hemichannels. Recent evidence suggests that inhibiting Cx43 hemichannel opening with Gap19 has antiarrhythmic effects. Here, we used multiple electrophysiology, imaging, and super-resolution techniques to understand and define the conditions underlying Cx43 hemichannel activation in ventricular cardiomyocytes, their contribution to diastolic Ca2+ release from the sarcoplasmic reticulum, and their impact on electrical stability. We showed that Cx43 hemichannels were activated during diastolic Ca2+ release in single ventricular cardiomyocytes and cardiomyocyte cell pairs from mice and pigs. This activation involved Cx43 hemichannel Ca2+ entry and coupling to Ca2+ release microdomains at the intercalated disc, resulting in enhanced Ca2+ dynamics. Hemichannel opening furthermore contributed to delayed afterdepolarizations and triggered action potentials. In single cardiomyocytes, cardiomyocyte cell pairs, and arterially perfused tissue wedges from failing human hearts, increased hemichannel activity contributed to electrical instability compared with nonfailing rejected donor hearts. We conclude that microdomain coupling between Cx43 hemichannels and Ca2+ release is a potentially novel, targetable mechanism of cardiac arrhythmogenesis in heart failure.

SERCA2a: a key protein in the Ca2+ cycle of the heart failure

Calcium ion (Ca²⁺) cycle plays a crucial role in the contraction and relaxation of cardiomyocytes. The sarcoplasmic reticulum (SR) acts as an organelle for storing Ca²⁺, which mediated the release and re-uptake of Ca²⁺ during contraction and relaxation. Disorders of SR function lead to the dysfunction of Ca²⁺ cycle and myocardial cell function. The sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2a (SERCA2a) acts as a subtype of SERCA expressed in the heart, which mediates the contraction of cardiomyocytes and Ca²⁺ in the cytoplasm to re-enter into the SR. The rate of uptake of Ca²⁺ by the SR determines the rate of myocardial relaxation. The regulation of SERCA2a activity controls the contractility and relaxation of the heart, affecting cardiac function. The expression and activity of SERCA2a are reduced in failing hearts. Gene therapy by increasing the expression of SERCA2a in the heart has been proven effective. In addition, SERCA2a is regulated by a variety of factors, including transmembrane micropeptides, protein kinases, and post-translational modifications (PTMs). In this review, we discuss the regulatory factors of SERCA2a and provide new insights into future treatments and the direction of heart failure research. In addition, gene therapy for SERCA2a has recently emerged as therapeutic option and hence will be discussed in this review.

The significance of left ventricular ejection time in heart failure with reduced ejection fraction

Left ventricular ejection time (LVET) is defined as the time interval from aortic valve opening to aortic valve closure, and is the phase of systole during which the left ventricle ejects blood into the aorta. LVET has been used for several decades to assess left ventricular function and contractility. However, there is a recent interest in LVET as a measure of therapeutic action for novel drugs in patients with heart failure with reduced ejection fraction (HFrEF), since LVET is shortened in these patients. This review provides an overview of the available information on LVET including methods of measuring LVET, mechanistic understanding of LVET, association of LVET with outcomes, mechanisms behind shortened LVET in HFrEF and the potential implications of drugs that affect and normalize LVET.

Comparative Genomics Reveals 13 Different Isoforms of Mytimycins (A-M) in Mytilus galloprovincialis

Mytimycins are cysteine-rich antimicrobial peptides that show antifungal properties. These peptides are part of the immune network that constitutes the defense system of the Mediterranean mussel (Mytilus galloprovincialis). The immune system of mussels has been increasingly studied in the last decade due to its great efficiency, since these molluscs, particularly resistant to adverse conditions and pathogens, are present all over the world, being considered as an invasive species. The recent sequencing of the mussel genome has greatly simplified the genetic study of some of its immune genes. In the present work, we describe a total of 106 different mytimycin variants in 16 individual mussel genomes. The 13 highly supported mytimycin clusters (A–M) identified with phylogenetic inference were found to be subject to the presence/absence variation, a widespread phenomenon in mussels. We also identified a block of conserved residues evolving under purifying selection, which may indicate the “functional core” of the mature peptide, and a conserved set of 10 invariable plus 6 accessory cysteines which constitute a plastic disulfide array. Finally, we extended the taxonomic range of distribution of mytimycins among Mytilida, identifying novel sequences in M. coruscus, M. californianus, P. viridis, L. fortunei, M. philippinarum, M. modiolus, and P. purpuratus.

In silico simulations reveal that RYR distribution affects the dynamics of calcium release in cardiac myocytes

The dyads of cardiac myocytes contain ryanodine receptors (RYRs) that generate calcium sparks upon activation. To test how geometric factors of RYR distribution contribute to the formation of calcium sparks, which cannot be addressed experimentally, we performed in silico simulations on a large set of models of calcium release sites (CRSs). Our models covered the observed range of RYR number, density, and spatial arrangement. The calcium release function of CRSs was modeled by RYR openings, with an open probability dependent on concentrations of free Ca2+ and Mg2+ ions, in a rapidly buffered system, with a constant open RYR calcium current. We found that simulations of spontaneous sparks by repeatedly opening one of the RYRs in a CRS produced three different types of calcium release events (CREs) in any of the models. Transformation of simulated CREs into fluorescence signals yielded calcium sparks with characteristics close to the observed ones. CRE occurrence varied broadly with the spatial distribution of RYRs in the CRS but did not consistently correlate with RYR number, surface density, or calcium current. However, it correlated with RYR coupling strength, defined as the weighted product of RYR vicinity and calcium current, so that CRE characteristics of all models followed the same state-response function. This finding revealed the synergy between structure and function of CRSs in shaping dyad function. Lastly, rearrangements of RYRs simulating hypothetical experiments on splitting and compaction of a dyad revealed an increased propensity to generate spontaneous sparks and an overall increase in calcium release in smaller and more compact dyads, thus underlying the importance and physiological role of RYR arrangement in cardiac myocytes.

Extra-skeletal effects of dietary calcium: Impact on the cardiovascular system, obesity, and cancer

Calcium is well known to be integral to bone and muscle health, with deleterious effects such as osteoporosis associated with inadequate calcium intake. Recent studies have also highlighted the significant effects of calcium in extra-musculoskeletal functioning, including the cardiovascular system, obesity, and cancer. Calcium impacts the cardiovascular system as an antagonist associated with a reduction in hypertension, increase vasodilation, and improvement in blood vessel function when obtained in the diet as an organic source, through food. However, the inorganic source of calcium, found in supplements, may be negatively associated with the cardiovascular system due to plaque deposits and atherogenesis when taken in excess. Some studies suggest that calcium intake may impact obesity by regulation of adipogenesis and reducing fat deposits with resulting weight loss. The pathogenesis of calcium for reducing obesity is thought to be related in part to its impact on gut microbiota profile, with the suggestion that calcium may have prebiotic properties. Animal and some human studies propose that calcium may also have a role in cancer prevention and/or treatment due to its function in the cell proliferation process and the impact on hormonal regulation, and thus warrants more investigations in the human population. Some prospective and small clinical studies suggest that calcium may be beneficial for colorectal cancer. Overall, emerging research in various areas continues to highlight the essentiality of dietary calcium for functioning at the molecular and biochemical level toward improvement in health and some chronic disease conditions.

Arrhythmogenic mechanisms of acute obstructive respiratory events in a porcine model of drug-induced long QT

BACKGROUND

Obstructive sleep apnea is associated with increased risk of sudden cardiac death.

OBJECTIVE

The purpose of this study was to elucidate changes in ventricular repolarization and electromechanical interaction during obstructive respiratory events simulated by intermittent negative upper airway pressure (INAP) in pigs. We also investigated the effect of a reduced repolarization reserve in drug-induced long QT (LQT) following INAP-induced changes in ventricular repolarization.

METHODS

In sedated spontaneously breathing pigs, 75 seconds of INAP was applied by a negative pressure device connected to the endotracheal tube. Ventricular electromechanical coupling was determined by the electromechanical window (EMW) before (pre-INAP), during (INAP), and after INAP (post-INAP). Incidence rates of premature ventricular contractions (PVCs) were measured respectively. A drug-induced LQT was modeled by treating the pigs with the hERG1 blocker dofetilide (DOF).

RESULTS

Whereas QT interval increased during and decreased after INAP (pre-INAP: 273 ± 5 ms; INAP 281 ± 6 ms; post-INAP 254 ± 9 ms), EMW shortened progressively throughout INAP and post-INAP periods (pre-INAP 81 ± 4 ms; post-INAP 44 ± 7 ms). DOF shortened EMW at baseline. Throughout INAP, EMW decreased in a comparable fashion as before DOF (pre-INAP/+DOF 61 ± 7 ms; post-INAP/+DOF 14 ± 9 ms) but resulted in shorter absolute EMW levels. Short EMW levels were associated with increased occurrence of PVCs (pre-INAP 7 ± 2 ms vs post-INAP 26 ± 6 ms; P = .02), which were potentiated in DOF pigs (pre-INAP/+DOF 5 ± 2 ms vs post-INAP/+DOF 40 ± 8 ms; P = .006). Administration of atenolol prevented post-INAP EMW shortening and decreased occurrence of PVCs.

CONCLUSION

Transient dissociation of ventricular electromechanical coupling during simulated obstructive respiratory events creates a dynamic ventricular arrhythmogenic substrate, which is sympathetically mediated and aggravated by drug-induced LQT.

An Overview of FGF-23 as a Novel Candidate Biomarker of Cardiovascular Risk

Fibroblast growth factor-23 (FGF)-23 is a phosphaturic hormone involved in mineral bone metabolism that helps control phosphate homeostasis and reduces 1,25-dihydroxyvitamin D synthesis. Recent data have highlighted the relevant direct FGF-23 effects on the myocardium, and high plasma levels of FGF-23 have been associated with adverse cardiovascular outcomes in humans, such as heart failure and arrhythmias. Therefore, FGF-23 has emerged as a novel biomarker of cardiovascular risk in the last decade. Indeed, experimental data suggest FGF-23 as a direct mediator of cardiac hypertrophy development, cardiac fibrosis and cardiac dysfunction via specific myocardial FGF receptor (FGFR) activation. Therefore, the FGF-23/FGFR pathway might be a suitable therapeutic target for reducing the deleterious effects of FGF-23 on the cardiovascular system. More research is needed to fully understand the intracellular FGF-23-dependent mechanisms, clarify the downstream pathways and identify which could be the most appropriate targets for better therapeutic intervention. This review updates the current knowledge on both clinical and experimental studies and highlights the evidence linking FGF-23 to cardiovascular events. The aim of this review is to establish the specific role of FGF-23 in the heart, its detrimental effects on cardiac tissue and the possible new therapeutic opportunities to block these effects.

Stress-driven cardiac calcium mishandling via a kinase-to-kinase crosstalk

Calcium homeostasis in the cardiomyocyte is critical to the regulation of normal cardiac function. Abnormal calcium dynamics such as altered uptake by the sarcoplasmic reticulum (SR) Ca²⁺-ATPase and increased diastolic SR calcium leak are involved in the development of maladaptive cardiac remodeling under pathological conditions. Ca²⁺/calmodulin-dependent protein kinase II-δ (CaMKIIδ) is a well-recognized key molecule in calcium dysregulation in cardiomyocytes. Elevated cellular stress is known as a common feature during pathological remodeling, and c-jun N-terminal kinase (JNK) is an important stress kinase that is activated in response to intrinsic and extrinsic stress stimuli. Our lab recently identified specific actions of JNK isoform 2 (JNK2) in CaMKIIδ expression, activation, and CaMKIIδ-dependent SR Ca²⁺ mishandling in the stressed heart. This review focuses on the current understanding of cardiac SR calcium handling under physiological and pathological conditions as well as the newly identified contribution of the stress kinase JNK2 in CaMKIIδ-dependent SR Ca²⁺ abnormal mishandling. The new findings identifying dual roles of JNK2 in CaMKIIδ expression and activation are also discussed in this review.

MICAL1 constrains cardiac stress responses and protects against disease by oxidizing CaMKII

Oxidant stress can contribute to health and disease. Here we show that invertebrates and vertebrates share a common stereospecific redox pathway that protects against pathological responses to stress, at the cost of reduced physiological performance, by constraining Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity. MICAL1, a methionine monooxygenase thought to exclusively target actin, and MSRB, a methionine reductase, control the stereospecific redox status of M308, a highly conserved residue in the calmodulin-binding (CaM-binding) domain of CaMKII. Oxidized or mutant M308 (M308V) decreased CaM binding and CaMKII activity, while absence of MICAL1 in mice caused cardiac arrhythmias and premature death due to CaMKII hyperactivation. Mimicking the effects of M308 oxidation decreased fight-or-flight responses in mice, strikingly impaired heart function in Drosophila melanogaster, and caused disease protection in human induced pluripotent stem cell-derived cardiomyocytes with catecholaminergic polymorphic ventricular tachycardia, a CaMKII-sensitive genetic arrhythmia syndrome. Our studies identify a stereospecific redox pathway that regulates cardiac physiological and pathological responses to stress across species.

Mitochondrial Ca2+, redox environment and ROS emission in heart failure: Two sides of the same coin?

Heart failure (HF) is a progressive, debilitating condition characterized, in part, by altered ionic equilibria, increased ROS production and impaired cellular energy metabolism, contributing to variable profiles of systolic and diastolic dysfunction with significant functional limitations and risk of premature death. We summarize current knowledge concerning changes of intracellular Na+ and Ca2+ control mechanisms during the disease progression and their consequences on mitochondrial Ca2+ homeostasis and the shift in redox balance. Absent existing biological data, our computational modeling studies advance a new 'in silico' analysis to reconcile existing opposing views, based on different experimental HF models, regarding variations in mitochondrial Ca2+ concentration that participate in triggering and perpetuating oxidative stress in the failing heart and their impact on cardiac energetics. In agreement with our hypothesis and the literature, model simulations demonstrate the possibility that the heart's redox status together with cytoplasmic Na+ concentrations act as regulators of mitochondrial Ca2+ levels in HF and of the bioenergetics response that will ultimately drive ATP supply and oxidative stress. The resulting model predictions propose future directions to study the evolution of HF as well as other types of heart disease, and to develop novel testable mechanistic hypotheses that may lead to improved therapeutics.

Altered calcium handling in cardiomyocytes from arginine-glycine amidinotransferase-knockout mice is rescued by creatine

The creatine kinase system facilitates energy transfer between mitochondria and the major ATPases in the heart. Creatine-deficient mice, which lack arginine-glycine amidinotransferase (AGAT) to synthesize creatine and homoarginine, exhibit reduced cardiac contractility. We studied how the absence of a functional CK system influences calcium handling in isolated cardiomyocytes from AGAT-knockouts and wild-type littermates as well as in AGAT-knockout mice receiving lifelong creatine supplementation via the food. Using a combination of whole cell patch clamp and fluorescence microscopy, we demonstrate that the L-type calcium channel (LTCC) current amplitude and voltage range of activation were significantly lower in AGAT-knockout compared with wild-type littermates. Additionally, the inactivation of LTCC and the calcium transient decay were significantly slower. According to our modeling results, these changes can be reproduced by reducing three parameters in knockout mice when compared with wild-type: LTCC conductance, the exchange constant of Ca2+ transfer between subspace and cytosol, and SERCA activity. Because tissue expression of LTCC and SERCA protein were not significantly different between genotypes, this suggests the involvement of posttranslational regulatory mechanisms or structural reorganization. The AGAT-knockout phenotype of calcium handling was fully reversed by dietary creatine supplementation throughout life. Our results indicate reduced calcium cycling in cardiomyocytes from AGAT-knockouts and suggest that the creatine kinase system is important for the development of calcium handling in the heart.NEW & NOTEWORTHY Creatine-deficient mice lacking arginine-glycine amidinotransferase exhibit compromised cardiac function. Here, we show that this is at least partially due to an overall slowing of calcium dynamics. Calcium influx into the cytosol via the L-type calcium current (LTCC) is diminished, and the rate of the sarcoendoplasmic reticulum calcium ATPase (SERCA) pumping calcium back into the sarcoplasmic reticulum is slower. The expression of LTCC and SERCA did not change, suggesting that the changes are regulatory.

Mitochondrial and Sarcoplasmic Reticulum Interconnection in Cardiac Arrhythmia

Ca²⁺ plays a pivotal role in mitochondrial energy production, contraction, and apoptosis. Mitochondrial Ca²⁺-targeted fluorescent probes have demonstrated that mitochondria Ca²⁺ transients are synchronized with Ca²⁺ fluxes occurring in the sarcoplasmic reticulum (SR). The presence of specialized proteins tethering SR to mitochondria ensures the local Ca²⁺ flux between these organelles. Furthermore, communication between SR and mitochondria impacts their functionality in a bidirectional manner. Mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniplex is essential for ATP production and controlled reactive oxygen species levels for proper cellular signaling. Conversely, mitochondrial ATP ensures the proper functioning of SR Ca²⁺-handling proteins, which ensures that mitochondria receive an adequate supply of Ca²⁺. Recent evidence suggests that altered SR Ca²⁺ proteins, such as ryanodine receptors and the sarco/endoplasmic reticulum Ca²⁺ ATPase pump, play an important role in maintaining proper cardiac membrane excitability, which may be initiated and potentiated when mitochondria are dysfunctional. This recognized mitochondrial role offers the opportunity to develop new therapeutic approaches aimed at preventing cardiac arrhythmias in cardiac disease.

Placental treatment improves cardiac tolerance to ischemia/reperfusion insult in adult male and female offspring exposed to prenatal hypoxia

Offspring born from complicated pregnancies are at greater risk of cardiovascular disease in adulthood. Prenatal hypoxia is a common pregnancy complication that results in placental oxidative stress and impairs fetal development. Adult offspring exposed to hypoxia during fetal life are more susceptible to develop cardiac dysfunction, and show decreased cardiac tolerance to an ischemia/reperfusion (I/R) insult. To improve offspring cardiac outcomes, we have assessed the use of a placenta-targeted intervention during hypoxic pregnancies, by encapsulating the mitochondrial antioxidant MitoQ into nanoparticles (nMitoQ). We hypothesized that maternal nMitoQ treatment during hypoxic pregnancies improves cardiac tolerance to I/R insult in adult male and female offspring. Pregnant Sprague-Dawley rats were exposed to normoxia (21 % O²) or hypoxia (11 % O²) from gestational day 15-20, after injection with 100 μL saline or nMitoQ (125 μM) on GD15 (n=6-8/group). Male and female offspring were aged to 4 months. Both male and female offspring from hypoxic pregnancies showed reduced cardiac tolerance to I/R (assessed ex vivo using the isolated working heart technique) which was ameliorated by nMitoQ treatment. To identify potential molecular mechanisms for the changes in cardiac tolerance to I/R, cardiac levels/phosphorylation of proteins important for intracellular Ca²⁺ cycling were assessed with Western blotting. In prenatally hypoxic male offspring, improved cardiac recovery from I/R by nMitoQ was accompanied by increased cardiac phospholamban and phosphatase 2Ce levels, and a trend to decreased Ca²⁺/calmodulin-dependent protein kinase IIδ phosphorylation. In contrast, in female offspring, nMitoQ treatment in hypoxic pregnancies increased phospholamban and protein kinase Cε phosphorylation. Maternal nMitoQ treatment improves cardiac tolerance to I/R insult in adult offspring and thus has the potential to improve the later-life trajectory of cardiovascular health of adult offspring born from pregnancies complicated by prenatal hypoxia.

The ForceLAB simulator: Application to the comparison of current models of cardiomyocyte contraction

Mathematical models are useful tools in the study of physiological phenomena. However, due to differences in assumptions and formulations, discrepancy in simulations may occur. Among the models for cardiomyocyte contraction based on Huxley's cross-bridge cycling, those proposed by Negroni and Lascano (NL) and Rice et al. (RWH) are the most frequently used. This study was aimed at developing a computational tool, ForceLAB, which allows implementing different contraction models and modifying several functional parameters. As an application, electrically-stimulated twitches triggered by an equal Ca²⁺ input and steady-state force x pCa relationship (pCa = -log of the molar free Ca²⁺ concentration) simulated with the NL and RWH models were compared. The equilibrium Ca²⁺-troponin C (TnC) dissociation constant (K_d) was modified by changing either the association (k_on) or the dissociation (k_off) rate constant. With the NL model, raising K_d by either maneuver decreased monotonically twitch amplitude and duration, as expected. With the RWH model, in contrast, the same K_d variation caused increase or decrease of peak force depending on which rate constant was modified. Additionally, force x pCa curves simulated using Ca²⁺ binding constants estimated in cardiomyocytes bearing wild-type and mutated TnC were compared to curves previously determined in permeabilized fibers. Mutations increased k_on and k_off, and decreased K_d. Both models produced curves fairly comparable to the experimental ones, although sensitivity to Ca²⁺ was greater, especially with RWH model. The NL model reproduced slightly better the qualitative changes associated with the mutations. It is expected that this tool can be useful for teaching and investigation.

Comprehensive Analysis of Cardiac Xeno-Graft Unveils Rejection Mechanisms

Porcine heart xenotransplantation is a potential treatment for patients with end-stage heart failure. To understand molecular mechanisms of graft rejection after heart transplantation, we transplanted a 31-day-old alpha-1,3-galactosyltransferase knockout (GTKO) porcine heart to a five-year-old cynomolgus monkey. Histological and transcriptome analyses were conducted on xenografted cardiac tissue at rejection (nine days after transplantation). The recipient monkey's blood parameters were analyzed on days -7, -3, 1, 4, and 7. Validation was conducted by quantitative real-time PCR (qPCR) with selected genes. A non-transplanted GTKO porcine heart from an age-matched litter was used as a control. The recipient monkey showed systemic inflammatory responses, and the rejected cardiac graft indicated myocardial infarction and cardiac fibrosis. The transplanted heart exhibited a total of 3748 differentially expressed genes compared to the non-transplanted heart transcriptome, with 2443 upregulated and 1305 downregulated genes. Key biological pathways involved at the terminal stage of graft rejection were cardiomyopathies, extracellular interactions, and ion channel activities. The results of qPCR evaluation were in agreement with the transcriptome data. Transcriptome analysis of porcine cardiac tissue at graft rejection reveals dysregulation of the key molecules and signaling pathways, which play relevant roles on structural and functional integrities of the heart.

The role of calcium homeostasis remodeling in inherited cardiac arrhythmia syndromes

Sudden cardiac death due to malignant ventricular arrhythmias remains the major cause of mortality in the postindustrial world. Defective intracellular Ca²⁺ homeostasis has been well established as a key contributing factor to the enhanced propensity for arrhythmia in acquired cardiac disease, such as heart failure or diabetic cardiomyopathy. More recent advances provide a strong basis to the emerging view that hereditary cardiac arrhythmia syndromes are accompanied by maladaptive remodeling of Ca²⁺ homeostasis which substantially increases arrhythmic risk. This brief review will focus on functional changes in elements of Ca²⁺ handling machinery in cardiomyocytes that occur secondary to genetic mutations associated with catecholaminergic polymorphic ventricular tachycardia, and long QT syndrome.

Continuous Aerobic Exercise Prevents Detrimental Remodeling and Right Heart Myocyte Contraction and Calcium Cycling Dysfunction in Pulmonary Artery Hypertension

ABSTRACT

Pulmonary artery hypertension (PAH) imposes right heart and lung detrimental remodeling which impairs cardiac contractility, physical effort tolerance, and survival. The effects of an early moderate-intensity continuous aerobic exercise training on the right ventricle and lung structure, and on contractility and the calcium (Ca2+) transient in isolated myocytes from rats with severe PAH induced by monocrotaline were analyzed. Rats were divided into control sedentary (CS), control exercise (CE), monocrotaline sedentary (MS), and monocrotaline exercise (ME) groups. Animals from control exercise and ME groups underwent a moderate-intensity aerobic exercise on a treadmill (60 min/d; 60% intensity) for 32 days, after a monocrotaline (60 mg/kg body weight i.p.) or saline injection. The pulmonary artery resistance was higher in MS than in control sedentary (1.36-fold) and was reduced by 39.39% in ME compared with MS. Compared with MS, the ME group presented reduced alveolus (17%) and blood vessel (46%) wall, fibrosis (25.37%) and type I collagen content (55.78%), and increased alveolus (52.96%) and blood vessel (146.97%) lumen. In the right ventricle, the ME group exhibited diminished hypertrophy index (25.53%) and type I collagen content (40.42%) and improved myocyte contraction [ie, reduced times to peak (29.27%) and to 50% relax (13.79%)] and intracellular Ca2+ transient [ie, decreased times to peak (16.06%) and to 50% decay (7.41%)] compared with MS. Thus, early moderate-intensity continuous aerobic exercise prevents detrimental remodeling in the right heart and lung increases in the pulmonary artery resistance and dysfunction in single myocyte contraction and Ca2+ cycling in this model.

Physiological and Pathological Relevance of Selective and Nonselective Ca2+ Channels in Skeletal and Cardiac Muscle

Contraction of the striated muscle is fundamental for human existence. The action of voluntary skeletal muscle enables activities such as breathing, establishing body posture, and diverse body movements. Additionally, highly precise motion empowers communication, artistic expression, and other activities that define everyday human life. The involuntary contraction of striated muscle is the core function of the heart and is essential for blood flow. Several ion channels are important in the transduction of action potentials to cytosolic Ca2+ signals that enable muscle contraction; however, other ion channels are involved in the progression of muscle pathologies that can impair normal life or threaten it. This chapter describes types of selective and nonselective Ca2+ permeable ion channels expressed in the striated muscle, their participation in different aspects of muscle excitation and contraction, and their relevance to the progression of some pathological states.

Non-canonical Molecular Targets for Novel Analgesics: Intracellular Calcium and HCN Channels

Pain is a prevalent biopsychosocial condition that poses a significant challenge to healthcare providers, contributes substantially to a disability, and is a major economic burden worldwide. An overreliance on opioid analgesics, which primarily target the μ-opioid receptor, has caused devastating morbidity and mortality in the form of misuse and overdose-related death. Thus, novel analgesic medications are needed that can effectively treat pain and provide an alternative to opioids. A variety of cellular ion channels contribute to nociception, the response of the sensory nervous system to a noxious stimulus that commonly leads to pain. Ion channels involved in nociception may provide a suitable target for pharmacologic modulation to achieve pain relief. This narrative review summarizes the evidence for two ion channels that merit consideration as targets for non-opioid pain medications: ryanodine receptors (RyRs), which are intracellular calcium channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which belong to the superfamily of voltage-gated K+ channels. The role of these channels in nociception and neuropathic pain is discussed and suitability as targets for novel analgesics and antihyperalgesics is considered.

Calcium mediated functional interplay between myocardial cells upon laser-induced single-cell injury: an in vitro study of cardiac cell death signaling mechanisms

Background

The electromechanical function of myocardial tissue depends on the intercellular communication between cardiomyocytes (CMs) as well as their crosstalk with other cell types. Cell injury, and subsequent death trigger inflammation as in myocardial infarction (MI) resulting in myocardial remodeling. Although mechanisms underlying myocardial cell death have been studied so far, the signaling events following single cell death and spontaneous response of connected cells in the myocardial tissue is still barely understood.

Methods

Here, we investigated the effect of laser-induced single cell death on Calcium (Ca²⁺) concentrations and transport in myocardial cell clusters in vitro. Spatial and temporal changes in intracellular Ca²⁺ concentrations [Ca²⁺]_i were studied using a fluorescent calcium indicator, Fluo-4AM. Spontaneous signaling events following cell death were studied in rat embryonic cardiomyocytes and non-myocytes using separate cell culture systems.

Results

Cell death triggered spontaneous increase in intracellular Ca²⁺ levels ([Ca²⁺]_i) of surrounding cells. The spread of the observed propagating Ca²⁺ signal was slow and sustained in myocytes while it was rapid and transient in fibroblasts (Fbs). Further, sustained high Ca²⁺ levels temporarily impaired the contractility in CMs. The cell-type specific effect of ablation was confirmed using separate cultures of CMs and Fbs. Comparing Ca²⁺ propagation speed in myocytes and fibroblasts, we argue for a diffusion-driven Ca²⁺ propagation in myocytes, but not in fibroblasts. Radial and sequential Ca²⁺ diffusion across the CMs through cell–cell contacts and presence of Cx43-based intercellular junctions indicated a gap junction flow of Ca²⁺.

Conclusions

These findings illustrate the spontaneous Ca²⁺-mediated functional interplay in myocardial cell clusters upon mechanical injury and, further, the difference in Ca²⁺ signaling in cardiomyocytes and fibroblasts.

Finely-Tuned Calcium Oscillations in Osteoclast Differentiation and Bone Resorption

Calcium (Ca²⁺) plays an important role in regulating the differentiation and function of osteoclasts. Calcium oscillations (Ca oscillations) are well-known phenomena in receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclastogenesis and bone resorption via calcineurin. Many modifiers are involved in the fine-tuning of Ca oscillations in osteoclasts. In addition to macrophage colony-stimulating factors (M-CSF; CSF-1) and RANKL, costimulatory signaling by immunoreceptor tyrosine-based activation motif-harboring adaptors is important for Ca oscillation generation and osteoclast differentiation. DNAX-activating protein of 12 kD is always necessary for osteoclastogenesis. In contrast, Fc receptor gamma (FcRγ) works as a key controller of osteoclastogenesis especially in inflammatory situation. FcRγ has a cofactor in fine-tuning of Ca oscillations. Some calcium channels and transporters are also necessary for Ca oscillations. Transient receptor potential (TRP) channels are well-known environmental sensors, and TRP vanilloid channels play an important role in osteoclastogenesis. Lysosomes, mitochondria, and endoplasmic reticulum (ER) are typical organelles for intracellular Ca²⁺ storage. Ryanodine receptor, inositol trisphosphate receptor, and sarco/endoplasmic reticulum Ca²⁺ ATPase on the ER modulate Ca oscillations. Research on Ca oscillations in osteoclasts has still many problems. Surprisingly, there is no objective definition of Ca oscillations. Causality between Ca oscillations and osteoclast differentiation and/or function remains to be examined.

Cardiac Complications of Propionic and Other Inherited Organic Acidemias

Clinical observations and experimental studies have determined that systemic acid-base disturbances can profoundly affect the heart. A wealth of information is available on the effects of altered pH on cardiac function but, by comparison, much less is known about the actions of the organic anions that accumulate alongside H⁺ ions in acidosis. In the blood and other body fluids, these organic chemical species can collectively reach concentrations of several millimolar in severe metabolic acidoses, as in the case of inherited organic acidemias, and exert powerful biological actions on the heart that are not intuitive to predict. Indeed, cardiac pathologies, such as cardiomyopathy and arrhythmia, are frequently reported in organic acidemia patients, but the underlying pathophysiological mechanisms are not well established. Research efforts in the area of organic anion physiology have increased dramatically in recent years, particularly for propionate, which accumulates in propionic acidemia, one of the commonest organic acidemias characterized by a high incidence of cardiac disease. This Review provides a comprehensive historical overview of all known organic acidemias that feature cardiac complications and a state-of-the-art overview of the cardiac sequelae observed in propionic acidemia. The article identifies the most promising candidates for molecular mechanisms that become aberrantly engaged by propionate anions (and its metabolites), and discusses how these may result in cardiac derangements in propionic acidemia. Key clinical and experimental findings are considered in the context of potential therapies in the near future.

Nitric Oxide and Mechano-Electrical Transduction in Cardiomyocytes

The ability§ of the heart to adapt to changes in the mechanical environment is critical for normal cardiac physiology. The role of nitric oxide is increasingly recognized as a mediator of mechanical signaling. Produced in the heart by nitric oxide synthases, nitric oxide affects almost all mechano-transduction pathways within the cardiomyocyte, with roles mediating mechano-sensing, mechano-electric feedback (via modulation of ion channel activity), and calcium handling. As more precise experimental techniques for applying mechanical stresses to cells are developed, the role of these forces in cardiomyocyte function can be further understood. Furthermore, specific inhibitors of different nitric oxide synthase isoforms are now available to elucidate the role of these enzymes in mediating mechano-electrical signaling. Understanding of the links between nitric oxide production and mechano-electrical signaling is incomplete, particularly whether mechanically sensitive ion channels are regulated by nitric oxide, and how this affects the cardiac action potential. This is of particular relevance to conditions such as atrial fibrillation and heart failure, in which nitric oxide production is reduced. Dysfunction of the nitric oxide/mechano-electrical signaling pathways are likely to be a feature of cardiac pathology (e.g., atrial fibrillation, cardiomyopathy, and heart failure) and a better understanding of the importance of nitric oxide signaling and its links to mechanical regulation of heart function may advance our understanding of these conditions.