Sarcoplasmic reticulum calcium mishandling: central tenet in heart failure?

Pathological mutations in the phospholamban cytoplasmic region affect its topology and dynamics modulating the extent of SERCA inhibition

Phospholamban (PLN) is a 52 amino acid regulin that allosterically modulates the activity of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in the heart muscle. In its unphosphorylated form, PLN binds SERCA within its transmembrane (TM) domains, approximately 20 Å away from the Ca2+ binding site, reducing SERCA's apparent Ca2+ affinity (pKCa) and decreasing cardiac contractility. During the enzymatic cycle, the inhibitory TM domain of PLN remains anchored to SERCA, whereas its cytoplasmic region transiently binds the ATPase's headpiece. Phosphorylation of PLN at Ser16 by protein kinase A increases the affinity of its cytoplasmic domain to SERCA, weakening the TM interactions with the ATPase, reversing its inhibitory function, and augmenting muscle contractility. How the structural changes caused by pathological mutations in the PLN cytoplasmic region are transmitted to its inhibitory TM domain is still unclear. Using solid-state NMR spectroscopy and activity assays, we analyzed the structural and functional effects of a series of mutations and their phosphorylated forms located in the PLN cytoplasmic region and linked to dilated cardiomyopathy. We found that these missense mutations affect the overall topology and dynamics of PLN and ultimately modulate its inhibitory potency. Also, the changes in the TM tilt angle and cytoplasmic dynamics of PLN caused by these mutations correlate well with the extent of SERCA inhibition. Our study unveils new molecular determinants for designing variants of PLN that outcompete endogenous PLN to regulate SERCA in a tunable manner.

Loss-of-function W4645R mutation in the RyR2-caffeine binding site: implications for synchrony and arrhythmogenesis

AIMS

Previous studies have identified RyR2 W4645R mutation, located in the caffeine-binding site, to associate with CPVT1 pathology. Caffeine binding to its site is thought to displace the carboxyl-terminal domain to Ca2+-binding, allowing the tryptophan residue (W4645) to regulate Ca2+ sensitivity of RyR2. To gain insights into regulation of RyR2 Ca2+-binding and its interaction with caffeine-binding site, we introduced W4645R-RyR2 point mutation via CRISPR/Cas9 gene-editing in human induced pluripotent stem cell-derived cardiomyocytes (hiPSCCMs) and characterized their Ca2+-signaling phenotype compared to WT hiPSCCMs.

METHODS AND RESULTS

W4645R-RyR2 cardiomyocytes had: (1) no significant change in ICa magnitude or voltage-dependence; (2) slightly reduced CICR; (3) altered relaxation kinetics of Ca2+-transients with no change in isoproterenol sensitivity; (4) complete loss of caffeine-triggered Ca2+ release; (5) larger SR Ca2+ leak resulting in 40 % lower SR Ca2+ content, as determined by myocytes' response to 4-CmC; (6) lower incidence of calcium sparks and asynchronous spontaneous SR Ca2+ releases.

CONCLUSIONS

W4645R-RyR2 mutation induces loss of caffeine-triggered SR Ca2+ release and enhances SR Ca2+ leak that underlie asynchronous spontaneous Ca2+ releases, triggering arrhythmia and impairing cardiac function.

Ion channel trafficking implications in heart failure

Heart failure (HF) is recognized as an epidemic in the contemporary world, impacting around 1%-2% of the adult population and affecting around 6 million Americans. HF remains a major cause of mortality, morbidity, and poor quality of life. Several therapies are used to treat HF and improve the survival of patients; however, despite these substantial improvements in treating HF, the incidence of HF is increasing rapidly, posing a significant burden to human health. The total cost of care for HF is USD 69.8 billion in 2023, warranting a better understanding of the mechanisms involved in HF. Among the most serious manifestations associated with HF is arrhythmia due to the electrophysiological changes within the cardiomyocyte. Among these electrophysiological changes, disruptions in sodium and potassium currents' function and trafficking, as well as calcium handling, all of which impact arrhythmia in HF. The mechanisms responsible for the trafficking, anchoring, organization, and recycling of ion channels at the plasma membrane seem to be significant contributors to ion channels dysfunction in HF. Variants, microtubule alterations, or disturbances of anchoring proteins lead to ion channel trafficking defects and the alteration of the cardiomyocyte's electrophysiology. Understanding the mechanisms of ion channels trafficking could provide new therapeutic approaches for the treatment of HF. This review provides an overview of the recent advances in ion channel trafficking in HF.

Insights into Organochlorine Pesticides Exposure in the Development of Cardiovascular Diseases: A Systematic Review

Many human diseases such as cancer, neurological diseases, autism and diabetes are associated with exposure to pesticides, especially organochlorine pesticides. However, pesticide exposure is also associated with cardiovascular disease (CVD) as the leading cause of death worldwide. In this systematic review, results on the link between organochlorine pesticide pollution and CVD were collected from databases (Medline (PubMed), Scopus and Science Direct) in May 2022 from studies published between 2010 and 2022. A total of 24 articles were selected for this systematic review. Sixteen articles were extracted by reviewers using a standardized form that included cross-sectional, cohort, and ecological studies that reported exposure to organochlorine pesticides in association with increased CVD risk. In addition, eight articles covering molecular mechanisms organochlorine pesticides and polychlorinated biphenyls (PCBs) on cardiovascular effects were retrieved for detailed evaluation. Based on the findings of the study, it seems elevated circulating levels of organochlorine pesticides and PCBs increase the risk of coronary heart disease, especially in early life exposure to these pesticides and especially in men. Changes in the regulatory function of peroxisome proliferator-activated γ receptor (PPARγ), reduction of paroxonase activity (PON1), epigenetic changes of histone through induction of reactive oxygen species, vascular endothelial inflammation with miR-expression 126 and miR-31, increased collagen synthesis enzymes in the extracellular matrix and left ventricular hypertrophy (LVH) and fibrosis are mechanisms by which PCBs increase the risk of CVD. According to this systematic review, organochlorine pesticide exposure is associated with increased risk of CVD and CVD mortality through the atherogenic and inflammatory molecular mechanism involving fatty acid and glucose metabolism.

Long-term changes in heart rate and electrical remodeling contribute to alternans formation in heart failure: a patient-specific in silico study

Individuals with chronic heart failure (CHF) have an increased risk of ventricular arrhythmias, which has been linked to pathological cellular remodeling and may also be mediated by changes in heart rate. Heart rate typically fluctuates on a timescale ranging from seconds to hours, termed heart rate variability (HRV). This variability is reduced in CHF, and this HRV reduction is associated with a greater risk for arrhythmias. Furthermore, variations in heart rate influence the formation of proarrhythmic alternans, a beat-to-beat alternation in the action potential duration (APD), or intracellular calcium (Ca). In this study, we investigate how long-term changes in heart rate and electrical remodeling associated with CHF influence alternans formation. We measure key statistical properties of the RR-interval sequences from ECGs of individuals with normal sinus rhythm (NSR) and CHF. Patient-specific RR-interval sequences and synthetic sequences (randomly generated to mimicking these statistical properties) are used as the pacing protocol for a discrete time-coupled map model that governs APD and intracellular Ca handling of a single cardiac myocyte, modified to account for pathological electrical remodeling in CHF. Patient-specific simulations show that beat-to-beat differences in APD vary temporally in both populations, with alternans formation more prevalent in CHF. Parameter studies using synthetic sequences demonstrate that increasing the autocorrelation time or mean RR-interval reduces APD alternations, whereas increasing the RR-interval standard deviation leads to higher alternans magnitudes. Importantly, we find that although both the CHF-associated changes in heart rate and electrical remodeling influence alternans formation, variations in heart rate may be more influential.NEW & NOTEWORTHY Using patient-specific data, we show that both the changes in heart rate and electrical remodeling associated with chronic heart failure influence the formation of proarrhythmic alternans in the heart.

Cardiac calcium regulation in human induced pluripotent stem cell cardiomyocytes: Implications for disease modeling and maturation

Human induced pluripotent stem cell cardiomyocytes (hiPSC-CMs) are based on ground-breaking technology that has significantly impacted cardiovascular research. They provide a renewable source of human cardiomyocytes for a variety of applications including in vitro disease modeling and drug toxicity testing. Cardiac calcium regulation plays a critical role in the cardiomyocyte and is often dysregulated in cardiovascular disease. Due to the limited availability of human cardiac tissue, calcium handling and its regulation have most commonly been studied in the context of animal models. hiPSC-CMs can provide unique insights into human physiology and pathophysiology, although a remaining limitation is the relative immaturity of these cells compared to adult cardiomyocytes Therefore, this field is rapidly developing techniques to improve the maturity of hiPSC-CMs, further establishing their place in cardiovascular research. This review briefly covers the basics of cardiomyocyte calcium cycling and hiPSC technology, and will provide a detailed description of our current understanding of calcium in hiPSC-CMs.

Long non-coding RNAs in the pathogenesis of heart failure: A literature review

Heart failure (HF) is a common cardiovascular disorder and a major cause of mortality and morbidity in older people. The mechanisms underlying HF are still not fully understood, restricting novel therapeutic target discovery and drug development. Besides, few drugs have been shown to improve the survival of HF patients. Increasing evidence suggests that long non-coding RNAs (lncRNAs) serve as a critical regulator of cardiac physiological and pathological processes, regarded as a new target of treatment for HF. lncRNAs are versatile players in the pathogenesis of HF. They can interact with chromatin, protein, RNA, or DNA, thereby modulating chromatin accessibility, gene expressions, and signaling transduction. In this review, we summarized the current knowledge on how lncRNAs involve in HF and categorized them into four aspects based on their biological functions, namely, cardiomyocyte contractility, cardiac hypertrophy, cardiac apoptosis, and myocardial fibrosis. Along with the extensive laboratory data, RNA-based therapeutics achieved great advances in recent years. These indicate that targeting lncRNAs in the treatment of HF may provide new strategies and address the unmet clinical needs.

Study on diverse pathological characteristics of heart failure in different stages based on proteomics

Heart failure is a process characterized by significant disturbance of protein turnover. To elucidate the alterations in cardiac protein expression during the various phases of heart failure and to understand the nature of the processes involved, we analysed the proteome in an established heart failure model at different time points to monitor thousands of different proteins simultaneously. Here, heart failure was induced by transverse aortic constriction (TAC) in KM mice. At 2, 4 and 12 weeks after operation, protein expression profiles were determined in sham‐operated (controls) and TAC mice, using label‐free quantitative proteomics, leading to identification and quantification of almost 4000 proteins. The results of the KEGG pathway enrichment analysis and GO function annotation revealed critical pathways associated with the transition from cardiac hypertrophy to heart failure, such as energy pathways and matrix reorganization. Our study suggests that in the pathophysiology of heart failure, alterations of protein groups related to cardiac energy substrate metabolism and cytoskeleton remodelling could play the more dominant roles for the signalling that eventually results in contractile dysfunction and heart failure.

Mechanisms for the development of heart failure and improvement of cardiac function by angiotensin-converting enzyme inhibitors

Angiotensin-converting enzyme (ACE) inhibitors, which prevent the conversion of angiotensin I to angiotensin II, are well-known for the treatments of cardiovascular diseases, such as heart failure, hypertension and acute coronary syndrome. Several of these inhibitors including captopril, enalapril, ramipril, zofenopril and imidapril attenuate vasoconstriction, cardiac hypertrophy and adverse cardiac remodeling, improve clinical outcomes in patients with cardiac dysfunction and decrease mortality. Extensive experimental and clinical research over the past 35 years has revealed that the beneficial effects of ACE inhibitors in heart failure are associated with full or partial prevention of adverse cardiac remodeling. Since cardiac function is mainly determined by coordinated activities of different subcellular organelles, including sarcolemma, sarcoplasmic reticulum, mitochondria and myofibrils, for regulating the intracellular concentration of Ca2+ and myocardial metabolism, there is ample evidence to suggest that adverse cardiac remodelling and cardiac dysfunction in the failing heart are the consequence of subcellular defects. In fact, the improvement of cardiac function by different ACE inhibitors has been demonstrated to be related to the attenuation of abnormalities in subcellular organelles for Ca2+-handling, metabolic alterations, signal transduction defects and gene expression changes in failing cardiomyocytes. Various ACE inhibitors have also been shown to delay the progression of heart failure by reducing the formation of angiotensin II, the development of oxidative stress, the level of inflammatory cytokines and the occurrence of subcellular defects. These observations support the view that ACE inhibitors improve cardiac function in the failing heart by multiple mechanisms including the reduction of oxidative stress, myocardial inflammation and Ca2+-handling abnormalities in cardiomyocytes.

The link between abnormalities of calcium handling proteins and catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT), a rare autosomal dominant or recessive disease, usually results in syncope or sudden cardiac death. Most CPVT patients do not show abnormal cardiac structure and electrocardiogram features and symptoms, usually onset during adrenergically mediated physiological conditions. CPVT tends to occur at a younger age and is not easy to be diagnosed and managed. The main cause of CPVT is associated with mishandling Ca²⁺ in cardiomyocytes. Intracellular Ca²⁺ is strictly controlled by a protein located in the sarcoplasm reticulum (SR), such as ryanodine receptor, histidine-rich Ca²⁺-binding protein, triadin, and junctin. Mutation in these proteins results in misfolding or malfunction of these proteins, thereby affecting their Ca²⁺-binding affinity, and subsequently disturbs Ca²⁺ homeostasis during excitation–contraction coupling (E-C coupling). Furthermore, transient disturbance of Ca²⁺ homeostasis increases membrane potential and causes Ca²⁺ store overload-induced Ca²⁺ release, which in turn leads to delayed after depolarization and arrhythmia. Previous studies have focused on the interaction between ryanodine receptors and protein kinase or phosphatase in the cytosol. However, recent studies showed the regulation signaling for ryanodine receptor not only from the cytosol but also within the SR. The changing of Ca²⁺ concentration is critical for protein interaction inside the SR which changes protein conformation to regulate the open probability of ryanodine receptors. Thus, it influences the threshold of Ca²⁺ released from the SR, making it easier to release Ca²⁺ during E-C coupling. In this review, we briefly discuss how Ca²⁺ handling protein variations affect the Ca²⁺ handling in CPVT.

Sympatho-adrenergic mechanisms in heart failure: new insights into pathophysiology

The sympathetic nervous system is activated in the setting of heart failure (HF) to compensate for hemodynamic instability. However, acute sympathetic surge or sustained high neuronal firing rates activates β-adrenergic receptor (βAR) signaling contributing to myocardial remodeling, dysfunction and electrical instability. Thus, sympatho-βAR activation is regarded as a hallmark of HF and forms pathophysiological basis for β-blocking therapy. Building upon earlier research findings, studies conducted in the recent decades have significantly advanced our understanding on the sympatho-adrenergic mechanism in HF, which forms the focus of this article. This review notes recent research progress regarding the roles of cardiac β2AR or α1AR in the failing heart, significance of β1AR-autoantibodies, and βAR signaling through G-protein independent signaling pathways. Sympatho-βAR regulation of immune cells or fibroblasts is specifically discussed. On the neuronal aspects, knowledge is assembled on the remodeling of sympathetic nerves of the failing heart, regulation by presynaptic α2AR of NE release, and findings on device-based neuromodulation of the sympathetic nervous system. The review ends with highlighting areas where significant knowledge gaps exist but hold promise for new breakthroughs.

Effects of omecamtiv mecarbil on failing human ventricular trabeculae and interaction with (-)-noradrenaline

Omecamtiv mecarbil (OM) is a novel medicine for systolic heart failure, targeting myosin to enhance cardiomyocyte performance. To assist translation to clinical practice we investigated OMs effect on explanted human failing hearts, specifically; contractile dynamics, interaction with the β₁–adrenoceptor (AR) agonist (−)‐noradrenaline and spontaneous contractions. Left and right ventricular trabeculae from 13 explanted failing hearts, and trabeculae from 58 right atrial appendages of non‐failing hearts, were incubated with or without a single concentration of OM for 120 min. Time to peak force (TPF) and 50% relaxation (t_50%) were recorded. In other experiments, trabeculae were observed for spontaneous contractions and cumulative concentration‐effect curves were established to (−)‐noradrenaline at β₁‐ARs in the absence or presence of OM. OM prolonged TPF and t_50% in ventricular trabeculae (600 nM, 2 µM, p < .001). OM had no significant inotropic effect but reduced time dependent deterioration in contractile strength compared to control (p < .001). OM did not affect the generation of spontaneous contractions. The potency of (−)‐noradrenaline (pEC₅₀ 6.05 ± 0.10), for inotropic effect, was unchanged in the presence of OM 600 nM or 2 µM. Co‐incubation with (−)‐noradrenaline reduced TPF and t_50%, reversing the negative diastolic effects of OM. OM, at both 600 nM and 2 µM, preserved contractile force in left ventricular trabeculae, but imparted negative diastolic effects in trabeculae from human failing heart. (−)‐Noradrenaline reversed the negative diastolic effects, co‐administration may limit the titration of inotropes by reducing the threshold for ischemic side effects.

Ryanodine Receptor Type 2: A Molecular Target for Dichlorodiphenyltrichloroethane- and Dichlorodiphenyldichloroethylene-Mediated Cardiotoxicity

Dichlorodiphenyltrichloroethane (DDT) and its metabolite dichlorodiphenyl-dichloroethylene (DDE) are ubiquitously found in the environment and linked to cardiovascular diseases-with a majority of the work focused on hypertension. Studies investigating whether DDx can interact with molecular targets on cardiac tissue to directly affect cardiac function are lacking. Therefore, we investigated whether o,p'-DDT, p,p'-DDT, o,p'-DDE, or p,p'-DDE (DDx, collectively) can directly alter the function of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) by assessing their effect(s) on hiPSC-CMs Ca2+ dynamics. DDx (0.1-10 µM) affected hiPSC-CMs synchronous Ca2+ oscillation frequency in a concentration-dependent manner, with p,p'-DDT and p,p'-DDE also decreasing Ca2+ stores. HEK-RyR2 cells cultured under antibiotic selection to induce expression of wild-type mouse ryanodine receptor type 2 (RyR2) are used to further investigate whether DDx alters hiPSC-CMs Ca2+ dynamics through engagement with RyR2, a protein critical for cardiac muscle excitation-contraction coupling (ECC). Acute treatment with 10 µM DDx failed to induce Ca2+ release in HEK293-RyR2, whereas pretreatment with DDx (0.1-10 µM) for 12- or 24-h significantly decreased sarcoplasmic reticulum Ca2+ stores in HEK-RyR2 cells challenged with caffeine (1 mM), an RyR agonist. [3H]ryanodine-binding analysis using murine cardiac RyR2 homogenates further confirmed that all DDx isomers (10 µM) can directly engage with RyR2 to favor an open (leaky) confirmation, whereas only the DDT isomers (10 µM) modestly (≤10%) inhibited SERCA2a activity. The data demonstrate that DDx increases heart rate and depletes Ca2+ stores in human cardiomyocytes through a mechanism that impairs RyR2 function and Ca2+ dynamics.

IMPACT STATEMENT

DDT/DDE interactions with RyR2 alter cardiomyocyte Ca2+ dynamics that may contribute to adverse cardiovascular outcomes associated with exposures.

Biophysics of human anatomy and physiology-a Special Issue in honor of Prof. Cristobal dos Remedios on the occasion of his 80th birthday

In 2001, Cristobal dos Remedios was made Professor of Anatomy (now emeritus) within Australia’s highest-ranked university (University of Sydney). For the majority of his career, he has examined the biomechanics and biophysics of human muscle contraction. To coincide with the occasion of his 80^th birthday, this Special Issue has commissioned a collection of review articles from experts exploring biophysical subjects within the general areas of human anatomy and physiology. After introducing the scope and contents of the Issue, we provide a short scientific biography, placing his scientific achievements within the context of the course of his life’s developments.