Cardiac-specific overexpression of phospholamban alters calcium kinetics and resultant cardiomyocyte mechanics in transgenic mice

ZBTB20 Regulates SERCA2a Activity and Myocardial Contractility Through Phospholamban

BACKGROUND

Intracellular Ca2+ cycling determines myocardial contraction and relaxation in response to physiological demands. SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a) is responsible for the sequestration of cytosolic Ca2+ into intracellular stores during cardiac relaxation, and its activity is reversibly inhibited by PLN (phospholamban). However, the regulatory hierarchy of SERCA2a activity remains unclear.

METHODS

Cardiomyocyte-specific ZBTB20 knockout mice were generated by crossing ZBTB20flox mice with Myh6-Cre mice. Echocardiography, blood pressure measurements, Langendorff perfusion, histological analysis and immunohistochemistry, quantitative reverse transcription-PCR, Western blot analysis, electrophysiological measurements, and chromatin immunoprecipitation assay were performed to clarify the phenotype and elucidate the molecular mechanisms.

RESULTS

Specific ablation of ZBTB20 in cardiomyocyte led to a significant increase in basal myocardial contractile parameters both in vivo and in vitro, accompanied by an impairment in cardiac reserve and exercise capacity. Moreover, the cardiomyocytes lacking ZBTB20 showed an increase in sarcoplasmic reticular Ca2+ content and exhibited a remarkable enhancement in both SERCA2a activity and electrically stimulated contraction. Mechanistically, PLN expression was dramatically reduced in cardiomyocytes at the mRNA and protein levels by ZBTB20 deletion or silencing, and PLN overexpression could largely restore the basal contractility in ZBTB20-deficient cardiomyocytes.

CONCLUSIONS

These data point to ZBTB20 as a fine-tuning modulator of PLN expression and SERCA2a activity, thereby offering new perspective on the regulation of basal contractility in the mammalian heart.

Gene therapy for heart failure: A novel treatment for the age old disease

Across the globe, cardiovascular disease (CVD) is the leading cause of mortality. According to reports, around 6.2 million people in the United states have heart failure. Current standards of care for heart failure can delay but not prevent progression of disease. Gene therapy is one of the novel treatment modalities that promises to fill this limitation in the current standard of care for Heart Failure. In this paper we performed an extensive search of the literature on various advances made in gene therapy for heart failure till date. We review the delivery methods, targets, current applications, trials, limitations and feasibility of gene therapy for heart failure. Various methods have been employed till date for administering gene therapies including but not limited to arterial and venous infusion, direct myocardial injection and pericardial injection. Various strategies such as AC6 expression, S100A1 protein upregulation, VEGF-B and SDF-1 gene therapy have shown promise in recent preclinical trials. Furthermore, few studies even show that stimulation of cardiomyocyte proliferation such as through cyclin A2 overexpression is a realistic avenue. However, a considerable number of obstacles need to be overcome for gene therapy to be part of standard treatment of care such as definitive choice of gene, gene delivery systems and a suitable method for preclinical trials and clinical trials on patients. Considering the challenges and taking into account the recent advances in gene therapy research, there are encouraging signs to indicate gene therapy for heart failure to be a promising treatment modality for the future. However, the time and feasibility of this option remains in a situation of balance.

Arrhythmogenic Cardiomyopathy: from Preclinical Models to Genotype-phenotype Correlation and Pathophysiology

Arrhythmogenic cardiomyopathy (ACM) is a hereditary myocardial disease characterized by the replacement of the ventricular myocardium with fibrous fatty deposits. ACM is usually inherited in an autosomal dominant pattern with variable penetrance and expressivity, which is mainly related to ventricular tachyarrhythmia and sudden cardiac death (SCD). Importantly, significant progress has been made in determining the genetic background of ACM due to the development of new techniques for genetic analysis. The exact molecular pathomechanism of ACM, however, is not completely clear and the genotype-phenotype correlations have not been fully elucidated, which are useful to predict the prognosis and treatment of ACM patients. Different gene-targeted and transgenic animal models, human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) models, and heterologous expression systems have been developed. Here, this review aims to summarize preclinical ACM models and platforms promoting our understanding of the pathogenesis of ACM and assess their value in elucidating the ACM genotype-phenotype relationship.

Salt and Gut Microbiota in Heart Failure

PURPOSE OF REVIEW

The role and underlying mechanisms mediated by dietary salt in modulating the gut microbiota and contributing to heart failure (HF) are not clear. This review summarizes the mechanisms of dietary salt and the gut-heart axis in HF.

RECENT FINDINGS

The gut microbiota has been implicated in several cardiovascular diseases (CVDs) including HF. Dietary factors including high consumption of salt play a role in influencing the gut microbiota, resulting in dysbiosis. An imbalance of microbial species due to a reduction in microbial diversity with accompanying immune cell activation has been implicated in the pathogenesis of HF via several mechanisms. The gut microbiota and gut-associated metabolites contribute to HF by reducing gut microbiota biodiversity and activating several signaling pathways. High dietary salt modulates the gut microbiota composition and exacerbate or induce HF by increasing the expression of the epithelial sodium/hydrogen exchanger isoform 3 in the gut, cardiac expression of beta myosin heavy chain, activation of the myocyte enhancer factor/nuclear factor of activated T cell, and salt-inducible kinase 1. These mechanisms explain the resulting structural and functional derangements in patients with HF.

Identification of hub genes and potential ceRNA networks of diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM), a common complication of diabetes, is defined as ventricular dysfunction in the absence of underlying heart disease. Noncoding RNAs (ncRNAs), including long noncoding RNAs (lncRNAs) and microRNAs (miRNAs), play a crucial role in the development of DCM. Weighted Gene Co-Expression Network Analysis (WGCNA) was used to identify key modules in DCM-related pathways. DCM-related miRNA-mRNA network and DCM-related ceRNA network were constructed by miRNA-seq to identify hub genes in these modules. We identified five hub genes that are associated with the onset of DCM, including Troponin C1 (Tnnc1), Phospholamban (Pln), Fatty acid binding proteins 3 (Fabp3), Popeye domain containing 2 (Popdc2), and Tripartite Motif-containing Protein 63 (Trim63). miRNAs that target the hub genes were mainly involved in TGF-β and Wnt signaling pathways. GO BP enrichment analysis found these miRNAs were involved in the signaling of TGF-β and glucose homeostasis. Q-PCR results found the gene expressions of Pln, Fabp3, Trim63, Tnnc1, and Popdc2 were significantly increased in DCM. Our study identified five hub genes (Tnnc1, Pln, Fabp3, Popdc2, Trim63) whose associated ceRNA networks are responsible for the onset of DCM.

Interaction of DWORF with SERCA and PLB as determined by EPR spectroscopy

Insufficient sarco/endoplasmic reticulum calcium ATPase (SERCA) activity significantly contributes to heart failure, which is a leading cause of death worldwide. A characteristic pathology of cardiac disease is the slow and incomplete Ca2+ removal from the myocyte cytoplasm in diastole, which is primarily driven by SERCA, the integral transmembrane Ca2+ pump. Phospholamban (PLB) allosterically inhibits SERCA by reducing its apparent Ca2+ affinity. Recently, the 34-codon novel dwarf open reading frame (DWORF) micropeptide has been identified as a muscle-specific SERCA effector, capable of reversing the inhibitory effects of PLB and independently activating SERCA in the absence of PLB. However, the structural basis for these functions has not yet been determined in a system of defined molecular components. We have used electron paramagnetic resonance (EPR) spectroscopy to investigate the protein-protein interactions of DWORF, co-reconstituted in proteoliposomes with SERCA and spin-labeled PLB. We analyzed the change of PLB rotational mobility in response to varying DWORF concentration, to quantify competitive binding of DWORF and PLB. We determined that DWORF competes with PLB for binding to SERCA at low [Ca2+], although the measured affinity of DWORF for SERCA is an order of magnitude weaker than that of PLB for SERCA, indicating cooperativity. The sensitivity of EPR to structural dynamics, using stereospecifically attached spin labels, allows us to obtain new information needed to refine the molecular model for regulation of SERCA activity, as needed for development of novel therapeutic remedies against cardiac pathologies.

Sarco/endoplasmic reticulum calcium ATPase activity is unchanged despite increased myofilament calcium sensitivity in Zucker type 2 diabetic fatty rat heart

Systolic and diastolic dysfunction in diabetes have frequently been associated with abnormal calcium (Ca²⁺) regulation. However, there is emerging evidence that Ca²⁺ mishandling alone is insufficient to fully explain diabetic heart dysfunction, with focus shifting to the properties of the myofilament proteins. Our aim was to examine the effects of diabetes on myofilament Ca²⁺ sensitivity and Ca²⁺ handling in left ventricular tissues isolated from the same type 2 diabetic rat hearts. We measured the force-pCa relationship in skinned left ventricular cardiomyocytes isolated from 20-week-old type 2 diabetic and non-diabetic rats. Myofilament Ca²⁺ sensitivity was greater in the diabetic relative to non-diabetic cardiomyocytes, and this corresponded with lower phosphorylation of cardiac troponin I (cTnI) at ser23/24 in the diabetic left ventricular tissues. Protein expression of sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA), phosphorylation of phospholamban (PLB) at Ser16, and SERCA/PLB ratio were lower in the diabetic left ventricular tissues. However, the maximum SERCA Ca²⁺ uptake rate was not different between the diabetic and non-diabetic myocardium. Our data suggest that impaired contractility in the diabetic heart is not caused by SERCA Ca²⁺ mishandling. This study highlights the important role of the cardiac myofilament and provides new insight on the pathophysiology of diabetic heart dysfunction.

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.

Impairment of the ER/mitochondria compartment in human cardiomyocytes with PLN p.Arg14del mutation

The phospholamban (PLN) p.Arg14del mutation causes dilated cardiomyopathy, with the molecular disease mechanisms incompletely understood. Patient dermal fibroblasts were reprogrammed to hiPSC, isogenic controls were established by CRISPR/Cas9, and cardiomyocytes were differentiated. Mutant cardiomyocytes revealed significantly prolonged Ca2+ transient decay time, Ca2+ -load dependent irregular beating pattern, and lower force. Proteomic analysis revealed less endoplasmic reticulum (ER) and ribosomal and mitochondrial proteins. Electron microscopy showed dilation of the ER and large lipid droplets in close association with mitochondria. Follow-up experiments confirmed impairment of the ER/mitochondria compartment. PLN p.Arg14del end-stage heart failure samples revealed perinuclear aggregates positive for ER marker proteins and oxidative stress in comparison with ischemic heart failure and non-failing donor heart samples. Transduction of PLN p.Arg14del EHTs with the Ca2+ -binding proteins GCaMP6f or parvalbumin improved the disease phenotype. This study identified impairment of the ER/mitochondria compartment without SR dysfunction as a novel disease mechanism underlying PLN p.Arg14del cardiomyopathy. The pathology was improved by Ca2+ -scavenging, suggesting impaired local Ca2+ cycling as an important disease culprit.

Arrhythmia Mechanism and Dynamics in a Humanized Mouse Model of Inherited Cardiomyopathy Caused by Phospholamban R14del Mutation

BACKGROUND

Arginine (Arg) 14 deletion (R14del) in the calcium regulatory protein phospholamban (hPLN^R14del) has been identified as a disease-causing mutation in patients with an inherited cardiomyopathy. Mechanisms underlying the early arrhythmogenic phenotype that predisposes carriers of this mutation to sudden death with no apparent structural remodeling remain unclear.

METHODS

To address this, we performed high spatiotemporal resolution optical mapping of intact hearts from adult knock-in mice harboring the human PLN^WT (wildtype [WT], n=12) or the heterozygous human PLN^R14del mutation (R14del, n=12) before and after ex vivo challenge with isoproterenol and rapid pacing.

RESULTS

Adverse electrophysiological remodeling was evident in the absence of significant structural or hemodynamic changes. R14del hearts exhibited increased arrhythmia susceptibility compared with wildtype. Underlying this susceptibility was preferential right ventricular action potential prolongation that was unresponsive to β-adrenergic stimulation. A steep repolarization gradient at the left ventricular/right ventricular interface provided the substrate for interventricular activation delays and ultimately local conduction block during rapid pacing. This was followed by the initiation of macroreentrant circuits supporting the onset of ventricular tachycardia. Once sustained, these circuits evolved into high-frequency rotors, which in their majority were pinned to the right ventricle. These rotors exhibited unique spatiotemporal dynamics that promoted their increased stability in R14del compared with wildtype hearts.

CONCLUSIONS

Our findings highlight the crucial role of primary electric remodeling caused by the hPLN^R14del mutation. These inherently arrhythmogenic features form the substrate for adrenergic-mediated VT at early stages of PLN^R14del induced cardiomyopathy.

Sodium Intake and Heart Failure

Sodium is an essential mineral and nutrient used in dietary practices across the world and is important to maintain proper blood volume and blood pressure. A high sodium diet is associated with increased expression of β—myosin heavy chain, decreased expression of α/β—myosin heavy chain, increased myocyte enhancer factor 2/nuclear factor of activated T cell transcriptional activity, and increased salt-inducible kinase 1 expression, which leads to alteration in myocardial mechanical performance. A high sodium diet is also associated with alterations in various proteins responsible for calcium homeostasis and myocardial contractility. Excessive sodium intake is associated with the development of a variety of comorbidities including hypertension, chronic kidney disease, stroke, and cardiovascular diseases. While the American College of Cardiology/American Heart Association/Heart Failure Society of America guidelines recommend limiting sodium intake to both prevent and manage heart failure, the evidence behind such recommendations is unclear. Our review article highlights evidence and underlying mechanisms favoring and contradicting limiting sodium intake in heart failure.

Ca2+ signaling of human pluripotent stem cells-derived cardiomyocytes as compared to adult mammalian cardiomyocytes

Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) have been extensively used for in vitro modeling of human cardiovascular disease, drug screening and pharmacotherapy, but little rigorous studies have been reported on their biophysical or Ca2+ signaling properties. There is also considerable concern as to the level of their maturity and whether they can serve as reliable models for adult human cardiac myocytes. Ultrastructural difference such as lack of t-tubular network, their polygonal shapes, disorganized sarcomeric myofilament, and their rhythmic automaticity, among others, have been cited as evidence for immaturity of hiPSC-CMs. In this review, we will deal with Ca2+ signaling, its regulation, and its stage of maturity as compared to the mammalian adult cardiomyocytes. We shall summarize the data on functional aspects of Ca2+signaling and its parameters that include: L-type calcium channel (Cav1.2), ICa-induced Ca2+release, CICR, and its parameters, cardiac Na/Ca exchanger (NCX1), the ryanodine receptors (RyR2), sarco-reticular Ca2+pump, SERCA2a/PLB, and the contribution of mitochondrial Ca2+ to hiPSC-CMs excitation-contraction (EC)-coupling as compared with adult mammalian cardiomyocytes. The comparative studies suggest that qualitatively hiPSC-CMs have similar Ca2+signaling properties as those of adult cardiomyocytes, but quantitative differences do exist. This review, we hope, will allow the readers to judge for themselves to what extent Ca2+signaling of hiPSC-CMs represents the adult form of this signaling pathway, and whether these cells can be used as good models of human cardiomyocytes.

The role of phospholamban and GSK3 in regulating rodent cardiac SERCA function

Cardiac contractile function is largely mediated by the regulation of Ca²⁺ cycling throughout the lifespan. The sarco(endo)plasmic reticulum Ca²⁺ ATPase (SERCA) pump is paramount to cardiac Ca²⁺ regulation, and it is well established that SERCA dysfunction pathologically contributes to cardiomyopathy and heart failure. Phospholamban (PLN) is a well-known inhibitor of the SERCA pump and its regulation of SERCA2a-the predominant cardiac SERCA isoform-contributes significantly to proper cardiac function. Glycogen synthase kinase 3 (GSK3) is a serine/threonine kinase involved in several metabolic pathways, and we and others have shown that it regulates SERCA function. In this mini-review, we highlight the underlying mechanisms behind GSK3's regulation of SERCA function specifically discussing changes in SERCA2a and PLN expression and its potential protection against oxidative stress. Ultimately, these recent findings that we discuss could have clinical implications in the treatment and prevention of cardiomyopathies and heart failure.

Low-dose lithium feeding increases the SERCA2a-to-phospholamban ratio, improving SERCA function in murine left ventricles

NEW FINDINGS

What is the central question of this study? Inhibition of glycogen synthase kinase-3 (GSK3) has been shown to improve cardiac SERCA2a function. Lithium can inhibit GSK3, but therapeutic doses used in treating bipolar disorder can have toxic effects. It has not been determined whether subtherapeutic doses of lithium can improve cardiac SERCA function. What is the main finding and its importance? Using left ventricles from wild-type mice, we found that subtherapeutic lithium feeding for 6 weeks decreased GSK3 activity and increased cardiac SERCA function compared with control-fed mice. These findings warrant the investigation of low-dose lithium feeding in preclinical models of cardiomyopathy and heart failure to determine the therapeutic benefit of GSK3 inhibition.

ABSTRACT

The sarco(endo)plasmic reticulum Ca²⁺ -ATPase (SERCA) pump is responsible for regulating calcium (Ca²⁺ ) within myocytes, with SERCA2a being the dominant isoform in cardiomyocytes. Its inhibitor, phospholamban (PLN), acts by decreasing the affinity of SERCA for Ca²⁺ . Changes in the SERCA2a:PLN ratio can cause Ca²⁺ dysregulation often seen in patients with dilated cardiomyopathy and heart failure. The enzyme glycogen synthase kinase-3 (GSK3) is known to downregulate SERCA function by decreasing the SERCA2a:PLN ratio. In this study, we sought to determine whether feeding mice low-dose lithium, a natural GSK3 inhibitor, would improve left ventricular SERCA function by altering the SERCA2a:PLN ratio. To this end, male wild-type C57BL/6J mice were fed low-dose lithium via drinking water (10 mg kg^-1  day^-1 LiCl for 6 weeks) and left ventricles were harvested. GSK3 activity was significantly reduced in LiCl-fed versus control-fed mice. The apparent affinity of SERCA for Ca²⁺ was also increased (pCa₅₀ ; control, 6.09 ± 0.03 versus LiCl, 6.26 ± 0.04, P < 0.0001) along with a 2.0-fold increase in SERCA2a:PLN ratio in LiCl-fed versus control-fed mice. These findings suggest that low-dose lithium supplementation can improve SERCA function by increasing the SERCA2a:PLN ratio. Future studies in murine preclinical models will determine whether GSK3 inhibition via low-dose lithium could be a potential therapeutic strategy for dilated cardiomyopathy and heart failure.

Dysregulation of Calcium Handling in Duchenne Muscular Dystrophy-Associated Dilated Cardiomyopathy: Mechanisms and Experimental Therapeutic Strategies

: Duchenne muscular dystrophy (DMD) is an X-linked recessive disease resulting in the loss of dystrophin, a key cytoskeletal protein in the dystrophin-glycoprotein complex. Dystrophin connects the extracellular matrix with the cytoskeleton and stabilizes the sarcolemma. Cardiomyopathy is prominent in adolescents and young adults with DMD, manifesting as dilated cardiomyopathy (DCM) in the later stages of disease. Sarcolemmal instability, leading to calcium mishandling and overload in the cardiac myocyte, is a key mechanistic contributor to muscle cell death, fibrosis, and diminished cardiac contractile function in DMD patients. Current therapies for DMD cardiomyopathy can slow disease progression, but they do not directly target aberrant calcium handling and calcium overload. Experimental therapeutic targets that address calcium mishandling and overload include membrane stabilization, inhibition of stretch-activated channels, ryanodine receptor stabilization, and augmentation of calcium cycling via modulation of the Serca2a/phospholamban (PLN) complex or cytosolic calcium buffering. This paper addresses what is known about the mechanistic basis of calcium mishandling in DCM, with a focus on DMD cardiomyopathy. Additionally, we discuss currently utilized therapies for DMD cardiomyopathy, and review experimental therapeutic strategies targeting the calcium handling defects in DCM and DMD cardiomyopathy.

Effects of feeding and digestion on myocardial contractility and expression of calcium-handling proteins in Burmese pythons (Python molurus)

Pythons are important models of studies on postprandial metabolism because their physiological responses are exacerbated when digesting large prey. Prior studies of these animals have shown hypertrophy of the cardiac tissue 2 to 3 days after feeding, coinciding with the peak of the specific dynamic action (SDA), but the consequences of this remodeling in myocardial contractility have not been studied, which is the purpose of this work. Specimens of Python molurus were divided into two groups: a Digesting group (2 days after feeding, at the peak of SDA), and a Fasting group (28 days after feeding). When compared to the Fasting group, the Digesting group showed higher relative ventricular mass and calcium-handling protein expression such as sarcoplasmic reticulum Ca²⁺-ATPase (SERCA), phospholamban (PLB), and the Na⁺/Ca²⁺ exchanger (NCX). Digesting pythons also exhibited significant increases in the cardiac contraction force (Fc), rates of force development and relaxation, and cardiac pumping capacity. Therefore, the higher SERCA, PLB and NCX expression levels increased cytosolic Ca²⁺ transient amplitude, improving myofilament force. These changes are crucial to maintain cardiac output and a relatively high and continuous blood flow required by metabolic expenditure that occurs in postprandial animals.

Newly Discovered Micropeptide Regulators of SERCA Form Oligomers but Bind to the Pump as Monomers

The recently-discovered single-span transmembrane proteins endoregulin (ELN), dwarf open reading frame (DWORF), myoregulin (MLN), and another-regulin (ALN) are reported to bind to the SERCA calcium pump in a manner similar to that of known regulators of SERCA activity, phospholamban (PLB) and sarcolipin (SLN). To determine how micropeptide assembly into oligomers affects the availability of the micropeptide to bind to SERCA in a regulatory complex, we used co-immunoprecipitation and fluorescence resonance energy transfer (FRET) to quantify micropeptide oligomerization and SERCA-binding. Micropeptides formed avid homo-oligomers with high-order stoichiometry (n > 2 protomers per homo-oligomer), but it was the monomeric form of all micropeptides that interacted with SERCA. In view of these two alternative binding interactions, we evaluated the possibility that oligomerization occurs at the expense of SERCA-binding. However, even the most avidly oligomeric micropeptide species still showed robust FRET with SERCA, and there was a surprising positive correlation between oligomerization affinity and SERCA-binding. This comparison of micropeptide family members suggests that the same structural determinants that support oligomerization are also important for binding to SERCA. Moreover, the unique oligomerization/SERCA-binding profile of DWORF is in harmony with its distinct role as a PLB-competing SERCA activator, in contrast to the inhibitory function of the other SERCA-binding micropeptides.

The sarcoplasmic reticulum and SERCA: a nexus for muscular adaptive thermogenesis

We are currently facing an "obesity epidemic" worldwide. Promoting inefficient metabolism in muscle represents a potential treatment for obesity and its complications. Sarco(endo)plasmic reticulum (SR) Ca²⁺-ATPase (SERCA) pumps in muscle are responsible for maintaining low cytosolic Ca²⁺ concentration through the ATP-dependent pumping of Ca²⁺ from the cytosol into the SR lumen. SERCA activity has the potential to be a critical regulator of body mass and adiposity given that it is estimated to contribute upwards of 20% of daily energy expenditure. More interestingly, this fraction can be modified physiologically in the face of stressors, such as ambient temperature and diet, through its physical interaction with several regulators known to inhibit Ca²⁺ uptake and muscle function. In this review, we discuss advances in our understanding of Ca²⁺-cycling thermogenesis within skeletal muscle, focusing on SERCA and its protein regulators, which were thought previously to only modulate muscular contractility. Novelty ATP consumption by SERCA pumps comprises a large proportion of resting energy expenditure in muscle and is dynamically regulated through interactions with small SERCA regulatory proteins. SERCA efficiency correlates significantly with resting metabolism, such that individuals with a higher resting metabolic rate have less energetically efficient SERCA Ca²⁺ pumping in muscle (i.e., lower coupling ratio). Futile Ca²⁺ cycling is a versatile heat generating mechanism utilized by both skeletal muscle and beige fat.

The intramembrane protease SPPL2c promotes male germ cell development by cleaving phospholamban

Signal peptide peptidase (SPP) and the four homologous SPP-like (SPPL) proteases constitute a family of intramembrane aspartyl proteases with selectivity for type II-oriented transmembrane segments. Here, we analyse the physiological function of the orphan protease SPPL2c, previously considered to represent a non-expressed pseudogene. We demonstrate proteolytic activity of SPPL2c towards selected tail-anchored proteins. Despite shared ER localisation, SPPL2c and SPP exhibit distinct, though partially overlapping substrate spectra and inhibitory profiles, and are organised in different high molecular weight complexes. Interestingly, SPPL2c is specifically expressed in murine and human testis where it is primarily localised in spermatids. In mice, SPPL2c deficiency leads to a partial loss of elongated spermatids and reduced motility of mature spermatozoa, but preserved fertility. However, matings of male and female SPPL2c-/- mice exhibit reduced litter sizes. Using proteomics we identify the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2)-regulating protein phospholamban (PLN) as a physiological SPPL2c substrate. Accumulation of PLN correlates with a decrease in intracellular Ca2+ levels in elongated spermatids that likely contribute to the compromised male germ cell differentiation and function of SPPL2c-/- mice.

Female Heart Health: Is GPER the Missing Link?

The G Protein-Coupled Estrogen Receptor (GPER) is a novel membrane-bound receptor that mediates non-genomic actions of the primary female sex hormone 17β-estradiol. Studies over the past two decades have elucidated the beneficial actions of this receptor in a number of cardiometabolic diseases. This review will focus specifically on the cardiac actions of GPER, since this receptor is expressed in cardiomyocytes as well as other cells within the heart and most likely contributes to estrogen-induced cardioprotection. Studies outlining the impact of GPER on diastolic function, mitochondrial function, left ventricular stiffness, calcium dynamics, cardiac inflammation, and aortic distensibility are discussed. In addition, recent data using genetic mouse models with global or cardiomyocyte-specific GPER gene deletion are highlighted. Since estrogen loss due to menopause in combination with chronological aging contributes to unique aspects of cardiac dysfunction in women, this receptor may provide novel therapeutic effects. While clinical studies are still required to fully understand the potential for pharmacological targeting of this receptor in postmenopausal women, this review will summarize the evidence gathered thus far on its likely beneficial effects.

The DWORF micropeptide enhances contractility and prevents heart failure in a mouse model of dilated cardiomyopathy

Calcium (Ca2+) dysregulation is a hallmark of heart failure and is characterized by impaired Ca2+ sequestration into the sarcoplasmic reticulum (SR) by the SR-Ca2+-ATPase (SERCA). We recently discovered a micropeptide named DWORF (DWarf Open Reading Frame) that enhances SERCA activity by displacing phospholamban (PLN), a potent SERCA inhibitor. Here we show that DWORF has a higher apparent binding affinity for SERCA than PLN and that DWORF overexpression mitigates the contractile dysfunction associated with PLN overexpression, substantiating its role as a potent activator of SERCA. Additionally, using a well-characterized mouse model of dilated cardiomyopathy (DCM) due to genetic deletion of the muscle-specific LIM domain protein (MLP), we show that DWORF overexpression restores cardiac function and prevents the pathological remodeling and Ca2+ dysregulation classically exhibited by MLP knockout mice. Our results establish DWORF as a potent activator of SERCA within the heart and as an attractive candidate for a heart failure therapeutic.

Phospholamban regulates nuclear Ca2+ stores and inositol 1,4,5-trisphosphate mediated nuclear Ca2+ cycling in cardiomyocytes

AIMS

Phospholamban (PLB) is the key regulator of the cardiac Ca²⁺ pump (SERCA2a)-mediated sarcoplasmic reticulum Ca²⁺ stores. We recently reported that PLB is highly concentrated in the nuclear envelope (NE) from where it can modulate perinuclear Ca²⁺ handling of the cardiomyocytes (CMs). Since inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) mediates nuclear Ca²⁺ release, we examined whether the nuclear pool of PLB regulates IP₃-induced nuclear Ca²⁺ handling.

METHODS AND RESULTS

Fluo-4 based confocal Ca²⁺ imaging was performed to measure Ca²⁺ dynamics across both nucleus and cytosol in saponin-permeabilized CMs isolated from wild-type (WT) or PLB-knockout (PLB-KO) mice. At diastolic intracellular Ca²⁺ ([Ca²⁺]_i = 100 nM), the Fab fragment of the monoclonal PLB antibody (anti-PLB Fab) facilitated the formation and increased the length of spontaneous Ca²⁺ waves (SCWs) originating from the nuclear region in CMs from WT but not from PLB-KO mice. We next examined nuclear Ca²⁺ activities at basal condition and after sequential addition of IP₃, anti-PLB Fab, and the IP₃R inhibitor 2-aminoethoxydiphenyl borate (2-APB) at a series of [Ca²⁺]_i. In WT mice, at 10 nM [Ca²⁺]_i where ryanodine receptor (RyR2) based spontaneous Ca²⁺ sparks rarely occurred, IP₃ increased fluorescence amplitude (F/F₀) of overall nuclear region to 1.19 ± 0.02. Subsequent addition of anti-PLB Fab significantly decreased F/F₀ to 1.09 ± 0.02. At 50 nM [Ca²⁺]_i, anti-PLB Fab not only decreased the overall nuclear F/F₀ previously elevated by IP₃, but also increased the amplitude and duration of spark-like nuclear Ca²⁺ release events. These nuclear Ca²⁺ releases were blocked by 2-APB. At 100 nM [Ca²⁺]_i, IP₃ induced short SCWs originating from nucleus. Anti-PLB Fab transformed those short waves into long SCWs with propagation from the nucleus into the cytosol. In contrast, neither nuclear nor cytosolic Ca²⁺ dynamics was affected by anti-PLB Fab in CMs from PLB-KO mice in all these conditions. Furthermore, in WT CMs pretreated with RyR2 blocker tetracaine, IP₃ and anti-PLB Fab still increased the magnitude of nuclear Ca²⁺ release but failed to regenerate SCWs. Finally, anti-PLB Fab increased low Ca²⁺ affinity mag-fluo 4 fluorescence intensity in the lumen of NE of nuclei isolated from WT but not in PLB-KO mice.

CONCLUSION

PLB regulates nuclear Ca²⁺ handling. By increasing Ca²⁺ uptake into lumen of the NE and perhaps other perinuclear membranes, the acute reversal of PLB inhibition decreases global Ca²⁺ concentration at rest in the nucleoplasm, and increases Ca²⁺ release into the nucleus, through mechanisms involving IP₃R and RyR2 in the vicinity.

Empagliflozin Improves Left Ventricular Diastolic Dysfunction in a Genetic Model of Type 2 Diabetes

PURPOSE

Cardiovascular (CV) diseases in type 2 diabetes (T2DM) represent an enormous burden with high mortality and morbidity. Sodium-glucose cotransporter 2 (SGLT2) inhibitors have recently emerged as a new antidiabetic class that improves glucose control, as well as body weight and blood pressure with no increased risk of hypoglycemia. The first CV outcome study terminated with empagliflozin, a specific SGLT2 inhibitor, has shown a reduction in CV mortality and in heart failure hospitalization, suggesting a beneficial impact on cardiac function which remains to be demonstrated. This study was designed to examine the chronic effect of empagliflozin on left ventricular (LV) systolic and diastolic functions in a genetic model of T2DM, ob/ob mice.

METHODS AND RESULTS

Cardiac phenotype was characterized by echocardiography, in vivo hemodynamics, histology, and molecular profiling. Our results demonstrate that empagliflozin significantly lowered HbA1c and slightly reduced body weight compared to vehicle treatment with no obvious changes in insulin levels. Empagliflozin also improved LV maximum pressure and in vivo indices of diastolic function. While systolic function was grossly not affected in both groups at steady state, response to dobutamine stimulation was significantly improved in the empagliflozin-treated group, suggesting amelioration of contractile reserve. This was paralleled by an increase in phospholamban (PLN) phosphorylation and increased SERCA2a/PLN ratio, indicative of enhanced SERCA2a function, further supporting improved cardiac relaxation and diastolic function. In addition, empagliflozin reconciled diabetes-associated increase in MAPKs and dysregulated phosphorylation of IRS1 and Akt, leading to improvement in myocardial insulin sensitivity and glucose utilization.

CONCLUSION

The data show that chronic treatment with empagliflozin improves diastolic function, preserves calcium handling and growth signaling pathways and attenuates myocardial insulin resistance in ob/ob mice, findings suggestive of a potential clinical utility for empagliflozin in the treatment of diastolic dysfunction.

Influence of Long-Term Salt Diets on Cardiac Ca2+ Handling and Contractility Proteins in Hypertensive Rats

BACKGROUND

High sodium intake contributes to the pathogenesis of hypertension and adversely affects cardiac function. Conversely, sodium reduction is associated with a blood pressure decrease and improved cardiovascular function. However, the mechanisms that underlie the cardiac effects induced by salt intake in hypertension have not been fully elucidated. Ca2+ handling is critical for efficient myocardial function; thus, we aimed to investigate the long-term effects of diets with different salt contents on cardiac function and Ca2+ handling proteins in spontaneously hypertensive rats (SHRs).

METHODS

Cardiac function was evaluated by catheterization. Ca2+ handling and contractile proteins were evaluated by immunoblotting in hearts from SHRs fed for 6 months with diets containing high (HS, 3%), low (LS, 0.03%), or normal salt content (NS, 0.3%). Diets were introduced immediately after weaning. Tail cuff pletismography was assessed at the 3rd and 7th months of follow-up.

RESULTS

Compared to the NS group, the HS group exhibited worsened hypertension, increased cardiac expression of β-myosin heavy chain (MHC), a decreased α/β-MHC ratio and reduced expression of both phospholamban (PLB) and Na+/Ca2+ exchanger (NCX). LS intake attenuated the blood pressure increase and left ventricle hypertrophy, slightly decreased the cardiac contractility and relaxation index, and increased the α/β-MHC ratio. These effects were accompanied by increased cardiac PLB expression and decreased Ca2+ L-type channel and NCX expression.

CONCLUSIONS

These findings indicate that the modulation of Ca2+ handling may be one of the molecular mechanisms underlying the effect of salt intake on myocardial function in hypertension.

Testosterone deficiency prevents left ventricular contractility dysfunction after myocardial infarction

Testosterone may affect myocardial contractility since its deficiency decreases the contraction and relaxation of the heart. Meanwhile, testosterone replacement therapy has raised concerns because it may worsen cardiac dysfunction and remodeling after myocardial infarction (MI). In this study, we evaluate cardiac contractility 60 days after MI in rats with suppressed testosterone. Male Wistar rats underwent bilateral orchidectomy one week before the ligation of the anterior descending left coronary artery. The animals were divided into orchidectomized (OCT); MI; orchidectomized + MI (OCT + MI); orchidectomized + MI + testosterone (OCT + MI + T) and control (Sham) groups. Eight weeks after MI, papillary muscle contractility was analyzed under increasing calcium (0.62, 1.25, 2.5 and 3.75 mM) and isoproterenol (10^-8 to 10^-2 M) concentrations. Ventricular myocytes were isolated for intracellular calcium measurements and assessment of Ca²⁺ handling proteins. Contractility was preserved in the orchidectomized animals after myocardial infarction and was reduced when testosterone was replaced (Ca²⁺ 3.75 mM: Sham: 608 ± 70 (n = 11); OCT: 590 ± 37 (n = 16); MI: 311 ± 33* (n = 9); OCT + MI: 594 ± 76 (n = 7); OCT + MI + T: 433 ± 38* (n=4), g/g *p < 0.05 vs Sham). Orchidectomy also increased the Ca²⁺ transient amplitude of the ventricular myocytes and SERCA-2a protein expression levels. PLB phosphorylation levels at Thr¹⁷ were not different in the orchidectomized animals compared to the Sham animals but were reduced after testosterone replacement. CAMKII phosphorylation and protein nitrosylation increased in the orchidectomized animals. Our results support the view that testosterone deficiency prevents MI contractility dysfunction by altering the key proteins involved in Ca²⁺ handling.

Insertional Mutagenesis Confounds the Mechanism of the Morbid Phenotype of a PLNR9C Transgenic Mouse Line

BACKGROUND

A mouse line with heterozygous transgenic expression of phospholamban carrying a substitution of cysteine for arginine 9 (TgPLN^R9C) under the control of α-myosin heavy chain (αMHC) promoter features dilated cardiomyopathy, heart failure, and premature death.

METHODS AND RESULTS

Determination of transgene chromosomal localization by conventional methods shows that in this line the transgenic array of 13 PLN^R9C expression cassettes, arranged in a head-to-tail tandem orientation, have integrated into the bidirectional promoter of the αMHC (Myh6) gene and the gene for the regulatory noncoding RNA Myheart (Mhrt), both of which are known to be involved in cardiac development and pathology. Expression of the noncoding RNA Mhrt in TgPLN^R9C mice exhibits profound deregulation, despite the presence of the second, intact allele.

CONCLUSIONS

The TgPLN^R9C mouse strain is, in the best case, a functionally ambiguous phenocopy of the human PLN^R9C heterozygote, because a similar constellation of genetically programmed events can not occur in a patient. Publications featuring "cardiac-specific overexpression" are focused on the phenotype and typically forgo the definition of the transgene integration site or transgene temporal expression profile, so caution should be exercised in attributing clinical relevance to pathologic phenomena observed in αMHC-driven transgenes.

The antiapoptotic protein HAX-1 mediates half of phospholamban's inhibitory activity on calcium cycling and contractility in the heart

The antiapoptotic protein HAX-1 (HS-associated protein X-1) localizes to sarcoplasmic reticulum (SR) in the heart and interacts with the small membrane protein phospholamban (PLN), inhibiting the cardiac sarco/endoplasmic reticulum calcium ATPase 2a (SERCA2a) in the regulation of overall calcium handling and heart muscle contractility. However, because global HAX-1 deletion causes early lethality, how much endogenous HAX-1 contributes to PLN's inhibitory activity on calcium cycling is unknown. We therefore generated a cardiac-specific and inducible knock-out mouse model. HAX-1 ablation in the adult heart significantly increased contractile parameters and calcium kinetics, associated with increased SR calcium load. These changes occurred without any changes in the protein expression of SERCA2a, PLN, and ryanodine receptor or in the PLN phosphorylation status. The enhanced calcium cycling in the HAX-1-depleted heart was mediated through increases in the calcium affinity of SERCA2a and reduced PLN-SERCA2a binding. Comparison of the HAX-1 deletion-induced stimulatory effects with those elicited by PLN ablation indicated that HAX-1 mediates ∼50% of the PLN-associated inhibitory effects in the heart. Stimulation with the inotropic and lusitropic agent isoproterenol eliminated the differences among wild-type, HAX-1-deficient, and PLN-deficient hearts, and maximally stimulated contractile and calcium kinetic parameters were similar among these three groups. Furthermore, PLN overexpression in the HAX-1-null cardiomyocytes did not elicit any inhibitory effects, indicating that HAX-1 may limit PLN activity. These findings suggest that HAX-1 is a major mediator of PLN's inhibitory activity and a critical gatekeeper of SR calcium cycling and contractility in the heart.

Effects of exercise training on excitation-contraction coupling, calcium dynamics and protein expression in the heart of the Neotropical fish Brycon amazonicus

Matrinxã (Brycon amazonicus) is a great swimming performance teleost fish from the Amazon basin. However, the possible cardiac adaptations of this ability are still unknown. Therefore, the aim of the present work was to investigate the effects of prolonged exercise (EX group - 60days under 0.4BL·s^-1) on ventricular contractility by (i) in-vitro analysis of contractility comparing the relative roles of sodium/calcium exchanger (NCX) and sarcoplasmic reticulum (SR) in the excitation-contraction (E-C) coupling and (ii) molecular analysis of NCX, sarcoplasmic reticulum Ca²⁺ ATPase (SERCA2) and phospholamban (PLB) expression and quantification. The exercise training significantly improved twitch tension, cardiac pumping capacity and the contraction rate when compared to controls (CT). Inhibition of the NCX function, replacing Na⁺ by Li⁺ in the physiological solutions, diminished cardiac contractility in the EX group, reduced all analyzed parameters under both high and low stimulation frequencies. The SR blockage, using 10μM ryanodine, caused ~50% tension reduction in CT at most analyzed frequencies while in EX, reductions (34-54%) were only found at higher frequencies. SR inhibition also decreased contraction and relaxation rates in both groups. Additionally, higher post-rest contraction values were recorded for EX, indicating an increase in SR Ca²⁺ loading. Higher NCX and PLB expression rates and lower SERCA2 rates were found in EX. Our data indicate that matrinxã presents a modulation in E-C coupling after exercise-training, enhancing the SR function under higher frequencies. This was the first study to functionally analyze the effects of swimming-induced exercise on fish cardiac E-C coupling.

Posttranslational modifications of calcium/calmodulin-dependent protein kinase IIδ and its downstream signaling in human failing hearts

BACKGROUND

In human failing hearts (HF) of different origin (coronary artery disease-CAD, dilated-DCM, restrictive and hypertrophic cardiomyopathy-OTHER), we investigated the active forms of Ca²⁺/calmodulin-dependent protein kinase IIδ (p-Thr²⁸⁷-CaMKIIδ, oxMet^281/282-CaMKIIδ) and their role in phenotypes of the disease.

METHODS AND RESULTS

Although basic diagnostic and clinical markers indicating the attenuated cardiac contractility and remodeling were comparable in HF groups, CaMKIIδ-mediated axis was different. P-Thr²⁸⁷-CaMKIIδ was unaltered in CAD group, whereas it was upregulated in non-ischemic cardiomyopathic groups. No correlation between the upregulated p-Thr²⁸⁷-CaMKIIδ and QT interval prolongation was detected. Unlike in DCM, oxMet^281/282-CaMKIIδ did not differ among HF groups. Independently of CaMKIIδ phosphorylation/oxidation, activation of its downstreams-phospholamban and cardiac myosin binding protein-C was significantly downregulated supporting both diminished cardiac lusitropy and inotropy in all hearts. Content of sarcoplasmic reticulum Ca²⁺-ATPase 2a in all HF was unchanged. Protein phosphatase1β was upregulated in CAD and DCM only, while 2A did not differ among groups.

CONCLUSION

This is the first demonstration that the posttranslational activation of CaMKIIδ differs in HF depending on etiology. Lower levels of downstream molecular targets of CaMKIIδ do not correlate with either activation of CaMKIIδ or the expression of major protein phosphatases in the HF. Thus, it is unlikely that these mechanisms exclusively underlie failing of the heart.

In vivo administration of urolithin A and B prevents the occurrence of cardiac dysfunction in streptozotocin-induced diabetic rats

Background

Emerging evidence suggests that specific (poly)phenols may constitute new preventative strategies to counteract cell oxidative stress and myocardial tissue inflammation, which have a key role in the patho-physiology of diabetic cardiomyopathy. In a rat model of early diabetes, we evaluated whether in vivo administration of urolithin A (UA) or urolithin B (UB), the main gut microbiota phenolic metabolites of ellagitannin-rich foods, can reduce diabetes-induced microenvironmental changes in myocardial tissue, preventing cardiac functional impairment.

Methods

Adult Wistar rats with streptozotocin-induced type-1 diabetes (n = 29) were studied in comparison with 10 control animals. Diabetic rats were either untreated (n  =  9) or subjected to daily i.p. injection of UA (n = 10) or UB (n = 10). After 3 weeks of hyperglycaemia, hemodynamics, cardiomyocyte contractile properties and calcium transients were measured to assess cardiac performance. The myocardial expression of the pro-inflammatory cytokine fractalkine and proteins involved in calcium dynamics (sarcoplasmic reticulum calcium ATPase, phospholamban and phosphorylated phospholamban) were evaluated by immunoblotting. Plasma, urine and tissue distribution of UA, UB and their phase II metabolites were determined.

Results

In vivo urolithin treatment reduced by approximately 30% the myocardial expression of the pro-inflammatory cytokine fractalkine, preventing the early inflammatory response of cardiac cells to hyperglycaemia. The improvement in myocardial microenvironment had a functional counterpart, as documented by the increase in the maximal rate of ventricular pressure rise compared to diabetic group (+18% and +31% in UA and UB treated rats, respectively), and the parallel reduction in the isovolumic contraction time (−12%). In line with hemodynamic data, both urolithins induced a recovery of cardiomyocyte contractility and calcium dynamics, leading to a higher re-lengthening rate (+21%, on average), lower re-lengthening times (−56%), and a more efficient cytosolic calcium clearing (−32% in tau values). UB treatment also increased the velocity of shortening (+27%). Urolithin metabolites accumulated in the myocardium, with a higher concentration of UB and UB-sulphate, potentially explaining the slightly higher efficacy of UB administration.

Conclusions

In vivo urolithin administration may be able to prevent the initial inflammatory response of myocardial tissue to hyperglycaemia and the negative impact of the altered diabetic milieu on cardiac performance.

Simulation of the effects of moderate stimulation/inhibition of the β1-adrenergic signaling system and its components in mouse ventricular myocytes

The β1-adrenergic signaling system is one of the most important protein signaling systems in cardiac cells. It regulates cardiac action potential duration, intracellular Ca2+concentration ([Ca2+]i) transients, and contraction force. In this paper, a comprehensive experimentally based mathematical model of the β1-adrenergic signaling system for mouse ventricular myocytes is explored to simulate the effects of moderate stimulations of β1-adrenergic receptors (β1-ARs) on the action potential, Ca2+and Na+dynamics, as well as the effects of inhibition of protein kinase A (PKA) and phosphodiesterase of type 4 (PDE4). Simulation results show that the action potential prolongations reach saturating values at relatively small concentrations of isoproterenol (∼0.01 μM), while the [Ca2+]itransient amplitude saturates at significantly larger concentrations (∼0.1–1.0 μM). The differences in the response of Ca2+and Na+fluxes to moderate stimulation of β1-ARs are also observed. Sensitivity analysis of the mathematical model is performed and the model limitations are discussed. The investigated model reproduces most of the experimentally observed effects of moderate stimulation of β1-ARs, PKA, and PDE4 inhibition on the L-type Ca2+current, [Ca2+]itransients, and the sarcoplasmic reticulum Ca2+load and makes testable predictions for the action potential duration and [Ca2+]itransients as functions of isoproterenol concentration.

Sex hormones in the cardiovascular system

Gender-associated differences in the development of cardiovascular diseases have been described in humans and animals. These differences could explain the low incidence of cardiovascular disease in women in the reproductive period, such as stroke, hypertension, and atherosclerosis. The cardiovascular protection observed in females has been attributed to the beneficial effects of estrogen on endothelial function. Besides estrogen, sex hormones are able to modulate blood pressure by acting on important systems as cardiovascular, renal, and neural. They can have complementary or antagonistic actions. For example, testosterone can raise blood pressure by stimulating the renin-angiotensin-aldosterone system, whereas estrogen alone or combined with progesterone has been associated with decreased blood pressure. The effects of testosterone in the development of cardiovascular disease are contradictory. Although some researchers suggest a positive effect, others indicate negative actions of testosterone. Estrogens physiologically stimulate the release of endothelium-derived vasodilator factors and inhibit the renin-angiotensin system. Although the cardioprotective effects of estrogen are widely appreciated, little is known about the effects of progesterone, which is commonly used in hormone replacement therapy. Progesterone has both vasodilatory and vasoconstrictive effects in the vasculature, depending on the location of the vessel and the level of exposure. Nevertheless, the mechanisms through which sex hormones modulate blood pressure have not been fully elucidated. Therefore, the characterization of those could lead to a better understanding of hypertension in women and men and perhaps to improved forms of therapy.

Sex Hormones Promote Opposite Effects on ACE and ACE2 Activity, Hypertrophy and Cardiac Contractility in Spontaneously Hypertensive Rats

Background

There is growing interest in sex differences and RAS components. However, whether gender influences cardiac angiotensin I-converting enzyme (ACE) and angiotensin-converting enzyme 2 (ACE2) activity is still unknown. In the present work, we determined the relationship between ACE and ACE2 activity, left ventricular function and gender in spontaneously hypertensive rats (SHRs).

Methodology / Principal Findings

Twelve-week-old female (F) and male (M) SHRs were divided into 2 experimental groups (n = 7 in each group): sham (S) and gonadectomized (G). Fifty days after gonadectomy, we measured positive and negative first derivatives (dP/dt maximum left ventricle (LV) and dP/dt minimum LV, respectively), hypertrophy (morphometric analysis) and ACE and ACE2 catalytic activity (fluorimetrically). Expression of calcium handling proteins was measured by western blot. Male rats exhibited higher cardiac ACE and ACE2 activity as well as hypertrophy compared to female rats. Orchiectomy decreased the activity of these enzymes and hypertrophy, while ovariectomy increased hypertrophy and ACE2, but did not change ACE activity. For cardiac function, the male sham group had a lower +dP/dt than the female sham group. After gonadectomy, the +dP/dt increased in males and reduced in females. The male sham group had a lower -dP/dt than the female group. After gonadectomy, the -dP/dt increased in the male and decreased in the female groups when compared to the sham group. No difference was observed among the groups in SERCA2a protein expression. Gonadectomy increased protein expression of PLB (phospholamban) and the PLB to SERCA2a ratio in female rats, but did not change in male rats.

Conclusion

Ovariectomy leads to increased cardiac hypertrophy, ACE2 activity, PLB expression and PLB to SERCA2a ratio, and worsening of hemodynamic variables, whereas in males the removal of testosterone has the opposite effects on RAS components.

In vivo model with targeted cAMP biosensor reveals changes in receptor-microdomain communication in cardiac disease

3',5'-cyclic adenosine monophosphate (cAMP) is an ubiquitous second messenger that regulates physiological functions by acting in distinct subcellular microdomains. Although several targeted cAMP biosensors are developed and used in single cells, it is unclear whether such biosensors can be successfully applied in vivo, especially in the context of disease. Here, we describe a transgenic mouse model expressing a targeted cAMP sensor and analyse microdomain-specific second messenger dynamics in the vicinity of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA). We demonstrate the biocompatibility of this targeted sensor and its potential for real-time monitoring of compartmentalized cAMP signalling in adult cardiomyocytes isolated from a healthy mouse heart and from an in vivo cardiac disease model. In particular, we uncover the existence of a phosphodiesterase-dependent receptor-microdomain communication, which is affected in hypertrophy, resulting in reduced β-adrenergic receptor-cAMP signalling to SERCA.

A novel human R25C-phospholamban mutation is associated with super-inhibition of calcium cycling and ventricular arrhythmia

AIMS

Depressed sarcoplasmic reticulum (SR) Ca(2+) cycling, a universal characteristic of human and experimental heart failure, may be associated with genetic alterations in key Ca(2+)-handling proteins. In this study, we identified a novel PLN mutation (R25C) in dilated cardiomyopathy (DCM) and investigated its functional significance in cardiomyocyte Ca(2+)-handling and contractility.

METHODS AND RESULTS

Exome sequencing identified a C73T substitution in the coding region of PLN in a family with DCM. The four heterozygous family members had implantable cardiac defibrillators, and three developed prominent ventricular arrhythmias. Overexpression of R25C-PLN in adult rat cardiomyocytes significantly suppressed the Ca(2+) affinity of SR Ca(2+)-ATPase (SERCA2a), resulting in decreased SR Ca(2+) content, Ca(2+) transients, and impaired contractile function, compared with WT-PLN. These inhibitory effects were associated with enhanced interaction of R25C-PLN with SERCA2, which was prevented by PKA phosphorylation. Accordingly, isoproterenol stimulation relieved the depressive effects of R25C-PLN in cardiomyocytes. However, R25C-PLN also elicited increases in the frequency of Ca(2+) sparks and waves as well as stress-induced aftercontractions. This was accompanied by increased Ca(2+)/calmodulin-dependent protein kinase II activity and hyper-phosphorylation of RyR2 at serine 2814.

CONCLUSION

The findings demonstrate that human R25C-PLN is associated with super-inhibition of SERCA2a and Ca(2+) transport as well as increased SR Ca(2+) leak, promoting arrhythmogenesis under stress conditions. This is the first mechanistic evidence that increased PLN inhibition may impact both SR Ca(2+) uptake and Ca(2+) release activities and suggests that the human R25C-PLN may be a prognostic factor for increased ventricular arrhythmia risk in DCM carriers.

Cardiac specific expression of threonine 5 to alanine mutant sarcolipin results in structural remodeling and diastolic dysfunction

The functional importance of threonine 5 (T5) in modulating the activity of sarcolipin (SLN), a key regulator of sarco/endoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA) pump was studied using a transgenic mouse model with cardiac specific expression of threonine 5 to alanine mutant SLN (SLNT5A). In these transgenic mice, the SLNT5A protein replaces the endogenous SLN in atria, while maintaining the total SLN content. The cardiac specific expression of SLNT5A results in severe cardiac structural remodeling accompanied by bi-atrial enlargement. Biochemical analyses reveal a selective downregulation of SR Ca²⁺ handling proteins and a reduced SR Ca²⁺ uptake both in atria and in the ventricles. Optical mapping analysis shows slower action potential propagation in the transgenic mice atria. Doppler echocardiography and hemodynamic measurements demonstrate a reduced atrial contractility and an impaired diastolic function. Together, these findings suggest that threonine 5 plays an important role in modulating SLN function in the heart. Furthermore, our studies suggest that alteration in SLN function can cause abnormal Ca²⁺ handling and subsequent cardiac remodeling and dysfunction.

Therapeutic Molecular Phenotype of β-Blocker-Associated Reverse-Remodeling in Nonischemic Dilated Cardiomyopathy

BACKGROUND

When β-blockers produce reverse-remodeling in idiopathic dilated cardiomyopathy, they partially reverse changes in fetal-adult/contractile protein, natriuretic peptide, SR-Ca(2+)-ATPase gene program constituents. The objective of the current study was to further test the hypothesis that reverse-remodeling is associated with favorable changes in myocardial gene expression by measuring additional contractile, signaling, and metabolic genes that exhibit a fetal/adult expression predominance, are thyroid hormone-responsive, and are regulated by β1-adrenergic receptor signaling. A secondary objective was to identify which of these putative regulatory networks is most closely associated with observed changes.

METHODS AND RESULTS

Forty-seven patients with idiopathic dilated cardiomyopathy (left ventricular ejection fraction, 0.24±0.09) were randomized to the adrenergic-receptor blockers metoprolol (β1-selective), metoprolol+doxazosin (β1/α1), or carvedilol (β1/β2/α1). Serial radionuclide ventriculography and endomyocardial biopsies were performed at baseline, 3, and 12 months. Expression of 50 mRNA gene products was measured by quantitative polymerase chain reaction. Thirty-one patients achieved left ventricular ejection fraction reverse-remodeling response defined as improvement by ≥0.08 at 12 months or by ≥0.05 at 3 months (Δ left ventricular ejection fraction, 0.21±0.10). Changes in gene expression in responders versus nonresponders were decreases in NPPA and NPPB and increases in MYH6, ATP2A2, PLN, RYR2, ADRA1A, ADRB1, MYL3, PDFKM, PDHX, and CPT1B. All except PDHX involved increase in adult or decrease in fetal cardiac genes, but 100% were concordant with changes predicted by inhibition of β1-adrenergic signaling.

CONCLUSIONS

In addition to known gene expression changes, additional calcium-handling, sarcomeric, adrenergic signaling, and metabolic genes were associated with reverse-remodeling. The pattern suggests a fetal-adult paradigm but may be because of reversal of gene expression controlled by a β1-adrenergic receptor gene network.

CLINICAL TRIAL REGISTRATION

URL: www.clinicaltrials.gov. Unique Identifier: NCT01798992.

Phospholamban interactome in cardiac contractility and survival: A new vision of an old friend

Depressed sarcoplasmic reticulum (SR) calcium cycling, reflecting impaired SR Ca-transport and Ca-release, is a key and universal characteristic of human and experimental heart failure. These SR processes are regulated by multimeric protein complexes, including protein kinases and phosphatases as well as their anchoring and regulatory subunits that fine-tune Ca-handling in specific SR sub-compartments. SR Ca-transport is mediated by the SR Ca-ATPase (SERCA2a) and its regulatory phosphoprotein, phospholamban (PLN). Dephosphorylated PLN is an inhibitor of SERCA2a and phosphorylation by protein kinase A (PKA) or calcium-calmodulin-dependent protein kinases (CAMKII) relieves these inhibitory effects. Recent studies identified additional regulatory proteins, associated with PLN, that control SR Ca-transport. These include the inhibitor-1 (I-1) of protein phosphatase 1 (PP1), the small heat shock protein 20 (Hsp20) and the HS-1 associated protein X-1 (HAX1). In addition, the intra-luminal histidine-rich calcium binding protein (HRC) has been shown to interact with both SERCA2a and triadin. Notably, there is physical and direct interaction between these protein players, mediating a fine-cross talk between SR Ca-uptake, storage and release. Importantly, regulation of SR Ca-cycling by the PLN/SERCA interactome does not only impact cardiomyocyte contractility, but also survival and remodeling. Indeed, naturally occurring variants in these Ca-cycling genes modulate their activity and interactions with other protein partners, resulting in depressed contractility and accelerated remodeling. These genetic variants may serve as potential prognostic or diagnostic markers in cardiac pathophysiology.

Calcium handling proteins: structure, function, and modulation by exercise

Heart failure is a serious public health issue with a growing prevalence, and it is related with the aging of the population. Hypertension is identified as the main precursor of left ventricular hypertrophy and therefore can lead to diastolic dysfunction and heart failure. Scientific studies have confirmed the beneficial effects of the physical exercise by reducing the blood pressure and improving the functional status of the heart in hypertension. Several proteins are involved in the mobilization of calcium during the coupling excitation-contraction process in the heart among those are sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban, calsequestrin, sodium-calcium exchanger, L-type calcium's channel, and ryanodine receptors. Our goal is to address the beneficial effects of exercise on the calcium handling proteins in a heart with hypertension.

Postextrasystolic blood pressure potentiation predicts poor outcome of cardiac patients

Background

Postextrasystolic blood pressure potentiation (PESP), the pulse wave augmentation after an extrasystolic beat, is typically enhanced in heart failure (HF) patients. This study prospectively tested the association of PESP and mortality in cardiac patients.

Methods and Results

Consecutive patients (n=941; mean age, 61 years; 19% female) presenting with acute myocardial infarction were enrolled between May 2000 and March 2005 and followed up until August 2010. The main study outcome was 5‐year all‐cause mortality. Patients underwent noninvasive 30‐minute recordings of ECG and continuous blood pressure. PESP presence was based on the ratio between the first postectopic pulse wave amplitude and the mean of the subsequent 9 pulse wave amplitudes. A ratio above 1 was prospectively defined as PESP present. Ventricular premature complexes (VPCs) suitable for PESP quantification were present in recordings of 220 patients. PESP was present in 62 of these patients. Patients without suitable VPCs were classified as PESP absent.

During the follow‐up, 72 patients died. Among the 220 patients in whom PESP was measurable, 27 died. Under univariable analysis, PESP was a significant predictor of death (P<0.001) as were GRACE score (P<0.001), left ventricular ejection fraction (LVEF) (P<0.001), and the number of recorded VPCs (P<0.001). Under multivariable analysis, PESP (P<0.001), GRACE score (P<0.001), and LVEF (P=0.001) were independently associated with outcome. The combination of PESP presence and LVEF ≤35% identified a subgroup of patients with a particularly high mortality of 46.7%. Separate validation reproduced the finding in an unrelated population of 146 HF patients.

Conclusions

PESP, which likely reflects abnormalities of myocardial calcium cycling, predicts the mortality risk in postinfarction patients.

Clinical Trial Registration

URL: ClinicalTrials.gov. Unique identifier: NCT00196274.

Nitric oxide synthase regulation of cardiac excitation-contraction coupling in health and disease

Significant advances in our understanding of the ability of nitric oxide synthases (NOS) to modulate cardiac function have provided key insights into the role NOS play in the regulation of excitation-contraction (EC) coupling in health and disease. Through both cGMP-dependent and cGMP-independent (e.g. S-nitrosylation) mechanisms, NOS have the ability to alter intracellular Ca(2+) handling and the myofilament response to Ca(2+), thereby impacting the systolic and diastolic performance of the myocardium. Findings from experiments using nitric oxide (NO) donors and NOS inhibition or gene deletion clearly implicate dysfunctional NOS as a critical contributor to many cardiovascular disease states. However, studies to date have only partially addressed NOS isoform-specific effects and, more importantly, how subcellular localization of NOS influences ion channels involved in myocardial EC coupling and excitability. In this review, we focus on the contribution of each NOS isoform to cardiac dysfunction and on the role of uncoupled NOS activity in common cardiac disease states, including heart failure, diabetic cardiomyopathy, ischemia/reperfusion injury and atrial fibrillation. We also review evidence that clearly indicates the importance of NO in cardioprotection. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

The role of CaMKII regulation of phospholamban activity in heart disease

Phospholamban (PLN) is a phosphoprotein in cardiac sarcoplasmic reticulum (SR) that is a reversible regulator of the Ca²⁺-ATPase (SERCA2a) activity and cardiac contractility. Dephosphorylated PLN inhibits SERCA2a and PLN phosphorylation, at either Ser¹⁶ by PKA or Thr¹⁷ by Ca²⁺-calmodulin-dependent protein kinase (CaMKII), reverses this inhibition. Through this mechanism, PLN is a key modulator of SR Ca²⁺ uptake, Ca²⁺ load, contractility, and relaxation. PLN phosphorylation is also the main determinant of β1-adrenergic responses in the heart. Although phosphorylation of Thr¹⁷ by CaMKII contributes to this effect, its role is subordinate to the PKA-dependent increase in cytosolic Ca²⁺, necessary to activate CaMKII. Furthermore, the effects of PLN and its phosphorylation on cardiac function are subject to additional regulation by its interacting partners, the anti-apoptotic HAX-1 protein and Gm or the anchoring unit of protein phosphatase 1. Regulation of PLN activity by this multimeric complex becomes even more important in pathological conditions, characterized by aberrant Ca²⁺-cycling. In this scenario, CaMKII-dependent PLN phosphorylation has been associated with protective effects in both acidosis and ischemia/reperfusion. However, the beneficial effects of increasing SR Ca²⁺ uptake through PLN phosphorylation may be lost or even become deleterious, when these occur in association with alterations in SR Ca²⁺ leak. Moreover, a major characteristic in human and experimental heart failure (HF) is depressed SR Ca²⁺ uptake, associated with decreased SERCA2a levels and dephosphorylation of PLN, leading to decreased SR Ca²⁺ load and impaired contractility. Thus, the strategy of altering SERCA2a and/or PLN levels or activity to restore perturbed SR Ca²⁺ uptake is a potential therapeutic tool for HF treatment. We will review here the role of CaMKII-dependent phosphorylation of PLN at Thr¹⁷ on cardiac function under physiological and pathological conditions.

SERCA2a: a prime target for modulation of cardiac contractility during heart failure

Heart failure is one of the leading causes of sudden death in developed countries. While current therapies are mostly aimed at mitigating associated symptoms, novel therapies targeting the subcellular mechanisms underlying heart failure are emerging. Failing hearts are characterized by reduced contractile properties caused by impaired Ca(2+) cycling between the sarcoplasm and sarcoplasmic reticulum (SR). Sarcoplasmic/ endoplasmic reticulum Ca(2+)ATPase 2a (SERCA2a) mediates Ca(2+) reuptake into the SR in cardiomyocytes. Of note, the expression level and/or activity of SERCA2a, translating to the quantity of SR Ca(2+) uptake, are significantly reduced in failing hearts. Normalization of the SERCA2a expression level by gene delivery has been shown to restore hampered cardiac functions and ameliorate associated symptoms in pre-clinical as well as clinical studies. SERCA2a activity can be regulated at multiple levels of a signaling cascade comprised of phospholamban, protein phosphatase 1, inhibitor-1, and PKCα. SERCA2 activity is also regulated by post-translational modifications including SUMOylation and acetylation. In this review, we will highlight the molecular mechanisms underlying the regulation of SERCA2a activity and the potential therapeutic modalities for the treatment of heart failure.

Calcium pumps: why so many?

Ca(2+)-ATPases (pumps) are key to the regulation of Ca(2+) in eukaryotic cells: nine are known today, belonging to three multigene families. The three endo(sarco)plasmic reticulum (SERCA) and the four plasma membrane (PMCA) pumps have been known for decades, the two Secretory Pathway Ca(2+) ATPase (SPCA) pumps have only become known recently. The number of pump isoforms is further increased by alternative splicing processes. The three pump types share the basic features of the catalytic mechanism, but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca(2+). The molecular understanding of the function of all pumps has received great impetus from the solution of the three-dimensional (3D) structure of one of them, the SERCA pump. This landmark structural advance has been accompanied by the emergence and rapid expansion of the area of pump malfunction. Most of the pump defects described so far are genetic and produce subtler, often tissue and isoform specific, disturbances that affect individual components of the Ca(2+)-controlling and/or processing machinery, compellingly indicating a specialized role for each Ca(2+) pump type and/or isoform.

ß-blocker timolol prevents arrhythmogenic Ca²⁺ release and normalizes Ca²⁺ and Zn²⁺ dyshomeostasis in hyperglycemic rat heart

Defective cardiac mechanical activity in diabetes results from alterations in intracellular Ca(2+) handling, in part, due to increased oxidative stress. Beta-blockers demonstrate marked beneficial effects in heart dysfunction with scavenging free radicals and/or acting as an antioxidant. The aim of this study was to address how β-blocker timolol-treatment of diabetic rats exerts cardioprotection. Timolol-treatment (12-week), one-week following diabetes induction, prevented diabetes-induced depressed left ventricular basal contractile activity, prolonged cellular electrical activity, and attenuated the increase in isolated-cardiomyocyte size without hyperglycemic effect. Both in vivo and in vitro timolol-treatment of diabetic cardiomyocytes prevented the altered kinetic parameters of Ca(2+) transients and reduced Ca(2+) loading of sarcoplasmic reticulum (SR), basal intracellular free Ca(2+) and Zn(2+) ([Ca(2+)]i and [Zn(2+)]i), and spatio-temporal properties of the Ca(2+) sparks, significantly. Timolol also antagonized hyperphosphorylation of cardiac ryanodine receptor (RyR2), and significantly restored depleted protein levels of both RyR2 and calstabin2. Western blot analysis demonstrated that timolol-treatment also significantly normalized depressed levels of some [Ca(2+)]i-handling regulators, such as Na(+)/Ca(2+) exchanger (NCX) and phospho-phospholamban (pPLN) to PLN ratio. Incubation of diabetic cardiomyocytes with 4-mM glutathione exerted similar beneficial effects on RyR2-macromolecular complex and basal levels of both [Ca(2+)]i and [Zn(2+)]i, increased intracellular Zn(2+) hyperphosphorylated RyR2 in a concentration-dependent manner. Timolol also led to a balanced oxidant/antioxidant level in both heart and circulation and prevented altered cellular redox state of the heart. We thus report, for the first time, that the preventing effect of timolol, directly targeting heart, seems to be associated with a normalization of macromolecular complex of RyR2 and some Ca(2+) handling regulators, and prevention of Ca(2+) leak, and thereby normalization of both [Ca(2+)]i and [Zn(2+)]i homeostasis in diabetic rat heart, at least in part by controlling the cellular redox status of hyperglycemic cardiomyocytes.

Carvedilol prevents ovariectomy-induced myocardial contractile dysfunction in female rat

Carvedilol has beneficial effects on cardiac function in patients with heart failure but its effect on ovariectomy-induced myocardial contractile dysfunction remains unclear. Estrogen deficiency induces myocardial contractile dysfunction and increases cardiovascular disease risk in postmenopausal women. Our aim was to investigate whether carvedilol, a beta receptor blocker, would prevent ovariectomy-induced myocardial contractile dysfunction. Female rats (8 weeks old) that underwent bilateral ovariectomy were randomly assigned to receive daily treatment with carvedilol (OVX+CAR, 20 mg/kg), placebo (OVX) and SHAM for 58 days. Left ventricle papillary muscle was mounted for isometric tension recordings. The inotropic response to Ca²⁺ (0.62 to 3.75 mM) and isoproterenol (Iso 10⁻⁸ to 10⁻² M) were assessed. Expression of calcium handling proteins was measured by western blot analysis. Carvedilol treatment in the OVX animals: prevented weight gain and slight hypertrophy, restored the reduced positive inotropic responses to Ca²⁺ and isoproterenol, prevented the reduction in SERCA2a expression, abolished the increase in superoxide anion production, normalized the increase in p22^phox expression, and decreased serum angiotensin converting enzyme (ACE) activity. This study demonstrated that myocardial contractile dysfunction and SERCA2a down regulation were prevented by carvedilol treatment. Superoxide anion production and NADPH oxidase seem to be involved in this response.