Endothelin-1-Stimulated InsP3-Induced Ca2+ Release Is a Nexus for Hypertrophic Signaling in Cardiac Myocytes

Phospholipase C epsilon 1 as a therapeutic target in cardiovascular diseases

BACKGROUND

Phospholipase C epsilon 1 (PLCε1) can hydrolyze phosphatidylinositol-4,5-bisphosphate and phosphatidylinositol-4-phosphate at the plasma membrane and perinuclear membrane in the cardiovascular system, producing lipid-derived second messengers. These messengers are considered prominent triggers for various signal transduction processes. Notably, diverse cardiac phenotypes have been observed in cardiac-specific and global Plce1 knockout mice under conditions of pathological stress. It is well established that the cardiac-specific Plce1 knockout confers cardioprotective benefits. Therefore, the development of tissue/cell-specific targeting approaches is critical for advancing therapeutic interventions.

AIM OF REVIEW

This review aims to distill the foundational biology and functional significance of PLCε1 in cardiovascular diseases, as well as to explore potential avenues for research and the development of novel therapeutic strategies targeting PLCε1.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Cardiovascular diseases remain the leading cause of morbidity and mortality worldwide, with incidence rates escalating annually. A comprehensive understanding of the multifaceted role of PLCε1 is essential for enhancing the diagnosis, management, and prognostic assessment of patients suffering from cardiovascular diseases.

Catecholamine treatment induces reversible heart injury and cardiomyocyte gene expression

BACKGROUND

Catecholamines are commonly used as therapeutic drugs in intensive care medicine to maintain sufficient organ perfusion during shock. However, excessive or sustained adrenergic activation drives detrimental cardiac remodeling and may lead to heart failure. Whether catecholamine treatment in absence of heart failure causes persistent cardiac injury, is uncertain. In this experimental study, we assessed the course of cardiac remodeling and recovery during and after prolonged catecholamine treatment and investigated the molecular mechanisms involved.

RESULTS

C57BL/6N wild-type mice were assigned to 14 days catecholamine treatment with isoprenaline and phenylephrine (IsoPE), treatment with IsoPE and subsequent recovery, or healthy control groups. IsoPE improved left ventricular contractility but caused substantial cardiac fibrosis and hypertrophy. However, after discontinuation of catecholamine treatment, these alterations were largely reversible. To uncover the molecular mechanisms involved, we performed RNA sequencing from isolated cardiomyocyte nuclei. IsoPE treatment resulted in a transient upregulation of genes related to extracellular matrix formation and transforming growth factor signaling. While components of adrenergic receptor signaling were downregulated during catecholamine treatment, we observed an upregulation of endothelin-1 and its receptors in cardiomyocytes, indicating crosstalk between both signaling pathways. To follow this finding, we treated mice with endothelin-1. Compared to IsoPE, treatment with endothelin-1 induced minor but longer lasting changes in cardiomyocyte gene expression. DNA methylation-guided analysis of enhancer regions identified immediate early transcription factors such as AP-1 family members Jun and Fos as key drivers of pathological gene expression following catecholamine treatment.

CONCLUSIONS

The results from this study show that prolonged catecholamine exposure induces adverse cardiac remodeling and gene expression before the onset of left ventricular dysfunction which has implications for clinical practice. The observed changes depend on the type of stimulus and are largely reversible after discontinuation of catecholamine treatment. Crosstalk with endothelin signaling and the downstream transcription factors identified in this study provide new opportunities for more targeted therapeutic approaches that may help to separate desired from undesired effects of catecholamine treatment.

Calcium signalling pathways in prostate cancer initiation and progression

Cancer cells proliferate, differentiate and migrate by repurposing physiological signalling mechanisms. In particular, altered calcium signalling is emerging as one of the most widespread adaptations in cancer cells. Remodelling of calcium signalling promotes the development of several malignancies, including prostate cancer. Gene expression data from in vitro, in vivo and bioinformatics studies using patient samples and xenografts have shown considerable changes in the expression of various components of the calcium signalling toolkit during the development of prostate cancer. Moreover, preclinical and clinical evidence suggests that altered calcium signalling is a crucial component of the molecular re-programming that drives prostate cancer progression. Evidence points to calcium signalling re-modelling, commonly involving crosstalk between calcium and other cellular signalling pathways, underpinning the onset and temporal progression of this disease. Discrete alterations in calcium signalling have been implicated in hormone-sensitive, castration-resistant and aggressive variant forms of prostate cancer. Hence, modulation of calcium signals and downstream effector molecules is a plausible therapeutic strategy for both early and late stages of prostate cancer. Based on this premise, clinical trials have been undertaken to establish the feasibility of targeting calcium signalling specifically for prostate cancer.

Nuclear Calcium in Cardiac (Patho)Physiology: Small Compartment, Big Impact

The nucleus of a cardiomyocyte has been increasingly recognized as a morphologically distinct and partially independent calcium (Ca²⁺) signaling microdomain, with its own Ca²⁺-regulatory mechanisms and important effects on cardiac gene expression. In this review, we (1) provide a comprehensive overview of the current state of research on the dynamics and regulation of nuclear Ca²⁺ signaling in cardiomyocytes, (2) address the role of nuclear Ca²⁺ in the development and progression of cardiac pathologies, such as heart failure and atrial fibrillation, and (3) discuss novel aspects of experimental methods to investigate nuclear Ca²⁺ handling and its downstream effects in the heart. Finally, we highlight current challenges and limitations and recommend future directions for addressing key open questions.

IP3R activity increases propensity of RyR-mediated sparks by elevating dyadic [Ca2＋]

Calcium (Ca^2＋) plays a critical role in the excitation contraction coupling (ECC) process that mediates the contraction of cardiomyocytes during each heartbeat. While ryanodine receptors (RyRs) are the primary Ca^2＋ channels responsible for generating the cell-wide Ca^2＋ transients during ECC, Ca^2＋ release, via inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) are also reported in cardiomyocytes to elicit ECC-modulating effects. Recent studies suggest that the localization of IP₃Rs at dyads grant their ability to modify the occurrence of Ca^2＋ sparks (elementary Ca^2＋ release events that constitute cell wide Ca^2＋ releases associated with ECC) which may underlie their modulatory influence on ECC. Here, we aim to uncover the mechanism by which dyad-localized IP₃Rs influence Ca^2＋ spark dynamics. To this end, we developed a mathematical model of the dyad that incorporates the behaviour of IP₃Rs, in addition to RyRs, to reveal the impact of their activity on local Ca^2＋ handling and consequent Ca^2＋ spark occurrence and its properties. Consistent with published experimental data, our model predicts that the propensity for Ca^2＋ spark formation increases in the presence of IP₃R activity. Our simulations support the hypothesis that IP₃Rs elevate Ca^2＋ in the dyad, sensitizing proximal RyRs towards activation and hence Ca^2＋ spark formation. The stochasticity of IP₃R gating is an important aspect of this mechanism. However, dyadic IP₃R activity lowers the Ca^2＋ available in the junctional sarcoplasmic reticulum (JSR) for release, thus resulting in Ca^2＋ sparks with similar durations but lower amplitudes.

The thromboxane receptor antagonist NTP42 promotes beneficial adaptation and preserves cardiac function in experimental models of right heart overload

Background

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by increased pulmonary artery pressure leading to right ventricular (RV) failure. While current PAH therapies improve patient outlook, they show limited benefit in attenuating RV dysfunction. Recent investigations demonstrated that the thromboxane (TX) A₂ receptor (TP) antagonist NTP42 attenuates experimental PAH across key hemodynamic parameters in the lungs and heart. This study aimed to validate the efficacy of NTP42:KVA4, a novel oral formulation of NTP42 in clinical development, in preclinical models of PAH while also, critically, investigating its direct effects on RV dysfunction.

Methods

The effects of NTP42:KVA4 were evaluated in the monocrotaline (MCT) and pulmonary artery banding (PAB) models of PAH and RV dysfunction, respectively, and when compared with leading standard-of-care (SOC) PAH drugs. In addition, the expression of the TP, the target for NTP42, was investigated in cardiac tissue from several other related disease models, and from subjects with PAH and dilated cardiomyopathy (DCM).

Results

In the MCT-PAH model, NTP42:KVA4 alleviated disease-induced changes in cardiopulmonary hemodynamics, pulmonary vascular remodeling, inflammation, and fibrosis, to a similar or greater extent than the PAH SOCs tested. In the PAB model, NTP42:KVA4 improved RV geometries and contractility, normalized RV stiffness, and significantly increased RV ejection fraction. In both models, NTP42:KVA4 promoted beneficial RV adaptation, decreasing cellular hypertrophy, and increasing vascularization. Notably, elevated expression of the TP target was observed both in RV tissue from these and related disease models, and in clinical RV specimens of PAH and DCM.

Conclusion

This study shows that, through antagonism of TP signaling, NTP42:KVA4 attenuates experimental PAH pathophysiology, not only alleviating pulmonary pathologies but also reducing RV remodeling, promoting beneficial hypertrophy, and improving cardiac function. The findings suggest a direct cardioprotective effect for NTP42:KVA4, and its potential to be a disease-modifying therapy in PAH and other cardiac conditions.

Inositol 1,4,5-trisphosphate receptors in cardiomyocyte physiology and disease

The contraction of cardiac muscle underlying the pumping action of the heart is mediated by the process of excitation-contraction coupling (ECC). While triggered by Ca²⁺ entry across the sarcolemma during the action potential, it is the release of Ca²⁺ from the sarcoplasmic reticulum (SR) intracellular Ca²⁺ store via ryanodine receptors (RyRs) that plays the major role in induction of contraction. Ca²⁺ also acts as a key intracellular messenger regulating transcription underlying hypertrophic growth. Although Ca²⁺ release via RyRs is by far the greatest contributor to the generation of Ca²⁺ transients in the cardiomyocyte, Ca²⁺ is also released from the SR via inositol 1,4,5-trisphosphate (InsP₃) receptors (InsP₃Rs). This InsP₃-induced Ca²⁺ release modifies Ca²⁺ transients during ECC, participates in directing Ca²⁺ to the mitochondria, and stimulates the transcription of genes underlying hypertrophic growth. Central to these specific actions of InsP₃Rs is their localization to responsible signalling microdomains, the dyad, the SR-mitochondrial interface and the nucleus. In this review, the various roles of InsP₃R in cardiac (patho)physiology and the mechanisms by which InsP₃ signalling selectively influences the different cardiomyocyte cell processes in which it is involved will be presented.

This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

A perinuclear calcium compartment regulates cardiac myocyte hypertrophy

The pleiotropic Ca2+/calmodulin-dependent phosphatase calcineurin is a key regulator of pathological cardiac myocyte hypertrophy. The selective activation of hypertrophic calcineurin signaling under stress conditions has been attributed to compartmentation of Ca2+ signaling in cardiac myocytes. Here, perinuclear signalosomes organized by the scaffold protein muscle A-Kinase Anchoring Protein β (mAKAPβ/AKAP6β) are shown to orchestrate local Ca2+ transients, inducing calcineurin-dependent NFATc nuclear localization and myocyte hypertrophy in response to β-adrenergic receptor activation. Fluorescent biosensors for Ca2+ and calcineurin and protein kinase A (PKA) activity, both diffusely expressed and localized by nesprin-1α to the nuclear envelope, are used to define an autonomous mAKAPβ signaling compartment in adult and neonatal rat ventricular myocytes. Notably, β-adrenergic-stimulated perinuclear Ca2+ and PKA and CaN activity transients depended upon mAKAPβ expression, while Ca2+ elevation and PKA and CaN activity in the cytosol were mAKAPβ independent. Buffering perinuclear cAMP and Ca2+ prevented calcineurin-dependent NFATc nuclear translocation and myocyte hypertrophy, without affecting cardiac myocyte contractility. Additional findings suggest that the perinuclear Ca2+ transients were mediated by signalosome-associated ryanodine receptors regulated by local PKA phosphorylation. These results demonstrate the existence of a functionally independent Ca2+ signaling compartment in the cardiac myocyte regulating hypertrophy and provide a premise for targeting mAKAPβ signalosomes to prevent selectively cardiac hypertrophy in disease.

The Nuclear Envelope as a Regulator of Immune Cell Function

The traditional view of the nuclear envelope (NE) was that it represented a relatively inert physical barrier within the cell, whose main purpose was to separate the nucleoplasm from the cytoplasm. However, recent research suggests that this is far from the case, with new and important cellular functions being attributed to this organelle. In this review we describe research suggesting an important contribution of the NE and its constituents in regulating the functions of cells of the innate and adaptive immune system. One of the standout properties of immune cells is their ability to migrate around the body, allowing them to carry out their physiological/pathophysiology cellular role at the appropriate location. This together with the physiological role of the tissue, changes in tissue matrix composition due to disease and aging, and the activation status of the immune cell, all result in immune cells being subjected to different mechanical forces. We report research which suggests that the NE may be an important sensor/transducer of these mechanical signals and propose that the NE is an integrator of both mechanical and chemical signals, allowing the cells of the innate immune system to precisely regulate gene transcription and functionality. By presenting this overview we hope to stimulate the interests of researchers into this often-overlooked organelle and propose it should join the ranks of mitochondria and phagosome, which are important organelles contributing to immune cell function.

The Cell Physiome: What Do We Need in a Computational Physiology Framework for Predicting Single-Cell Biology?

Modern biology and biomedicine are undergoing a big data explosion, needing advanced computational algorithms to extract mechanistic insights on the physiological state of living cells. We present the motivation for the Cell Physiome project: a framework and approach for creating, sharing, and using biophysics-based computational models of single-cell physiology. Using examples in calcium signaling, bioenergetics, and endosomal trafficking, we highlight the need for spatially detailed, biophysics-based computational models to uncover new mechanisms underlying cell biology. We review progress and challenges to date toward creating cell physiome models. We then introduce bond graphs as an efficient way to create cell physiome models that integrate chemical, mechanical, electromagnetic, and thermal processes while maintaining mass and energy balance. Bond graphs enhance modularization and reusability of computational models of cells at scale. We conclude with a look forward at steps that will help fully realize this exciting new field of mechanistic biomedical data science. Expected final online publication date for the Annual Review of Biomedical Data Science, Volume 5 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

MSK-Mediated Phosphorylation of Histone H3 Ser28 Couples MAPK Signalling with Early Gene Induction and Cardiac Hypertrophy

Heart failure is a leading cause of death that develops subsequent to deleterious hypertrophic cardiac remodelling. MAPK pathways play a key role in coordinating the induction of gene expression during hypertrophy. Induction of the immediate early gene (IEG) response including activator protein 1 (AP-1) complex factors is a necessary and early event in this process. How MAPK and IEG expression are coupled during cardiac hypertrophy is not resolved. Here, in vitro, in rodent models and in human samples, we demonstrate that MAPK-stimulated IEG induction depends on the mitogen and stress-activated protein kinase (MSK) and its phosphorylation of histone H3 at serine 28 (pH3S28). pH3S28 in IEG promoters in turn recruits Brg1, a BAF60 ATP-dependent chromatin remodelling complex component, initiating gene expression. Without MSK activity and IEG induction, the hypertrophic response is suppressed. These studies provide new mechanistic insights into the role of MAPK pathways in signalling to the epigenome and regulation of gene expression during cardiac hypertrophy.

Measuring hypertrophy in neonatal rat primary cardiomyocytes and human iPSC-derived cardiomyocytes

In the heart, left ventricular hypertrophy is initially an adaptive mechanism that increases wall thickness to preserve normal cardiac output and function in the face of coronary artery disease or hypertension. Cardiac hypertrophy develops in response to pressure and volume overload but can also be seen in inherited cardiomyopathies. As the wall thickens, it becomes stiffer impairing the distribution of oxygenated blood to the rest of the body. With complex cellular signalling and transcriptional networks involved in the establishment of the hypertrophic state, several model systems have been developed to better understand the molecular drivers of disease. Immortalized cardiomyocyte cell lines, primary rodent and larger animal models have all helped understand the pathological mechanisms underlying cardiac hypertrophy. Induced pluripotent stem cell-derived cardiomyocytes are also used and have the additional benefit of providing access to human samples with direct disease relevance as when generated from patients suffering from hypertrophic cardiomyopathies. Here, we briefly review in vitro and in vivo model systems that have been used to model hypertrophy and provide detailed methods to isolate primary neonatal rat cardiomyocytes as well as to generate cardiomyocytes from human iPSCs. We also describe how to model hypertrophy in a "dish" using gene expression analysis and immunofluorescence combined with automated high-content imaging.

IGFBP-4 Proteolysis by PAPP-A in a Primary Culture of Rat Neonatal Cardiomyocytes under Normal and Hypertrophic Conditions

Cardiovascular diseases (CVD) are among the leading causes of death and disability worldwide. Pregnancy-associated plasma protein-A (PAPP-A) is a matrix metalloprotease localized on the cell surface. One of the substrates that PAPP-A cleaves is the insulin-like growth factor binding protein-4 (IGFBP-4), a member of the family of proteins that bind insulin-like growth factor (IGF). Proteolysis of IGFBP-4 by PAPP-A occurs at a specific site resulting in formation of two proteolytic fragments - N-terminal IGFBP-4 (NT-IGFBP-4) and C-terminal IGFBP-4 (CT-IGFBP-4), and leads to the release of IGF activating various cellular processes including migration, proliferation, and cell growth. Increased levels of the proteolytic IGFBP-4 fragments correlate with the development of CVD complications and increased risk of death in patients with the coronary heart disease, acute coronary syndrome, and heart failure. However, there is no direct evidence that PAPP-A specifically cleaves IGFBP-4 in the cardiac tissue under normal and pathological conditions. In the present study, using a primary culture of rat neonatal cardiomyocytes as a model, we have demonstrated that: 1) proteolysis of IGFBP-4 by PAPP-A occurs in the conditioned medium of cardiomyocytes, 2) PAPP-A-specific IGFBP-4 proteolysis is increased when cardiomyocytes are transformed to a hypertrophic state. Thus, it can be assumed that the enhancement of IGFBP-4 cleavage by PAPP-A and hypertrophic changes in cardiomyocytes accompanying CVD are interrelated, and PAPP-A appears to be one of the activators of the IGF-dependent processes in normal and hypertrophic-state cardiomyocytes.

Effects and safety of Psilocybe cubensis and Panaeolus cyanescens magic mushroom extracts on endothelin-1-induced hypertrophy and cell injury in cardiomyocytes

Prevalence of major depression in people with chronic heart failure is higher than in normal populations. Depression in heart failure has become a major issue. Psilocybin-containing mushrooms commonly known as magic mushrooms, have been used since ancient times for their mind healing properties. Their safety in cardiovascular disease conditions is not fully known and may pose as a risk for users suffering from these illnesses. Study investigates the effects and safety of Psilocybe cubensis and Panaeolus cyanescens magic mushrooms use from genus Psilocybe and Panaeolus respectively, in a pathological hypertrophy conditions in which endothelin-1 disorder is a contributor to pathogenesis. We examined the effects of the mushrooms extracts on endothelin-1-induced hypertrophy and tumor necrosis factor-α (TNF- α)-induced cell injury in H9C2 cardiomyocytes. Mushrooms were oven dried and extracted with cold and boiling-hot water. H9C2 cardiomyocytes were induced with endothelin-1 prior to treatment with extracts over 48 h. Cell injury was stimulated with TNF-α. Results proposed that the water extracts of Panaeolus cyanescens and Psilocybe cubensis did not aggravate the pathological hypertrophy induced by endothelin-1 and also protected against the TNF-α-induced injury and cell death in concentrations used. Results support medicinal safe use of mushrooms under controlled conditions and cautioned use of higher concentrations.

Minimal contribution of IP3R2 in cardiac differentiation and derived ventricular-like myocytes from human embryonic stem cells

Type 2 inositol 1,4,5-trisphosphate receptor (IP₃R2) regulates the intracellular Ca²⁺ release from endoplasmic reticulum in human embryonic stem cells (hESCs), cardiovascular progenitor cells (CVPCs), and mammalian cardiomyocytes. However, the role of IP₃R2 in human cardiac development is unknown and its function in mammalian cardiomyocytes is controversial. hESC-derived cardiomyocytes have unique merits in disease modeling, cell therapy, and drug screening. Therefore, understanding the role of IP₃R2 in the generation and function of human cardiomyocytes would be valuable for the application of hESC-derived cardiomyocytes. In the current study, we investigated the role of IP₃R2 in the differentiation of hESCs to cardiomyocytes and in the hESC-derived cardiomyocytes. By using IP₃R2 knockout (IP₃R2KO) hESCs, we showed that IP₃R2KO did not affect the self-renewal of hESCs as well as the differentiation ability of hESCs into CVPCs and cardiomyocytes. Furthermore, we demonstrated the ventricular-like myocyte characteristics of hESC-derived cardiomyocytes. Under the α₁-adrenergic stimulation by phenylephrine (10 μmol/L), the amplitude and maximum rate of depolarization of action potential (AP) were slightly affected in the IP₃R2KO hESC-derived cardiomyocytes at differentiation day 90, whereas the other parameters of APs and the Ca²⁺ transients did not show significant changes compared with these in the wide-type ones. These results demonstrate that IP₃R2 has minimal contribution to the differentiation and function of human cardiomyocytes derived from hESCs, thus provide the new knowledge to the function of IP₃R2 in the generation of human cardiac lineage cells and in the early cardiomyocytes.

Cardiac CaMKIIδ and Wenxin Keli Prevents Ang II-Induced Cardiomyocyte Hypertrophy by Modulating CnA-NFATc4 and Inflammatory Signaling Pathways in H9c2 Cells

Previous studies have demonstrated that calcium-/calmodulin-dependent protein kinase II (CaMKII) and calcineurin A-nuclear factor of activated T-cell (CnA-NFAT) signaling pathways play key roles in cardiac hypertrophy (CH). However, the interaction between CaMKII and CnA-NFAT signaling remains unclear. H9c2 cells were cultured and treated with angiotensin II (Ang II) with or without silenced CaMKIIδ (siCaMKII) and cyclosporine A (CsA, a calcineurin inhibitor) and subsequently treated with Wenxin Keli (WXKL). Patch clamp recording was conducted to assess L-type Ca²⁺ current (I_Ca-L), and the expression of proteins involved in signaling pathways was measured by western blotting. Myocardial cytoskeletal protein and nuclear translocation of target proteins were assessed by immunofluorescence. The results indicated that siCaMKII suppressed Ang II-induced CH, as evidenced by reduced cell surface area and I_Ca-L. Notably, siCaMKII inhibited Ang II-induced activation of CnA and NFATc4 nuclear transfer. Inflammatory signaling was inhibited by siCaMKII and WXKL. Interestingly, CsA inhibited CnA-NFAT pathway expression but activated CaMKII signaling. In conclusion, siCaMKII may improve CH, possibly by blocking CnA-NFAT and MyD88 signaling, and WXKL has a similar effect. These data suggest that inhibiting CaMKII, but not CnA, may be a promising approach to attenuate CH and arrhythmia progression.

Ca2+ Release via IP3 Receptors Shapes the Cardiac Ca2+ Transient for Hypertrophic Signaling

Calcium (Ca²⁺) plays a central role in mediating both contractile function and hypertrophic signaling in ventricular cardiomyocytes. L-type Ca²⁺ channels trigger release of Ca²⁺ from ryanodine receptors for cellular contraction, whereas signaling downstream of G-protein-coupled receptors stimulates Ca²⁺ release via inositol 1,4,5-trisphosphate receptors (IP₃Rs), engaging hypertrophic signaling pathways. Modulation of the amplitude, duration, and duty cycle of the cytosolic Ca²⁺ contraction signal and spatial localization have all been proposed to encode this hypertrophic signal. Given current knowledge of IP₃Rs, we develop a model describing the effect of functional interaction (cross talk) between ryanodine receptor and IP₃R channels on the Ca²⁺ transient and examine the sensitivity of the Ca²⁺ transient shape to properties of IP₃R activation. A key result of our study is that IP₃R activation increases Ca²⁺ transient duration for a broad range of IP₃R properties, but the effect of IP₃R activation on Ca²⁺ transient amplitude is dependent on IP₃ concentration. Furthermore we demonstrate that IP₃-mediated Ca²⁺ release in the cytosol increases the duty cycle of the Ca²⁺ transient, the fraction of the cycle for which [Ca²⁺] is elevated, across a broad range of parameter values and IP₃ concentrations. When coupled to a model of downstream transcription factor (NFAT) activation, we demonstrate that there is a high correspondence between the Ca²⁺ transient duty cycle and the proportion of activated NFAT in the nucleus. These findings suggest increased cytosolic Ca²⁺ duty cycle as a plausible mechanism for IP₃-dependent hypertrophic signaling via Ca²⁺-sensitive transcription factors such as NFAT in ventricular cardiomyocytes.

The nuclear envelope: LINCing tissue mechanics to genome regulation in cardiac and skeletal muscle

Regulation of the genome is viewed through the prism of gene expression, DNA replication and DNA repair as controlled through transcription, chromatin compartmentalisation and recruitment of repair factors by enzymes such as DNA polymerases, ligases, acetylases, methylases and cyclin-dependent kinases. However, recent advances in the field of muscle cell physiology have also shown a compelling role for 'outside-in' biophysical control of genomic material through mechanotransduction. The crucial hub that transduces these biophysical signals is called the Linker of Nucleoskeleton and Cytoskeleton (LINC). This complex is embedded across the nuclear envelope, which separates the nucleus from the cytoplasm. How the LINC complex operates to mechanically regulate the many functions of DNA is becoming increasingly clear, and recent advances have provided exciting insight into how this occurs in cells from mechanically activated tissues such as skeletal and cardiac muscle. Nevertheless, there are still some notable shortcomings in our understanding of these processes and resolving these will likely help us understand how muscle diseases manifest at the level of the genome.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Fundamentals of Cellular Calcium Signaling: A Primer

Ionized calcium (Ca²⁺) is the most versatile cellular messenger. All cells use Ca²⁺ signals to regulate their activities in response to extrinsic and intrinsic stimuli. Alterations in cellular Ca²⁺ signaling and/or Ca²⁺ homeostasis can subvert physiological processes into driving pathological outcomes. Imaging of living cells over the past decades has demonstrated that Ca²⁺ signals encode information in their frequency, kinetics, amplitude, and spatial extent. These parameters alter depending on the type and intensity of stimulation, and cellular context. Moreover, it is evident that different cell types produce widely varying Ca²⁺ signals, with properties that suit their physiological functions. This primer discusses basic principles and mechanisms underlying cellular Ca²⁺ signaling and Ca²⁺ homeostasis. Consequently, we have cited some historical articles in addition to more recent findings. A brief summary of the core features of cellular Ca²⁺ signaling is provided, with particular focus on Ca²⁺ stores and Ca²⁺ transport across cellular membranes, as well as mechanisms by which Ca²⁺ signals activate downstream effector systems.

Examining Cardiomyocyte Dysfunction Using Acute Chemical Induction of an Ageing Phenotype

Much effort is focussed on understanding the structural and functional changes in the heart that underlie age-dependent deterioration of cardiac performance. Longitudinal studies, using aged animals, have pinpointed changes occurring to the contractile myocytes within the heart. However, whilst longitudinal studies are important, other experimental approaches are being advanced that can recapitulate the phenotypic changes seen during ageing. This study investigated the induction of an ageing cardiomyocyte phenotypic change by incubation of cells with hydroxyurea for several days ex vivo. Hydroxyurea incubation has been demonstrated to phenocopy age- and senescence-induced changes in neurons, but its utility for ageing studies with cardiac cells has not been examined. Incubation of neonatal rat ventricular myocytes with hydroxyurea for up to 7 days replicated specific aspects of cardiac ageing including reduced systolic calcium responses, increased alternans and a lesser ability of the cells to follow electrical pacing. Additional functional and structural changes were observed within the myocytes that pointed to ageing-like remodelling, including lipofuscin granule accumulation, reduced mitochondrial membrane potential, increased production of reactive oxygen species, and altered ultrastructure, such as mitochondria with disrupted cristae and disorganised myofibres. These data highlight the utility of alternative approaches for exploring cellular ageing whilst avoiding the costs and co-morbid factors that can affect longitudinal studies.

Crosstalk between FGF23- and angiotensin II-mediated Ca2+ signaling in pathological cardiac hypertrophy

Heart failure (HF) manifestation and progression are driven by systemic activation of neuroendocrine signaling cascades, such as the renin-angiotensin aldosterone system (RAAS). Fibroblast growth factor 23 (FGF23), an endocrine hormone, is linked to HF and cardiovascular mortality. It is also a mediator of left-ventricular hypertrophy (LVH). In vivo, high circulating levels of FGF23 are associated with an altered systemic RAAS response. FGF23 is proposed to trigger pathological signaling mediated by Ca2+-regulated transcriptional pathways. In the present study, we investigated Ca2+-dependent signaling of FGF23 in ventricular cardiomyocytes and its association with angiotensin II (ATII). In neonatal rat ventricular myocytes (NRVMs), both ATII and FGF23 induced hypertrophy as observed by an increase in cell area and hypertrophic gene expression. Furthermore, FGF23 activates nuclear Ca2+-regulated CaMKII-HDAC4 pathway, similar to ATII. In addition to a global increase in cytoplasmic Ca2+, FGF23, like ATII, induced inositol 1, 4, 5-triphosphate (IP3)-induced Ca2+ release from the nucleoplasmic Ca2+ store, associated with cellular hypertrophy. Interestingly, ATII receptor antagonist, losartan, significantly attenuated FGF23-induced changes in Ca2+ homeostasis and cellular hypertrophy suggesting an involvement of ATII receptor-mediated signaling. In addition, application of FGF23 increased intracellular expression of ATII peptide and its secretion in NRVMs, confirming the participation of ATII. In conclusion, FGF23 and ATII share a common mechanism of IP3-nuclear Ca2+-dependent cardiomyocyte hypertrophy. FGF23-mediated cellular hypertrophy is associated with increased production and secretion of ATII by cardiomyocytes. These findings indicate a pathophysiological role of the cellular angiotensin system in FGF23-induced hypertrophy in ventricular cardiomyocytes.

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.

Obstruction of ventricular Ca2+ -dependent arrhythmogenicity by inositol 1,4,5-trisphosphate-triggered sarcoplasmic reticulum Ca2+ release

KEY POINTS

Augmented inositol 1,4,5-trisphosphate (IP3 ) receptor (IP3 R2) expression has been linked to a variety of cardiac pathologies. Although cardiac IP3 R2 function has been in the focus of research for some time, a detailed understanding of its potential role in ventricular myocyte excitation-contraction coupling under pathophysiological conditions remains elusive. The present study focuses on mechanisms of IP3 R2-mediated sarcoplasmic reticulum (SR)-Ca2+ release in ventricular excitation-contraction coupling under IP3 R2-overexpressing conditions by studying intracellular Ca2+ events. We report that, upon IP3 R2 overexpression in ventricular myocytes, IP3 -induced Ca2+ release (IP3 ICR) modulates the SR-Ca2+ content via "eventless" SR-Ca2+ release, affecting the global SR-Ca2+ leak. Thus, IP3 R2 activation could act as a SR-Ca2+ gateway mechanism to escape ominous SR-Ca2+ overload. Our approach unmasks a so far unrecognized mechanism by which "eventless" IP3 ICR plays a protective role against ventricular Ca2+ -dependent arrhythmogenicity.

ABSTRACT

Augmented inositol 1,4,5-trisphosphate (IP3 ) receptor (IP3 R2) function has been linked to a variety of cardiac pathologies including cardiac arrhythmias. The functional role of IP3 -induced Ca2+ release (IP3 ICR) within ventricular excitation-contraction coupling (ECC) remains elusive. As part of pathophysiological cellular remodelling, IP3 R2s are overexpressed and have been repeatedly linked to enhanced Ca2+ -dependent arrhythmogenicity. In this study we test the hypothesis that an opposite scenario might be plausible in which IP3 ICR is part of an ECC protecting mechanism, resulting in a Ca2+ -dependent anti-arrhythmogenic response on the cellular scale. IP3 R2 activation was triggered via endothelin-1 or IP3 -salt application in single ventricular myocytes from a cardiac-specific IP3 R type 2 overexpressing mouse model. Upon IP3 R2 overexpression, IP3 R activation reduced Ca2+ -wave occurrence (46 vs. 21.72%; P < 0.001) while its block increased SR-Ca2+ content (∼29.4% 2-aminoethoxydiphenyl borate, ∼16.4% xestospongin C; P < 0.001), suggesting an active role of IP3 ICR in SR-Ca2+ content regulation and anti-arrhythmogenic function. Pharmacological separation of ryanodine receptor RyR2 and IP3 R2 functions and two-dimensional Ca2+ event analysis failed to identify local IP3 ICR events (Ca2+ puffs). SR-Ca2+ leak measurements revealed that under pathophysiological conditions, "eventless" SR-Ca2+ efflux via enhanced IP3 ICR maintains the SR-Ca2+ content below Ca2+ spark threshold, preventing aberrant SR-Ca2+ release and resulting in a protective mechanism against SR-Ca2+ overload and arrhythmias. Our results support a so far unrecognized modulatory mechanism in ventricular myocytes working in an anti-arrhythmogenic fashion.

Subcellular compartmentalization of proximal Gαq-receptor signaling produces unique hypertrophic phenotypes in adult cardiac myocytes

G protein-coupled receptors that signal through Gα_q (G_q receptors), such as α₁-adrenergic receptors (α₁-ARs) or angiotensin receptors, share a common proximal signaling pathway that activates phospholipase Cβ1 (PLCβ1), which cleaves phosphatidylinositol 4,5-bisphosphate (PIP₂) to produce inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol. Despite these common proximal signaling mechanisms, G_q receptors produce distinct physiological responses, yet the mechanistic basis for this remains unclear. In the heart, G_q receptors are thought to induce myocyte hypertrophy through a mechanism termed excitation-transcription coupling, which provides a mechanistic basis for compartmentalization of calcium required for contraction versus IP₃-dependent intranuclear calcium required for hypertrophy. Here, we identified subcellular compartmentalization of G_q-receptor signaling as a mechanistic basis for unique G_q receptor-induced hypertrophic phenotypes in cardiac myocytes. We show that α₁-ARs co-localize with PLCβ1 and PIP₂ at the nuclear membrane. Further, nuclear α₁-ARs induced intranuclear PLCβ1 activity, leading to histone deacetylase 5 (HDAC5) export and a robust transcriptional response (i.e. significant up- or down-regulation of 806 genes). Conversely, we found that angiotensin receptors localize to the sarcolemma and induce sarcolemmal PLCβ1 activity, but fail to promote HDAC5 nuclear export, while producing a transcriptional response that is mostly a subset of α₁-AR-induced transcription. In summary, these results link G_q-receptor compartmentalization in cardiac myocytes to unique hypertrophic transcription. They suggest a new model of excitation-transcription coupling in adult cardiac myocytes that accounts for differential G_q-receptor localization and better explains distinct physiological functions of G_q receptors.

Calcium Dynamics as a Machine for Decoding Signals

Calcium (Ca²⁺) is considered one of the most-important biological cations, because it is implicated in cell physiopathology and cell fate through a finely tuned signaling system. In support of this notion, Ca²⁺ is the primary driver of cell proliferation and cell growth; however, it is also intimately linked to cell death. Functional abnormalities or mutations in proteins that mediate Ca²⁺ homeostasis usually lead to a plethora of diseases and pathogenic states, including cancer, heart failure, diabetes, and neurodegenerative disease. In this review, we examine recent discoveries in the highly localized nature of Ca²⁺-dependent signal transduction and its roles in cell fate, inflammasome activation, and synaptic transmission.

Receptor- and cellular compartment-specific activation of the cAMP/PKA pathway by α1-adrenergic and ETA endothelin receptors

The signalling functions of many G protein-coupled receptors (GPCRs) expressed in the myocardium are incompletely understood. Among these are the endothelin receptor (ETR) family and α₁-adrenergic receptor (α₁-AR), which are thought to couple to the G protein Gαq. In this study, we used transcriptome analysis to compare the signalling networks downstream of these receptors in primary neonatal rat cardiomyocytes. This analysis indicated increased expression of target genes of cAMP responsive element modulator (CREM) after 24 h treatment with the α₁-AR agonist phenylephrine, but not the ETR agonist endothelin-1, suggesting a specific role for the α₁-AR in promoting cAMP production in cardiomyocytes. To validate the difference observed between these two GPCRs, we used heterologous expression of the receptors and genetically encoded biosensors in HEK 293 cell lines. We validated that both α_1A- and α_1B-AR subtypes were able to lead to the accumulation of cAMP in response to phenylephrine in both the nucleus and cytoplasm in a Gαs-dependent manner. However, the ETR subtype ETA did not affect cAMP levels in either compartment. All three receptors were coupled to Gαq signalling as expected. Further, we showed that activation of PKA in different compartments was α₁-AR subtype specific, with α_1B-AR able to activate PKA in the cytoplasm and nucleus and α_1A-AR only able to in the nucleus. We provide evidence for a pathway downstream of the α₁-AR, and show that distinct pools of a receptor lead to differential activation of downstream effector proteins dependent on their cellular compartment.

Vitamin D, reactive oxygen species and calcium signalling in ageing and disease

Vitamin D is a hormone that maintains healthy cells. It functions by regulating the low resting levels of cell signalling components such as Ca(2+) and reactive oxygen species (ROS). Its role in maintaining phenotypic stability of these signalling pathways depends on the ability of vitamin D to control the expression of those components that act to reduce the levels of both Ca(2+) and ROS. This regulatory role of vitamin D is supported by both Klotho and Nrf2. A decline in the vitamin D/Klotho/Nrf2 regulatory network may enhance the ageing process, and this is well illustrated by the age-related decline in cognition in rats that can be reversed by administering vitamin D. A deficiency in vitamin D has also been linked to two of the major diseases in man: heart disease and Alzheimer's disease (AD). In cardiac cells, this deficiency alters the Ca(2+) transients to activate the gene transcriptional events leading to cardiac hypertrophy and the failing heart. In the case of AD, it is argued that vitamin D deficiency results in the Ca(2+) landscape that initiates amyloid formation, which then elevates the resting level of Ca(2+) to drive the memory loss that progresses to neuronal cell death and dementia.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Functionally redundant control of cardiac hypertrophic signaling by inositol 1,4,5-trisphosphate receptors

Calcium plays an integral role to many cellular processes including contraction, energy metabolism, gene expression, and cell death. The inositol 1, 4, 5-trisphosphate receptor (IP₃R) is a calcium channel expressed in cardiac tissue. There are three IP₃R isoforms encoded by separate genes. In the heart, the IP₃R-2 isoform is reported to being most predominant with regards to expression levels and functional significance. The functional roles of IP₃R-1 and IP₃R-3 in the heart are essentially unexplored despite measureable expression levels. Here we show that all three IP₃Rs isoforms are expressed in both neonatal and adult rat ventricular cardiomyocytes, and in human heart tissue. The three IP₃R proteins are expressed throughout the cardiomyocyte sarcoplasmic reticulum. Using isoform specific siRNA, we found that expression of all three IP₃R isoforms are required for hypertrophic signaling downstream of endothelin-1 stimulation. Mechanistically, IP₃Rs specifically contribute to activation of the hypertrophic program by mediating the positive inotropic effects of endothelin-1 and leading to downstream activation of nuclear factor of activated T-cells. Our findings highlight previously unidentified functions for IP₃R isoforms in the heart with specific implications for hypertrophic signaling in animal models and in human disease.

Endothelin-1 promotes hypertrophic remodelling of cardiac myocytes by activating sustained signalling and transcription downstream of endothelin type A receptors

G-protein coupled receptor (GPCR) mediated activation of the MAPK signalling cascade is a key pathway in the induction of hypertrophic remodelling of the heart – a response to pathological cues including hypertension and myocardial infarction. While levels of pro-hypertrophic hormone agonists of GPCRs increase during periods of greater workload to enhance cardiac output, hypertrophy does not necessarily result. Here we investigated the relationship between the duration of exposure to the pro-hypertrophic GPCR agonist endothelin-1 (ET-1) and the induction of hypertrophic remodelling in neonatal rat ventricular myocytes (NRVM) and in the adult rat heart in vivo. Notably, a 15 min pulse of ET-1 was sufficient to induce markers of hypertrophy that were present when measured at 24 h in vivo and 48 h in vitro. The persistence of ET-1 action was insensitive to ET type A receptor (ET_A receptor) antagonism with BQ123. The extended effects of ET-1 were dependent upon sustained MAPK signalling and involved persistent transcription. Inhibitors of endocytosis however conferred sensitivity upon the hypertrophic response to BQ123, suggesting that endocytosis of ET_A receptors following ligand binding preserves their active state by protection against antagonist. Contrastingly, α₁ adrenergic-induced hypertrophic responses required the continued presence of agonist and were sensitive to antagonist. These studies shed new light on strategies to pharmacologically intervene in the action of different pro-hypertrophic mediators.

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Acute ET-1 exposure elicits a long-lasting cardiac myocyte hypertrophic response.

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ET-1 effects depend on persistent MAPK signalling and active transcription.

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ET-1 elicited hypertrophy is insensitive to subsequent ET_A receptor antagonism.

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Endocytosis inhibition potentiates ET-1-induction of hypertrophy markers.

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Endocytosis inhibition sensitises effects of ET-1 to ET_A receptor antagonist.

Emerin plays a crucial role in nuclear invagination and in the nuclear calcium transient

Alteration of the nuclear Ca²⁺ transient is an early event in cardiac remodeling. Regulation of the nuclear Ca²⁺ transient is partly independent of the cytosolic Ca²⁺ transient in cardiomyocytes. One nuclear membrane protein, emerin, is encoded by EMD, and an EMD mutation causes Emery-Dreifuss muscular dystrophy (EDMD). It remains unclear whether emerin is involved in nuclear Ca²⁺ homeostasis. The aim of this study is to elucidate the role of emerin in rat cardiomyocytes by means of hypertrophic stimuli and in EDMD induced pluripotent stem (iPS) cell-derived cardiomyocytes in terms of nuclear structure and the Ca²⁺ transient. The cardiac hypertrophic stimuli increased the nuclear area, decreased nuclear invagination, and increased the half-decay time of the nuclear Ca²⁺ transient in cardiomyocytes. Emd knockdown cardiomyocytes showed similar properties after hypertrophic stimuli. The EDMD-iPS cell-derived cardiomyocytes showed increased nuclear area, decreased nuclear invagination, and increased half-decay time of the nuclear Ca²⁺ transient. An autopsied heart from a patient with EDMD also showed increased nuclear area and decreased nuclear invagination. These data suggest that Emerin plays a crucial role in nuclear structure and in the nuclear Ca²⁺ transient. Thus, emerin and the nuclear Ca²⁺ transient are possible therapeutic targets in heart failure and EDMD.

Tissue Specificity: Store-Operated Ca2+ Entry in Cardiac Myocytes

Calcium (Ca²⁺) is a key regulator of cardiomyocyte contraction. The Ca²⁺ channels, pumps, and exchangers responsible for the cyclical cytosolic Ca²⁺ signals that underlie contraction are well known. In addition to those Ca²⁺ signaling components responsible for contraction, it has been proposed that cardiomyocytes express channels that promote the influx of Ca²⁺ from the extracellular milieu to the cytosol in response to depletion of intracellular Ca²⁺ stores. With non-excitable cells, this store-operated Ca²⁺ entry (SOCE) is usually easily demonstrated and is essential for prolonging cellular Ca²⁺ signaling and for refilling depleted Ca²⁺ stores. The role of SOCE in cardiomyocytes, however, is rather more elusive. While there is published evidence for increased Ca²⁺ influx into cardiomyocytes following Ca²⁺ store depletion, it has not been universally observed. Moreover, SOCE appears to be prominent in embryonic cardiomyocytes but declines with postnatal development. In contrast, there is overwhelming evidence that the molecular components of SOCE (e.g., STIM, Orai, and TRPC proteins) are expressed in cardiomyocytes from embryo to adult. Moreover, these proteins have been shown to contribute to disease conditions such as pathological hypertrophy, and reducing their expression can attenuate hypertrophic growth. It is plausible that SOCE might underlie Ca²⁺ influx into cardiomyocytes and may have important signaling functions perhaps by activating local Ca²⁺-sensitive processes. However, the STIM, Orai, and TRPC proteins appear to cooperate with multiple protein partners in signaling complexes. It is therefore possible that some of their signaling activities are not mediated by Ca²⁺ influx signals, but by protein-protein interactions.

The H3K9 dimethyltransferases EHMT1/2 protect against pathological cardiac hypertrophy

Cardiac hypertrophic growth in response to pathological cues is associated with reexpression of fetal genes and decreased cardiac function and is often a precursor to heart failure. In contrast, physiologically induced hypertrophy is adaptive, resulting in improved cardiac function. The processes that selectively induce these hypertrophic states are poorly understood. Here, we have profiled 2 repressive epigenetic marks, H3K9me2 and H3K27me3, which are involved in stable cellular differentiation, specifically in cardiomyocytes from physiologically and pathologically hypertrophied rat hearts, and correlated these marks with their associated transcriptomes. This analysis revealed the pervasive loss of euchromatic H3K9me2 as a conserved feature of pathological hypertrophy that was associated with reexpression of fetal genes. In hypertrophy, H3K9me2 was reduced following a miR-217-mediated decrease in expression of the H3K9 dimethyltransferases EHMT1 and EHMT2 (EHMT1/2). miR-217-mediated, genetic, or pharmacological inactivation of EHMT1/2 was sufficient to promote pathological hypertrophy and fetal gene reexpression, while suppression of this pathway protected against pathological hypertrophy both in vitro and in mice. Thus, we have established a conserved mechanism involving a departure of the cardiomyocyte epigenome from its adult cellular identity to a reprogrammed state that is accompanied by reexpression of fetal genes and pathological hypertrophy. These results suggest that targeting miR-217 and EHMT1/2 to prevent H3K9 methylation loss is a viable therapeutic approach for the treatment of heart disease.

Cardiac inositol 1,4,5-trisphosphate receptors

Calcium is a second messenger that regulates almost all cellular functions. In cardiomyocytes, calcium plays an integral role in many functions including muscle contraction, gene expression, and cell death. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are a family of calcium channels that are ubiquitously expressed in all tissues. In the heart, IP₃Rs have been associated with regulation of cardiomyocyte function in response to a variety of neurohormonal agonists, including those implicated in cardiac disease. Notably, IP₃R activity is thought to be essential for mediating the hypertrophic response to multiple stimuli including endothelin-1 and angiotensin II. In this review, we will explore the functional implications of IP₃R activity in the heart in health and disease.

Nuclear calcium is required for human T cell activation

Monaco et al. demonstrate that calcium signals in activated human T cells consist of a cytoplasmic and a nuclear component, which are both required for the immune response. Blockade of nuclear calcium signaling inhibits T cell activation and induces an anergy-like state.

Calcium signals in stimulated T cells are generally considered single entities that merely trigger immune responses, whereas costimulatory events specify the type of reaction. Here we show that the “T cell calcium signal” is a composite signal harboring two distinct components that antagonistically control genomic programs underlying the immune response. Using human T cells from healthy individuals, we establish nuclear calcium as a key signal in human T cell adaptogenomics that drives T cell activation and is required for signaling to cyclic adenosine monophosphate response element–binding protein and the induction of CD25, CD69, interleukin-2, and γ-interferon. In the absence of nuclear calcium signaling, cytosolic calcium activating nuclear factor of activated T cells translocation directed the genomic response toward enhanced expression of genes that negatively modulate T cell activation and are associated with a hyporesponsive state. Thus, nuclear calcium controls the T cell fate decision between a proliferative immune response and tolerance. Modulators of nuclear calcium–driven transcription may be used to develop a new type of pro-tolerance immunosuppressive therapy.

The Inositol Trisphosphate/Calcium Signaling Pathway in Health and Disease

Many cellular functions are regulated by calcium (Ca2+) signals that are generated by different signaling pathways. One of these is the inositol 1,4,5-trisphosphate/calcium (InsP3/Ca2+) signaling pathway that operates through either primary or modulatory mechanisms. In its primary role, it generates the Ca2+ that acts directly to control processes such as metabolism, secretion, fertilization, proliferation, and smooth muscle contraction. Its modulatory role occurs in excitable cells where it modulates the primary Ca2+ signal generated by the entry of Ca2+ through voltage-operated channels that releases Ca2+ from ryanodine receptors (RYRs) on the internal stores. In carrying out this modulatory role, the InsP3/Ca2+ signaling pathway induces subtle changes in the generation and function of the voltage-dependent primary Ca2+ signal. Changes in the nature of both the primary and modulatory roles of InsP3/Ca2+ signaling are a contributory factor responsible for the onset of a large number human diseases.

Regulation of phosphatidylinositol-specific phospholipase C at the nuclear envelope in cardiac myocytes

Phosphatidylinositol 4,5-bisphosphate hydrolysis at the plasma membrane by phospholipase C is one of the major hormone regulated intracellular signaling systems. The system generates the diffusible second messenger IP3 and the membrane bound messenger diacylglycerol. Spatial regulation of this system has been thought to be through specific subcellular distributions of the IP3 receptor or PKC. As is becoming increasingly apparent, receptor-stimulated signaling systems are also found at intracellular membranes. As discussed in this issue, G protein-coupled receptors have been identified at the nuclear envelope implying intracellular localization of the signaling systems that respond to G protein-coupled receptors. Here, we discuss the evidence for the existence of PLC signals that regulate nuclear processes, as well as the evidence for nuclear and nuclear envelope localization of PLC signaling components, and their implications for cardiac physiology and disease.

Tropisetron inhibits high glucose-induced calcineurin/NFAT hypertrophic pathway in H9c2 myocardial cells

OBJECTIVES

Cardiomyocyte hypertrophy is an important structural feature of diabetic cardiomyopathy. Calcineurin/nuclear factor of activated T-cell (NFAT) pathway plays a central role in the pathogenesis of cardiac hypertrophy. The purpose of this study was to investigate the effects of tropisetron, a novel calcineurin inhibitor, on high glucose (HG)-induced cardiomyocyte hypertrophy and its underlying mechanism.

METHODS

H9c2 myocardial cells were treated with tropisetron or cyclosporine A 1 h before exposure to HG for 48 h.

KEY FINDINGS

Exposure to HG resulted in enhanced cell size, protein content and atrial natriuretic peptide (ANP) protein expression. HG significantly increased Ca(2+) level, calcineurin expression and nuclear translocation of NFATc4. Both tropisetron and cyclosporine A markedly prevented the hypertrophic characteristic features, calcineurin overexpression and nuclear localization of NFATc4 while intracellular Ca(2+) was not affected.

CONCLUSION

Our results showed that tropisetron may have protective effects against HG-induced cardiomyocyte hypertrophy. The mechanism responsible for this beneficial effect seems to be, at least in part, blockade of calcineurin/NFAT signalling pathway.

Vitamin D: a custodian of cell signalling stability in health and disease

There is increasing evidence that a deficiency in vitamin D contributes to many human diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), hypertension and cardiovascular disease. The ability of vitamin D to maintain healthy cells seems to depend on its role as a guardian of phenotypic stability particularly with regard to the reactive oxygen species (ROS) and Ca2+ signalling systems. Vitamin D maintains the expression of those signalling components responsible for stabilizing the low-resting state of these two signalling pathways. This vitamin D signalling stability hypothesis proposes that vitamin D, working in conjunction with klotho and Nrf2 (nuclear factor-erythroid-2-related factor 2), acts as a custodian to maintain the normal function of the ROS and Ca2+ signalling pathways. A decline in vitamin D levels will lead to an erosion of this signalling stability and may account for why so many of the major diseases in man, which have been linked to vitamin D deficiency, are associated with a dysregulation in both ROS and Ca2+ signalling.

Cardiomyocyte Antihypertrophic Effect of Adipose Tissue Conditioned Medium from Rats and Its Abrogation by Obesity is Mediated by the Leptin to Adiponectin Ratio

White adipocytes are known to function as endocrine organs by secreting a plethora of bioactive adipokines which can regulate cardiac function including the development of hypertrophy. We determined whether adipose tissue conditioned medium (ATCM) generated from the epididymal regions of normal rats can affect the hypertrophic response of cultured rat ventricular myocytes to endothelin-1 (ET-1) administration. Myocytes were treated with ET-1 (10 nM) for 24 hours in the absence or presence of increasing ATCM concentrations. ATCM supressed the hypertrophic response to ET-1 in a concentration-dependent manner, an effect enhanced by the leptin receptor antagonist and attenuated by an antibody against the adiponectin AdipoR1 receptor. Antihypertrophic effects were also observed with ATCM generated from perirenal-derived adipose tissue. However, this effect was absent in ATCM from adipose tissue harvested from corpulent JCR:LA-cp rats. Detailed analyses of adipokine content in ATCM from normal and corpulent rats revealed no differences in the majority of products assayed, although a significant increase in leptin concentrations concomitant with decreased adiponectin levels was observed, resulting in a 11 fold increase in the leptin to adiponectin ratio in ATCM from JCR:LA-cp. The antihypertrophic effect of ATCM was associated with increased phosphorylation of AMP-activated protein kinase (AMPK), an effect abrogated by the AdipoR1 antibody. Moreover, the antihypertrophic effect of ATCM was mimicked by an AMPK activator. There was no effect of ET-1 on mitogen-activated protein kinase (MAPK) activities 24 hour after its addition either in the presence or absence of ATCM. Our study suggests that adipose tissue from healthy subjects exerts antihypertrophic effects via an adiponectin–dependent pathway which is impaired in obesity, most likely due to adipocyte remodelling resulting in enhanced leptin and reduced adiponectin levels.

Vitamin D cell signalling in health and disease

Vitamin D deficiency has been linked to many human diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), hypertension and cardiovascular disease. A Vitamin D phenotypic stability hypothesis, which is developed in this review, attempts to describe how this vital hormone acts to maintain healthy cellular functions. This role of Vitamin D as a guardian of phenotypic stability seems to depend on its ability to maintain the redox and Ca(2+) signalling systems. It is argued that its primary action is to maintain the expression of those signalling components responsible for stabilizing the low resting state of these two signalling pathways. This phenotypic stability role is facilitated through the ability of vitamin D to increase the expression of both Nrf2 and the anti-ageing protein Klotho, which are also major regulators of Ca(2+) and redox signalling. A decline in Vitamin D levels will lead to a decline in the stability of this regulatory signalling network and may account for why so many of the major diseases in man, which have been linked to vitamin D deficiency, are associated with a dysregulation in both ROS and Ca(2+) signalling.

Nuclear proton dynamics and interactions with calcium signaling

Biochemical signals acting on the nucleus can regulate gene expression. Despite the inherent affinity of nucleic acids and nuclear proteins (e.g. transcription factors) for protons, little is known about the mechanisms that regulate nuclear pH (pH_nuc), and how these could be exploited to control gene expression. Here, we show that pH_nuc dynamics can be imaged using the DNA-binding dye Hoechst 33342. Nuclear pores allow the passage of medium-sized molecules (calcein), but protons must first bind to mobile buffers in order to gain access to the nucleoplasm. Fixed buffering residing in the nucleus of permeabilized cells was estimated to be very weak on the basis of the large amplitude of pH_nuc transients evoked by photolytic H⁺-uncaging or exposure to weak acids/bases. Consequently, the majority of nuclear pH buffering is sourced from the cytoplasm in the form of mobile buffers. Effective proton diffusion was faster in nucleoplasm than in cytoplasm, in agreement with the higher mobile-to-fixed buffering ratio in the nucleus. Cardiac myocyte pH_nuc changed in response to maneuvers that alter nuclear Ca^2 + signals. Blocking Ca^2 + release from inositol-1,4,5-trisphosphate receptors stably alkalinized the nucleus. This Ca^2 +-pH interaction may arise from competitive binding to common chemical moieties. Competitive binding to mobile buffers may couple the efflux of Ca^2 +via nuclear pores with a counterflux of protons. This would generate a stable pH gradient between cytoplasm and nucleus that is sensitive to the state of nuclear Ca^2 + signaling. The unusual behavior of protons in the nucleus provides new mechanisms for regulating cardiac nuclear biology.

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Facilitated diffusion aboard mobile buffers is the only means by which protons enter the nucleus.

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The relative scarcity of fixed buffers residing in the nucleus accelerates proton diffusivity.

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Nuclear Ca^2 + signals can regulate nuclear pH and generate stable gradients relative to cytoplasm.