Local InsP3-dependent perinuclear Ca2+ signaling in cardiac myocyte excitation-transcription coupling

Modeling calcium regulation of contraction, energetics, signaling, and transcription in the cardiac myocyte

Calcium (Ca(2+)) plays many important regulatory roles in cardiac muscle cells. In the initial phase of the action potential, influx of Ca(2+) through sarcolemmal voltage-gated L-type Ca(2+) channels (LCCs) acts as a feed-forward signal that triggers a large release of Ca(2+) from the junctional sarcoplasmic reticulum (SR). This Ca(2+) drives heart muscle contraction and pumping of blood in a process known as excitation-contraction coupling (ECC). Triggered and released Ca(2+) also feed back to inactivate LCCs, attenuating the triggered Ca(2+) signal once release has been achieved. The process of ECC consumes large amounts of ATP. It is now clear that in a process known as excitation-energetics coupling, Ca(2+) signals exert beat-to-beat regulation of mitochondrial ATP production that closely couples energy production with demand. This occurs through transport of Ca(2+) into mitochondria, where it regulates enzymes of the tricarboxylic acid cycle. In excitation-signaling coupling, Ca(2+) activates a number of signaling pathways in a feed-forward manner. Through effects on their target proteins, these interconnected pathways regulate Ca(2+) signals in complex ways to control electrical excitability and contractility of heart muscle. In a process known as excitation-transcription coupling, Ca(2+) acting primarily through signal transduction pathways also regulates the process of gene transcription. Because of these diverse and complex roles, experimentally based mechanistic computational models are proving to be very useful for understanding Ca(2+) signaling in the cardiac myocyte.

The Golgi apparatus is a functionally distinct Ca2+ store regulated by the PKA and Epac branches of the β1-adrenergic signaling pathway

Ca(2+) release from the Golgi apparatus regulates key functions of the organelle, including vesicle trafficking. We found that the Golgi apparatus was the source of prolonged Ca(2+) release events that originated near the nuclei of primary cardiomyocytes. Golgi Ca(2+) release was unaffected by depletion of sarcoplasmic reticulum Ca(2+), and disruption of the Golgi apparatus abolished Golgi Ca(2+) release without affecting sarcoplasmic reticulum function, suggesting functional and spatial independence of Golgi and sarcoplasmic reticulum Ca(2+) stores. β1-Adrenoceptor stimulation triggers the production of the second messenger cAMP, which activates the Epac family of Rap guanine nucleotide exchange factors and the kinase PKA (protein kinase A). Phosphodiesterases (PDEs), including those in the PDE3 and PDE4 families, degrade cAMP. Activation of β1-adrenoceptors stimulated Golgi Ca(2+) release, an effect that required activation of Epac, PKA, and the kinase CaMKII. Inhibition of PDE3s or PDE4s potentiated β1-adrenergic-induced Golgi Ca(2+) release, which is consistent with compartmentalization of cAMP signaling near the Golgi apparatus. Interventions that stimulated Golgi Ca(2+) release appeared to increase the trafficking of vascular endothelial growth factor receptor-1 (VEGFR-1) from the Golgi apparatus to the surface membrane of cardiomyocytes. In cardiomyocytes from rats with heart failure, decreases in the abundance of PDE3s and PDE4s were associated with increased Golgi Ca(2+) release events. These data suggest that the Golgi apparatus is a focal point for β1-adrenergic-stimulated Ca(2+) signaling and that the Golgi Ca(2+) store functions independently from the sarcoplasmic reticulum and the global Ca(2+) transients that trigger contraction in cardiomyocytes.

Localized nuclear and perinuclear Ca(2+) signals in intact mouse skeletal muscle fibers

Nuclear Ca²⁺ is important for the regulation of several nuclear processes such as gene expression. Localized Ca²⁺ signals (LCSs) in skeletal muscle fibers of mice have been mainly studied as Ca²⁺ release events from the sarcoplasmic reticulum. Their location with regard to cell nuclei has not been investigated. Our study is based on the hypothesis that LCSs associated with nuclei are present in skeletal muscle fibers of adult mice. Therefore, we carried out experiments addressing this question and we found novel Ca²⁺ signals associated with nuclei of skeletal muscle fibers (with possibly attached satellite cells). We measured localized nuclear and perinuclear Ca²⁺ signals (NLCSs and PLCSs) alongside cytosolic localized Ca²⁺ signals (CLCSs) during a hypertonic treatment. We also observed NLCSs under isotonic conditions. The NLCSs and PLCSs are Ca²⁺ signals in the range of micrometer [FWHM (full width at half maximum): 2.75 ± 0.27 μm (NLCSs) and 2.55 ± 0.17 μm (PLCSs), S.E.M.]. Additionally, global nuclear Ca²⁺ signals (NGCSs) were observed. To investigate which type of Ca²⁺ channels contribute to the Ca²⁺ signals associated with nuclei in skeletal muscle fibers, we performed measurements with the RyR blocker dantrolene, the DHPR blocker nifedipine or the IP₃R blocker Xestospongin C. We observed Ca²⁺ signals associated with nuclei in the presence of each blocker. Nifedipine and dantrolene had an inhibitory effect on the fraction of fibers with PLCSs. The situation for the fraction of fibers with NLCSs is more complex indicating that RyR is less important for the generation of NLCSs compared to the generation of PLCSs. The fraction of fibers with NLCSs and PLCSs is not reduced in the presence of Xestospongin C. The localized perinuclear and intranuclear Ca²⁺ signals may be a powerful tool for the cell to regulate adaptive processes as gene expression. The intranuclear Ca²⁺ signals may be particularly interesting in this respect.

Role of sodium and calcium dysregulation in tachyarrhythmias in sudden cardiac death

Despite improvements in the therapy of underlying heart disease, sudden cardiac death is a major cause of death worldwide. Disturbed Na and Ca handling is known to be a major predisposing factor for life-threatening tachyarrhythmias. In cardiomyocytes, many ion channels and transporters, including voltage-gated Na and Ca channels, cardiac ryanodine receptors, Na/Ca-exchanger, and SR Ca-ATPase are involved in this regulation. We have learned a lot about the pathophysiological relevance of disturbed ion channel function from monogenetic disorders. Changes in the gating of a single ion channel and the activity of an ion pump suffice to dramatically increase the propensity for arrhythmias even in structurally normal hearts. Nevertheless, patients with heart failure with acquired dysfunction in many ion channels and transporters exhibit profound dysregulation of Na and Ca handling and Ca/calmodulin-dependent protein kinase and are especially prone to arrhythmias. A deeper understanding of the underlying arrhythmic principles is mandatory if we are to improve their outcome. This review addresses basic tachyarrhythmic mechanisms, the underlying ionic mechanisms and the consequences for ion homeostasis, and the situation in complex diseases like heart failure.

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.

Monovalent cationic channel activity in the inner membrane of nuclei from skeletal muscle fibers

Nuclear ion channels remain among the least studied and biophysically characterized channels. Although considerable progress has been made in characterizing calcium release channels in the nuclear membrane, very little is known regarding the properties of nuclear monovalent cationic channels. Here, we describe a method to isolate nuclei from adult skeletal muscle fibers that are suitable for electrophysiological experiments. Using this approach, we show for the first time, to our knowledge, that a nuclear monovalent cationic channel (NMCC) is prominently expressed in the inner membrane of nuclei isolated from flexor digitorum brevis skeletal muscle fibers of adult mice. In isotonic 140 mM KCl, the skeletal muscle NMCC exhibits a unitary conductance of ?160 pS and high, voltage-independent open probability. Based on single-channel reversal potential measurements, NMCCs are slightly more permeable to potassium ions over sodium (PK/PNa = 2.68 ± 0.21) and cesium (PK/PCs = 1.39 ± 0.03) ions. In addition, NMCCs do not permeate divalent cations, are inhibited by calcium ions, and demonstrate weak rectification in asymmetric Ca(2+)-containing solutions. Together, these studies characterize a voltage-independent NMCC in skeletal muscle, the properties of which are ideally suited to serve as a countercurrent mechanism during calcium release from the nuclear envelope.

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.

Endothelin signaling activates Mef2c expression in the neural crest through a MEF2C-dependent positive-feedback transcriptional pathway

Endothelin signaling is essential for neural crest development, and dysregulated Endothelin signaling is associated with several neural crest-related disorders, including Waardenburg and other syndromes. However, despite the crucial roles of this pathway in neural crest development and disease, the transcriptional effectors directly activated by Endothelin signaling during neural crest development remain incompletely elucidated. Here, we establish that the MADS box transcription factor MEF2C is an immediate downstream transcriptional target and effector of Endothelin signaling in the neural crest. We show that Endothelin signaling activates Mef2c expression in the neural crest through a conserved enhancer in the Mef2c locus and that CRISPR-mediated deletion of this Mef2c neural crest enhancer from the mouse genome abolishes Endothelin induction of Mef2c expression. Moreover, we demonstrate that Endothelin signaling activates neural crest expression of Mef2c by de-repressing MEF2C activity through a Calmodulin-CamKII-histone deacetylase signaling cascade. Thus, these findings identify a MEF2C-dependent, positive-feedback mechanism for Endothelin induction and establish MEF2C as an immediate transcriptional effector and target of Endothelin signaling in the neural crest.

Isolation and study of cardiac nuclei from canine myocardium and adult ventricular myocytes

The nuclear envelope encloses the genome as well as the molecular machinery responsible for both the replication and transcription of DNA as well as the maturation of nascent RNA. Recent studies ascribe a growing number of functions to the nuclear membrane, in addition to sequestering the DNA, through receptors and their effectors, ion channels, as well as ion pumps and transporters located within the nuclear membrane itself. Despite the obvious structural and functional importance of the nucleus, certain aspects remain poorly understood due to the challenges associated with its accessibility in vivo, as well as isolating nuclei intact and with sufficient purity from cardiac cells to permit studies in vitro. Here, we present a detailed protocol for isolation of intact nuclei from both myocardial tissue and freshly isolated adult ventricular cardiomyocytes. These methods are based on partial permeabilization of plasma membrane with digitonin and cell disruption, followed by differential and discontinuous sucrose density centrifugation. These preparations provide for rapid separation of nonnuclear membranes and cytosol from nuclei.

CaMKIIδ mediates β-adrenergic effects on RyR2 phosphorylation and SR Ca(2+) leak and the pathophysiological response to chronic β-adrenergic stimulation

Chronic activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) has been implicated in the deleterious effects of β-adrenergic receptor (β-AR) signaling on the heart, in part, by enhancing RyR2-mediated sarcoplasmic reticulum (SR) Ca(2+) leak. We used CaMKIIδ knockout (CaMKIIδ-KO) mice and knock-in mice with an inactivated CaMKII site S2814 on the ryanodine receptor type 2 (S2814A) to investigate the involvement of these processes in β-AR signaling and cardiac remodeling. Langendorff-perfused hearts from CaMKIIδ-KO mice showed inotropic and chronotropic responses to isoproterenol (ISO) that were similar to those of wild type (WT) mice; however, in CaMKIIδ-KO mice, CaMKII phosphorylation of phospholamban and RyR2 was decreased and isolated myocytes from CaMKIIδ-KO mice had reduced SR Ca(2+) leak in response to isoproterenol (ISO). Chronic catecholamine stress with ISO induced comparable increases in relative heart weight and other measures of hypertrophy from day 9 through week 4 in WT and CaMKIIδ-KO mice, but the development of cardiac fibrosis was prevented in CaMKIIδ-KO animals. A 4-week challenge with ISO resulted in reduced cardiac function and pulmonary congestion in WT, but not in CaMKIIδ-KO or S2814A mice, implicating CaMKIIδ-dependent phosphorylation of RyR2-S2814 in the cardiomyopathy, independent of hypertrophy, induced by prolonged β-AR stimulation.

Stimulus-dependent regulation of nuclear Ca2+ signaling in cardiomyocytes: a role of neuronal calcium sensor-1

In cardiomyocytes, intracellular calcium (Ca²⁺) transients are elicited by electrical and receptor stimulations, leading to muscle contraction and gene expression, respectively. Although such elevations of Ca²⁺levels ([Ca²⁺]) also occur in the nucleus, the precise mechanism of nuclear [Ca²⁺] regulation during different kinds of stimuli, and its relationship with cytoplasmic [Ca²⁺] regulation are not fully understood. To address these issues, we used a new region-specific fluorescent protein-based Ca²⁺ indicator, GECO, together with the conventional probe Fluo-4 AM. We confirmed that nuclear Ca²⁺ transients were elicited by both electrical and receptor stimulations in neonatal mouse ventricular myocytes. Kinetic analysis revealed that electrical stimulation-elicited nuclear Ca²⁺ transients are slower than cytoplasmic Ca²⁺ transients, and chelating cytoplasmic Ca²⁺ abolished nuclear Ca²⁺ transients, suggesting that nuclear Ca²⁺ are mainly derived from the cytoplasm during electrical stimulation. On the other hand, receptor stimulation such as with insulin-like growth factor-1 (IGF-1) preferentially increased nuclear [Ca²⁺] compared to cytoplasmic [Ca²⁺]. Experiments using inhibitors revealed that electrical and receptor stimulation-elicited Ca²⁺ transients were mainly mediated by ryanodine receptors and inositol 1,4,5-trisphosphate receptors (IP3Rs), respectively, suggesting different mechanisms for the two signals. Furthermore, IGF-1-elicited nuclear Ca²⁺ transient amplitude was significantly lower in myocytes lacking neuronal Ca²⁺ sensor-1 (NCS-1), a Ca²⁺ binding protein implicated in IP₃R-mediated pathway in the heart. Moreover, IGF-1 strengthened the interaction between NCS-1 and IP₃R. These results suggest a novel mechanism for receptor stimulation-induced nuclear [Ca²⁺] regulation mediated by IP3R and NCS-1 that may further fine-tune cardiac Ca²⁺ signal regulation.

Adaptive capacity of the right ventricle: why does it fail?

Only in recent years has the right ventricle (RV) function become appreciated to be equally important to the left ventricle (LV) function to maintain cardiac output. Right ventricular failure is, irrespectively of the etiology, associated with impaired exercise tolerance and poor survival. Since the anatomy and physiology of the RV is distinctly different than that of the LV, its adaptive mechanisms and the pathways involved are different as well. RV hypertrophy is an important mechanism of the RV to preserve cardiac output. This review summarizes the current knowledge on the right ventricle and its response to pathologic situations. We will focus on the adaptive capacity of the right ventricle and the molecular pathways involved, and we will discuss potential therapeutic interventions.

Novel Epac fluorescent ligand reveals distinct Epac1 vs. Epac2 distribution and function in cardiomyocytes

Exchange proteins directly activated by cAMP (Epac1 and Epac2) have been recently recognized as key players in β-adrenergic-dependent cardiac arrhythmias. Whereas Epac1 overexpression can lead to cardiac hypertrophy and Epac2 activation can be arrhythmogenic, it is unknown whether distinct subcellular distribution of Epac1 vs. Epac2 contributes to differential functional effects. Here, we characterized and used a novel fluorescent cAMP derivate Epac ligand 8-[Pharos-575]-2'-O-methyladenosine-3',5'-cyclic monophosphate (Φ-O-Me-cAMP) in mice lacking either one or both isoforms (Epac1-KO, Epac2-KO, or double knockout, DKO) to assess isoform localization and function. Fluorescence of Φ-O-Me-cAMP was enhanced by binding to Epac. Unlike several Epac-specific antibodies tested, Φ-O-Me-cAMP exhibited dramatically reduced signals in DKO myocytes. In WT, the apparent binding affinity (Kd = 10.2 ± 0.8 µM) is comparable to that of cAMP and nonfluorescent Epac-selective agonist 8-(4-chlorophenylthio)-2-O-methyladenosine-3'-,5'-cyclicmonophosphate (OMe-CPT). Φ-O-Me-cAMP readily entered intact myocytes, but did not activate PKA and its binding was competitively inhibited by OMe-CPT, confirming its Epac specificity. Φ-O-Me-cAMP is a weak partial agonist for purified Epac, but functioned as an antagonist for four Epac signaling pathways in myocytes. Epac2 and Epac1 were differentially concentrated along T tubules and around the nucleus, respectively. Epac1-KO abolished OMe-CPT-induced nuclear CaMKII activation and export of transcriptional regulator histone deacetylase 5. In conclusion, Epac1 is localized and functionally involved in nuclear signaling, whereas Epac2 is located at the T tubules and regulates arrhythmogenic sarcoplasmic reticulum Ca leak.

Chasing cardiac physiology and pathology down the CaMKII cascade

Calcium dynamics is central in cardiac physiology, as the key event leading to the excitation-contraction coupling (ECC) and relaxation processes. The primary function of Ca(2+) in the heart is the control of mechanical activity developed by the myofibril contractile apparatus. This key role of Ca(2+) signaling explains the subtle and critical control of important events of ECC and relaxation, such as Ca(2+) influx and SR Ca(2+) release and uptake. The multifunctional Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) is a signaling molecule that regulates a diverse array of proteins involved not only in ECC and relaxation but also in cell death, transcriptional activation of hypertrophy, inflammation, and arrhythmias. CaMKII activity is triggered by an increase in intracellular Ca(2+) levels. This activity can be sustained, creating molecular memory after the decline in Ca(2+) concentration, by autophosphorylation of the enzyme, as well as by oxidation, glycosylation, and nitrosylation at different sites of the regulatory domain of the kinase. CaMKII activity is enhanced in several cardiac diseases, altering the signaling pathways by which CaMKII regulates the different fundamental proteins involved in functional and transcriptional cardiac processes. Dysregulation of these pathways constitutes a central mechanism of various cardiac disease phenomena, like apoptosis and necrosis during ischemia/reperfusion injury, digitalis exposure, post-acidosis and heart failure arrhythmias, or cardiac hypertrophy. Here we summarize significant aspects of the molecular physiology of CaMKII and provide a conceptual framework for understanding the role of the CaMKII cascade on Ca(2+) regulation and dysregulation in cardiac health and disease.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Post-translational modifications regulate class IIa histone deacetylase (HDAC) function in health and disease

Class IIa histone deacetylases (HDACs4, -5, -7, and -9) modulate the physiology of the human cardiovascular, musculoskeletal, nervous, and immune systems. The regulatory capacity of this family of enzymes stems from their ability to shuttle between nuclear and cytoplasmic compartments in response to signal-driven post-translational modification. Here, we review the current knowledge of modifications that control spatial and temporal histone deacetylase functions by regulating subcellular localization, transcriptional functions, and cell cycle-dependent activity, ultimately impacting on human disease. We discuss the contribution of these modifications to cardiac and vascular hypertrophy, myoblast differentiation, neuronal cell survival, and neurodegenerative disorders.

G protein βγ subunits regulate cardiomyocyte hypertrophy through a perinuclear Golgi phosphatidylinositol 4-phosphate hydrolysis pathway

Gβγ regulation of the perinuclear Golgi PI4P pathway and a separate pathway at the PM is required for ET-1–stimulated hypertrophy, and the efficacy of Gβγ inhibition in preventing heart failure may be due, in part, to its blocking both of these pathways.

We recently identified a novel GPCR-dependent pathway for regulation of cardiac hypertrophy that depends on Golgi phosphatidylinositol 4-phosphate (PI4P) hydrolysis by a specific isoform of phospholipase C (PLC), PLCε, at the nuclear envelope. How stimuli are transmitted from cell surface GPCRs to activation of perinuclear PLCε is not clear. Here we tested the role of G protein βγ subunits. Gβγ inhibition blocked ET-1–stimulated Golgi PI4P depletion in neonatal and adult ventricular myocytes. Blocking Gβγ at the Golgi inhibited ET-1–dependent PI4P depletion and nuclear PKD activation. Translocation of Gβγ to the Golgi stimulated perinuclear Golgi PI4P depletion and nuclear PKD activation. Finally, blocking Gβγ at the Golgi or PM blocked ET-1–dependent cardiomyocyte hypertrophy. These data indicate that Gβγ regulation of the perinuclear Golgi PI4P pathway and a separate pathway at the PM is required for ET-1–stimulated hypertrophy, and the efficacy of Gβγ inhibition in preventing heart failure maybe due in part to its blocking both these pathways.

Assessing GPCR and G protein signaling to the nucleus in live cells using fluorescent biosensors

G protein-coupled receptor (GPCR) signaling cascades regulate a wide variety of cellular processes and feature prominently in many cardiovascular pathologies. As such they represent major drug targets and discovering novel aspects of GPCR signaling provide important opportunities to identify additional potential therapeutic approaches to reverse or prevent cardiac remodeling and failure. Monitoring cellular trafficking of signaling components and specific protein kinase activities using fluorescent biosensors has provided key insight into stress/GPCR-induced kinase signaling networks and their effect on cardiac gene expression. Herein we describe the protocols for the expression, visualization (by confocal microscopy), and interpretation of data obtained with such biosensors expressed in adult cardiomyocytes. Our focus is on the cellular trafficking of class II histone deacetylases (i.e., HDAC5) and on the FRET sensor (Camui) for calmodulin-dependent protein kinase II (CaMKII).

Measuring intranuclear and nuclear envelope [Ca(2+)] vs. cytosolic [Ca (2+)]

Nuclear Ca(2+) regulates key cellular processes, including gene expression, apoptosis, assembly of the nuclear envelope, and nucleocytoplasmic transport. Quantification of subcellularly resolved Ca(2+) signals is, therefore, essential for understanding physiological and pathological processes in various cell types. However, the properties of commonly used Ca(2+)-fluorescent indicators in intracellular compartments may differ, thus affecting the translation of Ca(2+)-dependent fluorescence changes into quantitative changes of Ca(2+) concentration. Here, we describe technical approaches for reliable subcellular quantification of [Ca(2+)] in the cytoplasm vs. the nucleus and the nuclear envelope by in situ calibration of fluorescein-derived fluorescent indicators Fluo-4 and Fluo-5N.

Revealing T-Tubules in Striated Muscle with New Optical Super-Resolution Microscopy Techniquess

The t-tubular system plays a central role in the synchronisation of calcium signalling and excitation-contraction coupling in most striated muscle cells. Light microscopy has been used for imaging t-tubules for well over 100 years and together with electron microscopy (EM), has revealed the three-dimensional complexities of the t-system topology within cardiomyocytes and skeletal muscle fibres from a range of species. The emerging super-resolution single molecule localisation microscopy (SMLM) techniques are offering a near 10-fold improvement over the resolution of conventional fluorescence light microscopy methods, with the ability to spectrally resolve nanometre scale distributions of multiple molecular targets. In conjunction with the next generation of electron microscopy, SMLM has allowed the visualisation and quantification of intricate t-tubule morphologies within large areas of muscle cells at an unprecedented level of detail. In this paper, we review recent advancements in the t-tubule structural biology with the utility of various microscopy techniques. We outline the technical considerations in adapting SMLM to study t-tubules and its potential to further our understanding of the molecular processes that underlie the sub-micron scale structural alterations observed in a range of muscle pathologies.

The type 2 inositol 1,4,5-trisphosphate receptor, emerging functions for an intriguing Ca²⁺-release channel

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) type 2 (IP3R2) is an intracellular Ca²⁺-release channel located on the endoplasmic reticulum (ER). IP3R2 is characterized by a high sensitivity to both IP3 and ATP and is biphasically regulated by Ca²⁺. Furthermore, IP3R2 is modulated by various protein kinases. In addition to its regulation by protein kinase A, IP3R2 forms a complex with adenylate cyclase 6 and is directly regulated by cAMP. Finally, in the ER, IP3R2 is less mobile than the other IP3R isoforms, while its functional properties appear dominant in heterotetramers. These properties make the IP3R2 a Ca²⁺ channel with exquisite properties for setting up intracellular Ca²⁺ signals with unique characteristics. IP3R2 plays a crucial role in the function of secretory cell types (e.g. pancreatic acinar cells, hepatocytes, salivary gland, eccrine sweat gland). In cardiac myocytes, the role of IP3R2 appears more complex, because, together with IP3R1, it is needed for normal cardiogenesis, while its aberrant activity is implicated in cardiac hypertrophy and arrhythmias. Most importantly, its high sensitivity to IP3 makes IP3R2 a target for anti-apoptotic proteins (e.g. Bcl-2) in B-cell cancers. Disrupting IP3R/Bcl-2 interaction therefore leads in those cells to increased Ca²⁺ release and apoptosis. Intriguingly, IP3R2 is not only implicated in apoptosis but also in the induction of senescence, another tumour-suppressive mechanism. These results were the first to unravel the physiological and pathophysiological role of IP3R2 and we anticipate that further progress will soon be made in understanding the function of IP3R2 in various tissues and organs.

Roles and post-translational regulation of cardiac class IIa histone deacetylase isoforms

Cardiomyocyte hypertrophy is an integral component of pathological cardiac remodelling in response to mechanical and chemical stresses in settings such as chronic hypertension or myocardial infarction. For hypertrophy to ensue, the pertinent mechanical and chemical signals need to be transmitted from membrane sensors (such as receptors for neurohormonal mediators) to the cardiomyocyte nucleus, leading to altered transcription of the genes that regulate cell growth. In recent years, nuclear histone deacetylases (HDACs) have attracted considerable attention as signal-responsive, distal regulators of the transcriptional reprogramming that in turn precipitates cardiomyocyte hypertrophy, with particular focus on the role of members of the class IIa family, such as HDAC4 and HDAC5. These histone deacetylase isoforms appear to repress cardiomyocyte hypertrophy through mechanisms that involve protein interactions in the cardiomyocyte nucleus, particularly with pro-hypertrophic transcription factors, rather than via histone deacetylation. In contrast, evidence indicates that class I HDACs promote cardiomyocyte hypertrophy through mechanisms that are dependent on their enzymatic activity and thus sensitive to pharmacological HDAC inhibitors. Although considerable progress has been made in understanding the roles of post-translational modifications (PTMs) such as phosphorylation, oxidation and proteolytic cleavage in regulating class IIa HDAC localisation and function, more work is required to explore the contributions of other PTMs, such as ubiquitination and sumoylation, as well as potential cross-regulatory interactions between distinct PTMs and between class IIa and class I HDAC isoforms.

An integrated mechanism of cardiomyocyte nuclear Ca(2+) signaling

In cardiomyocytes, Ca(2+) plays a central role in governing both contraction and signaling events that regulate gene expression. Current evidence indicates that discrimination between these two critical functions is achieved by segregating Ca(2+) within subcellular microdomains: transcription is regulated by Ca(2+) release within nuclear microdomains, and excitation-contraction coupling is regulated by cytosolic Ca(2+). Accordingly, a variety of agonists that control cardiomyocyte gene expression, such as endothelin-1, angiotensin-II or insulin-like growth factor-1, share the feature of triggering nuclear Ca(2+) signals. However, signaling pathways coupling surface receptor activation to nuclear Ca(2+) release, and the phenotypic responses to such signals, differ between agonists. According to earlier hypotheses, the selective control of nuclear Ca(2+) signals by activation of plasma membrane receptors relies on the strategic localization of inositol trisphosphate receptors at the nuclear envelope. There, they mediate Ca(2+) release from perinuclear Ca(2+) stores upon binding of inositol trisphosphate generated in the cytosol, which diffuses into the nucleus. More recently, identification of such receptors at nuclear membranes or perinuclear sarcolemmal invaginations has uncovered novel mechanisms whereby agonists control nuclear Ca(2+) release. In this review, we discuss mechanisms for the selective control of nuclear Ca(2+) signals with special focus on emerging models of agonist receptor activation.

Cx43 phosphorylation on S279/282 and intercellular communication are regulated by IP3/IP3 receptor signaling

Background

Inositol 1,4,5-trisphosphate receptor (IP₃R) plays a pivotal role in the Ca²⁺ release process in a variety of cell types. Additionally, IP₃R is distributed in ventricular intercalated discs, but its function(s) in this particular site remains unknown. Connexin (Cx43), the predominant gap junction (GJ) protein in ventricular myocardium, is linked to several signaling pathways that regulate Cx43 properties by (de)phosphorylation on multiple residues. Here, we investigated the regulatory role of IP₃R in cell-cell communication and the mechanism(s) underlying this effect.

Results

In neonatal rat and adult mouse ventricular myocytes IP₃R co-localized and co-immunoprecipitated with Cx43 in GJ plaques detected by immunostaining and western blot assays. Blocking IP₃R with antagonists or silencing pan-IP₃R expression with shRNA hindered the 6-carboxyfluorescein (6-CFDA) diffusion through GJs and desynchronized Ca²⁺ transients among confluent neonatal myocytes in culture, whereas stimulation of IP₃R with IP₃ ester or ATP exerted the opposite effect. Likewise, 6-CFDA propagation through GJs was modulated by IP₃R activation or inhibition in cell pairs of isolated adult cardiomyocytes. Furthermore, IP₃R activation or IP₃R suppression promoted or suppressed, respectively, Cx43 phosphorylation on S279/282. Site-directed mutagenesis indicated that expression of a mutant Cx43-S282A (alanine) inhibited S279/282 phosphorylation and GJ permeability, while the S279A mutant showed the opposite effect in ventricular myocytes. Expression of these mutants in HEK293 cells revealed that cells with a dual S279/282 mutation failed to express exogenous Cx43, whereas cells with a single S279 or S282 mutation displayed Cx43 overexpression with increased phosphorylation of S279/282 and promotion of intercellular communication.

Conclusions

These results demonstrated, for the first time, that IP₃R physically interacts with Cx43 and participates in the regulation of Cx43 phosphorylation on S279/282, thereby affecting GJ intercellular communication in ventricular myocytes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-014-0058-6) contains supplementary material, which is available to authorized users.

Early remodeling of perinuclear Ca2+ stores and nucleoplasmic Ca2+ signaling during the development of hypertrophy and heart failure

BACKGROUND

A hallmark of heart failure is impaired cytoplasmic Ca(2+) handling of cardiomyocytes. It remains unknown whether specific alterations in nuclear Ca(2+) handling via altered excitation-transcription coupling contribute to the development and progression of heart failure.

METHODS AND RESULTS

Using tissue and isolated cardiomyocytes from nonfailing and failing human hearts, as well as mouse and rabbit models of hypertrophy and heart failure, we provide compelling evidence for structural and functional changes of the nuclear envelope and nuclear Ca(2+) handling in cardiomyocytes as remodeling progresses. Increased nuclear size and less frequent intrusions of the nuclear envelope into the nuclear lumen indicated altered nuclear structure that could have functional consequences. In the (peri)nuclear compartment, there was also reduced expression of Ca(2+) pumps and ryanodine receptors, increased expression of inositol-1,4,5-trisphosphate receptors, and differential orientation among these Ca(2+) transporters. These changes were associated with altered nucleoplasmic Ca(2+) handling in cardiomyocytes from hypertrophied and failing hearts, reflected as increased diastolic Ca(2+) levels with diminished and prolonged nuclear Ca(2+) transients and slowed intranuclear Ca(2+) diffusion. Altered nucleoplasmic Ca(2+) levels were translated to higher activation of nuclear Ca(2+)/calmodulin-dependent protein kinase II and nuclear export of histone deacetylases. Importantly, the nuclear Ca(2+) alterations occurred early during hypertrophy and preceded the cytoplasmic Ca(2+) changes that are typical of heart failure.

CONCLUSIONS

During cardiac remodeling, early changes of cardiomyocyte nuclei cause altered nuclear Ca(2+) signaling implicated in hypertrophic gene program activation. Normalization of nuclear Ca(2+) regulation may therefore be a novel therapeutic approach to prevent adverse cardiac remodeling.

Calpain-dependent cleavage of junctophilin-2 and T-tubule remodeling in a mouse model of reversible heart failure

Background

A highly organized transverse tubule (T‐tubule) network is necessary for efficient Ca²⁺‐induced Ca²⁺ release and synchronized contraction of ventricular myocytes. Increasing evidence suggests that T‐tubule remodeling due to junctophilin‐2 (JP‐2) downregulation plays a critical role in the progression of heart failure. However, the mechanisms underlying JP‐2 dysregulation remain incompletely understood.

Methods and Results

A mouse model of reversible heart failure that is driven by conditional activation of the heterotrimeric G protein Gα_q in cardiac myocytes was used in this study. Mice with activated Gα_q exhibited disruption of the T‐tubule network and defects in Ca²⁺ handling that culminated in heart failure compared with wild‐type mice. Activation of Gα_q/phospholipase Cβ signaling increased the activity of the Ca²⁺‐dependent protease calpain, leading to the proteolytic cleavage of JP‐2. A novel calpain cleavage fragment of JP‐2 is detected only in hearts with constitutive Gαq signaling to phospholipase Cβ. Termination of the Gα_q signal was followed by normalization of the JP‐2 protein level, repair of the T‐tubule network, improvements in Ca²⁺ handling, and reversal of heart failure. Treatment of mice with a calpain inhibitor prevented Gα_q‐dependent JP‐2 cleavage, T‐tubule disruption, and the development of heart failure.

Conclusions

Disruption of the T‐tubule network in heart failure is a reversible process. Gα_q‐dependent activation of calpain and subsequent proteolysis of JP‐2 appear to be the molecular mechanism that leads to T‐tubule remodeling, Ca²⁺ handling dysfunction, and progression to heart failure in this mouse model.

Emerging roles of junctophilin-2 in the heart and implications for cardiac diseases

Cardiomyocytes rely on a highly specialized subcellular architecture to maintain normal cardiac function. In a little over a decade, junctophilin-2 (JPH2) has become recognized as a cardiac structural protein critical in forming junctional membrane complexes (JMCs), which are subcellular domains essential for excitation-contraction coupling within the heart. While initial studies described the structure of JPH2 and its role in anchoring junctional sarcoplasmic reticulum and transverse-tubule (T-tubule) membrane invaginations, recent research has an expanded role of JPH2 in JMC structure and function. For example, JPH2 is necessary for the development of postnatal T-tubule in mammals. It is also critical for the maintenance of the complex JMC architecture and stabilization of local ion channels in mature cardiomyocytes. Loss of this function by mutations or down-regulation of protein expression has been linked to hypertrophic cardiomyopathy, arrhythmias, and progression of disease in failing hearts. In this review, we summarize current views on the roles of JPH2 within the heart and how JPH2 dysregulation may contribute to a variety of cardiac diseases.

Toward a hierarchy of mechanisms in CaMKII-mediated arrhythmia

Calcium/calmodulin-dependent protein kinase II (CaMKII) activity has been shown to contribute to arrhythmogenesis in a remarkably broad range of cardiac pathologies. Several of these involve significant structural and electrophysiologic remodeling, whereas others are due to specific channelopathies, and are not typically associated with arrhythmogenic changes to protein expression or cellular and tissue structure. The ability of CaMKII to contribute to arrhythmia across such a broad range of phenotypes suggests one of two interpretations regarding the role of CaMKII in cardiac arrhythmia: (1) some CaMKII-dependent mechanism is a common driver of arrhythmia irrespective of the specific etiology of the disease, or (2) these different etiologies expose different mechanisms by which CaMKII is capable of promoting arrhythmia. In this review, we dissect the available mechanistic evidence to explore these two possibilities and discuss how the various molecular actions of CaMKII promote arrhythmia in different pathophysiologic contexts.

CaMKII regulation of cardiac ryanodine receptors and inositol triphosphate receptors

Ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3Rs) are structurally related intracellular calcium release channels that participate in multiple primary or secondary amplified Ca(2+) signals, triggering muscle contraction and oscillatory Ca(2+) waves, or activating transcription factors. In the heart, RyRs play an indisputable role in the process of excitation-contraction coupling as the main pathway for Ca(2+) release from sarcoplasmic reticulum (SR), and a less prominent role in the process of excitation-transcription coupling. Conversely, InsP3Rs are believed to contribute in subtle ways, only, to contraction of the heart, and in more important ways to regulation of transcription factors. Because uncontrolled activity of either RyRs or InsP3Rs may elicit life-threatening arrhythmogenic and/or remodeling Ca(2+) signals, regulation of their activity is of paramount importance for normal cardiac function. Due to their structural similarity, many regulatory factors, accessory proteins, and post-translational processes are equivalent for RyRs and InsP3Rs. Here we discuss regulation of RyRs and InsP3Rs by CaMKII phosphorylation, but touch on other kinases whenever appropriate. CaMKII is emerging as a powerful modulator of RyR and InsP3R activity but interestingly, some of the complexities and controversies surrounding phosphorylation of RyRs also apply to InsP3Rs, and a clear-cut effect of CaMKII on either channel eludes investigators for now. Nevertheless, some effects of CaMKII on global cellular activity, such as SR Ca(2+) leak or force-frequency potentiation, appear clear now, and this constrains the limits of the controversies and permits a more tractable approach to elucidate the effects of phosphorylation at the single channel level.

CaMKII-dependent responses to ischemia and reperfusion challenges in the heart

Ischemic heart disease is a leading cause of death, and there is considerable imperative to identify effective therapeutic interventions. Cardiomyocyte Ca²⁺ overload is a major cause of ischemia and reperfusion injury, initiating a cascade of events culminating in cardiomyocyte death, myocardial dysfunction, and occurrence of lethal arrhythmias. Responsive to fluctuations in intracellular Ca²⁺, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) has emerged as an enticing therapeutic target in the management of ischemic heart injury. CaMKII is activated early in ischemia and to a greater extent in the first few minutes of reperfusion, at a time when reperfusion arrhythmias are particularly prominent. CaMKII phosphorylates and upregulates many of the key proteins involved in intracellular Na⁺ and Ca²⁺ loading in ischemia and reperfusion. Experimentally, selective inhibition of CaMKII activity reduces cardiomyocyte death and arrhythmic incidence post-ischemia. New evidence is emerging that CaMKII actions in ischemia and reperfusion involve specific splice variant targeted actions, selective and localized post-translational modifications, and organelle-directed substrate interactions. A more complete mechanistic understanding of CaMKII mode of action in ischemia and reperfusion is required to optimize intervention opportunities. This review summarizes the current experimentally derived understanding of CaMKII participation in mediating the pathophysiology of the heart in ischemia and in reperfusion, and highlights priority future research directions.

Integrated mechanisms of CaMKII-dependent ventricular remodeling

CaMKII has been shown to be activated during different cardiac pathological processes, and CaMKII-dependent mechanisms contribute to pathological cardiac remodeling, cardiac arrhythmias, and contractile dysfunction during heart failure. Activation of CaMKII during cardiac stress results in a broad number of biological effects such as, on the one hand, acute effects due to phosphorylation of distinct cellular proteins as ion channels and calcium handling proteins and, on the other hand, integrative mechanisms by changing gene expression. This review focuses on transcriptional and epigenetic effects of CaMKII activation during chronic cardiac remodeling. Multiple mechanisms have been described how CaMKII mediates changes in cardiac gene expression. CaMKII has been shown to directly phosphorylate components of the cardiac gene regulation machinery. CaMKII phosphorylates several transcription factors such as CREB that induces the activation of specific gene programs. CaMKII activates transcriptional regulators also indirectly by phosphorylating histone deacetylases, especially HDAC4, which in turn inhibits transcription factors that drive cardiac hypertrophy, fibrosis, and dysfunction. Recent studies demonstrate that CaMKII also phosphorylate directly histones, which may contribute to changes in gene expression. These findings of CaMKII-dependent gene regulation during cardiac remodeling processes suggest novel strategies for CaMKII-dependent “transcriptional or epigenetic therapies” to control cardiac gene expression and function. Manipulation of CaMKII-dependent signaling pathways in the settings of pathological cardiac growth, remodeling, and heart failure represents an auspicious therapeutic approach.

Calcium and IP3 dynamics in cardiac myocytes: experimental and computational perspectives and approaches

Calcium plays a crucial role in excitation-contraction coupling (ECC), but it is also a pivotal second messenger activating Ca²⁺-dependent transcription factors in a process termed excitation-transcription coupling (ETC). Evidence accumulated over the past decade indicates a pivotal role of inositol 1,4,5-trisphosphate receptor (IP₃R)-mediated Ca²⁺ release in the regulation of cytosolic and nuclear Ca²⁺ signals. IP₃ is generated by stimulation of plasma membrane receptors that couple to phospholipase C (PLC), liberating IP₃ from phosphatidylinositol 4,5-bisphosphate (PIP₂). An intriguing aspect of IP₃ signaling is the presence of the entire PIP₂-PLC-IP₃ signaling cascade as well as the presence of IP₃Rs at the inner and outer membranes of the nuclear envelope (NE) which functions as a Ca²⁺ store. The observation that the nucleus is surrounded by its own putative Ca²⁺ store raises the possibility that nuclear IP₃-dependent Ca²⁺ release plays a critical role in ETC. This provides a potential mechanism of regulation that acts locally and autonomously from the global cytosolic Ca²⁺ signal underlying ECC. Moreover, there is evidence that: (i) the sarcoplasmic reticulum (SR) and NE are a single contiguous Ca²⁺ store; (ii) the nuclear pore complex is the major gateway for Ca²⁺ and macromolecules to pass between the cytosol and the nucleoplasm; (iii) the inner membrane of the NE hosts key Ca²⁺ handling proteins including the Na⁺/Ca²⁺ exchanger (NCX)/GM1 complex, ryanodine receptors (RyRs), nicotinic acid adenine dinucleotide phosphate receptors (NAADPRs), Na⁺/K⁺ ATPase, and Na⁺/H⁺ exchanger. Thus, it appears that the nucleus represents a Ca²⁺ signaling domain equipped with its own ion channels and transporters that allow for complex local Ca²⁺ signals. Many experimental and modeling approaches have been used for the study of intracellular Ca²⁺ signaling but the key to the understanding of the dual role of Ca²⁺ mediating ECC and ECT lays in quantitative differences of local [Ca²⁺] in the nuclear and cytosolic compartment. In this review, we discuss the state of knowledge regarding the origin and the physiological implications of nuclear Ca²⁺ transients in different cardiac cell types (adult atrial and ventricular myocytes) as well as experimental and mathematical approaches to study Ca²⁺ and IP₃ signaling in the cytosol and nucleus. In particular, we focus on the concept that highly localized Ca²⁺ signals are required to translocate and activate Ca²⁺-dependent transcription factors (e.g., nuclear factor of activated T-cells, NFAT; histone deacetylase, HDAC) through phosphorylation/dephosphorylation processes.

Control of cytoplasmic and nuclear protein kinase A by phosphodiesterases and phosphatases in cardiac myocytes

AIMS

The cAMP-dependent protein kinase (PKA) mediates β-adrenoceptor (β-AR) regulation of cardiac contraction and gene expression. Whereas PKA activity is well characterized in various subcellular compartments of adult cardiomyocytes, its regulation in the nucleus remains largely unknown. The aim of the present study was to compare the modalities of PKA regulation in the cytoplasm and nucleus of cardiomyocytes.

METHODS AND RESULTS

Cytoplasmic and nuclear cAMP and PKA activity were measured with targeted fluorescence resonance energy transfer probes in adult rat ventricular myocytes. β-AR stimulation with isoprenaline (Iso) led to fast cAMP elevation in both compartments, whereas PKA activity was fast in the cytoplasm but markedly slower in the nucleus. Iso was also more potent and efficient in activating cytoplasmic than nuclear PKA. Similar slow kinetics of nuclear PKA activation was observed upon adenylyl cyclase activation with L-858051 or phosphodiesterase (PDE) inhibition with 3-isobutyl-1-methylxantine. Consistently, pulse stimulation with Iso (15 s) maximally induced PKA and myosin-binding protein C phosphorylation in the cytoplasm, but marginally activated PKA and cAMP response element-binding protein phosphorylation in the nucleus. Inhibition of PDE4 or ablation of the Pde4d gene in mice prolonged cytoplasmic PKA activation and enhanced nuclear PKA responses. In the cytoplasm, phosphatase 1 (PP1) and 2A (PP2A) contributed to the termination of PKA responses, whereas only PP1 played a role in the nucleus.

CONCLUSION

Our study reveals a differential integration of cytoplasmic and nuclear PKA responses to β-AR stimulation in cardiac myocytes. This may have important implications in the physiological and pathological hypertrophic response to β-AR stimulation.

Endothelin-1 signalling controls early embryonic heart rate in vitro and in vivo

AIM

Spontaneous activity of embryonic cardiomyocytes originates from sarcoplasmic reticulum (SR) Ca(2+) release during early cardiogenesis. However, the regulation of heart rate during embryonic development is still not clear. The aim of this study was to determine how endothelin-1 (ET-1) affects the heart rate of embryonic mice, as well as the pathway through which it exerts its effects.

METHODS

The effects of ET-1 and ET-1 receptor inhibition on cardiac contraction were studied using confocal Ca(2+) imaging of isolated mouse embryonic ventricular cardiomyocytes and ultrasonographic examination of embryonic cardiac contractions in utero. In addition, the amount of ET-1 peptide and ET receptor a (ETa) and b (ETb) mRNA levels were measured during different stages of development of the cardiac muscle.

RESULTS

High ET-1 concentration and expression of both ETa and ETb receptors was observed in early cardiac tissue. ET-1 was found to increase the frequency of spontaneous Ca(2+) oscillations in E10.5 embryonic cardiomyocytes in vitro. Non-specific inhibition of ET receptors with tezosentan caused arrhythmia and bradycardia in isolated embryonic cardiomyocytes and in whole embryonic hearts both in vitro (E10.5) and in utero (E12.5). ET-1-mediated stimulation of early heart rate was found to occur via ETb receptors and subsequent inositol trisphosphate receptor activation and increased SR Ca(2+) leak.

CONCLUSION

Endothelin-1 is required to maintain a sufficient heart rate, as well as to prevent arrhythmia during early development of the mouse heart. This is achieved through ETb receptor, which stimulates Ca(2+) leak through IP3 receptors.

Calcineurin-NFATc regulates type 2 inositol 1,4,5-trisphosphate receptor (InsP3R2) expression during cardiac remodeling

In heart, the type 2 inositol 1,4,5-triphosphate receptor (InsP3R2) is the predominant isoform expressed and is localized in the nuclear membrane of ventricular myocytes. InsP3R2-mediated Ca(2+) release regulates hypertrophy specific gene expression by modulating CaMKIIδ, histone deacetylase, and calcineurin-NFATc signaling pathways. InsP3R2 protein is a hypertrophy specific marker and is overexpressed in heart failure animal models and in humans. However, the regulation of InsP3R2 mRNA and protein expression during cardiac hypertrophy and heart failure is not known. Here we show the transcriptional regulation of the Itpr2 gene in adult cardiomyocytes. Our data demonstrates that, InsP3R2 mRNA and protein expression is activated by hypertrophic agonists and attenuated by InsP3R inhibitors 2-aminoethoxyldiphenyl borate and xestospongin-C. The Itpr2 promoter is regulated by the calcineurin-NFATc signaling pathway. NFATc1 regulates Itpr2 gene expression by directly binding to the Itpr2 promoter. The calcineurin-NFATc mediated up-regulation of the Itpr2 promoter was attenuated by cyclosporine-A. InsP3R2 mRNA and protein expression was up-regulated in calcineurin-A transgenic mice and in human heart failure. Collectively, our data suggests that ITPR2 and hypertrophy specific gene expression is regulated, in part, by a positive feedback regulation between InsP3R2 and calcineurin-NFATc signaling pathways.

Spatially defined InsP3-mediated signaling in embryonic stem cell-derived cardiomyocytes

The functional role of inositol 1,4,5-trisphosphate (InsP3) signaling in cardiomyocytes is not entirely understood but it was linked to an increased propensity for triggered activity. The aim of this study was to determine how InsP3 receptors can translate Ca(2+) release into a depolarization of the plasma membrane and consequently arrhythmic activity. We used embryonic stem cell-derived cardiomyocytes (ESdCs) as a model system since their spontaneous electrical activity depends on InsP3-mediated Ca(2+) release. [InsP3]i was monitored with the FRET-based InsP3-biosensor FIRE-1 (Fluorescent InsP3 Responsive Element) and heterogeneity in sub-cellular [InsP3]i was achieved by targeted expression of FIRE-1 in the nucleus (FIRE-1nuc) or expression of InsP3 5-phosphatase (m43) localized to the plasma membrane. Spontaneous activity of ESdCs was monitored simultaneously as cytosolic Ca(2+) transients (Fluo-4/AM) and action potentials (current clamp). During diastole, the diastolic depolarization was paralleled by an increase of [Ca(2+)]i and spontaneous activity was modulated by [InsP3]i. A 3.7% and 1.7% increase of FIRE-1 FRET ratio and 3.0 and 1.5 fold increase in beating frequency was recorded upon stimulation with endothelin-1 (ET-1, 100 nmol/L) or phenylephrine (PE, 10 µmol/L), respectively. Buffering of InsP3 by FIRE-1nuc had no effect on the basal frequency while attenuation of InsP3 signaling throughout the cell (FIRE-1), or at the plasma membrane (m43) resulted in a 53.7% and 54.0% decrease in beating frequency. In m43 expressing cells the response to ET-1 was completely suppressed. Ca(2+) released from InsP3Rs is more effective than Ca(2+) released from RyRs to enhance INCX. The results support the hypothesis that in ESdCs InsP3Rs form a functional signaling domain with NCX that translates Ca(2+) release efficiently into a depolarization of the membrane potential.

Detection of differentially regulated subsarcolemmal calcium signals activated by vasoactive agonists in rat pulmonary artery smooth muscle cells

Intracellular calcium (Ca(2+)) plays pivotal roles in distinct cellular functions through global and local signaling in various subcellular compartments, and subcellular Ca(2+) signal is the key factor for independent regulation of different cellular functions. In vascular smooth muscle cells, subsarcolemmal Ca(2+) is an important regulator of excitation-contraction coupling, and nucleoplasmic Ca(2+) is crucial for excitation-transcription coupling. However, information on Ca(2+) signals in these subcellular compartments is limited. To study the regulation of the subcellular Ca(2+) signals, genetically encoded Ca(2+) indicators (cameleon), D3cpv, targeting the plasma membrane (PM), cytoplasm, and nucleoplasm were transfected into rat pulmonary arterial smooth muscle cells (PASMCs) and Ca(2+) signals were monitored using laser scanning confocal microscopy. In situ calibration showed that the Kd for Ca(2+) of D3cpv was comparable in the cytoplasm and nucleoplasm, but it was slightly higher in the PM. Stimulation of digitonin-permeabilized cells with 1,4,5-trisphosphate (IP3) elicited a transient elevation of Ca(2+) concentration with similar amplitude and kinetics in the nucleoplasm and cytoplasm. Activation of G protein-coupled receptors by endothelin-1 and angiotensin II preferentially elevated the subsarcolemmal Ca(2+) signal with higher amplitude in the PM region than the nucleoplasm and cytoplasm. In contrast, the receptor tyrosine kinase activator, platelet-derived growth factor, elicited Ca(2+) signals with similar amplitudes in all three regions, except that the rise-time and decay-time were slightly slower in the PM region. These data clearly revealed compartmentalization of Ca(2+) signals in the subsarcolemmal regions and provide the basis for further investigations of differential regulation of subcellular Ca(2+) signals in PASMCs.

Histone deacetylase signaling in cardioprotection

Cardiovascular disease (CVD) represents a major challenge for health care systems, both in terms of the high mortality associated with it and the huge economic burden of its treatment. Although CVD represents a diverse range of disorders, they share common compensatory changes in the heart at the structural, cellular, and molecular level that, in the long term, can become maladaptive and lead to heart failure. Treatment of adverse cardiac remodeling is therefore an important step in preventing this fatal progression. Although previous efforts have been primarily focused on inhibition of deleterious signaling cascades, the stimulation of endogenous cardioprotective mechanisms offers a potent therapeutic tool. In this review, we discuss class I and class II histone deacetylases, a subset of chromatin-modifying enzymes known to have critical roles in the regulation of cardiac remodeling. In particular, we discuss their molecular modes of action and go on to consider how their inhibition or the stimulation of their intrinsic cardioprotective properties may provide a potential therapeutic route for the clinical treatment of CVD.

Cardiac sarcoplasmic reticulum calcium leak: basis and roles in cardiac dysfunction

Synchronized SR calcium (Ca) release is critical to normal cardiac myocyte excitation-contraction coupling, and ideally this release shuts off completely between heartbeats. However, other SR Ca release events are referred to collectively as SR Ca leak (which includes Ca sparks and waves as well as smaller events not detectable as Ca sparks). Much, but not all, of the SR Ca leak occurs via ryanodine receptors and can be exacerbated in pathological states such as heart failure. The extent of SR Ca leak is important because it can (a) reduce SR Ca available for release, causing systolic dysfunction; (b) elevate diastolic [Ca]i, contributing to diastolic dysfunction; (c) cause triggered arrhythmias; and (d) be energetically costly because of extra ATP used to repump Ca. This review addresses quantitative aspects and manifestations of SR Ca leak and its measurement, and how leak is modulated by Ca, associated proteins, and posttranslational modifications in health and disease.

Roles for the sarco-/endoplasmic reticulum in cardiac myocyte contraction, protein synthesis, and protein quality control

Although the function of the sarcoplasmic/endoplasmic reticulum (SR/ER) in cardiac contractile calcium handling is well established, its roles in protein synthesis, folding, and quality control in cardiac myocytes are not as clear. This review explores evidence suggesting that, in cardiac myocytes, protein synthesis and contractile calcium handling may be physically and functionally integrated.

Three-dimensional reconstruction and analysis of the tubular system of vertebrate skeletal muscle

Skeletal muscle fibres are very large and elongated. In response to excitation there must be a rapid and uniform release of Ca(2+) throughout for contraction. To ensure a uniform spread of excitation throughout the fibre to all the Ca(2+) release sites, the muscle internalizes the plasma membrane, to form the tubular (t-) system. Hence the t-system forms a complex and dense network throughout the fibre that is responsible for excitation-contraction coupling and other signalling mechanisms. However, we currently do not have a very detailed view of this membrane network because of limitations in previously used imaging techniques to visualize it. In this study we serially imaged fluorescent dye trapped in the t-system of fibres from rat and toad muscle using the confocal microscope, and deconvolved and reconstructed these images to produce the first three-dimensional reconstructions of large volumes of the vertebrate t-system. These images showed complex arrangements of tubules that have not been described previously and also allowed the association of the t-system with cellular organelles to be visualized. There was a high density of tubules close to the nuclear envelope because of the close and parallel alignment of the long axes of the myofibrils and the nuclei. Furthermore local fluorescence intensity variations from sub-resolution tubules were converted to tubule diameters. Mean diameters of tubules were 85.9±6.6 and 91.2±8.2 nm, from rat and toad muscle under isotonic conditions, respectively. Under osmotic stress the distribution of tubular diameters shifted significantly in toad muscle only, with change specifically occurring in the transverse but not longitudinal tubules.