Endothelin-1 enhances nuclear Ca2+ transients in atrial myocytes through Ins(1,4,5)P3-dependent Ca2+ release from perinuclear Ca2+ stores

Store-Operated Calcium Entry Increases Nuclear Calcium in Adult Rat Atrial and Ventricular Cardiomyocytes

Store-operated calcium entry (SOCE) in cardiomyocytes may be involved in cardiac remodeling, but the underlying mechanisms remain elusive. We hypothesized that SOCE may increase nuclear calcium, which alters gene expression via calcium/calmodulin-dependent enzyme signaling, and elucidated the underlying cellular mechanisms. An experimental protocol was established in isolated adult rat cardiomyocytes to elicit SOCE by re-addition of calcium following complete depletion of sarcoplasmic reticulum (SR) calcium and to quantify SOCE in relation to the electrically stimulated calcium transient (CaT) measured in the same cell before SR depletion. Using confocal imaging, calcium changes were recorded simultaneously in the cytosol and in the nucleus of the cell. In ventricular myocytes, SOCE was observed in the cytosol and nucleus amounting to ≈15% and ≈25% of the respective CaT. There was a linear correlation between the SOCE-mediated calcium increase in the cytosol and nucleus. Inhibitors of TRPC or Orai channels reduced SOCE by ≈33-67%, whereas detubulation did not. In atrial myocytes, SOCE with similar characteristics was observed in the cytosol and nucleus. However, the SOCE amplitudes in atrial myocytes were ≈two-fold larger than in ventricular myocytes, and this was associated with ≈1.4- to 3.6-fold larger expression of putative SOCE proteins (TRPC1, 3, 6, and STIM1) in atrial tissue. The results indicated that SOCE in atrial and ventricular myocytes is able to cause robust calcium increases in the nucleus and that both TRPC and Orai channels may contribute to SOCE in adult cardiomyocytes.

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.

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’.

Measuring nuclear calcium and actin assembly in living cells

Nuclear calcium signalling has emerged as a critical mechanism regulating processes like chromatin organization and gene expression. Recently, we have shown that nuclear calcium elevation triggers rapid and transient actin filament assembly inside the nucleus. Here, we constructed and employed a nuclear-specific calcium sensor based upon the new generation of genetically encoded probes jGCaMP7f. By fusing a nuclear localization signal to jGCaMP7f, we achieved highly efficient nuclear-specific targeting. Comparing the jGCaMP7f-NLS probe with the previous GCaMP6f-NLS calcium sensor showed clearly that jGCaMP7f-NLS is more sensitive and reverses significantly quicker thereby reflecting rapid nuclear calcium transients in a closely physiological manner. We further confirm that nuclear calcium transients precede nuclear actin polymerization by several seconds. Our data show that calcium-triggered nuclear actin assembly in fibroblasts is independent of the actin nucleating Arp2/3 complex. Together, jGCaMP7f-NLS represents an easy to use, reliable and highly sensitive nuclear calcium sensor that allows to tightly interrogate real-time, spatiotemporal calcium signalling and calcium-elicited effects in the nucleus of living cells.

Angiotensin Receptors Heterodimerization and Trafficking: How Much Do They Influence Their Biological Function?

G-protein–coupled receptors (GPCRs) are targets for around one third of currently approved and clinical prescribed drugs and represent the largest and most structurally diverse family of transmembrane signaling proteins, with almost 1000 members identified in the human genome. Upon agonist stimulation, GPCRs are internalized and trafficked inside the cell: they may be targeted to different organelles, recycled back to the plasma membrane or be degraded. Once inside the cell, the receptors may initiate other signaling pathways leading to different biological responses. GPCRs’ biological function may also be influenced by interaction with other receptors. Thus, the ultimate cellular response may depend not only on the activation of the receptor from the cell membrane, but also from receptor trafficking and/or the interaction with other receptors. This review is focused on angiotensin receptors and how their biological function is influenced by trafficking and interaction with others receptors.

CaMKIIδC Drives Early Adaptive Ca2+ Change and Late Eccentric Cardiac Hypertrophy

Supplemental Digital Content is available in the text.

CaMKII (Ca²⁺-Calmodulin dependent protein kinase) δC activation is implicated in pathological progression of heart failure (HF) and CaMKIIδC transgenic mice rapidly develop HF and arrhythmias. However, little is known about early spatio-temporal Ca²⁺ handling and CaMKII activation in hypertrophy and HF.

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.

SERCA Activity Controls the Systolic Calcium Increase in the Nucleus of Cardiac Myocytes

In cardiomyocytes, nuclear calcium is involved in regulation of transcription and, thus, remodeling. The cellular mechanisms regulating nuclear calcium, however, remain elusive. Therefore, the aim of this study was to identify and characterize the factors that regulate nuclear calcium in cardiomyocytes. We focused on the roles of (1) the cytoplasmic calcium transient (CaT), (2) the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA), and (3) intracellular calcium stores for nuclear calcium handling. Experiments were performed on rat ventricular myocytes loaded with Fluo-4/AM. Subcellularly resolved CaTs were visualized using confocal microscopy. The cytoplasmic CaT was varied by reducing extracellular calcium (from 1.5 to 0.3 mM) or by exposure to isoprenaline (ISO, 10 nM). SERCA was blocked by thapsigargin (5 μM). There was a strict linear dependence of the nucleoplasmic CaT on the cytoplasmic CaT over a wide range of calcium concentrations. Increasing SERCA activity impaired, whereas decreasing SERCA activity augmented the systolic calcium increase in the nucleus. Perinuclear calcium store load, on the other hand, did not change with either 0.3 mM calcium or ISO and was not a decisive factor for the nucleoplasmic CaT. The results indicate, that the nucleoplasmic CaT is determined largely by the cytoplasmic CaT via diffusion of calcium through nuclear pores. They identify perinuclear SERCA activity, which limits the systolic calcium increase in the nucleus, as a novel regulator of the nuclear CaT in cardiac myocytes.

Sarcoplasmic reticulum Ca2+ -induced Ca2+ release regulates class IIa HDAC localization in mouse embryonic cardiomyocytes

In embryonic cardiomyocytes, sarcoplasmic reticulum (SR)‐derived Ca²⁺ release is required to induce Ca²⁺ oscillations for contraction and to control cardiac development through Ca²⁺‐activated pathways. Here, our aim was to study how SR Ca²⁺ release regulates cytosolic and nuclear Ca²⁺ distribution and the subsequent effects on the Ca²⁺‐dependent localization of class IIa histone deacetylases (HDAC) and cardiac‐specific gene expression in embryonic cardiomyocytes. Confocal microscopy was used to study changes in Ca²⁺‐distribution and localization of immunolabeled HDAC4 and HDAC5 upon changes in SR Ca²⁺ release in mouse embryonic cardiomyocytes. Dynamics of translocation were also observed with a confocal microscope, using HDAC5‐green fluorescent protein transfected myocytes. Expression of class IIa HDACs in differentiating myocytes and changes in cardiac‐specific gene expression were studied using real‐time quantitative PCR. Inhibition of SR Ca²⁺ release caused a significant decrease in intranuclear Ca²⁺ concentration, a rapid nuclear import of HDAC5 and subnuclear redistribution of HDAC4. Endogenous localization of HDAC5 and HDAC4 was mostly cytosolic and at the nuclear periphery, respectively. Downregulated expression of cardiac‐specific genes was also observed upon SR Ca²⁺ release inhibition. Electrical stimulation of sarcolemmal Ca²⁺ influx was not sufficient to rescue either the HDAC localization or the gene expression changes. SR Ca²⁺ release controls subcellular Ca²⁺ distribution and regulates localization of HDAC4 and HDAC5 in embryonic cardiomyocytes. Changes in SR Ca²⁺ release also caused changes in expression of the developmental phase‐specific genes, which may be due to the changes in HDAC‐localization.

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.

Role of inositol 1,4,5-trisphosphate receptor type 1 in ATP-induced nuclear Ca2+ signal and hypertrophy in atrial myocytes

Inositol 1,4,5-trisphosphate receptor type 1 (IP₃R1) is expressed in atrial muscle, but not in ventricle, and they are abundant in the perinucleus. We investigated the role of IP₃R1 in the regulations of local Ca²⁺ signal and cell size in HL-1 atrial myocytes under stimulation by IP₃-generating chemical messenger, ATP. Assessment of nuclear and cytosolic Ca²⁺ signal using confocal Ca²⁺ imaging revealed that IP₃ generation by ATP (1 mM) induced monophasic nuclear Ca²⁺ increase, followed by cytosolic Ca²⁺ oscillation. Genetic knock-down (KD) of IP₃R1 eliminated the monophasic nuclear Ca²⁺ signal and slowed the cytosolic Ca²⁺ oscillation upon ATP exposure. Prolonged application of ATP as well as other known hypertrophic agonists (endothelin-1 and phenylephrine) increased cell size in wild-type cells, but not in IP₃R1 KD cells. Our data indicate that IP₃R1 mediates sustained elevation in nuclear Ca²⁺ level and facilitates cytosolic Ca²⁺ oscillation upon external ATP increase, and further suggests possible role of nuclear IP₃R1 in atrial hypertrophy.

Inositol 1,4,5-trisphosphate-mediated sarcoplasmic reticulum-mitochondrial crosstalk influences adenosine triphosphate production via mitochondrial Ca2+ uptake through the mitochondrial ryanodine receptor in cardiac myocytes

AIMS

Elevated levels of inositol 1,4,5-trisphosphate (IP3) in adult cardiac myocytes are typically associated with the development of cardiac hypertrophy, arrhythmias, and heart failure. IP3 enhances intracellular Ca(2+ )release via IP3 receptors (IP3Rs) located at the sarcoplasmic reticulum (SR). We aimed to determine whether IP3-induced Ca(2+ )release affects mitochondrial function and determine the underlying mechanisms.

METHODS AND RESULTS

We compared the effects of IP3Rs- and ryanodine receptors (RyRs)-mediated cytosolic Ca(2+ )elevation achieved by endothelin-1 (ET-1) and isoproterenol (ISO) stimulation, respectively, on mitochondrial Ca(2+ )uptake and adenosine triphosphate (ATP) generation. Both ET-1 and isoproterenol induced an increase in mitochondrial Ca(2+ )(Ca(2 +) m) but only ET-1 led to an increase in ATP concentration. ET-1-induced effects were prevented by cell treatment with the IP3 antagonist 2-aminoethoxydiphenyl borate and absent in myocytes from transgenic mice expressing an IP3 chelating protein (IP3 sponge). Furthermore, ET-1-induced mitochondrial Ca(2+) uptake was insensitive to the mitochondrial Ca(2+ )uniporter inhibitor Ru360, however was attenuated by RyRs type 1 inhibitor dantrolene. Using real-time polymerase chain reaction, we detected the presence of all three isoforms of IP3Rs and RyRs in murine ventricular myocytes with a dominant presence of type 2 isoform for both receptors.

CONCLUSIONS

Stimulation of IP3Rs with ET-1 induces Ca(2+ )release from the SR which is tunnelled to mitochondria via mitochondrial RyR leading to stimulation of mitochondrial ATP production.

Nuclear calcium in cardiac myocytes

Calcium (Ca) is a universal second messenger involved in the regulation of various cellular processes, including electrical signaling, contraction, secretion, memory, gene transcription, and cell death. In heart, Ca governs cardiomyocyte contraction, is central in electrophysiological properties, and controls major signaling pathway implicated in gene transcription. How cardiomyocytes decode Ca signal to regulate gene expression without interfering with, or being controlled by, "contractile" Ca that floods the entire cytosol during each heartbeat is still elusive. In this review, we summarize recent findings on nuclear Ca regulation and its downstream signaling in cardiomyocytes. We will address difficulties in reliable quantification of nuclear Ca fluxes and discuss its role in the development and progression of cardiac hypertrophy and heart failure. We also point out key open questions to stimulate future work.

A stromal interaction molecule 1 variant up-regulates matrix metalloproteinase-2 expression by strengthening nucleoplasmic Ca2+ signaling

Very recent studies hold promise to reveal the role of stromal interaction molecule 1 (STIM1) in non-store-operated Ca2+ entry. Here we showed that in contrast to cytoplasmic membrane redistribution as previously noted, human umbilical vein endothelial STIM1 with a T-to-C nucleotide transition resulting in an amino acid substitution of leucine by proline in the signal peptide sequence translocated to perinuclear membrane upon intracellular Ca2+ depletion, amplified nucleoplasmic Ca2+ signaling through ryanodine receptor-dependent pathway, and enhanced the subsequent cAMP responsive element binding protein activity, matrix metalloproteinase-2 (MMP-2) gene expression, and endothelial tube forming. The abundance of mutated STIM1 and the MMP-2 expression were higher in native human umbilical vein endothelial cells of patients with gestational hypertension than controls and were significantly correlated with blood pressure. These findings broaden our understanding about structure-function bias of STIM1 and offer unique insights into its application in nucleoplasmic Ca2+, MMP-2 expression, endothelial dysfunction, and pathophysiological mechanism(s) of gestational hypertension.

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.

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.

Redox control of cardiac remodeling in atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice and is a potential cause of thromboembolic events. AF induces significant changes in the electrophysiological properties of atrial myocytes and causes alterations in the structure, metabolism, and function of the atrial tissue. The molecular basis for the development of structural atrial remodeling of fibrillating human atria is still not fully understood. However, increased production of reactive oxygen or nitrogen species (ROS/RNS) and the activation of specific redox-sensitive signaling pathways observed both in patients with and animal models of AF are supposed to contribute to development, progression and self-perpetuation of AF.

SCOPE OF REVIEW

The present review summarizes the sources and targets of ROS/RNS in the setting of AF and focuses on key redox-sensitive signaling pathways that are implicated in the pathogenesis of AF and function either to aggravate or protect from disease.

MAJOR CONCLUSIONS

NADPH oxidases and various mitochondrial monooxygenases are major sources of ROS during AF. Besides direct oxidative modification of e.g. ion channels and ion handling proteins that are crucially involved in action potential generation and duration, AF leads to the reversible activation of redox-sensitive signaling pathways mediated by activation of redox-regulated proteins including Nrf2, NF-κB, and CaMKII. Both processes are recognized to contribute to the formation of a substrate for AF and, thus, to increase AF inducibility and duration.

GENERAL SIGNIFICANCE

AF is a prevalent disease and due to the current demographic developments its socio-economic relevance will further increase. Improving our understanding of the role that ROS and redox-related (patho)-mechanisms play in the development and progression of AF may allow the development of a targeted therapy for AF that surpasses the efficacy of previous general anti-oxidative strategies. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

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.

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.

Urocortin 2 stimulates nitric oxide production in ventricular myocytes via Akt- and PKA-mediated phosphorylation of eNOS at serine 1177

Urocortin 2 (Ucn2) is a cardioactive peptide exhibiting beneficial effects in normal and failing heart. In cardiomyocytes, it elicits cAMP- and Ca(2+)-dependent positive inotropic and lusitropic effects. We tested the hypothesis that, in addition, Ucn2 activates cardiac nitric oxide (NO) signaling and elucidated the underlying signaling pathways and mechanisms. In isolated rabbit ventricular myocytes, Ucn2 caused concentration- and time-dependent increases in phosphorylation of Akt (Ser473, Thr308), endothelial NO synthase (eNOS) (Ser1177), and ERK1/2 (Thr202/Tyr204). ERK1/2 phosphorylation, but not Akt and eNOS phosphorylation, was suppressed by inhibition of MEK1/2. Increased Akt phosphorylation resulted in increased Akt kinase activity and was mediated by corticotropin-releasing factor 2 (CRF2) receptors (astressin-2B sensitive). Inhibition of phosphatidylinositol 3-kinase (PI3K) diminished both Akt as well as eNOS phosphorylation mediated by Ucn2. Inhibition of protein kinase A (PKA) reduced Ucn2-induced phosphorylation of eNOS but did not affect the increase in phosphorylation of Akt. Conversely, direct receptor-independent elevation of cAMP via forskolin increased phosphorylation of eNOS but not of Akt. Ucn2 increased intracellular NO concentration ([NO]i), [cGMP], [cAMP], and cell shortening. Inhibition of eNOS suppressed the increases in [NO]i and cell shortening. When both PI3K-Akt and cAMP-PKA signaling were inhibited, the Ucn2-induced increases in [NO]i and cell shortening were attenuated. Thus, in rabbit ventricular myocytes, Ucn2 causes activation of cAMP-PKA, PI3K-Akt, and MEK1/2-ERK1/2 signaling. The MEK1/2-ERK1/2 pathway is not required for stimulation of NO signaling in these cells. The other two pathways, cAMP-PKA and PI3K-Akt, converge on eNOS phosphorylation at Ser1177 and result in pronounced and sustained cellular NO production with subsequent stimulation of cGMP signaling.

The role of the paracrine/autocrine mediator endothelin-1 in regulation of cardiac contractility and growth

Endothelin-1 (ET-1) is a critical autocrine and paracrine regulator of cardiac physiology and pathology. Produced locally within the myocardium in response to diverse mechanical and neurohormonal stimuli, ET-1 acutely modulates cardiac contractility. During pathological cardiovascular conditions such as ischaemia, left ventricular hypertrophy and heart failure, myocyte expression and activity of the entire ET-1 system is enhanced, allowing the peptide to both initiate and maintain maladaptive cellular responses. Both the acute and chronic effects of ET-1 are dependent on the activation of intracellular signalling pathways, regulated by the inositol-trisphosphate and diacylglycerol produced upon activation of the ET(A) receptor. Subsequent stimulation of protein kinases C and D, calmodulin-dependent kinase II, calcineurin and MAPKs modifies the systolic calcium transient, myofibril function and the activity of transcription factors that coordinate cellular remodelling. The precise nature of the cellular response to ET-1 is governed by the timing, localization and context of such signals, allowing the peptide to regulate both cardiomyocyte physiology and instigate disease.

LINKED ARTICLES

This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca²⁺ in adult cardiac myocytes

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca(2+) signaling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with peptide: N-glycosidase F did not alter the electrophoretic mobility of ETB. The absence of N-glycosylation further indicates that these receptors did not originate at the cell surface. Intracellular photolysis of a caged ET-1 analog ([Trp-ODMNB(21)]ET-1) evoked an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was attenuated by inositol 1,4,5-trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and prevented by pre-treatment with ryanodine. A caged cell-permeable analog of the ETB-selective antagonist IRL-2500 blocked the ability of intracellular cET-1 to increase [Ca(2+)]n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffics directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulates nuclear Ca(2+) signaling.

The other side of cardiac Ca(2+) signaling: transcriptional control

Ca²⁺ is probably the most versatile signal transduction element used by all cell types. In the heart, it is essential to activate cellular contraction in each heartbeat. Nevertheless Ca²⁺ is not only a key element in excitation-contraction coupling (EC coupling), but it is also a pivotal second messenger in cardiac signal transduction, being able to control processes such as excitability, metabolism, and transcriptional regulation. Regarding the latter, Ca²⁺ activates Ca²⁺-dependent transcription factors by a process called excitation-transcription coupling (ET coupling). ET coupling is an integrated process by which the common signaling pathways that regulate EC coupling activate transcription factors. Although ET coupling has been extensively studied in neurons and other cell types, less is known in cardiac muscle. Some hints have been found in studies on the development of cardiac hypertrophy, where two Ca²⁺-dependent enzymes are key actors: Ca²⁺/Calmodulin kinase II (CaMKII) and phosphatase calcineurin, both of which are activated by the complex Ca²⁺/Calmodulin. The question now is how ET coupling occurs in cardiomyocytes, where intracellular Ca²⁺ is continuously oscillating. In this focused review, we will draw attention to location of Ca²⁺ signaling: intranuclear ([Ca²⁺]_n) or cytoplasmic ([Ca²⁺]_c), and the specific ionic channels involved in the activation of cardiac ET coupling. Specifically, we will highlight the role of the 1,4,5 inositol triphosphate receptors (IP₃Rs) in the elevation of [Ca²⁺]_n levels, which are important to locally activate CaMKII, and the role of transient receptor potential channels canonical (TRPCs) in [Ca²⁺]_c, needed to activate calcineurin (Cn).

Nuclear inositol 1,4,5-trisphosphate is a necessary and conserved signal for the induction of both pathological and physiological cardiomyocyte hypertrophy

It is well established that inositol 1,4,5-trisphosphate (IP3) dependent Ca(2+) signaling plays a crucial role in cardiomyocyte hypertrophy. However, it is not yet known whether nuclear IP3 represents a Ca(2+) mobilizing pathway involved in this process. The goal of the current work was to investigate the specific role of nuclear IP3 in cardiomyocyte hypertrophic response. In this work, we used an adenovirus construct that selectively buffers IP3 in the nuclear region of neonatal cardiomyocytes. We showed for the first time that nuclear IP3 mediates endothelin-1 (ET-1) induced hypertrophy. We also found that both calcineurin (Cn)/nuclear factor of activated T Cells (NFAT) and histone deacetylase-5 (HDAC5) pathways require nuclear IP3 to mediate pathological cardiomyocyte growth. Additionally, we found that nuclear IP3 buffering inhibited insulin-like growth factor-1 (IGF-1) induced hypertrophy and prevented reexpression of fetal gene program. Together, these results demonstrated that nuclear IP3 is an essential and a conserved signal for both pathological and physiological forms of cardiomyocyte hypertrophy.

Calsequestrin accumulation in rough endoplasmic reticulum promotes perinuclear Ca2+ release

Molecular mechanisms underlying Ca(2+) regulation by perinuclear endoplasmic/sarcoplasmic reticulum (ER/SR) cisternae in cardiomyocytes remain obscure. To investigate the mechanisms of changes in cardiac calsequestrin (CSQ2) trafficking on perinuclear Ca(2+) signaling, we manipulated the subcellular distribution of CSQ2 by overexpression of CSQ2-DsRed, which specifically accumulates in the perinuclear rough ER. Adult ventricular myocytes were infected with adenoviruses expressing CSQ2-DsRed, CSQ2-WT, or empty vector. We found that perinuclear enriched CSQ2-DsRed, but not normally distributed CSQ2-WT, enhanced nuclear Ca(2+) transients more potently than cytosolic Ca(2+) transients. Overexpression of CSQ2-DsRed produced more actively propagating Ca(2+) waves from perinuclear regions than did CSQ2-WT. Activities of the SR/ER Ca(2+)-ATPase and ryanodine receptor type 2, but not inositol 1,4,5-trisphosphate receptor type 2, were required for the generation of these perinuclear initiated Ca(2+) waves. In addition, CSQ2-DsRed was more potent than CSQ2-WT in inducing cellular hypertrophy in cultured neonatal cardiomyocytes. Our data demonstrate for the first time that CSQ2 retention in the rough ER/perinuclear region promotes perinuclear Ca(2+) signaling and predisposes to ryanodine receptor type 2-mediated Ca(2+) waves from CSQ2-enriched perinuclear compartments and myocyte hypotrophy. These findings provide new insights into the mechanism of CSQ2 in Ca(2+) homeostasis, suggesting that rough ER-localized Ca(2+) stores can operate independently in raising levels of cytosolic/nucleoplasmic Ca(2+) as a source of Ca(2+) for Ca(2+)-dependent signaling in health and disease.

Role of the nuclear envelope in calcium signalling

The endoplasmic reticulum (ER) is the major Ca(2+) store inside the cell. Its organisation in specialised subdomains allows the local delivery of Ca(2+) to specific cell areas on stimulation. The nuclear envelope (NE), which is continuous with the ER, has a double role: it insulates the nucleoplasm from the cytoplasm and it stores Ca(2+) around the nucleus. Furthermore, all the constituents of the signalling cascade leading to Ca(2+) mobilisation are found in the NE; this allows the nuclear Ca(2+) to be regulated autonomously. On the other hand, cytosolic Ca(2+) transients can propagate within the nucleus via the nuclear pore complex. The variations in nuclear Ca(2+) concentration are important for controlling gene transcription and progression in the cell cycle. Recent data suggest that invaginations of the NE modify the morphology of the nucleus and may affect Ca(2+) dynamics in the nucleus and regulate transcriptional activity.

Cardiomyocyte calcineurin signaling in subcellular domains: from the sarcolemma to the nucleus and beyond

The serine-threonine phosphatase calcineurin is activated in cardiac myocytes in the diseased heart and induces pathological hypertrophy. Calcineurin activity is mainly triggered by calcium/calmodulin binding but also through calpain mediated cleavage. How controlled calcineurin activation is possible in cardiac myocytes, which typically show a 10-fold difference in cytosolic calcium concentration with every heartbeat, has remained enigmatic. It is now emerging that calcineurin activation and signaling occur in subcellular microdomains, in which it is brought together with target proteins and exceedingly high concentrations of calcium in order to induce downstream signaling. We review current evidence of subcellular calcineurin mainly at the sarcolemma and the nucleus, but also in association with the sarcoplasmic reticulum and mitochondria. We also suggest that knowledge about subcellular signaling could help to develop inhibitors of calcineurin in specific microdomains to avoid side-effects that may arise from complete calcineurin inhibition.

Ca2+-induced PARP-1 activation and ANF expression are coupled events in cardiomyocytes

The nuclear protein PARP-1 [poly(ADP-ribose) polymerase-1] is activated in cardiomyocytes exposed to hypoxia causing DNA breaks. Unlike this stress-induced PARP-1 activation, our results provide evidence for Ca(2+)-induced PARP-1 activation in contracting newborn cardiomyocytes treated with growth factors and hormones that increased their contraction rate, induced intracellular Ca(2+) mobilization and its rhythmical and transient translocation into the nucleus. Furthermore, activated PARP-1 up-regulated the activity of phosphorylated ERK (extracellular-signal-regulated kinase) in the nucleus, promoting expression of the Elk1 target gene c-fos. Up-regulation of the transcription factor c-Fos/GATA-4 promoted ANF (atrial natriuretic factor) expression. Given that expression of ANF is known to be implicated in morphological changes, growth and development of cardiomyocytes, these results outline a PARP-1-dependent signal transduction mechanism that links contraction rate and Ca(2+) mobilization with the expression of genes underlying morphological changes in cardiomyocytes.

In situ calibration of nucleoplasmic versus cytoplasmic Ca²+ concentration in adult cardiomyocytes

Quantification of subcellularly resolved Ca²⁺ signals in cardiomyocytes is essential for understanding Ca²⁺ fluxes in excitation-contraction and excitation-transcription coupling. The properties of fluorescent indicators in intracellular compartments may differ, thus affecting the translation of Ca²⁺-dependent fluorescence changes into [Ca²⁺] changes. Therefore, we determined the in situ characteristics of a frequently used Ca²⁺ indicator, Fluo-4, and a ratiometric Ca²⁺ indicator, Asante Calcium Red, and evaluated their use for reporting and quantifying cytoplasmic and nucleoplasmic Ca²⁺ signals in isolated cardiomyocytes. Ca²⁺ calibration curves revealed significant differences in the apparent Ca²⁺ dissociation constants of Fluo-4 and Asante Calcium Red between cytoplasm and nucleoplasm. These parameters were used for transformation of fluorescence into nucleoplasmic and cytoplasmic [Ca²⁺]. Resting and diastolic [Ca²⁺] were always higher in the nucleoplasm. Systolic [Ca²⁺] was usually higher in the cytoplasm, but some cells (15%) exhibited higher systolic [Ca²⁺] in the nucleoplasm. Ca²⁺ store depletion or blockade of Ca²⁺ leak pathways eliminated the resting [Ca²⁺] gradient between nucleoplasm and cytoplasm, whereas inhibition of inositol 1,4,5-trisphosphate receptors by 2-APB reversed it. The results suggest the presence of significant nucleoplasmic-to-cytoplasmic [Ca²⁺] gradients in resting myocytes and during the cardiac cycle. Nucleoplasmic [Ca²⁺] in cardiomyocytes may be regulated via two mechanisms: diffusion from the cytoplasm and active Ca²⁺ release via inositol 1,4,5-trisphosphate receptors from perinuclear Ca²⁺ stores.

Structural evidence for perinuclear calcium microdomains in cardiac myocytes

At each heartbeat, cardiac myocytes are activated by a cytoplasmic Ca(2+) transient in great part due to Ca(2+) release from the sarcoplasmic reticulum via ryanodine receptors (RyRs) clustered within calcium release units (peripheral couplings/dyads). A Ca(2+) transient also occurs in the nucleoplasm, following the cytoplasmic transient with some delay. Under conditions where the InsP3 production is stimulated, these Ca(2+) transients are regulated actively, presumably by an additional release of Ca(2+) via InsP3 receptors (InsP3Rs). This raises the question whether InsP3Rs are appropriately located for this effect and whether sources of InsP3 and Ca(2+) are available for their activation. We have defined the structural basis for InsP3R activity at the nucleus, using immunolabeling for confocal microscopy and freeze-drying/shadowing, T tubule "staining" and thin sectioning for electron microscopy. By these means we establish the presence of InsP3R at the outer nuclear envelope and show a close spatial relationship between the nuclear envelope, T tubules (a likely source of InsP3) and dyads (the known source of Ca(2+)). The frequency, distribution and distance from the nucleus of T tubules and dyads appropriately establish local perinuclear Ca(2+) microdomains in cardiac myocytes.

Cardiac-targeted delivery of regulatory RNA molecules and genes for the treatment of heart failure

Ribonucleic acid (RNA) in its many facets of structure and function is becoming more fully understood, and, therefore, it is possible to design and use RNAs as valuable tools in molecular biology and medicine. Understanding of the role of RNAs within the cell has changed dramatically during the past few years. Therapeutic strategies based on non-coding regulatory RNAs include RNA interference (RNAi) for the silencing of specific genes, and microRNA (miRNA) modulations to alter complex gene expression patterns. Recent progress has allowed the targeting of therapeutic RNAi to the heart for the treatment of heart failure, and we discuss current strategies in this field. Owing to the peculiar biochemical properties of small RNA molecules, the actual therapeutic translation of findings in vitro or in cell cultures is more demanding than with small molecule drugs or proteins. The critical requirement for animal studies after pre-testing of RNAi tools in vitro likewise applies for miRNA modulations, which also have complex consequences for the recipient that are dependent on stability and distribution of the RNA tools. Problems in the field that are not yet fully solved are the prediction of targets and specificity of the RNA tools as well as their tissue-specific and regulatable expression. We discuss analogies and differences between regulatory RNA therapy and classical gene therapy, since recent breakthroughs in vector technology are of importance for both. Recent years have witnessed parallel progress in the fields of gene-based and regulatory RNA-based therapies that are likely to significantly expand the cardiovascular therapeutic repertoire within the next decade.

IP3-induced cytosolic and nuclear Ca2+ signals in HL-1 atrial myocytes: possible role of IP3 receptor subtypes

HL-1 cells are the adult cardiac cell lines available that continuously divide while maintaining an atrial phenotype. Here we examined the expression and localization of inositol 1,4,5-trisphosphate receptor (IP(3)R) subtypes, and investigated how pattern of IP(3)-induced subcellular local Ca(2+) signaling is encoded by multiple IP(3)R subtypes in HL-1 cells. The type 1 IP(3)R (IP(3)R1) was expressed in the perinucleus with a diffuse pattern and the type 2 IP(3)R (IP(3)R2) was expressed in the cytosol with a punctate distribution. Extracellular ATP (1 mM) elicited transient intracellular Ca(2+) releases accompanied by a Ca(2+) oscillation, which was eliminated by the blocker of IP(3)Rs, 2-APB, and attenuated by ryanodine. Direct introduction of IP(3) into the permeabilized cells induced Ca(2+) transients with Ca(2+) oscillations at [Symbol: see text] 20 muM of IP(3), which was removed by the inhibition of IP(3)Rs using 2-APB and heparin. IP(3)-induced local Ca(2+) transients contained two distinct time courses: a rapid oscillation and a monophasic Ca(2+) transient. The magnitude of Ca(2+) oscillation was significantly larger in the cytosol than in the nucleus, while the monophasic Ca(2+) transient was more pronounced in the nucleus. These results provide evidence for the molecular and functional expression of IP(3)R1 and IP(3)R2 in HL-1 cells, and suggest that such distinct local Ca(2+) signaling may be correlated with the punctate distribution of IP(3)R2s in the cytosol and the diffuse localization of IP(3)R1 in the peri-nucleus.

Na+-dependent SR Ca2+ overload induces arrhythmogenic events in mouse cardiomyocytes with a human CPVT mutation

AIMS

Mutations in the cardiac ryanodine receptor Ca(2+) release channel, RyR2, underlie catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited life-threatening arrhythmia. CPVT is triggered by spontaneous RyR2-mediated sarcoplasmic reticulum (SR) Ca(2+) release in response to SR Ca(2+) overload during beta-adrenergic stimulation. However, whether elevated SR Ca(2+) content--in the absence of protein kinase A activation--affects RyR2 function and arrhythmogenesis in CPVT remains elusive.

METHODS AND RESULTS

Isolated murine ventricular myocytes harbouring a human RyR2 mutation (RyR2(R4496C+/-)) associated with CPVT were investigated in the absence and presence of 1 micromol/L JTV-519 (RyR2 stabilizer) followed by 100 micromol/L ouabain intervention to increase cytosolic [Na(+)] and SR Ca(2+) load. Changes in membrane potential and intracellular [Ca(2+)] were monitored with whole-cell patch-clamping and confocal Ca(2+) imaging, respectively. At baseline, action potentials (APs), Ca(2+) transients, fractional SR Ca(2+) release, and SR Ca(2+) load were comparable in wild-type (WT) and RyR2(R4496C+/-) myocytes. Ouabain evoked significant increases in diastolic [Ca(2+)], peak systolic [Ca(2+)], fractional SR Ca(2+) release, and SR Ca(2+) content that were quantitatively similar in WT and RyR2(R4496C+/-) myocytes. Ouabain also induced arrhythmogenic events, i.e. spontaneous Ca(2+) waves, delayed afterdepolarizations and spontaneous APs, in both groups. However, the ouabain-induced increase in the frequency of arrhythmogenic events was dramatically larger in RyR2(R4496C+/-) when compared with WT myocytes. JTV-519 greatly reduced the frequency of ouabain-induced arrhythmogenic events.

CONCLUSION

The elevation of SR Ca(2+) load--in the absence of beta-adrenergic stimulation--is sufficient to increase the propensity for triggered arrhythmias in RyR2(R4496C+/-) cardiomyocytes. Stabilization of RyR2 by JTV-519 effectively reduces these triggered arrhythmias.

Endothelin receptors: what's new and what do we need to know?

Receptors are at the heart of how a molecule transmits a signal to a cell. Two receptor classes for endothelin (ET) are recognized, the ET(A) and ET(B) receptors. Intriguing questions have arisen in the field of ET receptor pharmacology, physiology, and function. For example, a host of pharmacological studies support the interaction of the ET(A) and ET(B) receptor in tissues (veins, arteries, bronchus, arterioles, esophagus), but yet few have been able to demonstrate direct ET(A)/ET(B) receptor interaction. Have we modeled this interaction wrong? Do we have a truly selective ET(A) receptor agonist such that we could selectively stimulate this important receptor? What can we learn from the recent phylogenic studies of the ET receptor family? Have we adequately addressed the number of biological molecules with which ET can interact to exert a biological effect? Recent mass spectrometry studies in our laboratory suggest that ET-1 interacts with other hereto unrecognized proteins. Biased ligands (ligands at the same receptor that elicit distinct signaling responses) have been discovered for other receptors. Do these exist for ET receptors and can we take advantage of this possibility in drug design? These and other questions will be posed in this minireview on topics on ET receptors.

An update on nuclear calcium signalling

Over the past 15 years or so, numerous studies have sought to characterise how nuclear calcium (Ca2+) signals are generated and reversed, and to understand how events that occur in the nucleoplasm influence cellular Ca2+ activity, and vice versa. In this Commentary, we describe mechanisms of nuclear Ca2+ signalling and discuss what is known about the origin and physiological significance of nuclear Ca2+ transients. In particular, we focus on the idea that the nucleus has an autonomous Ca2+ signalling system that can generate its own Ca2+ transients that modulate processes such as gene transcription. We also discuss the role of nuclear pores and the nuclear envelope in controlling ion flux into the nucleoplasm.

Increased InsP3Rs in the junctional sarcoplasmic reticulum augment Ca2+ transients and arrhythmias associated with cardiac hypertrophy

Cardiac hypertrophy is a growth response of the heart to increased hemodynamic demand or damage. Accompanying this heart enlargement is a remodeling of Ca(2+) signaling. Due to its fundamental role in controlling cardiomyocyte contraction during every heartbeat, modifications in Ca(2+) fluxes significantly impact on cardiac output and facilitate the development of arrhythmias. Using cardiomyocytes from spontaneously hypertensive rats (SHRs), we demonstrate that an increase in Ca(2+) release through inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) contributes to the larger excitation contraction coupling (ECC)-mediated Ca(2+) transients characteristic of hypertrophic myocytes and underlies the more potent enhancement of ECC-mediated Ca(2+) transients and contraction elicited by InsP(3) or endothelin-1 (ET-1). Responsible for this is an increase in InsP(3)R expression in the junctional sarcoplasmic reticulum. Due to their close proximity to ryanodine receptors (RyRs) in this region, enhanced Ca(2+) release through InsP(3)Rs served to sensitize RyRs, thereby increasing diastolic Ca(2+) levels, the incidence of extra-systolic Ca(2+) transients, and the induction of ECC-mediated Ca(2+) elevations. Unlike the increase in InsP(3)R expression and Ca(2+) transient amplitude in the cytosol, InsP(3)R expression and ECC-mediated Ca(2+) transients in the nucleus were not altered during hypertrophy. Elevated InsP(3)R2 expression was also detected in hearts from human patients with heart failure after ischemic dilated cardiomyopathy, as well as in aortic-banded hypertrophic mouse hearts. Our data establish that increased InsP(3)R expression is a general mechanism that underlies remodeling of Ca(2+) signaling during heart disease, and in particular, in triggering ventricular arrhythmia during hypertrophy.

Endoplasmic and sarcoplasmic reticulum in the heart

The concept of the presence of sarcoplasmic reticulum (SR) membrane in the heart is widely accepted and has been considered merely to be a different name for the endoplasmic reticulum (ER) in muscle tissues. Cardiac SR membranes are specialized in the regulation of Ca(2+) transport and control of excitation-contraction coupling. By contrast, the ER is responsible for protein synthesis, modification, secretion, lipid and steroid synthesis, and modulation of Ca(2+) signaling. Recent developments have indicated that functional changes in proteins or pathways normally associated with ER and not SR membrane impact cardiac development and pathology. Here, we propose that the SR and ER might be functionally distinct internal membrane compartments in cardiomyocytes.

Long-term cardiac-targeted RNA interference for the treatment of heart failure restores cardiac function and reduces pathological hypertrophy

BACKGROUND

RNA interference (RNAi) has the potential to be a novel therapeutic strategy in diverse areas of medicine. Here, we report on targeted RNAi for the treatment of heart failure, an important disorder in humans that results from multiple causes. Successful treatment of heart failure is demonstrated in a rat model of transaortic banding by RNAi targeting of phospholamban, a key regulator of cardiac Ca(2+) homeostasis. Whereas gene therapy rests on recombinant protein expression as its basic principle, RNAi therapy uses regulatory RNAs to achieve its effect.

METHODS AND RESULTS

We describe structural requirements to obtain high RNAi activity from adenoviral and adeno-associated virus (AAV9) vectors and show that an adenoviral short hairpin RNA vector (AdV-shRNA) silenced phospholamban in cardiomyocytes (primary neonatal rat cardiomyocytes) and improved hemodynamics in heart-failure rats 1 month after aortic root injection. For simplified long-term therapy, we developed a dimeric cardiotropic adeno-associated virus vector (rAAV9-shPLB) to deliver RNAi activity to the heart via intravenous injection. Cardiac phospholamban protein was reduced to 25%, and suppression of sacroplasmic reticulum Ca(2+) ATPase in the HF groups was rescued. In contrast to traditional vectors, rAAV9 showed high affinity for myocardium but low affinity for liver and other organs. rAAV9-shPLB therapy restored diastolic (left ventricular end-diastolic pressure, dp/dt(min), and tau) and systolic (fractional shortening) functional parameters to normal ranges. The massive cardiac dilation was normalized, and cardiac hypertrophy, cardiomyocyte diameter, and cardiac fibrosis were reduced significantly. Importantly, no evidence was found of microRNA deregulation or hepatotoxicity during these RNAi therapies.

CONCLUSIONS

Our data show for the first time the high efficacy of an RNAi therapeutic strategy in a cardiac disease.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

Inositol 1,4,5-trisphosphate (IP(3)) is a ubiquitous intracellular messenger regulating diverse functions in almost all mammalian cell types. It is generated by membrane receptors that couple to phospholipase C (PLC), an enzyme which liberates IP(3) from phosphatidylinositol 4,5-bisphosphate (PIP(2)). The major action of IP(3), which is hydrophilic and thus translocates from the membrane into the cytoplasm, is to induce Ca(2+) release from endogenous stores through IP(3) receptors (IP(3)Rs). Cardiac excitation-contraction coupling relies largely on ryanodine receptor (RyR)-induced Ca(2+) release from the sarcoplasmic reticulum. Myocytes express a significantly larger number of RyRs compared to IP(3)Rs (~100:1), and furthermore they experience substantial fluxes of Ca(2+) with each heartbeat. Therefore, the role of IP(3) and IP(3)-mediated Ca(2+) signaling in cardiac myocytes has long been enigmatic. Recent evidence, however, indicates that despite their paucity cardiac IP(3)Rs may play crucial roles in regulating diverse cardiac functions. Strategic localization of IP(3)Rs in cytoplasmic compartments and the nucleus enables them to participate in subsarcolemmal, bulk cytoplasmic and nuclear Ca(2+) signaling in embryonic stem cell-derived and neonatal cardiomyocytes, and in adult cardiac myocytes from the atria and ventricles. Intriguingly, expression of both IP(3)Rs and membrane receptors that couple to PLC/IP(3) signaling is altered in cardiac disease such as atrial fibrillation or heart failure, suggesting the involvement of IP(3) signaling in the pathology of these diseases. Thus, IP(3) exerts important physiological and pathological functions in the heart, ranging from the regulation of pacemaking, excitation-contraction and excitation-transcription coupling to the initiation and/or progression of arrhythmias, hypertrophy and heart failure.

Wide long lasting perinuclear Ca2+ release events generated by an interaction between ryanodine and IP3 receptors in canine Purkinje cells

The purpose of this study was to determine whether IP(3)Rs contribute to the generation of wide long lasting perinuclear Ca(2+) release events in canine Purkinje cells. Spontaneous Ca(2+) release events (elevations of basal [Ca(2+)] equivalent to F/F(0) 3.4SD over F(0)) were imaged using Fluo-4AM and 2D confocal microscope. Only cells free of Ca(2+) waves were analyzed. Subsarcolemmal region (SSL) was defined as 5 microm from cell edges. Core was the remaining cell. The majority of events (94%, 0.0035+/-0.0007 events (ev)/microm(2)/s, N=34 cells) were detected within a single frame (typical events, TE). However, a subpopulation (6.0%, 0.00022+/-0.00005 ev/microm(2)/s, N=41 cells: wide long lasting events, WLE) lasted for several frames, showed a greater spatial extent (51.0+/-3.9 vs. TE 9.0+/-0.3 microm(2), P<0.01) and higher amplitude (F/F(0) 1.38+/-0.02 vs. TE 1.20+/-0.003, P<0.01). WLE event rate was increased by phenylephrine (10 microM, P<0.01), inhibited by 2APB and U73122 (P<0.05), and abolished by tetracaine (1 mM) and ryanodine (100 microM). While SSL WLEs were scattered randomly, Core WLEs (n=69 events) were predominantly distributed longitudinally 18.2+/-1.6 microm from the center of nuclei. Immunocytochemistry showed that IP(3)R1s were located not only at SSL region but also near both ends of nucleus overlapping with RyRs. In Purkinje cells, wide long lasting Ca(2+) release events occur in SSL and in specific perinuclear regions. They are likely due to RyRs and IP(3)R1s evoked Ca(2+) release and may play a role in Ca(2+) dependent nuclear processes.