Epac enhances excitation-transcription coupling in cardiac myocytes

EPAC1 and 2 inhibit K+ currents via PLC/PKC and NOS/PKG pathways in rat ventricular cardiomyocytes

The exchange protein directly activated by cAMP (EPAC) has been implicated in cardiac proarrhythmic signaling pathways including spontaneous diastolic Ca2+ leak from sarcoplasmic reticulum and increased action potential duration (APD) in isolated ventricular cardiomyocytes. The action potential (AP) lengthening following acute EPAC activation is mainly due to a decrease of repolarizing steady-state K+ current (IKSS) but the mechanisms involved remain unknown. This study aimed to assess the role of EPAC1 and EPAC2 in the decrease of IKSS and to investigate the underlying signaling pathways. AP and K+ currents were recorded with the whole cell configuration of the patch-clamp technique in freshly isolated rat ventricular myocytes. EPAC1 and EPAC2 were pharmacologically activated with 8-(4-chlorophenylthio)-2'-O-methyl-cAMP acetoxymethyl ester (8-CPTAM, 10 µmol/L) and inhibited with R-Ce3F4 and ESI-05, respectively. Inhibition of EPAC1 and EPAC2 significantly decreased the effect of 8-CPTAM on APD and IKSS showing that both EPAC isoforms are involved in these effects. Unexpectedly, calmodulin-dependent protein kinase II (CaMKII) inhibition by AIP or KN-93, and Ca2+ chelation by intracellular BAPTA, did not impact the response to 8-CPTAM. However, inhibition of PLC/PKC and nitric oxide synthase (NOS)/PKG pathways partially prevents the 8-CPTAM-dependent decrease of IKSS. Finally, the cumulative inhibition of PKC and PKG blocked the 8-CPTAM effect, suggesting that these two actors work along parallel pathways to regulate IKSS upon EPAC activation. On the basis of such findings, we propose that EPAC1 and EPAC2 are involved in APD lengthening by inhibiting a K+ current via both PLC/PKC and NOS/PKG pathways. This may have pathological implications since EPAC is upregulated in diseases such as cardiac hypertrophy.NEW & NOTEWORHTY Exchange protein directly activated by cAMP (EPAC) proteins modulate ventricular electrophysiology at the cellular level. Both EPAC1 and EPAC2 isoforms participate in this effect. Mechanistically, PLC/PKC and nitric oxide synthase (NO)/PKG pathways are involved in regulating K+ repolarizing current whereas the well-known downstream effector of EPAC, calmodulin-dependent protein kinase II (CaMKII), does not participate. This may have pathological implications since EPAC is upregulated in diseases such as cardiac hypertrophy. Thus, EPAC inhibition may be a new approach to prevent arrhythmias under pathological conditions.

EPAC1 inhibition protects the heart from doxorubicin-induced toxicity

Anthracyclines, such as doxorubicin (Dox), are widely used chemotherapeutic agents for the treatment of solid tumors and hematologic malignancies. However, they frequently induce cardiotoxicity leading to dilated cardiomyopathy and heart failure. This study sought to investigate the role of the exchange protein directly activated by cAMP (EPAC) in Dox-induced cardiotoxicity and the potential cardioprotective effects of EPAC inhibition. We show that Dox induces DNA damage and cardiomyocyte cell death with apoptotic features. Dox also led to an increase in both cAMP concentration and EPAC1 activity. The pharmacological inhibition of EPAC1 (with CE3F4) but not EPAC2 alleviated the whole Dox-induced pattern of alterations. When administered in vivo, Dox-treated WT mice developed a dilated cardiomyopathy which was totally prevented in EPAC1 knock-out (KO) mice. Moreover, EPAC1 inhibition potentiated Dox-induced cell death in several human cancer cell lines. Thus, EPAC1 inhibition appears as a potential therapeutic strategy to limit Dox-induced cardiomyopathy without interfering with its antitumoral activity.

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.

3-Iodothyronamine, a trace amine-associated receptor agonist, regulates intracellular Ca2+ increases via CaMK II through Epac2 in rat cerebral arterioles

Trace amines (TAs) in the nervous system bind to TA-associated receptors (TAARs) and are involved in the regulation of monoaminergic functions. Among TAAR subtypes, TAAR1 has been implicated in the development of neurological disorders, such as schizophrenia. The present study investigated the effects of the TAAR1 agonist, 3-iodothyronamine (T1AM) on cerebral arterioles using fluctuations in the intracellular concentration of Ca2+ ([Ca2+]i) as an index of contractile responses. In cerebral arterioles, most of the TAAR agonists did not increase [Ca2+]i, while only T1AM elevated [Ca2+]i in vascular smooth muscle cells. This increase involved extracellular Ca2+ influx through T-type Ca2+ channels and inositol trisphosphate- and ryanodine-receptor-mediated Ca2+ release from intracellular stores. The inhibition of the cAMP sensor, exchange protein directly activated by cAMP (Epac) 2, and calmodulin kinase (CaMK) II strongly inhibited Ca2+ elevations. The present study revealed that T1AM acted not only on the TAAR1 receptor as previously suggested, but also on other G-protein coupled receptors and/or signal transduction systems to increase intracellular Ca2+ in cerebral arteriole smooth muscle cells. These results suggest that when using T1AM in clinical practice, attention should be paid to the early rise in blood pressure.

Spatial and temporal crosstalk between the cAMP and Ca2+ signaling systems

The ubiquitous secondary messengers, Ca²⁺ and cAMP, play a vital role in shaping a diverse array of physiological processes. More significantly, accumulating evidence over the past several decades underpin extensive crosstalk between these two canonical messengers in discrete sub-cellular nanodomains across various cell types. Within such specialized nanodomains, each messenger fine-tunes signaling to maintain homeostasis by manipulating the activities of cellular machinery accountable for the metabolism or activity of the complementary pathway. Interaction between these messengers is ensured by scaffolding proteins which tether components of the signaling machinery in close proximity. Disruption of dynamic communications between Ca²⁺ and cAMP at these loci consequently is linked to several pathological conditions. This review summarizes recent novel mechanisms underlying effective crosstalk between Ca²⁺ and cAMP in such nanodomains.

Cardioprotection of Immature Heart by Simultaneous Activation of PKA and Epac: A Role for the Mitochondrial Permeability Transition Pore

Metabolic and ionic changes during ischaemia predispose the heart to the damaging effects of reperfusion. Such changes and the resulting injury differ between immature and adult hearts. Therefore, cardioprotective strategies for adults must be tested in immature hearts. We have recently shown that the simultaneous activation of protein kinase A (PKA) and exchange protein activated by cAMP (Epac) confers marked cardioprotection in adult hearts. The aim of this study is to investigate the efficacy of this intervention in immature hearts and determine whether the mitochondrial permeability transition pore (MPTP) is involved. Isolated perfused Langendorff hearts from both adult and immature rats were exposed to global ischaemia and reperfusion injury (I/R) following control perfusion or perfusion after an equilibration period with activators of PKA and/or Epac. Functional outcome and reperfusion injury were measured and in parallel, mitochondria were isolated following 5 min of reperfusion to determine whether cardioprotective interventions involved changes in MPTP opening behaviour. Perfusion for 5 min preceding ischaemia of injury-matched adult and immature hearts with 5 µM 8-Br (8-Br-cAMP-AM), an activator of both PKA and Epac, led to significant reduction in post-reperfusion CK release and infarct size. Perfusion with this agent also led to a reduction in MPTP opening propensity in both adult and immature hearts. These data show that immature hearts are innately more resistant to I/R injury than adults, and that this is due to a reduced tendency of MPTP opening following reperfusion. Furthermore, simultaneous stimulation of PKA and Epac causes cardioprotection, which is additive to the innate resistance.

GRKs and Epac1 Interaction in Cardiac Remodeling and Heart Failure

β-adrenergic receptors (β-ARs) play a major role in the physiological regulation of cardiac function through signaling routes tightly controlled by G protein-coupled receptor kinases (GRKs). Although the acute stimulation of β-ARs and the subsequent production of cyclic AMP (cAMP) have beneficial effects on cardiac function, chronic stimulation of β-ARs as observed under sympathetic overdrive promotes the development of pathological cardiac remodeling and heart failure (HF), a leading cause of mortality worldwide. This is accompanied by an alteration in cAMP compartmentalization and the activation of the exchange protein directly activated by cAMP 1 (Epac1) signaling. Among downstream signals of β-ARs, compelling evidence indicates that GRK2, GRK5, and Epac1 represent attractive therapeutic targets for cardiac disease. Here, we summarize the pathophysiological roles of GRK2, GRK5, and Epac1 in the heart. We focus on their signalosome and describe how under pathological settings, these proteins can cross-talk and are part of scaffolded nodal signaling systems that contribute to a decreased cardiac function and HF development.

Nitration of protein phosphatase 2A increases via Epac1/PLCε/CaMKII/HDAC5/iNOS cascade in human endometrial stromal cell decidualization

Decidualization of the endometrial stroma is an essential differentiation process for embryo implantation and maintenance of pregnancy. We previously reported that protein phosphatase 2A (PP2A) acts as a key mediator during cAMP-induced decidualization of human endometrial stromal cells (hESCs). However, the mechanism underlying its activation has remained obscure in hESCs. In the present study, we aimed to reveal the mechanism that induces the nitration of PP2A catalytic subunit (PP2Ac) during cAMP-induced decidualization of hESCs. First, cAMP-induced PP2Ac nitration was significantly repressed using L-NAME, an inhibitor of nitric oxide synthase (NOS). Among several NOS isoforms, only inducible NOS (iNOS) was highly expressed in hESCs, indicating that iNOS directly induces the nitration of PP2Ac. Second, cAMP-induced iNOS expression and PP2Ac nitration were decreased by treatment with TSA, an inhibitor of histone deacetylase 5 (HDAC5). cAMP-induced phosphorylation of CaMKII and HDAC5 was suppressed by treatment with U73122 (an inhibitor of phospholipase C) or transfection of PLCε siRNA. Finally, small G protein Rap1 and its guanine nucleotide exchange factor Epac1 were found to be involved in cAMP-induced PP2A activation. Taken together, our results suggest that PP2Ac nitration during cAMP-induced decidualization of hESCs is induced through the Epac1-Rap1-PLCε-CaMKII-HDAC5-iNOS signaling pathway.

Role of EPAC1 Signalosomes in Cell Fate: Friends or Foes?

The compartmentation of signaling processes is accomplished by the assembly of protein complexes called signalosomes. These signaling platforms colocalize enzymes, substrates, and anchoring proteins into specific subcellular compartments. Exchange protein directly activated by cAMP 1 (EPAC1) is an effector of the second messenger, 3',5'-cyclic adenosine monophosphate (cAMP) that is associated with multiple roles in several pathologies including cardiac diseases. Both EPAC1 intracellular localization and molecular partners are key players in the regulation of cell fate, which may have important therapeutic potential. In this review, we summarize the recent findings on EPAC1 structure, regulation, and pharmacology. We describe the importance of EPAC1 subcellular distribution in its biological action, paying special attention to its nuclear localization and mechanism of action leading to cardiomyocyte hypertrophy. In addition, we discuss the role of mitochondrial EPAC1 in the regulation of cell death. Depending on the cell type and stress condition, we present evidence that supports either a protective or detrimental role of EPAC1 activation.

Activation of endogenous protein phosphatase 1 enhances the calcium sensitivity of the ryanodine receptor type 2 in murine ventricular cardiomyocytes

KEY POINTS

Increased protein phosphatase 1 (PP-1) activity has been found in end stage human heart failure. Although PP-1 has been extensively studied, a detailed understanding of its role in the excitation-contraction coupling mechanism, in normal and diseased hearts, remains elusive. The present study investigates the functional effect of the PP-1 activity on local Ca²⁺ release events in ventricular cardiomyocytes, by using an activating peptide (PDP3) for the stimulation of the endogenous PP-1 protein. We report that acute de-phosphorylation may increase the sensitivity of RyR2 channels to Ca²⁺ in situ, and that the RyR2-serine2808 phosphorylation site may mediate such a process. Our approach unmasks the functional importance of PP-1 in the regulation of RyR2 activity, suggesting a potential role in the generation of a pathophysiological sarcoplasmic reticulum Ca²⁺ leak in the diseased heart.

ABSTRACT

Changes in cardiac ryanodine receptor (RyR2) phosphorylation are considered to be important regulatory and disease related post-translational protein modifications. The extent of RyR2 phosphorylation is mainly determined by the balance of the activities of protein kinases and phosphatases, respectively. Increased protein phosphatase-1 (PP-1) activity has been observed in heart failure, although the regulatory role of this enzyme on intracellular Ca²⁺ handling remains poorly understood. To determine the physiological and pathophysiological significance of increased PP-1 activity, we investigated how the PP-1 catalytic subunit (PP-1c) alters Ca²⁺ sparks in permeabilized cardiomyocytes and we also applied a PP-1-disrupting peptide (PDP3) to specifically activate endogenous PP-1, including the one anchored on the RyR2 macromolecular complex. We compared wild-type and transgenic mice in which the usually highly phosphorylated site RyR2-S2808 has been ablated to investigate its involvement in RyR2 modulation (S2808A^+/+ ). In wild-type myocytes, PP-1 increased Ca²⁺ spark frequency by two-fold, followed by depletion of the sarcoplasmic reticulum Ca²⁺ store. Similarly, PDP3 transiently increased spark frequency and decreased sarcoplasmic reticulum Ca²⁺ load. RyR2 Ca²⁺ sensitivity, which was assessed by Ca²⁺ spark recovery analysis, was increased in the presence of PDP3 compared to a negative control peptide. S2808A^+/+ cardiomyocytes did not respond to both PP-1c and PDP3 treatment. Our results suggest an increased Ca²⁺ sensitivity of RyR2 upon de-phosphorylation by PP-1. Furthermore, we have confirmed the S2808 site as a target for PP-1 and as a potential link between RyR2s modulation and the cellular response.

The Epac1 Protein: Pharmacological Modulators, Cardiac Signalosome and Pathophysiology

The second messenger 3′,5′-cyclic adenosine monophosphate (cAMP) is one of the most important signalling molecules in the heart as it regulates many physiological and pathophysiological processes. In addition to the classical protein kinase A (PKA) signalling route, the exchange proteins directly activated by cAMP (Epac) mediate the intracellular functions of cAMP and are now emerging as a new key cAMP effector in cardiac pathophysiology. In this review, we provide a perspective on recent advances in the discovery of new chemical entities targeting the Epac1 isoform and illustrate their use to study the Epac1 signalosome and functional characterisation in cardiac cells. We summarize the role of Epac1 in different subcompartments of the cardiomyocyte and discuss how cAMP–Epac1 specific signalling networks may contribute to the development of cardiac diseases. We also highlight ongoing work on the therapeutic potential of Epac1-selective small molecules for the treatment of cardiac disorders.

Elevated cyclic-AMP represses expression of exchange protein activated by cAMP (EPAC1) by inhibiting YAP-TEAD activity and HDAC-mediated histone deacetylation

Ligand-induced activation of Exchange Protein Activated by cAMP-1 (EPAC1) is implicated in numerous physiological and pathological processes, including cardiac fibrosis where changes in EPAC1 expression have been detected. However, little is known about how EPAC1 expression is regulated. Therefore, we investigated regulation of EPAC1 expression by cAMP in cardiac fibroblasts. Elevation of cAMP using forskolin, cAMP-analogues or adenosine A2B-receptor activation significantly reduced EPAC1 mRNA and protein levels and inhibited formation of F-actin stress fibres. Inhibition of actin polymerisation with cytochalasin-D, latrunculin-B or the ROCK inhibitor, Y-27632, mimicked effects of cAMP on EPAC1 mRNA and protein levels. Elevated cAMP also inhibited activity of an EPAC1 promoter-reporter gene, which contained a consensus binding element for TEAD, which is a target for inhibition by cAMP. Inhibition of TEAD activity using siRNA-silencing of its co-factors YAP and TAZ, expression of dominant-negative TEAD or treatment with YAP-TEAD inhibitors, significantly inhibited EPAC1 expression. However, whereas expression of constitutively-active YAP completely reversed forskolin inhibition of EPAC1-promoter activity it did not rescue EPAC1 mRNA levels. Chromatin-immunoprecipitation detected a significant reduction in histone3-lysine27-acetylation at the EPAC1 proximal promoter in response to forskolin stimulation. HDAC1/3 inhibition partially reversed forskolin inhibition of EPAC1 expression, which was completely rescued by simultaneously expressing constitutively active YAP. Taken together, these data demonstrate that cAMP downregulates EPAC1 gene expression via disrupting the actin cytoskeleton, which inhibits YAP/TAZ-TEAD activity in concert with HDAC-mediated histone deacetylation at the EPAC1 proximal promoter. This represents a novel negative feedback mechanism controlling EPAC1 levels in response to cAMP elevation.

Sodium current inhibition following stimulation of exchange protein directly activated by cyclic-3',5'-adenosine monophosphate (Epac) in murine skeletal muscle

We investigated effects of pharmacological triggering of exchange protein directly activated by cyclic-3',5'-adenosine monophosphate (Epac) on Nav1.4 currents from intact murine (C67BL6) skeletal muscle fibres for the first time. This employed a loose patch clamp technique which examined ionic currents in response to superimposed 10-ms V1 steps to varying degrees of depolarisation, followed by V2 steps to a fixed, +100 mV depolarisation relative to resting membrane potential following 40 mV hyperpolarising prepulses of 50 ms duration. The activation and inactivation properties of the resulting Na+ membrane current densities revealed reduced maximum currents and steepnesses in their voltage dependences after addition of the Epac activator 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate (1 µM) to the bathing Krebs-Henseleit solutions. Contrastingly, voltages at half-maximal current and timecourses of currents obtained in response to the V1 depolarising steps were unchanged. These effects were abolished by further addition of the RyR-inhibitor dantrolene (10 µM). In contrast, challenge by dantrolene alone left both currents and their parameters intact. These effects of Epac activation in inhibiting skeletal muscle, Nav1.4, currents, complement similar effects previously reported in the homologous Nav1.5 in murine cardiomyocytes. They are discussed in terms of a hypothesis implicating Epac actions in increasing RyR-mediated SR Ca2+ release resulting in a Ca2+-mediated inhibition of Nav1.4. The latter effect may form the basis for Ca2+-dependent Na+ channel dysregulation in SCN4A channelopathies associated with cold- and K+-aggravated myotonias.

Gender-Dependent Alteration of Ca2+ and TNFα Signaling in db/db Mice, an Obesity-Linked Type 2 Diabetic Model

Cardiovascular complications are the primary death cause in type 2 diabetes, where inflammation can play a role. We, and others, have previously shown that, in diabetic cardiomyopathy, cardiac dysfunction is associated with Ca²⁺ mishandling. It is possible that diabetic cardiomyopathy differently affects men and women, as the latter present higher risk to develop heart failure and a higher plasmatic level of the pro-inflammatory cytokine, tumor necrosis factor alpha (TNFα), than men. However, the gender-dependent regulation of Ca²⁺ signaling in diabetes and its relationship with TNFα signaling are still unclear. Here, we analyzed TNFα signaling pathway and its role in Ca²⁺ signaling dysfunction in male and female rodent models of type 2 diabetes linked to obesity (db/db mice) using confocal microscopy in freshly isolated cardiomyocytes. TNFα increased [Ca²⁺]_i transient amplitude and accelerated its decay without affecting SR Ca²⁺ load or Ca²⁺ spark frequency in cells from control mice. All TNFα effects on Ca²⁺ handling were prevented by the inhibition of the ceramidase and the phospholipase A2 (PLA2). While the plasmatic level of TNFα was similar in male and female db/db mice, only male db/db hearts over-expressed both TNFα converting enzyme (TACE) and the protective TNFα receptors 2 (TNF-R2). TNFα receptor 1 (TNF-R1) expression, involved in negative inotropic response of TNFα, was unchanged in both male and female db/db mice compared to controls. We found that male db/db mice cardiomyocytes presented a decrease in [Ca²⁺]_i transient amplitude associated to a drop of sarcoplasmic reticulum Ca²⁺ load, not seen in female db/db mice. Interestingly, sustained incubation with TNFα did not restored Ca²⁺ signaling alteration observed in male db/db mice but still induces an increase in Ca²⁺ spark frequency as seen in control littermates. In cardiomyocytes from female db/db mice, TNFα had no visible effects on Ca²⁺ handling. In conclusion, our study shows that the alteration of Ca²⁺ signaling and TNFα, seen in db/db mice, is gender specific presenting an increase in TNFα cardio-protective pathway in male mice.

Fibroblast growth factor-23 promotes rhythm alterations and contractile dysfunction in adult ventricular cardiomyocytes

Background

Cardiac dysfunction and arrhythmia are common and onerous cardiovascular events in end-stage renal disease (ESRD) patients, especially those on dialysis. Fibroblast growth factor (FGF)-23 is a phosphate-regulating hormone whose levels dramatically increase as renal function declines. Beyond its role in phosphorus homeostasis, FGF-23 may elicit a direct effect on the heart. Whether FGF-23 modulates ventricular cardiac rhythm is unknown, prompting us to study its role on excitation–contraction (EC) coupling.

Methods

We examined FGF-23 in vitro actions on EC coupling in adult rat native ventricular cardiomyocytes using patch clamp and confocal microscopy and in vivo actions on cardiac rhythm using electrocardiogram.

Results

Compared with vehicle treatment, FGF-23 induced a significant decrease in rat cardiomyocyte contraction, L-type Ca2+ current, systolic Ca2+ transients and sarcoplasmic reticulum (SR) load and SR Ca2+-adenosine triphosphatase 2a pump activity. FGF-23 induced pro-arrhythmogenic activity in vitro and in vivo as automatic cardiomyocyte extracontractions and premature ventricular contractions. Diastolic spontaneous Ca2+ leak (sparks and waves) was significantly increased by FGF-23 via the calmodulin kinase type II (CaMKII)-dependent pathway related to hyperphosphorylation of ryanodine receptors at the CaMKII site Ser2814. Both contraction dysfunction and spontaneous pro-arrhythmic Ca2+ events induced by FGF-23 were blocked by soluble Klotho (sKlotho).

Conclusions

Our results show that FGF-23 reduces contractility and enhances arrhythmogenicity through intracellular Ca2+ mishandling. Blocking its actions on the heart by improving sKlotho bioavailability may enhance cardiac function and reduce arrhythmic events frequently observed in ESRD.

CaMKII signaling in heart diseases: Emerging role in diabetic cardiomyopathy

Calcium/calmodulin-dependent protein kinase II (CaMKII) is upregulated in diabetes and significantly contributes to cardiac remodeling with increased risk of cardiac arrhythmias. Diabetes is frequently associated with atrial fibrillation, coronary artery disease, and heart failure, which may further enhance CaMKII. Activation of CaMKII occurs downstream of neurohormonal stimulation (e.g. via G-protein coupled receptors) and involve various posttranslational modifications including autophosphorylation, oxidation, S-nitrosylation and O-GlcNAcylation. CaMKII signaling regulates diverse cellular processes in a spatiotemporal manner including excitation-contraction and excitation-transcription coupling, mechanics and energetics in cardiac myocytes. Chronic activation of CaMKII results in cellular remodeling and ultimately arrhythmogenic alterations in Ca²⁺ handling, ion channels, cell-to-cell coupling and metabolism. This review addresses the detrimental effects of the upregulated CaMKII signaling to enhance the arrhythmogenic substrate and trigger mechanisms in the heart. We also briefly summarize preclinical studies using kinase inhibitors and genetically modified mice targeting CaMKII in diabetes. The mechanistic understanding of CaMKII signaling, cardiac remodeling and arrhythmia mechanisms may reveal new therapeutic targets and ultimately better treatment in diabetes and heart disease in general.

Phosphorylation of the ryanodine receptor 2 at serine 2030 is required for a complete β-adrenergic response

Phosphorylation is thought to play a role in modulation of the ryanodine receptor 2 channel. Using a S2030A knock-in mouse model, Potenza et al. reveal that phosphorylation of RyR2-S2030 mediates channel regulation during the β-adrenergic response.

During physical exercise or stress, the sympathetic system stimulates cardiac contractility via β-adrenergic receptor (β-AR) activation, resulting in protein kinase A (PKA)–mediated phosphorylation of the cardiac ryanodine receptor RyR2. PKA-dependent “hyperphosphorylation” of the RyR2 channel has been proposed as a major impairment that contributes to progression of heart failure. However, the sites of PKA phosphorylation and their phosphorylation status in cardiac diseases are not well defined. Among the known RyR2 phosphorylation sites, serine 2030 (S2030) remains highly controversial as a site of functional impact. We examined the contribution of RyR2-S2030 to Ca²⁺ signaling and excitation–contraction coupling (ECC) in a transgenic mouse with an ablated RyR2-S2030 phosphorylation site (RyR2-S2030A^+/+). We assessed ECC gain by using whole-cell patch–clamp recordings and confocal Ca²⁺ imaging during β-ARs stimulation with isoproterenol (Iso) and consistent SR Ca²⁺ loading and L-type Ca²⁺ current (I_Ca) triggering. Under these conditions, ECC gain is diminished in mutant compared with WT cardiomyocytes. Resting Ca²⁺ spark frequency (CaSpF) with Iso is also reduced by mutation of S2030. In permeabilized cells, when SR Ca²⁺ pump activity is kept constant (using 2D12 antibody against phospholamban), cAMP does not change CaSpF in S2030A^+/+ myocytes. Using Ca²⁺ spark recovery analysis, we found that mutant RyR Ca²⁺ sensitivity is not enhanced by Iso application, contrary to WT RyRs. Furthermore, ablation of RyR2-S2030 prevents acceleration of Ca²⁺ waves and increases latency to the first spontaneous Ca²⁺ release after a train of stimulations during Iso treatment. Together, these results suggest that phosphorylation at S2030 may represent an important step in the modulation of RyR2 activity during β-adrenergic stimulation and a potential target for the development of new antiarrhythmic drugs.

Auto-inhibition of adenylyl cyclase 9 (AC9) by an isoform-specific motif in the carboxyl-terminal region

Trans-membrane adenylyl cyclase (tmAC) isoforms show markedly distinct regulatory properties that have not been fully explored. AC9 is highly expressed in vital organs such as the heart and the brain. Here, we report that the isoform-specific carboxyl-terminal domain (C2b) of AC9 inhibits the activation of the enzyme by Gs-coupled receptors (GsCR). In human embryonic kidney cells (HEK293) stably overexpressing AC9, cAMP production by AC9 induced upon the activation of endogenous β-adrenergic and prostanoid GsCRs was barely discernible. Cells expressing AC9 lacking the C2b domain showed a markedly enhanced cAMP response to GsCR. Subsequent studies of the response of AC9 mutants to the activation of GsCR revealed that residues 1268-1276 in the C2b domain were critical for auto-inhibition. Two main species of AC9 of 130 K and ≥ 170 K apparent molecular weight were observed on immunoblots of rodent and human myocardial membranes with NH₂-terminally directed anti-AC9 antibodies. The lower molecular weight AC9 band did not react with antibodies directed against the C2b domain. It was the predominant species of AC9 in rodent heart tissue and some of the human samples. There is a single gene for AC9 in vertebrates, moreover, amino acids 957-1353 of the COOH-terminus are encoded by a single exon with no apparent signs of mRNA splicing or editing making it highly unlikely that COOH-terminally truncated AC9 could arise through the processing or editing of mRNA. Thus, deductive reasoning leads to the suggestion that proteolytic cleavage of the C2b auto-inhibitory domain may govern the activation of AC9 by GsCR.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Epac Function and cAMP Scaffolds in the Heart and Lung

Evidence collected over the last ten years indicates that Epac and cAMP scaffold proteins play a critical role in integrating and transducing multiple signaling pathways at the basis of cardiac and lung physiopathology. Some of the deleterious effects of Epac, such as cardiomyocyte hypertrophy and arrhythmia, initially described in vitro, have been confirmed in genetically modified mice for Epac1 and Epac2. Similar recent findings have been collected in the lung. The following sections will describe how Epac and cAMP signalosomes in different subcellular compartments may contribute to cardiac and lung diseases.

Early effects of Epac depend on the fine-tuning of the sarcoplasmic reticulum Ca2+ handling in cardiomyocytes

In cardiac muscle, signaling through cAMP governs many fundamental cellular functions, including contractility, relaxation and automatism. cAMP cascade leads to the activation of the classic protein kinase A but also to the stimulation of the recently discovered exchange protein directly activated by cAMP (Epac). The role of Epac in the regulation of intracellular Ca2+ homeostasis and contractility in cardiac myocytes is still matter of debate. In this study we showed that the selective Epac activator, 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3', 5'-cyclic monophosphate (8-CPT), produced a positive inotropic effect when adult rat cardiac myocytes were stabilized at low [Ca2+]o (0.5mM), no changes at 1mM [Ca2+]o and a negative inotropic effect when [Ca2+]o was increased to 1.8mM. These effects were associated to parallel variations in sarcoplasmic reticulum (SR) Ca2+ content. At all [Ca2+]o studied, 8-CPT induced an increase in Ca2+ spark frequency and enhanced CaMKII autophosphorylation and the CaMKII-dependent phosphorylation of SR proteins: phospholamban (PLN, at Thr17 site) and ryanodine receptor (RyR2, at Ser2814 site). We used transgenic mice lacking PLN CaMKII phosphorylation site (PLN-DM) and knock-in mice with an inactivated CaMKII site S2814 on RyR2 (RyR2-S2814A) to investigate the involvement of these processes in the effects of Epac stimulation. In PLN-DM mice, 8-CPT failed to induce the positive inotropic effect at low [Ca2+]o and RyR2-S2814A mice showed no propensity to arrhythmic events when compared to wild type mice myocytes. We conclude that stimulation of Epac proteins could have either beneficial or deleterious effects depending on the steady-state Ca2+ levels at which the myocyte is functioning, favoring the prevailing mechanism of SR Ca2+ handling (uptake vs. leak) in the different situations.

Epac2-Rap1 Signaling Regulates Reactive Oxygen Species Production and Susceptibility to Cardiac Arrhythmias

AIMS

In the heart, β1-adrenergic signaling involves cyclic adenosine monophosphate (cAMP) acting via both protein kinase-A (PKA) and exchange protein directly activated by cAMP (Epac): a guanine nucleotide exchange factor for the small GTPase Rap1. Inhibition of Epac-Rap1 signaling has been proposed as a therapeutic strategy for both cancer and cardiovascular disease. However, previous work suggests that impaired Rap1 signaling may have detrimental effects on cardiac function. The aim of the present study was to investigate the influence of Epac2-Rap1 signaling on the heart using both in vivo and in vitro approaches.

RESULTS

Inhibition of Epac2 signaling induced early afterdepolarization arrhythmias in ventricular myocytes. The underlying mechanism involved an increase in mitochondrial reactive oxygen species (ROS) and activation of the late sodium current (INalate). Arrhythmias were blocked by inhibition of INalate or the mitochondria-targeted antioxidant, mitoTEMPO. In vivo, inhibition of Epac2 caused ventricular tachycardia, torsades de pointes, and sudden death. The in vitro and in vivo effects of Epac2 inhibition were mimicked by inhibition of geranylgeranyltransferase-1, which blocks interaction of Rap1 with downstream targets.

INNOVATION

Our findings show for the first time that Rap1 acts as a negative regulator of mitochondrial ROS production in the heart and that impaired Epac2-Rap1 signaling causes arrhythmias due to ROS-dependent activation of INalate. This has implications for the use of chemotherapeutics that target Epac2-Rap1 signaling. However, selective inhibition of INalate provides a promising strategy to prevent arrhythmias caused by impaired Epac2-Rap1 signaling.

CONCLUSION

Epac2-Rap1 signaling attenuates mitochondrial ROS production and reduces myocardial arrhythmia susceptibility. Antioxid. Redox Signal. 27, 117-132.

β-Adrenergic induced SR Ca2+ leak is mediated by an Epac-NOS pathway

Cardiac β-adrenergic receptors (β-AR) and Ca²⁺-Calmodulin dependent protein kinase (CaMKII) regulate both physiological and pathophysiological Ca²⁺ signaling. Elevated diastolic Ca²⁺ leak from the sarcoplasmic reticulum (SR) contributes to contractile dysfunction in heart failure and to arrhythmogenesis. β-AR activation is known to increase SR Ca²⁺ leak via CaMKII-dependent phosphorylation of the ryanodine receptor. Two independent and reportedly parallel pathways have been implicated in this β-AR-CaMKII cascade, one involving exchange protein directly activated by cAMP (Epac2) and another involving nitric oxide synthase 1 (NOS1). Here we tested whether Epac and NOS function in a single series pathway to increase β-AR induced and CaMKII-dependent SR Ca²⁺ leak. Leak was measured as both Ca²⁺ spark frequency and tetracaine-induced shifts in SR Ca²⁺, in mouse and rabbit ventricular myocytes. Direct Epac activation by 8-CPT (8-(4-chlorophenylthio)-2'-O-methyl-cAMP) mimicked β-AR-induced SR Ca²⁺ leak, and both were blocked by NOS inhibition. The same was true for myocyte CaMKII activation (assessed via a FRET-based reporter) and ryanodine receptor phosphorylation. Inhibitor and phosphorylation studies also implicated phosphoinositide 3-kinase (PI3K) and protein kinase B (Akt) downstream of Epac and above NOS activation in this pathway. We conclude that these two independently characterized parallel pathways function mainly via a single series arrangement (β-AR-cAMP-Epac-PI3K-Akt-NOS1-CaMKII) to mediate increased SR Ca²⁺ leak. Thus, for β-AR activation the cAMP-PKA branch effects inotropy and lusitropy (by effects on Ca²⁺ current and SR Ca²⁺-ATPase), this cAMP-Epac-NOS pathway increases pathological diastolic SR Ca²⁺leak. This pathway distinction may allow novel SR Ca²⁺ leak therapeutic targeting in treatment of arrhythmias in heart failure that spare the inotropic and lusitropic effects of the PKA branch.

β2-Adrenergic receptor activation mobilizes intracellular calcium via a non-canonical cAMP-independent signaling pathway

Beta adrenergic receptors (βARs) are G-protein-coupled receptors essential for physiological responses to the hormones/neurotransmitters epinephrine and norepinephrine which are found in the nervous system and throughout the body. They are the targets of numerous widely used drugs, especially in the case of the most extensively studied βAR, β₂AR, whose ligands are used for asthma and cardiovascular disease. βARs signal through Gα_s G-proteins and via activation of adenylyl cyclase and cAMP-dependent protein kinase, but some alternative downstream pathways have also been proposed that could be important for understanding normal physiological functioning of βAR signaling and its disruption in disease. Using fluorescence-based Ca²⁺ flux assays combined with pharmacology and gene knock-out methods, we discovered a previously unrecognized endogenous pathway in HEK-293 cells whereby β₂AR activation leads to robust Ca²⁺ mobilization from intracellular stores via activation of phospholipase C and opening of inositol trisphosphate (InsP₃) receptors. This pathway did not involve cAMP, Gα_s, or Gα_i or the participation of the other members of the canonical β₂AR signaling cascade and, therefore, constitutes a novel signaling mechanism for this receptor. This newly uncovered mechanism for Ca²⁺ mobilization by β₂AR has broad implications for adrenergic signaling, cross-talk with other signaling pathways, and the effects of βAR-directed drugs.

Functional and cardioprotective effects of simultaneous and individual activation of protein kinase A and Epac

Background and Purpose

Myocardial cAMP elevation confers cardioprotection against ischaemia/reperfusion (I/R) injury. cAMP activates two independent signalling pathways, PKA and Epac. This study investigated the cardiac effects of activating PKA and/or Epac and their involvement in cardioprotection against I/R.

Experimental Approach

Hearts from male rats were used either for determination of PKA and PKC activation or perfused in the Langendorff mode for either cardiomyocyte isolation or used to monitor functional activity at basal levels and after 30 min global ischaemia and 2 h reperfusion. Functional recovery and myocardial injury during reperfusion (LDH release and infarct size) were evaluated. Activation of PKA and/or Epac in perfused hearts was induced using cell permeable cAMP analogues in the presence or absence of inhibitors of PKA, Epac and PKC. H9C2 cells and cardiomyocytes were used to assess activation of Epac and effect on Ca²⁺ transients.

Key Results

Selective activation of either PKA or Epac was found to trigger a positive inotropic effect, which was considerably enhanced when both pathways were simultaneously activated. Only combined activation of PKA and Epac induced marked cardioprotection against I/R injury. This was accompanied by PKCε activation and repressed by inhibitors of PKA, Epac or PKC.

Conclusion and Implications

Simultaneous activation of both PKA and Epac induces an additive inotropic effect and confers optimal and marked cardioprotection against I/R injury. The latter effect is mediated by PKCε activation. This work has introduced a new therapeutic approach and targets to protect the heart against cardiac insults.

The role of Epac in the heart

As one of the most important second messengers, 3',5'-cyclic adenosine monophosphate (cAMP) mediates various extracellular signals including hormones and neurotransmitters, and induces appropriate responses in diverse types of cells. Since cAMP was formerly believed to transmit signals through only two direct target molecules, protein kinase A and the cyclic nucleotide-gated channel, the sensational discovery in 1998 of another novel direct effecter of cAMP [exchange proteins directly activated by cAMP (Epac)] attracted a great deal of scientific interest in cAMP signaling. Numerous studies on Epac have since disclosed its important functions in various tissues in the body. Recently, observations of genetically manipulated mice in various pathogenic models have begun to reveal the in vivo significance of previous in vitro or cellular-level findings. Here, we focused on the function of Epac in the heart. Accumulating evidence has revealed that both Epac1 and Epac2 play important roles in the structure and function of the heart under physiological and pathological conditions. Accordingly, developing the ability to regulate cAMP-mediated signaling through Epac may lead to remarkable new therapies for the treatment of cardiac diseases.

Cyclic AMP Sensor EPAC Proteins and Their Role in Cardiovascular Function and Disease

cAMP is a universal second messenger that plays central roles in cardiovascular regulation influencing gene expression, cell morphology, and function. A crucial step toward a better understanding of cAMP signaling came 18 years ago with the discovery of the exchange protein directly activated by cAMP (EPAC). The 2 EPAC isoforms, EPAC1 and EPAC2, are guanine-nucleotide exchange factors for the Ras-like GTPases, Rap1 and Rap2, which they activate independently of the classical effector of cAMP, protein kinase A. With the development of EPAC pharmacological modulators, many reports in the literature have demonstrated the critical role of EPAC in the regulation of various cAMP-dependent cardiovascular functions, such as calcium handling and vascular tone. EPAC proteins are coupled to a multitude of effectors into distinct subcellular compartments because of their multidomain architecture. These novel cAMP sensors are not only at the crossroads of different physiological processes but also may represent attractive therapeutic targets for the treatment of several cardiovascular disorders, including cardiac arrhythmia and heart failure.

Recent advances in understanding cardiac contractility in health and disease

The aim of this review is to provide the reader with a synopsis of some of the emerging ideas and experimental findings in cardiac physiology and pathophysiology that were published in 2015. To provide context for the non-specialist, a brief summary of cardiac contraction and calcium (Ca) regulation in the heart in health and disease is provided. Thereafter, some recently published articles are introduced that indicate the current thinking on (1) the Ca regulatory pathways modulated by Ca/calmodulin-dependent protein kinase II, (2) the potential influences of nitrosylation by nitric oxide or S-nitrosated proteins, (3) newly observed effects of reactive oxygen species (ROS) on contraction and Ca regulation following myocardial infarction and a possible link with changes in mitochondrial Ca, and (4) the effects of some of these signaling pathways on late Na current and pro-arrhythmic afterdepolarizations as well as the effects of transverse tubule disturbances.

New pharmacologic interventions to increase cardiac contractility: challenges and opportunities

PURPOSE OF REVIEW

The most extensively studied inotropic agents in patients with heart failure are phosphodiesterase (PDE) 3 inhibitors, which increase contractility by raising intracellular cyclic adenosine monophosphate content. In clinical trials, the inotropic benefits of these agents have been outweighed by an increase in sudden cardiac death. Here, I review recent findings that help explain what are likely to be distinct mechanisms involved in the beneficial and adverse effects of PDE3 inhibition.

RECENT FINDINGS

The proapoptotic consequences of PDE3 inhibition are becoming more apparent. Moreover, it has also become clear that individual PDE3 isoforms in cardiac myocytes are selectively regulated to interact with different proteins in different intracellular compartments. The beneficial and adverse effects of PDE3 inhibition may thus be attributable to the inhibition of different isoforms in different intracellular domains. In particular, PDE3A1 has been shown to interact directly with sarcoplasmic/endoplasmic reticulum Ca ATPase (SERCA2) in the sarcoplasmic reticulum through a phosphorylation of a site in its unique N-terminal domain, making it possible that this isoform can be selectively targeted to increase intracellular Ca cycling.

SUMMARY

Conventional PDE3 inhibitors target several functionally distinct isoforms of these enzymes. Isoform-selective and/or compartment-selective targeting of PDE3, through its protein-protein interactions, may produce the inotropic benefits of PDE3 inhibition without the adverse consequences.

[Molecular determinants of pathological cardiac remodeling: the examples of Epac and Carabin]

Physical exercise or hypertension requires that the heart increases its hemodynamic work. However, this adaptation is based on distinct cardiac remodelling according to the physiological or pathological origin of the stress. As shown here with two examples, understanding the molecular events leading to cardiac remodeling may offer new opportunities for the development of therapies for heart failure. The recently described Epac1 protein is an effector of the second messenger cAMP. Following a pathological stress, the cAMP-binding protein Epac1 induces cardiac hypertrophy and fibrosis as well as alteration of calcium cycling suggesting that Epac1 pharmacological inhibition may be of therapeutic value. Furthermore, the protein carabin is an important regulator of several effectors of pathological cardiac remodelling. Experimental manipulation of carabin expression profoundly alters the development of heart failure.

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.

Structure and functional roles of Epac2 (Rapgef4)

Epac (exchange protein activated by cyclic-AMP) 2 is a direct target of 3'-5'-cyclic adenosine monophosphate (cAMP) and is involved in cAMP-mediated signal transduction through activation of the Ras-like small GTPase Rap. Crystallographic analyses revealed that activation of Epac2 by cAMP is accompanied by dynamic structural changes. Epac2 is expressed mainly in brain, neuroendocrine and endocrine tissues, and is involved in diverse cellular functions in the tissues. In this review, we summarize the structure and function of Epac2. We also discuss the physiological and pathophysiological roles of Epac2, and the possibility of Epac2 as a therapeutic target.

Reconciling depressed Ca2+ sparks occurrence with enhanced RyR2 activity in failing mice cardiomyocytes

Abnormalities in cardiomyocyte Ca2+ handling contribute to impaired contractile function in heart failure (HF). Experiments on single ryanodine receptors (RyRs) incorporated into lipid bilayers have indicated that RyRs from failing hearts are more active than those from healthy hearts. Here, we analyzed spontaneous Ca2+ sparks (brief, localized increased in [Ca2+]i) to evaluate RyR cluster activity in situ in a mouse post-myocardial infarction (PMI) model of HF. The cardiac ejection fraction of PMI mice was reduced to ∼30% of that of sham-operated (sham) mice, and their cardiomyocytes were hypertrophied. The [Ca2+]i transient amplitude and sarcoplasmic reticulum (SR) Ca2+ load were decreased in intact PMI cardiomyocytes compared with those from sham mice, and spontaneous Ca2+ sparks were less frequent, whereas the fractional release and the frequency of Ca2+ waves were both increased, suggesting higher RyR activity. In permeabilized cardiomyocytes, in which the internal solution can be controlled, Ca2+ sparks were more frequent in PMI cells (under conditions of similar SR Ca2+ load), confirming the enhanced RyR activity. However, in intact cells from PMI mice, the Ca2+ sparks frequency normalized by the SR Ca2+ load in that cell were reduced compared with those in sham mice, indicating that the cytosolic environment in intact cells contributes to the decrease in Ca2+ spark frequency. Indeed, using an internal "failing solution" with less ATP (as found in HF), we observed a dramatic decrease in Ca2+ spark frequency in permeabilized PMI and sham myocytes. In conclusion, our data show that, even if isolated RyR channels show more activity in HF, concomitant alterations in intracellular media composition and SR Ca2+ load may mask these effects at the Ca2+ spark level in intact cells. Nonetheless, in this scenario, the probability of arrhythmogenic Ca2+ waves is enhanced, and they play a potential role in the increase in arrhythmia events in HF patients.

Proarrhythmic effect of sustained EPAC activation on TRPC3/4 in rat ventricular cardiomyocytes

The Exchange Protein directly Activated by cAMP (EPAC) participates to the pathological signaling of cardiac hypertrophy and heart failure, in which the role of Ca(2+) entry through the Transient Receptor Potential Canonical (TRPC) channels begin to be appreciated. Here we studied whether EPAC activation could influence the activity and/or expression of TRPC channels in cardiac myocytes. In adult rat ventricular myocytes treated for 4 to 6h with the selective EPAC activator, 8-pCPT (10μM), we observed by Fluo-3 confocal fluorescence a Store-Operated Ca(2+) Entry (SOCE) like-activity, which was blunted by co-incubation with EPAC inhibitors (ESI-05 and CE3F4 at 10 μM). This SOCE-like activity, which was very small in control incubated cells, was sensitive to 30-μM SKF-96365. Molecular screening showed a specific upregulation of TRPC3 and C4 protein isoforms after 8-pCPT treatment. Moreover, sustained EPAC activation favored proarrhythmic Ca(2+) waves, which were reduced either by co-incubation with EPAC inhibitors or bath perfusion with TRPC inhibitors. Our study provides the first evidence that sustained selective EPAC activation leads to an increase in TRPC3 and C4 protein expression and induces a proarrhythmic SOCE-like activity in adult rat ventricular cardiomyocytes, which might be of importance during the development of cardiac diseases.

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.

Epac proteins: specific ligands and role in cardiac remodelling

Epacs (exchange proteins directly activated by cAMP) act as guanine-nucleotide-exchange factors for the Ras-like small G-proteins Rap1 and Rap2, and are now recognized as incontrovertible factors leading to complex and diversified cAMP signalling pathways. Given the critical role of cAMP in the regulation of cardiac function, several studies have investigated the functional role of Epacs in the heart, providing evidence that Epacs modulate intracellular Ca2+ and are involved in several cardiac pathologies such as cardiac hypertrophy and arrhythmia. The present review summarizes recent data on the Epac signalling pathway and its role in cardiac pathophysiology. We also discuss recent advances in the discovery of novel pharmacological modulators of Epacs that were identified by high-throughput screening and their therapeutic potential for the treatment of cardiac disorders.

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.

Exchange protein directly activated by cAMP 1 promotes autophagy during cardiomyocyte hypertrophy

AIMS

Stimulation of β-adrenergic receptors (β-AR) increases cAMP production and contributes to the pathogenesis of cardiac hypertrophy and failure through poorly understood mechanisms. We previously demonstrated that Exchange protein directly activated by cAMP 1 (Epac1)-induced hypertrophy in primary cardiomyocytes. Among the mechanisms triggered by cardiac stress, autophagy has been highlighted as a protective or harmful response. Here, we investigate whether Epac1 promotes cardiac autophagy and how altered autophagy has an impact on Epac1-induced cardiomyocyte hypertrophy.

METHODS AND RESULTS

We reported that direct stimulation of Epac1 with the agonist, Sp-8-(4-chlorophenylthio)-2'-O-methyl-cAMP (Sp-8-pCPT) promoted autophagy activation in neonatal cardiomyocytes. Stimulation of β-AR with isoprenaline (ISO) mimicked the effect of Epac1 on autophagy markers. Conversely, the induction of autophagy flux following ISO treatment was prevented in cardiomyocytes pre-treated with a selective inhibitor of Epac1, CE3F4. Importantly, we found that Epac1 deletion in mice protected against β-AR-induced cardiac remodelling and prevented the induction of autophagy. The signalling mechanisms underlying Epac1-induced autophagy involved a Ca(2+)/calmodulin-dependent kinase kinase β (CaMKKβ)/AMP-dependent protein kinase (AMPK) pathway. Finally, we provided evidence that pharmacological inhibition of autophagy using 3-methyladenine (3-MA) or down-regulation of autophagy-related protein 5 (Atg5) significantly potentiated Epac1-promoted cardiomyocyte hypertrophy.

CONCLUSION

Altogether, these findings demonstrate that autophagy is an adaptive response to antagonize Epac1-promoted cardiomyocyte hypertrophy.

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.

Inflammation and cardiac dysfunction during sepsis, muscular dystrophy, and myocarditis

Inflammation plays an important role in cardiac dysfunction under different situations. Acute systemic inflammation occurring in patients with severe burns, trauma, and inflammatory diseases causes cardiac dysfunction, which is one of the leading causes of mortality in these patients. Acute sepsis decreases cardiac contractility and impairs myocardial compliance. Chronic inflammation such as that occurring in Duchenne muscular dystropshy and myocarditis may cause adverse cardiac remodeling including myocyte hypertrophy and death, fibrosis, and altered myocyte function. However, the underlying cellular and molecular mechanisms for inflammatory cardiomyopathy are still controversial probably due to multiple factors involved. Potential mechanisms include the change in circulating blood volume; a direct inhibition of myocyte contractility by cytokines (tumor necrosis factor (TNF)-α, interleukin (IL)-1β); abnormal nitric oxide and reactive oxygen species (ROS) signaling; mitochondrial dysfunction; abnormal excitation-contraction coupling; and reduced calcium sensitivity at the myofibrillar level and blunted β-adrenergic signaling. This review will summarize recent advances in diagnostic technology, mechanisms, and potential therapeutic strategies for inflammation-induced cardiac dysfunction.

Select 3',5'-cyclic nucleotide phosphodiesterases exhibit altered expression in the aged rodent brain

3',5'-cyclic nucleotide phosphodiesterases (PDEs) are the only known enzymes to compartmentalize cAMP and cGMP, yet little is known about how PDEs are dynamically regulated across the lifespan. We mapped mRNA expression of all 21 PDE isoforms in the adult rat and mouse central nervous system (CNS) using quantitative polymerase chain reaction (qPCR) and in situ hybridization to assess conservation across species. We also compared PDE mRNA and protein in the brains of old (26 months) versus young (5 months) Sprague-Dawley rats, with select experiments replicated in old (9 months) versus young (2 months) BALB/cJ mice. We show that each PDE isoform exhibits a unique expression pattern across the brain that is highly conserved between rats, mice, and humans. PDE1B, PDE1C, PDE2A, PDE4A, PDE4D, PDE5A, PDE7A, PDE8A, PDE8B, PDE10A, and PDE11A showed an age-related increase or decrease in mRNA expression in at least 1 of the 4 brain regions examined (hippocampus, cortex, striatum, and cerebellum). In contrast, mRNA expression of PDE1A, PDE3A, PDE3B, PDE4B, PDE7A, PDE7B, and PDE9A did not change with age. Age-related increases in PDE11A4, PDE8A3, PDE8A4/5, and PDE1C1 protein expression were confirmed in hippocampus of old versus young rodents, as were age-related increases in PDE8A3 protein expression in the striatum. Age-related changes in PDE expression appear to have functional consequences as, relative to young rats, the hippocampi of old rats demonstrated strikingly decreased phosphorylation of GluR1, CaMKIIα, and CaMKIIβ, decreased expression of the transmembrane AMPA regulatory proteins γ2 (a.k.a. stargazin) and γ8, and increased trimethylation of H3K27. Interestingly, expression of PDE11A4, PDE8A4/5, PDE8A3, and PDE1C1 correlate with these functional endpoints in young but not old rats, suggesting that aging is not only associated with a change in PDE expression but also a change in PDE compartmentalization.

Mechanisms of altered Ca²⁺ handling in heart failure

Ca²⁺ plays a crucial role in connecting membrane excitability with contraction in myocardium. The hallmark features of heart failure are mechanical dysfunction and arrhythmias; defective intracellular Ca²⁺ homeostasis is a central cause of contractile dysfunction and arrhythmias in failing myocardium. Defective Ca²⁺ homeostasis in heart failure can result from pathological alteration in the expression and activity of an increasingly understood collection of Ca²⁺ homeostatic and structural proteins, ion channels, and enzymes. This review focuses on the molecular mechanisms of defective Ca²⁺ cycling in heart failure and considers how fundamental understanding of these pathways may translate into novel and innovative therapies.

Targeting protein-protein interactions within the cyclic AMP signaling system as a therapeutic strategy for cardiovascular disease

The cAMP signaling system can trigger precise physiological cellular responses that depend on the fidelity of many protein-protein interactions, which act to bring together signaling intermediates at defined locations within cells. In the heart, cAMP participates in the fine control of excitation-contraction coupling, hence, any disregulation of this signaling cascade can lead to cardiac disease. Due to the ubiquitous nature of the cAMP pathway, general inhibitors of cAMP signaling proteins such as PKA, EPAC and PDEs would act non-specifically and universally, increasing the likelihood of serious 'off target' effects. Recent advances in the discovery of peptides and small molecules that disrupt the protein-protein interactions that underpin cellular targeting of cAMP signaling proteins are described and discussed.

Exchange protein activated by cAMP (Epac) induces vascular relaxation by activating Ca2+-sensitive K+ channels in rat mesenteric artery

Vasodilator-induced elevation of intracellular cyclic AMP (cAMP) is a central mechanism governing arterial relaxation but is incompletely understood due to the diversity of cAMP effectors. Here we investigate the role of the novel cAMP effector exchange protein directly activated by cAMP (Epac) in mediating vasorelaxation in rat mesenteric arteries. In myography experiments, the Epac-selective cAMP analogue 8-pCPT-2-O-Me-cAMP-AM (5 μM, subsequently referred to as 8-pCPT-AM) elicited a 77.6 ± 7.1% relaxation of phenylephrine-contracted arteries over a 5 min period (mean ± SEM; n = 6). 8-pCPT-AM induced only a 16.7 ± 2.4% relaxation in arteries pre-contracted with high extracellular K(+) over the same time period (n = 10), suggesting that some of Epac's relaxant effect relies upon vascular cell hyperpolarization. This involves Ca(2+)-sensitive, large-conductance K(+) (BK(Ca)) channel opening as iberiotoxin (100 nM) significantly reduced the ability of 8-pCPT-AM to reverse phenylephrine-induced contraction (arteries relaxed by only 35.0 ± 8.5% over a 5 min exposure to 8-pCPT-AM, n = 5; P < 0.05). 8-pCPT-AM increased Ca(2+) spark frequency in Fluo-4-AM-loaded mesenteric myocytes from 0.045 ± 0.008 to 0.103 ± 0.022 sparks s(-1) μm(-1) (P < 0.05) and reversibly increased both the frequency (0.94 ± 0.25 to 2.30 ± 0.72 s(-1)) and amplitude (23.9 ± 3.3 to 35.8 ± 7.7 pA) of spontaneous transient outward currents (STOCs) recorded in isolated mesenteric myocytes (n = 7; P < 0.05). 8-pCPT-AM-activated STOCs were sensitive to iberiotoxin (100 nM) and to ryanodine (30 μM). Current clamp recordings of isolated myocytes showed a 7.9 ± 1.0 mV (n = 10) hyperpolarization in response to 8-pCPT-AM that was sensitive to iberiotoxin (n = 5). Endothelial disruption suppressed 8-pCPT-AM-mediated relaxation in phenylephrine-contracted arteries (24.8 ± 4.9% relaxation after 5 min of exposure, n = 5; P < 0.05), as did apamin and TRAM-34, blockers of Ca(2+)-sensitive, small- and intermediate-conductance K(+) (SK(Ca) and IK(Ca)) channels, respectively, and N(G)-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase (NOS). In Fluo-4-AM-loaded mesenteric endothelial cells, 8-pCPT-AM induced a sustained increase in global Ca(2+). Our data suggest that Epac hyperpolarizes smooth muscle by (1) increasing localized Ca(2+) release from ryanodine receptors (Ca(2+) sparks) to activate BK(Ca) channels, and (2) endothelial-dependent mechanisms involving the activation of SK(Ca)/IK(Ca) channels and NOS. Epac-mediated smooth muscle hyperpolarization will limit Ca(2+) entry via voltage-sensitive Ca(2+) channels and represents a novel mechanism of arterial relaxation.

Opposing HDAC4 nuclear fluxes due to phosphorylation by β-adrenergic activated protein kinase A or by activity or Epac activated CaMKII in skeletal muscle fibres

Class IIa histone deacetylases (HDACs) move between skeletal muscle fibre cytoplasm and nuclei in response to various stimuli, suppressing activity of the exclusively nuclear transcription factor Mef2. Protein kinase A (PKA) phosphorylates class IIa HDACs in cardiac muscle, resulting in HDAC nuclear accumulation, but this has not been examined in skeletal muscle. Using HDAC4-green fluorescent protein (HDAC4-GFP) expressed in isolated skeletal muscle fibres, we now show that activation of PKA by the beta-receptor agonist isoproterenol or dibutyryl (Db) cAMP causes a steady HDAC4-GFP nuclear influx. The beta-receptor blocker propranolol or PKA inhibitor Rp-cAMPS blocks the effects of isoproterenol on the nuclear influx of HDAC4-GFP, and Rp-cAMPS blocks the effects of Db cAMP. The HDAC4-GFP construct having serines 265 and 266 replaced with alanines, HDAC4 (S265/266A)-GFP, did not respond to beta-receptor or PKA activation. Immunoprecipitation results show that HDAC4-GFP is a substrate of PKA, but HDAC4 (S265/266A)-GFP is not, implicating HDAC4 serines 265/266 as the site(s) phosphorylated by PKA. During 10 Hz trains of muscle fibre electrical stimulation, the nuclear efflux rate of HDAC4-GFP, but not of HDAC4 (S265/266)-GFP, was decreased by PKA activation, directly demonstrating antagonism between the effects of fibre stimulation and beta-adrenergic activation of PKA on HDAC4 nuclear fluxes. 8-CPT, a specific activator of Epac, caused nuclear efflux of HDAC4-GFP, opposite to the effect of PKA. Db cAMP increased both phosphorylated PKA and GTP-bound Rap1. Our results demonstrate that the PKA and CaMKII pathways play important opposing roles in skeletal muscle gene expression by oppositely affecting the subcellular localization of HDAC4.

Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions

Since the discovery nearly 60 years ago, cAMP is envisioned as one of the most universal and versatile second messengers. The tremendous feature of cAMP to tightly control highly diverse physiologic processes, including calcium homeostasis, metabolism, secretion, muscle contraction, cell fate, and gene transcription, is reflected by the award of five Nobel prizes. The discovery of Epac (exchange protein directly activated by cAMP) has ignited a new surge of cAMP-related research and has depicted novel cAMP properties independent of protein kinase A and cyclic nucleotide-gated channels. The multidomain architecture of Epac determines its activity state and allows cell-type specific protein-protein and protein-lipid interactions that control fine-tuning of pivotal biologic responses through the "old" second messenger cAMP. Compartmentalization of cAMP in space and time, maintained by A-kinase anchoring proteins, phosphodiesterases, and β-arrestins, contributes to the Epac signalosome of small GTPases, phospholipases, mitogen- and lipid-activated kinases, and transcription factors. These novel cAMP sensors seem to implement certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Agonists and antagonists selective for Epac are developed and will support further studies on the biologic net outcome of the activation of Epac. This will increase our current knowledge on the pathophysiology of devastating diseases, such as diabetes, cognitive impairment, renal and heart failure, (pulmonary) hypertension, asthma, and chronic obstructive pulmonary disease. Further insights into the cAMP dynamics executed by the Epac signalosome will help to optimize the pharmacological treatment of these diseases.

Epac2 mediates cardiac β1-adrenergic-dependent sarcoplasmic reticulum Ca2+ leak and arrhythmia

BACKGROUND

β-Adrenergic receptor (β-AR) activation can provoke cardiac arrhythmias mediated by cAMP-dependent alterations of Ca(2+) signaling. However, cAMP can activate both protein kinase A and an exchange protein directly activated by cAMP (Epac), but their functional interaction is unclear. In heart, selective Epac activation can induce potentially arrhythmogenic sarcoplasmic reticulum (SR) Ca(2+) release that involves Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) effects on the ryanodine receptor (RyR).

METHODS AND RESULTS

We tested whether physiological β-AR activation causes Epac-mediated SR Ca(2+) leak and arrhythmias and whether it requires Epac1 versus Epac2, β(1)-AR versus β(2)-AR, and CaMKIIδ-dependent phosphorylation of RyR2-S2814. We used knockout (KO) mice for Epac1, Epac2, or both. All KOs exhibited unaltered basal cardiac function, Ca(2+) handling, and hypertrophy in response to pressure overload. However, SR Ca(2+) leak induced by the specific Epac activator 8-CPT in wild-type mice was abolished in Epac2-KO and double-KO mice but was unaltered in Epac1-KO mice. β-AR-induced arrhythmias were also less inducible in Epac2-KO versus wild-type mice. β-AR activation with protein kinase A inhibition mimicked 8-CPT effects on SR Ca(2+) leak and was prevented by blockade of β(1)-AR but not β(2)-AR. CaMKII inhibition (KN93) and genetic ablation of either CaMKIIδ or CaMKII phosphorylation on RyR2-S2814 prevented 8-CPT-induced SR Ca(2+) leak.

CONCLUSIONS

β(1)-AR activates Epac2 to induce SR Ca(2+) leak via CaMKIIδ-dependent phosphorylation of RyR2-S2814. This pathway contributes to β-AR-induced arrhythmias and reduced cardiac function.