Developmental changes in gene expression of Epac and its upregulation in myocardial hypertrophy

Differential changes in cyclic adenosine 3′‐5′ monophosphate ( cAMP ) effectors and major Ca 2+ handling proteins during diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is associated with differential and time-specific regulation of β-adrenergic receptors and cardiac cyclic nucleotide phosphodiesterases with consequences for total cyclic adenosine 3'-5' monophosphate (cAMP) levels. We aimed to investigate whether these changes are associated with downstream impairments in cAMP and Ca²⁺ signalling in a type 1 diabetes (T1D)-induced DCM model. T1D was induced in adult male rats by streptozotocin (65 mg/kg) injection. DCM was assessed by cardiac structural and molecular remodelling. We delineated sequential changes affecting the exchange protein (Epac1/2), cAMP-dependent protein kinase A (PKA) and Ca²⁺ /Calmodulin-dependent kinase II (CaMKII) at 4, 8 and 12 weeks following diabetes, by real-time quantitative PCR and western blot. Expression of Ca²⁺ ATPase pump (SERCA2a), phospholamban (PLB) and Troponin I (TnI) was also examined. Early upregulation of Epac1 transcripts was noted in diabetic hearts at Week 4, followed by increases in Epac2 mRNA, but not protein levels, at Week 12. Expression of PKA subunits (RI, RIIα and Cα) remained unchanged regardless of the disease stage, whereas CaMKII increased at Week 12 in DCM. Moreover, PLB transcripts were upregulated in diabetic hearts, whereas SERCA2a and TnI gene expression was unchanged irrespective of the disease evolution. PLB phosphorylation at threonine-17 was increased in DCM, whereas phosphorylation of both PLB at serine-16 and TnI at serine-23/24 was unchanged. We show for the first time differential and time-specific regulations in cardiac cAMP effectors and Ca²⁺ handling proteins, data that may prove useful in proposing new therapeutic approaches in T1D-induced DCM.

cAMP-Epac1 signaling is activated in DDAVP-induced endolymphatic hydrops of guinea pigs

OBJECTIVE

To explore whether Cyclic Adenosine Monophosphate (cAMP)-Epac1 signaling is activated in 1-Desamino-8-D-arginine-Vasopressin-induced Endolymphatic Hydrops (DDAVP-induced EH) and to provide new insight for further in-depth study of DDAVP-induced EH.

METHODS

Eighteen healthy, red-eyed guinea pigs (36 ears) weighing 200-350 g were randomly divided into three groups: the control group, which received intraperitoneal injection of sterile saline (same volume as that in the other two groups) for 7 consecutive days; the DDAVP-7d group, which received intraperitoneal injection of 10 mg/mL/kg DDAVP for 7 consecutive days; and the DDAVP-14d group, which received intraperitoneal injection of 10 μg/mL/kg DDAVP for 14 consecutive days. After successful modeling, all animals were sacrificed, and cochlea tissues were collected to detect the mRNA and protein expression of the exchange protein directly activated by cAMP-1 and 2 (Epac1, Epac2), and Repressor Activator Protein-1 (Rap1) by Reverse Transcription (RT)-PCR and western blotting, respectively.

RESULTS

Compared to the control group, the relative mRNA expression of Epac1, Epac2, Rap1A, and Rap1B in the cochlea tissue of the DDAVP-7d group was significantly higher (p <  0.05), while no significant difference in Rap1 GTPase activating protein (Rap1gap) mRNA expression was found between the two groups. The relative mRNA expression of Epac1, Rap1A, Rap1B, and Rap1gap in the cochlea tissue of the DDAVP-14d group was significantly higher than that of the control group (p <  0.05), while no significant difference in Epac2 mRNA expression was found between the DDAVP-14d and control groups. Comparison between the DDAVP-14d and DDAVP-7d groups showed that the DDAVP-14d group had significantly lower Epac2 and Rap1A (p <  0.05) and higher Rap1gap (p < 0.05) mRNA expression in the cochlea tissue than that of the DDAVP-7d group, while no significant differences in Epac1 and Rap1B mRNA expression were found between the two groups. Western blotting showed that Epac1 protein expression in the cochlea tissue was the highest in the DDAVP-14d group, followed by that in the DDAVP-7d group, and was the lowest in the control group, showing significant differences between groups (p <  0.05); Rap1 protein expression in the cochlea tissue was the highest in the DDAVP-7d group, followed by the DDAVP-14d group, and was the lowest in the control group, showing significant differences between groups (p <  0.05); no significant differences in Epac2 protein expression in the cochlea tissue were found among the three groups.

CONCLUSION

DDAVP upregulated Epac1 protein expression in the guinea pig cochlea, leading to activation of the inner ear cAMP-Epac1 signaling pathway. This may be an important mechanism by which DDAVP regulates endolymphatic metabolism to induce EH and affect inner ear function. OXFORD CENTRE FOR EVIDENCE-BASED MEDICINE 2011 LEVELS OF EVIDENCE: Level 5.

Basic Research Approaches to Evaluate Cardiac Arrhythmia in Heart Failure and Beyond

Patients with heart failure often develop cardiac arrhythmias. The mechanisms and interrelations linking heart failure and arrhythmias are not fully understood. Historically, research into arrhythmias has been performed on affected individuals or in vivo (animal) models. The latter however is constrained by interspecies variation, demands to reduce animal experiments and cost. Recent developments in in vitro induced pluripotent stem cell technology and in silico modelling have expanded the number of models available for the evaluation of heart failure and arrhythmia. An agnostic approach, combining the modalities discussed here, has the potential to improve our understanding for appraising the pathology and interactions between heart failure and arrhythmia and can provide robust and validated outcomes in a variety of research settings. This review discusses the state of the art models, methodologies and techniques used in the evaluation of heart failure and arrhythmia and will highlight the benefits of using them in combination. Special consideration is paid to assessing the pivotal role calcium handling has in the development of heart failure and arrhythmia.

Molecules linked to Ras signaling as therapeutic targets in cardiac pathologies

Abstract

The Ras family of small Guanosine Triphosphate (GTP)-binding proteins (G proteins) represents one of the main components of intracellular signal transduction required for normal cardiac growth, but is also critically involved in the development of cardiac hypertrophy and heart failure. The present review provides an update on the role of the H-, K- and N-Ras genes and their related pathways in cardiac diseases. We focus on cardiac hypertrophy and heart failure, where Ras has been studied the most. We also review other cardiac diseases, like genetic disorders related to Ras. The scope of the review extends from fundamental concepts to therapeutic applications. Although the three Ras genes have a nearly identical primary structure, there are important functional differences between them: H-Ras mainly regulates cardiomyocyte size, whereas K-Ras regulates cardiomyocyte proliferation. N-Ras is the least studied in cardiac cells and is less associated to cardiac defects. Clinically, oncogenic H-Ras causes Costello syndrome and facio-cutaneous-skeletal syndromes with hypertrophic cardiomyopathy and arrhythmias. On the other hand, oncogenic K-Ras and alterations of other genes of the Ras-Mitogen-Activated Protein Kinase (MAPK) pathway, like Raf, cause Noonan syndrome and cardio-facio-cutaneous syndromes characterized by cardiac hypertrophy and septal defects. We further review the modulation by Ras of key signaling pathways in the cardiomyocyte, including: (i) the classical Ras-Raf-MAPK pathway, which leads to a more physiological form of cardiac hypertrophy; as well as other pathways associated with pathological cardiac hypertrophy, like (ii) The SAPK (stress activated protein kinase) pathways p38 and JNK; and (iii) The alternative pathway Raf-Calcineurin-Nuclear Factor of Activated T cells (NFAT). Genetic alterations of Ras isoforms or of genes in the Ras-MAPK pathway result in Ras-opathies, conditions frequently associated with cardiac hypertrophy or septal defects among other cardiac diseases. Several studies underline the potential role of H- and K-Ras as a hinge between physiological and pathological cardiac hypertrophy, and as potential therapeutic targets in cardiac hypertrophy and failure.

Graphic abstract

The Ras (Rat Sarcoma) gene family is a group of small G proteins

Ras is regulated by growth factors and neurohormones affecting cardiomyocyte growth and hypertrophy

Ras directly affects cardiomyocyte physiological and pathological hypertrophy

Genetic alterations of Ras and its pathways result in various cardiac phenotypes

Ras and its pathway are differentially regulated in acquired heart disease

Ras modulation is a promising therapeutic target in various cardiac conditions.

Exchange proteins directly activated by cAMP mediate cardiac repolarization and arrhythmogenesis during chronic heart failure

Most sudden cardiac death in chronic heart failure (CHF) is caused by malignant ventricular arrhythmia (VA); however, the molecular mechanism remains unclear. This study aims to explore the effect of exchange proteins directly activated by cAMP (Epac) on VA in CHF and the potential molecular mechanism. Transaortic constriction was performed to prepare CHF guinea pigs. Epac activation model was obtained with 8-pCPT administration. Programmed electrical stimulation (PES) was performed to detect effective refractory period (ERP) or induce VA. Isolated adult cardiomyocytes were treated with 8-pCPT and (or) the Epac inhibitor. Cellular electrophysiology was examined by whole-cell patch clamp. With Epac activation, corrected QT duration was lengthened by 12.6%. The 8-pCPT increased action potential duration (APD) (APD50: 236.9 ± 18.07 ms vs. 328.8 ± 11.27 ms, p < 0.05; APD90: 264.6 ± 18.22 ms vs. 388.6 ± 6.47 ms, p < 0.05) and decreased rapid delayed rectifier potassium (IKr) current (tail current density: 1.1 ± 0.08 pA/pF vs. 0.7 ± 0.03 pA/pF, p < 0.05). PES induced more malignant arrhythmias in the 8-pCPT group than in the control group (3/4 vs. 0/8, p < 0.05). The selective Epac1 inhibitor CE3F4 rescued the drop in IKr after 8-pCPT stimulation (tail current density: 0.5 ± 0.02 pA/pF vs. 0.6 ± 0.03 pA/pF, p < 0.05). In conclusion, Epac1 regulates IKr, APD, and ERP in guinea pigs, which could contribute to the proarrhythmic effect of Epac1 in CHF.

Cardiac cAMP-PKA Signaling Compartmentalization in Myocardial Infarction

Under physiological conditions, cAMP signaling plays a key role in the regulation of cardiac function. Activation of this intracellular signaling pathway mirrors cardiomyocyte adaptation to various extracellular stimuli. Extracellular ligand binding to seven-transmembrane receptors (also known as GPCRs) with G proteins and adenylyl cyclases (ACs) modulate the intracellular cAMP content. Subsequently, this second messenger triggers activation of specific intracellular downstream effectors that ensure a proper cellular response. Therefore, it is essential for the cell to keep the cAMP signaling highly regulated in space and time. The temporal regulation depends on the activity of ACs and phosphodiesterases. By scaffolding key components of the cAMP signaling machinery, A-kinase anchoring proteins (AKAPs) coordinate both the spatial and temporal regulation. Myocardial infarction is one of the major causes of death in industrialized countries and is characterized by a prolonged cardiac ischemia. This leads to irreversible cardiomyocyte death and impairs cardiac function. Regardless of its causes, a chronic activation of cardiac cAMP signaling is established to compensate this loss. While this adaptation is primarily beneficial for contractile function, it turns out, in the long run, to be deleterious. This review compiles current knowledge about cardiac cAMP compartmentalization under physiological conditions and post-myocardial infarction when it appears to be profoundly impaired.

Cytoskeleton regulators CAPZA2 and INF2 associate with CFTR to control its plasma membrane levels under EPAC1 activation

Cystic Fibrosis (CF), the most common lethal autosomic recessive disorder among Caucasians, is caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) protein, a cAMP-regulated chloride channel expressed at the apical surface of epithelial cells. Cyclic AMP regulates both CFTR channel gating through a protein kinase A (PKA)-dependent process and plasma membane (PM) stability through activation of the exchange protein directly activated by cAMP1 (EPAC1). This cAMP effector, when activated promotes the NHERF1:CFTR interaction leading to an increase in CFTR at the PM by decreasing its endocytosis. Here, we used protein interaction profiling and bioinformatic analysis to identify proteins that interact with CFTR under EPAC1 activation as possible regulators of this CFTR PM anchoring. We identified an enrichment in cytoskeleton related proteins among which we characterized CAPZA2 and INF2 as regulators of CFTR trafficking to the PM. We found that CAPZA2 promotes wt-CFTR trafficking under EPAC1 activation at the PM whereas reduction of INF2 levels leads to a similar trafficking promotion effect. These results suggest that CAPZA2 is a positive regulator and INF2 a negative one for the increase of CFTR at the PM after an increase of cAMP and concomitant EPAC1 activation. Identifying the specific interactions involving CFTR and elicited by EPAC1 activation provides novel insights into late CFTR trafficking, insertion and/or stabilization at the PM and highlighs new potential therapeutic targets to tackle CF disease.

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.

Loss of EPAC2 alters dendritic spine morphology and inhibitory synapse density

EPAC2 is a guanine nucleotide exchange factor that regulates GTPase activity of the small GTPase Rap and Ras and is highly enriched at synapses. Activation of EPAC2 has been shown to induce dendritic spine shrinkage and increase spine motility, effects that are necessary for synaptic plasticity. These morphological effects are dysregulated by rare mutations of Epac2 associated with autism spectrum disorders. In addition, EPAC2 destabilizes synapses through the removal of synaptic GluA2/3-containing AMPA receptors. Previous work has shown that Epac2 knockout mice (Epac2^−/−) display abnormal social interactions, as well as gross disorganization of the frontal cortex and abnormal spine motility in vivo. In this study we sought to further understand the cellular consequences of knocking out Epac2 on the development of neuronal and synaptic structure and organization of cortical neurons. Using primary cortical neurons generated from Epac2^+/+ or Epac2^−/− mice, we confirm that EPAC2 is required for cAMP-dependent spine shrinkage. Neurons from Epac2^−/− mice also displayed increased synaptic expression of GluA2/3-containing AMPA receptors, as well as of the adhesion protein N-cadherin. Intriguingly, analysis of excitatory and inhibitory synaptic proteins revealed that loss of EPAC2 resulted in altered expression of vesicular GABA transporter (VGAT) but not vesicular glutamate transporter 1 (VGluT1), indicating an altered ratio of excitatory and inhibitory synapses onto neurons. Finally, examination of cortical neurons located within the anterior cingulate cortex further revealed subtle deficits in the establishment of dendritic arborization in vivo. These data provide evidence that loss of EPAC2 enhances the stability of excitatory synapses and increases the number of inhibitory inputs.

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EPAC2 is required for cAMP-dependent spine remodeling.

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Loss of EPAC2 results in over-stabilized excitatory synapses.

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Loss of EPAC2 results in an increase in inhibitory input onto neurons.

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EPAC2 is required for correct dendritic arborization and spine formation in vivo.

PDE4 and Epac1 Synergistically Promote Rectal Carcinoma via the cAMP Pathway

Objective

To assess the expression levels of exchange protein 1 directly activated by cAMP (Epac1) and phosphodiesterase 4 (PDE4) in rectal carcinoma, and their associations with clinicopathological indexes. In addition, the associations of PDE4 and Epac1 with A-kinase anchor protein 95, connexin 43, cyclin D1, and cyclin E1 were evaluated.

Methods

The PV-9000 two-step immunohistochemistry method was used to determine protein expression in 44 rectal carcinoma tissue samples and 16 paracarcinoma tissue specimens.

Results

The positive rate of PDE4 protein expression in rectal carcinoma tissues was higher than that of paracarcinoma tissues (59.09% vs. 12.5%, P < 0.05). Similar findings were obtained for Epac1 (55% vs. 6.25%, P < 0.05). No significant associations of PDE4 and Epac1 with degree of differentiation, histological type, and lymph node metastasis were found in rectal carcinoma (P > 0.05). Correlations between PDE4 and Epac1, PDE4 and Cx43, PDE4 and cyclin E1, and Epac1 and Cx43 were observed (all P < 0.05). There was no correlation between the other protein pairs examined (P > 0.05).

Conclusion

PDE4 and Epac1 expression levels are increased in rectal carcinoma tissues, suggesting that the two proteins may be involved in the development of this malignancy. Meanwhile, correlations between PDE4 and Epac1, PDE4 and Cx43, PDE4 and cyclin E1, and Epac1 and Cx43 suggested synergistic effects of these proteins in promoting rectal carcinoma.

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.

Molecular pathways of oestrogen receptors and β-adrenergic receptors in cardiac cells: Recognition of their similarities, interactions and therapeutic value

Oestrogen receptors (ERs) and β-adrenergic receptors (βARs) play important roles in the cardiovascular system. Moreover, these receptors are expressed in cardiac myocytes and vascular tissues. Numerous experimental observations support the hypothesis that similarities and interactions exist between the signalling pathways of ERs (ERα, ERβ and GPR30) and βARs (β₁ AR, β₂ AR and β₃ AR). The recently discovered oestrogen receptor GPR30 shares structural features with the βARs, and this forms the basis for the interactions and functional overlap. GPR30 possesses protein kinase A (PKA) phosphorylation sites and PDZ binding motifs and interacts with A-kinase anchoring protein 5 (AKAP5), all of which enable its interaction with the βAR pathways. The interactions between ERs and βARs occur downstream of the G-protein-coupled receptor, through the G_αs and G_αi proteins. This review presents an up-to-date description of ERs and βARs and demonstrates functional synergism and interactions among these receptors in cardiac cells. We explore their signalling cascades and the mechanisms that orchestrate their interactions and propose new perspectives on the signalling patterns for the GPR30 based on its structural resemblance to the βARs. In addition, we explore the relevance of these interactions to cell physiology, drugs (especially β-blockers and calcium channel blockers) and cardioprotection. Furthermore, a receptor-independent mechanism for oestrogen and its influence on the expression of βARs and calcium-handling proteins are discussed. Finally, we highlight promising therapeutic avenues that can be derived from the shared pathways, especially the phosphatidylinositol-3-OH kinase (PI3K/Akt) pathway.

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.

Growth hormone-releasing hormone attenuates cardiac hypertrophy and improves heart function in pressure overload-induced heart failure

Pathological cardiac hypertrophy, characterized by heart growth in response to pressure or volume overload, such as in the setting of hypertension, is the main risk factor for heart failure (HF). The identification of therapeutic strategies to prevent or reverse cardiac hypertrophy is therefore a priority for curing HF. It is known that growth hormone-releasing hormone (GHRH) displays cardioprotective functions; however, its therapeutic potential in hypertrophy and HF is unknown. Here we show that GHRH reduces cardiomyocyte hypertrophy in vitro through inhibition of hypertrophic pathways. In vivo, the GHRH analog MR-409 attenuates cardiac hypertrophy in mice subjected to transverse aortic constriction and improves cardiac function. These findings suggest therapeutic use of GHRH analogs for treatment of pathological cardiac hypertrophy and HF.

It has been shown that growth hormone-releasing hormone (GHRH) reduces cardiomyocyte (CM) apoptosis, prevents ischemia/reperfusion injury, and improves cardiac function in ischemic rat hearts. However, it is still not known whether GHRH would be beneficial for life-threatening pathological conditions, like cardiac hypertrophy and heart failure (HF). Thus, we tested the myocardial therapeutic potential of GHRH stimulation in vitro and in vivo, using GHRH or its agonistic analog MR-409. We show that in vitro, GHRH(1-44)NH₂ attenuates phenylephrine-induced hypertrophy in H9c2 cardiac cells, adult rat ventricular myocytes, and human induced pluripotent stem cell-derived CMs, decreasing expression of hypertrophic genes and regulating hypertrophic pathways. Underlying mechanisms included blockade of Gq signaling and its downstream components phospholipase Cβ, protein kinase Cε, calcineurin, and phospholamban. The receptor-dependent effects of GHRH also involved activation of Gα_s and cAMP/PKA, and inhibition of increase in exchange protein directly activated by cAMP1 (Epac1). In vivo, MR-409 mitigated cardiac hypertrophy in mice subjected to transverse aortic constriction and improved cardiac function. Moreover, CMs isolated from transverse aortic constriction mice treated with MR-409 showed improved contractility and reversal of sarcolemmal structure. Overall, these results identify GHRH as an antihypertrophic regulator, underlying its therapeutic potential for HF, and suggest possible beneficial use of its analogs for treatment of pathological cardiac hypertrophy.

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.

Exchange proteins directly activated by cAMP (EPACs): Emerging therapeutic targets

Exchange proteins directly activated by cAMP (EPACs) are critical cAMP-dependent signaling pathway mediators. The discovery of EPAC proteins has significantly facilitated understanding on cAMP-dependent signaling pathway and efforts along this line open new avenues for developing novel therapeutics for cancer, diabetes, heart failure, inflammation, infections, neurological disorders and other human diseases. Over the past decade, important progress has been made in the identification of EPAC agonists, antagonists and their biological and pharmacological applications. In this review, we briefly summarize recently reported novel functions of EPACs and the discovery of their small molecule modulators. The challenges and future perspectives are also discussed.

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.

Long-term consumption of an obesogenic high fat diet prior to ischemia-reperfusion mediates cardioprotection via Epac1-dependent signaling

BACKGROUND

Obesity is still considered a risk factor for cardiovascular disease, although more recent knowledge also suggests obesity to be associated with reduced morbidity and mortality - the "obesity paradox". This study explores if long-term feeding of an obesogenic high fat diet renders the myocardium less susceptible to ischemic-reperfusion induced injury via Epac-dependent signaling.

METHODS

Wild type (wt), Epac1 (Epac1^-/-) and Epac2 (Epac2^-/-) deficient mice were fed a high fat (HFD) or normal chow diet (ND) for 33 ± 1 weeks. Six experimental groups were included: (1) control wt ND (wt ND), (2) control wt HFD (wt HFD), (3) Epac1^-/- mice on ND (Epac1^-/-ND), (4) Epac1^-/- mice on HFD (Epac1^-/-HFD), (5) Epac2^-/- mice on ND (Epac2^-/-ND), and (6) Epac2^-/- mice on HFD (Epac2^-/-HFD). Isolated ex vivo mice hearts were perfused in a constant pressure Langendorff mode, and exposed to 30min of global ischemia (GI) and 60min of reperfusion. Endpoints were infarct size and functional recovery.

RESULTS

All groups fed a HFD presented with significantly enhanced body weight, visceral fat content and reduced glucose clearance compared to corresponding ND groups. Although the HFD cohorts presented with an overall comparable systemic capability to clear glucose, the Epac1^-/- HFD group presented with glucose levels slightly above the human diabetes criteria at the end of the intraperitoneal glucose tolerance test (ipGTT). Moreover, the HFD significantly reduced infarct size in both wild type (wt HFD 41.3 ± 5.5% vs. wt ND 58.0 ± 9.8%, p < 0.05) and Epac2^-/- cohorts (Epac2^-/-HFD 34.4 ± 7.2% vs. Epac2^-/-ND 56.5 ± 3.8%, p < 0.05). Interestingly, however, the HFD did not reduce infarct size in Epac1^-/- deficient mice hearts (Epac1^-/-HFD 65.1 ± 5.1% vs. Epac1^-/-ND 56.1 ± 3.5%, ns.).

CONCLUSION

Epac1-dependent signaling is involved in mediating the cardioprotection afforded by long-term feeding of an obesogenic high fat diet in mice hearts.

Epac1 interacts with importin β1 and controls neurite outgrowth independently of cAMP and Rap1

Exchange protein directly activated by cAMP-1 (Epac1) is a cAMP sensor that regulates multiple cellular functions including cellular migration, proliferation and differentiation. Classically, Epac1 is thought to exert its effects through binding of cAMP leading to a conformational change in Epac1 and its accumulation at the plasma membrane (PM) where it activates Rap1. In search for regulators of Epac1 activity, we show here that importin β1 (impβ1) is an Epac1 binding partner that prevents PM accumulation of Epac1. We demonstrate that in the absence of impβ1, endogenous as well as overexpressed Epac1 accumulate at the PM. Moreover, agonist-induced PM translocation of Epac1 leads to dissociation of Epac1 from impβ1. Localization of Epac1 at the PM in the absence of impβ1, requires residue R82 in its DEP domain. Notably, the PM accumulation of Epac1 in the absence of impβ1 does not require binding of cAMP to Epac1 and does not result in Rap1 activation. Functionally, PM accumulation of Epac1, an Epac1 mutant deficient in cAMP binding, or an Epac1 mutant tethered to the PM, is sufficient to inhibit neurite outgrowth. In conclusion, we uncover a cAMP-independent function of Epac1 at the PM and demonstrate that impβ1 controls subcellular localization of Epac1.

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.

EPAC1 activation by cAMP stabilizes CFTR at the membrane by promoting its interaction with NHERF1

Cyclic AMP (cAMP) activates protein kinase A (PKA) but also the guanine nucleotide exchange factor 'exchange protein directly activated by cAMP' (EPAC1; also known as RAPGEF3). Although phosphorylation by PKA is known to regulate CFTR channel gating - the protein defective in cystic fibrosis - the contribution of EPAC1 to CFTR regulation remains largely undefined. Here, we demonstrate that in human airway epithelial cells, cAMP signaling through EPAC1 promotes CFTR stabilization at the plasma membrane by attenuating its endocytosis, independently of PKA activation. EPAC1 and CFTR colocalize and interact through protein adaptor NHERF1 (also known as SLC9A3R1). This interaction is promoted by EPAC1 activation, triggering its translocation to the plasma membrane and binding to NHERF1. Our findings identify a new CFTR-interacting protein and demonstrate that cAMP activates CFTR through two different but complementary pathways - the well-known PKA-dependent channel gating pathway and a new mechanism regulating endocytosis that involves EPAC1. The latter might constitute a novel therapeutic target for treatment of cystic fibrosis.

Functionalized N,N-Diphenylamines as Potent and Selective EPAC2 Inhibitors

N,N-Diphenylamines were discovered as potent and selective EPAC2 inhibitors. A study was conducted to determine the structure-activity relationships in a series of inhibitors of which several compounds displayed submicromolar potencies. Selectivity over the related EPAC1 protein was also demonstrated. Computational modeling reveals an allosteric site that is distinct from the cAMP binding domain shared by both EPAC isoforms, providing a theory with regards to subtype selectivity.

Disruption of Epac1 protects the heart from adenylyl cyclase type 5-mediated cardiac dysfunction

Type 5 adenylyl cyclase (AC5) plays an important role in the development of chronic catecholamine stress-induced heart failure and arrhythmia in mice. Epac (exchange protein activated by cAMP), which is directly activated by cAMP independent of protein kinase A, has been recently identified as a novel mediator of cAMP signaling in the heart. However, the role of Epac in AC5-mediated cardiac dysfunction and arrhythmias remains poorly understood. We therefore generated AC5 transgenic mice (AC5TG) with selective disruption of the Epac1 gene (AC5TG-Epac1KO), and compared their phenotypes with those of AC5TG after chronic isoproterenol (ISO) infusion. Decreased cardiac function as well as increased susceptibility to pacing-induced atrial fibrillation (AF) in response to ISO were significantly attenuated in AC5TG-Epac1KO mice, compared to AC5TG mice. Increased cardiac apoptosis and cardiac fibrosis were also concomitantly attenuated in AC5TG-Epac1KO mice compared to AC5TG mice. These findings indicate that Epac1 plays an important role in AC5-mediated cardiac dysfunction and AF susceptibility.

Role of EPAC in cAMP-Mediated Actions in Adrenocortical Cells

Adrenocorticotropic hormone regulates adrenal steroidogenesis mainly via the intracellular signaling molecule cAMP. The effects of cAMP are principally relayed by activating protein kinase A (PKA) and the more recently discovered exchange proteins directly activated by cAMP 1 and 2 (EPAC1 and EPAC2). While the intracellular roles of PKA have been extensively studied in steroidogenic tissues, those of EPACs are only emerging. EPAC1 and EPAC2 are encoded by the genes RAPGEF3 and RAPGEF4, respectively. Whereas EPAC1 is ubiquitously expressed, the expression of EPAC2 is more restricted, and typically found in endocrine tissues. Alternative promoter usage of RAPGEF4 gives rise to three different isoforms of EPAC2 that vary in their N-termini (EPAC2A, EPAC2B, and EPAC2C) and that exhibit distinct expression patterns. EPAC2A is expressed in the brain and pancreas, EPAC2B in steroidogenic cells of the adrenal gland and testis, and EPAC2C has until now only been found in the liver. In this review, we discuss current knowledge on EPAC expression and function with focus on the known roles of EPAC in adrenal gland physiology.

Sex Steroids Influence Brain-Derived Neurotropic Factor Secretion From Human Airway Smooth Muscle Cells

Brain derived neurotropic factor (BDNF) is emerging as an important player in airway inflammation, remodeling, and hyperreactivity. Separately, there is increasing evidence that sex hormones contribute to pathophysiology in the lung. BDNF and sex steroid signaling are thought to be intricately linked in the brain. There is currently little information on BDNF and sex steroid interactions in the airway but is relevant to understanding growth factor signaling in the context of asthma in men versus women. In this study, we assessed the effect of sex steroids on BDNF expression and secretion in human airway smooth muscle (ASM). Human ASM was treated with estrogen (E2 ) or testosterone (T, 10 nM each) and intracellular BDNF and secreted BDNF measured. E2 and T significantly reduced secretion of BDNF; effects prevented by estrogen and androgen receptor inhibitor, ICI 182,780 (1 μM), and flutamide (10 μM), respectively. Interestingly, no significant changes were observed in intracellular BDNF mRNA or protein expression. High affinity BDNF receptor, TrkB, was not altered by E2 or T. E2 (but not T) significantly increased intracellular cyclic AMP levels. Notably, Epac1 and Epac2 expression were significantly reduced by E2 and T. Furthermore, SNARE complex protein SNAP25 was decreased. Overall, these novel data suggest that physiologically relevant concentrations of E2 or T inhibit BDNF secretion in human ASM, suggesting a potential interaction of sex steroids with BDNF in the airway that is different from brain. The relevance of sex steroid-BDNF interactions may lie in their overall contribution to airway diseases such as asthma.

cAMP-mediated secretion of brain-derived neurotrophic factor in developing airway smooth muscle

Moderate hyperoxic exposure in preterm infants contributes to subsequent airway dysfunction and to risk of developing recurrent wheeze and asthma. The regulatory mechanisms that can contribute to hyperoxia-induced airway dysfunction are still under investigation. Recent studies in mice show that hyperoxia increases brain-derived neurotrophic factor (BDNF), a growth factor that increases airway smooth muscle (ASM) proliferation and contractility. We assessed the mechanisms underlying effects of moderate hyperoxia (50% O2) on BDNF expression and secretion in developing human ASM. Hyperoxia increased BDNF secretion, but did not alter endogenous BDNF mRNA or intracellular protein levels. Exposure to hyperoxia significantly increased [Ca2+]i responses to histamine, an effect blunted by the BDNF chelator TrkB-Fc. Hyperoxia also increased ASM cAMP levels, associated with reduced PDE4 activity, but did not alter protein kinase A (PKA) activity or adenylyl cyclase mRNA levels. However, 50% O2 increased expression of Epac2, which is activated by cAMP and can regulate protein secretion. Silencing RNA studies indicated that Epac2, but not Epac1, is important for hyperoxia-induced BDNF secretion, while PKA inhibition did not influence BDNF secretion. In turn, BDNF had autocrine effects of enhancing ASM cAMP levels, an effect inhibited by TrkB and BDNF siRNAs. Together, these novel studies suggest that hyperoxia can modulate BDNF secretion, via cAMP-mediated Epac2 activation in ASM, resulting in a positive feedback effect of BDNF-mediated elevation in cAMP levels. The potential functional role of this pathway is to sustain BDNF secretion following hyperoxic stimulus, leading to enhanced ASM contractility and proliferation.

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.

Exchange protein directly activated by cAMP encoded by the mammalian rapgef3 gene: Structure, function and therapeutics

Mammalian exchange protein directly activated by cAMP isoform 1 (EPAC1), encoded by the RAPGEF3 gene, is one of the two-membered family of cAMP sensors that mediate the intracellular functions of cAMP by acting as guanine nucleotide exchange factors for the Ras-like Rap small GTPases. Extensive studies have revealed that EPAC1-mediated cAMP signaling is highly coordinated spatiotemporally through the formation of dynamic signalosomes by interacting with a diverse array of cellular partners. Recent functional analyses of genetically engineered mouse models further suggest that EPAC1 functions as an important stress response switch and is involved in pathophysiological conditions of cardiac stresses, chronic pain, cancer and infectious diseases. These findings, coupled with the development of EPAC specific small molecule modulators, validate EPAC1 as a promising target for therapeutic interventions.

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Human gene RAPGEF3 encodes for EPAC1 protein.

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Along with PKA, CNG & HCN, EPAC is an important cAMP sensor.

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Selective modulators of EPAC1 have been developed for use as pharmacological probes.

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Formation of EPAC1 signalosomes allows spatiotemporal control of cAMP signaling.

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EPAC1 is implicated in major pathophysiological conditions and is an attractive therapeutic target.

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.

Epac is required for GLP-1R-mediated inhibition of oxidative stress and apoptosis in cardiomyocytes

Although the cardioprotective effects of glucagon-like peptide-1 and its analogs have been reported, the exact mechanisms of the glucagon-like peptide-1 receptor (GLP-1R) signaling pathway in the heart are still unclear. Activation of the GLP-1R has been shown to increase cAMP levels, thus eliciting protein kinase A- and exchange protein activated by cAMP (Epac)-dependent signaling pathways in pancreatic β-cells. However, which pathway plays an important role in the antioxidant and antiapoptotic effects of GLP-1R activation in the heart is not known. In this study, we demonstrated that stimulation of GLP-1Rs with exendin-4 attenuated H2O2-induced reactive oxygen species production and increased the synthesis of antioxidant enzymes, catalase, glutathione peroxidase-1, and manganese superoxide dismutase that is dependent on Epac. Additionally, exendin-4 has an antiapoptotic effect by decreasing a number of apoptotic cells, inhibiting caspase-3 activity, and enhancing the expression of antiapoptotic protein B-cell lymphoma 2, which is mediated through both protein kinase A- and Epac-dependent pathways. These data indicate a critical role for Epac in GLP-1R-mediated cardioprotection.

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.

Allosteric inhibition of Epac: computational modeling and experimental validation to identify allosteric sites and inhibitors

Epac, a guanine nucleotide exchange factor for the low molecular weight G protein Rap, is an effector of cAMP signaling and has been implicated to have roles in numerous diseases, including diabetes mellitus, heart failure, and cancer. We used a computational molecular modeling approach to predict potential binding sites for allosteric modulators of Epac and to identify molecules that might bind to these regions. This approach revealed that the conserved hinge region of the cyclic nucleotide-binding domain of Epac1 is a potentially druggable region of the protein. Using a bioluminescence resonance energy transfer-based assay (CAMYEL, cAMP sensor using YFP-Epac-Rluc), we assessed the predicted compounds for their ability to bind Epac and modulate its activity. We identified a thiobarbituric acid derivative, 5376753, that allosterically inhibits Epac activity and used Swiss 3T3 and HEK293 cells to test the ability of this compound to modulate the activity of Epac and PKA, as determined by Rap1 activity and vasodilator-stimulated phosphoprotein phosphorylation, respectively. Compound 5376753 selectively inhibited Epac in biochemical and cell migration studies. These results document the utility of a computational approach to identify a domain for allosteric regulation of Epac and a novel compound that prevents the activation of Epac1 by cAMP.

Exchange protein directly activated by cAMP mediates slow delayed-rectifier current remodeling by sustained β-adrenergic activation in guinea pig hearts

RATIONALE

β-Adrenoceptor activation contributes to sudden death risk in heart failure. Chronic β-adrenergic stimulation, as occurs in patients with heart failure, causes potentially arrhythmogenic reductions in slow delayed-rectifier K(+) current (IKs).

OBJECTIVE

To assess the molecular mechanisms of IKs downregulation caused by chronic β-adrenergic activation, particularly the role of exchange protein directly activated by cAMP (Epac).

METHODS AND RESULTS

Isolated guinea pig left ventricular cardiomyocytes were incubated in primary culture and exposed to isoproterenol (1 μmol/L) or vehicle for 30 hours. Sustained isoproterenol exposure decreased IKs density (whole cell patch clamp) by 58% (P<0.0001), with corresponding decreases in potassium voltage-gated channel subfamily E member 1 (KCNE1) mRNA and membrane protein expression (by 45% and 51%, respectively). Potassium voltage-gated channel, KQT-like subfamily, member 1 (KCNQ1) mRNA expression was unchanged. The β1-adrenoceptor antagonist 1-[2-((3-Carbamoyl-4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol dihydrochloride (CGP-20712A) prevented isoproterenol-induced IKs downregulation, whereas the β2-antagonist ICI-118551 had no effect. The selective Epac activator 8-pCPT-2'-O-Me-cAMP decreased IKs density to an extent similar to isoproterenol exposure, and adenoviral-mediated knockdown of Epac1 prevented isoproterenol-induced IKs/KCNE1 downregulation. In contrast, protein kinase A inhibition with a cell-permeable highly selective peptide blocker did not affect IKs downregulation. 1,2-Bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetate-AM acetoxymethyl ester (BAPTA-AM), cyclosporine, and inhibitor of nuclear factor of activated T cell (NFAT)-calcineurin association-6 (INCA6) prevented IKs reduction by isoproterenol and INCA6 suppressed isoproterenol-induced KCNE1 downregulation, consistent with signal-transduction via the Ca(2+)/calcineurin/NFAT pathway. Isoproterenol induced nuclear NFATc3/c4 translocation (immunofluorescence), which was suppressed by Epac1 knockdown. Chronic in vivo administration of isoproterenol to guinea pigs reduced IKs density and KCNE1 mRNA and protein expression while inducing cardiac dysfunction and action potential prolongation. Selective in vivo activation of Epac via sp-8-pCPT-2'-O-Me-cAMP infusion decreased IKs density and KCNE1 mRNA/protein expression.

CONCLUSIONS

Prolonged β1-adrenoceptor stimulation suppresses IKs by downregulating KCNE1 mRNA and protein via Epac-mediated Ca(2+)/calcineurin/NFAT signaling. These results provide new insights into the molecular basis of K(+) channel remodeling under sustained adrenergic stimulation.

Blocking of exchange proteins directly activated by cAMP leads to reduced replication of Middle East respiratory syndrome coronavirus

The outbreak of Middle East respiratory syndrome coronavirus (MERS-CoV) infections and diseases represents a potential threat for worldwide spread and requires development of effective therapeutic strategies. In this study, we revealed a novel positive function of an exchange protein directly activated by cyclic AMP 1 (cAMP-1; Epac-1) on MERS-CoV replication. Specifically, we have shown that Epac-specific inhibitor treatment or silencing Epac-1 gene expression rendered cells resistant to viral infection. We believe Epac-1 inhibitors deserve further study as potential therapeutic agents for MERS-CoV infection.

Recent advances in the discovery of small molecules targeting exchange proteins directly activated by cAMP (EPAC)

3',5'-Cyclic adenosine monophosphate (cAMP) is a pivotal second messenger that regulates numerous biological processes under physiological and pathological conditions, including cancer, diabetes, heart failure, inflammation, and neurological disorders. In the past, all effects of cAMP were initially believed to be mediated by protein kinase A (PKA) and cyclic nucleotide-regulated ion channels. Since the discovery of exchange proteins directly activated by cyclic adenosine 5'-monophosphate (EPACs) in 1998, accumulating evidence has demonstrated that the net cellular effects of cAMP are also regulated by EPAC. The pursuit of the biological functions of EPAC has benefited from the development and applications of a growing number of pharmacological probes targeting EPACs. In this review, we seek to provide a concise update on recent advances in the development of chemical entities including various membrane-permeable analogues of cAMP and newly discovered EPAC-specific ligands from high throughput assays and hit-to-lead optimizations.

EPAC expression and function in cardiac fibroblasts and myofibroblasts

In the heart, cardiac fibroblasts (CF) and cardiac myofibroblasts (CMF) are the main cells responsible for wound healing after cardiac insult. Exchange protein activated by cAMP (EPAC) is a downstream effector of cAMP, and it has been not completely studied on CF. Moreover, in CMF, which are the main cells responsible for cardiac healing, EPAC expression and function are unknown. We evaluated in both CF and CMF the effect of transforming growth factor β1 (TGF-β1) on EPAC-1 expression. We also studied the EPAC involvement on collagen synthesis, adhesion, migration and collagen gel contraction.

METHOD

Rat neonatal CF and CMF were treated with TGF-β1 at different times and concentrations. EPAC-1 protein levels and Rap1 activation were measured by western blot and pull down assay respectively. EPAC cellular functions were determined by adhesion, migration and collagen gel contraction assay; and collagen expression was determined by western blot.

RESULTS

TGF-β1 through Smad and JNK significantly reduced EPAC-1 expression in CF, while in CMF this cytokine increased EPAC-1 expression through ERK1/2, JNK, p38, AKT and Smad3. EPAC activation was able to induce higher Rap1-GTP levels in CMF than in CF. EPAC and PKA, both cAMP effectors, promoted CF and CMF adhesion on fibronectin, as well as CF migration; however, this effect was not observed in CMF. EPAC but not PKA activation mediated collagen gel contraction in CF, while in CMF both PKA and EPAC mediated collagen gel contraction. Finally, the EPAC and PKA activation reduced collagen synthesis in CF and CMF.

CONCLUSION

TGF-β1 differentially regulates the expression of EPAC in CF and CMF; and EPAC regulates differentially CF and CMF functions associated with cardiac remodeling.

Role of HIF-1α-activated Epac1 on HSC-mediated neuroplasticity in stroke model

Exchange protein activated by cAMP-1 (Epac1) plays an important role in cell proliferation, cell survival and neuronal signaling, and activation of Epac1 in endothelial progenitor cells increases their homing to ischemic muscles and promotes neovascularization in a model of hind limb ischemia. Moreover, upregulation of Epac1 occurs during organ development and in diseases such as myocardial hypertrophy, diabetes, and Alzheimer's disease. We report here that hypoxia upregulated Epac1 through HIF-1α induction in the CD34-immunosorted human umbilical cord blood hematopoietic stem cells (hUCB(34)). Importantly, implantation of hUCB(34) subjected to hypoxia-preconditioning (HP-hUCB(34)) improved stroke outcome, more than did implantation of untreated hUCB(34), in rodents subjected to cerebral ischemia, and this required Epac1-to-matrix metalloprotease (MMP) signaling. This improved therapeutic efficacy correlated with better engraftment and differentiation of these cells in the ischemic host brain. In addition, more than did implantation of untreated HP-hUCB(34), implantation of HP-hUCB(34) improved cerebral blood flow into the ischemic brain via induction of angiogenesis, facilitated proliferation/recruitment of endogenous neural progenitor cells in the ischemic brain, and promoted neurite outgrowth following cerebral ischemia. Consistent with our proposed role of Epac1-to-MMP signaling in hypoxia-preconditioning, the above mentioned effects of implanting HP-hUCB(34) could be abolished by pharmacological inhibition and genetic disruption/deletion of Epac1 or MMPs. We have discovered a HIF-1α-to-Epac1-to-MMP signaling pathway that is required for the improved therapeutic efficacy resulting from hypoxia preconditioning of hUCB(34) in vitro prior to their implantation into the host brain in vivo.

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.

Epac activator critically regulates action potential duration by decreasing potassium current in rat adult ventricle

Sympathetic stimulation is an important modulator of cardiac function via the classic cAMP-dependent signaling pathway, PKA. Recently, this paradigm has been challenged by the discovery of a family of guanine nucleotide exchange proteins directly activated by cAMP (Epac), acting in parallel to the classic signaling pathway. In cardiac myocytes, Epac activation is known to modulate Ca(2+) cycling yet their actions on cardiac ionic currents remain poorly characterized. This study attempts to address this paucity of information using the patch clamp technique to record action potential (AP) and ionic currents on rat ventricular myocytes. Epac was selectively activated by 8-CPT-AM (acetoxymethyl ester form of 8-CPT). AP amplitude, maximum depolarization rate and resting membrane amplitude were unaltered by 8-CPT-AM, strongly suggesting that Na(+) current and inward rectifier K(+) current are not regulated by Epac. In contrast, AP duration was significantly increased by 8-CPT-AM (prolongation of duration at 50% and 90% of repolarization by 41±10% and 43±8% respectively, n=11). L-type Ca(2+) current density was unaltered by 8-CPT-AM (n=16) so this cannot explain the action potential lengthening. However, the steady state component of K(+) current was significantly inhibited by 8-CPT-AM (-38±6%, n=15), while the transient outward K(+) current was unaffected by 8-CPT-AM. These effects were PKA-independent since they were observed in the presence of PKA inhibitor KT5720. Isoprenaline (100nM) induced a significant prolongation of AP duration, even in the presence of KT5720. This study provides the first evidence that the cAMP-binding protein Epac critically modulates cardiac AP duration by decreasing steady state K(+) current. These observations may be relevant to diseases in which Epac is upregulated, like cardiac hypertrophy.

Epac in cardiac calcium signaling

Epac, exchange protein directly activated by cAMP, is emerging as a new regulator of cardiac physiopathology. Although its effects are much less known than the classical cAMP effector, PKA, several studies have investigated the cardiac role of Epac, providing evidences that Epac modulates intracellular Ca(2+). In one of the first analyses, it was shown that Epac can increase the frequency of spontaneous Ca(2+) oscillations in cultured rat cardiomyocytes. Later on, in adult cardiomyocytes, it was shown that Epac can induce sarcoplasmic reticulum (SR) Ca(2+) release in a PKA independent manner. The pathway identified involved phospholipase C (PLC) and Ca(2+)/calmodulin kinase II (CaMKII). The latter phosphorylates the ryanodine receptor (RyR), increasing the Ca(2+) spark probability. The RyR, Ca(2+) release channel located in the SR membrane, is a key element in the excitation-contraction coupling. Thus Epac participates in the excitation-contraction coupling. Moreover, by inducing RyR phosphorylation, Epac is arrhythmogenic. A detailed analysis of Ca(2+) mobilization in different microdomains showed that Epac preferently elevated Ca(2+) in the nucleoplasm ([Ca(2+)]n). This effect, besides PLC and CaMKII, required inositol 1,4,5 trisphosphate receptor (IP3R) activation. IP3R is other Ca(2+) release channel located mainly in the perinuclear area in the adult ventricular myocytes, where it has been shown to participate in the excitation-transcription coupling (the process by which Ca(2+) activates transcription). If Epac activation is maintained for some time, the histone deacetylase (HDAC) is translocated out of the nucleus de-repressing the transcription factor myocyte enhancer factor (MEF2). These evidences also pointed to Epac role in activating the excitation-transcription coupling. In fact, it has been shown that Epac induces cardiomyocyte hypertrophy. Epac activation for several hours, even before the cell hypertrophies, induces a profound modulation of the excitation-contraction coupling: increasing the [Ca(2+)]i transient amplitude and cellular contraction. Thus Epac actions are rapid but time and microdomain dependent in the cardiac myocyte. Taken together the results collected indicate that Epac may have an important role in the cardiac response to stress.

Social, communication, and cortical structural impairments in Epac2-deficient mice

Deficits in social and communication behaviors are common features of a number of neurodevelopmental disorders. However, the molecular and cellular substrates of these higher order brain functions are not well understood. Here we report that specific alterations in social and communication behaviors in mice occur as a result of loss of the EPAC2 gene, which encodes a protein kinase A-independent cAMP target. Epac2-deficient mice exhibited robust deficits in social interactions and ultrasonic vocalizations, but displayed normal olfaction, working and reference memory, motor abilities, anxiety, and repetitive behaviors. Epac2-deficient mice displayed abnormal columnar organization in the anterior cingulate cortex, a region implicated in social behavior in humans, but not in somatosensory cortex. In vivo two-photon imaging revealed reduced dendritic spine motility and density on cortical neurons in Epac2-deficient mice, indicating deficits at the synaptic level. Together, these findings provide novel insight into the molecular and cellular substrates of social and communication behavior.

Distinct PKA and Epac compartmentalization in airway function and plasticity

Asthma and chronic obstructive pulmonary disease (COPD) are obstructive lung diseases characterized by airway obstruction, airway inflammation and airway remodelling. Next to inflammatory cells and airway epithelial cells, airway mesenchymal cells, including airway smooth muscle cells and (myo)fibroblasts, substantially contribute to disease features by the release of inflammatory mediators, smooth muscle contraction, extracellular matrix deposition and structural changes in the airways. Current pharmacological treatment of both diseases intends to target the dynamic features of the endogenous intracellular suppressor cyclic AMP (cAMP). This review will summarize our current knowledge on cAMP and will emphasize on key discoveries and paradigm shifts reflecting the complex spatio-temporal nature of compartmentalized cAMP signalling networks in health and disease. As airway fibroblasts and airway smooth muscle cells are recognized as central players in the development and progression of asthma and COPD, we will focus on the role of cAMP signalling in their function in relation to airway function and plasticity. We will recapture on the recent identification of cAMP-sensing multi-protein complexes maintained by cAMP effectors, including A-kinase anchoring proteins (AKAPs), proteins kinase A (PKA), exchange protein directly activated by cAMP (Epac), cAMP-elevating seven-transmembrane (7TM) receptors and phosphodiesterases (PDEs) and we will report on findings indicating that the pertubation of compartmentalized cAMP signalling correlates with the pathopysiology of obstructive lung diseases. Future challenges include studies on cAMP dynamics and compartmentalization in the lung and the development of novel drugs targeting these systems for therapeutic interventions in chronic obstructive inflammatory diseases.

Sustained Epac activation induces calmodulin dependent positive inotropic effect in adult cardiomyocytes

Cardiac actions of Epac (exchange protein directly activated by cAMP) are not completely elucidated. Epac induces cardiomyocytes hypertrophy, Ca(2+)/calmodulin protein kinase II (CaMKII) and excitation-transcription coupling in rat cardiac myocytes. Here we aimed to elucidate the pathway cascade involved in Epac sustained actions, as during the initiation of hypertrophy development, where β-adrenergic signaling is chronically stimulated. Rats were treated with the Epac selective activator 8-pCPT during 4 weeks and Ca(2+) signaling was analyzed in isolated cardiac myocytes by confocal microscopy. We observed a positive inotropic effect manifested by increased [Ca(2+)](i) transient amplitudes. In order to further analyze these actions, we incubated adult cardiomyocytes in the presence of 8-pCPT. The effects were similar to those obtained in-vivo and are blunted by Epac1 knock down. Interestingly, the increase in [Ca(2+)] transients was abolished by protein synthesis blockade or when the downstream effectors of calmodulin (CaMKII or calcineurin) were inhibited, pointing to calmodulin (CaM) as an important downstream protein in Epac sustained actions. In fact, CaM expression was enhanced by 8-pCPT treatment in isolated cells, as found by Western blots. Moreover, the 8-pCPT-induced, PKA-independent, positive inotropic effect was favored by enhanced extracellular Ca(2+) influx via L-type Ca(2+) channels. However, 8-pCPT also induced aberrant Ca(2+) release as Ca(2+) waves and extra [Ca(2+)](i) transients, suggesting proarrhythmic effect. These results provide new insights regarding Epac cardiac actions, suggesting an important role in the initial compensation of the heart to pathological stimuli during the initiation of cardiac hypertrophy, favoring contraction but also arrhythmia risk.

Role of Epac in brain and heart

Epacs (exchange proteins directly activated by cAMP) are guanine-nucleotide-exchange factors for the Ras-like small GTPases Rap1 and Rap2. Epacs were discovered in 1998 as new sensors for the second messenger cAMP acting in parallel to PKA (protein kinase A). As cAMP regulates many important physiological functions in brain and heart, the existence of Epacs raises many questions regarding their role in these tissues. The present review focuses on the biological roles and signalling pathways of Epacs in neurons and cardiac myocytes. We discuss the potential involvement of Epacs in the manifestation of cardiac and central diseases such as cardiac hypertrophy and memory disorders.