Ras is required for the cyclic AMP-dependent activation of Rap1 via Epac2

Protein SUMOylation promotes cAMP-independent EPAC1 activation

Protein SUMOylation is a prevalent stress-response posttranslational modification crucial for maintaining cellular homeostasis. Herein, we report that protein SUMOylation modulates cellular signaling mediated by cAMP, an ancient and universal stress-response second messenger. We identify K561 as a primary SUMOylation site in exchange protein directly activated by cAMP (EPAC1) via site-specific mapping of SUMOylation using mass spectrometry. Sequence and site-directed mutagenesis analyses reveal that a functional SUMO-interacting motif in EPAC1 is required for the binding of SUMO-conjugating enzyme UBC9, formation of EPAC1 nuclear condensate, and EPAC1 cellular SUMOylation. Heat shock-induced SUMO modification of EPAC1 promotes Rap1/2 activation in a cAMP-independent manner. Structural modeling and molecular dynamics simulation studies demonstrate that SUMO substituent on K561 of EPAC1 promotes Rap1 interaction by increasing the buried surface area between the SUMOylated receptor and its effector. Our studies identify a functional SUMOylation site in EPAC1 and unveil a novel mechanism in which SUMOylation of EPAC1 leads to its autonomous activation. The findings of SUMOylation-mediated activation of EPAC1 not only provide new insights into our understanding of cellular regulation of EPAC1 but also will open up a new field of experimentation concerning the cross-talk between cAMP/EPAC1 signaling and protein SUMOylation, two major cellular stress response pathways, during cellular homeostasis.

Protein SUMOylation promotes cAMP-independent EPAC1 activation

Exchange protein directly activated by cAMP (EPAC1) mediates the intracellular functions of a critical stress-response second messenger, cAMP. Herein, we report that EPAC1 is a cellular substrate of protein SUMOylation, a prevalent stress-response posttranslational modification. Site-specific mapping of SUMOylation by mass spectrometer leads to identifying K561 as a primary SUMOylation site in EPAC1. Sequence and site-directed mutagenesis analyses reveal a functional SUMO-interacting motif required for cellular SUMOylation of EPAC1. SUMO modification of EPAC1 mediates its heat shock-induced Rap1/2 activation in a cAMP-independent manner. Structural modeling and molecular dynamics simulation studies demonstrate that SUMO substituent on K561 of EPAC1 promotes Rap1 interaction by increasing the buried surface area between the SUMOylated receptor and its effector. Our studies identify a functional SUMOylation site in EPAC1 and unveil a novel mechanism in which SUMOylation of EPAC1 leads to its autonomous activation. The findings of SUMOylation-mediated activation of EPAC1 not only provide new insights into our understanding of cellular regulation of EPAC1 but also will open up a new field of experimentation concerning the cross-talk between cAMP/EPAC1 signaling and protein SUMOylation, two major cellular stress response pathways, during cellular homeostasis.

cAMP Signaling in Cancer: A PKA-CREB and EPAC-Centric Approach

Cancer is one of the most common causes of death globally. Despite extensive research and considerable advances in cancer therapy, the fundamentals of the disease remain unclear. Understanding the key signaling mechanisms that cause cancer cell malignancy may help to uncover new pharmaco-targets. Cyclic adenosine monophosphate (cAMP) regulates various biological functions, including those in malignant cells. Understanding intracellular second messenger pathways is crucial for identifying downstream proteins involved in cancer growth and development. cAMP regulates cell signaling and a variety of physiological and pathological activities. There may be an impact on gene transcription from protein kinase A (PKA) as well as its downstream effectors, such as cAMP response element-binding protein (CREB). The position of CREB downstream of numerous growth signaling pathways implies its oncogenic potential in tumor cells. Tumor growth is associated with increased CREB expression and activation. PKA can be used as both an onco-drug target and a biomarker to find, identify, and stage tumors. Exploring cAMP effectors and their downstream pathways in cancer has become easier using exchange protein directly activated by cAMP (EPAC) modulators. This signaling system may inhibit or accelerate tumor growth depending on the tumor and its environment. As cAMP and its effectors are critical for cancer development, targeting them may be a useful cancer treatment strategy. Moreover, by reviewing the material from a distinct viewpoint, this review aims to give a knowledge of the impact of the cAMP signaling pathway and the related effectors on cancer incidence and development. These innovative insights seek to encourage the development of novel treatment techniques and new approaches.

Origin and Isoform Specific Functions of Exchange Proteins Directly Activated by cAMP: A Phylogenetic Analysis

Exchange proteins directly activated by cAMP (EPAC1 and EPAC2) are one of the several families of cellular effectors of the prototypical second messenger cAMP. To understand the origin and molecular evolution of EPAC proteins, we performed a comprehensive phylogenetic analysis of EPAC1 and EPAC2. Our study demonstrates that unlike its cousin PKA, EPAC proteins are only present in multicellular Metazoa. Within the EPAC family, EPAC1 is only associated with chordates, while EPAC2 spans the entire animal kingdom. Despite a much more contemporary origin, EPAC1 proteins show much more sequence diversity among species, suggesting that EPAC1 has undergone more selection and evolved faster than EPAC2. Phylogenetic analyses of the individual cAMP binding domain (CBD) and guanine nucleotide exchange (GEF) domain of EPACs, two most conserved regions between the two isoforms, further reveal that EPAC1 and EPAC2 are closely clustered together within both the larger cyclic nucleotide receptor and RAPGEF families. These results support the notion that EPAC1 and EPAC2 share a common ancestor resulting from a fusion between the CBD of PKA and the GEF from RAPGEF1. On the other hand, the two terminal extremities and the RAS-association (RA) domains show the most sequence diversity between the two isoforms. Sequence diversities within these regions contribute significantly to the isoform-specific functions of EPACs. Importantly, unique isoform-specific sequence motifs within the RA domain have been identified.

Identification of novel Epac2 antagonists through in silico and in vitro analyses

cAMP-dependent guanine nucleotide exchange factor (Epac) is a key regulator in signal transduction and represents an excellent drug target to be investigated against various diseases. To date, very few modulators selective for Epac are available; however, there is still an unmet need of isoform-selective inhibitors. In the present study, ligand-based pharmacophores were designed to investigating structurally diverse molecules as Epac2 inhibitors. Pharmacophore models were developed using reported allosteric site inhibitors. The developed models were used to screen 95 thousand compounds from the National Cancer Institute (NCI), Maybride, and our in-house ICCBS Database. The binding mode and efficiency of the screened hits was investigated using molecular docking simulation on the allosteric site of Epac2 apo-protein (PDB ID: 2BYV) followed by ADMET (Absorption, Distribution, Metabolism, Excretion, and Toxicity) profiling Furthermore, obtained in silico screened hits were subjected to in vitro assay for insulin secretion. We identified, three lead molecules RDR02145, AAK-399, and AAD-026 reducing, insulin secretion. Remarkably, a higher inhibitory effect on insulin secretion was observed in AAK-399, and AAD-026 as compared to that of standard Epac2 non-competitive allosteric site inhibitor, MAY0132. Furthermore, Dynamic simulation studies of lead compounds proved the structural stability of the Epac2 auto-inhibited state. These findings underline the potential of these compounds as valuable pharmacological tools for designing future selective probes to inhibit the Epac-mediated signaling pathway.

The Role of Epac in Cancer Progression

Cancer continues to be a prime contributor to global mortality. Despite tremendous research efforts and major advances in cancer therapy, much remains to be learned about the underlying molecular mechanisms of this debilitating disease. A better understanding of the key signaling events driving the malignant phenotype of cancer cells may help identify new pharmaco-targets. Cyclic adenosine 3',5'-monophosphate (cAMP) modulates a plethora of biological processes, including those that are characteristic of malignant cells. Over the years, most cAMP-mediated actions were attributed to the activity of its effector protein kinase A (PKA). However, studies have revealed an important role for the exchange protein activated by cAMP (Epac) as another effector mediating the actions of cAMP. In cancer, Epac appears to have a dual role in regulating cellular processes that are essential for carcinogenesis. In addition, the development of Epac modulators offered new routes to further explore the role of this cAMP effector and its downstream pathways in cancer. In this review, the potentials of Epac as an attractive target in the fight against cancer are depicted. Additionally, the role of Epac in cancer progression, namely its effect on cancer cell proliferation, migration/metastasis, and apoptosis, with the possible interaction of reactive oxygen species (ROS) in these phenomena, is discussed with emphasis on the underlying mechanisms and pathways.

EPAC in Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are major components of blood vessels. They regulate physiological functions, such as vascular tone and blood flow. Under pathological conditions, VSMCs undergo a remodeling process known as phenotypic switching. During this process, VSMCs lose their contractility and acquire a synthetic phenotype, where they over-proliferate and migrate from the tunica media to the tunica interna, contributing to the occlusion of blood vessels. Since their discovery as effector proteins of cyclic adenosine 3′,5′-monophosphate (cAMP), exchange proteins activated by cAMP (EPACs) have been shown to play vital roles in a plethora of pathways in different cell systems. While extensive research to identify the role of EPAC in the vasculature has been conducted, much remains to be explored to resolve the reported discordance in EPAC’s effects. In this paper, we review the role of EPAC in VSMCs, namely its regulation of the vascular tone and phenotypic switching, with the likely involvement of reactive oxygen species (ROS) in the interplay between EPAC and its targets/effectors.

EPAC2: A new and promising protein for glioma pathogenesis and therapy

Gliomas are prime brain cancers which are initiated by malignant modification of neural stem cells, progenitor cells and differentiated glial cells such as astrocyte, oligodendrocyte as well as ependymal cells. Exchange proteins directly activated by cAMP (EPACs) are crucial cyclic adenosine 3’,5’-monophosphate (cAMP)-determined signaling pathways. Cyclic AMP-intermediated signaling events were utilized to transduce protein kinase A (PKA) leading to the detection of EPACs or cAMP-guanine exchange factors (cAMP-GEFs). EPACs have been detected as crucial proteins associated with the pathogenesis of neurological disorders as well as numerous human diseases. EPAC proteins have two isoforms. These isoforms are EPAC1 and EPAC2. EPAC2 also known as Rap guanine nucleotide exchange factor 4 (RAPGEF4) is generally expression in all neurites. Higher EAPC2 levels was detected in the cortex, hippocampus as well as striatum of adult mouse brain. Activation as well as over-secretion of EPAC2 triggers apoptosis in neurons and EPAC-triggered apoptosis was intermediated via the modulation of Bcl-2 interacting member protein (BIM). EPAC2 secretory levels has proven to be more in low-grade clinical glioma than high-grade clinical glioma. This review therefore explores the effects of EPAC2/RAPGEF4 on the pathogenesis of glioma instead of EPAC1 because EPAC2 and not EPAC1 is predominately expressed in the brain. Therefore, EPAC2 is most likely to modulate glioma pathogenesis rather than EPAC1.

Epidermal growth factor receptor controls glycogen phosphorylase in T cells through small GTPases of the RAS family

We recently uncovered a regulatory pathway of the muscle isoform of glycogen phosphorylase (PYGM) that plays an important role in regulating immune function in T cells. Here, using various enzymatic, pulldown, and immunoprecipitation assays, we describe signaling cross-talk between the small GTPases RAS and RAP1A, member of RAS oncogene family (RAP1) in human Kit 225 lymphoid cells, which, in turn, is regulated by the epidermal growth factor receptor (EGFR). We found that this communication bridge is essential for glycogen phosphorylase (PYG) activation through the canonical pathway because this enzyme is inactive in the absence of adenylyl cyclase type 6 (ADCY6). PYG activation required stimulation of both exchange protein directly activated by cAMP 2 (EPAC2) and RAP1 via RAS and ADCY6 phosphorylation, with the latter being mediated by Raf-1 proto-oncogene, Ser/Thr kinase (RAF1). Consistent with this model, PYG activation was EGFR-dependent and may be initiated by the constitutively active form of RAS. Consequently, PYG activation in Kit 225 T cells could be blocked with specific inhibitors of RAS, EPAC, RAP1, RAF1, ADCY6, and cAMP-dependent protein kinase. Our results establish a new paradigm for the mechanism of PYG activation, which depends on the type of receptor involved.

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.

Recruitment of Epac2A to Insulin Granule Docking Sites Regulates Priming for Exocytosis

Epac is a cAMP-activated guanine nucleotide exchange factor that mediates cAMP signaling in various types of cells, including β-cells, where it is involved in the control of insulin secretion. Upon activation, the protein redistributes to the plasma membrane, but the underlying molecular mechanisms and functional consequences are unclear. Using quantitative high-resolution microscopy, we found that cAMP elevation caused rapid binding of Epac2A to the β-cell plasma membrane, where it accumulated specifically at secretory granules and rendered them more prone to undergo exocytosis. cAMP-dependent membrane binding required the high-affinity cyclic nucleotide-binding (CNB) and Ras association domains, but not the disheveled-Egl-10-pleckstrin domain. Although the N-terminal low-affinity CNB domain (CNB-A) was dispensable for the translocation to the membrane, it was critical for directing Epac2A to the granule sites. Epac1, which lacks the CNB-A domain, was recruited to the plasma membrane but did not accumulate at granules. We conclude that Epac2A controls secretory granule release by binding to the exocytosis machinery, an effect that is enhanced by prior cAMP-dependent accumulation of the protein at the plasma membrane.

Regulation of Rap GTPases in mammalian neurons

Small GTPases are central regulators of many cellular processes. The highly conserved Rap GTPases perform essential functions in the mammalian nervous system during development and in mature neurons. During neocortical development, Rap1 is required to regulate cadherin- and integrin-mediated adhesion. In the adult nervous system Rap1 and Rap2 regulate the maturation and plasticity of dendritic spine and synapses. Although genetic studies have revealed important roles of Rap GTPases in neurons, their regulation by guanine nucleotide exchange factors (GEFs) that activate them and GTPase activating proteins (GAPs) that inactivate them by stimulating their intrinsic GTPase activity is just beginning to be explored in vivo. Here we review how GEFs and GAPs regulate Rap GTPases in the nervous system with a focus on their in vivo function.

Role of exchange protein directly activated by cAMP (EPAC1) in breast cancer cell migration and apoptosis

Despite the current progress in cancer research and therapy, breast cancer remains the leading cause of mortality among half a million women worldwide. Migration and invasion of cancer cells are associated with prevalent tumor metastasis as well as high mortality. Extensive studies have powerfully established the role of prototypic second messenger cAMP and its two ubiquitously expressed intracellular cAMP receptors namely the classic protein kinaseA/cAMP-dependent protein kinase (PKA) and the more recently discovered exchange protein directly activated by cAMP/cAMP-regulated guanine nucleotide exchange factor (EPAC/cAMP-GEF) in cell migration, cell cycle regulation, and cell death. Herein, we performed the analysis of the Cancer Genome Atlas (TCGA) dataset to evaluate the essential role of cAMP molecular network in breast cancer. We report that EPAC1, PKA, and AKAP9 along with other molecular partners are amplified in breast cancer patients, indicating the importance of this signaling network. To evaluate the functional role of few of these proteins, we used pharmacological modulators and analyzed their effect on cell migration and cell death in breast cancer cells. Hence, we report that inhibition of EPAC1 activity using pharmacological modulators leads to inhibition of cell migration and induces cell death. Additionally, we also observed that the inhibition of EPAC1 resulted in disruption of its association with the microtubule cytoskeleton and delocalization of AKAP9 from the centrosome as analyzed by in vitro imaging. Finally, this study suggests for the first time the mechanistic insights of mode of action of a primary cAMP-dependent sensor, Exchange protein activated by cAMP 1 (EPAC1), via its interaction with A-kinase anchoring protein 9 (AKAP9). This study provides a new cell signaling cAMP-EPAC1-AKAP9 direction to the development of additional biotherapeutics for breast cancer.

Phosphorylation of Rap1 by cAMP-dependent Protein Kinase (PKA) Creates a Binding Site for KSR to Sustain ERK Activation by cAMP

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1 have been proposed to mediate this activation. Using HEK293 cells as a model system, we have recently shown that both Ras and Rap1 are required for cAMP signaling to ERKs. However, cAMP-dependent Ras signaling to ERKs is transient and rapidly terminated by PKA phosphorylation of the Raf isoforms C-Raf and B-Raf. In contrast, cAMP-dependent Rap1 signaling to ERKs and Rap1 is potentiated by PKA. We show that this is due to sustained binding of B-Raf to Rap1. One of the targets of PKA is Rap1 itself, directly phosphorylating Rap1a on serine 180 and Rap1b on serine 179. We show that these phosphorylations create potential binding sites for the adaptor protein 14-3-3 that links Rap1 to the scaffold protein KSR. These results suggest that Rap1 activation of ERKs requires PKA phosphorylation and KSR binding. Because KSR and B-Raf exist as heterodimers within the cell, this binding also brings B-Raf to Rap1, allowing Rap1 to couple to ERKs through B-Raf binding to Rap1 independently of its Ras-binding domain.

Protein Kinase A-independent Ras Protein Activation Cooperates with Rap1 Protein to Mediate Activation of the Extracellular Signal-regulated Kinases (ERK) by cAMP

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1, have been proposed to mediate this activation, with either Ras or Rap1 acting in distinct cell types. Using Hek293 cells, we show that both Ras and Rap1 are required for cAMP signaling to ERKs. The roles of Ras and Rap1 were distinguished by their mechanism of activation, dependence on the cAMP-dependent protein kinase (PKA), and the magnitude and kinetics of their effects on ERKs. Ras was required for the early portion of ERK activation by cAMP and was activated independently of PKA. Ras activation required the Ras/Rap guanine nucleotide exchange factor (GEF) PDZ-GEF1. Importantly, this action of PDZ-GEF1 was disrupted by mutation within its putative cyclic nucleotide-binding domain within PDZ-GEF1. Compared with Ras, Rap1 activation of ERKs was of longer duration. Rap1 activation was dependent on PKA and required Src family kinases and the Rap1 exchanger C3G. This is the first report of a mechanism for the cooperative actions of Ras and Rap1 in cAMP activation of ERKs. One physiological role for the sustained activation of ERKs is the transcription and stabilization of a range of transcription factors, including c-FOS. We show that the induction of c-FOS by cAMP required both the early and sustained phases of ERK activation, requiring Ras and Rap1, as well as for each of the Raf isoforms, B-Raf and C-Raf.

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.

Activation of MEK/ERK Signaling by PACAP in Guinea Pig Cardiac Neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) signaling can increase guinea pig cardiac neuron excitability in part through extracellular signal-regulated kinase (ERK) activation. The present study examined the PACAP receptors and signaling cascades that stimulate guinea pig cardiac neuron ERK signaling using confocal microscopy to quantify PACAP-induced neuronal phosphorylated ERK (pERK) immunoreactivity. PACAP and maxadilan, but not vasoactive intestinal polypeptide (VIP), increased cardiac neuron pERK, implicating primary roles for PACAP-selective PAC1 receptor (Adcyap1r1) signaling rather than VPAC receptors (Vipr1 and Vipr2) in the generation of cardiac neuron pERK. The adenylyl cyclase (AC) activator forskolin, but not the protein kinase C (PKC) activator phorbol myristate acetate (PMA), increased pERK. Also, Bim1 did not blunt PACAP activation of pERK. Together, the results suggest PAC1 receptor signal transduction via Gs/adenylyl cyclase (AC)/cAMP rather than Gq/phospholipase C (PLC) generated neuronal pERK. Activator and inhibitor studies suggested that the PACAP-mediated pERK activation was PKA-dependent rather than an exchange protein directly activated by a cAMP (EPAC), PKA-independent mechanism. The PACAP-induced pERK was inhibited by the clathrin inhibitor Pitstop2 to block receptor internalization and endosomal signaling. We propose that the PACAP-mediated MEK/ERK activation in cardiac neurons involves both AC/cAMP/PKA signaling and PAC1 receptor internalization/activation of signaling endosomes.

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.

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.

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.

Identification and validation of modulators of exchange protein activated by cAMP (Epac) activity: structure-function implications for Epac activation and inhibition

The signaling molecule cAMP primarily mediates its effects by activating PKA and/or exchange protein activated by cAMP (Epac). Epac has been implicated in many responses in cells, but its precise roles have been difficult to define in the absence of Epac inhibitors. Epac, a guanine nucleotide exchange factor for the low molecular weight G protein Rap, is directly activated by cAMP. Using a bioluminescence resonance energy transfer-based assay (CAMYEL) to examine modulators of Epac activity, we took advantage of its intramolecular movement that occurs upon cAMP binding to assess Epac activation. We found that the use of CAMYEL can detect the binding of cAMP analogs to Epac and their modulation of its activity and can distinguish between agonists (cAMP), partial agonists (8-chlorophenylthio-cAMP), and super agonists (8-chlorophenylthio-2'-O-Me-cAMP). The CAMYEL assay can also identify competitive and uncompetitive Epac inhibitors, e.g. (Rp)-cAMPS and CE3F4, respectively. To confirm the results with the CAMYEL assay, we used Swiss 3T3 cells and assessed the ability of cyclic nucleotide analogs to modulate the activity of Epac or PKA, determined by Rap1 activity or VASP phosphorylation, respectively. We used computational molecular modeling to analyze the interaction of analogs with Epac1. The results reveal a rapid means to identify modulators (potentially including allosteric inhibitors) of Epac activity that also provides insight into the mechanisms of Epac activation and inhibition.

Antidiabetic sulfonylureas and cAMP cooperatively activate Epac2A

Sulfonylureas are widely used drugs for treating insulin deficiency in patients with type 2 diabetes. Sulfonylureas bind to the regulatory subunit of the pancreatic β cell potassium channel that controls insulin secretion. Sulfonylureas also bind to and activate Epac2A, a member of the Epac family of cyclic adenosine monophosphate (cAMP)-binding proteins that promote insulin secretion through activation of the Ras-like guanosine triphosphatase Rap1. Using molecular docking simulation, we identified amino acid residues in one of two cyclic nucleotide-binding domains, cNBD-A, in Epac2A predicted to mediate the interaction with sulfonylureas. We confirmed the importance of the identified residues by site-directed mutagenesis and analysis of the response of the mutants to sulfonylureas using two assays: changes in fluorescence resonance energy transfer (FRET) of an Epac2A-FRET biosensor and direct sulfonylurea-binding experiments. These residues were also required for the sulfonylurea-dependent Rap1 activation by Epac2A. Binding of sulfonylureas to Epac2A depended on the concentration of cAMP and the structures of the drugs. Sulfonylureas and cAMP cooperatively activated Epac2A through binding to cNBD-A and cNBD-B, respectively. Our data suggest that sulfonylureas stabilize Epac2A in its open, active state and provide insight for the development of drugs that target Epac2A.

Protein kinase A-dependent phosphorylation of Rap1 regulates its membrane localization and cell migration

The small G protein Rap1 can mediate "inside-out signaling" by recruiting effectors to the plasma membrane that signal to pathways involved in cell adhesion and cell migration. This action relies on the membrane association of Rap1, which is dictated by post-translational prenylation as well as by a stretch of basic residues within its carboxyl terminus. One feature of this stretch of acidic residues is that it lies adjacent to a functional phosphorylation site for the cAMP-dependent protein kinase PKA. This phosphorylation has two effects on Rap1 action. One, it decreases the level of Rap1 activity as measured by GTP loading and the coupling of Rap1 to RapL, a Rap1 effector that couples Rap1 GTP loading to integrin activation. Two, it destabilizes the membrane localization of Rap1, promoting its translocation into the cytoplasm. These two actions, decreased GTP loading and decreased membrane localization, are related, as the translocation of Rap1-GTP into the cytoplasm is associated with its increased GTP hydrolysis and inactivation. The consequences of this phosphorylation in Rap1-dependent cell adhesion and cell migration were also examined. Active Rap1 mutants that lack this phosphorylation site had a minimal effect on cell adhesion but strongly reduced cell migration, when compared with an active Rap1 mutant that retained the phosphorylation site. This suggests that optimal cell migration is associated with cycles of Rap1 activation, membrane egress, and inactivation, and requires the regulated phosphorylation of Rap1 by PKA.

Ras-mutant cancer cells display B-Raf binding to Ras that activates extracellular signal-regulated kinase and is inhibited by protein kinase A phosphorylation

The small G protein Ras regulates proliferation through activation of the mitogen-activated protein (MAP) kinase (ERK) cascade. The first step of Ras-dependent activation of ERK signaling is Ras binding to members of the Raf family of MAP kinase kinase kinases, C-Raf and B-Raf. Recently, it has been reported that in melanoma cells harboring oncogenic Ras mutations, B-Raf does not bind to Ras and does not contribute to basal ERK activation. For other types of Ras-mutant tumors, the relative contributions of C-Raf and B-Raf are not known. We examined non-melanoma cancer cell lines containing oncogenic Ras mutations and express both C-Raf and B-Raf isoforms, including the lung cancer cell line H1299 cells. Both B-Raf and C-Raf were constitutively bound to oncogenic Ras and contributed to Ras-dependent ERK activation. Ras binding to B-Raf and C-Raf were both subject to inhibition by the cAMP-dependent protein kinase PKA. cAMP inhibited the growth of H1299 cells and Ras-dependent ERK activation via PKA. PKA inhibited the binding of Ras to both C-Raf and B-Raf through phosphorylations of C-Raf at Ser-259 and B-Raf at Ser-365, respectively. These studies demonstrate that in non-melanocytic Ras-mutant cancer cells, Ras signaling to B-Raf is a significant contributor to ERK activation and that the B-Raf pathway, like that of C-Raf, is a target for inhibition by PKA. We suggest that cAMP and hormones coupled to cAMP may prove useful in dampening the effects of oncogenic Ras in non-melanocytic cancer cells through PKA-dependent actions on B-Raf as well as C-Raf.

DNA methylation of alternative promoters directs tissue specific expression of Epac2 isoforms

Epac 1 and Epac 2 (Epac1/2; exchange factors directly activated by cAMP) are multidomain proteins that mediate cellular responses upon activation by the signaling molecule cAMP. Epac1 is ubiquitously expressed, whereas Epac2 exhibits a restricted expression pattern. The gene encoding Epac2 gives rise to at least three protein isoforms (Epac2A, Epac2B and Epac2C) that exhibit confined tissue and cell specific expression profiles. Here, we describe alternative promoter usage for the different isoforms of Epac2, and demonstrate that the activity of these promoters depend on the DNA methylation status. Bisulfite sequencing demonstrated that the level of methylation of the promoters in different tissues correlates with Epac2 isoform expression. The presented data indicate that the tissue-specific expression of the Epac2 isoforms is epigenetically regulated, and identify tissue-specific differentially methylated promoter regions within the Epac2 locus that are essential for its transcriptional control.

Spatial control of Epac2 activity by cAMP and Ca2+-mediated activation of Ras in pancreatic β cells

The cAMP (adenosine 3',5'-monophosphate)-activated guanine nucleotide exchange factor (GEF) Epac2 is an important mediator of cAMP-dependent processes in multiple cell types. We used real-time confocal and total internal reflection fluorescence microscopy to examine the spatiotemporal regulation of Epac2, which is a GEF for the guanosine triphosphatase (GTPase) Rap. We demonstrated that increases in the concentration of cAMP triggered the translocation of Epac2 from the cytoplasm to the plasma membrane in insulin-secreting β cells. Glucose-induced oscillations of the submembrane concentration of cAMP were associated with cyclic translocation of Epac2, and this translocation could be amplified by increases in the cytoplasmic Ca(2+) concentration. Analyses of Epac2 mutants identified the high-affinity cAMP-binding and the Ras association domains as crucial for the translocation. Expression of a dominant-negative Ras mutant reduced Epac2 translocation, and Ca(2+)-dependent oscillations in Ras activity synchronized with Epac2 translocation in single β cells. The cyclic translocation of Epac2 was accompanied by oscillations of Rap GTPase activity at the plasma membrane, and expression of an inactive Rap1B mutant decreased insulin secretion. Thus, Epac2 localization is dynamically controlled by cAMP as well as by Ca(2+)-mediated activation of Ras. These results help to explain how oscillating signals can produce pulses of insulin release from pancreatic β cells.

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.

Specific interactions between Epac1, β-arrestin2 and PDE4D5 regulate β-adrenergic receptor subtype differential effects on cardiac hypertrophic signaling

β1 and β2 adrenergic receptors (βARs) are highly homologous but fulfill distinct physiological and pathophysiological roles. Here we show that both βAR subtypes activate the cAMP-binding protein Epac1, but they differentially affect its signaling. The distinct effects of βARs on Epac1 downstream effectors, the small G proteins Rap1 and H-Ras, involve different modes of interaction of Epac1 with the scaffolding protein β-arrestin2 and the cAMP-specific phosphodiesterase (PDE) variant PDE4D5. We found that β-arrestin2 acts as a scaffold for Epac1 and is necessary for Epac1 coupling to H-Ras. Accordingly, knockdown of β-arrestin2 prevented Epac1-induced histone deacetylase 4 (HDAC4) nuclear export and cardiac myocyte hypertrophy upon β1AR activation. Moreover, Epac1 competed with PDE4D5 for interaction with β-arrestin2 following β2AR activation. Dissociation of the PDE4D5-β-arrestin2 complex allowed the recruitment of Epac1 to β2AR and induced a switch from β2AR non-hypertrophic signaling to a β1AR-like pro-hypertrophic signaling cascade. These findings have implications for understanding the molecular basis of cardiac myocyte remodeling and other cellular processes in which βAR subtypes exert opposing effects.

Regulation of the inflammatory response of vascular endothelial cells by EPAC1

Life-threatening diseases of the cardiovascular system, like atherosclerosis, are exacerbated by unwanted inflammation within the structures of large blood vessels. This inflammation involves increased permeability of the vascular endothelial cells (VECs) that form the lining of blood vessels, leading to exaggerated extravasation of blood components and accumulation of fluid in the extravascular space. This results in tissue dysfunction and increased secretion of chemokines that attract leukocytes and monocytes to the inflamed endothelium. Cyclic AMP is synthesized in VECs in response to endogenous Gs-coupled receptors and is known to limit cytokine action and reduce endothelial hyperpermeability induced by multiple pro-inflammatory stimuli. The mechanisms underlying this anti-inflammatory action of cyclic AMP are now being elucidated and it is becoming clear that the cyclic AMP sensor, exchange protein activated by cyclic AMP (EPAC1), appears to play a key role in suppressing unwanted inflammation. EPAC1 mediates at least three anti-inflammatory pathways in VECs by down-regulating inflammatory signalling through the induction of the suppressors of cytokine signalling 3 (SOCS-3) gene, limiting integrin-dependent vascular permeability and enhancing endothelial barrier function through the stabilization of VE-cadherin junctions. Given that manipulation of cellular cyclic AMP levels currently forms the basis of many effective pharmaceuticals and that EPAC1 is involved in multiple anti-inflammatory protective processes in VECs, does this make EPAC1 an attractive target for the development of activators capable of eliciting a coordinated programme of 'protection' against the development of endothelial dysfunction? Here we discuss whether EPAC1 represents an attractive therapeutic target for limiting endothelial dysfunction associated with cardiovascular diseases like atherosclerosis. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

Neuronally expressed Ras-family GTPase Di-Ras modulates synaptic activity in Caenorhabditis elegans

Ras-family GTPases regulate a wide variety of cellular functions including cell growth and differentiation. Di-Ras, which belongs to a distinct subfamily of Ras-family GTPases, is expressed predominantly in brain, but the role of Di-Ras in nervous systems remains totally unknown. Here, we report that the Caenorhabditis elegans Di-Ras homologue drn-1 is expressed specifically in neuronal cells and involved in synaptic function at neuromuscular junctions. Loss of function of drn-1 conferred resistance to the acetylcholinesterase inhibitor aldicarb and partially suppressed the aldicarb-hypersensitive phenotypes of heterotrimeric G-protein mutants, in which acetylcholine release is up-regulated. drn-1 mutants displayed no apparent defects in the axonal distribution of the membrane-bound second messenger diacylglycerol (DAG), which is a key stimulator of acetylcholine release. Finally, we have identified EPAC-1, a C. elegans Epac homologue, as a binding partner for DRN-1. Deletion mutants of epac-1 displayed an aldicarb-resistant phenotype as drn-1 mutants. Genetic analysis of drn-1 and epac-1 showed that they acted in the same pathway to control acetylcholine release. Furthermore, DRN-1 and EPAC-1 were co-immunoprecipitated. These findings suggest that DRN-1 may function cooperatively with EPAC-1 to modulate synaptic activity in C. elegans.

Cell signalling in insulin secretion: the molecular targets of ATP, cAMP and sulfonylurea

Clarification of the molecular mechanisms of insulin secretion is crucial for understanding the pathogenesis and pathophysiology of diabetes and for development of novel therapeutic strategies for the disease. Insulin secretion is regulated by various intracellular signals generated by nutrients and hormonal and neural inputs. In addition, a variety of glucose-lowering drugs including sulfonylureas, glinide-derivatives, and incretin-related drugs such as dipeptidyl peptidase IV (DPP-4) inhibitors and glucagon-like peptide 1 (GLP-1) receptor agonists are used for glycaemic control by targeting beta cell signalling for improved insulin secretion. There has been a remarkable increase in our understanding of the basis of beta cell signalling over the past two decades following the application of molecular biology, gene technology, electrophysiology and bioimaging to beta cell research. This review discusses cell signalling in insulin secretion, focusing on the molecular targets of ATP, cAMP and sulfonylurea, an essential metabolic signal in glucose-induced insulin secretion (GIIS), a critical signal in the potentiation of GIIS, and the commonly used glucose-lowering drug, respectively.

IP3-dependent intracellular Ca2+ release is required for cAMP-induced c-fos expression in hippocampal neurons

Ca(2+) and cAMP are widely used in concert by neurons to relay signals from the synapse to the nucleus, where synaptic activity modulates gene expression required for synaptic plasticity. Neurons utilize different transcriptional regulators to integrate information encoded in the spatiotemporal dynamics and magnitude of Ca(2+) and cAMP signals, including some that are Ca(2+)-responsive, some that are cAMP-responsive and some that detect coincident Ca(2+) and cAMP signals. Because Ca(2+) and cAMP can influence each other's amplitude and spatiotemporal characteristics, we investigated how cAMP acts to regulate gene expression when increases in intracellular Ca(2+) are buffered. We show here that cAMP-mobilizing stimuli are unable to induce expression of the immediate early gene c-fos in hippocampal neurons in the presence of the intracellular Ca(2+) buffer BAPTA-AM. Expression of enzymes that attenuate intracellular IP(3) levels also inhibited cAMP-dependent c-fos induction. Synaptic activity induces c-fos transcription through two cis regulatory DNA elements - the CRE and the SRE. We show here that in response to cAMP both CRE-mediated and SRE-mediated induction of a luciferase reporter gene is attenuated by IP(3) metabolizing enzymes. Furthermore, cAMP-induced nuclear translocation of the CREB coactivator TORC1 was inhibited by depletion of intracellular Ca(2+) stores. Our data indicate that Ca(2+) release from IP(3)-sensitive pools is required for cAMP-induced transcription in hippocampal neurons.

An autism-associated variant of Epac2 reveals a role for Ras/Epac2 signaling in controlling basal dendrite maintenance in mice

The architecture of dendritic arbors determines circuit connectivity, receptive fields, and computational properties of neurons, and dendritic structure is impaired in several psychiatric disorders. While apical and basal dendritic compartments of pyramidal neurons are functionally specialized and differentially regulated, little is known about mechanisms that selectively maintain basal dendrites. Here we identified a role for the Ras/Epac2 pathway in maintaining basal dendrite complexity of cortical neurons. Epac2 is a guanine nucleotide exchange factor (GEF) for the Ras-like small GTPase Rap, and it is highly enriched in the adult mouse brain. We found that in vivo Epac2 knockdown in layer 2/3 cortical neurons via in utero electroporation reduced basal dendritic architecture, and that Epac2 knockdown in mature cortical neurons in vitro mimicked this effect. Overexpression of an Epac2 rare coding variant, found in human subjects diagnosed with autism, also impaired basal dendritic morphology. This mutation disrupted Epac2's interaction with Ras, and inhibition of Ras selectively interfered with basal dendrite maintenance. Finally, we observed that components of the Ras/Epac2/Rap pathway exhibited differential abundance in the basal versus apical dendritic compartments. These findings define a role for Epac2 in enabling crosstalk between Ras and Rap signaling in maintaining basal dendrite complexity, and exemplify how rare coding variants, in addition to their disease relevance, can provide insight into cellular mechanisms relevant for brain connectivity.

The progesterone receptor as a transcription factor regulates phospholipase D1 expression through independent activation of protein kinase A and Ras during 8-Br-cAMP-induced decidualization in human endometrial stromal cells

Decidualization is a biological and morphological process occurring in hES (human endometrial stromal) cells. Previously, we reported that PLD1 (phospholipase D1) plays an important role in cAMP-induced decidualization of hES cells. In the present study, we focused on how PLD1 expression is up-regulated during decidualization. Treatment with PKA (protein kinase A) inhibitors (Rp-cAMP or H89) or a Ras inhibitor (manumycin) partially inhibited PLD1 expression and decidua formation in response to cAMP treatment. Interestingly, dual inhibition of PKA and Ras completely inhibited PLD1 expression and cAMP-induced decidualization. These results suggest that PLD1 expression during decidualization is controlled additively by PKA and Ras. The use of inhibitors showed that extracellular-signal-regulated kinase, a downstream effector of Ras, was required for PLD activation and the morphological changes during decidualization, but not for the increase in PLD1 protein. Next, to investigate the regulator of the PLD1 gene at the transcriptional level, a promoter assay using deletion mutants of the PLD1 promoter was performed; the result indicated that PR (progesterone receptor) was a possible regulator of the PLD1 gene. In addition, chromatin immunoprecipitation assays on the PLD1 promoter identified PR as a transcription factor for PLD1 expression during 8-Br-cAMP-induced decidualization. Taken together, our findings demonstrate that PKA and Ras are novel regulators of PLD1 expression and also identify PR as a transcription factor for PLD1 expression during the decidualization of hES cells.

Regulating Rap small G-proteins in time and space

Signaling by the small G-protein Rap is under tight regulation by its GEFs and GAPs. These are multi-domain proteins that are themselves controlled by distinct upstream pathways, and thus couple different extra- and intracellular cues to Rap. The individual RapGEFs and RapGAPs are, in addition, targeted to specific cellular locations by numerous anchoring mechanisms and, consequently, may control different pools of Rap. Here, we review the various activating signals and targeting mechanisms of these proteins and discuss their contribution to the spatiotemporal regulation and biological functions of the Rap proteins.

Reelin, Rap1 and N-cadherin orient the migration of multipolar neurons in the developing neocortex

Projection neurons migrate from the ventricular zone to the neocortical plate during mouse brain development. Their overall movement is radial, but they become multipolar and move non-radially in the intermediate zone. Here we show that Reelin, the Rap1 GTPase, and N-cadherin (NCad) are important for multipolar neurons to polarize their migration towards the cortical plate. Inhibition and rescue experiments indicate that Reelin regulates migration through Rap1 and Akt, and that Rap1-regulated GTPases, RalA/B, Rac1 and Cdc42, are also involved. We find that Rap1 regulates plasma membrane localization of N-cadherin, and N-cadherin rescues radial polarization when Rap1 is inhibited. Curiously, inhibition of Rap1 or N-cadherin has little effect on glia-dependent locomotion. We propose a multi-step mechanism in which Reelin activates Rap1, Rap1 up-regulates N-cadherin, and N-cadherin is needed to orient cell migration.

The gsp oncogene disrupts Ras/ERK-dependent prolactin gene regulation in gsp inducible somatotroph cell line

The MAPK ERK1/2 cascade regulates all the critical cellular functions, and in many pathological situations, these regulatory processes are perturbed. It has been clearly established that this cascade is an integrative point in the control of the pituitary functions exerted by various extracellular signals. In particular, ERK1/2 cross talk with the cAMP pathway is determinant in the control of somatolactotroph hormonal secretion exerted via neuropeptide receptors. GH-secreting adenomas are characterized by frequent cAMP pathway alterations, such as constitutive activation of the α-subunit of the heterotrimeric Gs protein (the gsp oncogene), overexpression of Gsα, and changes in the protein kinase A regulatory subunits. However, it has not yet been established exactly how these alterations result in GH-secreting adenomas or how the ERK1/2 cascade contributes to the process of GH-secreting adenoma tumorigenesis. In this study on the conditional gsp-oncogene-expressing GH4C1 cell line, expression of the gsp oncogene, which was observed in up to 40% of GH-secreting adenomas, was found to induce sustained ERK1/2 activation, which required activation of the protein kinase A and the GTPases Ras and Rap1. All these signaling components contribute to the chronic activation of the human prolactin promoter. The data obtained here show that Ras plays a crucial role in these processes: in a physiopathological context, i.e. in the presence of the gsp oncogene, it switched from being a repressor of the cAMP/ protein kinase A ERK-sensitive prolactin gene control exerted by neuropeptides to an activator of the prolactin promoter.

A noise-reduction GWAS analysis implicates altered regulation of neurite outgrowth and guidance in autism

BACKGROUND

Genome-wide Association Studies (GWAS) have proved invaluable for the identification of disease susceptibility genes. However, the prioritization of candidate genes and regions for follow-up studies often proves difficult due to false-positive associations caused by statistical noise and multiple-testing. In order to address this issue, we propose the novel GWAS noise reduction (GWAS-NR) method as a way to increase the power to detect true associations in GWAS, particularly in complex diseases such as autism.

METHODS

GWAS-NR utilizes a linear filter to identify genomic regions demonstrating correlation among association signals in multiple datasets. We used computer simulations to assess the ability of GWAS-NR to detect association against the commonly used joint analysis and Fisher's methods. Furthermore, we applied GWAS-NR to a family-based autism GWAS of 597 families and a second existing autism GWAS of 696 families from the Autism Genetic Resource Exchange (AGRE) to arrive at a compendium of autism candidate genes. These genes were manually annotated and classified by a literature review and functional grouping in order to reveal biological pathways which might contribute to autism aetiology.

RESULTS

Computer simulations indicate that GWAS-NR achieves a significantly higher classification rate for true positive association signals than either the joint analysis or Fisher's methods and that it can also achieve this when there is imperfect marker overlap across datasets or when the closest disease-related polymorphism is not directly typed. In two autism datasets, GWAS-NR analysis resulted in 1535 significant linkage disequilibrium (LD) blocks overlapping 431 unique reference sequencing (RefSeq) genes. Moreover, we identified the nearest RefSeq gene to the non-gene overlapping LD blocks, producing a final candidate set of 860 genes. Functional categorization of these implicated genes indicates that a significant proportion of them cooperate in a coherent pathway that regulates the directional protrusion of axons and dendrites to their appropriate synaptic targets.

CONCLUSIONS

As statistical noise is likely to particularly affect studies of complex disorders, where genetic heterogeneity or interaction between genes may confound the ability to detect association, GWAS-NR offers a powerful method for prioritizing regions for follow-up studies. Applying this method to autism datasets, GWAS-NR analysis indicates that a large subset of genes involved in the outgrowth and guidance of axons and dendrites is implicated in the aetiology of autism.

Dual regulation of RA-RhoGAP activity by phosphatidic acid and Rap1 during neurite outgrowth

During neurite outgrowth, Rho small G protein activity is spatiotemporally regulated to organize the neurite sprouting, extension, and branching. We have previously identified a potent Rho GTPase-activating protein (GAP), RA-RhoGAP, as a direct downstream target of Rap1 small G protein in the neurite outgrowth. In addition to the Ras-associating (RA) domain for Rap1 binding, RA-RhoGAP has the pleckstrin homology (PH) domain for lipid binding. Here, we showed that phosphatidic acid (PA) bound to the PH domain and enhanced GAP activity for Rho. RA-RhoGAP induced extension of neurite in a diacylglycerol kinase-mediated synthesis of the PA-dependent manner. Knockdown of RA-RhoGAP reduced the diacylglycerol kinase-induced neurite extension. In contrast to the effect of the RA domain, the PH domain was specifically involved in the neurite extension, not in the sprouting and branching. These results indicate that PA and Rap1 cooperatively regulate RA-RhoGAP activity for promoting neurite outgrowth.

Radixin assembles cAMP effectors Epac and PKA into a functional cAMP compartment: role in cAMP-dependent cell proliferation

cAMP is an ubiquitous second messenger. Localized areas with high cAMP concentration, i.e. cAMP microdomains, provide an elegant mechanism to generate signaling specificity and transduction efficiency. However, the mechanisms underlying cAMP effector targeting into these compartments is still unclear. Here we report the identification of radixin as a scaffolding unit for both cAMP effectors, Epac and PKA. This complex localizes in a submembrane compartment where cAMP synthesis occurs. Compartment disruption by shRNA and dominant negative approaches negatively affects cAMP action. Inhibition can be rescued by expression of Rap1b, a substrate for both Epac1 and PKA, but only in its GTP-bound and phosphorylated state. We propose that radixin scaffolds both cAMP effectors in a functional cAMP-sensing compartment for efficient signal transduction, using Rap1 as a downstream signal integrator.

New perspectives in cyclic AMP-mediated axon growth and guidance: The emerging epoch of Epac

In the search for a cure to brain and spinal cord injury much has been learned about the inhibitory environment of the central nervous system (CNS), and yet a clinical therapy remains elusive. In recent years great advances have been made in understanding intracellular molecular mechanisms that transduce cell surface receptor-mediated signals that neurons receive from their environment. Many of these signalling pathways share common mechanisms, which presents the possibility that manipulating activities of key cell signalling molecules such as those regulated by 3'-5'-cyclic adenosine monophosphate (cAMP) might allow axons to simultaneously overcome the inhibitory effects of a number of extracellular ligands. The identification of Epac, a novel direct intracellular target for cAMP, has opened up a new avenue of research that is beginning to explain how cAMP can mediate a range of neuronal functions including distinct axon growth and guidance decisions. With current research tools that allow more specific activation of proteins or knock-down of their expression, as well as quantitation of protein activities in live cells, it is already becoming clear that Epac plays highly important roles in the development and function of the nervous system. Here, we focus on emerging evidence that Epac mediates cAMP-regulated axon growth and chemoattraction, and thus represents a novel target for overcoming axon growth inhibition and promoting CNS regeneration.

Spatial regulation of cyclic AMP-Epac1 signaling in cell adhesion by ERM proteins

Epac1 is a guanine nucleotide exchange factor for the small G protein Rap and is involved in membrane-localized processes such as integrin-mediated cell adhesion and cell-cell junction formation. Cyclic AMP (cAMP) directly activates Epac1 by release of autoinhibition and in addition induces its translocation to the plasma membrane. Here, we show an additional mechanism of Epac1 recruitment, mediated by activated ezrin-radixin-moesin (ERM) proteins. Epac1 directly binds with its N-terminal 49 amino acids to ERM proteins in their open conformation. Receptor-induced activation of ERM proteins results in increased binding of Epac1 and consequently the clustered localization of Epac1 at the plasma membrane. Deletion of the N terminus of Epac1, as well as disruption of the Epac1-ERM interaction by an interfering radixin mutant or small interfering RNA (siRNA)-mediated depletion of the ERM proteins, impairs Epac1-mediated cell adhesion. We conclude that ERM proteins are involved in the spatial regulation of Epac1 and cooperate with cAMP- and Rap-mediated signaling to regulate adhesion to the extracellular matrix.

The interaction of Epac1 and Ran promotes Rap1 activation at the nuclear envelope

Epac1 (exchange protein directly activated by cyclic AMP [cAMP]) couples intracellular cAMP to the activation of Rap1, a Ras family GTPase that regulates cell adhesion, proliferation, and differentiation. Using mass spectrometry, we identified the small G protein Ran and Ran binding protein 2 (RanBP2) as potential binding partners of Epac1. Ran is a small G protein best known for its role in nuclear transport and can be found at the nuclear pore through its interaction with RanBP2. Here we demonstrate that Ran-GTP and Epac1 interact with each other in vivo and in vitro. This binding requires a previously uncharacterized Ras association (RA) domain in Epac1. Surprisingly, the interaction of Epac1 with Ran is necessary for the efficient activation of Rap1 by Epac1. We propose that Ran and RanBP2 anchor Epac1 to the nuclear pore, permitting cAMP signals to activate Rap1 at the nuclear envelope.

Pituitary adenylate cyclase-activating polypeptide (PACAP)/PAC1HOP1 receptor activation coordinates multiple neurotrophic signaling pathways: Akt activation through phosphatidylinositol 3-kinase gamma and vesicle endocytosis for neuronal survival

MAPK and Akt pathways are predominant mediators of trophic signaling for many neuronal systems. Among the vasoactive intestinal peptide/secretin/glucagon family of related peptides, pituitary adenylate cyclase-activating polypeptide (PACAP) binding to specific PAC(1) receptor isoforms can engage multiple signaling pathways and promote neuroprotection through mechanisms that are not well understood. Using a primary sympathetic neuronal system, the current studies demonstrate that PACAP activation of PAC(1)HOP1 receptors engages both MAPK and Akt neurotrophic pathways in an integrated program to facilitate neuronal survival after growth factor withdrawal. PACAP not only stimulated prosurvival ERK1/2 and ERK5 activation but also abrogated SAPK/JNK and p38 MAPK signaling in parallel. In contrast to the potent and rapid effects of PACAP in ERK1/2 phosphorylation, PACAP stimulated Akt phosphorylation in a late phase of PAC(1)HOP1 receptor signaling. From inhibitor and immunoprecipitation analyses, the PACAP/PAC(1)HOP1 receptor-mediated Akt responses did not represent transactivation mechanisms but appeared to depend on G alpha(q)/phosphatidylinositol 3-kinase gamma activity and vesicular internalization pathways. Phosphatidylinositol 3-kinase gamma-selective inhibitors blocked PACAP-stimulated Akt phosphorylation in primary neuronal cultures and in PAC(1)HOP1-overexpressing cell lines; RNA interference-mediated knockdown of the receptor effectors attenuated PACAP-mediated Akt activation. Similarly, perturbation of endocytic pathways also blocked Akt phosphorylation. Between ERK and Akt pathways, PACAP-stimulated Akt signaling was the primary cascade that attenuated cultured neuron apoptosis after growth factor withdrawal. The partitioning of PACAP-mediated Akt signaling in endosomes may be a key mechanism contributing to the high spatial and temporal specificity in signal transduction necessary for survival pathways.

Epac: defining a new mechanism for cAMP action

cAMP is a second messenger that is essential for relaying hormonal responses in many biological processes. The discovery of the cAMP target Epac explained various effects of cAMP that could not be attributed to the established targets PKA and cyclic nucleotide-gated ion channels. Epac1 and Epac2 function as guanine nucleotide exchange factors for the small G protein Rap. cAMP analogs that selectively activate Epac have helped to reveal a role for Epac in processes ranging from insulin secretion to cardiac contraction and vascular permeability. Advances in the understanding of the activation mechanism of Epac and its regulation by diverse anchoring mechanisms have helped to elucidate the means by which cAMP fulfills these functions via Epac.

The role of Epac proteins, novel cAMP mediators, in the regulation of immune, lung and neuronal function

Chronic degenerative inflammatory diseases, such as chronic obstructive pulmonary disease and Alzheimer's dementia, afflict millions of people around the world, causing death and debilitation. Despite the global impact of these diseases, there have been few innovative breakthroughs into their cause, treatment or cure. As with many debilitating disorders, chronic degenerative inflammatory diseases may be associated with defective or dysfunctional responses to second messengers, such as cyclic adenosinemonophosphate (cAMP). The identification of the cAMP-activated guanine nucleotide exchange factors for Ras-like GTPases, Epac1 (also known as cAMP-GEF-I) and Epac2 (also known as cAMP-GEF-II), profoundly altered the prevailing assumptions concerning cAMP signalling, which until then had been solely associated with protein kinase A (PKA). Studies of the molecular mechanisms of Epac-related signalling have demonstrated that these novel cAMP sensors regulate many physiological processes either alone and/or in concert with PKA. These include calcium handling, cardiac and smooth muscle contraction, learning and memory, cell proliferation and differentiation, apoptosis, and inflammation. The diverse signalling properties of cAMP might be explained by spatio-temporal compartmentalization, as well as A-kinase anchoring proteins, which seem to coordinate Epac signalling networks. Future research should focus on the Epac-regulated dynamics of cAMP, and, hopefully, the development of compounds that specifically interfere with the Epac signalling system in order to determine the precise significance of Epac proteins in chronic degenerative inflammatory disorders.

Direct spatial control of Epac1 by cyclic AMP

Epac1 is a guanine nucleotide exchange factor (GEF) for the small G protein Rap and is directly activated by cyclic AMP (cAMP). Upon cAMP binding, Epac1 undergoes a conformational change that allows the interaction of its GEF domain with Rap, resulting in Rap activation and subsequent downstream effects, including integrin-mediated cell adhesion and cell-cell junction formation. Here, we report that cAMP also induces the translocation of Epac1 toward the plasma membrane. Combining high-resolution confocal fluorescence microscopy with total internal reflection fluorescence and fluorescent resonance energy transfer assays, we observed that Epac1 translocation is a rapid and reversible process. This dynamic redistribution of Epac1 requires both the cAMP-induced conformational change as well as the DEP domain. In line with its translocation, Epac1 activation induces Rap activation predominantly at the plasma membrane. We further show that the translocation of Epac1 enhances its ability to induce Rap-mediated cell adhesion. Thus, the regulation of Epac1-Rap signaling by cAMP includes both the release of Epac1 from autoinhibition and its recruitment to the plasma membrane.