Differential roles of Epac in regulating cell death in neuronal and myocardial cells

Beyond PDE4 inhibition: A comprehensive review on downstream cAMP signaling in the central nervous system

Cyclic adenosine monophosphate (cAMP) is a key second messenger that regulates signal transduction pathways pivotal for numerous biological functions. Intracellular cAMP levels are spatiotemporally regulated by their hydrolyzing enzymes called phosphodiesterases (PDEs). It has been shown that increased cAMP levels in the central nervous system (CNS) promote neuroplasticity, neurotransmission, neuronal survival, and myelination while suppressing neuroinflammation. Thus, elevating cAMP levels through PDE inhibition provides a therapeutic approach for multiple CNS disorders, including multiple sclerosis, stroke, spinal cord injury, amyotrophic lateral sclerosis, traumatic brain injury, and Alzheimer's disease. In particular, inhibition of the cAMP-specific PDE4 subfamily is widely studied because of its high expression in the CNS. So far, the clinical translation of full PDE4 inhibitors has been hampered because of dose-limiting side effects. Hence, focusing on signaling cascades downstream activated upon PDE4 inhibition presents a promising strategy, offering novel and pharmacologically safe targets for treating CNS disorders. Yet, the underlying downstream signaling pathways activated upon PDE(4) inhibition remain partially elusive. This review provides a comprehensive overview of the existing knowledge regarding downstream mediators of cAMP signaling induced by PDE4 inhibition or cAMP stimulators. Furthermore, we highlight existing gaps and future perspectives that may incentivize additional downstream research concerning PDE(4) inhibition, thereby providing novel therapeutic approaches for CNS disorders.

EPAC1 inhibition protects the heart from doxorubicin-induced toxicity

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

Epac as a tractable therapeutic target

In 1957, cyclic adenosine monophosphate (cAMP) was identified as the first secondary messenger, and the first signaling cascade discovered was the cAMP-protein kinase A (PKA) pathway. Since then, cAMP has received increasing attention given its multitude of actions. Not long ago, a new cAMP effector named exchange protein directly activated by cAMP (Epac) emerged as a critical mediator of cAMP's actions. Epac mediates a plethora of pathophysiologic processes and contributes to the pathogenesis of several diseases such as cancer, cardiovascular disease, diabetes, lung fibrosis, neurological disorders, and others. These findings strongly underscore the potential of Epac as a tractable therapeutic target. In this context, Epac modulators seem to possess unique characteristics and advantages and hold the promise of providing more efficacious treatments for a wide array of diseases. This paper provides an in-depth dissection and analysis of Epac structure, distribution, subcellular compartmentalization, and signaling mechanisms. We elaborate on how these characteristics can be utilized to design specific, efficient, and safe Epac agonists and antagonists that can be incorporated into future pharmacotherapeutics. In addition, we provide a detailed portfolio for specific Epac modulators highlighting their discovery, advantages, potential concerns, and utilization in the context of clinical disease entities.

Enhanced cGMP Interactor Rap Guanine Exchange Factor 4 (EPAC2) Expression and Activity in Degenerating Photoreceptors: A Neuroprotective Response?

The disease retinitis pigmentosa (RP) leads to photoreceptor degeneration by a yet undefined mechanism(s). In several RP mouse models (i.e., rd mice), a high cyclic GMP (cGMP) level within photoreceptors is detected, suggesting that cGMP plays a role in degeneration. The rap guanine exchange factor 4 (EPAC2) is activated by cyclic AMP (cAMP) and is an accepted cGMP-interacting protein. It is unclear whether and how cGMP interacts with EPAC2 in degenerating photoreceptors; we therefore investigated EPAC2 expression and interactions with cGMP and cAMP in retinas of the rd1 and rd10 models for retinal degeneration. EPAC2 expression in the photoreceptor layer increased significantly during rd1 and rd10 degeneration, and an increase in EPAC2 interactions with cGMP but not cAMP in the rd1 was also seen via a proximity ligation assay on histological sections. Retinal explant cultures revealed that pharmacological inhibition of the EPAC2 activity reduced the photoreceptor layer thickness in the rd10 retina, suggesting that EPAC2 inhibition promotes degeneration. Taken together, our results support the hypothesis that high degeneration-related cGMP leads to increased EPAC2 and cGMP interactions, inhibiting EPAC2. By inference, EPAC2 could have neuroprotective capacities that may be exploited in the future.

Analysis and Identification Genetic Effect of SARS-CoV-2 Infections to Alzheimer's Disease Patients by Integrated Bioinformatics

BACKGROUND

COVID-19 pandemic is a global crisis which results in millions of deaths and causes long-term neurological sequelae, such as Alzheimer's disease (AD).

OBJECTIVE

We aimed to explore the interaction between COVID-19 and AD by integrating bioinformatics to find the biomarkers which lead to AD occurrence and development with COVID-19 and provide early intervention.

METHODS

The differential expressed genes (DEGs) were found by GSE147507 and GSE132903, respectively. The common genes between COVID-19 and AD were identified. Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and protein-protein interactions (PPI) network analysis were carried out. Hub genes were found by cytoscape. A multivariate logistic regression model was constructed. NetworkAnalyst was used for the analysis of TF-gene interactions, TF-miRNA coregulatory network, and Protein-chemical Interactions.

RESULTS

Forty common DEGs for AD and COVID-19 were found. GO and KEGG analysis indicated that the DEGs were enriched in the calcium signal pathway and other pathways. A PPI network was constructed, and 5 hub genes were identified (ITPR1, ITPR3, ITPKB, RAPGEF3, MFGE8). Four hub genes (ITPR1, ITPR3, ITPKB, RAPGEF3) which were considered as important factors in the development of AD that were affected by COVID-19 were shown by nomogram. Utilizing NetworkAnalyst, the interaction network of 4 hub genes and TF, miRNA, common AD risk genes, and known compounds is displayed, respectively.

CONCLUSION

COVID-19 patients are at high risk of developing AD. Vaccination is required. Four hub genes can be considered as biomarkers for prediction and treatment of AD development caused by COVID-19. Compounds with neuroprotective effects can be used as adjuvant therapy for COVID-19 patients.

Broad Impact of Exchange Protein Directly Activated by cAMP 2 (EPAC2) on Respiratory Viral Infections

The recently discovered exchange protein directly activated by cAMP (EPAC), compared with protein kinase A (PKA), is a fairly new family of cAMP effectors. Soon after the discovery, EPAC has shown its significance in many diseases including its emerging role in infectious diseases. In a recent study, we demonstrated that EPAC, but not PKA, is a promising therapeutic target to regulate respiratory syncytial virus (RSV) replication and its associated inflammation. In mammals, there are two isoforms of EPAC-EPAC1 and EPAC2. Unlike other viruses, including Middle East respiratory syndrome coronavirus (MERS-CoV) and Ebola virus, which use EPAC1 to regulate viral replication, RSV uses EPAC2 to control its replication and associated cytokine/chemokine responses. To determine whether EPAC2 protein has a broad impact on other respiratory viral infections, we used an EPAC2-specific inhibitor, MAY0132, to examine the functions of EPAC2 in human metapneumovirus (HMPV) and adenovirus (AdV) infections. HMPV is a negative-sense single-stranded RNA virus belonging to the family Pneumoviridae, which also includes RSV, while AdV is a double-stranded DNA virus. Treatment with an EPAC1-specific inhibitor was also included to investigate the impact of EPAC1 on these two viruses. We found that the replication of HMPV, AdV, and RSV and the viral-induced immune mediators are significantly impaired by MAY0132, while an EPAC1-specific inhibitor, CE3F4, does not impact or slightly impacts, demonstrating that EPAC2 could serve as a novel common therapeutic target to control these viruses, all of which do not have effective treatment and prevention strategies.

Cell-Type Apoptosis in Lung during SARS-CoV-2 Infection

The SARS-CoV-2 pandemic has inspired renewed interest in understanding the fundamental pathology of acute respiratory distress syndrome (ARDS) following infection. However, the pathogenesis of ARDS following SRAS-CoV-2 infection remains largely unknown. In the present study, we examined apoptosis in postmortem lung sections from COVID-19 patients and in lung tissues from a non-human primate model of SARS-CoV-2 infection, in a cell-type manner, including type 1 and 2 alveolar cells and vascular endothelial cells (ECs), macrophages, and T cells. Multiple-target immunofluorescence assays and Western blotting suggest both intrinsic and extrinsic apoptotic pathways are activated during SARS-CoV-2 infection. Furthermore, we observed that SARS-CoV-2 fails to induce apoptosis in human bronchial epithelial cells (i.e., BEAS2B cells) and primary human umbilical vein endothelial cells (HUVECs), which are refractory to SARS-CoV-2 infection. However, infection of co-cultured Vero cells and HUVECs or Vero cells and BEAS2B cells with SARS-CoV-2 induced apoptosis in both Vero cells and HUVECs/BEAS2B cells but did not alter the permissiveness of HUVECs or BEAS2B cells to the virus. Post-exposure treatment of the co-culture of Vero cells and HUVECs with a novel non-cyclic nucleotide small molecule EPAC1-specific activator reduced apoptosis in HUVECs. These findings may help to delineate a novel insight into the pathogenesis of ARDS following SARS-CoV-2 infection.

Epac: new emerging cAMP-binding protein

The well-known second messenger cyclic adenosine monophosphate (cAMP) regulates the morphology and physiology of neurons and thus higher cognitive brain functions. The discovery of exchange protein activated by cAMP (Epac) as a guanine nucleotide exchange factor for Rap GTPases has shed light on protein kinase A (PKA)-independent functions of cAMP signaling in neural tissues. Studies of cAMP-Epac-mediated signaling in neurons under normal and disease conditions also revealed its diverse contributions to neurodevelopment, synaptic remodeling, and neurotransmitter release, as well as learning, memory, and emotion. In this mini-review, the various roles of Epac isoforms, including Epac1 and Epac2, highly expressed in neural tissues are summarized, and controversies or issues are highlighted that need to be resolved to uncover the critical functions of Epac in neural tissues and the potential for a new therapeutic target of mental disorders.

Neuronal Epac1 mediates retinal neurodegeneration in mouse models of ocular hypertension

Progressive loss of retinal ganglion cells (RGCs) leads to irreversible visual deficits in glaucoma. Here, we found that the level of cyclic AMP and the activity and expression of its mediator Epac1 were increased in retinas of two mouse models of ocular hypertension. Genetic depletion of Epac1 significantly attenuated ocular hypertension–induced detrimental effects in the retina, including vascular inflammation, neuronal apoptosis and necroptosis, thinning of ganglion cell complex layer, RGC loss, and retinal neuronal dysfunction. With bone marrow transplantation and various Epac1 conditional knockout mice, we further demonstrated that Epac1 in retinal neuronal cells (especially RGCs) was responsible for their death. Consistently, pharmacologic inhibition of Epac activity prevented RGC loss. Moreover, in vitro study on primary RGCs showed that Epac1 activation was sufficient to induce RGC death, which was mechanistically mediated by CaMKII activation. Taken together, these findings indicate that neuronal Epac1 plays a critical role in retinal neurodegeneration and suggest that Epac1 could be considered a target for neuroprotection in glaucoma.

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

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

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.

Recent Advances in EPAC-Targeted Therapies: A Biophysical Perspective

The universal second messenger cAMP regulates diverse intracellular processes by interacting with ubiquitously expressed proteins, such as Protein Kinase A (PKA) and the Exchange Protein directly Activated by cAMP (EPAC). EPAC is implicated in multiple pathologies, thus several EPAC-specific inhibitors have been identified in recent years. However, the mechanisms and molecular interactions underlying the EPAC inhibition elicited by such compounds are still poorly understood. Additionally, being hydrophobic low molecular weight species, EPAC-specific inhibitors are prone to forming colloidal aggregates, which result in non-specific aggregation-based inhibition (ABI) in aqueous systems. Here, we review from a biophysical perspective the molecular basis of the specific and non-specific interactions of two EPAC antagonists-CE3F4R, a non-competitive inhibitor, and ESI-09, a competitive inhibitor of EPAC. Additionally, we discuss the value of common ABI attenuators (e.g., TX and HSA) to reduce false positives at the expense of introducing false negatives when screening aggregation-prone compounds. We hope this review provides the EPAC community effective criteria to evaluate similar compounds, aiding in the optimization of existing drug leads, and informing the development of the next generation of EPAC-specific inhibitors.

Role of PI3K/Akt and MEK/ERK Signalling in cAMP/Epac-Mediated Endothelial Barrier Stabilisation

Background and Aims

Activation of the cAMP/Epac signalling stabilises endothelial barrier function. Moreover, its activation is accompanied by an activation of PI3K/Akt and MEK/ERK signalling in diverse cell types but their impact on endothelial barrier function is largely unknown. Here the role of PI3K/Akt and MEK/ERK signalling in cAMP/Epac-mediated endothelial barrier stabilisation was analysed.

Methods

Endothelial barrier function was analysed in cultured human umbilical vein endothelial cells (HUVECs) by measuring flux of albumin. A modified cAMP analogue 8-pCPT-2′-O-Me-cAMP (Epac agonist) was used to specifically activate cAMP/Epac signalling.

Results

Epac agonist reduces the basal and attenuates thrombin-induced endothelial hyperpermeability accompanied by an activation of PI3K/Akt and MEK/ERK signalling. The qPCR data demonstrate HUVECs express PI3Kα, PI3Kβ, and PI3Kγ but not PI3Kδ isoforms. The western blot data demonstrate Epac agonist activates PI3Kα and PI3Kβ isoforms. Inhibition of MEK/ERK but not PI3K/Akt pathway potentiates the endothelial barrier protective effects of cAMP/Epac signalling. Inhibition of MEK/ERK signalling in the presence of Epac agonist induces a reorganisation of actin cytoskeleton to the cell periphery, enhanced VE-cadherin localisation at cell-cell junctions, and dephosphorylation of myosin light chains (MLC) but not inhibition of RhoA/Rock signalling. Moreover, Epac agonist promotes endothelial cell (EC) survival via reduction in activities of pro-apoptotic caspases in a PI3K/Akt and MEK/ERK signalling-dependent manner.

Conclusion

Our data demonstrate that the Epac agonist simultaneously activates diverse signalling pathways in ECs, which may have differential effects on endothelial barrier function. It activates PI3K/Akt and MEK/ERK signalling which mainly govern its pro-survival effects on ECs. Inhibition of MEK/ERK but not PI3K/Akt signalling enhances barrier stabilising and barrier protective effects of cAMP/Epac activation.

Chemical Compounds Used In This Study

8-pCPT-2′-O-Me-cAMP (PubChem CID: 9913268); Akt inhibitor VIII (PubChem CID: 10196499); AS-252424 (PubChem CID: 11630874); IC-87114 (PubChem CID: 9908783); PD 98059 (PubChem CID: 4713); PIK-75 (PubChem CID: 10275789); TGX-221 (PubChem CID: 9907093); Thrombin (PubChem CID: 90470996); U0126 (PubChem CID: 3006531); Wortmannin (PubChem CID: 312145).

Mice depleted for Exchange Proteins Directly Activated by cAMP (Epac) exhibit irregular liver regeneration in response to partial hepatectomy

The exchange proteins directly activated by cAMP 1 and 2 (Epac1 and Epac2) are expressed in a cell specific manner in the liver, but their biological functions in this tissue are poorly understood. The current study was undertaken to begin to determine the potential roles of Epac1 and Epac2 in liver physiology and disease. Male C57BL/6J mice in which expression of Epac1 and/or Epac2 are deleted, were subjected to partial hepatectomy and the regenerating liver was analyzed with regard to lipid accumulation, cell replication and protein expression. In response to partial hepatectomy, deletion of Epac1 and/or Epac2 led to increased hepatocyte proliferation 36 h post surgery, and the transient steatosis observed in wild type mice was virtually absent in mice lacking both Epac1 and Epac2. The expression of the protein cytochrome P4504a14, which is implicated in hepatic steatosis and fibrosis, was substantially reduced upon deletion of Epac1/2, while a number of factors involved in lipid metabolism were significantly decreased. Moreover, the number of Küpffer cells was affected, and Epac2 expression was increased in the liver of wild type mice in response to partial hepatectomy, further supporting a role for these proteins in liver function. This study establishes hepatic phenotypic abnormalities in mice deleted for Epac1/2 for the first time, and introduces Epac1/2 as regulators of hepatocyte proliferation and lipid accumulation in the regenerative process.

Orally active Epac inhibitor reverses mechanical allodynia and loss of intraepidermal nerve fibers in a mouse model of chemotherapy-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major side effect of cancer treatment that significantly compromises quality of life of cancer patients and survivors. Identification of targets for pharmacological intervention to prevent or reverse CIPN is needed. We investigated exchange protein regulated by cAMP (Epac) as a potential target. Epacs are cAMP-binding proteins known to play a pivotal role in mechanical allodynia induced by nerve injury and inflammation. We demonstrate that global Epac1-knockout (Epac1-/-) male and female mice are protected against paclitaxel-induced mechanical allodynia. In addition, spinal cord astrocyte activation and intraepidermal nerve fiber (IENF) loss are significantly reduced in Epac1-/- mice as compared to wild-type mice. Moreover, Epac1-/- mice do not develop the paclitaxel-induced deficits in mitochondrial bioenergetics in the sciatic nerve that are a hallmark of CIPN. Notably, mice with cell-specific deletion of Epac1 in Nav1.8-positive neurons (N-Epac1-/-) also show reduced paclitaxel-induced mechanical allodynia, astrocyte activation, and IENF loss, indicating that CIPN develops downstream of Epac1 activation in nociceptors. The Epac-inhibitor ESI-09 reversed established paclitaxel-induced mechanical allodynia in wild-type mice even when dosing started 10 days after completion of paclitaxel treatment. In addition, oral administration of ESI-09 suppressed spinal cord astrocyte activation in the spinal cord and protected against IENF loss. Ex vivo, ESI-09 blocked paclitaxel-induced abnormal spontaneous discharges in dorsal root ganglion neurons. Collectively, these findings implicate Epac1 in nociceptors as a novel target for treatment of CIPN. This is clinically relevant because ESI-09 has the potential to reverse a debilitating and long-lasting side effect of cancer treatment.

Epac1 deficiency inhibits basic fibroblast growth factor-mediated vascular smooth muscle cell migration

Vascular smooth muscle cell (VSMC) migration and the subsequent intimal thickening play roles in vascular restenosis. We previously reported that an exchange protein activated by cAMP 1 (Epac1) promotes platelet-derived growth factor (PDGF)-induced VSMC migration and intimal thickening. Because basic fibroblast growth factor (bFGF) also plays a pivotal role in restenosis, we examined whether Epac1 was involved in bFGF-mediated VSMC migration. bFGF-induced lamellipodia formation and migration were significantly decreased in VSMCs obtained from Epac1-/- mice compared to those in Epac1+/+-VSMCs. The bFGF-induced phosphorylation of Akt and glycogen synthase kinase 3β (GSK3β), which play a role in bFGF-induced cell migration, was attenuated in Epac1-/--VSMCs. Intimal thickening induced by the insertion of a large wire was attenuated in Epac1-/- mice, and was accompanied by the decreased phosphorylation of GSK3β. These data suggest that Epac1 deficiency attenuates bFGF-induced VSMC migration, possibly via Akt/GSK3β pathways.

Inhibition of EPAC2 Attenuates Intracerebral Hemorrhage-Induced Secondary Brain Injury via the p38/BIM/Caspase-3 Pathway

Exchange proteins directly activated by cAMP (EPACs) are critical cAMP-dependent signaling pathway intermediaries that have been implicated in the pathogenesis of several human diseases, particularly neurological disorders. However, their pathogenic role in secondary brain injury (SBI) induced by intracranial hemorrhage (ICH) is unknown. The aim of this study was to examine the effects of EPAC2 on ICH-induced SBI and its underlying mechanisms. An in vivo ICH model was established in Sprague-Dawley rats by autologous blood injection. In addition, rat primary cortical neuronal cultures were exposed to oxyhemoglobin to simulate ICH in vitro. The function of EPAC2 in SBI induced by ICH was studied using the EPAC2-specific inhibitor ESI-05. In this study, we found that EPAC2 protein expression was significantly increased in the ICH models in vitro and in vivo. Furthermore, EPAC2 activation was inhibited by ESI-05 under ICH conditions. Inhibition of EPAC2 decreased the apoptosis rate of nerve cells in the cortex accompanied by a corresponding decrease in the protein expression of phosphorylated p38, Bcl-2-like protein 11 (BIM), and caspase-3. In summary, this study showed that inhibition of EPAC2 activation by ESI-05 suppressed SBI induced by ICH via the p38/BIM/caspase-3 signaling pathway.

Deletion of Rap1b, but not Rap1a or Epac1, Reduces Protein Kinase A-Mediated Thyroid Cancer

BACKGROUND

Thyroid cancer is an emerging health problem in the United States and worldwide. With incidence rates of thyroid cancer rapidly rising, the need to develop new treatment options is becoming a priority, and understanding the molecular mechanisms of this disease is crucial to furthering these efforts. Thyroid growth is driven by the TSH/cAMP/PKA signaling pathway, and it has previously been shown that activation of PKA through genetic ablation of the regulatory subunit Prkar1a (Prkar1a KO) is sufficient to cause follicular thyroid cancer in mouse models. cAMP also activates the Epac proteins and their downstream effectors, Rap1a and Rap1b.

METHODS

Previously, the authors' laboratory generated a mouse model of follicular thyroid cancer by conferring thyroid-specific deletion of Prkar1a (R1a-TpoKO). To probe the roles of other components of the PKA signaling system in the development of thyroid cancer, this study deleted Rap1 and Epac1 in the setting of the Prkar1a knockout.

RESULTS

Deletion of Rap1 significantly decreases thyroid size and cancer incidence in Prkar1a KO thyroids. Further, isoform-specific ablation of Rap1a and Rap1b implicates Rap1b as the downstream effector of PKA during thyroid carcinogenesis. In vivo modeling provides definitive evidence that Epac1 plays little role in thyroid proliferation and is dispensable for thyroid carcinogenesis arising from the deletion of Prkar1a.

CONCLUSIONS

This study demonstrate that PKA signaling to Rap1b is a key signaling node for follicular thyroid carcinogenesis, while Epac1 activity is not required for tumor development. This work sheds new light on the pathways involved in FTC development and identifies a possible target for the development of new therapies in the treatment of FTC.

Spinal protein phosphatase 1 constrains respiratory plasticity after sustained hypoxia

Plasticity is an important aspect of the neural control of breathing. One well-studied form of respiratory plasticity is phrenic long-term facilitation (pLTF) induced by acute intermittent but not sustained hypoxia. Okadaic acid-sensitive protein phosphatases (PPs) differentially regulate phrenic nerve activity with intermittent vs. sustained hypoxia, at least partially accounting for pLTF pattern sensitivity. However, okadaic acid inhibits multiple serine/threonine phosphatases, and the relevant phosphatase (PP1, PP2A, PP5) for pLTF pattern sensitivity has not been identified. Here, we demonstrate that sustained hypoxia (25 min, 9-10.5% O₂) elicits phrenic motor facilitation in rats pretreated with bilateral intrapleural injections of small interfering RNAs (siRNAs; Accell-modified to preferentially transfect neurons, 3.33 μM, 3 days) targeting PP1 mRNA (48 ± 14% change from baseline, n = 6) but not PP2A (14 ± 9% baseline, n = 6) or nontargeting siRNAs (4 ± 10% baseline, n = 7). In time control rats (no hypoxia) treated with siRNAs ( n = 6), no facilitation was evident (-9 ± 9% baseline). siRNAs had no effect on the hypoxic phrenic response. Immunohistochemistry revealed PP1 and PP2A protein in identified phrenic motoneurons. Although PP1 and PP2A siRNAs significantly decreased PP1 and PP2A mRNA in PC12 cell cultures, we were not able to verify "knockdown" in vivo after siRNA treatment. On the other hand, PP1 and PP2A siRNAs significantly decreased PP1 and PP2A mRNA in PC12 cell cultures, verifying the intended siRNA effects. In conclusion, PP1 (not PP2A) is the relevant okadaic acid-sensitive phosphatase constraining phrenic motor facilitation after sustained hypoxia and likely contributing to pLTF pattern sensitivity. NEW & NOTEWORTHY This study demonstrates that the relevant okadaic acid-sensitive Ser/Thr protein phosphatase (PP) constraining facilitation after sustained hypoxia is PP1 and not PP2A. It suggests that PP1 may be critical in the pattern sensitivity of hypoxia-induced phrenic motor plasticity.

Deletion of exchange proteins directly activated by cAMP (Epac) causes defects in hippocampal signaling in female mice

Previous studies demonstrate essential roles for the exchange proteins directly activated by cAMP 1 and 2 (Epac1 and Epac2; here collectively referred to as Epac) in the brain. In the hippocampus, Epac contributes to the control of neuronal growth and differentiation and has been implicated in memory and learning as well as in anxiety and depression. In the present study we address the hypothesis that Epac affects hippocampal cellular responses to acute restraint stress. Stress causes activation of the hypothalamus-pituitary-adrenal (HPA)-axis, and glucocorticoid receptor (GR) signaling is essential for proper feedback regulation of the stress response, both in the brain and along the HPA axis. In the hippocampus, GR expression is regulated by cAMP and the brain enriched micro RNA miR-124. Epac has been associated with miR-124 expression in hippocampal neurons, but not in regulation of GR. We report that hippocampal expression of Epac1 and Epac2 increased in response to acute stress in female wild type mice. In female mice genetically deleted for Epac, nuclear translocation of GR in response to restraint stress was significantly delayed, and moreover, miR-124 expression was decreased in these mice. Male mice lacking Epac also showed abnormalities in miR-124 expression, but the phenotype was less profound than in females. Serum corticosterone levels were slightly altered immediately after stress in both male and female mice deleted for Epac. The presented data indicate that Epac1 and Epac2 are involved in controlling cellular responses to acute stress in the mouse hippocampus and provide novel insights into the underlying transcriptional and signaling networks. Interestingly, we observe sex specific differences when Epac is deleted. As the incidence and prevalence of stress-related diseases are higher in women than in men, the Epac knockout models might serve as genetic tools to further elucidate the cellular mechanisms underlying differences between male and female with regard to regulation of stress.

Cyclase-associated protein 1 (CAP1) is a prenyl-binding partner of Rap1 GTPase

Rap1 proteins are members of the Ras subfamily of small GTPases involved in many biological responses, including adhesion, cell proliferation, and differentiation. Like all small GTPases, they work as molecular allosteric units that are active in signaling only when associated with the proper membrane compartment. Prenylation, occurring in the cytosol, is an enzymatic posttranslational event that anchors small GTPases at the membrane, and prenyl-binding proteins are needed to mask the cytoplasm-exposed lipid during transit to the target membrane. However, several of these proteins still await discovery. In this study, we report that cyclase-associated protein 1 (CAP1) binds Rap1. We found that this binding is GTP-independent, does not involve Rap1's effector domain, and is fully contained in its C-terminal hypervariable region (HVR). Furthermore, Rap1 prenylation was required for high-affinity interactions with CAP1 in a geranylgeranyl-specific manner. The prenyl binding specifically involved CAP1's C-terminal hydrophobic β-sheet domain. We present a combination of experimental and computational approaches, yielding a model whereby the high-affinity binding between Rap1 and CAP1 involves electrostatic and nonpolar side-chain interactions between Rap1's HVR residues, lipid, and CAP1 β-sheet domain. The binding was stabilized by the lipid insertion into the β-solenoid whose interior was occupied by nonpolar side chains. This model was reminiscent of the recently solved structure of the PDEδ-K-Ras complex; accordingly, disruptors of this complex, e.g. deltarasin, blocked the Rap1-CAP1 interaction. These findings indicate that CAP1 is a geranylgeranyl-binding partner of Rap1.

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.

Alleviation of thalamic pain by cilostazol administration: a case report

Thalamic pain is severe and treatment-resistant; however, there are few available options for improving thalamic pain. This study demonstrated that thalamic pain was alleviated by administration of cilostazol, suggesting that cilostazol may be a candidate for treating thalamic pain.

Elevated aggressive behavior in male mice with thyroid-specific Prkar1a and global Epac1 gene deletion

Alterations in circulating thyroid hormone concentrations are associated with several psychological and behavioral disorders. In humans, behavioral disorders such as anxiety, depression, and attention-deficit hyperactivity disorder can be associated with thyroid disease. The Tpo-Cre;Prkar1aflox/flox;Epac1-/- (R1A-Epac1KO) mice, originally bred to investigate the role of exchange protein directly activated by cAMP (Epac1) in follicular thyroid cancer, displayed self-mutilating and aggressive behaviors during casual observation. To assess these atypical responses, behavioral testing was conducted with the R1A-Epac1KO mice, as well as their single knockout counterparts, the thyroid-specific Prkar1a-/- and global Epac1-/- mice. Mice of all three genotypes demonstrated increased aggressive behavior against an intruder mouse. In addition, Epac1-/- mice increased response to an auditory stimulus, and the Prkar1a-/- and R1A-Epac1KO mice increased swimming behavior in the Porsolt forced swim test. Both Prkar1a-/- mice and R1A-Epac1KO mice have increased circulating thyroxine and corticosterone concentrations. Although hyperthyroidism has not been previously associated with aggression, increased thyroid hormone signaling might contribute to the increased aggressive response to the intruder mouse, as well as the increased swimming response. Mice with a genetic background of Tpo-Cre;Prkar1aflox/flox;Epac1-/- are aggressive, and both the thyroid-specific knockout of Prkar1a and global knockout of Epac1 likely contribute to this aggressive behavior. This study supports the hypothesis that altered thyroid signaling and aggressive behavior are linked.

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.

Role of phosphodiesterase 4 expression in the Epac1 signaling-dependent skeletal muscle hypertrophic action of clenbuterol

Clenbuterol (CB), a selective β2-adrenergic receptor (AR) agonist, induces muscle hypertrophy and counteracts muscle atrophy. However, it is paradoxically less effective in slow-twitch muscle than in fast-twitch muscle, though slow-twitch muscle has a greater density of β-AR We recently demonstrated that Epac1 (exchange protein activated by cyclic AMP [cAMP]1) plays a pivotal role in β2-AR-mediated masseter muscle hypertrophy through activation of the Akt and calmodulin kinase II (CaMKII)/histone deacetylase 4 (HDAC4) signaling pathways. Here, we investigated the role of Epac1 in the differential hypertrophic effect of CB using tibialis anterior muscle (TA; typical fast-twitch muscle) and soleus muscle (SOL; typical slow-twitch muscle) of wild-type (WT) and Epac1-null mice (Epac1KO). The TA mass to tibial length (TL) ratio was similar in WT and Epac1KO at baseline and was significantly increased after CB infusion in WT, but not in Epac1KO The SOL mass to TL ratio was also similar in WT and Epac1KO at baseline, but CB-induced hypertrophy was suppressed in both mice. In order to understand the mechanism involved, we measured the protein expression levels of β-AR signaling-related molecules, and found that phosphodiesterase 4 (PDE4) expression was 12-fold greater in SOL than in TA These results are consistent with the idea that increased PDE4-mediated cAMP hydrolysis occurs in SOL compared to TA, resulting in a reduced cAMP concentration that is insufficient to activate Epac1 and its downstream Akt and CaMKII/HDAC4 hypertrophic signaling pathways in SOL of WT This scenario can account for the differential effects of CB on fast- and slow-twitch muscles.

Anxiety and depression with neurogenesis defects in exchange protein directly activated by cAMP 2-deficient mice are ameliorated by a selective serotonin reuptake inhibitor, Prozac

Intracellular cAMP and serotonin are important modulators of anxiety and depression. Fluoxetine, a selective serotonin reuptake inhibitor (SSRI) also known as Prozac, is widely used against depression, potentially by activating cAMP response element-binding protein (CREB) and increasing brain-derived neurotrophic factor (BDNF) through protein kinase A (PKA). However, the role of Epac1 and Epac2 (Rap guanine nucleotide exchange factors, RAPGEF3 and RAPGEF4, respectively) as potential downstream targets of SSRI/cAMP in mood regulations is not yet clear. Here, we investigated the phenotypes of Epac1 (Epac1(-/-)) or Epac2 (Epac2(-/-)) knockout mice by comparing them with their wild-type counterparts. Surprisingly, Epac2(-/-) mice exhibited a wide range of mood disorders, including anxiety and depression with learning and memory deficits in contextual and cued fear-conditioning tests without affecting Epac1 expression or PKA activity. Interestingly, rs17746510, one of the three single-nucleotide polymorphisms (SNPs) in RAPGEF4 associated with cognitive decline in Chinese Alzheimer's disease (AD) patients, was significantly correlated with apathy and mood disturbance, whereas no significant association was observed between RAPGEF3 SNPs and the risk of AD or neuropsychiatric inventory scores. To further determine the detailed role of Epac2 in SSRI/serotonin/cAMP-involved mood disorders, we treated Epac2(-/-) mice with a SSRI, Prozac. The alteration in open field behavior and impaired hippocampal cell proliferation in Epac2(-/-) mice were alleviated by Prozac. Taken together, Epac2 gene polymorphism is a putative risk factor for mood disorders in AD patients in part by affecting the hippocampal neurogenesis.

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.

Dipeptidyl Peptidase 4 Inhibition Alleviates Shortage of Circulating Glucagon-Like Peptide-1 in Heart Failure and Mitigates Myocardial Remodeling and Apoptosis via the Exchange Protein Directly Activated by Cyclic AMP 1/Ras-Related Protein 1 Axis

BACKGROUND

Ample evidence demonstrates cardiovascular protection by incretin-based therapy using dipeptidyl peptidase 4 inhibitor (DPP4i) and glucagon-like peptide-1 (GLP-1) under either diabetic or nondiabetic condition. Their action on myocardium is mediated by the cyclic AMP (cAMP) signal; however, the pathway remains uncertain. This study was conducted to address the effect of DPP4i/GLP-1/cAMP axis on cardiac dysfunction and remodeling induced by pressure overload (thoracic aortic constriction [TAC]) independently of diabetes mellitus.

METHODS AND RESULTS

DPP4i (alogliptin, 10 mg/kg per day for 4 weeks) prevented TAC-induced contractile dysfunction, remodeling, and apoptosis of myocardium in a GLP-1 receptor antagonist (exendin [9-39])-sensitive fashion. In TAC, circulating level of GLP-1 (in pmol/L; 0.86 ± 0.10 for TAC versus 2.13 ± 0.54 for sham control) unexpectedly declined and so did the myocardial cAMP concentration (in pmol/mg protein; 33.0 ± 1.4 for TAC versus 42.2 ± 1.5 for sham). Alogliptin restored the decline in the GLP-1/cAMP levels observed in TAC, thereby augmented cAMP signaling effectors (protein kinase A [PKA] and exchange protein directly activated by cAMP 1 [EPAC1]). In vitro assay revealed distinct roles of PKA and EPAC1 in cardiac apoptosis. EPAC1 promoted cardiomyocyte survival via concomitant increase in B cell lymphoma-2 (Bcl-2) expression and activation of small G protein Ras-related protein 1 (Rap1) in a cAMP dose-dependent and PKA-independent fashion.

CONCLUSIONS

DPP4i restores cardiac remodeling and apoptosis caused by the pathological decline in circulating GLP-1 in response to pressure overload. EPAC1 is essential for cardiomyocyte survival via the cAMP/Rap1 activation independently of PKA.

Critical role for Epac1 in inflammatory pain controlled by GRK2-mediated phosphorylation of Epac1

cAMP signaling plays a key role in regulating pain sensitivity. Here, we uncover a previously unidentified molecular mechanism in which direct phosphorylation of the exchange protein directly activated by cAMP 1 (EPAC1) by G protein kinase 2 (GRK2) suppresses Epac1-to-Rap1 signaling, thereby inhibiting persistent inflammatory pain. Epac1(-/-) mice are protected against inflammatory hyperalgesia in the complete Freund's adjuvant (CFA) model. Moreover, the Epac-specific inhibitor ESI-09 inhibits established CFA-induced mechanical hyperalgesia without affecting normal mechanical sensitivity. At the mechanistic level, CFA increased activity of the Epac target Rap1 in dorsal root ganglia of WT, but not of Epac1(-/-), mice. Using sensory neuron-specific overexpression of GRK2 or its kinase-dead mutant in vivo, we demonstrate that GRK2 inhibits CFA-induced hyperalgesia in a kinase activity-dependent manner. In vitro, GRK2 inhibits Epac1-to-Rap1 signaling by phosphorylation of Epac1 at Ser-108 in the Disheveled/Egl-10/pleckstrin domain. This phosphorylation event inhibits agonist-induced translocation of Epac1 to the plasma membrane, thereby reducing Rap1 activation. Finally, we show that GRK2 inhibits Epac1-mediated sensitization of the mechanosensor Piezo2 and that Piezo2 contributes to inflammatory mechanical hyperalgesia. Collectively, these findings identify a key role of Epac1 in chronic inflammatory pain and a molecular mechanism for controlling Epac1 activity and chronic pain through phosphorylation of Epac1 at Ser-108. Importantly, using the Epac inhibitor ESI-09, we validate Epac1 as a potential therapeutic target for chronic pain.

Exchange Protein Directly Activated by cAMP (EPAC) Regulates Neuronal Polarization through Rap1B

Acquisition of neuronal polarity is a complex process involving cellular and molecular events. The second messenger cAMP is involved in axonal specification through activation of protein kinase A. However, an alternative cAMP-dependent mechanism involves the exchange protein directly activated by cAMP (EPAC), which also responds to physiological changes in cAMP concentration, promoting activation of the small Rap GTPases. Here, we present evidence that EPAC signaling contributes to axon specification and elongation. In primary rat hippocampal neurons, EPAC isoforms were expressed differentially during axon specification. Furthermore, 8-pCPT, an EPAC pharmacological activator, and genetic manipulations of EPAC in neurons induced supernumerary axons indicative of Rap1b activation. Moreover, 8-pCPT-treated neurons expressed ankyrin G and other markers of mature axons such as synaptophysin and axonal accumulation of vGLUT1. In contrast, pharmacological inhibition of EPAC delayed neuronal polarity. Genetic manipulations to inactivate EPAC1 using either shRNA or neurons derived from EPAC1 knock-out (KO) mice led to axon elongation and polarization defects. Interestingly, multiaxonic neurons generated by 8-pCPT treatments in wild-type neurons were not found in EPAC1 KO mice neurons. Altogether, these results propose that EPAC signaling is an alternative and complementary mechanism for cAMP-dependent axon determination.

SIGNIFICANCE STATEMENT

This study identifies the guanine exchange factor responsible for Rap1b activation during neuronal polarization and provides an alternate explanation for cAMP-dependent acquisition of neuronal polarity.

Effects of exogenous neurotrophin-3 on myocyte apoptosis and Ca(2+)-ATP enzyme levels following nerve injury in rats

This study aims to determine the influence of neurotrophin-3 (NT-3) plasmids on neuronal apoptosis and Ca(2+)-ATP enzyme levels in injured muscles. We also investigated the mechanism underlying the role of NT-3 in delaying muscle atrophy following a peripheral nerve injury. Sixty adult Wistar rats were used to generate the peripheral nerve injury models. The rats were randomly assigned to the saline and NT-3 groups. Related indicators, such as caspase-3 protein expression, skeletal muscle cell apoptosis, and Ca(2+)-ATP enzyme expression were quantified. The expression levels of caspase-3 and the histone-muscle cell apoptosis rate in the NT-3 group decreased at different post-operative times following peripheral nerve injury, whereas NT-3 expression and the sarcoplasmic reticulum Ca(2+)-ATP enzyme levels increased. Statistically significant differences were observed in the NT-3 group as compared to the saline group (P < 0.05). NT-3 mitigated muscle atrophy following peripheral nerve damage by inhibiting caspase-3 gene expression and increasing Ca(2+)-ATP enzymatic activity, ultimately reducing muscle apoptosis.

Epac1 Deficiency Attenuated Vascular Smooth Muscle Cell Migration and Neointimal Formation

OBJECTIVE

Vascular smooth muscle cell (SMC) migration causes neointima, which is related to vascular remodeling after mechanical injury and atherosclerosis development. We previously reported that an exchange protein activated by cAMP (Epac) 1 was upregulated in mouse arterial neointima and promoted SMC migration. In this study, we examined the molecular mechanisms of Epac1-induced SMC migration and the effect of Epac1 deficiency on vascular remodeling in vivo.

APPROACH AND RESULTS

Platelet-derived growth factor-BB promoted a 2-fold increase in SMC migration in a primary culture of aortic SMCs obtained from Epac1(+/+) mice (Epac1(+/+)-ASMCs), whereas there was only a 1.2-fold increase in Epac1(-/-)-ASMCs. The degree of platelet-derived growth factor-BB-induced increase in intracellular Ca(2+) was smaller in Fura2-labeled Epac1(-/-)-ASMCs than in Epac1(+/+)-ASMCs. In Epac1(+/+)-ASMCs, an Epac-selective cAMP analog or platelet-derived growth factor-BB increased lamellipodia accompanied by cofilin dephosphorylation, which is induced by Ca(2+) signaling, whereas these effects were rarely observed in Epac1(-/-)-ASMCs. Furthermore, 4 weeks after femoral artery injury, prominent neointima were formed in Epac1(+/+) mice, whereas neointima formation was significantly attenuated in Epac1(-/-) mice in which dephosphorylation of cofilin was inhibited. The chimeric mice generated by bone marrow cell transplantation from Epac1(+/+) into Epac1(-/-) mice and vice versa demonstrated that the genetic background of vascular tissues, including SMCs rather than of bone marrow-derived cells affected Epac1-mediated neointima formation.

CONCLUSIONS

These data suggest that Epac1 deficiency attenuates neointima formation through, at least in part, inhibition of SMC migration, in which a decrease in Ca(2+) influx and a suppression of cofilin-mediated lamellipodia formation occur.

Mechanisms of cyclic AMP/protein kinase A- and glucocorticoid-mediated apoptosis using S49 lymphoma cells as a model system

Cyclic AMP/protein kinase A (cAMP/PKA) and glucocorticoids promote the death of many cell types, including cells of hematopoietic origin. In wild-type (WT) S49 T-lymphoma cells, signaling by cAMP and glucocorticoids converges on the induction of the proapoptotic B-cell lymphoma-family protein Bim to produce mitochondria-dependent apoptosis. Kin(-), a clonal variant of WT S49 cells, lacks PKA catalytic (PKA-Cα) activity and is resistant to cAMP-mediated apoptosis. Using sorbitol density gradient fractionation, we show here that in kin(-) S49 cells PKA-Cα is not only depleted but the residual PKA-Cα mislocalizes to heavier cell fractions and is not phosphorylated at two conserved residues (Ser(338) or Thr(197)). In WT S49 cells, PKA-regulatory subunit I (RI) and Bim coimmunoprecipitate upon treatment with cAMP analogs and forskolin (which increases endogenous cAMP concentrations). By contrast, in kin(-) cells, expression of PKA-RIα and Bim is prominently decreased, and increases in cAMP do not increase Bim expression. Even so, kin(-) cells undergo apoptosis in response to treatment with the glucocorticoid dexamethasone (Dex). In WT cells, glucorticoid-mediated apoptosis involves an increase in Bim, but in kin(-) cells, Dex-promoted cell death appears to occur by a caspase 3-independent apoptosis-inducing factor pathway. Thus, although cAMP/PKA-Cα and PKA-R1α/Bim mediate apoptotic cell death in WT S49 cells, kin(-) cells resist this response because of lower levels of PKA-Cα and PKA-RIα subunits as well as Bim. The findings for Dex-promoted apoptosis imply that these lymphoma cells have adapted to selective pressure that promotes cell death by altering canonical signaling pathways.

Epac2-deficiency leads to more severe retinal swelling, glial reactivity and oxidative stress in transient middle cerebral artery occlusion induced ischemic retinopathy

Ischemia occurs in diabetic retinopathy with neuronal loss, edema, glial cell reactivity and oxidative stress. Epacs, consisting of Epac1 and Epac2, are cAMP mediators playing important roles in maintenance of endothelial barrier and neuronal functions. To investigate the roles of Epacs in the pathogenesis of ischemic retinopathy, transient middle cerebral artery occlusion (tMCAO) was performed on Epac1-deficient (Epac1 (-/-)) mice, Epac2-deficient (Epac2 (-/-)) mice, and their wild type counterparts (Epac1 (+/+) and Epac2 (+/+)). Two-hour occlusion and 22-hour reperfusion were conducted to induce ischemia/reperfusion injury to the retina. After tMCAO, the contralateral retinae displayed similar morphology between different genotypes. Neuronal loss, retinal edema and increase in immunoreactivity for aquaporin 4 (AQP4), glial fibrillary acidic protein (GFAP), peroxiredoxin 6 (Prx6) were observed in ipsilateral retinae. Epac2 (-/-) ipsilateral retinae showed more neuronal loss in retinal ganglion cell layer, increased retinal thickness and stronger immunostaining of AQP4, GFAP, and Prx6 than those of Epac2 (+/+). However, Epac1 (-/-) ipsilateral retinae displayed similar pathology as those in Epac1 (+/+) mice. Our observations suggest that Epac2-deficiency led to more severe ischemic retinopathy after retinal ischemia/reperfusion injury.

Neuroglobin protects cardiomyocytes against apoptosis and cardiac hypertrophy induced by isoproterenol in rats

Neuroglobin (Ngb) is well known as a physiological role in oxygen homeostasis of neurons and perhaps a protective role against hypoxia and oxidative stress. In this study, we found that Ngb is expressed in rat heart tissues and it is related to isoproterenol induced cardiac hypertrophy. Moreover, overexpression or knock-down of Ngb influences the expression of hypertrophic markers ANP and BNP and the ratio of hypertrophic cells in rat H9c2 myoblasts when isoproterenol treatment. The Annexin V-FITC/PI Staining, Western blot and qPCR analysis showed that the involvement in p53-mediated apoptosis of cardiomyocytes of Ngb is might be the mechanism. This protein could prevent the cells against ROS and POS-induced apoptosis not only in nervous systems but also in cardiomyocytes. From the results, it is concluded that Ngb is a promising protectant in the cardiac hypertrophy, it may be a candidate target to cardiac hypertrophy for clinic treatment.

Prostaglandin E2 EP2 receptor deletion attenuates intracerebral hemorrhage-induced brain injury and improves functional recovery

Intracerebral hemorrhage (ICH) is a devastating type of stroke characterized by bleeding into the brain parenchyma and secondary brain injury resulting from strong neuroinflammatory responses to blood components. Production of prostaglandin E₂ (PGE₂) is significantly upregulated following ICH and contributes to this inflammatory response in part through its E prostanoid receptor subtype 2 (EP2). Signaling through the EP2 receptor has been shown to affect outcomes of many acute and chronic neurological disorders; although, not yet explored in the context of ICH. Wildtype (WT) and EP2 receptor knockout (EP2^−/−) mice were subjected to ICH, and various anatomical and functional outcomes were assessed by histology and neurobehavioral testing, respectively. When compared with age-matched WT controls, EP2^−/− mice had 41.9 ± 4.7% smaller ICH-induced brain lesions and displayed significantly less ipsilateral hemispheric enlargement and incidence of intraventricular hemorrhage. Anatomical outcomes correlated with improved functional recovery as identified by neurological deficit scoring. Histological staining was performed to begin investigating the mechanisms involved in EP2-mediated neurotoxicity after ICH. EP2^−/− mice exhibited 45.5 ± 5.8% and 41.4 ± 8.1% less blood and ferric iron accumulation, respectively. Furthermore, significantly less striatal and cortical microgliosis, striatal and cortical astrogliosis, blood–brain barrier breakdown, and peripheral neutrophil infiltration were seen in EP2^−/− mice. This study is the first to suggest a deleterious role for the PGE₂-EP2 signaling axis in modulating brain injury, inflammation, and functional recovery following ICH. Targeting the EP2 G protein-coupled receptor may represent a new therapeutic avenue for the treatment of hemorrhagic stroke.

Neuroprotective role of prostaglandin PGE2 EP2 receptor in hemin-mediated toxicity

Heme (Fe(2+) protoporphyrin IX) and hemin (Fe(3+)), the prosthetic group of hemoprotein, are cytotoxic due to their ability to contribute to the production of reactive oxygen species, increased intracellular calcium levels, and stimulate glutamate-mediated excitotoxicity. Previous work by our group showed that blockade of the prostaglandin E2 (PGE2)-EP1 receptor reduced hemin-induced cytotoxicity in primary cortical neuronal cultures. However, the role of the prostaglandin E2 (PGE2)-EP2 receptor in hemin neurotoxicity remains unclear. Activation of the EP2 receptor in neurons results in increased cyclic AMP (cAMP) and protein kinase A signaling; therefore, we hypothesized that the activation of the EP2 receptor decreases hemin neurotoxicity. Using postnatal primary cortical neurons cultured from wildtype-control (WT) and EP2(-/-) mice, we investigated the role of the EP2 receptor in hemin neurotoxicity by monitoring cell survival with the Calcein-AM live-cell and lactate dehydrogenase assays. MitoTracker staining was also performed to determine how mitochondria were affected by hemin. Hemin neurotoxicity in EP2(-/-) neurons was 37.2 ± 17.0% greater compared to WT neurons. Of interest, cotreatment with the EP2 receptor agonist, butaprost (1 and 10 μM), significantly attenuated hemin neurotoxicity by 55.7 ± 21.1% and 60.1 ± 14.8%, respectively. To further investigate signaling mechanisms related to EP2 receptor mediating cytoprotection, neurons were cotreated with hemin and activators/inhibitors of both the cAMP-protein kinase A/exchange protein directly activated by cAMP (Epac) pathways. Forskolin, a cAMP activator, and 8-pCPT-cAMP, an Epac activator, both attenuated hemin neurotoxicity by 78.8 ± 22.2% and 58.4 ± 9.8%, respectively, as measured using the lactate dehydrogenase assay. Together, the results reveal that activation of the EP2 receptor is protective against hemin neurotoxicity in vitro and these findings suggest that neuroprotection occurs through the cAMP-Epac pathway in neuronal cultures. Therefore, activation of the EP2 receptor could be used to minimize neuronal damage following exposure to supraphysiological levels of hemin.

Role of cyclic AMP sensor Epac1 in masseter muscle hypertrophy and myosin heavy chain transition induced by β2-adrenoceptor stimulation

The predominant isoform of β-adrenoceptor (β-AR) in skeletal muscle is β2-AR and that in the cardiac muscle is β1-AR. We have reported that Epac1 (exchange protein directly activated by cAMP 1), a new protein kinase A-independent cAMP sensor, does not affect cardiac hypertrophy in response to pressure overload or chronic isoproterenol (isoprenaline) infusion. However, the role of Epac1 in skeletal muscle hypertrophy remains poorly understood. We thus examined the effect of disruption of Epac1, the major Epac isoform in skeletal muscle, on masseter muscle hypertrophy induced by chronic β2-AR stimulation with clenbuterol (CB) in Epac1-null mice (Epac1KO). The masseter muscle weight/tibial length ratio was similar in wild-type (WT) and Epac1KO at baseline and was significantly increased in WT after CB infusion, but this increase was suppressed in Epac1KO. CB treatment significantly increased the proportion of myosin heavy chain (MHC) IIb at the expense of that of MHC IId/x in both WT and Epac1KO, indicating that Epac1 did not mediate the CB-induced MHC isoform transition towards the faster isoform. The mechanism of suppression of CB-mediated hypertrophy in Epac1KO is considered to involve decreased activation of Akt signalling. In addition, CB-induced histone deacetylase 4 (HDAC4) phosphorylation on serine 246 mediated by calmodulin kinase II (CaMKII), which plays a role in skeletal muscle hypertrophy, was suppressed in Epac1KO. Our findings suggest that Epac1 plays a role in β2-AR-mediated masseter muscle hypertrophy, probably through activation of both Akt signalling and CaMKII/HDAC4 signalling.

Epac1-dependent phospholamban phosphorylation mediates the cardiac response to stresses

PKA phosphorylates multiple molecules involved in calcium (Ca2+) handling in cardiac myocytes and is considered to be the predominant regulator of β-adrenergic receptor-mediated enhancement of cardiac contractility; however, recent identification of exchange protein activated by cAMP (EPAC), which is independently activated by cAMP, has challenged this paradigm. Mice lacking Epac1 (Epac1 KO) exhibited decreased cardiac contractility with reduced phospholamban (PLN) phosphorylation at serine-16, the major PKA-mediated phosphorylation site. In Epac1 KO mice, intracellular Ca2+ storage and the magnitude of Ca2+ movement were decreased; however, PKA expression remained unchanged, and activation of PKA with isoproterenol improved cardiac contractility. In contrast, direct activation of EPAC in cardiomyocytes led to increased PLN phosphorylation at serine-16, which was dependent on PLC and PKCε. Importantly, Epac1 deletion protected the heart from various stresses, while Epac2 deletion was not protective. Compared with WT mice, aortic banding induced a similar degree of cardiac hypertrophy in Epac1 KO; however, lack of Epac1 prevented subsequent cardiac dysfunction as a result of decreased cardiac myocyte apoptosis and fibrosis. Similarly, Epac1 KO animals showed resistance to isoproterenol- and aging-induced cardiomyopathy and attenuation of arrhythmogenic activity. These data support Epac1 as an important regulator of PKA-independent PLN phosphorylation and indicate that Epac1 regulates cardiac responsiveness to various stresses.

Balancing GRK2 and EPAC1 levels prevents and relieves chronic pain

Chronic pain is a major clinical problem, yet the mechanisms underlying the transition from acute to chronic pain remain poorly understood. In mice, reduced expression of GPCR kinase 2 (GRK2) in nociceptors promotes cAMP signaling to the guanine nucleotide exchange factor EPAC1 and prolongs the PGE2-induced increase in pain sensitivity (hyperalgesia). Here we hypothesized that reduction of GRK2 or increased EPAC1 in dorsal root ganglion (DRG) neurons would promote the transition to chronic pain. We used 2 mouse models of hyperalgesic priming in which the transition from acute to chronic PGE2-induced hyperalgesia occurs. Hyperalgesic priming with carrageenan induced a sustained decrease in nociceptor GRK2, whereas priming with the PKCε agonist ΨεRACK increased DRG EPAC1. When either GRK2 was increased in vivo by viral-based gene transfer or EPAC1 was decreased in vivo, as was the case for mice heterozygous for Epac1 or mice treated with Epac1 antisense oligodeoxynucleotides, chronic PGE2-induced hyperalgesia development was prevented in the 2 priming models. Using the CFA model of chronic inflammatory pain, we found that increasing GRK2 or decreasing EPAC1 inhibited chronic hyperalgesia. Our data suggest that therapies targeted at balancing nociceptor GRK2 and EPAC1 levels have promise for the prevention and treatment of chronic pain.

Prostaglandin receptor EP2 in the crosshairs of anti-inflammation, anti-cancer, and neuroprotection

Modulation of a specific prostanoid synthase or receptor provides therapeutic alternatives to nonsteroidal anti-inflammatory drugs (NSAIDs) for treating pathological conditions governed by cyclooxygenase-2 (COX-2 or PTGS2). Among the COX-2 downstream signaling pathways, the prostaglandin E2 (PGE2) receptor EP2 subtype (PTGER2) is emerging as a crucial mediator of many physiological and pathological events. Genetic ablation strategies and recent advances in chemical biology provide tools for a better understanding of EP2 signaling. In the brain, the EP2 receptor modulates some beneficial effects, including neuroprotection, in acute models of excitotoxicity, neuroplasticity, and spatial learning via cAMP-PKA signaling. Conversely, EP2 activation accentuates chronic inflammation mainly through the cAMP-Epac pathway, likely contributing to delayed neurotoxicity. EP2 receptor activation also engages β-arrestin in a G-protein-independent pathway that promotes tumor cell growth and migration. Understanding the conditions under which multiple EP2 signaling pathways are engaged might suggest novel therapeutic strategies to target this key inflammatory prostaglandin receptor.

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.

A role for Piezo2 in EPAC1-dependent mechanical allodynia

Aberrant mechanosensation has an important role in different pain states. Here we show that Epac1 (cyclic AMP sensor) potentiation of Piezo2-mediated mechanotransduction contributes to mechanical allodynia. Dorsal root ganglia Epac1 mRNA levels increase during neuropathic pain, and nerve damage-induced allodynia is reduced in Epac1-/- mice. The Epac-selective cAMP analogue 8-pCPT sensitizes mechanically evoked currents in sensory neurons. Human Piezo2 produces large mechanically gated currents that are enhanced by the activation of the cAMP-sensor Epac1 or cytosolic calcium but are unaffected by protein kinase C or protein kinase A and depend on the integrity of the cytoskeleton. In vivo, 8-pCPT induces long-lasting allodynia that is prevented by the knockdown of Epac1 and attenuated by mouse Piezo2 knockdown. Piezo2 knockdown also enhanced thresholds for light touch. Finally, 8-pCPT sensitizes responses to innocuous mechanical stimuli without changing the electrical excitability of sensory fibres. These data indicate that the Epac1-Piezo2 axis has a role in the development of mechanical allodynia during neuropathic pain.

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.

Multiple facets of cAMP signalling and physiological impact: cAMP compartmentalization in the lung

Therapies involving elevation of the endogenous suppressor cyclic AMP (cAMP) are currently used in the treatment of several chronic inflammatory disorders, including chronic obstructive pulmonary disease (COPD). Characteristics of COPD are airway obstruction, airway inflammation and airway remodelling, processes encompassed by increased airway smooth muscle mass, epithelial changes, goblet cell and submucosal gland hyperplasia. In addition to inflammatory cells, airway smooth muscle cells and (myo)fibroblasts, epithelial cells underpin a variety of key responses in the airways such as inflammatory cytokine release, airway remodelling, mucus hypersecretion and airway barrier function. Cigarette smoke, being next to environmental pollution the main cause of COPD, is believed to cause epithelial hyperpermeability by disrupting the barrier function. Here we will focus on the most recent progress on compartmentalized signalling by cAMP. In addition to G protein-coupled receptors, adenylyl cyclases, cAMP-specific phospho-diesterases (PDEs) maintain compartmentalized cAMP signalling. Intriguingly, spatially discrete cAMP-sensing signalling complexes seem also to involve distinct members of the A-kinase anchoring (AKAP) superfamily and IQ motif containing GTPase activating protein (IQGAPs). In this review, we will highlight the interaction between cAMP and the epithelial barrier to retain proper lung function and to alleviate COPD symptoms and focus on the possible molecular mechanisms involved in this process. Future studies should include the development of cAMP-sensing multiprotein complex specific disruptors and/or stabilizers to orchestrate cellular functions. Compartmentalized cAMP signalling regulates important cellular processes in the lung and may serve as a therapeutic target.

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.