Function of Adenylyl Cyclase in Heart: the AKAP Connection

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

POPDC1 scaffolds a complex of adenylyl cyclase 9 and the potassium channel TREK-1 in heart

The establishment of macromolecular complexes by scaffolding proteins is key to the local production of cAMP by anchored adenylyl cyclase (AC) and the subsequent cAMP signaling necessary for cardiac functions. We identify a novel AC scaffold, the Popeye domain-containing (POPDC) protein. The POPDC family of proteins is important for cardiac pacemaking and conduction, due in part to their cAMP-dependent binding and regulation of TREK-1 potassium channels. We show that TREK-1 binds the AC9:POPDC1 complex and copurifies in a POPDC1-dependent manner with AC9 activity in heart. Although the AC9:POPDC1 interaction is cAMP-independent, TREK-1 association with AC9 and POPDC1 is reduced upon stimulation of the β-adrenergic receptor (βAR). AC9 activity is required for βAR reduction of TREK-1 complex formation with AC9:POPDC1 and in reversing POPDC1 enhancement of TREK-1 currents. Finally, deletion of the gene-encoding AC9 (Adcy9) gives rise to bradycardia at rest and stress-induced heart rate variability, a milder phenotype than the loss of Popdc1 but similar to the loss of Kcnk2 (TREK-1). Thus, POPDC1 represents a novel adaptor for AC9 interactions with TREK-1 to regulate heart rate control.

Preferential Expression of Ca2+-Stimulable Adenylyl Cyclase III in the Supraventricular Area, Including Arrhythmogenic Pulmonary Vein of the Rat Heart

Ectopic excitability in pulmonary veins (PVs) is the major cause of atrial fibrillation. We previously reported that the inositol trisphosphate receptor in rat PV cardiomyocytes cooperates with the Na+-Ca2+ exchanger to provoke ectopic automaticity in response to norepinephrine. Here, we focused on adenylyl cyclase (AC) as another effector of norepinephrine stimulation. RT-PCR, immunohistochemistry, and Western blotting revealed that the abundant expression of Ca2+-stimulable AC3 was restricted to the supraventricular area, including the PVs. All the other AC isotypes hardly displayed any region-specific expressions. Immunostaining of isolated cardiomyocytes showed an enriched expression of AC3 along the t-tubules in PV myocytes. The cAMP-dependent response of L-type Ca2+ currents in the PV and LA cells is strengthened by the 0.1 mM intracellular Ca2+ condition, unlike in the ventricular cells. The norepinephrine-induced automaticity of PV cardiomyocytes was reversibly suppressed by 100 µM SQ22536, an adenine-like AC inhibitor. These findings suggest that the specific expression of AC3 along t-tubules may contribute to arrhythmogenic automaticity in rat PV cardiomyocytes.

Subcellular Propagation of Cardiomyocyte β-Adrenergic Activation of Calcium Uptake Involves Internal β-Receptors and AKAP7

β-adrenergic receptor (β-AR) signaling in cardiac myocytes is central to cardiac function, but spatiotemporal activation within myocytes is unresolved. In rabbit ventricular myocytes, β-AR agonists or high extracellular [Ca] were applied locally at one end, to measure β-AR signal propagation as Ca-transient (CaT) amplitude and sarcoplasmic reticulum (SR) Ca uptake. High local [Ca]_o, increased CaT amplitude under the pipette faster than did ISO, but was also more spatially restricted. Local isoproterenol (ISO) or norepinephrine (NE) increased CaT amplitude and SR Ca uptake, that spread along the myocyte to the unexposed end. Thus, local [Ca]_i decline kinetics reflect spatio-temporal progression of β-AR end-effects in myocytes. To test whether intracellular β-ARs contribute to this response, we used β-AR-blockers that are membrane permeant (propranolol) or not (sotalol). Propranolol completely blocked NE-dependent CaT effects. However, blocking surface β-ARs only (sotalol) suppressed only ∼50% of the NE-induced increase in CaT peak and rate of [Ca]_i decline, but these changes spread more gradually than NE alone. We also tested whether A-kinase anchoring protein 7γ (AKAP7γ; that interacts with phospholamban) is mobile, such that it might contribute to intracellular spatial propagation of β-AR signaling. We found AKAP7γ to be highly mobile using fluorescence recovery after photobleach of GFP tagged AKAP7γ, and that PKA activation accelerated AKAP7γ-GFP wash-out upon myocyte saponin-permeabilization, suggesting increased AKAP7γ mobility. We conclude that local β-AR activation can activate SR Ca uptake at remote myocyte sites, and that intracellular β-AR and AKAP7γ mobility may play a role in this spread of activation.

A Tale of Two New Targets for Hypertriglyceridaemia: Which Choice of Therapy?

Angiopoietin-like protein 3 (ANGPTL3) and apolipoprotein C-III (apoC-III) are novel metabolic targets for correcting hypertriglyceridaemia (HTG). As a background to their potential clinical use, we review the metabolic aetiology of HTG, particular abnormalities in triglyceride-rich lipoproteins (TRLs) and their role in atherosclerotic cardiovascular disease (ASCVD) and acute pancreatitis. Molecular and cardiometabolic aspects of ANGPTL3 and apoC-III, as well as inhibition of these targets with monoclonal antibody and nucleic acid therapies, are summarized as background information to descriptions and analyses of recent clinical trials. These studies suggest that ANGPTL3 and apoC-III inhibitors are equally potent in lowering elevated plasma triglycerides and TRLs across a wide range of concentrations, with possibly greater efficacy with inhibition of apoC-III. ANGPTL3 inhibition may, however, have the advantage of greater lowering of plasma LDL cholesterol and could specifically address elevated LDL cholesterol in familial hypercholesterolaemia refractory to standard drug therapies. Large clinical outcome trials in relevant populations are still required to confirm the long-term efficacy, safety and cost effectiveness of these potent agents for mitigating the complications of HTG. Beyond targeting severe chylomicronaemia in the prevention of acute pancreatitis, both agents could be useful in addressing residual risk of ASCVD due to TRLs in patients receiving best standard of care, including behavioural modifications, statins, ezetimibe, fibrates and proprotein convertase subtilisin/kexin type 9 inhibitors.

Emerging Role of cAMP/AMPK Signaling

The 5′-Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a natural energy sensor in mammalian cells that plays a key role in cellular and systemic energy homeostasis. At the cellular level, AMPK supports numerous processes required for energy and redox homeostasis, including mitochondrial biogenesis, autophagy, and glucose and lipid metabolism. Thus, understanding the pathways regulating AMPK activity is crucial for developing strategies to treat metabolic disorders. Mounting evidence suggests the presence of a link between cyclic AMP (cAMP) and AMPK signaling. cAMP signaling is known to be activated in circumstances of physiological and metabolic stress due to the release of stress hormones, such as adrenaline and glucagon, which is followed by activation of membrane-bound adenylyl cyclase and elevation of cellular cAMP. Because the majority of physiological stresses are associated with elevated energy consumption, it is not surprising that activation of cAMP signaling may promote AMPK activity. Aside from the physiological role of the cAMP/AMPK axis, numerous reports have suggested its role in several pathologies, including inflammation, ischemia, diabetes, obesity, and aging. Furthermore, novel reports have provided more mechanistic insight into the regulation of the cAMP/AMPK axis. In particular, the role of distinct cAMP microdomains generated by soluble adenylyl cyclase in regulating basal and induced AMPK activity has recently been demonstrated. In the present review, we discuss current advances in the understanding of the regulation of the cAMP/AMPK axis and its role in cellular homeostasis and explore some translational aspects.

The Role of POPDC Proteins in Cardiac Pacemaking and Conduction

The Popeye domain-containing (POPDC) gene family, consisting of Popdc1 (also known as Bves), Popdc2, and Popdc3, encodes transmembrane proteins abundantly expressed in striated muscle. POPDC proteins have recently been identified as cAMP effector proteins and have been proposed to be part of the protein network involved in cAMP signaling. However, their exact biochemical activity is presently poorly understood. Loss-of-function mutations in animal models causes abnormalities in skeletal muscle regeneration, conduction, and heart rate adaptation after stress. Likewise, patients carrying missense or nonsense mutations in POPDC genes have been associated with cardiac arrhythmias and limb-girdle muscular dystrophy. In this review, we introduce the POPDC protein family, and describe their structure function, and role in cAMP signaling. Furthermore, the pathological phenotypes observed in zebrafish and mouse models and the clinical and molecular pathologies in patients carrying POPDC mutations are described.

Physiological Roles of Mammalian Transmembrane Adenylyl Cyclase Isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors. The transmembrane ACs display varying expression patterns across tissues, giving potential for them having a wide array of physiologic roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gα_s so it was long assumed that all ACs are activated by Gα_s-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

AKAP79 enables calcineurin to directly suppress protein kinase A activity

Interplay between the second messengers cAMP and Ca2+ is a hallmark of dynamic cellular processes. A common motif is the opposition of the Ca2+-sensitive phosphatase calcineurin and the major cAMP receptor, protein kinase A (PKA). Calcineurin dephosphorylates sites primed by PKA to bring about changes including synaptic long-term depression (LTD). AKAP79 supports signaling of this type by anchoring PKA and calcineurin in tandem. In this study, we discovered that AKAP79 increases the rate of calcineurin dephosphorylation of type II PKA regulatory subunits by an order of magnitude. Fluorescent PKA activity reporter assays, supported by kinetic modeling, show how AKAP79-enhanced calcineurin activity enables suppression of PKA without altering cAMP levels by increasing PKA catalytic subunit capture rate. Experiments with hippocampal neurons indicate that this mechanism contributes toward LTD. This non-canonical mode of PKA regulation may underlie many other cellular processes.

Dual Activation of Phosphodiesterase 3 and 4 Regulates Basal Cardiac Pacemaker Function and Beyond

The sinoatrial (SA) node is the physiological pacemaker of the heart, and resting heart rate in humans is a well-known risk factor for cardiovascular disease and mortality. Consequently, the mechanisms of initiating and regulating the normal spontaneous SA node beating rate are of vital importance. Spontaneous firing of the SA node is generated within sinoatrial nodal cells (SANC), which is regulated by the coupled-clock pacemaker system. Normal spontaneous beating of SANC is driven by a high level of cAMP-mediated PKA-dependent protein phosphorylation, which rely on the balance between high basal cAMP production by adenylyl cyclases and high basal cAMP degradation by cyclic nucleotide phosphodiesterases (PDEs). This diverse class of enzymes includes 11 families and PDE3 and PDE4 families dominate in both the SA node and cardiac myocardium, degrading cAMP and, consequently, regulating basal cardiac pacemaker function and excitation-contraction coupling. In this review, we will demonstrate similarities between expression, distribution, and colocalization of various PDE subtypes in SANC and cardiac myocytes of different species, including humans, focusing on PDE3 and PDE4. Here, we will describe specific targets of the coupled-clock pacemaker system modulated by dual PDE3 + PDE4 activation and provide evidence that concurrent activation of PDE3 + PDE4, operating in a synergistic manner, regulates the basal cardiac pacemaker function and provides control over normal spontaneous beating of SANCs through (PDE3 + PDE4)-dependent modulation of local subsarcolemmal Ca²⁺ releases (LCRs).

Dual Activation of Phosphodiesterase 3 and 4 Regulates Basal Cardiac Pacemaker Function and Beyond

The sinoatrial (SA) node is the physiological pacemaker of the heart, and resting heart rate in humans is a well-known risk factor for cardiovascular disease and mortality. Consequently, the mechanisms of initiating and regulating the normal spontaneous SA node beating rate are of vital importance. Spontaneous firing of the SA node is generated within sinoatrial nodal cells (SANC), which is regulated by the coupled-clock pacemaker system. Normal spontaneous beating of SANC is driven by a high level of cAMP-mediated PKA-dependent protein phosphorylation, which rely on the balance between high basal cAMP production by adenylyl cyclases and high basal cAMP degradation by cyclic nucleotide phosphodiesterases (PDEs). This diverse class of enzymes includes 11 families and PDE3 and PDE4 families dominate in both the SA node and cardiac myocardium, degrading cAMP and, consequently, regulating basal cardiac pacemaker function and excitation-contraction coupling. In this review, we will demonstrate similarities between expression, distribution, and colocalization of various PDE subtypes in SANC and cardiac myocytes of different species, including humans, focusing on PDE3 and PDE4. Here, we will describe specific targets of the coupled-clock pacemaker system modulated by dual PDE3 + PDE4 activation and provide evidence that concurrent activation of PDE3 + PDE4, operating in a synergistic manner, regulates the basal cardiac pacemaker function and provides control over normal spontaneous beating of SANCs through (PDE3 + PDE4)-dependent modulation of local subsarcolemmal Ca²⁺ releases (LCRs).

Cross-Talk Between the Adenylyl Cyclase/cAMP Pathway and Ca2+ Homeostasis

Cyclic AMP and Ca²⁺ are the first second or intracellular messengers identified, unveiling the cellular mechanisms activated by a plethora of extracellular signals, including hormones. Cyclic AMP generation is catalyzed by adenylyl cyclases (ACs), which convert ATP into cAMP and pyrophosphate. By the way, Ca²⁺, as energy, can neither be created nor be destroyed; Ca²⁺ can only be transported, from one compartment to another, or chelated by a variety of Ca²⁺-binding molecules. The fine regulation of cytosolic concentrations of cAMP and free Ca²⁺ is crucial in cell function and there is an intimate cross-talk between both messengers to fine-tune the cellular responses. Cancer is a multifactorial disease resulting from a combination of genetic and environmental factors. Frequent cases of cAMP and/or Ca²⁺ homeostasis remodeling have been described in cancer cells. In those tumoral cells, cAMP and Ca²⁺ signaling plays a crucial role in the development of hallmarks of cancer, including enhanced proliferation and migration, invasion, apoptosis resistance, or angiogenesis. This review summarizes the cross-talk between the ACs/cAMP and Ca²⁺ intracellular pathways with special attention to the functional and reciprocal regulation between Orai1 and AC8 in normal and cancer cells.

Cardiac cAMP-PKA Signaling Compartmentalization in Myocardial Infarction

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

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

Cell Communications among Microorganisms, Plants, and Animals: Origin, Evolution, and Interplays

Cellular communications play pivotal roles in multi-cellular species, but they do so also in uni-cellular species. Moreover, cells communicate with each other not only within the same individual, but also with cells in other individuals belonging to the same or other species. These communications occur between two unicellular species, two multicellular species, or between unicellular and multicellular species. The molecular mechanisms involved exhibit diversity and specificity, but they share common basic features, which allow common pathways of communication between different species, often phylogenetically very distant. These interactions are possible by the high degree of conservation of the basic molecular mechanisms of interaction of many ligand-receptor pairs in evolutionary remote species. These inter-species cellular communications played crucial roles during Evolution and must have been positively selected, particularly when collectively beneficial in hostile environments. It is likely that communications between cells did not arise after their emergence, but were part of the very nature of the first cells. Synchronization of populations of non-living protocells through chemical communications may have been a mandatory step towards their emergence as populations of living cells and explain the large commonality of cell communication mechanisms among microorganisms, plants, and animals.

Therapeutic Implications for PDE2 and cGMP/cAMP Mediated Crosstalk in Cardiovascular Diseases

Phosphodiesterases (PDEs) are the principal superfamily of enzymes responsible for degrading the secondary messengers 3',5'-cyclic nucleotides cAMP and cGMP. Their refined subcellular localization and substrate specificity contribute to finely regulate cAMP/cGMP gradients in various cellular microdomains. Redistribution of multiple signal compartmentalization components is often perceived under pathological conditions. Thereby PDEs have long been pursued as therapeutic targets in diverse disease conditions including neurological, metabolic, cancer and autoimmune disorders in addition to numerous cardiovascular diseases (CVDs). PDE2 is a unique member of the broad family of PDEs. In addition to its capability to hydrolyze both cAMP and cGMP, PDE2 is the sole isoform that may be allosterically activated by cGMP increasing its cAMP hydrolyzing activity. Within the cardiovascular system, PDE2 serves as an integral regulator for the crosstalk between cAMP/cGMP pathways and thereby may couple chronically adverse augmented cAMP signaling with cardioprotective cGMP signaling. This review provides a comprehensive overview of PDE2 regulatory functions in multiple cellular components within the cardiovascular system and also within various subcellular microdomains. Implications for PDE2- mediated crosstalk mechanisms in diverse cardiovascular pathologies are discussed highlighting the prospective use of PDE2 as a potential therapeutic target in cardiovascular disorders.

Autonomic modulation of sinoatrial node: Role of pacemaker current and calcium sensitive adenylyl cyclase isoforms

This article reviews work over the past three decades that is related to the contribution of the pacemaker current, I_f, to basal and autonomically regulated spontaneous rate in the sinoatrial node. It also addresses how the actions of the pacemaker current relate to those of Ca homeostasis with respect to basal and autonomically regulated rhythm. In this regard, it explores the relative contributions of Ca-sensitive and Ca-insensitive isoforms of adenylyl cyclase to sinoatrial node automaticity. The latter studies include previously unpublished work making use of mice in which both the type 1 and type 8 Ca-sensitive adenylyl cyclase isoforms were knocked out. These studies indicate that the pacemaker current and the L-type Ca current are distinctly influenced by Ca-sensitive and insensitive adenylyl cyclase isoforms.

Nanometric targeting of type 9 adenylyl cyclase in heart

Adenylyl cyclases (ACs) convert ATP into the classical second messenger cyclic adenosine monophosphate (cAMP). Cardiac ACs, specifically AC5, AC6, and AC9, regulate cAMP signaling controlling functional outcomes such as heart rate, contractility and relaxation, gene regulation, stress responses, and glucose and lipid metabolism. With so many distinct functional outcomes for a single second messenger, the cell creates local domains of cAMP signaling to correctly relay signals. Targeting of ACs to A-kinase anchoring proteins (AKAPs) not only localizes ACs, but also places them within signaling nanodomains, where cAMP levels and effects can be highly regulated. Here we will discuss the recent work on the structure, regulation and physiological functions of AC9 in the heart, where it accounts for <3% of total AC activity. Despite the small contribution of AC9 to total cardiac cAMP production, AC9 binds and regulates local PKA phosphorylation of Yotiao-IKs and Hsp20, demonstrating a role for nanometric targeting of AC9.

The role of A-kinase anchoring proteins in cardiac oxidative stress

Cardiac stress initiates a pathological remodeling process that is associated with cardiomyocyte loss and fibrosis that ultimately leads to heart failure. In the injured heart, a pathologically elevated synthesis of reactive oxygen species (ROS) is the main driver of oxidative stress and consequent cardiomyocyte dysfunction and death. In this context, the cAMP-dependent protein kinase (PKA) plays a central role in regulating signaling pathways that protect the heart against ROS-induced cardiac damage. In cardiac cells, spatiotemporal regulation of PKA activity is controlled by A-kinase anchoring proteins (AKAPs). This family of scaffolding proteins tether PKA and other transduction enzymes at subcellular microdomains where they can co-ordinate cellular responses regulating oxidative stress. In this review, we will discuss recent literature illustrating the role of PKA and AKAPs in modulating the detrimental impact of ROS production on cardiac function.

Imaging cAMP nanodomains in the heart

3′-5′-cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger that modulates multiple cellular functions. It is now well established that cAMP can mediate a plethora of functional effects via a complex system of local regulatory mechanisms that result in compartmentalized signalling. The use of fluorescent probes to monitor cAMP in intact, living cells have been instrumental in furthering our appreciation of this ancestral and ubiquitous pathway and unexpected details of the nano-architecture of the cAMP signalling network are starting to emerge. Recent evidence shows that sympathetic control of cardiac contraction and relaxation is achieved via generation of multiple, distinct pools of cAMP that lead to differential phosphorylation of target proteins localized only tens of nanometres apart. The specific local control at these nanodomains is enabled by a distinct signalosome where effectors, targets, and regulators of the cAMP signal are clustered. In this review, we focus on recent advances using targeted fluorescent reporters for cAMP and how they have contributed to our current understanding of nanodomain cAMP signalling in the heart. We briefly discuss how this information can be exploited to design novel therapies and we highlight some of the questions that remain unanswered.

Whole-Cell cAMP and PKA Activity are Epiphenomena, Nanodomain Signaling Matters

Novel targeted fluorescent biosensors provide key insights into very local nanodomains of cAMP and PKA activity, and how they respond differently to β-adrenergic activation in cardiac myocytes. This unique spatiotemporal detail in living cells is not available with biochemical measurements of total cellular cAMP and PKA, and provides unique physiological insights.

The Role of Cyclic AMP Signaling in Cardiac Fibrosis

Myocardial stress and injury invariably promote remodeling of the cardiac tissue, which is associated with cardiomyocyte death and development of fibrosis. The fibrotic process is initially triggered by the differentiation of resident cardiac fibroblasts into myofibroblasts. These activated fibroblasts display increased proliferative capacity and secrete large amounts of extracellular matrix. Uncontrolled myofibroblast activation can thus promote heart stiffness, cardiac dysfunction, arrhythmias, and progression to heart failure. Despite the well-established role of myofibroblasts in mediating cardiac disease, our current knowledge on how signaling pathways promoting fibrosis are regulated and coordinated in this cell type is largely incomplete. In this respect, cyclic adenosine monophosphate (cAMP) signaling acts as a major modulator of fibrotic responses activated in fibroblasts of injured or stressed hearts. In particular, accumulating evidence now suggests that upstream cAMP modulators including G protein-coupled receptors, adenylyl cyclases (ACs), and phosphodiesterases (PDEs); downstream cAMP effectors such as protein kinase A (PKA) and the guanine nucleotide exchange factor Epac; and cAMP signaling organizers such as A-kinase anchoring proteins (AKAPs) modulate a variety of fundamental cellular processes involved in myocardial fibrosis including myofibroblast differentiation, proliferation, collagen secretion, and invasiveness. The current review will discuss recent advances highlighting the role of cAMP and AKAP-mediated signaling in regulating pathophysiological responses controlling cardiac fibrosis.

The Role of the Popeye Domain Containing Gene Family in Organ Homeostasis

The Popeye domain containing (POPDC) gene family consists of POPDC1 (also known as BVES), POPDC2 and POPDC3 and encodes a novel class of cyclic adenosine monophosphate (cAMP) effector proteins. Despite first reports of their isolation and initial characterization at the protein level dating back 20 years, only recently major advances in defining their biological functions and disease association have been made. Loss-of-function experiments in mice and zebrafish established an important role in skeletal muscle regeneration, heart rhythm control and stress signaling. Patients suffering from muscular dystrophy and atrioventricular block were found to carry missense and nonsense mutations in either of the three POPDC genes, which suggests an important function in the control of striated muscle homeostasis. However, POPDC genes are also expressed in a number of epithelial cells and function as tumor suppressor genes involved in the control of epithelial structure, tight junction formation and signaling. Suppression of POPDC genes enhances tumor cell proliferation, migration, invasion and metastasis in a variety of human cancers, thus promoting a malignant phenotype. Moreover, downregulation of POPDC1 and POPDC3 expression in different cancer types has been associated with poor prognosis. However, high POPDC3 expression has also been correlated to poor clinical prognosis in head and neck squamous cell carcinoma, suggesting that POPDC3 potentially plays different roles in the progression of different types of cancer. Interestingly, a gain of POPDC1 function in tumor cells inhibits cell proliferation, migration and invasion thereby reducing malignancy. Furthermore, POPDC proteins have been implicated in the control of cell cycle genes and epidermal growth factor and Wnt signaling. Work in tumor cell lines suggest that cyclic nucleotide binding may also be important in epithelial cells. Thus, POPDC proteins have a prominent role in tissue homeostasis and cellular signaling in both epithelia and striated muscle.

Airway Exposure to Modified Multi-walled Carbon Nanotubes Perturbs Cardiovascular Adenosinergic Signaling in Mice

The broad list of commercial applications for multi-walled carbon nanotubes (MWCNT) can be further expanded with the addition of various surface chemistry modifications. For example, standard commercial grade MWCNT (C-grade) can be carboxylated (COOH) or nitrogen-doped (N-doped) to suite specific utilities. We previously reported dose-dependent expansions of cardiac ischemia/reperfusion (I/R) injury, 24 h after intratracheal instillation of C-grade, COOH, or N-doped MWCNT in mice. Here, we have tested the hypothesis that airway exposure to MWCNT perturbs cardiovascular adenosinergic signaling, which could contribute to exacerbation of cardiac I/R injury. 100 µL of Vehicle or identical suspension volumes containing 100 µg of C-grade, COOH, or N-doped MWCNT were instilled into the trachea of CD-1 ICR mice. 1 day later, we measured cyclic adenosine monophosphate (cAMP) concentrations in cardiac tissue and evaluated arterial adenosinergic smooth muscle signaling mechanisms related to nitric oxide synthase (NOS) and cyclooxygenase (COX) in isolated aortic tissue. We also verified cardiac I/R injury expansion and examined both lung histology and bronchoalveolar lavage fluid cellularity in MWCNT exposed mice. Myocardial cAMP concentrations were reduced (p < 0.05) in the C-grade group by 17.4% and N-doped group by 13.7% compared to the Vehicle group. Curve fits to aortic ring 2-Cl-Adenosine concentration responses were significantly greater in the MWCNT groups vs. the Vehicle group. Aortic constrictor responses were more pronounced with NOS inhibition and were abolished with COX inhibition. These findings indicate that addition of functional chemical moieties on the surface of MWCNT may alter the biological responses to exposure by influencing cardiovascular adenosinergic signaling and promoting cardiac injury.

The Popeye domain containing gene family encoding a family of cAMP-effector proteins with important functions in striated muscle and beyond

The Popeye domain containing (POPDC) gene family encodes a novel class of membrane-bound cyclic AMP effector proteins. POPDC proteins are abundantly expressed in cardiac and skeletal muscle. Consistent with its predominant expression in striated muscle, Popdc1 and Popdc2 null mutants in mouse and zebrafish develop cardiac arrhythmia and muscular dystrophy. Likewise, mutations in POPDC genes in patients have been associated with cardiac arrhythmia and muscular dystrophy phenotypes. A membrane trafficking function has been identified in this context. POPDC proteins have also been linked to tumour formation. Here, POPDC1 plays a role as a tumour suppressor by limiting c-Myc and WNT signalling. Currently, a common functional link between POPDC's role in striated muscle and as a tumour suppressor is lacking. We also discuss several alternative working models to better understand POPDC protein function.

Aspects of astrocytic cAMP signaling with an emphasis on the putative power of compartmentalized signals in health and disease

This review discusses aspects of known and putative compartmentalized 3',5'-cyclic adenosine monophosphate (cAMP) signaling in astrocytes, a cell type that has turned out to be a key player in brain physiology and pathology. cAMP has attracted less attention than Ca²⁺ in recent years, but could turn out to rival Ca²⁺ in its potential to drive cellular functions and responses to intra- and extracellular cues. Further, Ca²⁺ and cAMP are known to engage in extensive crosstalk and cAMP signals often take place within subcellular compartments revolving around multi-protein signaling complexes; however, we know surprisingly little about this in astrocytes. Here, we review aspects of astrocytic cAMP signaling, provide arguments for an increased interest in this subject, suggest possible future research directions within the field, and discuss putative drug targets.

Direct stimulation of adenylyl cyclase 9 by the fungicide imidazole miconazole

In mammals, nine genes encode trans-membrane adenylyl cyclase (tmAC) isoforms that synthesize the intracellular messenger compound cAMP from ATP. As cAMP is produced in virtually all types of cell, isoform-selective modulators of tmAC would have major research and therapeutic potential. This study investigated the effects of fungicide imidazoles previously shown to suppress cAMP production in various tissues on the activities of tmAC isoforms AC1, 2, or 9 stably expressed in human embryonic kidney 293 cells. Intact cells, as well as crude membranes, were exposed to various imidazoles or known stimulators of tmAC and the ensuing changes in the production of cAMP analyzed. In crude membranes, the activity of AC9 in the presence of GDP-β-S was enhanced by miconazole with an EC₅₀ of ~ 8 μM, while AC1 and AC2 were inhibited with an IC₅₀ of ~ 20 μM. Clotrimazole (10-100 μM) was an inhibitor of all the ACs tested. Substrate saturation analysis indicated that miconazole increased the V_max of AC9 by 3-fold while having no effect on the K_m. In intact cells, the effect of miconazole on cAMP production through AC9 was additive with that of isoproterenol. The stimulation of cAMP production by miconazole was inhibited by Ca²⁺, and this could be prevented by the calcineurin blocker FK506. In sum, activation of AC9 by miconazole is through a mechanism distinct from that of forskolin, activated G proteins, or the COOH-terminal mediated autoinhibition. However, it is subject to the AC9 isoform-specific inhibition by Ca²⁺/calcineurin. Differential modulation of mammalian tmAC paralogs appears to be achievable by an imidazole with phenylated side chains. Optimization of the lead compound and exploration of the underlying mechanism(s) of action in more detail could exploit this further.

Functional Significance of the Adcy10-Dependent Intracellular cAMP Compartments

Mounting evidence confirms the compartmentalized structure of evolutionarily conserved 3'⁻5'-cyclic adenosine monophosphate (cAMP) signaling, which allows for simultaneous participation in a wide variety of physiological functions and ensures specificity, selectivity and signal strength. One important player in cAMP signaling is soluble adenylyl cyclase (sAC). The intracellular localization of sAC allows for the formation of unique intracellular cAMP microdomains that control various physiological and pathological processes. This review is focused on the functional role of sAC-produced cAMP. In particular, we examine the role of sAC-cAMP in different cellular compartments, such as cytosol, nucleus and mitochondria.

The Popeye Domain Containing Genes and Their Function as cAMP Effector Proteins in Striated Muscle

The Popeye domain containing (POPDC) genes encode transmembrane proteins, which are abundantly expressed in striated muscle cells. Hallmarks of the POPDC proteins are the presence of three transmembrane domains and the Popeye domain, which makes up a large part of the cytoplasmic portion of the protein and functions as a cAMP-binding domain. Interestingly, despite the prediction of structural similarity between the Popeye domain and other cAMP binding domains, at the protein sequence level they strongly differ from each other suggesting an independent evolutionary origin of POPDC proteins. Loss-of-function experiments in zebrafish and mouse established an important role of POPDC proteins for cardiac conduction and heart rate adaptation after stress. Loss-of function mutations in patients have been associated with limb-girdle muscular dystrophy and AV-block. These data suggest an important role of these proteins in the maintenance of structure and function of striated muscle cells.