Cyclic AMP compartments and signaling specificity: role of cyclic nucleotide phosphodiesterases

Vinpocetine alleviated alveolar epithelial cells injury in experimental pulmonary fibrosis by targeting PPAR-γ/NLRP3/NF-κB and TGF-β1/Smad2/3 pathways

This study aimed to investigate the potential anti-fibrotic activity of vinpocetine in an experimental model of pulmonary fibrosis by bleomycin and in the MRC-5 cell line. Pulmonary fibrosis was induced in BALB/c mice by oropharyngeal aspiration of a single dose of bleomycin (5 mg/kg). The remaining induced animals received a daily dose of pirfenidone (as a standard anti-fibrotic drug) (300 mg/kg/PO) and vinpocetine (20 mg/kg/PO) on day 7 of the induction till the end of the experiment (day 21). The results of the experiment revealed that vinpocetine managed to alleviate the fibrotic endpoints by statistically improving (P ≤ 0.05) the weight index, histopathological score, reduced expression of fibrotic-related proteins in immune-stained lung sections, as well as fibrotic markers measured in serum samples. It also alleviated tissue levels of oxidative stress and inflammatory and pro-fibrotic mediators significantly elevated in bleomycin-only induced animals (P ≤ 0.05). Vinpocetine managed to express a remarkable attenuating effect in pulmonary fibrosis both in vivo and in vitro either directly by interfering with the classical TGF-β1/Smad2/3 signaling pathway or indirectly by upregulating the expression of Nrf2 enhancing the antioxidant system, activating PPAR-γ and downregulating the NLRP3/NF-κB pathway making it a candidate for further clinical investigation in cases of pulmonary fibrosis.

cAMP signaling: a remarkably regional affair

Louis Pasteur once famously said 'in the fields of observation chance favors only the prepared mind'. Much of chance is being in the right place at the right time. This is particularly true in the crowded molecular environment of the cell where being in the right place is often more important than timing. Although Brownian motion argues that enzymes will eventually bump into substrates, this probability is greatly enhanced if both molecules reside in the same subcellular compartment. However, activation of cell signaling enzymes often requires the transmission of chemical signals from extracellular stimuli to intracellular sites of action. This review highlights new developments in our understanding of cAMP generation and the 3D utilization of this second messenger inside cells.

Spatial organization of adenylyl cyclase and its impact on dopamine signaling in neurons

The cAMP cascade is widely recognized to transduce its physiological effects locally through spatially limited cAMP gradients. However, little is known about how the adenylyl cyclase enzymes, which initiate cAMP gradients, are localized. Here we answer this question in physiologically relevant striatal neurons and delineate how AC localization impacts downstream signaling functions. We show that the major striatal AC isoforms are differentially sorted between ciliary and extraciliary domains of the plasma membrane, and that AC9 is uniquely targeted to endosomes. We identify key sorting determinants in the N-terminal cytoplasmic domain responsible for isoform-specific localization. We also show that AC9-containing endosomes accumulate activated dopamine receptors and form an elaborately intertwined network with juxtanuclear PKA stores bound to Golgi membranes. Finally, we show that endosomal localization is critical for AC9 to selectively elevate PKA activity in the nucleus relative to the cytoplasm. These results reveal a precise spatial landscape of the cAMP cascade in neurons and a key role of AC localization in directing downstream signal transduction to the nucleus.

Next Generation PDE4 Inhibitors that Selectively Target PDE4B/D Subtypes: A Narrative Review

For decades, topical corticosteroids have been the mainstay of treatment for mild-to-moderate inflammatory skin diseases, even though only short-term use is approved for these agents and systemic inflammation is not addressed. Increased understanding of the immunopathogenesis of these conditions, especially for psoriasis and atopic dermatitis, has facilitated the development of antibody-based drugs that neutralize single key cytokines or their associated receptors, such as interleukin (IL)-17A/F, IL-23, and IL-17RA in psoriasis and IL-13 and IL-4Rα in atopic dermatitis. However, oral therapy is still preferred by many patients owing to the ease of use and needle-free administration. Phosphodiesterase 4 (PDE4) inhibitors have been approved for both oral and topical use for inflammatory skin diseases. In this review, we present a summary of an emerging class of selective PDE4B/D inhibitors under clinical development and compare the differences in selectivity of this new generation of PDE4 inhibitors with the less selective currently approved PDE4 inhibitors.

Fluorescent biosensor imaging meets deterministic mathematical modelling: quantitative investigation of signalling compartmentalization

Cells execute specific responses to diverse environmental cues by encoding information in distinctly compartmentalized biochemical signalling reactions. Genetically encoded fluorescent biosensors enable the spatial and temporal monitoring of signalling events in live cells. Temporal and spatiotemporal computational models can be used to interpret biosensor experiments in complex biochemical networks and to explore hypotheses that are difficult to test experimentally. In this review, we first provide brief discussions of the experimental toolkit of fluorescent biosensors as well as computational basics with a focus on temporal and spatiotemporal deterministic models. We then describe how we used this combined approach to identify and investigate a protein kinase A (PKA) - cAMP - Ca2+ oscillatory circuit in MIN6 β cells, a mouse pancreatic β cell system. We describe the application of this combined approach to interrogate how this oscillatory circuit is differentially regulated in a nano-compartment formed at the plasma membrane by the scaffolding protein A kinase anchoring protein 79/150. We leveraged both temporal and spatiotemporal deterministic models to identify the key regulators of this oscillatory circuit, which we confirmed with further experiments. The powerful approach of combining live-cell biosensor imaging with quantitative modelling, as discussed here, should find widespread use in the investigation of spatiotemporal regulation of cell signalling.

Cyclic AMP but Not Calmodulin as a Potential Wasoconstrictor in Simulated Reperfusion

The phenomena of ischemia and reperfusion are associated with the pathological background of cardiovascular diseases. Ischemia is initiated by ischemia reperfusion injury (IRI), which involves disruption of intracellular signaling pathways and causes cell death. The aim of this study was to assess the reactivity of vascular smooth muscle cells in the conditions of induced ischemia and reperfusion, and to determine the mechanisms leading to contractility disorders. This study was conducted using classical pharmacometric methods on an isolated model of the rat caudal artery. The experiment consisted of the analysis of the final and initial perfusate pressure measurements after induction of arterial contraction with phenylephrine in the presence of forskolin and A7 hydrochloride, two ligands modifying the contractility of vascular smooth muscle cells (VSMC). The pharmacometric analysis showed that in simulated reperfusion, cyclic nucleotides have a vasoconstrictive effect, and calmodulin has a vasodilating effect. The responsiveness of vascular smooth muscle cells to the vasopressor effects of α1-adrenomimetics during reperfusion may change uncontrollably, and the effects of secondary messengers may be counter physiological. Further studies are needed to evaluate the function of other second messengers on VSMCs in the process of ischemia and reperfusion.

Arrestin-dependent nuclear export of phosphodiesterase 4D promotes GPCR-induced nuclear cAMP signaling required for learning and memory

G protein-coupled receptors (GPCRs) promote the expression of immediate early genes required for learning and memory. Here, we showed that β2-adrenergic receptor (β2AR) stimulation induced the nuclear export of phosphodiesterase 4D5 (PDE4D5), an enzyme that degrades the second messenger cAMP, to enable memory consolidation. We demonstrated that the endocytosis of β2AR phosphorylated by GPCR kinases (GRKs) mediated arrestin3-dependent nuclear export of PDE4D5, which was critical for promoting nuclear cAMP signaling and gene expression in hippocampal neurons for memory consolidation. Inhibition of the arrestin3-PDE4D5 association prevented β2AR-induced nuclear cAMP signaling without affecting receptor endocytosis. Direct PDE4 inhibition rescued β2AR-induced nuclear cAMP signaling and ameliorated memory deficits in mice expressing a form of the β2AR that could not be phosphorylated by GRKs. These data reveal how β2AR phosphorylated by endosomal GRK promotes the nuclear export of PDE4D5, leading to nuclear cAMP signaling, changes in gene expression, and memory consolidation. This study also highlights the translocation of PDEs as a mechanism to promote cAMP signaling in specific subcellular locations downstream of GPCR activation.

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

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

A multiscale model of the regulation of aquaporin 2 recycling

The response of cells to their environment is driven by a variety of proteins and messenger molecules. In eukaryotes, their distribution and location in the cell are regulated by the vesicular transport system. The transport of aquaporin 2 between membrane and storage region is a crucial part of the water reabsorption in renal principal cells, and its malfunction can lead to Diabetes insipidus. To understand the regulation of this system, we aggregated pathways and mechanisms from literature and derived three models in a hypothesis-driven approach. Furthermore, we combined the models to a single system to gain insight into key regulatory mechanisms of Aquaporin 2 recycling. To achieve this, we developed a multiscale computational framework for the modeling and simulation of cellular systems. The analysis of the system rationalizes that the compartmentalization of cAMP in renal principal cells is a result of the protein kinase A signalosome and can only occur if specific cellular components are observed in conjunction. Endocytotic and exocytotic processes are inherently connected and can be regulated by the same protein kinase A signal.

Characterization of the cAMP phosphodiesterase domain in plant adenylyl cyclase/cAMP phosphodiesterase CAPE from the liverwort Marchantia polymorpha

Cyclic AMP (cAMP) acts as a second messenger and is involved in the regulation of various physiological responses. Recently, we identified the cAMP-synthesis/hydrolysis enzyme CAPE, which contains the two catalytic domains adenylyl cyclase (AC) and cAMP phosphodiesterase (PDE) from the liverwort Marchantia polymorpha. Here we characterize the PDE domain of M. polymorpha CAPE (MpCAPE-PDE) using the purified protein expressed in E. coli. The K_m and V_max of MpCAPE-PDE were 30 µM and 5.8 nmol min^-1 mg^-1, respectively. Further, we investigated the effect of divalent cations on PDE activity and found that Ca²⁺ enhanced PDE activity, suggesting that Ca²⁺ may be involved in cAMP signaling through the regulation of PDE activity of CAPE. Among the PDE inhibitors tested, only dipyridamole moderately inhibited PDE activity by approximately 40% at high concentrations. Conversely, 3-isobutyl-1-methylxanthine (IBMX) did not inhibit PDE activity.

Measurement of agonist-induced Ca2+ signals in human airway smooth muscle cells using excitation scan-based hyperspectral imaging and image analysis approaches

Ca²⁺ and cAMP are ubiquitous second messengers known to differentially regulate a variety of cellular functions over a wide range of timescales. Studies from a variety of groups support the hypothesis that these signals can be localized to discrete locations within cells, and that this subcellular localization is a critical component of signaling specificity. However, to date, it has been difficult to track second messenger signals at multiple locations. To overcome this limitation, we utilized excitation scan-based hyperspectral imaging approaches to track second messenger signals as well as labeled cellular structures and/or proteins in the same cell. We have previously reported that hyperspectral imaging techniques improve the signal-to-noise ratios of both fluorescence measurements, and are thus well suited for the measurement of localized Ca²⁺ signals. We investigated the spatial spread and intensities of agonist-induced Ca²⁺ signals in primary human airway smooth muscle cells (HASMCs) using the Ca²⁺ indicator Cal520. We measured responses triggered by three agonists, carbachol, histamine, and chloroquine. We utilized custom software coded in MATLAB and Python to assess agonist induced changes in Ca²⁺ levels. Software algorithms removed the background and applied correction coefficients to spectral data prior to linear unmixing, spatial and temporal filtering, adaptive thresholding, and automated region of interest (ROI) detection. All three agonists triggered transient Ca²⁺ responses that were spatially and temporally complex. We are currently analyzing differences in both ROI area and intensity distributions triggered by these agonists. This work was supported by NIH awards P01HL066299, K25HL136869, and R01HL137030 and NSF award MRI1725937.

Phosphoprotein Phosphatase 1 but Not 2A Activity Modulates Coupled-Clock Mechanisms to Impact on Intrinsic Automaticity of Sinoatrial Nodal Pacemaker Cells

Spontaneous AP (action potential) firing of sinoatrial nodal cells (SANC) is critically dependent on protein kinase A (PKA) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-dependent protein phosphorylation, which are required for the generation of spontaneous, diastolic local Ca²⁺ releases (LCRs). Although phosphoprotein phosphatases (PP) regulate protein phosphorylation, the expression level of PPs and phosphatase inhibitors in SANC and the impact of phosphatase inhibition on the spontaneous LCRs and other players of the oscillatory coupled-clock system is unknown. Here, we show that rabbit SANC express both PP1, PP2A, and endogenous PP inhibitors I-1 (PPI-1), dopamine and cyclic adenosine 3′,5′-monophosphate (cAMP)-regulated phosphoprotein (DARPP-32), kinase C-enhanced PP1 inhibitor (KEPI). Application of Calyculin A, (CyA), a PPs inhibitor, to intact, freshly isolated single SANC: (1) significantly increased phospholamban (PLB) phosphorylation (by 2–3-fold) at both CaMKII-dependent Thr¹⁷ and PKA-dependent Ser¹⁶ sites, in a time and concentration dependent manner; (2) increased ryanodine receptor (RyR) phosphorylation at the Ser²⁸⁰⁹ site; (3) substantially increased sarcoplasmic reticulum (SR) Ca²⁺ load; (4) augmented L-type Ca²⁺ current amplitude; (5) augmented LCR’s characteristics and decreased LCR period in intact and permeabilized SANC, and (6) increased the spontaneous basal AP firing rate. In contrast, the selective PP2A inhibitor okadaic acid (100 nmol/L) had no significant effect on spontaneous AP firing, LCR parameters, or PLB phosphorylation. Application of purified PP1 to permeabilized SANC suppressed LCR, whereas purified PP2A had no effect on LCR characteristics. Our numerical model simulations demonstrated that PP inhibition increases AP firing rate via a coupled-clock mechanism, including respective increases in the SR Ca²⁺ pumping rate, L-type Ca²⁺ current, and Na⁺/Ca²⁺-exchanger current. Thus, PP1 and its endogenous inhibitors modulate the basal spontaneous firing rate of cardiac pacemaker cells by suppressing SR Ca²⁺ cycling protein phosphorylation, the SR Ca²⁺ load and LCRs, and L-type Ca²⁺ current.

β-eudesmol but not atractylodin exerts an inhibitory effect on CFTR-mediated chloride transport in human intestinal epithelial cells

Oriental herbal medicine with the two bioactive constituents, β-eudesmol (BE) and atractylodin (AT), has been used as a remedy for gastrointestinal disorders. There was no scientific evidence reporting their antidiarrheal effect and underpinning mechanisms. Therefore, we aimed to investigate the anti-secretory activity of these two compounds in vitro. The inhibitory effect of BE and AT on cAMP-induced Cl^- secretion was evaluated by Ussing chamber in human intestinal epithelial (T84) cells. Short-circuit current (I_SC) and apical Cl^- current (I_Cl-) were measured after adding indirect and direct cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel activator. MTT assay was used to determine cellular cytotoxicity. Protein-ligand interaction was investigated by in silico molecular docking analysis. BE, but not AT concentration-dependently (IC₅₀ of ~1.05 µM) reduced cAMP-mediated, CFTR_inh-172 inhibitable Cl^- secretion as determined by transepithelial I_SC across a monolayer of T84 cells. Potency of CFTR-mediated I_Cl- inhibition by BE did not change with the use of different CFTR activators suggesting a direct blockage of the channel active site(s). Pretreatment with BE completely prevented cAMP-induced I_Cl-. Furthermore, BE at concentrations up to 200 µM (24 h) had no effect on T84 cell viability. In silico studies indicated that BE could best dock onto dephosphorylated structure of CFTR at ATP-binding pockets in nucleotide-binding domain (NBD) 2 region. These findings provide the first evidence for the anti-secretory effect of BE involving inhibition of CFTR function. BE represents a promising candidate for the therapeutic or prophylactic intervention of diarrhea resulted from intestinal hypersecretion of Cl^.

Cyclic nucleotide signaling and pacemaker activity

The sinoatrial node (SAN) is the natural pacemaker of the heart, producing the electrical impulse that initiates every heart beat. Its activity is tightly controlled by the autonomic nervous system, and by circulating and locally released factors. Neurohumoral regulation of heart rate plays a crucial role in the integration of vital functions and influences behavior and ability to respond to changing environmental conditions. At the cellular level, modulation of SAN activity occurs through intracellular signaling pathways involving cyclic nucleotides: cyclic AMP (cAMP) and cyclic GMP (cGMP). In this Review, dedicated to Professor Dario DiFrancesco and his accomplishements in the field of cardiac pacemaking, we summarize all findings on the role of cyclic nucleotides signaling in regulating the key actors of cardiac automatism, and we provide an up-to-date review on cAMP- and cGMP-phosphodiesterases (PDEs), compellingly involved in this modulation.

Drug Repositioning for the Treatment of Hematologic Disease: Limits, Challenges and Future Perspectives

Drug repositioning is a strategy to identify new uses for approved or investigational drugs that are used off-label outside the scope of the original medical indication. In this review, we report the most relevant studies about drug repositioning in hematology, reporting the signalling pathways and molecular targets of these drugs, and describing the biological mechanisms which are responsible for their anticancer effects. Although the majority of studies on drug repositioning in hematology concern acute myeloid leukemia and multiple myeloma, numerous studies are present in the literature on the possibility of using these drugs also in other hematological diseases, such as acute lymphoblastic leukemia, chronic myeloid leukemia, and lymphomas. Numerous anti-infectious drugs and chemical entities used for the therapy of neurological or endocrine diseases, oral antidiabetics, statins and medications used to treat high blood pressure and heart failure, bisphosphonate and natural substance such as artemisin and curcumin, have found a place in the treatment of hematological diseases. Moreover, several molecules drastically reversed the resistance of the tumor cells to the chemotherapeutic drugs both in vitro and in vivo.

Development of Phosphodiesterase-Protein-Kinase Complexes as Novel Targets for Discovery of Inhibitors with Enhanced Specificity

Phosphodiesterases (PDEs) hydrolyze cyclic nucleotides to modulate multiple signaling events in cells. PDEs are recognized to actively associate with cyclic nucleotide receptors (protein kinases, PKs) in larger macromolecular assemblies referred to as signalosomes. Complexation of PDEs with PKs generates an expanded active site that enhances PDE activity. This facilitates signalosome-associated PDEs to preferentially catalyze active hydrolysis of cyclic nucleotides bound to PKs and aid in signal termination. PDEs are important drug targets, and current strategies for inhibitor discovery are based entirely on targeting conserved PDE catalytic domains. This often results in inhibitors with cross-reactivity amongst closely related PDEs and attendant unwanted side effects. Here, our approach targeted PDE–PK complexes as they would occur in signalosomes, thereby offering greater specificity. Our developed fluorescence polarization assay was adapted to identify inhibitors that block cyclic nucleotide pockets in PDE–PK complexes in one mode and disrupt protein-protein interactions between PDEs and PKs in a second mode. We tested this approach with three different systems—cAMP-specific PDE8–PKAR, cGMP-specific PDE5–PKG, and dual-specificity RegA–R_D complexes—and ranked inhibitors according to their inhibition potency. Targeting PDE–PK complexes offers biochemical tools for describing the exquisite specificity of cyclic nucleotide signaling networks in cells.

Distribution of adenylyl cyclase/cAMP phosphodiesterase gene, CAPE, in streptophytes reproducing via motile sperm

We recently isolated a novel adenylyl cyclase/cAMP phosphodiesterase gene from the liverwort, Marchantia polymorpha. The protein encoded by this gene has a class III adenylyl cyclase (AC) in the C-terminal domain and class I phosphodiesterase (PDE) in the N-terminal domain; therefore, we named it CAPE (COMBINED AC with PDE). CAPE protein is likely involved in spermatogenesis and sperm motility due to its tissue-specific expression pattern in M. polymorpha and the distribution of CAPE genes in streptophytes. However, little is known about the distribution of CAPE in gymnosperms that use motile sperm for fertilization, such as cycads and ginkgo. The present study aimed to isolate CAPE genes from the cycad, Cycas revoluta, the ginkgo, Ginkgo biloba, and the hornwort, Anthoceros agerestis. Sequences with high homology to CAPE were obtained from these species. Our analyses revealed that all plant taxonomic groups reproducing via motile sperm possessed CAPE, whereas those that do not produce motile sperm did not possess CAPE, with one exception in gymnosperm Cupressales. The phylogenic distribution of CAPE almost corresponds to the evolutionary history of motile sperm production and further suggests that CAPE may be involved in sexual reproduction process using motile sperm in streptophytes.

Phosphodiesterases Expression during Murine Cardiac Development

3′-5′ cyclic nucleotide phosphodiesterases (PDEs) are a large family of enzymes playing a fundamental role in the control of intracellular levels of cAMP and cGMP. Emerging evidence suggested an important role of phosphodiesterases in heart formation, but little is known about the expression of phosphodiesterases during cardiac development. In the present study, the pattern of expression and enzymatic activity of phosphodiesterases was investigated at different stages of heart formation. C57BL/6 mice were mated and embryos were collected from 14.5 to 18.5 days of development. Data obtained by qRT-PCR and Western blot analysis showed that seven different isoforms are expressed during heart development, and PDE1C, PDE2A, PDE4D, PDE5A and PDE8A are modulated from E14.5 to E18.5. In heart homogenates, the total cAMP and cGMP hydrolytic activity is constant at the evaluated times, and PDE4 accounts for the majority of the cAMP hydrolyzing ability and PDE2A accounts for cGMP hydrolysis. This study showed that a subset of PDEs is expressed in developing mice heart and some of them are modulated to maintain constant nucleotide phosphodiesterase activity in embryonic and fetal heart.

Compartmentation of β2 -adrenoceptor stimulated cAMP responses by phosphodiesterase types 2 and 3 in cardiac ventricular myocytes

BACKGROUND AND PURPOSE

In cardiac myocytes, cyclic AMP (cAMP) produced by both β₁ - and β₂ -adrenoceptors increases L-type Ca²⁺ channel activity and myocyte contraction. However, only cAMP produced by β₁ -adrenoceptors enhances myocyte relaxation through phospholamban-dependent regulation of the sarco/endoplasmic reticulum Ca²⁺ ATPase 2 (SERCA2). Here we have tested the hypothesis that stimulation of β₂ -adrenoceptors produces a cAMP signal that is unable to reach SERCA2 and determine what role, if any, phosphodiesterase (PDE) activity plays in this compartmentation.

EXPERIMENTAL APPROACH

The cAMP responses produced by β₁ -and β₂ -adrenoceptor stimulation were studied in adult rat ventricular myocytes using two different fluorescence resonance energy transfer (FRET)-based biosensors, the Epac2-camps, which is expressed uniformly throughout the cytoplasm of the entire cell and the Epac2-αKAP, which is targeted to the SERCA2 signalling complex.

KEY RESULTS

Selective activation of β₁ - or β₂ -adrenoceptors produced cAMP responses detected by Epac2-camps. However, only stimulation of β₁ -adrenoceptors produced a cAMP response detected by Epac2-αKAP. Yet, stimulation of β₂ -adrenoceptors was able to produce a cAMP signal detected by Epac2-αKAP in the presence of selective inhibitors of PDE2 or PDE3, but not PDE4.

CONCLUSION AND IMPLICATIONS

These results support the conclusion that cAMP produced by β₂ -adrenoceptor stimulation was not able to reach subcellular locations where the SERCA2 pump is located. Furthermore, this compartmentalized response is due at least in part to PDE2 and PDE3 activity. This discovery could lead to novel PDE-based therapeutic treatments aimed at correcting cardiac relaxation defects associated with certain forms of heart failure.

The basics of mitochondrial cAMP signalling: Where, when, why

Cytosolic cAMP signalling in live cells has been extensively investigated in the past, while only in the last decade the existence of an intramitochondrial autonomous cAMP homeostatic system began to emerge. Thanks to the development of novel tools to investigate cAMP dynamics and cAMP/PKA-dependent phosphorylation within the matrix and in other mitochondrial compartments, it is now possible to address directly and in intact living cells a series of questions that until now could be addressed only by indirect approaches, in isolated organelles or through subcellular fractionation studies. In this contribution we discuss the mechanisms that regulate cAMP dynamics at the surface and inside mitochondria, and its crosstalk with organelle Ca²⁺ handling. We then address a series of still unsolved questions, such as the intramitochondrial localization of key elements of the cAMP signaling toolkit, e.g., adenylate cyclases, phosphodiesterases, protein kinase A (PKA) and Epac. Finally, we discuss the evidence for and against the existence of an intramitochondrial PKA pool and the functional role of cAMP increases within the organelle matrix.

Spatially compartmentalized phase regulation of a Ca2+-cAMP-PKA oscillatory circuit

Signaling networks are spatiotemporally organized to sense diverse inputs, process information, and carry out specific cellular tasks. In β cells, Ca2+, cyclic adenosine monophosphate (cAMP), and Protein Kinase A (PKA) exist in an oscillatory circuit characterized by a high degree of feedback. Here, we describe a mode of regulation within this circuit involving a spatial dependence of the relative phase between cAMP, PKA, and Ca2+. We show that in mouse MIN6 β cells, nanodomain clustering of Ca2+-sensitive adenylyl cyclases (ACs) drives oscillations of local cAMP levels to be precisely in-phase with Ca2+ oscillations, whereas Ca2+-sensitive phosphodiesterases maintain out-of-phase oscillations outside of the nanodomain. Disruption of this precise phase relationship perturbs Ca2+ oscillations, suggesting the relative phase within an oscillatory circuit can encode specific functional information. This work unveils a novel mechanism of cAMP compartmentation utilized for localized tuning of an oscillatory circuit and has broad implications for the spatiotemporal regulation of signaling networks.

Role of Phosphodiesterase in the Biology and Pathology of Diabetes

Glucose metabolism is the initiator of a large number of molecular secretory processes in β cells. Cyclic nucleotides as a second messenger are the main physiological regulators of these processes and are functionally divided into compartments in pancreatic cells. Their intracellular concentration is limited by hydrolysis led by one or more phosphodiesterase (PDE) isoenzymes. Literature data confirmed multiple expressions of PDEs subtypes, but the specific roles of each in pancreatic β-cell function, particularly in humans, are still unclear. Isoforms present in the pancreas are also found in various tissues of the body. Normoglycemia and its strict control are supported by the appropriate release of insulin from the pancreas and the action of insulin in peripheral tissues, including processes related to homeostasis, the regulation of which is based on the PDE- cyclic AMP (cAMP) signaling pathway. The challenge in developing a therapeutic solution based on GSIS (glucose-stimulated insulin secretion) enhancers targeted at PDEs is the selective inhibition of their activity only within β cells. Undeniably, PDEs inhibitors have therapeutic potential, but some of them are burdened with certain adverse effects. Therefore, the chance to use knowledge in this field for diabetes treatment has been postulated for a long time.

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.

Phase Separation of a PKA Regulatory Subunit Controls cAMP Compartmentation and Oncogenic Signaling

The fidelity of intracellular signaling hinges on the organization of dynamic activity architectures. Spatial compartmentation was first proposed over 30 years ago to explain how diverse G protein-coupled receptors achieve specificity despite converging on a ubiquitous messenger, cyclic adenosine monophosphate (cAMP). However, the mechanisms responsible for spatially constraining this diffusible messenger remain elusive. Here, we reveal that the type I regulatory subunit of cAMP-dependent protein kinase (PKA), RIα, undergoes liquid-liquid phase separation (LLPS) as a function of cAMP signaling to form biomolecular condensates enriched in cAMP and PKA activity, critical for effective cAMP compartmentation. We further show that a PKA fusion oncoprotein associated with an atypical liver cancer potently blocks RIα LLPS and induces aberrant cAMP signaling. Loss of RIα LLPS in normal cells increases cell proliferation and induces cell transformation. Our work reveals LLPS as a principal organizer of signaling compartments and highlights the pathological consequences of dysregulating this activity architecture.

PAN-selective inhibition of cAMP-phosphodiesterase 4 (PDE4) induces gastroparesis in mice

Inhibitors of cAMP-phosphodiesterase 4 (PDE4) exert a number of promising therapeutic benefits, but adverse effects, in particular emesis and nausea, have curbed their clinical utility. Here, we show that PAN-selective inhibition of PDE4, but not inhibition of PDE3, causes a time- and dose-dependent accumulation of chow in the stomachs of mice fed ad libitum without changing the animals' food intake or the weight of their intestines, suggesting that PDE4 inhibition impairs gastric emptying. Indeed, PDE4 inhibition induced gastric retention in an acute model of gastric motility that traces the passage of a food bolus through the stomach over a 30 minutes time period. In humans, abnormal gastric retention of food is known as gastroparesis, a syndrome predominated by nausea (>90% of cases) and vomiting (>80% of cases). We thus explored the abnormal gastric retention induced by PDE4 inhibition in mice under the premise that it may represent a useful correlate of emesis and nausea. Delayed gastric emptying was produced by structurally distinct PAN-PDE4 inhibitors including Rolipram, Piclamilast, Roflumilast, and RS25344, suggesting that it is a class effect. PDE4 inhibitors induced gastric retention at similar or below doses commonly used to induce therapeutic benefits (e.g., 0.04 mg/kg Rolipram), thus mirroring the narrow therapeutic window of PDE4 inhibitors in humans. YM976, a PAN-PDE4 inhibitor that does not efficiently cross the blood-brain barrier, induced gastroparesis only at significantly higher doses (≥1 mg/kg). This suggests that PDE4 inhibition may act in part through effects on the autonomic nervous system regulation of gastric emptying and that PDE4 inhibitors that are not brain-penetrant may have an improved safety profile. The PDE4 family comprises four subtypes, PDE4A, B, C, and D. Selective ablation of any of these subtypes in mice did not induce gastroparesis per se, nor did it protect from PAN-PDE4 inhibitor-induced gastroparesis, indicating that gastric retention may result from the concurrent inhibition of multiple PDE4s. Thus, potentially, any of the four PDE4 subtypes may be targeted individually for therapeutic benefits without inducing nausea or emesis. Acute gastric retention induced by PDE4 inhibition is alleviated by treatment with the widely used prokinetic Metoclopramide, suggesting a potential of this drug to alleviate the side effects of PDE4 inhibitors. Finally, given that the cause of gastroparesis remains largely idiopathic, our findings open the possibility that a physiologic or pathophysiologic downregulation of PDE4 activity/expression may be causative in a subset of patients.

Genetic manipulation of cyclic nucleotide signaling during hippocampal neuroplasticity and memory formation

Decades of research have underscored the importance of cyclic nucleotide signaling in memory formation and synaptic plasticity. In recent years, several new genetic techniques have expanded the neuroscience toolbox, allowing researchers to measure and modulate cyclic nucleotide gradients with high spatiotemporal resolution. Here, we will provide an overview of studies using genetic approaches to interrogate the role cyclic nucleotide signaling plays in hippocampus-dependent memory processes and synaptic plasticity. Particular attention is given to genetic techniques that measure real-time changes in cyclic nucleotide levels as well as newly-developed genetic strategies to transiently manipulate cyclic nucleotide signaling in a subcellular compartment-specific manner with high temporal resolution.

Priming the Proteasome to Protect against Proteotoxicity

Increased proteotoxic stress (IPTS) resulting from the increased production or decreased removal of abnormally folded proteins is recognized as an important pathogenic factor for a large group of highly disabling and life-threatening human diseases, such as neurodegenerative disorders and many heart diseases. The proteasome is pivotal to the timely removal of abnormal proteins but its functional capacity often becomes inadequate in the disease conditions; consequently, proteasome functional insufficiency in return exacerbates IPTS. Recent research in proteasome biology reveals that the proteasome can be activated by endogenous protein kinases, making it possible to pharmacologically prime the proteasome for treating diseases with IPTS.

The dual phosphodiesterase 3/4 inhibitor RPL554 stimulates rare class III and IV CFTR mutants

Over 2,000 mutations have been reported in the cystic fibrosis transmembrane conductance regulator (cftr) gene, many of which cause disease but are rare and have no effective treatment. Thus, there is an unmet need for new, mutation-agnostic therapies for cystic fibrosis (CF). Phosphodiesterase (PDE) inhibitors are one such class of therapeutics that have been shown to elevate intracellular cAMP levels and stimulate CFTR-dependent anion secretion in human airway epithelia; however, the number of people with CF that could be helped by PDE inhibitors remains to be determined. Here we used Fisher rat thyroid (FRT) cells stably transduced with rare human CFTR mutants and studied their responsiveness to the dual phosphodiesterase 3/4 inhibitor RPL554 (Verona Pharma). Through its inhibitory effect on PDE4D, we find that RPL554 can elevate intracellular cAMP leading to a potentiation of forskolin-stimulated current mediated by R334W, T338I, G551D, and S549R mutants of CFTR when used alone or in combination with CFTR modulators. We also were able to reproduce these effects of RPL554 on G551D-CFTR when it was expressed in primary human bronchial epithelial cells, indicating that RPL554 would have stimulatory effects on rare CFTR mutants in human airways and validating FRT cells as a model for PDE inhibitor studies. Furthermore, we provide biochemical evidence that VX-809 causes surprisingly robust correction of several class III and IV CFTR mutants. Together, our findings further support the therapeutic potential of RPL554 for patients with CF with class III/IV mutations and emphasize the potential of PDEs as potential drug targets that could benefit patients with CF.

Elevated intracellular cAMP concentration mediates growth suppression in glioma cells

Supressed levels of intracellular cAMP have been associated with malignancy. Thus, elevating cAMP through activation of adenylyl cyclase (AC) or by inhibition of phosphodiesterase (PDE) may be therapeutically beneficial. Here, we demonstrate that elevated cAMP levels suppress growth in C6 cells (a model of glioma) through treatment with forskolin, an AC activator, or a range of small molecule PDE inhibitors with differing selectivity profiles. Forskolin suppressed cell growth in a PKA-dependent manner by inducing a G₂/M phase cell cycle arrest. In contrast, trequinsin (a non-selective PDE2/3/7 inhibitor), not only inhibited cell growth via PKA, but also stimulated (independent of PKA) caspase-3/-7 and induced an aneuploidy phenotype. Interestingly, a cocktail of individual PDE 2,3,7 inhibitors suppressed cell growth in a manner analogous to forskolin but not trequinsin. Finally, we demonstrate that concomitant targeting of both AC and PDEs synergistically elevated intracellular cAMP levels thereby potentiating their antiproliferative actions.

In vitro profiling of opioid ligands using the cAMP formation inhibition assay and the β-arrestin2 recruitment assay: No two ligands have the same profile

Previously, we showed that no two of seven opioids administered by the intracerebroventricular route had the same potency rank order for evoking antinociception, constipation and respiratory depression in rats. To gain insight at the cellular level, this study was designed to systematically investigate the activity profiles of six commonly used opioid ligands using the forskolin-stimulated cAMP assay and a β-arrestin2 recruitment assay in cultured HEK-293 cells transfected with MOP(μ), DOP(δ) or KOP(κ) receptors(-r). Morphine was a potent agonist at the MOP-r in the cAMP assay whereas it was a weak agonist at the KOP-r and DOP-r. Oxycodone had moderate efficacy and low potency at the MOP-r. Buprenorphine was a potent MOP-r and DOP-r agonist; its efficacy rank order was DOP > MOP > KOP. Fentanyl was a potent agonist at the MOP-r; its efficacy rank order was MOP > DOP > KOP. For DPDPE, its agonist efficacy was confined to the DOP-r, whereas for U69593, its efficacy rank order was KOP>> MOP. For the β-arrestin2 assay, fentanyl had full efficacy at the MOP-r whereas morphine and oxycodone were weak with insignificant efficacy at DOP and KOP receptors. Buprenorphine did not recruit β-arrestin2 at all three opioid-receptors. DPDPE and U69593 had full efficacy for β-arrestin2 recruitment to the DOP-r and KOP-r respectively. Despite the low efficacy and potency of morphine, oxycodone and buprenorphine in recruiting β-arrestin2 to the MOP-r herein, these opioids all evoked respiratory depression and constipation in rats. Together, our findings discount a key role for β-arrestin2 recruitment at the MOP-r in evoking opioid-related side-effects.

Signal Transduction Pathways as Therapeutic Target for Chagas Disease

Trypanosomatids are a group of flagellated unicellular eukaryotes, causing serious human diseases including Chagas disease (Trypanosoma cruzi), sleeping sickness (Trypanosoma brucei spp.) and Leishmaniasis (Leishmania spp.). The second messenger cAMP is involved in numerous and fundamental processes in these parasites including differentiation between stages, proliferation, osmoregulation, oxidative stress and quorum sensing. Interestingly, its signaling pathway is quite different from that of mammals, including structurally different adenylyl cyclases, the shortage of orthologous effector proteins and the absence of G-protein-coupled-receptors, among others. These characteristics make the proteins involved in these transduction pathways good candidates for therapeutic targets. However, the identification of new unknown druggable targets involves extensive research time and is economically very expensive, making difficult the transition from basic research to the clinical phase. Trypanosomatid PDEs have characteristic binding pockets that allow for a differential inhibition from their human orthologs. Modification in the approved drugs for human to convert them into trypanocidal treatments could lead to more effective therapies, shorter lab time and lower costs. In view of the fact that kinetoplastid PDEs are highly conserved with their mammalian counterparts, and since there are already numerous drugs on the market against human PDEs, the drug repositioning approach is highly promising. The development of new technologies, higher government and industrial involvement and more scientists committed to basic investigation, are the key to ultimately find an effective treatment and cure for the neglected tropical diseases.

Oscillatory cAMP signaling rapidly alters H3K4 methylation

This work explores how GPCR-cAMP signaling dynamically influences histone methylation by altering intracellular labile Fe(II) levels and subsequently modulating histone demethylase activity.

Epigenetic variation reflects the impact of a dynamic environment on chromatin. However, it remains elusive how environmental factors influence epigenetic events. Here, we show that G protein–coupled receptors (GPCRs) alter H3K4 methylation via oscillatory intracellular cAMP. Activation of Gs-coupled receptors caused a rapid decrease of H3K4me3 by elevating cAMP, whereas stimulation of Gi-coupled receptors increased H3K4me3 by diminishing cAMP. H3K4me3 gradually recovered towards baseline levels after the removal of GPCR ligands, indicating that H3K4me3 oscillates in tandem with GPCR activation. cAMP increased intracellular labile Fe(II), the cofactor for histone demethylases, through a non-canonical cAMP target—Rap guanine nucleotide exchange factor-2 (RapGEF2), which subsequently enhanced endosome acidification and Fe(II) release from the endosome via vacuolar H⁺-ATPase assembly. Removing Fe(III) from the media blocked intracellular Fe(II) elevation after stimulation of Gs-coupled receptors. Iron chelators and inhibition of KDM5 demethylases abolished cAMP-mediated H3K4me3 demethylation. Taken together, these results suggest a novel function of cAMP signaling in modulating histone demethylation through labile Fe(II).

When trafficking and signaling mix: How subcellular location shapes G protein-coupled receptor activation of heterotrimeric G proteins

G protein-coupled receptors (GPCRs) physically connect extracellular information with intracellular signal propagation. Membrane trafficking plays a supportive role by "bookending" signaling events: movement through the secretory pathway delivers GPCRs to the cell surface where receptors can sample the extracellular environment, while endocytosis and endolysosomal membrane trafficking provide a versatile system to titrate cellular signaling potential and maintain homeostatic control. Recent evidence suggests that, in addition to these important effects, GPCR trafficking actively shapes the cellular signaling response by altering the location and timing of specific receptor-mediated signaling reactions. Here, we review key experimental evidence underlying this expanding view, focused on GPCR signaling mediated through activation of heterotrimeric G proteins located in the cytoplasm. We then discuss lingering and emerging questions regarding the interface between GPCR signaling and trafficking.

A two-dimensional finite element model of cyclic adenosine monophosphate (cAMP) intracellular signaling

In this work, we present a two-dimensional finite element analysis (FEA) model that describes fundamental intracellular signals of cyclic adenosine monophosphate (cAMP) in a general fashion. The model was subsequently solved numerically and the results were displayed in forms of time-course plots of cAMP concentration at a cellular location or color-filled contour maps of cAMP signal distribution within the cell at specific time points. Basic intracellular cAMP signaling was described in this model so it can be numerically validated by verifying its numerical results against available analytical solutions and against results obtained from other numerical techniques reported in the literature. This is the first important step before the model can be expanded in future work. Model simulations demonstrate that under certain conditions, sustained cAMP concentrations can be formed within endothelial cells (ECs), similar to those observed in rat pulmonary microvascular ECs. Spatial and temporal cAMP dynamic simulations indicated that the proposed FEA model is an effective tool for the study of the kinetics and spatial spread of second messenger signaling and can be expanded to simulate second messenger signals in the pulmonary vasculature.

Geometric Control of Frequency Modulation of cAMP Oscillations due to Calcium in Dendritic Spines

The spatiotemporal regulation of cyclic adenosine monophosphate (cAMP) and its dynamic interactions with other second messengers such as calcium are critical features of signaling specificity required for neuronal development and connectivity. cAMP is known to contribute to long-term potentiation and memory formation by controlling the formation and regulation of dendritic spines. Despite the recent advances in biosensing techniques for monitoring spatiotemporal cAMP dynamics, the underlying molecular mechanisms that attribute to the subcellular modulation of cAMP remain unknown. In this work, we model the spatiotemporal dynamics of calcium-induced cAMP signaling pathway in dendritic spines. Using a three-dimensional reaction-diffusion model, we investigate the effect of different spatial characteristics of cAMP dynamics that may be responsible for subcellular regulation of cAMP concentrations. Our model predicts that the volume/surface ratio of the spine, regulated through the spine head size, spine neck size, and the presence of physical barriers (spine apparatus), is an important regulator of cAMP dynamics. Furthermore, localization of the enzymes responsible for the synthesis and degradation of cAMP in different compartments also modulates the oscillatory patterns of cAMP through exponential relationships. Our findings shed light on the significance of complex geometric and localization relationships for cAMP dynamics in dendritic spines.

Cannabinoid Receptor 2 (CB2) Signals via G-alpha-s and Induces IL-6 and IL-10 Cytokine Secretion in Human Primary Leukocytes

Cannabinoid receptor 2 (CB₂) is a promising therapeutic target for immunological modulation. There is, however, a deficit of knowledge regarding CB₂ signaling and function in human primary immunocompetent cells. We applied an experimental paradigm which closely models the in situ state of human primary leukocytes (PBMC; peripheral blood mononuclear cells) to characterize activation of a number of signaling pathways in response to a CB₂-selective ligand (HU308). We observed a "lag" phase of unchanged cAMP concentration prior to development of classically expected Gα_i-mediated inhibition of cAMP synthesis. Application of G protein inhibitors revealed that this apparent lag was a result of counteraction of Gα_i effects by concurrent Gα_s activation. Monitoring downstream signaling events showed that activation of p38 was mediated by Gα_i, whereas ERK1/2 and Akt phosphorylation were mediated by Gα_i-coupled βγ. Activation of CREB integrated multiple components; Gα_s and βγ mediated ∼85% of the response, while ∼15% was attributed to Gα_i. Responses to HU308 had an important functional outcome-secretion of interleukins 6 (IL-6) and 10 (IL-10). IL-2, IL-4, IL-12, IL-13, IL-17A, MIP-1α, and TNF-α were unaffected. IL-6/IL-10 induction had a similar G protein coupling profile to CREB activation. All response potencies were consistent with that expected for HU308 acting via CB₂. Additionally, signaling and functional effects were completely blocked by a CB₂-selective inverse agonist, giving additional evidence for CB₂ involvement. This work expands the current paradigm regarding cannabinoid immunomodulation and reinforces the potential utility of CB₂ ligands as immunomodulatory therapeutics.

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.

Phosphodiesterase 3B (PDE3B) antagonizes the anti-angiogenic actions of PKA in human and murine endothelial cells

Recent reports show that protein kinase A (PKA), but not exchange protein activated by cAMP (EPAC), acts in a cell autonomous manner to constitutively reduce the angiogenic sprouting capacity of murine and human endothelial cells. Specificity in the cellular actions of individual cAMP-effectors can be achieved when a cyclic nucleotide phosphodiesterase (PDE) enzyme acts locally to control the "pool" of cAMP that activates the cAMP-effector. Here, we examined whether PDEs coordinate the actions of PKA during endothelial cell sprouting. Inhibiting each of the cAMP-hydrolyzing PDEs expressed in human endothelial cells revealed that phosphodiesterase 3 (PDE3) inhibition with cilostamide reduced angiogenic sprouting in vitro, while inhibitors of PDE2 and PDE4 family enzymes had no such effect. Identifying a critical role for PDE3B in the anti-angiogenic effects of cilostamide, silencing this PDE3 variant, but not PDE3A, markedly impaired sprouting. Importantly, using both in vitro and ex vivo models of angiogenesis, we show the hypo-sprouting phenotype induced by PDE3 inhibition or PDE3B silencing was reversed by PKA inhibition. Examination of the individual cellular events required for sprouting revealed that PDE3B and PKA each regulated angiogenic sprouting by controlling the invasive capacity of endothelial cells, more specifically, by regulating podosome rosette biogenesis and matrix degradation. In support of the idea that PDE3B acts to inhibit angiogenic sprouting by limiting PKA-mediated reductions in active cdc42, the effects of PDE3B and/or PKA on angiogenic sprouting were negated in cells with reduced cdc42 expression or activity. Since PDE3B and PKA were co-localized in a perinuclear region in human ECs, could be co-immunoprecipitated from lysates of these cells, and silencing PDE3B activated the perinuclear pool of PKA in these cells, we conclude that PDE3B-mediated hydrolysis of cAMP acts to limit the anti-angiogenic potential of PKA in ECs.

Dexamethasone Inhibits Synergistic Induction of PDE4B Expression by Roflumilast and Bacterium NTHi

Phosphodiesterase 4B (PDE4B) plays an important role in inflammation. Recently we have reported that roflumilast as a PDE4-selective inhibitor, synergizes with nontypeable Haemophilus influenzae (NTHi) to up-regulate PDE4B expression in vitro and in vivo. Clinical evidence and our previous results suggest that synergistic induction of PDE4B could be counterproductive for suppressing inflammation or may contribute to tolerance to roflumilast. We thus investigated if dexamethasone inhibits the synergistic induction of PDE4B by roflumilast and NTHi as well as inflammation. Here, dexamethasone markedly suppressed the synergistic induction of PDE4B in human lung epithelial cells and in vivo. We also found that dexamethasone further suppressed NTHi-induced inflammatory response in vitro and in vivo. Moreover, Compound A, as a dissociating non-steroidal glucocorticoid receptor (GR) ligand, inhibited the synergistic induction of PDE4B, thereby suggesting the requirement of dexamethasone-mediated GR activation in the suppression of PDE4B expression. Taken together, our data suggest that dexamethasone may help attenuate inflammation and tolerance through suppressing the PDE4B expression in chronic obstructive pulmonary disease (COPD) patients using roflumilast.

Dual Activation of Phosphodiesterases 3 and 4 Regulates Basal Spontaneous Beating Rate of Cardiac Pacemaker Cells: Role of Compartmentalization?

Spontaneous firing of sinoatrial (SA) node cells (SANCs) is regulated by cyclic adenosine monophosphate (cAMP)-mediated, protein kinase A (PKA)-dependent (cAMP/PKA) local subsarcolemmal Ca²⁺ releases (LCRs) from ryanodine receptors (RyR). The LCRs occur during diastolic depolarization (DD) and activate an inward Na⁺/Ca²⁺ exchange current that accelerates the DD rate prompting the next action potential (AP). Basal phosphodiesterases (PDEs) activation degrades cAMP, reduces basal cAMP/PKA-dependent phosphorylation, and suppresses normal spontaneous firing of SANCs. The cAMP-degrading PDE1, PDE3, and PDE4 represent major PDE activities in rabbit SANC, and PDE inhibition by 3-isobutyl-1-methylxanthine (IBMX) increases spontaneous firing of SANC by ∼50%. Though inhibition of single PDE1–PDE4 only moderately increases spontaneous SANC firing, dual PDE3 + PDE4 inhibition produces a synergistic effect hastening the spontaneous SANC beating rate by ∼50%. Here, we describe the expression and distribution of different PDE subtypes within rabbit SANCs, several specific targets (L-type Ca²⁺ channels and phospholamban) regulated by basal concurrent PDE3 + PDE4 activation, and critical importance of RyR Ca²⁺ releases for PDE-dependent regulation of spontaneous SANC firing. Colocalization of PDE3 and PDE4 beneath sarcolemma or in striated patterns inside SANCs strongly suggests that PDE-dependent regulation of cAMP/PKA signaling might be executed at the local level; this idea, however, requires further verification.

Norepinephrine stimulates glycogenolysis in astrocytes to fuel neurons with lactate

The mechanism of rapid energy supply to the brain, especially to accommodate the heightened metabolic activity of excited states, is not well-understood. We explored the role of glycogen as a fuel source for neuromodulation using the noradrenergic stimulation of glia in a computational model of the neural-glial-vasculature ensemble (NGV). The detection of norepinephrine (NE) by the astrocyte and the coupled cAMP signal are rapid and largely insensitive to the distance of the locus coeruleus projection release sites from the glia, implying a diminished impact for volume transmission in high affinity receptor transduction systems. Glucosyl-conjugated units liberated from glial glycogen by NE-elicited cAMP second messenger transduction winds sequentially through the glycolytic cascade, generating robust increases in NADH and ATP before pyruvate is finally transformed into lactate. This astrocytic lactate is rapidly exported by monocarboxylate transporters to the associated neuron, demonstrating that the astrocyte-to-neuron lactate shuttle activated by glycogenolysis is a likely fuel source for neuromodulation and enhanced neural activity. Altogether, the energy supply for both astrocytes and neurons can be supplied rapidly by glycogenolysis upon neuromodulatory stimulus.

Although efficient compared to computers, the human brain utilizes energy at 10-fold the rate of other organs by mass. How the brain is supplied with sufficient on-demand energy to support its activity in the absence of neuronal storage capacity remains unknown. Neurons are not capable of meeting their own energy requirements, instead energy supply in the brain is managed by an oligocellular cartel composed of neurons, glia and the local vasculature (NGV), wherein glia can provide the ergogenic metabolite lactate to the neuron in a process called the astrocyte-to-neuron shuttle (ANLS). The only means of energy storage in the brain is glycogen, a polymerized form of glucose that is localized largely to astrocytes, but its exact role and conditions of use are not clear. In this computational model we show that neuromodulatory stimulation by norepinephrine induces astrocytes to recover glucosyl subunits from glycogen for use in a glycolytic process that favors the production of lactate. The ATP and NADH produced support metabolism in the astrocyte while the lactate is exported to feed the neuron. Thus, rapid energy demands by both neurons and glia in a stimulated brain can be met by glycogen mobilization.

Active 3'-5' cyclic nucleotide phosphodiesterases are present in detergent-resistant membranes of mural granulosa cells

Lipids rafts are specialised membrane microdomains involved in cell signalling that can be isolated as detergent-resistant membranes (DRMs). The second messenger cyclic AMP (cAMP) has a central role in cell signalling in the ovary and its degradation is carried out by the phosphodiesterase (PDE) enzyme family. We hypothesised that PDEs could be functionally present in the lipid rafts of porcine mural granulosa cell membranes. PDE6C, PDE8A and PDE11A were detected by dot blot in the DRMs and the Triton-soluble fraction of the mural granulosa cells membrane and the cytosol. As shown by immunocytochemistry, PDEs showed clear immunostaining in mural granulosa cell membranes and the cytosol. Interestingly, cAMP-PDE activity was 18 times higher in the DRMs than in the Triton-soluble fraction of cell membranes and was 7.7 times higher in the cytosol than in the DRMs. cAMP-PDE activity in mural granulosa cells was mainly contributed by the PDE8 and PDE11 families. This study shows that PDEs from the PDE8 and PDE11 families are present in mural granulosa cells and that the cAMP-PDE activity is mainly contributed by the cytosol. In the cell membrane, the cAMP-PDE activity is mainly contributed by the DRMs. In addition, receptors for prostaglandin E2 and LH, two G-protein-coupled receptors, are present in lipid rafts and absent from the non-raft fraction of the granulosa cell membrane. These results suggest that in these cells, the lipid rafts exist as a cell-signalling platform and PDEs are one of the key enzyme families present in the raft.

The Development of Compartmentation of cAMP Signaling in Cardiomyocytes: The Role of T-Tubules and Caveolae Microdomains

3′-5′-cyclic adenosine monophosphate (cAMP) is a signaling messenger produced in response to the stimulation of cellular receptors, and has a myriad of functional applications depending on the cell type. In the heart, cAMP is responsible for regulating the contraction rate and force; however, cAMP is also involved in multiple other functions. Compartmentation of cAMP production may explain the specificity of signaling following a stimulus. In particular, transverse tubules (T-tubules) and caveolae have been found to be critical structural components for the spatial confinement of cAMP in cardiomyocytes, as exemplified by beta-adrenergic receptor (β-ARs) signaling. Pathological alterations in cardiomyocyte microdomain architecture led to a disruption in compartmentation of the cAMP signal. In this review, we discuss the difference between atrial and ventricular cardiomyocytes in respect to microdomain organization, and the pathological changes of atrial and ventricular cAMP signaling in response to myocyte dedifferentiation. In addition, we review the role of localized phosphodiesterase (PDE) activity in constraining the cAMP signal. Finally, we discuss microdomain biogenesis and maturation of cAMP signaling with the help of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Understanding these mechanisms may help to overcome the detrimental effects of pathological structural remodeling.

Compartmentalized cAMP Signaling Associated With Lipid Raft and Non-raft Membrane Domains in Adult Ventricular Myocytes

Aim: Confining cAMP production to discrete subcellular locations makes it possible for this ubiquitous second messenger to elicit unique functional responses. Yet, factors that determine how and where the production of this diffusible signaling molecule occurs are incompletely understood. The fluid mosaic model originally proposed that signal transduction occurs through random interactions between proteins diffusing freely throughout the plasma membrane. However, it is now known that the movement of membrane proteins is restricted, suggesting that the plasma membrane is segregated into distinct microdomains where different signaling proteins can be concentrated. In this study, we examined what role lipid raft and non-raft membrane domains play in compartmentation of cAMP signaling in adult ventricular myocytes.

Methods and Results: The freely diffusible fluorescence resonance energy transfer-based biosensor Epac2-camps was used to measure global cytosolic cAMP responses, while versions of the probe targeted to lipid raft (Epac2-MyrPalm) and non-raft (Epac2-CAAX) domains were used to monitor local cAMP production near the plasma membrane. We found that β-adrenergic receptors, which are expressed in lipid raft and non-raft domains, produce cAMP responses near the plasma membrane that are distinctly different from those produced by E-type prostaglandin receptors, which are expressed exclusively in non-raft domains. We also found that there are differences in basal cAMP levels associated with lipid raft and non-raft domains, and that this can be explained by differences in basal adenylyl cyclase activity associated with each of these membrane environments. In addition, we found evidence that phosphodiesterases 2, 3, and 4 work together in regulating cAMP activity associated with both lipid raft and non-raft domains, while phosphodiesterase 3 plays a more prominent role in the bulk cytoplasmic compartment.

Conclusion: These results suggest that different membrane domains contribute to the formation of distinct pools of cAMP under basal conditions as well as following receptor stimulation in adult ventricular myocytes.

Adenosine Augmentation Evoked by an ENT1 Inhibitor Improves Memory Impairment and Neuronal Plasticity in the APP/PS1 Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive impairment and synaptic dysfunction. Adenosine is an important homeostatic modulator that controls the bioenergetic network in the brain through regulating receptor-evoked signaling pathways, bioenergetic machineries, and epigenetic-mediated gene regulation. Equilibrative nucleoside transporter 1 (ENT1) is a major adenosine transporter that recycles adenosine from the extracellular space. In the present study, we report that a small adenosine analogue (designated J4) that inhibited ENT1 prevented the decline in spatial memory in an AD mouse model (APP/PS1). Electrophysiological and biochemical analyses further demonstrated that chronic treatment with J4 normalized the impaired basal synaptic transmission and long-term potentiation (LTP) at Schaffer collateral synapses as well as the aberrant expression of synaptic proteins (e.g., NR2A and NR2B), abnormal neuronal plasticity-related signaling pathways (e.g., PKA and GSK3β), and detrimental elevation in astrocytic A_2AR expression in the hippocampus and cortex of APP/PS1 mice. In conclusion, our findings suggest that modulation of adenosine homeostasis by J4 is beneficial in a mouse model of AD. Our study provides a potential therapeutic strategy to delay the progression of AD.

cAMPr: A single-wavelength fluorescent sensor for cyclic AMP

Genetically encoded fluorescent sensors enable cell-specific measurements of ions and small molecules in real time. Cyclic adenosine monophosphate (cAMP) is one of the most important signaling molecules in virtually all cell types and organisms. We describe cAMPr, a new single-wavelength cAMP sensor. We developed cAMPr in bacteria and embryonic stem cells and validated the sensor in mammalian neurons in vitro and in Drosophila circadian pacemaker neurons in intact brains. Comparison with other single-wavelength cAMP sensors showed that cAMPr improved the quantitative detection of cAMP abundance. In addition, cAMPr is compatible with both single-photon and two-photon imaging. This enabled us to use cAMPr together with the red fluorescent Ca²⁺ sensor RCaMP1h to simultaneously monitor Ca²⁺ and cAMP in Drosophila brains. Thus, cAMPr is a new and versatile genetically encoded cAMP sensor.

Adenylyl cyclase 1 as a major isoform to generate cAMP signaling for apoA-1-mediated cholesterol efflux pathway

HDL apoA-1-mediated cholesterol efflux pathway requires multiple cellular proteins and signal transduction processes, including adenylyl cyclase (AC)/cAMP signaling. Due to the existence of multiple transmembrane AC isoforms, it was not known how many AC isoforms are expressed and which ones are essential for cholesterol efflux in macrophage foam cells. These questions were investigated in THP-1 macrophages in this study. Quantitative RT-PCR detected mRNAs for all nine transmembrane AC isoforms, but only the mRNA and protein of the AC1 isoform were consistently upregulated by cholesterol loading and apoA-1. AC1 shRNA interference decreased AC1 mRNA and protein levels, resulting in reduction of apoA-1-mediated cAMP production and cholesterol efflux, while the intracellular cholesterol levels remained high. Confocal microscopy showed that apoA-1 promoted translocation of cholesterol and formation of cholesterol-apoA-1 complexes (protrusions) on the cholesterol-loaded macrophage surface. AC1 shRNA-interfered macrophages showed no translocation of cholesterol to the cell surface. AC1 shRNA interference also disrupted cellular localization of the intracellular cholesterol indicator protein adipophillin, and the expression as well as surface translocation of ABCA1. Together, our results show that AC1 is a major isoform for apoA-1-activated cAMP signaling to promote cholesterol transport and exocytosis to the surface of THP-1 macrophage foam cells.

Phosphodiesterase Diversity and Signal Processing Within cAMP Signaling Networks

A large number of neuromodulators activate G-protein coupled receptors (GPCRs) and mediate their cellular actions via the regulation of intracellular cAMP, the small highly diffusible second messenger. In fact, in the same neuron several different GPCRs can regulate cAMP with seemingly identical timecourses that give rise to distinct signaling outcomes, suggesting that cAMP does not have equivalent access to all its downstream effectors and may exist within defined intracellular pools or domains. cAMP compartmentalization is the process that allows the neuron to differentially interpret these various intracellular cAMP signals into cellular response. The molecular mechanisms that give rise to cAMP compartmentalization are not fully understood, but it is thought that phosphodiesterases (PDEs), the enzymes that degrade cAMP, significantly contribute to this process. PDEs, as the sole mechanism of signal termination for cAMP, hold great promise as therapeutic targets for pathologies that are due to the dysregulation of intracellular cAMP signaling. Due to their diverse catalytic activity, regulation and localization each PDE subtype expressed in a given neuron may have a distinct role on downstream signaling.