Receptor-associated independent cAMP nanodomains mediate spatiotemporal specificity of GPCR signaling.

G protein-coupled receptors (GPCRs) relay extracellular stimuli into specific cellular functions. Cells express many different GPCRs, but all these GPCRs signal to only a few second messengers such as cAMP. It is largely unknown how cells distinguish between signals triggered by different GPCRs to orchestrate their complex functions. Here, we demonstrate that individual GPCRs signal via receptor-associated independent cAMP nanodomains (RAINs) that constitute self-sufficient, independent cell signaling units. Low concentrations of glucagon-like peptide 1 (GLP-1) and isoproterenol exclusively generate highly localized cAMP pools around GLP-1- and ß2-adrenergic receptors, respectively, which are protected from cAMP originating from other receptors and cell compartments. Mapping local cAMP concentrations with engineered GPCR nanorulers reveals gradients over only tens of nanometers that define the size of individual RAINs. The coexistence of many such RAINs allows a single cell to operate thousands of independent cellular signals simultaneously, rather than function as a simple "on/off" switch.

Integrated Proteomics Unveils Nuclear PDE3A2 as a Regulator of Cardiac Myocyte Hypertrophy.

BACKGROUND: Signaling by cAMP is organized in multiple distinct subcellular nanodomains regulated by cAMP-hydrolyzing PDEs (phosphodiesterases). Cardiac ß-adrenergic signaling has served as the prototypical system to elucidate cAMP compartmentalization. Although studies in cardiac myocytes have provided an understanding of the location and properties of a handful of cAMP subcellular compartments, an overall view of the cellular landscape of cAMP nanodomains is missing. METHODS: Here, we combined an integrated phosphoproteomics approach that takes advantage of the unique role that individual PDEs play in the control of local cAMP, with network analysis to identify previously unrecognized cAMP nanodomains associated with ß-adrenergic stimulation. We then validated the composition and function of one of these nanodomains using biochemical, pharmacological, and genetic approaches and cardiac myocytes from both rodents and humans. RESULTS: We demonstrate the validity of the integrated phosphoproteomic strategy to pinpoint the location and provide critical cues to determine the function of previously unknown cAMP nanodomains. We characterize in detail one such compartment and demonstrate that the PDE3A2 isoform operates in a nuclear nanodomain that involves SMAD4 (SMAD family member 4) and HDAC-1 (histone deacetylase 1). Inhibition of PDE3 results in increased HDAC-1 phosphorylation, leading to inhibition of its deacetylase activity, derepression of gene transcription, and cardiac myocyte hypertrophic growth. CONCLUSIONS: We developed a strategy for detailed mapping of subcellular PDE-specific cAMP nanodomains. Our findings reveal a mechanism that explains the negative long-term clinical outcome observed in patients with heart failure treated with PDE3 inhibitors.

Activation of IP3R in atrial cardiomyocytes leads to generation of cytosolic cAMP.

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia. Excessive stimulation of the inositol (1,4,5)-trisphosphate (IP3) signaling pathway has been linked to AF through abnormal calcium handling. However, little is known about the mechanisms involved in this process. We expressed the fluorescence resonance energy transfer (FRET)-based cytosolic cyclic adenosine monophosphate (cAMP) sensor EPAC-SH187 in neonatal rat atrial myocytes (NRAMs) and neonatal rat ventricular myocytes (NRVMs). In NRAMs, the addition of the α1-agonist, phenylephrine (PE, 3 µM), resulted in a FRET change of 21.20 ± 7.43%, and the addition of membrane-permeant IP3 derivative 2,3,6-tri-O-butyryl-myo-IP3(1,4,5)-hexakis(acetoxymethyl)ester (IP3-AM, 20 µM) resulted in a peak of 20.31 ± 6.74%. These FRET changes imply an increase in cAMP. Prior application of IP3 receptor (IP3R) inhibitors 2-aminoethyl diphenylborinate (2-APB, 2.5 µM) or Xestospongin-C (0.3 µM) significantly inhibited the change in FRET in NRAMs in response to PE. Xestospongin-C (0.3 µM) significantly inhibited the change in FRET in NRAMs in response to IP3-AM. The FRET change in response to PE in NRVMs was not inhibited by 2-APB or Xestospongin-C. Finally, the localization of cAMP signals was tested by expressing the FRET-based cAMP sensor, AKAP79-CUTie, which targets the intracellular surface of the plasmalemma. We found in NRAMs that PE led to FRET change corresponding to an increase in cAMP that was inhibited by 2-APB and Xestospongin-C. These data support further investigation of the proarrhythmic nature and components of IP3-induced cAMP signaling to identify potential pharmacological targets.NEW & NOTEWORTHY This study shows that indirect activation of the IP3 pathway in atrial myocytes using phenylephrine and direct activation using IP3-AM leads to an increase in cAMP and is in part localized to the cell membrane. These changes can be pharmacologically inhibited using IP3R inhibitors. However, the cAMP rise in ventricular myocytes is independent of IP3R calcium release. Our data support further investigation into the proarrhythmic nature of IP3-induced cAMP signaling.

Multi-Compartment, Early Disruption of cGMP and cAMP Signalling in Cardiac Myocytes from the mdx Model of Duchenne Muscular Dystrophy.

Duchenne muscular dystrophy (DMD) is the most frequent and severe form of muscular dystrophy. The disease presents with progressive body-wide muscle deterioration and, with recent advances in respiratory care, cardiac involvement is an important cause of morbidity and mortality. DMD is caused by mutations in the dystrophin gene resulting in the absence of dystrophin and, consequently, disturbance of other proteins that form the dystrophin-associated protein complex (DAPC), including neuronal nitric oxide synthase (nNOS). The molecular mechanisms that link the absence of dystrophin with the alteration of cardiac function remain poorly understood but disruption of NO-cGMP signalling, mishandling of calcium and mitochondrial disturbances have been hypothesized to play a role. cGMP and cAMP are second messengers that are key in the regulation of cardiac myocyte function and disruption of cyclic nucleotide signalling leads to cardiomyopathy. cGMP and cAMP signals are compartmentalised and local regulation relies on the activity of phosphodiesterases (PDEs). Here, using genetically encoded FRET reporters targeted to distinct subcellular compartments of neonatal cardiac myocytes from the DMD mouse model mdx, we investigate whether lack of dystrophin disrupts local cyclic nucleotide signalling, thus potentially providing an early trigger for the development of cardiomyopathy. Our data show a significant alteration of both basal and stimulated cyclic nucleotide levels in all compartments investigated, as well as a complex reorganization of local PDE activities.

Control of ßAR- and N-methyl-D-aspartate (NMDA) Receptor-Dependent cAMP Dynamics in Hippocampal Neurons.

Norepinephrine, a neuromodulator that activates ß-adrenergic receptors (ßARs), facilitates learning and memory as well as the induction of synaptic plasticity in the hippocampus. Several forms of long-term potentiation (LTP) at the Schaffer collateral CA1 synapse require stimulation of both ßARs and N-methyl-D-aspartate receptors (NMDARs). To understand the mechanisms mediating the interactions between ßAR and NMDAR signaling pathways, we combined FRET imaging of cAMP in hippocampal neuron cultures with spatial mechanistic modeling of signaling pathways in the CA1 pyramidal neuron. Previous work implied that cAMP is synergistically produced in the presence of the ßAR agonist isoproterenol and intracellular calcium. In contrast, we show that when application of isoproterenol precedes application of NMDA by several minutes, as is typical of ßAR-facilitated LTP experiments, the average amplitude of the cAMP response to NMDA is attenuated compared with the response to NMDA alone. Models simulations suggest that, although the negative feedback loop formed by cAMP, cAMP-dependent protein kinase (PKA), and type 4 phosphodiesterase may be involved in attenuating the cAMP response to NMDA, it is insufficient to explain the range of experimental observations. Instead, attenuation of the cAMP response requires mechanisms upstream of adenylyl cyclase. Our model demonstrates that Gs-to-Gi switching due to PKA phosphorylation of ßARs as well as Gi inhibition of type 1 adenylyl cyclase may underlie the experimental observations. This suggests that signaling by ß-adrenergic receptors depends on temporal pattern of stimulation, and that switching may represent a novel mechanism for recruiting kinases involved in synaptic plasticity and memory.

Phosphorylation of ezrin on Thr567 is required for the synergistic activation of cell spreading by EPAC1 and protein kinase A in HEK293T cells.

Recent studies have demonstrated that the actin binding protein, ezrin, and the cAMP-sensor, EPAC1, cooperate to induce cell spreading in response to elevations in intracellular cAMP. To investigate the mechanisms underlying these effects we generated a model of EPAC1-dependent cell spreading based on the stable transfection of EPAC1 into HEK293T (HEK293T-EPAC1) cells. We found that direct activation of EPAC1 with the EPAC-selective analogue, 8-pCPT-2'-O-Me-cAMP (007), promoted cell spreading in these cells. In addition, co-activation of EPAC1 and PKA, with a combination of the adenylate cyclase activator, forskolin, and the cAMP phosphodiesterase inhibitor, rolipram, was found to synergistically enhance cell spreading, in association with cortical actin bundling and mobilisation of ezrin to the plasma membrane. PKA activation was also associated with phosphorylation of ezrin on Thr567, as detected by an electrophoretic band mobility shift during SDS-PAGE. Inhibition of PKA activity blocked ezrin phosphorylation and reduced the cell spreading response to cAMP elevation to levels induced by EPAC1-activation alone. Transfection of HEK293T-EPAC1 cells with inhibitory ezrin mutants lacking the key PKA phosphorylation site, ezrin-Thr567Ala, or the ability to associate with actin, ezrin-Arg579Ala, promoted cell arborisation and blocked the ability of EPAC1 and PKA to further promote cell spreading. The PKA phospho-mimetic mutants of ezrin, ezrin-Thr567Asp had no effect on EPAC1-driven cell spreading. Our results indicate that association of ezrin with the actin cytoskeleton and phosphorylation on Thr567 are required, but not sufficient, for PKA and EPAC1 to synergistically promote cell spreading following elevations in intracellular cAMP.

Autocatalytic cleavage of Clostridium difficile toxin B.

Clostridium difficile, the causative agent of nosocomial antibiotic-associated diarrhoea and pseudomembranous colitis, possesses two main virulence factors: the large clostridial cytotoxins A and B. It has been proposed that toxin B is cleaved by a cytosolic factor of the eukaryotic target cell during its cellular uptake. Here we report that cleavage of not only toxin B, but also all other large clostridial cytotoxins, is an autocatalytic process dependent on host cytosolic inositolphosphate cofactors. A covalent inhibitor of aspartate proteases, 1,2-epoxy-3-(p-nitrophenoxy)propane, completely blocked toxin B function on cultured cells and was used to identify its catalytically active protease site. To our knowledge this is the first report on a bacterial toxin that uses eukaryotic signals for induced autoproteolysis to deliver its toxic domain into the cytosol of target cells. On the basis of our data, we present an integrated model for the uptake and inositolphosphate-induced activation of toxin B.

cAMP Compartmentalisation in Human Myometrial Cells.

Preterm birth is the leading cause of childhood mortality and morbidity. A better understanding of the processes that drive the onset of human labour is essential to reduce the adverse perinatal outcomes associated with dysfunctional labour. Beta-mimetics, which activate the myometrial cyclic adenosine monophosphate (cAMP) system, successfully delay preterm labour, suggesting a key role for cAMP in the control of myometrial contractility; however, the mechanisms underpinning this regulation are incompletely understood. Here we used genetically encoded cAMP reporters to investigate cAMP signalling in human myometrial smooth muscle cells at the subcellular level. We found significant differences in the dynamics of the cAMP response in the cytosol and at the plasmalemma upon stimulation with catecholamines or prostaglandins, indicating compartment-specific handling of cAMP signals. Our analysis uncovered significant disparities in the amplitude, kinetics, and regulation of cAMP signals in primary myometrial cells obtained from pregnant donors compared with a myometrial cell line and found marked response variability between donors. We also found that in vitro passaging of primary myometrial cells had a profound impact on cAMP signalling. Our findings highlight the importance of cell model choice and culture conditions when studying cAMP signalling in myometrial cells and we provide new insights into the spatial and temporal dynamics of cAMP in the human myometrium.

Disrupted propionate metabolism evokes transcriptional changes in the heart by increasing histone acetylation and propionylation.

Propiogenic substrates and gut bacteria produce propionate, a post-translational protein modifier. In this study, we used a mouse model of propionic acidaemia (PA) to study how disturbances to propionate metabolism result in histone modifications and changes to gene expression that affect cardiac function. Plasma propionate surrogates were raised in PA mice, but female hearts manifested more profound changes in acyl-CoAs, histone propionylation and acetylation, and transcription. These resulted in moderate diastolic dysfunction with raised diastolic Ca2+, expanded end-systolic ventricular volume and reduced stroke volume. Propionate was traced to histone H3 propionylation and caused increased acetylation genome-wide, including at promoters of Pde9a and Mme, genes related to contractile dysfunction through downscaled cGMP signaling. The less severe phenotype in male hearts correlated with ß-alanine buildup. Raising ß-alanine in cultured myocytes treated with propionate reduced propionyl-CoA levels, indicating a mechanistic relationship. Thus, we linked perturbed propionate metabolism to epigenetic changes that impact cardiac function.

Micro-2D Cell Culture for cAMP Measurements Using FRET Reporters in Human iPSC-Derived Cardiomyocytes.

In the last years human induced pluripotent stem cell-derived cardiomyocytes (hIPS-CMs) have emerged as a promising alternative to rodent-derived cardiomyocytes. However, as the differentiation process is lengthy and commercially available cells are expensive, the cell number is limited. Here we provide detailed information on how to scale down 2D cell cultures of hIPS-CMs for the purpose of cAMP FRET measurements, thereby extending the number of possible experiments by more than tenfold. Crucial factors like cell density or cell number to culturing media volume can be maintained exactly as under normal culturing conditions and existing equipment does not need to be modified.The chapter covers the preparation of downscaled cell culture vessels, coating and seeding procedures, transduction or transfection of the cells with a genetically encoded cAMP FRET sensor, performing real-time cAMP FRET measurements with this sensor and the analysis of generated imaging data. Numbers for seeding areas, seeding densities, coating volumes and concentrations, media volumes, and concentrations of reagents are given as guidelines.

Phosphodiesterase 2A2 regulates mitochondria clearance through Parkin-dependent mitophagy.

Programmed degradation of mitochondria by mitophagy, an essential process to maintain mitochondrial homeostasis, is not completely understood. Here we uncover a regulatory process that controls mitophagy and involves the cAMP-degrading enzyme phosphodiesterase 2A2 (PDE2A2). We find that PDE2A2 is part of a mitochondrial signalosome at the mitochondrial inner membrane where it interacts with the mitochondrial contact site and organizing system (MICOS). As part of this compartmentalised signalling system PDE2A2 regulates PKA-mediated phosphorylation of the MICOS component MIC60, resulting in modulation of Parkin recruitment to the mitochondria and mitophagy. Inhibition of PDE2A2 is sufficient to regulate mitophagy in the absence of other triggers, highlighting the physiological relevance of PDE2A2 in this process. Pharmacological inhibition of PDE2 promotes a 'fat-burning' phenotype to retain thermogenic beige adipocytes, indicating that PDE2A2 may serve as a novel target with potential for developing therapies for metabolic disorders.

Troponin destabilization impairs sarcomere-cytoskeleton interactions in iPSC-derived cardiomyocytes from dilated cardiomyopathy patients.

The sarcomeric troponin-tropomyosin complex is a critical mediator of excitation-contraction coupling, sarcomeric stability and force generation. We previously reported that induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from patients with a dilated cardiomyopathy (DCM) mutation, troponin T (TnT)-R173W, display sarcomere protein misalignment and impaired contractility. Yet it is not known how TnT mutation causes dysfunction of sarcomere microdomains and how these events contribute to misalignment of sarcomeric proteins in presence of DCM TnT-R173W. Using a human iPSC-CM model combined with CRISPR/Cas9-engineered isogenic controls, we uncovered that TnT-R173W destabilizes molecular interactions of troponin with tropomyosin, and limits binding of PKA to local sarcomere microdomains. This attenuates troponin phosphorylation and dysregulates local sarcomeric microdomains in DCM iPSC-CMs. Disrupted microdomain signaling impairs MYH7-mediated, AMPK-dependent sarcomere-cytoskeleton filament interactions and plasma membrane attachment. Small molecule-based activation of AMPK can restore TnT microdomain interactions, and partially recovers sarcomere protein misalignment as well as impaired contractility in DCM TnT-R173W iPSC-CMs. Our findings suggest a novel therapeutic direction targeting sarcomere- cytoskeleton interactions to induce sarcomere re-organization and contractile recovery in DCM.

Quantification and Comparison of Signals Generated by Different FRET-Based cAMP Reporters.

A variety of FRET-based biosensors are currently in use for real-time monitoring of dynamic changes of intracellular cAMP. Due to differences in sensor properties, unique features of the cell type under examination and diverse specifications of the imaging setups in different laboratories, data generated using these sensors may not be immediately comparable within the same study or across studies. To facilitate comparison, often FRET data are normalized and expressed as fractional change of the maximal FRET response at sensor saturation. However, this approach may lead to misinterpretation of the underlying cAMP change. In this chapter, we provide examples of the problems that may arise when using normalized FRET data and present a method based on the conversion of FRET ratio changes into actual cAMP concentrations that mitigates these issues.

RNase-induced apoptosis: fate of calcium-activated potassium channels.

The connection between the action of microbial RNases and Ca2+-activated K+ (KCa) channels was investigated in human embryo kidney cells HEKhSK4 artificially expressing the channels. These channels protected HEKhSK4 cells from apoptosis induced by binase and 5K charge reversal mutant of RNase Sa. After the first 24h, potassium current increased without increase in intracellular Ca2+, and mitochondrial potential remained high. After 72 h, the concentration of calcium increased and mitochondria lost their potential. Whole-cell recordings of membrane currents through KCa channels in RNase-treated cells demonstrated a biphasic pattern: initially their activity in cell population increased, peaked at 24h, and then gradually decreased. In each individual cell we observed either an increase of the amplitude of KCa current, or a complete shutdown of the channels. The activity of KCa channels could be restored by removing RNases from the media. Based on this pattern and especially its timing, we hypothesize that toxic RNases downregulate KCa channels at the level of transcription or translation. Our results indicate that new anticancer agents could be created on the basis of microbial RNases targeting KCa channels.

Increase in Ca2+ current by sustained cAMP levels enhances proliferation rate in GH3 cells.

AIMS: Ca2+ and cAMP are important intracellular modulators. In order to generate intracellular signals with various amplitudes, as well as different temporal and spatial properties, a tightly and precise control of these modulators in intracellular compartments is necessary. The aim of this study was to evaluate the effects of elevated and sustained cAMP levels on voltage-dependent Ca2+ currents and proliferation in pituitary tumor GH3 cells. MAIN METHODS: Effect of long-term exposure to forskolin and dibutyryl-cyclic AMP (dbcAMP) on Ca2+ current density and cell proliferation rate were determined by using the whole-cell patch-clamp technique and real time cell monitoring system. The cAMP levels were assayed, after exposing transfected GH3 cells with the EPAC-1 cAMP sensor to forskolin and dbcAMP, by FRET analysis. KEY FINDINGS: Sustained forskolin treatment (24 and 48h) induced a significant increase in total Ca2+ current density in GH3 cells. Accordingly, dibutyryl-cAMP incubation (dbcAMP) also elicited increase in Ca2+ current density. However, the maximum effect of dbcAMP occurred only after 72h incubation, whereas forskolin showed maximal effect at 48h. FRET-experiments confirmed that the time-course to elevate intracellular cAMP was distinct between forskolin and dbcAMP. Mibefradil inhibited the fast inactivating current component selectively, indicating the recruitment of T-type Ca2+ channels. A significant increase on cell proliferation rate, which could be related to the elevated and sustained intracellular levels of cAMP was observed. SIGNIFICANCE: We conclude that maintaining high levels of intracellular cAMP will cause an increase in Ca2+ current density and this phenomenon impacts proliferation rate in GH3 cells.

Why Escherichia coli alpha-hemolysin induces calcium oscillations in mammalian cells--the pore is on its own.

Escherichia coli alpha-hemolysin (HlyA), archetype of a bacterial pore-forming toxin, has been reported to deregulate physiological Ca2+ channels, thus inducing periodic low-frequency Ca2+ oscillations that trigger transcriptional processes in mammalian cells. The present study was undertaken to delineate the mechanisms underlying the Ca2+ oscillations. Patch-clamp experiments were combined with single cell measurements of intracellular Ca2+ and with flowcytometric analyses. Application of HlyA at subcytocidal concentrations provoked Ca2+ oscillations in human renal and endothelial cells. However, contrary to the previous report, the phenomenon could not be inhibited by the Ca2+ channel blocker nifedipine and Ca2+ oscillations showed no constant periodicity at all. Ca2+ oscillations were dependent on the pore-forming activity of HlyA: application of a nonhemolytic but bindable toxin had no effect. Washout experiments revealed that Ca2+ oscillations could not be maintained in the absence of toxin in the medium. Analogously, propidium iodide flux into cells occurred in the presence of HlyA, but cells rapidly became impermeable toward the dye after toxin washout, indicating resealing or removal of the membrane lesions. Finally, patch-clamp experiments revealed temporal congruence between pore formation and Ca2+ influx. We conclude that the nonperiodic Ca2+ oscillations induced by HlyA are not due to deregulation of physiological Ca2+ channels but derive from pulsed influxes of Ca2+ as a consequence of formation and rapid closure of HlyA pores in mammalian cell membranes.

Activation of PKA in cell requires higher concentration of cAMP than in vitro: implications for compartmentalization of cAMP signalling.

cAMP is a ubiquitous second messenger responsible for the cellular effects of multiple hormones and neurotransmitters via activation of its main effector, protein kinase A (PKA). Multiple studies have shown that the basal concentration of cAMP in several cell types is about 1 µM. This value is well above the reported concentration of cAMP required to half-maximally activate PKA, which measures in the 100-300 nM range. Several hypotheses have been suggested to explain this apparent discrepancy including inaccurate measurements of intracellular free cAMP, inaccurate measurement of the apparent activation constant of PKA or shielding of PKA from bulk cytosolic cAMP via localization of the enzyme to microdomains with lower basal cAMP concentration. However, direct experimental evidence in support of any of these models is limited and a firm conclusion is missing. In this study we use multiple FRET-based reporters for the detection of cAMP and PKA activity in intact cells and we establish that the sensitivity of PKA to cAMP is almost twenty times lower when measured in cell than when measured in vitro. Our findings have important implications for the understanding of compartmentalized cAMP signalling.

FRET biosensor uncovers cAMP nano-domains at ß-adrenergic targets that dictate precise tuning of cardiac contractility.

Compartmentalized cAMP/PKA signalling is now recognized as important for physiology and pathophysiology, yet a detailed understanding of the properties, regulation and function of local cAMP/PKA signals is lacking. Here we present a fluorescence resonance energy transfer (FRET)-based sensor, CUTie, which detects compartmentalized cAMP with unprecedented accuracy. CUTie, targeted to specific multiprotein complexes at discrete plasmalemmal, sarcoplasmic reticular and myofilament sites, reveals differential kinetics and amplitudes of localized cAMP signals. This nanoscopic heterogeneity of cAMP signals is necessary to optimize cardiac contractility upon adrenergic activation. At low adrenergic levels, and those mimicking heart failure, differential local cAMP responses are exacerbated, with near abolition of cAMP signalling at certain locations. This work provides tools and fundamental mechanistic insights into subcellular adrenergic signalling in normal and pathological cardiac function.

Sustained exposure to catecholamines affects cAMP/PKA compartmentalised signalling in adult rat ventricular myocytes.

In the heart compartmentalisation of cAMP/protein kinase A (PKA) signalling is necessary to achieve a specific functional outcome in response to different hormonal stimuli. Chronic exposure to catecholamines is known to be detrimental to the heart and disrupted compartmentalisation of cAMP signalling has been associated to heart disease. However, in most cases it remains unclear whether altered local cAMP signalling is an adaptive response, a consequence of the disease or whether it contributes to the pathogenetic process. We have previously demonstrated that isoforms of PKA expressed in cardiac myocytes, PKA-I and PKA-II, localise to different subcellular compartments and are selectively activated by spatially confined pools of cAMP, resulting in phosphorylation of distinct downstream targets. Here we investigate cAMP signalling in an in vitro model of hypertrophy in primary adult rat ventricular myocytes. By using a real time imaging approach and targeted reporters we find that that sustained exposure to catecholamines can directly affect cAMP/PKA compartmentalisation. This appears to involve a complex mechanism including both changes in the subcellular localisation of individual phosphodiesterase (PDE) isoforms as well as the relocalisation of PKA isoforms. As a result, the preferential coupling of PKA subsets with different PDEs is altered resulting in a significant difference in the level of cAMP the kinase is exposed to, with potential impact on phosphorylation of downstream targets.

A novel approach combining real-time imaging and the patch-clamp technique to calibrate FRET-based reporters for cAMP in their cellular microenvironment.

Fluorescence resonance energy transfer (FRET)-based reporters are invaluable tools to study spatiotemporal aspects of cyclic adenosine monophosphate (cAMP) signaling and compartmentalization in living cells. These sensors allow estimation of relative changes of cAMP levels in real-time and intact cells. However, one of their major shortcomings is that they do not easily allow direct measurement of cAMP concentrations. This is mainly due to the fact that the methods to calibrate these sensors in their physiological microenvironment are not generally available. All published approaches to calibrate FRET-based reporters rely at least in part on data derived under nonphysiological conditions. Here, we present a protocol to calibrate FRET reporters completely "in cell." We introduce a combination of FRET imaging of cAMP and the whole-cell patch-clamp techniques to microinfuse or dilute intracellular cAMP to known concentrations. This method represents a general tool to accurately estimate intracellular cAMP concentrations by allocating concentration values to FRET ratio changes.