Analysis of compartmentalized cAMP: a method to compare signals from differently targeted FRET reporters

Phosphodiesterase type 4 anchoring regulates cAMP signaling to Popeye domain-containing proteins

Cyclic AMP is a ubiquitous second messenger used to transduce intracellular signals from a variety of Gs-coupled receptors. Compartmentalisation of protein intermediates within the cAMP signaling pathway underpins receptor-specific responses. The cAMP effector proteins protein-kinase A and EPAC are found in complexes that also contain phosphodiesterases whose presence ensures a coordinated cellular response to receptor activation events. Popeye domain containing (POPDC) proteins are the most recent class of cAMP effectors to be identified and have crucial roles in cardiac pacemaking and conduction. We report the first observation that POPDC proteins exist in complexes with members of the PDE4 family in cardiac myocytes. We show that POPDC1 preferentially binds the PDE4A sub-family via a specificity motif in the PDE4 UCR1 region and that PDE4s bind to the Popeye domain of POPDC1 in a region known to be susceptible to a mutation that causes human disease. Using a cell-permeable disruptor peptide that displaces the POPDC1-PDE4 complex we show that PDE4 activity localized to POPDC1 modulates cycle length of spontaneous Ca²⁺ transients firing in intact mouse sinoatrial nodes.

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POPDC1 forms a complex with type 4 phosphodiesterases (PDE4s) in cardiac myocytes.

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POPDC1 binds PDE4 enzymes in the Upstream Conserved Region 1 (UCR1) domain.

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The PDE4 binding motif within the Popeye domain lies in a region that harbours a mutation, which underpins human disease.

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Disruption of the POPDC1-PDE4 complex modulates the cycle length of spontaneous Ca2+ transients in the sinoatrial node.

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Disruption of the POPDC1-PDE4 complex causes a significant prolongation of the action potential repolarization phase.

Subcellular Organization of the cAMP Signaling Pathway

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

Establishing a sensitive fluorescence-based quantification method for cyclic nucleotides

BACKGROUND

Approximately 40% of prescribed drugs exert their activity via GTP-binding protein-coupled receptors (GPCRs). Once activated, these receptors cause transient changes in the concentration of second messengers, e.g., cyclic adenosine 3',5'-monophosphate (cAMP). Specific and efficacious genetically encoded biosensors have been developed to monitor cAMP fluctuations with high spatial and temporal resolution in living cells or tissue. A well characterized biosensor for cAMP is the Förster resonance energy transfer (FRET)-based Epac1-camps protein. Pharmacological characterization of newly developed ligands acting at GPCRs often includes numerical quantification of the second messenger amount that was produced.

RESULTS

To quantify cellular cAMP concentrations, we bacterially over-expressed and purified Epac1-camps and applied the purified protein in a cell-free detection assay for cAMP in a multi-well format. We found that the biosensor can detect as little as 0.15 pmol of cAMP, and that the sensitivity is not impaired by non-physiological salt concentrations or pH values. Notably, the assay tolerated desiccation and storage of the protein without affecting Epac1-camps cyclic nucleotide sensitivity.

CONCLUSIONS

We found that determination cAMP in lysates obtained from cell assays or tissue samples by purified Epac1-camps is a robust, fast, and sensitive assay suitable for routine and high throughput analyses.

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.

Pseudoscaffolds and anchoring proteins: the difference is in the details

Pseudokinases and pseudophosphatases possess the ability to bind substrates without catalyzing their modification, thereby providing a mechanism to recruit potential phosphotargets away from active enzymes. Since many of these pseudoenzymes possess other characteristics such as localization signals, separate catalytic sites, and protein-protein interaction domains, they have the capacity to influence signaling dynamics in local environments. In a similar manner, the targeting of signaling enzymes to subcellular locations by A-kinase-anchoring proteins (AKAPs) allows for precise and local control of second messenger signaling events. Here, we will discuss how pseudoenzymes form 'pseudoscaffolds' and compare and contrast this compartment-specific regulatory role with the signal organization properties of AKAPs. The mitochondria will be the focus of this review, as they are dynamic organelles that influence a broad range of cellular processes such as metabolism, ATP synthesis, and apoptosis.

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.

Simultaneous assessment of cAMP signaling events in different cellular compartments using FRET-based reporters

Several aspects of the cAMP signaling cascade, including the levels of the messenger itself and the activity of its main effector protein kinase A (PKA), can be measured in living cells, thanks to genetically encoded probes based on fluorescence resonance energy transfer (FRET). While these biosensors enable the assessment of cAMP or PKA activity with great spatial and temporal resolution, concomitant events triggered by the same stimuli at the same or other cellular compartments are not easily assessed. In this chapter we present a simple approach that allows the simultaneous measurement of cAMP and its actions in subcellular compartments of neighboring cells. As proof of principle, we compare cAMP signals and PKA activity in the cytosol of neighboring HEK cells. We propose that this flexible and powerful method can significantly improve the direct comparison of cAMP signals and their action in specific cellular domains.

Development of computational models of cAMP signaling

Despite the growing evidence defining the cAMP signaling network as a master regulator of cellular function in a number of tissues, regulatory feedback loops, signal compartmentalization, as well as cross-talk with other signaling pathways make understanding the emergent properties of cAMP cellular action a daunting task. Dynamical models of signaling that combine quantitative rigor with molecular details can contribute valuable mechanistic insight into the complexity of intracellular cAMP signaling by complementing and guiding experimental efforts. In this chapter, we review the development of cAMP computational models. We describe how features of the cAMP network can be represented and review the types of experimental data useful in modeling cAMP signaling. We also compile a list of published cAMP models that can aid in the development of novel dynamical models of cAMP signaling.

cAMP signalling meets mitochondrial compartments

Mitochondria are highly dynamic organelles comprising at least three distinct areas, the OMM (outer mitochondrial membrane), the IMS (intermembrane space) and the mitochondrial matrix. Physical compartmentalization allows these organelles to host different functional domains and therefore participate in a variety of important cellular actions such as ATP synthesis and programmed cell death. In a surprising homology, it is now widely accepted that the ubiquitous second messenger cAMP uses the same stratagem, compartmentalization, in order to achieve the characteristic functional pleiotropy of its pathway. Accumulating evidence suggests that all the main mitochondrial compartments contain segregated cAMP cascades; however, the regulatory properties and functional significance of such domains are not fully understood and often remain controversial issues. The present mini-review discusses our current knowledge of how the marriage between mitochondrial and cAMP compartmentalization is achieved and its effects on the biology of the cell.

In vivo model with targeted cAMP biosensor reveals changes in receptor-microdomain communication in cardiac disease

3',5'-cyclic adenosine monophosphate (cAMP) is an ubiquitous second messenger that regulates physiological functions by acting in distinct subcellular microdomains. Although several targeted cAMP biosensors are developed and used in single cells, it is unclear whether such biosensors can be successfully applied in vivo, especially in the context of disease. Here, we describe a transgenic mouse model expressing a targeted cAMP sensor and analyse microdomain-specific second messenger dynamics in the vicinity of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA). We demonstrate the biocompatibility of this targeted sensor and its potential for real-time monitoring of compartmentalized cAMP signalling in adult cardiomyocytes isolated from a healthy mouse heart and from an in vivo cardiac disease model. In particular, we uncover the existence of a phosphodiesterase-dependent receptor-microdomain communication, which is affected in hypertrophy, resulting in reduced β-adrenergic receptor-cAMP signalling to SERCA.

cAMP signaling in subcellular compartments

In the complex microcosm of a cell, information security and its faithful transmission are critical for maintaining internal stability. To achieve a coordinated response of all its parts to any stimulus the cell must protect the information received from potentially confounding signals. Physical segregation of the information transmission chain ensures that only the entities able to perform the encoded task have access to the relevant information. The cAMP intracellular signaling pathway is an important system for signal transmission responsible for the ancestral 'flight or fight' response and involved in the control of critical functions including frequency and strength of heart contraction, energy metabolism and gene transcription. It is becoming increasingly apparent that the cAMP signaling pathway uses compartmentalization as a strategy for coordinating the large number of key cellular functions under its control. Spatial confinement allows the formation of cAMP signaling "hot spots" at discrete subcellular domains in response to specific stimuli, bringing the information in proximity to the relevant effectors and their recipients, thus achieving specificity of action. In this report we discuss how the different constituents of the cAMP pathway are targeted and participate in the formation of cAMP compartmentalized signaling events. We illustrate a few examples of localized cAMP signaling, with a particular focus on the nucleus, the sarcoplasmic reticulum and the mitochondria. Finally, we discuss the therapeutic potential of interventions designed to perturb specific cAMP cascades locally.

Förster resonance energy transfer - an approach to visualize the spatiotemporal regulation of macromolecular complex formation and compartmentalized cell signaling

BACKGROUND

Signaling messengers and effector proteins provide an orchestrated molecular machinery to relay extracellular signals to the inside of cells and thereby facilitate distinct cellular behaviors. Formations of intracellular macromolecular complexes and segregation of signaling cascades dynamically regulate the flow of a biological process.

SCOPE OF REVIEW

In this review, we provide an overview of the development and application of FRET technology in monitoring cyclic nucleotide-dependent signalings and protein complexes associated with these signalings in real time and space with brief mention of other important signaling messengers and effector proteins involved in compartmentalized signaling.

MAJOR CONCLUSIONS

The preciseness, rapidity and specificity of cellular responses indicate restricted alterations of signaling messengers, particularly in subcellular compartments rather than globally. Not only the physical confinement and selective depletion, but also the intra- and inter-molecular interactions of signaling effectors modulate the direction of signal transduction in a compartmentalized fashion. To understand the finer details of various intracellular signaling cascades and crosstalk between proteins and other effectors, it is important to visualize these processes in live cells. Förster Resonance Energy Transfer (FRET) has been established as a useful tool to do this, even with its inherent limitations.

GENERAL SIGNIFICANCE

FRET technology remains as an effective tool for unraveling the complex organization and distribution of various endogenous signaling proteins, as well as the spatiotemporal dynamics of second messengers inside a single cell to distinguish the heterogeneity of cell signaling under normal physiological conditions and during pathological events.