The protocol presented in this study illustrates the effectiveness of the cAMP Difference Detector In Situ in measuring cAMP through two methods. One method utilizes a 96-well plate reading spectrophotometer with HEK-293 cells. The other method demonstrates individual HASM cells under a fluorescent microscope.
cAMP Difference Detector In Situ (cADDis) is a novel biosensor that allows for the continuous measurement of cAMP levels in living cells. The biosensor is created from a circularly permuted fluorescent protein linked to the hinge region of Epac2. This creates a single fluorophore biosensor that displays either increased or decreased fluorescence upon binding of cAMP. The biosensor exists in red and green upward versions, as well as green downward versions, and several red and green versions targeted to subcellular locations. To illustrate the effectiveness of the biosensor, the green downward version, which decreases in fluorescence upon cAMP binding, was used. Two protocols using this sensor are demonstrated: one utilizing a 96-well plate reading spectrophotometer compatible with high-throughput screening and another utilizing single-cell imaging on a fluorescent microscope. On the plate reader, HEK-293 cells cultured in 96-well plates were stimulated with 10 µM forskolin or 10 nM isoproterenol, which induced rapid and large decreases in fluorescence in the green downward version. The biosensor was used to measure cAMP levels in individual human airway smooth muscle (HASM) cells monitored under a fluorescent microscope. The green downward biosensor displayed similar responses to populations of cells when stimulated with forskolin or isoproterenol. This single-cell assay allows visualization of the biosensor location at 20x and 40x magnification. Thus, this cAMP biosensor is sensitive and flexible, allowing real-time measurement of cAMP in both immortalized and primary cells, and with single cells or populations of cells. These attributes make cADDis a valuable tool for studying cAMP signaling dynamics in living cells.
Adenosine 3′,5′-cyclic monophosphate, cAMP, plays a central role in cellular communication and the coordination of various physiological processes. cAMP acts as a second messenger, relaying external signals from hormones, neurotransmitters, or other extracellular molecules to initiate a cascade of intracellular events1. Moreover, cAMP is intricately involved in various signaling pathways, including those associated with G-protein-coupled receptors (GPCRs) and adenylyl cyclases. Understanding the role of cAMP in cellular signaling is fundamental to unraveling the complex mechanisms that underlie normal cellular functions and the development of potential therapies for a wide range of medical conditions2.
In the past, various methods have been employed to measure cAMP directly or indirectly. These included radiolabeling of cellular ATP pools followed by column purification, HPLC, radioimmunoassays, and enzyme-linked immunoassays1,2. These legacy assays are limited by the fact that they are end-point measures, requiring a large number of samples to construct time-dependent responses. More recently, Fluorescence Resonance Energy Transfer (FRET) sensors were developed to create assays in living cells, producing real-time, dynamic data and allowing sensors to be targeted in different subcellular locations3. FRET leverages two fluorophores, one fluorescent donor, and one fluorescent acceptor that when in close proximity, the acceptor fluorophore will be excited by the donor fluorescent output. The two fluorophores most used are cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) since these have compatible excitation and emission properties. In addition to CFP and YFP, the utilization of the green fluorescent protein (GFP) and red fluorescent protein (RFP) is commonly used for FRET biosensors. cAMP FRET biosensors operate by having a donor and acceptor on opposite ends of the Epac2 cAMP binding protein. cAMP binding alters the confirmation of Epac and increases the distance between donor and acceptor fluorophores3,4. This conformational change is detected by a loss of FRET, that is, the excitation of the acceptor fluorophore by energy transferred from the donor fluorophore drops3. While a seemingly simple process, there are an abundance of limitations and issues with the FRET biosensor for cAMP research5. One of which is the selection of fluorescent proteins, for example, GFP, which can dimerize naturally, thus reducing sensitivity6. FRET-based cAMP biosensors have been targeted to specific microdomains7, but there may be limitations owing to the large size of a construct with two fluorophores6. Another significant issue is the low signal-to-noise ratio of FRET signals resulting from the overlap between excitation and emission of the fluorophore, resulting in high sampling frequency and complicating analysis of the results4,5.
Most recently, the novel biosensor (cAMP Difference Detector In Situ), cADDis has solved these and other limitations when it comes to studying the regulation of cAMP signaling8. One important improvement is the dependence on a single fluorophore. This allows for a rapid and efficient signal with a wider dynamic range and a high signal-to-noise ratio. As a result, there can be more accuracy as there is a less broad scope of wavelengths to comb through8. Like FRET probes, the biosensor has been targeted to subcellular locations, allowing research into the compartmentation theory and exploration in lipid rafts and non-rafts, and other subcellular domains9. Perhaps most important is the suitability of a single fluorophore biosensor for high-throughput screening, which has improved sensitivity and reproducibility over FRET-based biosensors. The biosensor is packaged into a BacMam vector for easy transduction of a wide array of cell types and precise control over protein expression.
Expression control via the BacMam vector can be particularly useful in assays using GPCR orthologs from different species to facilitate the interpretation of data from animal studies. Furthermore, control over receptor expression is critical for measuring the different degrees of drug efficacy (e.g., inverse agonists and partial agonists), and low levels of receptor expression are useful to mimic the low levels found in animal tissue. BacMam is a baculovirus vector that has been modified to transduce mammalian cells such as primary cell cultures and HEK-293 lines10. Dominant selectable markers allow for BacMam to provide more stability over traditional plasmid infections11. Such selective promoters allow for more efficient gene delivery and expression. In addition, adding trichostatin A (a histone deacetylase inhibitor) enhances the reporter protein levels11. Expression levels can be controlled via the titer of the BacMam virus used and should be optimized for each cell type. In the case of this biosensor, a red or green fluorescent protein is linked to the Epac at the N- and C-termini. When cAMP binds, a conformational change in the biosensor moves amino acids adjacent to the fluorescent protein. Such a shift moves the absorbance from the anionic state to the neutral state at 400 nm, thus decreasing the fluorescence.
There are 90-120 GPCRs expressed in a single cell that respond to a wide variety of neurohumoral signals12. Therefore, it can be hypothesized that at least several dozen GPCRs per cell can stimulate or inhibit cAMP through Gs or Gi coupling, respectively. While there has been progress in monitoring this second messenger in real-time, such as FRET, more efficient methods are needed. The methodology for monitoring the synthesis and degradation of cAMP signals using cADDis in real-time is presented here. The change in fluorescence can be monitored in real-time using a fluorescence plate reader for high throughput assays or using a fluorescent microscope for single-cell assays. These methods are useful for a wide array of biological questions regarding GPCR signaling via cAMP.
The details of all the reagents and equipment used for the study are listed in the Table of Materials.
1. Plate reading spectrophotometer high-throughput assay
2. Single-cell assay using an inverted fluorescent microscope
The present study validated the cytosolic biosensor in both plate reader and microscope assays. Once cells expressed the biosensor, they were stimulated with either 10 µM forskolin (a direct activator of adenylyl cyclase), 10 nM isoproterenol (an agonist at ß1AR and ß2AR), or vehicle (Figure 1). The subsequent changes in fluorescence, indicative of cAMP production, were captured every 30 s.
The data was transformed as the change in fluorescence from the initial fluorescence (ΔF/F0). A one-site decay model was applied to the fluorescence data to quantify the temporal changes in cAMP levels across the different conditions (Figure 2 and Figure 3). The parameters of these decay curves can be used to quantify the response to a given concentration of the drug. The product of the decay rate (k) and the plateau as a single value that quantifies the drug response was used, and these were used to create concentration-response curves when multiple concentrations of the same drug are applied13.
On the plate reader, upon stimulation with 10 µM forskolin, the cytosolic sensor showed a sizable decrease in fluorescence intensity within seconds and progressed over many minutes (from the baseline 0 ΔF/Fo at 200 s to -0.7 ΔF/Fo at 600 s) in HEK-293 cells (Figure 2A). Given the green sensor's design, a decrease in fluorescence indicates an increase in cAMP production, indicating that forskolin caused a uniform increase in cAMP levels in the cytosol. Stimulating HEK-293 cells with a sub-maximal concentration of isoproterenol (10 nM) led to rapid but smaller decreases in the biosensor fluorescence (-0.3 ΔF/Fo at 600 s, Figure 2A). When these lower concentrations of isoproterenol (10 nM and 100 nM) were examined in a single well of cells over time, oscillations of cAMP levels were observed in HEK-293 cells (Figure 2B). The periodicity of these oscillations was consistently around 200 s. The routine data analysis includes fitting the response to a single site decay model, which averages these oscillations (see connecting line). Averaging multiple experiments together typically led to these oscillations becoming obscured in the data. Nonetheless, the biosensor displays a sensitivity and rapid kinetics that allow the observation of cAMP oscillations.
cADDis can also be measured using a fluorescence microscope. This approach allows monitoring of cAMP in single cells and can also be adapted to visualize biosensors that are targeted to different subcellular locations. In the present study, the biosensor was used in primary human airway smooth muscle (HASM) cells. Stimulation of HASM with either vehicle, 10 µM forskolin, or 10 nM isoproterenol leads to an observable decrease in fluorescence intensity over time (from the baseline 0 ΔF/Fo at 120 s to -0.9 ΔF/Fo at 410 s) (Figure 3) (Video 1, Video 2, and Video 3). Fluorescence is evenly distributed throughout the cytosol of cells and excludes the nucleus (see the time-lapse videos). Thus, forskolin and isoproterenol produced a similar rapid and sizable decrease in fluorescence HASM cells. These data demonstrate the ability to use the biosensor in primary cell cultures.
Figure 1: Schematic representation of the biosensor protocol steps. Initially, culture cells are split and resuspended with fresh media, then infected with the BacMam virus carrying the cADDis sensors. Post-transfection cells are stimulated with pharmacological agents triggering cellular pathways that alter cAMP. Changes in the fluorescence intensity in response to the varying cAMP concentration are captured using a plate reading spectrophotometer (A) or fluorescence microscope (B), providing a real-time quantification of the cAMP dynamics. Please click here to view a larger version of this figure.
Figure 2: Real-time monitoring of cAMP in live HEK-293 cells on a plate reading spectrophotometer. Fluorescence of HEK-293 cells on 96-well plates expressing the biosensor was measured over time. After baseline fluorescence was established, fluorescence decay was monitored for 7 min. (A) Fluorescence decay after the addition of either 10 µM forskolin or 10 nM of isoproterenol is plotted as the mean ± SEM of 10-15 experiments. (B) Fluorescence decay oscillations are shown by plotting a single experiment after the addition of either 10 nM or 100 nM of isoproterenol. Data in both graphs is fit to a single site decay model (connecting line). Please click here to view a larger version of this figure.
Figure 3: Real-time monitoring of cAMP in live HASM cells on a fluorescence microscope. Fluorescence of HASM cells on plated 35 mm dishes expressing the biosensor was measured over time. After baseline values were established, fluorescence decay was measured every 30 s for 20 min. The indicated agent was added at 120 s (arrow). The biosensor fluorescence decay curves in response to either vehicle, 10 µM forskolin, or 10 nM isoproterenol. Data from a single experiment is shown with a connecting line. Time-lapse videos of cells treated with vehicle (control), forskolin, or isoproterenol are included to provide a visual representation of the fluorescent responses of the biosensor. Please click here to view a larger version of this figure.
Video 1: Time-lapse video of real-time monitoring of cAMP in live HASM cells in response to the vehicle. Fluorescence decay curves of the biosensor in response to the vehicle (control). Each frame was captured every 30 s for 20 min. Scale bar: 40 μM. Please click here to download this Video.
Video 2: Time-lapse video of real-time monitoring of cAMP in live HASM cells in response to 10 µM forskolin. Fluorescence decay curves of the biosensor in response to 10 µM forskolin. Each frame was captured every 30 s for 20 min. Scale bar: 40 μM. Please click here to download this Video.
Video 3: Time-lapse video of real-time monitoring of cAMP in live HASM cells in response to 10 nM isoproterenol. Fluorescence decay curves of the biosensor in response to 10 nM isoproterenol. Each frame was captured every 30 s for 20 min. Scale bar: 40 μM. Please click here to download this Video.
Media formulation | |
HASM medium | |
Name | Volume |
Ham's F-12K | 419.65 mL |
FBS | 50 mL |
HEPES (1M) | 12.5 mL |
Sodium hydroxide solution | 6 mL |
L-glutamine 200 mM (100X) | 5 mL |
Calcium chloride (1 M) | 0.850 mL |
Antibiotic-Antimycotic (100X) | 5 mL |
Primocin | 1 mL |
HEK medium | |
Name | Volume |
DMEM (1x) | 444 mL |
FBS | 50 mL |
Antibiotic-Antimycotic (100X) | 5 mL |
Primocin | 1 mL |
Table 1: Media formulation of HASM medium and HEK medium.
Accurate and sensitive measurement of cAMP is crucial for understanding its role in various cellular processes and for studying the activity of cAMP-dependent signaling pathways. There are several methods commonly employed to measure cAMP levels, including ELISA, radioimmunoassay, FRET biosensors, and the GloSensor cAMP assay14,15,16,17,18. Each cAMP assay has strengths and weaknesses. The protocol allows the real-time detection and monitoring, ranging from minutes to hours, and measurement of cAMP dynamics within living cells without the need to include phosphodiesterase inhibitors. This latter point is critical for researchers interested in studying the role of phosphodiesterase isoforms in regulating cAMP signaling19. The biosensor is available commercially (see Table of Materials) in multiple versions: a green fluorophore that decreases fluorescence upon cAMP binding, a red fluorophore that exhibits increased fluorescence upon cAMP binding, and a green fluorophore that exhibits increased fluorescence upon cAMP binding. The downward version of the biosensor is most useful since the main limiting factor is the level of expression of the biosensor, and this version will display maximal fluorescence at baseline prior to starting an experiment, alleviating the need to assess biosensor expression by adding a maximal cAMP stimulus. The biosensor is also available in versions that are targeted to membrane microdomains, primary cilia, or to the nucleus. These different versions enable measurements of cAMP simultaneously in cytosolic and membrane/cilial compartments within the same cell. These novel tools are uniquely adept at the visualization and quantification of cAMP dynamics across different cellular components, a rapidly evolving and important area of study20.
The biosensors offer a straightforward method to effectively monitor real-time cAMP dynamics, enabling researchers to observe rapid changes in cAMP levels with exceptional temporal resolution. One of the key strengths of the biosensor lies in its high sensitivity, which allows for the detection of both subtle and significant variations in cAMP concentrations8. An example of this in the cAMP oscillations is observed at low levels of isoproterenol stimulus in HEK-293 cells with no phosphodiesterase inhibitor present (Figure 2B). This oscillatory behavior is not understood and appears unique to these cells, but the sensitivity and rapid kinetics of the biosensor enable their detection. It should be noted, however, that the biosensor can be readily saturated, so this biosensor can be limited in quantifying the differences between medium and large cAMP signals. This limiting feature is particularly relevant when employing phosphodiesterase inhibitors that cause global increases in cAMP levels when a receptor stimulus is also present. While we present the infection of HASM in this study, it's important to acknowledge the challenge associated with infecting primary cells using the BacMam vector. The Conditions should be optimized for individual cell types in order to achieve sufficient expression of the biosensor. Nonetheless, the sensitivity and scalability of the biosensor make it well-suited for high-throughput assays.
Another improvement of using this biosensor over FRET-based sensors is the minimization of photobleaching. Many fluorescent biosensors are subject to photobleaching following prolonged and repeated excitation. The protocols described here involve short-duration excitations that are spaced 30 s apart. The biosensor is also extremely bright, thus limiting potential photobleaching for this type of analysis. Photobleaching would be observed during the initial baseline read as a drift. If baseline drift were to occur, one could subtract the drifting baseline from the data before fitting the curve or increase the sampling interval to reduce the frequency of excitation.
Like other cAMP biosensors, this biosensor has high specificity for binding to cAMP. Consequently, the signals generated by this assay directly correlate to cAMP levels, significantly reducing the potential interference from other cellular components and providing reliable results. Furthermore, the versatility of the biosensor is worth highlighting. While the GloSensor assay is widely used in the field, it does have certain limitations. For instance, relying on transient transfection may result in inefficiencies in specific cell types, and the analysis of multiple parameters or processes within the same cell is a challenge. Additionally, the assay relies on luciferase enzymes that can be influenced by certain compounds, potentially leading to compromised result accuracy21. On the other hand, the biosensor, can be employed in various cell types and experimental systems, including adherent and suspension cells. Its compatibility with diverse cellular contexts allows researchers to investigate cAMP dynamics across different biological systems22,23,24. The integration of this biosensor assay with fluorescence microscopy enhances its utility even further since cell morphological information can also be gathered25,26,27.
This assay provides researchers with great flexibility in experimental design, enabling a wide range of investigations related to cAMP signaling. It serves multiple purposes, allowing for the measurement of basal cAMP levels, the study of cAMP fluctuations in response to diverse stimuli or treatments, and the exploration of cAMP dynamics in various physiological or pathological conditions. One of the key strengths of the assay is its compatibility with other techniques, which allows for a more comprehensive understanding of cAMP signaling. By combining the biosensor with methods such as immunofluorescence staining or genetic manipulation10, researchers can gain deeper insights into the precise spatial and temporal regulation of cAMP within specific cellular compartments or signaling pathways. Through this integration, researchers can investigate the diverse functions of cAMP in various contexts, leading to valuable insights into the complexities of cAMP-mediated cellular processes. Employing these techniques allows researchers to visualize and quantify cAMP levels at both the individual cell and population level, offering a comprehensive understanding of its distribution within the cell, including the identification of cAMP pools. As a result, the combination of these methods enables the acquisition of precise data, contributing significantly to our knowledge of cAMP signaling.
While traditional methods have been valuable for cAMP analysis, the introduction of cADDis represents a promising advancement in this field. Its specificity, sensitivity, and real-time capabilities make it a valuable addition to the researcher's toolkit, particularly due to its compatibility with living cells. With the ability to monitor cAMP levels in real-time and its high sensitivity, the biosensor offers a significant advantage in various experimental settings. This progress contributes to a deeper understanding of the dynamic nature of cAMP signaling and its functional implications in both physiological and pathological processes.
The authors have nothing to disclose.
This study was supported by the National Heart, Lung, and Blood Institute (NHLBI) (HL169522).
96-well plate (clear) | Fisherbrand | 21-377-203 | |
35 mm dish | Greiner Bio-One | 627870 | Cell culture dishes with glass bottom |
96-well plate | Corning | 3904 | Black with clear flat bottom |
Antibiotic-Antimycotic (100x) | Gibco | 15240062 | For HEK and HASM media |
BZ-X fluorescence microscope | Keyence | ||
Calcium chloride (IM) | Quality Biological Inc | E506 | For HASM media |
Centrifuge tube (15 mL) | Thermo Scientific | 339651 | |
DMEM (1x) | Gibco | 11965092 | HEK media |
DPBS with Mg2+ and Ca2+ | Gibco | 14040-133 | |
DPBS without Mg2+ and Ca2+ | Corning | 14040-133 | |
Fetal Bovine Serum (FBS) | R&D systems | S11195 | For HEK and HASM media |
Forskolin | Millipore | 344270 | Drug |
Green Down cADDis cAMP Assay Kit | Montana Molecular | #D0200G | Reagent |
Ham's F-12K | Gibco | 21127022 | For HASM media |
HEPES (1M) | Gibco | 15630080 | For HASM media |
Isoproterenol | Sigma | I6504 | Drug |
L-glutamine 200 mM (100x) | Gibco | 25030-081 | For HASM media |
Microcentrifuge tube (2 mL) | Eppendorf | 22363352 | |
Primocin | Invitrogen | ant-pm-1 | Antibiotic for HASM media |
RNAse away | Thermo Scientific | 700511 | Reagent |
Sodium hydroxide solution | Sigma | S2770 | For HASM media |
Spectrmax M5 plate reader | Molecular Devices | ||
Trichostatin A | TCI America | T2477 | Reagent |
Trypsin EDTA | Gibco | 25200-056 | Reagent |