HitHunter cAMP XS+ assay for GPCR screening using the PHERAstar FS

Lindy Kauffman Sherrylyn De La Llera DiscoverX Corporation 01/2006
  • Directly assess either Gi- or Gs- coupled GPCRs in cells
  • Chemiluminescence detection reduces interference from fluorescent compounds
  • Assay miniaturization up to 1536-well plate format

Table of contents


G protein-coupled receptors (GPCRs) make up a major family of cell surface receptors which mediate intercellular communication and GPCRs are a rich source of “druggable” targets. It has been estimated that 50% of prescription drugs interact with GPCRs. With the increasing popularity of functional assays for high throughput screening, an increasing need arises for robust second messenger assays that reflect GPCR activation and that are readily amenable for miniaturization. GPCRs that modulate adenylyl cyclase activity upon agonist stimulation and, consequently, cellular cyclic adenosine monophosphate (cAMP) levels, via the G protein Gs or Gi, form a subset of therapeutic targets. cAMP acts at several downstream targets including ion channels, kinases that modulate gene transcription, and cell metabolism. Changes in the  intracellular cAMP levels correlate with GPCR activation and therefore measurement of cAMP is a simple functional assay frequently utilized in the HTS laboratory.


DiscoverX supplies several homogenous cAMP assays based on chemiluminescence to fit the various needs of the HTS customer.

HitHunter® cAMP XS+, is specifically targeted to  provide enhanced performance and usability. In this application note we describe a cAMP assay based on the enzyme fragmentation complementation (EFC) of ß-galactosidase(ß-gal) measured on BMG LABTECH’s PHERAstar® FS multi-mode microplate reader.


Assay Principle

The DiscoverX EFC technology is based on two fragments of E. coli ß-galactosidase (ß-gal), a large protein fragment (enzyme acceptor, EA) and a small peptide fragment (enzyme donor, ED). Separately these fragments are inactive, but in solution they rapidly complement (recombine) to form active ß-gal enzyme which can hydrolyze substrate to produce luminescence. In this assay cAMP from cell lysates and ED-labeled cAMP (ED-cAMP) compete for antibody binding sites. Steric hindrance of EFC occurs when the antibody binds the ED-conjugated cAMP such that little or no active ß-gal is formed. However, free cAMP generated by the cell can competitively displace the ED-conjugated cAMP from the antibody and allows the ED-conjugate to freely complement EA and thus generate a signal  directly proportional to the amount of cAMP present in the cell (figure 1).

Materials & Methods

BMG LABTECH’s PHERAstar FS combines rapid plate reading necessary for HTS with the enhanced performance and sensitivity needed to read small liquid volumes. The PHERAstar FS was run in luminescence mode for the monitoring of the HitHunter® cAMP XS+ demo kit (DiscoverX Corporation) containing following reagents:


1) cAMP XS+ Lysis Buffer
2) cAMP XS+ EA Reagent
3) cAMP XS+ ED Reagent
4) cAMP XS+ Antibody Reagent
5) cAMP XS+ Standard (250 μM) 6a) Galacton-Star®
6b) Emerald-II™



HitHunter® cAMP XS+ is a single set of reagents which can be used in two different protocols, depending on the needs of the end user. 


  • In the three reagent addition protocol cells are plated  and induced in PBS or media, and cell induction is  followed by three reagent additions and two incubation steps.
  • In the two reagent addition protocol, cells are plated  and induced in a diluted antibody solution, and cell  induction is followed by two reagent additions and  two incubation steps.


The table below outlines the volumes and procedure for the cAMP XS+ three reagent addition protocol standard curve in a 384-well plate format.


Table 1: cAMP XS+ Three Reagent Addition Protocol: Standard curve.


384-well Plate cAMP XS+ Three Reagent Addition Protocol: Standard Curve
Step 1 10 μL of diluted standard 5 μL PBS
Step 2 5 μL Antibody Reagent
Step 3 20 μL ED/Lysis/CL Working Solution
Step 4 Incubate 1 hour at room temperature
Step 5 20 μL EA Reagent
Step 6 Incubate at least 1 hour at room temperature, then read luminescence signal on PHERAstar FS



The standard curve for DiscoverX’s cAMP XS+ kit was  run according to the package insert protocol in white 384-well plates (COSTAR) with 60 μL total assay volume. Chemiluminescence was read on the PHERAstar FS two hours after the addition of the last reagent using a measurement time of 1 second. 

Results & Discussion

DiscoverX’s HitHunter® cAMP XS+ assay was prepared in 384-well format and standard curves were run on  BMG LABTECH’s PHERAstar FS in luminescence mode. Cyclic AMP standard detection reagents were added according to the assay protocols (two and three reagent addition protocols) and chemiluminescence for both  was read two hours after the addition of the last reagent (figure 2). EC50 values were calculated using Graphpad Prism software.


GPCRs are important targets in the HTS drug discovery approach and GPCR signalling can be examined by  direct quantitation of cAMP by applying the HitHunter® cAMP XS+ kit. Excellent results were achieved on the PHERAstar FS multimode microplate reader which is designed to read all leading HTS detection modes in all formats up to 1536. The high degree of sensitivity, easy-to-use software, robust hardware, and optimized detection systems make the PHERAstar FS ideal for GPCR analyses in the high-throughput assay environment.

The PHERAstar FS was run in luminescence mode for the HitHunter® cAMP assay which uses enzyme fragment complementation (EFC) for sensitive detection and fewer false positives. This assay is a useful tool for those customers screening difficult targets with low basal levels of cAMP (cell lysate, tissue, or serum), or low levels of cAMP response. The chemiluminescence  signal is robust and assures minimal interference from library compounds. The stable signal allows flexibility in read time, i.e. the assay can be read the same day or after an overnight incubation.

While there are several cAMP assays currently available, most of them are not scalable for miniaturization into the 1,536-well format employed for automated high throughput screening of large chemical libraries. DiscoverX offers several different configurations of this assay based on different applications, readouts, sample and cell types. For more information on DiscoverX assays please refer to the web site: www.discoverx.com

HitHunter, DiscoverX and the DiscoverX logo are registered trademarks of the DiscoverX Corporation.