A Homogeneous Enzyme Fragment Complementation Cyclic AMP Screen for GPCR Agonists

Abstract

In the new high-throughput screening (HTS) campaign, receptor functional assays, 3’,5’-cyclic adenosine mono-phosphate (cAMP), intracellular [Ca2+]i, phosphatidylinositol turnover, and reporter-based assays are being used as primary screens as they are now developed as homogeneous and automation-friendly assays. FlashPlate assay and scintillation proximity assay using radiolabeled cAMP have been used for measuring cAMP. A nonradioactive homogeneous HTS assay using HitHunter™ enzyme fragment complementation (EFC) technology was evaluated for measuring cAMP in adherent and suspension cells overexpressing a Gαs-coupled receptor. In the EFC-cAMP assay, the β-galactosidase (β-gal) donor fragment-cAMP (ED-cAMP) conjugate complements with the β-gal enzyme acceptor (EA) fragment to form an active β-gal enzyme. Binding of ED-cAMP conjugate to the anti-cAMP antibody prevents its complementation with the EA fragment to form an active enzyme. Cyclic AMP in the samples compete with ED-cAMP to bind to the anti-cAMP antibody, thus increasing the free ED-cAMP that can complement with the EA fragment to form an active enzyme that is assayed with a luminescent substrate. Thus, this assay results in a positive signal unlike other technologies, wherein the signal is completed by cAMP in the sample. Glucagon-like peptide (GLP)-1 binds to GLP-1 receptor (with a Kd of 0.2 nM) signals through Gαsto activate adenylate cyclase, which results in an increase of intracellular cAMP (EC50of 0.3 nM). GLP-1 stimulation of cAMP levels measured by the EFC method was similar in both adherent and suspension cell formats (EC50~0.3 nM) at different cell numbers. The assay was further validated with forskolin, exendin, and several active GLP-1 peptide analogues. The stimulation of cAMP by GLP-1 and forskolin was effectively inhibited by the adenylate cyclase inhibitors MDL-12330A and SQ-22536, confirming that the increased cAMP is through the AC pathway. The assay tolerates dimethyl sulfoxide (DMSO) up to 10%, and tartrazine does not interfere with the assay with the adherent cells up to 1 mM and affects minimally up to 10 μM in suspension cells. The assay is very robust, with a Z value of 0.7 to 0.8. The assay was validated with several plates of low molecular weight nonpeptide compounds and peptide agonists with different potencies. The suspension cell protocol is a robust homogeneous assay that involves fewer steps than the adherent cell protocol and is suitable for HTS. The cAMP assay using EFC technology is advantageous in that it has a greater dynamic range of detection; is nonradioactive, very sensitive, robust; has minimal interference from DMSO and colored compounds; and is amenable for automation. An added advantage of this assay is that the cAMP is measured as a positive signal, thereby reducing the incidence of false positives.