Development of a cyclic adenosine monophosphate assay for Gi-coupled G protein-coupled receptors by utilizing the endogenous calcitonin activity in Chinese hamster ovary cells

Assay for Detecting Gαi-Mediated Decreases in cAMP in Living Cells

Cell-based assays to detect Gαi signaling are often indirect, frequently involve complex pharmacological interventions, and are usually blind to the kinetics of the signaling. Our goal was to develop a simple, direct measure of Gαi signaling in living cells. We previously reported our fluorescent cADDis assay and showed that it reliably detects Gαs-mediated increases in cAMP levels. Agonists that stimulate a Gs-coupled receptor produce changes in the intensity of bright green or red fluorescent protein sensors that can be followed over time using automated fluorescence plate readers or fluorescence imaging systems. Since the cADDis sensors can monitor Gαs-mediated increases in adenylyl cyclase activity, in theory they should also be capable of detecting Gαi-mediated decreases. Here we apply our green fluorescent cADDis sensor to the detection of Gαi-mediated inhibition of adenylyl cyclase activity. We validated and optimized the assay in living HEK 293T cells using several known Gαi-coupled receptors and agonists, and we report robust Z' statistics and consistent EC₅₀ responses.

Forskolin-free cAMP assay for Gi-coupled receptors

G protein-coupled receptors (GPCRs) represent the most successful receptor family for treating human diseases. Many are poorly characterized with few ligands reported or remain completely orphans. Therefore, there is a growing need for screening-compatible and sensitive assays. Measurement of intracellular cyclic AMP (cAMP) levels is a validated strategy for measuring GPCRs activation. However, agonist ligands for Gi-coupled receptors are difficult to track because inducers such as forskolin (FSK) must be used and are sources of variations and errors. We developed a method based on the GloSensor system, a kinetic assay that consists in a luciferase fused with cAMP binding domain. As a proof of concept, we selected the succinate receptor 1 (SUCNR1 or GPR91) which could be an attractive drug target. It has never been validated as such because very few ligands have been described. Following analyses of SUCNR1 signaling pathways, we show that the GloSensor system allows real time, FSK-free detection of an agonist effect. This FSK-free agonist signal was confirmed on other Gi-coupled receptors such as CXCR4. In a test screening on SUCNR1, we compared the results obtained with a FSK vs FSK-free protocol and were able to identify agonists with both methods but with fewer false positives when measuring the basal levels. In this report, we validate a cAMP-inducer free method for the detection of Gi-coupled receptors agonists compatible with high-throughput screening. This method will facilitate the study and screening of Gi-coupled receptors for active ligands.

STC1 interference on calcitonin family of receptors signaling during osteoblastogenesis via adenylate cyclase inhibition

Stanniocalcin 1 (STC1) and calcitonin gene-related peptide (CGRP) are involved in bone formation/remodeling. Here we investigate the effects of STC1 on functional heterodimer complex CALCRL/RAMP1, expression and activity during osteoblastogenesis. STC1 did not modify CALCRL and ramp1 gene expression during osteoblastogenesis when compared to controls. However, plasma membrane spatial distribution of CALCRL/RAMP1 was modified in 7-day pre-osteoblasts exposed to either CGRP or STC1, and both peptides induced CALCRL and RAMP1 assembly. CGRP, but not STC1 stimulated cAMP accumulation in 7-day osteoblasts and in CALCRL/RAMP1 transfected HEK293 cells. Furthermore, STC1 inhibited forskolin stimulated cAMP accumulation of HEK293 cells, but not in CALCRL/RAMP1 transfected HEK293 cells. However, STC1 inhibited cAMP accumulation in calcitonin receptor (CTR) HEK293 transfected cells stimulated by calcitonin. In conclusion, STC1 signals through inhibitory G-protein modulates CGRP receptor spatial localization during osteoblastogenesis and may function as a regulatory factor interacting with calcitonin peptide members during bone formation.

Constitutive activity in cannabinoid receptors

The cannabinoid receptors are G protein-coupled receptors activated by endocannabinoids or exogenous agonist such as tetrahydrocannabinol. Upon agonist binding, cannabinoid receptors will activate G proteins of the Gi family, which in turn inhibits adenylyl cyclase. Recently, inverse agonists and neutral antagonist for cannabinoid receptors have been discovered, demonstrating constitutive activity of the cannabinoid receptors. This chapter will discuss the current state of the art and provide a framework for evaluating constitutive receptor activity and distinguishing constitutive receptor activity from constitutive endogenous agonist tone.

A real-time, fluorescence-based assay for measuring μ-opioid receptor modulation of adenylyl cyclase activity in Chinese hamster ovary cells

Inhibition of adenylyl cyclase (AC) activity is frequently used to measure µ-opioid receptor (MOR) activation. We sought to develop a simple, rapid assay of AC activity in whole cells that could be used to study MOR signaling. Chinese hamster ovary cells expressing human MOR (CHO-MOR cells) were grown in 96-well plates and loaded with membrane potential-sensitive fluorescent dye. CHO-MOR cells were treated with the AC activator forskolin (FSK), with or without simultaneous application of MOR agonists, and the resulting change in fluorescence was measured. CHO-MOR cells hyperpolarized in response to application of FSK (pEC₅₀, 7.3) or calcitonin (pEC₅₀, 9.4). A submaximally effective concentration of FSK (300 nM) caused a 52% ± 2% decrease in fluorescence. Simultaneous application of the opioids DAMGO (pEC₅₀, 7.4; E(max), 56%), morphine (pEC₅₀, 7.0; E(max), 61%); and buprenorphine (pEC₅₀, 8.6; E(max), 24%) inhibited the FSK response in a dose-dependent manner while having no effect by themselves. The effects of DAMGO were blocked by pertussis toxin. This assay represents a simple, robust method for real-time observation of AC inhibition by MOR in CHO cells. It represents an appealing alternative to end-point assays that rely on cAMP accumulation and can avoid potential confounds associated with rapid desensitization of MOR signaling.