Luciferase-based GloSensor™ cAMP assay: Temperature optimization and application to cell-based kinetic studies

G protein-coupled receptors (GPCRs) are an important receptor superfamily and common therapeutic targets. The second messenger cyclic adenosine monophosphate (cAMP) is a key mediator in many GPCR signaling pathways. Monitoring intracellular cAMP levels can help identify orthosteric agonists and antagonists, as well as allosteric modulators. In this regard, luminescence-based biosensors have revolutionized our ability to monitor GPCR signaling kinetics. The GloSensor™ cAMP assay enables real-time monitoring of signaling downstream of many GPCRs. However, it is crucial to optimize assay conditions such as temperature. As well, it has not been reported whether the effects of temperature on biosensor activity are reversible. Here, we describe the temperature sensitivity and reversibility of the GloSensor™ cAMP assay, and which GloSensor™ version is optimal for measuring cytosolic cAMP. We also present a detailed protocol for monitoring cAMP levels in live cells expressing endogenous or exogenous GPCRs. Temperature optimization studies were carried out using HEK293H cells transiently transfected with the adenosine receptor A2a and the GloSensor™ plasmid (pGloSensor-20F or -22F). We found that preincubation and luminescence reading at room temperature were optimal as compared to higher temperatures. As well, the GloSensor-22F biosensor had a superior signal-to-background ratio and the effect of temperature on biosensor activity was reversible. However, thermal instability of the biosensor may pose a problem for in vivo studies. Nevertheless, the GloSensor™ cAMP assay can be applied to analyze signaling by a wide range of GPCRs for drug discovery purposes.

Rapid, Direct SMOOTHENED Activity Assays in Live Cells Using cAMP-Based Conformational Sensors

Communication between PATCHED1 (PTCH1) and SMOOTHENED (SMO) is fundamental to Hedgehog (Hh) signal transduction in development and disease. We describe a real-time cell-based SMO functional assay based on SMO activity-dependent changes in cellular cAMP concentrations. This assay is capable of detecting changes in SMO conformation within minutes of PTCH1 inactivation by Hh ligands. As a result, it expands the range of experimental perturbations that can be used to dissect PTCH1-SMO communication, enabling a deeper mechanistic understanding of a longstanding mystery in Hh signal transduction.

Development of a High-Throughput Screening-Compatible Assay for Discovery of GPR3 Inverse Agonists Using a cAMP Biosensor

While G-protein-coupled receptors (GPCRs) represent the largest class of cell surface proteins, there are ≥100 orphan GPCRs whose endogenous ligands are unknown. Accordingly, these could prove to be potential therapeutic targets for the pharmaceutical intervention of various diseases. Constitutively active orphan GPCRs are activated without ligands; thus, inverse agonists may be very useful pharmacological tools for inhibiting constitutive activity. However, in general, inverse agonist screening is considered more difficult to perform with high quality than antagonist screening, particularly due to the narrow assay window. We developed a high-throughput screening (HTS)-compatible assay to identify inverse agonists of GPR3. GPR3 is expressed in the central nervous system (CNS) and is known to be related to Alzheimer's disease and other CNS diseases. The GPR3 inducible cell line was established using T-REx 293 cells that stably expressed the tetracycline repressor protein, and the cAMP biosensor, GloSensor, was stably co-expressed. After optimization of the induction level of GPR3 and assay conditions, the GloSensor assay showed an approximately 20-fold signal-to-background ratio and high sensitivity. Using the HTS method, we successfully screened a library of hundreds of thousands of compounds for the inhibition of constitutive activity with good quality and excellent reproducibility. Finally, 35 compounds were identified as GPR3 selective inverse agonists. This inverse agonist screening approach using GloSensor in combination with the inducible expression of orphan GPCR indicates universal applicability to the search for inverse agonists of constitutively active orphan GPCRs.

Increased luminescence of the GloSensor cAMP assay in LβT2 cells does not correlate with cAMP accumulation under low pH conditions

Cyclic adenosine monophosphate (cAMP) plays a pivotal role in gonadotrope responses in the pituitary. Gonadotropin-releasing hormone (GnRH) mediated synthesis and secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are regulated by both the G_s/cAMP and G_q/Ca²⁺ signaling pathways. Pituitary adenylate cyclase-activating polypeptide (PACAP) also regulates GnRH responsiveness in gonadotropes through the PACAP receptor, which activates the G_s/cAMP signaling pathway. Therefore, measuring intracellular cAMP levels is important for elucidating the molecular mechanisms of FSH and LH synthesis and secretion in gonadotropes. The GloSensor cAMP assay is useful for detecting cAMP levels in intact, living cells. In this study, we found that increased GloSensor luminescence intensity did not correlate with cAMP accumulation in LβT2 cells under low pH conditions. This result indicates that cell type and condition must be considered when using GloSensor cAMP.

Imaging Proteolytic Activities in Mouse Models of Cancer

Proteases are "protein-cleaving" enzymes, which, in addition to their non-specific degrading function, also catalyze the highly specific and regulated process of proteolytic processing, thus regulating multiple biological functions. Alterations in proteolytic activity occur during pathological conditions such as cancer. One of the major deregulated classes of proteases in cancer is caspases, the proteolytic initiators and mediators of the apoptotic machinery. The ability to image apoptosis noninvasively in living cells and animal models of cancer can not only provide new insight into the biological basis of the disease but can also be used as a quantitative tool to screen and evaluate novel therapeutic strategies. Optical molecular imaging such as bioluminescence-based genetically engineered biosensors has been developed in our laboratory and exploited to study protease activity in animal models with a high signal to noise. Using the circularly permuted form of firefly luciferase, we have developed a reporter for Caspase 3/7, referred to as Caspase 3/7 GloSensor. Here, we discuss the use of the Caspase 3/7 GloSensor for imaging apoptotic activity in mouse xenografts and genetically engineered mouse models of cancer and present the potential of this powerful platform technology to image the proteolytic activity of numerous other proteases.