GPCRs (G-protein-coupled receptors) are the largest and most diverse family of membrane proteins that transduce extracellular stimuli into intracellular signals. Their association with several human diseases has made them a hot topic of research in the pharmaceutical world. They regulate a wide variety of physiological processes and are also well-validated ‘druggable’ targets. Currently around 475 drugs, acting on 108 unique GPCR targets have been approved by the FDA and many more are in clinical trials. Read our blog below to learn more about GPCRs and their role in drug discovery.
GPCRs: An Overview
GPCRs are the largest class of cell surface receptors in humans. They detect molecules outside cells and activate internal signal transduction pathways leading to cellular responses. They are also known as seven-transmembrane receptors (7 TMRs) as they pass through the cell membrane seven times. There are around 800 GPCRs identified in humans, out of which nearly half are responsible for sensory functions mediating olfaction, taste, light perception and pheromone signalling. The remaining non-sensory GPCRs facilitate signalling by ligands that include growth factors, hormones and other endogenous molecules.
GPCRs are activated by ligands or other signal mediators, creating a conformational change in the receptor, which further activates a G protein. The ultimate effect depends on the type of G protein activated. G proteins are subsequently inactivated by GTPase activating proteins, known as RGS (Regulators of G protein Signalling) proteins. There are two principal signal transduction pathways involving GPCRs:
- the cAMPsignal pathway
- the phosphatidylinositolsignal pathway
GPCRs are commonly classified into 6 groups based on sequence homology and functional similarity:
- Class A: Rhodopsin-like family
- Class B: Secretin receptor family
- Class C: Metabotropic glutamate, GABA receptors, calcium- sensing receptors
- Class D: Fungal mating pheromone receptors
- Class E: cAMP receptors
- Class F: Frizzled/Smoothened receptors
Despite the similarities, individual GPCRs show unique combinations of signal-transduction activities involving multiple G-protein subtypes, and G-protein-independent signalling pathways as well as complex regulatory processes. Due to their substantial involvement in human pathophysiology and their pharmacological tractability, GPCRs are considered major drug targets and have been subjects of considerable research.
GPCRs as drug targets:
GPCRs are involved in a wide variety of human physiological processes, including growth, metabolism and homeostasis. They are associated with a broad spectrum of diseases and are targets of approximately 34% of all modern drugs including angiotensin receptor blockers (ARBs) for hypertension, antihistamines for allergy, bronchodilators for asthma, and H2 blockers for acid reflux. According to latest reports, the global sales volume for these drugs is estimated to be 180 billion US dollars as of 2018.
GPCR targeting drugs include both agonists and antagonists that work on nearly every major organ system such as the respiratory, cardiovascular, central nervous system (CNS), metabolic and urogenital systems. Over the last few years, there has been an increased focus on targeting GPCRs in oncology as they regulate many signal transduction pathways that are relevant in cancer cells, e.g. EGFR/Ras (proliferation), ATF4/CHOP (cell stress), chemokine (metastasis) and p53 (apoptosis) signalling. GPCRs are less frequently mutated as compared to other molecules in oncological pathways, which is why traditionally, lesser importance was given to them. However, there is an increased recognition that pharmacological engagement of GPCRs presents an opportunity to effectively block diverse tumorigenic signals.
Whilst developing a compound for a GPCR target, several parameters such as binding affinity, selectivity, kinetics of compound binding, etc have to be kept in mind. Promega has developed NanoBRETTM technology to study the binding interactions (Kon and Koff) of compounds with a GPCR target in living cells. Read below to learn more.
Aurelia Bioscience has performed their studies on a 3D electrospun scaffold format that allows them to manipulate adherent cells as though they are reagents. In collaboration with Promega, they used NanoBRETTM GPCR reagents to examine the binding kinetics of fluorescently tagged propranolol to β2MAR tracer in a 3D scaffold format. They studied the use of this tracer to measure the competitive binding of known β2MAR compounds and measured the rate at which these compounds displace the tracer.
This format has significant advantages over traditional methods of compound profiling in 2D well plates: cells adhere to scaffolds and thus, can be effectively moved between plates, replacing buffers without washing and providing the ability to use ‘assay ready compound plates’; cells can be cryo-preserved on scaffolds, then defrosted and used repeatedly from the same stock or batch, therefore improving data consistency. This technology can be applied to other GPCR’s with the benefits of
- Highly specific detection of binding interactions due to the inherent distance constrains of BRET
- Small and minimally interfering bio-luminescent peptide tag
- Selective cell surface detection
- Non-radioactive and homogeneous live cell assay
- High throughput –can be used for compound profiling in HTS formats
Are you working with GPCRs and want to discuss your pre-clinical research? Get in touch with our experienced team of scientists and take your project to the next level!