G protein-coupled receptors (GPCRs) are important mediators in cellular signaling and are common targets of drug action. GPCRs are responsible for the transduction of extracellular signals into intracellular signals, mediated by G proteins of four types: Gs, Gi, Gq, and G12/13. A thorough understanding of a signaling pathway involves determining which G protein is coupled to a signal-activated GPCR. In this project, a technique called Bioluminescence Resonance Energy Transfer (BRET) was used to measure the interaction between an activated GPCR from the serotonin (or hydroxytryptamine, 5-HT) receptor family, and G proteins from each subtype. The cDNA for serotonin receptors 5-HT1E�and 5-HT2C�was fused with the gene for a luminescent protein called Nanoluciferase (Nluc). Then, the receptor-Nluc DNA along with DNA containing a G protein tagged with a fluorescent protein (Venus) was transfected into mammalian cells for expression. Data from BRET assays suggest that the 5-HT1Ereceptor couples to the Go and Gi subclasses of G proteins upon serotonin activation while the 5-HT2C�receptor couples to the Gq class of G proteins. Profiling serotonin receptors will deepen our understanding of serotonin receptors, associated diseases, and the drugs that target them.
GPCRs play a major role in cell signaling through their interactions with heterotrimeric G proteins. In conventional models of GPCR-G protein coupling, agonist binding promotes a conformational change within the receptor, which then associates with G proteins, facilitating the exchange of GDP for GTP. GTP-bound G proteins dissociate from the receptor and exert their effects on downstream signaling molecules. Previous studies suggest that serotonin 5HT7 receptors associate with Gs�heterotrimers prior to agonist binding, and that 5HT7-Gs�complexes dissociate after the G protein is activated. Here we study this unconventional mode of coupling using bioluminescence resonance energy transfer (BRET) between luciferase-tagged 5HT7 receptors and Gs�heterotrimers labeled with Venus. Our results confirm that 5HT7 receptors interact with inactive (GDP-bound) Gs�heterotrimers in the absence of an agonist, and that this interaction is stabilized by the inverse agonist methiothepin. Stimulation with the endogenous agonist serotonin (5HT) decreased BRET between 5HT7 receptors and Gs, indicating that the activation of the receptor leads to 5HT7-Gscomplex dissociation. Interestingly, Gs�activation was not required for complex dissociation. These results are consistent with the hypothesis that 5HT7 receptors couple to Gs�heterotrimers via an unconventional mechanism involving ligand-sensitive complexes of receptors and inactive Gs.
GPCRs are receptors that act in signal transduction pathways via guanosine nucleotide-binding proteins (G proteins). Extracellular ligands act on GPCRs resulting in activation of one or more G protein subtypes (Gs, Gi/o, Gq/11 and G12/13) affecting the concentration of intracellular second messenger molecules ultimately altering cellular function. Cellular responses to external signals are typically studied indirectly by measuring concentration changes in second messengers. However, this approach can be problematic as many GPCRs can activate multiple G protein subtypes, and many second messenger pathways engage in crosstalk. To address this issue, we used Bioluminescence Resonance Energy Transfer (BRET) to directly measure coupling between 5-hydroxytryptamine (5-HT; serotonin) receptors and different G protein family subtypes. We co-transfected cells with plasmid DNA encoding the 5-HT2B or 5-HT4 receptors fused to the bioluminescent protein nanoluciferase (NLuc) as well as plasmid DNA containing G protein subtypes fused to the fluorescent protein Venus. In BRET assays, we found that mGsq couples to 5-HT2B and mGscouples with 5-HT4 in response to 5-HT activation. These results are consistent with the literature. Interestingly, initial studies suggest that activated 5-HT4 shows secondary coupling to mGsi highlighting the potential novel signaling pathways that can be elucidated using this technique.
The experiment discusses the role of inverse agonist binding to receptors and how its effect cell signaling. The specific receptors that was focused on in the project was histamine receptor H1 (HRH1) and histamine receptor H2 (HRH2) which are types of G-protein coupled receptors (GPCR). Both receptors are activated when a ligand, specifically a histamine molecule, which binds to the receptor and activates the signaling pathway within the cell. The main protein within the signaling pathway is the G-protein which helps the cascade effect of the signal to other molecules. G-proteins are activated through GTP. An inverse agonist works like an agonist but will have an opposite end effect within the cell. It was originally thought that inverse agonist works the same way as an agonist to recruit a GTP and activate a G-protein for signaling. The experiment being tests tries to explain the opposite that the inverse agonist could activate the protein without GTP and continue to have its effect on the cell. Human embryonic cells were transfected with plasmids that contain sequences for the receptors and the G-protein, which were also tagged with a fluorophore to measure any bioluminescence with interaction of G-protein and the receptor when the ligands binds. From collecting data from the bioluminescence effect, it shows that there is an interaction a receptor and G-protein complex when the inverse agonist is bound.
G protein-coupled receptors (GPCRs) are receptors involved in signal transduction, a process for converting extracellular signals into internal messages to elicit a cellular response. Signal transduction pathways involve activating various G protein subtypes (Gs, Gi/o, Gq/11 and G12/13) which typically lead to second messenger production. Traditionally, second messenger concentration assays are used to identify GPCR coupling with G protein(s), but they are not efficient in profiling GPCRs since they compare the concentrations from different downstream signals. Instead, novel tools, such as Bioluminescence Resonance Energy Transfer (BRET) and mini G (mG) proteins, can be used to profile GPCRs. BRET is a technique that provides quantitative data when protein-protein interaction occurs and requires the proteins of interest to be fused with either a bioluminescent protein or fluorescent protein. In this study, we used mG proteins representing each G protein subtype to identify 5-hydroxytryptamine (5-HT; serotonin) receptor coupling upon serotonin stimulation. Through BRET assays, we determined that both the 5-HT1D and 5-HT1F receptors couple primarily with the mGsiand mGo classes of mG proteins. This supports previous studies that these receptors couple to Gi/o proteins and suggests that the use of mG proteins in BRET assays is an effective tool for GPCR profiling.
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