Current Pharmaceutical Biotechnology - Volume 15, Issue 10, 2014

Signals relayed through G protein-coupled receptors (GPCR) play pivotal roles in human physiology and are important drug targets. About 40% of all GPCRs couple to the heterotrimeric G protein Gq. Biochemical studies as well as crystallography have improved our understanding of GqPCRs and their downstream partners. Here we focus on the "functional imaging" tools that have been developed to visualize, dissect and quantify signalling processes at the single living cell level. We provide an overview of the most important developments in readout of signalling by FRET and BRET, as well as of the labelling strategies commonly used to visualize proteins in living cells. In addition, tools that allow manipulation of individual steps, including chemically inducible dimerization and optogenetic tools are covered. Together, these developments will provide unprecedented insights in GqPCR signalling in living cells and model organisms.

As the largest family of integral membrane proteins, G-protein-coupled receptors (GPCRs) comprise the largest class of therapeutic targets that aimed approximately 40% of modern medicinal drugs. Understanding the agonist/ antagonist mechanism, as well as the signal transduction of the GPCRs, is pivotal in drug discovery and new therapeutic strategy development. In the past few years, determination of high-resolution crystal structures of GPCRs from different subfamilies laid a solid foundation for both experimental and computational studies on GPCR-related diseases. Dopamine and serotonin receptors that belong to class A GPCRs play key roles in psychotic disorders, such as schizophrenia. As a robust approach, computer-aided drug design (CADD) has been demonstrated to be a powerful tool to discover novel drugs against these disorders and to help understand the activation mechanism of related receptors. Herein, we reviewed the recent progresses on CADD-based drug discovery, agonist/antagonist mechanism, and agonist-induced signaling mechanism in dopamine and serotonin receptors.

GPCRs are a major family of homologous proteins and are key mediators of the effects of numerous endogenous neurotransmitters, hormones, cytokines, therapeutic drugs, and drugs-of-abuse. Despite the enormous amount of research on the pharmacological and biochemical properties of GPCRs, there is surprisingly little information on GPCR dimer structure and function in primary cell culture or in vivo. We have used two novel approaches to develop methods to detect and study GPCR dimer function: FCS/PCH and “inactivation-reactivation”. This review will focus on the data we have developed and our interpretations of those data. Using FCS/PCH 5-HT2C receptors have been detected directly and appear to exist as dimers, consistent with the inactivation-reactivation data on 5-HT7 and 5-HT2A receptors. Studies of the 5-HT7 and 5-HT2A serotonin receptors have revealed that binding of a pseudo-irreversible antagonist (“inactivator”) to one of the orthosteric sites of a homodimer abolishes all receptor activity, and subsequent binding of a competitive antagonist to the orthosteric site of the second protomer releases the inactivator, allowing the receptor to return to an active state. This approach demonstrates allosteric crosstalk between protomers of native GPCR homodimers, indicating that GPCRs do exist and function as homodimers in both recombinant cells and rat primary astrocytes. This technique can be applied universally using intact recombinant or primary cells in culture, membrane homogenate preparations and, potentially, in vivo. This approach can be applied to heterodimers as well as homodimers and may aid in the development of novel drugs with heterodimer selectivity.

Chemokine receptors are G protein-coupled receptors that contain seven trans-membrane domains. CXCR4 and CCR5 as major co-receptors for HIV-1 entry into host cells are implicated in cancer and inflammation. They have been attractive targets for the pharmaceutical industry basing on their roles in HIV disease. Homology modeling, molecular docking, molecular dynamics, Molecular Mechanics/Generalized Born Surface Area and many other computational methods are applied to illustrate the structure, function and binding site of GPCR. Moreover, the high resolution crystal structures of CXCR4 and CCR5 have provided extremely valuable structural information and receptor activation mechanisms, enable structure-based drug discovery for the treatment of HIV-1 infection. We also describe the recent progress about the small molecule antagonists of CXCR4 and CCR5 and the interaction between GPCR and their ligands predicted by molecular docking and molecular dynamics methods. Future research questions and further investigations are outlined to highlight some researches that may be relevant to the advancement of therapies targeting the important receptor related with HIV.

Detection of tastes is critical for animals. Sweet, umami and bitter taste are mediated by G-protein-coupled receptors that are expressed in the taste receptor cells. TAS1Rs which belong to class C G-protein-coupled receptors form heterodimeric complexes to function as sweet (TAS1R2 + TAS1R3) or umami (TAS1R1 + TAS1R3) taste receptors. Umami taste is also considered to be mediated by mGluRs. TAS2Rs belong to class A G-protein-coupled receptors and are responsible for bitter taste. After activation of these receptors, their second messenger pathways lead to depolarization and intracellular calcium increase in taste receptor cells. Then, transmitter is released from taste receptor cells leading to activation of taste nerve fibers and taste information is sent to the central nervous system. Recent studies on heterologous expression system and molecular modeling lead to better understanding of binding site of TAS1Rs and TAS2Rs and molecular mechanisms for interaction between taste substances and these receptors. TAS1Rs and TAS2Rs have multiple and single binding sites for structurally diverse ligands, respectively. Sensitivities of these receptors are known to differ among individuals, strains, and species. In addition, some species abolish these receptors and signaling molecules. Here we focus on structure, function, signaling, polymorphism, and molecular evolution of the taste G-protein-coupled receptors.

G-protein-coupled receptors (GPCRs) represent the main family of cell surface receptors and are virtually expressed in all eukaryotic cells. Interestingly, a large number of clinically used drugs exert their pharmacological effect via a GPCR, thus it seems crucial to deeply understand the biology of these receptors. The study of GPCR activation and signaling has been classically performed by physiological, biochemical and pharmacological approaches using radioactivity-based tools. However, apart from the potential hazards of radioisotope handling and environmental burden, these approaches have some technical limitations. Therefore, the development of fluorescence-based techniques in general and fluorescence and bioluminescence resonance energy transfer (FRET and BRET) in particular have revolutionized the way to study GPCR functioning both in vitro and in vivo. Indeed, these techniques allow the characterization and visualization of all the individual GPCR signaling steps (i.e. ligand binding, receptor activation, G-protein coupling, G-protein activation, GPCR desensitization) with high temporal and spatial resolution. Here, we review the use and impact of fluorescent-based methodologies on the deciphering of GPCR biology.

G protein-coupled receptors (GPCRs), a family of seven-transmembrane receptors, are among the most important drug targets with over half of all marketed drugs targeting the family. However, only a handful of easily druggable GPCRs are successfully targeted by pharmaceuticals. Efforts to shift this intensive focus to other, more recalcitrant GPCR targets will increasingly draw on new information such as structural details, which have until recently proven tremendously challenging to gather for this class of protein receptors due to the difficulties in obtaining diffraction-quality crystals. Recently, the development and application of lipidic cubic phase (LCP) technology has reduced one major hurdle for crystallization of GPCRs, with 22 unique receptors being structurally characterized from LCP grown crystals over the span of seven years. This review focuses on the technological improvements for LCP that have led to its successful utilization on the GPCR family, including the most recent combination of LCP with the X-ray free-electron laser that dramatically reduces requirements on crystal size, and holds significant promise for shortening timelines for structure determination and for accessing previously unattainable structures such as those of signaling complexes.

G-protein coupled receptors (GPCRs) are proteins of the plasma membrane, which are characterized by seven membrane-spanning segments (TMs). GPCRs play an important role in almost all of our physiological and pathophysiological conditions by interacting with a large variety of ligands and stimulating different G-proteins and signaling cascades. By playing a key role in the function of our body and being involved in the pathophysiology of many disorders, GPCRs are very important therapeutic targets. Determination of the structure and function of GPCRs could advance the design of novel receptor-specific drugs against various diseases. A powerful method to obtain structural and functional information for GPCRs is the cysteine substituted accessibility method (SCAM). SCAM is used to systematically map the TM residues of GPCRs and determine their functional role. SCAM can also be used to determine differences in the structures of the TMs in different functional states of GPCRs.

Kir3 (or GIRK) channels have been known for nearly three decades to be activated by direct interactions with the βγ subunits of heterotrimeric G (Gαβγ) proteins in a membrane-delimited manner. Gα also interacts with GIRK channels and since PTX-sensitive Gα subunits show higher affinity of interaction they confer signaling specificity to G Protein- Coupled Receptors (GPCRs) that normally couple to these G protein subunits. In heterologous systems, overexpression of non PTX-sensitive Gα subunits scavenges the available Gβγ and biases GIRK activation through GPCRs that couple to these Gα subunits. Moreover, all Kir channels rely on their direct interactions with the phospholipid PIP2 to maintain their activity. Thus, signals that activate phospholipase C (e.g. through Gq signaling) to hydrolyze PIP2 result in inhibition of Kir channel activity. In this review, we illustrate with experiments performed in Xenopus oocytes that Kir channels can be used efficiently as reporters of GPCR function through Gi, Gs or Gq signaling. The membrane-delimited nature of this expression system makes it highly efficient for constructing dose-response curves yielding highly reproducible apparent affinities of different ligands for each GPCR tested.

Growing experimental evidences suggest that dimerization and oligomerization are important for G Protein- Coupled Receptors (GPCRs) function. The detailed structural information of dimeric/oligomeric GPCRs would be very important to understand their function. Although it is encouraging that recently several experimental GPCR structures in oligomeric forms have appeared, experimental determination of GPCR structures in oligomeric forms is still a big challenge, especially in mimicking the membrane environment. Therefore, development of computational approaches to predict dimerization of GPCRs will be highly valuable. In this review, we summarize computational approaches that have been developed and used for modeling of GPCR dimerization. In addition, we introduce a novel two-dimensional Brownian Dynamics based protein docking approach, which we have recently adapted, for GPCR dimer prediction.