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G protein-coupled receptors, known familiarly as GPCRs, or as 7TMRs (seven transmembrane receptors) comprise a large protein family of integral membrane proteins that possess seven membrane-spanning domains or transmembrane helices that sense molecules outside the cell and activate signal transduction pathways inside the cell and, ultimately, cellular responses. Although a number of receptor classes exist in humans, by far the most abundant and therapeutically relevant is represented by the GPCR class. There are more than 250 “known” GPCRs, for which the endogenous ligand has been identified, while receptors for which the endogenous ligand has not been identified are referred to as "orphan" receptors. There are still more than 150 of these orphan receptors without considering the large group (almost 400 receptors) dedicated to olfaction. G protein-coupled receptors are found only in eukaryotes, including yeast, plants, choanoflagellates and animals. The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. There are different binding domains depending upon each receptor and its ligands, that are located in the N terminal end and/or within the transmembrane domain. In addition, a series of allosteric binding sites have been identified for GPCRs. It is believed that a GPCR exists in a conformational equilibrium between active and inactive biophysical states. The binding of ligands to the receptor may shift the equilibrium toward the active receptor states. There are different types of orthosteric ligands (binding to the site of the endogenous ligand): agonists are ligands that shift the equilibrium in favor of active states; inverse agonists are ligands that shift the equilibrium in favour of inactive states; and neutral antagonists are ligands that do not affect the equilibrium. In addition, it is now evident that GPCRs possess additional, extracellular, allosteric binding sites that can be recognized by a variety of small molecule modulator ligands. Allosteric modulators modulate positively (PAM) or negatively (NAM) the activity of orthosteric agonists. Moreover some PAMs can act as allosteric agonists and PAM or NAM activity can be blocked by neutral allosteric modulators. It is not yet known exactly how the active and inactive states differ from each other.
The transduction of the agonist signal through the membrane by the receptor is not completely understood. It is known that the inactive G protein is bound to the receptor in its inactive state. Once the ligand is recognized, the receptor shifts conformation and thus mechanically activates the G protein, which detaches from the receptor. The receptor can now either activate another G protein or switch back to its inactive state. Activated G proteins are bound to GTP. Further signal transduction depends on the type of G protein. The enzyme adenylate cyclase (Figure 2, green protein in panel C) is an example of a cellular protein that can be regulated by a G protein, in this case the G protein Gs. Adenylate cyclase activity is activated when it binds to a subunit of the activated G protein. Activation of adenylate cyclase ends when the G protein returns to the GDP-bound state.
In addition, GPCRs are able to signal without G proteins. GPCRs interact with beta arrestin-mediated uncoupling of G-protein-mediated signaling. It therefore seems likely that some mechanisms previously believed to be purely related to receptor desensitisation are actually examples of receptors switching their signaling pathway rather than simply being switched off.
GPCRs become desensitized when exposed to their ligand for a prolonged period of time. There are two recognized forms of desensitization: 1) homologous desensitization, in which the activated GPCR is down-regulated; and 2) heterologous desensitization, wherein the activated GPCR causes down-regulation of a different GPCR. The key reaction of this down-regulation is the phosphorylation of the intracellular (or cytoplasmic) receptor domain by protein kinases, like GRK or protein kinase A.
It is generally accepted that G-protein-coupled receptors can form homo- and/or heterodimers and possibly more complex oligomeric structures, and indeed heterodimerization has been shown to be essential for the function of receptors such as the metabotropic GABA(B) or TAR receptors.. It is also unclear what the functional significance of oligomerization might be, although it is thought that the phenomenon may contribute to the pharmacological heterogeneity of GPCRs in a manner not previously anticipated.
GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:
1. the visual sense: the opsins use a photoisomerization reaction to translate electromagnetic radiation into cellular signals. Rhodopsin, for example, uses the conversion of 11-cis-retinal to all-trans-retinal for this purpose;
2. the sense of smell: receptors of the olfactory epithelium bind odorants (olfactory receptors) and pheromones in animals(vomeronasal receptors);
3. behavioral and mood regulation: receptors in the mammalian brain bind several different neurotransmitters, including serotonin, dopamine, GABA, and glutamate;
4. regulation of immune system activity and inflammation: chemokine and chemoattractants receptors bind ligands that mediate intercellular communication between cells of the immune system; receptors such as histamine receptors bind inflammatory mediators and engage target cell types in the inflammatory response :
5. autonomic nervous system transmission: both the sympathetic and parasympathetic nervous systems are regulated by GPCR pathways, responsible for control of many automatic functions of the body such as blood pressure, heart rate, and digestive processes.
GPCRs have proved to be very successful drug targets for the pharmaceutical industry. It is usually accepted that 30%–40% of marketed drugs target directly or indirectly less than 50 of the 400 non olfactory GPCRs. The top 100 brand name prescription drugs in 1999 included the following drugs interacting with GPCRs: Claritin® (allergies) Prozac® (depression) Vasotec® (hypertension) Paxil® (depression) Zoloft® (depression) Zyprexa® (psychotic disorder) Cozaar® (hypertension) Imitrex® (migraine) Zantac® (reflux) Propulsid® (reflux disease) Risperdal® (schizophrenia) Serevent® (asthma) Pepcid® (reflux) Gaster® (ulcers) Atrovent® (bronchospasm) Effexor® (depression) Depakote® (epilepsy) Cardura® (prostatic hypertrophy) Allegra® (allergies) Lupron® (prostate cancer) Zoladex® (prostate cancer) Diprivan® (anesthesia) BuSpar® (anxiety) Ventolin® (bronchospasm) Hytrin® (hypertension) Wellbutrin® (depression) Zyrtec® (rhinitis) Plavix® (MI/stroke) Toprol-XL® (hypertension) Tenormin® (angina) Xalatan® (glaucoma) Singulair® (asthma) Diovan® (hypertension) Hamal® (prostatic hyperplasia) (Med Ad News 1999 Data).
The increasing knowledge of GPCRs and their ligands (chemical ligand space) associated with increasing number of screening strategies enables novel drug design strategies to accelerate the finding and optimization of GPCR leads: The crystal structure of rhodopsin provides the first three-dimensional GPCR information, which now supports homology modeling studies and structure-based drug design approaches within the GPCR target family. On the other hand, the classical ligand-based design approaches (for example, virtual screening, pharmacophore modeling, quantitative structure-activity relationship (QSAR)) are still powerful methods for lead finding and optimization. In addition, the cross-target analysis of GPCR ligands has revealed more and more common structural motifs and three-dimensional pharmacophores. Such GPCR privileged structural motifs have been successfully used by many pharmaceutical companies to design and synthesize combinatorial libraries, which are subsequently tested against novel GPCR targets for lead finding. In the near future, structural biology and chemogenomics might allow the mapping of the ligand binding to the receptor. The linking of chemical and biological spaces will aid in generating lead-finding libraries, which are tailor-made for their respective receptor. In addition, the recent development of allosteric modulators opens new avenues for high selectivity, novel modes of efficacy and may lead to novel therapeutic agents for the treatment of multiple psychiatric and neurological human disorders.
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