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Abstract
As our knowledge and understanding of the way in which GPCRs operate continues to grow rapidly, many new opportunities are emerging to develop novel therapeutic agents. This themed issue of the British Journal of Pharmacology contains a series of papers that cover recent developments and identify approaches that may help determine future directions. Many of these papers contain material that was presented at the 6th International Molecular Pharmacology of G Protein‐Coupled Receptors meeting held at the Monash Institute of Pharmaceutical Sciences in Melbourne Australia in late 2010.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein‐Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue‐6 . To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue‐5/issuetoc
... Most of the cellular and molecular mechanisms involved in RXFP1-mediated cancer promotion have been established in breast, thyroid, and prostate cancer models. For more detailed reviews on relaxin-like peptides and their cognate receptors, the reader is referred to recent excellent reviews (1)(2)(3)(4)(5). Increased expression of relaxin-like peptides has been detected in breast cancer (6,7). ...
The relaxin-like RXFP1 ligand–receptor system has important functions in tumor growth and tissue invasion. Recently, we have identified the secreted protein, CTRP8, a member of the C1q/tumor necrosis factor-related protein (CTRP) family, as a novel ligand of the relaxin receptor, RXFP1, with functions in brain cancer. Here, we review the role of CTRP members in cancers cells with particular emphasis on CTRP8 in glioblastoma.
The study of P2X receptors has long been handicapped by a poverty of small molecule tools that serve as selective agonists and antagonists. There has been progress, particularly in the past 10 years, as cell-based high throughput screening methods were applied, together with large chemical libraries. This has delivered some drug-like molecules in several chemical classes that target selectively P2X1, P2X3 or P2X7 receptors. Some of these are, or have been, in clinical trials for rheumatoid arthritis, pain and cough. Current preclinical research programs are studying P2X receptor involvement in pain, inflammation, osteoporosis, multiple sclerosis, spinal cord injury and bladder dysfunction. The determination of the atomic structure of P2X receptors in closed and open (ATP-bound) states by X-ray crystallography is now allowing new approaches by molecular modelling. This is supported by a large body of previous work using mutagenesis and functional expression, and is now being supplemented by molecular dynamic simulations and in silico ligand docking. These approaches should lead to P2X receptors soon taking their place alongside other ion channel proteins as a therapeutically important drug targets.
Seven transmembrane receptors (7TMRs) are nature's prototype allosteric proteins made to bind molecules at one location to subsequently change their shape to affect the binding of another molecule at another location. This paper attempts to describe the divergent 7TMR behaviours (i.e. third party allostery, receptor oligomerization, biased agonism) observed in pharmacology in terms of a homogeneous group of allosteric behaviours. By considering the bodies involved as a vector defined by a modulator, conduit and guest, these activities can all be described by a simple model of functional allostery made up of the Ehlert allosteric model and the Black/Leff operational model. It will be shown how this model yields parameters that can be used to characterize the activity of any ligand or protein producing effect through allosteric interaction with a 7TMR.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein‐Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue‐6 . To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue‐5/issuetoc
GPCRs exhibit a common architecture of seven transmembrane helices (TMs) linked by intracellular loops and extracellular loops (ECLs). Given their peripheral location to the site of G‐protein interaction, it might be assumed that ECL segments merely link the important TMs within the helical bundle of the receptor. However, compelling evidence has emerged in recent years revealing a critical role for ECLs in many fundamental aspects of GPCR function, which supported by recent GPCR crystal structures has provided mechanistic insights. This review will present current understanding of the key roles of ECLs in ligand binding, activation and regulation of both family A and family B GPCRs.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein‐Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue‐6 . To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue‐5/issuetoc
GPCRs represent the largest family of integral membrane proteins and were first identified as receptor proteins that couple via heterotrimeric G-proteins to regulate a vast variety of effector proteins to modulate cellular function. It is now recognized that GPCRs interact with a myriad of proteins that not only function to attenuate their signalling but also function to couple these receptors to heterotrimeric G-protein-independent signalling pathways. In addition, intracellular and transmembrane proteins associate with GPCRs and regulate their processing in the endoplasmic reticulum, trafficking to the cell surface, compartmentalization to plasma membrane microdomains, endocytosis and trafficking between intracellular membrane compartments. The present review will overview the functional consequence of β-arrestin, receptor activity-modifying proteins (RAMPS), regulators of G-protein signalling (RGS), GPCR-associated sorting proteins (GASPs), Homer, small GTPases, PSD95/Disc Large/Zona Occludens (PDZ), spinophilin, protein phosphatases, calmodulin, optineurin and Src homology 3 (SH3) containing protein interactions with GPCRs.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein-Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-6. To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue-5/issuetoc
G protein‐coupled chemokine receptors and their peptidergic ligands are interesting therapeutic targets due to their involvement in various immune‐related diseases, including rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, chronic obstructive pulmonary disease, HIV‐1 infection and cancer. To tackle these diseases, a lot of effort has been focused on discovery and development of small‐molecule chemokine receptor antagonists. This has been rewarded by the market approval of two novel chemokine receptor inhibitors, AMD3100 (CXCR4) and Maraviroc (CCR5) for stem cell mobilization and treatment of HIV‐1 infection respectively. The recent GPCR crystal structures together with mutagenesis and pharmacological studies have aided in understanding how small‐molecule ligands interact with chemokine receptors. Many of these ligands display behaviour deviating from simple competition and do not interact with the chemokine binding site, providing evidence for an allosteric mode of action. This review aims to give an overview of the evidence supporting modulation of this intriguing receptor family by a range of ligands, including small molecules, peptides and antibodies. Moreover, the computer‐assisted modelling of chemokine receptor–ligand interactions is discussed in view of GPCR crystal structures. Finally, the implications of concepts such as functional selectivity and chemokine receptor dimerization are considered.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein‐Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue‐6 . To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue‐5/issuetoc
An exciting aspect of the heptahelical orexin receptor 1 (OX(1)R) has emerged recently, when it was shown that it drives apoptosis in human colon cancer cell lines. Here we review recent findings related to the role of OX(1)R in colorectal cancers and the unexpected mechanism whereby this G protein-coupled receptor works. The OX(1)R is aberrantly expressed at all steps of primary colorectal tumour progression and after local (lymph node) or distant (liver, lung) metastasis. No OX(1)R is detected in normal colonic epithelial cells. Treatment of human colon cancer cells in culture with orexins promotes robust apoptosis and subsequent reduction of growth including in cells that are resistant to 5-fluorouracil, the most commonly used drug in chemotherapy. When human colon cancer cells are xenografted in nude mice, treatment with orexins dramatically slows tumour growth and even reverses the development of established tumours. Thus, OX(1)R agonists might be novel candidates for colon cancer therapy. Activation of OX(1)R drives apoptosis through G(q) protein but independently of classical Gα(q) activation of phospholipase C. In fact, it is the freed βγ dimer of G(q) that plays a pivotal role by stimulating Src-tyrosine kinase. This results in phosphorylation of two immunoreceptor tyrosine-based inhibitory motifs (ITIM) in OX(1)R and subsequent recruitment by OX(1)R of the phosphotyrosine phosphatase SHP-2, which is activated thereby. Downstream events include release of cytochrome c from mitochondria and activation of caspase-3 and caspase-7. The role of ITIMs in OX(1)R-driven apoptosis represents a new paradigm of G protein-coupled receptor signalling.
Agonists acting on µ-opioid receptors (MOR) are very effective analgesics but cause tolerance during long-term or repeated exposure. Intensive efforts have been made to find novel opioid agonists that are efficacious analgesics but can elude the signalling events that cause tolerance. µ-Opioid agonists differentially couple to downstream signalling mechanisms. Some agonists, such as enkephalins, D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), methadone and sufentanyl are efficacious at mediating G-protein and effector coupling, as well as triggering MOR regulatory events that include MOR phosphorylation, β-arrestin binding, receptor endocytosis and recycling. By contrast, morphine and closely related alkaloids can mediate efficacious MOR-effector coupling but poorly trigger receptor regulation. Several models have been proposed to relate differential MOR regulation by different opioids with their propensity to cause tolerance. Most are based on dogma that β-arrestin-2 (βarr-2) binding causes MOR desensitization and/or that MOR endocytosis and recycling are required for receptor resensitization. This review will examine some of these notions in light of recent evidence establishing that MOR dephosphorylation and resensitization do not require endocytosis. Recent evidence from opioid-treated animals also suggests that impaired MOR-effector coupling is driven, at least in part, by enhanced desensitization, as well as impaired resensitization that appears to be βarr-2 dependent. Better understanding of how chronic exposure to opioids alters receptor regulatory mechanisms may facilitate the development of effective analgesics that produce limited tolerance.
The classical second messenger cAMP is important in diverse physiological processes, where its spatial and temporal compartmentalization allows precise control over multiple cellular events. Within this context, G‐protein‐coupled receptors (GPCRs) govern specialized pools of cAMP, which are functionally specific for the unique cellular effects attributed to a particular system. The relaxin receptor, RXFP1, is a GPCR that exerts pleiotropic physiological effects including a potent anti‐fibrotic response, increased cancer metastases, and has efficacy as a vasodilator in heart failure. On a cellular level, relaxin stimulation of RXFP1 results in the activation of multiple G‐protein pathways affecting cAMP accumulation. Specificity and diversity in the cAMP signal generated by RXFP1 is controlled by differential G‐protein coupling dependent upon the background of cellular expression, and cAMP compartmentalization. Further complexity in cAMP signalling results from the constitutive assembly of an RXFP1–signalosome, which specifically responds to low concentrations of relaxin, and activates a distinct cAMP pathway. The RXFP1–signalosome is a higher‐order protein complex that facilitates receptor sensitivity to attomolar concentration of peptide, exhibits constitutive activity and dual coupling to G‐proteins and β‐arrestins and reveals a concentration‐biased agonism mediated by relaxin. The specific and directed formation of GPCR‐centered signalosomes allows an even greater spatial and temporal control of cAMP, thus rationalizing the considerable physiological scope of this ubiquitous second messenger.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein‐Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue‐6 . To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue‐5/issuetoc
G‐protein‐coupled receptors [GPCRs, also known as 7‐transmembrane (7‐TM) receptors] comprise the largest family of membrane receptors in humans and other species and, in addition, represent the greatest number of current drug targets. In this article, we review methods to define GPCR expression and data indicating that individual cells express >100 different GPCRs. Results from studies that have quantified expression of these receptors lead us to conclude that the optimal GPCRs may not be currently used as therapeutic targets. We propose that studies of GPCR expression in individual cells will likely reveal new insights regarding cellular physiology and therapeutic approaches. Findings that define and characterize the most highly expressed GPCRs thus have important potential in terms of identifying new drug targets and novel therapies directed at a wide range of clinical disorders.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein‐Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue‐6 . To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue‐5/issuetoc
Calcium‐sensing receptors (CaSR) are integral to regulation of systemic Ca ²⁺ homeostasis. Altered expression levels or mutations in CaSR cause Ca ²⁺ handling diseases. CaSR is regulated by both endogenous allosteric modulators and allosteric drugs, including the first Food and Drug Administration‐approved allosteric agonist, Cinacalcet HCl (Sensipar®). Recent studies suggest that allosteric modulators not only alter function of plasma membrane‐localized CaSR, but regulate CaSR stability at the endoplasmic reticulum. This brief review summarizes our current understanding of the role of membrane‐permeant allosteric agonists in cotranslational stabilization of CaSR, and highlights additional, indirect, signalling‐dependent role(s) for membrane‐impermeant allosteric drugs. Overall, these studies suggest that allosteric drugs act at multiple cellular organelles to control receptor abundance and hence function, and that drug hydrophobicity can bias the relative contributions of plasma membrane and intracellular organelles to CaSR abundance and signalling.
LINKED ARTICLES This article is part of a themed section on the Molecular Pharmacology of G Protein‐Coupled Receptors (GPCRs). To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue‐6 . To view the 2010 themed section on the same topic visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2010.159.issue‐5/issuetoc
Lifting the lid on G-protein-coupled receptors: the role of extracellular loops
- M Wheatley
- D Wootten
- M T Conner
- J Simms
- R Kendrick
- R T Logan
Wheatley M, Wootten D, Conner MT, Simms J, Kendrick R,
Logan RT et al. (2012). Lifting the lid on G-protein-coupled
receptors: the role of extracellular loops. Br J Pharmacol 165:
1688-1703.
Lifting the lid on G-protein-coupled receptors: the role of extracellular loops
- Wheatley