Graeme Milligan

Identification of a serotonin/glutamate receptor complex implicated in psychosis

The psychosis associated with schizophrenia is characterized by alterations in sensory processing and perception. Some antipsychotic drugs were identified by their high affinity for serotonin 5-HT2A receptors (2AR). Drugs that interact with metabotropic glutamate receptors (mGluR) also have potential for the treatment of schizophrenia. The effects of hallucinogenic drugs, such as psilocybin and lysergic acid diethylamide, require the 2AR and resemble some of the core symptoms of schizophrenia. Here we show that the mGluR2 interacts through specific transmembrane helix domains with the 2AR, a member of an unrelated G-protein-coupled receptor family, to form functional complexes in brain cortex. The 2AR–mGluR2 complex triggers unique cellular responses when targeted by hallucinogenic drugs, and activation of mGluR2 abolishes hallucinogen-specific signalling and behavioural responses. In post-mortem human brain from untreated schizophrenic subjects, the 2AR is upregulated and the mGluR2 is downregulated, a pattern that could predispose to psychosis. These regulatory changes indicate that the 2AR–mGluR2 complex may be involved in the altered cortical processes of schizophrenia, and this complex is therefore a promising new target for the treatment of psychosis.

Presynaptic Control of Striatal Glutamatergic Neurotransmission by Adenosine A1–A2A Receptor Heteromers

The functional role of heteromers of G-protein-coupled receptors is a matter of debate. In the present study, we demonstrate that heteromerization of adenosine A1 receptors (A1Rs) and A2A receptors (A2ARs) allows adenosine to exert a fine-tuning modulation of glutamatergic neurotransmission. By means of coimmunoprecipitation, bioluminescence and time-resolved fluorescence resonance energy transfer techniques, we showed the existence of A1R–A2AR heteromers in the cell surface of cotransfected cells. Immunogold detection and coimmunoprecipitation experiments indicated that A1R and A2AR are colocalized in the same striatal glutamatergic nerve terminals. Radioligand-binding experiments in cotransfected cells and rat striatum showed that a main biochemical characteristic of the A1R–A2AR heteromer is the ability of A2AR activation to reduce the affinity of the A1R for agonists. This provides a switch mechanism by which low and high concentrations of adenosine inhibit and stimulate, respectively, glutamate release. Furthermore, it is also shown that A1R–A2AR heteromers constitute a unique target for caffeine and that chronic caffeine treatment leads to modifications in the function of the A1R–A2AR heteromer that could underlie the strong tolerance to the psychomotor effects of caffeine.

Tailoring cAMP-signalling responses through isoform multiplicity

Multiple forms of cAMP phosphodiesterases (PDE), adenylate cyclase and protein kinase A (PKA) allow cells to tailor the responsiveness of the cAMP-signalling system and to allow for its dynamic adjustment. Multiple forms of these enzymes confer spatial and temporal characteristics on cAMP signalling so as to affect compartmentalised responses within a single cell type. The ability to breach the PKA activation threshold can depend upon either or both the activation of adenylate cyclase and inhibition of specific PDE isoforms.

G-protein-coupled receptor mas is a physiological antagonist of the angiotensin II type 1 receptor

Background—We previously identified the G-protein–coupled receptor Mas, encoded by the Mas proto-oncogene, as an endogenous receptor for the heptapeptide angiotensin-(1-7); however, the receptor is also suggested to be involved in actions of angiotensin II. We therefore tested whether this could be mediated indirectly through an interaction with the angiotensin II type 1 receptor, AT1. Methods and Results—In transfected mammalian cells, Mas was not activated by angiotensin II; however, AT1 receptor–mediated, angiotensin II–induced production of inositol phosphates and mobilization of intracellular Ca2+ was diminished by 50% after coexpression of Mas, despite a concomitant increase in angiotensin II binding capacity. Mas and the AT1 receptor formed a constitutive hetero-oligomeric complex that was unaffected by the presence of agonists or antagonists of the 2 receptors. In vivo, Mas acts as an antagonist of the AT1 receptor; mice lacking the Mas gene show enhanced angiotensin II–mediated vasoconstriction in mesenteric microvessels. Conclusions—These results demonstrate that Mas can hetero-oligomerize with the AT1 receptor and by so doing inhibit the actions of angiotensin II. This is a novel demonstration that a G-protein–coupled receptor acts as a physiological antagonist of a previously characterized receptor. Consequently, the AT1-Mas complex could be of great importance as a target for pharmacological intervention in cardiovascular diseases.

THE DYNAMIC ROLE OF PALMITOYLATION IN SIGNAL TRANSDUCTION

Guanine nucleotide binding protein (G protein)-linked receptors, the alpha-subunits of heterotrimeric G proteins and members of the Src family of nonreceptor tyrosine kinases are among many polypeptides that are posttranslationally modified by the addition of palmitate, a long-chain fatty acid. Attachment of palmitate to these proteins is dynamic and may be regulated by their activation. The presence of palmitate appears to play a key role in the membrane localization of either the entire polypeptide or parts of it, and may regulate the interactions of these polypeptides with other proteins.

G Protein–Coupled Receptor Oligomerization Revisited: Functional and Pharmacological Perspectives

Most evidence indicates that, as for family C G protein–coupled receptors (GPCRs), family A GPCRs form homo- and heteromers. Homodimers seem to be a predominant species, with potential dynamic formation of higher-order oligomers, particularly tetramers. Although monomeric GPCRs can activate G proteins, the pentameric structure constituted by one GPCR homodimer and one heterotrimeric G protein may provide a main functional unit, and oligomeric entities can be viewed as multiples of dimers. It still needs to be resolved if GPCR heteromers are preferentially heterodimers or if they are mostly constituted by heteromers of homodimers. Allosteric mechanisms determine a multiplicity of possible unique pharmacological properties of GPCR homomers and heteromers. Some general mechanisms seem to apply, particularly at the level of ligand-binding properties. In the frame of the dimer-cooperativity model, the two-state dimer model provides the most practical method to analyze ligand–GPCR interactions when considering receptor homomers. In addition to ligand-binding properties, unique properties for each GPCR oligomer emerge in relation to different intrinsic efficacy of ligands for different signaling pathways (functional selectivity). This gives a rationale for the use of GPCR oligomers, and particularly heteromers, as novel targets for drug development. Herein, we review the functional and pharmacological properties of GPCR oligomers and provide some guidelines for the application of discrete direct screening and high-throughput screening approaches to the discovery of receptor-heteromer selective compounds.

Building a new conceptual framework for receptor heteromers

Receptor heteromers constitute a new area of research that is reshaping our thinking about biochemistry, cell biology, pharmacology and drug discovery. In this commentary, we recommend clear definitions that should facilitate both information exchange and research on this growing class of transmembrane signal transduction units and their complex properties. We also consider research questions underlying the proposed nomenclature, with recommendations for receptor heteromer identification in native tissues and their use as targets for drug development.

G protein-coupled receptor hetero-dimerization: contribution to pharmacology and function.

The concept that G protein‐coupled receptors (GPCRs) can form hetero‐dimers or hetero‐oligomers continues to gain experimental support. However, with the exception of the GABAB receptor and the sweet and umami taste receptors few reported examples meet all of the criteria suggested in a recent International Union of Basic and Clinical Pharmacology sponsored review ( Pin et al., 2007 ) that should be required to define distinct and physiologically relevant receptor species. Despite this, there are many examples in which pairs of co‐expressed GPCRs reciprocally modulate their function, trafficking and/or ligand pharmacology. Such data are at least consistent with physical interactions between the receptor pairs. In recent times, it has been suggested that specific GPCR hetero‐dimer or hetero‐oligomer pairs may represent key molecular targets of certain clinically effective, small molecule drugs and there is growing interest in efforts to identify ligands that may modulate hetero‐dimer function selectively. The current review summarizes key recent developments in these topics.

Heterotrimeric G-proteins: a short history

Some 865 genes in man encode G‐protein‐coupled receptors (GPCRs). The heterotrimeric guanine nucleotide‐binding proteins (G‐proteins) function to transduce signals from this vast panoply of receptors to effector systems including ion channels and enzymes that alter the rate of production, release or degradation of intracellular second messengers. However, it was not until the 1970s that the existence of such transducing proteins was even seriously suggested. Combinations of bacterial toxins that mediate their effects via covalent modification of the α‐subunit of certain G‐proteins and mutant cell lines that fail to generate cyclic AMP in response to agonists because they either fail to express or express a malfunctional G‐protein allowed their identification and purification. Subsequent to initial cloning efforts, cloning by homology has defined the human G‐proteins to derive from 35 genes, 16 encoding α‐subunits, five β and 14 γ. All function as guanine nucleotide exchange on–off switches and are mechanistically similar to other proteins that are enzymic GTPases. Although not readily accepted initially, it is now well established that β/γ complexes mediate as least as many functions as the α‐subunits. The generation of chimeras between different α‐subunits defined the role of different sections of the primary/secondary sequence and crystal structures and cocrystals with interacting proteins have given detailed understanding of their molecular structure and basis of function. Finally, further modifications of such chimeras have generated a range of G‐protein α‐subunits with greater promiscuity to interact across GPCR classes and initiated the use of such modified G‐proteins in drug discovery programmes.

International Union of Pharmacology. LXXI. Free Fatty Acid Receptors FFA1, -2, and -3: Pharmacology and Pathophysiological Functions

Identification of G protein-coupled receptors that are activated by free fatty acids has led to considerable interest in their pharmacology and function because of the wide range of normal physiology and disease states in which fatty acids have been implicated. Free fatty acid receptor (FFA) 1 is activated by medium- to long-chain fatty acids and is expressed in the insulin-producing β-cells of the pancreas. Activation of FFA1 has been proposed to mediate fatty acid augmentation of glucose-stimulated insulin secretion although it is unclear whether the known long-term detrimental effects of β-cell exposure to high levels of fatty acids are also mediated through this receptor. The related receptors FFA2 and FFA3 are both activated by short-chain fatty acids although they have key differences in the signaling pathways they activate and tissue expression pattern. The aim of this review is to provide a comprehensive overview of the current understanding of the pharmacology and physiological role of these fatty acid receptors.