G protein-coupled receptor dimerisation: Molecular basis and relevance to function

Targeting trafficking as a therapeutic avenue for misfolded GPCRs leading to endocrine diseases

G protein-coupled receptors (GPCRs) are plasma membrane proteins associated with an array of functions. Mutations in these receptors lead to a number of genetic diseases, including diseases involving the endocrine system. A particular subset of loss-of-function mutant GPCRs are misfolded receptors unable to traffic to their site of function (i.e. the cell surface plasma membrane). Endocrine disorders in humans caused by GPCR misfolding include, among others, hypo- and hyper-gonadotropic hypogonadism, morbid obesity, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, X-linked nephrogenic diabetes insipidus, congenital hypothyroidism, and familial glucocorticoid resistance. Several in vitro and in vivo experimental approaches have been employed to restore function of some misfolded GPCRs linked to endocrine disfunction. The most promising approach is by employing pharmacological chaperones or pharmacoperones, which assist abnormally and incompletely folded proteins to refold correctly and adopt a more stable configuration to pass the scrutiny of the cell's quality control system, thereby correcting misrouting. This review covers the most important aspects that regulate folding and traffic of newly synthesized proteins, as well as the experimental approaches targeted to overcome protein misfolding, with special focus on GPCRs involved in endocrine diseases.

Characterisation of agonist signalling profiles and agonist-dependent antagonism at PACAP-responsive receptors: Implications for drug discovery

BACKGROUND AND PURPOSE

The pituitary adenylate cyclase-activating peptide (PACAP) family is of clinical interest for the treatment of migraine. These peptides activate three different PACAP-responsive class B G protein-coupled receptors: the PAC₁ , VPAC₁ and VPAC₂ receptors. The PAC₁ receptor may be alternatively spliced, generating variants that can differ in their pharmacological or signalling profiles. To inform drug discovery efforts targeting migraine, we need to better understand how the different PACAP-responsive receptors signal and how effectively these responses can be blocked by antagonists.

EXPERIMENTAL APPROACH

The signalling profiles of the human PAC_1n , PAC_1s , VPAC₁ and VPAC₂ receptors were examined in transfected Cos7 cells for cAMP, IP₁ , pAkt, pERK and pCREB. Biased signalling was then quantified. The ability of antagonists to block PACAP-38, PACAP-27 or VIP stimulated cAMP accumulation at PACAP-responsive receptors was also determined.

KEY RESULTS

PACAP-responsive receptors exhibited varied pharmacological profiles but activated signalling in a similar manner. The PAC_1n and PAC_1s receptors displayed distinct pharmacology. At the PAC_1s receptor, VIP and PHM were more potent than at the PAC_1n receptor. PACAP-responsive receptors displayed agonist-dependent antagonism where PACAP-38 was less effectively antagonised compared to PACAP-27 and VIP.

CONCLUSIONS AND IMPLICATIONS

The distinct pharmacological profile displayed by the PAC_1s receptor suggests that it can act as a dual receptor for VIP and PACAP. Furthermore, the effectiveness of blocking a signalling pathway can be influenced by which endogenous PACAP family agonist is present. These effects have potential implications for the development and effectiveness of drugs targeting the PACAP system.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Glycoprotein G-protein Coupled Receptors in Disease: Luteinizing Hormone Receptors and Follicle Stimulating Hormone Receptors

Signal transduction by luteinizing hormone receptors (LHRs) and follicle-stimulating hormone receptors (FSHRs) is essential for the successful reproduction of human beings. Both receptors and the thyroid-stimulating hormone receptor are members of a subset of G-protein coupled receptors (GPCRs) described as the glycoprotein hormone receptors. Their ligands, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and a structurally related hormone produced in pregnancy, human chorionic gonadotropin (hCG), are large protein hormones that are extensively glycosylated. Although the primary physiologic functions of these receptors are in ovarian function and maintenance of pregnancy in human females and spermatogenesis in males, there are reports of LHRs or FSHRs involvement in disease processes both in the reproductive system and elsewhere. In this review, we evaluate the aggregation state of the structure of actively signaling LHRs or FSHRs, their functions in reproduction as well as summarizing disease processes related to receptor mutations affecting receptor function or expression in reproductive and non-reproductive tissues. We will also present novel strategies for either increasing or reducing the activity of LHRs signaling. Such approaches to modify signaling by glycoprotein receptors may prove advantageous in treating diseases relating to LHRs or FSHRs function in addition to furthering the identification of new strategies for modulating GPCR signaling.

Signaling activations through G-protein-coupled-receptor aggregations

Eukaryotic cells transmit extracellular signal information to cellular interiors through the formation of a ternary complex made up of a ligand (or agonist), G-protein, and G-protein-coupled receptor (GPCR). Previously formalized theories of ternary complex formation have mainly assumed that observable states of receptors can only take the form of monomers. Here, we propose a multiary complex model of GPCR signaling activations via the vector representation of various unobserved aggregated receptor states. Our results from model simulations imply that receptor aggregation processes can govern cooperative effects in a regime inaccessible by previous theories. In particular, we show how the affinity of ligand-receptor binding can be largely varied by various oligomer formations in the low concentration range of G-protein stimulus.

Pharmacological Treatment of Anxiety Disorders: The Role of the HPA Axis

Stress in general, and early life stress in particular, has been associated with the development of anxiety and mood disorders. The molecular, biological and psychological links between stress exposure and the pathogenesis of anxiety and mood disorders have been extensively studied, resulting in the search of novel psychopharmacological strategies aimed at targets of the hypothalamic-pituitary-adrenal (HPA) axis. Hyperactivity of the HPA axis has been observed in certain subgroups of patients with anxiety and mood disorders. In addition, the effects of different anti-anxiety agents on various components of the HPA axis has been investigated, including benzodiazepines, tricyclic antidepressants (TCAs), and selective serotonin reuptake inhibitors (SSRIs). For example, benzodiazepines, including clonazepam and alprazolam, have been demonstrated to reduce the activity of corticotrophin releasing factor (CRF) neurons in the hypothalamus. TCAs and SSRIs are also effective anti-anxiety agents and these may act, in part, by modulating the HPA axis. In this regard, the SSRI escitalopram inhibits CRF release in the central nucleus of the amygdala, while increasing glucocorticoid receptor (GRs) density in the hippocampus and hypothalamus. The molecular effects of these anti-anxiety agents in the regulation of the HPA axis, taken together with their clinical efficacy, may provide further understanding about the role of the HPA axis in the pathophysiology of mood and anxiety disorders, paving the way for the development of novel therapeutic strategies.

G-protein signaling is required for increasing germline stem cell division frequency in response to mating in Drosophila males

Adult stem cells divide to renew the stem cell pool and replenish specialized cells that are lost due to death or usage. However, little is known about the mechanisms regulating how stem cells adjust to a demand for specialized cells. A failure of the stem cells to respond to this demand can have serious consequences, such as tissue loss, or prolonged recovery post injury. Here, we challenge the male germline stem cells (GSCs) of Drosophila melanogaster for the production of specialized cells, sperm cells, using mating experiments. We show that repeated mating reduced the sperm pool and increased the percentage of GSCs in M- and S-phase of the cell cycle. The increase in dividing GSCs depended on the activity of the highly conserved G-proteins. Germline expression of RNA-Interference (RNA-i) constructs against G-proteins, or a dominant negative G-protein eliminated the increase in GSC division frequency in mated males. Consistent with a role for the G-proteins in regulating GSC division frequency, RNA-i against seven out of 35 G-protein coupled receptors (GPCRs) within the germline cells also eliminated the capability of males to increase the numbers of dividing GSCs in response to mating.

Diffuse or hitch a ride: how photoreceptor lipidated proteins get from here to there

Photoreceptors are polarized neurons, with specific subcellular compartmentalization and unique requirements for protein expression and trafficking. Each photoreceptor contains an outer segment (OS) where vision begins, an inner segment (IS) where protein synthesis occurs and a synaptic terminal for signal transmission to second-order neurons. The OS is a large, modified primary cilium attached to the IS by a slender connecting cilium (CC), the equivalent of the transition zone (TZ). Daily renewal of ~10% of the OS requires massive protein biosynthesis in the IS with reliable transport and targeting pathways. Transport of lipidated (‘sticky’) proteins depends on solubilization factors, phosphodiesterase δ (PDEδ) and uncoordinated protein-119 (UNC119), and the cargo dispensation factor (CDF), Arf-like protein 3-guanosine triphosphate (ARL3-GTP). As PDE6 and transducin still reside prominently in the OS of PDEδ and UNC119 germline knockout mice, respectively, we propose the existence of an alternate trafficking pathway, whereby lipidated proteins migrate in rhodopsin-containing vesicles of the secretory pathway.

2016 Philip S. Portoghese Medicinal Chemistry Lectureship: Designing Bivalent or Bitopic Molecules for G-Protein Coupled Receptors. The Whole Is Greater Than the Sum of Its Parts

The genesis of designing bivalent or bitopic molecules that engender unique pharmacological properties began with Portoghese's work directed toward opioid receptors, in the early 1980s. This strategy has evolved as an attractive way to engineer highly selective compounds for targeted G-protein coupled receptors (GPCRs) with optimized efficacies and/or signaling bias. The emergence of X-ray crystal structures of many GPCRs and the identification of both orthosteric and allosteric binding sites have provided further guidance to ligand drug design that includes a primary pharmacophore (PP), a secondary pharmacophore (SP), and a linker between them. It is critical to note the synergistic relationship among all three of these components as they contribute to the overall interaction of these molecules with their receptor proteins and that strategically designed combinations have and will continue to provide the GPCR molecular tools of the future.

Serotonin receptor oligomerization regulates cAMP-based signaling

Protein-protein interaction is often investigated using quantitative molecular microscopy with Förster resonant energy transfer (FRET). Here, we combined 'linear unmixing FRET' (lux-FRET) with the simultaneous application of a FRET-based biosensor for cAMP to investigate the oligomerization between the 5-HT₇ receptor (5-HT₇R, also known as HTR7) and the 5-HT_1A receptor (5-HT_1AR, also known as HTR1A) and its importance for cAMP signaling. We found that the 5-HT₇R not only stimulates cAMP production, but also forms hetero-oligomers with 5-HT_1AR, which blocks the inhibitory effect of the latter. 5-HT₇R signaling, however, is not affected by this hetero-oligomerization. By modeling the kinetics of intracellular cAMP level changes in relation to the 5-HT₇R:5-HT_1AR stoichiometry, we were able to decipher the complex signaling characteristics of endogenous serotonin receptors in cultured hippocampal neurons. Our findings indicate that serotonergic signaling is not only modulated by the concentration of an individual receptor but also by its specific interaction with other receptors in endogenous systems. We conclude that the regulated ratio of serotonin receptors in immature and mature neurons may be critically involved in both the onset and response to treatments of psychiatric diseases, such as anxiety and depression.

Biological and Biochemical Basis of the Differential Efficacy of First and Second Generation Somatostatin Receptor Ligands in Neuroendocrine Neoplasms

Endogenous somatostatin shows anti-secretory effects in both physiological and pathological settings, as well as inhibitory activity on cell growth. Since somatostatin is not suitable for clinical practice, researchers developed synthetic somatostatin receptor ligands (SRLs) to overcome this limitation. Currently, SRLs represent pivotal tools in the treatment algorithm of neuroendocrine tumors (NETs). Octreotide and lanreotide are the first-generation SRLs developed and show a preferential binding affinity to somatostatin receptor (SST) subtype 2, while pasireotide, which is a second-generation SRL, has high affinity for multiple SSTs (SST₅ > SST₂ > SST₃ > SST₁). A number of studies demonstrated that first-generation and second-generation SRLs show distinct functional properties, besides the mere receptor affinity. Therefore, the aim of the present review is to critically review the current evidence on the biological effects of SRLs in pituitary adenomas and neuroendocrine tumors, by mainly focusing on the differences between first-generation and second-generation ligands.

Peeking at G-protein-coupled receptors through the molecular dynamics keyhole

Molecular dynamics is a state of the art computational tool for the investigation of biophysics phenomenon at a molecular scale, as it enables the modeling of dynamic processes, such as conformational motions, molecular solvation and ligand binding. The recent advances in structural biology have led to a bloom in published G-protein-coupled receptor structures, representing a solid and valuable resource for molecular dynamics studies. During the last decade, indeed, a plethora of physiological and pharmacological facets of this membrane protein superfamily have been addressed by means of molecular dynamics simulations, including the activation mechanism, allosterism and, very recently, biased signaling. Here, we try to recapitulate some of the main contributions that molecular dynamics has recently produced in the field.

Reinterpreting anomalous competitive binding experiments within G protein-coupled receptor homodimers using a dimer receptor model

An increasing number of G protein-coupled receptors (GPCRs) have been reported to be expressed in the plasma membrane as dimers. Since most ligand binding data are currently fitted by classical equations developed only for monomeric receptors, the interpretation of data could be misleading in the presence of GPCR dimers. On the other hand, the equations developed from dimer receptor models assuming the existence of two orthosteric binding sites within the dimeric molecule offer the possibility to directly calculate macroscopic equilibrium dissociation constants for the two sites, an index of cooperativity (D_C) that reflects the molecular communication within the dimer and, importantly, a constant of radioligand-competitor allosteric interaction (K_DAB) in competitive assays. Here, we provide a practical way to fit competitive binding data that allows the interpretation of apparently anomalous results, such as competition curves that could be either bell-shaped, monophasic or biphasic depending on the assay conditions. The consideration of a radioligand-competitor allosteric interaction allows fitting these curve patterns both under simulation conditions and in real radioligand binding experiments, obtaining competitor affinity parameters closer to the actual values. Our approach is the first that, assuming the formation of receptor homodimers, is able to explain several experimental results previously considered erroneous due to their impossibility to be fitted. We also deduce the radioligand concentration responsible for the conversion of biphasic to monophasic or to bell-shaped curves in competitive radioligand binding assays. In conclusion, bell-shaped curves in competitive binding experiments constitute evidence for GPCR homodimerization.

GPR37 and GPR37L1 differently interact with dopamine 2 receptors in live cells

Receptor-receptor interactions are essential to fine tune receptor responses and new techniques enable closer characterization of the interactions between involved proteins directly in the plasma membrane. Fluorescence cross-correlation spectroscopy (FCCS), which analyses concurrent movement of bound molecules with single-molecule detection limit, was here used to, in live N2a cells, study interactions between the Parkinson's disease (PD) associated orphan receptor GPR37, its homologue GPR37L1, and the two splice variants of the dopamine 2 receptor (D2R). An interaction between GPR37 and both splice forms of D2R was detected. 4-phenylbutyrate (4-PBA), a neuroprotective chemical chaperone known to increase GPR37 expression at the cell surface, increased the fraction of interacting molecules. The interaction was also increased by pramipexole, a D2R agonist commonly used in the treatment of PD, indicating a possible clinically relevance. Cross-correlation, indicating interaction between GPR37L1 and the short isoform of D2R, was also detected. However, this interaction was not changed with 4-PBA or pramipexole treatment. Overall, these data provide further evidence that heteromeric GPR37-D2R exist and can be pharmacologically modulated, which is relevant for the treatment of PD.

This article is part of the Special Issue entitled ‘Receptor heteromers and their allosteric receptor-receptor interactions’.

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GPCR interaction is studied with fluorescence cross-correlation spectroscopy.

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Interaction between GPR37 and both isoforms of D2R is detected in live cells.

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GPR37's homologue GPR37L1 is detected to interact with D2RS in live cells.

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GPR37-D2R interaction is increased by D2-like agonist and 4-PBA treatment.

GPCRs and Signal Transducers: Interaction Stoichiometry

Until the late 1990s, class A G protein-coupled receptors (GPCRs) were believed to function as monomers. Indirect evidence that they might internalize or even signal as dimers has emerged, along with proof that class C GPCRs are obligatory dimers. Crystal structures of GPCRs and their much larger binding partners were consistent with the idea that two receptors might engage a single G protein, GRK, or arrestin. However, recent biophysical, biochemical, and structural evidence invariably suggests that a single GPCR binds G proteins, GRKs, and arrestins. Here we review existing evidence of the stoichiometry of GPCR interactions with signal transducers and discuss potential biological roles of class A GPCR oligomers, including proposed homo- and heterodimers.

The Dynamics of GPCR Oligomerization and Their Functional Consequences

The functional importance of G protein-coupled receptor (GPCR) oligomerization remains controversial. Although obligate dimers of class C GPCRs are well accepted, the generalizability of this phenomenon is still strongly debated with respect to other classes of GPCRs. In this review, we focus on understanding the organization and dynamics between receptor equivalents and their signaling partners in oligomeric receptor complexes, with a view toward integrating disparate viewpoints into a unified understanding. We discuss the nature of functional oligomeric entities, and how asymmetries in receptor structure and function created by oligomers might have implications for receptor function as allosteric machines and for future drug discovery.

Intracellular Trafficking of Gonadotropin Receptors in Health and Disease

Gonadotropin receptors belong to the highly conserved subfamily of the G protein-coupled receptor (GPCR) superfamily, the so-called Rhodopsin-like family (class A), which is the largest class of GPCRs and currently a major drug target. Both the follicle-stimulating hormone receptor (FSHR) and the luteinizing hormone/chorionic gonadotropin hormone receptor (LHCGR) are mainly located in the gonads where they play key functions associated to essential reproductive functions. As any other protein, gonadotropin receptors must be properly folded into a mature tertiary conformation compatible with quaternary assembly and endoplasmic reticulum export to the cell surface plasma membrane. Several primary and secondary structural features, including presence of particular amino acid residues and short motifs and in addition, posttranslational modifications, regulate intracellular trafficking of gonadotropin receptors to the plasma membrane as well as internalization and recycling of the receptor back to the cell surface after activation by agonist. Inactivating mutations of gonadotropin receptors may derive from receptor misfolding and lead to absent or reduced plasma membrane expression of the altered receptor, thereby manifesting an array of phenotypical abnormalities mostly characterized by reproductive failure and/or abnormal or absence of development of secondary sex characteristics. In this chapter we review the structural requirements necessary for intracellular trafficking of the gonadotropin receptors, and describe how mutations in these receptors may lead to receptor misfolding and disease in humans.

Dynamic roles for the N-terminus of the yeast G protein-coupled receptor Ste2p

The Saccharomyces cerevisiae α-factor receptor Ste2p has been used extensively as a model to understand the molecular mechanism of signal transduction by G protein-coupled receptors (GPCRs). Single and double cysteine mutants of Ste2p were created and served as surrogates to detect intramolecular interactions and dimerization of Ste2p using disulfide cross-linking methodology. When a mutation was introduced into the phylogenetically conserved tyrosine residue at position 26 (Y26C) in the N-terminus of Ste2p, dimerization was increased greatly. The amount of dimer formed by this Y26C mutant was greatly reduced by ligand binding even though the ligand binding site is far removed from the N-terminus; the lowering of the dimer formation was consistent with a conformational change in the N-terminus of the receptor upon activation. Dimerization was decreased by double mutations Y26C/V109C or Y26C/T114C indicating that Y26 is in close proximity to V109 and T114 of extracellular loop 1 in native Ste2p. Combined with earlier studies, these results indicate previously unrecognized roles for the N-terminus of Ste2p, and perhaps of GPCRs in general, and reveal a specific N-terminus residue or region, that is involved in GPCR signaling, intrareceptor interactions, and receptor dimerization.

Homodimerization enhances both sensitivity and dynamic range of the ligand-binding domain of type 1 metabotropic glutamate receptor

Cooperativity in ligand binding is a key emergent property of protein oligomers. Positive cooperativity (higher affinity for subsequent binding events than for initial binding) is frequent. However, the symmetrically homodimeric ligand-binding domain (LBD) of metabotropic glutamate receptor type 1 exhibits negative cooperativity. To investigate its origin and functional significance, we measured the response to glutamate in vitro of wild-type and C140S LBD as a function of the extent of dimerization. Our results indicate that homodimerization enhances the affinity of the first, but not the second, binding site, relative to the monomer, giving the dimeric receptor both greater sensitivity and a broader dynamic range.

What Do We Really Know About 5-HT1A Receptor Signaling in Neuronal Cells?

Serotonin (5-HT) is a neurotransmitter that plays an important role in neuronal plasticity. Variations in the levels of 5-HT at the synaptic cleft, expression or dysfunction of 5-HT receptors may alter brain development and predispose to various mental diseases. Here, we review the transduction pathways described in various cell types transfected with recombinant 5-HT_1A receptor (5-HT_1AR), specially contrasting with those findings obtained in neuronal cells. The 5-HT_1AR is detected in early stages of neural development and is located in the soma, dendrites and spines of hippocampal neurons. The 5-HT_1AR differs from other 5-HT receptors because it is coupled to different pathways, depending on the targeted cell. The signaling pathway associated with this receptor is determined by G_α isoforms and some cascades involve βγ signaling. The activity of 5-HT_1AR usually promotes a reduction in neuronal excitability and firing, provokes a variation in cAMP and Ca²⁺, levels which may be linked to specific types of behavior and cognition. Furthermore, evidence indicates that 5-HT_1AR induces neuritogesis and synapse formation, probably by modulation of the neuronal cytoskeleton through MAPK and phosphoinositide-3-kinase (PI3K)-Akt signaling pathways. Advances in understanding the actions of 5-HT_1AR and its association with different signaling pathways in the central nervous system will reveal their pivotal role in health and disease.

Immunomodulatory effects mediated by serotonin

Serotonin (5-HT) induces concentration-dependent metabolic effects in diverse cell types, including neurons, entherochromaffin cells, adipocytes, pancreatic beta-cells, fibroblasts, smooth muscle cells, epithelial cells, and leukocytes. Three classes of genes regulating 5-HT function are constitutively expressed or induced in these cells: (a) membrane proteins that regulate the response to 5-HT, such as SERT, 5HTR-GPCR, and the 5HT₃-ion channels; (b) downstream signaling transduction proteins; and (c) enzymes controlling 5-HT metabolism, such as IDO and MAO, which can generate biologically active catabolites, including melatonin, kynurenines, and kynurenamines. This review covers the clinical and experimental mechanisms involved in 5-HT-induced immunomodulation. These mechanisms are cell-specific and depend on the expression of serotonergic components in immune cells. Consequently, 5-HT can modulate several immunological events, such as chemotaxis, leukocyte activation, proliferation, cytokine secretion, anergy, and apoptosis. The effects of 5-HT on immune cells may be relevant in the clinical outcome of pathologies with an inflammatory component. Major depression, fibromyalgia, Alzheimer disease, psoriasis, arthritis, allergies, and asthma are all associated with changes in the serotonergic system associated with leukocytes. Thus, pharmacological regulation of the serotonergic system may modulate immune function and provide therapeutic alternatives for these diseases.

G protein-coupled receptors in cardiac biology: old and new receptors

G protein-coupled receptors (GPCRs) are seven-transmembrane-spanning proteins that mediate cellular and physiological responses. They are critical for cardiovascular function and are targeted for the treatment of hypertension and heart failure. Nevertheless, current therapies only target a small fraction of the cardiac GPCR repertoire, indicating that there are many opportunities to investigate unappreciated aspects of heart biology. Here, we offer an update on the contemporary view of GPCRs and the complexities of their signalling, and review the roles of the 'classical' GPCRs in cardiovascular physiology and disease. We then provide insights into other GPCRs that have been less extensively studied in the heart, including orphan, odorant and taste receptors. We contend that these novel cardiac GPCRs contribute to heart function in health and disease and thereby offer exciting opportunities to therapeutically modulate heart function.

Dimer-dependent intrinsic/basal activity of the class B G protein-coupled receptor PAC1 promotes cellular anti-apoptotic activity through Wnt/β-catenin pathways that are associated with dimer endocytosis

The high expression of PACAP (pituitary adenylate cyclase-activating polypeptide)-preferring receptor PAC1 is associated with nerve injury and tumors. Our previous report (Yu R, et al. PLoS One 2012; 7: e51811) confirmed the dimerization of PAC1 and found that the M-PAC1 mutation in the N-terminal first Cys/Ala lost the ability to form dimers. In this study, Chinese hamster ovary (CHO-K1) cells overexpressing wild-type PAC1 (PAC1-CHO) had significantly higher anti-apoptotic activities against serum withdrawal-induced apoptosis associated with a lower caspase 3 activity and a higher Bcl-2 level in a ligand-independent manner than those of CHO cells overexpressing the mutant M-PAC1 (M-PAC1-CHO). PAC1-CHO had significantly higher β-catenin, cyclin D1 and c-myc levels corresponding to the Wnt/β-catenin signal than did M-PAC1-CHO. In addition, the Wnt/β-catenin pathway inhibitor XAV939 significantly inhibited the anti-apoptotic activities of PAC1-CHO. Top-flash assays demonstrated that PAC1-CHO had a significantly stronger Wnt/β-catenin signal than did M-PAC1-CHO. Acetylcysteine (NAC) as an inhibitor of the dimerization of PAC1 inhibited the anti-apoptotic activities that were endowed by PAC1 and decreased the Wnt/β-catenin signal in Top-flash assays. In the PAC1 Tet (tetracycline)-on inducible gene expression system by doxycycline (Dox), higher expression levels of PAC1 resulted in higher anti-apoptotic activities that were associated with a stronger Wnt/β-catenin signal. A similar correlation was also found with the down-regulation of PAC1 in the Neuro2a neuroblastoma cell. BiFC combined with fluorescence confocal imaging indicated that during serum-withdrawal-induced apoptosis, PAC1 dimers displayed significant endocytosis. These findings indicate that PAC1 has ligand-independent and dimer-dependent intrinsic/basal activity, conferring cells with anti-apoptotic activities against serum withdrawal, which is involved in the Wnt/β-catenin signal and is associated with the endocytosis of PAC1 dimers. The discovery and study of the dimer-dependent basal activity of PAC1 not only help us understand the physiological and pathological role of PAC1 but also promote the development of drugs targeting PAC1.

Synthesis and binding profile of haloperidol-based bivalent ligands targeting dopamine D(2)-like receptors

Homodimers of dopamine D2-like receptors are suggested to be of particular importance in the pathophysiology of schizophrenia and, thus, serve as promising targets for the discovery of atypical antipsychotics. This study describes the development of a series of novel bivalent molecules with a pharmacophore derived from the dopamine receptor antagonist haloperidol. These dimers were investigated in comparison to their monomeric analogues for their D2long, D2short, D3, and D4 receptor binding and the ability to bridge two neighboring receptor protomers. Radioligand binding studies provided diagnostic insights when Hill slopes close to two for the bivalent ligand 13 incorporating 22 spacer atoms and a comparative analysis with monovalent control ligands indicated a bivalent binding mode with a simultaneous occupancy of two neighboring binding sites.

Physiology, signaling, and pharmacology of opioid receptors and their ligands in the gastrointestinal tract: current concepts and future perspectives

Opioid receptors are widely distributed in the human body and are crucially involved in numerous physiological processes. These include pain signaling in the central and the peripheral nervous system, reproduction, growth, respiration, and immunological response. Opioid receptors additionally play a major role in the gastrointestinal (GI) tract in physiological and pathophysiological conditions. This review discusses the physiology and pharmacology of the opioid system in the GI tract. We additionally focus on GI disorders and malfunctions, where pathophysiology involves the endogenous opioid system, such as opioid-induced bowel dysfunction, opioid-induced constipation or abdominal pain. Based on recent reports in the field of pharmacology and medicinal chemistry, we will also discuss the opportunities of targeting the opioid system, suggesting future treatment options for functional disorders and inflammatory states of the GI tract.

Transitioning pharmacoperones to therapeutic use: in vivo proof-of-principle and design of high throughput screens

A pharmacoperone (from "pharmacological chaperone") is a small molecule that enters cells and serves as molecular scaffolding in order to cause otherwise-misfolded mutant proteins to fold and route correctly within the cell. Pharmacoperones have broad therapeutic applicability since a large number of diseases have their genesis in the misfolding of proteins and resultant misrouting within the cell. Misrouting may result in loss-of-function and, potentially, the accumulation of defective mutants in cellular compartments. Most known pharmacoperones were initially derived from receptor antagonist screens and, for this reason, present a complex pharmacology, although these are highly target specific. In this summary, we describe efforts to produce high throughput screens that identify these molecules from chemical libraries as well as a mouse model which provides proof-of-principle for in vivo protein rescue using existing pharmacoperones.

Novel structural and functional insights into M3 muscarinic receptor dimer/oligomer formation

Class A G protein-coupled receptors (GPCRs) are able to form homodimers and/or oligomeric arrays. We recently proposed, based on bioluminescence resonance energy transfer studies with the M3 muscarinic receptor (M3R), a prototypic class A GPCR, that the M3R is able to form multiple, structurally distinct dimers that are probably transient in nature (McMillin, S. M., Heusel, M., Liu, T., Costanzi, S., and Wess, J. (2011) J. Biol. Chem. 286, 28584-28598). To provide more direct experimental support for this concept, we employed a disulfide cross-linking strategy to trap various M3R dimeric species present in a native lipid environment (transfected COS-7 cells). Disulfide cross-linking studies were carried out with many mutant M3Rs containing single cysteine (Cys) substitutions within two distinct cytoplasmic M3R regions, the C-terminal portion of the second intracellular loop (i2) and helix H8 (H8). The pattern of cross-links that we obtained, in combination with molecular modeling studies, was consistent with the existence of two structurally distinct M3R dimer interfaces, one involving i2/i2 contacts (TM4-TM5-i2 interface) and the other one characterized by H8-H8 interactions (TM1-TM2-H8 interface). Specific H8-H8 disulfide cross-links led to significant impairments in M3R-mediated G protein activation, suggesting that changes in the structural orientation or mobility of H8 are critical for efficient receptor-G protein coupling. Our findings provide novel structural and functional insights into the mechanisms involved in M3R dimerization (oligomerization). Because the M3R shows a high degree of sequence similarity with many other class A GPCRs, our findings should be of considerable general interest.

Heterodimerization between the lutropin and follitropin receptors is associated with an attenuation of hormone-dependent signaling

The LH receptor (LHR) and FSH receptor (FSHR) are each G protein-coupled receptors that play critical roles in reproductive endocrinology. Each of these receptors has previously been shown to self-associate into homodimers and oligomers shortly after their biosynthesis. As shown herein using bioluminescence resonance energy transfer to detect protein-protein interactions, our data show that the LHR and FSHR, when coexpressed in the same cells, specifically heterodimerize with each other. Further experiments confirm that at least a portion of the cellular LHR/FSHR heterodimers are present on the cell surface and are functional. We then sought to ascertain what effects, if any, heterodimerization between the LHR and FSHR might have on signaling. It was observed that when the LHR was expressed under conditions promoting the heterodimerization with FSHR, LH or human chorionic gonadotropin (hCG) stimulation of Gs was attenuated. Conversely, when the FSHR was expressed under conditions promoting heterodimerization with the LHR, FSH-stimulated Gs activation was attenuated. These results demonstrate that the coexpression of the LHR and FSHR enables heterodimerizaton between the 2 gonadotropin receptors and results in an attenuation of signaling through each receptor.

The pituitary-interrenal axis of fish: a review focusing on the lamprey and flounder

In fish, the pituitary-interrenal axis is associated with stress response and a variety of biological processes such as metabolism, immune response, and growth. The major hormones involved in this axis are adrenocorticotropic hormone (ACTH), released from the pars distalis of the pituitary gland, and corticosteroid, released from the interrenal gland that is embedded in the head kidney in ray-finned fish. The ACTH signal, by which corticosteroid release is stimulated, is transmitted by melanocortin (MC) receptors on interrenal cells. Thus, the interaction of ACTH and MC receptors is the pivotal event for interrenal cells. Knowledge about ACTH and MC receptors in lamprey, cartilaginous fish, and ray-finned fish is available, and it suggests the pituitary-interrenal axis was established early in vertebrate evolution. Moreover, the data, including our recent results from flounders and lampreys, provide interesting features about ligand-receptor interactions. This review focuses on the characteristics of ACTH, the proopiomelanocortin gene encoding ACTH, and the MC receptor, and it is mostly based on the results of our investigations.

Functional significance of serotonin receptor dimerization

The original model of G-protein activation by a single G-protein-coupled receptor (GPCR) is giving way to a new model, wherein two protomers of a GPCR dimer interact with a single G-protein. This article will review the evidence suggesting that 5-HT receptors form dimers/oligomers and will compare the findings with the results obtained from the studies with other biogenic amine receptors. Topics to be covered include the origin or biogenesis of dimer formation, potential dimer interface(s), and oligomer size (dimer vs. tetramer or higher order). The functional significance will be discussed in terms of G-protein activation following ligand binding to one or two protomers in a dimeric structure, the formation of heterodimers, and the development of bivalent ligands.

Arrest Functions of the MIF Ligand/Receptor Axes in Atherogenesis

Macrophage migration inhibitory factor (MIF) has been defined as an important chemokine-like function (CLF) chemokine with an essential role in monocyte recruitment and arrest. Adhesion of monocytes to the vessel wall and their transendothelial migration are critical in atherogenesis and many other inflammatory diseases. Chemokines carefully control all steps of the monocyte recruitment process. Those chemokines specialized in controlling arrest are typically immobilized on the endothelial surface, mediating the arrest of rolling monocytes by chemokine receptor-triggered pathways. The chemokine receptor CXCR2 functions as an important arrest receptor on monocytes. An arrest function has been revealed for the bona fide CXCR2 ligands CXCL1 and CXCL8, but genetic studies also suggested that additional arrest chemokines are likely to be involved in atherogenic leukocyte recruitment. While CXCR2 is known to interact with numerous CXC chemokine ligands, the CLF chemokine MIF, which structurally does not belong to the CXC chemokine sub-family, was surprisingly identified as a non-cognate ligand of CXCR2, responsible for critical arrest functions during the atherogenic process. MIF was originally identified as macrophage migration inhibitory factor (this function being eponymous), but is now known as a potent inflammatory cytokine with CLFs including chemotaxis and leukocyte arrest. This review will cover the mechanisms underlying these functions, including MIF’s effects on LFA1 integrin activity and signal transduction, and will discuss the structural similarities between MIF and the bona fide CXCR2 ligand CXCL8 while emphasizing the structural differences. As MIF also interacts with CXCR4, a chemokine receptor implicated in CXCL12-elicited lymphocyte arrest, the arrest potential of the MIF/CXCR4 axis will also be scrutinized as well as the recently identified role of pericyte MIF in attracting leukocytes exiting through venules as part of the pericyte “motility instruction program.”

Structural and biophysical characterisation of G protein-coupled receptor ligand binding using resonance energy transfer and fluorescent labelling techniques

The interaction between ligands and the G protein-coupled receptors (GPCRs) to which they bind has long been the focus of intensive investigation. The signalling cascades triggered by receptor activation, due in most cases to ligand binding, are of great physiological and medical importance; indeed, GPCRs are targeted by in excess of 30% of small molecule therapeutic medicines. Attempts to identify further pharmacologically useful GPCR ligands, for receptors with known and unknown endogenous ligands, continue apace. In earlier days direct assessment of such interactions was restricted largely to the use of ligands incorporating radioactive isotope labels as this allowed detection of the ligand and monitoring its interaction with the GPCR. This use of such markers has continued with the development of ligands labelled with fluorophores and their application to the study of receptor-ligand interactions using both light microscopy and resonance energy transfer techniques, including homogenous time-resolved fluorescence resonance energy transfer. Details of ligand-receptor interactions via X-ray crystallography are advancing rapidly as methods suitable for routine production of substantial amounts and stabilised forms of GPCRs have been developed and there is hope that this may become as routine as the co-crystallisation of serine/threonine kinases with ligands, an approach that has facilitated widespread use of rapid structure-based ligand design. Conformational changes involved in the activation of GPCRs, widely predicted by biochemical and biophysical means, have inspired the development of intramolecular FRET-based sensor forms of GPCRs designed to investigate the events following ligand binding and resulting in a signal propagation across the cell membrane. Finally, a number of techniques are emerging in which ligand-GPCR binding can be studied in ways that, whilst indirect, are able to monitor its results in an unbiased and integrated manner. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

Homodimerization of adenosine A₁ receptors in brain cortex explains the biphasic effects of caffeine

Using bioluminescence resonance energy transfer and proximity ligation assays, we obtained the first direct evidence that adenosine A₁ receptors (A₁Rs) form homomers not only in cell cultures but also in brain cortex. By radioligand binding experiments in the absence or in the presence of the A₁Rs allosteric modulator, adenosine deaminase, and by using the two-state dimer receptor model to fit binding data, we demonstrated that the protomer-protomer interactions in the A₁R homomers account for some of the pharmacological characteristics of agonist and antagonist binding to A₁Rs. These pharmacological properties include the appearance of cooperativity in agonist binding, the change from a biphasic saturation curve to a monophasic curve in self-competition experiments and the molecular cross-talk detected when two different specific molecules bind to the receptor. In this last case, we discovered that caffeine binding to one protomer increases the agonist affinity for the other protomer in the A₁R homomer, a pharmacological characteristic that correlates with the low caffeine concentrations-induced activation of agonist-promoted A₁R signaling. This pharmacological property can explain the biphasic effects reported at low and high concentration of caffeine on locomotor activity.

GPCR oligomerization and receptor trafficking

The effects of oligomerization of G protein-coupled receptors (GPCRs) upon their trafficking around the cell are considerable, and this raises the potential of significant impact upon the use of existing pharmacological agents and the development of new ones. Herein, we describe a number of different techniques that can be used to study receptor dimerization/oligomerization and trafficking, beginning with a cellular system which allows the expression of two GPCRs simultaneously, one under inducible control. Subsequently, we describe means to visualize and monitor the movement of GPCRs within the cell, detect oligomerization by both resonance energy transfer and more traditional biochemical approaches, and to measure the internalization of GPCRs as part of the process of receptor regulation.

D2-like dopamine and β-adrenergic receptors form a signaling complex that integrates Gs- and Gi-mediated regulation of adenylyl cyclase

β-Adrenergic receptors (βAR) and D(2)-like dopamine receptors (which include D(2)-, D(3)- and D(4)-dopamine receptors) activate G(s) and G(i), the stimulatory and inhibitory heterotrimeric G proteins, respectively, which in turn regulate the activity of adenylyl cyclase (AC). β(2)-Adrenergic receptors (β(2)AR) and D(4)-dopamine receptors (D(4)DR) co-immunoprecipitated when co-expressed in HEK 293 cells, suggesting the existence of a signaling complex containing both receptors. In order to determine if these receptors are closely associated with each other, and with other components involved in G protein-mediated signal transduction, β(2)AR, D(4)DR, G protein subunits (Gα(i1) and the Gβ(1)γ(2) heterodimer) and AC were tagged so that bioluminescence resonance energy transfer (BRET) could be used to monitor their interactions. All of the tagged proteins retained biological function. For the first time, FlAsH-labeled proteins were used in BRET experiments as fluorescent acceptors for the energy transferred from Renilla luciferase-tagged donor proteins. Our experiments revealed that β(2)AR, D(4)DR, G proteins and AC were closely associated in a functional signaling complex in cellulo. Furthermore, BRET experiments indicated that although activation of G(i) caused a conformational change within the heterotrimeric protein, it did not cause the Gβγ heterodimer to dissociate from the Gα(i1) subunit. Evidence for the presence of a signaling complex in vivo was obtained by purifying βAR from detergent extracts of mouse brain with alprenolol-Sepharose and showing that the precipitate also contained both D(2)-like dopamine receptors and AC.

Cysteinyl leukotriene 2 receptor on dendritic cells negatively regulates ligand-dependent allergic pulmonary inflammation

Cysteinyl leukotrienes (cys-LTs) can mediate Th2 immunity to the house dust mite, Dermatophagoides farinae, via the type 1 receptor CysLT(1)R on dendritic cells (DCs). However, the role of the homologous type 2 receptor CysLT(2)R in Th2 immunity is unknown. D. farinae sensitization and challenge of CysLT(2)R-deficient mice showed a marked augmentation of eosinophilic pulmonary inflammation, serum IgE, and Th2 cytokines. Wild-type (WT) mice sensitized by adoptive transfer of D. farinae-pulsed CysLT(2)R-deficient bone marrow-derived DCs (BMDCs) also had a marked increase in D. farinae-elicited eosinophilic lung inflammation and Th2 cytokines in restimulated hilar nodes. This response was absent in mice sensitized with D. farinae-pulsed BMDCs lacking leukotriene C(4) synthase (LTC(4)S), CysLT(1)R, or both CysLT(2)R/LTC(4)S, suggesting that CysLT(2)R negatively regulates LTC(4)S- and CysLT(1)R-dependent DC-mediated sensitization. CysLT(2)R-deficient BMDCs had increased CysLT(1)R-dependent LTD(4)-induced ERK phosphorylation, whereas N-methyl LTC(4) activation of CysLT(2)R on WT BMDCs reduced such signaling. Activation of endogenously expressed CysLT(1)R and CysLT(2)R occurred over an equimolar range of LTD(4) and N-methyl LTC(4), respectively. Although the baseline expression of cell surface CysLT(1)R was not increased on CysLT(2)R-deficient BMDCs, it was upregulated at 24 h by a pulse of D. farinae, compared with WT or CysLT(2)R/LTC(4)S-deficient BMDCs. Importantly, treatment with N-methyl LTC(4) reduced D. farinae-induced CysLT(1)R expression on WT BMDCs. Thus, CysLT(2)R negatively regulates the development of cys-LT-dependent Th2 pulmonary inflammation by inhibiting both CysLT(1)R signaling and D. farinae-induced LTC(4)S-dependent cell surface expression of CysLT(1)R on DCs. Furthermore, these studies highlight how the biologic activity of cys-LTs can be tightly regulated by competition between these endogenously expressed receptors.

Emerging opportunities for allosteric modulation of G-protein coupled receptors

Their ubiquitous nature, wide cellular distribution and versatile molecular recognition and signalling help make G-protein binding receptors (GPCRs) the most important class of membrane proteins in clinical medicine, accounting for ∼40% of all current therapeutics. A large percentage of current drugs target the endogenous ligand binding (orthosteric) site, which are structurally and evolutionarily conserved, particularly among members of the same GPCR subfamily. With the recent advances in GPCR X-ray crystallography, new opportunities for developing novel subtype selective drugs have emerged. Given the increasing recognition that the extracellular surface conformation changes in response to ligand binding, it is likely that all GPCRs possess an allosteric site(s) capable of regulating GPCR signalling. Allosteric sites are less structurally conserved than their corresponding orthosteric site and thus provide new opportunities for the development of more selective drugs. Constitutive oligomerisation (dimerisation) identified in many of the GPCRs investigated, adds another dimension to the structural and functional complexity of GPCRs. In this review, we compare 60 crystal structures of nine GPCR subtypes (rhodopsin, ß₂-AR, ß₁-AR, A(2a)-AR, CXCR4, D₃R, H₁R, M₂R, M₃R) across four subfamilies of Class A GPCRs, and discuss mechanisms involved in receptor activation and potential allosteric binding sites across the highly variable extracellular surface of these GPCRs. This analysis has identified a new extracellular salt bridge (ESB-2) that might be exploited in the design of allosteric modulators.

Fluorescence/bioluminescence resonance energy transfer techniques to study G-protein-coupled receptor activation and signaling

Fluorescence and bioluminescence resonance energy transfer (FRET and BRET) techniques allow the sensitive monitoring of distances between two labels at the nanometer scale. Depending on the placement of the labels, this permits the analysis of conformational changes within a single protein (for example of a receptor) or the monitoring of protein-protein interactions (for example, between receptors and G-protein subunits). Over the past decade, numerous such techniques have been developed to monitor the activation and signaling of G-protein-coupled receptors (GPCRs) in both the purified, reconstituted state and in intact cells. These techniques span the entire spectrum from ligand binding to the receptors down to intracellular second messengers. They allow the determination and the visualization of signaling processes with high temporal and spatial resolution. With these techniques, it has been demonstrated that GPCR signals may show spatial and temporal patterning. In particular, evidence has been provided for spatial compartmentalization of GPCRs and their signals in intact cells and for distinct physiological consequences of such spatial patterning. We review here the FRET and BRET technologies that have been developed for G-protein-coupled receptors and their signaling proteins (G-proteins, effectors) and the concepts that result from such experiments.

Regulation of GIP and GLP1 receptor cell surface expression by N-glycosylation and receptor heteromerization

In response to a meal, Glucose-dependent Insulinotropic Polypeptide (GIP) and Glucagon-like Peptide-1 (GLP-1) are released from gut endocrine cells into the circulation and interact with their cognate G-protein coupled receptors (GPCRs). Receptor activation results in tissue-selective pleiotropic responses that include augmentation of glucose-induced insulin secretion from pancreatic beta cells. N-glycosylation and receptor oligomerization are co-translational processes that are thought to regulate the exit of functional GPCRs from the ER and their maintenance at the plasma membrane. Despite the importance of these regulatory processes, their impact on functional expression of GIP and GLP-1 receptors has not been well studied. Like many family B GPCRs, both the GIP and GLP-1 receptors possess a large extracellular N-terminus with multiple consensus sites for Asn-linked (N)-glycosylation. Here, we show that each of these Asn residues is glycosylated when either human receptor is expressed in Chinese hamster ovary cells. N-glycosylation enhances cell surface expression and function in parallel but exerts stronger control over the GIP receptor than the GLP-1 receptor. N-glycosylation mainly lengthens receptor half-life by reducing degradation in the endoplasmic reticulum. N-glycosylation is also required for expression of the GIP receptor at the plasma membrane and efficient GIP potentiation of glucose-induced insulin secretion from the INS-1 pancreatic beta cell line. Functional expression of a GIP receptor mutant lacking N-glycosylation is rescued by co-expressed wild type GLP1 receptor, which, together with data obtained using Bioluminescence Resonance Energy Transfer, suggests formation of a GIP-GLP1 receptor heteromer.

Functional relevance of μ-δ opioid receptor heteromerization: a role in novel signaling and implications for the treatment of addiction disorders: from a symposium on new concepts in mu-opioid pharmacology

Morphine and other opiates are among the most widely prescribed and clinically useful medications for the treatment of chronic pain. However, the applicability of these compounds has been severely hampered by the rapid development of tolerance and physical dependence that typically accompanies their repeated use. A growing body of evidence has implicated the regulated functioning of μ-δ opioid receptor heteromers in both the modulation of morphine-mediated antinociception, and in the limitation of undesirable side effects resulting from chronic opiate exposure. Moreover, μ-δ heteromers exhibit unique ligand binding characteristics and signaling properties, indicating that pharmacological targeting of the μ-δ heteromer may represent a novel therapeutic approach for the management of chronic pain and addiction disorders. Therefore, the present review will attempt to summarize the latest relevant findings regarding the regulation and functional characteristics of the μ-δ heteromer both in vitro and in vivo.

Melanocortin systems on pigment dispersion in fish chromatophores

α-Melanocyte-stimulating hormone (α-MSH) is responsible for pigment dispersion in the chromatophores of fish and other tetrapods such as amphibians and reptiles. Recently, we discovered that α-MSH did not always stimulate pigment dispersion because this hormonal peptide exerted no effects on the melanophores of flounders. We assumed that the reduction of α-MSH activity was related to the co-expression of different α-MSH receptor subtypes - termed melanocortin receptors (MCR) - a member of G-protein-coupled receptors (GPCR) - based on several reports demonstrating that GPCR forms heterodimers with various properties that are distinct from those of the corresponding monomers. In this review, we summarize the relationships between the pigment-dispersing activity of α-MSH-related peptides, molecular forms of α-MSH-related peptides, and mcr subtypes expressed in fish chromatophores.

Functional homomers and heteromers of dopamine D2L and D3 receptors co-exist at the cell surface

Human dopamine D(2long) and D(3) receptors were modified by N-terminal addition of SNAP or CLIP forms of O(6)-alkylguanine-DNA-alkyltransferase plus a peptide epitope tag. Cells able to express each of these four constructs only upon addition of an antibiotic were established and used to confirm regulated and inducible control of expression, the specificity of SNAP and CLIP tag covalent labeling reagents, and based on homogenous time-resolved fluorescence resonance energy transfer, the presence of cell surface D(2long) and D(3) receptor homomers. Following constitutive expression of reciprocal constructs, potentially capable of forming and reporting the presence of cell surface D(2long)-D(3) heteromers, individual clones were assessed for levels of expression of the constitutively expressed protomer. This was unaffected by induction of the partner protomer and the level of expression of the partner required to generate detectable cell surface D(2long)-D(3) heteromers was defined. Such homomers and heteromers were found to co-exist and using a reconstitution of function approach both homomers and heteromers of D(2long) and D(3) receptors were shown to be functional, potentially via trans-activation of associated G protein. These studies demonstrate the ability of dopamine D(2long) and D(3) receptors to form both homomers and heteromers, and show that in cells expressing each subtype a complex mixture of homomers and heteromers co-exists at steady state. These data are of potential importance both to disorders in which D(2long) and D(3) receptors are implicated, like schizophrenia and Parkinson disease, and also to drugs exerting their actions via these sites.