Cooperative conformational changes in a G-protein-coupled receptor dimer, the leukotriene B(4) receptor BLT1

Dimerization of β2-adrenergic receptor is responsible for the constitutive activity subjected to inverse agonism

Dimerization of beta 2-adrenergic receptor (β₂-AR) has been observed across various physiologies. However, the function of dimeric β₂-AR is still elusive. Here, we revealed that dimerization of β₂-AR is responsible for the constitutive activity of β₂-AR generating inverse agonism. Using a co-immunoimmobilization assay, we found that transient β₂-AR dimers exist in a resting state, and the dimer was disrupted by the inverse agonists. A Gα_s preferentially interacts with dimeric β₂-AR, but not monomeric β₂-AR, in a resting state, resulting in the production of a resting cAMP level. The formation of β₂-AR dimers requires cholesterol on the plasma membrane. The cholesterol did not interfere with the agonist-induced activation of monomeric β₂-AR, unlike the inverse agonists, implying that the cholesterol is a specific factor regulating the dimerization of β₂-AR. Our model not only shows the function of dimeric β₂-AR but also provides a molecular insight into the mechanism of the inverse agonism of β₂-AR.

BRET Analysis of GPCR Dimers in Neurons and Non-Neuronal Cells: Evidence for Inactive, Agonist, and Constitutive Conformations

G-protein-coupled receptors (GPCRs) are dimeric proteins, but the functional consequences of the process are still debated. Active GPCR conformations are promoted either by agonists or constitutive activity. Inverse agonists decrease constitutive activity by promoting inactive conformations. The histamine H₃ receptor (H₃R) is the target of choice for the study of GPCRs because it displays high constitutive activity. Here, we study the dimerization of recombinant and brain H₃R and explore the effects of H₃R ligands of different intrinsic efficacy on dimerization. Co-immunoprecipitations and Western blots showed that H₃R dimers co-exist with monomers in transfected HEK 293 cells and in rodent brains. Bioluminescence energy transfer (BRET) analysis confirmed the existence of spontaneous H₃R dimers, not only in living HEK 293 cells but also in transfected cortical neurons. In both cells, agonists and constitutive activity of the H₃R decreased BRET signals, whereas inverse agonists and GTPγS, which promote inactive conformations, increased BRET signals. These findings show the existence of spontaneous H₃R dimers not only in heterologous systems but also in native tissues, which are able to adopt a number of allosteric conformations, from more inactive to more active states.

Exploring the Activation Mechanism of a Metabotropic Glutamate Receptor Homodimer via Molecular Dynamics Simulation

Metabotropic glutamate receptors of class C GPCRs exist as constitutive dimers, which play important roles in activating excitatory synapses of the central nervous system. However, the activation mechanism induced by agonists has not been clarified in experiments. To address the problem, we used microsecond all-atom molecular dynamics (MD) simulation couple with protein structure network (PSN) to explore the glutamate-induced activation for the mGluR1 homodimer. The results indicate that glutamate binding stabilizes not only the closure of Venus flytrap domains but also the polar interaction of LB2-LB2, in turn keeping the extracelluar domain in the active state. The activation of the extracelluar domain drives transmembrane domains (TMDs) of the two protomers closer and induces asymmetric activation for the TMD domains of the two protomers. One protomer with lower binding affinity to the agonist is activated, while the other protomer with higher binding energy is still in the inactive state. The PSN analysis identifies the allosteric regulation pathway from the ligand-binding pocket in the extracellular domain to the G-protein binding site in the intracellular TMD region and further reveals that the asymmetric activation is attributed to a combination of trans-pathway and cis-pathway regulations from two glumatates, rather than a single activation pathway. These observations could provide valuable molecular information for understanding of the structure and the implications in drug efficacy for the class C GPCR dimers.

GHSR-D2R heteromerization modulates dopamine signaling through an effect on G protein conformation

The growth hormone secretagogue receptor (GHSR) and dopamine receptor (D2R) have been shown to oligomerize in hypothalamic neurons with a significant effect on dopamine signaling, but the molecular processes underlying this effect are still obscure. We used here the purified GHSR and D2R to establish that these two receptors assemble in a lipid environment as a tetrameric complex composed of two each of the receptors. This complex further recruits G proteins to give rise to an assembly with only two G protein trimers bound to a receptor tetramer. We further demonstrate that receptor heteromerization directly impacts on dopamine-mediated Gi protein activation by modulating the conformation of its α-subunit. Indeed, association to the purified GHSR:D2R heteromer triggers a different active conformation of Gαi that is linked to a higher rate of GTP binding and a faster dissociation from the heteromeric receptor. This is an additional mechanism to expand the repertoire of GPCR signaling modulation that could have implications for the control of dopamine signaling in normal and physiopathological conditions.

Allosteric signaling through an mGlu2 and 5-HT2A heteromeric receptor complex and its potential contribution to schizophrenia

Heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) can form multiprotein complexes (heteromers), which can alter the pharmacology and functions of the constituent receptors. Previous findings demonstrated that the Gq/11-coupled serotonin 5-HT2A receptor and the Gi/o-coupled metabotropic glutamate 2 (mGlu2) receptor-GPCRs that are involved in signaling alterations associated with psychosis-assemble into a heteromeric complex in the mammalian brain. In single-cell experiments with various mutant versions of the mGlu2 receptor, we showed that stimulation of cells expressing mGlu2-5-HT2A heteromers with an mGlu2 agonist led to activation of Gq/11 proteins by the 5-HT2A receptors. For this crosstalk to occur, one of the mGlu2 subunits had to couple to Gi/o proteins, and we determined the relative location of the Gi/o-contacting subunit within the mGlu2 homodimer of the heteromeric complex. Additionally, mGlu2-dependent activation of Gq/11, but not Gi/o, was reduced in the frontal cortex of 5-HT2A knockout mice and was reduced in the frontal cortex of postmortem brains from schizophrenic patients. These findings offer structural insights into this important target in molecular psychiatry.

Differential Contribution of BLT1 and BLT2 to Leukotriene B4-Induced Human NK Cell Cytotoxicity and Migration

Accumulating evidence indicates that leukotriene B₄ (LTB₄) via its receptors BLT₁ and/or BLT₂ (BLTRs) could have an important role in regulating infection, tumour progression, inflammation, and autoimmune diseases. In the present study, we showed that LTB₄ not only augments cytotoxicity by NK cells but also induces their migration. We found that approximately 30% of fresh NK cells express BLT₁, 36% express BLT₂, and 15% coexpress both receptors. The use of selective BLTR antagonists indicated that BLT₁ was involved in both LTB₄-induced migration and cytotoxicity, whereas BLT₂ was involved exclusively in NK cell migration, but only in response to higher concentrations of LTB₄. BLT₁ and BLT₂ expression increased after activation of NK cells with IL-2 and IL-15. These changes of BLTR expression by cytokines were reflected in enhanced NK cell responses to LTB₄. Our findings suggest that BLT₁ and BLT₂ play differential roles in LTB₄-induced modulation of NK cell activity.

The specific monomer/dimer equilibrium of the corticotropin-releasing factor receptor type 1 is established in the endoplasmic reticulum

G protein-coupled receptors (GPCRs) represent the most important drug targets. Although the smallest functional unit of a GPCR is a monomer, it became clear in the past decades that the vast majority of the receptors form dimers. Only very recently, however, data were presented that some receptors may in fact be expressed as a mixture of monomers and dimers and that the interaction of the receptor protomers is dynamic. To date, equilibrium measurements were restricted to the plasma membrane due to experimental limitations. We have addressed the question as to where this equilibrium is established for the corticotropin-releasing factor receptor type 1. By developing a novel approach to analyze single molecule fluorescence cross-correlation spectroscopy data for intracellular membrane compartments, we show that the corticotropin-releasing factor receptor type 1 has a specific monomer/dimer equilibrium that is already established in the endoplasmic reticulum (ER). It remains constant at the plasma membrane even following receptor activation. Moreover, we demonstrate for seven additional GPCRs that they are expressed in specific but substantially different monomer/dimer ratios. Although it is well known that proteins may dimerize in the ER in principle, our data show that the ER is also able to establish the specific monomer/dimer ratios of GPCRs, which sheds new light on the functions of this compartment.

Heterodimerization with Its splice variant blocks the ghrelin receptor 1a in a non-signaling conformation: a study with a purified heterodimer assembled into lipid discs

Heterodimerization of G protein-coupled receptors has an impact on their signaling properties, but the molecular mechanisms underlying heteromer-directed selectivity remain elusive. Using purified monomers and dimers reconstituted into lipid discs, we explored how dimerization impacts the functional and structural behavior of the ghrelin receptor. In particular, we investigated how a naturally occurring truncated splice variant of the ghrelin receptor exerts a dominant negative effect on ghrelin signaling upon dimerization with the full-length receptor. We provide direct evidence that this dominant negative effect is due to the ability of the non-signaling truncated receptor to restrict the conformational landscape of the full-length protein. Indeed, associating both proteins within the same disc blocks all agonist- and signaling protein-induced changes in ghrelin receptor conformation, thus preventing it from activating its cognate G protein and triggering arrestin 2 recruitment. This is an unambiguous demonstration that allosteric conformational events within dimeric assemblies can be directly responsible for modulation of signaling mediated by G protein-coupled receptors.

High constitutive activity is an intrinsic feature of ghrelin receptor protein: a study with a functional monomeric GHS-R1a receptor reconstituted in lipid discs

Despite its central role in signaling and the potential therapeutic applications of inverse agonists, the molecular mechanisms underlying G protein-coupled receptor (GPCR) constitutive activity remain largely to be explored. In this context, ghrelin receptor GHS-R1a is a peculiar receptor in the sense that it displays a strikingly high, physiologically relevant, constitutive activity. To identify the molecular mechanisms responsible for this high constitutive activity, we have reconstituted a purified GHS-R1a monomer in a lipid disc. Using this reconstituted system, we show that the isolated ghrelin receptor per se activates G(q) in the absence of agonist, as assessed through guanosine 5'-O-(thiotriphosphate) binding experiments. The measured constitutive activity is similar in its extent to that observed in heterologous systems and in vivo. This is the first direct evidence for the high constitutive activity of the ghrelin receptor being an intrinsic property of the protein rather than the result of influence of its cellular environment. Moreover, we show that the isolated receptor in lipid discs recruits arrestin-2 in an agonist-dependent manner, whereas it interacts with μ-AP2 in the absence of ligand or in the presence of ghrelin. Of importance, these differences are linked to ligand-specific GHS-R1a conformations, as assessed by intrinsic fluorescence measurements. The distinct ligand requirements for the interaction of purified GHS-R1a with arrestin and AP2 provide a new rationale to the differences in basal and agonist-induced internalization observed in cells.

Asymmetry of GPCR oligomers supports their functional relevance

G protein-coupled receptors (GPCRs) can exist as dimers or as larger oligomeric clusters that enable intercommunication between different receptor protomers within the same complex. This phenomenon is observed at three distinct levels: (i) at the level of ligand binding where the activation of one protomer can allosterically inhibit or facilitate ligand binding to the second protomer; (ii) at the level of ligand-induced conformational switches, which occur between transmembrane domains of the two protomers; and (iii) within GPCR-associated protein complexes, either directly at the level of GPCR-interacting proteins or at further downstream levels of the complex. Intercommunication at these different levels introduces asymmetry within GPCR dimers wherein each protomer fulfills its specific task. In this review, we discuss how the asymmetric behavior of GPCRs highlights the advantage of oligomeric receptor organization and supports the functional relevance of GPCR dimerization.

Allostery at G protein-coupled receptor homo- and heteromers: uncharted pharmacological landscapes

For many years seven transmembrane domain G protein-coupled receptors (GPCRs) were thought to exist and function exclusively as monomeric units. However, evidence both from native cells and heterologous expression systems has demonstrated that GPCRs can both traffic and signal within higher-order complexes. As for other protein-protein interactions, conformational changes in one polypeptide, including those resulting from binding of pharmacological ligands, have the capacity to alter the conformation and therefore the response of the interacting protein(s), a process known as allosterism. For GPCRs, allosterism across homo- or heteromers, whether dimers or higher-order oligomers, represents an additional topographical landscape that must now be considered pharmacologically. Such effects may offer the opportunity for novel therapeutic approaches. Allosterism at GPCR heteromers is particularly exciting in that it offers additional scope to provide receptor subtype selectivity and tissue specificity as well as fine-tuning of receptor signal strength. Herein, we introduce the concept of allosterism at both GPCR homomers and heteromers and discuss the various questions that must be addressed before significant advances can be made in drug discovery at these GPCR complexes.

Trans-activation between 7TM domains: implication in heterodimeric GABAB receptor activation

Seven-transmembrane domain (7TM) receptors have important functions in cell-cell communication and can assemble into dimers or oligomers. Such complexes may allow specific functional cross-talk through trans-activation of interacting 7TMs, but this hypothesis requires further validation. Herein, we used the GABAB receptor, which is composed of two distinct subunits, GABAB1, which binds the agonist, and GABAB2, which activates G proteins, as a model system. By using a novel orthogonal-labelling approach compatible with time-resolved FRET and based on ACP- and SNAP-tag technologies to verify the heterodimerization of wild-type and mutated GABAB subunits, we demonstrate the existence of a direct allosteric coupling between the 7TMs of GABAB heterodimers. Indeed, a GABAB receptor, in which the GABAB2 extracellular domain was deleted, was still capable of activating G proteins. Furthermore, synthetic ligands for the GABAB2 7TM could increase agonist affinity at the GABAB1 subunit in this mutated receptor. In addition to bringing new information on GABAB receptor activation, these data clearly demonstrate the existence of direct trans-activation between the 7TM of two interacting proteins.

Leukotriene BLT2 receptor monomers activate the G(i2) GTP-binding protein more efficiently than dimers

Accumulating evidence indicates that G protein-coupled receptors can assemble as dimers/oligomers but the role of this phenomenon in G protein coupling and signaling is not yet clear. We have used the purified leukotriene B(4) receptor BLT2 as a model to investigate the capacity of receptor monomers and dimers to activate the adenylyl cyclase inhibitory G(i2) protein. For this, we overexpressed the recombinant receptor as inclusion bodies in the Escherichia coli prokaryotic system, using a human alpha(5) integrin as a fusion partner. This strategy allowed the BLT2 as well as several other G protein-coupled receptors from different families to be produced and purified in large amounts. The BLT2 receptor was then successfully refolded to its native state, as measured by high-affinity LTB(4) binding in the presence of the purified G protein G alpha(i2). The receptor dimer, in which the two protomers displayed a well defined parallel orientation as assessed by fluorescence resonance energy transfer, was then separated from the monomer. Using two methods of receptor-catalyzed guanosine 5'-3-O-(thio)triphosphate binding assay, we clearly demonstrated that monomeric BLT2 stimulates the purified G alpha(i2) beta(1) gamma(2) protein more efficiently than the dimer. These data suggest that assembly of two BLT2 protomers into a dimer results in the reduced ability to signal.

GPCR homomers and heteromers: a better choice as targets for drug development than GPCR monomers?

G protein-coupled receptors (GPCR) are targeted by many therapeutic drugs marketed to fight against a variety of diseases. Selection of novel lead compounds are based on pharmacological parameters obtained assuming that GPCR are monomers. However, many GPCR are expressed as dimers/oligomers. Therefore, drug development may consider GPCR as homo- and hetero-oligomers. A two-state dimer receptor model is now available to understand GPCR operation and to interpret data obtained from drugs interacting with dimers, and even from mixtures of monomers and dimers. Heteromers are distinct entities and therefore a given drug is expected to have different affinities and different efficacies depending on the heteromer. All these concepts would lead to broaden the therapeutic potential of drugs targeting GPCRs, including receptor heteromer-selective drugs with a lower incidence of side effects, or to identify novel pharmacological profiles using cell models expressing receptor heteromers.

Allosteric communication between protomers of dopamine class A GPCR dimers modulates activation

A major obstacle to understanding the functional importance of dimerization between Class A G protein-coupled receptors (GPCRs) has been the methodological limitation in achieving control of the identity of the components comprising the signaling unit. We have developed a functional complementation assay that enables such control and illustrate it for the human dopamine D2 receptor. The minimal signaling unit, two receptors and a single G protein, is maximally activated by agonist binding to a single protomer, which suggests an asymmetrical activated dimer. Inverse agonist binding to the second protomer enhances signaling, whereas agonist binding to the second protomer blunts signaling. Ligand-independent constitutive activation of the second protomer also inhibits signaling. Thus, GPCR dimer function can be modulated by the activity state of the second protomer, which for a heterodimer may be altered in pathological states. Our novel methodology also makes possible the characterization of signaling from a defined heterodimer unit.

Amphipol-assisted in vitro folding of G protein-coupled receptors

G protein-coupled receptors (GPCRs) regulate numerous physiological functions. The primary difficulty presented by their study in vitro is to obtain them in sufficient amounts under a functional and stable form. Escherichia coli is a host of choice for producing recombinant proteins for structural studies. However, the insertion of GPCRs into its plasma membrane usually results in bacterial death. An alternative approach consists of targeting recombinant receptors to inclusion bodies, where they accumulate without affecting bacterial growth, and then folding them in vitro. This approach, however, stumbles over the very low folding yields typically achieved, whether in detergent solutions or in detergent-lipid mixtures. Here, we show that synthetic polymers known as amphipols provide a highly efficient medium for folding GPCRs. Using a generic protocol, we have folded four class A GPCRs to their functional state, as evidenced by the binding of their respective ligands. This strategy thus appears to have the potential to be generalized to a large number of GPCRs. These data are also of interest from a more fundamental point of view: they indicate that the structural information stored in the sequence of these four receptors allows them to reach their correct three-dimensional structure in an environment that bears no similarity, beyond the amphiphilic character, to lipid bilayers.

G-protein-coupled receptor heteromers or how neurons can display differently flavoured patterns in response to the same neurotransmitter

It is becoming accepted that G-protein-coupled receptors (GPCRs) arrange in the neuronal membrane into homo- and hetero-oligomers and, therefore, these complexes mediate neurotransmission. New models are then needed to understand GPCR operation and predict the consequences of GPCR homo- or hetero-oligomerization. Although there is not any unifying theory addressing how hetero-oligomerization occurs, recent models have been devised to understand the thermodynamics of binding of neurotransmitters to GPCRs and the allosteric protomer-protomer interactions involved in neurotransmitter-mediated activation of GPCRs. Although a model to predict how signalling is produced via homo- or hetero-oligomerization is lacking, functional data show that receptor oligomers exist to produce a variety of effects in neurons in response to a single neurotransmitter.

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 GABA(B) 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.

G protein activation by the leukotriene B4 receptor dimer. Evidence for an absence of trans-activation

There is compelling evidence that G protein-coupled receptors exist as homo- and heterodimers, but the way these assemblies function at the molecular level remains unclear. We used here the purified leukotriene B(4) receptor BLT1 stabilized in its dimeric state to analyze how a receptor dimer activates G proteins. For this, we produced heterodimers between the wild-type BLT1 and a BLT1/ALXR chimera. The latter is no longer activated by leukotriene B(4) but is still activated by ALXR agonists. In this heterodimer, agonist binding to either one of the two protomers induced asymmetric conformational changes within the receptor dimer. Of importance, no G protein activation was observed when using a dimer where the ligand-loaded protomer was not able to trigger GDP/GTP exchange due to specific mutations in its third intracellular loop, establishing that the conformation of the agonist-free protomer is not competent for G protein activation. Taken together, these data indicate that although ligand binding to one protomer in the heterodimer is associated with cross-conformational changes, a trans-activation mechanism where the ligand-free subunit would trigger GDP/GTP exchange cannot be considered in this case for G protein activation. This observation sheds light into the way GPCR dimers, in particular heterodimers, could activate their cognate G proteins.

Conformational cross-talk between alpha2A-adrenergic and mu-opioid receptors controls cell signaling

Morphine, a powerful analgesic, and norepinephrine, the principal neurotransmitter of sympathetic nerves, exert major inhibitory effects on both peripheral and brain neurons by activating distinct cell-surface G protein-coupled receptors-the mu-opioid receptor (MOR) and alpha2A-adrenergic receptor (alpha2A-AR), respectively. These receptors, either singly or as a heterodimer, activate common signal transduction pathways mediated through the inhibitory G proteins (G(i) and G(o)). Using fluorescence resonance energy transfer microscopy, we show that in the heterodimer, the MOR and alpha2A-AR communicate with each other through a cross-conformational switch that permits direct inhibition of one receptor by the other with subsecond kinetics. We discovered that morphine binding to the MOR triggers a conformational change in the norepinephrine-occupied alpha2A-AR that inhibits its signaling to G(i) and the downstream MAP kinase cascade. These data highlight a new mechanism in signal transduction whereby a G protein-coupled receptor heterodimer mediates conformational changes that propagate from one receptor to the other and cause the second receptor's rapid inactivation.

Dimerization of the thyrotropin-releasing hormone receptor potentiates hormone-dependent receptor phosphorylation

The G protein-coupled thyrotropin (TSH)-releasing hormone (TRH) receptor forms homodimers. Regulated receptor dimerization increases TRH-induced receptor endocytosis. These studies test whether dimerization increases receptor phosphorylation, which could potentiate internalization. Phosphorylation at residues 355-365, which is critical for internalization, was measured with a highly selective phospho-site-specific antibody. Two strategies were used to drive receptor dimerization. Dimerization of a TRH receptor-FK506-binding protein (FKBP) fusion protein was stimulated by a dimeric FKBP ligand. The chemical dimerizer caused a large increase in TRH-dependent phosphorylation within 1 min, whereas a monomeric FKBP ligand had no effect. The dimerizer did not alter phoshorylation of receptors lacking the FKBP domain. Dimerization of receptors containing an N-terminal HA epitope also was induced with anti-HA antibody. Anti-HA IgG strongly increased TRH-induced phosphorylation, whereas monomeric Fab fragments had no effect. Anti-HA antibody did not alter phosphorylation in receptors lacking an HA tag. Furthermore, two phosphorylation-defective TRH receptors functionally complemented one another and permitted phosphorylation. Receptors with a D71A mutation in the second transmembrane domain do not signal, whereas receptors with four Ala mutations in the 355-365 region signal normally but lack phosphorylation sites. When D71A- and 4Ala-TRH receptors were expressed alone, neither underwent TRH-dependent phosphorylation. When they were expressed together, D71A receptor was phosphorylated by G protein-coupled receptor kinases in response to TRH. These results suggest that the TRH receptor is phosphorylated preferentially when it is in dimers or when preexisting receptor dimers are driven into microaggregates. Increased receptor phosphorylation may amplify desensitization.

Computational studies of Family A and Family B GPCRs

A full picture of the similarities between Family A and Family B GPCRs (G-protein coupled receptors) has been frustrated by the lack of clear homology between the respective sequences. Here, we review previous computational studies on GPCR dimerization in which the putative dimerization interfaces have been analysed using entropy, the ET (evolutionary trace) method and related methods. The results derived from multiple sequence alignments of Family A subfamilies have been mapped on to the rhodopsin crystal structure using standard alignments. Similarly, the results for the Family B alignments have been mapped on to the rhodopsin crystal structure using the 'cold-spot' alignment. For both Family A and Family B GPCRs, the sequence analysis indicates that there are functional sites on essentially all transmembrane helices, consistent with the parallel daisy chain model of GPCR oligomerization in which each GPCR makes interactions with a number of neighbouring GPCRs. The results are not too sensitive to the quality of the alignment. Molecular Dynamics simulations of the activation process within a single transmembrane bundle of the rhodopsin and the beta(2)-adrenergic receptor have been reviewed; the key observation, which is consistent with other computational studies, is that there is a translation and bending of helix 6, which contributes to a significant opening out of the intracellular face of the receptor, as shown in the accompanying movies. The simulations required the application of specific experiment-derived harmonic and half-harmonic distance restraints and so the application of such simulations to Family B GPCRs requires considerable care because of the alignment problem. Thus, in order to address the alignment problem, we have exploited the observation that GCR1, a plant GPCR, has homology with Family A, Family B and Family E GPCRs. The resulting alignment for transmembrane helix 3 is presented.

Partial agonist actions of aripiprazole and the candidate antipsychotics S33592, bifeprunox, N-desmethylclozapine and preclamol at dopamine D2Lreceptors are modified by co-transfection of D3receptors: potential role of heterodimer formation

Aripiprazole and the candidate antipsychotics, S33592, bifeprunox, N-desmethylclozapine (NDMC) and preclamol, are partial agonists at D(2) receptors. Herein, we examined their actions at D(2L) and D(3) receptors expressed separately or together in COS-7 cells. In D(2L) receptor-expressing cells co-transfected with (D(3) receptor-insensitive) chimeric adenylate cyclase-V/VI, drugs reduced forskolin-stimulated cAMP production by approximately 20% versus quinpirole (48%). Further, quinpirole-induced inhibition was blunted by aripiprazole and S33592, confirming partial agonist properties. In cells co-transfected with equal amounts of D(2L)and D(3) receptors (1 : 1), efficacies of aripiprazole and S33592 were attenuated. Further, in cells co-transfected with D(2L) and an excess of D(3) receptors (1 : 3), aripiprazole and S33592 were completely inactive, and they abolished the actions of quinpirole. Likewise, bifeprunox, NDMC and preclamol lost agonist properties in cells co-transfected with D(2L)and D(3) receptors. Accordingly, at split D(2trunk)/D(3tail) and D(3trunk)/D(2tail) chimeras, agonist actions of quinpirole were blocked by aripiprazole and S33592 that, like bifeprunox, NDMC and preclamol, were inactive alone. Conversely, when a 12 amino acid sequence in the third intracellular loop of D(3) receptors was replaced by the homologous sequence of D(2L) receptors, aripiprazole, S33592, bifeprunox, NDMC and preclamol inhibited cAMP formation by approximately 20% versus quinpirole (42%). Moreover, at D(2L) receptor-expressing cells co-transfected with modified D(3i3(D2)) receptors, drugs behaved as partial agonists. To summarize, low efficacy agonist actions of aripiprazole, S33592, bifeprunox, NDMC and preclamol at D(2L) receptors are abrogated upon co-expression of D(3) receptors, probably due to physical association and weakened coupling efficacy. These findings have implications for the functional profiles of antipsychotics.

Allosteric enhancers, allosteric agonists and ago-allosteric modulators: where do they bind and how do they act?

Many small-molecule agonists also display allosteric properties. Such ago-allosteric modulators act as co-agonists, providing additive efficacy--instead of partial antagonism--and they can affect--and often improve--the potency of the endogenous agonist. Surprisingly, the apparent binding sites of several ordinary allosteric enhancers and ago-allosteric modulators seem to overlap with those of the endogenous agonists. Different molecular scenarios are proposed to explain this discrepancy from classical allosteric models. In one scenario, the ago-allosteric modulator can interchange between different binding modes. In another, dimeric, receptor scenario, the endogenous agonist binds to one protomer while the ago-allosteric modulator binds to the other, 'allosteric' protomer. It is suggested that testing for ago-allosteric properties should be an integral part of the agonist drug discovery process because a compound that acts with--rather than against--the endogenous agonist could be an optimal agonist drug.

Allosteric properties of G protein-coupled receptor oligomers

Allosteric regulation of ligand binding is a well-established mechanism regulating the function of G protein-coupled receptors (GPCR). Allosteric modulators have been considered so far as molecules binding to an allosteric site, distinct from that of the reference ligand (orthosteric site), and able to modulate the binding affinity at the orthosteric site and/or the signaling properties resulting from orthosteric site occupancy. Given that most GPCR are known to form dimers or higher order oligomers, we explored whether allosteric interactions could also occur between protomers within oligomeric arrays, thereby influencing binding and signaling receptor properties. Two main conclusions emerged from such studies. First, allosteric modulators can affect one receptor by binding to another receptor within a dimeric or oligomeric complex. Second, allosteric modulators might act on a given receptor by targeting the "orthosteric site" in another receptor of the complex. Allosteric regulation within di(oligo)mers thus implies that the pharmacological properties of a given receptor subtype can be influenced by the array of dimerization partners coexpressed in each particular cell type. Ligands could thus act as agonists or antagonists on 1 receptor, while modulating allosterically the function of a variety of other receptors to which they do not bind directly. Allosteric regulation across GPCR oligomeric interfaces is expected to greatly influence the practice of pharmacology. It will likely affect the design of drug discovery programs, which rely mostly on the overexpression of the receptor of interest in a cell line, thereby focusing on homo-oligomers and ignoring the potential effects of other partners.

Serotonin 5-HT(2C) receptor homodimerization is not regulated by agonist or inverse agonist treatment

Serotonin 5-HT(2C) receptors represent targets for therapeutics aimed at treating anxiety, depression, schizophrenia, and obesity. Previously, we demonstrated that 5-HT(2C) receptors function as homodimers. Herein, we investigated the effect of agonist and inverse agonist treatment on the homodimer status of two naturally occurring 5-HT(2C) receptor isoforms, one without basal activity (VGV) and one with constitutive activity (INI) with respect to Galpha(q) signaling. Cyan- and yellow-fluorescent proteins were used to monitor VGV and INI homodimer formation by western blot, and in living cells using bioluminescence and fluorescence resonance energy transfer (BRET and FRET). Western blots of solubilized membrane proteins revealed equal proportions of homodimeric receptor species from HEK293 cells transfected with either the VGV or INI isoform in the absence and presence of 5-HT. BRET ratios measured in HEK293 cells transfected with the VGV or INI isoform were the same and were not modulated by 5-HT. Similarly, FRET efficiencies were the same regardless of whether measured in cells expressing the VGV or INI isoform in the absence or presence of 5-HT or clozapine. The results indicate that serotonin 5-HT(2C) receptors form homodimers regardless of whether they are in an inactive or active conformation and are not regulated by drug treatment.

New tensio-active molecules stabilize a human G protein-coupled receptor in solution

Structural characterization of membrane proteins is hampered by the instability of the isolated proteins in detergent solutions. Here, we describe a new class of phospholipid-like surfactants that stabilize the G protein-coupled receptor, BLT1. These compounds, called C(13)U(9), C(13)U(19), C(15)U(25) and C(17)U(16), were synthesized by radical polymerization of Tris(hydroxymethyl) acrylamidomethane in the presence of thioglycerol, first endowed with two hydrocarbon chains with variable lengths (13-17 carbon atoms), as transfer reagent. C(13)U(19), C(17)U(16) or C(15)U(25) significantly enhanced the stability of BLT1 in solution compared to what was obtained with common detergents. These molecules therefore represent a promising step towards the structural characterization of BLT1 and possibly other membrane proteins.

Transducin activation by nanoscale lipid bilayers containing one and two rhodopsins

Nanodiscs are nanometer scale planar membranes of controlled size that are rendered soluble in aqueous solution via an encircling amphipathic membrane scaffold protein "belt" (Bayburt, T. H., Grinkova, Y. V., and Sligar, S. G. (2002) Nano. Lett. 2, 853-856). Integral membrane proteins can be self-assembled into the Nanodisc bilayer with defined stoichiometry, which allows an unprecedented opportunity to investigate the nature of the oligomerization state of a G-protein-coupled receptor and its coupling to heterotrimeric G-proteins. We generated Nanodiscs having one and two rhodopsins present in the 10-nm-diameter lipid bilayer domain. Efficient transducin activation and isolation of a high affinity transducin-metarhodopsin II complex was demonstrated for a monodisperse and monomeric receptor. A population of Nanodiscs containing two rhodopsins was generated using an increased ratio of receptor to membrane scaffold protein in the self-assembly mixture. The two-rhodopsin population was isolated and purified by density gradient centrifugation. Interestingly, in this case, only one of the two receptors present in the Nanodisc was able to form a stable metarhodopsin II-G-protein complex. Thus there is clear evidence that a monomeric rhodopsin is capable of full coupling to transducin. Importantly, presumably due to steric interactions, it appears that only a single receptor in the Nanodiscs containing two rhodopsins can interact with G-protein. These results have important implications for the stoichiometry of receptor-G-protein coupling and cross talk in signaling pathways.

International Union of Basic and Clinical Pharmacology. LXVII. Recommendations for the Recognition and Nomenclature of G Protein-Coupled Receptor Heteromultimers

G protein-coupled receptors (GPCRs) have long been considered to be monomeric membrane proteins. Although numerous recent studies have indicated that GPCRs can form multimeric complexes, the functional and pharmacological consequences of this phenomenon have remained elusive. With the discovery that the functional GABA(B) receptor is an obligate heterodimer and with the use of energy transfer technologies, it is now accepted that GPCRs can form heteromultimers. In some cases, specific properties of such heteromers not shared by their respective homomers have been reported. Although in most cases these properties have only been observed in heterologous expression systems, there are a few reports describing data consistent with such heteromultimeric GPCR complexes also existing in native tissues. The present article illustrates well-documented examples of such native multimeric complexes, lists a number of recommendations for recognition and acceptance of such multimeric receptors, and gives recommendations for their nomenclature.

Allosteric Modulation of G Protein–Coupled Receptors

The past decade has witnessed a significant growth in the identification of allosteric modulators of G protein-coupled receptors (GPCRs), i.e., ligands that interact with binding sites that are topographically distinct from the orthosteric site recognized by the receptor's endogenous agonist. Because of their ability to modulate receptor conformations in the presence of orthosteric ligand, allosteric modulators can "fine-tune" classical pharmacological responses. This is advantageous in terms of a potential for engendering greater GPCR subtype-selectivity, but represents a significant challenge for detecting and validating allosteric behaviors. Although allosteric sites need not have evolved to accommodate endogenous ligands, there are a number of examples of where such modulators have been shown to contribute to physiological or pathophysiological processes. Studies are also beginning to unravel the structural basis of allosteric modulation of GPCRs. It remains to be determined whether such modulation represents interactions within monomers versus across dimers.

Activation of G protein-coupled receptors

G protein-coupled receptors (GPCRs) mediate responses to hormones and neurotransmitters, as well as the senses of sight, smell, and taste. These remarkably versatile signaling molecules respond to structurally diverse ligands. Many GPCRs couple to multiple G protein subtypes, and several have been shown to activate G protein-independent signaling pathways. Drugs acting on GPCRs exhibit efficacy profiles that may differ for different signaling cascades. The functional plasticity exhibited by GPCRs can be attributed to structural flexibility and the existence of multiple ligand-specific conformational states. This chapter will review our current understanding of the mechanism by which agonists bind and activate GPCRs.

Asymmetric conformational changes in a GPCR dimer controlled by G-proteins

G-protein-coupled receptors (GPCRs) are key players in cell communication. Although long considered as monomeric, it now appears that these heptahelical proteins can form homo- or heterodimers. Here, we analyzed the conformational changes in each subunit of a receptor dimer resulting from agonist binding to either one or both subunits by measuring the fluorescent properties of a leukotriene B(4) receptor dimer with a single 5-hydroxytryptophan-labeled protomer. We show that a receptor dimer with only a single agonist-occupied subunit can trigger G-protein activation. We also show that the two subunits of the receptor dimer in the G-protein-coupled state differ in their conformation, even when both are liganded by the agonist. No such asymmetric conformational changes are observed in the absence of G-protein, indicating that the interaction of the G-protein with the receptor dimer brings specific constraints that prevent a symmetric functioning of this dimer. These data open new options for the differential signaling properties of GPCR dimers.

G protein coupled receptor structure and activation

G protein coupled receptors (GPCRs) are remarkably versatile signaling molecules. The members of this large family of membrane proteins are activated by a spectrum of structurally diverse ligands, and have been shown to modulate the activity of different signaling pathways in a ligand specific manner. In this manuscript I will review what is known about the structure and mechanism of activation of GPCRs focusing primarily on two model systems, rhodopsin and the beta(2) adrenoceptor.

Ago-allosteric modulation and other types of allostery in dimeric 7TM receptors

Conventionally, an allosteric modulator is neutral in respect of efficacy and binds to a receptor site distant from the orthosteric site of the endogenous agonist. However, recently compounds being ago-allosteric modulators have been described i.e., compounds acting both as agonists on their own and as enhancers for the endogenous agonists in both increasing agonist potency and providing additive efficacy-superagonism. The additive efficacy can also be observed with agonists, which are neutral or even negative modulators of the potency of the endogenous ligand. Based on the prevailing dimeric concept for 7TM receptors, it is proposed that the ago-allosteric modulators bind in the orthosteric binding site, but-importantly-in the "other" or allosteric protomer of the dimer. Hereby, they can act both as additive co-agonists, and through intermolecular cooperative effects between the protomers, they may influence the potency of the endogenous agonist. It is of interest that at least some endogenous agonists can only occupy one protomer of a dimeric 7TM receptor complex at a time and thereby they leave the orthosteric binding site in the allosteric protomer free, potentially for binding of exogenous, allosteric modulators. If the allosteric modulator is an agonist, it is an ago-allosteric modulator; if it is neutral, it is a classical enhancer. Molecular mapping in hetero-dimeric class-C receptors, where the endogenous agonist clearly binds only in one protomer, supports the notion that allosteric modulators can act through binding in the "other" protomer. It is suggested that for the in vivo, clinical setting a positive ago-allosteric modulator should be the preferred agonist drug.

Residues from Transmembrane Helices 3 and 5 Participate in Leukotriene B4 Binding to BLT1

Leukotrienes are inflammatory mediators that bind to seven transmembrane, G-protein-coupled receptors (GPCRs). Here we examine residues from transmembrane helices 3 and 5 of the leukotriene B4 (LTB4) receptor BLT1 to elucidate how these residues are involved in ligand binding. We have selected these residues on the basis of (1) amino acid sequence analysis, (2) receptor binding and activation studies with a variety of leukotriene-like ligands and recombinant BLT1 receptors, (3) previously published recombinant BLT1 mutants, and (4) a computed model of the active structure of the BLT1 receptor. We propose that LTB4 binds with the polar carboxylate group of LTB4 near the extracellular surface of BLT1 and with the hydrophobic LTB4 tail pointing into the transmembrane regions of the receptor protein. The carboxylate group and the two hydroxyls of LTB4 interact with Arg178 and Glu185 in transmembrane helix 5. Residues from transmembrane helix 3, Val105 and Ile108, also line the pocket deeper inside the receptor. LTB4 is becoming increasingly important as an immunomodulator during a number of pathologies, including atherosclerosis. Detailed information about the LTB4 binding mechanism, and the receptor residues involved, will hopefully aid in the design of new immunomodulatory drugs.

Targeting opioid receptor heterodimers: strategies for screening and drug development

G-protein-coupled receptors are a major target for the development of new marketable drugs. A growing number of studies have shown that these receptors could bind to their ligands, signal, and be internalized as dimers. Most of the evidence comes from in vitro studies, but recent studies using animal models support an important role for dimerization in vivo and in human pathologies. It is therefore becoming highly relevant to include dimerization in screening campaigns: the increased complexity reached by the ability to target 2 receptors should lead to the identification of more specific hits that could be developed into drugs with fewer side effects. In this review, we have summarized results from a series of studies characterizing the properties of G-protein-coupled receptor dimers using both in vitro and in vivo systems. Since opioid receptors exist as dimers and heterodimerization modulates their pharmacology, we have used them as a model system to develop strategies for the identification of compounds that will specifically bind and activate opioid receptor heterodimers: such compounds could represent the next generation of pain relievers with decreased side effects, including reduced drug abuse liability.

Functional and structural characterization of rhodopsin oligomers

A major question in G protein-coupled receptor signaling concerns the quaternary structure required for signal transduction. Do these transmembrane receptors function as monomers, dimers, or larger oligomers? We have investigated the oligomeric state of the model G protein-coupled receptor rhodopsin (Rho), which absorbs light and initiates a phototransduction-signaling cascade that forms the basis of vision. In this study, different forms of Rho were isolated using gel filtration techniques in mild detergents, including n-dodecyl-beta-D-maltoside, n-tetradecyl-beta-D-maltoside, and n-hexadecyl-beta-D-maltoside. The quaternary structure of isolated Rho was determined by transmission electron microscopy, demonstrating that in micelles containing n-dodecyl-beta-D-maltoside, Rho exists as a mixture of monomers and dimers whereas in n-tetradecyl-beta-D-maltoside and n-hexadecyl-beta-D-maltoside Rho forms higher ordered structures. Especially in n-hexadecyl-beta-D-maltoside, most of the particles are present in tightly packed rows of dimers. The oligomerization of Rho seems to be important for interaction with its cognate G protein, transducin. Although the activated Rho (Meta II) monomer or dimers are capable of activating the G protein, transducin, the activation process is much faster when Rho exists as organized dimers. Our studies provide direct comparisons between signaling properties of Meta II in different quaternary complexes.

Crosstalk in G protein-coupled receptors: changes at the transmembrane homodimer interface determine activation

Functional crosstalk between G protein-coupled receptors in a homo- or heterodimeric assembly likely involves conformational changes at the dimer interface, but the nature of this interface is not yet established, and the dynamic changes have not yet been identified. We have mapped the homodimer interface in the dopamine D2 receptor over the entire length of the fourth transmembrane segment (TM4) by crosslinking of substituted cysteines. Their susceptibilities to crosslinking are differentially altered by the presence of agonists and inverse agonists. The TM4 dimer interface in the inverse agonist-bound conformation is consistent with the dimer of the inactive form of rhodopsin modeled with constraints from atomic force microscopy. Crosslinking of a different set of cysteines in TM4 was slowed by inverse agonists and accelerated in the presence of agonists; crosslinking of the latter set locks the receptor in an active state. Thus, a conformational change at the TM4 dimer interface is part of the receptor activation mechanism.

Expression of 5-oxoETE receptor in prostate cancer cells: critical role in survival

Previously, we reported that metabolism of arachidonic acid through the 5-lipoxygenase (5-LOX) pathway plays an important role in the survival and growth of human prostate cancer cells. Inhibition of 5-LOX by pharmacological inhibitors triggers apoptosis in prostate cancer cells within hours of treatment, which is prevented by the metabolites of arachidonate 5-lipoxygenase, 5(S)-hydroxyeicosatetraenoic acid (5(S)-HETE), and its dehydrogenated derivative, 5-oxoeicosatetraenoic acid (5-oxoETE). These findings suggested that 5-lipoxygenase metabolites are critical survival factors of prostate cancer cells. However, molecular mechanisms by which 5(S)-HETE and its derivative 5-oxoETE exert their effects on prostate cancer cell survival are yet to be understood. Here, we report that human prostate cancer cells differentially express a G-protein-coupled 5-oxoETE receptor (5-oxoER) in them. Blocking expression of 5-oxoER by short-interfering RNA (siRNA) significantly reduced the viability of prostate cancer cells, suggesting that 5-oxoER is critical for prostate cancer cell survival, and that the 5-LOX metabolite, 5-oxoETE, controls survival of prostate cancer cells through its own G-protein-coupled receptor, 5-oxoER.