Oligomerisation of G-protein-coupled receptors

Dysfunctional Heteroreceptor Complexes as Novel Targets for the Treatment of Major Depressive and Anxiety Disorders

Among mental diseases, major depressive disorder (MDD) and anxiety deserve a special place due to their high prevalence and their negative impact both on society and patients suffering from these disorders. Consequently, the development of novel strategies designed to treat them quickly and efficiently, without or at least having limited side effects, is considered a highly important goal. Growing evidence indicates that emerging properties are developed on recognition, trafficking, and signaling of G-protein coupled receptors (GPCRs) upon their heteromerization with other types of GPCRs, receptor tyrosine kinases, and ionotropic receptors such as N-methyl-D-aspartate (NMDA) receptors. Therefore, to develop new treatments for MDD and anxiety, it will be important to identify the most vulnerable heteroreceptor complexes involved in MDD and anxiety. This review focuses on how GPCRs, especially serotonin, dopamine, galanin, and opioid heteroreceptor complexes, modulate synaptic and volume transmission in the limbic networks of the brain. We attempt to provide information showing how these emerging concepts can contribute to finding new ways to treat both MDD and anxiety disorders.

G protein-coupled receptor-effector macromolecular membrane assemblies (GEMMAs)

G protein-coupled receptors (GPCRs) are the largest group of receptors involved in cellular signaling across the plasma membrane and a major class of drug targets. The canonical model for GPCR signaling involves three components - the GPCR, a heterotrimeric G protein and a proximal plasma membrane effector - that have been generally thought to be freely mobile molecules able to interact by 'collision coupling'. Here, we synthesize evidence that supports the existence of GPCR-effector macromolecular membrane assemblies (GEMMAs) comprised of specific GPCRs, G proteins, plasma membrane effector molecules and other associated transmembrane proteins that are pre-assembled prior to receptor activation by agonists, which then leads to subsequent rearrangement of the GEMMA components. The GEMMA concept offers an alternative and complementary model to the canonical collision-coupling model, allowing more efficient interactions between specific signaling components, as well as the integration of the concept of GPCR oligomerization as well as GPCR interactions with orphan receptors, truncated GPCRs and other membrane-localized GPCR-associated proteins. Collision-coupling and pre-assembled mechanisms are not exclusive and likely both operate in the cell, providing a spectrum of signaling modalities which explains the differential properties of a multitude of GPCRs in their different cellular environments. Here, we explore the unique pharmacological characteristics of individual GEMMAs, which could provide new opportunities to therapeutically modulate GPCR signaling.

Heteromerization of G2A and OGR1 enhances proton sensitivity and proton-induced calcium signals

Proton-sensing G-protein-coupled receptors (GPCRs; OGR1, GPR4, G2A, TDAG8), with full activation at pH 6.4 ∼ 6.8, are important to pH homeostasis, immune responses and acid-induced pain. Although G2A mediates the G13-Rho pathway in response to acid, whether G2A activates Gs, Gi or Gq proteins remains debated. In this study, we examined the response of this fluorescence protein-tagged OGR1 family to acid stimulation in HEK293T cells. G2A did not generate detectable intracellular calcium or cAMP signals or show apparent receptor redistribution with moderate acid (pH ≥ 6.0) stimulation but reduced cAMP accumulation under strong acid stimulation (pH ≤ 5.5). Surprisingly, coexpression of OGR1- and G2A-enhanced proton sensitivity and proton-induced calcium signals. This alteration is attributed to oligomerization of OGR1 and G2A. The oligomeric potential locates receptors at a specific site, which leads to enhanced proton-induced calcium signals through channels.

Alarmin function of cathelicidin antimicrobial peptide LL37 through IL-36γ induction in human epidermal keratinocytes

Several dermatoses, including psoriasis, atopic dermatitis, and rosacea, alter the expression of the innate immune effector human cathelicidin antimicrobial peptide (CAMP). To elucidate the roles of aberrant CAMP in dermatoses, we performed cDNA array analysis in CAMP-stimulated human epidermal keratinocytes, the primary cells responding to innate immune stimuli and a major source of CAMP LL37 in skin. Among LL37-inducible genes, IL-1 cluster genes, particularly IL36G, are of interest because we observed coordinate increases in CAMP and IL-36γ in the lesional skin of psoriasis, whereas virtually no CAMP or IL-36γ was observed in nonlesional skin and normal skin. The production and release of IL-36γ were up to 20-30 ng/ml in differentiated keratinocytes cultured in high-calcium media. G-protein inhibitor pertussis toxin and p38 inhibitor suppressed IL-36γ induction by LL37. As an alarmin, LL37 induces chemokines, including CXCL1, CXCL8/IL8, CXCL10/IP-10, and CCL20/MIP3a, and IL-36 (10-100 ng/ml) augments the production of these chemokines by LL37. Pretreatment with small interfering RNA against IL36γ and IL-36R IL36R/IL1RL2 and IL1RAP suppressed LL37-dependent IL8, CXCL1, CXCL10/IP10, and CCL20 production in keratinocytes, suggesting that the alarmin function of LL37 was partially dependent on IL-36γ and its receptors. Counting on CAMP induction in innate stimuli, such as in infection and wounding, IL-36γ induction by cathelicidin would explain the mechanism of initiation of skin inflammation and occasional exacerbations of psoriasis and skin diseases by general infection.

Role of chemokines in proteinuric kidney disorders

Experimental and human studies have shown that proteinuria contributes to the progression of renal disease. Overexposure to filtered proteins promotes the expression and release of chemokines by tubular epithelial cells, thus leading to inflammatory cell recruitment and renal impairment. This review focuses on recent progress in cellular and molecular understanding of the role of chemokines in the pathogenesis of proteinuria-induced renal injury, as well as their clinical implications and therapeutic potential.

The fall and rise of pharmacology--(re-)defining the discipline?

Pharmacology is an integrative discipline that originated from activities, now nearly 7000 years old, to identify therapeutics from natural product sources. Research in the 19th Century that focused on the Law of Mass Action (LMA) demonstrated that compound effects were dose-/concentration-dependent eventually leading to the receptor concept, now a century old, that remains the key to understanding disease causality and drug action. As pharmacology evolved in the 20th Century through successive biochemical, molecular and genomic eras, the precision in understanding receptor function at the molecular level increased and while providing important insights, led to an overtly reductionistic emphasis. This resulted in the generation of data lacking physiological context that ignored the LMA and was not integrated at the tissue/whole organism level. As reductionism became a primary focus in biomedical research, it led to the fall of pharmacology. However, concerns regarding the disconnect between basic research efforts and the approval of new drugs to treat 21st Century disease tsunamis, e.g., neurodegeneration, metabolic syndrome, etc. has led to the reemergence of pharmacology, its rise, often in the semantic guise of systems biology. Against a background of limited training in pharmacology, this has resulted in issues in experimental replication with a bioinformatics emphasis that often has a limited relationship to reality. The integration of newer technologies within a pharmacological context where research is driven by testable hypotheses rather than technology, together with renewed efforts in teaching pharmacology, is anticipated to improve the focus and relevance of biomedical research and lead to novel therapeutics that will contain health care costs.

G protein activation by serotonin type 4 receptor dimers: evidence that turning on two protomers is more efficient

The discovery that class C G protein-coupled receptors (GPCRs) function as obligatory dimeric entities has generated major interest in GPCR oligomerization. Oligomerization now appears to be a common feature among all GPCR classes. However, the functional significance of this process remains unclear because, in vitro, some monomeric GPCRs, such as rhodopsin and β(2)-adrenergic receptors, activate G proteins. By using wild type and mutant serotonin type 4 receptors (5-HT(4)Rs) (including a 5-HT(4)-RASSL) expressed in COS-7 cells as models of class A GPCRs, we show that activation of one protomer in a dimer was sufficient to stimulate G proteins. However, coupling efficiency was 2 times higher when both protomers were activated. Expression of combinations of 5-HT(4), in which both protomers were able to bind to agonists but only one could couple to G proteins, suggested that upon agonist occupancy, protomers did not independently couple to G proteins but rather that only one G protein was activated. Coupling of a single heterotrimeric G(s) protein to a receptor dimer was further confirmed in vitro, using the purified recombinant WT RASSL 5-HT(4)R obligatory heterodimer. These results, together with previous findings, demonstrate that, differently from class C GPCR dimers, class A GPCR dimers have pleiotropic activation mechanisms.

Refining efficacy: allosterism and bias in G protein-coupled receptor signaling

Receptors on the surface of cells function as conduits for information flowing between the external environment and the cell interior. Since signal transduction is based on the physical interaction of receptors with both extracellular ligands and intracellular effectors, ligand binding must produce conformational changes in the receptor that can be transmitted to the intracellular domains accessible to G proteins and other effectors. Classical models of G protein-coupled receptor (GPCR) signaling envision receptor conformations as highly constrained, wherein receptors exist in equilibrium between single "off" and "on" states distinguished by their ability to activate effectors, and ligands act by perturbing this equilibrium. In such models, ligands can be classified based upon two simple parameters; affinity and efficacy, and ligand activity is independent of the assay used to detect the response. However, it is clear that GPCRs assume multiple conformations, any number of which may be capable of interacting with a discrete subset of possible effectors. Both orthosteric ligands, molecules that occupy the natural ligand-binding pocket, and allosteric modulators, small molecules or proteins that contact receptors distant from the site of ligand binding, have the ability to alter the conformational equilibrium of a receptor in ways that affect its signaling output both qualitatively and quantitatively. In this context, efficacy becomes pluridimensional and ligand classification becomes assay dependent. A more complete description of ligand-receptor interaction requires the use of multiplexed assays of receptor activation and screening assays may need to be tailored to detect specific efficacy profiles.

Refining efficacy: exploiting functional selectivity for drug discovery

Early models of G protein-coupled receptor (GPCR) activation envisioned the receptor in equilibrium between unique "off" and "on" states, wherein ligand binding affected signaling by increasing or decreasing the fraction of receptors in the active conformation. It is now apparent that GPCRs spontaneously sample multiple conformations, any number of which may couple to one or more downstream effectors. Such "multistate" models imply that the receptor-ligand complex, not the receptor alone, defines which active receptor conformations predominate. "Functional selectivity" refers to the ability of a ligand to activate only a subset of its receptor's signaling repertoire. There are now numerous examples of ligands that "bias" receptor coupling between different G protein pools and non-G protein effectors such as arrestins. The type 1 parathyroid hormone receptor (PTH(1)R) is a particularly informative example, not only because of the range of biased effects that have been produced, but also because the actions of many of these ligands have been characterized in vivo. Biased PTH(1)R ligands can selectively couple the PTH(1)R to G(s) or G(q/11) pathways, with or without activating arrestin-dependent receptor desensitization and signaling. These reagents have provided insight into the contribution of different signaling pathways to PTH action in vivo and suggest it may be possible to exploit ligand bias to uncouple the anabolic effects of PTH(1)R from its catabolic and calcitropic effects. Whereas conventional agonists and antagonists only modulate the quantity of efficacy, functionally selective ligands qualitatively change GPCR signaling, offering the prospect of drugs with improved therapeutic efficacy or reduced side effects.

Subtle conformational changes between CX3CR1 genetic variants as revealed by resonance energy transfer assays

The chemokine CX3CL1 is expressed as a membrane protein that forms a potent adhesive pair with its unique receptor CX3CR1. This receptor has 3 natural variants, V249-T280 (VT), I249-T280 (IT), and I249-M280 (IM), whose relative frequencies are significantly associated with the incidence of various inflammatory diseases. To assess the adhesive potency of CX3CR1 and the molecular diversity of its variants, we assayed their clustering status and their possible structural differences by fluorescence/bioluminescence resonance energy transfer (FRET or BRET) techniques. FRET assays by flow cytometry showed that the CX3CR1 variants cluster, in comparison with appropriate controls. BRET assays showed low nonspecific signals for VT and IT variants and high specific signals for IM, and thus pointed out a structural difference in this variant. We used molecular modeling to show how natural point mutations of CX3CR1 affect the packing of the 6th and 7th helices of this G-protein coupled receptor. Moreover, we found that the BRET technique is sensitive enough to detect these tiny changes. Consistently with our previous finding that CX3CL1 aggregates, our data here indicate that CX3CR1 clustering may contribute to the adhesiveness of the CX3CL1-CX3CR1 pair and may thus represent a new target for anti-inflammatory therapies.

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.

Brain receptor mosaics and their intramembrane receptor-receptor interactions: molecular integration in transmission and novel targets for drug development

The concept of intramembrane receptor-receptor interactions and evidence for their existence was introduced by Agnati and Fuxe in 1980/81 suggesting the existence of heteromerization of receptors. In 1982, they proposed the existence of aggregates of multiple receptors in the plasma membrane and coined the term receptor mosaics (RM). In this way, cell signaling becomes a branched process beginning at the level of receptor recognition at the plasma membrane where receptors can directly modify the ligand recognition and signaling capacity of the receptors within a RM. Receptor-receptor interactions in RM are classified as operating either with classical cooperativity, when consisting of homomers or heteromers of similar receptor subtypes having the same transmitter, or non-classical cooperativity, when consisting of heteromers. It has been shown that information processing within a RM depends not only on its receptor composition, but also on the topology and the order of receptor activation determined by the concentrations of the ligands and the receptor properties. The general function of RM has also been demonstrated to depend on allosteric regulators (e.g., homocysteine) of the receptor subtypes present. RM as integrative nodes for receptor-receptor interactions in conjunction with membrane associated proteins may form horizontal molecular networks in the plasma membrane coordinating the activity of multiple effector systems modulating the excitability and gene expression of the cells. The key role of electrostatic epitope-epitope interactions will be discussed for the formation of the RM. These interactions probably represent a general molecular mechanism for receptor-receptor interactions and, without a doubt, indicate a role for phosphorylation-dephosphorylation events in these interactions. The novel therapeutic aspects given by the RMs will be discussed in the frame of molecular neurology and psychiatry and combined drug therapy appears as the future way to go.

Heptahelical terpsichory. Who calls the tune?

The discovery that arrestins can function as ligand-regulated signaling scaffolds has revealed a previously unappreciated level of complexity in G protein-coupled receptor (GPCR) signal transduction. Because arrestin-bound GPCRs are uncoupled from G proteins, arrestin binding can be viewed as switching receptors between two temporally and spatially distinct signaling modes. Recent work has established two factors that underscore this duality of GPCR signaling and suggest it may ultimately have therapeutic significance. The first is that signaling by receptor-arrestin "signalsomes" does not require heterotrimeric G protein activation. The second is that arrestin-dependent signals can be initiated by pathway-specific "biased agonists," creating the potential for drugs that selectively modulate different aspects of GPCR function. Currently, however, little is known about the physiological relevance of G protein-independent signals at the cellular or whole animal levels, and additional work is needed to determine whether arrestin pathway-selective drugs will find clinical application.

Binding of orthosteric ligands to the allosteric site of the M(2) muscarinic cholinergic receptor

The M(2) muscarinic receptor has two topographically distinct sites: the orthosteric site and an allosteric site recognized by compounds such as gallamine. It also can exhibit cooperative effects in the binding of orthosteric ligands, presumably to the orthosteric sites within an oligomer. Such effects would be difficult to interpret, however, if those ligands also bound to the allosteric site. Monomers of the hemagglutinin (HA)- and FLAG-tagged human M(2) receptor therefore have been purified from coinfected Sf9 cells and examined for any effect of the antagonist N-methyl scopolamine or the agonist oxotremorine-M on the rate at which N-[(3)H]methyl scopolamine dissociates from the orthosteric site (k(obsd)). The predominantly monomeric status was confirmed by coimmunoprecipitation and by cross-linking with bis(sulfosuccinimidyl)suberate. Both N-methyl scopolamine and oxotremorine-M acted in a cooperative manner to decrease k(obsd) by 4.5- and 9.1-fold, respectively; the corresponding estimates of affinity (log K(L)) are -2.55 +/- 0.13 and -2.29 +/- 0.14. Gallamine and the allosteric ligand obidoxime decreased k(obsd) by more than 100-fold (log K(L) = -4.12 +/- 0.04) and by only 1.1-fold (log K(L) = -1.73 +/- 0.91), respectively. Obidoxime reversed the effect of N-methyl scopolamine, oxotremorine-M, and gallamine in a manner that could be described by a model in which all four ligands compete for a common allosteric site. Ligands generally assumed to be exclusively orthosteric therefore can act at the allosteric site of the M(2) receptor, albeit at comparatively high concentrations.

Mechanism of dimerization of the human melanocortin 1 receptor

The melanocortin 1 receptor (MC1R) is a dimeric G protein-coupled receptor expressed in melanocytes, where it regulates the amount and type of melanins produced and determines the tanning response to ultraviolet radiation. We have studied the mechanisms of MC1R dimerization. Normal dimerization of a deleted mutant lacking the seventh transmembrane fragment and the C-terminal cytosolic extension excluded coiled-coil interactions as the basis of dimerization. Conversely, the electrophoretic pattern of wild type receptor and several Cys-->Ala mutants showed that four disulfide bonds are established between the monomers. Disruption of any of these bonds abolished MC1R function, but only the one involving Cys35 was essential for traffic to the plasma membrane. A quadruple Cys35-267-273-275Ala mutant migrating as a monomer in SDS-PAGE in the absence of reducing agents was able to dimerize with WT, suggesting that in addition to disulfide bond formation, dimerization involves non-covalent interactions, likely of domain swap type.

Night blindness-associated mutations in the ligand-binding, cysteine-rich, and intracellular domains of the metabotropic glutamate receptor 6 abolish protein trafficking

Mutations in the GRM6 gene, which encodes the metabotropic glutamate receptor 6 (mGluR6), lead to autosomal recessive congenital stationary night blindness (CSNB), which is characterized by loss of night vision due to a defect in signal transmission from photoreceptor to the adjacent ON-bipolar cells in the retina. So far, the sequence variations that have been described in six different families include nonsense, frameshift, and missense mutations. Here we investigated the impact of missense mutations in the ligand-binding domain, a conserved cysteine-rich domain, and the intracellular domain on the localization of the protein. We visualized and discriminated between surface and intracellular protein. Here we demonstrate that the wild-type (wt) protein localizes to the cell surface, and to endoplasmic reticulum (ER) and Golgi compartments. This also holds true for a mGluR6 variant containing a polymorphic, nondisease-associated amino acid exchange in the ligand-binding domain. In contrast, all disease-associated missense mutations lead to retention of the protein in the ER, while dimerization seems not to be affected. This is the first report that shows that CSNB-associated mutations in three different domains of mGluR6 abolish proper protein trafficking. We propose that the ligand-binding and the poorly characterized cysteine-rich domains, in addition to the intracellular domains, have a pivotal role in correct trafficking of metabotropic glutamate receptors to the cell surface.

On the role of receptor–receptor interactions and volume transmission in learning and memory

Learning and memory seem to be inherent to a biological neural network. To emerge, they need an extensive functional connectivity, enabling a large repertoire of possible responses to stimuli, and sensitivity of the connectivity to activity, allowing for the selection of adaptive responses. According to the classical view about the organization of the CNS, the connectivity issue is realized by the huge amount of synaptic contacts each neuron establishes, while the adaptation of the network to specific tasks is obtained by mechanisms of activity-dependent synaptic plasticity. The discovery of direct receptor-receptor interactions at the level of the plasma membrane and the existence in the brain of two main modes of communication, the wiring transmission (such as the synaptic transmission) and the volume transmission (based on the diffusion of signals in the extracellular space), provided a broader view of the functional organization of the CNS with potential important consequences on the understanding of learning and memory processes. Owing to receptor-receptor interactions clusters of receptors, the receptor mosaics (RM), can be formed at the plasma membrane where they can work as collective functional units. As a consequence, the connections between the cells become themselves networks (molecular networks) able to adapt their function according to the stimuli they receive. Learning, therefore, can occur also at the level of RMs. Thus, memory formation seems not only to be a distributed process, but also to follow a hierarchical morpho-functional organization. Furthermore, the combination of the two different forms of transmission could allow processes of correlation and coordination to be established between networks and network elements without the need of additional physical connections, leading to a significant increase of the degrees of freedom available to the CNS for learning.

Cholesterol as a determinant of cooperativity in the M2 muscarinic cholinergic receptor

M2 muscarinic receptor extracted from Sf9 cells in cholate-NaCl differs from that extracted from porcine sarcolemma. The latter has been shown to exhibit an anomalous pattern in which the capacity for N-[3H]methylscopolamine (NMS) is only 50% of that for [3H]quinuclidinylbenzilate (QNB), yet unlabeled NMS exhibits high affinity for all of the sites labeled by [3H]QNB. The effects can be explained in terms of cooperativity within a receptor that is at least tetravalent [Park PS, Sum CS, Pawagi AB, Wells JW. Cooperativity and oligomeric status of cardiac muscarinic cholinergic receptors. Biochemistry 2002;41:5588-604]. In contrast, M2 receptor extracted from Sf9 membranes exhibited no shortfall in the capacity for [3H]NMS at either 30 or 4 degrees C, although there was a time-dependent inactivation during incubation with [3H]NMS at 30 degrees C; also, any discrepancies in the affinity of NMS were comparatively small. The level of cholesterol in Sf9 membranes was only 4% of that in sarcolemmal membranes, and it was increased to about 100% by means of cholesterol-methyl-beta-cyclodextrin. M2 receptors extracted from treated Sf9 membranes were stable at 30 and 4 degrees C and resembled those from heart. Cholesterol induced a marked heterogeneity detected in the binding of both radioligands, including a shortfall in the apparent capacity for [3H]NMS, and there were significant discrepancies in the apparent affinity of NMS as estimated directly and via the inhibition of [3H]QNB. The data can be described quantitatively in terms of cooperative effects among six or more interacting sites. Cholesterol therefore appears to promote cooperativity in the binding of antagonists to the M2 muscarinic receptor.

A high-throughput method for detection of protein self-association and second virial coefficient using size-exclusion chromatography through simultaneous measurement of concentration and scattered light intensity

PURPOSE

To characterize protein self-association along with second virial coefficient (a measure of solution nonideality) using size-exclusion chromatography (SEC) utilizing a novel flow cell that is capable of simultaneously measuring protein concentration and scattered light intensity.

METHODS

beta-lactoglobulin A (beta Lg), known to exhibit NaCl-dependent monomer-dimer equilibrium at pH 3.0, was used as the model protein. A range of concentrations and corresponding scattered light intensities, obtained in the eluting peak from a single protein injection, in different solution conditions, were used to generate the Debye plots [Formula: see text]. The Debye light scattering equation was modified to include the monomer-dimer equilibrium model and the second virial coefficient to analyze the data obtained.

RESULTS

Debye plots of beta Lg, while linear at pH 2.3, 0 M NaCl (pure monomer) and at pH 3.0, 1 M NaCl (pure dimer), showed curvature at pH 3.0, for varying NaCl concentrations (0.02-0.5 M). The curvature was indicative of the association behavior of this protein. The modified Debye light scattering equation, when fit onto the nonlinear Debye plots, yielded apparent K (a) values ranging from 10(2) to 10(5) M(-1) under various solution conditions. The apparent K (a) values obtained from this method followed similar trend to those reported in literature.

CONCLUSIONS

SEC combined with simultaneous detection of scattered light intensity and concentration provides a rapid means of detection of protein self-association. The short duration of sample detection and analysis combined with SEC makes this method a useful tool for high-throughput characterization of protein association during early stages of protein formulation.

Recovery of oligomers and cooperativity when monomers of the M2 muscarinic cholinergic receptor are reconstituted into phospholipid vesicles

FLAG- and HA-tagged M2 muscarinic receptors from coinfected Sf9 cells have been purified in digitonin-cholate and reconstituted into phospholipid vesicles. The purified receptor was predominantly monomeric: it showed no detectable coimmunoprecipitation; it migrated as a monomer during electrophoresis before or after cross-linking with bis(sulfosuccinimidyl)suberate; and it bound agonists and antagonists in a manner indicative of identical and mutually independent sites. Receptor cross-linked after reconstitution or after reconstitution and subsequent solubilization in digitonin-cholate migrated almost exclusively as a tetramer. The binding properties of the reconstituted receptor mimicked those reported previously for cardiac muscarinic receptors. The apparent capacity for N-[3H]methylscopolamine (NMS) was only 60% of that for [3H]quinuclidinylbenzilate (QNB), yet binding at saturating concentrations of [3H]QNB was inhibited fully and in a noncompetitive manner at comparatively low concentrations of unlabeled NMS. Reconstitution of the receptor with a saturating quantity of functional G proteins led to the appearance of three classes of sites for the agonist oxotremorine-M in assays with [3H]QNB; GMP-PNP caused an apparent interconversion from highest to lowest affinity and the concomitant emergence of a fourth class of intermediate affinity. All of the data can be described quantitatively in terms of cooperativity among four interacting sites, presumably within a tetramer; the effect of GMP-PNP can be accommodated as a shift in the distribution of tetramers between two states that differ in their cooperative properties. Monomers of the M2 receptor therefore can be assembled into tetramers with binding properties that closely resemble those of the muscarinic receptor in myocardial preparations.

Dimerization of the lutropin receptor: insights from computational modeling

A computational approach based upon rigid-body docking, ad hoc filtering, and cluster analysis has been carried out to predict likely interfaces in LHR homodimers. Quaternary structure predictions emphasize the role of helices 4, 5 and 6, with prominence to helix 4, in mediating inter-monomer interactions. Intermolecular interactions essentially involve the transmembrane domains rather than the hydrophilic loops and do not implicate disulfide bridges.Collectively, molecular dynamics simulations on the isolated receptor and computational modeling of LHR homodimerization suggest that mutation-induced LHR activation favors H4-H4 contacts involving the highly conserved W491 from both the receptors monomers.

Inactive and active states and supramolecular organization of GPCRs: insights from computational modeling

Herein we make an overview of the results of our computational experiments aimed at gaining insight into the molecular mechanisms of GPCR functioning either in their normal conditions or when hit by gain-of-function or loss-of-function mutations. Molecular simulations of a number of GPCRs in their wild type and mutated as well as free and ligand-bound forms were instrumental in inferring the structural features, which differentiate the mutation- and ligand-induced active from the inactive states. These features essentially reside in the interaction pattern of the E/DRY arginine and in the degree of solvent exposure of selected cytosolic domains. Indeed, the active states differ from the inactive ones in the weakening of the interactions made by the highly conserved arginine and in the increase in solvent accessibility of the cytosolic interface between helices 3 and 6. Where possible, the structural hallmarks of the active and inactive receptor states are translated into molecular descriptors useful for in silico functional screening of novel receptor mutants or ligands. Computational modeling of the supramolecular organization of GPCRs and their intracellular partners is the current challenge toward a deep understanding of their functioning mechanisms.

A boolean network modelling of receptor mosaics relevance of topology and cooperativity

In the last five years data have been obtained showing that a functional cross-talk among G Protein Coupled receptors (GPCR) exists at the plasma membrane level where they can dimerise and are able to generate high order oligomers. These findings are in agreement with the receptor mosaic (RM) hypothesis that claims the existence of clusters of receptor proteins at the plasma membrane level, where they establish mutual interactions and work as 'intelligent interfaces' between the extra-cellular and the intra-cellular environments. Individual receptor dimers can be considered to have two stable conformational states with respect to the macromolecular effectors: one active, one inactive. Owing to receptor-receptor interactions, however, a state change of a given receptor will change the probability of changing the state for the adjacent receptors in the RM and the effect will propagate throughout the cluster, leading to a complex cooperative behaviour. In this study we explore the properties of a RM on the basis of an equivalence with a Boolean network, a mathematical framework able to describe how complex properties may emerge from systems characterized by deterministic local interactions of many simple components acting in parallel. Computer simulations of receptor clusters arranged according to topologies consistent with available experimental ultrastructural data were performed. They indicated that RMs after a stimulation can achieve a limited number of specific temporary equilibrium configurations (attractors), characterized by the presence of receptor units frozen in the active state. They could be interpreted as a form of information storage and a role of RM in learning and memory could be hypothesized. Moreover, they seem to be at the basis of very common 'macroscopical' properties of a receptor system, such as a sigmoidal response curve to an extracellular ligand, the sensitivity of the mosaic being modulated by changes in the topology and/or in the level of cooperativity among receptors.

Heterodimerization and surface localization of G protein coupled receptors

G protein coupled receptors (GPCRs) are one of the largest human gene families, and are targets for many important therapeutic drugs. Over the last few years, there has been a major paradigm shift in our understanding of how these receptors function. Formerly, GPCRs were thought to exist as monomers that, upon agonist occupation, activated a heterotrimeric G protein to alter the concentrations of specific second messengers. Until recently, this relatively linear cascade has been the standard paradigm for signaling by these molecules. However, it is now clear that this model is not adequate to explain many aspects of GPCR function. We now know that many, if not most, GPCRs form homo- and/or hetero-oligomeric complexes and interact directly with intracellular proteins in addition to G proteins. It now appears that many GPCRs may not function independently, but might more accurately be described as subunits of large multi-protein signaling complexes. These observations raise many important new questions; some of which include: (1) how many functionally and pharmacologically distinct receptor subtypes exist in vivo? (2) Which GPCRs physically associate, and in what stochiometries? (3) What are the roles of individual subunits in binding ligand and activating responses? (4) Are the pharmacological or signaling properties of GPCR heterodimers different from monomers? Since these receptors are the targets for a large number of clinically useful compounds, such information is likely to be of direct therapeutic importance, both in understanding how existing drugs work, but also in discovering novel compounds to treat disease.

Computational methods in drug design: modeling G protein-coupled receptor monomers, dimers, and oligomers

G protein-coupled receptors (GPCRs) are membrane proteins that serve as very important links through which cellular signal transduction mechanisms are activated. Many vital physiological events such as sensory perception, immune defense, cell communication, chemotaxis, and neurotransmission are mediated by GPCRs. Not surprisingly, GPCRs are major targets for drug development today. Most modeling studies in the GPCR field have focused upon the creation of a model of a single GPCR (ie, a GPCR monomer) based upon the crystal structure of the Class A GPCR, rhodopsin. However, the emerging concept of GPCR dimerization has challenged our notions of the monomeric GPCR as functional unit. Recent work has shown not only that many GPCRs exist as homo- and heterodimers but also that GPCR oligomeric assembly may have important functional roles. This review focuses first on methodology for the creation of monomeric GPCR models. Special emphasis is given to the identification of localized regions where the structure of a GPCR may diverge from that of bovine rhodopsin. The review then focuses on GPCR dimers and oligomers and the bioinformatics methods available for identifying homo- and heterodimer interfaces.

Molecular mechanism of 7TM receptor activation--a global toggle switch model

The multitude of chemically highly different agonists for 7TM receptors apparently do not share a common binding mode or active site but nevertheless act through induction of a common molecular activation mechanism. A global toggle switch model is proposed for this activation mechanism to reconcile the accumulated biophysical data supporting an outward rigid-body movement of the intracellular segments, as well as the recent data derived from activating metal ion sites and tethered ligands, which suggests an opposite, inward movement of the extracellular segments of the transmembrane helices. According to this model, a vertical see-saw movement of TM-VI-and to some degree TM-VII-around a pivot corresponding to the highly conserved prolines will occur during receptor activation, which may involve the outer segment of TM-V in an as yet unclear fashion. Small-molecule agonists can stabilize such a proposed active conformation, where the extracellular segments of TM-VI and -VII are bent inward toward TM-III, by acting as molecular glue deep in the main ligand-binding pocket between the helices, whereas larger agonists, peptides, and proteins can stabilize a similar active conformation by acting as Velcro at the extracellular ends of the helices and the connecting loops.

Melanocortin-1 receptor structure and functional regulation

The melanogenic actions of the melanocortins are mediated by the melanocortin-1 receptor (MC1R). MC1R is a member of the G-protein-coupled receptors (GPCR) superfamily expressed in cutaneous and hair follicle melanocytes. Activation of MC1R by adrenocorticotrophin or alpha-melanocyte stimulating hormone is positively coupled to the cAMP signaling pathway and leads to a stimulation of melanogenesis and a switch from the synthesis of pheomelanins to the production of eumelanic pigments. The functional behavior of the MC1R agrees with emerging concepts in GPCR signaling including dimerization, coupling to more than one signaling pathway and a high agonist-independent constitutive activity accounting for inverse agonism phenomena. In addition, MC1R displays unique properties such as an unusually high number of natural variants often associated with clearly visible phenotypes and the occurrence of endogenous peptide antagonists. Therefore MC1R is an ideal model to study GPCR function. Here we review our current knowledge of MC1R structure and function, with emphasis on information gathered from the analysis of natural variants. We also discuss recent data on the regulation of MC1R function by paracrine and endocrine factors and by external stimuli such as ultraviolet light.

Dimerization of the Human Melanocortin 1 Receptor: Functional Consequences and Dominant-Negative Effects

The melanocortin 1 receptor (MC1R), a G(S)-protein-coupled receptor (GPCR), is a key regulator of proliferation and differentiation of epidermal melanocytes, and a determinant of human skin phototype and cancer risk. Homodimerization has been demonstrated for several GPCRs, but little information is available for MC1R. SDS-PAGE analysis of melanoma cells and heterologous cells expressing epitope-tagged MC1R revealed dimeric and oligomeric species in detergent-solubilized extracts, confirmed by co-immunoprecipitation of differentially tagged MC1R forms. Dimerization occurs early during MC1R biosynthesis, and is seen for mutants displaying intracellular retention. These mutants exerted dominant-negative effects on wild-type (WT) MC1R. Conversely, partial functional trans-complementation of selected loss-of-function mutants was observed. WT-MC1R lacks cooperativity in agonist binding, yet coexpression of WT and a C-terminal deletion mutant yielded a form of different pharmacological properties. The natural diminished function alleles R151C, R160W, and D294H, associated with red hair, displayed dimerization and heterodimerization with WT. Coexpression of WT and R151C or R160W reduced the density of binding sites on the plasma membrane of transfected cells, whereas D294H mediated a dominant-negative effect on functional coupling to adenylyl cyclase. Therefore, subtle changes of functional properties may be associated with different MC1R haplotypes, contributing to the complexity of skin phenotype.

Design and synthesis of specific probes for human 5-HT4 receptor dimerization studies

Recently, human 5-HT4 receptors have been demonstrated to form constitutive dimers in living cells. To evaluate the role of dimerization on the 5-HT4 receptor function, we investigated the conception and the synthesis of bivalent molecules able to influence the dimerization process. Their conception is based on a model of the 5-HT4 receptor dimer derived from protein/protein docking experiments. These bivalent ligands are constituted by two ML10302 units, a specific 5-HT4 ligand, linked through a spacer of different sizes and natures. These synthesized bivalent ligands were evaluated in binding assays and cyclic AMP production on the 5-HT4(e/g) receptor isoform stably transfected in C6 glial cells. Our data showed that bivalent ligands conserved a similar affinity compared to the basal ML10302 unit. Nevertheless, according to the nature and the size of the spacer, the pharmacological profile of ML10302 is more or less conserved. In view of the interest of bivalent ligands for investigating the GPCR dimerization process, these 5-HT4 specific bivalent ligands constitute valuable pharmacological tools for the study of 5-HT4 receptor dimerization.

A constitutively active GPCR retains its G protein specificity and the ability to form dimers

G protein-coupled receptors (GPCRs) are cell surface proteins which help to regulate the physiology of all the major organ systems within higher eukaryotes. They are stimulated by multiple ligands and activate a range of effector molecules to bring about changes in cell behaviour. The use of constitutively active mutants (CAMs) of GPCRs has enabled a better understanding of receptor activation as CAMs exhibit ligand-independent signalling negating the use of ligands. Here we introduce the fission yeast Schizosaccharomyces pombe as a host for producing CAMs, by describing the isolation and characterization of constitutive mutants of the P-factor receptor (Mam2). One mutant Mam2[P261L] contained a single-amino-acid substitution (Pro261 to Leu) within a region of high homology in GPCRs. Substitution of this proline leads to an 18-fold increase in ligand-independent signalling. We utilized Mam2[P261L] to investigate CAM activity by demonstrating that Mam2[P261L] is efficiently trafficked to the cell surface where it can form fully functional oligomeric complexes with the native receptor. Mam2[P261L] also retains the G protein specificity (RG-profile) of the native receptor and only induces constitutive signalling in the same G proteins. Finally, evidence is provided to indicate that CAM activity results from a reduction in the kinetics of G protein binding. This is the first time that S. pombe has been utilized for isolating and characterizing CAMs and the techniques employed will complement the current systems available for studying these important receptors.

A serotonin receptor antagonist induces oocyte maturation in both frogs and mice: evidence that the same G protein-coupled receptor is responsible for maintaining meiosis arrest in both species

Accumulating evidence has indicated that vertebrate oocytes are arrested at late prophase (G2 arrest) by a G protein coupled receptor (GpCR) that activates adenylyl cyclases. However, the identity of this GpCR or its regulation in G2 oocytes is unknown. We demonstrated that ritanserin (RIT), a potent antagonist of serotonin receptors 5-HT2R and 5-HT7R, released G2 arrest in denuded frog oocytes, as well as in follicle-enclosed mouse oocytes. In contrast to RIT, several other serotonin receptor antagonists (mesulergine, methiothepine, and risperidone) had no effect on oocyte maturation. The unique ability of RIT, among serotonergic antagonists, to induce GVBD did not match the antagonist profile of any known serotonin receptors including Xenopus 5-HT7R, the only known G(s)-coupled serotonin receptor cloned so far in this species. Unexpectedly, injection of x5-HT7R mRNA in frog oocytes resulted in hormone-independent frog oocyte maturation. The addition of exogenous serotonin abolished x5-HT7R-induced oocyte maturation. Furthermore, the combination of x5-HT7R and exogenous serotonin potently inhibited progesterone-induced oocyte maturation. These results provide the first evidence that a G-protein coupled receptor related to 5-HT7R may play a pivotal role in maintaining G2 arrest in vertebrate oocytes.

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

BACKGROUND

We previously identified the G-protein-coupled receptor Mas, encoded by the Mas proto-oncogene, as an endogenous receptor for the heptapeptide angiotensin-(1-7); however, the receptor is also suggested to be involved in actions of angiotensin II. We therefore tested whether this could be mediated indirectly through an interaction with the angiotensin II type 1 receptor, AT1.

METHODS AND RESULTS

In transfected mammalian cells, Mas was not activated by angiotensin II; however, AT1 receptor-mediated, angiotensin II-induced production of inositol phosphates and mobilization of intracellular Ca2+ was diminished by 50% after coexpression of Mas, despite a concomitant increase in angiotensin II binding capacity. Mas and the AT1 receptor formed a constitutive hetero-oligomeric complex that was unaffected by the presence of agonists or antagonists of the 2 receptors. In vivo, Mas acts as an antagonist of the AT1 receptor; mice lacking the Mas gene show enhanced angiotensin II-mediated vasoconstriction in mesenteric microvessels.

CONCLUSIONS

These results demonstrate that Mas can hetero-oligomerize with the AT1 receptor and by so doing inhibit the actions of angiotensin II. This is a novel demonstration that a G-protein-coupled receptor acts as a physiological antagonist of a previously characterized receptor. Consequently, the AT1-Mas complex could be of great importance as a target for pharmacological intervention in cardiovascular diseases.

Novel clusters of receptors for sphingosine-1-phosphate, sphingosylphosphorylcholine, and (lyso)-phosphatidic acid: new receptors for "old" ligands

The (lyso)phospholipid mediators sphingosine-1-phosphate (S1P), lysophosphatidic acid (LPA), sphingosylphosphorylcholine (SPC), and phosphatidic acid (PA) regulate diverse cellular responses such as proliferation, survival and death, cytoskeletal rearrangements, cell motility, and differentiation among many others. Signaling is complex and many signaling events are mediated through the activation of cell surface seven transmembrane (7TM) G protein coupled receptors. Five high affinity receptors for S1P have been identified so far and named S1P(1, 2,3,4,5) (formerly referred to as endothelial differentiation gene (edg)1, 5, 3, 6, 8). Recently, the orphan receptor GPR63 was identified a low affinity S1P receptor structurally distant from the S1P(1-5) family. The orphan GPR3, 6, 12 cluster, phylogenetically related to the edg and melanocortin receptors appears to be subject to modulation by S1P and SPC although all three receptors are strong constitutive stimulators of the Galphas-adenylyl cyclase (AC) pathway and would not require additional ligand stimulation but rather inverse agonism to control activity. Ovarian cancer G protein coupled receptor 1 (OGR1) and GPR4, two structurally closely related receptors were assigned in functional and binding studies as high affinity molecular targets for SPC. Very recently, however, both OGR1 and GPR4 were described as receptors endowed with the ability to signal cells in response to protons. LPA exerts its biological effects through the activation of G protein coupled LPA(1-3) receptors (formerly referred to as edg2, 4, 7). A fourth high affinity LPA receptor has been identified: P2Y9 (GPR23) structurally related to nucleotide receptors and phylogenetically quite distant from the high affinity LPA(1-3) cluster. This review attempts to give an overview about the existing families of lysophosholipid receptors and the spectrum of lipid agonists they use as high or low affinity ligands to relay extracellular signals into intracellular responses. Recently deorphaned lipid receptors, within and outside the known lipid receptor clusters will receive particular attention.

Oligomerization of G protein-coupled receptors: past, present, and future

G protein-coupled receptor (GPCR)-mediated signal transduction has been studied for more than a century. Despite the intense focus on this class of proteins, a molecular understanding of what constitutes the functional form of the receptor is still uncertain. GPCRs have traditionally been conceptualized as monomeric proteins, and this view has changed little over the years until relatively recently. Recent biochemical and biophysical studies have challenged this traditional concept, and point instead to a mechanistic view of signal transduction wherein the receptor functions as an oligomer. Cooperative interactions within such an oligomeric array may be critical for the propagation of an external signal across the cell membrane and to the G protein, and may therefore underlie the mechanistic basis of signaling.

Applications of bioluminescence- and fluorescence resonance energy transfer to drug discovery at G protein-coupled receptors

Bioluminescence (BRET)- and fluorescence resonance energy transfer (FRET) techniques have become integral approaches in studies of protein-protein interactions in living cells. They rely on non-radiative transfer of energy between donor and acceptor species that can be appended to the proteins of interest. These techniques display exquisite dependence on distance and orientation between the energy transfer partners. This means they are well suited to measure both small conformational changes in response to ligand binding between partner proteins that remain within a complex or more extensive translocations of proteins between cellular compartments that occur in response to cellular challenge. Introduction of both energy donor and acceptor into a single polypeptide can also allow the detection of ligand-induced conformational switches in monomeric proteins in the millisecond time scale. Many of these approaches are amenable to high throughput screening and the drug discovery process. G protein-coupled receptors (GPCRs) represent a key drug target class. Specific applications of resonance energy transfer techniques to the identification of ligands for this class of protein are highlighted to illustrate general principles.

The Composition of the β-2 Adrenergic Receptor Oligomer Affects Its Membrane Trafficking after Ligand-Induced Endocytosis

The beta-2 adrenergic receptor (B2AR) is well known to form oligomeric complexes in vivo, but the functional significance of this process is not fully understood. The present results identify an effect of oligomerization of the human B2AR on the membrane trafficking of receptors after agonist-induced endocytosis in stably transfected human embryonic kidney 293 cells. A sequence present in the cytoplasmic tail of the B2AR has been shown previously to be required for efficient recycling of internalized receptors. Mutation of this sequence was observed to inhibit recycling not only of the receptor containing the mutation but also of the coexpressed wild-type B2AR. Coexpression of recycling-defective mutant B2ARs also enhanced proteolytic degradation of the wild-type B2AR after agonist-induced endocytosis, consistent with trafficking of both receptors to lysosomes in an oligomeric complex. Coexpression of the delta opioid receptor (DOR) at similar levels produced a much smaller effect on endocytic trafficking of the B2AR, even though DOR traverses a similar membrane pathway as recycling-defective mutant B2ARs. Biochemical studies confirmed that B2AR/B2AR-ala homomeric complexes form more readily than DOR/B2AR heteromers in expression-matched cell clones and support the hypothesis that B2AR/B2AR-ala complexes are not disrupted by agonist. These results suggest that a significant fraction of B2ARs exists in oligomeric complexes after ligand-induced endocytosis and that the composition of the oligomeric complex influences the sorting of endocytosed receptors between functionally distinct recycling and degradative membrane pathways.

Heterodimerization and cross-desensitization between the mu-opioid receptor and the chemokine CCR5 receptor

Cross-desensitization between micro-opioid receptor agonists and CC chemokines was shown to occur in immune cells and in the central nervous system. However, these cells do not permit examination of potential mechanisms at cellular levels due to low levels and mixed populations of receptors. In this study, we investigated possible interactions and biochemical mechanisms of cross-desensitization between the mu-opioid and chemokine CCR5 receptors coexpressed in Chinese hamster ovary (CHO) cells. Hemagglutinin (HA)-tagged micro-opioid receptor coimmunoprecipitated with FLAG (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys)-tagged chemokine receptor CCR5 in cells expressing the two receptors, but not in a mixture of cells transfected with one of the two receptors, indicating that the two receptors form heterodimers. Treatment with the mu-opioid receptor agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin), the chemokine RANTES (Regulated on Activation, Normal T cell-Expressed and -Secreted) (CCL5), or both, did not affect the level of coimmunoprecipitation. DAMGO and RANTES (CCL5) induced chemotaxis in CHO cells coexpressing both receptors, and preincubation with either DAMGO or RANTES (CCL5) profoundly inhibited chemotaxis caused by the other. DAMGO pretreatment enhanced phosphorylation of the chemokine CCR5 receptor and reduced RANTES (CCL5)-promoted [35S]GTP gamma S binding. Conversely, RANTES (CCL5) preincubation slightly increased phosphorylation of the mu-opioid receptor and significantly reduced DAMGO-induced [35S]GTP gamma S binding. These results indicate that activation of either receptor affected G protein coupling of the other, likely due to enhanced phosphorylation of the receptor. Heterodimerization between the two receptors may contribute to the observed cross-desensitization.

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

We have used an isolated receptor, the leukotriene B(4) receptor BLT1, to analyze the mechanism of receptor activation in a G-protein-coupled receptor dimer. The isolated receptor is essentially a dimer whether the agonist is present or not, provided the detergent used stabilizes the inactive dimeric assembly. We have produced a receptor mutant where Cys(97) in the third transmembrane domain has been replaced by a serine. This mutation leads to an approximately 100-fold decrease in the affinity for the agonist. 5-Hydroxytryptophan has then been introduced at position 234 in the C97A mutant sixth transmembrane domain. Agonist binding to the labeled receptor is associated with variations in the fluorescence properties of 5-hydroxytryptophan due to specific agonist-induced conformational changes. The C97A mutant labeled with 5-hydroxytryptophan has then been associated with a wild-type receptor in a dimeric complex that has been subsequently purified. The purified complex activates its G-protein partner in a similar manner as the wild-type homodimer. Due to the difference in the affinity for the agonist between the wild-type and mutant protomers in this dimer, we have been able to reach a state where one of the protomers, the mutant, is in its unliganded state, whereas the other, the wild type, is loaded with the agonist. We show that agonist binding to the wild-type receptor induces specific changes in the conformation of the unliganded protomer, as evidenced by the variations in the emission of the 5-hydroxytryptophan residue in the mutant receptor. These data provide a direct demonstration for agonist-induced cooperative conformational changes in a GPCR dimer.

High-affinity interactions between human alpha1A-adrenoceptor C-terminal splice variants produce homo- and heterodimers but do not generate the alpha1L-adrenoceptor

Using combinations of bioluminescence resonance energy transfer, time-resolved fluorescence resonance energy transfer and the functional complementation of pairs of inactive receptor-G protein fusion proteins, the human alpha(1A-1)-adrenoceptor was shown to form homodimeric/oligomeric complexes when expressed in human embryonic kidney (HEK) 293 cells. Saturation bioluminescence resonance energy transfer studies indicated the alpha(1A-1)-adrenoceptor homodimer interactions to be high affinity and some 75 times greater than interactions between the alpha(1A-1)-adrenoceptor and the delta opioid peptide receptor. Only a fraction of the alpha(1A-1)-adrenoceptors was at the plasma membrane of HEK293 cells at steady state. However, dimers of alpha(1A-1)-adrenoceptors were also present in intracellular membranes, and the dimer status of those delivered to the cell surface was unaffected by the presence of agonist. Splice variation can generate at least three forms of the human alpha(1A-1)-adrenoceptor with differences limited to the C-terminal tail. Each of the alpha(1A-1), alpha(1A-2a), and alpha(1A-3a)-adrenoceptor splice variants formed homodimers/oligomers, and all combinations of these splice variants were able to generate heterodimeric/oligomeric interactions. Despite the coexpression of these splice variants in human tissues that possess the pharmacologically defined alpha(1L)-adrenoceptor binding site, coexpression of any pair in HEK293 cells failed to generate ligand binding characteristic of the alpha(1L)-adrenoceptor.

Protease-Activated Receptors (PAR1 and PAR2) Contribute to Tumor Cell Motility and Metastasis

The effects of the pleiotropic serine protease thrombin on tumor cells are commonly thought to be mediated by the thrombin receptor protease-activated receptor 1 (PAR1). We demonstrate here that PAR1 activation has a role in experimental metastasis using the anti-PAR1 antibodies ATAP2 and WEDE15, which block PAR1 cleavage and activation. Thrombin also stimulates chemokinesis of human melanoma cells toward fibroblast conditioned media and soluble matrix proteins. Thrombin-enhanced migration is abolished by anti-PAR1 antibodies, demonstrating that PAR1 cleavage and activation are required. The PAR1-specific agonist peptide TFLLRNPNDK, however, does not stimulate migration, indicating that PAR1 activation is not sufficient. In contrast, a combination of TFLLRNPNDK and the PAR2 agonist peptide SLIGRL mimics the thrombin effect on migration, whereas PAR2 agonist alone has no effect. Agonist peptides for the thrombin receptors PAR3 and PAR4 used alone or with PAR1 agonist also have no effect. Similarly, activation of PAR1 and PAR2 also enhances chemokinesis of prostate cancer cells. Desensitization with PAR2 agonist abolishes thrombin-enhanced cell motility, demonstrating that thrombin acts through PAR2. PAR2 is cleaved by proteases with trypsin-like specificity but not by thrombin. Thrombin enhances migration in the presence of a cleavage-blocking anti-PAR2 antibody, suggesting that thrombin activates PAR2 indirectly and independent of receptor cleavage. Treatment of melanoma cells with trypsin or PAR2 agonist peptide enhances experimental metastasis. Together, these data confirm a role for PAR1 in migration and metastasis and demonstrate an unexpected role for PAR2 in thrombin-dependent tumor cell migration and in metastasis.

Hetero-oligomerization between β2- and β3-Adrenergic Receptors Generates a β-Adrenergic Signaling Unit with Distinct Functional Properties

The ability of the closely related beta(2)- and beta(3)-adrenergic receptors (AR) to form hetero-oligomers was assessed by bioluminescence resonance energy transfer. Quantitative bioluminescence resonance energy transfer titration curves revealed that the beta(2)AR has identical propensity to hetero-oligomerize with the beta(3)AR than to form homo-oligomers. To determine the influence of heterooligomerization, a HEK293 cell line stably expressing an excess of beta(3)AR over beta(2)AR was generated so that all beta(2)AR are engaged in hetero-oligomerization with beta(3)AR, providing a tool to study the effect of hetero-oligomerization on beta(2)AR function in the absence of any beta(2)AR homooligomer. The hetero-oligomerization had no effect on the ligand binding properties of various beta(2)AR ligands and did not affect the potency of isoproterenol to stimulate adenylyl cyclase. Despite the unaltered ligand binding properties of the beta(2/3)AR hetero-oligomer, the stable association of the beta(2)AR with the beta(3)AR completely blocked agonist-stimulated internalization of the beta(2)AR. Given that the beta(3)AR is resistant to agonist-promoted endocytosis, the results indicate that the beta(3)AR acted as a dominant negative of the beta(2)AR endocytosis process. Consistent with this notion, the beta(2/3)AR hetero-oligomer displayed a lower propensity to recruit beta-arrestin-2 than the beta(2)AR. The hetero-oligomerization also led to a change in G protein coupling selectivity. Indeed, in contrast to beta(2)AR and beta(3)AR, which regulate adenylyl cyclase and extracellular signal-regulated kinase activity through a coupling to G(s) and G(i/o), no G(i/o) coupling was observed for the beta(2/3)AR hetero-oligomer. Together, these results demonstrate that hetero-oligomerization between beta(2)AR and beta(3)AR forms a beta-adrenergic signaling unit that possesses unique functional properties.