The structure of α- and β-adrenergic receptors

G Protein–Coupled Receptor Oligomerization

The cardiovascular system is richly endowed with G protein–coupled receptors (GPCRs), members of the largest family of plasma membrane-localized receptors. During the last 10 years, it has become increasingly clear that many, if not all, GPCRs function in oligomeric complexes, as either homo- or hetero-oligomers. This review explores the mechanistic implications of GPCR dimerization and/or oligomerization on receptor activation and interactions with G proteins. The effects of GPCR oligomerization on receptor pharmacology, GPCR-mediated signaling, and potential contributions to GPCR crosstalk will be considered in the context of receptors important in the cardiovascular system. Our evolving understanding of the structural and functional consequences of GPCR oligomerization may provide novel and more selective sites for pharmacological tuning of cardiovascular function.

Heterodimerization of calcium sensing receptors with metabotropic glutamate receptors in neurons

Calcium sensing (CaR) and Group I metabotropic glutamate receptors exhibit overlapping expression patterns in brain, and share common signal transduction pathways. To determine whether CaR and Group I metabotropic glutamate receptors (mGluRs) (mGluR1alpha and mGluR5) can form heterodimers, we immunoprecipitated CaR from bovine brain and observed co-precipitation of mGluR1alpha. CaR and mGluR1alpha co-localize in hippocampal and cerebellar neurons, but are expressed separately in other brain regions. In vitro transfection studies in HEK-293 cells established the specificity and disulfide-linked nature of the CaR:mGluR1alpha (CaR:mGluR5) interactions. CaR:mGluR1alpha (CaR:mGluR5) heterodimers exhibit altered trafficking via Homer 1c when compared with CaR:CaR homodimers. CaR becomes sensitive to glutamate-mediated internalization when present in CaR:mGluR1alpha heterodimers. These results demonstrate cross-family covalent heterodimerization of CaR with Group I mGluRs, and increase the potential role(s) for CaR in modulating neuronal function.

Dimerization of the calcium-sensing receptor occurs within the extracellular domain and is eliminated by Cys --> Ser mutations at Cys101 and Cys236

Calcium-sensing receptors are present in membranes as dimers that can be reduced to monomers with sufhydryl reagents. All studies were carried out on the human calcium-sensing receptor tagged at the carboxyl terminus with green fluorescent protein (hCaR-GFP) to permit identification and localization of expressed proteins. Truncations containing either the extracellular agonist binding domain plus transmembrane helix 1 (ECD/TMH1-GFP) or the transmembrane domain plus the intracellular carboxyl terminus (TMD/carboxyl terminus-GFP) were used to identify the dimerization domain. ECD/TMH1-GFP was a dimer in the absence of reducing reagents, whereas TMD/carboxyl-terminal GFP was a monomer in the absence or presence of reducing agents, suggesting that dimerization occurs via the ECD. To identify the residue(s) involved in dimerization within the ECD, cysteine --> serine point mutations were made in residues that are conserved between hCaR and metabotropic glutamate receptors. Mutations at positions 60 and 131 were expressed at levels comparable to wild type in HEK 293 cells, had minimal effects on hCaR function, and did not eliminate dimerization, whereas mutations at positions 101 and 236 greatly decreased receptor expression and resulted in significant amounts of monomer in the absence of reducing agents. The double point mutant hCaR(C101S/C236S)-GFP was expressed more robustly than either C101S or C236S and covalent dimerization was eliminated. hCaR(C101S/C236S)-GFP had a decreased affinity for extracellular Ca2+ and slower response kinetics upon increases or decreases in agonist concentration. These results suggest that covalent, disulfide bond-mediated dimerization of the calcium-sensing receptor contributes to stabilization of the ECD and to acceleration of the transitions between inactive and active receptor conformations.

Synthetic rat V1a vasopressin receptor fragments interfere with vasopressin binding via specific interaction with the receptor

To study the vasopressin receptor domains involved in the hormonal binding, we synthesized natural and modified fragments of V1a vasopressin receptor and tested their abilities to affect hormone-receptor interactions. Natural fragments mimicking the external loops one, two, and three were able to inhibit specific vasopressin binding to V1a receptor. In contrast, the natural N-terminal part of the V1a vasopressin receptor was found inactive. One fragment, derived from the external second loop and containing an additional C-terminal cysteine amide, was able to fully inhibit the specific binding of both labeled vasopressin agonist and antagonist to rat liver V1a vasopressin receptor and the vasopressin-sensitive phospholipase C of WRK1 cells. The peptide-mediated inhibition involved specific interactions between the V1a receptor and synthetic V1a vasopressin receptor fragment since 1) it was dependent upon the vasopressin receptor subtype tested (Ki(app) for the peptide: 3.7, 14.6, and 64.5 microM for displacing [3H]vasopressin from rat V1a, V1b, and V2 receptors, respectively; 2) it was specific and did not affect sarcosin 1-angiotensin II binding to rat liver membranes; 3) it was not mimicked by vasopressin receptor unrelated peptides exhibiting putative detergent properties; and 4) no direct interaction between [3H]vasopressin and synthetic peptide linked to an affinity chromatography column could be observed. Such an inhibition affected both the maximal binding capacity of the V1a vasopressin receptor and its affinity for the labeled hormone, depending upon the dose of synthetic peptide used and was partially irreversible. Structure-activity studies using a serie of synthetic fragments revealed the importance of their size and cysteinyl composition. These data indicate that some peptides mimicking extracellular loops of the V1a vasopressin receptor may interact with the vasopressin receptor itself and modify its coupling with phospholipase C.

A peptide derived from a beta2-adrenergic receptor transmembrane domain inhibits both receptor dimerization and activation

One of the assumptions of the mobile receptor hypothesis as it relates to G protein-coupled receptors is that the stoichiometry of receptor, G protein, and effector is 1:1:1 (Bourne, H. R., Sanders, D. A., and McCormick, F.(1990) Nature 348, 125-132). Many studies on the cooperativity of agonist binding are incompatible with this notion and have suggested that both G proteins and their associated receptors can be oligomeric. However, a clear physical demonstration that G protein-coupled receptors can indeed interact as dimers and that such interactions may have functional consequences was lacking. Here, using differential epitope tagging we demonstrate that beta2-adrenergic receptors do form SDS-resistant homodimers and that transmembrane domain VI of the receptor may represent part of an interface for receptor dimerization. The functional importance of dimerization is supported by the observation that a peptide derived from this domain that inhibits dimerization also inhibits beta-adrenergic agonist-promoted stimulation of adenylyl cyclase activity. Moreover, agonist stimulation was found to stabilize the dimeric state of the receptor, while inverse agonists favored the monomeric species, which suggests that interconversion between monomeric and dimeric forms may be important for biological activity.

Cooperativity manifest in the binding properties of purified cardiac muscarinic receptors

Muscarinic receptors were solubilized from porcine atria in digitonin-cholate and were purified by chromatography on DEAE-Sepharose and 3-(2'-aminobenzhydryloxy)tropane-Sepharose. The product identified on Western blots migrated with an apparent molecular mass of 60-75 kDa, with additional bands indicative of homotrimers (190 kDa) and homotetramers (240 kDa). Receptor eluted from the affinity column was accompanied by a mixture of guanyl nucleotide-binding proteins (G-proteins) identified on Western blots as Gi1/2, G(o), Gq/11, and Gs (preparation M2G); the G-proteins were largely removed by further processing on hydroxyapatite (preparation M2). Solubilized purified receptors bound muscarinic ligands in an apparently cooperative manner. In studies at equilibrium, the antagonists [3H]AF-DX 384, N-[3H]methylscopolamine (NMS), and [3H]quinuclidinylbenzilate (QNB) revealed Hill coefficients between about 0.8 and 1.2. Also, the apparent capacity for [3H]QNB exceeded that for [3H]AF-DX 384 and [3H]NMS by about 1.5-fold in M2 and by 2-fold in M2G. Binding to M2G at high concentrations of [3H]QNB was fully inhibited by unlabeled NMS, which therefore affected sites not labeled at similar concentrations of [3H]NMS. Oxotremorine-M displayed a biphasic inhibitory effect on the binding of [3H]AF-DX 384 in M2 and M2G, suggesting that multiple states of affinity are intrinsic to the receptor; 5'-guanylylimidodiphosphate was without appreciable effect in M2 but resulted in a bell-shaped binding profile for the agonist in M2G. All of the data can be described in terms of cooperative interactions within a receptor that is at least tetravalent and presumably an oligomer. In the context of the model, copurifying G-proteins and guanyl nucleotides serve to regulate the degree of cooperativity between successive equivalents of muscarinic ligands.

Effects of adenyl nucleotides and carbachol on cooperative interactions among G proteins

Muscarinic agonists and adenyl nucleotides are noncompetitive modulators of sites labeled by [35S]GTP gamma S in washed cardiac membranes from Syrian golden hamsters. Specific binding of the radioligand and its inhibition by either GTP gamma S or GDP reveals three states of affinity for guanyl nucleotides. In the absence of adenyl nucleotide, carbachol promotes an apparent interconversion of sites from higher to lower affinity for GDP; the effect recalls that of guanyl nucleotides on the binding of agonists to muscarinic receptors. In the presence of 0.1 mM ATP gamma S, the binding of [35S]GTP gamma S is increased at concentrations up to about 50 nM and decreased at higher concentrations. At a radioligand concentration of 160 pM, binding exhibits a bell-shaped dependence on the concentration of both ATP gamma S and AMP-PNP; with ADP and ATP, there is a second increase in bound [35S]GTP gamma S at the highest concentrations of adenyl nucleotide. ATP gamma S and AMP-PNP also modulate the effect of GDP, which itself emerges as a cooperative process: that is, binding of the radioligand in the presence of AMP-PNP exhibits a bell-shaped dependence on the concentration of GDP; moreover, the GDP-dependent increase in bound [35S]GTP gamma S is enhanced by carbachol. The interactions among GDP, GTP gamma S, and carbachol can be rationalized quantitatively in terms of a cooperative model involving two sites tentatively identified as G proteins. Both GTP gamma S and GDP exhibit negative homotropic cooperativity; carbachol enhances the homotropic cooperativity of GDP and induces or enhances positive heterotropic cooperativity between GDP and [35S]GTP gamma S. An analogous mechanism may underlie the guanyl nucleotide-dependent binding of agonists to muscarinic receptors. The data suggest that the binding properties of G proteins and their associated receptors reflect cooperative effects within heterooligomeric arrays; agonist-induced changes in cooperativity may facilitate the exchange of GTP for bound GDP and thereby constitute the mechanism of G protein activation in vivo.

Molecular receptors and their potential for artificial transduction

The thesis is presented that molecular receptor physical chemistry offers an interesting model for the design of biosensors with respect to chemical recognition and transduction. In order to appreciate the desirable features of this system and the inherent difficulties, the structures and binding state energetics of molecular receptors are considered via a direct comparison with enzyme chemistry. Detailed analyses of the candidate species nicotinic and beta-adrenergic receptors are provided to illustrate the complexity of molecular receptor binding properties. A revised ternary-complex model, which combines enzymatic and receptor energetics, is proposed to explain the free-energy changes which drive ligand-binding reactions of coupled systems. Throughout this discussion the relevance of the various arguments to applications in biosensor development is highlighted. Finally, a brief appraisal of attempts to produce devices ready for marriage to molecular receptor material is presented.

Effects of deoxyribonuclease I and neuraminidase treatments on the specific binding of 3H-prazosin and 3H-quinuclidinyl benzilate (3H-QNB) to alpha-adrenergic and muscarinic receptors in rat myocardial membranes

The effects of neuraminidase and deoxyribonuclease I (DNase) treatments on the specific binding of 3H-prazosin and 3H-quinuclidinyl benzilate (3H-QNB) to alpha-adrenoceptors and muscarinic cholinergic receptors in the membrane of the rat myocardium was examined, and the dissociation constant (Kd) and the maximum number of binding sites (Bmax) was analyzed using the method of Scatchard analysis. Although no changes in Kd values were observed when DNA or sialic acid was removed from cardiac muscles by treatments with DNase or neuraminidase, the Bmax values of alpha-adrenergic and muscarinic receptors were markedly decreased after treatment with DNase, while neuraminidase treatment induced an increase in the Bmax values of the alpha-adrenoceptors. The possibility that these results provide the basis for elucidation of the characteristics of these receptors in the rat myocardium was discussed.