High Efficacy but Low Potency of δ-Opioid Receptor-G Protein Coupling in Brij-58-Treated, Low-Density Plasma Membrane Fragments

Principal Findings

HEK293 cells stably expressing PTX-insensitive δ-opioid receptor-Gi1α (C³⁵¹I) fusion protein were homogenized, treated with low concentrations of non-ionic detergent Brij-58 at 0°C and fractionated by flotation in sucrose density gradient. In optimum range of detergent concentrations (0.025–0.05% w/v), Brij-58-treated, low-density membranes exhibited 2-3-fold higher efficacy of DADLE-stimulated, high-affinity [³²P]GTPase and [³⁵S]GTPγS binding than membranes of the same density prepared in the absence of detergent. The potency of agonist DADLE response was significantly decreased. At high detergent concentrations (>0.1%), the functional coupling between δ-opioid receptors and G proteins was completely diminished. The same detergent effects were measured in plasma membranes isolated from PTX-treated cells. Therefore, the effect of Brij-58 on δ-opioid receptor-G protein coupling was not restricted to the covalently bound G_i1α within δ-opioid receptor-G_i1α fusion protein, but it was also valid for PTX-sensitive G proteins of G_i/G_o family endogenously expressed in HEK293 cells. Characterization of the direct effect of Brij-58 on the hydrophobic interior of isolated plasma membranes by steady-state anisotropy of diphenylhexatriene (DPH) fluorescence indicated a marked increase of membrane fluidity. The time-resolved analysis of decay of DPH fluorescence by the “wobble in cone” model of DPH motion in the membrane indicated that the exposure to the increasing concentrations of Brij-58 led to a decreased order and higher motional freedom of the dye.

Summary

Limited perturbation of plasma membrane integrity by low concentrations of non-ionic detergent Brij-58 results in alteration of δ-OR-G protein coupling. Maximum G protein-response to agonist stimulation (efficacy) is increased; affinity of response (potency) is decreased. The total degradation plasma membrane structure at high detergent concentrations results in diminution of functional coupling between δ-opioid receptors and G proteins.

Recent advances in drug action and therapeutics: relevance of novel concepts in G-protein-coupled receptor and signal transduction pharmacology

PROBLEM STATEMENT

During especially the past two decades many discoveries in biological sciences, and in particular at the molecular and genetic level, have greatly impacted on our knowledge and understanding of drug action and have helped to develop new drugs and therapeutic strategies. Furthermore, many exciting new drugs acting via novel pharmacological mechanisms are expected to be in clinical use in the not too distant future.

SCOPE AND CONTENTS OF REVIEW

In this educational review, these concepts are explained and their relevance illustrated by examples of drugs used commonly in the clinical setting, with special reference to the pharmacology of G-protein-coupled receptors. The review also addresses the basic theoretical concepts of full and partial agonism, neutral antagonism, inverse agonism and protean and ligand-selective agonism, and the relevance of these concepts in current rational drug therapy. Moreover, the mechanisms whereby receptor signalling (and eventually response to drugs) is fine-tuned, such as receptor promiscuity, agonist-directed trafficking of receptor signalling, receptor trafficking, receptor 'cross-talk' and regulators of G-protein signalling (RGSs) are discussed, from theory to proposed therapeutic implications.

CONCLUSIONS

It is concluded that the understanding of molecular receptor and signal transduction pharmacology enables clinicians to improve their effective implementation of current and future pharmacotherapy, ultimately enhancing the quality of life of their patients.

A highly effective dominant negative alpha s construct containing mutations that affect distinct functions inhibits multiple Gs-coupled receptor signaling pathways

To investigate the subcellular organization of receptor-G protein signaling pathways, a robust dominant negative alpha(s) mutant containing substitutions that alter distinct functions was produced and tested for its effects on G(s)-coupled receptor activity in HEK-293 cells. Mutations in the alpha3beta5 loop region, which increase receptor affinity, decrease receptor-mediated activation, and impair activation of adenylyl cyclase, were combined with G226A, which increases affinity for betagamma, and A366S, which decreases affinity for GDP. This triple alpha(s) mutant can inhibit signaling to G(s) from the luteinizing hormone receptor by 97% and from the calcitonin receptor by 100%. In addition, this alpha(s) mutant blocks all signaling from the calcitonin receptor to G(q). These results lead to two conclusions about receptor-G protein signaling. First, individual receptors have access to multiple types of G proteins in HEK-293 cell membranes. Second, different G protein alpha subunits can compete with each other for binding to the same receptor. This dominant negative alpha(s) construct will be useful for determining interrelationships among distinct receptor-G protein interactions in a wide variety of cells and tissues.

Relationship between rate and extent of G protein activation: comparison between full and partial opioid agonists

Opioid agonists acting at their receptors alter intracellular events by initiating activation of various types of Gi/Go proteins. This can be measured by the binding of the stable GTP analog [(35)S]guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS). In this study agonist efficacy is defined by the degree to which an opioid stimulates the binding of [(35)S]GTPgammaS. This allows for a definition of full and partial agonists; a full agonist causing a greater stimulation of [(35)S]GTPgammaS binding than a partial agonist. The hypothesis that the rate of agonist-stimulated [(35)S]GTPgammaS binding is dependent upon agonist efficacy was tested using membranes from C6 glioma cells expressing mu- or delta-opioid receptors. At maximal concentrations the rate of agonist-stimulated [(35)S]GTPgammaS binding followed the efficacy of mu-agonists in stimulating [(35)S]GTPgammaS binding, i.e., [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin > morphine > meperidine > butorphanol > nalbuphine. At submaximal concentrations of mu- or delta-full agonists the [(35)S]GTPgammaS association rate was also reduced, such that the rate of [(35)S]GTPgammaS binding correlated with the extent of [(35)S]GTPgammaS bound, whether this binding was stimulated by a full agonist or a partial agonist. Agonists also stimulated [(35)S]GTPgammaS dissociation, showing that binding of this stable nucleotide was reversible. Comparison of the delta-agonists [D-Ser(2),Leu(5)]-enkephalin-Thr and (+/-)-4-((alpha-R*)-alpha-((2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxylbenzyl)-N,N-diethylbenzamide, a compound with slow dissociation kinetics, showed the measured rate of G protein activation was not influenced by the agonist switching between receptors. The results are consistent with the idea that the active state(s) of the receptor induced by full or partial agonists is the same, but the number of activated receptors determines the rate of G protein activation.

G(q/11) and G(i/o) activation profiles in CHO cells expressing human muscarinic acetylcholine receptors: dependence on agonist as well as receptor-subtype

1. Profiles of G protein activation have been assessed using a [35S]-GTPgammaS binding/immunoprecipitation strategy in Chinese hamster ovary cells expressing either M1, M2, M3 or M4 muscarinic acetylcholine (mACh) receptor subtypes, where expression levels of M1 and M3, or M2 and M4 receptors were approximately equal. 2. Maximal [35S]-GTPgammaS binding to G(q/11)alpha stimulated by M1/M3 receptors, or G(i1-3)alpha stimulated by M2/M4 receptors occurred within approximately 2 min of agonist addition. The increases in G(q/11)alpha-[35S]-GTPgammaS binding after M1 and M3 receptor stimulation differed substantially, with M1 receptors causing a 2-3 fold greater increase in [35S]-GTPgammaS binding and requiring 5 fold lower concentrations of methacholine to stimulate a half-maximal response. 3. Comparison of M2 and M4 receptor-mediated G(i1-3)alpha-[35S]-GTPgammaS binding also revealed differences, with M2 receptors causing a greater increase in G(i1-3)alpha activation and requiring 10 fold lower concentrations of methacholine to stimulate a half-maximal response. 4. Comparison of methacholine- and pilocarpine-mediated effects revealed that the latter partial agonist is more effective in activating G(i3)alpha compared to G(i1/2)alpha for both M2 and M4 receptors. More marked agonist/partial agonist differences were observed with respect to M1/M3-mediated stimulations of G(q/11)alpha- and G(i1-3)alpha-[35S]-GTPgammaS binding. Whereas coupling to these Galpha subclasses decreased proportionately for M1 receptor stimulation by these agonists, pilocarpine possesses a greater intrinsic activity at M3 receptors for G(i)alpha versus G(q/11)alpha activation. 5. These data demonstrate that mACh receptor subtype and the nature of the agonist used govern the repertoire of G proteins activated. They also provide insights into how the diversity of coupling can be pharmacologically exploited, and provide a basis for a better understanding of how multiple receptor subtypes can be differentially regulated.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

The C terminus of the Saccharomyces cerevisiae alpha-factor receptor contributes to the formation of preactivation complexes with its cognate G protein

Binding of the alpha-factor pheromone to its G-protein-coupled receptor (encoded by STE2) activates the mating pathway in MATa yeast cells. To investigate whether specific interactions between the receptor and the G protein occur prior to ligand binding, we analyzed dominant-negative mutant receptors that compete with wild-type receptors for G proteins, and we analyzed the ability of receptors to suppress the constitutive signaling activity of mutant Galpha subunits in an alpha-factor-independent manner. Although the amino acid substitution L236H in the third intracellular loop of the receptor impairs G-protein activation, this substitution had no influence on the ability of the dominant-negative receptors to sequester G proteins or on the ability of receptors to suppress the GPA1-A345T mutant Galpha subunit. In contrast, removal of the cytoplasmic C-terminal domain of the receptor eliminated both of these activities even though the C-terminal domain is unnecessary for G-protein activation. Moreover, the alpha-factor-independent signaling activity of ste2-P258L mutant receptors was inhibited by the coexpression of wild-type receptors but not by coexpression of truncated receptors lacking the C-terminal domain. Deletion analysis suggested that the distal half of the C-terminal domain is critical for sequestration of G proteins. The C-terminal domain was also found to influence the affinity of the receptor for alpha-factor in cells lacking G proteins. These results suggest that the C-terminal cytoplasmic domain of the alpha-factor receptor, in addition to its role in receptor downregulation, promotes the formation of receptor-G-protein preactivation complexes.

Activation of G protein by opioid receptors: role of receptor number and G-protein concentration

The collision-coupling model for receptor-G-protein interaction predicts that the rate of G-protein activation is dependent on receptor density, but not G-protein levels. C6 cells expressing mu- or delta-opioid receptors, or SH-SY5Y cells, were treated with beta-funaltrexamine (mu) or naltrindole-5'-isothiocyanate (delta) to decrease receptor number. The time course of full or partial agonist-stimulated ¿35SGTPgammaS binding did not vary in C6 cell membranes containing <1-25 pmol/mg mu-opioid receptor, or 1. 4-4.3 pmol/mg delta-opioid receptor, or in SHSY5Y cells containing 0. 16-0.39 pmol/mg receptor. The association of ¿35SGTPgammaS binding was faster in membranes from C6mu cells than from C6delta cells. A 10-fold reduction in functional G-protein, following pertussis toxin treatment, lowered the maximal level of ¿35SGTPgammaS binding but not the association rate. These data indicate a compartmentalization of opioid receptors and G protein at the cell membrane.

G(i) activator region of alpha(2A)-adrenergic receptors: distinct basic residues mediate G(i) versus G(s) activation

The structural determinants of G protein coupling versus activation by G protein-coupled receptors are not well understood. We examine the role of two distinct basic regions in the carboxyl terminal portion of the third intracellular loop of the alpha(2A)-adrenergic receptor to dissect these aspects of function. Changing three arginines to alanines by mutagenesis and stable expression in Chinese hamster ovary-K1 cells impaired the alpha(2)-adrenergic receptor G(s)-mediated stimulation of cyclic AMP (cAMP) accumulation, whereas G(i)-mediated inhibition was normal. When two (B2) or three (B3) basic residues closer to transmembrane span 6 were mutated to alanine, normal ligand binding was observed, but G(i)-mediated inhibition of cAMP accumulation showed 20-fold and 50-fold decreases in agonist potency for the B2 and B3 mutants, respectively. Surprisingly, a normal G(s) response was seen for the B2 mutant, and the B3 mutant showed only a 6-fold decrease in agonist potency. Mutation of both the three alanines and B3 residues to alanines showed a 200-fold decrease in agonist potency for G(i)-mediated inhibition of cAMP accumulation, whereas the G(s) response was nearly completely eliminated. The three basic residues (which include the BB of the BBXXF motif) play a role as G(i) activators rather than in receptor-G protein coupling, because high-affinity agonist binding is intact. Thus, we have identified three basic residues required for activation of G(i) but not required for receptor-G protein coupling. Also, distinct basic residues are required for optimal G(i) and G(s) responses, defining a microspecificity determinant within the carboxyl terminal portion of the third intracellular loop of the alpha(2a) adrenergic receptor.

Effects of reciprocal chimeras between the C-terminal portion of third intracellular loops of the human dopamine D2 and D3 receptors

The dopamine D3 receptor is a member of the G protein-coupled superfamily of receptors. However, its coupling with intracellular events is still not well understood. We have performed chimera constructions in which amino acid residues located in a region of the receptor involved in the coupling with second messengers (the C-terminal portion of the third intracellular loop) have been exchanged between dopamine D2 and D3 receptors. Chimera constructions did not modify substantially the pharmacological profiles, nor G protein coupling, as compared to their respective wild-type receptors. However, the D2 receptor chimera, containing the C-terminal portion of the third intracellular loop of the D3 receptor, has a lower potency to inhibit cyclic AMP production. The reciprocal construction generated a D3 receptor that is fully coupled to this second messenger pathway whereas, the native D3 receptor is uncoupled to this pathway in our transfected cells. These results suggest that the sequence selected is important for specific coupling characteristics shown by these two dopamine receptor homologues.