Allosteric modulation of binding properties between units of chemokine receptor homo- and hetero-oligomers

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

The belief that G protein-coupled receptors exist and function as monomeric, non-interacting species has been largely supplanted in recent years by evidence, derived from a range of approaches, that indicate they can form dimers and/or higher-order oligomeric complexes. Key roles for receptor homo-dimerisation include effective quality control of protein folding prior to plasma membrane delivery and interactions with hetero-trimeric G proteins. Growing evidence has also indicated the potential for many co-expressed G protein-coupled receptors to form hetero-dimers/oligomers. The relevance of this to physiology and function is only beginning to be unravelled but may offer great potential for more selective therapeutic intervention.

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

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

Direct Assessment of CXCR4 Mutant Conformations Reveals Complex Link between Receptor Structure and Gαi Activation

Ligand binding to G protein-coupled receptors (GPCRs) is thought to induce changes in receptor conformation that translate into activation of downstream effectors. The link between receptor conformation and activity is still insufficiently understood, as current models of GPCR activation fail to take an increasing amount of experimental data into account. To elucidate structure-function relationships in GPCR activation, we used bioluminescence resonance energy transfer to directly assess the conformation of mutants of the chemokine receptor CXCR4. We analyzed substitutions in the arginine cage DRY motif and in the conserved asparagine N(3.35)119, which are pivotal molecular switches for receptor conformation and activation. G(alpha)(i) activation of the mutants was either similar to wild-type CXCR4 (D133N, Y135A, and N119D) or resulted in loss of activity (R134A and N119K). Mutant N119S was constitutively active but further activated by agonist. Bioluminescence resonance energy transfer analysis suggested no simple correlation between conformational changes in response to ligand binding and activation of G(alpha)(i) by the mutants. Different conformations of active receptors were detected (for wild-type CXCR4, D133N, and N119S), suggesting that different receptor conformations are able to trigger G(alpha)(i) activity. Several conformations were also found for inactive mutants. These data provide biophysical evidence for different receptor conformations being active with respect to a single readout. They support models of GPCR structure-activity relationships that take this conformational flexibility of active receptors into account.

Allosteric Modulation of G Protein–Coupled Receptors

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

Deorphanization of G-Protein-Coupled Receptors

G-protein-coupled receptors constitute one of the major families of drug targets. Orphan receptors, for which the ligands and function are still unknown, are an attractive set of future targets for presently unmet medical needs. Screening strategies have been developed over the years in order to identify the natural ligands of these receptors. Natural or chimeric G-proteins that can redirect the natural coupling of receptors toward intracellular calcium release are frequently used. Potential problems include poor expression or trafficking to the cell surface, constitutive activity of the receptors, or the presence of endogenous receptors in the cell types used for functional expression, leading to nonspecific responses. Many orphan receptors characterized over the last 10 years have been associated with previously known bioactive molecules. However, new and unpredicted biological mediators have also been purified from complex biological sources. A few old and recent examples, including nociceptin, chemerin, and the F2L peptide are illustrated. Future challenges for the functional characterization of the remaining orphan receptors include the potential requirement of specific proteins necessary for quality control, trafficking or coupling of specific receptors, the possible formation of obligate heterodimers, and the possibility that some constitutively active receptors may lack ligands or respond only to inverse agonists. Adapted expression and screening strategies will be needed to deal with these issues.

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

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

Orexin-1 receptor-cannabinoid CB1 receptor heterodimerization results in both ligand-dependent and -independent coordinated alterations of receptor localization and function

Following inducible expression in HEK293 cells, the human orexin-1 receptor was targeted to the cell surface but became internalized following exposure to the peptide agonist orexin A. By contrast, constitutive expression of the human cannabinoid CB1 receptor resulted in a predominantly punctate, intracellular distribution pattern consistent with spontaneous, agonist-independent internalization. Expression of the orexin-1 receptor in the presence of the CB1 receptor resulted in both receptors displaying the spontaneous internalization phenotype. Single cell fluorescence resonance energy transfer imaging indicated the two receptors were present as heterodimers/oligomers in intracellular vesicles. Addition of the CB1 receptor antagonist SR-141716A to cells expressing only the CB1 receptor resulted in re-localization of the receptor to the cell surface. Although SR-141716A has no significant affinity for the orexin-1 receptor, in cells co-expressing the CB1 receptor, the orexin-1 receptor was also re-localized to the cell surface by treatment with SR-141716A. Treatment of cells co-expressing the orexin-1 and CB1 receptors with the orexin-1 receptor antagonist SB-674042 also resulted in re-localization of both receptors to the cell surface. Treatment with SR-141716A resulted in decreased potency of orexin A to activate the mitogen-activated protein kinases ERK1/2 only in cells co-expressing the two receptors. Treatment with SB-674042 also reduced the potency of a CB1 receptor agonist to phosphorylate ERK1/2 only when the two receptors were co-expressed. These studies introduce an entirely novel pharmacological paradigm, whereby ligands modulate the function of receptors for which they have no significant inherent affinity by acting as regulators of receptor heterodimers.

Human T-cell leukemia virus type-1 Tax oncoprotein regulates G-protein signaling

Human T-cell leukemia virus type-1 (HTLV-1) is associated with adult T-cell leukemia (ATL) and neurological syndromes. HTLV-1 encodes the oncoprotein Tax-1, which modulates viral and cellular gene expression leading to T-cell transformation. Guanine nucleotide-binding proteins (G proteins) and G protein-coupled receptors (GPCRs) constitute the largest family of membrane proteins known and are involved in the regulation of most biological functions. Here, we report an interaction between HTLV-1 Tax oncoprotein and the G-protein beta subunit. Interestingly, though the G-protein beta subunit inhibits Tax-mediated viral transcription, Tax-1 perturbs G-protein beta subcellular localization. Functional evidence for these observations was obtained using conditional Tax-1-expressing transformed T-lymphocytes, where Tax expression correlated with activation of the SDF-1/CXCR4 axis. Our data indicated that HTLV-1 developed a strategy based on the activation of the SDF-1/CXCR4 axis in the infected cell; this could have tremendous implications for new therapeutic strategies.

Multiple drugbinding sites on the R482G isoform of the ABCG2 transporter

BACKGROUND & PURPOSE

Drug-resistant cancer cells frequently display efflux pumps such as P-glycoprotein (P-gp), the multidrug resistance associated protein (MRP1) or the transporter ABCG2. These transporters are each capable of mediating the active efflux of numerous anticancer drugs and display relatively distinct substrate preferences. The last, most recently discovered member, ABCG2, plays a major role in resistance in several types of cancer and the precise pharmacology of this multidrug transporter remain unresolved as does the nature of substrate binding.

EXPERIMENTAL APPROACH

Plasma membranes from insect cells expressing ABCG2 were used to characterise binding of [3H]daunomycin to the multidrug transporter. The kinetics of association and dissociation for this substrate and several other compounds were also determined in this experimental system.

KEY RESULTS

The dissociation constant for [3H]daunomycin binding was 564 +/- 57 nM and a Hill slope of 1.4 suggested cooperative binding. Doxorubicin, prazosin and daunomycin completely displaced the binding of radioligand, while mitoxantrone and Hoechst 33342 produced only a partial displacement. Analysis of the dissociation rates revealed that [3H]daunomycin and doxorubicin bind to multiple sites on the transporter.

CONCLUSIONS

Both kinetic and equilibrium data support the presence of at least two symmetric drug binding sites on ABCG2, which is distinct from the asymmetry observed for P-gp. The data provide the first molecular details underlying the mechanism by which this transporter is capable of interacting with multiple substrates.

Probing the existence of G protein-coupled receptor dimers by positive and negative ligand-dependent cooperative binding

An increasing amount of ligand binding data on G protein-coupled receptors (GPCRs) is not compatible with the prediction of the simple mass action law. This may be related to the propensity of most GPCRs, if not all, to oligomerize. Indeed, one of the consequences of receptor oligomerization could be a possible cross-talk between the protomers, which in turn could lead to negative or positive cooperative ligand binding. We prove here that this can be demonstrated experimentally. Saturation, dissociation, and competition binding experiments were performed on vasopressin and oxytocin receptors expressed in Chinese hamster ovary or COS-7 cells. Linear, concave, and convex Scatchard plots were then obtained, depending on the ligand used. Moreover, some competition curves exhibited an increase of the radiotracer binding for low concentrations of competitors, suggesting a cooperative binding process. These data demonstrate that various vasopressin analogs display either positive or negative cooperative binding. Because positive cooperative binding cannot be explained without considering receptor as multivalent, these binding data support the concept of GPCR dimerization process. The results, which are in good accordance with the predictions of previous mathematical models, suggest that binding experiments can be used to probe the existence of receptor dimers.