Conformational antibody binding to a native, cell-free expressed GPCR in block copolymer membranes.

G-protein coupled receptors (GPCRs) play a key role in physiological processes and are attractive drug targets. Their biophysical characterization is, however, highly challenging because of their innate instability outside a stabilizing membrane and the difficulty of finding a suitable expression system. We here show the cell-free expression of a GPCR, CXCR4, and its direct embedding in diblock copolymer membranes. The polymer-stabilized CXCR4 is readily immobilized onto biosensor chips for label-free binding analysis. Kinetic characterization using a conformationally sensitive antibody shows the receptor to exist in the correctly folded conformation, showing binding behaviour that is commensurate with heterologously expressed CXCR4.

CXCR7/CXCR4 heterodimer constitutively recruits beta-arrestin to enhance cell migration.

G protein-coupled receptor hetero-oligomerization is emerging as an important regulator of ligand-dependent transmembrane signaling, but precisely how receptor heteromers affect receptor pharmacology remains largely unknown. In this study, we have attempted to identify the functional significance of the heteromeric complex between CXCR4 and CXCR7 chemokine receptors. We demonstrate that co-expression of CXCR7 with CXCR4 results in constitutive recruitment of ß-arrestin to the CXCR4·CXCR7 complex and simultaneous impairment of G(i)-mediated signaling. CXCR7/CXCR4 co-expression also results in potentiation of CXCL12 (SDF-1)-mediated downstream ß-arrestin-dependent cell signaling pathways, including ERK1/2, p38 MAPK, and SAPK as judged from the results of experiments using siRNA knockdown to deplete ß-arrestin. Interestingly, CXCR7/CXCR4 co-expression enhances cell migration in response to CXCL12 stimulation. Again, inhibition of ß-arrestin using either siRNA knockdown or a dominant negative mutant abrogates the enhanced CXCL12-dependent migration of CXCR4/CXCR7-expressing cells. These results show how CXCR7, which cannot signal directly through G protein-linked pathways, can nevertheless affect cellular signaling networks by forming a heteromeric complex with CXCR4. The CXCR4·CXCR7 heterodimer complex recruits ß-arrestin, resulting in preferential activation of ß-arrestin-linked signaling pathways over canonical G protein pathways. CXCL12-dependent signaling of CXCR4 and its role in cellular physiology, including cancer metastasis, should be evaluated in the context of potential functional hetero-oligomerization with CXCR7.

Cell surface targeting of mu-delta opioid receptor heterodimers by RTP4.

Mu opioid receptors are G protein-coupled receptors that mediate the pain-relieving effects of clinically used analgesics, such as morphine. Accumulating evidence shows that mu-delta opioid heterodimers have a pharmacologic profile distinct from those of the mu or delta homodimers. Because the heterodimers exhibit distinct signaling properties, the protein and mechanism regulating their levels have significant effects on morphine-mediated physiology. We report the characterization of RTP4, a Golgi chaperone, as a regulator of the levels of heterodimers at the cell surface. We show that the association with RTP4 protects mu-delta receptors from ubiquitination and degradation. This leads to increases in surface heterodimer levels, thereby affecting signaling. Thus, the oligomeric organization of opioid receptors is controlled by RTP4, and this governs their membrane targeting and functional activity. This work is the first report of the identification of a chaperone involved in the regulation of the biogenesis of a family A GPCR heterodimer. The identification of such factors as RTP4 controlling dimerization will provide insight into the regulation of heterodimers in vivo. This has implications in the modulation of pharmacology of their endogenous ligands, and in the development of drugs with specific therapeutic effects.

Post-activation-mediated changes in opioid receptors detected by N-terminal antibodies.

The majority of studies examining activity-induced conformational changes in G protein-coupled receptors have focused on transmembrane helices or intracellular regions. Relatively few studies have examined the involvement of the extracellular region in general and the N-terminal region in particular in this process. To begin to address this, we generated a series of antibodies to the N-terminal region of opioid receptors. Characterization of these antibodies revealed that they differentially recognize activated receptors. Recently, we generated monoclonal antibodies that recognize regions proximal to glycosylation sites in the receptor N terminus. Characterization of these antibodies revealed that agonist treatment leads to a decrease in epitope recognition by the antibody presumably because of a movement of the region of the N terminus proximal to glycosylation sites. The time course of the decrease in antibody recognition suggested that it could be due to a post-activation-mediated event. Examination of the involvement of receptor residues in the C-tail and beta-arrestin binding using site-directed mutagenesis and cells or tissues lacking beta-arrestin 2 suggests a role for these desensitization-related mechanisms in governing antibody binding to the receptor. Thus, these N-terminally directed antibodies can differentially recognize post-activation-mediated changes in the C-terminal (intracellular) region of the receptor. Therefore, these conformation-sensitive antibodies represent powerful reagents to probe receptor activation states and provide a potential tool for identifying and characterizing new compounds of therapeutic interest.

Conformation state-sensitive antibodies to G-protein-coupled receptors.

A growing body of evidence indicates that G-protein-coupled receptors undergo complex conformational changes upon agonist activation. It is likely that the extracellular region, including the N terminus, undergoes activation-dependent conformational changes. We examined this by generating antibodies to regions within the N terminus of micro-opioid receptors. We find that antibodies to the midportion of the N-terminal tail exhibit enhanced recognition of activated receptors, whereas those to the distal regions do not. The enhanced recognition is abolished upon treatment with agents that block G-protein coupling or deglycosylate the receptor. This suggests that the N-terminal region of mu receptors undergoes conformational changes following receptor activation that can be selectively detected by these region-specific antibodies. We used these antibodies to characterize micro receptor type-specific ligands and find that the antibodies accurately differentiate ligands with varying efficacies. Next, we examined if these antibodies can be used to investigate the extent and duration of activation of endogenous receptors. We find that peripheral morphine administration leads to a time-dependent increase in antibody binding in the striatum and prefrontal cortex with a peak at about 30 min, indicating that these antibodies can be used to probe the spatio-temporal dynamics of native mu receptors. Finally, we show that this strategy of targeting the N-terminal region to generate receptor conformation-specific antisera can be applied to other G(alpha)(i)-coupled (delta-opioid, CB1 cannabinoid, alpha(2A)-adrenergic) as well as G(alpha)(s)-(beta(2)-adrenergic) and G(alpha)(q)-coupled (AT1 angiotensin) receptors. Taken together, these studies describe antisera as tools that allow, for the first time, studies probing differential conformation states of G-protein-coupled receptors, which could be used to identify molecules of therapeutic interest.

Peptidomics of Cpefat/fat mouse hypothalamus and striatum: effect of chronic morphine administration.

Chronic morphine administration is known to affect several neuropeptide systems, and this could contribute to the behavioral effects of opiates. To quantitate global changes in neuropeptide levels upon chronic morphine administration, we took advantage of a method that allows selective isolation of neuropeptides from brains of mice lacking carboxypeptidase E (Cpefat/fat mice), a critical enzyme in the generation of many neuroendocrine peptides. We used a differential labeling procedure with stable isotopic tags and mass spectrometry to quantitate the relative changes in a number of hypothalamic and striatal peptides in Cpefat/fat mice chronically treated with morphine. A total of 27 distinct peptides were detected in hypothalamus and striatum. Of these, 27 were identified by mass spectrometry-based sequencing, 1 was tentatively identified by the mass and charge, and 9 were not identified. The identified peptides included fragments of proenkephalin, prothyrotropin-releasing hormone, secretogranin II, chromogranin Aand B, protachykinin B, provasopressin, promelanin concentrating hormone, and pro-SAAS. Upon morphine administration, although the levels of most of the peptides were unaltered (within a factor of 1.3 to 0.7 compared with saline control), the levels of a small number of peptides did show consistent changes (increased or decreased by 1.3-fold or more) in hypothalamus and/or striatum. Taken together, these results provide interesting insights into endogenous neuropeptide systems that are modulated by morphine and suggest further experiments to link candidate peptides with long-term effects of morphine.

Targeting opioid receptor heterodimers: strategies for screening and drug development.

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

Opioid receptor random mutagenesis reveals a mechanism for G protein-coupled receptor activation.

The high resolution structure of rhodopsin has greatly enhanced current understanding of G protein-coupled receptor (GPCR) structure in the off-state, but the activation process remains to be clarified. We investigated molecular mechanisms of delta-opioid receptor activation without a preconceived structural hypothesis. Using random mutagenesis of the entire receptor, we identified 30 activating point mutations. Three-dimensional modeling revealed an activation path originating from the third extracellular loop and propagating through tightly packed helices III, VI and VII down to a VI-VII cytoplasmic switch. N- and C-terminal determinants also influence receptor activity. Findings for this therapeutically important receptor may apply to other GPCRs that respond to diffusible ligands.