Molecular Pharmacology

O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=40 SRC="FIGDIR/small/570073v1_ufig1.gif" ALT="Figure 1"> View larger version (13K): org.highwire.dtl.DTLVardef@198d7f5org.highwire.dtl.DTLVardef@18a3d9forg.highwire.dtl.DTLVardef@d3c9e8org.highwire.dtl.DTLVardef@6ca5f2_HPS_FORMAT_FIGEXP M_FIG C_FIG Desensitisation of the mu-opioid receptor (MOR) is proposed to underlie the initiation of opioid analgesic tolerance and previous work has shown that agonist-induced phosphorylation of the MOR C-tail contributes to this desensitisation. Moreover, we and others have shown that phosphorylation is important for {beta}-arrestin recruitment to the receptor, and that ligands of different efficacies induce distinct patterns, or barcodes, of receptor phosphorylation. Within the MOR C-tail, the 370TREHPSTANT379 motif harbours Ser/Thr residues important for these regulatory functions. 375Ser acts as a primary phosphorylation site of a ligand-dependent, hierarchical, and sequential process, whereby flanking 370Thr, 376Thr and 379Thr residues can get subsequently phosphorylated. Here we used HEK293 GRK KO cells, in combination with phosphosite specific antibodies and site-directed mutagenesis of the MOR, to evaluate the contribution of the different GRK subfamilies to ligand-induced phosphorylation barcodes and {beta}-arrestin2 recruitment. We show that both GRK subfamilies (GRK2/3 and GRK5/6) promote phosphorylation of Thr370 and Ser375. However, only GRK2/3 induce phosphorylation of Thr376 and Thr379, which is required to promote robust {beta}-arrestin recruitment to the receptor. Moreover, while DAMGO and fentanyl can engage all kinases to promote Thr370 and Ser375 phosphorylation, under endogenous GRK expression conditions, morphine-induced phosphorylation of these residues is specifically mediated by GRK5/6. These data provide insight into the mechanisms of MOR regulation and suggest that the cellular complement of the different GRK subfamilies plays an important role in determining the tissue responses of distinct opioid agonists.

Camelid single-domain antibody fragments (nanobodies) offer the specificity of an antibody in a single 15kDa immunoglobulin domain. Their small size allows for easy genetic manipulation of the nanobody sequence to incorporate protein tags, facilitating their use as biochemical probes. The nanobody VUN400, which recognises the second extracellular loop of the human CXCR4 chemokine receptor, was used as a probe to monitor specific CXCR4 conformations. VUN400 was fused via its C-terminus to the 11-amino acid HiBiT tag (VUN400-HiBiT) which complements to LgBiT protein, forming a full length functional NanoLuc luciferase. Here, complemented luminescence was used to detect VUN400-HiBiT binding to CXCR4 receptors expressed in living HEK293 cells. VUN400-HiBiT binding to CXCR4 could be prevented by orthosteric and allosteric ligands, allowing VUN400-HiBiT to be used as a probe to detect specific conformations of CXCR4. These data demonstrate that the high specificity offered by extracellular-targeted nanobodies can be utilised to probe receptor pharmacology.

The dopamine D1 receptor (D1R) has fundamental roles in voluntary movement and memory and is a validated drug target for neurodegenerative and neuropsychiatric disorders. However, previously developed D1R selective agonists possess a catechol moiety which displays poor pharmacokinetic properties. The first selective non-catechol D1R agonists were recently discovered and unexpectedly many of these ligands showed G protein biased signaling. Here, we investigate both catechol and non-catechol D1R agonists to validate potential biased signaling and examine if this impacts agonist-induced D1R endocytosis. We determined that most, but not all, non-catechol agonists display G protein biased signaling at the D1R and have reduced or absent {beta}-arrestin recruitment. A notable exception was compound (Cmpd) 19, a non-catechol agonist with full efficacy at both D1R-G protein or D1R-{beta}-arrestin pathways. In addition, the catechol ligand A-77636 was a highly potent, super agonist for D1R-{beta}-arrestin activity. When examined for agonist-induced D1R endocytosis, balanced agonists SKF-81297 and Cmpd 19 induced robust D1R endocytosis while the G protein biased agonists did not. The {beta}-arrestin super agonist, A-77636, showed significantly increased D1R endocytosis. Moreover, {beta}-arrestin recruitment efficacy of tested agonists strongly correlated with total D1R endocytosis. Taken together, these results indicate the degree of D1R signaling functional selectivity profoundly impacts D1R endocytosis regardless of pharmacophore. The range of functional selectivity of these D1R agonists will provide valuable tools to further investigate D1R signaling, trafficking and therapeutic potential. Significance StatementThe D1R is a validated therapeutic target and the recently discovered non-catechol D1R agonists have translational potential. We have systematically characterized several structurally distinct D1R agonists including non-catechols with balanced or G protein biased activity. When examined for agonist-induced D1R endocytosis, balanced agonists induced robust D1R endocytosis while G protein biased agonists did not. These results indicate the degree of D1R signaling functional selectivity profoundly impacts receptor endocytosis. This work also independently validates agonist tools to further investigate D1R activation in basic and translational research.

IntroductionThe {beta}2-adrenoceptor ({beta}2AR) is a class A G protein-coupled receptor (GPCR). It is therapeutically relevant in asthma, whereby {beta}2AR agonists relieve bronchoconstriction. The {beta}2AR is a prototypical GPCR for structural and biophysical studies. However, the molecular basis of agonist efficacy at the {beta}2AR is not understood. We hypothesized that the kinetics of ligand binding and GPCR-G protein interactions could play a role in ligand efficacy. We characterised the molecular pharmacology of a range of {beta}2AR agonists and examined the correlation between ligand and mini-Gs binding kinetics and efficacy. MethodsWe used a Time-resolved Fluorescence Resonance Energy Transfer (TR-FRET) based competition ligand binding assay to measure the affinity and residence times of a range of {beta}2AR agonists binding to the human {beta}2AR. TR-FRET between Lumi4-Tb3+ on the N terminus of the {beta}2AR and fluorescent CA200693 (S)-propranolol-green was measured using a PHERAstar FSX. The ability of these {beta}2AR agonists to activate the heterotrimeric Gs protein was measured using the CASE Gs protein biosensor. This assay senses a reduction in NanoBRET between the nano-luciferase (nLuc) donor on the G subunit and Venus acceptor on the G{psi}, on receptor activation, quantified using the operational model of agonism. NanoBRET was also used to measure interactions between DDM solubilised {beta}2AR-nLuc and purified Venus-mini-Gs. A large excess of unlabelled mini-Gs was used to dissociate the {beta}2AR-nLuc: Venus-mini-Gs complex. ResultsCharacterisation of the molecular pharmacology of seven {beta}2AR agonists showed a broad range of ligand binding affinities (pKi = 4.4 {+/-} 0.09 to 9.2 {+/-} 0.08) and kinetics parameters. There was no correlation between ligand residence times and their ability (log{tau} ) to activate the Gs protein (R2=0.26, p=0.29). However, there were statistically significant differences in the association rate (kon (fast)) (3.36{+/-}0.64x105 to 9.19{+/-} 0.42x105) and affinity (Kd) values of mini-Gs binding to the agonist-{beta}2AR complex (pKd =6.0 to 6.7). Both an increase in ligand driven mini-Gs kon(fast) rate and associated increase in mini-Gs pKd for the receptor, were moderately correlated with efficacy (log{tau} ) (R2 =0.58 and R2 =0.50 respectively). ConclusionsThese data support a model in which agonists of increased efficacy cause the {beta}2AR to adopt a conformation that is more likely to recruit G protein. Conversely, these data did not support a role for agonist binding kinetics in the molecular basis of efficacy.

Opioids reduce severe pain, but persistent use is compromised by tolerance, attenuated by either {beta}-arrestin2 depletion, prompting development of biased opioids limited by partial efficacy, or c-Src kinase inhibitors, potentially acting through off-target effects. We tested eCF506, a conformationally selective c-Src inhibitor, on morphine antinociception and examined its effect on receptor signaling and {beta}-arrestin2 recruitment. Oral eCF506 inhibited morphine tolerance in C57BL/6J mice. Exposure of PathHunter CHO cells to eCF506 did not affect inhibition of cAMP accumulation by the agonist, DAMGO, but reduced {beta}-arrestin2 recruitment. This effect, mimicked by targeted degradation of c-Src, occurred through inhibition of c-Src catalytic function as evidenced by its diminution by the catalytically inactive Src250-536(K298M) construct. This mutant also restricted the effect of c-Src inhibitors on {beta}-arrestin2 recruitment. eCF506 additionally increased surface expression of receptors and limited their internalization by endomorphin-2 but did not alter DAMGO-evoked GRK-mediated receptor phosphorylation. These findings suggest that eCF506 prolongs opioid antinociception by inducing signalling bias, diminishing {beta}-arrestin2-mediated receptor regulation.

G protein-coupled receptors (GPCRs) mediate diverse signaling outputs through their proximal transducers: G proteins, GRKs, and {beta}-arrestins. Although ligand bias at chemokine receptors (CKRs), where ligands for the same receptor display distinct signaling patterns, is well recognized, receptor bias, where the same agonist at different receptors yields distinct transducer engagement, remains poorly understood. We compared endogenous chemokine ligands (CXCL1, CXCL5, CXCL7, CXCL8) at the highly homologous CXCR1 and CXCR2 receptors using biosensor assays to measure Gi activation, {beta}-arrestin1/2 recruitment, GRK2/3/5/6 translocation, and receptor internalization. Our data reveal qualitatively different signaling patterns, most notably where CXCL1 acts as a G protein-biased partial agonist at CXCR1 but as a balanced full agonist at CXCR2. These signaling differences correlate with receptor internalization but not subcellular ERK activation patterns measured using compartment-specific biosensors. Collectively, our findings demonstrate receptor bias in CKR signaling, transducer activation, and compartmentalized kinase activation in translating chemokine identity into discrete functional outcomes. SIGNIFICANCE STATEMENTChemokine ligand bias, where different ligands for the same receptor display different signaling patterns, is now well appreciated. However, there are only few examples of receptor bias, where the same agonist generates distinct signaling profiles at different receptors. Here we used biosensors and compartmental ERK biosensors, to show that CXCL1, CXCL5, CXCL7, and CXCL8 differentially engage G proteins, {beta}-arrestins, and GRKs, at CXCR1 and CXCR2. This work provides mechanistic insight into CXCR1/CXCR2 signaling diversity.

CXCL17 is the most recently described chemokine. It is principally expressed by mucosal tissues, where it facilitates chemotaxis of monocytes, dendritic cells, and macrophages and has antimicrobial properties. CXCL17 is also implicated in the pathology of inflammatory disorders and progression of several cancers, as well as being highly upregulated during viral infections of the lung. However, the exact role of CXCL17 in health and disease is largely unknown, mainly due to a lack of known molecular targets mediating CXCL17 functional responses. Using a range of bioluminescence resonance energy transfer (BRET) based assays, here we demonstrate that CXCL17 inhibits CXCR4-mediated signalling and ligand binding. Moreover, CXCL17 interacts with neuropillin-1, a VEGFR2 co-receptor. Additionally, we find CXCL17 only inhibits CXCR4 ligand binding in intact cells and demonstrate that this effect is mimicked by known glycosaminoglycan binders, surfen and protamine sulfate. This indicates that CXCL17 inhibits CXCR4 by a unique mechanism of action that potentially requires the presence of a glycosaminoglycan containing accessory protein. Altogether, our results reveal that CXCL17 is an endogenous inhibitor of CXCR4 and represents an important discovery in our understanding of the (patho) physiological functions of CXCL17 and regulation of CXCR4 signalling.

Gastric inhibitory polypeptide (GIP) receptor is a class B1 GPCR, that responds to GIP and physiologically potentiates glucose-stimulated insulin secretion. Like most class B1 GPCRs, GIPR has been shown to interact with RAMPs, yet the effects of RAMPs on its signalling and trafficking remain poorly understood. We demonstrate that RAMPs modulate G protein activation and GIPR internalisation profiles. RAMP3 reduced GIPR Gs activation and cAMP production but retained GIPR at the cell surface, and this was associated with prolonged ERK1/2 phosphorylation and {beta}-arrestin association. By contrast, RAMP1/2 reduced Gq/11/15 activation of the GIPR. Through knockout mice studies, we show that RAMP1 is important to the normal physiological functioning of GIPR to regulate blood glucose levels. Thus, RAMPs act on G protein/{beta}-arrestin complexes, having both acute and chronic effects on GIPR function, while this study also raises the possibility of a more general role of RAMP3 to enhance GPCR plasma membrane localisation.

The histamine H3 receptor (H3R) regulates as a presynaptic G protein-coupled receptor the release of histamine and other neurotransmitters in the brain, and is consequently a potential therapeutic target for neuronal disorders. The human H3R encodes for seven splice variants that vary in the length of intracellular loop 3 and/or the C-terminal tail but are all able to induce heterotrimeric Gi protein signaling. The last two decades H3R drug discovery and lead optimization has been exclusively focused on the 445 amino acids-long reference isoform H3R-445. In this study, we pharmacologically characterized for the first time all seven H3R isoforms by determining their binding affinities for reference histamine H3 receptor agonists and inverse agonists. The H3R-453, H3R-415, and H3R-413 isoforms display similar binding affinities for all ligands as the H3R-445. However, increased agonist binding affinities were observed for the three shorter isoforms H3R-329, H3R-365, and H3R-373, whereas inverse agonists such as the approved anti-narcolepsy drug pitolisant (Wakix(R)) displayed significantly decreased binding affinities for the latter two isoforms. This opposite change in binding affinity of agonist versus inverse agonists on H3R-365 and H3R-373 is associated with their higher constitutive activity in a cAMP biosensor assay as compared to the other 5 isoforms. The observed differences in pharmacology between longer and shorter H3R isoforms should be considered in future drug discovery programs.

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and are a common drug target. They can be stabilised in different conformational states by ligands to activate multiple transducers and effectors leading to a variety of cellular responses. The potential of agonists to activate select pathways has important implications for drug discovery. Thus, there is a clear need to profile the initial GPCR signal transduction event, activation of G proteins, to enhance understanding of receptor coupling and guide drug design. The BRET-based biosensor suite, TRUPATH, was recently developed to enable quantification of the activation profiles of all non-visual G proteins (excluding Golf and G14) and has since been utilised in numerous studies. However, it fails to detect Gq/11 activation for a number of GPCRs previously reported to display promiscuous secondary coupling to Gq/11. Here we report modifications to the Gq and G11 biosensors in the switch I region that prevent intrinsic GTPase activity (R183C/Q). Except for the PAC1R, substitution with cancer-associated mutations, Cys or Gln, significantly increased sensitivity to allow detection of robust, reliable, and representative Gq/11 responses to Class B1 GPCRs. We also demonstrate the utility of these modified biosensors for promiscuously coupled class A GPCR that have primary Gs-coupling. Thus, we propose that modification to Gq/11 may also be necessary in other biosensor systems to enable detection of Gq/11 activation.

The 26S proteasome is an essential regulator of protein homeostasis and a clinically validated therapeutic target in multiple myeloma (MM). Rapaprotin, a novel macrocycle identified from a rapamycin-inspired rapafucin library, disrupts 26S proteasome function by inducing disassembly of the 19S regulatory particle in the 26S proteasome, leading to apoptosis in MM cells. Its bioactivation requires prolyl endopeptidase (PREP)-mediated cleavage to generate Rapaprotin-L, a negatively charged, linear metabolite with potent proteasome-disassembly activity. Using the PRISM cancer cell line profiling platform, we identified high P-glycoprotein (P-gp/ABCB1) expression as a major determinant of Rapaprotin resistance in solid tumor cell lines. Efflux assays confirmed Rapaprotin-L, but not its parent Rapaprotin, as a high-affinity P-gp substrate. Co-treatment with the third-generation P-gp inhibitor tariquidar restored the intracellular accumulation of Rapaprotin-L, reinstating proteasome inhibition and consequent apoptosis of Rapaprotin-resistant colorectal cancer cell lines. Strong synergy between Rapaprotin and tariquidar was observed in a 3D spheroid model. These results establish P-gp as a key mediator of resistance to Rapaprotin and identify a rare example of a negatively charged Rapaprotin-L as a P-gp substrate. Together, these findings expand the potential therapeutic scope of Rapaprotin beyond hematologic malignancies to a broader range of solid tumors.

ATP-binding cassette (ABC) transporters expressed at the blood-brain barrier (BBB) impede delivery of therapeutic agents to the brain, including agents to treat neurodegenerative diseases and primary and metastatic brain cancers. Two transporters, P-glycoprotein (P-gp, ABCB1) and ABCG2, are highly expressed at the BBB and are responsible for the efflux of numerous clinically useful chemotherapeutic agents, including irinotecan, paclitaxel, and doxorubicin. Based on a previous mouse model, we have generated transgenic zebrafish in which expression of NanoLuciferase (NanoLuc) is controlled by the promoter of glial fibrillary acidic protein, leading to expression in zebrafish glia. To identify agents that disrupt the BBB, including inhibitors of ABCB1 and ABCG2, we identified NanoLuc substrates that are also transported by P-gp, ABCG2, and their zebrafish homologs. These substrates will elevate the amount of bioluminescent light produced in the transgenic zebrafish with BBB disruption. We transfected HEK293 cells with NanoLuc and either human ABCB1, ABCG2, or their zebrafish homologs Abcb4 or Abcg2a, respectively, and expressed at the zebrafish BBB. We evaluated the luminescence of ten NanoLuc substrates, then screened the eight brightest to determine which are most efficiently effluxed by the ABC transporters. We identified one substrate efficiently pumped out by ABCB1, two by Abcb4, six by ABCG2, and four by Abcg2a. These data will aid in the development of a transgenic zebrafish model of the BBB to identify novel BBB disruptors and should prove useful in the development of other animal models that use NanoLuc as a reporter. Significance StatementThe ATP-Binding Cassette (ABC) transporters ABCB1 and ABCG2 at the blood-brain barrier (BBB) hinder pharmacological treatment of brain-related diseases. Consequently, there is a need for tools to identify BBB disruptors. We conducted a screen of ten NanoLuciferase substrates, identifying the brightest and those that were transported by human and zebrafish ABC transporters at the BBB. This work supports and complements our development of a transgenic zebrafish model, in which NanoLuciferase is expressed within glial cells, enabling detection of BBB disruption.

Animal use in research extends beyond the use of animal models to study physiology and disease. Many aspects of in vitro research use reagents derived from animals, most prolifically the use of foetal bovine serum (FBS) in growth media for cellular models. With the aim to reduce animal use, we investigated the effect of reduced FBS culture conditions on cell growth, as well as different stages of G protein-coupled receptor (GPCR) signalling, a wide area of research which might therefore impact many groups. We identified little differences on cell growth or GPCR signalling when reducing culture FBS percentage from 10% to 5%, using assays ranging from receptor activation to downstream transcription factors stimulation. In addition to diminishing animal use, the reduction of FBS use will also have economic and environmental benefits, which we hope will be of benefit to the wider research community.

APOE4, the major genetic risk factor for Alzheimers disease (AD), and ABCA1, required for lipidation of APOE are gene products of the liver X receptor (LXR) receptor. LXR agonists have been validated in animal models as therapeutics for AD, atherosclerosis, and many other diseases. Clinical progress has been thwarted by unwanted hepatic lipogenesis. Structurally diverse LXR ligands were profiled in coregulator TR-FRET (CRT) assays analyzing ligand-induced coactivator recruitment, coactivator selectivity, corepressor dissociation, and LXR isoform selectivity. A multiplex CRT assay was developed to measure synchronous ligand-induced displacement of corepressor by coactivator. Potency for coactivator recruitment to LXR{beta} correlated with induction of ABCA1 in human astrocytoma cells. Correlation with lipogenic activation of sterol response element (SRE) in hepatocarcinoma cells, was more complex. CRT response was diverse revealing ligands with theoretical full agonist, partial agonist, antagonist, and inverse agonist, and other signatures within the same chemical series, suggesting the scope for precision CRT to guide nonlipogenic LXR agonist design.

G protein coupled receptors (GPCRs) are the largest family of signalling proteins and highly successful drug targets. Most GPCR drugs interact with a binding pocket for the natural ligand, typically near the extracellular region of the transmembrane domains. Advancements in structural biology have identified additional allosteric binding sites in other parts of these receptors. Allosteric sites provide theoretical advantages, including the ability to modulate natural ligand function, and there is a need for better ways to study how ligands interact with these binding sites. We have developed an approach to study multiple ligands binding to the same receptor at the same time based on sequential resonance energy transfer between two fluorescent ligands bound to a GPCR. We use this approach to identify novel ligand pharmacology and understand binding kinetics to the FFA1 free fatty acid receptor, a clinically relevant receptor. This novel method will aid development of new GPCR drugs.

Nonsense mutations, which occur in ~11% of patients with genetic disorders, introduce premature termination codons (PTCs) that lead to truncated proteins and promote nonsense-mediated mRNA decay. Aminoglycosides such as gentamicin and G418 permit PTC readthrough and so may address this problem. However, their effects are variable between patients, making clinical use of aminoglycosides challenging. In this study, we addressed the hypothesis that TRP non-selective cation channels contribute to the variable effect of aminoglycosides by controlling their cellular uptake. To attempt to identify the channel type involved, we tested AC1903, a 2-aminobenzimidazole derivative recently reported to selectively inhibit TRPC5 cation channels. AC1903 consistently suppressed G418 uptake and G418-induced PTC readthrough in the DMS-114 cell line and patient-derived JEB01 keratinocytes. In an effort to validate the suggested role of TRPC5, we tested an independent and more potent inhibitor called Pico145, which affects channels containing TRPC1, TRPC4 and TRPC5 but not other TRPCs or other channels. Unexpectedly, Pico145 was completely without effect, suggesting that AC1903 may work through other or additional targets. Consistent with this suggestion, AC1903 inhibited multiple TRPC channels including homomeric TRPC3, TRPC4, TRPC5, TRPC6 as well as concatemeric TRPC4-C1 and TRPC5-C1 channels, all with low micromolar IC50 values. It also inhibited TRPV4 channels but had weak or no effects on TRPV1 and no effect on another non-selective cation channel, PIEZO1. Overall, our study reveals a suppressor of aminoglycoside-mediated PTC readthrough (i.e., AC1903) but suggests that this compound has previously unrecognised effects. These effects require further investigation to determine the molecular mechanism by which AC1903 suppresses aminoglycoside uptake and PTC readthrough.

Glucocorticoids are steroid hormones that are essential for life in mammals. Therapeutically, they are some of the most cost-effective drugs for the treatment of inflammatory diseases ranging from skin rashes to COVID-19, but their use is limited by adverse effects. Glucocorticoids exert their effects via the glucocorticoid receptor, a type I nuclear hormone receptor which modulates gene expression. The transcriptional activity of some related, but nuclear restricted, type II nuclear hormone receptors can be enhanced by a family of intracellular transport proteins, the fatty acid binding proteins (FABPs). We find that the transcriptional activity of the GR can be altered by a sub-set of FABP family members dependent on the GR-ligand. The ability of some FABPs to selectively promote or limit the transcriptional activity of the GR in a ligand-dependent manner could facilitate the discovery of drugs that narrow GR activity to only the desired subset of therapeutically relevant genes.

Free fatty acid receptor 1 (FFA1 or GPR40), activated by medium- and long-chain fatty acids, amplifies glucose-stimulated insulin secretion, making it a promising target for type 2 diabetes. Radioligand studies revealed distinct binding sites for partial and full agonists, with full agonists showing positive cooperativity. Functional assays demonstrated positive cooperativity between agonists and varying interactions with the endogenous fatty acid DHA. These findings suggest three allosterically linked binding sites on FFA1, with activation influenced by key arginine residues. Potent ligands with strong cooperativity hold significant therapeutic potential.

Background and PurposeMetabotropic glutamate receptors (mGlus) are obligate dimer G protein coupled receptors that can all homodimerize and heterodimerize in select combinations. Responses of mGlu heterodimers to selective ligands, including orthosteric agonists and allosteric modulators, are largely unknown. Experimental ApproachThe pharmacological properties of each group II and III mGlu homodimer (except mGlu6) and several heterodimers were examined when stochastically assembled in HEK293T cells, or specifically measured using an improved G protein mediated BRET assay employing complimented fragments of NanoLuciferase. ResultsStochastically assembled receptors adopted unique signaling characteristics. Some favored the potency, efficacy or signaling kinetics of a dominant subunit, while others exhibited blended profiles reflective of a combination of homo- and heterodimers at various ratios of expressed receptor. Finally, group II and III mGlu dimers were examined for responses to selective agonists and allosteric modulators. Effects of glutamate and selective group II and III orthosteric agonists were found to result in unique concentration response profiles when examining each combination of group II and II mGlu. Effects of select allosteric modulators were examined for each mGlu2 containing dimer as well as several group III dimer pairs. Likewise, allosteric modulator effects were often unique across dimers containing the targeted subunit of the ligand being tested. ConclusionsResults demonstrate that mGlu dimers respond uniquely to selective ligands, and show that the mGlu family is not governed by generalizable rules dictating consequences of dimeric subunit interactions leading to signaling consequences.

Chemokine receptor 4 (CXCR4) is a member of the chemokine receptor exclusively activated by the chemokine CXCL12. While CXCR4 regulates numerous physiological processes associated with cell migration and embryogenesis, its overexpression has been involved in various cancer types. Studies suggest that intracellular CXCR4 expression rather than CXCR4-operated signaling underlies its pro-tumorigenic functions. Given the role of GPCR interacting proteins in their trafficking and subcellular localization, we characterized the CXCR4 interactome using an affinity purification coupled to mass spectrometry (AP-MS) strategy. The most abundant protein identified in the CXCR4 interactome is Ephrin B1, a member of the Ephrin protein family that shares several functions with CXCR4, such as the regulation of cell migration and proliferation. Further studies showed that interaction between CXCR4 and Ephrin B1 is direct and enhanced by treating cell with CXCL12. They also indicated that Ephrin B1 prevents CXCR4 N-glycosylation, decreases CXCR4 cell surface expression and consistently inhibits CXCL12-induced CXCR4 coupling to Gi1-3 and the recruitment of {beta}-arrestins 1 and 2. Conversely, Ephrin B1 signals to Erk1,2 through CXCR4 activation and mediates the decrease in Death Receptor 5 expression elicited by intracellular CXCR4. Collectively, these findings identify Ephrin B1 as a key mediator of CXCR4 tumorigenic signaling.