Molecular insights into the biased signaling mechanism of the µ-opioid receptor.

GPCR functional selectivity opens new opportunities for the design of safer drugs. Ligands orchestrate GPCR signaling cascades by modulating the receptor conformational landscape. Our study provides insights into the dynamic mechanism enabling opioid ligands to preferentially activate the G protein over the ß-arrestin pathways through the µ-opioid receptor (µOR). We combine functional assays in living cells, solution NMR spectroscopy, and enhanced-sampling molecular dynamic simulations to identify the specific µOR conformations induced by G protein-biased agonists. In particular, we describe the dynamic and allosteric communications between the ligand-binding pocket and the receptor intracellular domains, through conserved motifs in class A GPCRs. Most strikingly, the biased agonists trigger µOR conformational changes in the intracellular loop 1 and helix 8 domains, which may impair ß-arrestin binding or signaling. The findings may apply to other GPCR families and provide key molecular information that could facilitate the design of biased ligands.

Gαs is dispensable for ß-arrestin coupling but dictates GRK selectivity and is predominant for gene expression regulation by ß2-adrenergic receptor.

ß-arrestins play a key role in G protein-coupled receptor (GPCR) internalization, trafficking, and signaling. Whether ß-arrestins act independently of G protein-mediated signaling has not been fully elucidated. Studies using genome-editing approaches revealed that whereas G proteins are essential for mitogen-activated protein kinase activation by GPCRs., ß-arrestins play a more prominent role in signal compartmentalization. However, in the absence of G proteins, GPCRs may not activate ß-arrestins, thereby limiting the ability to distinguish G protein from ß-arrestin-mediated signaling events. We used ß2-adrenergic receptor (ß2AR) and its ß2AR-C tail mutant expressed in human embryonic kidney 293 cells wildtype or CRISPR-Cas9 gene edited for Gαs, ß-arrestin1/2, or GPCR kinases 2/3/5/6 in combination with arrestin conformational sensors to elucidate the interplay between Gαs and ß-arrestins in controlling gene expression. We found that Gαs is not required for ß2AR and ß-arrestin conformational changes, ß-arrestin recruitment, and receptor internalization, but that Gαs dictates the GPCR kinase isoforms involved in ß-arrestin recruitment. By RNA-Seq analysis, we found that protein kinase A and mitogen-activated protein kinase gene signatures were activated by stimulation of ß2AR in wildtype and ß-arrestin1/2-KO cells but absent in Gαs-KO cells. These results were validated by re-expressing Gαs in the corresponding KO cells and silencing ß-arrestins in wildtype cells. These findings were extended to cellular systems expressing endogenous levels of ß2AR. Overall, our results support that Gs is essential for ß2AR-promoted protein kinase A and mitogen-activated protein kinase gene expression signatures, whereas ß-arrestins initiate signaling events modulating Gαs-driven nuclear transcriptional activity.

Recurrent high-impact mutations at cognate structural positions in class A G protein-coupled receptors expressed in tumors.

G protein-coupled receptors (GPCRs) are the largest family of human proteins. They have a common structure and, signaling through a much smaller set of G proteins, arrestins, and effectors, activate downstream pathways that often modulate hallmark mechanisms of cancer. Because there are many more GPCRs than effectors, mutations in different receptors could perturb signaling similarly so as to favor a tumor. We hypothesized that somatic mutations in tumor samples may not be enriched within a single gene but rather that cognate mutations with similar effects on GPCR function are distributed across many receptors. To test this possibility, we systematically aggregated somatic cancer mutations across class A GPCRs and found a nonrandom distribution of positions with variant amino acid residues. Individual cancer types were enriched for highly impactful, recurrent mutations at selected cognate positions of known functional motifs. We also discovered that no single receptor drives this pattern, but rather multiple receptors contain amino acid substitutions at a few cognate positions. Phenotypic characterization suggests these mutations induce perturbation of G protein activation and/or ß-arrestin recruitment. These data suggest that recurrent impactful oncogenic mutations perturb different GPCRs to subvert signaling and promote tumor growth or survival. The possibility that multiple different GPCRs could moonlight as drivers or enablers of a given cancer through mutations located at cognate positions across GPCR paralogs opens a window into cancer mechanisms and potential approaches to therapeutics.

Constitutive signal bias mediated by the human GHRHR splice variant 1.

Alternative splicing of G protein-coupled receptors has been observed, but their functions are largely unknown. Here, we report that a splice variant (SV1) of the human growth hormone-releasing hormone receptor (GHRHR) is capable of transducing biased signal. Differing only at the receptor N terminus, GHRHR predominantly activates Gs while SV1 selectively couples to ß-arrestins. Based on the cryogenic electron microscopy structures of SV1 in the apo state or GHRH-bound state in complex with the Gs protein, molecular dynamics simulations reveal that the N termini of GHRHR and SV1 differentiate the downstream signaling pathways, Gs versus ß-arrestins. As suggested by mutagenesis and functional studies, it appears that GHRH-elicited signal bias toward ß-arrestin recruitment is constitutively mediated by SV1. The level of SV1 expression in prostate cancer cells is also positively correlated with ERK1/2 phosphorylation but negatively correlated with cAMP response. Our findings imply that constitutive signal bias may be a mechanism that ensures cancer cell proliferation.

G protein-coupled receptor kinase phosphorylation of distal C-tail sites specifies ßarrestin1-mediated signaling by chemokine receptor CXCR4.

G protein-coupled receptor (GPCR) kinases (GRKs) and arrestins mediate GPCR desensitization, internalization, and signaling. The spatial pattern of GPCR phosphorylation is predicted to trigger these discrete GRK and arrestin-mediated functions. Here, we provide evidence that distal carboxyl-terminal tail (C-tail), but not proximal, phosphorylation of the chemokine receptor CXCR4 specifies ßarrestin1 (ßarr1)-dependent signaling. We demonstrate by pharmacologic inhibition of GRK2/3-mediated phosphorylation of the chemokine receptor CXCR4 coupled with site-directed mutagenesis and bioluminescence resonance energy transfer approaches that distal, not proximal, C-tail phosphorylation sites are required for recruitment of the adaptor protein STAM1 (signal-transducing adaptor molecule) to ßarr1 and focal adhesion kinase phosphorylation but not extracellular signal-regulated kinase 1/2 phosphorylation. In addition, we show that GPCRs that have similarly positioned C-tail phosphoresidues are also able to recruit STAM1 to ßarr1. However, although necessary for some GPCRs, we found that distal C-tail sites might not be sufficient to specify recruitment of STAM1 to ßarr1 for other GPCRs. In conclusion, this study provides evidence that distal C-tail phosphorylation sites specify GRK-ßarrestin-mediated signaling by CXCR4 and other GPCRs.

MRAP2 inhibits ß-arrestin recruitment to the ghrelin receptor by preventing GHSR1a phosphorylation.

The melanocortin receptor accessory protein 2 (MRAP2) is essential for several physiological functions of the ghrelin receptor growth hormone secretagogue receptor 1a (GHSR1a), including increasing appetite and suppressing insulin secretion. In the absence of MRAP2, GHSR1a displays high constitutive activity and a weak G-protein-mediated response to ghrelin and readily recruits ß-arrestin. In the presence of MRAP2, however, G-protein-mediated signaling via GHSR1a is strongly dependent on ghrelin stimulation and the recruitment of ß-arrestin is significantly diminished. To better understand how MRAP2 modifies GHSR1a signaling, here we investigated the role of several phosphorylation sites within the C-terminal tail and third intracellular loop of GHSR1a, as well as the mechanism behind MRAP2-mediated inhibition of ß-arrestin recruitment. We show that Ser252 and Thr261 in the third intracellular loop of GHSR1a contribute to ß-arrestin recruitment, whereas the C-terminal region is not essential for ß-arrestin interaction. Additionally, we found that MRAP2 inhibits GHSR1a phosphorylation by blocking the interaction of GRK2 and PKC with the receptor. Taken together, these data suggest that MRAP2 alters GHSR1a signaling by directly impacting the phosphorylation state of the receptor and that the C-terminal tail of GHSR1a prevents rather than contribute to ß-arrestin recruitment.

Phenylalanine 193 in Extracellular Loop 2 of the ß 2-Adrenergic Receptor Coordinates ß-Arrestin Interaction.

G protein-coupled receptors (GPCRs) transduce a diverse variety of extracellular stimuli into intracellular signaling. These receptors are the most clinically productive drug targets at present. Despite decades of research on the signaling consequences of molecule-receptor interactions, conformational components of receptor-effector interactions remain incompletely described. The ß 2-adrenergic receptor (ß 2AR) is a prototypical and extensively studied GPCR that can provide insight into this aspect of GPCR signaling thanks to robust structural data and rich pharmacopeia. Using bioluminescence resonance energy transfer -based biosensors, second messenger assays, and biochemical techniques, we characterize the properties of ß 2AR-F193A. This single point mutation in extracellular loop 2 of the ß 2AR is sufficient to intrinsically bias the ß 2AR away from ß-arrestin interaction and demonstrates altered regulatory outcomes downstream of this functional selectivity. This study highlights the importance of extracellular control of intracellular response to stimuli and suggests a previously undescribed role for the extracellular loops of the receptor and the extracellular pocket formed by transmembrane domains 2, 3, and 7 in GPCR regulation that may contribute to biased signaling at GPCRs. SIGNIFICANCE STATEMENT: The role of extracellular G protein-coupled receptor (GPCR) domains in mediating intracellular interactions is poorly understood. We characterized the effects of extracellular loop mutations on agonist-promoted interactions of GPCRs with G protein and ß-arrestin. Our studies reveal that F193 in extracellular loop 2 in the ß2-adrenergic receptor mediates interactions with G protein and ß-arrestin with a biased loss of ß-arrestin binding. These results provide new insights on the role of the extracellular domain in differentially modulating intracellular interactions with GPCRs.

Genetic and biased agonist-mediated reductions in ß-arrestin recruitment prolong cAMP signaling at glucagon family receptors.

Receptors for the peptide hormones glucagon-like peptide-1 (GLP-1R), glucose-dependent insulinotropic polypeptide (GIPR), and glucagon (GCGR) are important regulators of insulin secretion and energy metabolism. GLP-1R agonists have been successfully deployed for the treatment of type 2 diabetes, but it has been suggested that their efficacy is limited by target receptor desensitization and downregulation due to recruitment of ß-arrestins. Indeed, recently described GLP-1R agonists with reduced ß-arrestin-2 recruitment have delivered promising results in preclinical and clinical studies. We therefore aimed to determine if the same phenomenon could apply to the closely related GIPR and GCGR. In HEK293 cells depleted of both ß-arrestin isoforms the duration of G protein-dependent cAMP/PKA signaling was increased in response to the endogenous ligand for each receptor. Moreover, in wildtype cells, "biased" GLP-1, GCG, and GIP analogs with selective reductions in ß-arrestin-2 recruitment led to reduced receptor endocytosis and increased insulin secretion over a prolonged stimulation period, although the latter effect was only seen at high agonist concentrations. Biased GCG analogs increased the duration of cAMP signaling, but this did not lead to increased glucose output from hepatocytes. Our study provides a rationale for the development of GLP-1R, GIPR, and GCGR agonists with reduced ß-arrestin recruitment, but further work is needed to maximally exploit this strategy for therapeutic purposes.

The short third intracellular loop and cytoplasmic tail of bitter taste receptors provide functionally relevant GRK phosphorylation sites in TAS2R14.

For most G protein-coupled receptors, the third intracellular loop (IL3) and carboxy-terminal tail (CT) are sites for G protein-coupled receptor kinase (GRK)-mediated phosphorylation, leading to ß-arrestin binding and agonist-specific desensitization. These regions of bitter taste receptors (TAS2Rs) are extremely short compared with the superfamily, and their function in desensitization is unknown. TAS2R14 expressed on human airway smooth muscle cells relax the cell, suggesting a novel target for bronchodilators. To assess IL3 and CT in agonist-promoted TAS2R14 desensitization (tachyphylaxis), we generated fusion proteins of both the WT sequence and Ala substituted for Ser/Thr in the IL3 and CT sequences. In vitro, activated GRK2 phosphorylated WT IL3 and WT CT proteins but not Ala-substituted forms. TAS2R14s with mutations in IL3 (IL-5A), CT (CT-5A), and in both regions (IL/CT-10A) were expressed in human embryonic kidney 293T cells. IL/CT-10A and CT-5A failed to undergo desensitization of the intracellular calcium response compared with WT, indicating that functional desensitization by GRK phosphorylation is at residues in the CT. Desensitization of TAS2R14 was blocked by GRK2 knockdown in human airway smooth muscle cells. Receptor:ß-arrestin binding was absent in IL/CT-10A and CT-5A and reduced in IL-5A, indicating a role for IL3 phosphorylation in the ß-arrestin interaction for this function. Agonist-promoted internalization of IL-5A and CT-5A receptors was impaired, and they failed to colocalize with early endosomes. Thus, agonist-promoted functional desensitization of TAS2R14 occurs by GRK phosphorylation of CT residues and ß-arrestin binding. However, ß-arrestin function in the internalization and trafficking of the receptor also requires GRK phosphorylation of IL3 residues.

Artificial signaling in mammalian cells enabled by prokaryotic two-component system.

Augmenting live cells with new signal transduction capabilities is a key objective in genetic engineering and synthetic biology. We showed earlier that two-component signaling pathways could function in mammalian cells, albeit while losing their ligand sensitivity. Here, we show how to transduce small-molecule ligands in a dose-dependent fashion into gene expression in mammalian cells using two-component signaling machinery. First, we engineer mutually complementing truncated mutants of a histidine kinase unable to dimerize and phosphorylate the response regulator. Next, we fuse these mutants to protein domains capable of ligand-induced dimerization, which restores the phosphoryl transfer in a ligand-dependent manner. Cytoplasmic ligands are transduced by facilitating mutant dimerization in the cytoplasm, while extracellular ligands trigger dimerization at the inner side of a plasma membrane. These findings point to the potential of two-component regulatory systems as enabling tools for orthogonal signaling pathways in mammalian cells.

The nonselective cation channel TRPV4 inhibits angiotensin II receptors.

G-protein-coupled receptors (GPCRs) are a ubiquitously expressed family of receptor proteins that regulate many physiological functions and other proteins. They act through two dissociable signaling pathways: the exchange of GDP to GTP by linked G-proteins and the recruitment of ß-arrestins. GPCRs modulate several members of the transient receptor potential (TRP) channel family of nonselective cation channels. How TRP channels reciprocally regulate GPCR signaling is less well-explored. Here, using an array of biochemical approaches, including immunoprecipitation and fluorescence, calcium imaging, phosphate radiolabeling, and a ß-arrestin-dependent luciferase assay, we characterize a GPCR-TRP channel pair, angiotensin II receptor type 1 (AT1R), and transient receptor potential vanilloid 4 (TRPV4), in primary murine choroid plexus epithelial cells and immortalized cell lines. We found that AT1R and TRPV4 are binding partners and that activation of AT1R by angiotensin II (ANGII) elicits ß-arrestin-dependent inhibition and internalization of TRPV4. Activating TRPV4 with endogenous and synthetic agonists inhibited angiotensin II-mediated G-protein-associated second messenger accumulation, AT1R receptor phosphorylation, and ß-arrestin recruitment. We also noted that TRPV4 inhibits AT1R phosphorylation by activating the calcium-activated phosphatase calcineurin in a Ca2+/calmodulin-dependent manner, preventing ß-arrestin recruitment and receptor internalization. These findings suggest that when TRP channels and GPCRs are co-expressed in the same tissues, many of these channels can inhibit GPCR desensitization.

Angiotensin II type 1 receptor variants alter endosomal receptor-ß-arrestin complex stability and MAPK activation.

The angiotensin II (AngII) type 1 receptor (AT1R), a member of the G protein-coupled receptor (GPCR) family, signals through G proteins and ß-arrestins, which act as adaptors to regulate AT1R internalization and mitogen-activated protein kinase (MAPK) ERK1/2 activation. ß-arrestin-dependent ERK1/2 regulation is the subject of important studies because its spatiotemporal control remains poorly understood for many GPCRs, including AT1R. To study the link between ß-arrestin-dependent trafficking and ERK1/2 signaling, we investigated three naturally occurring AT1R variants that show distinct receptor-ß-arrestin interactions: A163T, T282M, and C289W. Using bioluminescence resonance energy transfer (BRET)-based and conformational fluorescein arsenical hairpin-BRET sensors coupled with high-resolution fluorescence microscopy, we show that all AT1R variants form complexes with ß-arrestin2 at the plasma membrane and efficiently internalize into endosomes upon AngII stimulation. However, mutant receptors imposed distinct conformations in ß-arrestin2 and differentially impacted endosomal trafficking and MAPK signaling. Notably, T282M accumulated in endosomes, but its ability to form stable complexes following internalization was reduced, markedly impairing its ability to co-traffic with ß-arrestin2. We also found that despite ß-arrestin2 overexpression, T282M's and C289W's residency with ß-arrestin2 in endosomes was greatly reduced, leading to decreased ß-arrestin-dependent ERK1/2 activation, faster recycling of receptors to the plasma membrane, and impaired AngII-mediated proliferation. Our findings reveal that naturally occurring AT1R variants alter the patterns of receptor/ß-arrestin2 trafficking and suggest conformationally dependent ß-arrestin-mediated MAPK activation as well as endosomal receptor-ß-arrestin complex stabilization in the mitogenic response of AT1R.

Amino terminal recognition by a CCR6 chemokine receptor antibody blocks CCL20 signaling and IL-17 expression via ß-arrestin.

BACKGROUND: CCR6 chemokine receptor is an important target in inflammatory diseases. Th17 cells express CCR6 and a number of inflammatory cytokines, including IL-17 and IL-22, which are involved in the propagation of inflammatory immune responses. CCR6 antagonist would be a potential treatment for inflammatory diseases such as psoriasis or rheumatoid arthritis. The aim of this study is to develop an antagonistic monoclonal antibody (mAb) against human CCR6 receptor (hCCR6). RESULTS: We generate monoclonal antibodies against hCCR6 immunizing Balb/c mice with hCCR6 overexpressing cells. The antibodies were tested by flow cytometry for specific binding to hCCR6, cloned by limiting dilution and resulted in the isolation and purification monoclonal antibody 1C6. By ELISA and flow cytometry, was determined that the antibody obtained binds to hCCR6 N-terminal domain. The ability of 1C6 to neutralize hCCR6 signaling was tested and we determined that 1C6 antibody were able to block response in ß-arrestin recruitment assay with IC50 10.23 nM, but did not inhibit calcium mobilization. In addition, we found in a chemotaxis assay that 1C6 reduces the migration of hCCR6 cells to their ligand CCL20. Finally, we determined by RT-qPCR that the expression of IL-17A in Th17 cells treated with 1C6 was inhibited. CONCLUSIONS: In the present study, we applied whole cell immunization for successfully obtain an antibody that is capable to neutralize hCCR6 signaling and to reduce hCCR6 cells migration and IL-17 expression. These results provide an efficient approach to obtain therapeutic potential antibodies in the treatment of CCR6-mediated inflammatory diseases.

Beta-arrestins operate an on/off control switch for focal adhesion kinase activity.

Focal adhesion kinase (FAK) regulates key biological processes downstream of G protein-coupled receptors (GPCRs) in normal and cancer cells, but the modes of kinase activation by these receptors remain unclear. We report that after GPCR stimulation, FAK activation is controlled by a sequence of events depending on the scaffolding proteins ß-arrestins and G proteins. Depletion of ß-arrestins results in a marked increase in FAK autophosphorylation and focal adhesion number. We demonstrate that ß-arrestins interact directly with FAK and inhibit its autophosphorylation in resting cells. Both FAK-ß-arrestin interaction and FAK inhibition require the FERM domain of FAK. Following the stimulation of the angiotensin receptor AT1AR and subsequent translocation of the FAK-ß-arrestin complex to the plasma membrane, ß-arrestin interaction with the adaptor AP-2 releases inactive FAK from the inhibitory complex, allowing its activation by receptor-stimulated G proteins and activation of downstream FAK effectors. Release and activation of FAK in response to angiotensin are prevented by an AP-2-binding deficient ß-arrestin and by a specific inhibitor of ß-arrestin/AP-2 interaction; this inhibitor also prevents FAK activation in response to vasopressin. This previously unrecognized mechanism of FAK regulation involving a dual role of ß-arrestins, which inhibit FAK in resting cells while driving its activation at the plasma membrane by GPCR-stimulated G proteins, opens new potential therapeutic perspectives in cancers with up-regulated FAK.

Loss of APJ mediated ß-arrestin signalling improves high-fat diet induced metabolic dysfunction but does not alter cardiac function in mice.

Apelin receptor (APJ) is a G protein-coupled receptor that contributes to many physiological processes and is emerging as a therapeutic target to treat a variety of diseases. For most disease indications the role of G protein vs ß-arrestin signalling in mitigating disease pathophysiology remains poorly understood. This hinders the development of G protein biased APJ agonists, which have been proposed to have several advantages over balanced APJ signalling agonists. To elucidate the contribution of APJ ß-arrestin signalling, we generated a transgenic mouse harbouring a point mutation (APJ I107A) that maintains full G protein activity but fails to recruit ß-arrestin following receptor activation. APJ I107A mutant mice did not alter cardiac function at rest, following exercise challenge or in response to pressure overload induced cardiac hypertrophy. Additionally, APJ I107A mice have comparable body weights, plasma glucose and lipid levels relative to WT mice when fed a chow diet. However, APJ I107A mice showed significantly lower body weight, blood insulin levels, improved glucose tolerance and greater insulin sensitivity when fed a high-fat diet. Furthermore, loss of APJ ß-arrestin signalling also affected fat composition and the expression of lipid metabolism related genes in adipose tissue from high-fat fed mice. Taken together, our results suggest that G protein biased APJ activation may be more effective for certain disease indications given that loss of APJ mediated ß-arrestin signalling appears to mitigate several aspects of diet induced metabolic dysfunction.

Nitric Oxide and S-Nitrosylation in Cardiac Regulation: G Protein-Coupled Receptor Kinase-2 and ß-Arrestins as Targets.

Cardiac diseases including heart failure (HF), are the leading cause of morbidity and mortality globally. Among the prominent characteristics of HF is the loss of ß-adrenoceptor (AR)-mediated inotropic reserve. This is primarily due to the derangements in myocardial regulatory signaling proteins, G protein-coupled receptor (GPCR) kinases (GRKs) and ß-arrestins (ß-Arr) that modulate ß-AR signal termination via receptor desensitization and downregulation. GRK2 and ß-Arr2 activities are elevated in the heart after injury/stress and participate in HF through receptor inactivation. These GPCR regulators are modulated profoundly by nitric oxide (NO) produced by NO synthase (NOS) enzymes through S-nitrosylation due to receptor-coupled NO generation. S-nitrosylation, which is NO-mediated modification of protein cysteine residues to generate an S-nitrosothiol (SNO), mediates many effects of NO independently from its canonical guanylyl cyclase/cGMP/protein kinase G signaling. Herein, we review the knowledge on the NO system in the heart and S-nitrosylation-dependent modifications of myocardial GPCR signaling components GRKs and ß-Arrs.

Opioid Addiction and Opioid Receptor Dimerization: Structural Modeling of the OPRD1 and OPRM1 Heterodimer and Its Signaling Pathways.

Opioid addiction is a complex phenomenon with genetic, social, and other components. Due to such complexity, it is difficult to interpret the outcome of clinical studies, and thus, mutations found in individuals with these addictions are still not indisputably classified as opioid addiction-causing variants. Here, we computationally investigated two such mutations, A6V and N40D, found in the mu opioid receptor gene OPRM1. The mutations are located in the extracellular domain of the corresponding protein, which is important to the hetero-dimerization of OPRM1 with the delta opioid receptor protein (OPRD1). The hetero-dimerization of OPRD1-OPRM1 affects the signaling pathways activated by opioids and natural peptides and, thus, could be considered a factor contributing to addiction. In this study, we built four 3D structures of molecular pathways, including the G-protein signaling pathway and the ß-arrestin signaling pathway of the heterodimer of OPRD1-OPRM1. We also analyzed the effect of mutations of A6V and N40D on the stability of individual OPRM1/OPRD1 molecules and the OPRD1-OPRM1 heterodimer with the goal of inferring their plausible linkage with opioid addiction. It was found that both mutations slightly destabilize OPRM1/OPRD1 monomers and weaken their association. Since hetero-dimerization is a key step for signaling processes, it is anticipated that both mutations may be causing increased addiction risk.

Partial ligand-receptor engagement yields functional bias at the human complement receptor, C5aR1.

The human complement component, C5a, binds two different seven-transmembrane receptors termed C5aR1 and C5aR2. C5aR1 is a prototypical G-protein-coupled receptor that couples to the Gαi subfamily of heterotrimeric G-proteins and ß-arrestins (ßarrs) following C5a stimulation. Peptide fragments derived from the C terminus of C5a can still interact with the receptor, albeit with lower affinity, and can act as agonists or antagonists. However, whether such fragments might display ligand bias at C5aR1 remains unexplored. Here, we compare C5a and a modified C-terminal fragment of C5a, C5apep, in terms of G-protein coupling, ßarr recruitment, endocytosis, and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase activation at the human C5aR1. We discover that C5apep acts as a full agonist for Gαi coupling as measured by cAMP response and extracellular signal-regulated kinase 1/2 phosphorylation, but it displays partial agonism for ßarr recruitment and receptor endocytosis. Interestingly, C5apep exhibits full-agonist efficacy with respect to inhibiting lipopolysaccharide-induced interleukin-6 secretion in human macrophages, but its ability to induce human neutrophil migration is substantially lower compared with C5a, although both these responses are sensitive to pertussis toxin treatment. Taken together, our data reveal that compared with C5a, C5apep exerts partial efficacy for ßarr recruitment, receptor trafficking, and neutrophil migration. Our findings therefore uncover functional bias at C5aR1 and also provide a framework that can potentially be extended to chemokine receptors, which also typically interact with chemokines through a biphasic mechanism.

ß-Adrenergic receptor structure and function: molecular insights guiding the development of novel therapeutic strategies to treat malignancy.

ß adrenergic receptors mediate effects via activation of G proteins, transactivation of membrane growth factor receptors, or ß adrenergic receptor-ß arrestin-facilitated scaffold-mediated signaling. Agonist occupancy of the ß adrenergic receptor induces desensitization by promoting ß adrenergic receptor kinase phosphorylation of the carboxyl terminal domain, facilitating binding of the amino terminal of the ß arrestin, which sterically inhibits interactions between ß adrenergic receptors and G proteins and induces clathrin-coated pit-mediated receptor endocytosis. Scaffold formation promoted by ß arrestin binding to the ß adrenergic receptor activates extracellular regulated kinase 1/2 in a manner which elicits cytosolic retention of, and prevents promotion of nuclear transcriptional activity by, mitogen-activated protein kinase. The ß adrenergic receptor kinase also interacts with a yet to be determined microsomal membrane protein via high-affinity electrostatic interactions. We evaluate ß adrenergic receptor structure, function, and downstream signaling and ß arrestin-mediated desensitization, receptor endocytosis, and scaffold-facilitated signal transduction in order to illumine therapeutic strategies designed to modulate these pathways. We trust these approaches may arm us with the capacity to selectively modulate signal transduction pathways regulating cellular proliferation, immunogenicity, angiogenesis, and invasive and metastatic potential implicated in cancer initiation, promotion, and progression.

Receptor selectivity between the G proteins Gα12 and Gα13 is defined by a single leucine-to-isoleucine variation.

Despite recent advances in structural definition of GPCR-G protein complexes, the basis of receptor selectivity between G proteins remains unclear. The Gα12 and Gα13 subtypes together form the least studied group of heterotrimeric G proteins. G protein-coupled receptor 35 (GPR35) has been suggested to couple efficiently to Gα13 but weakly to Gα12. Using combinations of cells genome-edited to not express G proteins and bioluminescence resonance energy transfer-based sensors, we confirmed marked selectivity of GPR35 for Gα13. Incorporating Gα12/Gα13 chimeras and individual residue swap mutations into these sensors defined that selectivity between Gα13 and Gα12 was imbued largely by a single leucine-to-isoleucine variation at position G.H5.23. Indeed, leucine could not be substituted by other amino acids in Gα13 without almost complete loss of GPR35 coupling. The critical importance of leucine at G.H5.23 for GPR35-G protein interaction was further demonstrated by introduction of this leucine into Gαq, resulting in the gain of coupling to GPR35. These studies demonstrate that Gα13 is markedly the most effective G protein for interaction with GPR35 and that selection between Gα13 and Gα12 is dictated largely by a single conservative amino acid variation.-Mackenzie, A. E., Quon, T., Lin, L.-C., Hauser, A. S., Jenkins, L., Inoue, A., Tobin, A. B., Gloriam, D. E., Hudson, B. D., Milligan, G. Receptor selectivity between the G proteins Gα12 and Gα13 is defined by a single leucine-to-isoleucine variation.