Arrestin-dependent internalization of rhodopsin-like G protein-coupled receptors.

The internalization of G protein-coupled receptors (GPCRs) is an important mechanism regulating the signal strength and limiting the opportunity of receptor activation. Based on the importance of GPCRs, the detailed knowledge about the regulation of signal transduction is crucial. Here, current knowledge about the agonist-induced, arrestin-dependent internalization process of rhodopsin-like GPCRs is reviewed. Arrestins are conserved molecules that act as key players within the internalization process of many GPCRs. Based on highly conserved structural characteristics within the rhodopsin-like GPCRs, the identification of arrestin interaction sites in model systems can be compared and used for the investigation of internalization processes of other receptors. The increasing understanding of this essential regulation mechanism of receptors can be used for drug development targeting rhodopsin-like GPCRs. Here, we focus on the neuropeptide Y receptor family, as these receptors transmit various physiological processes such as food intake, energy homeostasis, and regulation of emotional behavior, and are further involved in pathophysiological processes like cancer, obesity and mood disorders. Hence, this receptor family represents an interesting target for the development of novel therapeutics requiring the understanding of the regulatory mechanisms influencing receptor mediated signaling.

Biased agonists at the human Y1 receptor lead to prolonged membrane residency and extended receptor G protein interaction.

Functionally selective ligands to address specific cellular responses downstream of G protein-coupled receptors (GPCR) open up new possibilities for therapeutics. We designed and characterized novel subtype- and pathway-selective ligands. Substitution of position Q34 of neuropeptide Y to glycine (G34-NPY) results in unprecedented selectivity over all other YR subtypes. Moreover, this ligand displays a significant bias towards activation of the Gi/o pathway over recruitment of arrestin-3. Notably, no bias is observed for an established Y1R versus Y2R selective ligand carrying a proline at position 34 (F7,P34-NPY). Next, we investigated the spatio-temporal signaling at the Y1R and demonstrated that G protein-biased ligands promote a prolonged localization at the cell membrane, which leads to enhanced G protein signaling, while endosomal receptors do not contribute to cAMP signaling. Thus, spatial components are critical for the signaling of the Y1R that can be modulated by tailored ligands and represent a novel mode for biased pathways.

Different mode of arrestin-3 binding at the human Y1 and Y2 receptor.

GPCR internalization, which is induced by arrestin recruitment, is an important mechanism for the regulation of signaling and receptor quantity at the cell surface. In this study, differences in the mechanism of arrestin-3 (arr-3) recruitment to the neuropeptide Y1 and Y2 receptor were identified. These receptors play an essential role in the regulation of feeding, energy homeostasis and cancer. The Y1R displays high affinity to arr-3, which induces rapid internalization of the arrestin/receptor complex. In contrast, the Y2R has a lower affinity for arr-3. Internalization is induced by arrestin binding, but arr-3 is released from the receptor and remains at the membrane while the receptor internalizes. Moreover, the deletion of the finger loop region of arr-3 reduces its agonist-dependent recruitment to the Y2R significantly, but not to the Y1R suggesting different binding conformations. For the first time, the formation of a supercomplex consisting of Y receptor, Gα0 protein and arrestin was studied by BRET-assay. We demonstrated that the Y1R is able to bind Gα0 protein as well as arr-3 simultaneously and internalizes as a supercomplex. For the Y2R no supercomplex formation was observed. By substituting the C-terminus or specific residues within the intracellular loop 1 and 2 of the receptors, the arr-3 recruitment of the Y1R and Y2R can be switched. Thus, we shed light on the specific spatio-temporal distribution of Gα0 protein and arrestin in response to Y1 versus Y2 receptor activation and identified the molecular determinants.

G Protein Preassembly Rescues Efficacy of W6.48 Toggle Mutations in Neuropeptide Y2 Receptor.

Ligand binding and pathway-specific activation of G protein-coupled receptors is currently being studied with great effort. Individual answers may depend on the nature of the ligands and the effector pathway. Recently, we have presented a detailed model of neuropeptide Y bound to the Y2R. Accordingly, the C-terminal part of the peptide binds deeply in the transmembrane bundle and brings the side chain of the most essential Y36 in close proximity to W6.48 Here, we investigate the role of this interaction for ligand binding and activation of this receptor. BRET sensors were used for detailed investigation of effector coupling and led to the identification of preassembly of the Y2R-Gi complex. It further confirmed ligand-dependent recruitment of arrestin3. Using equally sensitive readouts for Gi activation and arrestin recruitment as well as quantification with operational models of agonism allowed us to identify a strong inherent bias for Gi activation over arrestin3 recruitment for the wild-type receptor. By systematic mutagenesis, we found that W6.48 does not contribute to the binding affinity, but acts as an allosteric connector to couple ligand binding to Gi activation and arrestin3 recruitment. However, even mutagenesis to a small threonine did not lead to a complete loss of signaling. Interestingly, signaling was restored to wild-type levels by ligands that contain a naphthylalanine as the C-terminal residue instead of Y36 Steric and polar contributions of W6.48 for the activation of the receptor are discussed in the context of different mechanisms of G protein coupling and arrestin recruitment.

C-terminal motif of human neuropeptide Y4 receptor determines internalization and arrestin recruitment.

The human neuropeptide Y4 receptor is a rhodopsin-like G protein-coupled receptor (GPCR), which contributes to anorexigenic signals. Thus, this receptor is a highly interesting target for metabolic diseases. As GPCR internalization and trafficking affect receptor signaling and vice versa, we aimed to investigate the molecular mechanism of hY4R desensitization and endocytosis. The role of distinct segments of the hY4R carboxyl terminus was investigated by fluorescence microscopy, binding assays, inositol turnover experiments and bioluminescence resonance energy transfer assays to examine the internalization behavior of hY4R and its interaction with arrestin-3. Based on results of C-terminal deletion mutants and substitution of single amino acids, the motif 7.78EESEHLPLSTVHTEVSKGS7.96 was identified, with glutamate, threonine and serine residues playing key roles, based on site-directed mutagenesis. Thus, we identified the internalization motif for the human neuropeptide Y4 receptor, which regulates arrestin-3 recruitment and receptor endocytosis.

A G Protein-biased Designer G Protein-coupled Receptor Useful for Studying the Physiological Relevance of Gq/11-dependent Signaling Pathways.

Designerreceptorsexclusivelyactivated by adesignerdrug (DREADDs) are clozapine-N-oxide-sensitive designer G protein-coupled receptors (GPCRs) that have emerged as powerful novel chemogenetic tools to study the physiological relevance of GPCR signaling pathways in specific cell types or tissues. Like endogenous GPCRs, clozapine-N-oxide-activated DREADDs do not only activate heterotrimeric G proteins but can also trigger ß-arrestin-dependent (G protein-independent) signaling. To dissect the relative physiological relevance of G protein-mediatedversusß-arrestin-mediated signaling in different cell types or physiological processes, the availability of G protein- and ß-arrestin-biased DREADDs would be highly desirable. In this study, we report the development of a mutationally modified version of a non-biased DREADD derived from the M3muscarinic receptor that can activate Gq/11with high efficacy but lacks the ability to interact with ß-arrestins. We also demonstrate that this novel DREADD is activein vivoand that cell type-selective expression of this new designer receptor can provide novel insights into the physiological roles of G protein (Gq/11)-dependentversusß-arrestin-dependent signaling in hepatocytes. Thus, this novel Gq/11-biased DREADD represents a powerful new tool to study the physiological relevance of Gq/11-dependent signaling in distinct tissues and cell types, in the absence of ß-arrestin-mediated cellular effects. Such studies should guide the development of novel classes of functionally biased ligands that show high efficacy in various pathophysiological conditions but display a reduced incidence of side effects.

Peptide modifications differentially alter G protein-coupled receptor internalization and signaling bias.

Although G protein-coupled receptors (GPCRs) are targeted by more clinically used drugs than any other type of protein, their ligand development is particularly challenging. Humans have four neuropeptide Y receptors: hY1R and hY5R are orexigenic, while hY2R and hY4R are anorexigenic, and represent important anti-obesity drug targets. We show for the first time that PEGylation and lipidation, chemical modifications that prolong the plasma half-lives of peptides, confer additional benefits. Both modifications enhance pancreatic polypeptide preference for hY2R/hY4R over hY1R/hY5R. Lipidation biases the ligand towards arrestin recruitment and internalization, whereas PEGylation confers the opposite bias. These effects were independent of the cell system and modified residue. We thus provide novel insights into the mode of action of peptide modifications and open innovative venues for generating peptide agonists with extended therapeutic potential.

Mutations in arrestin-3 differentially affect binding to neuropeptide Y receptor subtypes.

Based on the identification of residues that determine receptor selectivity in arrestins and the phylogenetic analysis of the arrestin (arr) family, we introduced fifteen mutations of receptor-discriminator residues in arr-3, which were identified previously using mutagenesis, in vitro binding, and BRET-based recruitment assay in intact cells. The effects of these mutations were tested using neuropeptide Y receptors Y1R and Y2R. NPY-elicited arr-3 recruitment to Y1R was not affected by these mutations, or even alanine substitution of all ten residues (arr-3-NCA), which prevented arr-3 binding to other receptors tested so far. However, NCA and two other mutations prevented agonist-independent arr-3 pre-docking to Y1R. In contrast, eight out of 15 mutations significantly reduced agonist-dependent arr-3 recruitment to Y2R. NCA eliminated arr-3 binding to active Y2R, whereas Tyr239Thr reduced it ~7-fold. Thus, manipulation of key residues on the receptor-binding surface generates arr-3 with high preference for Y1R over Y2R. Several mutations differentially affect arr-3 pre-docking and agonist-induced recruitment. Thus, arr-3 recruitment to the receptor involves several mechanistically distinct steps. Targeted mutagenesis can fine-tune arrestins directing them to specific receptors and particular activation states of the same receptor.