Molecular determinants underlying the formation of stable intracellular G protein-coupled receptor-beta-arrestin complexes after receptor endocytosis*

Cell-type-specific pathways of neurotensin endocytosis

The neurotensin receptor subtype 1 (NTS1) is a G-protein-coupled receptor (GPCR) mediating a large number of central and peripheral effects of neurotensin. Upon stimulation, NTS1 is rapidly internalized and targeted to lysosomes. This process depends on the interaction of the phosphorylated receptor with beta-arrestin. Little is known about other accessory endocytic proteins potentially involved. Here, we investigated the involvement of dynamin, amphiphysin, and intersectin in the internalization of NTS1 receptor-ligand complexes in transfected COS-7 and HEK 293 cells, by using the transferrin receptor as an internal control for the constitutive endocytic pathway. We found that NTS1 endocytosis was not only arrestin-dependent, but also dynamin-dependent in both COS-7 and HEK 293 cells, whereas internalization of the transferrin receptor was independent of arrestin but required dynamin. Overexpression of the SH3 domain of amphiphysin II had no effect on receptor internalization in either cell type. By contrast, overexpression of full-length intersectin or of its SH3 domain (but not of its EH domain) inhibited NTS1 internalization in COS-7 but not in HEK 293 cells. This difference between COS-7 and HEK 293 cells was not attributable to differences in endogenous intersectin levels between the two cell lines. Indeed, the same constructs inhibited transferrin endocytosis equally well in COS-7 and HEK 293 cells. However, immunogold electron microscopy revealed that internalized NTS1 receptors were associated with clathrin-coated pits in COS-7 cells but with smooth vesicles in HEK 293 cells, suggesting that NTS1 internalization proceeds via different endocytic pathways in these two cell types.

A Ser/Thr cluster within the C-terminal domain of the rat prostaglandin receptor EP3alpha is essential for agonist-induced phosphorylation, desensitization and internalization

Two isoforms of the rat prostaglandin E(2) receptor, rEP3alpha-R and rEP3beta-R, differ only in their C-terminal domain. To analyze the function of the rEP3-R C-terminal domain in agonist induced desensitization, a cluster of Ser/Thr residues in the C-terminal domain of the rEP3alpha-R was mutated to Ala and both isoforms and the receptor mutant (rEP3alpha-ST341-349A-R) were stably expressed in HEK293 cells. All rEP3-R receptors showed a similar ligand-binding profile. They were functionally coupled to Gi and reduced forskolin-induced cAMP-formation. Repeated exposure of cells expressing the rEP3alpha-R isoform to PGE(2) reduced the agonist induced inhibition of forskolin-stimulated cAMP-formation by 50% and led to internalization of the receptor to intracellular endocytotic vesicles. By contrast, Gi-response as well as plasma membrane localization of the rEP3beta-R and the rEP3alpha-ST341-349A-R were not affected by prior agonist-stimulation. Agonist-stimulation of HEK293-rEP3alpha-R cells induced a time- and dose-dependent phosphorylation of the receptor most likely by G protein-coupled receptor kinases and not by protein kinase A or protein kinase C. By contrast, upon agonist-stimulation the rEP3beta-R was not phosphorylated and the rEP3alpha-ST341-349A-R was phosphorylated only weakly. These results led to the hypothesis that agonist-induced desensitization of the rEP3alpha-R isoform is mediated most likely by a GRK-dependent phosphorylation of Ser/Thr residues 341-349. Phosphorylation then initiates uncoupling of the receptor from Gi protein and receptor internalization.

A beta-arrestin binding determinant common to the second intracellular loops of rhodopsin family G protein-coupled receptors

beta-Arrestins have been shown to inhibit competitively G protein-dependent signaling and to mediate endocytosis for many of the hundreds of nonvisual rhodopsin family G protein-coupled receptors (GPCR). An open question of fundamental importance concerning the regulation of signal transduction of several hundred rhodopsin-like GPCRs is how these receptors of limited sequence homology, when considered in toto, can all recruit and activate the two highly conserved beta-arrestin proteins as part of their signaling/desensitization process. Although the serine and threonine residues that form GPCR kinase phosphorylation sites are common beta-arrestin-associated receptor determinants regulating receptor desensitization and internalization, the agonist-activated conformation of a GPCR probably reveals the most fundamental determinant mediating the GPCR and arrestin interaction. Here we identified a beta-arrestin binding determinant common to the rhodopsin family GPCRs formed from the proximal 10 residues of the second intracellular loop. We demonstrated by both gain and loss of function studies for the serotonin 2C, beta2-adrenergic, alpha2a)adrenergic, and neuropeptide Y type 2 receptors that the highly conserved amino acids, proline and alanine, naturally occurring in rhodopsin family receptors six residues distal to the highly conserved second loop DRY motif regulate beta-arrestin binding and beta-arrestin-mediated internalization. In particular, as demonstrated for the beta2 AR, this occurs independently of changes in GPCR kinase phosphorylation. These results suggest that a GPCR conformation directed by the second intracellular loop, likely using the loop itself as a binding patch, may function as a switch for transitioning beta-arrestin from its inactive form to its active receptor-binding state.

High-throughput screening of G protein-coupled receptor antagonists using a bioluminescence resonance energy transfer 1-based beta-arrestin2 recruitment assay

In this study, the authors developed HEK293 cell lines that stably coexpressed optimal amounts of beta-arrestin2-Rluc and VENUS fusions of G protein-coupled receptors (GPCRs) belonging to both class A and class B receptors, which include receptors that interact transiently or stably with beta-arrestins. This allowed the use of a bioluminescence resonance energy transfer (BRET) 1- beta-arrestin2 translocation assay to quantify receptor activation or inhibition. One of the developed cell lines coexpressing CCR5-VENUS and beta-arrestin2- Renilla luciferase was then used for high-throughput screening (HTS) for antagonists of the chemokine receptor CCR5, the primary co-receptor for HIV. A total of 26,000 compounds were screened for inhibition of the agonist-promoted beta-arrestin2 recruitment to CCR5, and 12 compounds were found to specifically inhibit the agonist-induced beta-arrestin2 recruitment to CCR5. Three of the potential hits were further tested using other functional assays, and their abilities to inhibit CCR5 agonist-promoted signaling were confirmed. This is the 1st study describing a BRET1-beta-arrestin recruitment assay in stable mammalian cells and its successful application in HTS for GPCRs antagonists.

Composition and function of g protein-coupled receptor signalsomes controlling mitogen-activated protein kinase activity

Seven membrane-spanning G protein-coupled receptors (GPCRs) function as ligand-activated guanine nucleotide exchange factors for heterotrimeric guanine nucleotide-binding (G) proteins that relay extracellular stimuli by activating intracellular effector enzymes or ion channels. Recent work, however, has shown that GPCRs also participate in numerous other protein-protein interactions that generate intracellular signals in conjunction with, or even independent of, G-protein activation. Nowhere has the importance of protein complex assembly in GPCR signaling been demonstrated more clearly than in the control of the spatial and temporal activity of the extracellular signal-regulated kinase (ERK1/2) mitogen-activated protein (MAP) kinase cascade. ERK1/2 activation by GPCRs often involves cross talk with classical receptor tyrosine kinases or focal adhesion complexes, which scaffold the assembly of a Ras activation complex. Even more surprising is the phenomenon of G protein-independent signaling using beta-arrestins, proteins originally characterized for their role in homologous GPCR desensitization, as scaffolds for the assembly of a multiprotein signalsome directly upon the GPCR. Although both forms of signaling lead to MAP kinase activation, the pathways appear to be functionally, as well as mechanistically, distinct. Transactivated receptor tyrosine kinases mediate rapid and transient MAP kinase activation that favors nuclear translocation of the kinases and transcriptional activation. In contrast, beta-arrestin-dependent signaling produces a slower and more sustained increase in MAP kinase activity that is often restricted to the cytosol. Together, these highly organized signaling complexes dictate the location, duration, and ultimate function of GPCR-stimulated MAP kinase activity.

Characterization of agonist-induced motilin receptor trafficking and its implications for tachyphylaxis

The motilin receptor (MR) is a member of the seven-transmembrane receptor family and is expressed throughout the gastrointestinal tract of humans and other species. Motilin, the natural MR peptide ligand, has profound stimulatory effects on gastrointestinal contractility, indicating a therapeutic potential for MR modulators. However, long-term clinical use of certain MR agonists is limited by tachyphylaxis, a reduced responsiveness to repeated compound exposure. This study was meant to characterize the ligand-induced endocytosis of MR and to test whether receptor trafficking contributes to tachyphylaxis. A cell-based assay was developed by fusing a green fluorescent protein (GFP) moiety to the motilin receptor, and high-content biology instrumentation was used to quantify time and dose dependence of MR-GFP endocytosis. Maximal internalization of MR-GFP was induced after 45 min of constant exposure to 80 nM motilin. This process was disrupted by nocodazole, suggesting an essential role for microtubules. Internalized MR-GFP vesicles disappeared within 15 to 45 min of motilin withdrawal but did not overlap with the lysosomal compartment, indicating that MR-GFP escaped degradation and was recycled back to the plasma membrane. It is noteworthy that the kinetics of MR-GFP redistribution varied substantially when stimulated with motilin, erythromycin, 6,9-hemiacetal 8,9-anhydro-4''-deoxy-3'-N-desmethyl-3'-N-ethylerythromycin B (ABT-229), or N-[(1S)-1-[[[(1S)-1-(aminocarbonyl)-3-phenylpropyl]amino]carbonyl]-3-phenylpropyl]-2'-(1,3-benzodioxol-5-ylmethyl)tetrahydro-1',3'-dioxo-spiro[piperidine-4,5'(6'H)-[1H][1,2,4]triazolo[1,2-a]pyridazine]-8'-carboxamide (BMS-591348) at equipotent doses for Ca(2+)-mobilization. Retardation of the intracellular MR-GFP sorting cycle seemed to correlate with the tachyphylaxis-inducing properties of each compound, but not its EC(50). These results indicate that MR internalization, desensitization, and resensitization are ligand-dependent and that appropriate screening strategies may enable the development of small molecule agonists with ideal combinations of these distinct properties.

Pleiotropic AT1 receptor signaling pathways mediating physiological and pathogenic actions of angiotensin II

Angiotensin II (Ang II) activates a wide spectrum of signaling responses via the AT1 receptor (AT1R) that mediate its physiological control of blood pressure, thirst, and sodium balance and its diverse pathological actions in cardiovascular, renal, and other cell types. Ang II-induced AT1R activation via Gq/11 stimulates phospholipases A2, C, and D, and activates inositol trisphosphate/Ca2+ signaling, protein kinase C isoforms, and MAPKs, as well as several tyrosine kinases (Pyk2, Src, Tyk2, FAK), scaffold proteins (G protein-coupled receptor kinase-interacting protein 1, p130Cas, paxillin, vinculin), receptor tyrosine kinases, and the nuclear factor-kappaB pathway. The AT1R also signals via Gi/o and G11/12 and stimulates G protein-independent signaling pathways, such as beta-arrestin-mediated MAPK activation and the Jak/STAT. Alterations in homo- or heterodimerization of the AT1R may also contribute to its pathophysiological roles. Many of the deleterious actions of AT1R activation are initiated by locally generated, rather than circulating, Ang II and are concomitant with the harmful effects of aldosterone in the cardiovascular system. AT1R-mediated overproduction of reactive oxygen species has potent growth-promoting, proinflammatory, and profibrotic actions by exerting positive feedback effects that amplify its signaling in cardiovascular cells, leukocytes, and monocytes. In addition to its roles in cardiovascular and renal disease, agonist-induced activation of the AT1R also participates in the development of metabolic diseases and promotes tumor progression and metastasis through its growth-promoting and proangiogenic activities. The recognition of Ang II's pathogenic actions is leading to novel clinical applications of angiotensin-converting enzyme inhibitors and AT1R antagonists, in addition to their established therapeutic actions in essential hypertension.

Monitoring agonist-promoted conformational changes of beta-arrestin in living cells by intramolecular BRET

Recruitment of beta-arrestin (beta-arr) to agonist-stimulated G-protein-coupled receptors (GPCRs) has a crucial role in controlling signalling efficacy and selectivity. When translocated to the receptor, beta-arr is believed to undergo important conformational rearrangement necessary for its downstream actions. To probe these changes in living cells, we constructed an intramolecular bioluminescence resonance energy transfer (BRET)-based biosensor, in which beta-arr is sandwiched between the Renilla luciferase (Luc) and the yellow fluorescent protein (YFP). We show that the intramolecular BRET between Luc and YFP was significantly increased following GPCR activation, suggesting a conformational rearrangement bringing the amino terminus and carboxyl terminus of beta-arr in closer proximity. Kinetic analysis showed that this conformational change follows the initial beta-arr/receptor engagement. In addition to providing new insights into the agonist-induced conformational rearrangements of beta-arr in living cells, the double-brilliance beta-arr offers a universal biosensor for GPCR activation, allowing the study of native receptors in large-scale screening analysis.

Chemokine receptor internalization and intracellular trafficking

The internalization and intracellular trafficking of chemokine receptors have important implications for the cellular responses elicited by chemokine receptors. The major pathway by which chemokine receptors internalize is the clathrin-mediated pathway, but some receptors may utilize lipid rafts/caveolae-dependent internalization routes. This review discusses the current knowledge and controversies regarding these two different routes of endocytosis. The functional consequences of internalization and the regulation of chemokine receptor recycling will also be addressed. Modifications of chemokine receptors, such as palmitoylation, ubiquitination, glycosylation, and sulfation, may also impact trafficking, chemotaxis and signaling. Finally, this review will cover the internalization and trafficking of viral and decoy chemokine receptors.

Involvement of a cytoplasmic-tail serine cluster in urotensin II receptor internalization

Most G-protein-coupled receptors that undergo agonist-dependent internalization require the presence of specific cytoplasmic-tail residues to initiate interactions with proteins of the endocytic machinery. Here we show that the UT receptor (urotensin II receptor) undergoes internalization, and that specific serine residues of the receptor's cytoplasmic tail participate in this process. We first observed a time-dependent increase in internalization of the UT receptor expressed in COS-7 cells following binding of the agonist urotensin II. This sequestration was significantly reduced in the presence of sucrose, demonstrating that the agonist-activated UT receptor is internalized in part by clathrin-coated pits. Moreover, the sequestered receptor was co-localized in endocytic vesicles with beta-arrestin1 and beta-arrestin2. To assess whether specific regions of the receptor's cytoplasmic tail were involved in internalization, five UT receptor mutants were constructed. In four constructs the receptor's cytoplasmic tail was truncated at various positions (UTDelta367, UTDelta363, UTDelta350 and UTDelta336), and in the other four adjacent serine residues at positions 364-367 were replaced by Ala (Mut4S). Each mutant, except UTDelta367, demonstrated a significantly reduced internalization rate, thereby revealing the importance of specific serine residues within the cytoplasmic tail of the UT receptor for its ability to be internalized efficiently.

A novel diuretic hormone receptor in Drosophila: evidence for conservation of CGRP signaling

The Drosophila orphan G protein-coupled receptor encoded by CG17415 is related to members of the calcitonin receptor-like receptor (CLR) family. In mammals, signaling from CLR receptors depend on accessory proteins, namely the receptor activity modifying proteins (RAMPs) and receptor component protein (RCP). We tested the possibility that this Drosophila CLR might also require accessory proteins for proper function and we report that co-expression of the mammalian or Drosophila RCP or mammalian RAMPs permitted neuropeptide diuretic hormone 31 (DH31) signaling from the CG17415 receptor. RAMP subtype expression did not alter the pharmacological profile of CG17415 activation. CG17415 antibodies revealed expression within the principal cells of Malpighian tubules, further implicating DH31 as a ligand for this receptor. Immunostaining in the brain revealed an unexpected convergence of two distinct DH signaling pathways. In both the larval and adult brain, most DH31 receptor-expressing neurons produce the neuropeptide corazonin, and also express the CRFR-related receptor CG8422, which is a receptor for the neuropeptide diuretic hormone 44 (DH44). There is extensive convergence of CRF and CGRP signaling within vertebrates and we report a striking parallel in Drosophila involving DH44 (CRF) and DH31 (CGRP). Therefore, it appears that both the molecular details as well as the functional organization of CGRP signaling have been conserved.

ATP stimulates GRK-3 phosphorylation and beta-arrestin-2-dependent internalization of P2X7 receptor

The objective of this study was to understand the mechanisms involved in P2X(7) receptor activation. Treatments with ATP or with the P2X(7) receptor-specific ligand 2',3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) induced pore formation, but the effect was slower in CaSki cells expressing endogenous P2X(7) receptor than in human embryonic kidney (HEK)-293 cells expressing exogenous P2X(7) receptor (HEK-293-hP2X(7)-R). In both types of cells Western blots revealed expression of three forms of the receptor: the functional 85-kDa form present mainly in the membrane and 65- and 18-kDa forms expressed in both the plasma membrane and the cytosol. Treatments with ATP transiently decreased the 85-kDa form and increased the 18-kDa form in the membrane, suggesting internalization, degradation, and recycling of the receptor. In CaSki cells ATP stimulated phosphorylation of the 85-kDa form on tyrosine and serine residues. Phosphorylation on threonine residues increased with added ATP, and it increased ATP requirements for phosphorylation on tyrosine and serine residues, suggesting a dominant-negative effect. In both CaSki and in HEK-293-hP2X(7)-R cells ATP also increased binding of the 85-kDa form to G protein-coupled receptor kinase (GRK)-3, beta-arrestin-2, and dynamin, and it stimulated beta-arrestin-2 redistribution into submembranous regions of the cell. These results suggest a novel mechanism for P2X(7) receptor action, whereby activation involves a GRK-3-, beta-arrestin-2-, and dynamin-dependent internalization of the receptor into clathrin domains, followed in part by receptor degradation as well as receptor recycling into the plasma membrane.

Inhibition of chemoattractant N-formyl peptide receptor trafficking by active arrestins

Recent studies have highlighted the emergence of a class of G protein-coupled receptors that are internalized in an arrestin-independent manner. In addition to demonstrating that the N-formyl peptide receptor belongs in this family, we have recently shown that recycling of the receptor requires the presence of arrestins. To further elucidate mechanisms of arrestin-dependent regulation of G protein-coupled receptor processing, we examined the effects of altering the receptor-arrestin complex on ternary complex formation and cellular trafficking of the N-formyl peptide receptor by studying two active arrestin-2 mutants (truncated arrestin-2 [1-382], and arrestin-2 I386A, V387A, F388A). Complexes between the N-formyl peptide receptor and active arrestins exhibited higher affinity in vitro than the complex between the N-formyl peptide receptor and wild-type arrestin and furthermore were observed in vivo by colocalization studies using confocal microscopy. To assess the effects of these altered interactions on receptor trafficking, we demonstrated that active, but not wild-type, arrestin expression retards N-formyl peptide receptor internalization. Furthermore, expression of arrestin-2 I386A/V387A/F388A but not arrestin-2 [1-382] inhibited recycling of the N-formyl peptide receptor, reflecting an expanded role for arrestins in G protein-coupled receptor processing and trafficking. Whereas the extent of N-formyl peptide receptor phosphorylation had no effect on the inhibition of internalization, N-formyl peptide receptor recycling was restored when the receptor was only partially phosphorylated. These results indicate not only that a functional interaction between receptor and arrestin is required for recycling of certain G protein-coupled receptors, such as the N-formyl peptide receptor, but that the pattern of receptor phosphorylation further regulates this process.

Uncoupling and endocytosis of 5-hydroxytryptamine 4 receptors. Distinct molecular events with different GRK2 requirements

The 5-hydroxytryptamine type 4 receptors (5-HT4Rs) are involved in memory, cognition, feeding, respiratory control, and gastrointestinal motility through activation of a G(s)/cAMP pathway. We have shown that 5-HT4R undergoes rapid and profound homologous uncoupling in neurons. However, no significant uncoupling was observed in COS-7 or HEK293 cells, which expressed either no or a weak concentration of GRK2, respectively. High expression of GRK2 in neurons is likely to be the reason for this difference because overexpression of GRK2 in COS-7 and HEK293 cells reproduced rapid and profound uncoupling of 5-HT4R. We have also shown, for the first time, that GRK2 requirements for uncoupling and endocytosis were very different. Indeed, beta-arrestin/dynamin-dependent endocytosis was observed in HEK293 cells without any need of GRK2 overexpression. In addition to this difference, uncoupling and beta-arrestin/dynamin-dependent endocytosis were mediated through distinct mechanisms. Neither uncoupling nor beta-arrestin/dynamin-dependent endocytosis required the serine and threonine residues localized within the specific C-terminal domains of the 5-HT4R splice variants. In contrast, a cluster of serines and threonines, common to all variants, was an absolute requirement for beta-arrestin/dynamin-dependent receptor endocytosis, but not for receptor uncoupling. Furthermore, beta-arrestin/dynamin-dependent endocytosis and uncoupling were dependent on and independent of GRK2 kinase activity, respectively. These results clearly demonstrate that the uncoupling and endocytosis of 5-HT4R require different GRK2 concentrations and involve distinct molecular events.

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.

The oxytocin receptor antagonist atosiban inhibits cell growth via a "biased agonist" mechanism

In human myometrial cells, the promiscuous coupling of the oxytocin receptors (OTRs) to G(q) and G(i) leads to contraction. However, the activation of OTRs coupled to different G protein pathways can also trigger opposite cellular responses, e.g. OTR coupling to G(i) inhibits, whereas its coupling to G(q) stimulates, cell proliferation. Drug analogues capable of promoting a selective receptor-G protein coupling may be of great pharmacological and clinical importance because they may target only one specific signal transduction pathway. Here, we report that atosiban, an oxytocin derivative that acts as a competitive antagonist on OTR/G(q) coupling, displays agonistic properties on OTR/G(i) coupling, as shown by specific (35)S-labeled guanosine 5'-3-O-(thio) trisphosphate ([(35)S]GTPgammaS) binding. Moreover, atosiban, by acting on a G(i)-mediated pathway(,) inhibits cell growth of HEK293 and Madin-Darby canine kidney cells stably transfected with OTRs and of DU145 prostate cancer cells expressing endogenous OTRs. Notably, atosiban leads to persistent ERK1/2 activation and p21(WAF1/CIP1) induction, the same signaling events leading to oxytocin-mediated cell growth inhibition via a G(i) pathway. Finally, atosiban exposure did not cause OTR internalization and led to only a modest decrease (20%) in the number of high affinity cell membrane OTRs, two observations consistent with the finding that atosiban did not lead to any desensitization of the oxytocin-induced activation of the G(q)-phospholipase C pathway. Taken together, these observations indicate that atosiban acts as a "biased agonist" of the human OTRs and thus belongs to the class of compounds capable of selectively discriminating only one among the multiple possible active conformations of a single G protein-coupled receptor, thereby leading to the selective activation of a unique intracellular signal cascade.

C5L2 is a functional receptor for acylation-stimulating protein

C5L2 binds acylation-stimulating protein (ASP) with high affinity and is expressed in ASP-responsive cells. Functionality of C5L2 has not yet been demonstrated. Here we show that C5L2 is expressed in human subcutaneous and omental adipose tissue in both preadipocytes and adipocytes. In mice, C5L2 is expressed in all adipose tissues, at levels comparable with other tissues. Stable transfection of human C5L2 cDNA into HEK293 cells results in ASP stimulation of triglyceride synthesis (TGS) (193 +/- 33%, 5 microM ASP, p < 0.001, where basal = 100%) and glucose transport (168 +/- 21%, 10 microM ASP, p < 0.001). C3a similarly stimulates TGS (163 +/- 12%, p < 0.001), but C5a and C5a des-Arg have no effect. The ASP mechanism is to increase Vmax of glucose transport (149%) and triglyceride (TG) synthesis activity (165%) through increased diacylglycerolacyltransferase activity (200%). Antisense oligonucleotide down-regulation of C5L2 in human skin fibroblasts decreases cell surface C5L2 (down to 54 +/- 4% of control, p < 0.001, comparable with nonimmune background). ASP response is coordinately lost (basal TGS = 14.6 +/- 1.6, with ASP = 21.0 +/- 1.4 (144%), with ASP + oligonucleotides = 11.0 +/- 0.8 pmol of TG/mg of cell protein, p < 0.001). In mouse 3T3-L1 preadipocytes, antisense oligonucleotides decrease C5L2 expression to 69.5 +/- 0.5% of control, p < 0.001 (comparable with nonimmune) with a loss of ASP stimulation (basal TGS = 22.4 +/- 2.9, with ASP = 39.6 +/- 8.8 (177%), with ASP + oligonucleotides = 25.3 +/- 3.0 pmol of TG/mg of cell protein, p < 0.001). C5L2 down-regulation and decreased ASP response correlate (r = 0.761, p < 0.0001 for HSF and r = 0.451, p < 0.05 for 3T3-L1). In HEK-hC5L2 expressing fluorescently tagged beta-arrestin, ASP induced beta-arrestin translocation to the plasma membrane and formation of endocytic complexes concurrently with increased phosphorylation of C5L2. This is the first demonstration that C5L2 is a functional receptor, mediating ASP triglyceride stimulation.

The glucagon-like peptide-2 receptor C terminus modulates beta-arrestin-2 association but is dispensable for ligand-induced desensitization, endocytosis, and G-protein-dependent effector activation

Classic models of receptor desensitization and internalization have been largely based on the behavior of Family A G-protein-coupled receptors (GPCRs). The glucagon-like peptide-2 receptor (GLP-2R) is a member of the Family B glucagon-secretin GPCR family, which exhibit significant sequence and structural differences from the Family A receptors in their intracellular and extracellular domains. To identify structural motifs that regulate GLP-2R signaling and cell surface receptor expression, we analyzed the functional properties of a series of mutant GLP-2Rs. The majority of the C-terminal receptor tail was dispensable for GLP-2-induced cAMP accumulation, ERK1/2 activation, and endocytosis in transfected cells. However, progressive truncation of the C terminus reduced cell surface receptor expression, altered agonist-induced GLP-2R trafficking, and abrogated protein kinase A-mediated heterologous receptor desensitization. Elimination of the distal 21 amino acids of the receptor was sufficient to promote constitutive receptor internalization and prevent agonist-induced recruitment of beta-arrestin-2. Site-directed mutagenesis identified specific amino acid residues within the distal GLP-2R C terminus that mediate the stable association with beta-arrestin-2. Surprisingly, although the truncated mutant receptors failed to interact with beta-arrestin-2, they underwent homologous desensitization and subsequent resensitization with kinetics similar to that observed with the wild-type GLP-2R. Our data suggest that, although the GLP-2R C terminus is not required for coupling to cellular machinery regulating signaling or desensitization, it may serve as a sorting signal for intracellular trafficking. Taken together with the previously demonstrated clathrin and dynamin-independent, lipid-raft-dependent pathways for internalization, our data suggest that GLP-2 receptor signaling has evolved unique structural and functional mechanisms for control of receptor trafficking, desensitization, and resensitization.

Internal PDZ ligands: novel endocytic recycling motifs for G protein-coupled receptors

Internalization, recycling and lysosomal sorting are key processes that regulate the temporal and spatial signaling of G protein-coupled receptors (GPCRs). Interactions between GPCR intracytosolic sorting signals and adaptor proteins facilitate trafficking through the endocytic pathway. To date only a few sorting signals and molecules that regulate GPCR trafficking have been identified. A study reported in the May 2005 issue of Molecular Pharmacology has now identified an internal PDZ ligand motif that seems to regulate efficient recycling of the ET(A) endothelin receptor. This finding now expands the diversity of GPCR sorting motifs to include internal and C-terminal PDZ ligands, tyrosine-based motifs, and lysine residues capable of being ubiquitinated.

beta-Arrestins bind and decrease cell-surface abundance of the Na+/H+ exchanger NHE5 isoform

The neuronal Na(+)/H(+) exchanger NHE5 isoform not only resides in the plasma membrane but also accumulates in recycling vesicles by means of clathrin-mediated endocytosis. To further investigate the underlying molecular mechanisms, a human brain cDNA library was screened for proteins that interact with the cytoplasmic C-terminal region of NHE5 by using yeast two-hybrid methodology. One candidate cDNA identified by this procedure encoded beta-arrestin2, a specialized adaptor/scaffolding protein required for internalization and signaling of members of the G protein-coupled receptor superfamily. Direct interaction between the two proteins was demonstrated in vitro by GST fusion protein pull-down assays. Sequences within the N-terminal receptor activation-recognition domain and the C-terminal secondary receptor-binding domain of beta-arrestin2 conferred strong binding to the C terminus of NHE5. Full-length NHE5 and beta-arrestin2 also associated in intact cells, as revealed by their coimmunoprecipitation from extracts of transfected CHO cells. Moreover, ectopic expression of both proteins caused a redistribution of beta-arrestin2 from the cytoplasm to vesicles containing NHE5, and significantly decreased the abundance of the transporter at the cell surface. Comparable results were also obtained for the beta-arrestin1 isoform. These data reveal a broader role for arrestins in the trafficking of integral plasma membrane proteins than previously recognized.

Adaptors for clathrin coats: structure and function

Clathrin-coated vesicles (CCVs) are responsible for the transport of proteins between various compartments of the secretory and endocytic systems. Clathrin forms a scaffold around these vesicles that is linked to membranes by clathrin adaptors. The adaptors simultaneously bind to clathrin and to transmembrane proteins and/or phospholipids and can also interact with each other and with other components of the CCV formation machinery. The result is a collection of proteins that can make multiple, moderate strength (microM Kd) interactions and thereby establish the dynamic regulatable networks to drive vesicle genesis at the correct time and place in the cell. This review focuses on the structure of clathrin adaptors and how these structures provide functional information on the mechanism of CCV formation.

G protein-coupled receptor kinase regulates dopamine D3 receptor signaling by modulating the stability of a receptor-filamin-beta-arrestin complex. A case of autoreceptor regulation

In addition to its postsynaptic role, the dopamine D3 receptor (D3R) serves as a presynaptic autoreceptor, where it provides continuous feedback regulation of dopamine release at nerve terminals for processes as diverse as emotional tone and locomotion. D3R signaling ability is supported by an association with filamin (actin-binding protein 280), which localizes the receptor with G proteins in plasma membrane lipid rafts but is not appreciably antagonized in a classical sense by the ligand-mediated activation of G protein-coupled receptor kinases (GRKs) and beta-arrestins. In this study, we investigate GRK-mediated regulation of D3R.filamin complex stability and its effect on D3R.G protein signaling potential. Studies in HEK-293 cells show that in the absence of agonist the D3R immunoprecipitates in a complex containing both filamin A and beta-arrestin2. Moreover, the filamin directly interacts with beta-arrestin2 as assessed by immunoprecipitation and yeast two-hybrid studies. With reductions in basal GRK2/3 activity, an increase in the basal association of filamin A and beta-arrestin2 with D3R is observed. Conversely, increases in the basal GRK2/3 activity result in a reduction in the interaction between the D3R and filamin but a relative increase in the agonist-mediated interaction between beta-arrestin2 and the D3R. Our data suggest that the D3R, filamin A, and beta-arrestin form a signaling complex that is destabilized by agonist- or expression-mediated increases in GRK2/3 activity. These findings provide a novel GRK-based mechanism for regulating D3R signaling potential and provide insight for interpreting D3R autoreceptor behavior.

Activity-dependent internalization of smoothened mediated by beta-arrestin 2 and GRK2

Binding of Sonic Hedgehog (Shh) to Patched (Ptc) relieves the latter's tonic inhibition of Smoothened (Smo), a receptor that spans the cell membrane seven times. This initiates signaling which, by unknown mechanisms, regulates vertebrate developmental processes. We find that two molecules interact with mammalian Smo in an activation-dependent manner: G protein-coupled receptor kinase 2 (GRK2) leads to phosphorylation of Smo, and beta-arrestin 2 fused to green fluorescent protein interacts with Smo. These two processes promote endocytosis of Smo in clathrin-coated pits. Ptc inhibits association of beta-arrestin 2 with Smo, and this inhibition is relieved in cells treated with Shh. A Smo agonist stimulated and a Smo antagonist (cyclopamine) inhibited both phosphorylation of Smo by GRK2 and interaction of beta-arrestin 2 with Smo. beta-Arrestin 2 and GRK2 are thus potential mediators of signaling by activated Smo.

Beta-arrestin- and G protein receptor kinase-mediated calcium-sensing receptor desensitization

Extracellular calcium rapidly controls PTH secretion through binding to the G protein-coupled calcium-sensing receptor (CASR) expressed in parathyroid glands. Very little is known about the regulatory proteins involved in desensitization of CASR. G protein receptor kinases (GRK) and beta-arrestins are important regulators of agonist-dependent desensitization of G protein-coupled receptors. In the present study, we investigated their role in mediating agonist-dependent desensitization of CASR. In heterologous cell culture models, we found that the transfection of GRK4 inhibits CASR signaling by enhancing receptor phosphorylation and beta-arrestin translocation to the CASR. In contrast, we found that overexpression of GRK2 desensitizes CASR by classical mechanisms as well as through phosphorylation-independent mechanisms involving disruption of Galphaq signaling. In addition, we observed lower circulating PTH levels and an attenuated increase in serum PTH after hypocalcemic stimulation in beta-arrestin2 null mice, suggesting a functional role of beta-arrestin2-dependent desensitization pathways in regulating CASR function in vivo. We conclude that GRKs and beta-arrestins play key roles in regulating CASR responsiveness in parathyroid glands.

Beta-arrestin binding to the beta2-adrenergic receptor requires both receptor phosphorylation and receptor activation

Homologous desensitization of beta2-adrenergic receptors has been shown to be mediated by phosphorylation of the agonist-stimulated receptor by G-protein-coupled receptor kinase 2 (GRK2) followed by binding of beta-arrestins to the phosphorylated receptor. Binding of beta-arrestin to the receptor is a prerequisite for subsequent receptor desensitization, internalization via clathrin-coated pits, and the initiation of alternative signaling pathways. In this study we have investigated the interactions between receptors and beta-arrestin2 in living cells using fluorescence resonance energy transfer. We show that (a) the initial kinetics of beta-arrestin2 binding to the receptor is limited by the kinetics of GRK2-mediated receptor phosphorylation; (b) repeated stimulation leads to the accumulation of GRK2-phosphorylated receptor, which can bind beta-arrestin2 very rapidly; and (c) the interaction of beta-arrestin2 with the receptor depends on the activation of the receptor by agonist because agonist withdrawal leads to swift dissociation of the receptor-beta-arrestin2 complex. This fast agonist-controlled association and dissociation of beta-arrestins from prephosphorylated receptors should permit rapid control of receptor sensitivity in repeatedly stimulated cells such as neurons.

Development and validation of algorithms for measuring G-protein coupled receptor activation in cells using the LSC-based imaging cytometer platform

BACKGROUND

A cell-based assay system (Transfluor) has been developed for measurement of G-protein coupled receptor (GPCR) activity by using cells transfected to express a fusion protein of arrestin plus green fluorescent protein (GFP) and the target GPCR. Upon agonist stimulation, the arrestin-GFP translocates to and binds the activated GPCR at the plasma membrane. The receptor/arrestin-GFP complexes then localize in clathrin-coated pits and/or intracellular vesicles. This redistribution of arrestin-GFP into condensed fluorescent spots is useful for visually monitoring the active status of GPCRs and its quantitation is possible with certain types of digital image analysis systems.

METHODS

We designed two lines of image processing algorithms to carry out quantitative measurement of the arrestin-GFP movement on an inverted version of laser scanning cytometry (iCyte) as an imaging platform. We used a cell line expressing arrestin-GFP and the wild-type beta2-adrenergic receptor or a modified version of this receptor with enhanced affinity for arrestin. Each cell line was challenged with various concentrations of agonist.

RESULTS

A dose-dependent signal was measured and half-maximal effective concentration values were obtained that agreed well with results determined by other methods previously reported.

CONCLUSIONS

The results indicate that the combination of Transfluor, iCyte, and our algorithms is suitable for robust and pharmacologically relevant GPCR ligand exploration.

beta-arrestin-1 competitively inhibits insulin-induced ubiquitination and degradation of insulin receptor substrate 1

beta-arrestin-1 is an adaptor protein that mediates agonist-dependent internalization and desensitization of G-protein-coupled receptors (GPCRs) and also participates in the process of heterologous desensitization between receptor tyrosine kinases and GPCR signaling. In the present study, we determined whether beta-arrestin-1 is involved in insulin-induced insulin receptor substrate 1 (IRS-1) degradation. Overexpression of wild-type (WT) beta-arrestin-1 attenuated insulin-induced degradation of IRS-1, leading to increased insulin signaling downstream of IRS-1. When endogenous beta-arrestin-1 was knocked down by transfection of beta-arrestin-1 small interfering RNA, insulin-induced IRS-1 degradation was enhanced. Insulin stimulated the association of IRS-1 and Mdm2, an E3 ubiquitin ligase, and this association was inhibited to overexpression of WT beta-arrestin-1, which led by decreased ubiquitin content of IRS-1, suggesting that both beta-arrestin-1 and IRS-1 competitively bind to Mdm2. In summary, we have found the following: (i) beta-arrestin-1 can alter insulin signaling by inhibiting insulin-induced proteasomal degradation of IRS-1; (ii) beta-arrestin-1 decreases the rate of ubiquitination of IRS-1 by competitively binding to endogenous Mdm2, an E3 ligase that can ubiquitinate IRS-1; (iii) dephosphorylation of S412 on beta-arrestin and the amino terminus of beta-arrestin-1 are required for this effect of beta-arrestin on IRS-1 degradation; and (iv) inhibition of beta-arrestin-1 leads to enhanced IRS-1 degradation and accentuated cellular insulin resistance.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

beta-arrestins: traffic cops of cell signaling

Once thought to function only in the desensitization of seven membrane spanning receptors (7MSRs), the ubiquitous beta-arrestin molecules are increasingly appreciated to play important roles in the endocytosis and signaling of these receptors. These functions reflect the ability of the beta-arrestins to bind an ever-growing list of signaling and endocytic elements, often in an agonist-dependent fashion. One heavily studied system is that leading to MAP kinase activation via beta-arrestin-mediated scaffolding of these pathways in a receptor-dependent fashion. The beta-arrestins are also found to be involved in the regulation of novel receptor systems, such as Frizzled and TGFbeta receptors.

Characterization of glucagon-like peptide-1 receptor beta-arrestin 2 interaction: a high-affinity receptor phenotype

To dissect the interaction between beta-arrestin ((beta)arr) and family B G protein-coupled receptors, we constructed fusion proteins between the glucagon-like peptide 1 receptor and (beta)arr2. The fusion constructs had an increase in apparent affinity selectively for glucagon, suggesting that (beta)arr2 interaction locks the receptor in a high-affinity conformation, which can be explored by some, but not all, ligands. The fusion constructs adopted a signaling phenotype governed by the tethered (beta)arr2 with an attenuated G protein-mediated cAMP signal and a higher maximal internalization compared with wild-type receptors. This distinct phenotype of the fusion proteins can not be mimicked by coexpressing wild-type receptor with (beta)arr2. However, when the wild-type receptor was coexpressed with both (beta)arr2 and G protein-coupled receptor kinase 5, a phenotype similar to that observed for the fusion constructs was observed. We conclude that the glucagon-like peptide 1 fusion construct mimics the natural interaction of the receptor with (beta)arr2 with respect to binding peptide ligands, G protein-mediated signaling and internalization, and that this distinct molecular phenotype is reminiscent of that which has previously been characterized for family A G protein-coupled receptors, suggesting similarities in the effect of (beta)arr interaction between family A and B receptors also at the molecular level.

Thrombin induces endocytosis of endoglin and type-II TGF-beta receptor and down-regulation of TGF-beta signaling in endothelial cells

Thrombin activates protease-activated receptor 1 (PAR1) on endothelial cells (ECs) and is critical for angiogenesis and vascular development. However, the mechanism underlying the proangiogenic effect of thrombin has not been elucidated yet. Here, we report the discovery of a novel functional link between thrombin-PAR1 and transforming growth factor-beta (TGF-beta) signaling pathways. We showed that thrombin via PAR1 induced the internalization of endoglin and type-II TGF-beta receptor (TbetaRII) but not type-I receptors in human ECs. This effect was mediated by protein kinase C-zeta (PKC-zeta) since specific inhibition of PKC-zeta caused an aggregation of endoglin or TbetaRII on cell surface and blocked their internalization by thrombin. Furthermore, acute and long-term pretreatment of ECs with thrombin or PAR1 peptide agonist suppressed the TGF-beta-induced serine phosphorylation of Smad2, a critical mediator of TGF-beta signaling. Moreover, activation of PAR1 led to a profound and spread cytosolic clustering formation of Smad2/3 and markedly prevented Smad2/3 nuclear translocation evoked by TGF-beta1. Since TGF-beta plays a crucial role in the resolution phase of angiogenesis, the down-regulation of TGF-beta signaling by thrombin-PAR1 pathway may provide a new insight into the mechanism of the proangiogenic effect of thrombin.

Internalization of the human CRF receptor 1 is independent of classical phosphorylation sites and of beta-arrestin 1 recruitment

The corticotropin releasing factor receptor 1 (CRFR1) belongs to the superfamily of G-protein coupled receptors. Though CRF is involved in the aetiology of several stress-related disorders, including depression and anxiety, details of CRFR1 regulation such as internalization remain uncharacterized. In the present study, agonist-induced internalization of CRFR1 in HEK293 cells was visualized by confocal microscopy and quantified using the radioligand 125I-labelled sauvagine. Recruitment of beta-arrestin 1 in response to receptor activation was demonstrated by confocal microscopy. The extent of 125I-labelled sauvagine stimulated internalization was significantly impaired by sucrose, indicating the involvement of clathrin-coated pits. No effect on the extent of internalization was observed in the presence of the second messenger dependent kinase inhibitors H-89 and staurosporine, indicating that cAMP-dependent protein kinase and protein kinase C are not prerequisites for CRFR1 internalization. Surprisingly, deletion of all putative phosphorylation sites in the C-terminal tail, as well as a cluster of putative phosphorylation sites in the third intracellular loop, did not affect receptor internalization. However, these mutations almost abolished the recruitment of beta-arrestin 1 following receptor activation. In conclusion, we demonstrate that CRFR1 internalization is independent of phosphorylation sites in the C-terminal tail and third intracellular loop, and the degree of beta-arrestin 1 recruitment.

Stable Interaction between β-Arrestin 2 and Angiotensin Type 1A Receptor Is Required for β-Arrestin 2-mediated Activation of Extracellular Signal-regulated Kinases 1 and 2

Binding of beta-arrestins to seven-membrane-spanning receptors (7MSRs) not only leads to receptor desensitization and endocytosis but also elicits additional signaling processes. We recently proposed that stimulation of the angiotensin type 1A (AT(1A)) receptor results in independent beta-arrestin 2- and G protein-mediated extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation. Here we utilize two AT(1A) mutant receptors to study these independent pathways, one truncated at residue 324, thus removing all potential carboxyl-terminal phosphorylation sites, and the other bearing four mutations in the serine/threonine-rich clusters in the carboxyl terminus. As assessed by confocal microscopy, the two mutant receptors interacted with beta-arrestin 2-green fluorescent protein with much lower affinity than did the wild-type receptor. In addition, the mutant receptors more robustly stimulated G protein-mediated inositol phosphate production. Approximately one-half of the wild-type AT(1A) receptor-stimulated ERK1/2 activation was via a beta-arrestin 2-dependent pathway (suppressed by beta-arrestin 2 small interfering RNA), whereas the rest was mediated by a G protein-dependent pathway (suppressed by protein kinase C inhibitor). ERK1/2 activation by the mutant receptors was insensitive to beta-arrestin 2 small interfering RNA but was reduced more than 80% by a protein kinase C inhibitor. The biochemical consequences of ERK activation by the G protein and beta-arrestin 2-dependent pathways were also distinct. G-protein-mediated ERK activation enhanced the transcription of early growth response 1, whereas beta-arrestin 2-dependent ERK activation did not. In addition, stimulation of the truncated AT(1A) mutant receptor caused significantly greater early growth response 1 transcription than did the wild-type receptor. These findings demonstrate how the ability of receptors to interact with beta-arrestins determines both the mechanism of ERK activation as well as the physiological consequences of this activation.

Kinetics of G-protein-coupled receptor signalling and desensitization

The kinetics of G-protein-coupled receptor activation and deactivation has, so far, been measured only indirectly, most frequently by assessing the production of various second messengers. We have developed methods based on fluorescence resonance energy transfer to quantify the kinetics of receptor activation by agonist (measured as conformational change in the receptor), the kinetics of G-protein activation (measured as G-protein subunit rearrangement) and the kinetics of receptor inactivation by arrestins (measured as receptor–arrestin interaction). Using these methods, we show that receptor activation by agonists and signalling to G-proteins occur on the subsecond time scale, whereas receptor desensitization is limited by receptor phosphorylation and proceeds more slowly.

Multiple independent functions of arrestins in the regulation of protease-activated receptor-2 signaling and trafficking

The irreversible proteolytic nature of protease-activated receptor-2 (PAR2) activation suggests that mechanism(s) responsible for termination of receptor signaling are critical determinants of the magnitude and duration of PAR2-elicited cellular responses. Rapid desensitization of activated G-protein-coupled receptors (GPCRs) involves both phosphorylation and binding of arrestins. Arrestins also function as scaffolds and transducers of mitogen-activated protein (MAP) kinase signaling cascades. The PAR2 cytoplasmic tail (C-tail) contains multiple sites of phosphorylation and may be an important determinant for arrestin interaction. Desensitization and internalization of activated PAR2 were markedly impaired in arrestin-deficient cells compared with wild-type control cells. PAR2 C-tail truncation mutants displayed normal agonist-induced internalization, caused rapid distribution of betaarr2-GFP to the plasma membrane, and desensitized in an arrestin-dependent manner similar to that of wild-type PAR2. It is interesting that PAR2 C-tail mutants lost the capacity to stably associate with arrestins and consequently, redistributed to endocytic vesicles without betaarr2-GFP, whereas internalized wild-type PAR2 remained stably associated with betaarr2-GFP in endosomes. Moreover, activated PAR2 caused rapid and prolonged activation of endogenous extracellular signal-regulated kinase (ERK1/2). It was striking that in arrestin-deficient cells, activated PAR2 induced an initial peak in ERK1/2 activity that rapidly declined. The inability of internalized PAR2 C-tail mutants to stably associate with arrestins also resulted in loss of prolonged ERK2 activation. Thus, the PAR2 C-tail regulates the stability of arrestin interaction and kinetics of ERK1/2 activation but is not essential for desensitization or internalization. These findings further suggest that the diverse functions of arrestins in regulating PAR2 signaling and trafficking are controlled by multiple independent interactions involving both the intracellular loops and the C-tail.

Respiratory responses to intermittent hypoxia in unsedated piglets: relation to substance P binding in brainstem

Respiratory responses to single intermittent hypoxia (5 min 21% O(2), 5 min 8% O(2) X6) in 5-6, 10-11, 21-22 and 26-27 day-old piglets, and to recurrent six daily intermittent hypoxia in 10-11 and 26-27 day-old piglets were assessed. Substance P binding in the piglets' brainstem immediately after the last hypoxic episode was measured. All piglets hyperventilated during hypoxia. Weight adjusted inspired ventilation, tidal volume and instantaneous flow decreased with age. The oldest piglets uniquely displayed attenuated ventilation and tidal volume during the sixth versus first hypoxic episode with single intermittent hypoxia, and reduced inspired ventilation and tidal volume during the first hypoxic episode on the sixth daily hypoxia compared to single hypoxia. By contrast, substance P binding was greatly reduced in the solitary, hypoglossal, paraambigual and lateral reticular brainstem nuclei of both younger and older piglets following either single or recurrent intermittent hypoxia. Thus, the reduction in membrane-bound neurokinin receptors by intermittent hypoxia, presumably consequent to endogenously released substance P, does not exclusively determine whether the ventilatory response to that hypoxia will be attenuated or not.

Identification of a Ser/Thr cluster in the C-terminal domain of the human prostaglandin receptor EP4 that is essential for agonist-induced beta-arrestin1 recruitment but differs from the apparent principal phosphorylation site

hEP4-R (human prostaglandin E2 receptor, subtype EP4) is a G(s)-linked heterotrimeric GPCR (G-protein-coupled receptor). It undergoes agonist-induced desensitization and internalization that depend on the presence of its C-terminal domain. Desensitization and internalization of GPCRs are often linked to agonist-induced beta-arrestin complex formation, which is stabilized by phosphorylation. Subsequently beta-arrestin uncouples the receptor from its G-protein and links it to the endocytotic machinery. The C-terminal domain of hEP4-R contains 38 Ser/Thr residues that represent potential phosphorylation sites. The present study aimed to analyse the relevance of these Ser/Thr residues for agonist-induced phosphorylation, interaction with beta-arrestin and internalization. In response to agonist treatment, hEP4-R was phosphorylated. By analysis of proteolytic phosphopeptides of the wild-type receptor and mutants in which groups of Ser/Thr residues had been replaced by Ala, the principal phosphorylation site was mapped to a Ser/Thr-containing region comprising residues 370-382, the presence of which was necessary and sufficient to obtain full agonist-induced phosphorylation. A cluster of Ser/Thr residues (Ser-389-Ser-390-Thr-391-Ser-392) distal to this site, but not the principal phosphorylation site, was essential to allow agonist-induced recruitment of beta-arrestin1. However, phosphorylation greatly enhanced the stability of the beta-arrestin1-receptor complexes. For maximal agonist-induced internalization, phosphorylation of the principal phosphorylation site was not required, but both beta-arrestin1 recruitment and the presence of Ser/Thr residues in the distal half of the C-terminal domain were necessary.

Constitutive desensitization: a new paradigm for g protein-coupled receptor regulation

GPCRs are a large family of cell-surface proteins that regulate many important biochemical pathways and physiological responses. The isolation and characterization of GPCRs represent one of the more remarkable success stories that occurred during the revolution in biology of the last quarter century. Of the many discoveries that originated in the laboratory of Robert Lefkowitz at Duke University concerning GPCR regulation, none is more fundamental than the elucidation of the families of GRKs and arrestin proteins that terminate GPCR signaling. In this essay, we will discuss how advances in microscopy and biology have made the visualization of GPCR, GRK, and arrestin activity possible in single cells. Additionally, we will discuss how imaging studies using arrestins and a naturally occurring mutant of the vasopressin receptor led to the recognition of a novel phenotypic receptor behavior, in which the receptor desensitizes in the absence of agonist. We have termed this process constitutive desensitization, and this unexpected receptor property suggests that it may be possible to develop novel classes of signal-inhibiting drugs distinct from conventional antagonists.

Molecular identification and characterization of three isoforms of tachykinin NK1-like receptors in the cane toadBufo marinus

The tachykinin peptide bufokinin, isolated from the cane toad intestine, is important in intestinal and cardiovascular regulation in the toad. In this study, three tachykinin NK1-like receptor isoforms, bNK1-A, bNK1-B, and bNK1-C, encoding proteins of 309, 390, and 371 amino acids, respectively, were cloned from the toad brain and intestine. These isoforms differ only at the intracellular COOH terminus. The bNK1-A and bNK1-B isoforms are similar to the truncated and full-length forms of the mammalian NK1receptor, whereas bNK1-C is unique and does not correspond to any previously described receptor. RT-PCR studies demonstrated that three isoform transcripts are widely distributed in the toad with high expression in gut, spinal cord, brain, lung, and skeletal muscle. When expressed in COS-7 cells, bufokinin showed similar high affinity (IC500.6–0.8 nM) in competing for125I-labeled Bolton-Hunter bufokinin binding at all receptors, but the binding affinities of substance P (SP) and neurokinin A (NKA) were very different at each isoform. When expressed in Xenopus oocytes, the truncated isoform, bNK1-A, was inactive, whereas bNK1-B and bNK1-C produced changes in chloride current when stimulated by tachykinins (minimum concentrations: bufokinin, 0.1 nM; SP, 1 nM; and NKA, 10 nM). A marked desensitization of the response was seen to subsequent applications of tachykinins, as experienced by the mammalian NK1receptor. In summary, our study describing three isoforms of NK1-like receptor from the toad suggests that the alternative splicing of NK1receptor is a physiologically conserved mechanism and raises a fundamental question as to the physiological role of each isoform.

Novel roles for arrestins in the post-endocytic trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane signaling molecules in the human genome. As such, they interact with numerous intracellular molecules, which can act either to propagate or curtail signaling from the receptor. Their primary mode of cellular activation occurs through heterotrimeric G proteins, which in turn can activate a wide spectrum of effector molecules, including phosphodiesterases, phospholipases, adenylyl cyclases and ion channels. Active GPCRs are also the target of G protein-coupled receptor kinases, which phosphorylate the receptors culminating in the binding of the protein arrestin. This results in rapid desensitization through inhibition of G protein binding, as well as novel mechanisms of cellular activation that involve the scaffolding of cellular kinases to GPCR-arrestin complexes. Arrestins can also serve to mediate the internalization of certain GPCRs, a process which plays an important role in regulating cellular activity both by mediating long-term desensitization through down regulation (degradation) of receptors and by recycling desensitized receptors back to the cell surface to initiate additional rounds of signaling. The mechanisms that regulate the subsequent intracellular trafficking of GPCRs following internalization are largely unknown. Recently however, it has become clear that the pattern of receptor phosphorylation and subsequent binding of arrestin play a critical role in the intracellular trafficking of internalized receptors, thereby dictating the ultimate fate of the receptor. In addition, arrestins have now been shown to be required for the recycling of GPCRs that are capable of internalizing through arrestin-independent mechanisms. This review will summarize recent advances in our understanding of the roles of arrestins in post-endocytic GPCR trafficking.

Membrane trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) modulate diverse physiological and behavioral signaling pathways by virtue of changes in receptor activation and inactivation states. Functional changes in receptor properties include dynamic interactions with regulatory molecules and trafficking to various cellular compartments at various stages of the life cycle of a GPCR. This review focuses on trafficking of GPCRs to the cell surface, stabilization there, and agonist-regulated turnover. GPCR interactions with a variety of newly revealed partners also are reviewed with the intention of provoking further analysis of the relevance of these interactions in GPCR trafficking, signaling, or both. The disease consequences of mislocalization of GPCRs also are described.

The molecular acrobatics of arrestin activation

Arrestin proteins play a key role in desensitizing G-protein-coupled receptors and re-directing their signaling to alternative pathways. The precise timing of arrestin binding to the receptor and its subsequent dissociation is ensured by its exquisite selectivity for the activated phosphorylated form of the receptor. The interaction between arrestin and the receptor involves the engagement of arrestin sensor sites that discriminate between active and inactive and phosphorylated and unphosphorylated forms of the receptor. This initial interaction is followed by a global conformational rearrangement of the arrestin molecule in the process of its transition into the high-affinity receptor-binding state that brings additional binding sites into action. In this article, we discuss the molecular mechanisms that underlie the sequential multi-site binding that ensures arrestin selectivity for the active phosphoreceptor and high fidelity of signal regulation by arrestin proteins.

Recycling and resensitization of the neurokinin 1 receptor. Influence of agonist concentration and Rab GTPases

Substance P (SP) induces endocytosis and recycling of the neurokinin 1 receptor (NK1R) in endothelial cells and spinal neurons at sites of inflammation and pain, and it is thus important to understand the mechanism and function of receptor trafficking. We investigated how the SP concentration affects NK1R trafficking and determined the role of Rab GTPases in trafficking. NK1R trafficking was markedly influenced by the SP concentration. High SP (10 nM) induced translocation of the NK1R and beta-arrestin 1 to perinuclear sorting endosomes containing Rab5a, where NK1R remained for >60 min. Low SP (1 nM) induced translocation of the NK1R to early endosomes located immediately beneath the plasma membrane that also contained Rab5a and beta-arrestin 1, followed by rapid recycling of the NK1R. Overexpression of Rab5a promoted NK1R translocation to perinuclear sorting endosomes, whereas the GTP binding-deficient mutant Rab5aS34N caused retention of the NK1R in superficial early endosomes. NK1R translocated from superficial early endosomes to recycling endosomes containing Rab4a and Rab11a, and Rab11aS25N inhibited NK1R recycling. Rapid NK1R recycling coincided with resensitization of SP-induced Ca2+ mobilization and with the return of surface SP binding sites. Resensitization was minimally affected by inhibition of vacuolar H(+)-ATPase and phosphatases but was markedly suppressed by disruption of Rab4a and Rab11a. Thus, whereas beta-arrestins mediate NK1R endocytosis, Rab5a regulates translocation between early and sorting endosomes, and Rab4a and Rab11a regulate trafficking through recycling endosomes. We have thus identified a new function of Rab5a as a control protein for directing concentration-dependent trafficking of the NK1R into different intracellular compartments and obtained evidence that Rab4a and Rab11a contribute to G-protein-coupled receptor recycling from early endosomes.

Differential β-Arrestin Trafficking and Endosomal Sorting of Somatostatin Receptor Subtypes

The physiological responses of somatostatin are mediated by five different G protein-coupled receptors. Although agonist-induced endocytosis of the various somatostatin receptor subtypes (sst(1)-sst(5)) has been studied in detail, little is known about their postendocytic trafficking. Here we show that somatostatin receptors profoundly differ in patterns of beta-arrestin mobilization and endosomal sorting. The beta-arrestin-dependent trafficking of the sst(2A) somatostatin receptor resembled that of a class B receptor in that upon receptor activation, beta-arrestin and the receptor formed stable complexes and internalized together into the same endocytic vesicles. This pattern was dependent on GRK2 (G protein-coupled receptor kinase 2)-mediated phosphorylation of a cluster of phosphate acceptor sites within the cytoplasmic tail of the sst(2A) receptor. Unlike other class B receptors, however, the sst(2A) receptor was rapidly resensitized and recycled to the plasma membrane. The beta-arrestin mobilization of the sst(3) and the sst(5) somatostatin receptors resembled that of a class A receptor in that upon receptor activation, beta-arrestin and the receptor formed relatively unstable complexes that dissociated at or near the plasma membrane. Consequently, beta-arrestin was excluded from sst(3)-containing vesicles. Unlike other class A receptors, a large proportion of sst(3) receptors was subject to ubiquitin-dependent lysosomal degradation and did not rapidly recycle to the plasma membrane. The sst(4) somatostatin receptor is unique in that it did not exhibit agonist-dependent receptor phosphorylation and beta-arrestin recruitment. Together, these findings may provide important clues about the regulation of receptor responsiveness during long-term administration of somatostatin analogs.

Real-Time Detection of Interactions between the Human Oxytocin Receptor and G Protein-Coupled Receptor Kinase-2

Although the oxytocin receptor (OTR) mediates many important functions including uterine contractions, milk ejection, and maternal behavior, the mechanisms controlling agonist-induced OTR desensitization have remained unclear, and attempts to demonstrate involvement of a G protein-coupled receptor kinase (GRK) have so far failed. Using the OTR as a model, we demonstrate here directly for the first time the dynamics of agonist-induced interactions of a GRK with a G protein-coupled receptor in real time, using time-resolved bioluminescence resonance energy transfer. GRK2/receptor interactions started within 4 sec, peaked at 10 sec, and decreased to less than 40% within 8 min. By contrast, β-arrestin/OTR interactions initiated only at 10 sec, reached plateau levels at 120 sec, but remained stable with little decrease thereafter. Physical GRK2/OTR association was further demonstrated by coimmunoprecipitation of endogenous GRK2 with activated OTR. In COS-7 cells, which express low levels of GRK2 and β-arrestin, overexpression of GRK2 and β-arrestin increased receptor phosphorylation, desensitization, and internalization to the high levels observed in human embryonic kidney 293 cells. By contrast, specific inhibition of endogenous GRK2 by dominant-negative mutants robustly inhibited OTR phosphorylation and internalization as well as arrestin/OTR interactions. These data characterize the temporal and causal relationship of GRK-2/OTR and β-arrestin/OTR interactions and establish GRK/OTR interaction as a prerequisite for β-arrestin-mediated OTR desensitization.

Different regulation of human delta-opioid receptors by SNC-80 [(+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide] and endogenous enkephalins

Among the different mechanisms underlying opioid tolerance, receptor desensitization would represent a major cellular adaptation process in which the role of receptor internalization is still a matter of debate. In the present study, we examined desensitization of the human delta-opioid receptor (hDOR) produced by endogenous opioid peptides Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) and Met-enkephalin (Tyr-Gly-Gly-Phe-Met), and the contribution of internalization in this process. Results obtained with natural peptides were compared with those produced by a synthetic opioid agonist, SNC-80 [(+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide). After a 30-min treatment, we observed a different regulation of hDOR between agonists. SNC-80 produced a stronger and faster desensitization and was associated with a loss of opioid binding sites by 50%. SNC-80 also caused a marked hDOR down-regulation by 30% as observed by Western blot. Immunocytochemistry revealed that SNC-80 induced a complete redistribution of hDOR from cell surface into intracellular compartments, whereas a partial internalization was visualized upon enkephalin exposure. In contrast, a stronger hDOR recycling and resensitization were measured after enkephalin treatment compared with SNC-80. These data strongly suggested a differential sorting of the internalized receptors caused by enkephalins and SNC-80 that was further confirmed by chloroquine as a lysosomal degradation blocker and monensin as a recycling endosome inhibitor. Finally, by preventing hDOR internalization with 0.5 M sucrose, we demonstrated that hDOR internalization contributes partially to desensitization. In conclusion, hDOR desensitization depends both on its internalization and its sorting either to the recycling pathway or to lysosomes.

Differential desensitization, receptor phosphorylation, beta-arrestin recruitment, and ERK1/2 activation by the two endogenous ligands for the CC chemokine receptor 7

Many members of the chemokine receptor family of G protein-coupled receptors utilize multiple endogenous ligands. However, differences between the signaling properties of multiple chemokines through a single receptor have yet to be well characterized. In this study we investigated the early signaling events of CCR7 initiated by its two endogenous ligands, CCL19 and CCL21. Both CCL19 and CCL21 induce G protein activation and calcium mobilization with equal potency. However, only activation by CCL19, not CCL21, promotes robust desensitization of endogenous CCR7 in the human T cell lymphoma cell line H9. Desensitization occurs through the induction of receptor phosphorylation and beta-arrestin recruitment (shown in HEK293 cells expressing CCR7-FLAG). The sites of CCL19-induced phosphorylation were mapped by mutating to alanines the serines and threonines found within kinase phosphorylation consensus sequences in the carboxyl terminus of CCR7. A cluster of sites, including Thr-373-376 and Ser-378 is important for CCL19-mediated phosphorylation of the receptor, whereas residues serine 356, 357, 364, and 365 are important for basal receptor phosphorylation by protein kinase C. Activation of CCR7 by both ligands leads to signaling to the ERK1/2 mitogen-activated protein kinase pathway. However, CCL19 promotes 4-fold more ERK1/2 phosphorylation than does CCL21. The mechanism by which CCL19 activates ERK1/2 was determined to be beta-arrestin-dependent, because it is reduced both by depletion of beta-arrestin-2 with small interfering RNA and by elimination of the phosphorylation sites in the tail of the receptor. Taken together, these findings demonstrate that CCL19 and CCL21 place CCR7 in functionally distinct conformations that are independent of their G protein-coupling potency: one that allows the efficient desensitization of the receptor and activation of ERK1/2, and another that is impaired in these functions.

Drosophila CG8422 encodes a functional diuretic hormone receptor

Diuretic hormone 44 (DH) is a bioactive neuropeptide that mediates osmotic balance in a wide variety of insects through increases in cAMP. It is structurally similar to mammalian corticotrophin releasing factor (CRF) peptides. In the moth Manduca and the cricket Acheta, functional studies have shown that its cognate receptor (DH-R) is related to the mammalian CRF receptor. The Drosophila genome contains two genes (CG8422 and CG12370) orthologous to Manduca and Acheta DH-Rs. Here, we present multiple lines of evidence to support the hypothesis that the orphan CG8422 G-protein-coupled receptor is a functional DH-R. When expressed in mammalian cells, CG8422 conferred selective sensitivity to DH, as indicated by translocation of a beta-arrestin-2-GFP reporter from the cytoplasm to the cell membrane. Consistent with its in vivo activities in other insects, DH activation of CG8422 elicited increases in a cAMP reporter system (CRE-luciferase), with an EC(50) of 1.7 nmol l(-1). CG8422 activation by DH also led to increases in intracellular calcium but at substantially higher doses (EC(50) approximately 300 nmol l(-1)). By microarray analysis, the CG8422 transcript was detectable in Drosophila head mRNA of different genotypes and under different environmental conditions. The identification of a Drosophila receptor for the DH neuropeptide provides a basis for genetic analysis of this critical factor's roles in maintaining physiological homeostasis.

Regulation of Angiotensin II Type 1A Receptor Intracellular Retention, Degradation, and Recycling by Rab5, Rab7, and Rab11 GTPases

Previous studies have demonstrated that the interaction of the angiotensin II type 1A receptor (AT(1A)R) carboxyl-terminal tail with Rab5a may modulate Rab5a activity, leading to the homotypic fusion of endocytic vesicles. Therefore, we have investigated whether AT(1A)R/Rab5a interactions mediate the retention of AT(1A)R.beta-arrestin complexes in early endosomes and whether the overexpression of Rab7 and Rab11 GTPases influences AT(1A)R lysosomal degradation and plasma membrane recycling. We found that internalized AT(1A)R was retained in Rab5a-positive early endosomes and was neither targeted to lysosomes nor recycled back to the cell surface, whereas a mutant defective in Rab5a binding, AT(1A)R-(1-349), was targeted to lysosomes for degradation. However, the loss of Rab5a binding to the AT(1A)R carboxyl-terminal tail did not promote AT(1A)R recycling. Rather, it was the stable binding of beta-arrestin to the AT(1A)R that prevented, at least in part, AT(1A)R recycling. The overexpression of wild-type Rab7 and Rab7-Q67L resulted in both increased AT(1A)R degradation and AT(1A)R targeting to lysosomes. The Rab7 expression-dependent transition of "putative" AT(1A)R.beta-arrestin complexes to late endosomes was blocked by the expression of dominant-negative Rab5a-S34N. Rab11 overexpression established AT(1A)R recycling and promoted the redistribution of AT(1A)R.beta-arrestin complexes from early to recycling endosomes. Taken together, our data suggest that Rab5, Rab7, and Rab11 work in concert with one another to regulate the intracellular trafficking patterns of the AT(1A)R.

Regulation of MAP kinase signaling modules by scaffold proteins in mammals

The mitogen-activated protein kinase (MAPK) group of serine/threonine protein kinases mediates the response of cells to many extracellular stimuli such as cytokines and growth factors. These protein kinases include the extracellular signal-regulated protein kinases (ERK) and two stress-activated protein kinases (SAPK), the c-Jun N-terminal kinases (JNK), and the p38 MAPK. The enzymes are evolutionarily conserved and are activated by a common mechanism that involves a protein kinase cascade. Scaffold proteins have been proposed to interact with MAPK pathway components to create a functional signaling module and to control the specificity of signal transduction. Here we critically evaluate the evidence that supports a physiologically relevant role of MAPK scaffold proteins in mammals.