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

Mechanisms of melanocortin-2 receptor (MC2R) internalization and recycling in human embryonic kidney (hek) cells: identification of Key Ser/Thr (S/T) amino acids

ACTH is the most important stimulus of the adrenal cortex. The precise molecular mechanisms underlying the ACTH response are not yet clarified. The functional ACTH receptor includes melanocortin-2 receptor (MC2R) and MC2R accessory proteins (MRAP). In human embryonic kidney 293/Flp recombinase target cells expressing MC2R, MRAP1 isoforms, and MRAP2, we found that ACTH induced a concentration-dependent and arrestin-, clathrin-, and dynamin-dependent MC2R/MRAP1 internalization, followed by intracellular colocalization with Rab (Ras-like small guanosine triphosphate enzyme)4-, Rab5-, and Rab11-positive recycling endosomes. Preincubation of cells with monensin and brefeldin A revealed that 28% of the internalized receptors were recycled back to the plasma membrane and participated in total accumulation of cAMP. Moreover, certain intracellular Ser and Thr (S/T) residues of MC2R were found to play important roles not only in plasma membrane targeting and function but also in promoting receptor internalization. The S/T residues T131, S140, T204, and S280 were involved in MRAP1-independent cell-surface MC2R expression. Other mutants (S140A, S208A, and S202D) had lower cell-surface expressions in absence of MRAPβ. In addition, T143A and T147D drastically impaired cell-surface expression and function, whereas T131A, T131D, and S280D abrogated MC2R internalization. Thus, the modification of MC2R intracellular S/T residues may positively or negatively regulate its plasma membrane expression and the capacity of ACTH to induce cAMP accumulation. Mutations of T131, T143, T147, and S280 into either A or D had major repercussions on cell-surface expression, cAMP accumulation, and/or internalization parameters, pointing mostly to the second intracellular loop as being crucial for MC2R expression and functional regulation.

Role of Beta-arrestin 2 downstream of dopamine receptors in the Basal Ganglia

Multifunctional scaffolding protein beta-arrestins (βArr) and the G protein-receptor kinases are involved in the desensitization of several G protein-coupled receptors (GPCR). However, arrestins can also contribute to GPCR signaling independently from G proteins. In this review, we focus on the role of βArr in the regulation of dopamine receptor functions in the striatum. First, we present in vivo evidence supporting a role for these proteins in the regulation of dopamine receptor desensitization. Second, we provide an overview of the roles of βArr2 in the regulation of extracellular-signal-regulated kinases/MAP kinases and Akt/GSK3 signaling pathways downstream of the D1 and D2 dopamine receptors. Thereafter, we examine the possible involvement of βArr-mediated signaling in the action of dopaminergic drugs used for the treatment of mental disorders. Finally, we focus on different potential cellular proteins regulated by βArr-mediated signaling which could contribute to the regulation of behavioral responses to dopamine. Overall, the identification of a cell signaling function for βArr downstream of dopamine receptors underscores the intricate complexity of the intertwined mechanisms regulating and mediating cell signaling in the basal ganglia. Understanding these mechanisms may lead to a better comprehension of the several roles played by these structures in the regulation of mood and to the development of new psychoactive drugs having better therapeutic efficacy.

Differential requirements for clathrin endocytic pathway components in cellular entry by Ebola and Marburg glycoprotein pseudovirions

Clathrin-mediated endocytosis was previously implicated as one of the cellular pathways involved in filoviral glycoprotein mediated viral entry into target cells. Here we have further dissected the requirements for different components of this pathway in Ebola versus Marburg virus glycoprotein (GP) mediated viral infection. Although a number of these components were involved in both cases; Ebola GP-dependent viral entry specifically required the cargo recognition proteins Eps15 and DAB2 as well as the clathrin adaptor protein AP-2. In contrast, Marburg GP-mediated infection was independent of these three proteins and instead required beta-arrestin 1 (ARRB1). These findings have revealed an unexpected difference between the clathrin pathway requirements for Ebola GP versus Marburg GP pseudovirion infection. Anthrax toxin also uses a clathrin-, and ARRB1-dependent pathway for cellular entry, indicating that the mechanism used by Marburg GP pseudovirions may be more generally important for pathogen entry.

Rapid dephosphorylation of G protein-coupled receptors by protein phosphatase 1β is required for termination of β-arrestin-dependent signaling

Termination of signaling of activated G protein-coupled receptors (GPCRs) is essential for maintenance of cellular homeostasis. It is well established that β-arrestin redistributes to phosphorylated GPCRs and thereby facilitates desensitization of classical G protein-dependent signaling. β-Arrestin in turn serves as a scaffold to initiate a second wave of signaling. Here, we report a molecular mechanism that regulates the termination of unconventional β-arrestin-dependent GPCR signaling. We identify protein phosphatase 1β (PP1β) as a phosphatase for the cluster of phosphorylated threonines ((353)TTETQRT(359)) within the sst(2A) somatostatin receptor carboxyl terminus that mediates β-arrestin binding using siRNA knock-down screening. We show that PP1β-mediated sst(2A) dephosphorylation is initiated directly after receptor activation at or near the plasma membrane. As a functional consequence of diminished PP1β activity, we find that somatostatin- and substance P-induced but not epidermal growth factor-induced ERK activation was aberrantly enhanced and prolonged. Thus, we demonstrate a novel mechanism for fine tuning unconventional β-arrestin-dependent GPCR signaling in that recruitment of PP1β to activated GPCRs facilitates GPCR dephosphorylation and, hence, leads to disruption of the β-arrestin-GPCR complex.

Protein phosphatase 2A mediates resensitization of the neurokinin 1 receptor

Activated G protein-coupled receptors (GPCRs) are phosphorylated and interact with β-arrestins, which mediate desensitization and endocytosis. Endothelin-converting enzyme-1 (ECE-1) degrades neuropeptides in endosomes and can promote recycling. Although endocytosis, dephosphorylation, and recycling are accepted mechanisms of receptor resensitization, a large proportion of desensitized receptors can remain at the cell surface. We investigated whether reactivation of noninternalized, desensitized (phosphorylated) receptors mediates resensitization of the substance P (SP) neurokinin 1 receptor (NK(1)R). Herein, we report a novel mechanism of resensitization by which protein phosphatase 2A (PP2A) is recruited to dephosphorylate noninternalized NK(1)R. A desensitizing concentration of SP reduced cell-surface SP binding sites by only 25%, and SP-induced Ca(2+) signals were fully resensitized before cell-surface binding sites started to recover, suggesting resensitization of cell-surface-retained NK(1)R. SP induced association of β-arrestin1 and PP2A with noninternalized NK(1)R. β-Arrestin1 small interfering RNA knockdown prevented SP-induced association of cell-surface NK(1)R with PP2A, indicating that β-arrestin1 mediates this interaction. ECE-1 inhibition, by trapping β-arrestin1 in endosomes, also impeded SP-induced association of cell-surface NK(1)R with PP2A. Resensitization of NK(1)R signaling required both PP2A and ECE-1 activity. Thus, after stimulation with SP, PP2A interacts with noninternalized NK(1)R and mediates resensitization. PP2A interaction with NK(1)R requires β-arrestin1. ECE-1 promotes this process by releasing β-arrestin1 from NK(1)R in endosomes. These findings represent a novel mechanism of PP2A- and ECE-1-dependent resensitization of GPCRs.

The arrestin fold: variations on a theme

Endocytosis of ligand-activated plasma membrane receptors has been shown to contribute to the regulation of their downstream signaling. β-arrestins interact with the phosphorylated tail of activated receptors and act as scaffolds for the recruitment of adaptor proteins and clathrin, that constitute the machinery used for receptor endocytosis. Visual- and β-arrestins have a two-lobe, immunoglobulin-like, β-strand sandwich structure. The recent resolution of the crystal structure of VPS26, one of the retromer subunits, unexpectedly evidences an arrestin fold in this protein, which is otherwise unrelated to arrestins. From a functional point of view, VPS26 is involved in the retrograde transport of the mannose 6-P receptor from the endosomes to the trans-Golgi network. In addition to the group of genuine arrestins and Vps26, mammalian cells harbor a vast repertoire of proteins that are related to arrestins on the basis of their PFAM Nter and Cter arrestin- domains, which are named Arrestin Domain- Containing proteins (ADCs). The biological role of ADC proteins is still poorly understood. The three subfamilies have been merged into an arrestin-related protein clan.

This paper provides an overall analysis of arrestin clan proteins. The structures and functions of members of the subfamilies are reviewed in mammals and model organisms such as Drosophila, Caenorhabditis, Saccharomyces and Dictyostelium.

Arrestins differentially regulate histamine- and oxytocin-evoked phospholipase C and mitogen-activated protein kinase signalling in myometrial cells

BACKGROUND AND PURPOSE

The uterotonins oxytocin and histamine, mediate contractile signals through specific G protein-coupled receptors, a process which is tightly controlled during gestation to prevent preterm labour. We previously identified G protein-coupled receptor kinase (GRK)2 and GRK6 as respective cardinal negative regulators of histamine H(1) and oxytocin receptor signalling. GRK-mediated phosphorylation promotes arrestin recruitment, not only desensitizing receptors but activating an increasing number of diverse signalling pathways. Here we investigate potential roles that arrestins play in the regulation of myometrial oxytocin/histamine H(1) receptor signalling.

EXPERIMENTAL APPROACH

Endogenous arrestins2 and 3 were specifically depleted using RNA-interference in a human myometrial cell line and the consequences of this for G protein-coupled receptor-mediated signalling were assessed using Ca(2+) /inositol 1,4,5-trisphophate imaging and standard mitogen-activated protein kinase (MAPK) assays.

KEY RESULTS

Depletion of arrestin3, but not arrestin2 enhanced and prolonged H(1) receptor-stimulated Ca(2+) responses, whilst depletion of either arrestin increased oxytocin receptor responses. Arrestin3 depletion decreased H(1) receptor desensitization, whilst removal of either arrestin isoform was equally effective in preventing oxytocin receptor desensitization. Following arrestin3 depletion oxytocin-induced phospho-extracellular signal-regulated kinase1/2 signals were diminished and histamine-stimulated signals virtually absent, whereas depletion of arrestin2 augmented extracellular signal-regulated kinase1/2 responses to each agonist. Conversely, depletion of arrestin3 enhanced p38 signals to each agonist, whilst arrestin2 suppression increased oxytocin-, but not histamine-induced p38 MAPK responses.

CONCLUSIONS AND IMPLICATIONS

Arrestin proteins are key regulators of H(1) and oxytocin receptor desensitization, and play integral roles mediating uterotonin-stimulated MAPK-signalling. These data provide insights into the in situ regulation of these receptor subtypes and may inform pathophysiological functioning in preterm labour.

Down-modulation of the G-protein-coupled estrogen receptor, GPER, from the cell surface occurs via a trans-Golgi-proteasome pathway

GPER is a G(s)-coupled seven-transmembrane receptor that has been linked to specific estrogen binding and signaling activities that are manifested by plasma membrane-associated enzymes. However, in many cell types, GPER is predominately localized to the endoplasmic reticulum (ER), and only minor amounts of receptor are detectable at the cell surface, an observation that has caused controversy regarding its role as a plasma membrane estrogen receptor. Here, we show that GPER constitutively buds intracellularly into EEA-1+ endosomes from clathrin-coated pits. Nonvisual arrestins-2/-3 do not co-localize with GPER, and expression of arrestin-2 dominant-negative mutants lacking clathrin- or β-adaptin interaction sites fails to block GPER internalization suggesting that arrestins are not involved in GPER endocytosis. Like β1AR, which recycles to the plasma membrane, GPER co-traffics with transferrin+, Rab11+ recycling endosomes. However, endocytosed GPER does not recycle to the cell surface, but instead returns to the trans-Golgi network (TGN) and does not re-enter the ER. GPER is ubiquitinated at the cell surface, exhibits a short half-life (t½;) <1 h), and is protected from degradation by the proteasome inhibitor, MG132. Disruption of the TGN by brefeldin A induces the accumulation of endocytosed GPER in Rab11+ perinuclear endosomes and prevents GPER degradation. Our results provide an explanation as to why GPER is not readily detected on the cell surface in some cell types and further suggest that TGN serves as the checkpoint for degradation of endocytosed GPER.

β-Arrestin-mediated receptor trafficking and signal transduction

β-Arrestins function as endocytic adaptors and mediate trafficking of a variety of cell-surface receptors, including seven-transmembrane receptors (7TMRs). In the case of 7TMRs, β-arrestins carry out these tasks while simultaneously inhibiting upstream G-protein-dependent signaling and promoting alternate downstream signaling pathways. The mechanisms by which β-arrestins interact with a continuously expanding ensemble of protein partners and perform their multiple functions including trafficking and signaling are currently being uncovered. Molecular changes at the level of protein conformation as well as post-translational modifications of β-arrestins probably form the basis for their dynamic interactions during receptor trafficking and signaling. It is becoming increasingly evident that β-arrestins, originally discovered as 7TMR adaptor proteins, indeed have much broader and more versatile roles in maintaining cellular homeostasis. In this review paper, we assess the traditional and novel functions of β-arrestins and discuss the molecular attributes that might facilitate multiple interactions in regulating cell signaling and receptor trafficking.

Temporal profiling of orexin receptor-arrestin-ubiquitin complexes reveals differences between receptor subtypes

Orexin G protein-coupled receptors (OxRs) and their cognate agonists have been implicated in a number of disorders since their recent discovery, ranging from narcolepsy to formation of addictive behavior. Bioluminescence resonance energy transfer assays of agonist-occupied OxRs provided evidence for a strong dose-dependent interaction with both trafficking proteins β-arrestin 1 and 2 that required unusually high agonist concentrations compared with inositol phosphate signaling. This appears to be reflected in functional differences in potency with respect to orexin A (OxA) and OxR2-dependent ERK1/2 phosphorylation after 90 min compared with 2 min, potentially consistent with β-arrestin-mediated versus G protein-mediated signaling, respectively. Furthermore, extended bioluminescence resonance energy transfer kinetic data monitoring OxA-dependent receptor-β-arrestin and β-arrestin-ubiquitin proximity suggested subtype-specific differences in receptor trafficking, with OxR2 activation resulting in more sustained receptor-β-arrestin-ubiquitin complex formation than elicited by OxR1 activation. Enzyme-linked immunosorbent assay (ELISA) data also revealed that OxR1 underwent significantly more rapid recycling compared with OxR2. Finally, we have observed sustained OxA-dependent ERK1/2 phosphorylation in the presence of OxR2 compared with OxR1. Although both OxR subtypes could be classified as class B receptors for β-arrestin usage based on the initial strength of interaction with both β-arrestins, our temporal profiling revealed tangible differences between OxR subtypes. Consequently, OxR1 appears to fit uneasily into the commonly used β-arrestin classification scheme. More importantly, it is hoped that this improved profiling capability, enabling the subtleties of protein complex formation, stability, and duration to be assessed in live cells, will help unlock the therapeutic potential of targeting these receptors.

Ser352 and Ser354 in the carboxyl terminus of the human P2Y(1) receptor are required for agonist-promoted phosphorylation and internalization in MDCK cells

BACKGROUND AND PURPOSE

The P2Y(1) receptor promotes chloride secretion in epithelial cells, a process critical for regulation of extracellular ion and fluid levels. Here we have examined the role of phosphorylation in agonist-induced internalization of P2Y(1) receptors.

EXPERIMENTAL APPROACH

A high-affinity radiolabelled antagonist, MRS2500, was used to quantify cell surface-binding sites of P2Y(1) receptors in Madin-Darby canine kidney (MDCK) epithelial cells, following exposure to agonists. The regions in the carboxyl terminus involved in both agonist-induced internalization of the receptor and its phosphorylation were identified by mutational analysis.

KEY RESULTS

Endogenous and stably expressed recombinant P2Y(1) receptors rapidly internalized with similar time courses in response to agonist in MDCK cells, ensuring that the levels of recombinant receptor achieved by retroviral infection did not adversely affect function of the internalization machinery. Four protein kinase C inhibitors of varying specificity did not affect internalization of recombinant receptors. Agonist-promoted internalization of a series of truncated P2Y(1) receptors identified a region between residues 349 and 359 in the carboxyl terminus as critical for regulation. Two amino acids within this region, Ser352 and Ser354, were shown to be both necessary and sufficient for agonist-promoted receptor phosphorylation and internalization.

CONCLUSIONS AND IMPLICATIONS

Our results firmly establish Ser352 and Ser354 in the carboxyl terminus of P2Y(1) receptors as critical residues for agonist-induced receptor internalization in MDCK cells. As the mechanism mediating this regulation requires phosphorylation of these key residues, the relevant receptor-regulated protein kinase can now be identified.

The physiology, signaling, and pharmacology of dopamine receptors

G protein-coupled dopamine receptors (D1, D2, D3, D4, and D5) mediate all of the physiological functions of the catecholaminergic neurotransmitter dopamine, ranging from voluntary movement and reward to hormonal regulation and hypertension. Pharmacological agents targeting dopaminergic neurotransmission have been clinically used in the management of several neurological and psychiatric disorders, including Parkinson's disease, schizophrenia, bipolar disorder, Huntington's disease, attention deficit hyperactivity disorder (ADHD(1)), and Tourette's syndrome. Numerous advances have occurred in understanding the general structural, biochemical, and functional properties of dopamine receptors that have led to the development of multiple pharmacologically active compounds that directly target dopamine receptors, such as antiparkinson drugs and antipsychotics. Recent progress in understanding the complex biology of dopamine receptor-related signal transduction mechanisms has revealed that, in addition to their primary action on cAMP-mediated signaling, dopamine receptors can act through diverse signaling mechanisms that involve alternative G protein coupling or through G protein-independent mechanisms via interactions with ion channels or proteins that are characteristically implicated in receptor desensitization, such as β-arrestins. One of the future directions in managing dopamine-related pathologic conditions may involve a transition from the approaches that directly affect receptor function to a precise targeting of postreceptor intracellular signaling modalities either directly or through ligand-biased signaling pharmacology. In this comprehensive review, we discuss dopamine receptor classification, their basic structural and genetic organization, their distribution and functions in the brain and the periphery, and their regulation and signal transduction mechanisms. In addition, we discuss the abnormalities of dopamine receptor expression, function, and signaling that are documented in human disorders and the current pharmacology and emerging trends in the development of novel therapeutic agents that act at dopamine receptors and/or on related signaling events.

β-Arrestin mediates oxytocin receptor signaling, which regulates uterine contractility and cellular migration

Desensitization of the oxytocin receptor (OXTR) in the setting of prolonged oxytocin exposure may lead to dysfunctional labor, which increases the risk for cesarean delivery, and uterine atony, which may result in postpartum hemorrhage. The molecular mechanism for OXTR desensitization is through the agonist-mediated recruitment of the multifunctional protein β-arrestin. In addition to its desensitizing function, β-arrestins have recently been shown to simultaneously activate downstream signaling. We tested whether oxytocin stimulation promotes β-arrestin-mediated OXTR desensitization in vivo and activates β-arrestin-mediated mitogen-activated protein kinase (MAPK) growth signaling. Uterine muscle strips isolated from wild-type mice exhibited diminished uterine contractility following repeated exposure to oxytocin, whereas uterine muscle strips from β-arrestin-1 and β-arrestin-2 knockout mice showed no desensitization. Utilizing siRNA knockdown of β-arrestin-1 and β-arrestin-2 in HEK-293 cells expressing the OXTR, we demonstrated oxytocin-mediated MAPK signaling that was dependent on β-arrestin-1 and β-arrestin-2. Wild-type and β-arrestin-1 and β-arrestin-2 knockout mice receiving intravenous oxytocin also demonstrated oxytocin-mediated MAPK signaling that was dependent on β-arrestin-1 and β-arrestin-2. Finally, to test the significance of β-arrestin-mediated signaling from the OXTR, HEK-293 cells expressing the OXTR showed β-arrestin-dependent proliferation in a cell migration assay following oxytocin treatment. In conclusion, β-arrestin is a multifunctional scaffold protein that mediates both desensitization of the OXTR, leading to decreases in uterine contractility, and MAPK growth signaling following stimulation by oxytocin. The development of unique OXTR ligands that prevent receptor desensitization may be a novel approach in the treatment of adverse clinical events secondary to prolonged oxytocin therapy.

CK2 phosphorylation of an acidic Ser/Thr di-isoleucine motif in the Na+/H+ exchanger NHE5 isoform promotes association with beta-arrestin2 and endocytosis

Internalization of the Na(+)/H(+) exchanger NHE5 into recycling endosomes is enhanced by the endocytic adaptor proteins β-arrestin1 and -2, best known for their preferential recognition of ligand-activated G protein-coupled receptors (GPCRs). However, the mechanism underlying their atypical association with non-GPCRs, such as NHE5, is unknown. In this study, we identified a highly acidic, serine/threonine-rich, di-isoleucine motif (amino acids 697-723) in the cytoplasmic C terminus of NHE5 that is recognized by β-arrestin2. Gross deletions of this site decreased the state of phosphorylation of NHE5 as well as its binding and responsiveness to β-arrestin2 in intact cells. More refined in vitro analyses showed that this site was robustly phosphorylated by the acidotropic protein kinase CK2, whereas other kinases, such as CK1 or the GPCR kinase GRK2, were considerably less potent. Simultaneous mutation of five Ser/Thr residues within 702-714 to Ala ((702)ST/AA(714)) abolished phosphorylation and binding of β-arrestin2. In transfected cells, the CK2 catalytic α subunit formed a complex with NHE5 and decreased wild-type but not (702)ST/AA(714) NHE5 activity, further supporting a regulatory role for this kinase. The rate of internalization of (702)ST/AA(714) was also diminished and relatively insensitive to overexpression of β-arrestin2. However, unlike in vitro, this mutant retained its ability to form a complex with β-arrestin2 despite its lack of responsiveness. Additional mutations of two di-isoleucine-based motifs (I697A/L698A and I722A/I723A) that immediately flank the acidic cluster, either separately or together, were required to disrupt their association. These data demonstrate that discrete elements of an elaborate sorting signal in NHE5 contribute to β-arrestin2 binding and trafficking along the recycling endosomal pathway.

Oxytocin exposure during labor among women with postpartum hemorrhage secondary to uterine atony

OBJECTIVE

We sought to determine if women with severe postpartum hemorrhage (PPH) secondary to uterine atony received greater amounts of oxytocin during labor compared to women without PPH.

STUDY DESIGN

Subjects with severe PPH secondary to uterine atony, who received a blood transfusion, were compared to matched controls. Total oxytocin exposure was calculated as the area under the concentration curve (mU/min*min). Variables were compared using paired t test, χ², and logistic regression.

RESULTS

Women with severe PPH had a mean oxytocin area under the curve of 10,054 mU compared to 3762 mU in controls (P < .001). After controlling for race, body mass index, admission hematocrit, induction status, magnesium therapy, and chorioamnionitis using logistic regression, oxytocin area under the curve continued to predict severe PPH.

CONCLUSION

Women with severe PPH secondary to uterine atony were exposed to significantly more oxytocin during labor compared to matched controls.

Therapeutic potential of β-arrestin- and G protein-biased agonists

Members of the seven-transmembrane receptor (7TMR), or G protein-coupled receptor (GPCR), superfamily represent some of the most successful targets of modern drug therapy, with proven efficacy in the treatment of a broad range of human conditions and disease processes. It is now appreciated that β-arrestins, once viewed simply as negative regulators of traditional 7TMR-stimulated G protein signaling, act as multifunctional adapter proteins that regulate 7TMR desensitization and trafficking and promote distinct intracellular signals in their own right. Moreover, several 7TMR biased agonists, which selectively activate these divergent signaling pathways, have been identified. Here we highlight the diversity of G protein- and β-arrestin-mediated functions and the therapeutic potential of selective targeting of these in disease states.

Regulation of membrane cholecystokinin-2 receptor by agonists enables classification of partial agonists as biased agonists

Given the importance of G-protein-coupled receptors as pharmacological targets in medicine, efforts directed at understanding the molecular mechanism by which pharmacological compounds regulate their presence at the cell surface is of paramount importance. In this context, using confocal microscopy and bioluminescence resonance energy transfer, we have investigated internalization and intracellular trafficking of the cholecystokinin-2 receptor (CCK2R) in response to both natural and synthetic ligands with different pharmacological features. We found that CCK and gastrin, which are full agonists on CCK2R-induced inositol phosphate production, rapidly and abundantly stimulate internalization. Internalized CCK2R did not rapidly recycle to plasma membrane but instead was directed to late endosomes/lysosomes. CCK2R endocytosis involves clathrin-coated pits and dynamin and high affinity and prolonged binding of β-arrestin1 or -2. Partial agonists and antagonists on CCK2R-induced inositol phosphate formation and ERK1/2 phosphorylation did not stimulate CCK2R internalization or β-arrestin recruitment to the CCK2R but blocked full agonist-induced internalization and β-arrestin recruitment. The extreme C-terminal region of the CCK2R (and more precisely phosphorylatable residues Ser(437)-Xaa(438)-Thr(439)-Thr(440)-Xaa(441)-Ser(442)-Thr(443)) were critical for β-arrestin recruitment. However, this region and β-arrestins were dispensable for CCK2R internalization. In conclusion, this study allowed us to classify the human CCK2R as a member of class B G-protein-coupled receptors with regard to its endocytosis features and identified biased agonists of the CCK2R. These new important insights will allow us to investigate the role of internalized CCK2R·β-arrestin complexes in cancers expressing this receptor and to develop new diagnosis and therapeutic strategies targeting this receptor.

Role of ßarrestins in bradykinin B2 receptor-mediated signalling

G protein-coupled receptors (GPCRs) can engage multiple pathways to activate ERK1/2 via both G proteins and/or ßarrestin. Receptor recruitment of ßarrestin is also important for GPCR desensitization, internalization and resensitization. Modulation of the receptor/ßarrestin interaction through modification of either component would presumably alter the output generated by receptor activation. Here we examined how ßarrestins regulate bradykinin (BK) B2 receptor (B2R) signalling and desensitization by either truncating ßarrestin1 or ßarrestin2 or by alanine substitution of a serine/threonine cluster in the C-terminal tail of B2R (B2R-4A), conditions which all affect the avidity of the B2R/ßarrestin complex. We first demonstrate that BK-mediated ERK1/2 activation is biphasic containing an early peak (between 2-5min) followed by sustained activation for at least 60min. The early but not the sustained phase was predictably affected by inhibition of either Gαq/11 or Gαi/o, whereas loss of ßarrestin2 but not ßarrestin1 resulted in diminished prolonged ERK1/2 activation. ßarrestin2's role was further examined using a truncation mutant with augmented avidity for the agonist-occupied receptor, revealing an increase in both immediate and extended ERK1/2 signalling. We also show that ERK1/2 is recruited to the B2R/ßarrestin complex on endosomes as well as the plasma membrane. Moreover, we investigated ßarrestin's role using the B2R-4A, which is deficient in ßarrestin binding and does not internalize. We show that ERK1/2 signalling downstream of the receptor is entirely G protein-dependent and receptor-mediated intracellular calcium mobilization studies revealed a lack of desensitization. Functionally, the lack of desensitization resulted in increased cell growth and migration compared to the wild-type receptor, which was sensitive to MEK inhibition. These results highlight ßarrestin's crucial role in the maintenance of proper B2R signalling.

Role of PTH1R internalization in osteoblasts and bone mass using a phosphorylation-deficient knock-in mouse model

Phosphorylation, internalization, and desensitization of G protein-coupled receptors, such as the parathyroid hormone (PTH) and PTH-related peptide (PTHrP) receptor (PTH1R), are well characterized and known to regulate the cellular responsiveness in vitro. However, the role of PTH1R receptor phosphorylation in bone formation and osteoblast functions has not yet been elucidated. In previous studies, we demonstrated impaired internalization and sustained cAMP stimulation of a phosphorylation-deficient (pd) PTH1R in vitro, and exaggerated cAMP and calcemic responses to s.c. PTH infusion in pdPTH1R knock-in mouse model. In this study, we examined the impact of impaired PTH1R phosphorylation on the skeletal phenotype of mice maintained on normal, low, and high calcium diets. The low calcium diet moderately reduced (P<0.05) bone volume and trabecular number, and increased trabecular spacing in both wild-type (WT) and pd mice. The effects, however, seem to be less pronounced in the female pd compared to WT mice. In primary calvarial osteoblasts isolated from 2-week-old pd or WT mice, PTH and PTHrP decreased phosphorylated extracellular signal-regulated kinases 1/2 (pERK1/2), a member of mitogen-activated protein kinase, and cyclin D1, a G₁/S phase cyclin, in vitro. In contrast to WT osteoblasts, down-regulation of cyclin D1 was sustained for longer periods of time in osteoblasts isolated from the pd mice. Our results suggest that adaptive responses of intracellular signaling pathways in the pd mice may be important for maintaining bone homeostasis.

Biased signaling of the angiotensin II type 1 receptor can be mediated through distinct mechanisms

BACKGROUND

Seven transmembrane receptors (7TMRs) can adopt different active conformations facilitating a selective activation of either G protein or β-arrestin-dependent signaling pathways. This represents an opportunity for development of novel therapeutics targeting selective biological effects of a given receptor. Several studies on pathway separation have been performed, many of these on the Angiotensin II type 1 receptor (AT1R). It has been shown that certain ligands or mutations facilitate internalization and/or recruitment of β-arrestins without activation of G proteins. However, the underlying molecular mechanisms remain largely unresolved. For instance, it is unclear whether such selective G protein-uncoupling is caused by a lack of ability to interact with G proteins or rather by an increased ability of the receptor to recruit β-arrestins. Since uncoupling of G proteins by increased ability to recruit β-arrestins could lead to different cellular or in vivo outcomes than lack of ability to interact with G proteins, it is essential to distinguish between these two mechanisms.

METHODOLOGY/PRINCIPAL FINDINGS

We studied five AT1R mutants previously published to display pathway separation: D74N, DRY/AAY, Y292F, N298A, and Y302F (Ballesteros-Weinstein numbering: 2.50, 3.49-3.51, 7.43, 7.49, and 7.53). We find that D74N, DRY/AAY, and N298A mutants are more prone to β-arrestin recruitment than WT. In contrast, receptor mutants Y292F and Y302F showed impaired ability to recruit β-arrestin in response to Sar1-Ile4-Ile8 (SII) Ang II, a ligand solely activating the β-arrestin pathway.

CONCLUSIONS/SIGNIFICANCE

Our analysis reveals that the underlying conformations induced by these AT1R mutants most likely represent principally different mechanisms of uncoupling the G protein, which for some mutants may be due to their increased ability to recruit β-arrestin2. Hereby, these findings have important implications for drug discovery and 7TMR biology and illustrate the necessity of uncovering the exact molecular determinants for G protein-coupling and β-arrestin recruitment, respectively.

Role of ubiquitination in endocytic trafficking of G-protein-coupled receptors

Lysyl ubiquitination has long been known to target cytoplasmic proteins for proteasomal degradation, and there is now extensive evidence that ubiquitination functions in vacuolar/lysosomal targeting of membrane proteins from both the biosynthetic and endocytic pathways. G-protein-coupled receptors (GPCRs) represent the largest and most diverse family of membrane proteins, whose function is of fundamental importance both physiologically and therapeutically. In this review, we discuss the role of ubiquitination in the vacuolar/lysosomal downregulation of GPCRs through the endocytic pathway, with a primary focus on lysosomal trafficking in mammalian cells. We will summarize evidence indicating that mammalian GPCRs are regulated by ubiquitin-dependent mechanisms conserved in budding yeast, and then consider evidence for additional ubiquitin-dependent and -independent regulation that may be specific to animal cells.

Differential signaling of the endogenous agonists at the beta2-adrenergic receptor

The concept of "functional selectivity" or "biased signaling" suggests that a ligand can have distinct efficacies with regard to different signaling pathways. We have investigated the question of whether biased signaling may be related to distinct agonist-induced conformational changes in receptors using the β(2)-adrenergic receptor (β(2)AR) and its two endogenous ligands epinephrine and norepinephrine as a model system. Agonist-induced conformational changes were determined in a fluorescently tagged β(2)AR FRET sensor. In this β(2)AR sensor, norepinephrine caused signals that amounted to only ≈50% of those induced by epinephrine and the standard "full" agonist isoproterenol. Furthermore, norepinephrine-induced changes in the β(2)AR FRET sensor were slower than those induced by epinephrine (rate constants, 47 versus 128 ms). A similar partial β(2)AR activation signal was revealed for the synthetic agonists fenoterol and terbutaline. However, norepinephrine was almost as efficient as epinephrine (and isoproterenol) in causing activation of G(s) and adenylyl cyclase. In contrast, fenoterol was quite efficient in triggering β-arrestin2 recruitment to the cell surface and its interaction with β(2)AR, as well as internalization of the receptors, whereas norepinephrine caused partial and slow changes in these assays. We conclude that partial agonism of norepinephrine at the β(2)AR is related to the induction of a different active conformation and that this conformation is efficient in signaling to G(s) and less efficient in signaling to β-arrestin2. These observations extend the concept of biased signaling to the endogenous agonists of the β(2)AR and link it to distinct conformational changes in the receptor.

Ligand-supported purification of the urotensin-II receptor

A crucial limitation for structural and biophysical analysis of G protein-coupled receptors (GPCRs) is the inherent challenge of purifying and stabilizing these receptors in an active (agonist-bound) conformation. Peptide ligands, such as the vasoactive, cyclic hormone urotensin-II (U-II), may provide new purification tools, via high affinity, pseudo-irreversible binding suitable for ligand-based affinity purification. We show that the U-II receptor (UT) is resistant to desensitization as a result of low phosphorylation and diminished endocytosis. UT also displays an unusual proclivity to remain active with vasoconstriction sustained despite extensive washout of the ligand. To exploit these properties for ligand-supported purification, we modified the U-II ligand by attaching a biotin moiety and spacer arm to the N terminus, creating a novel affinity ligand (Bio-U-II) to interface with streptavidin media. Bio-U-II bound to UT with pharmacological properties analogous to those of the unmodified U-II ligand (high-affinity, pseudo-irreversible binding). The prebinding of Bio-U-II to UT (before exposure to detergent) facilitated specific capture of UT by stabilizing the receptor structure during solubilization with detergent. Solubilization of UT with the most compatible detergent, n-dodecyl β-d-maltoside, was dependent on the critical micelle concentration, and Gα(q/11) protein was copurified with captured Bio-U-II-UT complexes. Furthermore, captured Bio-U-II-UT complexes were resistant to dissociation at elevated temperatures, suggesting that UT is relatively thermostable, making it an ideal candidate for future structural and biophysical studies. This work demonstrates the utility of pseudo-irreversible ligands to support the purification of a GPCR during detergent extraction, resulting in the first successful purification of the UT.

Beyond desensitization: physiological relevance of arrestin-dependent signaling

Heptahelical G protein-coupled receptors are the most diverse and therapeutically important family of receptors in the human genome. Ligand binding activates heterotrimeric G proteins that transmit intracellular signals by regulating effector enzymes or ion channels. G protein signaling is terminated, in large part, by arrestin binding, which uncouples the receptor and G protein and targets the receptor for internalization. It is clear, however, that heptahelical receptor signaling does not end with desensitization. Arrestins bind a host of catalytically active proteins and serve as ligand-regulated scaffolds that recruit protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into the receptor-arrestin complex. Although many of these arrestin-bound effectors serve to modulate G protein signaling, degrading second messengers and regulating endocytosis and trafficking, other signals seem to extend beyond the receptor-arrestin complex to regulate such processes as protein translation and gene transcription. Although these findings have led to a re-envisioning of heptahelical receptor signaling, little is known about the physiological roles of arrestin-dependent signaling. In vivo, the duality of arrestin function makes it difficult to dissociate the consequences of arrestin-dependent desensitization from those that might be ascribed to arrestin-mediated signaling. Nonetheless, recent evidence generated using arrestin knockouts, G protein-uncoupled receptor mutants, and arrestin pathway-selective "biased agonists" is beginning to reveal that arrestin signaling plays important roles in the retina, central nervous system, cardiovascular system, bone remodeling, immune system, and cancer. Understanding the signaling roles of arrestins may foster the development of pathway-selective drugs that exploit these pathways for therapeutic benefit.

Differential expression of new splice variants of the neurotensin receptor 1 gene in human prostate cancer cell lines

Neurotensin is a neuroendocrine peptide acting as a trophic factor in a variety of cells in vivo but it can also function as an autocrine growth factor in human prostate cancer cells in vitro. In addition, the high-affinity G protein-coupled NT receptor (NTS1) is overexpressed in prostate cancer cell lines. Increasing evidence argues for a direct correlation between specific alternative splice variants and cancer. We detected four splice variants of the NTS1 receptor in human prostate cancer cell lines. These isoforms include one or more exons skipping as well as an alternative 5' splice donor site and are expressed in the late-stage androgen independent prostate cancer cell lines PC3 and DU145, but not in the early-stage androgen-sensitive LNCaP or in normal prostate tissue, which only express the normal transcript. This result shows new splice variants of NTS1 for the first time. The differential expression observed among prostate cancer cell lines and normal prostate tissue opens the interesting possibility of a new role of NT/NTS1 pathway in prostate cancer.

Pasireotide and octreotide stimulate distinct patterns of sst2A somatostatin receptor phosphorylation

Pasireotide (SOM230) is currently under clinical evaluation as a successor compound to octreotide for the treatment of acromegaly, Cushing's disease, and carcinoid tumors. Whereas octreotide acts primarily via the sst(2A) somatostatin receptor, pasireotide was designed to exhibit octreotide-like sst(2A) activity combined with enhanced binding to other somatostatin receptor subtypes. In the present study, we used phophosite-specific antibodies to examine agonist-induced phosphorylation of the rat sst(2A) receptor. We show that somatostatin and octreotide stimulate the complete phosphorylation of a cluster of four threonine residues within the cytoplasmic (353)TTETQRT(359) motif in a variety of cultured cell lines in vitro as well as in intact animals in vivo. This phosphorylation was mediated by G protein-coupled receptor kinases (GRK) 2 and 3 and followed by rapid cointernalization of the receptor and ss-arrestin into the same endocytic vesicles. In contrast, pasireotide failed to promote substantial phosphorylation and internalization of the rat sst(2A) receptor. In the presence of octreotide or SS-14, SOM230 showed partial agonist behavior, inhibiting phosphorylation, and internalization of sst(2A). Upon overexpression of GRK2 or GRK3, pasireotide stimulated selective phosphorylation of Thr356 and Thr359 but not of Thr353 or Thr354 within the (353)TTETQRT(359) motif. Pasireotide-mediated phosphorylation led to the formation of relatively unstable beta-arrestin-sst(2A) complexes that dissociated at or near the plasma membrane. Thus, octreotide and pasireotide are equally active in inducing classical G protein-dependent signaling via the sst(2A) somatostatin receptor. Yet, we find that they promote strikingly different patterns of sst(2A) receptor phosphorylation and, hence, stimulate functionally distinct pools of beta-arrestin.

Characterization of C-terminal tail determinants involved in CRTH2 receptor trafficking: identification of a recycling motif

The molecular mechanisms regulating the trafficking of the CRTH2 receptor are poorly understood. In the present study, we characterize C-terminal tail determinants involved in the agonist-induced trafficking of the CRTH2 receptor for prostaglandin D(2). Our results showed that progressive deletion of C-terminal tail residues from amino acid 395 up to 337 gradually impaired CRTH2 internalization by approximately 50% as measured by ELISA in HEK293 cells. Surprisingly, further deletion of the C-tail to amino acid 328 or 317 resulted in receptor mutants displaying internalization similar to the wild-type receptor. Individual mutations of Asp(330), Ser(331), Glu(332), and Leu(333) to Ala in the C-tail of the full length receptor resulted in a 45% increase in internalization of the receptor mutants relative to the wild-type receptor. Pretreatment with the recycling inhibitor monensin increased internalization of the wild-type receptor but did not affect that of the D330A, S331A, E332A and L333A mutants, indicating that these residues are part of a recycling motif. Further experiments revealed that Asp(330), Ser(331) and Glu(332) are not only involved in receptor recycling, but are also required for promotion of CRTH2 internalization by GRK2 and GRK5. Site-directed mutagenesis identified Thr(347) as a major site for PKC-induced internalization of the receptor. Confocal microscopy revealed that arrestin-3 dissociated from the receptor after agonist stimulation and internalization, suggesting that CRTH2 is a class A G protein-coupled receptor. Our study identified specific amino acids in the CRTH2 receptor C-tail implicated in the agonist-induced internalization and the recycling of the receptor.

Functional differences between two CRF-related diuretic hormone receptors in Drosophila

In Drosophila, two related G-protein-coupled receptors are members of the corticotropin releasing factor (CRF) receptor subfamily. We have previously reported that one of these receptors, encoded by CG8422 is a functional receptor for a diuretic hormone, DH(44). Here, we report that the other CRF receptor subfamily member, encoded by CG12370, is also a receptor for the DH(44) neuropeptide. The lines of evidence to support this identification include increases in cAMP levels due to CG12370 receptor activation and the recruitment of beta-arrestin-GFP to the plasma membrane in response to DH(44) application. We compared these features of the receptors DH44-R2 (encoded by CG12370) and DH44-R1 (encoded by CG8422) and found fundamental differences in signaling, association with the arrestins, and peptide sensitivity. We found that the sensitivity of DH44-R2 to the DH(44) peptide is lower than that of DH44-R1, specifically an estimated EC(50) of 7.98E-07 moll(-1) for DH(44) by DH44-R2 to an EC(50) of 5.12E-09 moll(-1) by DH44-R1 and found that previous reports on the sensitivity of the tubule to DH(44) is in agreement with our measurements of DH44-R2 sensitivity. We employed a specific RNAi construct to selectively knock-down DH44-R2 expression and this led to heightened sensitivity to osmotic challenges. The functional characterization of this diuretic hormone receptor in Drosophila demonstrates a high degree of conservation of CRF-like signaling.

Bimolecular fluorescence complementation: lighting up seven transmembrane domain receptor signalling networks

There is increasing complexity in the organization of seven transmembrane domain (7TM) receptor signalling pathways, and in the ability of their ligands to modulate and direct this signalling. Underlying these events is a network of protein interactions between the 7TM receptors themselves and associated effectors, such as G proteins and beta-arrestins. Bimolecular fluorescence complementation, or BiFC, is a technique capable of detecting these protein-protein events essential for 7TM receptor function. Fluorescent proteins, such as those from Aequorea victoria, are split into two non-fluorescent halves, which then tag the proteins under study. On association, these fragments refold and regenerate a mature fluorescent protein, producing a BiFC signal indicative of complex formation. Here, we review the experimental criteria for successful application of BiFC, considered in the context of 7TM receptor signalling events such as receptor dimerization, G protein and beta-arrestin signalling. The advantages and limitations of BiFC imaging are compared with alternative resonance energy transfer techniques. We show that the essential simplicity of the fluorescent BiFC measurement allows high-content and advanced imaging applications, and that it can probe more complex multi-protein interactions alone or in combination with resonance energy transfer. These capabilities suggest that BiFC techniques will become ever more useful in the analysis of ligand and 7TM receptor pharmacology at the molecular level of protein-protein interactions.

Amphioxus (Branchiostoma floridae) has orthologs of vertebrate odorant receptors

Background

A common feature of chemosensory systems is the involvement of G protein-coupled receptors (GPCRs) in the detection of environmental stimuli. Several lineages of GPCRs are involved in vertebrate olfaction, including trace amine-associated receptors, type 1 and 2 vomeronasal receptors and odorant receptors (ORs). Gene duplication and gene loss in different vertebrate lineages have lead to an enormous amount of variation in OR gene repertoire among species; some fish have fewer than 100 OR genes, while some mammals possess more than 1000. Fascinating features of the vertebrate olfactory system include allelic exclusion, where each olfactory neuron expresses only a single OR gene, and axonal guidance where neurons expressing the same receptor project axons to common glomerulae. By identifying homologous ORs in vertebrate and in non-vertebrate chordates, we hope to expose ancestral features of the chordate olfactory system that will help us to better understand the evolution of the receptors themselves and of the cellular components of the olfactory system.

Results

We have identified 50 full-length and 11 partial ORs in Branchiostoma floridae. No ORs were identified in Ciona intestinalis. Phylogenetic analysis places the B. floridae OR genes in a monophyletic clade with the vertebrate ORs. The majority of OR genes in amphioxus are intronless and many are also tandemly arrayed in the genome. By exposing conserved amino acid motifs and testing the ability of those motifs to discriminate between ORs and non-OR GPCRs, we identified three OR-specific amino acid motifs common in cephalochordate, fish and mammalian and ORs.

Conclusion

Here, we show that amphioxus has orthologs of vertebrate ORs. This conclusion demonstrates that the receptors, and perhaps other components of vertebrate olfaction, evolved at least 550 million years ago. We have also identified highly conserved amino acid motifs that may be important for maintaining receptor conformation or regulating receptor activity. We anticipate that the identification of vertebrate OR orthologs in amphioxus will lead to an improved understanding of OR gene family evolution, OR gene function, and the mechanisms that control cell-specific expression, axonal guidance, signal transduction and signal integration.

CXCR2 inverse agonism detected by arrestin redistribution

To study CXCR2 modulated arrestin redistribution, the authors employed arrestin as a fusion protein containing either the Aequorea victoria-derived enhanced green fluorescent protein (EGFP) or a recently developed mutant of eqFP611, a red fluorescent protein derived from Entacmaea quadricolor. This mutant, referred to as RFP611, had earlier been found to assume a dimeric quarternary structure. It was therefore employed in this work as a "tandem" (td) construct for pseudo-monomeric fusion protein labeling. Both arrestin fusion proteins, containing either td-RFP611 (Arr-td-RFP611) or enhanced green fluorescent protein (EGFP; Arr-EGFP), were found to colocalize with internalized fluorescently labeled Gro-alpha a few minutes after Gro-alpha addition. Intriguingly, however, Arr-td-RFP611 and Arr-EGFP displayed distinct cellular distribution patterns in the absence of any CXCR2-activating ligand. Under these conditions, Arr-td-RFP611 showed a largely homogeneous cytosolic distribution, whereas Arr-EGFP segregated, to a large degree, into granular spots. These observations indicate a higher sensitivity of Arr EGFP to the constitutive activity of CXCR2 and, accordingly, an increased arrestin redistribution to coated pits and endocytic vesicles. In support of this interpretation, the authors found the known CXCR2 antagonist Sch527123 to act as an inverse agonist with respect to Arr-EGFP redistribution. The inverse agonistic properties of Sch527123 were confirmed in vitro in a guanine nucleotide binding assay, revealing an IC(50) value similar to that observed for Arr-EGFP redistribution. Thus, the redistribution assay, when based on Arr-EGFP, enables the profiling of antagonistic test compounds with respect to inverse agonism. When based on Arr-td-RFP611, the assay may be employed to study CXCR2 agonism or neutral antagonism.

Regulation of oxytocin receptor responsiveness by G protein-coupled receptor kinase 6 in human myometrial smooth muscle

Oxytocin plays an important role in the progression, timing, and modulation of uterine contraction during labor and is widely used as an uterotonic agent. We investigated the mechanisms regulating oxytocin receptor (OTR) signaling in human primary myometrial smooth muscle cells and the ULTR cell-line. Oxytocin produced concentration-dependent increases in both total [(3)H]inositol phosphate accumulation and intracellular Ca(2+) concentration ([Ca(2+)](i)); however, responses were greater and more reproducible in the ULTR cell line. Assessment of phospholipase C activity in single cells revealed that the OTR desensitizes rapidly (within 5 min) in the presence of oxytocin (100 nm). To characterize OTR desensitization further, cells were stimulated with a maximally effective concentration of oxytocin (100 nm, 30 sec) followed by a variable washout period and a second identical application of oxytocin. This brief exposure to oxytocin caused a marked decrease (>70%) in OTR responsiveness to rechallenge and was fully reversed by increasing the time period between agonist challenges. To assess involvement of G protein-coupled receptor kinases (GRKs) in OTR desensitization, cells were transfected with small interfering RNAs to cause specific > or =75% knockdown of GRKs 2, 3, 5, or 6. In both primary myometrial and ULTR cells, knockdown of GRK6 largely prevented oxytocin-induced OTR desensitization; in contrast, selective depletion of GRKs 2, 3, or 5 was without effect. These data indicate that GRK6 recruitment is a cardinal effector of OTR responsiveness and provide mechanistic insight into the likely in vivo regulation of OTR signaling in uterine smooth muscle.

Agonist-biased signaling via proteinase activated receptor-2: differential activation of calcium and mitogen-activated protein kinase pathways

We evaluated the ability of different trypsin-revealed tethered ligand (TL) sequences of rat proteinase-activated receptor 2 (rPAR(2)) and the corresponding soluble TL-derived agonist peptides to trigger agonist-biased signaling. To do so, we mutated the proteolytically revealed TL sequence of rPAR(2) and examined the impact on stimulating intracellular calcium transients and mitogen-activated protein (MAP) kinase. The TL receptor mutants, rPAR(2)-Leu(37)Ser(38), rPAR(2)-Ala(37-38), and rPAR(2)-Ala(39-42) were compared with the trypsin-revealed wild-type rPAR(2) TL sequence, S(37)LIGRL(42)-. Upon trypsin activation, all constructs stimulated MAP kinase signaling, but only the wt-rPAR(2) and rPAR(2)-Ala(39-42) triggered calcium signaling. Furthermore, the TL-derived synthetic peptide SLAAAA-NH2 failed to cause PAR(2)-mediated calcium signaling but did activate MAP kinase, whereas SLIGRL-NH2 triggered both calcium and MAP kinase signaling by all receptors. The peptides AAIGRL-NH2 and LSIGRL-NH2 triggered neither calcium nor MAP kinase signals. Neither rPAR(2)-Ala(37-38) nor rPAR(2)-Leu(37)Ser(38) constructs recruited beta-arrestins-1 or -2 in response to trypsin stimulation, whereas both beta-arrestins were recruited to these mutants by SLIGRL-NH2. The lack of trypsin-triggered beta-arrestin interactions correlated with impaired trypsin-activated TL-mutant receptor internalization. Trypsin-stimulated MAP kinase activation by the TL-mutated receptors was not blocked by inhibitors of Galpha(i) (pertussis toxin), Galpha(q) [N-cyclohexyl-1-(2,4-dichlorophenyl)-1,4-dihydro-6-methylindeno[1,2-c]pyrazole-3-carboxamide (GP2A)], Src kinase [4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]-pyrimidine (PP1)], or the epidermal growth factor (EGF) receptor [4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline (AG1478)], but was inhibited by the Rho-kinase inhibitor (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide, 2HCl (Y27362). The data indicate that the proteolytically revealed TL sequence(s) and the mode of its presentation to the receptor (tethered versus soluble) can confer biased signaling by PAR(2), its arrestin recruitment, and its internalization. Thus, PAR(2) can signal to multiple pathways that are differentially triggered by distinct proteinase-revealed TLs or by synthetic signal-selective activating peptides.

Illuminating the life of GPCRs

The investigation of biological systems highly depends on the possibilities that allow scientists to visualize and quantify biomolecules and their related activities in real-time and non-invasively. G-protein coupled receptors represent a family of very dynamic and highly regulated transmembrane proteins that are involved in various important physiological processes. Since their localization is not confined to the cell surface they have been a very attractive "moving target" and the understanding of their intracellular pathways as well as the identified protein-protein-interactions has had implications for therapeutic interventions. Recent and ongoing advances in both the establishment of a variety of labeling methods and the improvement of measuring and analyzing instrumentation, have made fluorescence techniques to an indispensable tool for GPCR imaging. The illumination of their complex life cycle, which includes receptor biosynthesis, membrane targeting, ligand binding, signaling, internalization, recycling and degradation, will provide new insights into the relationship between spatial receptor distribution and function. This review covers the existing technologies to track GPCRs in living cells. Fluorescent ligands, antibodies, auto-fluorescent proteins as well as the evolving technologies for chemical labeling with peptide- and protein-tags are described and their major applications concerning the GPCR life cycle are presented.

Angiotensin II-induced cyclooxygenase 2 expression in rat aorta vascular smooth muscle cells does not require heterotrimeric G protein activation

Angiotensin II (AngII) initiates cellular effects via its G protein-coupled angiotensin 1 (AT(1)) receptor (AT(1)R). Previously, we showed that AngII-induced expression of the prostanoid-producing enzyme cyclooxygenase 2 (COX-2) was dependent upon nuclear trafficking of activated AT(1)R. In the present study, mastoparan (an activator of G proteins), suramin (an inhibitor of G proteins), 1-[6-[[17beta-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122; a specific inhibitor of phospholipase C), and sarcosine(1)-Ile(4)-Ile(8)-AngII (SII-AngII; a G protein-independent AT(1)R agonist) were used to determine the involvement of G proteins and AT(1A)R trafficking in AngII-stimulated COX-2 protein expression in human embryonic kidney-293 cells stably expressing AT(1A)/green fluorescent protein receptors and cultured vascular smooth muscle cells, respectively. Mastoparan alone stimulated release of intracellular calcium and increased COX-2 expression. Preincubation with mastoparan inhibited AngII-induced calcium signaling without altering AngII-induced AT(1A)R trafficking, p42/44 extracellular signal-regulated kinase (ERK) activation, or COX-2 expression. Suramin or U73122 had no significant effect on their own; they did not inhibit AngII-induced AT(1A)R trafficking, p42/44 ERK activation, or COX-2 expression; but they did inhibit AngII-induced calcium responses. SII-AngII stimulated AT(1A)R trafficking and increased COX-2 protein expression without activating intracellular calcium release. These data suggest that G protein activation results in increased COX-2 protein expression, but AngII-induced COX-2 expression seems to occur independently of G protein activation.

Beta-arrestin-based Bret2 screening assay for the "non"-beta-arrestin binding CB1 receptor

CB1 receptor (CB1R) antagonists have been demonstrated to be effective in treating obesity and related disorders. This study has been focused on establishing a beta-arrestin 2-based screening assay for the CB1R using BRET2 technology. When the existing BRET2 screening platform was applied to the CB1R, the authors discovered that the receptor interacted weakly with beta-arrestin 2, resulting in unsatisfactory assay performance. To enhance the beta-arrestin binding capacity, they replaced the C-terminal tail of the CB1R with tails from either the V2 or BRS3 receptors, both of which interact strongly with beta-arrestin 2. Using this chimeric approach, the authors screened a small compound library and identified 21 antagonist and inverse agonist hits with IC50 and EC50 values ranging from 0.3 nM to 7.5 microM. Both primary and secondary screening were performed with Z'>0.5, suggesting that the assay is a robust and cost-effective alternative to existing cell-based assays.

Adaptor protein-2 interaction with arrestin regulates GPCR recycling and apoptosis

G protein-coupled receptors (GPCRs) are integral to cellular function in nearly all physiologic and many pathologic processes. GPCR signaling represents an intricate balance between receptor activation, inactivation (desensitization, internalization and degradation) and resensitization (recycling and de novo synthesis). Complex formation between phosphorylated GPCRs, arrestins and an ever-increasing number of effector molecules is known to regulate cellular function. Previous studies have demonstrated that, although N-formyl peptide receptor (FPR) internalization occurs in the absence of arrestins, FPR recycling is arrestin-dependent. Furthermore, FPR stimulation in the absence of arrestins leads to receptor accumulation in perinuclear endosomes and apoptosis. In this study, we show that the interaction of GPCR-bound arrestin with adaptor protein-2 (AP-2) is a critical anti-apoptotic event. In addition, AP-2 associates with the receptor-arrestin complex in perinuclear endosomes and is required for proper post-endocytic GPCR trafficking. Finally, we observed that depletion of endogenous AP-2 results in the initiation of apoptosis upon stimulation of multiple GPCRs, including P2Y purinergic receptors and CXCR2, but not CXCR4. We propose a model in which the abnormal accumulation of internalized GPCR-arrestin complexes in recycling endosomes, resulting from defective arrestin-AP-2 interactions, leads to the specific initiation of aberrant signaling pathways and apoptosis.

Neurokinin 1 receptors regulate morphine-induced endocytosis and desensitization of mu-opioid receptors in CNS neurons

mu-Opioid receptors (MORs) are G-protein-coupled receptors (GPCRs) that mediate the physiological effects of endogenous opioid neuropeptides and opiate drugs such as morphine. MORs are coexpressed with neurokinin 1 receptors (NK1Rs) in several regions of the CNS that control opioid dependence and reward. NK1R activation affects opioid reward specifically, however, and the cellular basis for this specificity is unknown. We found that ligand-induced activation of NK1Rs produces a cell-autonomous and nonreciprocal inhibition of MOR endocytosis induced by diverse opioids. Studies using epitope-tagged receptors expressed in cultured striatal neurons and a neuroblastoma cell model indicated that this heterologous regulation is mediated by NK1R-dependent sequestration of arrestins on endosome membranes. First, endocytic inhibition mediated by wild-type NK1Rs was overcome in cells overexpressing beta-arrestin2, a major arrestin isoform expressed in striatum. Second, NK1R activation promoted sequestration of beta-arrestin2 on endosomes, whereas MOR activation did not. Third, heterologous inhibition of MOR endocytosis was prevented by mutational disruption of beta-arrestin2 sequestration by NK1Rs. NK1R-mediated regulation of MOR trafficking was associated with reduced opioid-induced desensitization of adenylyl cyclase signaling in striatal neurons. Furthermore, heterologous regulation of MOR trafficking was observed in both amygdala and locus ceruleus neurons that naturally coexpress these receptors. These results identify a cell-autonomous mechanism that may underlie the highly specific effects of NK1R on opioid signaling and suggest, more generally, that receptor-specific trafficking of arrestins may represent a fundamental mechanism for coordinating distinct GPCR-mediated signals at the level of individual CNS neurons.

Prolonged RXFP1 and RXFP2 signaling can be explained by poor internalization and a lack of beta-arrestin recruitment

Relaxin induces sustained physiological responses, which brings into question the deactivation processes typical of most G protein-coupled receptors (GPCR) for its receptor, relaxin family peptide receptor 1 (RXFP1). Here, we examined relaxin-dependent phosphorylation of RXFP1 and the related insulin-like peptide 3 (INSL3) receptor, RXFP2, as well as the capacity of these receptors to recruit beta-arrestins and internalize in response to ligand stimulation. We confirmed in human embryonic kidney (HEK)-293T cells, expressing RXFP1 or RXFP2, that both receptors elicit prolonged cAMP responses up to 6 h after stimulation. Receptors immunoprecipitated from (32)P metabolically labeled cells were used to investigate the agonist-specific phosphorylation. Rapid and robust receptor phosphorylation was not observed for either RXFP1 or RXFP2, although some (32)P-incorporation was observed at 30 min; however, this was not statistically significant. In accord with this result, RXFP1 and RXFP2 demonstrated poor internalization in response to relaxin or INSL3, as compared with the angiotensin II type 1 receptor (AT(1)R), which undergoes rapid and robust phosphorylation and internalization in response to angiotensin II. Additionally, coexpression of GPCR kinases has no effect on the rate of internalization for either RXFP1 or RXFP2. Confocal microscopy was used to follow the trafficking of green fluorescent protein-labeled beta-arrestins after receptor activation. Neither RXFP1 nor RXFP2 activation results in recruitment of beta-arrestins to the cell surface, whereas AT(1)R rapidly recruits both beta-arrestins-1 and -2. The apparent lack of classical regulation for RXFP1 and RXFP2 provides the molecular basis for the prolonged signaling and physiological actions of relaxin and related peptides.

Intracellular trafficking of the human oxytocin receptor: evidence of receptor recycling via a Rab4/Rab5 "short cycle"

As in the case of most G protein-coupled receptors, agonist stimulation of human oxytocin receptors (OTRs) leads to desensitization and internalization; however, little is known about the subsequent intracellular OTR trafficking, which is crucial for reestablishing agonist responsiveness. We examined receptor resensitization by first using HEK293T cells stably expressing human OTRs. Upon agonist activation, the receptors were almost completely sequestered inside intracellular compartments that were not labeled by lysosomal markers, thus indicating that the internalized receptors were not sorted to these degrading organelles. Binding and fluorescence assays showed that almost 85% of the receptors had returned to the cell surface after 4 h, by which time cell responsiveness to the agonist was also completely restored, as shown by measuring phospholipase C activation. Similar results were also obtained in the presence of cycloheximide, thus indicating that receptor recycling and not de novo receptor synthesis was responsible for the resensitization. Notably, very similar internalization and recycling kinetics were observed in endogenous OTRs expressed on myometrial cells. We also investigated the role of beta-arrestin2 in OTR recycling as these receptors have been previously classified as slowly or nonrecycling receptors on the basis of their stable association with this interacting protein. Our data suggest that the stable OTR/beta-arrestin2 interaction plays an important role in determining the rate of recycling of human OTRs, but does not determine the fate of endocytosed receptors. Subsequent investigations of receptor recycling pathways showed that OTRs localize in vesicles containing the Rab5 and Rab4 small GTPases (markers of the "short cycle"), whereas there was no colocalization with Rab11 (a marker of the "long cycle") or Rab7 (a marker of vesicles directed to endosomal/lysosomal compartments). Taken together, these data indicate that OTRs are capable of very efficient and complete resensitization due to receptor recycling via the short cycle.

G-protein-coupled receptor-focused drug discovery using a target class platform approach

In recent years, several large pharmaceutical companies have taken a novel approach to drug discovery biology and chemistry in that they channel their efforts with respect to particular target classes, such as G-protein-coupled receptors (GPCRs), toward dedicated, specialized teams. Benefits of such an organizational structure are the prospects of establishing several target-family-specific experimental techniques and skill sets, thereby enabling a comprehensive functional profiling of drug candidates in different pharmacological respects. In this context, the recently increased number of reports on GPCR ligand-biased signaling has further spurred the efforts in the pharmaceutical industry toward broader biological characterization of the test compounds, for example employing high-content screening to analyze different GPCR ligand-induced signaling pathways. The knowledge of the disease-relevant functional properties of the small molecule GPCR ligands enables target-specific chemical optimization and GPCR-subclass-directed library design. In the case of GPCRs, where little--although at present slowly expanding--structural information on the targets is available, the modeling of GPCR structures crucially depends on biological validation (typically supported by site-directed mutagenesis of the GPCR ligand binding site). In this review, we aim to recapitulate efforts in the pharmaceutical industry to address GPCR-directed drug discovery in a target-class-directed platform approach: establishing GPCR-specific biological assay panels and creating computational chemistry methods for finding and optimizing small molecules modulating the activity of GPCRs.

Glucagon receptor recycling: role of carboxyl terminus, beta-arrestins, and cytoskeleton

Glucagon receptor (GR) activity and expression are altered in several diseases, including Type 2 diabetes. Previously, we investigated the mechanism of GR desensitization and internalization. The present study focused on the fate of internalized GR. Using both hamster hepatocytes and human embryonic kidney (HEK)-293 cells, we showed that internalized GR recycled to the plasma membrane within 30-60 min following stimulation of the cells with 100 nM glucagon. In HEK-293 cells and during recycling, GR colocalized with Rab4, Rab11, beta-arrestin1, beta-arrestin2, and actin filaments, in the cytosolic and/or perinuclear domains. Glucagon treatment triggered redistribution of actin filaments from the plasma membrane to the cytosol. GR coimmunoprecipitated with beta-actin in both hepatocytes and HEK-293 cells. Downregulation of beta-arrestin1 and beta-arrestin2 or disruption of the cytoskeleton inhibited recycling, but not internalization of GR. Deletion of the GR carboxyl-terminal 70 amino acids abolished internalization of GR in response to glucagon while deletion of the last 40 amino acids only did not affect GR internalization and recycling. After exposure of the cells to either high concentrations or prolonged duration of glucagon, GR colocalized with lysosomes. GR degradation was inhibited by lysosomal, but not proteosomal, inhibitors. In conclusion, GR recycles through Rab4- and Rab11- positive vesicles. The actin cytoskeleton, beta-arrestin1, beta-arrestin2, and the receptor's carboxyl terminus are involved in recycling. Prolonged stimulation with glucagon targets GR for degradation in lysosomes. Therefore, the present study provides a better understanding of the GR recycling mechanism, which could become useful in the treatment of certain diseases, including diabetes.

Phosphorylation state of mu-opioid receptor determines the alternative recycling of receptor via Rab4 or Rab11 pathway

Agonist-induced phosphorylation, internalization, and intracellular trafficking of G protein-coupled receptors are critical in regulating both cellular responsiveness and signal transduction. The current study investigated the role of receptor phosphorylation state in regulation of agonist-induced internalization and intracellular trafficking of mu-opioid receptor (MOR). Our results showed that after agonist stimulation, the recycle of a mutant MOR that lacks the C-terminal residues after Asn(362) (MOR362T) was greatly decreased, whereas a C-terminal phosphorylation sites-mutated MOR (MOR3A), which is deficient in agonist-induced phosphorylation recycled back to the membrane at a level comparable to that of the wild-type receptor, however, interestingly at a slower rate. Inhibition of functions of either Rab4 or Rab11 by dominant-negative mutants and small interfering RNA both significantly impaired the recycling of the wild-type MOR, whereas the recycling of the phosphorylation-deficient mutant was only inhibited by the dominant-negative mutant and small interfering RNA of Rab11, suggesting that the recycling of nonphosphorylated MOR is exclusively via Rab11-mediated pathway. Furthermore, phosphorylated MOR was observed accumulated in Rab5- and Rab4-, but not Rab11-positive vesicles. Our data indicate that both phosphorylated and nonphosphorylated MOR internalize via Rab5-dependent pathway after agonist stimulation, and the phosphorylated and nonphosphorylated MORs recycle through distinct vesicular trafficking pathways mediated by Rab4 and Rab11, respectively, which may ultimately lead to differential cellular responsiveness or downstream signaling.