Association of beta-Arrestin 1 with the type 1A angiotensin II receptor involves phosphorylation of the receptor carboxyl terminus and correlates with receptor internalization

Genetic Deletion of β-Arrestin 2 From the Subfornical Organ and Other Periventricular Nuclei in the Brain Alters Fluid Homeostasis and Blood Pressure

BACKGROUND

ANG (angiotensin II) elicits dipsogenic and pressor responses via activation of the canonical Gαq (G-protein component of the AT1R [angiotensin type 1 receptor])-mediated AT1R in the subfornical organ. Recently, we demonstrated that ARRB2 (β-arrestin 2) global knockout mice exhibit a higher preference for salt and exacerbated pressor response to deoxycorticosterone acetate salt. However, whether ARRB2 within selective neuroanatomical nuclei alters physiological responses to ANG is unknown. Therefore, we hypothesized that ARRB2, specifically in the subfornical organ, counterbalances maladaptive dipsogenic and pressor responses to the canonical AT1R signaling.

METHODS

Male and female Arrb2FLOX mice received intracerebroventricular injection of either adeno-associated virus (AAV)-Cre-GFP (green fluorescent protein) to induce brain-specific deletion of ARRB2 (Arrb2ICV-Cre). Arrb2FLOX mice receiving ICV-AAV-GFP were used as control (Arrb2ICV-Control). Infection with ICV-AAV-Cre primarily targeted the subfornical organ with few off targets. Fluid intake was evaluated using the 2-bottle choice paradigm with 1 bottle containing water and 1 containing 0.15 mol/L NaCl.

RESULTS

Arrb2ICV-Cre mice exhibited a greater pressor response to acute ICV-ANG infusion. At baseline conditions, Arrb2ICV-Cre mice exhibited a significant increase in saline intake compared with controls, resulting in a saline preference. Furthermore, when mice were subjected to water-deprived or sodium-depleted conditions, which would naturally increase endogenous ANG levels, Arrb2ICV-Cre mice exhibited elevated saline intake.

CONCLUSIONS

Overall, these data indicate that ARRB2 in selective cardiovascular nuclei in the brain, including the subfornical organ, counterbalances canonical AT1R responses to both exogenous and endogenous ANG. Stimulation of the AT1R/ARRB axis in the brain may represent a novel strategy to treat hypertension.

Insights Into the Role of Angiotensin-II AT1 Receptor-Dependent β-Arrestin Signaling in Cardiovascular Disease

β-arrestins are a family of intracellular signaling proteins that play a key role in regulating the activity of G protein-coupled receptors. The angiotensin-II type 1 receptor is an important G protein-coupled receptor involved in the regulation of cardiovascular function and has been implicated in the progression of cardiovascular diseases. In addition to canonical G protein signaling, G protein-coupled receptors including the angiotensin-II type 1 receptor can signal via β-arrestin. Dysregulation of β-arrestin signaling has been linked to several cardiovascular diseases including hypertension, atherosclerosis, and heart failure. Understanding the role of β-arrestins in these conditions is critical to provide new therapeutic targets for the treatment of cardiovascular disease. In this review, we will discuss the beneficial and maladaptive physiological outcomes of angiotensin-II type 1 receptor-dependent β-arrestin activation in different cardiovascular diseases.

Molecular Effects of Auto-Antibodies on Angiotensin II Type 1 Receptor Signaling and Cell Proliferation

The angiotensin II (Ang II) type 1 receptor (AT1R) is involved in the regulation of blood pressure (through vasoconstriction) and water and ion homeostasis (mediated by interaction with the endogenous agonist). AT1R can also be activated by auto-antibodies (AT1R-Abs), which are associated with manifold diseases, such as obliterative vasculopathy, preeclampsia and systemic sclerosis. Knowledge of the molecular mechanisms related to AT1R-Abs binding and associated signaling cascade (dys-)regulation remains fragmentary. The goal of this study was, therefore, to investigate details of the effects of AT1R-Abs on G-protein signaling and subsequent cell proliferation, as well as the putative contribution of the three extracellular receptor loops (ELs) to Abs-AT1R signaling. AT1R-Abs induced nuclear factor of activated T-cells (NFAT) signaling, which reflects Gq/11 and Gi activation. The impact on cell proliferation was tested in different cell systems, as well as activation-triggered receptor internalization. Blockwise alanine substitutions were designed to potentially investigate the role of ELs in AT1R-Abs-mediated effects. First, we demonstrate that Ang II-mediated internalization of AT1R is impeded by binding of AT1R-Abs. Secondly, exclusive AT1R-Abs-induced Gq/11 activation is most significant for NFAT stimulation and mediates cell proliferation. Interestingly, our studies also reveal that ligand-independent, baseline AT1R activation of Gi signaling has, in turn, a negative effect on cell proliferation. Indeed, inhibition of Gi basal activity potentiates proliferation triggered by AT1R-Abs. Finally, although AT1R containing EL1 and EL3 blockwise alanine mutations were not expressed on the human embryonic kidney293T (HEK293T) cell surface, we at least confirmed that parts of EL2 are involved in interactions between AT1R and Abs. This current study thus provides extended insights into the molecular action of AT1R-Abs and associated mechanisms of interrelated pathogenesis.

The endocytosis of oxidized LDL via the activation of the angiotensin II type 1 receptor

Arrestin-dependent activation of a G-protein-coupled receptor (GPCR) triggers endocytotic internalization of the receptor complex. We analyzed the interaction between the pattern recognition receptor (PRR) lectin-like oxidized low-density lipoprotein (oxLDL) receptor (LOX-1) and the GPCR angiotensin II type 1 receptor (AT1) to report a hitherto unidentified mechanism whereby internalization of the GPCR mediates cellular endocytosis of the PRR ligand. Using genetically modified Chinese hamster ovary cells, we found that oxLDL activates Gαi but not the Gαq pathway of AT1 in the presence of LOX-1. Endocytosis of the oxLDL-LOX-1 complex through the AT1-β-arrestin pathway was demonstrated by real-time imaging of the membrane dynamics of LOX-1 and visualization of endocytosis of oxLDL. Finally, this endocytotic pathway involving GPCR kinases (GRKs), β-arrestin, and clathrin is relevant in accumulating oxLDL in human vascular endothelial cells. Together, our findings indicate that oxLDL activates selective G proteins and β-arrestin-dependent internalization of AT1, whereby the oxLDL-LOX-1 complex undergoes endocytosis.

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The binding of oxidized LDL (oxLDL) to LOX-1 induces selective activation of AT1

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oxLDL and angiotensin II additively or competitively activate AT1 in different cells

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oxLDL promotes β-arrestin-dependent internalization of oxLDL-LOX-1-AT1 complex

Genetic code expansion and photocross-linking identify different β-arrestin binding modes to the angiotensin II type 1 receptor

The angiotensin II (AngII) type 1 receptor (AT1R) is a member of the G protein-coupled receptor (GPCR) family and binds β-arrestins (β-arrs), which regulate AT1R signaling and trafficking. These processes can be biased by different ligands or mutations in the AGTR1 gene. As for many GPCRs, the exact details for AT1R-β-arr interactions driven by AngII or β-arr-biased ligands remain largely unknown. Here, we used the amber-suppression technology to site-specifically introduce the unnatural amino acid (UAA) p-azido-l-phenylalanine (azF) into the intracellular loops (ICLs) and the C-tail of AT1R. Our goal was to generate competent photoreactive receptors that can be cross-linked to β-arrs in cells. We performed UV-mediated photolysis of 25 different azF-labeled AT1Rs to cross-link β-arr1 to AngII-bound receptors, enabling us to map important contact sites in the C-tail and in the ICL2 and ICL3 of the receptor. The extent of AT1R-β-arr1 cross-linking among azF-labeled receptors differed, revealing variability in β-arr's contact mode with the different AT1R domains. Moreover, the signature of ligated AT1R-β-arr complexes from a subset of azF-labeled receptors also differed between AngII and β-arr-biased ligand stimulation of receptors and between azF-labeled AT1R bearing and that lacking a bias signaling mutation. These observations further implied distinct interaction modalities of the AT1R-β-arr1 complex in biased signaling conditions. Our findings demonstrate that this photocross-linking approach is useful for understanding GPCR-β-arr complexes in different activation states and could be extended to study other protein-protein interactions in cells.

The Role of β-Arrestin Proteins in Organization of Signaling and Regulation of the AT1 Angiotensin Receptor

AT1 angiotensin receptor plays important physiological and pathophysiological roles in the cardiovascular system. Renin-angiotensin system represents a target system for drugs acting at different levels. The main effects of ATR1 stimulation involve activation of Gq proteins and subsequent IP3, DAG, and calcium signaling. It has become evident in recent years that besides the well-known G protein pathways, AT1R also activates a parallel signaling pathway through β-arrestins. β-arrestins were originally described as proteins that desensitize G protein-coupled receptors, but they can also mediate receptor internalization and G protein-independent signaling. AT1R is one of the most studied receptors, which was used to unravel the newly recognized β-arrestin-mediated pathways. β-arrestin-mediated signaling has become one of the most studied topics in recent years in molecular pharmacology and the modulation of these pathways of the AT1R might offer new therapeutic opportunities in the near future. In this paper, we review the recent advances in the field of β-arrestin signaling of the AT1R, emphasizing its role in cardiovascular regulation and heart failure.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

A synthetic intrabody-based selective and generic inhibitor of GPCR endocytosis

Beta-arrestins (βarrs) critically mediate desensitization, endocytosis and signalling of G protein-coupled receptors (GPCRs), and they scaffold a large number of interaction partners. However, allosteric modulation of their scaffolding abilities and direct targeting of their interaction interfaces to modulate GPCR functions selectively have not been fully explored yet. Here we identified a series of synthetic antibody fragments (Fabs) against different conformations of βarrs from phage display libraries. Several of these Fabs allosterically and selectively modulated the interaction of βarrs with clathrin and ERK MAP kinase. Interestingly, one of these Fabs selectively disrupted βarr-clathrin interaction, and when expressed as an intrabody, it robustly inhibited agonist-induced endocytosis of a broad set of GPCRs without affecting ERK MAP kinase activation. Our data therefore demonstrate the feasibility of selectively targeting βarr interactions using intrabodies and provide a novel framework for fine-tuning GPCR functions with potential therapeutic implications.

Heterologous phosphorylation-induced formation of a stability lock permits regulation of inactive receptors by β-arrestins

β-Arrestins are key regulators and signal transducers of G protein-coupled receptors (GPCRs). The interaction between receptors and β-arrestins is generally believed to require both receptor activity and phosphorylation by GPCR kinases. In this study, we investigated whether β-arrestins are able to bind second messenger kinase-phosphorylated, but inactive receptors as well. Because heterologous phosphorylation is a common phenomenon among GPCRs, this mode of β-arrestin activation may represent a novel mechanism of signal transduction and receptor cross-talk. Here we demonstrate that activation of protein kinase C (PKC) by phorbol myristate acetate, G_q/11-coupled GPCR, or epidermal growth factor receptor stimulation promotes β-arrestin2 recruitment to unliganded AT₁ angiotensin receptor (AT₁R). We found that this interaction depends on the stability lock, a structure responsible for the sustained binding between GPCRs and β-arrestins, formed by phosphorylated serine-threonine clusters in the receptor's C terminus and two conserved phosphate-binding lysines in the β-arrestin2 N-domain. Using improved FlAsH-based serine-threonine clusters β-arrestin2 conformational biosensors, we also show that the stability lock not only stabilizes the receptor-β-arrestin interaction, but also governs the structural rearrangements within β-arrestins. Furthermore, we found that β-arrestin2 binds to PKC-phosphorylated AT₁R in a distinct active conformation, which triggers MAPK recruitment and receptor internalization. Our results provide new insights into the activation of β-arrestins and reveal their novel role in receptor cross-talk.

Structure-activity relationship study of angiotensin II analogs in terms of β-arrestin-dependent signaling to aldosterone production

The known angiotensin II (AngII) physiological effect of aldosterone synthesis and secretion induction, a steroid hormone that contributes to the pathology of postmyocardial infarction (MI) heart failure (HF), is mediated by both Gq/11 proteins and β-arrestins, both of which couple to the AngII type 1 receptors (AT1Rs) of adrenocortical zona glomerulosa (AZG) cells. Over the past several years, AngII analogs with increased selectivity ("bias") toward β-arrestin-dependent signaling at the AT1R have been designed and described, starting with SII, the gold-standard β-arrestin-"biased" AngII analog. In this study, we examined the relative potencies of an extensive series of AngII peptide analogs at relative activation of G proteins versus β-arrestins by the AT1R. The major structural difference of these peptides from SII was their varied substitutions at position 5, rather than position 4 of native AngII. Three of them were found biased for β-arrestin activation and extremely potent at stimulating aldosterone secretion in AZG cells in vitro, much more potent than SII in that regard. Finally, the most potent of these three ([Sar(1), Cys(Et)(5), Leu(8)]-AngII, CORET) was further examined in post-MI rats progressing to HF and overexpressing adrenal β-arrestin1 in vivo. Consistent with the in vitro studies, CORET was found to exacerbate the post-MI hyperaldosteronism, and, consequently, cardiac function of the post-MI animals in vivo. Finally, our data suggest that increasing the size of position 5 of the AngII peptide sequence results in directly proportional increases in AT1R-dependent β-arrestin activation. These findings provide important insights for AT1R pharmacology and future AngII-targeted drug development.

Identification of Distinct Conformations of the Angiotensin-II Type 1 Receptor Associated with the Gq/11 Protein Pathway and the β-Arrestin Pathway Using Molecular Dynamics Simulations

Biased signaling represents the ability of G protein-coupled receptors to engage distinct pathways with various efficacies depending on the ligand used or on mutations in the receptor. The angiotensin-II type 1 (AT1) receptor, a prototypical class A G protein-coupled receptor, can activate various effectors upon stimulation with the endogenous ligand angiotensin-II (AngII), including the Gq/11 protein and β-arrestins. It is believed that the activation of those two pathways can be associated with distinct conformations of the AT1 receptor. To verify this hypothesis, microseconds of molecular dynamics simulations were computed to explore the conformational landscape sampled by the WT-AT1 receptor, the N111G-AT1 receptor (constitutively active and biased for the Gq/11 pathway), and the D74N-AT1 receptor (biased for the β-arrestin1 and -2 pathways) in their apo-forms and in complex with AngII. The molecular dynamics simulations of the AngII-WT-AT1, N111G-AT1, and AngII-N111G-AT1 receptors revealed specific structural rearrangements compared with the initial and ground state of the receptor. Simulations of the D74N-AT1 receptor revealed that the mutation stabilizes the receptor in the initial ground state. The presence of AngII further stabilized the ground state of the D74N-AT1 receptor. The biased agonist [Sar(1),Ile(8)]AngII also showed a preference for the ground state of the WT-AT1 receptor compared with AngII. These results suggest that activation of the Gq/11 pathway is associated with a specific conformational transition stabilized by the agonist, whereas the activation of the β-arrestin pathway is linked to the stabilization of the ground state of the receptor.

Investigation of the fate of type I angiotensin receptor after biased activation

Biased agonism on the type I angiotensin receptor (AT1-R) can achieve different outcomes via activation of G protein-dependent and -independent cellular responses. In this study, we investigated whether the biased activation of AT1-R can lead to different regulation and intracellular processing of the receptor. We analyzed β-arrestin binding, endocytosis, and subsequent trafficking steps, such as early and late phases of recycling of AT1-R in human embryonic kidney 293 cells expressing wild-type or biased mutant receptors in response to different ligands. We used Renilla luciferase-tagged receptors and yellow fluorescent protein-tagged β-arrestin2, Rab5, Rab7, and Rab11 proteins in bioluminescence resonance energy transfer measurements to follow the fate of the receptor after stimulation. We found that not only is the signaling of the receptor different upon using selective ligands, but the fate within the cells is also determined by the type of the stimulation. β-arrestin binding and the internalization kinetics of the angiotensin II-stimulated AT1-R differed from those stimulated by the biased agonists. Similarly, angiotensin II-stimulated wild-type AT1-R showed differences compared with a biased mutant AT1-R (DRY/AAY AT1-R) with regards to β-arrestin binding and endocytosis. We found that the differences in the internalization kinetics of the receptor in response to biased agonist stimulation are due to the differences in plasma membrane phosphatidylinositol 4,5-bisphosphate depletion. Moreover, the stability of the β-arrestin binding is a major determinant of the later fate of the internalized AT1-R receptor.

Identification of serine 348 on the apelin receptor as a novel regulatory phosphorylation site in apelin-13-induced G protein-independent biased signaling

Phosphorylation plays vital roles in the regulation of G protein-coupled receptor (GPCR) functions. The apelin and apelin receptor (APJ) system is involved in the regulation of cardiovascular function and central control of body homeostasis. Here, using tandem mass spectrometry, we first identified phosphorylated serine residues in the C terminus of APJ. To determine the role of phosphorylation sites in APJ-mediated G protein-dependent and -independent signaling and function, we induced a mutation in the C-terminal serine residues and examined their effects on the interaction between APJ with G protein or GRK/β-arrestin and their downstream signaling. Mutation of serine 348 led to an elimination of both GRK and β-arrestin recruitment to APJ induced by apelin-13. Moreover, APJ internalization and G protein-independent ERK signaling were also abolished by point mutation at serine 348. In contrast, this mutant at serine residues had no demonstrable impact on apelin-13-induced G protein activation and its intracellular signaling. These findings suggest that mutation of serine 348 resulted in inactive GRK/β-arrestin. However, there was no change in the active G protein thus, APJ conformation was biased. These results provide important information on the molecular interplay and impact of the APJ function, which may be extrapolated to design novel drugs for cardiac hypertrophy based on this biased signal pathway.

Chemical crosslinkers enhance detection of receptor interactomes

Receptor function is dependent on interaction with various intracellular proteins that ensure the localization and signaling of the receptor. While a number of approaches have been optimized for the isolation, purification, and proteomic characterization of receptor-protein interaction networks (interactomes) in cells, the capture of receptor interactomes and their dynamic properties remains a challenge. In particular, the study of interactome components that bind to the receptor with low affinity or can rapidly dissociate from the macromolecular complex is difficult. Here we describe how chemical crosslinking (CC) can aid in the isolation and proteomic analysis of receptor-protein interactions. The addition of CC to standard affinity purification and mass spectrometry protocols boosts the power of protein capture within the proteomic assay and enables the identification of specific binding partners under various cellular and receptor states. The utility of CC in receptor interactome studies is highlighted for the nicotinic acetylcholine receptor as well as several other receptor types. A better understanding of receptors and their interactions with proteins spearheads molecular biology, informs an integral part of bench medicine which helps in drug development, drug action, and understanding the pathophysiology of disease.

A functional siRNA screen identifies genes modulating angiotensin II-mediated EGFR transactivation

The angiotensin type 1 receptor (AT1R) transactivates the epidermal growth factor receptor (EGFR) to mediate cellular growth, however, the molecular mechanisms involved have not yet been resolved. To address this, we performed a functional siRNA screen of the human kinome in human mammary epithelial cells that demonstrate a robust AT1R-EGFR transactivation. We identified a suite of genes encoding proteins that both positively and negatively regulate AT1R-EGFR transactivation. Many candidates are components of EGFR signalling networks, whereas others, including TRIO, BMX and CHKA, have not been previously linked to EGFR transactivation. Individual knockdown of TRIO, BMX or CHKA attenuated tyrosine phosphorylation of the EGFR by angiotensin II stimulation, but this did not occur following direct stimulation of the EGFR with EGF, indicating that these proteins function between the activated AT1R and the EGFR. Further investigation of TRIO and CHKA revealed that their activity is likely to be required for AT1R-EGFR transactivation. CHKA also mediated EGFR transactivation in response to another G protein-coupled receptor (GPCR) ligand, thrombin, indicating a pervasive role for CHKA in GPCR-EGFR crosstalk. Our study reveals the power of unbiased, functional genomic screens to identify new signalling mediators important for tissue remodelling in cardiovascular disease and cancer.

Disruption of parathyroid hormone and parathyroid hormone-related peptide receptor phosphorylation prolongs ERK1/2 MAPK activation and enhances c-fos expression

Previous studies have demonstrated that parathyroid hormone (PTH) binding to the PTH/PTH-related peptide receptor (PPR) stimulates G protein coupling, receptor phosphorylation, β-arrestin translocation, and internalization of the ligand/receptor complex. The extracellular signal-regulated mitogen-activated protein kinases 1/2 (ERK1/2 MAPK) are downstream effectors of PPR. In the current study, we investigated the role of PPR phosphorylation in the PTH regulation of the ERK1/2 MAPK pathway. Short treatment with PTH (0-40 min) of LLCP-K(1) cells stably expressing a wild-type (WT) or a phosphorylation-deficient (PD) PPR (WT-PPR or PD-PPR cells, respectively) results in similar activation of ERK1/2. Interestingly, PTH stimulation of ERK1/2 in the WT-PPR cells then decreases as a result of longer PTH (60 min) treatment, and inhibition of ERK1/2 by PTH is observed at 90 min. Strikingly, the PD-PPR cells exhibit prolonged ERK1/2 activation up to 90 min of PTH treatment. An ERK1/2-dependent increase in c-fos expression is observed in the PD-PPR cells. Subsequently, c-fos expression in the WT-PPR and PD-PPR cells was markedly attenuated by a specific ERK1/2 pathway inhibitor. Further investigations revealed that PTH treatment causes a robust recruitment of a green fluorescent protein-tagged β-arrestin2 (β-arrestin2-GFP) in the WT-PPR cells. In contrast, β-arrestin2 recruitment was reduced in the PD-PPR cells. Importantly, expression of a receptor phosphorylation-independent β-arrestin2 (R169E) in the PD-PPR cells restored the biphasic effect of PTH on ERK1/2 as in the WT-PPR cells. The study reports a novel role for receptor phosphorylation and β-arrestin2 in the subsequent inhibition of the ERK1/2 pathway and in control of gene expression.

Mapping of the localization of type 1 angiotensin receptor in membrane microdomains using bioluminescence resonance energy transfer-based sensors

Initiation and termination of signaling of the type I angiotensin receptor (AT(1)-R) can lead to dynamic changes in its localization in plasma membrane microdomains. Several markers were recently developed to investigate membrane microdomains. Here, we used several YFP-labeled fusion constructs (i.e. raft or non-raft plasma membrane markers) to analyze the agonist-induced changes in compartmentalization of AT(1)-R, including internalization or lateral movement between plasma membrane compartments in response to stimulation using bioluminescence resonance energy transfer measurements. Our data demonstrate that angiotensin II (AngII) stimulus changes the microdomain localization of wild type or mutated (DRY → AAY or TSTS → AAAA) AT(1)-Rs co-expressed with the fluorescent probes in HEK293 cells. The comparison of the trafficking of AT(1)-R upon AngII stimulus with those of [Sar(1),Ile(8)]AngII or [Sar(1),Ile(4),Ile(8)]AngII stimulus revealed different types of changes, depending on the nature of the ligand. The observed changes in receptor compartmentalization of the AT(1)-R are strikingly different from those of 5HT-2C and EGF receptors, which demonstrate the usefulness of the bioluminescence resonance energy transfer-based measurements in the investigation of receptor trafficking in the plasma membrane in living cell experiments.

Nitric oxide inhibits the expression of AT1 receptors in neurons

We have previously observed an increased of angiotensin II (ANG II) type 1 receptor (AT(1)R) with enhanced AT(1)R-mediated sympathetic outflow and concomitant downregulation of neuronal nitric oxide (NO) synthase (nNOS) with reduced NO-mediated inhibition from the paraventricular nucleus (PVN) in rats with heart failure. To test the hypothesis that NO exerts an inhibitory effect on AT(1)R expression in the PVN, we used primary cultured hypothalamic cells of neonatal rats and neuronal cell line NG108-15 as in vitro models. In hypothalamic primary culture, NO donor sodium nitroprusside (SNP) induced dose-dependent decreases in mRNA and protein of AT(1)R (10(-5) M SNP, AT(1)R protein was 10 ± 2% of control level) while NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) induced dose-dependent increases in mRNA and protein levels of AT(1)R (10(-5) M l-NMMA, AT(1)R protein was 148 ± 8% of control level). Similar effects of SNP and l-NMMA on AT(1)R expression were also observed in NG108-15 cell line (10(-6) M SNP, AT(1)R protein was 30 ± 4% of control level while at the dose of 10(-6) M l-NMMA, AT(1)R protein was 171 ± 15% of the control level). Specific inhibition of nNOS, using antisense, caused an increase in AT(1)R expression while overexpression of nNOS, using adenoviral gene transfer (Ad.nNOS), caused an inhibition of AT(1)R expression in NG108 cells. Antisense nNOS transfection augmented the increase while Ad.nNOS infection blunted the increase in intracellular calcium concentration in response to ANG II treatment in NG108 cells. In addition, downregulation of AT(1)R mRNA as well as protein level in neuronal cell line in response to S-nitroso-N-acetyl pencillamine (SNAP) treatment was blocked by protein kinase G (PKG) inhibitor, while the peroxynitrite scavenger deforxamine had no effect. These results suggest that NO acts as an inhibitory regulator of AT(1)R expression and the activation of PKG is the required step in the regulation of AT(1)R gene expression via cGMP-dependent signaling pathway.

Helix 8 plays a crucial role in bradykinin B(2) receptor trafficking and signaling

Upon activation the human bradykinin B(2) receptor (B(2)R) acts as guanine nucleotide exchange factor for the G proteins G(q/11) and G(i). Thereafter, it gets phosphorylated by G protein-coupled receptor kinases (GRKs) and recruits β-arrestins, which block further G protein activation and promote B(2)R internalization via clathrin-coated pits. As for most G protein-coupled receptors of family A, an intracellular helix 8 after transmembrane domain 7 is also predicted for the B(2)R. We show here that disruption of helix 8 in the B(2)R by either C-terminal truncation or just by mutation of a central amino acid (Lys-315) to a helix-breaking proline resulted in strong reduction of surface expression. Interestingly, this malfunction could be overcome by the addition of the membrane-permeable B(2)R antagonist JSM10292, suggesting that helix 8 has a general role for conformational stabilization that can be accounted for by an appropriate antagonist. Intriguingly, an intact helix 8, but not the C terminus with its phosphorylation sites, was indispensable for receptor sequestration and for interaction of the B(2)R with GRK2/3 and β-arrestin2 as shown by co-immunoprecipitation. Recruitment of β-arrestin1, however, required the presence of the C terminus. Taken together, our results demonstrate that helix 8 of the B(2)R plays a crucial role not only in efficient trafficking to the plasma membrane or the activation of G proteins but also for the interaction of the B(2)R with GRK2/3 and β-arrestins. Additional data obtained with chimera of B(2)R with other G protein-coupled receptors of family A suggest that helix 8 might have similar functions in other GPCRs as well.

New insights and perspectives on intrarenal renin-angiotensin system: focus on intracrine/intracellular angiotensin II

Although renin, the rate-limiting enzyme of the renin-angiotensin system (RAS), was first discovered by Robert Tigerstedt and Bergman more than a century ago, the research on the RAS still remains stronger than ever. The RAS, once considered to be an endocrine system, is now widely recognized as dual (circulating and local/tissue) or multiple hormonal systems (endocrine, paracrine and intracrine). In addition to the classical renin/angiotensin I-converting enzyme (ACE)/angiotensin II (Ang II)/Ang II receptor (AT₁/AT₂) axis, the prorenin/(Pro)renin receptor (PRR)/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, and the Ang IV/AT₄/insulin-regulated aminopeptidase (IRAP) axis have recently been discovered. Furthermore, the roles of the evolving RAS have been extended far beyond blood pressure control, aldosterone synthesis, and body fluid and electrolyte homeostasis. Indeed, novel actions and underlying signaling mechanisms for each member of the RAS in physiology and diseases are continuously uncovered. However, many challenges still remain in the RAS research field despite of more than one century's research effort. It is expected that the research on the expanded RAS will continue to play a prominent role in cardiovascular, renal and hypertension research. The purpose of this article is to review the progress recently being made in the RAS research, with special emphasis on the local RAS in the kidney and the newly discovered prorenin/PRR/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, the Ang IV/AT₄/IRAP axis, and intracrine/intracellular Ang II. The improved knowledge of the expanded RAS will help us better understand how the classical renin/ACE/Ang II/AT₁ receptor axis, extracellular and/or intracellular origin, interacts with other novel RAS axes to regulate blood pressure and cardiovascular and kidney function in both physiological and diseased states.

Role of Phosphorylation in the Control of Clathrin-Mediated Internalization of GPCR

The process by which G protein-coupled receptors (GPCRs) are internalized through the clathrin-coated vesicles involves interactions of multifunctional adaptor proteins. These interactions are tightly controlled by phosphorylation and dephosphorylation mechanisms resulting in the regulation of receptor endocytosis. However, the identities of the kinases involved in this process remained largely unknown until recently. This paper discusses advances in our knowledge of the important role played by protein phosphorylation in the regulation of the endocytic machinery and how phosphorylation controls the coated vesicle cycle.

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.

A dopamine D2 receptor mutant capable of G protein-mediated signaling but deficient in arrestin binding

Arrestins mediate G protein-coupled receptor desensitization, internalization, and signaling. Dopamine D(2) and D(3) receptors have similar structures but distinct characteristics of interaction with arrestins. The goals of this study were to compare arrestin-binding determinants in D(2) and D(3) receptors other than phosphorylation sites and to create a D(2) receptor that is deficient in arrestin binding. We first assessed the ability of purified arrestins to bind to glutathione transferase (GST) fusion proteins containing the receptor third intracellular loops (IC3). Arrestin3 bound to IC3 of both D(2) and D(3) receptors, with the affinity and localization of the binding site indistinguishable between the receptor subtypes. Mutagenesis of the GST-IC3 fusion proteins identified an important determinant of the binding of arrestin3 in the N-terminal region of IC3. Alanine mutations of this determinant (IYIV212-215) in the full-length D(2) receptor generated a signaling-biased receptor with intact ligand binding and G-protein coupling and activation, but deficient in receptor-mediated arrestin3 translocation to the membrane, agonist-induced receptor internalization, and agonist-induced desensitization in human embryonic kidney 293 cells. This mutation also decreased arrestin-dependent activation of extracellular signal-regulated kinases. The finding that nonphosphorylated D(2)-IC3 and D(3)-IC3 have similar affinity for arrestin is consistent with previous suggestions that the differential effects of D(2) and D(3) receptor activation on membrane translocation of arrestin and receptor internalization are due, at least in part, to differential phosphorylation of the receptors. In addition, these results imply that the sequence IYIV212-215 at the N terminus of IC3 of the D(2) receptor is a key element of the arrestin binding site.

Site-specific cleavage of G protein-coupled receptor-engaged beta-arrestin. Influence of the AT1 receptor conformation on scissile site selection

The discovery of beta-arrestin-related approximately 46-kDa polypeptide in transfected cells and mouse hearts led us to examine angiotensin II type 1 receptor (AT(1)R)-dependent proteolytic cleavage of beta-arrestin(s). Receptor-ligand induced proteolysis of beta-arrestin(s) is novel, especially in the endocrine system, since proteolytic and/or splice variants of nonvisual arrestins are unknown. We used a strategy to retrieve AT(1)R-engaged isoforms of beta-arrestin 1 to confirm direct interaction of fragments with this G protein-coupled receptor and determine cleavage sites. Here we show that the angiotensin II-AT(1)R complex is associated with full-length and approximately 46-kDa beta-arrestin forms. Mass spectrometric analysis of the AT(1)R-associated short form suggested a scissile site located within the Arg(363)-Arg(393) region in the bovine beta-arrestin 1. Edman degradation analysis of a beta-arrestin 1 C-terminal fragment fused to enhanced green fluorescent protein confirmed the major cleavage to be after Phe(388) and a minor cleavage after Asn(375). Rather unexpectedly, the inverse agonist EXP3174-bound AT(1)R generated different fragmentation of bovine beta-arrestin 1, at Pro(276). The angiotensin II-induced cleavage is independent of inositol 1,4,5-trisphosphate- and Ca(2+)-mediated signaling pathways. The proteolysis of beta-arrestin 2 occurs, but the pattern is more complex. Our findings suggest that beta-arrestin cleavage upon AT(1)R stimulation is a part of the unraveling beta-arrestin-mediated G protein-coupled receptor signaling diversity.

Brain angiotensin receptors and binding proteins

This review addresses classical and novel aspects of the brain angiotensin system. The brain contains both the AT1 and AT2 angiotensin II (Ang II) receptor subtypes which are well-characterized guanine nucleotide binding protein (G protein)-coupled receptors (GPCRs). Like other GPCRs, novel signal transduction pathways and protein interactions are being described for Ang II receptors. For brain AT1 receptors, there is a controversy regarding the identity of the active angiotensin peptide in the brain which is addressed in this review. This review also summarizes a recent discovery of a novel, membrane-bound, non-AT1, non-AT2 binding site for angiotensin peptides that appears to be brain-specific. This binding site is unmasked by a limited concentration range of the organometallic sulfhydryl-reactive agent p-chloromercuribenzoic acid (PCMB) suggesting that functional expression of this binding site may depend on the redox state of the milieu of the brain. While this binding site has similarities to a previously described soluble angiotensin-binding protein found in liver that is unmasked by PCMB, it has many different characteristics. The possible functional significance of this novel non-AT1, non-AT2 binding site for angiotensin peptides as a mediator of non-traditional actions of Ang II in the brain, e.g., stimulation of dopamine release from the striatum, as a peptidase, or as a clearance receptor, and the importance of the state of the internal environment of the brain to its function is reviewed.

The third intracellular loop of the human somatostatin receptor 5 is crucial for arrestin binding and receptor internalization after somatostatin stimulation

Somatostatin (SS) is a widely distributed polypeptide that exerts inhibitory effects on hormone secretion and cell proliferation by interacting with five different receptors (SST1-SST5). Beta-arrestins have been implicated in regulating SST internalization, but the structural domains mediating this effect are largely unknown. The aim of this study was to characterize the intracellular mechanisms responsible for internalization of human SST5 in the rat pituitary cell line GH3 and to identify the SST5 structural domains involved in this process. To this purpose we evaluated, by fluorescence microscopy and biochemical assay, the ability of wild-type, progressive C-terminal truncated and third cytoplasmatic loop mutants SST5-DsRed to associate with beta-arrestin-enhanced green fluorescent protein and to internalize under SS28 stimulation. The truncated mutants were comparable to the wild-type receptor with respect to recruitment of beta-arrestin-2 and internalization, whereas the third loop mutants R240W, S242A, and T247A showed the abolishment or reduction of arrestin association and a significant reduction of receptor internalization (14.4%, 29%, and 30.9% vs. 52.4% of wild type) and serine phosphorylation upon SS28 stimulation. Moreover, we evaluated the ability of simultaneous mutation of these three residues (R240, S242, and T247) and C-terminal truncated receptors to internalize. The progressive truncation of the C-terminal tail resulted in a progressive increased internalization (21.6%, 36.7%, and 41%, respectively) with respect to the full-length total third-loop mutant (15%). In conclusion, our results indicate the SST5 third intracellular loop as an important mediator of beta-arrestin/receptor interaction and receptor internalization, whereas they suggest that residues 328-347 within the C terminus may play an inhibitory role in receptor internalization.

S-nitrosylation of cysteine 289 of the AT1 receptor decreases its binding affinity for angiotensin II

1. Nitric oxide (NO) is known to affect the properties of various proteins via the S-nitrosylation of cysteine residues. This study evaluated the direct effects of the NO donor sodium nitroprusside (SNP) on the pharmacological properties of the AT1 receptor for angiotensin II expressed in HEK-293 cells. 2. SNP dose-dependently decreased the binding affinity of the AT1 receptor without affecting its total binding capacity. This modulatory effect was reversed within 5 min of removing SNP. 3. The effect of SNP was not modified in the presence of the G protein uncoupling agent GTPgammaS or the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. 4. The binding properties of a mutant AT1 receptor in which all five cysteine residues within the transmembrane domains had been replaced by serine was not affected by SNP. Systematic analysis of mutant AT1 receptors revealed that cysteine 289 conferred the sensitivity to SNP. 5. These results suggest that NO decreased the binding affinity of the AT1 receptor by S-nitrosylation of cysteine 289. This modulatory mechanism may be particularly relevant in pathophysiological situations where the beneficial effects of NO oppose the deleterious effects of angiotensin II.

Type 1 angiotensin receptor pharmacology: signaling beyond G proteins

Drugs that inhibit the production of angiotensin II (AngII) or its access to the type 1 angiotensin receptor (AT₁R) are prescribed to alleviate high blood pressure and its cardiovascular complications. Accordingly, much research has focused on the molecular pharmacology of AT₁R activation and signaling. An emerging theme is that the AT₁R generates G protein dependent as well as independent signals and that these transduction systems separately contribute to AT₁R biology in health and disease. Regulatory molecules termed arrestins are central to this process as is the capacity of AT₁R to crosstalk with other receptor systems, such as the widely studied transactivation of growth factor receptors. AT₁R function can also be modulated by polymorphisms in the AGTR gene, which may significantly alter receptor expression and function; a capacity of the receptor to dimerize/oligomerize with altered pharmacology; and by the cellular environment in which the receptor resides. Together, these aspects of the AT₁R “flavour” the response to angiotensin; they may also contribute to disease, determine the efficacy of current drugs and offer a unique opportunity to develop new therapeutics that antagonize only selective facets of AT₁R function.

The structural basis of arrestin-mediated regulation of G-protein-coupled receptors

The 4 mammalian arrestins serve as almost universal regulators of the largest known family of signaling proteins, G-protein-coupled receptors (GPCRs). Arrestins terminate receptor interactions with G proteins, redirect the signaling to a variety of alternative pathways, and orchestrate receptor internalization and subsequent intracellular trafficking. The elucidation of the structural basis and fine molecular mechanisms of the arrestin-receptor interaction paved the way to the targeted manipulation of this interaction from both sides to produce very stable or extremely transient complexes that helped to understand the regulation of many biologically important processes initiated by active GPCRs. The elucidation of the structural basis of arrestin interactions with numerous non-receptor-binding partners is long overdue. It will allow the construction of fully functional arrestins in which the ability to interact with individual partners is specifically disrupted or enhanced by targeted mutagenesis. These "custom-designed" arrestin mutants will be valuable tools in defining the role of various interactions in the intricate interplay of multiple signaling pathways in the living cell. The identification of arrestin-binding sites for various signaling molecules will also set the stage for designing molecular tools for therapeutic intervention that may prove useful in numerous disorders associated with congenital or acquired disregulation of GPCR signaling.

Multifaceted roles of beta-arrestins in the regulation of seven-membrane-spanning receptor trafficking and signalling

Beta-arrestins are cytosolic proteins that bind to activated and phosphorylated G-protein-coupled receptors [7MSRs (seven-membrane-spanning receptors)] and uncouple them from G-protein-mediated second messenger signalling pathways. The binding of beta-arrestins to 7MSRs also leads to new signals via activation of MAPKs (mitogen-activated protein kinases) such as JNK3 (c-Jun N-terminal kinase 3), ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38 MAPKs. By binding to endocytic proteins [clathrin, AP2 (adapter protein 2), NSF (N -ethylmaleimide-sensitive fusion protein) and ARF6 (ADP-ribosylation factor 6)], beta-arrestins also serve as adapters to link the receptors to the cellular trafficking machinery. Agonist-promoted ubiquitination of beta-arrestins is a prerequisite for their role in receptor internalization, as well as a determinant of the differing trafficking patterns of distinct classes of receptors. Recently, beta-arrestins have also been implicated as playing novel roles in cellular chemotaxis and apoptosis. By virtue of their ability to bind, in a stimulus-dependent fashion, to 7MSRs as well as to different classes of cellular proteins, beta-arrestins serve as versatile adapter proteins that regulate the signalling and trafficking of the receptors.

Differential PI 3-kinase dependence of early and late phases of recycling of the internalized AT1 angiotensin receptor

Agonist-induced endocytosis and processing of the G protein-coupled AT1 angiotensin II (Ang II) receptor (AT1R) was studied in HEK 293 cells expressing green fluorescent protein (GFP)- or hemagglutinin epitope-tagged forms of the receptor. After stimulation with Ang II, the receptor and its ligand colocalized with Rab5-GFP and Rab4-GFP in early endosomes, and subsequently with Rab11-GFP in pericentriolar recycling endosomes. Inhibition of phosphatidylinositol (PI) 3-kinase by wortmannin (WT) or LY294002 caused the formation of large endosomal vesicles of heterogeneous Rab composition, containing the ligand-receptor complex in their limiting membranes and in small associated vesicular structures. In contrast to Alexa(R)-transferrin, which was mainly found in small vesicles associated with the outside of large vesicles in WT-treated cells, rhodamine-Ang II was also segregated into small internal vesicles. In cells labeled with 125I-Ang II, WT treatment did not impair the rate of receptor endocytosis, but significantly reduced the initial phase of receptor recycling without affecting its slow component. Similarly, WT inhibited the early, but not the slow, component of the recovery of AT1R at the cell surface after termination of Ang II stimulation. These data indicate that internalized AT1 receptors are processed via vesicles that resemble multivesicular bodies, and recycle to the cell surface by a rapid PI 3-kinase-dependent recycling route, as well as by a slower pathway that is less sensitive to PI 3-kinase inhibitors.