Residues Val254, His256, and Phe259 of the angiotensin II AT1 receptor are not involved in ligand binding but participate in signal transduction

Angiotensin II cyclic analogs as tools to investigate AT1R biased signaling mechanisms

G protein coupled receptors (GPCRs) produce pleiotropic effects by their capacity to engage numerous signaling pathways once activated. Functional selectivity (also called biased signaling), where specific compounds can bring GPCRs to adopt conformations that enable selective receptor coupling to distinct signaling pathways, continues to be significantly investigated. However, an important but often overlooked aspect of functional selectivity is the capability of ligands such as angiotensin II (AngII) to adopt specific conformations that may preferentially bind to selective GPCRs structures. Understanding both receptor and ligand conformation is of the utmost importance for the design of new drugs targeting GPCRs. In this study, we examined the properties of AngII cyclic analogs to impart biased agonism on the angiotensin type 1 receptor (AT₁R). Positions 3 and 5 of AngII were substituted for cysteine and homocysteine residues ([Sar¹Hcy^3,5]AngII, [Sar¹Cys³Hcy⁵]AngII and [Sar¹Cys^3,5]AngII) and the resulting analogs were evaluated for their capacity to activate the Gq/11, G12, Gi2, Gi3, Gz, ERK and β-arrestin (βarr) signaling pathways via AT₁R. Interestingly, [Sar¹Hcy^3,5]AngII exhibited potency and full efficacy on all pathways tested with the exception of the Gq pathway. Molecular dynamic simulations showed that the energy barrier associated with the insertion of residue Phe⁸ of AngII within the hydrophobic core of AT₁R, associated with Gq/11 activation, is increased with [Sar¹Hcy^3,5]AngII. These results suggest that constraining the movements of molecular determinants within a given ligand by introducing cyclic structures may lead to the generation of novel ligands providing more efficient biased agonism.

A fluorimetric binding assay for angiotensin II and kinin receptors

Angiotensin II (AngII) and kinins (bradykinin (BK) and des-Arg9-bradykinin (DBK)), are potent agents involved in the maintenance of blood pressure and several biological activities, and their better understanding is important to produce new drugs aimed to control arterial blood pressure. Previous studies on ligand-receptor binding have been based on radioactive methods, which led us to study a new method based on the fluorimetric method. A lanthanide attached to the N-terminal segment of the peptide (AngII, BK and DBK), which produces a time-resolved-fluorescent ligand, was used in a binding test with CHO cells expressing the AT1, AT2, B1 or B2 receptors in comparison with the same cell line tested with the radioactive ligand. Our findings indicated that the non-radioactive method provided a comparable result for the angiotensin receptors. On the other hand, the kinin receptors showed a slight reduction in the binding affinity, probably due to the linkage at the N-terminal segment and/or to the lower biological stability associated to the high temperature (37°C) used for the fluorimetric method, while the radioactive one was at 4°C. We can conclude that a time-resolved fluorescence assay would provide a sensitive method as an alternative tool for receptor studies.

By interacting with the C-terminal Phe of apelin, Phe255 and Trp259 in helix VI of the apelin receptor are critical for internalization

Apelin is the endogenous ligand of the orphan seven-transmembrane domain (TM) G protein-coupled receptor APJ. Apelin is involved in the regulation of body fluid homeostasis and cardiovascular functions. We previously showed the importance of the C-terminal Phe of apelin 17 (K17F) in the hypotensive activity of this peptide. Here, we show either by deleting the Phe residue (K16P) or by substituting it by an Ala (K17A), that it plays a crucial role in apelin receptor internalization but not in apelin binding or in Gα(i)-protein coupling. Then we built a homology three-dimensional model of the human apelin receptor using the cholecystokinin receptor-1 model as a template, and we subsequently docked K17F into the binding site. We visualized a hydrophobic cavity at the bottom of the binding pocket in which the C-terminal Phe of K17F was embedded by Trp(152) in TMIV and Trp(259) and Phe(255) in TMVI. Using molecular modeling and site-directed mutagenesis studies, we further showed that Phe(255) and Trp(259) are key residues in triggering receptor internalization without playing a role in apelin binding or in Gα(i)-protein coupling. These findings bring new insights into apelin receptor activation and show that Phe(255) and Trp(259), by interacting with the C-terminal Phe of the pyroglutamyl form of apelin 13 (pE13F) or K17F, are crucial for apelin receptor internalization.

Structural analysis of three peptides related to the transmambranic helix VI of AT1 receptor

INTRODUCTION

Angiotensin II (AII) is the main active product of the renin angiotensin system. Better known effects of AII are via AT1 receptor (AT1R). Expression of AT1R mutants (L265D and L262D) in CHO cells increased cAMP formation when compared to CHO cells expressing the wild type (WT) AT1R. Morphological transformation of CHO cells transfected with mutants correlated with their increased cAMP formation. DNA synthesis was inhibited in these cells too, indicating that cAMP promotes inhibitory effects on transfected CHO cells growth and causes their morphological change from a tumorigenic phenotype to a non-tumorigenic one.

OBJECTIVES

To assess the importance of leucine 262 and 265 in determining AT1R structure by means of a comparative structural analysis of two mutant peptides and of a wild-type fragment.

METHODOLOGY

Three peptides had their conformation compared by circular dichroism (CD): L262D(259-272), L265D(259-272) (mutants) and WT(260-277).

RESULTS

Secondary structures were: beta-turn for WT and L262D and random coil for L265D.

CONCLUSIONS

Strong correlation was found in the results of biochemical, cellular and structural approaches used to compare WT AT1R to mutant types. Random coil structure of the L265D mutant may be a key point to explain those changes observed in biochemical (binding and signal transduction) and proliferation assays (Correa et al., 2005). beta-Turn formation is an important step during early protein folding and this secondary simple structure is present in L262D and WT, but not in L265D. Therefore, leucine 265 seems to play a crucial role in determining an entirely functional AT1R.

The Angiotensin II AT1 Receptor Structure-Activity Correlations in the Light of Rhodopsin Structure

The most prevalent physiological effects of ANG II, the main product of the renin-angiotensin system, are mediated by the AT1 receptor, a rhodopsin-like AGPCR. Numerous studies of the cardiovascular effects of synthetic peptide analogs allowed a detailed mapping of ANG II's structural requirements for receptor binding and activation, which were complemented by site-directed mutagenesis studies on the AT1 receptor to investigate the role of its structure in ligand binding, signal transduction, phosphorylation, binding to arrestins, internalization, desensitization, tachyphylaxis, and other properties. The knowledge of the high-resolution structure of rhodopsin allowed homology modeling of the AT1 receptor. The models thus built and mutagenesis data indicate that physiological (agonist binding) or constitutive (mutated receptor) activation may involve different degrees of expansion of the receptor's central cavity. Residues in ANG II structure seem to control these conformational changes and to dictate the type of cytosolic event elicited during the activation. 1) Agonist aromatic residues (Phe8 and Tyr4) favor the coupling to G protein, and 2) absence of these residues can favor a mechanism leading directly to receptor internalization via phosphorylation by specific kinases of the receptor's COOH-terminal Ser and Thr residues, arrestin binding, and clathrin-dependent coated-pit vesicles. On the other hand, the NH2-terminal residues of the agonists ANG II and [Sar1]-ANG II were found to bind by two distinct modes to the AT1 receptor extracellular site flanked by the COOH-terminal segments of the EC-3 loop and the NH2-terminal domain. Since the [Sar1]-ligand is the most potent molecule to trigger tachyphylaxis in AT1 receptors, it was suggested that its corresponding binding mode might be associated with this special condition of receptors.

Model of the whole rat AT1 receptor and the ligand-binding site

We present a three-dimensional model of the rat type 1 receptor (AT1) for the hormone angiotensin II (Ang II). Ang II and the AT1 receptor play a critical role in the cell-signaling process responsible for the actions of renin-angiotensin system in the regulation of blood pressure, water-electrolyte homeostasis and cell growth. Development of improved therapeutics would be significantly enhanced with the availability of a 3D-structure model for the AT1 receptor and of the binding site for agonists and antagonists. This model was constructed using a combination of computation and homology-modeling techniques starting with the experimentally determined three-dimensional structure of bovine rhodopsin (PDB#1F88) as a template. All 359 residues and two disulfide bonds in the rat AT1 receptor have been accounted for in this model. Ramachandran-map analysis and a 1 nanosecond molecular dynamics simulation of the solvated receptor with and without the bound ligand, Ang II, lend credence to the validity of the model. Docking calculations were performed with the agonist, Ang II and the antihypertensive antagonist, losartan. [Figure: see text].

Tachyphylactic properties of angiotensin II analogs with bulky and hydrophobic substituents at the N-terminus

Tachyphylaxis, defined as the acute loss of response of some smooth muscles upon repeated stimulations with angiotensin II (Ang II), has been shown to be dependent mainly on the N-terminal region of the ligand. To further study the structural requirements for the induction of tachyphylaxis we have synthesized Ang II analogs containing the bulky and very lipophilic substituents 9-fluorenylmethyloxycarbonyl (Fmoc) and 9-fluorenylmethyl ester (OFm) at the alpha-amino (Nalpha-Fmoc-Ang II) or the beta-carboxyl ([Asp(OFm)1]-Ang II) groups of the Asp1 residue, respectively. In binding assays with Chinese hamster ovary cells transfected with the AT1 Ang II receptor, Nalpha-Fmoc-Ang II bound with high affinity, whereas [Asp(OFm)1]-Ang II showed lower affinity. In biological assays, these two analogs were full agonists and showed 30 and 3%, respectively, of the Ang II potency in contracting the guinea-pig ileum smooth muscle. The two analogs induced tachyphylaxis, in spite of the lack of a free amino group in Nalpha-Fmoc-Ang II. Thus, analogs with Fmoc- or OFm-type groups coupled to the Asp1 residue, whether at the amino or carboxyl functions, induce tachyphylaxis through an unreported mechanism. Based in these findings and those available from the literature, an alternate molecular interaction mode between Ang II N-terminal portion and the AT1 receptor is proposed to explain the tachyphylactic phenomenon.

Comparison of the solution structures of angiotensin I & II. Implication for structure-function relationship

Conformational analysis of angiotensin I (AI) and II (AII) peptides has been performed through 2D 1H-NMR spectroscopy in dimethylsulfoxide and 2,2,2-trifluoroethanol/H2O. The solution structural models of AI and AII have been determined in dimethylsulfoxide using NOE distance and 3JHNHalpha coupling constants. Finally, the AI family of models resulting from restrained energy minimization (REM) refinement, exhibits pairwise rmsd values for the family ensemble 0.26 +/- 0.13 A, 1.05 +/- 0.23 A, for backbone and heavy atoms, respectively, and the distance penalty function is calculated at 0.075 +/- 0.006 A2. Comparable results have been afforded for AII ensemble (rmsd values 0.30 +/- 0.22 A, 1.38 +/- 0.48 A for backbone and heavy atoms, respectively; distance penalty function is 0.029 +/- 0.003 A2). The two peptides demonstrate similar N-terminal and different C-terminal conformation as a consequence of the presence/absence of the His9-Leu10 dipeptide, which plays an important role in the different biological function of the two peptides. Other conformational variations focused on the side-chain orientation of aromatic residues, which constitute a biologically relevant hydrophobic core and whose inter-residue contacts are strong in dimethylsulfoxide and are retained even in mixed organic-aqueous media. Detailed analysis of the peptide structural features attempts to elucidate the conformational role of the C-terminal dipeptide to the different binding affinity of AI and AII towards the AT1 receptor and sets the basis for understanding the factors that might govern free- or bound-depended AII structural differentiation.

Interaction of a non-peptide agonist with angiotensin II AT1 receptor mutants

To identify residues of the rat AT1A angiotensin II receptor involved with signal transduction and binding of the non-peptide agonist L-162,313 (5,7-dimethyl-2-ethyl-3-[[4-[2(n-butyloxycarbonylsulfonamido)-5-isobutyl-3-thienyl]phenyl]methyl]imidazol[4,5,6]-pyridine) we have performed ligand binding and inositol phosphate turnover assays in COS-7 cells transiently transfected with the wild-type and mutant forms of the receptor. Mutant receptors bore modifications in the extracellular region: T88H, Y92H, G1961, G196W, and D278E. Compound L-162,313 displaced [125I]-Sar1,Leu8-AngII from the mutants G196I and G196W with IC50 values similar to that of the wild-type. The affinity was, however, slightly affected by the D278E mutation and more significantly by the T88H and Y92H mutations. In inositol phosphate turnover assays, the ability of L-162,313 to trigger the activation cascade was compared with that of angiotensin II. These assays showed that the G196W mutant reached a relative maximum activation exceeding that of the wild-type receptor; the efficacy was slightly reduced in the G1961 mutant and further reduced in the T88H, Y92H, and D278E mutants. Our data suggest that residues of the extracellular domain of the AT1 receptor are involved in the binding of the non-peptide ligand, or in a general receptor activation phenomenon that involves conformational modifications for a preferential binding of agonists or antagonists.

Relevant role of Leu265 in helix VI of the angiotensin AT1 receptor in agonist binding and activity

The finding of critical residues for angiotensin II (AII) binding and receptor signalling in helices V and VI led us to assess if, in this region of the receptor, aliphatic side chains might play a role in the agonist-mediated mechanism. Two mutations of the angiotensin AT1 receptor were designed to explore a possible role of a leucine at two positions, Leu265 and Leu268. Thus two mutants, L265D and L268D, were prepared through single substitutions of Leu265, located in the C-terminal region of transmembrane VI (TM-VI), and Leu268, in the adjoining region of the third extracellular loop (EC-3), for an aspartyl residue, and were stably transfected into Chinese hamster ovary (CHO) cells. Ligand-binding experiments and the functional assays determining inositol phosphate (IP) production were performed in these cells expressing these mutants. No significant changes were found in the binding affinity for the ligands, AII, DuP753, and [Sar1Leu8]AII in the mutant L268D. Moreover, the relative potency and the maximum effect on IP production of this mutant were similar to those of the wild-type receptor. In contrast, L265D mutant AT1 receptor, located within the transmembrane domain, markedly decreased binding affinity and ability to stimulate phosphatidylinositol turnover. Our results suggest that the hydrophobic side chain of Leu265, at the C-terminal portion of the AT1's TM-VI, but not Leu268, which belongs to the EC-3 loop, might interact with the AII molecule. On the other side, the aliphatic side chain of Leu265 may be involved in the formation of the ligand binding sites through allosteric effects, thus helping to stabilize the receptor structure around the agonist binding site for full activity.

3D model for TM region of the AT-1 receptor in complex with angiotensin II independently validated by site-directed mutagenesis data

A three-dimensional model of the complex of angiotensin II (AII) with the transmembrane (TM) region of the angiotensin II receptor of type 1 (the AT-1 receptor) was obtained by molecular modeling procedures employing structural homology to the X-ray structure of rhodopsin. Since the modeling procedure considered only steric and energy considerations without prior knowledge of the experimental results of site-directed mutagenesis, the results with receptor mutants could be used for independent validation of the model. Indeed, the model brings in contact the residues of AII responsible for agonistic activity, Tyr(4), His(6), and Phe(8), with many residues of AT-1 involved in signal transduction according to site-directed mutagenesis. The model also predicts the existence of several possible conformational pathways for transferring the binding signal through the TM region of AT-1 to the intracellular loops interacting with the G-protein.

Ligand-induced endocytosis and nuclear localization of angiotensin II receptors expressed in CHO cells

A construct (AT1R-NF) containing a "Flag" sequence added to the N-terminus of the rat AT1 receptor was stably expressed in Chinese hamster ovary cells and quantified in the cell membrane by confocal microscopy after reaction with a fluorescein-labeled anti-Flag monoclonal antibody. Angiotensin II bound to AT1R-NF and induced endocytosis with a half-time of 2 min. After 60-90 min, fluorescence accumulated around the cell nucleus, suggesting migration of the ligand-receptor complex to the nuclear membrane. Angiotensin antagonists also induced endocytosis, suggesting that a common step in the transduction signal mechanism occurring after ligand binding may be responsible for the ligand-receptor complex internalization.

The angiotensin II type 1 receptor and receptor-associated proteins

The mechanisms of regulation, activation and signal transduction of the angiotensin II (Ang II) type 1 (AT1) receptor have been studied extensively in the decade after its cloning. The AT1 receptor is a major component of the renin-angiotensin system (RAS). It mediates the classical biological actions of Ang II. Among the structures required for regulation and activation of the receptor, its carboxyl-terminal region plays crucial roles in receptor internalization, desensitization and phosphorylation. The mechanisms involved in heterotrimeric G-protein coupling to the receptor, activation of the downstream signaling pathway by G proteins and the Ang II signal transduction pathways leading to specific cellular responses are discussed. In addition, recent work on the identification and characterization of novel proteins associated with carboxyl-terminus of the AT1 receptor is presented. These novel proteins will advance our understanding of how the receptor is internalized and recycled as they provide molecular mechanisms for the activation and regulation of G-protein-coupled receptors.

Mutational analysis of the interaction of the N- and C-terminal ends of angiotensin II with the rat AT(1A) receptor

1. The role of different residues of the rat AT(1A) receptor in the interaction with the N- and C-terminal ends of angiotensin II (AngII) was studied by determining ligand binding and production of inositol phosphates (IP) in COS-7 cells transiently expressing the following AT(1A) mutants: T88H, Y92H, G196I, G196W and D278E. 2. G196W and G196I retained significant binding and IP-production properties, indicating that bulky substituents in position 196 did not affect the interaction of AngII's C-terminal carboxyl with Lys(199) located three residues below. 3. Although the T88A mutation did not affect binding, the T88H mutant had greatly decreased affinity for AngII, suggesting that substitution of Thr(88) by His might hinder binding through an indirect effect. 4. The Y92H mutation caused loss of affinity for AngII that was much less pronounced than that reported for Y92A, indicating that His in that position can fulfil part of the requirements for binding. 5. Replacing Asp(278) by Glu caused a much smaller reduction in affinity than replacing it by Ala, indicating the importance of Asp's beta-carboxyl group for AngII binding. 6. Mutations in residues Thr(88), Tyr(92) and Asp(278) greatly reduced affinity for AngII but not for Sar(1) Leu(8)-AngII, suggesting unfavourable interactions between these residues and AngII's aspartic acid side-chain or N-terminal amino group, which might account for the proposed role of the N-terminal amino group of AngII in the agonist-induced desensitization (tachyphylaxis) of smooth muscles.

Structure of two fragments of the third cytoplasmic loop of the rat angiotensin II AT1A receptor. Implications with respect to receptor activation and G-protein selection and coupling

The structural bases that render the third intracellular loop (i3) of the rat angiotensin II AT1A receptor one of the cytoplasmic domains responsible for G-protein coupling are still unknown. The three-dimensional structures of two overlapping peptides mapping the entire i3 loop and shown to differently interact with purified G-proteins have been obtained by simulated annealing calculations, using NMR-derived constraints collected in 70% water/30% trifluoroethanol solution. While the NH2-terminal half, Ni3, residues 213-231, adopts a stable amphipathic alpha-helix, extending over almost the entire peptide, a more flexible conformation is found for the COOH-terminal half, Ci3, residues 227-242. For this peptide, a cis-trans isomerization around the Lys6-Pro7 peptide bond generates two exchanging isomers adopting similar conformations, with an alpha-helix spanning from Asn9 to Ile15 and a poorly defined NH2 terminus. A quite distinct structural organization is found for the sequence EIQKN, common to Ni3 and Ci3. The data do suggest that the extension and orientation of the amphipathic alpha-helix, present in the proximal part of i3, may be modulated by the distal part of the loop itself through the Pro233 residue. A molecular model where this possibility is considered as a mechanism for G-protein selection and coupling is presented.