Mutagenesis and the molecular modeling of the rat angiotensin II receptor (AT1)

Principle role of the (pro)renin receptor system in cardiovascular and metabolic diseases: An update

(Pro)renin receptor (PRR), along with its soluble form, sPRR, functions not only as a crucial activator of the local renin-angiotensin system but also engages with and activates various angiotensin II-independent signaling pathways, thus playing complex and significant roles in numerous physiological and pathophysiological processes, including cardiovascular and metabolic disorders. This article reviews current knowledge on the intracellular partners of the PRR system and explores its physiological and pathophysiological impacts on cardiovascular diseases as well as conditions related to glucose and lipid metabolism, such as hypertension, heart disease, liver disease, diabetes, and diabetic complications. Targeting the PRR system could emerge as a promising therapeutic strategy for treating these conditions. Elevated levels of circulating sPRR might indicate the severity of these diseases, potentially serving as a biomarker for diagnosis and prognosis in clinical settings. A comprehensive understanding of the functions and regulatory mechanisms of the PRR system could facilitate the development of novel therapeutic approaches for the prevention and management of cardiovascular and metabolic diseases.

Characterization of the First Animal Toxin Acting as an Antagonist on AT1 Receptor

The renin-angiotensin system (RAS) is one of the main regulatory systems of cardiovascular homeostasis. It is mainly composed of angiotensin-converting enzyme (ACE) and angiotensin II receptors AT1 and AT2. ACE and AT1 are targets of choice for the treatment of hypertension, whereas the AT2 receptor is still not exploited due to the lack of knowledge of its physiological properties. Peptide toxins from venoms display multiple biological functions associated with varied chemical and structural properties. If Brazilian viper toxins have been described to inhibit ACE, no animal toxin is known to act on AT1/AT2 receptors. We screened a library of toxins on angiotensin II receptors with a radioligand competition binding assay. Functional characterization of the selected toxin was conducted by measuring second messenger production, G-protein activation and β-arrestin 2 recruitment using bioluminescence resonance energy transfer (BRET) based biosensors. We identified one original toxin, A-CTX-cMila, which is a 7-residues cyclic peptide from Conus miliaris with no homology sequence with known angiotensin peptides nor identified toxins, displaying a 100-fold selectivity for AT1 over AT2. This toxin shows a competitive antagonism mode of action on AT1, blocking Gαq, Gαi3, GαoA, β-arrestin 2 pathways and ERK_1/2 activation. These results describe the first animal toxin active on angiotensin II receptors.

Soluble (pro)renin receptor induces endothelial dysfunction and hypertension in mice with diet-induced obesity via activation of angiotensin II type 1 receptor

Until now, renin-angiotensin system (RAS) hyperactivity was largely thought to result from angiotensin II (Ang II)-dependent stimulation of the Ang II type 1 receptor (AT1R). Here we assessed the role of soluble (pro)renin receptor (sPRR), a product of site-1 protease-mediated cleavage of (pro)renin receptor (PRR), as a possible ligand of the AT1R in mediating: (i) endothelial cell dysfunction in vitro and (ii) arterial dysfunction in mice with diet-induced obesity. Primary human umbilical vein endothelial cells (HUVECs) treated with a recombinant histidine-tagged sPRR (sPRR-His) exhibited IκBα degradation concurrent with NF-κB p65 activation. These responses were secondary to sPRR-His evoked elevations in Nox4-derived H2O2 production that resulted in inflammation, apoptosis and reduced NO production. Each of these sPRR-His-evoked responses was attenuated by AT1R inhibition using Losartan (Los) but not ACE inhibition using captopril (Cap). Further mechanistic exploration revealed that sPRR-His activated AT1R downstream Gq signaling pathway. Immunoprecipitation coupled with autoradiography experiments and radioactive ligand competitive binding assays indicate sPRR directly interacts with AT1R via Lysine199 and Asparagine295. Important translational relevance was provided by findings from obese C57/BL6 mice that sPRR-His evoked endothelial dysfunction was sensitive to Los. Besides, sPRR-His elevated blood pressure in obese C57/BL6 mice, an effect that was reversed by concurrent treatment with Los but not Cap. Collectively, we provide solid evidence that the AT1R mediates the functions of sPRR during obesity-related hypertension. Inhibiting sPRR signaling should be considered further as a potential therapeutic intervention in the treatment and prevention of cardiovascular disorders involving elevated blood pressure.

Quantum binding energy features of the drug olmesartan bound to angiotensin type-1 receptors in the therapeutics of stroke

We investigated the binding energies of 105 residues within a 10 Å pocket radius, predicted the energetic relevance of olmesartan regions, and the influence of individual protein segments on OLM -AT1 binding.

In silico prediction of ARB resistance: A first step in creating personalized ARB therapy

Angiotensin II type 1 receptor (AT₁R) blockers (ARBs) are among the most prescribed drugs. However, ARB effectiveness varies widely, which may be due to non-synonymous single nucleotide polymorphisms (nsSNPs) within the AT₁R gene. The AT₁R coding sequence contains over 100 nsSNPs; therefore, this study embarked on determining which nsSNPs may abrogate the binding of selective ARBs. The crystal structure of olmesartan-bound human AT₁R (PDB:4ZUD) served as a template to create an inactive apo-AT₁R via molecular dynamics simulation (n = 3). All simulations resulted in a water accessible ligand-binding pocket that lacked sodium ions. The model remained inactive displaying little movement in the receptor core; however, helix 8 showed considerable flexibility. A single frame representing the average stable AT₁R was used as a template to dock Olmesartan via AutoDock 4.2, MOE, and AutoDock Vina to obtain predicted binding poses and mean Boltzmann weighted average affinity. The docking results did not match the known pose and affinity of Olmesartan. Thus, an optimization protocol was initiated using AutoDock 4.2 that provided more accurate poses and affinity for Olmesartan (n = 6). Atomic models of 103 of the known human AT₁R polymorphisms were constructed using the molecular dynamics equilibrated apo-AT₁R. Each of the eight ARBs was then docked, using ARB-optimized parameters, to each polymorphic AT₁R (n = 6). Although each nsSNP has a negligible effect on the global AT₁R structure, most nsSNPs drastically alter a sub-set of ARBs affinity to the AT₁R. Alterations within N298 –L314 strongly effected predicted ARB affinity, which aligns with early mutagenesis studies. The current study demonstrates the potential of utilizing in silico approaches towards personalized ARB therapy. The results presented here will guide further biochemical studies and refinement of the model to increase the accuracy of the prediction of ARB resistance in order to increase overall ARB effectiveness.

The term "personalized medicine" was coined at the turn of the century, but most medicines currently prescribed are based on disease categories and occasionally racial demographics, not personalized attributes. In cardiovascular medicine, the personalization of medication is minimal, despite the fact that not all patients respond equally to common cardiovascular medications. Here we chose one prominent cardiovascular drug target, the angiotensin receptor, and, using computer modeling, created preliminary models of over 100 known alterations to the angiotensin receptor to determine if the alterations changed the ability of clinically used drugs to interact with the angiotensin receptor. The strength of interaction was compared to the wild-type angiotensin receptor, generating a map predicting which alteration affected which drug(s). It is expected that in the future, sequencing of drug targets can be used to compare a patient’s result to a map similar to what is provided in this manuscript to choose the optimal medication based on the patient’s genetics. Such a process has the potential to facilitate the personalization of current medication therapy.

Biased Agonist TRV027 Determinants in AT1R by Molecular Dynamics Simulations

Functional selectivity is a phenomenon observed in G protein-coupled receptors in which intermediate active-state conformations are stabilized by mutations or ligand binding, resulting in different sets of signaling pathways. Peptides capable of selectively activating β-arrestin, known as biased agonists, have already been characterized in vivo and could correspond to a new therapeutic approach for treatment of cardiovascular diseases. Despite the potential of biased agonism, the mechanism involved in selective signaling remains unclear. In this work, molecular dynamics simulations were employed to compare the conformational profile of the angiotensin II type 1 receptor (AT1R) crystal bound to angiotensin II, bound to the biased ligand TRV027, and in the apo form. Our results show that both ligands induce changes near the NPxxY motif in transmembrane domain 7 that are related to receptor activation. However, the biased ligand does not cause the rotamer toggle alternative positioning and displays an exclusive hydrogen-bonding pattern. Our work sheds light on the biased agonism mechanism and will help in the future design of novel biased agonists for AT1R.

Homocysteine directly interacts and activates the angiotensin II type I receptor to aggravate vascular injury

Hyperhomocysteinemia (HHcy) is a risk factor for various cardiovascular diseases. However, the mechanism underlying HHcy-aggravated vascular injury remains unclear. Here we show that the aggravation of abdominal aortic aneurysm by HHcy is abolished in mice with genetic deletion of the angiotensin II type 1 (AT1) receptor and in mice treated with an AT1 blocker. We find that homocysteine directly activates AT1 receptor signalling. Homocysteine displaces angiotensin II and limits its binding to AT1 receptor. Bioluminescence resonance energy transfer analysis reveals distinct conformational changes of AT1 receptor upon binding to angiotensin II and homocysteine. Molecular dynamics and site-directed mutagenesis experiments suggest that homocysteine regulates the conformation of the AT1 receptor both orthosterically and allosterically by forming a salt bridge and a disulfide bond with its Arg¹⁶⁷ and Cys²⁸⁹ residues, respectively. Together, these findings suggest that strategies aimed at blocking the AT1 receptor may mitigate HHcy-associated aneurysmal vascular injuries.

High homocysteine plasma levels are associated with cardiovascular diseases. Here, Li and colleagues find that homocysteine aggravates vascular injury by direct binding to the angiotensin II type 1 receptor (AT1R), identifying AT1R inhibition as a potential strategy to counteract the deleterious vascular effects of hyperhomocysteinemia.

Divergent Spatiotemporal Interaction of Angiotensin Receptor Blocking Drugs with Angiotensin Type 1 Receptor

Crystal structures of the human angiotensin II type 1 receptor (AT₁R) complex with the antihypertensive agent ZD7155 (PDB id: 4YAY ) and the blood pressure medication Benicar (PDB id: 4ZUD ) showed that binding poses of both antagonists are similar. This finding implies that clinically used angiotensin receptor blocking (ARB) drugs may interact in a similar fashion. However, clinically observed differences in pharmacological and therapeutic efficacies of ARBs lead to the question of whether the dynamic interactions of AT₁R with ARBs vary. To address this, we performed induced-fit docking (IFD) of eight clinically used ARBs to AT₁R followed by 200 ns molecular dynamic (MD) simulation. The experimental K_i values for ARBs correlated remarkably well with calculated free energy with R² = 0.95 and 0.70 for AT₁R-ARB models generated respectively by IFD and MD simulation. The eight ARB-AT₁R complexes share a common set of binding residues. In addition, MD simulation results validated by mutagenesis data discovered distinctive spatiotemporal interactions that display unique bonding between an individual ARB and AT₁R. These findings provide a reasonably broader picture reconciling the structure-based observations with clinical studies reporting efficacy variations for ARBs. The unique differences unraveled for ARBs in this study will be useful for structure-based design of the next generation of more potent and selective ARBs.

Role of basic amino acids of the human angiotensin type 1 receptor in the binding of the non-peptide antagonist candesartan

To explain the insurmountable/long-lasting binding of biphenyltetrazole-containing AT1-receptor antagonists such as candesartan, to the human angiotensin II type 1-receptor, a model is proposed in which the basic amino acids Lys199 and Arg 167 of the receptor interact respectively with the carboxylate and the tetrazole group of the antagonists. To validate this model, we have investigated the impact of substitution of Lys199 by Ala or Gln and of Arg167 by Ala on the binding properties of [3H]candesartan and on competition binding by candesartan, EXP3174, irbesartan, losartan, angiotensin II (Ang II) and [Sar1-Ile8]angiotensin. Our results indicate that both amino acids play an important role in the AT1-receptor ligand binding. Whereas the negative charge of Lys 199 is involved in an ionic bond with the end-standing carboxylate group of the peptide ligands, its polarity also contributes to the non-peptide antagonist binding. Substitution of Arg167 by Ala completely abolished [3H]Ang II, as well as [3H] candesartan, binding. Whereas these results are in line with the proposed model, it cannot be excluded that both amino acid residues are important for the structural integrity of the AT1-receptor with respect to its ligand binding properties.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

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.

New structural insights into the apelin receptor: identification of key residues for apelin binding

Apelin is the endogenous ligand of the orphan 7-transmembrane domain GPCR APJ, now named the apelin receptor (ApelinR). Apelin plays a prominent role in body fluid and cardiovascular homeostasis. To better understand the structural organization of the ApelinR, we built 3 homology 3-dimensional (3D) models of the human ApelinR using the validated cholecystokinin receptor-1 3D model or the X-ray structures of the β2-adrenergic and CXCR4 receptors as templates. Docking of the pyroglutamyl form of apelin 13 (pE13F) into these models revealed the conservation at the bottom of the binding site of a hydrophobic cavity in which the C-terminal Phe of pE13F was embedded. In contrast, at the top of the binding site, depending on the model, different interactions were visualized between acidic residues of the ApelinR and the basic residues of pE13F. Using site-directed mutagenesis, we showed that Asp 92, Glu 172, and Asp 282 of rat ApelinR are key residues in apelin binding by interacting with Lys 8, Arg 2, and Arg 4 of pE13F, respectively. These residues are only seen in the CXCR4-based ApelinR 3D model, further validating this model. These findings bring new insights into the structural organization of the ApelinR and the mode of apelin binding.

Structural determinants for binding, activation, and functional selectivity of the angiotensin AT1 receptor

The renin-angiotensin system (RAS) plays an important role in the pathophysiology of cardiovascular disorders. Pharmacologic interventions targeting the RAS cascade have led to the discovery of renin inhibitors, angiotensin-converting enzyme inhibitors, and AT(1) receptor blockers (ARBs) to treat hypertension and some cardiovascular and renal disorders. Mutagenesis and modeling studies have revealed that differential functional outcomes are the results of multiple active states conformed by the AT(1) receptor upon interaction with angiotensin II (Ang II). The binding of agonist is dependent on both extracellular and intramembrane regions of the receptor molecule, and as a consequence occupies more extensive area of the receptor than a non-peptide antagonist. Both agonist and antagonist bind to the same intramembrane regions to interfere with each other's binding to exhibit competitive, surmountable interaction. The nature of interactions with the amino acids in the receptor is different for each of the ARBs given the small differences in the molecular structure between drugs. AT(1) receptors attain different conformation states after binding various Ang II analogues, resulting in variable responses through activation of multiple signaling pathways. These include both classical and non-classical pathways mediated through growth factor receptor transactivations, and provide cross-communication between downstream signaling molecules. The structural requirements for AT(1) receptors to activate extracellular signal-regulated kinases 1 and 2 through G proteins, or G protein-independently through β-arrestin, are different. We review the structural and functional characteristics of Ang II and its analogs and antagonists, and their interaction with amino acid residues in the AT(1) receptor.

Insights into AT1 receptor activation through AngII binding studies

This study investigates the binding of angiotensin II (AngII) to the angiotensin II type 1 receptor (AT1R), taking into consideration several known activation elements that have been observed for G-protein-coupled receptors (GPCRs). In order to determine the crucial interactions of AngII upon binding, several MD simulations were implemented using AngII conformations derived from experimental data (NMR ROEs) and in silico flexible docking methodologies. An additional goal was to simulate the induced activation mechanism and examine the already known structural rearrangements of GPCRs upon activation. Performing MD simulations to the AT1R - AngII - lipids complex, a series of dynamic changes in the topology of AngII and the intracellular part of the receptor were observed. Overall, the present study proposes a complete binding profile of AngII to the AT1R, as well as the key transitional elements of the receptor and the agonist peptide upon activation through NMR and in silico studies.

Angiotensin type 1 receptor resistance to blockade in the opossum proximal tubule cell due to variations in the binding pocket

Blockade of the angiotensin (ANG) II receptor type 1 (AT(1)R) with angiotensin receptor blockers (ARBs) is widely used in the treatment of hypertension. However, ARBs are variably effective in reducing blood pressure, likely due, in part, to polymorphisms in the ARB binding pocket of the AT(1)R. Therefore, we need a better understanding of variations/polymorphisms that alter binding of ARBs in heterogeneous patient populations. The opossum proximal tubule cell (OKP) line is commonly used in research to evaluate renal sodium handling and therefore blood pressure. Investigating this issue, we found natural sequence variations in the opossum AT(1)R paralleling those observed in the human AT(1)R. Therefore, we posited that these sequence variations may explain ARB resistance. We demonstrate that OKP cells express AT(1)R mRNA, bind (125)I-labeled ANG II, and exhibit ANG II-induced phosphorylation of Jak2. However, Jak2 phosphorylation is not inhibited by five different ARBs commonly used to treat hypertension. Additionally, nonradioactive ANG II competes (125)I-ANG II efficiently, whereas a 10-fold molar excess of olmesartan and the ANG II receptor type 2 blocker PD-123319 is unable to block (125)I-ANG II binding. In contrast, ANG II binding to OKP cells stably expressing rat AT(1A)Rs, which have a conserved AT(1)R-binding pocket with human AT(1)R, is efficiently inhibited by olmesartan. A novel observation was that resistance to ARB binding to opossum AT(1)Rs correlates with variations from the human receptor at positions 108, 163, 192, and 198 within the ARB-binding pocket. These observations highlight the potential utility of evaluating AT(1)R polymorphisms within the ARB-binding pocket in various hypertensive populations.

Structure of the human angiotensin II type 1 (AT1) receptor bound to angiotensin II from multiple chemoselective photoprobe contacts reveals a unique peptide binding mode

Breakthroughs in G protein-coupled receptor structure determination based on crystallography have been mainly obtained from receptors occupied in their transmembrane domain core by low molecular weight ligands, and we have only recently begun to elucidate how the extracellular surface of G protein-coupled receptors (GPCRs) allows for the binding of larger peptide molecules. In the present study, we used a unique chemoselective photoaffinity labeling strategy, the methionine proximity assay, to directly identify at physiological conditions a total of 38 discrete ligand/receptor contact residues that form the extracellular peptide-binding site of an activated GPCR, the angiotensin II type 1 receptor. This experimental data set was used in homology modeling to guide the positioning of the angiotensin II (AngII) peptide within several GPCR crystal structure templates. We found that the CXC chemokine receptor type 4 accommodated the results better than the other templates evaluated; ligand/receptor contact residues were spatially grouped into defined interaction clusters with AngII. In the resulting receptor structure, a β-hairpin fold in extracellular loop 2 in conjunction with two extracellular disulfide bridges appeared to open and shape the entrance of the ligand-binding site. The bound AngII adopted a somewhat vertical binding mode, allowing concomitant contacts across the extracellular surface and deep within the transmembrane domain core of the receptor. We propose that such a dualistic nature of GPCR interaction could be well suited for diffusible linear peptide ligands and a common feature of other peptidergic class A GPCRs.

Structure and reorientational dynamics of angiotensin I and II: a microscopic physical insight

We present a study of structural analysis and reorientational dynamics of Angiotensin I (AngI) and Angiotensin II (AngII) in aqueous solution. AngI is a decapeptide that acts as a precursor to the octapeptide AngII in the Renin-Angiotensin-Aldosterone system for blood pressure regulation. Experimental structural characterization of these peptides, carried out with circular dichroism and infrared spectroscopy, showed that the angiotensins are mostly disordered but exhibit a measurable population of ordered structures at room temperature. Interestingly, these change from the unordered polyproline-like conformation for AngI to a more compact and ordered conformation for AngII as the length of the peptide is decreased. Anisotropy decay measurements with picosecond time resolution indicate slower overall tumbling and a greater amplitude of internal motion in AngI compared to AngII, consistent with more compact and less flexible structure of the active form of the peptide. To model the microscopic behavior of the peptides, 2-μs molecular dynamics simulation trajectories were generated for AngI and AngII, at 300 K using the OPLS-AA potential and SPC water. The structures sampled in the simulations mostly agree with the experimental results, showing the prevalence of disordered structures, turns, and polyproline helices. Additionally, the computational results predict fewer sampled conformations, tighter side-chain packing and marked increase of Phe8 solvent accessibility upon AngI truncation to AngII. Our combined approach of experiment and extensive computer simulation thus yields new information on the conformational dynamics of the angiotensins, helping provide insight into the structural basis for the potency of AngI relative to AngII.

Modeling of human prokineticin receptors: interactions with novel small-molecule binders and potential off-target drugs

BACKGROUND AND MOTIVATION

The Prokineticin receptor (PKR) 1 and 2 subtypes are novel members of family A GPCRs, which exhibit an unusually high degree of sequence similarity. Prokineticins (PKs), their cognate ligands, are small secreted proteins of ∼80 amino acids; however, non-peptidic low-molecular weight antagonists have also been identified. PKs and their receptors play important roles under various physiological conditions such as maintaining circadian rhythm and pain perception, as well as regulating angiogenesis and modulating immunity. Identifying binding sites for known antagonists and for additional potential binders will facilitate studying and regulating these novel receptors. Blocking PKRs may serve as a therapeutic tool for various diseases, including acute pain, inflammation and cancer.

METHODS AND RESULTS

Ligand-based pharmacophore models were derived from known antagonists, and virtual screening performed on the DrugBank dataset identified potential human PKR (hPKR) ligands with novel scaffolds. Interestingly, these included several HIV protease inhibitors for which endothelial cell dysfunction is a documented side effect. Our results suggest that the side effects might be due to inhibition of the PKR signaling pathway. Docking of known binders to a 3D homology model of hPKR1 is in agreement with the well-established canonical TM-bundle binding site of family A GPCRs. Furthermore, the docking results highlight residues that may form specific contacts with the ligands. These contacts provide structural explanation for the importance of several chemical features that were obtained from the structure-activity analysis of known binders. With the exception of a single loop residue that might be perused in the future for obtaining subtype-specific regulation, the results suggest an identical TM-bundle binding site for hPKR1 and hPKR2. In addition, analysis of the intracellular regions highlights variable regions that may provide subtype specificity.

Mass spectrometric characterization of modifications to angiotensin II by lipid peroxidation products, 4-oxo-2(E)-nonenal and 4-hydroxy-2(E)-nonenal

The octapeptide angiotensin II (Ang II; Asp(1)-Arg(2)-Val(3)-Tyr(4)-Ile(5)-His(6)-Pro(7)-Phe(8)) is the primary active hormone of the renin/angiotensin system (RAS) and has been implicated in various cardiovascular diseases. Numerous structure-activity relationship studies have identified Asp(1), Arg(2), and His(6) of Ang II to be critical for its biological activity and receptor binding. From the reactions of Ang II with lipid peroxidation-derived aldehydes, 4-oxo-2(E)-nonenal (ONE) or 4-hydroxy-2(E)-nonenal (HNE), we have identified the major modifications to the N-terminus, Asp(1), Arg(2), and His(6) of Ang II by liquid chromatography/mass spectrometry (LC/MS) and matrix-assisted laser desorption ionization-time-of-flight/MS (MALDI-TOF/MS). The identities of ONE- and HNE-modified Ang II were confirmed by tandem mass spectrometry (MS/MS) and postsource decay (PSD)-TOF/MS before and after the reaction with sodium borohydride. In the reaction with ONE, a pyruvamide-Ang II that formed via oxidative decarboxylation of N-terminal Asp was detected as the most abundant product after 48 h of incubation. It was followed by Arg-modified [Arg(2)(ONE-H(2)O)]-Ang II and the N-terminal-modified 4-ketoamide form of [N-ONE]-Ang II. The Michael addition products of [His(6)(HNE)]-Ang II were the most abundant products in the beginning of the reaction with HNE, followed by the dehydrated Michael addition products of [His(6)(HNE-H(2)O)]-Ang II. [His(6)(HNE)]-Ang II was dehydrated to [His(6)(HNE-H(2)O)]-Ang II during the prolonged incubation, and [His(6)(HNE-H(2)O)]-Ang II became the major products after 7 days. The model reactions of N(α)-tert-butoxycarbonyl (tBoc)-Arg with ONE and tBoc-His with HNE were performed and compared with the Ang II reaction. tBoc-Arg readily reacted with ONE to produce a compound analogous to [Arg(2)(ONE-H(2)O)]-Ang II, which confirmed Arg as one of the important target nucleophiles of ONE. However, tBoc-His exclusively formed a Michael addition product upon the reaction with HNE. The unexpected formation of [His(6)(HNE-H(2)O)]-Ang II can be explained by the proximity of His(6) to C-terminal carboxylate in the specific conformation of Ang II, which facilitates the dehydration of Michael addition products. Therefore, our results suggest a possible discrepancy in the adduction chemistry of ONE and HNE for model amino acids and endogenous bioactive peptides, which is governed by the microenvironment of peptides, such as the specific amino acid sequence and conformation. Such stable ONE- and HNE-derived modifications to Ang II could potentially modulate its functions in vivo by disrupting the interaction with Ang II type 1 (AT(1)) receptor and/or inhibiting the enzyme activity of aminopeptidase A (APA), which cleaves the N-terminal Asp residue of Ang II to generate Ang III.

Role of the second disulfide bridge (Cys(18)-Cys(274)) in stabilizing the inactive AT₁ receptor

Previous research showed that disruption of the Cys(18)-Cys(274) bond in the angiotensin II (AngII) AT₁ receptor mutant (C18S), expressed in CHO cells, causes an increase in the basal activity and attenuation of the maximum response to AngII. In addition, this mutant was mostly intracellularly distributed. Our aim was to investigate whether the intracellular presence of the mutant was due to a constitutive internalization or to a defective maturation of the receptor. The first hypothesis was assessed by pretreating the cells with losartan or [Sar¹Leu⁸]-AngII, specific AT₁ receptor antagonists, a maneuver to revert the receptor internalization. The second hypothesis was tested using calnexin, an endoplasmic reticulum marker. We found that treatment with AT₁ receptor antagonists causes an increase in the binding ability of the mutant to AngII. Furthermore, whereas the maximum effect is increased, it reduces the enhanced basal levels of IP₃. The hypothesis for a lack of maturation of the mutant receptor was ruled out because calnexin was poorly colocalized with the intracellular C18S receptor. Our results suggest that the mutation of the AT₁ receptor leads to a conformational structure similar to that of the active mode of the AT₁ receptor, favoring its internalization in the absence of the agonist.

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.

Ligand-specific conformation of extracellular loop-2 in the angiotensin II type 1 receptor

The orientation of the second extracellular loop (ECL2) is divergent in G-protein coupled receptor (GPCR) structures determined. This discovery provoked the question, is the ECL2 conformation differentially regulated in the GPCRs that respond to diffusible ligands? We have determined the conformation of the ECL2 of the angiotensin II type 1 receptor by reporter-cysteine accessibility mapping in different receptor states (i.e. empty, agonist-bound and antagonist-bound). We introduced cysteines at each position of ECL2 of an N-terminal epitope-tagged receptor surrogate lacking all non-essential cysteines and then measured reaction of these with a cysteine-reactive biotin probe. The ability of biotinylated mutant receptors to react with a steptavidin-HRP-conjugated antibody was used as the basis for examining differences in accessibility. Two segments of ECL2 were accessible in the empty receptor, indicating an open conformation of ECL2. These segments were inaccessible in the ligand-bound states of the receptor. Using the accessibility constraint, we performed molecular dynamics simulation to predict ECL2 conformation in different states of the receptor. Analysis suggested that a lid conformation similar to that of ECL2 in rhodopsin was induced upon binding both agonist and antagonist, but exposing different accessible segments delimited by the highly conserved disulfide bond. Our study reveals the ability of ECL2 to interact with diffusing ligands and to adopt a ligand-specific lid conformation, thus, slowing down dissociation of ligands when bound. Distinct conformations induced by the bound agonist and the antagonist around the conserved disulfide bond suggest an important role for this disulfide bond in producing different functional states of the receptor.

The Active and the Inactive Form of the hAT1Receptor Have an Identical Ligand-Binding Environment: An MPA Study on a Constitutively Active Angiotensin II Receptor Mutant

Several models of activation mechanisms were proposed for G protein-coupled receptors (GPCRs), yet no direct methods exist for their elucidation. The availability of constitutively active mutants has given an opportunity to study active receptor conformations within acceptable limits using models such as the angiotensin II type 1 (AT1)1 receptor mutant N111G-hAT1 which displays an important constitutive activity. Recently, by using methionine proximity assay, we showed for the hAT1 receptor that TMD III, VI, and VII form the ligand-binding pocket of the C-terminal amino acid of an antagonistic AngII analogue. In the present contribution, we investigated whether the same residues would also constitute the ligand-binding contacts in constitutively activated mutant (CAM) receptors. For this purpose, the same Met mutagenesis strategy was carried out on the N111G double mutants. Analysis of 43 receptors mutants in the N111G-hAT1 series, photolabeled and CNBr digested, showed that there were only subtle structural changes between the wt-receptor and its constitutively active form.

Identification of Tyr(290(6.58)) of the human gonadotropin-releasing hormone (GnRH) receptor as a contact residue for both GnRH I and GnRH II: importance for high-affinity binding and receptor activation

Molecular modeling showed interactions of Tyr (290(6.58)) in transmembrane domain 6 of the GnRH receptor with Tyr (5) of GnRH I, and His (5) of GnRH II. The wild-type receptor exhibited high affinity for [Phe (5)]GnRH I and [Tyr (5)]GnRH II, but 127- and 177-fold decreased affinity for [Ala (5)]GnRH I and [Ala (5)]GnRH II, indicating that the aromatic ring in position 5 is crucial for receptor binding. The receptor mutation Y290F decreased affinity for GnRH I, [Phe (5)]GnRH I, GnRH II and [Tyr (5)]GnRH II, while Y290A and Y290L caused larger decreases, suggesting that both the para-OH and aromatic ring of Tyr (290(6.58)) are important for binding of ligands with aromatic residues in position 5. Mutating Tyr (290(6.58)) to Gln increased affinity for Tyr (5)-containing GnRH analogues 3-12-fold compared with the Y290A and Y290L mutants, suggesting a hydrogen-bond between Gln of the Y290Q mutant and Tyr (5) of GnRH analogues. All mutations had small effects on affinity of GnRH analogues that lack an aromatic residue in position 5. These results support direct interactions of the Tyr (290(6.58)) side chain with Tyr (5) of GnRH I and His (5) of GnRH II. Tyr (290(6.58)) mutations, except for Y290F, caused larger decreases in GnRH potency than affinity, indicating that an aromatic ring is important for the agonist-induced receptor conformational switch.

Photoaffinity labeling and its application in structural biology

This review contains a brief consideration of some theoretical aspects of photoaffinity (photoreactive) labeling (PAL), and the most widely used photoreactive groups, such as arylazide, benzophenone, and 3-(trifluoromethyl)-3-phenyldiazirine, are characterized in comparison. Experimental methodology is described, including modern approaches of mass spectrometry for analysis of cross-linking products between the photoreactive probes and biomolecules. Examples of PAL application in diverse fields of structural biology during the last five-ten years are presented. Potential drug targets, transport processes, stereochemistry of interaction of G-protein-coupled receptors with ligands, as well as structural changes in nicotinic acetylcholine receptor are considered. Applications of photoaffinity ganglioside and phospholipid probes for studying biological membranes and of nucleotide probes in investigations of replicative and transcriptional complexes, as well as photoaffinity glycoconjugates for detecting carbohydrate-binding proteins are covered. In combination with modern techniques of instrumental analysis and computer-aided modeling, PAL remains the most important approach in studies on the organization of biological systems.

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.

Ligand-supported homology modeling of the human angiotensin II type 1 (AT1) receptor: Insights into the molecular determinants of telmisartan binding

Angiotensin II type 1 (AT(1)) receptor belongs to the super-family of G-protein-coupled receptors, and antagonists of the AT(1) receptor are effectively used in the treatment of hypertension. To understand the molecular interactions of these antagonists, such as losartan and telmisartan, with the AT(1) receptor, a homology model of the human AT(1) (hAT(1)) receptor with all connecting loops was constructed from the 2.6 A resolution crystal structure (PDB i.d., 1L9H) of bovine rhodopsin. The initial model generated by MODELLER was subjected to a stepwise ligand-supported model refinement. This protocol involved initial docking of non-peptide AT(1) antagonists in the putative binding site, followed by several rounds of iterative energy minimizations and molecular dynamics simulations. The final model was validated based on its correlation with several structure-activity relationships and site-directed mutagenesis data. The final model was also found to be in agreement with a previously reported AT(1) antagonist pharmacophore model. Docking studies were performed for a series of non-peptide AT(1) receptor antagonists in the active site of the final hAT(1) receptor model. The docking was able to identify key molecular interactions for all the AT(1) antagonists studied. Reasonable correlation was observed between the interaction energy values and the corresponding binding affinities of these ligands, providing further validation for the model. In addition, an extensive unrestrained molecular dynamics simulation showed that the docking-derived bound pose of telmisartan is energetically stable. Knowledge gained from the present studies can be used in structure-based drug design for developing novel ligands for the AT(1) receptor.

Role of the Cys18–Cys274 disulfide bond and of the third extracellular loop in the constitutive activation and internalization of angiotensin II type 1 receptor

An insertion of residues in the third extracellular loop and a disulfide bond linking this loop to the N-terminal domain were identified in a structural model of a G-protein coupled receptor specific to angiotensin II (AT1 receptor), built in homology to the seven-transmembrane-helix bundle of rhodopsin. Both the insertion and the disulfide bond were located close to an extracellular locus, flanked by the second extracellular loop (EC-2), the third extracellular loop (EC-3) and the N-terminal domain of the receptor; they contained residues identified by mutagenesis studies to bind the angiotensin II N-terminal segment (residues D1 and R2). It was postulated that the insertion and the disulfide bond, also found in other receptors such as those for bradykinin, endothelin, purine and other ligands, might play a role in regulating the function of the AT1 receptor. This possibility was investigated by assaying AT1 forms devoid of the insertion and with mutations to Ser on both positions of Cys residues forming the disulfide bond. Binding and activation experiments showed that abolition of this bond led to constitutive activation, decay of agonist binding and receptor activation levels. Furthermore, the receptors thus mutated were translocated to cytosolic environments including those in the nucleus. The receptor form with full deletion of the EC-3 loop residue insertion, displayed a wild type receptor behavior.

Unusually Weak Binding Interactions in Tetrazole−Amidine Complexes

[reaction: see text] Tetrazoles frequently replace carboxylic acids in pharmaceutical drugs. However, while the binding modes of tetrazolate and carboxylate anions in amidinium complexes turns out to be similar, the association constant of the former is 2-3 orders of magnitude smaller in DMSO. Crystal structures revealed that the N...H-N hydrogen bonds in amidinium tetrazolates are bent (162 degrees and 169 degrees ) and noticeably longer (N...N 2.96 A) than corresponding hydrogen bonds in both amidinium carboxylates and ammonium tetrazolates.

Molecular mechanisms of constitutive activity: mutations at position 111 of the angiotensin AT1 receptor

A possible molecular mechanism for the constitutive activity of mutants of the angiotensin type 1 receptor (AT1) at position 111 was suggested by molecular modeling. This involves a cascade of conformational changes in spatial positions of side chains along transmembrane helix (TM3) from L112 to Y113 to F117, which in turn, results in conformational changes in TM4 (residues I152 and M155) leading to the movement of TM4 as a whole. The mechanism is consistent with the available data of site-directed mutagenesis, as well as with correct predictions of constitutive activity of mutants L112F and L112C. It was also predicted that the double mutant N111G/L112A might possess basal constitutive activity comparable with that of the N111G mutant, whereas the double mutants N111G/Y113A, N111G/F117A, and N111G/I152A would have lower levels of basal activity. Experimental studies of the above double mutants showed significant constitutive activity of N111G/L112A and N111G/F117A. The basal activity of N111G/I152A was higher than expected, and that of N111G/Y113A was not determined due to poor expression of the mutant. The proposed mechanism of constitutive activity of the AT(1) receptor reveals a novel nonsimplistic view on the general problem of constitutive activity, and clearly demonstrates the inherent complexity of the process of G protein-coupled receptor (GPCR) activation.

A Review of the Structural and Functional Features of Olmesartan Medoxomil, An Angiotensin Receptor Blocker

The angiotensin II (A-II) type 1 (AT1) receptor-mediated effects of A-II play a key role in the pathophysiology of hypertension. Effective inhibition of A-II is provided by the latest class of antihypertensive medications, the AT1 receptor blockers (ARBs). These orally available agents were developed around a common imidazole-based structural core. The most recent member of this drug class to be approved by the Food and Drug Administration, olmesartan medoxomil, contains unique features that may explain its clinical efficacy. Key structural elements of olmesartan medoxomil include a hydroxyalkyl substituent at the imidazole 4-position and a hydrolyzable ester group at the imidazole 5-position. Inter- and intramolecular hydrogen bonding involving these groups may contribute to the potentiation of antagonist activity. After oral administration, olmesartan medoxomil is deesterified in the intestinal tract to produce the active metabolite olmesartan, which undergoes no additional metabolic change. The marked antihypertensive efficacy of olmesartan medoxomil may result from a unique pharmacological interaction of the drug with the AT1 receptor, resulting in a potent, long-lasting, dose-dependent blockade of A-II. This review article characterizes the structural features of olmesartan that may be responsible for its clinical efficacy. Inferential pharmacological studies compare and contrast the effects of olmesartan to those of other ARBs in comparable preclinical animal models.

A rhodopsin exhibiting binding ability to agonist all-trans-retinal

Rhodopsins are the members of the family of G protein-coupled receptors that have diverged from ligand-binding receptors into photoreceptive pigments. Vertebrate rhodopsins are able to bind the inverse agonist 11-cis-retinal but are unable to bind the agonist all-trans-retinal, indicating that vertebrate rhodopsin changed its binding ability during the course of molecular evolution. Here, we show that unlike vertebrate rhodopsin, amphioxus rhodopsin is still able to bind the agonist all-trans-retinal. The opsin of amphioxus rhodopsin can also bind 11-cis-retinal to form a photoreceptive pigment that can convert to a red-shifted photoproduct through cis-trans isomerization of the chromophore upon photon absorption. The red-shifted photoproduct is the stable G protein activating state. Incubation of the opsin with all-trans-retinal produces a G protein activating state that is spectroscopically and biochemically indistinguishable from the red-shifted photoproduct, indicating that the opsin possesses agonist-binding ability. The opsin exhibits an approximately 50-fold higher affinity for 11-cis-retinal than for all-trans-retinal, and mutational analyses revealed that Trp-265 situated in helix VI is important for the increase in binding affinity to 11-cis-retinal. These properties of amphioxus rhodopsin suggest that an ancestral rhodopsin increased the affinity for 11-cis-retinal by rearrangement of a structure including Trp-265 to act as a photoreceptor. In addition, an additional mechanism was acquired in vertebrate rhodopsin to prevent completely the binding of exogenous all-trans-retinal during molecular evolution.

Sarcosine1,glycine8 angiotensin II is an AT1 angiotensin II receptor subtype selective antagonist

Studies predating the discovery of the two major subtypes of angiotensin II (Ang II) receptors, AT1 and AT2, revealed anomalous characteristics of sarcosine1,glycine8 Ang II (Sar1,Gly8 Ang II). It competed poorly for 125I-Ang II binding in bovine brain but potently antagonized dipsogenic responses to intracerebroventricularly administered Ang II. Subsequent recognition that bovine brain contains AT(2) receptors, while dipsogenic responses to Ang II are mediated by AT1 receptors, suggests that Sar1,Gly(8) Ang II is AT1 selective. Sar1,Gly8 Ang II competed for 125I-sarcosine1,isoleucine8 Ang II binding to AT1 receptors in pituitary, liver and adrenal (the latter with the AT2 selective antagonist PD 123,319) with Ki's of 0.66, 1.40 and 1.36 nM, respectively. In contrast, the Ki of Sar1,Gly8 Ang II for AT2 receptors in rat adrenal (with the selective AT1 antagonist losartan) was 52 nM. 125I-Sar1,Gly8 Ang II (0.5-3 nM) bound to AT1 receptors in pituitary, liver, heart, adrenal, and hypothalamic membranes with high affinity (Kd=0.43, 1.6, 2.3, 0.96 and 1.8 nM, respectively), but showed no saturable binding to the adrenal AT2 receptor. 125I-Sar1,Gly8 Ang II selectively labeled AT1 receptors in sections of adrenal using receptor autoradiography. Thus, binding studies reveal Sar1,Gly8 Ang II to be the first angiotensin peptide analog to show AT1 receptor selectivity. 125I-Sar1,Gly8 Ang II offers a new means to selectively radiolabel AT1 receptors and may help to characterize ligand docking sites and agonist switches for AT1 versus AT2 receptors.

Losartan's molecular basis of interaction with membranes and AT1 receptor

Physicochemical methods were used to study the thermal and dynamic changes caused by losartan in the membrane bilayers. In addition, molecular modeling was implemented to explore its topography both in membranes and AT(1) receptor. Its incorporation resulted in the modification of thermal profile of dipalmitoyl phosphatidylcholine (DPPC) bilayers in a concentration dependent way up to 20mol% as it is depicted from the combination of differential scanning calorimetry (DSC) and MAS data. In particular, the presence of losartan caused lowering of the phase transition temperature and abolishment of the pretransition. T(1) experiments revealed the location of the drug into the membrane bilayers. The use of a combination of biophysical methods along with docking experiments brought out a possible two-step mechanism which involves incorporation of losartan at the interface of membrane bilayers and diffusion in the upper parts of AT(1) receptor helices IV-VII.

Gonadotropin-releasing hormone receptors

GnRH and its analogs are used extensively for the treatment of hormone-dependent diseases and assisted reproductive techniques. They also have potential as novel contraceptives in men and women. A thorough delineation of the molecular mechanisms involved in ligand binding, receptor activation, and intracellular signal transduction is kernel to understanding disease processes and the development of specific interventions. Twenty-three structural variants of GnRH have been identified in protochordates and vertebrates. In many vertebrates, three GnRHs and three cognate receptors have been identified with distinct distributions and functions. In man, the hypothalamic GnRH regulates gonadotropin secretion through the pituitary GnRH type I receptor via activation of G(q). In-depth studies have identified amino acid residues in both the ligand and receptor involved in binding, receptor activation, and translation into intracellular signal transduction. Although the predominant coupling of the type I GnRH receptor in the gonadotrope is through productive G(q) stimulation, signal transduction can occur via other G proteins and potentially by G protein-independent means. The eventual selection of intracellular signaling may be specifically directed by variations in ligand structure. A second form of GnRH, GnRH II, conserved in all higher vertebrates, including man, is present in extrahypothalamic brain and many reproductive tissues. Its cognate receptor has been cloned from various vertebrate species, including New and Old World primates. The human gene homolog of this receptor, however, has a frame-shift and stop codon, and it appears that GnRH II signaling occurs through the type I GnRH receptor. There has been considerable plasticity in the use of different GnRHs, receptors, and signaling pathways for diverse functions. Delineation of the structural elements in GnRH and the receptor, which facilitate differential signaling, will contribute to the development of novel interventive GnRH analogs.

Angiotensin II inhibitory peptide found in the receptor sequence using peptide array

Peptide array consisting of hundreds of peptides spatially addressed and synthesized on a cellulose membrane support was used to screen ligand-inhibitory peptides. As a model, angiotensin II (Ang II), a significant peptide related to the treatment of cardiovascular diseases, was chosen as the target ligand. Peptide arrays covering the Ang II receptor type 1 sequence were prepared, and peptide domains with high affinity to the Ang II fluorescein conjugate were investigated. The peptide (VVIVIY) within the first transmembrane region exhibited the highest affinity to Ang II. The synthesized soluble VVIVIY peptide had an 84% inhibitory effect on Ang II-induced aorta contraction. These results indicate that our screening strategy utilizing peptide array is an effective approach for the peptide drug development.

Effect of 3-5 monocyclizations of angiotensin II and 4-aminoPhe6-Ang II on AT2 receptor affinity

The endogenous angiotensin II (Ang II) and the synthetic AT(2) selective agonist 4-aminoPhe(6)-Ang II respond very differently to identical cyclizations. Cyclizations of Ang II by thioacetalization, involving the 3 and 5 amino acid residue side chains, provided ligands with almost equipotent binding affinities to Ang II at the AT(2) receptor. In contrast, the same cyclization procedures applied on the AT(2) selective 4-aminoPhe(6)-Ang II delivered significantly less potent AT(2) receptor ligands, although the AT(2)/AT(1) selectivity was still very high. The fact that different structure-activity relationships are observed after imposing conformational restrictions on Ang II and 4-aminoPhe(6)-Ang II, respectively, suggests that the peptides, despite large similarities might adopt quite different backbone conformations when binding to the AT(2) receptor.

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.

Peptide and nonpeptide antagonist interaction with constitutively active human AT1 receptors

Wild type human AT(1) receptors (WT-AT(1)) and mutant receptors, in which Asn(111) was replaced by glycine (N111G), alanine (N111A) and serine (N111S), or in which Asp(281) was replaced by alanine (D281A) or in which N111G and D281A replacements were combined, were transiently expressed in CHO-K1 cells. While the biphenyltetrazole compound candesartan dissociated slowly and behaved as an insurmountable antagonist for WT-AT(1), it dissociated swiftly and only produced a rightward shift of the angiotensin Ang II- and -IV dose-response curves for inositol phosphate (IP) accumulation in cells expressing N111G. [3H]candesartan competition binding yielded the same potency order of the related biphenyltetrazoles for WT-AT(1) and mutated receptors, i.e. candesartan>EXP3174>irbesartan>losartan. Affinities were equal for WT-AT(1) and D281A and 40- to 400-fold lower for all Asn(111) mutants. Mutations did not affect the affinity of the peptide antagonist [Sar(1)Ile(8)]Ang II (SARILE). Basal IP accumulation in cells with WT-AT(1) was not affected by any biphenyltetrazole antagonists and was increased by SARILE to 19% of the maximal Ang II stimulation. Basal IP accumulation was higher for cells expressing the Asn(111)-mutated receptors. For N111G, this accumulation was partially inhibited by all the biphenyltetrazoles upon long-term (18hr) exposure. In these cells SARILE produced the same maximal stimulation as Ang II. Asn(111)-mutated AT(1) receptors are thought to mimic the pre-activated state of the wild type receptor and comparing the efficacy and affinity of ligands for such mutated receptors facilitate the distinction of partial (SARILE) and inverse (biphenyltetrazoles) agonists from true antagonists.

Downstream coding region determinants of bacterio-opsin, muscarinic acetylcholine receptor and adrenergic receptor expression in Halobacterium salinarum

The aim of this work is to develop a prokaryotic system capable of expressing membrane-bound receptors in quantities suitable for biochemical and biophysical studies. Our strategy exploits the endogenous high-level expression of the membrane protein bacteriorhodopsin (BR) in the Archaeon Halobacterium salinarum. We attempted to express the human muscarinic acetylcholine (M(1)) and adrenergic (a2b) receptors by fusing the coding region of the m1 and a2b genes to nucleotide sequences known to direct bacterio-opsin (bop) gene transcription. The fusions included downstream modifications to produce non-native carboxyl-terminal amino acids useful for protein identification and purification. bop mRNA and BR accumulation were found to be tightly coupled and the carboxyl-terminal coding region modifications perturbed both. m1 and a2b mRNA levels were low, and accumulation was sensitive to both the extent of the bop gene fusion and the specific carboxyl-terminal coding sequence modifications included. Functional a2b adrenergic receptor expression was observed to be dependent on the downstream coding region. This work demonstrates that a critical determinant of expression resides in the downstream coding region of the wild-type bop gene and manipulation of the downstream coding region of heterologous genes may affect their potential for expression in H. salinarum.

Angiotensin II vs its type I antagonists: conformational requirements for receptor binding assessed from NMR spectroscopic and receptor docking experiments

The conformations of three angiotensin II (AII) peptide antagonists ([Sar1]-AII(1-7)-NH(2), [Sar1,Val5,Ala8]-AII and the AII antipeptide, [Glu1,Gly2,Val5,Val8]-AII) were assessed in a lipid medium. A common backbone turn was identified through modeling and spectroscopic studies. The His6 residue acted as a pivoting point beyond which each peptide adopted two distinct conformations. One principle conformer resembled that previously determined for AII while the other was designated as an AII antagonist like conformer. A computational overlay between the nonpeptide antagonist, Losartan, and both the AII and the AII like conformation of [Sar1,Val5,Ala8]-AII revealed common pharmacophoric points with RMS deviations between 1 and 1.5 A. Both the AII conformer and the AII antagonist like conformer of [Sar1,Val5,Ala8]-AII were docked into a model of the AT(1) receptor. Receptor residue Phe289 and Asp281 provided good contact points for both peptides. Some differences were also noted. The terminal carboxyl of AII contacted Lys199 of the receptor while that of [Sar1,Val5,Ala8]-AII bridged Arg23 at the top of helix 1. The Asp1 side chain of AII interacted with His183 of the receptor.

Aliphatic amino acids in helix VI of the AT(1) receptor play a relevant role in agonist binding and activity

Angiotensin II (AII) AT(1) receptor mutants with replacements of aliphatic amino acids in the distal region of helix VI and the adjoining region of the third extracellular loop (EC-3) were expressed in Chinese hamster ovary (CHO) cells to determine their role in ligand binding and activation. The triple mutant [L262D, L265D, L268D]AT(1) (L3D) showed a marked reduction in affinity for AII and for non-peptide (losartan) and peptide ([Sar(1)Leu(8) ]AII) antagonists; in functional assays using inositol phosphate (IP) accumulation, the relative potency and the maximum effect of AII were reduced in L3D. Replacement of Leu(268) (in EC-3) and Leu(262) (in the transmembrane domain) by aspartyl residues did not cause significant changes in the receptor's affinity for the ligands and in IP production. In contrast, the point mutation L265D, at helix VI, markedly decreased affinity and ability to stimulate phosphatidylinositol turnover. Molecular modeling of the AT(1) receptor based on a recent crystal structure of rhodopsin, suggests that the side chain of Leu(265) but not that of Leu(262) is facing a cleft between helices V and VI and interacts with the lipid bilayer, thus helping to stabilize the receptor structure near the Lys(199) residue of helix V in the agonist binding site which is necessary for full activity.

Identification by site-directed mutagenesis of residues involved in ligand recognition and activation of the human A3 adenosine receptor

Ligand recognition has been extensively explored in G protein-coupled A(1), A(2A), and A(2B) adenosine receptors but not in the A(3) receptor, which is cerebroprotective and cardioprotective. We mutated several residues of the human A(3) adenosine receptor within transmembrane domains 3 and 6 and the second extracellular loop, which have been predicted by previous molecular modeling to be involved in the ligand recognition, including His(95), Trp(243), Leu(244), Ser(247), Asn(250), and Lys(152). The N250A mutant receptor lost the ability to bind both radiolabeled agonist and antagonist. The H95A mutation significantly reduced affinity of both agonists and antagonists. In contrast, the K152A (EL2), W243A (6.48), and W243F (6.48) mutations did not significantly affect the agonist binding but decreased antagonist affinity by approximately 3-38-fold, suggesting that these residues were critical for the high affinity of A(3) adenosine receptor antagonists. Activation of phospholipase C by wild type (WT) and mutant receptors was measured. The A(3) agonist 2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine stimulated phosphoinositide turnover in the WT but failed to evoke a response in cells expressing W243A and W243F mutant receptors, in which agonist binding was less sensitive to guanosine 5'-gamma-thiotriphosphate than in WT. Thus, although not important for agonist binding, Trp(243) was critical for receptor activation. The results were interpreted using a rhodopsin-based model of ligand-A(3) receptor interactions.

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.

Role of Asp297 of the AT2 receptor in high-affinity binding to different peptide ligands

To determine how ligand-receptor interaction is affected by the charges of the amino acids at position 2 of the ligands and position 297 of the AT2 receptor, we generated the Asp297Lys mutant of AT2 and a ligand SarAsp(2)Ile. Asp297Lys mutant lost affinity to Ang II and SarIle however retained partial affinity to 125I-CGP42112A. The SarAsp(2)Ile had high affinity to Asp297Lys (IC(50)3.5nM) and partial affinity to the AT2 (IC(50)15nM). Therefore, not only the charge, but also the length of the side arms of the amino acids at position 2 of the ligand and position 297 of the receptor affect their interaction.