Differentiation between binding sites for angiotensin II and nonpeptide antagonists on the angiotensin II type 1 receptors

Structural basis for GPCR signaling by small polar versus large lipid metabolites-discovery of non-metabolite ligands

Key metabolites act through specific G protein-coupled receptors (GPCRs) as extracellular signals of fuel availability and metabolic stress. Here, we focus on the succinate receptor SUCNR1/GPR91 and the long chain fatty acid receptor FFAR1/GPR40, for which 3D structural information is available. Like other small polar acidic metabolites, succinate is excreted from the cell by transporter proteins to bind to an extracellular, solvent-exposed pocket in SUCNR1. Non-metabolite pharmacological tool compounds are currently being designed based on the structure of the SUCNR1 binding pocket. In FFAR1, differently signaling lipid mimetics bind in two distinct membrane-exposed sites corresponding to each of the lipid bilayer leaflets. Conceivably endogenous lipid ligands gain access to these sites by way of the membrane and probably occupy both sites under physiological circumstances. Design of polar agonists for a dynamic, solvent-exposed pocket in FFAR1 underlines the possibility of structure-based approaches for development of novel tool compounds even in lipid sensing metabolite GPCRs.

Mechanism of Hormone Peptide Activation of a GPCR: Angiotensin II Activated State of AT1R Initiated by van der Waals Attraction

We present a succession of structural changes involved in hormone peptide activation of a prototypical GPCR. Microsecond molecular dynamics simulation generated conformational ensembles reveal propagation of structural changes through key "microswitches" within human AT₁R bound to native hormone. The endocrine octa-peptide angiotensin II (AngII) activates AT₁R signaling in our bodies which maintains physiological blood pressure, electrolyte balance, and cardiovascular homeostasis. Excessive AT₁R activation is associated with pathogenesis of hypertension and cardiovascular diseases which are treated by sartan drugs. The mechanism of AT₁R inhibition by sartans has been elucidated by 2.8 Å X-ray structures, mutagenesis, and computational analyses. Yet, the mechanism of AT₁R activation by AngII is unclear. The current study delineates an activation scheme initiated by AngII binding. A van der Waals "grasp" interaction between Phe8^AngII with Ile288^7.39 in AT₁R induced mechanical strain pulling Tyr292^7.43 and breakage of critical interhelical H-bonds, first between Tyr292^7.43 and Val108^3.32 and second between Asn111^3.35 and Asn295^7.46. Subsequently changes are observed in conserved microswitches DRY^TM3, Yx7K(R)^TM5, CWxP^TM6, and NPxxY^TM7 in AT₁R. Activating the microswitches in the intracellular region of AT₁R may trigger formation of the G-protein binding pocket as well as exposure of helix-8 to cytoplasm. Thus, the active-like conformation of AT₁R is initiated by the van der Waals interaction of Phe8^AngII with Ile288^7.39, followed by systematic reorganization of critical interhelical H-bonds and activation of microswitches.

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.

Structure-Function Basis of Attenuated Inverse Agonism of Angiotensin II Type 1 Receptor Blockers for Active-State Angiotensin II Type 1 Receptor

Ligand-independent signaling by the angiotensin II type 1 receptor (AT1R) can be activated in clinical settings by mechanical stretch and autoantibodies as well as receptor mutations. Transition of the AT1R to the activated state is known to lower inverse agonistic efficacy of clinically used AT1R blockers (ARBs). The structure-function basis for reduced efficacy of inverse agonists is a fundamental aspect that has been understudied not only in relation to the AT1R but also regarding other homologous receptors. Here, we demonstrate that the active-state transition in the AT1R indeed attenuates an inverse agonistic effect of four biphenyl-tetrazole ARBs through changes in specific ligand-receptor interactions. In the ground state, tight interactions of four ARBs with a set of residues (Ser109(TM3), Phe182(ECL2), Gln257(TM6), Tyr292(TM7), and Asn295(TM7)) results in potent inverse agonism. In the activated state, the ARB-AT1R interactions shift to a different set of residues (Val108(TM3), Ser109(TM3), Ala163(TM4), Phe182(ECL2), Lys199(TM5), Tyr292(TM7), and Asn295(TM7)), resulting in attenuated inverse agonism. Interestingly, V108I, A163T, N295A, and F182A mutations in the activated state of the AT1R shift the functional response to the ARB binding toward agonism, but in the ground state the same mutations cause inverse agonism. Our data show that the second extracellular loop is an important regulator of the functional states of the AT1R. Our findings suggest that the quest for discovering novel ARBs, and improving current ARBs, fundamentally depends on the knowledge of the unique sets of residues that mediate inverse agonistic potency in the two states of the AT1R.

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.

Understanding the hysteresis loop conundrum in pharmacokinetic/pharmacodynamic relationships

Hysteresis loops are phenomena that sometimes are encountered in the analysis of pharmacokinetic and pharmacodynamic relationships spanning from pre-clinical to clinical studies. When hysteresis occurs it provides insight into the complexity of drug action and disposition that can be encountered. Hysteresis loops suggest that the relationship between drug concentration and the effect being measured is not a simple direct relationship, but may have an inherent time delay and disequilibrium, which may be the result of metabolites, the consequence of changes in pharmacodynamics or the use of a non-specific assay or may involve an indirect relationship. Counter-clockwise hysteresis has been generally defined as the process in which effect can increase with time for a given drug concentration, while in the case of clockwise hysteresis the measured effect decreases with time for a given drug concentration. Hysteresis loops can occur as a consequence of a number of different pharmacokinetic and pharmacodynamic mechanisms including tolerance, distributional delay, feedback regulation, input and output rate changes, agonistic or antagonistic active metabolites, uptake into active site, slow receptor kinetics, delayed or modified activity, time-dependent protein binding and the use of racemic drugs among other factors. In this review, each of these various causes of hysteresis loops are discussed, with incorporation of relevant examples of drugs demonstrating these relationships for illustrative purposes. Furthermore, the effect that pharmaceutical formulation has on the occurrence and potential change in direction of the hysteresis loop, and the major pharmacokinetic / pharmacodynamic modeling approaches utilized to collapse and model hysteresis are detailed.

Angiotensin II contractile effects in mouse colon: role for pre- and post-junctional AT(1A) receptors

AIM

This study investigates whether a local renin-angiotensin system (RAS) exists in mouse colon and whether angiotensin II (Ang II) may play a role in the regulation of the contractile activity.

METHODS

Isometric recordings were performed in vitro on the longitudinal muscle of mouse proximal and distal colon. Transcripts encoding for RAS components were investigated by RT-PCR.

RESULTS

Ang II caused, in both preparations, a concentration-dependent contractile effect, antagonized by losartan, AT(1) receptor antagonist, but not by PD123319, AT(2) receptor antagonist. The combination of losartan plus PD123319 caused no change on the Ang II-induced contraction than losartan alone. Tetrodotoxin, neural blocker, reduced the contractile response to Ang II in the proximal colon, whilst the response was abolished in the distal colon. In both preparations, atropine, muscarinic receptor antagonist, or SR140333, NK(1) receptor antagonist, reduced the Ang II responses. Ondansetron, 5-HT(3) receptor antagonist, SR48968, NK(2) receptor antagonist, or hexamethonium, nicotinic receptor antagonist, were ineffective. The joint application of atropine and SR140333 produced no additive effect. Atropine reduced NK(1) -induced contraction. Transcripts encoding RAS components were detected in the colon samples. However, just AT(1A) mRNA was expressed in both preparations, and AT(2) mRNA was expressed only in the distal colon.

CONCLUSION

In the murine colon, local RAS may play a significant role in the control of contractile activity. Ang II positively modulates the spontaneous contractile activity via activation of post-junctional and pre-junctional AT(1A) receptors, the latter located on the enteric neurones, modulating the release of tachykinins and acetylcholine.

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.

Small-molecule agonists for the glucagon-like peptide 1 receptor

The peptide hormone glucagon-like peptide (GLP)-1 has important actions resulting in glucose lowering along with weight loss in patients with type 2 diabetes. As a peptide hormone, GLP-1 has to be administered by injection. Only a few small-molecule agonists to peptide hormone receptors have been described and none in the B family of the G protein coupled receptors to which the GLP-1 receptor belongs. We have discovered a series of small molecules known as ago-allosteric modulators selective for the human GLP-1 receptor. These compounds act as both allosteric activators of the receptor and independent agonists. Potency of GLP-1 was not changed by the allosteric agonists, but affinity of GLP-1 for the receptor was increased. The most potent compound identified stimulates glucose-dependent insulin release from normal mouse islets but, importantly, not from GLP-1 receptor knockout mice. Also, the compound stimulates insulin release from perfused rat pancreas in a manner additive with GLP-1 itself. These compounds may lead to the identification or design of orally active GLP-1 agonists.

Molecular Mechanism Underlying Inverse Agonist of Angiotensin II Type 1 Receptor

To delineate the molecular mechanism underlying the inverse agonist activity of olmesartan, a potent angiotensin II type 1 (AT1) receptor antagonist, we performed binding affinity studies and an inositol phosphate production assay. Binding affinity of olmesartan and its related compounds to wild-type and mutant AT1 receptors demonstrated that interactions between olmesartan and Tyr113, Lys199, His256, and Gln257 in the AT1 receptor were important. The inositol phosphate production assay of olmesartan and related compounds using mutant receptors indicated that the inverse agonist activity required two interactions, that between the hydroxyl group of olmesartan and Tyr113 in the receptor and that between the carboxyl group of olmesartan and Lys199 and His256 in the receptor. Gln257 was found to be important for the interaction with olmesartan but not for the inverse agonist activity. Based on these results, we constructed a model for the interaction between olmesartan and the AT1 receptor. Although the activation of G protein-coupled receptors is initiated by anti-clockwise rotation of transmembrane (TM) III and TM VI followed by changes in the conformation of the receptor, in this model, cooperative interactions between the hydroxyl group and Tyr113 in TM III and between the carboxyl group and His256 in TM VI were essential for the potent inverse agonist activity of olmesartan. We speculate that the specific interaction of olmesartan with these two TMs is essential for stabilizing the AT1 receptor in an inactive conformation. A better understanding of the molecular mechanisms of the inverse agonism could be useful for the development of new G protein-coupled receptor antagonists with inverse agonist activity.

Molecular mechanism of 7TM receptor activation--a global toggle switch model

The multitude of chemically highly different agonists for 7TM receptors apparently do not share a common binding mode or active site but nevertheless act through induction of a common molecular activation mechanism. A global toggle switch model is proposed for this activation mechanism to reconcile the accumulated biophysical data supporting an outward rigid-body movement of the intracellular segments, as well as the recent data derived from activating metal ion sites and tethered ligands, which suggests an opposite, inward movement of the extracellular segments of the transmembrane helices. According to this model, a vertical see-saw movement of TM-VI-and to some degree TM-VII-around a pivot corresponding to the highly conserved prolines will occur during receptor activation, which may involve the outer segment of TM-V in an as yet unclear fashion. Small-molecule agonists can stabilize such a proposed active conformation, where the extracellular segments of TM-VI and -VII are bent inward toward TM-III, by acting as molecular glue deep in the main ligand-binding pocket between the helices, whereas larger agonists, peptides, and proteins can stabilize a similar active conformation by acting as Velcro at the extracellular ends of the helices and the connecting loops.

On the Mechanism of Interaction of Potent Surmountable and Insurmountable Antagonists with the Prostaglandin D2 Receptor CRTH2

Chemoattractant receptor-homologous molecule expressed on T helper 2 cells (CRTH2) has attracted interest as a potential therapeutic target in inflammatory diseases. Ramatroban, a thromboxane A2 receptor antagonist with clinical efficacy in allergic rhinitis, was recently found to also display potent CRTH2 antagonistic activity. Here, we present the pharmacological profile of three ramatroban analogs that differ chemically from ramatroban by either a single additional methyl group (TM30642), or an acetic acid instead of a propionic acid side chain (TM30643), or both modifications (TM30089). All three compounds bound to human CRTH2 stably expressed in human embryonic kidney 293 cells with nanomolar affinity. [3H]Prostaglandin D2 (PGD2) saturation analysis reveals that ramatroban and TM30642 decrease PGD2 affinity, whereas TM30643 and TM30089 exclusively depress ligand binding capacity (Bmax). Each of the three compounds acted as potent CRTH2 antagonists, yet the nature of their antagonism differed markedly. In functional assays measuring inhibition of PGD2-mediated 1) guanosine 5'-O-(3-thio)triphosphate binding, 2) beta-arrestin translocation, and 3) shape change of human eosinophils endogenously expressing CRTH2, ramatroban, and TM30642 produced surmountable antagonism and parallel rightward shifts of the PGD2 concentration-response curves. For TM30643 and TM30089, this shift was accompanied by a progressive reduction of maximal response. Binding analyses indicated that the functional insurmountability of TM30643 and TM30089 was probably related to long-lasting CRTH2 inhibition mediated via the orthosteric site of the receptor. A mechanistic understanding of insurmountability of CRTH2 antagonists could be fundamental for development of this novel class of anti-inflammatory drugs.

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].

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.

Structural biology of G protein-coupled receptors

More than 60% of the current drugs are based on G protein-coupled receptors. Paradoxically, high-resolution structures are not available to facilitate rational drug design. Difficulties in expression, purification, and crystallization of these transmembrane receptors are the reasons for the low success rate. Recent individual and network-based technology development has significantly improved our knowledge of structural biology and might soon bring a major breakthrough in this area.

“Network Leaning” as a Mechanism of Insurmountable Antagonism of the Angiotensin II Type 1 Receptor by Non-peptide Antagonists

A mechanistic understanding of the insurmountable antagonism of the angiotensin II type 1 (AT(1)) receptor could be fundamental in the quest for discovery and improvement of drugs. Candesartan and EXP3174 are competitive, reversible insurmountable antagonists of the AT(1) receptor. They contain di-acidic substitutions, whereas the surmountable antagonist, losartan, contains only one acidic group. We tested the hypothesis that these two classes of ligands interact with the AT(1) receptor through similar but not identical bonds and that the differences in the acid-base group contacts are critical for insurmountable antagonism. By pharmacological characterization of site-directed AT(1) receptor mutants expressed in COS1 cells we show that specific interactions with Gln(257) in transmembrane 6 distinguishes insurmountable antagonists and that abolishing these interactions transforms insurmountable to surmountable antagonism. In the Q257A mutant, the dissociation rate of [(3)H]candesartan is 2.8-fold more than the rate observed with wild type, and the association rate was reduced 4-fold lower than the wild type. The pattern of antagonism of angiotensin II concentration-response in the Q257A mutant pretreated with EXP3174 and candesartan is surmountable. We propose that leaning ability of insurmountable antagonists on Gln(257) in the wild-type receptor is the basis of an antagonist-mediated conformational transition, which is responsible for both slow dissociation and inhibition of maximal IP response.

Recognition of Privileged Structures by G-Protein Coupled Receptors

Privileged structures are ligand substructures that are widely used to generate high-affinity ligands for more than one type of receptor. To explain this, we surmised that there must be some common feature in the target proteins. For a set of class A GPCRs, we found a good correlation between conservation patterns of residues in the ligand binding pocket and the privileged structure fragments in class A GPCR ligands. A major part of interior surface of the common ligand binding pocket of class A receptors, identified in many GPCRs, is lined with variable residues that are responsible for selectivity in ligand recognition, while other regions, typically located deeper into the binding pocket, are more conserved and retain a predominantly hydrophobic and aromatic character. The latter is reflected in the chemical nature of most GPCR privileged structures and is proposed to be the common feature that is recognized by the privileged structures. Further, we find that this subpocket is conserved even in distant orthologs within the class A family. Three pairs of ligands recognizing widely different receptor types were docked into receptor models of their target receptors utilizing available structure- activity relationships and mutagenesis data. For each pair of ligands, the ligand-receptor complexes reveal that the nature of the privileged structure binding pocket is conserved between the two complexes, in support of our hypothesis. Only part of the privileged structures can be accommodated within the conserved subpocket. Some contacts are established between the privileged structure and the nonconserved parts of the binding pocket. This implies that any one particular privileged structure can target only a subset of receptors, those complementary to the full privileged structure. Our hypothesis leads to a valuable novelty in that ligand libraries can be designed without any foreknowledge of the structure of the endogenous ligand, which in turn means that even orphan receptors can in principle now be addressed as potential drug targets.

Structural determinants for the activation mechanism of the angiotensin II type 1 receptor differ for phosphoinositide hydrolysis and mitogen-activated protein kinase pathways

While the mechanism whereby the angiotensin II type 1 receptor (AT(1) receptor) activates its classical effector phospholipase C-beta (PLC-beta) has largely been elucidated, there is little consensus on how this receptor activates a more recently identified effector, the p42/44 mitogen-activated protein kinases (p42/44(MAPK)). Using transfected COS-1 cells, we investigated the activation of this signaling pathway at the receptor level itself. Previous mutational studies that relied on phosphoinositide turnover as an index of receptor activation have indicated that key residues in the second and seventh transmembrane domains participate in AT(1) receptor activation mechanisms. Thus, we introduced a variety of mutations-AT(1)[D74N], AT(1)[Y292F], AT(1)[N295S], and AT(1)[AT(2) TM7], which is composed of a chimeric substitution of the AT(1) seventh transmembrane domain with its AT(2) counterpart. These mutations that strongly diminished the receptor's ability to activate PLC-beta had little to no effect on its ability to activate p42/44(MAPK), which not only suggests that p42/44(MAPK) does not exclusively lie downstream of the G-protein G(q)/PLC-beta pathway but also indicates that more than one activation state may exist for the AT(1) receptor. The failure of a protein kinase C inhibitor to block AT(1) receptor activation of p42/44(MAPK) further corroborated evidence that the receptor's activation of p42/44(MAPK) is largely independent of the G(q)/PLC-beta/PKC pathway. Taken together, the experimental evidence strongly suggests that the mechanism whereby the AT(1) receptor activates p42/44(MAPK) is fundamentally different from that for PLC-beta, even at the level of the receptor itself.

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.

Conformational features of a synthetic model of the first extracellular loop of the angiotensin II AT1A receptor

The angiotensin II AT1A receptor belongs to the G-protein coupled receptors (GPCRs). Like other membrane proteins, GPCRs are not easily amenable to direct structure determination by the currently available methods. The peptide encompassing the putative first extracellular loop of AT1A (residues Thr88-Leu100, el1) has been synthesized along with a cyclic model where the linear peptide has been covalently linked to a template designed to keep the distance between the peptide termini as expected in the receptor. The conformational features of the two molecules have been studied using circular dichroism and NMR techniques. The region W94PFG97 forms a type-II beta-turn and undergoes a Trp-Pro peptide bond cis-trans isomerization in both peptides confirming that these characteristics are intrinsic to el1. In addition, the presence of the spacer seems to modulate the flexibility of the peptide.

Biophysical methods to study ligand-receptor interactions of neuropeptide Y

Neuropeptide Y (NPY) is a 36 amino acids peptide amide that was isolated for the first time almost 20 years ago from porcine brain. NPY displays a multiplicity of physiological effects that are transmitted by at least six G-protein coupled receptors (GPCRs) named Y(1), Y(2), Y(3), Y(4), Y(5), and y(6). Because of the difficulty in obtaining high-resolution crystallographic structures from GPCRs that all belong to seven transmembrane helices proteins, a variety of biophysical methods have been applied in order to characterize the interaction of ligand and receptor. In this review article we present the most relevant outcomes of the studies performed in this field by our group and others. The use of photoaffinity labeling allowed the molecular characterization of the Y(2) receptor. The concerted application of molecular modeling and mutagenesis studies led to a model for the interaction of the natural agonist and nonpeptide antagonists with the Y(1) receptor. The three-dimensional (3D) structure and dynamics of micelle-bound NPY and their implications for receptor selection have been studied by NMR. The characterization of the tertiary and quaternary structure of the NPY dimer in solution at millimolar concentrations has been performed by NMR and extended to physiologically relevant concentrations by fluorescence resonance energy transfer (FRET) experiments performed with fluorescence-labeled analogues.

Cross-talk between AT(1) and AT(2) angiotensin receptors in rat anococcygeus smooth muscle

Schild regressions for the selective AT(1) and AT(2) receptor antagonists, losartan and PD123319 (S-[+]-1-[(4-dimethylamino]-3-methylphenyl)methyl]-5-[diphenylacetyl]-4,5,6,7-tetrahydro-1H-imidazol[4,5-c]pyridine-6-carboxilic acid), respectively, were calculated to analyze the heterogeneity of receptor populations in the rat anococcygeus muscle. For a one-receptor system, the Schild regression has a slope of unity and an intercept of K(B) for competitive antagonists. However, in a two-receptor system, a deviation from the single-receptor plot will occur. This is predicated on the assumption that the secondary receptor is less sensitive to the antagonist than the primary receptor. Results showed that the Schild regression for losartan did not produce a slope of unity, and PD123319 did not produce any effect. However, tissue incubation with losartan plus PD123319 resulted in a Schild regression that has a slope of unity and a pK(B) of 9.32. In the presence of prazosin, an alpha(1)-adrenoceptor antagonist, losartan did not produce any effect. Conversely, PD123319 enhanced the angiotensin II (Ang II)-induced contraction in a concentration-dependent fashion, suggesting an inhibitory AT(2)-mediated effect. This effect was confirmed with assays that showed a relaxant response induced by Ang II on precontracted tissues incubated with prazosin. PD123319 and N(G)-nitro-L-arginine methyl ester [nitric-oxide (NO) synthase inhibitor)] markedly inhibited the relaxant response of Ang II. In contrast, losartan did not produce any significant effect. Consequently, results show that the mechanism underlying the AT(2)-mediated effect is highly dependent on NO generation. Results indicate the presence of a heterogeneous angiotensin receptor population in the rat anococcygeus muscle following a negative cross-talk relationship between the AT(1) and AT(2) subtypes.

In vitro and in vivo characterization of the activity of telmisartan: an insurmountable angiotensin II receptor antagonist

In vitro studies have shown that telmisartan is an insurmountable angiotensin II subtype-1 (AT1) receptor antagonist. Herein, the molecular basis of this insurmountable antagonism has been investigated in vitro, and the effect of telmisartan has been compared in vivo with that of irbesartan and candesartan. Association and dissociation kinetics of telmisartan to AT1 receptors have been characterized in vitro on rat vascular smooth muscle cells (RVSMC) expressing solely the AT1 receptor subtype. In a second set of experiments, the antagonistic efficacy of single intravenous doses (0.1, 0.3, and 1 mg/kg) of telmisartan was compared with that of irbesartan (0.3, 1.0, 3.0, and 10.0 mg/kg) and candesartan (0.3 and 1 mg/kg) in conscious, normotensive, male Wistar rats. The results show that the specific binding of [(3)H]telmisartan to the surface of living RVSMC is saturable and increases quickly to reach equilibrium within 1 h. Telmisartan dissociates very slowly from the receptor with a dissociation half-life (t(1/2)) of 75 min, which is comparable with candesartan and almost 5 times slower than angiotensin II (AngII). In vivo, telmisartan blunts the blood pressure response to exogenous AngII dose dependently. The blockade is long lasting and remains significant at 24 h at doses >0.1 mg/kg. Ex vivo assessment of the AT1 receptor blockade using an in vitro AngII receptor binding assay shows similar results. When administered intravenously in rats, telmisartan is 10-fold more potent than irbesartan and comparable to candesartan. Taken together, our in vitro data show that the insurmountable antagonism of telmisartan is due at least in part to its very slow dissociation from AT1 receptors.

Site-directed mutagenesis of the gerbil and human angiotensin II AT(1) receptors identifies amino acid residues attributable to the binding affinity for the nonpeptidic antagonist losartan

Gerbil angiotensin II AT(1) receptors have more than 90% amino acid sequence homology with human AT(1) receptors and similar affinity for the natural peptide agonist angiotensin II. However, their binding affinity for the biphenylimidazole AT(1) receptor antagonist losartan is greatly reduced compared with the hAT(1) receptor (400 times lower for the gAT(1A) receptor and 40 times lower for the gAT(1B) receptor cloned here). Gain- and loss-of-function site-directed mutagenesis revealed that in gerbil and human AT(1) receptors, the amino acid most important for losartan binding is located in position 108, followed by 107, both in transmembrane (TM) III. In both gerbil and human AT(1) receptors, the effect of G107S and I108V mutants is cumulative. Mutation L195M in TM V is very important, when combined with mutations G107S and I108V, for both gerbil and human AT(1) receptors. In the gerbil, less important amino acids are located in positions 150/151 (TM IV) and 177 in the extracellular loop 2. The study of gerbil natural mutants allowed us to advance our understanding of amino acids selectively involved in the determination of antagonist affinity for gerbil and, most importantly, for human angiotensin II AT(1) receptors.

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.

A two-state receptor model for the interaction between angiotensin II type 1 receptors and non-peptide antagonists

The interaction between non-peptide antagonists and the human angiotensin II type 1 (AT1) receptor in CHO-K1 cells was investigated by incubating the cells with antagonist, followed by a brief exposure to angiotensin II and measurement of the resulting inositol phosphate accumulation. The experimental data, expressed either as angiotensin II concentration-response curves or as antagonist concentration-inhibition curves, were in good agreement with computer-generated data according to a single-state model for the surmountable antagonist losartan and according to a two-step, two-state receptor model for the insurmountable antagonists candesartan, EXP3174, and irbesartan. Experimental and computer-generated data concerning the simultaneous exposure of the receptors to EXP3174 and losartan indicated that losartan produced a concentration-dependent restoration of the maximal response (angiotensin II concentration-response curves) as well as a rightward shift of the insurmountable portion of the EXP3174 inhibition curves, thus counteracting the higher-affinity EXP3174 binding. In conclusion, these findings provide further support for the concept that insurmountable and surmountable AT1 antagonists are mutually competitive and that insurmountable antagonist-receptor complexes may adopt different states.

Comparative analysis of amphibian and mammalian angiotensin receptors

Amphibian angiotensin receptors (xAT receptors) share many similarities with mammalian type 1 angiotensin receptors (AT(1) receptors). Both xAT and AT(1) receptors belong to the super family of seven transmembrane spanning G protein-coupled receptors and share approximately 60% amino acid homology. Highly stable secondary structure in the 5' leader sequences and the presence of the mRNA destabilizing sequence (AUUUA) in the 3' untranslated region (3'UTR) of the xAT and AT(1) receptor mRNAs suggest similar mechanisms exist for regulating gene expression. Amphibian and mammalian AT receptors bind angiotensin with equivalent affinities but show marked differences in their affinities towards mammalian AT(1) receptor subtype selective non-peptide ligands. Both xAT and AT(1) receptors couple to G proteins and to the phospholipase C (PLC) signal transduction pathway. Mammalian AT(1) receptors play a key role in maintaining blood pressure and fluid homeostasis and there is considerable evidence that xAT receptors play a similarly important role in amphibians. This review focuses on the comparison of amphibian xAT receptors with mammalian AT(1) receptors in terms of their structure, pharmacology, signaling, and function.

Does angiotensin II have a significant tonic action on cardiovascular neurons in the rostral and caudal VLM?

The peptidic ANG II receptor antagonists [Sar(1),Ile(8)]ANG II (sarile) or [Sar(1),Thr(8)]ANG II (sarthran) are known to decrease arterial pressure and sympathetic activity when injected into the rostral part of the ventrolateral medulla (VLM). In anesthetized rabbits and rats, the profound depressor and sympathoinhibitory response after bilateral microinjections of sarile or sarthran into the rostral VLM was unchanged after prior selective blockade of angiotensin type 1 (AT(1)) and ANG-(1---7) receptors, although this abolished the effects of exogenous ANG II. Unlike the neuroinhibitory compounds muscimol or lignocaine, microinjections of sarile in the rostral VLM did not affect respiratory activity. Sarile or sarthran in the caudal VLM resulted in a large pressor and sympathoexcitatory response, which was also unaffected by prior blockade of AT(1) and ANG-(1---7) receptors. The results indicate that the peptidic ANG receptor antagonists profoundly inhibit the tonic activity of cardiovascular but not respiratory neurons in the VLM and that these effects are independent of ANG II or ANG-(1---7) receptors.

Lys(199) mutation of the human angiotensin type 1 receptor differentially affects the binding of surmountable and insurmountable non-peptide antagonists

Many slow dissociating (insurmountable) non-peptide angiotensin type 1 receptor (AT1) antagonists contain,besides the acidic biphenyltetrazole substructure of losartan, a second acidic group to stabilise antagonist-receptor complexes. To investigate the involved basic amino-acids of the human AT1-receptor, wild-type and mutant receptors were transiently transfected in CHO-K1 cells and characterised by [3H]candesartan binding. Lys199-->Gln substitution decreased the affinity 45-fold for candesartan (95% insurmountable),18-fold for EXP3174 (70% insurmountable), 10-fold for irbesartan (40% insurmountable) and 5-fold for losartan (surmountable). His256 -->Ala substitution had only minor effects. This suggests that Lys199 is important for the tight binding of non-peptide antagonists.

Comparative pharmacodynamics and pharmacokinetics of candesartan and losartan in man

The angiotensin II antagonistic effects of candesartan and losartan were compared in-vivo after single and repeated doses. Effects were related to antagonistic activity in plasma. In this double-blind, crossover study, 12 healthy male volunteers received, in random order, daily oral doses of 8 mg candesartan cilexetil or 50 mg losartan for seven days. On day 1 and day 8, dynamics and kinetics were assessed up to 48 h after dosing. Antagonistic effect was determined from the antagonist-induced rightward shifts of the diastolic blood pressure response curves to exogenously administered angiotensin II measured as the dose ratio (DR). The antagonistic activity in plasma was measured using an ex-vivo/in-vitro radioreceptor assay. Specific high-performance liquid chromatography assays determined plasma concentrations of candesartan, losartan and its active metabolite EXP-3174. The pharmacokinetic properties of candesartan and losartan were comparable and antagonistic activity in plasma almost identical (ratio candesartan: losartan = 0.97 and 1-2 after single and multiple doses, respectively). However, the antagonistic effects of candesartan and losartan in-vivo were quite different. Twenty-four hours after single dosing with candesartan a clinically relevant rightward shift in the angiotensin II dose-response curve (DR= 3.2) occurred that was more pronounced than that following losartan administration (DR=2.1, ratio candesartan: losartan= 1.65). Twenty-four hours after multiple doses of candesartan or losartan, the values of the DR were 4.8 and 2.3, respectively (ratio candesartan: losartan = 1.94). The values of DR for candesartan were significantly higher compared with losartan between 6 and 36h after a single dose and between 3 and 24 h post-dose following multiple dose administration. A counter-clockwise hysteresis was apparent between antagonistic activity in plasma and antagonistic effect. Despite equivalent angiotensin II antagonistic activity in plasma, the pharmacodynamic effect of candesartan cilexetil was greater than that of losartan. Candesartan appeared to have a slower off-rate from the angiotensin AT1-receptor compared with losartan, nevertheless differences in distributional phenomena or the extent of insurmountable antagonistic activity cannot be ruled out.

Reversible and syntopic interaction between angiotensin receptor antagonists on Chinese hamster ovary cells expressing human angiotensin II type 1 receptors

Evidence for a competitive type of interaction between angiotensin II type 1 (AT(1)) antagonists on Chinese hamster ovary cells expressing the human AT(1) receptor (CHO-AT(1)) was obtained by analyzing the binding of [(3)H]-2-ethoxy-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]-1H- ben zimidazoline-7-carboxylic acid ([(3)H]candesartan) and by measuring the AT-induced production of inositol phosphates. The AT(1) antagonists candesartan, 2-n-butyl-4-chloro-1-[(2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]+ ++imid azole-5-carboxylic acid (EXP3174), or 2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)bip hen yl- 4-yl)methyl]imidazole (losartan) produced a concentration-dependent increase in the apparent K(d) values of [(3)H]candesartan in saturation binding experiments, while the B(max) values were unchanged. Furthermore, the dissociation rate of the radioligand initiated by 1 microM unlabelled candesartan was not changed in the presence of 10 microM losartan, 10 microM EXP3174, or 10 microM irbesartan (2-n-butyl-4-spirocyclopentane-1-[(2'-(1H-tetrazol-5-yl)b iph enyl-4-yl) methyl]2-imidazolin-5-one)). Preincubation of the CHO-AT(1) cells with candesartan, EXP3174, and irbesartan caused a reduction in the maximal AT-induced inositol mono-, bis-, and trisphosphate production. This insurmountable effect was reversed in the presence of 1 microM losartan. In line with this finding, the insurmountable antagonist concentration-inhibition curves at 10 microM AT were shifted to the right in the presence of losartan. For candesartan this effect was concentration-dependent, yielding a pK(B) value for losartan of 7.7, which is similar to the pK(B) from previously obtained AT concentration-response curves. Finally, the dissociation rate of candesartan, EXP3174, irbesartan, and losartan was determined by measuring the recovery of AT responses after antagonist pretreatment and washing of the cells with medium containing 1 microM losartan to prevent re-association of the insurmountable antagonists. In addition, similar kinetic data were obtained from the slowing of the [(3)H]candesartan association rate to antagonist preincubated cells.

Molecular characterization of the ligand-receptor interaction of the neuropeptide Y family

Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) belong to the NPY hormone family and activate a class of receptors called the Y-receptors, and also belong to the large superfamily of the G-protein coupled receptors. Structure-affinity and structure-activity relationship studies of peptide analogs, combined with studies based on site-directed mutagenesis and anti-receptor antibodies, have given insight into the individual characterization of each receptor subtype relative to its interaction with the ligand, as well as to its biological function. A number of selective antagonists at the Y1-receptor are available whose structures resemble that of the C-terminus of NPY. Some of these compounds, like BIBP3226, BIBO3304 and GW1229, have recently been used for in vivo investigations of the NPY-induced increase in food intake. Y2-receptor selective agonists are the analog cyclo-(28/32)-Ac-[Lys28-Glu32]-(25-36)-pNPY and the TASP molecule containing two units of the NPY segment 21-36. Now the first antagonist with nanomolar affinity for the Y2-receptor is also known, BIIE0246. So far, the native peptide PP has been shown to be the most potent ligand at the Y4-receptor. However, by the design of PP/NPY chimera, some analogs have been found that bind not only to the Y4-, but also to the Y5-receptor with subnanomolar affinities, and are as potent as NPY at the Y1-receptor. For the characterization of the Y5-receptor in vitro and in vivo, a new class of highly selective agonists is now available. This consists of analogs of NPY and of PP/NPY chimera which all contain the motif Ala31-Aib32. This motif has been shown to induce a 3(10)-helical turn in the region 28-31 of NPY and is suggested to be the key motif for high Y5-receptor selectivity. The results of feeding experiments in rats treated with the first highly specific Y5-receptor agonists support the hypothesis that this receptor plays a role in the NPY-induced stimulation of food intake. In conclusion, the selective compounds for the different Y receptor subtypes known so far are promising tools for a better understanding of the physiological properties of the hormones of the NPY family and related receptors.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

Specific nonpeptide photoprobes as tools for the structural study of the angiotensin II AT(1) receptor

The aim of this work was to obtain photoactivatable nonpeptide antagonists of the angiotensin II AT(1) receptor. Based on structure-function relationships, two chemical structures as well as appropriate synthetic schemes were chosen as a frame for the design of radiolabeled azido probes. The feasibility of the strategy was first assessed by the synthesis of two tritiated ligands 21 and 22 possessing a high affinity for the AT(1) receptor and a low nonspecific binding to membrane or cell preparations. We then prepared two unlabeled azido derivatives 7 and 14 which retained a fairly high affinity for the AT(1) receptor. The latter compound proved to be suitable for receptor irreversible labeling and was prepared in its tritiated form 28. This tritiated azido nonpeptide probe displayed a K(d) value of 11.8 nM and a low nonspecific binding. It was suitable for specific and efficient covalent labeling of the recombinant AT(1A) receptor stably expressed in CHO cells. The electrophoretic pattern of the specifically labeled entity was strictly identical to that of purified receptor photolabeled with a biotinylated peptidic photoactivatable probe. This new tool should be useful for the mapping of the nonpeptide receptor binding site. These potential applications are discussed in light of the current knowledge of molecular mechanisms of G-protein coupled receptor activation and inactivation.

Regulation of angiotensin II type 1 (AT1) receptor function

The type 1 angiotensin receptor (AT1) mediates the important biological actions of the peptide hormone, angiotensin II (AngII), by activating an array of intracellular signaling pathways. The unique temporal arrangement and duration of AngII-stimulated signals suggests a hierarchy of post-AT1 receptor binding events that permits activation of selective effector pathways. Moreover, it predicts that the coupling of AT1 receptors is tightly regulated, allowing cells to differentiate acute responses from those requiring longer periods of stimulation. Recent studies have concentrated on delineating the molecular processes involved in modulating AT1 receptor activity. In addition to AT1 receptor modification (phosphorylation), trafficking (internalization and degradation) and interaction with regulatory intracellular proteins, other processes may include receptor dimerization, cross-regulation by other receptor systems, and receptor isomerization between activated and non-activated forms. This review focuses on recent advances in this area of research, highlighting directions for future investigation.

Molecular cloning and pharmacological characterization of an atypical gerbil angiotensin II type-1 receptor and its mRNA expression in brain and peripheral tissues

In the gerbil brain, most of the [125I]Sarcosine1-Angiotensin II binding sites are atypical, not sensitive to displacement with selective Angiotensin II AT1 and AT2 receptor ligands. A similar atypical binding profile exists in the gerbil kidney, where binding is highly expressed. We isolated a 2197 base pair clone from a gerbil kidney cDNA library which encodes a 359 amino acid protein with higher than 90% homology to other mammalian angiotensin II AT1 receptors. When expressed in COS-7 cells, stimulation by Angiotensin II of both the cloned gerbil receptor or the human AT1 receptor enhanced IP3 production to a similar degree. In COS-7 cells, the gerbil receptor also had a ligand affinity profile similar to that of the human AT1 receptor, but showed greatly reduced affinity for losartan (IC50=3480+/-174 nM). In the gerbil brain, in situ hybridization revealed receptor mRNA in circumventricular organs, selective hypothalamic, midbrain and brain stem areas, and in the hippocampus, where high mRNA expression was detected in the stratum pyramidale of the CA1 and CA2 subfields, and in the stratum granulosum of the dentate gyrus. The expression pattern of receptor mRNA corresponded well with that of atypical [125I]Sar1-Ang II binding. In situ hybridization and Southern blot experiments using riboprobes against the open reading frame and the 3'-untranslated region of the cloned gerbil Ang II receptor cDNA suggest that gerbils have, like other rodents, two AT1 receptor subtypes. The receptor mRNA distribution of the cloned gerbil Ang II receptor corresponds to the distribution of AT1A receptors described in other rodent species.

Dependence of AT1 angiotensin receptor function on adjacent asparagine residues in the seventh transmembrane helix

For several G protein-coupled receptors, amino acids in the seventh transmembrane helix have been implicated in ligand binding and receptor activation. The function of this region in the AT1 angiotensin receptor was further investigated by mutation of two conserved polar residues (Asn294 and Asn295) and the adjacent Phe293 residue. Analysis of the properties of the mutant receptors expressed in COS-7 cells revealed that alanine replacement of Phe293 had no major effect on AT1 receptor function. Substitution of the adjacent Asn294 residue with alanine (N294A) reduced receptor binding affinities for angiotensin II, two nonpeptide agonists (L-162,313 and L-163,491), and the AT1-selective nonpeptide antagonist losartan but not that for the peptide antagonist [Sar1, Ile8]angiotensin II. The N294A receptor also showed impaired G protein coupling and severely attenuated inositol phosphate generation. In contrast, alanine replacement of Asn295 decreased receptor binding affinities for all angiotensin II ligands but did not impair signal transduction. Additional substitutions of Asn295 with a variety of amino acids did not identify specific structural elements for ligand binding. These findings indicate that Asn295 is required for the integrity of the intramembrane binding pocket of the AT1a receptor but is not essential for signal generation. They also demonstrate the importance of transmembrane helices in the formation of the binding site for nonpeptide AT1 receptor agonists. We conclude that the Asn294 residue of the AT1 receptor is an essential determinant of receptor activation and that the adjacent Asn295 residue is required for normal ligand binding.

Mutagenesis and modeling of the neurotensin receptor NTR1. Identification of residues that are critical for binding SR 48692, a nonpeptide neurotensin antagonist

The two neurotensin receptor subtypes known to date, NTR1 and NTR2, belong to the family of G-protein-coupled receptors with seven putative transmembrane domains (TM). SR 48692, a nonpeptide neurotensin antagonist, is selective for the NTR1. In the present study we attempted, through mutagenesis and computer-assisted modeling, to identify residues in the rat NTR1 that are involved in antagonist binding and to provide a tentative molecular model of the SR 48692 binding site. The seven putative TMs of the NTR1 were defined by sequence comparison and alignment of bovine rhodopsin and G-protein-coupled receptors. Thirty-five amino acid residues within or flanking the TMs were mutated to alanine. Additional mutations were performed for basic residues. The wild type and mutant receptors were expressed in COS M6 cells and tested for their ability to bind 125I-NT and [3H]SR 48692. A tridimensional model of the SR 48692 binding site was constructed using frog rhodopsin as a template. SR 48692 was docked into the receptor, taking into account the mutagenesis data for orienting the antagonist. The model shows that the antagonist binding pocket lies near the extracellular side of the transmembrane helices within the first two helical turns. The data identify one residue in TM 4, three in TM 6, and four in TM 7 that are involved in SR 48692 binding. Two of these residues, Arg327 in TM 6 and Tyr351 in TM 7, play a key role in antagonist/receptor interactions. The former appears to form an ionic link with the carboxylic group of SR 48692, as further supported by structure-activity studies using SR 48692 analogs. The data also show that the agonist and antagonist binding sites in the rNTR1 are different and help formulate hypotheses as to the structural basis for the selectivity of SR 48692 toward the NTR1 and NTR2.

Role of the amino terminus in ligand binding for the angiotensin II type 2 receptor

Key amino terminal residues in type 1 (AT1) angiotensin II (AngII) receptors are not conserved within type 2 (AT2) receptors. We therefore characterized amino terminal mutants that are transiently expressed in COS-3 membranes. AT2 amino terminal deletion drastically reduced affinity for AngII, suggesting its importance for this subtype. AT1-AT2 amino terminal exchanges retained wild type AngII affinities (Kd ranging from 3-5 nM), indicating compensation despite substantial sequence dissimilarities. Finally, binding of AT2 selective ligands (CGP42112A and PD123319) was not dependent on the amino terminus.

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

The role of the external third of helix VI of the angiotensin II (AII) AT1 receptor for the interaction with its ligand and for the subsequent signal transduction was investigated by individually replacing residues 252-256 by Ala, and residues 259 or 261 by Tyr, and permanently transfecting the resulting mutants to Chinese hamster ovary (CHO) cells. Binding experiments showed no great changes in affinity of any of the mutants for AII, [Sar1]-AII, or [Sar1, Leu8]-AII, but the affinity for the nonpeptide antagonist DuP753 was significantly decreased. The inositol phosphate response to AII was remarkably decreased in mutants V254A, H256A, and F259Y. These results indicate that AT1 residues Val254, His256, and Phe259 are not involved in ligand binding but participate in signal transduction. Based in these results and in others from the literature, it is suggested that, in addition to the His256 imidazole ring, the Phe259 aromatic ring interacts with the AII's Phe8, thus contributing to the signal-triggering mechanism.