The conformational change responsible for AT1 receptor activation is dependent upon two juxtaposed asparagine residues on transmembrane helices III and VII

The non-biphenyl-tetrazole angiotensin AT1 receptor antagonist eprosartan is a unique and robust inverse agonist of the active state of the AT1 receptor

BACKGROUND AND PURPOSE

Conditions such as hypertension and renal allograft rejection are accompanied by chronic, agonist-independent, signalling by angiotensin II AT1 receptors. The current treatment paradigm for these diseases entails the preferred use of inverse agonist AT1 receptor blockers (ARBs). However, variability in the inverse agonist activities of common biphenyl-tetrazole ARBs for the active state of AT1 receptors often leads to treatment failure. Therefore, characterization of robust inverse agonist ARBs for the active state of AT1 receptors is necessary.

EXPERIMENTAL APPROACH

To identify the robust inverse agonist for active state of AT1 receptors and its molecular mechanism, we performed site-directed mutagenesis, competition binding assay, inositol phosphate production assay and molecular modelling for both ground-state wild-type AT1 receptors and active-state N111G mutant AT1 receptors.

KEY RESULTS

Although candesartan and telmisartan exhibited weaker inverse agonist activity for N111G- compared with WT-AT1 receptors, only eprosartan exhibited robust inverse agonist activity for both N111G- and WT- AT1 receptors. Specific ligand-receptor contacts for candesartan and telmisartan are altered in the active-state N111G- AT1 receptors compared with the ground-state WT-AT1 receptors, suggesting an explanation of their attenuated inverse agonist activity for the active state of AT1 receptors. In contrast, interactions between eprosartan and N111G-AT1 receptors were not significantly altered, and the inverse agonist activity of eprosartan was robust.

CONCLUSIONS AND IMPLICATIONS

Eprosartan may be a better therapeutic option than other ARBs. Comparative studies investigating eprosartan and other ARBs for the treatment of diseases caused by chronic, agonist-independent, AT1 receptor activation are warranted.

Distinctions between non-peptide angiotensin II AT1-receptor antagonists

A far-reaching understanding of the molecular action mechanism of AT1-receptor antagonists (AIIAs) was obtained by using CHO cells expressing transfected human AT 1-receptors. In this model, direct [3H]-antagonist binding and inhibition of agonist-induced responses (inositol phosphate accumulation) can be measured under identical experimental conditions. Whereas preincubation with a surmountable AIIA (losartan) causes parallel shifts of the angiotensin II (Ang II) concentration-response curve, insurmountable antagonists also cause partial (i.e., 30% for irbesartan, 50% for valsartan, 70% for EXP3174,) to almost complete (95% for candesartan) reductions of the maximal response. The main conclusions are that all investigated antagonists are competitive with respect to Ang II. They bind to a common or overlapping site on the receptor in a mutually exclusive way. Insurmountable inhibition is related to the slow dissociation rate of the antagonist-receptor complex (t 1/2 of 7 minutes for irbesartan, 17 minutes for valsartan, 30 minutes for EXP3174 and 120 minutes for candesartan). Antagonist-bound AT1-receptors can adopt a fast and a slow reversible state. This is responsible for the partial nature of the insurmountable inhibition. The long-lasting effect of candesartan, the active metabolite of candesartan cilexetil, in vascular smooth muscle contraction studies, as well as in in vivo experiments on rat and in clinical studies, is compatible with its slow dissociation from, and continuous recycling between AT1-receptors. This recycling, or `rebinding' takes place because of the very high affinity of candesartan for the AT1-receptor.

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.

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.

Construction of a yeast-based signaling biosensor for human angiotensin II type 1 receptor via functional coupling between Asn295-mutated receptor and Gpa1/Gi3 chimeric Gα

Angiotensin II (Ang II) type 1 receptor (AGTR1) is a G-protein-coupled receptor (GPCR). Its natural ligand, Ang II, is an important effector molecule controlling blood pressure and volume in the cardiovascular system, and is consequently involved in various diseases such as hypertension and heart failure. Thus, the signaling mediator, AGTR1, is a significant molecular target in medicinal and therapeutic fields. Yeast is a useful organism for sensing GPCR signaling because it provides a simplified version of the complicated machinery used by mammalian cells for signal transduction. Although yeast cells can successfully transmit a signal through a variety of human GPCRs expressed in the cell membrane, there have been no reports of the functional activation of AGTR1-mediated signaling in yeast cells. In the present study, we introduced a single mutation into human AGTR1 and used yeast-human chimeric Gα to exert the functional activation of AGTR1 in yeast cells. The engineered yeast cells expressing AGTR1 mutated at Asn295 and the chimeric Gα successfully transmitted the signal inside the yeast cells in response to Ang II peptide and its analogs (Ang III and Ang IV peptides) added to the assay medium. Further, we demonstrated that the autocrine Ang II peptide and its analog, produced and secreted by the engineered yeast cells, could by themselves promote AGTR1-mediated signaling. This means that screening for agonistic peptides with various sequences from a self-produced genetic library would be a viable strategy. Thus, the constructed yeast biosensor, integrating an Asn295-mutated AGTR1 receptor, will be valuable in the design of drugs to treat AGTR1-related diseases.

Constitutive activity in the angiotensin II type 1 receptor: discovery and applications

The pathophysiological actions of the renin-angiotensin system hormone, angiotensin II (AngII), are mainly mediated by the AngII type 1 (AT1) receptor, a GPCR. The intrinsic spontaneous activity of the AT1 receptor in native tissues is difficult to detect due to its low expression levels. However, factors such as the membrane environment, interaction with autoantibodies, and mechanical stretch are known to increase G protein signaling in the absence of AngII. Naturally occurring and disease-causing activating mutations have not been identified in AT1 receptor. Constitutively active mutants (CAMs) of AT1 receptor have been engineered using molecular modeling and site-directed mutagenesis approaches among which substitution of Asn(111) in the transmembrane helix III with glycine or serine results in the highest basal activity of the receptor. Transgenic animal models expressing the CAM AT1 receptors that mimic various in vivo disease conditions have been useful research tools for discovering the pathophysiological role of AT1 receptor and evaluating the therapeutic potential of inverse agonists. This chapter summarizes the studies on the constitutive activity of AT1 receptor in recombinant as well as physiological systems. The impact of the availability of CAM AT1 receptors on our understanding of the molecular mechanisms underlying receptor activation and inverse agonism is described.

Critical hydrogen bond formation for activation of the angiotensin II type 1 receptor

G protein-coupled receptors contain selectively important residues that play central roles in the conformational changes that occur during receptor activation. Asparagine 111 (N111(3.35)) is such a residue within the angiotensin II type 1 (AT(1)) receptor. Substitution of N111(3.35) for glycine leads to a constitutively active receptor, whereas substitution for tryptophan leads to an inactivable receptor. Here, we analyzed the AT(1) receptor and two mutants (N111G and N111W) by molecular dynamics simulations, which revealed a novel molecular switch involving the strictly conserved residue D74(2.50). Indeed, D74(2.50) forms a stable hydrogen bond (H-bond) with the residue in position 111(3.35) in the wild-type and the inactivable receptor. However, in the constitutively active mutant N111G-AT(1) receptor, residue D74 is reoriented to form a new H-bond with another strictly conserved residue, N46(1.50). When expressed in HEK293 cells, the mutant N46G-AT(1) receptor was poorly activable, although it retained a high binding affinity. Interestingly, the mutant N46G/N111G-AT(1) receptor was also inactivable. Molecular dynamics simulations also revealed the presence of a cluster of hydrophobic residues from transmembrane domains 2, 3, and 7 that appears to stabilize the inactive form of the receptor. Whereas this hydrophobic cluster and the H-bond between D74(2.50) and W111(3.35) are more stable in the inactivable N111W-AT(1) receptor, the mutant N111W/F77A-AT(1) receptor, designed to weaken the hydrophobic core, showed significant agonist-induced signaling. These results support the potential for the formation of an H-bond between residues D74(2.50) and N46(1.50) in the activation of the AT(1) receptor.

Design, synthesis, and docking studies of novel benzimidazoles for the treatment of metabolic syndrome

In addition to lowering blood pressure, telmisartan, an angiotensin (AT(1)) receptor blocker, has recently been shown to exert pleiotropic effects as a partial agonist of nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma). On the basis of these findings and docking pose similarity between telmisartan and rosiglitazone in PPAR gamma active site, two classes of benzimidazole derivatives were designed and synthesized as dual PPAR gamma agonist/angiotensin II antagonists for the possible treatment of metabolic syndrome. Compound 4, a bisbenzimidazole derivative showed the best affinity for the AT(1) receptor with a K(i) = 13.4 nM, but it was devoid of PPAR gamma activity. On the other hand 9, a monobenzimidazole derivative, showed the highest activity in PPAR gamma transactivation assay (69% activation) with no affinity for the AT(1) receptor. Docking studies lead to the designing of a molecule with dual activity, 10, with moderate PPARgamma activity (29%) and affinity for the AT(1) receptor (K(i) = 2.5 microM).

Analysis of transmembrane domains 1 and 4 of the human angiotensin II AT1 receptor by cysteine-scanning mutagenesis

The octapeptide hormone angiotensin II (AngII) exerts a wide variety of cardiovascular effects through the activation of the AT(1) receptor, which belongs to the G protein-coupled receptor superfamily. Like other G protein-coupled receptors, the AT(1) receptor possesses seven transmembrane domains that provide structural support for the formation of the ligand-binding pocket. Here, we investigated the role of the first and fourth transmembrane domains (TMDs) in the formation of the binding pocket of the human AT(1) receptor using the substituted-cysteine accessibility method. Each residue within the Phe-28((1.32))-Ile-53((1.57)) fragment of TMD1 and Leu-143((4.40))-Phe-170((4.67)) fragment of TMD4 was mutated, one at a time, to a cysteine. The resulting mutant receptors were expressed in COS-7 cells, which were subsequently treated with the charged sulfhydryl-specific alkylating agent methanethiosulfonate ethylammonium (MTSEA). This treatment led to a significant reduction in the binding affinity of TMD1 mutants M30C((1.34))-AT(1) and T33C((1.37))-AT(1) and TMD4 mutant V169C((4.66))-AT(1). Although this reduction in binding of the TMD1 mutants was maintained when examined in a constitutively active receptor (N111G-AT(1)) background, we found that V169C((4.66))-AT(1) remained unaffected when treated with MTSEA compared with untreated in this context. Moreover, the complete loss of binding observed for R167C((4.64))-AT(1) was restored upon treatment with MTSEA. Our results suggest that the extracellular portion of TMD1, particularly residues Met-30((1.34)) and Thr-33((1.37)), as well as residues Arg-167((4.64)) and Val-169((4.66)) at the junction of TMD4 and the second extracellular loop, are important binding determinants within the AT(1) receptor binding pocket but that these TMDs undergo very little movement, if at all, during the activation process.

The fifth transmembrane domain of angiotensin II Type 1 receptor participates in the formation of the ligand-binding pocket and undergoes a counterclockwise rotation upon receptor activation

The octapeptide hormone angiotensin II exerts a wide variety of cardiovascular effects through the activation of the angiotensin II Type 1 (AT(1)) receptor, which belongs to the G protein-coupled receptor superfamily. Like other G protein- coupled receptors, the AT(1) receptor possesses seven transmembrane domains that provide structural support for the formation of the ligand-binding pocket. The role of the fifth transmembrane domain (TMD5) was investigated using the substituted cysteine accessibility method. All of the residues within Thr-190 to Leu-217 region were mutated one at a time to cysteine, and after expression in COS-7 cells, the mutant receptors were treated with the sulfhydryl-specific alkylating agent methanethiosulfonate-ethylammonium (MTSEA). MTSEA reacts selectively with water-accessible, free sulfhydryl groups of endogenous or introduced point mutation cysteines. If a cysteine is found in the binding pocket, the covalent modification will affect the binding kinetics of the ligand. MTSEA substantially decreased the binding affinity of L197C-AT(1), N200C-AT(1), I201C-AT(1), G203C-AT(1), and F204C-AT(1) mutant receptors, which suggests that these residues orient themselves within the water-accessible binding pocket of the AT(1) receptor. Interestingly, this pattern of acquired MTSEA sensitivity was altered for TMD5 reporter cysteines engineered in a constitutively active N111G-AT(1) receptor background. Indeed, mutant I201C-N111G-AT(1) became more sensitive to MTSEA, whereas mutant G203C-N111G-AT(1) lost some sensitivity. Our results suggest that constitutive activation of AT(1) receptor causes an apparent counterclockwise rotation of TMD5 as viewed from the extracellular side.

Activation induces structural changes in the liganded angiotensin II type 1 receptor

The octapeptide hormone angiotensin II (AngII) binds to and activates the human angiotensin II type 1 receptor (hAT(1)) of the G protein-coupled receptor class A family. Several activation mechanisms have been proposed for this family, but they have not yet been experimentally validated. We previously used the methionine proximity assay to show that 11 residues in transmembrane domain (TMD) III, VI, and VII of the hAT(1) receptor reside in close proximity to the C-terminal residue of AngII. With the exception of a single change in TMD VI, the same contacts are present on N111G-hAT(1), a constitutively active mutant; this N111G-hAT(1) is a model for the active form of the receptor. In this study, two series of 53 individual methionine mutations were constructed in TMD I, II, IV, and V on both receptor forms. The mutants were photolabeled with a neutral antagonist, (125)I-[Sar(1),p-benzoyl-L-Phe(8)]AngII, and the resulting complexes were digested with cyanogen bromide. Although no new contacts were found for the hAT(1) mutants, two were found in the constitutively active mutants, Phe-77 in TMD II and Asn-200 in TMD V. To our knowledge, this is the first time that a direct ligand contact with TMD II and TMD V has been reported. These contact point differences were used to identify the structural changes between the WT-hAT(1) and N111G-hAT(1) complexes through homology-based modeling and restrained molecular dynamics. The model generated revealed an important structural rearrangement of several TMDs from the basal to the activated form in the WT-hAT(1) receptor.

Angiotensin II AT1 receptor constitutive activation: from molecular mechanisms to pathophysiology

Mutations activating the angiotensin II AT(1) receptor are important to identify and characterize because they give access to the activation mechanisms of this G protein coupled receptor and help to characterize the signaling pathways and the potential pathophysiology of this receptor. The different constitutively activated mutations of the AT(1) receptor are mostly localized in transmembrane domains (TM) and their characterization demonstrated that release of intramolecular constraints and movements among these TM are a necessary step for receptor activation. These mutations constitutively activate Gq linked signaling pathways, receptor internalization and maybe the G protein-independent signaling pathways. Expression of such mutations in mice is linked to hypertension and cardiovascular diseases, but such natural mutations have not been identified in human pathology.

Sartan-AT1 receptor interactions: in vitro evidence for insurmountable antagonism and inverse agonism

Sartans are non-peptide AT(1) receptor antagonists used to treat hypertension and related pathologies. Their effects on the G protein-dependent responses of angiotensin II (Ang II) were the same in vascular tissues and in isolated cell systems. All are competitive but, when pre-incubated, they act surmountably (only rightward shift of the Ang II concentration-response curve) or insurmountably (also decreasing the maximal response). Insurmountable behaviour reflects the formation of tight sartan-receptor complexes; it is often partial due to the co-existence of tight and loose complexes. Their ratio positively correlates with the dissociation half-life of the tight complexes and depends on the sartan: i.e. candesartan>olmesartan>telmisartan approximately equal EXP3174>valsartan>irbesartan>>losartan. When AT(1) receptors display sufficient basal activity (in case of receptor over-expression, mutation and, especially, tissue stretching) sartans may also act as inverse agonists. This rather affects long-term, G protein-independent hypertrophic responses leading to cardiovascular remodelling.

Mechanisms and functions of agonist-independent activation in the angiotensin II type 1 receptor

The angiotensin II (AngII) type 1 (AT(1)) receptor is a seven-transmembrane G protein-coupled receptor, and is involved in regulating the physiological and pathological process of the cardiovascular system. Systemically and locally generated AngII has agonistic action on AT(1) receptor, but recent studies have demonstrated that AT(1) receptor inherently shows spontaneous activity even in the absence of AngII. Furthermore, mechanical stress can activate AT(1) receptor by inducing conformational switch without the involvement of AngII, and induce cardiac hypertrophy in vivo. These agonist-independent activities of AT(1) receptor can be inhibited by inverse agonists, but not by neutral antagonists. Considerable attention has been directed to molecular mechanisms and clinical implications of agonist-independent AT(1) receptor activation, and inverse agonist activity emerges as an important pharmacological parameter for AT(1) receptor blockers that will improve efficacy and expand therapeutic potentials in cardiovascular medicine.

The second transmembrane domain of the human type 1 angiotensin II receptor participates in the formation of the ligand binding pocket and undergoes integral pivoting movement during the process of receptor activation

The octapeptide hormone angiotensin II (AngII) exerts a wide variety of cardiovascular effects through the activation of the angiotensin II type-1 (AT(1)) receptor, which belongs to the G protein-coupled receptor superfamily. Like other G protein-coupled receptors, the AT(1) receptor possesses seven transmembrane domains that provide structural support for the formation of the ligand-binding pocket. In order to identify those residues in the second transmembrane domain (TMD2) that contribute to the formation of the binding pocket of the AT(1) receptor, we used the substituted cysteine accessibility method. All of the residues within the Leu-70 to Trp-94 region were mutated one at a time to a cysteine, and, after expression in COS-7 cells, the mutant receptors were treated with the sulfhydryl-specific alkylating agent methanethiosulfonate-ethylammonium (MTSEA). MTSEA reacts selectively with water-accessible, free sulfhydryl groups of endogenous or introduced point mutation cysteines. If a cysteine is found in the binding pocket, the covalent modification will affect the binding kinetics of the ligand. MTSEA substantially decreased the binding affinity of D74C-AT(1), L81C-AT(1), A85C-AT(1), T88C-AT(1), and A89C-AT(1) mutant receptors, which suggests that these residues orient themselves within the water-accessible binding pocket of the AT(1) receptor. Interestingly, this pattern of acquired MTSEA sensitivity was altered for TMD2 reporter cysteines engineered in a constitutively active N111G-AT(1) receptor background. Indeed, mutant D74C-N111G-AT(1) became insensitive to MTSEA, whereas mutant L81C-N111G-AT(1) lost some sensitivity and mutant V86C-N111G-AT(1) became sensitive to MTSEA. Our results suggest that constitutive activation of the AT(1) receptor causes TMD2 to pivot, bringing the top of TMD2 closer to the binding pocket and pushing the bottom of TMD2 away from the binding pocket.

Angiotensin II type-1 receptor activation in the adult heart causes blood pressure-independent hypertrophy and cardiac dysfunction

AIMS

Sustained hypertension leads to cardiac hypertrophy that can progress, through pathological remodelling, to heart failure. Abnormality of the renin-angiotensin system (RAS) has been strongly implicated in this process. Although hypertrophy in human is an established risk factor independent of blood pressure (BP), separation of remodelling in response to local cues within the differentiated myocardium from that related to pressure overload is unresolved. This study aimed to clarify the role of local RAS activity, specifically in the adult heart, in modulating cardiac hypertrophy and pathological remodelling.

METHODS AND RESULTS

Transgenic mice with inducible cardiomyocyte-specific expression of a wild-type or N111G mutant form of the human angiotensin II (Ang II) type-1 receptor (hAT1R) were generated. The wild-type receptor is primarily stimulated by Ang II. In contrast, the N111G receptor can also be fully stimulated by the Ang II derivative, Ang IV, at levels that do not stimulate the wild-type receptor. The unique properties of these models were used to investigate the myocardial growth, remodelling and functional responses to hAT1R stimulation, specifically in adult cardiomyocytes, under normal conditions and following Ang IV infusion. Low-level expression of wild-type or N111G hAT1R at the cardiomyocyte membrane, from the onset of adolescence, induced enhanced myocyte growth and associated cardiac hypertrophy in the adult. This was not associated with change in resting BP or heart rate, measured by longitudinal telemetric analysis, and did not progress to pathological remodelling or heart failure. However, selective activation of cardiomyocyte-specific N111G receptors by Ang IV peptide infusion induced adverse ventricular remodelling within 4 weeks. This was characterized by increased interstitial fibrosis, dilatation of the left ventricle, and impaired cardiac function.

CONCLUSION

Low-level local AT1R activity in differentiated myocardium causes compensated cardiac hypertrophy, that is, increased myocardial mass but with the retention of normal function, whereas short-term increased stimulation induces cardiac dysfunction with dilatation, reduced ejection fraction, and increased fibrosis in the absence of change in systemic BP.

Structural impact of the E113Q counterion mutation on the activation and deactivation pathways of the G protein-coupled receptor rhodopsin

Disruption of an interhelical salt bridge between the retinal protonated Schiff base linked to H7 and Glu113 on H3 is one of the decisive steps during activation of rhodopsin. Using previously established stabilization strategies, we engineered a stabilized E113Q counterion mutant that converted rhodopsin to a UV-absorbing photoreceptor with deprotonated Schiff base and allowed reconstitution into native-like lipid membranes. Fourier-transform infrared difference spectroscopy reveals a deprotonated Schiff base in the photoproducts of the mutant up to the active state Meta II, the absence of the classical pH-dependent Meta I/Meta II conformational equilibrium in favor of Meta II, and an anticipation of active state features under conditions that stabilize inactive photoproduct states in wildtype rhodopsin. Glu181 on extracellular loop 2, is found to be unable to maintain a counterion function to the Schiff base on the activation pathway of rhodopsin in the absence of the primary counterion, Glu113. The Schiff base becomes protonated in the transition to Meta III. This protonation is, however, not associated with a deactivation of the receptor, in contrast to wildtype rhodopsin. Glu181 is suggested to be the counterion in the Meta III state of the mutant and appears to be capable of stabilizing a protonated Schiff base in Meta III, but not of constraining the receptor in an inactive conformation.

An ion-responsive motif in the second transmembrane segment of rhodopsin-like receptors

A L(M)xxxD(N, E) motif (x=a non-ionic amino acid residue, most frequently A, S, L or F; small capitals indicating a minor representation) is found in the second transmembrane (tm2) segment of most G-protein coupling metazoan receptors of the rhodopsin family (Rh-GPCRs). Changes in signal transduction, agonist binding and receptor cycling are known for numerous receptors bearing evolved or experimentally introduced mutations in this tm2 motif, especially of its aspartate residue. The [Na(+)] sensitivity of the receptor-agonist interaction relates to this aspartate in a number of Rh-GPCRs. Native non-conservative mutations in the tm2 motif only rarely coincide with significant changes in two other ubiquitous features of the rhodopsin family, the seventh transmembrane N(D)PxxY(F) motif and the D(E)RY(W,F) or analogous sequence at the border of the third transmembrane helix and the second intracellular loop. Native tm2 mutations with Rh-GPCRs frequently result in constitutive signaling, and with visual opsins also in shifts to short-wavelength sensitivity. Substitution of a strongly basic residue for the tm2 aspartate in Taste-2 receptors could be connected to a lack of sodium sensing by these receptors. These properties could be consistent with ionic interactions, and even of ion transfer, that involve the tm2 motif. A decrease in cation sensing by this motif is usually connected to an enhanced constitutive interaction of the mutated receptors with cognate G- proteins, and also relates to both the constitutive and the overall activity of the short-wavelength opsins.

Conformational complexity of G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) are remarkably versatile signaling molecules. Members of this large family of membrane proteins respond to structurally diverse ligands and mediate most transmembrane signal transduction in response to hormones and neurotransmitters, and in response to the senses of sight, smell and taste. Individual GPCRs can signal through several G-protein subtypes and through G-protein-independent pathways, often in a ligand-specific manner. This functional plasticity can be attributed to structural flexibility of GPCRs and the ability of ligands to induce or to stabilize ligand-specific conformations. Here, we review what has been learned about the dynamic nature of the structure and mechanism of GPCR activation, primarily focusing on spectroscopic studies of purified human beta2 adrenergic receptor.

Manifold active-state conformations in GPCRs: Agonist-activated constitutively active mutant AT1receptor preferentially couples to Gq compared to the wild-type AT1receptor

The angiotensin II type I (AT(1)) receptor mediates regulation of blood pressure and water-electrolyte balance by Ang II. Substitution of Gly for Asn(111) of the AT(1) receptor constitutively activates the receptor leading to Gq-coupled IP(3) production independent of Ang II binding. The Ang II-activated conformation of the AT1(N111G) receptor was proposed to be similar to that of the wild-type AT(1) receptor, although, various aspects of the Ang II-induced conformation of this constitutively active mutant receptor have not been systematically studied. Here, we provide evidence that the conformation of the active state of the wild-type and the constitutively active AT(1) receptors are different. Upon Ang II binding an activated conformation of the wild-type AT(1) receptor activates G protein and recruits beta-arrestin. In contrast, the agonist-bound AT1(N111G) mutant receptor preferentially couples to Gq and is inadequate in beta-arrestin recruitment.

Activation of the Angiotensin II Type 1 Receptor Leads to Movement of the Sixth Transmembrane Domain: Analysis by the Substituted Cysteine Accessibility Method

The role of transmembrane domain six (TMD6) of the angiotensin II type 1 receptor, which is predicted to undergo conformational changes after agonist binding, was investigated using the substituted-cysteine accessibility method. Each residue in the Lys240-Leu265 fragment was mutated, one at a time, to a cysteine. The resulting mutants were expressed in COS-7 cells, which were subsequently treated with the charged sulfhydryl-specific alkylating agent methanethiosulfonate-ethylammonium (MTSEA). This treatment led to a significant reduction in binding of (125)I-[Sar(1),Ile(8)]AngII to the F249C, H256C, T260C, and V264C mutant receptors, suggesting that these residues orient themselves within the water-accessible binding pocket of the AT(1) receptor. It is noteworthy that this pattern of acquired MTSEA sensitivity was altered for TMD6 cysteines engineered in a constitutively active AT(1) receptor. Indeed, mutant F249C was insensitive to MTSEA treatment, whereas the sensitivity of mutant V264C decreased. Under these conditions, one other mutant, F261C, was found to be sensitive to MTSEA treatment. Our results suggest that constitutive activation of the AT(1) receptor causes TMD6 to pivot. This movement moves the top (extracellular side) of TMD6 toward the binding pocket and simultaneously distances the bottom (intracellular side) away from the binding pocket. Using this approach, we identified key elements within TMD6 that contribute to the activation of class A GPCRs through structural rearrangements.

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.

Activation of G protein-coupled receptors

G protein-coupled receptors (GPCRs) mediate responses to hormones and neurotransmitters, as well as the senses of sight, smell, and taste. These remarkably versatile signaling molecules respond to structurally diverse ligands. Many GPCRs couple to multiple G protein subtypes, and several have been shown to activate G protein-independent signaling pathways. Drugs acting on GPCRs exhibit efficacy profiles that may differ for different signaling cascades. The functional plasticity exhibited by GPCRs can be attributed to structural flexibility and the existence of multiple ligand-specific conformational states. This chapter will review our current understanding of the mechanism by which agonists bind and activate GPCRs.

Angiotensinergic stimulation of vascular endothelium in mice causes hypotension, bradycardia, and attenuated angiotensin response

It is not clear whether endothelial cell (EC) activation by the hormone angiotensin II (Ang II) modulates contraction of vascular smooth muscle cells (VSMCs) in the vasculature and whether impairment of this regulation in vivo contributes to hypertension. Delineation of the actions of Ang II through the type 1 receptor (AT1R) on ECs in the blood vessels has been a challenging problem because of the predominance of the AT1R functions in VSMCs that lie underneath the endothelium. We have obviated this limitation by generating transgenic (TG) mice engineered to target expression of the constitutively active N111G mutant AT1R only in ECs. In these TG mice, the enhanced angiotensinergic signal in ECs without infusion of Ang II resulted in hypotension and bradycardia. The pressor response to acute infusion of Ang II was significantly reduced. Increased expression of endothelial nitric oxide synthase and production of hypotensive mediators, nitric oxide and cyclic guanosine monophosphate, cause these phenotypes. Hypotension and bradycardia observed in the TG mice could be rescued by treatment with an AT1R-selective antagonist. Our results imply that the Ang II action by means of EC-AT1R is antagonistic to vasoconstriction in general, and it may moderate the magnitude of functional response to Ang II in VSMCs. This control mechanism in vivo most likely is a determinant of altered hemodynamic regulation involved in endothelial dysfunction in hypertensive cardiovascular disease.

Interactions between Conserved Residues in Transmembrane Helices 2 and 7 during Angiotensin AT1Receptor Activation

Site-directed mutagenesis studies and independent molecular modeling studies were combined to investigate the network of inter-residue interactions within the transmembrane region of the angiotensin AT(1a) receptor. Site-directed mutagenesis was focused on residues Tyr292, Asn294, Asn295, and Asn298 in transmembrane helix 7, and the conserved Asp74 in helix 2 and other polar residues. Functional interactions between pairs of residues were evaluated by determining the effects of single and double-reciprocal mutations on agonist-induced AT(1a) receptor activation. Replacement of Tyr292 by aspartate in helix 7 abolished radioligand binding to both Y292D and D74Y/Y292D mutant receptors. Reciprocal mutations of Asp74/Asn294, Ser115/Asn294, Ser252/Asn294, and Asn298/Sen115 caused additive impairment of function, suggesting that these pairs of residues make independent contributions to AT(1a) receptor activation. In contrast, mutations of the Asp74/Tyr298 pair revealed that the D74N/N298D reciprocal mutation substantially increased the impaired inositol phosphate responses of the D74N and N298D receptors. Extensive molecular modeling yielded 3D models of the TM region of the AT(1) receptor and the mutants as well as of their complexes with angiotensin II, which were used to rationalize the possible reasons of impairing of function of some mutants. These data indicate that Asp74 and Asn298 are not optimally positioned for direct strong interaction in the resting conformation of the AT(1a) receptor. Balance of interactions between residues in helix 2 (as D74) and helix 7 (as N294, N295 and N298) in the AT(1) receptors, however, has a crucial role both in determining their functional activity and levels of their expression.

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.

Identification of Structural Determinants for G Protein-Independent Activation of Mitogen-Activated Protein Kinases in the Seventh Transmembrane Domain of the Angiotensin II Type 1 Receptor

Although the intrareceptor mechanisms whereby the angiotensin II (AngII) type 1 receptor activates phospholipase C (PLC) have been extensively investigated, analogous studies of signaling through mitogen-activated protein kinases (MAPK) have been lacking. We investigated MAPK activation and traditional G(q)/PLC signaling in transfected cells using AngII and the signaling selective agonist [Sar(1),Ile(4),Ile(8)] AngII (SII). SII stimulated MAPK without inositol trisphosphate (IP(3)) production and thereby stabilizes an activated receptor state linked to G protein-independent MAPK signaling. Using receptor mutagenesis, we focused on the seventh transmembrane domain and identified three key residues-Tyr(292), Phe(293), and Thr(287). At least three distinct activated states were revealed: 1) an AngII-stabilized state linked to G(q)/PLC signaling, 2) an AngII-stabilized state connected to G protein-independent MAPK activation, and 3) a SII-stabilized state associated with G protein-independent MAPK signaling. The mutant Y292F failed to exhibit AngII-induced IP(3) turnover yet remained capable of AngII-induced MAPK activation. SII failed to stimulate MAPK in Y292F-transfected cells. Thus, Tyr(292) is a key epitope for activated states 1 and 3 but not required for activated state 2. Although the F293L mutant retained normal AngII responses, it also showed an IP(3) response to SII, indicating that Phe(293) may be involved in constraining the receptor to its inactive state. Mutations of Thr(287) abolished all SII-induced signaling without affecting any AngII responses. Thr(287) therefore represents a key residue for a SII-stabilized activated state. Taken together, the data identified a novel structural requirement (Thr(287)) for the SII-stabilized activated state and redefined the mechanistic roles for Tyr(292) and Phe(293).

Proposal of a New Binding Orientation for Non-Peptide AT1 Antagonists: Homology Modeling, Docking and Three-Dimensional Quantitative Structure−Activity Relationship Analysis

A three-dimensional model of the AT1 receptor was constructed by means of a homology modeling procedure, using the X-ray structure of bovine rhodopsin as the initial template and taking into account the available site-directed mutagenesis data. The docking of losartan and its active metabolite EXP3174, followed by 1 ns of molecular dynamics (MD) simulation inserted into the phospholipid bilayer, suggested a different binding orientation for these antagonists from those previously proposed. Furthermore, the docking of several non-peptide antagonists was used as an alignment tool for the development of a three-dimensional quantitative structure-activity relationship (3D-QSAR) model, and the good results confirmed our binding hypothesis and the reliability of the model.

Synthesis, binding studies and in vivo biological evaluation of novel non-peptide antihypertensive analogues

AT(1) antagonists (SARTANs) constitute the last generation of drugs for the treatment of hypertension, designed and synthesized to mimic the C-terminal segment of the vasoconstrictive hormone angiotensin II (AngII). They exert their action by blocking the binding of AngII on the AT(1) receptor. Up to date eight AT(1) antagonists have been approved for the regulation of high blood pressure. Although these molecules share common structural features and are designed to act under the same mechanism, they have differences in their pharmacological profiles and antihypertensive efficacy. Thus, there is still a need for novel analogues with better pharmacological and financial profiles. An example of a novel synthetic non peptide AT(1) antagonist which devoids the classical template of SARTANs is MM1. In vivo studies showed that MMK molecules, which fall in the same class of MM1, had a significant antihypertensive (40-80% compared to the drug losartan) activity. However, in vitro affinity studies showed that losartan has considerably higher affinity. The theoretical docking studies showed that MM1 acts on the same site of the receptor as losartan. They exert hydrophobic interactions with amino acid Val108 of the third helix of the AT(1) receptor and other hydrophobic amino acids in spatial vicinity. In addition, losartan favours multiple hydrogen bondings between its tetrazole group with Lys199. These additional interactions may in part explain its higher in vitro binding affinity.

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.

Conformational induction is the key process for activation of the AT1 receptor

It is currently unclear whether activation of the AT1 receptor by agonists involves conformational selection or induction. We evaluated the pharmacological properties of wild type and N111G CAM human AT1 receptors stably expressed in HEK293 cells. Although [Sar1]-Ang II and Ang IV were full agonists at both receptors, the potency of Ang IV was 280-fold lower at the wild type receptor. [Sar1, Ile8]-Ang II was only a full agonist at the N111G CAM AT1 receptor. [Sar1]-Ang II and [Sar1, Ile8]-Ang II displayed similar high affinity binding to both receptors. In contrast, Ang IV displayed low affinity binding to the wild type and high affinity binding to the N111G CAM AT1 receptor. Based on these observations we provide strong evidence that conformational induction is the key process for activation of the AT1 receptor. Only by the creation of CAMs can conformational selection be envisaged to take place.

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.

Constitutive activity of human angiotensin II type-1 receptors by Gq overexpression

We have developed an inducible HEK293/Tet-On cell line that transiently expresses both FLAG-tagged human angiotensin II type-I receptors (FLAG-hAT(1)R) and G(q)alpha G protein subunits in response to doxycycline. High and tightly regulated levels of FLAG-hAT(1)R (740+/-57 fmol/mg protein) and G(q)alpha (36-fold increase compared with non-induced cells) overexpression were consistently achieved. We investigated the possibility of using an inducible system to increase the proportion of constitutively active wild-type FLAG-hAT(1)Rs by overexpressing G(q)alpha. Following doxycycline treatment, we observed no significant change in the apparent binding affinity or potency (coupling efficiency) of angiotensin II, though significant increases in the intrinsic activity of several partial agonists were observed, indicative of constitutive activity. DUP753 (10 microM), a suggested inverse agonist, did not inhibit the enhanced level of basal (agonist-independent) activity. The data suggest that the resting equilibrium of hAT(1) receptors between the inactive (R) and active (R*) forms is predominantly weighted towards the inactive conformation.

Characterization of determinants of ligand binding to the nicotinic acid receptor GPR109A (HM74A/PUMA-G)

The G-protein-coupled receptor GPR109A (HM74A/PUMA-G) has recently been shown to function as a receptor for nicotinic acid (niacin) and to mediate its antilipolytic effects. Nicotinic acid is able to strongly raise plasma levels of high-density lipoprotein cholesterol, a property that distinguishes nicotinic acid from other lipid-lowering drugs. To investigate the structural determinants of GPR109A ligand binding, we performed site-directed mutagenesis of putative ligand binding residues combined with generation of chimeric receptors consisting of GPR109A and its close relative GPR109B, which does not bind nicotinic acid. We could identify Asn86/Trp91 [transmembrane helix (TMH) 2/extracellular loop (ECL) 1], Arg111 (TMH3), Ser178 (ECL2), Phe276 (TMH7), and Tyr284 (TMH7) as amino acid residues critical for binding of nicotinic acid. Together with data from molecular modeling studies, our data suggest that the ligand binding pocket for nicotinic acid of GPR109A is distinct from that of most other group A receptors. Although Arg111 at TMH3 serves as the basic anchor point for the carboxylate ligands, the ring system of nicotinic acid is embedded between Trp91 at the junction TMH2/ECL1 and Phe276/Tyr284 at TMH7. The heterocyclic ring is also bound to Ser178 at ECL2 via an H-bond. These data will facilitate the design of new antidyslipidemic drugs acting via GPR109A.

G protein-coupled receptors: a count of 1001 conformations

G protein-coupled receptors (GPCRs) were initially regarded to adopt an inactive and an active conformation and to activate a single type of G protein. Studies with recombinant cell systems have led to a more complex picture. First, GPCRs can activate distinct G protein species. Second, GPCR multistate models have been invoked to explain their complex behaviour in the presence of agonists, antagonists and other binding partners. The occurrence of intermediate receptor conformational states during GPCR activation and antagonist binding is suggested by fluorescence measurements and studies with constitutively active receptor mutants and insurmountable antagonists. Different agonists may trigger distinct effector pathways through a single receptor by dictating its preference for certain G proteins (i.e. 'agonist trafficking'). Structural modification and exogenous and endogenous (e.g. other cellular proteins, lipids) allosteric modulators also affect ligand-GPCR interaction and receptor activation. These new developments in GPCR research could lead to the development of more selective therapeutic drugs.

Constitutive activity of endogenous receptors by inducible Gq overexpression

We have developed an inducible cell line that transiently expresses Gq alpha G protein subunits in response to doxycycline. HEK293/Tet-On pBI(Gq alpha) cells worked consistently, achieving high and tightly regulated levels of Gq alpha overexpression (38-fold increase compared with non-induced cells). We investigated the possibility of using an inducible system to increase the proportion of constitutively active endogenously expressed G protein-coupled receptors (GPCRs) by overexpressing Gq alpha. Not only did we observe an increase in basal activity following doxycycline treatment, but also increased intrinsic activity of agonists such as carbachol, endothelin, lysophosphatidic acid (LPA), and bradykinin. Furthermore, carbachol and LPA potency increased following Gq alpha overexpression, as did the intrinsic activity of the partial agonist pilocarpine, observations indicative of constitutive activity. An inducible cell line, transiently expressing G proteins, can therefore be employed to induce constitutive activity of endogenously expressed GPCRs. This model system could be used to identify clinically important inverse agonists.

Single Mutations at Asn295and Leu305in the Cytoplasmic Half of Transmembrane α-Helix Domain 7 of the AT1Receptor Induce Promiscuous Agonist Specificity for Angiotensin II Fragments: A Pseudo-Constitutive Activity

The most striking feature of a G protein-coupled receptor (GPCR) is its highly exclusive agonist specificity. This feature guarantees that a GPCR recognizes only its specific native agonist(s). In this study, we showed that two point mutations of N295S and L305Q enabled the AT(1) receptors to recognize multiple Ang II fragments. Similar to the well established constitutively active AT(1) mutant receptor N111G, the mutations of N295S and L305Q induced an increased production of basal inositol 1,4,5-phosphates in the absence of exogenous Ang II when expressed in HEK293 cells. Distinct from the N111G, however, is the fact that the increased basal activity disappeared in COS-7 cells because of the lack of endogenous Ang II fragments produced by the cells-a pseudo-constitutive activity. It is surprising that the Ang II analog [Sar(1),Ile(4),Ile(8)]Ang II and the native angiotensin II fragments Ang 1-7, Ang IV, and Ang 5-8, which are inactive in activating the wild-type receptor, activated N295S and L305Q. Results generated by lowering the Na(+) concentration suggest that the mutant N295S and L305Q may be trapped in neutral conformational states (R(N)). These data allow us to identify for the first time a novel pattern of GPCR mutations with a broad spectrum of agonist specificity, suggesting possible existence of functional GPCRs in nature that are activated through conformational "selection" rather than "induction" mechanisms.

Ligand binding and functional properties of human angiotensin AT1 receptors in transiently and stably expressed CHO-K1 cells

Chinese Hamster Ovary Cells (CHO-K1) were transiently and stably transfected to express the human angiotensin AT(1) receptor. Cell surface receptor expression was maximal 2 days after transient transfection. Their pharmacological and signalling properties differed from stably expressed receptors. Receptor reserve was significant in the transient cells but not in stable cells, explaining the higher potency of angiotensin II and the lower degree of insurmountable inhibition by candesartan in the transient cells. [Sar(1)Ile(8)]angiotensin II (sarile) is a potent angiotensin AT(1) receptor antagonist for the stable cells but is a partial agonist, producing 19% of the maximal response by angiotensin II, in transient cells. Internalization of [(3)H]angiotensin II and [(125)I]sarile (i.e., acid-resistant binding) was more pronounced in stable cells. CHO-K1 cells were also transiently transfected with the enhanced green fluorescence-AT(1) receptor gene. Confocal microscopy revealed rapid internalization induced by angiotensin II and sarile but not by candesartan. The above disparities may result from differences in receptor maturation and/or cellular surrounding.

Analysis of the third transmembrane domain of the human type 1 angiotensin II receptor by cysteine scanning mutagenesis

Activation of G protein-coupled receptors by agonists involves significant movement of transmembrane domains (TMD) following agonist binding. The underlying structural mechanism by which receptor activation takes place is largely unknown but can be inferred by detecting variability within the environment of the ligand-binding pocket, which is a water-accessible crevice surrounded by the seven TMD helices. Using the substituted-cysteine accessibility method, we identified the residues within the third TMD of the wild-type angiotensin II (AT1) receptor that contribute to the formation of the binding site pocket. Each residue within the Ile103-Tyr127 region was mutated one at a time to a cysteine. Treating the A104C, N111C, and L112C mutant receptors with the charged sulfhydryl-specific alkylating agent methanethiosulfonate-ethylammonium (MTSEA) strongly inhibited ligand binding, which suggests that these residues orient themselves within the water-accessible binding pocket of the AT1 receptor. Interestingly, this pattern of acquired MTSEA sensitivity was altered for TMD3 reporter cysteines engineered in a constitutively active AT1 receptor. Indeed, two additional mutants (S109C and V116C) were found to be sensitive to MTSEA treatment. Our results suggest that constitutive activation of the AT1 receptor causes a minor counterclockwise rotation of TMD3, thereby exposing residues, which are not present in the inactive state, to the binding pocket. This pattern of accessibility of residues in the TMD3 of the AT1 receptor parallels that of homologous residues in rhodopsin. This study identified key elements of TMD3 that contribute to the activation of class A G protein-coupled receptors through structural rearrangements.

Down-regulation of inositol 1,4,5-trisphosphate receptor in cells stably expressing the constitutively active angiotensin II N111G-AT(1) receptor

The diverse cellular changes brought about by the expression of a constitutively active receptor are poorly understood. QBI-human embryonic kidney 293A cells stably expressing the constitutively active N111G-AT(1) receptor (N111G cells) showed elevated levels of inositol phosphates and frequent spontaneous intracellular Ca(2+) oscillations. Interestingly, Ca(2+) transients triggered with maximal doses of angiotensin II were much weaker in N111G cells than in wild-type cells. These blunted responses were observed independently of the presence or absence of extracellular Ca(2+) and were also obtained when endogenous muscarinic and purinergic receptors were activated, revealing a heterologous desensitization process. The desensitized component of the Ca(2+) signaling cascade was neither the G protein G(q) nor phospholipase C. The intracellular Ca(2+) store of N111G cells and their mechanism of Ca(2+) entry also appeared to be intact. The most striking adaptive response of N111G cells was a down-regulation of their inositol 1,4,5-trisphosphate receptor (IP(3)R) as revealed by reduced IP(3)-induced Ca(2+) release, lowered [(3)H]IP(3) binding capacity, diminished IP(3)R immunoreactivity, and accelerated IP(3)R degradation involving the lysosomal pathway. Treatment with the inverse agonist EXP3174 reversed the desensitized phenotype of N111G cells. Down-regulation of IP(3)R represents a reversible adaptive response to protect cells against the adverse effects of constitutively active Ca(2+)-mobilizing receptors.