Angiotensin I-converting enzyme acts primarily as a dipeptidyl carboxypeptidase and is involved in the metabolism of two major vasoactive peptides, angiotensin II and bradykinin. ACE is a membrane-bound enzyme and is orientated such that the catalytic sites are exposed at the extracellular surface of the cell. Due to the association of ACE with the plasma membrane of vascular endothelial cells, the enzyme displays an ubiquitous tissue distribution. The sequence identity between the N- and C-domains is more than 60% over a stretch of 357 amino acids. Each of the two homologous domains contain the consensus sequence found at the active sites of thermolysin, neutral endopeptidase and other zinc peptidases. ACE is the only zinc metallopeptidase known to date to possess two active sites. Some other enzymes contain two active sites and are truly bifunctional since they exhibit distinct functions as, for example, the brush-border hydrolases sucrase-isomaltase and lactase-phlorizin hydrolase.

Objective: Familial Hyperkalemic Hypertension (FHHt) is caused by mutations in WNK1, WNK4, KLHL3 or CUL3 (cullin-3). Patients with CUL3 mutation display a more severe phenotype. The mechanisms associated with this severity remain unclear. Design and method: All CUL3 mutations result in the skipping of exon 9. We have generated a mouse model of “Cul3-FHHt” by deleting Cul3 exon 9. Results: RT-PCR proved that the exon skipping occurred as expected in the kidney of Cul3+/d9 mice. They developed the classical FHHt features, which were all corrected by hydrochlorothiazide administration, a blocker of the Na+-Cl- cotransporter NCC. Accordingly, NCC expression and phosphorylation were increased in Cul3+/d9 mice. Cul3+/d9 mice are smaller and lighter than control littermates. Preliminary results suggest that this could be due to the hyperkalemia and/or metabolic acidosis. Cul3 serves as a scaffold for RING-type E3 ubiquitin-ligase complexes. It interacts with KLHL3, which recruits the substrates (such as WNK1/4) for ubiquitination. The consequences of exon 9 skipping on Cul3 activity are still debated. It could result in an increased degradation of KLHL3 and thus decreased recruitment and degradation of the substrates. However, the expression level of KLHL3 was similar in Cul3+/d9 and control mice. Conclusions: As in humans, the phenotype of Cul3-FHHt mice is more severe than that of the WNK1-FHHt mice we previously described. Two hypotheses have been proposed: a broader dysfunction of the distal nephron or an increased vascular reactivity. Further studies of the Cul3+/d9 mice are required to define the causes of this severity.

We conducted the present study to determine whether the angiotensin II type I receptor (AT,) gene might be implicated in human essential hypertension by using case-control and linkage studies. The entire coding and 3' untranslated regions of the AT, receptor gene (2.2 kb) were amplified by polymerase chain reaction and submitted to single-strand conformation polymorphism in 60 hypertensive subjects with a familial susceptibility. We identified five polymorphisms (T-»C, A-*G, A" ^C, G "^T, and A'-»G). However, no mutations that alter the encoded amino acid sequence were detected. A case-control study performed on white hypertensive (n=206; blood pressure, 168±16/103±9 mm Hg) and normotensive (n=298; blood pressure, 122±10/75±9 mm Hg) subjects using three of five polymorphisms showed a significant increase in allelic freHuman essential hypertension is thought to result from the interaction of environmental and genetic factors, with approximately 30% of the interindividual variability in blood pressure being genetically determined. The renin-angiotensin system is an important component of blood pressure regulation, playing roles in saltwater homeostasis and vascular tone, and has been suspected to be involved in hypertension. Indeed, evidence for a genetic linkage of human essential hypertension to the angiotensinogen locus was recently obtained in an extensive collaborative study. However, linkage and association studies of the human renin and angiotensin I (Ang I)-converting enzyme loci have given negative results. Ang II receptors, which mediate the vasoconstrictive and salt-conserving actions of the renin-angiotensin system, also represent interesting candidate genes for essential hypertension. Two subtypes of cell surface receptors have been identified (AT, and AT2) using ligand binding studies. In humans, the AT, receptor is present predominantly in vascular smooth muscle cells, and the AT2 receptor is present in the uterus, brain, and adrenal medulla. Both subtypes are also expressed in the adrenal cortex and kidney. The ATi receptor, through which are exerted most of the actions of Ang II, is a G protein-coupled receptor spanning seven transReceived January 31, 1994; accepted in revised form March 30, 1994. From INSERM U36 (A.B., E.D., X.J., I.F., A.C., E.C., P.C., F.S.), College de France; INSERM U258 (L.T.), Hopital Broussais; and INSERM SC7 (F.C.), Paris, France. Correspondence to Dr F. Soubrier, INSERM U36, College de France, 3 rue d'Ulm, 75005, Paris, France. © 1994 American Heart Association, Inc. quency of C" in hypertensive subjects (0.36 versus 0.28 for normotensive subjects, *-=6.8, P<.01). Frequencies for the alleles of the other two polymorphisms (T^C, A'^G) were similar in both groups. We performed a linkage study using the affected sib pair method and a highly polymorphic marker of the AT, receptor gene. There was no evidence for linkage in 267 sib pairs analyzed from 138 pedigrees. These findings would be compatible with a common variant of the AT, receptor imparting a small effect on blood pressure; further studies will be needed to address this possibility. (Hypertension. 1994;24:63-69.)

The renin-angiotensin-aldosterone system (RAAS) is central to cardiovascular and renal physiology. However, there is no animal model in which the activation of the RAAS only reflects the activation of the angiotensin II (ANG II) AT1 receptor. As a first step to developing such a model, we characterized a gain-of-function mutant of the mouse AT1A receptor. This mutant carries two mutations: N111S predicted to activate the receptor constitutively and a COOH-terminal deletion, delta329, expected to reduce receptor internalization and desensitization. We expressed this double mutant (AT1A-N111S/delta329) in heterologous cells. It showed a pharmacological profile consistent with that of other constitutively active mutants. Furthermore, it increased basal production of inositol phosphates, as well as basal cytosolic and nuclear ERK activities. Basal proliferation of cells expressing the mutant was also greater than that of the wild type. The double mutant was poorly internalized and failed to recruit beta-arrestin 2 in the presence of ANG II. It also showed hypersensitive and hyperreactive responses to ANG II for both inositol phosphate production and ERK activation. The additivity of the phenotypes of the two mutations makes this mutant an appropriate candidate to test the physiological consequences of the AT1A receptor activation itself in transgenic animal models.

Les techniques de biologie utilisées en endocrinologie sont indissociables de la clinique : dosage immunologique et biologie moléculaire sont aujourd'hui réalisés en routine. En comprendre les principes est essentiel pour une bonne interprétation des résultats. Les outils de pharmacologie sont utiles en endocrinologie afin de caractériser le devenir d'une hormone dans l'organisme après sa sécrétion, et l'action de cette hormone sur ses cibles.

Les récepteurs hormonaux membranaires sont répartis en trois grandes familles : les récepteurs couplés aux protéines G, les récepteurs tyrosine kinase et les récepteurs des cytokines. Chacune de ces familles est caractérisée par des propriétés structurales communes et des mécanismes de transduction du signal particuliers.

As constitutively active mutants (CAMs) mimic an active conformation, they can be used to characterize the process of G protein-coupled receptor activation. Here, we used CAMs to study the link between activation and internalization of the angiotensin II AT1A receptor. The cellular localization of fluorescently tagged N111A, I245T, and L305Q mutants was determined by confocal microscopy. In the absence of ligand, CAMs were mostly located in intracellular vesicles, whereas the wild-type AT1A was found at the cell surface. After 2 h incubation with inverse agonist, losartan, CAMs were translocated to the plasma membrane. Similar observations were made in H295, a human adrenocortical cell line which expresses physiologically the AT1 receptor. This phenomenon, which was not dependent on protein synthesis and the pharmacology and kinetics of which were similar to the recycling of the wild-type receptor, was called “externalization”. After externalization and losartan removal, the L305Q CAM underwent rapid ligand-independent endocytocis, with the same kinetics and temperature sensitivity as the angiotensin II-induced internalization of the wild-type AT1A. Moreover, the addition of a second mutation known to block internalization (Δ329 truncation) prevented intracellular localization of the CAM. These data show that AT1A CAMs are constitutively and permanently internalized and recycled. This mechanism is different from the down-regulation observed for CAMs of other G protein-coupled receptors and thus defines a new paradigm for the cellular regulation of CAMs.

In the past decade, the concept of constitutive activity has profoundly modified our understanding of G protein-coupled-receptors (GPCRs). Here, we review the contribution of constitutively active mutants (CAMs) to our understanding of three aspects of GPCR physiopathology: (1) GPCR activation is a complex mechanism involving both the release of inactive state conformational constraints, mimicked by most CAMs, and the creation of new interactions that stabilize the active state and are mimicked by a restricted set of CAMs; (2) GPCR phosphorylation, internalization and desensitization processes are activated by receptor conformations, which partly overlap those activating G protein; (3) natural CAMs, mostly affecting GPCRs of the endocrine system, are found in several hereditary and acquired diseases, including cancers. One major remaining question is how CAMs recapitulate the different structural modifications of the agonist-induced active conformation(s) of the wild-type receptor. This characterization is a prerequisite for further use of CAMs as ligand-free models of active GPCRs in structural, cellular and physiological studies.

The angiotensin II (Ang II) AT(1A) receptor was tagged at its C terminus with the enhanced green fluorescent protein (EGFP), and the corresponding chimeric cDNA was expressed in HEK-293 cells, This tagged receptor presents wild-type pharmacological and signaling properties and can be immunodetected by Western blotting and immunoprecipitation using EGFP antibodies, Therefore, this EGFP-tagged AT(1A) receptor is the perfect tool for analyzing in parallel the subcellular distributions of the receptor and its interacting G protein and their trafficking using confocal microscopy. Morphological observation of both the fluorescent receptor and its cognate G alphaq/11 protein, identified by indirect immunofluorescence, and the development of a specific software for digital image analysis together allow examination and quantification of the cellular distribution of these proteins before and after the binding of different agonist or antagonist ligands, These observations result in several conclusions: 1) Expression of increasing amounts of the AT(1A) receptor at the cell surface is associated with a progressive recruitment of the cytosolic G alphaq/11 protein at the membrane; 2) Internalization of the EGFP-tagged AT(1A) induced by peptide ligands but not nonpeptide ligands is accompanied by a G alphaq/11 protein intracellular translocation, which presents a similar kinetic pattern but occurs predominantly in a different compartment; and 3) This G alphaq/11 protein cellular translocation is dependent on receptor internalization process, but not G protein coupling and signal transduction mechanisms, as assessed by pharmacological data using agonists and antagonists and the characterization of AT(1A) receptor mutants ((DN)-N-74 and Delta 329) for which the coupling and internalization functions are modified.

The constitutive activation of G-protein-coupled receptors is a major new approach to investigating their physiopathology and pharmacology. A large number of spontaneous and site-directed mutations resulting in constitutive activity have been identified, but systematic mapping of the amino acids involved for a given receptor would be extremely useful for complete elucidation of the molecular mechanisms underlying its activation. We carried out such mapping for the angiotensin II type 1A (AT(1A)) receptor by screening a randomly mutated cDNA library after expressing the mutated clones in eukaryotic cells. To test the AT(1A) mutants generated, we developed an original, specific, and highly sensitive assay based on the properties of CGP42112A. This classical AT(2) agonist is a weak partial agonist of the wild-type AT(1A) receptor and becomes a full agonist for constitutively active AT(1A) mutants, as shown experimentally and in allostery-based theoretical models. Activation of the mutated receptors by CGP42112A was monitored by using the bioluminescent protein aequorin, a very sensitive and specific sensor of intracellular calcium mobilization. The screening of 4,800 clones, providing an exhaustive coverage of all of the mutations generated, led to the identification of 16 mutations in sequences encoding the transmembrane domains that were responsible for high sensitivity to CGP42112A. The constitutive activity was confirmed by agonist-independent production of inositol phosphates, which showed that at least half of the clones had significantly increased basal activity. These data demonstrate that this new type of approach is very efficient for the systematic identification of constitutively active mutants of G-protein-coupled receptors.

The gene of the mouse V3/V1b receptor was identified by homology cloning. One of the genomic clones contained the entire coding sequence. The cDNA presented high identity with rat (92%) and human (84%) sequences. Southern blot analysis indicated the existence of a single gene. Tissue distribution was studied by RT-PCR. The major site of expression was the pituitary. A faint signal was also present in hypothalamus, brain, adrenal, pancreas and colon. The mouse corticotroph cell line, AtT20, did not express the transcript. In order to confirm the identity of the sequence, the V3/V1b receptor cDNA was cloned and stably expressed in CHO-AA8 Tet-Off cells under the control of tetracycline. When transfected cells were treated with arginine vasopressin (AVP), inositol phosphate production increased in a dose-dependent manner, indicating that the V3/V1b receptor couples to phospholipase C. Moreover, AVP did not stimulate cAMP production. Binding studies with [3H]AVP indicated that the affinity of the mouse V3/V1b receptor (Kd=0.5 nM) is similar to that reported for rat and human receptors. The rank order of potency established in competition binding experiments with different analogues was representative of a V3/V1b profile, distinct from V1a and V2. However, significant differences were found between human and mouse receptors tested in parallel. Thus the pharmacology of V3/V1b receptors can not be transposed among different species.

The angiotensin II (AngII) AT1 receptor is a seven-transmembrane domain receptor coupled to a Gq/11 protein and phospholipase C, but also to other G proteins and to several tyrosine kinase pathways. These signaling pathways transduce inside the cells the classical actions of AngII (vasoconstriction, aldosterone secretion, etc.), but also the mitogenic action of this vasoactive peptide. In the past 5 yr, site-directed mutagenesis has elucidated the molecular determinants of the AngII and nonpeptidic analogue-binding sites together with those of G protein interaction. In addition, these studies have demonstrated that modifications of the specific interactions between transmembrane domains are responsible for the activation of the receptor. Therefore, several mutations of these domains are able to block the receptor in active or inactive states. Finally, these mutagenesis studies identify two interesting phenotypes of the AT1 receptor. (1) A carboxy-terminal truncation of the AT1 receptor produces a mutant that is unable to be internalized and desensitized and therefore is functionally hyper-reactive. (2) A replacement of the distal part of the third intracellular loop of the AT1 receptor by the homologous segment of the beta2-adrenergic receptor produces a mutant coupled to both Gq and Gs proteins, which is unable to transduce the mitogenic action of AngII.

To study the role of transmembrane (TM) domains interactions in the activation of the insulin receptor, we have replaced the insulin receptor TM domain with that of glycophorin A (GpA), an erythrocyte protein that spontaneously forms detergent-resistant dimers through TM-TM interactions. Insulin receptor cDNA sequences with the TM domain replaced by that of GpA were constructed and stably transfected in CHO cells. Insulin binding to cells and solubilized receptors was not modified. Electrophoresis after partial reduction of disulfide bonds revealed an altered structure for the soluble chimeric receptors, seen as an altered mobility apparently due to increased interactions between the beta subunits of the receptor. Insulin signaling was markedly decreased for cells transfected with chimeric receptors compared with cells transfected with normal receptors. A decrease in insulin-induced receptor kinase activity was observed for solubilized chimeric receptors. In conclusion, substitution by the native GpA TM domain of the insulin receptor results in structurally modified chimeric receptors that are unable to transmit the insulin signal properly. It is hypothesized that this substitution may impose structural constraints that prevent the proper changes in conformation necessary for activation of the receptor kinase. Other mutants modifying the structure or the membrane orientation of the glycophorin A TM domain are required to better understand these constraints.

The structural determinants of the rat angiotensin (ANG) II AT1A receptor involved in receptor internalization, desensitization, and activation are investigated by producing six mutants that had progressively larger deletions of the cytoplasmic tail (-13, -19, -24, -31, -46, and -56 residues, respectively). After stable transfection of the cDNAs into Chinese hamster ovary cells, all mutants, except the most truncated, exhibit normal [Sar1]ANG II affinities [dissociation constant (Kd) = 0.19-0.70 nM] compared with the wild-type (WT) receptor (Kd = 0.62 nM) and are able to activate a Gq/11 protein and a phospholipase C as measured by the ANG II-induced inositol phosphate (IP) turnover in the different clones. However, one of these mutants, delta 329 (deletion of 31 residues), exhibits a peculiar phenotype. This mutant shows a reduced ligand-induced internalization as measured by the acid-washing procedure (only 32% of receptors are internalized vs. 83% for WT). Moreover, the delta 329 mutant is less desensitized by a pretreatment with either ANG II (15% desensitization of ANG II-stimulated IP turnover vs. 60% for WT receptor) or the phorbol ester phorbol 12-myristate 13-acetate (no desensitization vs. 29% for WT receptor). These functional modifications of the delta 329 mutant are associated with the transduction of an amplified signal as demonstrated on both IP turnover and an integrated physiological effect of ANG II. Taken together, these data indicate that the sequence 329SLSTKMS335 of the rat AT1A receptor is involved in both receptor internalization and desensitization. This is the first demonstration that a desensitization- and internalization-defective AT1A receptor mutant is also hyperreactive and mediates augmented cellular responses.

We have examined the expression profiles of the different vasopressin receptors (V1, V2, V3) that can be expressed in the three different types of tumors associated with Cushing's syndrome. V3 (V1b) receptor cDNA was cloned from a pituitary tumor responsible for Cushing's disease. We show that it is overexpressed in these tumors and can respond to DD-AVP. High expression of the V3 receptor on highly differentiated, ACTH-secreting, bronchial carcinoid tumors explain why these non-pituitary tumors occasionally respond to vasopressin, mimicking a "pituitary-like" behavior. A retrospective analysis showed that vasopressin induced an ACTH-independent cortisol rise in 27% of the adrenocortical tumors responsible for Cushing's syndrome. V1 mRNA was detected in normal adrenal cortex and in all tumors. Adenomas had significantly higher levels than carcinomas. V1 mRNA levels were higher in responders than in non-responders. One adenoma which had a brisk cortisol response in vivo, also had in vitro cortisol responses that were inhibited by a specific V1 antagonist. In situ hybridization showed the presence of V1 mRNA in the normal human adrenal cortex where the signal predominated in the compact cells of the zona reticularis. A positive signal was also present in the tumors with high V1 mRNA levels determined by RT-PCR; its distribution pattern was heterogeneous and showed preferential association with compact cells. High-and not ectopic-expression of the V1 receptor occurs in a minority of adrenal cortical tumors which become directly responsive to vasopressin stimulation.

The V2 vasopressin and the AT1A angiotensin II receptors are respectively coupled to the adenylyl cyclase and the phosphoinositide pathways. The cross-talk between these two receptors and their transduction pathways were investigated in CHO cells transfected with cDNA of both AT1A and V2 receptors. In these cells, angiotensin II induced an increase in intracellular calcium, and vasopressin a rise in intracellular cAMP accumulation. The simultaneous addition of angiotensin II and vasopressin potentiated the production of cAMP by the V2 receptor. This potentiation was dose-dependent and, at a concentration of 10(-7) M angiotensin II, the accumulation of cAMP was 4-fold greater than that induced by 10(-7) M vasopressin alone. Such cross-talk occurred in the presence and absence of cyclic nucleotide phosphodiesterase inhibitors, indicating that inhibition of phosphodiesterase activity was not the principal cause of potentiation. This was confirmed by the absence of calcium-inhibitable isoforms of phosphodiesterases in CHO cells. The addition of angiotensin II to forskolin, which stimulates the adenylyl cyclase, did not modify the production of cAMP. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), partially mimicked, and staurosporine, an inhibitor of PKC, partially inhibited the effect of angiotensin II on vasopressin. Chelation of intracellular calcium with BAPTA-AM markedly reduced the potentiation of V2 receptor by angiotensin II. However, increase in intracellular calcium with thapsigargin did not modify the cAMP accumulation induced by vasopressin. It was concluded that, in CHO cells, activation of the AT1A receptor by angiotensin II potentiates the V2 receptor through activation of protein kinase C in the presence of intracellular calcium at a step located between the receptor and the adenylyl cyclase.

We cloned by interaction with the β-subunit of the insulin receptor the rat variant of the human adapter Grb14 (rGrb14). rGrb14 is specifically expressed in rat insulin-sensitive tissues and in the brain. The binding of rGrb14 to insulin receptors is insulin-dependent in vivo in Chinese hamster ovary (CHO) cells overexpressing both proteins and importantly, in rat liver expressing physiological levels of proteins. However, rGrb14 is not a substrate of the tyrosine kinase of the receptor. In the two-hybrid system, two domains of rGrb14 can mediate the interaction with insulin receptors: the Src homology 2 (SH2) domain and a region between the PH and SH2 domains that we named PIR (forphosphorylated insulin receptor-interactingregion). In vitro interaction assays using deletion mutants of rGrb14 show that the PIR, but not the SH2 domain, is able to coprecipitate insulin receptors, suggesting that the PIR is the major binding domain of rGrb14. The interaction between rGrb14 and the insulin receptors is almost abolished by mutating tyrosine residue Tyr1150 or Tyr1151 of the receptor. The overexpression of rGrb14 in CHO-IR cells decreases insulin stimulation of both DNA and glycogen synthesis. These effects are accompanied by a decrease in insulin-stimulated tyrosine phosphorylation of IRS-1, but insulin receptor autophosphorylation is unaltered. These findings suggest that rGrb14 could be a new downstream signaling component of the insulin-mediated pathways.