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

Presence of functional angiotensin II receptor and angiotensin converting enzyme in the aorta of the snake Bothrops jararaca

AIM

Angiotensin II (Ang II) interacts with AT(1) and AT(2) receptors and, in some vertebrates, with an Ang II binding site showing low affinity for AT(1) and AT(2) receptor antagonists. This study was carried out to characterize the Ang II receptor, and the presence of an angiotensin-converting enzyme (ACE) in the aorta of the Bothrops jararaca snake.

MAIN METHOD

Contraction induced by Ang I or II in aortic ring from the snake was evaluated in the absence or in the presence of ACE-blocker or Ang II antagonists.

KEY FINDINGS

Ang II analogs, modified at positions 1 and 5, induced vasoconstriction with differences in their potencies. The relative rank order was: [Asp(1), Val(5)] Ang II=[Asp(1), Ile(5)] Ang II>>>[Asn(1), Val(5)] Ang II. ACE-like activity was detected, as well as an Ang II receptor with low affinity for AT(1) and AT(2) selective receptor antagonists (pK(B) values of 5.62±0.23 and 5.08±0.25). A disulfide reducing agent almost abolished the Ang II effect, while an alpha adrenoceptor antagonist, or removing the endothelium, did not modify the Ang II effect. These results indicate that the B. jararaca aorta has an Ang II receptor pharmacologically distinct from AT(1) and AT(2) receptors, and the vasoconstrictor effect observed is independent of catecholamine or endothelium modulation. ACE and the AT receptor in the aorta of B. jararaca may be part of a tissue renin-angiotensin system.

SIGNIFICANCE

The data contribute to the knowledge of the renin-angiotensin system in vertebrate species, and provide insight into the understanding of snake Ang II receptor characteristics and diversity.

Characteristics of neural and humoral systems involved in the regulation of blood pressure in snakes

Cardiovascular function is affected by many mechanisms, including the autonomic system, the kallikrein-kinin system (KKS), the renin-angiotensin system (RAS) and the endothelin system. The function of these systems seems to be fairly well preserved throughout the vertebrate scale, but evolution required several adaptations. Snakes are particularly interesting for studies related to the cardiovascular function because of their elongated shape, their wide variation in size and length, and because they had to adapt to extremely different habitats and gravitational influences. To keep the normal cardiovascular control the snakes developed anatomical and functional adaptations and interesting structural peculiarities are found in their autonomic, KKS, RAS and endothelin systems. Our laboratory has characterized some biochemical, pharmacological and physiological properties of these systems in South American snakes. This review compares the components and function of these systems in snakes and other vertebrates, and focuses on differences found in snakes, related with receptor or ligand structure and/or function in autonomic system, RAS and KKS, absence of components in KKS and the intriguing identity between a venom and a plasma component in the endothelin system.

Molecular cloning, characterization, and distribution of the gerbil angiotensin II AT2 receptor

We isolated a cDNA clone encoding the gerbil AT2 receptor (gAT2) gene from a gerbil adrenal gland cDNA library. The full-length cDNA contains a 1,089-bp open reading frame encoding 363 amino acid residues with 90.9, 96.1, and 95.6% identity with the human (hAT2), rat (rAT2), and mouse AT2 (mAT2) receptors, respectively. There are at least seven nonconserved amino acids in the NH2-terminal domain and in positions Val196, Val217, and Met293, important for angiotensin (ANG) II but not for CGP-42112 binding. Displacement studies in adrenal sections revealed that affinity of the gAT2 receptor was 10-20 times lower for ANG II, ANG III, and PD-123319 than was affinity of the rAT2 receptor. The affinity of each receptor remained the same for CGP-42112. When transfected into COS-7 cells, the gAT2 receptor shows affinity for ANG II that is three times lower than that shown by the hAT2 receptor, whereas affinities for ANG III and the AT2 receptor ligands CGP-42112 and PD-123319 were similar. Autoradiography in sections of the gerbil head showed higher binding in muscles, retina, skin, and molars at embryonic day 19 than at 1 wk of age. In situ hybridization and emulsion autoradiography revealed that at embryonic day 19 the gAT2 receptor mRNA was highly localized to the base of the dental papilla of maxillary and mandibular molars. Our results suggest selective growth-related functions in late gestation and early postnatal periods for the gAT2 receptor and provide an essential basis for future mutagenesis studies to further define structural requirements for agonist binding.

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.

Increased AT(1) receptors in adrenal gland of AT(2) receptor gene-disrupted mice

Angiotensin II (Ang II) AT(2) receptor-gene disrupted mice have increased systemic blood pressure and response to exogenous Angiotensin II. To clarify the mechanism of these changes, we studied adrenal AT(1) receptor expression and mRNA by receptor autoradiography and in situ hybridization in female AT(2) receptor-gene disrupted mice (agtr 2-/-) and wild-type controls (agtr 2+/+). We found high expression of AT(1) receptor binding and mRNA in adrenal zona glomerulosa of female wild-type mice. AT(2) receptors and mRNA were highly expressed in adrenal medulla of wild-type mice, but were not detected in zona glomerulosa. There was no AT(2) receptor binding or mRNA in adrenal glands of AT(2) receptor-gene disrupted mice. In these animals, AT(1) receptor binding and mRNA were increased in adrenal zona glomerulosa and AT(1) receptor mRNA was increased in the adrenal medulla when compared with wild-type animals.The present data support the hypothesis of an interaction or cross talk between AT(2) and AT(1) receptors in adrenal gland. The significant increase in AT(1) receptor expression in the absence of AT(2) receptor transcription may be partially responsible for the increased blood pressure and for the enhanced response to exogenously administered Angiotensin II in this model.

Peripheral administration of an angiotensin II AT(1) receptor antagonist decreases the hypothalamic-pituitary-adrenal response to isolation Stress

Angiotensin II, which stimulates AT(1) receptors, is a brain and peripheral stress hormone. We pretreated rats with the AT(1) receptor antagonist candesartan for 13 d via sc-implanted osmotic minipumps, followed by 24-h isolation in individual metabolic cages. We measured angiotensin II receptor-type binding and mRNAs and tyrosine hydroxylase mRNA by quantitative autoradiography and in situ hybridization, catecholamines by HPLC, and hormones by RIA. Isolation increased AT(1) receptor binding in hypothalamic paraventricular nucleus as well as anterior pituitary ACTH, and decreased posterior pituitary AVP. Isolation stress also increased AT(1) receptor binding and AT(1B) mRNA in zona glomerulosa and AT(2) binding in adrenal medulla, adrenal catecholamines, tyrosine hydroxylase mRNA, aldosterone, and corticosterone. Candesartan blocked AT(1) binding in paraventricular nucleus and adrenal gland; prevented the isolation-induced alterations in pituitary ACTH and AVP and in adrenal corticosterone, aldosterone, and catecholamines; abolished the increase in AT(2) binding in adrenal medulla; and substantially decreased urinary AVP, corticosterone, aldosterone, and catecholamines during isolation. Peripheral pretreatment with an AT(1) receptor antagonist blocks brain and peripheral AT(1) receptors and inhibits the hypothalamic-pituitary-adrenal response to stress, suggesting a physiological role for peripheral and brain AT(1) receptors during stress and a possible beneficial effect of AT(1) antagonism in stress-related disorders.

Intracellular Angiotensin II and cell growth of vascular smooth muscle cells

1. We recently demonstrated that intracellular application of Angiotensin II (Angiotensin II(intr)) induces rat aorta contraction independent of plasma membrane Angiotensin II receptors. In this study we investigated the effects of Angiotensin II(intr) on cell growth in A7r5 smooth muscle cells. 2. DNA-synthesis was increased dose-dependently by liposomes filled with Angiotensin II as measured by [(3)H]-thymidine incorporation at high (EC(50)=27+/-6 pM) and low (EC(50)=14+/-5 nM) affinity binding sites with increases in E(max) of 58+/-4 and 37+/-4% above quiescent cells, respectively. Cell growth was corroborated by an increase in cell number. 3. Extracellular Angiotensin II (10 pM - 10 microM) did not modify [(3)H]-thymidine incorporation. 4. Growth effects of Angiotensin II(intr) mediated via high affinity sites were inhibited by liposomes filled with 1 microM of the non-peptidergic antagonists losartan (AT(1)-receptor) or PD123319 (AT(2)-receptor) or with the peptidergic agonist CGP42112A (AT(2)-receptor). E(max) values were decreased to 30+/-3, 29+/-4 and 4+/-2%, respectively, without changes in EC(50). The Angiotensin II(intr) effect via low affinity sites was only antagonized by CGP42112A (E(max)=11+/-3%), while losartan and PD123319 increased E(max) to 69+/-4%. Intracellular applications were ineffective in the absence of Angiotensin II(intr). 5. Neither intracellular nor extracellular Angiotensin I (1 microM) were effective. 6. The Angiotensin II(intr) induced growth response was blocked by selective inhibition of phosphatidyl inositol 3-kinase (PI-3K) by wortmannin (1 microM) and of the mitogen-activated protein kinase (MAPK/ERK) pathway by PD98059 (1 microM) to 61+/-14 and 4+/-8% of control, respectively. 7. These data demonstrate that Angiotensin II(intr) induces cell growth through atypical AT-receptors via a PI-3K and MAPK/ERK -sensitive pathway.

Angiotensin receptor(s) in fowl

The cloning of the avian Ang II receptor shows that it is molecularly close to the AT(1)-type mammalian receptor. However, pharmacological characterization in transfected cells shows that, even though the avian receptor is coupled to the phospholipase C, as is the AT(1), its profile of specificity towards antagonists appears different from that of the two angiotensin II mammalian receptor types. The fowl Ang II receptor mRNA is expressed in classical adult target organs for Ang II and, interestingly, also in endothelial cells, but not in vascular smooth muscle cells. In the endothelial cells, it may mediate the peculiar vasorelaxation effect of Ang II already reported in the chicken. The recent description of the expression pattern in the chick embryo shows that the avian Ang II receptor is expressed in many different mesenchymal tissues, a feature which is the signature of the AT(2) mammalian receptor. Altogether, these data imply that the avian Ang II receptor is an atypical receptor that cannot be readily classified as either of the two mammalian Ang II receptor types and, therefore, reinforce the evidence for another Ang II receptor in the avian class.

Molecular cloning of a ferret angiotensin II AT(1) receptor reveals the importance of position 163 for Losartan binding

A complementary DNA for the angiotensin II (AngII) type 1 (AT(1)) receptor from Mustela putorius furo (ferret) was isolated from a ferret atria cDNA library. The cDNA encodes a protein (fAT(1)) of 359 amino acids having high homologies (93-99%) to other mammalian AT(1) receptor counterparts. When fAT(1) was expressed in COS-7 cells and photoaffinity labeled with the photoactive analogue (125)I-¿Sar(1), Bpa(8)AngII, a protein of 100 kDa was detected by autoradiography. The formation of this complex was specific since it was abolished in the presence of the AT(1) non-peptidic antagonist L-158,809. Functional analysis indicated that the fAT(1) receptor efficiently coupled to phospholipase C as demonstrated by an increase in inositol phosphate production following stimulation with AngII. Binding studies revealed that the fAT(1) receptor had a high affinity for the peptide antagonist ¿Sar(1), Ile(8)AngII (K(d) of 5. 8+/-1.4 nM) but a low affinity for the AT(1) selective non-peptidic antagonist DuP 753 (K(d) of 91+/-15.6 nM). Interestingly, when we substituted Thr(163) with an Ala residue, which occupies this position in many mammalian AT(1) receptors, we restored the high affinity of this receptor for Dup 753 (11.7+/-5.13 nM). These results suggest that position 163 of the AT(1) receptor does not contribute to the overall binding of peptidic ligands but that certain non-peptidic antagonists such as Dup 753 are clearly dependent on this position for efficient binding.

Gerbil angiotensin II AT1 receptors are highly expressed in the hippocampus and cerebral cortex during postnatal development

Increasing evidence suggests that Angiotensin II, classically known from its many effects regulating salt and water homeostasis, is also involved in brain development and cognitive functions through activation of AT1 Angiotensin II receptors. The recently cloned gerbil AT1 receptor is expressed in brain areas controlling hydro-mineral homeostasis, and particularly highly expressed in limbic areas such as the hippocampal formation. We quantified the gerbil AT1 receptor messenger RNA expression and receptor binding by quantitative in situ hybridization and receptor autoradiography, respectively, in the hippocampal formation and cerebral cortex of gerbils during postnatal development. The receptor messenger RNA and binding were present from birth and showed a gradual and sustained increase through postnatal maturation in the CA1 and CA2 regions of the hippocampus and in the dentate gyrus. Conversely, in the CA3 region, no binding was detected while receptor messenger RNA peaked at 15 days after birth and disappeared in the adult. The highest receptor messenger RNA expression and binding were found in the septomedial portions of the CA1 region and at septal levels of the CA2 region. We detected the highest receptor messenger RNA expression at postnatal day one in the frontolateral pole of the cerebral hemispheres. In these areas, and in the frontoparietal and insular cortex, receptor messenger RNA dramatically decreased during postnatal life. Similarly, we found receptor messenger RNA expression in the cingulate, retrosplenial, perirhinal and infralimbic cortex with higher values during the first two weeks of development and decreased expression in the adult. However, receptor binding in the cerebral cortex, did not decrease during postnatal life. The differential profile of receptor messenger RNA expression and binding in the gerbil cortex and hippocampus during postnatal maturation suggest a role for AT1 receptors in the development and function of the corticohippocampal system.

Emerging features of brain angiotensin receptors

In mammalian brain, angiotensin II AT1 and AT2 receptor subtypes are apparently expressed only in neurons and not in glia. AT1 and AT2 receptor subtypes are sometimes closely associated, but apparently expressed in different neurons. Brain AT1/AT2 interactions may occur in selective cases as inter-neuron cross talk. There are two AT1 isoforms in rodents. AT1A, which predominates, and AT1B. There are also important inter-species differences in receptor expression. Relative lack of amino acid conservation in the gerbil gAT1A receptor substantially decreases affinity for the AT1 antagonists. AT1 receptors are expressed in brain areas regulating autonomic and hormonal responses. AT1A receptors are heterogeneously regulated in a number of experimental conditions. In specific areas, AT1A receptors are not normally expressed, but are induced under influence of reproductive hormones in dopaminergic neurons. There are AT1 and AT2 receptors also in areas related to limbic, sensory and motor functions and their expression is developmentally regulated. A picture is emerging of widespread, neuronally localized, heterogeneously regulated, closely associated brain angiotensin receptor subtypes, modulating multiple functions including neuroendocrine and autonomic responses, stress, cerebrovascular flow, and perhaps brain maturation, neuronal plasticity, memory and behavior.

Ischemia-induced neuronal cell loss is associated with loss of atypical angiotensin type-1 receptor expression in the gerbil hippocampal formation

The hippocampal formation of Mongolian gerbils expresses high amounts of atypical angiotensin II type-1 receptors. We studied the expression of these receptors by in situ hybridization using specific [35S]-labeled riboprobes and by receptor autoradiography using [125I]Sarcosine1-angiotensin II. Angiotensin II receptor mRNA was found in the pyramidal cell layer of the CA1, CA2 and CA3 subfields, with the highest expression in the CA2 subfield, and in the granular cell layer of the dentate gyrus. Angiotensin II binding was detected in the stratum oriens and stratum radiatum of the CA1 and CA2 subfields, in the stratum oriens of the CA3 subfield, and in the molecular layer of the dentate gyrus. We then studied the effect of ischemia on hippocampal angiotensin II receptor expression, 1, 4 and 15 days after bilateral occlusion of the common carotid arteries for 5 min. No changes in angiotensin II receptor mRNA or binding were detected 1 day after ischemia. Delayed, progressive loss of angiotensin II mRNA and binding occurred 4 and 15 days after ischemia, in the CA1, CA2 and CA3 subfields. The decline was faster in the CA1 subfield, and paralleled the loss of neurons after ischemia. In the dentate gyrus, angiotensin II receptor mRNA and angiotensin II binding were not changed when compared to sham operated controls. The decrease of angiotensin II receptor expression may reflect the loss of angiotensin II receptor-producing neurons rather than a down-regulation of receptor expression.