Immunohistochemical localization of type-II (AT2) angiotensin receptors with a polyclonal antibody against a peptide from the C-terminal tail

Angiotensin II AT2 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

Immunohistochemical Localization of AT1a, AT1b, and AT2 Angiotensin II Receptor Subtypes in the Rat Adrenal, Pituitary, and Brain with a Perspective Commentary

Angiotensin II increases blood pressure and stimulates thirst and sodium appetite in the brain. It also stimulates secretion of aldosterone from the adrenal zona glomerulosa and epinephrine from the adrenal medulla. The rat has 3 subtypes of angiotensin II receptors: AT1a, AT1b, and AT2. mRNAs for all three subtypes occur in the adrenal and brain. To immunohistochemically differentiate these receptor subtypes, rabbits were immunized with C-terminal fragments of these subtypes to generate receptor subtype-specific antibodies. Immunofluorescence revealed AT1a and AT2 receptors in adrenal zona glomerulosa and medulla. AT1b immunofluorescence was present in the zona glomerulosa, but not the medulla. Ultrastructural immunogold labeling for the AT1a receptor in glomerulosa and medullary cells localized it to plasma membrane, endocytic vesicles, multivesicular bodies, and the nucleus. AT1b and AT2, but not AT1a, immunofluorescence was observed in the anterior pituitary. Stellate cells were AT1b positive while ovoid cells were AT2 positive. In the brain, neurons were AT1a, AT1b, and AT2 positive, but glia was only AT1b positive. Highest levels of AT1a, AT1b, and AT2 receptor immunofluorescence were in the subfornical organ, median eminence, area postrema, paraventricular nucleus, and solitary tract nucleus. These studies complement those employing different techniques to characterize Ang II receptors.

Impact of cell type and epitope tagging on heterologous expression of G protein-coupled receptor: a systematic study on angiotensin type II receptor

Despite heterologous expression of epitope-tagged GPCR is widely adopted for functional characterization, there is lacking of systematic analysis of the impact of expression host and epitope tag on GPCR expression. Angiotensin type II (AT2) receptor displays agonist-dependent and -independent activities, coupling to a spectrum of signaling molecules. However, consensus has not been reached on the subcellular distributions, signaling cascades and receptor-mediated actions. To examine the contributions of host cell and epitope tag on receptor expression and activity, epitope-tagged AT2 receptor variants were transiently or stably expressed in HEK293, CHO-K1 and PC12 cells. The epitope-tagged AT2 receptor variants were detected both on the cell membrane and in the perinuclear region. In transiently transfected HEK293 cells, Myc-AT2 existed predominantly as monomer. Additionally, a ladder of ubiquitinated AT2 receptor proteins was detected. By contrast, stably expressed epitope-tagged AT2 receptor variants existed as both monomer and high molecular weight complexes, and the latter was enriched in cell surface. Glycosylation promoted cell surface expression of Myc-AT2 but had no effect on AT2-GFP in HEK293 cells. In cells that stably expressed Myc-AT2, serum starvation induced apoptosis in CHO-K1 cells but not in HEK293 or PC12 cells. Instead, HEK293 and PC12 cells stably expressing Myc-AT2 exhibited partial cell cycle arrest with cells accumulating at G1 and S phases, respectively. Taken together, these results suggest that expression levels, subcellular distributions and ligand-independent constitutive activities of AT2 receptor were cell type-dependent while posttranslational processing of nascent AT2 receptor protein was modulated by epitope tag and mode of expression.

A peripherally administered, centrally acting angiotensin II AT2 antagonist selectively increases brain AT1 receptors and decreases brain tyrosine hydroxylase transcription, pituitary vasopressin and ACTH

The physiological actions of brain Angiotensin II AT(2) receptors and their relationship to Angiotensin II AT(1) receptors remain controversial. To further clarify their role, we determined to what extent systemic administration of an AT(2) receptor antagonist affected AT(2) receptor binding within the brain and the expression of AT(1) receptors. For this purpose, we subcutaneously administered the AT(2) receptor antagonist PD123319 (1 mg/kg/day) to adult male rats for two weeks via osmotic minipumps. We also studied the content of pituitary adrenocorticotropic hormone and vasopressin, representative of hypothalamic-pituitary-adrenal axis activation, and the tyrosine hydroxylase gene expression in the locus coeruleus as a measure of central norepinephrine function. We found significant decreases in AT(2) receptor binding in brain areas inside the blood brain barrier, the inferior olive and the locus coeruleus. AT(2) receptor blockade increased AT(1) receptor binding and mRNA expression not only in the subfornical organ and the median eminence, situated outside the blood brain barrier, but also in the hypothalamic paraventricular nucleus, located inside the blood brain barrier. These changes paralleled decreased expression of tyrosine hydroxylase mRNA in the locus coeruleus and decreased pituitary adrenocorticotropic and vasopressin content. Our results demonstrate that sustained peripheral administration of an AT(2) antagonist decreases binding to brain AT(2) receptors, indicating that this drug is a useful tool for the study of their central role. AT(2) receptor activity inhibition up-regulates AT(1) receptor expression in specific brain areas. Blockade of brain AT(2) receptors is compatible with enhanced hypothalamic-pituitary-adrenal axis and decreased central sympathetic system activity.

Microinjection of angiotensin II in the caudal ventrolateral medulla induces hyperalgesia

Nociceptive transmission from the spinal cord is controlled by supraspinal pain modulating systems that include the caudal ventrolateral medulla (CVLM). The neuropeptide angiotensin II (Ang II) has multiple effects in the CNS and at the medulla oblongata. Here we evaluated the expression of angiotensin type 1 (AT(1)) receptors in spinally-projecting CVLM neurons, and tested the effect of direct application of exogenous Ang II in the CVLM on nociceptive behaviors. Although AT(1)-immunoreactive neurons occurred in the CVLM, only 3% of AT(1)-positive neurons were found to project to the dorsal horn, using double-immunodetection of the retrograde tracer cholera toxin subunit B. In behavioral studies, administration of Ang II (100 pmol) in the CVLM gave rise to hyperalgesia in both the tail-flick and formalin tests. This hyperalgesia was significantly attenuated by local administration of the AT(1) antagonist losartan. The present study demonstrates that Ang II can act on AT(1) receptors in the CVLM to modulate nociception. The effect on spinal nociceptive processing is likely indirect, since few AT(1)-expressing CVLM neurons were found to project to the spinal cord. The renin-angiotensin system may also play a role in other supraspinal areas implicated in pain modulation.

Identification and characterization of surrogate peptide ligand for orphan G protein-coupled receptor mas using phage-displayed peptide library

In the present study, a phage-displayed random peptide library was used to identify surrogate peptide ligands for orphan GPCR mas. Sequence analysis of the isolated phage clones indicated a selective enrichment of some peptide sequences. Moreover, multiple alignments of the isolated phage clones gave two conserved peptide motifs from which we synthesized peptide MBP7 for further evaluation. Characterization of the representative phage clones and the synthetic peptide MBP7 by immunocytochemistry revealed a strong punctate cell surface staining in CHO cells expressing mas-GFP fusion protein. The isolated phage clones and synthetic peptide MBP7 induced mas internalization in a stable CHO cell clone (MC0M80) over-expressing mas. In addition, MBP7-stimulated phospholipase C activity and intracellular calcium mobilization in these same cells. In summary, we have demonstrated a systematic approach to derive surrogate peptide ligands for orphan GPCRs. With this technique, we have identified two conserved peptide motifs which allow us to identify potential protein partners for mas, and have generated a peptide agonist MBP7 which will be invaluable for functional characterization of the mas oncogene.

Immunohistochemical colocalization of type II angiotensin receptors with somatostatin in rat pancreas

Earlier studies indicate that binding sites of type II angiotensin (AT2) receptors are detected all over the pancreas, as well as in the pancreatic exocrine cell line AR4-2J. However, lack of corresponding functional AT2 receptor responses can be detected in the exocrine pancreas. The aim of present study is to determine the protein expression of AT2 receptors in the pancreas by probing with an AT2 receptor-specific antibody, and to examine the role of AT2 receptors in the regulation of pancreatic endocrine hormone release. In Western protein analysis of adult rat tissues, expression of AT2 receptor-immunoreactive bands of 56, 68, and 78 kDa was detected in the adrenal, kidney, liver, salivary glands, and pancreas. In adult rat pancreas, strong immunoreactivity was detected on cells that were located at the outer region of Langerhans islets. Immunohistochemical studies indicated that AT2 receptors colocalized with somatostatin-producing cells in the endocrine pancreas. Consistent with the findings in adult pancreas, abundant expression of AT2 receptors was also detected in immortalized rat pancreatic endocrinal cells lines RIN-m and RIN-14B. To examine the role of AT2 receptors on somatostatin secretion in the pancreas, angiotensin-stimulated somatostatin release from pancreatic RIN-14B cells was studied by an enzyme immunoassay in the absence or presence of various subtype-selective angiotensin analogues. There was a basal release of somatostatin from RIN-14B cells at a rate of 8.72 +/- 4.21 ng/10(6) cells (n = 7). Angiotensin II (1 nM-10 microM) stimulated a biphasic somatostatin release in a dose-dependent manner with an apparent EC50 value of 49.3 +/- 25.9 nM (n = 5), and reached maximal release at 1 microM angiotensin II (982 +/- 147.34% over basal secretion; n = 5). Moreover, the AT2 receptor-selective angiotensin analogue, CGP42112, was 1000 times more potent than the AT1 receptor-selective angiotensin analogue, losartan, in inhibiting angiotensin II-stimulated somatostatin release. These results suggest that angiotensin may modulate pancreatic hormone release via regulation of somatostatin secretion.

Localization of AT(2) receptors in the nucleus of the solitary tract of spontaneously hypertensive and Wistar Kyoto rats using [125I] CGP42112: upregulation of a non-angiotensin II binding site following unilateral nodose ganglionectomy

We have examined the binding distribution of a selective AT(2) receptor ligand [125I] CGP42112 in the brain of adult Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). AT(2) receptor localization was also examined in the rat brainstem following unilateral nodose ganglionectomy. Specific [125I] CGP42112 binding was observed in discrete brain regions from both rat strains, including the nucleus of the solitary tract (NTS), and did not differ between WKY and SHR. [125I] CGP42112 binding in the NTS revealed an AT(2) receptor component that was displaceable by PD 123319 and Ang II (50-58%), as well as a non-angiotensin II receptor component (42-49%). Following unilateral nodose ganglionectomy, [125I] CGP42112 binding density on the denervated side of the NTS was increased approximately two-fold in both WKY and SHR. This increased [125I] CGP42112 binding density in the ipsilateral NTS was comprised of a greater non-angiotensin II component than that observed in the sham groups, since only approximately 30% was displaced by PD123319 and angiotensin II. Furthermore, [125I] CGP42112 also revealed high binding density on the denervated side in the dorsal motor nucleus and the nucleus ambiguus in both WKY and SHR. AT(2) receptor immunoreactivity was also visualised in the NTS of sham operated rats, but was not observed in the dorsal motor nucleus or the nucleus ambiguus, nor was it up-regulated following nodose ganglionectomy. These results demonstrate, for the first time, an AT(2) receptor binding site in the NTS, as well as a non-angiotensin II [125I] CGP42112 binding site. These studies also demonstrate that nodose ganglionectomy represents a useful model in which to study a non-angiotensin II [125I] CGP42112 binding site that is up-regulated following degeneration of afferent vagal nerves.

Immunocytochemical localization of angiotensin II receptor subtypes 1 and 2 in the porcine fetal, prepubertal and postpubertal ovary

There is considerable evidence for a mammalian ovarian renin-angiotensin system, which may influence ovulation, angiogenesis and steroidogenesis via the autocrine and/or paracrine actions of the biologically active product of the cascade, angiotensin II (AngII). There are two characterized AngII receptors--type 1, AT1 and type 2, AT2. We report the localization of these receptor subtypes within porcine fetal, prepubertal and postpubertal ovaries. Positive staining for AT1 and AT2 receptors was observed in egg nests in all fetal ovaries studied, as well as in a defined two-cell layer at the ovarian periphery. In prepubertal tissue, positive AT1 and AT2 staining was localized to granulosa cells adjacent to the basement membrane of pre-antral and antral follicles, with no staining in the thecal layer. There was immunostaining for both receptors in prepubertal oocytes and zona pellucida. In postpubertal tissue, positive AT1 and AT2 immunostaining was localized to areas of putative neovascularization, the zona pellucida and the oocyte. Further AT1 staining was located to the postpubertal antral follicle granulosa cells. The results indicate that there are higher densities of AT1 receptors than AT2 receptors in the porcine fetal, prepubertal and postpubertal ovary, and this has profound implications for the role of AngII in ovarian development.

Immunocytochemical localization of angiotensin II receptor subtypes and angiotensin II with monoclonal antibodies in the rat adrenal gland

Angiotensin II (Ang II), a major regulator of cardiovascular function and body fluid homeostasis, mediates its biological actions via two subtypes of G protein-coupled receptors, termed AT(1) and AT(2). The primary goal of this study was to raise monoclonal anti-peptide antibodies specific to angiotensin AT(1)- and AT(2)-receptor subtypes and to Ang II itself and using these monoclonal antibodies to determine the intraadrenal localization of AT(1) and AT(2) receptors and Ang II in male adult rats. Immunocytochemistry unambiguously demonstrates a regional colocalization of Ang II and angiotensin II receptors in the adrenal gland. The novel antibodies localized Ang II and the AT(1) receptors to the zona glomerulosa of the cortex and to the medulla whereas AT(2) receptors were limited to the medulla. The specificity of immunostaining was documented by pre-adsorption of the antibody with the immunogenic peptide. Our data underscore that AT(1) appears to mediate most of the physiological actions of Ang II in adrenal. Western blot analysis of rat adrenal protein extracts using AT(1) antibody showed a predominant 73-kDa band and a weaker 97-kDa immunoreactive band corresponding to glycosylated forms of the AT(1) receptor. Immunostaining with anti-AT(2) yielded one major immunoreactive band of 73-kDa size and one additional fainter band of 120 kDa. These antibodies may prove of value in unraveling the subcellular localization and intracellular effector pathways of AT(1) and AT(2).

The localization and expression of the renin-angiotensin system in the human placenta throughout pregnancy

All the components of the renin-angiotensin system (RAS), including the AT(1)receptor, have previously been shown to be present in the human term placenta. However, the presence of the RAS components has not been fully investigated in the human placenta throughout pregnancy. The aim of this study was to examine the localization of the angiotensin receptors AT(1)and AT(2)using immunocytochemistry and the expression of prorenin, angiotensinogen and the AT(1)and AT(2)receptor mRNA using RT-PCR in the human placenta in the first, second and third trimesters of pregnancy. Localization of the AT(1)receptor was shown throughout gestation in the syncytiotrophoblast, cytotrophoblast, Hofbauer cells and the fetal vascular endothelium. Expression of mRNA for prorenin, angiotensinogen and the AT(1)receptor was shown in the placenta throughout gestation. However, localization or mRNA expression of the AT(2)receptor was not detected in any of the placental samples studied. These results clearly show the expression of a majority of the components of the RAS in the placenta from early gestation onwards. Therefore, these results suggest that the RAS may have a role in the human placenta throughout gestation.