Genetic deletion of angiotensin AT2 receptor leads to increased cell numbers in different brain structures of mice

The effect of angiotensin AT1A inactivation on innate and learned fear responses in mice and its relationship to blood pressure

Angiotensin AT₁ receptors are implicated in behavioral and physiological processes associated with fear and stress. However, the precise role of AT₁ receptors in modulating fear-related behavior and its relation to their physiological effects remains unclear. Here, we examined innate and learned fear responses and their relationship to cardiovascular arousal in AT_1A receptor knockout (AT_1A^-/-) mice. Using synchronized video and blood pressure telemetry, we found that, in a novel test environment, AT_1A^-/- mice showed reduced neophobia but a similar rise in blood pressure, as compared to AT_1A^+/+ mice. In response to a discrete threat, footshock, both flight behavior and cardiovascular arousal were decreased in AT_1A^-/- mice. Reduced flight behavior was also observed in AT_1A^-/- mice in the elevated T-maze test. During fear conditioning, the immediate freezing response to the first shock, but not the rate of freezing acquisition was decreased in AT_1A^-/- mice. Likewise, AT_1A^-/- mice showed reduced freezing and pressor responses to the first re-exposure, but normal rate of freezing extinction over subsequent trials. Similarly, in the elevated T-maze, the rates of avoidance acquisition and escape learning remained unchanged in AT_1A^-/- mice. Finally, after re-exposure, AT_1A^-/- mice displayed altered c-Fos expression, compared to AT_1A^+/+ mice, in the hypothalamus and periaqueductal gray but not in fear-related limbic-cortical areas, nor in medullary nuclei that convey visceral afferent information. We conclude that AT_1A receptor knockout reduces innate fear responses, without affecting learning efficiency in mice. These effects are dissociable from cardiovascular effects and likely reflect altered neurotransmission in hypothalamic-midbrain defense regions.

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.

Reporter mouse strain provides a novel look at angiotensin type-2 receptor distribution in the central nervous system

Angiotensin-II acts at its type-1 receptor (AT1R) in the brain to regulate body fluid homeostasis, sympathetic outflow and blood pressure. However, the role of the angiotensin type-2 receptor (AT2R) in the neural control of these processes has received far less attention, largely because of limited ability to effectively localize these receptors at a cellular level in the brain. The present studies combine the use of a bacterial artificial chromosome transgenic AT2R-enhanced green fluorescent protein (eGFP) reporter mouse with recent advances in in situ hybridization (ISH) to circumvent this obstacle. Dual immunohistochemistry (IHC)/ISH studies conducted in AT2R-eGFP reporter mice found that eGFP and AT2R mRNA were highly co-localized within the brain. Qualitative analysis of eGFP immunoreactivity in the brain then revealed localization to neurons within nuclei that regulate blood pressure, metabolism, and fluid balance (e.g., NTS and median preoptic nucleus [MnPO]), as well as limbic and cortical areas known to impact stress responding and mood. Subsequently, dual IHC/ISH studies uncovered the phenotype of specific populations of AT2R-eGFP cells. For example, within the NTS, AT2R-eGFP neurons primarily express glutamic acid decarboxylase-1 (80.3 ± 2.8 %), while a smaller subset express vesicular glutamate transporter-2 (18.2 ± 2.9 %) or AT1R (8.7 ± 1.0 %). No co-localization was observed with tyrosine hydroxylase in the NTS. Although AT2R-eGFP neurons were not observed within the paraventricular nucleus (PVN) of the hypothalamus, eGFP immunoreactivity is localized to efferents terminating in the PVN and within GABAergic neurons surrounding this nucleus. These studies demonstrate that central AT2R are positioned to regulate blood pressure, metabolism, and stress responses.

Systemic angiotensin II and exercise-induced neurogenesis in adult rat hippocampus

Physical exercise is a robust stimulus that enhances hippocampal neurogenesis via cell proliferation in rodents. We examined the role of systemic angiotensin (Ang) peptides in exercise-dependent enhancement of neurogenesis in the adult rat hippocampus. Plasma angiotensin peptide concentration increased rapidly in response to 30 min of treadmill exercise. After undertaking this exercise once daily for a week, the number of proliferating cells in the hippocampus, identified by 5-bromo-2'-deoxyuridine (BrdU) incorporation, had increased compared with controls. To mimic the increase in plasma Ang peptide concentrations brought about by exercise, rats were injected with 10(-5)M Ang II once daily for a week. The number of BrdU-incorporating cells and of doublecortin (DCX)-expressing immature neurons in the hippocampus rose approximately 1.5 and 1.9-fold compared with controls, respectively. The effects were completely abolished by an Ang II receptor subtype 1 antagonist losartan. These findings, taken together, suggest that an increased concentrations of Ang peptides in the systemic circulation during exercise may promote neurogenesis in the adult rat hippocampus.

Effects of the angiotensin-(1-7) receptor Mas on cell proliferation and on the population of doublecortin positive cells within the dentate gyrus and the piriform cortex

Aside from the well-known biologically active angiotensin II, other biologically active angiotensins have been discovered, including angiotensin IV and angiotensin-(1-7). Some years ago, we and others discovered that the Mas proto-oncogene encodes a G protein-coupled receptor being essential for angiotensin-(1-7) signaling. Mas is not only expressed in the periphery but also within the brain, e.g. in the dentate gyrus (DG) and the piriform cortex (PC). Since the DG is capable of adult neurogenesis, we examined the impact of a deletion of Mas upon adult neurogenesis. Deletion of Mas did not alter cell proliferation in the adult DG (as monitored with phosphohistone H3) and did not alter cell death (as monitored with activated Caspase 3). However, Mas deficiency resulted in an increase in the number of doublecortin (DCX) positive cells, indicating that lack of Mas increases the number of this cell population. Concerning the PC, it is discussed whether adult neurogenesis occurs under physiological conditions in this area. We could demonstrate that Mas deficiency has an impact on cell division and on the population of DCX-positive cells within the PC. Since Mas is not expressed before birth within the brain, our data may suggest that adult hippocampal neurogenesis and neurogenesis occurring during prenatal development share several common mechanisms, but are, at least in part, differentially regulated. Moreover, since deficiency for Mas increases the numbers of DCX-positive young neurons, blockage of Mas might be beneficial in stimulating neurogenesis in adults.

A systematic review to investigate whether Angiotensin-(1-7) is a promising therapeutic target in human heart failure

Context. Heart failure (HF) is a common condition causing much morbidity and mortality despite major advances in pharmacological and device therapies. Preclinical data suggest a cardioprotective role of Angiotensin-(1-7) in animal models of HF. Objective. Perform a systematic review on the effects of Angiotensin-(1-7) on humans, focusing on HF. Results. 39 studies were included in the review (4 in human HF and (35) in non-HF patients). There is only one intervention study on 8 patients with human HF, using Angiotensin-(1-7), with forearm blood flow (FBF) as the endpoint. Angiotensin-(1-7) caused no significant effect on FBF in this HF study but caused vasodilation in 3 out of 4 non-HF studies. In one other non-HF study, Angiotensin-(1-7) infusion led to a significant increase in blood pressure in normal men; however, effects were <0.03% that of angiotensin II. Cardioprotective effects seen in non-HF studies include for instance beneficial actions against atherosclerosis and myocardial fibrosis. Conclusions. The main finding of our systematic review is that Angiotensin-(1-7) plays an important cardioprotective role in HF in animals and in patients without heart failure. More research is required to test the hypothesis that Angiotensin-(1-7) benefits patients with heart failure.

Renin Angiotensin System in Cognitive Function and Dementia

Angiotensin II represents a key molecule in hypertension and cerebrovascular pathology. By promoting inflammation and oxidative stress, enhanced Ang II levels accelerate the onset and progression of cell senescence. Sustained activation of RAS promotes end-stage organ injury associated with aging and results in cognitive impairment and dementia. The discovery of the angiotensin-converting enzyme ACE2-angiotensin (1–7)-Mas receptor axis that exerts vasodilator, antiproliferative, and antifibrotic actions opposed to those of the ACE-Ang II-AT1 receptor axis has led to the hypothesis that a decrease in the expression or activity of angiotensin (1–7) renders the systems more susceptible to the pathological actions of Ang II. Given the successful demonstration of beneficial effects of increased expression of ACE2/formation of Ang1–7/Mas receptor binding and modulation of Mas expression in animal models in containing cerebrovascular pathology in hypertensive conditions and aging, one could reasonably hope for analogous effects regarding the prevention of cognitive decline by protecting against hypertension and cerebral microvascular damage. Upregulation of ACE2 and increased balance of Ang 1–7/Ang II, along with positive modulation of Ang II signaling through AT2 receptors and Ang 1–7 signaling through Mas receptors, may be an appropriate strategy for improving cognitive function and treating dementia.

Angiotensin II, a Neuropeptide at the Frontier between Endocrinology and Neuroscience: Is There a Link between the Angiotensin II Type 2 Receptor and Alzheimer's Disease?

Amyloid-β peptide deposition, abnormal hyperphosphorylation of tau, as well as inflammation and vascular damage, are associated with the development of Alzheimer's disease (AD). Angiotensin II (Ang II) is a peripheral hormone, as well as a neuropeptide, which binds two major receptors, namely the Ang II type 1 receptor (AT1R) and the type 2 receptor (AT2R). Activation of the AT2R counteracts most of the AT1R-mediated actions, promoting vasodilation, decreasing the expression of pro-inflammatory cytokines, both in the brain and in the cardiovascular system. There is evidence that treatment with AT1R blockers (ARBs) attenuates learning and memory deficits. Studies suggest that the therapeutic effects of ARBs may reflect this unopposed activation of the AT2R in addition to the inhibition of the AT1R. Within the context of AD, modulation of AT2R signaling could improve cognitive performance not only through its action on blood flow/brain microcirculation but also through more specific effects on neurons. This review summarizes the current state of knowledge and potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT2R activation by non-peptide and highly selective agonists, acting on neuronal plasticity, could represent new pharmacological tools that may help improve impaired cognitive performance in AD and other neurological cognitive disorders.

The day-night difference of blood pressure is increased in AT(1A)-receptor knockout mice on a high-sodium diet

BACKGROUND

Abnormal circadian variation of blood pressure (BP) increases cardiovascular risk. In this study, we examined the influence of angiotensin AT(1A) receptors on circadian BP variation, and specifically on its behavioral activity-related and -unrelated components.

METHODS

BP and locomotor activity were recorded by radiotelemetry in AT(1A)-receptor knockout mice (AT(1A)(-/-)) and their wild-type controls (AT(1A)(+/+)) placed on a normal-salt diet (NSD) or high-salt diet (HSD, 3.1% Na).

RESULTS

The 24-h BP was lower in AT(1A)(-/-) than AT(1A)(+/+) mice on a NSD (92 +/- 2 and 118 +/- 2 mm Hg, respectively), whereas the day-night BP difference (DeltaDNBP) was similar between groups (11 +/- 2 and 12 +/- 1 mm Hg, respectively). HSD increased BP by 20 +/- 2 mm Hg and DeltaDNBP by 7 +/- 1 mm Hg in AT(1A)(-/-) mice, without affecting these parameters much in AT(1A)(+/+) mice. The DeltaDNBP increase in AT(1A)(-/-) mice was caused by nondipping BP during the inactive late-dark period. Conversely, BP rise associated with circadian behavioral activation during the early dark period was not altered by HSD in AT(1A)(-/-) mice. The BP change associated with spontaneous ultradian activity-inactivity bouts was also similar between strains on HSD as was the BP rise associated with induced (cage-switch) behavioral activity. Ganglionic or alpha(1)-adrenergic blockade decreased BP in both strains; HSD did not affect this response in AT(1A)(-/-), but abolished it in AT(1A)(+/+) mice.

CONCLUSIONS

AT(1A)-receptor deficiency, when combined with HSD, can increase circadian BP difference in mice. This increase is mediated principally by activity-unrelated factors, such as the nonsuppressibility of basal resting sympathetic tone by HSD, thus suggesting a form of salt-/volume-dependent hypertension.

Sex-different effect of angiotensin II type 2 receptor on ischemic brain injury and cognitive function

We previously reported that angiotensin II type 2 (AT(2)) receptor signaling prevents neural damage and cognitive impairment after focal cerebral ischemia. We investigated the possible roles of the AT(2) receptor in the sex difference, focusing on cognitive function and ischemic brain damage using AT(2) receptor-deficient mice (Agtr2(-)). In Agtr2(-), spatial memory evaluated by the Morris water maze test was impaired in female compared with that in male Agtr2(-) and female wild-type (Agtr2(+)), while no significant sex-different change was observed in Agtr2(+). Interestingly, bromodeoxyuridine incorporation assay showed a significant decrease of hippocampal neurogenesis in female Agtr2(-) compared with that in female Agtr2(+). In contrast, ischemic area after middle cerebral artery (MCA) occlusion was significantly increased in male compared with female mice in Agtr2(-), while no significant sex-different change was observed in Agtr2(+). Male Agtr2(-) mice showed higher AT(1) receptor expression and significantly impaired cerebral blood flow (CBF) in the ipsilateral side 24 hours after MCA occlusion compared with female Agtr2(-) mice. In conclusion, deletion of the AT(2) receptor showed a sex-different effect such as a severe cognitive impairment with a decrease of hippocampal neurogenesis in females and a larger ischemic brain damage with a decrease of CBF in males.

Prenatal blockade of Ang II receptors affects neonatal rat hindbrain structure and receptor localization

The development of knock-out mice for Angiotensin II (Ang II) AT(2) receptors, which exhibited altered exploratory behavior, prompted us to investigate the cerebellum and brainstem. We evaluated the effect of stimulation/inhibition of Ang II receptors on hindbrain development, in offspring (postnatal days P0, P8) of pregnant rats treated during late pregnancy (Ang II, Losartan or PD123319, 1 mg/kg/day). Receptor localization by autoradiography showed in P0 and P8 hindbrains, that most structures expressed AT(2) subtype: cerebellar cortex, cerebellar nuclei, genu facial nucleus, inferior colicullus, inferior olive. In the cerebellar cortex, [(125)I]Ang II AT(2) binding was predominant, while low AT(1) binding was observed in adjacent layers of the cerebellar cortex. Blockade of AT(2) receptors with PD123319 increased binding in cerebellar nuclei (p<0.05) and brainstem nuclei at P0, P8, in correlation with increased AT(2) receptor expression by RT-PCR. The enlarged external granular layer (EGL) in PD123319-treated P0 pups contrast with the significant decrease in Ang II binding (p<0.001) in the cerebellar cortex. Blockade of AT(2) receptors during late pregnancy seems to arrest cerebellar cortex development in P0 animals. On the contrary, increased AT(2) binding was observed in cerebellar cortex and DTg nucleus in PD123319-treated P8 animals (p<0.001). Ang II treatment leads to increased binding in the brainstem. In spite of the low doses of Ang II antagonists used, treatments were performed during a time-frame critical for hindbrain development, leading to remarkable effects. The present study makes a contribution to understand the role of Ang II receptors during hindbrain development.

Candidate Agtr2 influenced genes and pathways identified by expression profiling in the developing brain of Agtr2(-/y) mice

Intellectual disability (ID) is a common developmental disability observed in 1 to 3% of the human population. A possible role for the Angiotensin II type 2 receptor (AGTR2) in brain function, affecting learning, memory, and behavior, has been suggested in humans and rodents. Mice lacking the Agtr2 gene (Agtr2(-/y)) showed significant impairment in their spatial memory and exhibited abnormal dendritic spine morphology. To identify Agtr2 influenced genes and pathways, we performed whole genome microarray analysis on RNA isolated from brains of Agtr2(-/y) and control male mice at embryonic day 15 (E15) and postnatal day one (P1). The gene expression profiles of the Agtr2(-/y) brain samples were significantly different when compared to profiles of the age-matched control brains. We identified 62 differently expressed genes (p< or =0.005) at E15 and in P1 brains of the Agtr2(-/y) mice. We verified the differential expression of several of these genes in brain samples using quantitative RT-PCR. Differentially expressed genes encode molecules involved in multiple cellular processes including microtubule functions associated with dendritic spine morphology. This study provides insight into Agtr2 influenced candidate genes and suggests that expression dysregulation of these genes may modulate Agtr2 actions in the brain that influences learning and memory.

AT2 receptors: functional relevance in cardiovascular disease

The renin angiotensin system (RAS) is intricately involved in normal cardiovascular homeostasis. Excessive stimulation by the octapeptide angiotensin II contributes to a range of cardiovascular pathologies and diseases via angiotensin type 1 receptor (AT₁R) activation. On the other hand, tElsevier Inc.he angiotensin type 2 receptor (AT₂R) is thought to counter-regulate AT₁R function. In this review, we describe the enhanced expression and function of AT₂R in various cardiovascular disease settings. In addition, we illustrate that the RAS consists of a family of angiotensin peptides that exert cardiovascular effects that are often distinct from those of Ang II. During cardiovascular disease, there is likely to be an increased functional importance of AT₂R, stimulated by Ang II, or even shorter angiotensin peptide fragments, to limit AT₁R-mediated overactivity and cardiovascular pathologies.

Cardiovascular phenotype of mice lacking all three subtypes of angiotensin II receptors

Angiotensin II activates two distinct receptors, the angiotensin II receptors type 1 (AT(1)) and type 2 (AT(2)). In rodents, two AT(1) subtypes were identified (AT(1a) and AT(1b)). To determine receptor-specific functions and possible angiotensin II effects independent of its three known receptors we generated mice deficient in either one of the angiotensin II receptors, in two, or in all three (triple knockouts). Triple knockouts were vital and fertile, but survival was impaired. Hypotension and renal histological abnormalities in triple knockouts were comparable to those in mice lacking both AT(1) subtypes. All combinations lacking AT(1a) were distinguished by reduced heart rate. AT(1a) deletion impaired the in vivo pressor response to angiotensin II bolus injection, whereas deficiency for AT(1b) and/or AT(2) had no effect. However, the additional lack of AT(1b) in AT(1a)-deficient mice further impaired the vasoconstrictive capacity of angiotensin II. Although general vasoconstrictor properties were not changed, angiotensin II failed to alter blood pressure in triple knockouts, indicating that there are no other receptors involved in direct angiotensin II pressor effects. Our data identify mice deficient in all three angiotensin II receptors as an ideal tool to better understand the structure and function of the renin-angiotensin system and to search for angiotensin II effects independent of AT(1) and AT(2).

Impaired spatial memory and altered dendritic spine morphology in angiotensin II type 2 receptor-deficient mice

Mental retardation is the most frequent cause of serious handicap in children and young adults. Mutations in the human angiotensin II type 2 receptor (AT2) have been implicated in X-linked forms of mental retardation. We here demonstrate that mice lacking the AT2 receptor gene are significantly impaired in their performance in a spatial memory task and in a one-way active avoidance task. As no difference was observed between the genotypes in fear conditioning, the detected deficit in spatial memory may not relate to fear. Notably, receptor knockout mice showed increased motility in an activity meter and elevated plus maze. Importantly, these mice are characterized by abnormal dendritic spine morphology and length, both features also found to be associated with some cases of mental retardation. These findings suggest a crucial role of AT2 in normal brain function and that dysfunction of the receptor has impact on brain development and ultrastructural morphology with distinct consequences on learning and memory.

Angiotensin II Type-2 Receptor Stimulation Prevents Neural Damage by Transcriptional Activation of Methyl Methanesulfonate Sensitive 2

The molecular mechanisms of the contribution of angiotensin II type-1 receptor blockers to neuronal protection are still unclear. Here, we investigated the effect of angiotensin II type-2 (AT2) receptor stimulation on neurons and cognitive function involving a new neuroprotective factor, methyl methanesulfonate sensitive 2 (MMS2). Angiotensin II treatment of neurospheres enhanced their differentiation and increased MMS2 expression. Knockdown of the MMS2 gene by small interference RNA (siRNA) significantly reduced the number of neurospheres, with loss of sphere formation. An angiotensin II type-1 receptor blocker, valsartan, enhanced such neurosphere differentiation and MMS2 induction, whereas an AT2 receptor antagonist, PD123319, inhibited them. After mice underwent permanent middle cerebral artery occlusion, AT2 receptor mRNA expression was significantly increased in the ischemic side of the brain. Passive avoidance rate to evaluate cognitive function was significantly impaired in AT2 receptor null (Agtr2-) mice compared with wild-type mice. Treatment with valsartan prevented the cognitive decline in wild-type mice, but this effect was weaker in Agtr2- mice. In ischemic brain regions, MMS2 was increased in wild-type mice, but not in Agtr2- mice. Valsartan also enhanced MMS2 expression to a greater degree in wild-type mice. Finally, intracerebroventricular administration of MMS2 siRNA showed more impaired avoidance rate after middle cerebral artery occlusion compared with that in control siRNA-transfected mice. These findings experimentally support the clinical evidence and indicate a unique mechanism of the AT2 receptor in brain protection.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Central angiotensin II controls alcohol consumption via its AT1 receptor

Pharmacological and genetic manipulations of the renin-angiotensin system (RAS) have been found to alter the voluntary consumption of alcohol. Here we characterize the role of central angiotensin II (Ang II) in alcohol intake first by using transgenic rats that express an antisense RNA against angiotensinogen and consequently have reduced Ang II levels exclusively in the central nervous system [TGR(ASrAOGEN)680]. These rats consumed markedly less alcohol in comparison to their wild-type controls. Second, Spirapril, an inhibitor of the angiotensin-converting enzyme (ACE), which passes the blood-brain barrier, did not influence the alcohol consumption in the TGR(ASrAOGEN)680, but it significantly reduced alcohol intake in wild-type rats. Studies in knockout mice indicated that the central effect of Ang II on alcohol consumption is mediated by the angiotensin receptor AT1 whereas the AT2 receptor and the bradykinin B2 receptor are not involved. Furthermore, the dopamine concentration in the ventral tegmental area (VTA) is markedly reduced in rats with low central Ang II, strengthening our hypothesis of a role of dopaminergic transmission in Ang II-controlled alcohol preference. Our results indicate that a distinct drug-mediated control of the central RAS could be a promising therapy for alcohol disease.

Angiotensin-(1–7) enhances LTP in the hippocampus through the G-protein-coupled receptor Mas

The renin-angiotensin system not only plays a critical role in blood pressure control but is also involved in learning and memory mechanisms. In addition to angiotensin (Ang) II, Ang-(1-7) may also have important biological activities in the brain. Here, we show for the first time that Ang-(1-7) enhances long-term potentiation (LTP) in the CA1 region of the hippocampus. Our studies with AT1 receptor antagonists and selective Ang-(1-7) receptor antagonists demonstrate the existence of a distinct Ang-(1-7) receptor in the brain, the G-protein-coupled receptor Mas, encoded by the Mas protooncogene. We also show that the genetic deletion of this receptor abolishes the Ang-(1-7)-induced enhancement of LTP. Thus, we firstly demonstrate that Ang-(1-7) influences the induction of LTP in limbic structures implicating its distinct function in learning and memory mechanisms; secondly, we have identified Mas as a functional receptor for Ang-(1-7) in the brain.

Non-syndromic X-linked mental retardation: From a molecular to a clinical point of view

This review focuses on the 19 identified genes involved in X-linked "non-syndromic" mental retardation (MR) and defines the signaling pathways in which they are involved, focusing on emerging common mechanisms. The majority of proteins are involved in three distinct pathways: (1) Rho GTPases pathway modulating neuronal differentiation and synaptic plasticity; (2) Rab GTPases pathway regulating synaptic vesicle cycling; (3) gene expression regulation. The function of four proteins (ACSL4, AT2, SLC6A8, and SAP102) could not be reconciled to a common pathway. From a clinical point of view, the review discusses whether some common dysmorphic features can be identified even in non-syndromic MR patients and whether it is correct to maintain the distinction between "non-syndromic" and "syndromic" MR.

Increased angiotensin-(1-7)-forming activity in failing human heart ventricles: evidence for upregulation of the angiotensin-converting enzyme Homologue ACE2

BACKGROUND

The formation of angiotensin-(1-7) from either angiotensin (Ang) I or Ang II in failing human hearts is not well understood.

METHODS AND RESULTS

Angiotensinase activity in left and right ventricular membranes from 14 idiopathic dilated cardiomyopathy (IDC), 8 primary pulmonary hypertension (PPH), and 13 nonfailing human hearts was measured with either 125I-Ang I or 125I-Ang II as substrate. Ang-(1-7)-forming activity from 125I-Ang I was inhibited by thiorphan. With 125I-Ang II as substrate, Ang-(1-7) formation was inhibited by the ACE2-specific inhibitor C16. Western blotting with an anti-ACE2 antibody confirmed the presence of ACE2. Angiotensinase activity with 125I-Ang I as substrate was increased in failing IDC left ventricles (LVs) compared with nonfailing LVs (P<0.001). Ang-(1-7)-forming activity with 125I-Ang II as substrate was increased in both failing LVs and right ventricles (RVs) of IDC hearts and only in failing RVs of PPH hearts (PPH LV, 51.12+/-5.25; PPH RV, 89.97+/-11.21; IDC LV, 139.7+/-21.96; and IDC RV, 192.7+/-5.43; NF LV, 32.89+/-5.38; NF RV 40.49+/-10.66 fmol/min per milligram (P<0.05 PPH RV versus PPH LV; P<0.05 PPH RV versus NF RV; P<0.001 IDC LV versus NF LV; P<0.001 IDC RV versus NF RV).

CONCLUSIONS

Ang-(1-7)-forming activity from both Ang I and Ang II was increased in failing human heart ventricles but was mediated by at least two different angiotensinases. The first, which demonstrated substrate preference for Ang I, was neutral endopeptidase (NEP)-like. The second was ACE2, as demonstrated by Western blotting and inhibition of activity with C16.

Angiotensin II accelerates functional recovery in the rat sciatic nerve in vivo: role of the AT2 receptor and the transcription factor NF-kappaB

The AT2 receptor regulates several functions of nerve cells, e.g., ionic fluxes, cell differentiation, and axonal regeneration, but also modulates programmed cell death. We tested the hypothesis that angiotensin II (ANG II) via its AT2 receptor not only promotes regeneration but also functional recovery after sciatic nerve crush in adult rats. ANG II (10(-7), 10(-9), 10(-11) M) applied locally via osmotic minipumps promoted functional recovery with maximal effects after the lowest concentration. The toe spread distance as a parameter for re-innervation after 20 days was significantly (P<0.01) greater (10.2+/-10.27 mm) compared with the control group (8.73+/-0.16 mm). The response to local electrical stimulation (return of sensorimotor function) was reduced to 14.6 days vs. 17.9 days in the control group (P<0.01). The AT2 receptor antagonist PD 123319 administered alone or in combination with ANG II completely prevented the ANG II-induced recovery, whereas the AT1 receptor antagonist losartan had no effect. Furthermore, ANG II induces, via the AT2 receptor, activation of the transcription factor NF-kappaB in Schwann cells. Histological criteria, morphometric analyses, and electron microscopy confirmed the functional data. These results are the first to present direct evidence for an involvement of the AT2 receptor and NF-kappaB in peripheral nerve regeneration.

AngII induces transient phospholipase D activity in the H295R glomerulosa cell model

In this report we demonstrate that in human adrenocortical carcinoma NCI H295R cells, a model for adrenal glomerulosa cells, PLD was activated both by AngII and protein kinase C (PKC)-activating phorbol 12-myristate 13-acetate (PMA). However, while PMA triggered sustained PLD activation, AngII induced transient PLD activation, in contrast to results in bovine glomerulosa cells in primary culture. Despite the transient effect of AngII on PLD activity, PLD-derived lipid signals were required for maximal AngII-elicited aldosterone secretion. AngII-induced PLD activation was inhibited by PKC inhibitors, but not by tyrosine kinase or calcium/calmodulin-dependent kinase inhibitors or a calmodulin antagonist. Both AngII- and PMA-stimulated PLD activity was enhanced by phosphoinositide 3-kinase (PI3K) inhibitors. Akt, a downstream protein kinase activated by the products of PI3K, was constitutively active in H295R cells, and this activity was blocked by PI3K inhibitors. These results suggested that in H295R adrenocortical carcinoma cells, AngII-induced PLD activation was promoted by PKC and inhibited by the constitutively active PI3K pathway.

Age-related decline in the tyrosine hydroxylase-immunoreactive innervation of the amygdala and dentate gyrus in mice

Numbers of catecholaminergic neurons are known to decline with aging. Whether projections of these neurons to the forebrain are similarly affected is not known. High densities of tyrosine hydroxylase-immunoreactive (TH-ir) fibers are found in the hippocampal formation (CA1-3, dentate gyrus) and in the amygdala of normal adult mice. We report here that densities of TH-ir fibers in the amygdala and hippocampus in aged mice (21-26 months) decrease dramatically and in a subregion-specific fashion. There is a reduction of 35% in the dentate gyrus, while hippocampal regions CA1 through CA3 are almost entirely spared. In the amygdala the lateral, basolateral, basomedial, and central nucleus were affected, with fiber reduction ranging from 19% to 34%. These results indicate that the age-related decline of TH-ir catecholaminergic cell bodies in the substantia nigra and the ventral tegmental area induces substantial losses of TH-ir fibers in the amygdala and dentate gyrus, but not in other areas of the hippocampal formation. This suggests that region-specific factors may be implicated in the regulation of maintenance vs. degeneration of TH-ir fibers during aging.