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

Effect of AT1 receptor blockade on cardiovascular outcome after cardiac arrest: an experimental study in rats

Angiotensin II receptor 1(AT1) antagonists are beneficial in focal ischemia/reperfusion (I/R). However, in cases of global I/R, such as cardiac arrest (CA), AT1 blocker's potential benefits are still unknown. Wistar male rats were allocated into four groups: Control group (CG)-animals submitted to CA by ventricular fibrillation induced by direct electrical stimulation for 3 min, and anoxia for 5 min; Group AT1 (GAT1)-animals subjected to CA and treated with 0.2 mg/kg of candesartan diluted in dimethylsulfoxide (DMSO) (0.1%); Vehicle Group (VG): animals subjected to CA and treated with 0.2 ml/kg of DMSO and Sham group (SG)-animals submitted to surgical interventions, without CA. Cardiopulmonary resuscitation consisted of group medications, chest compressions, ventilation, epinephrine (20 mcg/kg) and defibrillation. The animals were observed up to 4 h after spontaneous circulation (ROSC) return, and survival rates, hemodynamic variables, histopathology, and markers of tissue injury were analyzed. GAT1 group had a higher rate of ROSC (62.5% vs. 42.1%, p < 0.0001), survival (100% vs. 62.5%, p = 0.027), lower incidence of arrhythmia after 10 min of ROSC (10% vs. 62.5%, p = 0.000), and lower neuronal and cardiac injury scores on histology evaluation (p = 0.025 and p = 0.0052, respectively) than GC group. The groups did not differ regarding CA duration, number of adrenaline doses, or number of defibrillations. AT1 receptor blockade with candesartan yielded higher rates of ROSC and survival, in addition to neuronal and myocardial protection.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Recent Advances in the Endogenous Brain Renin-Angiotensin System and Drugs Acting on It

The RAS (renin-angiotensin system) is the part of the endocrine system that plays a prime role in the control of essential hypertension. Since the discovery of brain RAS in the seventies, continuous efforts have been put by the scientific committee to explore it more. The brain has shown the presence of various components of brain RAS such as angiotensinogen (AGT), converting enzymes, angiotensin (Ang), and specific receptors (ATR). AGT acts as the precursor molecule for Ang peptides—I, II, III, and IV—while the enzymes such as prorenin, ACE, and aminopeptidases A and N synthesize it. AT1, AT2, AT4, and mitochondrial assembly receptor (MasR) are found to be plentiful in the brain. The brain RAS system exhibits pleiotropic properties such as neuroprotection and cognition along with regulation of blood pressure, CVS homeostasis, thirst and salt appetite, stress, depression, alcohol addiction, and pain modulation. The molecules acting through RAS predominantly ARBs and ACEI are found to be effective in various ongoing and completed clinical trials related to cognition, memory, Alzheimer's disease (AD), and pain. The review summarizes the recent advances in the brain RAS system highlighting its significance in pathophysiology and treatment of the central nervous system-related disorders.

Renin-Angiotensin System Alterations in the Human Alzheimer's Disease Brain

BACKGROUND

Understanding Alzheimer's disease (AD) in terms of its various pathophysiological pathways is essential to unravel the complex nature of the disease process and identify potential therapeutic targets. The renin-angiotensin system (RAS) has been implicated in several brain diseases, including traumatic brain injury, ischemic stroke, and AD.

OBJECTIVE

This study was designed to evaluate the protein expression levels of RAS components in postmortem cortical and hippocampal brain samples obtained from AD versus non-AD individuals.

METHODS

We analyzed RAS components in the cortex and hippocampus of postmortem human brain samples by western blotting and immunohistochemical techniques in comparison with age-matched non-demented controls.

RESULTS

The expression of AT1R increased in the hippocampus, whereas AT2R expression remained almost unchanged in the cortical and hippocampal regions of AD compared to non-AD brains. The Mas receptor was downregulated in the hippocampus. We also detected slight reductions in ACE-1 protein levels in both the cortex and hippocampus of AD brains, with minor elevations in ACE-2 in the cortex. We did not find remarkable differences in the protein levels of angiotensinogen and Ang II in either the cortex or hippocampus of AD brains, whereas we observed a considerable increase in the expression of brain-derived neurotrophic factor in the hippocampus.

CONCLUSION

The current findings support the significant contribution of RAS components in AD pathogenesis, further suggesting that strategies focusing on the AT1R and AT2R pathways may lead to novel therapies for the management of AD.

The Renin-Angiotensin System Modulates Dopaminergic Neurotransmission: A New Player on the Scene

Parkinson’s disease (PD) is an extrapyramidal disorder characterized by neuronal degeneration in several regions of the peripheral and central nervous systems. It is the second most frequent neurodegenerative disease after Alzheimer’s. It has become a major health problem, affecting 1% of the world population over 60 years old and 3% of people beyond 80 years. The main histological findings are intracellular Lewy bodies composed of misfolded α-synuclein protein aggregates and loss of dopaminergic neurons in the central nervous system. Neuroinflammation, apoptosis, mitochondrial dysfunction, altered calcium homeostasis, abnormal protein degradation, and synaptic pathobiology have been put forward as mechanisms leading to cell death, α-synuclein deposition, or both. A progressive loss of dopaminergic neurons in the substantia nigra late in the neurodegeneration leads to developing motor symptoms like bradykinesia, tremor, and rigidity. The renin–angiotensin system (RAS), which is involved in regulating blood pressure and body fluid balance, also plays other important functions in the brain. The RAS is involved in the autocrine and paracrine regulation of the nigrostriatal dopaminergic synapses. Dopamine depletion, as in PD, increases angiotensin II expression, which stimulates or inhibits dopamine synthesis and is released via AT1 or AT2 receptors. Furthermore, angiotensin II AT1 receptors inhibit D1 receptor activation allosterically. Therefore, the RAS may have an important modulating role in the flow of information from the brain cortex to the basal ganglia. High angiotensin II levels might even aggravate neurodegeneration, activating the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, which leads to increased reactive oxygen species production.

Association between Use of Angiotensin-converting Enzyme Inhibitors or Angiotensin Receptor Blockers and Postoperative Delirium

Background

Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers improve cognitive function. The authors therefore tested the primary hypothesis that preoperative use of angiotensin inhibitors is associated with less delirium in critical care patients. Post hoc, the association between postoperative use of angiotensin system inhibitors and delirium was assessed.

Methods

The authors conducted a single-site cohort study of adults admitted to Cleveland Clinic critical care units after noncardiac procedures between 2013 and 2018 who had at least one Confusion Assessment Method delirium assessment. Patients with preexisting dementia, Alzheimer’s disease or other cognitive decline, and patients who had neurosurgical procedures were excluded. For the primary analysis, the confounder-adjusted association between preoperative angiotensin inhibitor use and the incidence of postoperative delirium was assessed. Post hoc, the confounder-adjusted association between postoperative angiotensin system inhibitor use and the incidence of delirium was assessed.

Results

The incidence of delirium was 39% (551 of 1,396) among patients who were treated preoperatively with angiotensin system inhibitors and 39% (1,344 of 3,468) in patients who were not. The adjusted odds ratio of experiencing delirium during critical care was 0.98 (95% CI, 0.86 to 1.10; P = 0.700) for preoperative use of angiotensin system inhibitors versus control. Delirium was observed in 23% (100 of 440) of patients who used angiotensin system inhibitors postoperatively before intensive care discharge, and in 41% (1,795 of 4,424) of patients who did not (unadjusted P &lt; 0.001). The confounder-adjusted odds ratio for experiencing delirium in patients who used angiotensin system inhibitors postoperatively was 0.55 (95% CI, 0.43 to 0.72; P &lt; 0.001).

Conclusions

Preoperative use of angiotensin system inhibitors is not associated with reduced postoperative delirium. In contrast, treatment during intensive care was associated with lower odds of delirium. Randomized trials of postoperative angiotensin-converting enzymes inhibitors and angiotensin receptor blockers seem justified.

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Serum Level of the Angiotensin-Converting Enzyme in Patients with Idiopathic Acute Optic Neuritis: A Case-Control Study

PURPOSE

To evaluate the serum level of angiotensin-converting enzyme (ACE) as an important component of the renin-angiotensin system (RAS) in optic neuritis (ON) compared to the healthy control group in the context investigating the possible role of ACE in ON pathogenesis.

METHODS

This case-control study was conducted on patients with ON and healthy controls. Serum ACE levels were assessed and compared between the two groups by using commercially available kits by ELISA for ACE.

RESULTS

Sixty-five ON patients (75.4% female, mean age 29.70 ± 8.30 years) and 65 controls (75.4% female, mean age 29.66 ± 8.36 years) were enrolled. The median serum ACE levels were 33.5 U/L (range: 25-540) and 26 U/L (range: 22.3-72) for the ON patients and controls, respectively. Serum ACE levels were significantly higher in the patients than in the control group (P < 0.001). High level of serum ACE (defined as a serum ACE >65 U/L) was present in 9 (13.8%) patients with ON and 2 (3.1%) controls.

CONCLUSION

Our results indicated that the serum level of ACE appeared to be significantly higher in acute ON than in normal controls.

Role of brain renin angiotensin system in neurodegeneration: An update

Renin angiotensin system (RAS) is an endocrine system widely known for its physiological roles in electrolyte homeostasis, body fluid volume regulation and cardiovascular control in peripheral circulation. However, brain RAS is an independent form of RAS expressed locally in the brain, which is known to be involved in brain functions and disorders. There is strong evidence for a major involvement of excessive brain angiotensin converting enzyme (ACE)/Angiotensin II (Ang II)/Angiotensin type-1 receptor (AT-1R) axis in increased activation of oxidative stress, apoptosis and neuroinflammation causing neurodegeneration in several brain disorders. Numerous studies have demonstrated strong neuroprotective effects by blocking AT1R in these brain disorders. Additionally, the angiotensin converting enzyme 2 (ACE2)/Angiotensin (1–7)/Mas receptor (MASR), is another axis of brain RAS which counteracts the damaging effects of ACE/Ang II/AT1R axis on neurons in the brain. Thus, angiotensin II receptor blockers (ARBs) and activation of ACE2/Angiotensin (1–7)/MASR axis may serve as an exciting and novel method for neuroprotection in several neurodegenerative diseases. Here in this review article, we discuss the expression of RAS in the brain and highlight how altered RAS level may cause neurodegeneration. Understanding the pathophysiology of RAS and their links to neurodegeneration has enormous potential to identify potentially effective pharmacological tools to treat neurodegenerative diseases in the brain.

Cerebral ischemia induced inflammatory response and altered glutaminergic function mediated through brain AT1 and not AT2 receptor

In the present study, we investigated the effects of angiotensin (Ang II) receptor blockers in cerebral ischemia by administration of telmisartan (AT₁ blocker) and/or PD123319 (AT₂ blocker) in global ischemic mice model. The neuroprotective effect of AT antagonists was evaluated through monitoring muscle co-ordination and cerebral blood perfusion in ischemic mice. Gene expression studies (NF-κB, GSK-3β, EAAT-2, AT₁ & AT₂ receptors) and staining of brain regions with cresyl violet, GFAP, synaptophysin and NSE methods were carried out in to understand the molecular mechanisms. Further, the brain glutamate, cytokines, and Ang II peptide levels were evaluated and their correlation with EAAT-2 mRNA expression was performed. Our results indicate that the induction of ischemia elevates brain Ang II, cytokines, and glutamate levels and reduced muscle co-ordination and cerebral blood perfusion. The expressions of NF-κB, GSK-3β and AT₁ were significantly increased, whereas, EAAT-2 expression was decreased. Blocking of AT₁ receptors by telmisartan (TM) reversed the detrimental responses of cerebral ischemia and restored the cerebral blood flow denoting blockade of Ang II/AT₁ pathway is beneficial in ischemia, whereas, blockade of AT₂ receptors by PD123319 (PD) increased the ischemic injury in mice. This vulnerable effect of PD may be attributed through augmenting the Ang II/AT₁ dependent cytokines mediated glutamate transporter (EAAT-2) dysfunction. Interestingly, the beneficial effects of AT₁ blocker was remarkably antagonized by AT₂ blocker in most of the parameters studied in ischemic conditions. Also, the expression of AT₂ receptors was significantly increased compared to that of AT₁ receptors upon ischemic induction. It denotes that the endogenous Ang II predominantly acts on AT₂ receptor, thereby promoting its own mRNA transcription. Hence, the increased expression of AT₂ receptors in ischemic condition could be used as target protein for therapeutic benefit.

The renin angiotensin system and the brain: New developments

The traditional renin-angiotensin system (RAS) is indispensable system in adjusting sodium homeostasis, body fluid volume, and controlling arterial blood pressure. The key elements are renin splitting inactive angiotensinogen to yield angiotensin (Ang-I). Ang-1 is then changed by angiotensin-1 converting enzyme (ACE) into angiotensin II (Ang-II). Using PubMed, Google Scholar, and other means, we searched the peer-reviewed literature from 1990 to 2013 for articles on newly discovered findings related to the RAS, especially focusing on how the system influences the central nervous system (CNS). The classical RAS is now considered to be only part of the picture; the discovery of additional RAS pathways in the brain and elsewhere has yielded a vastly improved understanding of how the RAS influences the CNS. Newly discovered effects of the RAS on brain tissue include neuroprotection, cognition, and cerebral vasodilation. A number of brain biochemical pathways are influenced by the brain RAS. Within various pathways, there are potential opportunities for classical pharmacologic interventions as well as the possibility of controlling gene expression.

Alternative renin-angiotensin system pathways in adipose tissue and their role in the pathogenesis of obesity

Adipose tissue expresses all the renin-angiotensin system (RAS) components that play an important role in the adipogenesis, lipid and glucose metabolism regulation in an auto/paracrine manner. The classical RAS has been found to be over-activated during the adipose tissue enlargement, thus elevated generation of angiotensin II (Ang II) may contribute to the obesity pathogenesis. The contemporary view on the RAS has become more complex with the discovery of alternative pathways, including angiotensin-converting enzyme 2 (ACE2)/angiotensin (Ang)-(1-7)/Mas receptor, (pro)renin receptor, as well as angiotensin IV(Ang IV)/AT4 receptor. Ang-(1-7) via Mas receptor counteracts with most of the deleterious effects of the Ang II-mediated by AT1 receptor implying its beneficial role in the glucose and lipid metabolism, oxidative stress, inflammation, and insulin resistance. Pro(renin) receptor may play a role (at least partial) in the pathogenesis of the obesity by increasing the local production of Ang II in adipose tissue as well as triggering signal transduction independently of Ang II. In this review, modulation of alternative RAS pathways in adipose tissue during obesity is discussed and the involvement of Ang-(1-7), (pro)renin and AT4 receptors in the regulation of adipose tissue homeostasis and insulin resistance is summarized.

The High Blood Pressure-Malaria Protection Hypothesis

RATIONALE

A recently proposed hypothesis states that malaria may contribute to hypertension in endemic areas,¹ but the role of angiotensin II (Ang II), a major regulator of blood pressure, was not considered. Elevated levels of Ang II may confer protection against malaria morbidity and mortality, providing an alternative explanation for hypertension in malaria endemic areas.

OBJECTIVE

To discuss a possible alternative cause for hypertension in populations who have been under the selective pressure of malaria.

METHODS AND RESULTS

We reviewed published scientific literature for studies that could establish a link between Ang II and malaria. Both genetic and functional studies suggested that high levels of Ang II may confer protection against cerebral malaria by strengthening the integrity of the endothelial brain barrier. We also describe strong experimental evidence supporting our hypothesis through genetic, functional, and interventional studies.

CONCLUSIONS

A causal association between high levels of Ang II and protection from malaria pathogenesis can provide a likely explanation for the increased prevalence in hypertension observed in populations of African and South Asian origin. Furthermore, this potential causative connection might also direct unique approaches for the effective treatment of cerebral malaria.

Telmisartan prevention of LPS-induced microglia activation involves M2 microglia polarization via CaMKKβ-dependent AMPK activation

Brain inflammation plays an important role in the pathophysiology of many psychiatric and neurological diseases. During brain inflammation, microglia cells are activated, producing neurotoxic molecules and neurotrophic factors depending on their pro-inflammatory M1 and anti-inflammatory M2 phenotypes. It has been demonstrated that Angiotensin II type 1 receptor blockers (ARBs) ameliorate brain inflammation and reduce M1 microglia activation. The ARB telmisartan suppresses glutamate-induced upregulation of inflammatory genes in cultured primary neurons. We wished to clarify whether telmisartan, in addition, prevents microglia activation through polarization to an anti-inflammatory M2 phenotype. We found that telmisartan promoted M2 polarization and reduced M1 polarization in LPS-stimulated BV2 and primary microglia cells, effects partially dependent on PPARγ activation. The promoting effects of telmisartan on M2 polarization, were attenuated by an AMP-activated protein kinase (AMPK) inhibitor or AMPK knockdown, indicating that AMPK activation participates on telmisartan effects. Moreover, in LPS-stimulated BV2 cells, telmisartan enhancement of M2 gene expression was prevented by the inhibitor STO-609 and siRNA of calmodulin-dependent protein kinase kinase β (CaMKKβ), an upstream kinase of AMPK. Furthermore, telmisartan enhanced brain AMPK activation and M2 gene expression in a mouse model of LPS-induced neuroinflammation. In addition, telmisartan reduced the LPS-induced sickness behavior in this in vivo model, and this effect was prevented by prior administration of an AMPK inhibitor. Our results indicate that telmisartan can be considered as a novel AMPK activator, suppressing microglia activation by promoting M2 polarization. Telmisartan may provide a novel, safe therapeutic approach to treat brain disorders associated with enhanced inflammation.

Angiotensin II in inflammation, immunity and rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune inflammatory disease that is characterized by increased cardiovascular morbidity and mortality, independent of the traditional risk factors for cardiovascular disease. Although classically known for its role in the regulation of circulatory homeostasis, angiotensin II (Ang II) is recognized to act as a powerful proinflammatory mediator. Some research has showed that Ang II plays important roles in autoimmune diseases, including RA, systemic lupus erythematosus and multiple sclerosis. Ang II blockers prove effective in reducing inflammation and autoimmunity in rheumatic diseases and their relative safety, together with their effects for reducing the cardiovascular disease risk, suggest that Ang II blockers may at least act as effective adjunctive therapy for disease control in patients with RA. The present review focuses systematically on the potential impact of Ang II and its receptors on inflammation and immunomodulation in patients with RA.

Candesartan restores pressure-induced vasodilation and prevents skin pressure ulcer formation in diabetic mice

Background

Angiotensin II type 1 receptor (AT1R) blockers have beneficial effects on neurovascular complications in diabetes and in organ’s protection against ischemic episodes. The present study examines whether the AT1R blocker candesartan (1) has a beneficial effect on diabetes-induced alteration of pressure-induced vasodilation (PIV, a cutaneous physiological neurovascular mechanism which could delay the occurrence of tissue ischemia), and (2) could be protective against skin pressure ulcer formation.

Methods

Male Swiss mice aged 5–6 weeks were randomly assigned to four experimental groups. In two groups, diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ, 200 mg.kg⁻¹). After 6 weeks, control and STZ mice received either no treatment or candesartan (1 mg/kg-daily in drinking water) during 2 weeks. At the end of treatment (8 weeks of diabetes duration), C-fiber mediated nociception threshold, endothelium-dependent vasodilation and PIV were assessed. Pressure ulcers (PUs) were then induced by pinching the dorsal skin between two magnetic plates for three hours. Skin ulcer area development was assessed during three days, and histological examination of the depth of the skin lesion was performed at day three.

Results

After 8 weeks of diabetes, the skin neurovascular functions (C-fiber nociception, endothelium-dependent vasodilation and PIV) were markedly altered in STZ-treated mice, but were fully restored by treatment with candesartan. Whereas in diabetes mice exposure of the skin to pressure induced wide and deep necrotic lesions, treatment with candersartan restored their ability to resist to pressure-induced ulceration as efficiently as the control mice.

Conclusion

Candesartan decreases the vulnerability to pressure-induced ulceration and restores skin neurovascular functions in mice with STZ-induced established diabetes.

The protective arms of the renin-angiontensin system in stroke

It is quite well established that activation of the so-called protective arms of the renin-angiotensin system (RAS), involving both AT2 and Mas receptors, provides a counter-regulatory role to AT1 receptor overactivity that may drive pathological changes in the cardiovascular system. In this brief review, we will focus on recent evidence that identifies at least three different pathways that may be effective in the setting of stroke and may be complementary with AT1 receptor blockade. Such mechanisms include AT2 receptor stimulation, Mas receptor stimulation and insulin-regulated aminopeptidase blockade. This report highlights recent data demonstrating striking neuroprotective effects in preclinical models of stroke targeting each of these pathways, which may pave the way for translational opportunities in this field.

Interplay between brain stem angiotensins and monocyte chemoattractant protein-1 as a novel mechanism for pressor response after ischemic stroke

Pressor response after stroke commonly leads to early death or susceptibility to stroke recurrence, and detailed mechanisms are still lacking. We assessed the hypothesis that the renin-angiotensin system contributes to pressor response after stroke by differential modulation of the pro-inflammatory chemokine monocyte chemoattractant protein-1 (MCP-1) in the rostral ventrolateral medulla (RVLM), a key brain stem site that maintains blood pressure. We also investigated the beneficial effects of a novel renin inhibitor, aliskiren, against stroke-elicited pressor response. Experiments were performed in male adult Sprague-Dawley rats. Stroke induced by middle cerebral artery occlusion elicited significant pressor response, accompanied by activation of angiotensin II (Ang II)/type I receptor (AT1R) and AT2R signaling, depression of Ang-(1-7)/MasR and Ang IV/AT4R cascade, alongside augmentation of MCP-1/C-C chemokine receptor 2 (CCR2) signaling and neuroinflammation in the RVLM. Stroke-elicited pressor response was significantly blunted by antagonism of AT1R, AT2R or MCP-1/CCR2 signaling, and eliminated by applying Ang-(1-7) or Ang IV into the RVLM. Furthermore, stroke-activated MCP-1/CCR2 signaling was enhanced by AT1R and AT2R activation, and depressed by Ang-(1-7)/MasR and Ang IV/AT4R cascade. Aliskiren inhibited stroke-elicited pressor response via downregulating MCP-1/CCR2 activity and reduced neuroinflammation in the RVLM; these effects were potentiated by Ang-(1-7) or Ang IV. We conclude that whereas Ang II/AT1R or Ang II/AT2R signaling in the brain stem enhances, Ang-(1-7)/MasR or Ang IV/AT4R antagonizes pressor response after stroke by differential modulations of MCP-1 in the RVLM. Furthermore, combined administration of aliskiren and Ang-(1-7) or Ang IV into the brain stem provides more effective amelioration of stroked-induced pressor response.

Improvement in spatial memory dysfunction by telmisartan through reduction of brain angiotensin II and oxidative stress in experimental uremic mice

AIMS

We previously reported that chronic uremia induces spatial working memory dysfunction in mice, and that it is attributed to cerebral oxidative stress. The source of oxidative stress was considered to be uremic toxins, but this remains unclear. In the present study, we examined whether the brain renin-angiotensin system was activated in the CKD mouse model, and whether it contributed to cognitive impairment.

MAIN METHODS

CKD was induced in 8-week-old male mice by 5/6 nephrectomy. Mice were divided into four groups: control mice administered tap water (Cont-V), control mice treated with 0.5mg/kg/day telmisartan, an angiotensin II (AII) receptor blocker, for 8 weeks (Cont-T), CKD mice administered tap water (CKD-V), and CKD mice treated with 0.5 mg/kg/day telmisartan for 8 weeks (CKD-T). After the treatment period, a radial arm water maze (RAWM) test was performed, and angiotensin II (AII) concentrations and markers of oxidative stress were measured in the brains of mice.

KEY FINDINGS

Errors in the RAWM test were more frequent in the CKD-V group than in the Cont-V group. In addition, errors in the CKD-T group were comparable to control mice. Tissue brain AII concentrations were greater in the CKD-V group compared with the other groups. Oxidative DNA damage and lipid peroxidation in the brain were also greater in the CKD-V group compared with the other groups.

SIGNIFICANCE

Our results suggest that brain AII levels were exaggerated in CKD mice, and that this contributes to cognitive impairment through oxidative stress.

Angiotensin receptor type 2 activation induces neuroprotection and neurogenesis after traumatic brain injury

Angiotensin II receptor type 2 (AT(2)) agonists have been shown to limit brain ischemic insult and to improve its outcome. The activation of AT(2) was also linked to induced neuronal proliferation and differentiation in vitro. In this study, we examined the therapeutic potential of AT(2) activation following traumatic brain injury (TBI) in mice, a brain pathology that displays ischemia-like secondary damages. The AT(2) agonist CGP42112A was continuously infused immediately after closed head injury (CHI) for 3 days. We have followed the functional recovery of the injured mice for 35 days post-CHI, and evaluated cognitive function, lesion volume, molecular signaling, and neurogenesis at different time points after the impact. We found dose-dependent improvement in functional recovery and cognitive performance after CGP42112A treatment that was accompanied by reduced lesion volume and induced neurogenesis in the neurogenic niches of the brain and also in the injury region. At the cellular/molecular level, CGP42112A induced early activation of neuroprotective kinases protein kinase B (Akt) and extracellular-regulated kinases ½ (ERK½), and the neurotrophins nerve growth factor and brain-derived neurotrophic factor; all were blocked by treatment with the AT(2) antagonist PD123319. Our results suggest that AT(2) activation after TBI promotes neuroprotection and neurogenesis, and may be a novel approach for the development of new drugs to treat victims of TBI.

Possible synergistic effect of direct angiotensin II type 2 receptor stimulation by compound 21 with memantine on prevention of cognitive decline in type 2 diabetic mice

Type 2 diabetes mellitus (T2DM) is known to be associated with increased risk of cognitive impairment including Alzheimer disease. Recent studies have suggested an interaction between angiotensin II and N-methyl-d-aspartic acid (NMDA) glutamate receptors. We previously reported that stimulation of the angiotensin II type 2 (AT2) receptor exerts brain protective effects. A newly developed AT2 receptor agonist, compound 21 (C21), has enabled examination of the direct effect of AT2 receptor stimulation in vivo. Accordingly, we examined the possible synergistic effect of C21 and memantine on cognitive impairment in T2DM mice, KKAy. KKAy were divided into four groups; (1) control, (2) treatment with C21 (10 μg/kg/day), (3) treatment with memantine (20mg/kg/day), and (4) treatment with both for 4 weeks, and subjected to Morris water maze tasks. Treatment with C21 or memantine alone at these doses tended to shorten escape latency compared to that in the control group. C21 treatment increased cerebral blood flow (CBF), but memantine did not influence CBF. Treatment with C21 or C21 plus memantine increased hippocampal field-excitatory postsynaptic potential (f-EPSP). Moreover, treatment with memantine or C21 increased acetylcholine level, which was lower in KKAy than in wild-type mice, and C21 plus memantine treatment enhanced memantine or C21-induced acetylcholine secretion. This study provides an insight into new approaches to understand the interaction of angiotensin II and neurotransmitters. We can anticipate a new therapeutic approach against cognitive decline using C21 and memantine.

Telmisartan ameliorates glutamate-induced neurotoxicity: roles of AT(1) receptor blockade and PPARγ activation

Sartans (Angiotensin II AT(1) Receptor Blockers, ARBs) are powerful neuroprotective agents in vivo and protect against IL-1β neurotoxicity in vitro. The purpose of this research was to determine the extent of sartan neuroprotection against glutamate excitotoxicity, a common cause of neuronal injury and apoptosis. The results show that sartans are neuroprotective, significantly reducing glutamate-induced neuronal injury and apoptosis in cultured rat primary cerebellar granule cells (CGCs). Telmisartan was the most potent sartan studied, with an order of potency telmisartan > candesartan > losartan > valsartan. Mechanisms involved reduction of pro-apoptotic caspase-3 activation, protection of the survival PI3K/Akt/GSK-3β pathway and prevention of glutamate-induced ERK1/2 activation. NMDA receptor stimulation was essential for glutamate-induced cell injury and apoptosis. Participation of AT(1A) receptor was supported by glutamate-induced upregulation of AT(1A) gene expression and AT(1) receptor binding. Conversely, AT(1B) or AT(2) receptors played no role. Glutamate-induced neuronal injury and the neuroprotective effect of telmisartan were decreased, but not abolished, in CGCs obtained from AT(1A) knock-out mice. This indicates that although AT(1) receptors are necessary for glutamate to exert its full neurotoxic potential, part of the neuroprotective effect of telmisartan is independent of AT(1) receptor blockade. PPARγ activation was also involved in the neuroprotective effects of telmisartan, as telmisartan enhanced PPARγ nuclear translocation and the PPARγ antagonist GW9662 partially reversed the neuroprotective effects of telmisartan. The present results substantiate the therapeutic use of sartans, in particular telmisartan, in neurodegenerative diseases and traumatic brain disorders where glutamate neurotoxicity plays a significant role.

Promoter methylation represses AT2R gene and increases brain hypoxic-ischemic injury in neonatal rats

Perinatal nicotine exposure downregulated angiotensin II type 2 receptor (AT2R) in the developing brain and increased brain vulnerability to hypoxic-ischemic injury in male neonatal rats. We tested the hypothesis that site-specific CpG methylation at AT2R gene promoter contributes to the increased vulnerability of brain injury in the neonate. Nicotine was administered to pregnant rats from day 4 of gestation to day 10 after birth. Brain hypoxic-ischemic injury was induced in day 10 male pups. CpG methylation at AT2R promoter was determined in the brain by quantitative methylation-specific PCR. Nicotine exposure significantly increased the methylation of a single CpG-52 locus near the TATA-box at AT2R promoter. Electrophoretic mobility shift assay indicated that the methylation of CpG-52 significantly decreased the binding affinity of TATA-binding protein (TBP). Chromatin immunoprecipitation assay further demonstrated an increase in the binding of a methyl-binding protein and a decrease in TBP binding to AT2R promoter in vivo in neonatal brains of nicotine-treated animals. This resulted in AT2R gene repression in the brain. Intracerebroventricular administration of a demethylating agent 5-aza-2'-deoxycytidine abrogated the enhanced methylation of CpG-52, rescued the TBP binding, and restored AT2R gene expression. Of importance, 5-aza-2'-deoxycytidine reversed the nicotine-increased vulnerability of brain hypoxic-ischemic injury in the neonate. The finding provides mechanistic evidence of increased promoter methylation and resultant AT2R gene repression in the developing brain linking perinatal stress and a pathophysiological consequence of heightened vulnerability of brain hypoxic-ischemic encephalopathy in the neonate.

Vascular aspects of cognitive impairment and dementia

Hypertension and stroke are highly prevalent risk factors for cognitive impairment and dementia. Alzheimer's disease (AD) and vascular dementia (VaD) are the most common forms of dementia, and both conditions are preceded by a stage of cognitive impairment. Stroke is a major risk factor for the development of vascular cognitive impairment (VCI) and VaD; however, stroke may also predispose to AD. Hypertension is a major risk factor for stroke, thus linking hypertension to VCI and VaD, but hypertension is also an important risk factor for AD. Reducing these two major, but modifiable, risk factors-hypertension and stroke-could be a successful strategy for reducing the public health burden of cognitive impairment and dementia. Intake of long-chain omega-3 polyunsaturated fatty acids (LC-n3-FA) and the manipulation of factors involved in the renin-angiotensin system (e.g. angiotensin II or angiotensin-converting enzyme) have been shown to reduce the risk of developing hypertension and stroke, thereby reducing dementia risk. This paper will review the research conducted on the relationship between hypertension, stroke, and dementia and also on the impact of LC-n3-FA or antihypertensive treatments on risk factors for VCI, VaD, and AD.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

A modern understanding of the traditional and nontraditional biological functions of angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

Roles of Brain Angiotensin II in Cognitive Function and Dementia

The brain renin-angiotensin system (RAS) has been highlighted as having a pathological role in stroke, dementia, and neurodegenerative disease. Particularly, in dementia, epidemiological studies indicate a preventive effect of RAS blockade on cognitive impairment in Alzheimer disease (AD). Moreover, basic experiments suggest a role of brain angiotensin II in neural injury, neuroinflammation, and cognitive function and that RAS blockade attenuates cognitive impairment in rodent dementia models of AD. Therefore, RAS regulation is expected to have therapeutic potential for AD. Here, we discuss the role of angiotensin II in cognitive impairment and AD. Angiotensin II binds to the type 2 receptor (AT₂) and works mainly by binding with the type 1 receptor (AT₁). AT₂ receptor signaling plays a role in protection against multiple-organ damage. A direct AT₂ receptor agonist is now available and is expected to reduce inflammation and oxidative stress and enhance cell differentiation. We and other groups reported that AT₂ receptor activation enhances neuronal differentiation and neurite outgrowth in the brain. Here, we also review the effect of the AT₂ receptor on cognitive function. RAS modulation may be a new therapeutic option for dementia including AD in the future.

Angiotensin II Type 2 Receptor Stimulation Initiated After Stroke Causes Neuroprotection in Conscious Rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT 2 R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT 2 R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT 2 R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32±13 mm 3 versus vehicle, 170±49 mm 3 ; P <0.05) and improved motor function. Furthermore, we have demonstrated that AT 2 R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT 2 R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT 2 R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

Angiotensin II Type 2 Receptor Stimulation Initiated After Stroke Causes Neuroprotection in Conscious Rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT 2 R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT 2 R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT 2 R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32±13 mm 3 versus vehicle, 170±49 mm 3 ; P <0.05) and improved motor function. Furthermore, we have demonstrated that AT 2 R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT 2 R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT 2 R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

Angiotensin II Type 2 Receptor Stimulation Initiated After Stroke Causes Neuroprotection in Conscious Rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT 2 R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT 2 R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT 2 R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32±13 mm 3 versus vehicle, 170±49 mm 3 ; P <0.05) and improved motor function. Furthermore, we have demonstrated that AT 2 R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT 2 R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT 2 R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

Angiotensin II Type 2 Receptor Stimulation Initiated After Stroke Causes Neuroprotection in Conscious Rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT 2 R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT 2 R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT 2 R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32±13 mm 3 versus vehicle, 170±49 mm 3 ; P <0.05) and improved motor function. Furthermore, we have demonstrated that AT 2 R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT 2 R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT 2 R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

Angiotensin II Type 2 Receptor Stimulation Initiated After Stroke Causes Neuroprotection in Conscious Rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT 2 R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT 2 R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT 2 R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32±13 mm 3 versus vehicle, 170±49 mm 3 ; P <0.05) and improved motor function. Furthermore, we have demonstrated that AT 2 R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT 2 R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT 2 R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

Angiotensin II Type 2 Receptor Stimulation Initiated After Stroke Causes Neuroprotection in Conscious Rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT 2 R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT 2 R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT 2 R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32±13 mm 3 versus vehicle, 170±49 mm 3 ; P <0.05) and improved motor function. Furthermore, we have demonstrated that AT 2 R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT 2 R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT 2 R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

Angiotensin II Type 2 Receptor Stimulation Initiated After Stroke Causes Neuroprotection in Conscious Rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT 2 R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT 2 R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT 2 R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32±13 mm 3 versus vehicle, 170±49 mm 3 ; P <0.05) and improved motor function. Furthermore, we have demonstrated that AT 2 R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT 2 R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT 2 R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

Angiotensin II type 2 receptor stimulation initiated after stroke causes neuroprotection in conscious rats

We have demonstrated previously that pretreatment with an angiotensin II type 2 receptor (AT(2)R) agonist is neuroprotective against a subsequent stroke independent of any changes in blood pressure. Therefore, in the current study, we have examined the potential neuroprotective effect of AT(2)R stimulation initiated after stroke induction to mimic the clinical setting. Intracerebroventricular administration of the AT(2)R agonist CGP42112 was commenced 6 hours after an ischemic stroke had been induced in conscious spontaneously hypertensive rats. CGP42112 given over 4 doses in the same rats (3 µg/kg per dose centrally) at 6, 24, 48, and 72 hours after stroke induction reduced total infarct volume (32 ± 13 mm(3) versus vehicle, 170 ± 49 mm(3); P<0.05) and improved motor function. Furthermore, we have demonstrated that AT(2)R stimulation after stroke increased neuronal survival, decreased apoptosis, and caused an increase in the number of activated microglia in the core region of damage. The effects of CGP42112 were partially reversed with the coadministration of an AT(2)R antagonist, PD123319. Thus, the current study has shown for the first time that delayed central AT(2)R stimulation after a cerebral incident is neuroprotective in a conscious rat model of stroke.

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.

Neuroprotective effect of an angiotensin receptor type 2 agonist following cerebral ischemia in vitro and in vivo

Background

Intracerebral administration of the angiotensin II type 2 receptor (AT₂R) agonist, CGP42112, is neuroprotective in a rat model of ischemic stroke. To explore further its possible cellular target(s) and therapeutic utility, we firstly examined whether CGP42112 may exert direct protective effects on primary neurons following glucose deprivation in vitro. Secondly, we tested whether CGP42112 is effective when administered systemically in a mouse model of cerebral ischemia.

Methods

Primary cortical neurons were cultured from E17 C57Bl6 mouse embryos for 9 d, exposed to glucose deprivation for 24 h alone or with drug treatments, and percent cell survival assessed using trypan blue exclusion. Ischemic stroke was induced in adult male C57Bl6 mice by middle cerebral artery occlusion for 30 min, followed by reperfusion for 23.5 h. Neurological assessment was performed and then mice were euthanized and infarct and edema volume were analysed.

Results

During glucose deprivation, CGP42112 (1x10^-8 M and 1x10^-7 M) reduced cell death by ~30%, an effect that was prevented by the AT₂R antagonist, PD123319 (1x10^-6 M). Neuroprotection by CGP42112 was lost at a higher concentration (1x10^-6 M) but was unmasked by co-application with the AT₁R antagonist, candesartan (1x10^-7 M). By contrast, Compound 21 (1x10^-8 M to 1x10^-6 M), a second AT₂R agonist, had no effect on neuronal survival. Mice treated with CGP42112 (1 mg/kg i.p.) after cerebral ischemia had improved functional outcomes over vehicle-treated mice as well as reduced total and cortical infarct volumes.

Conclusions

These results indicate that CGP42112 can directly protect neurons from ischemia-like injury in vitro via activation of AT₂Rs, an effect opposed by AT₁R activation at high concentrations. Furthermore, systemic administration of CGP42112 can reduce functional deficits and infarct volume following cerebral ischemia in vivo.

Effect of angiotensin II type 2 receptor on stroke, cognitive impairment and neurodegenerative diseases

Here, we briefly review the role of the renin-angiotensin system (RAS) in cognitive impairment and neurodegenerative disease, mainly discussing our experimental studies on the angiotensin II type 2 (AT(2)) receptor. Ischemic brain damage is enhanced in mice with overexpression of angiotensin II, with reduced cerebral blood flow in the penumbra and an increase in oxidative stress in the ischemic area. Angiotensin II binds two types of receptors, type 1 (AT(1)) and type 2 (AT(2)). Our previous experiments showed that AT(1) receptor signaling has a harmful effect, and AT(2) receptor signaling has a protective effect on the brain after stroke. AT(2) receptor signaling in bone marrow stromal cells or hematopoietic cells was shown to prevent ischemic brain damage after middle cerebral artery occlusion. In contrast, AT(2) receptor signaling also affects cognitive function. We showed that direct stimulation of the AT(2) receptor by a newly generated direct AT(2) receptor agonist, Compound 21 (C21), enhanced cognitive function in wild-type (C57BL6) mice and an Alzheimer's disease mouse model with intracerebroventricular injection of amyloid β (1-40). Finally, we carried out clinical research by investigating the levels of RAS components in patients with neurodegenerative diseases. We observed a reduction of angiotensin II and angiotensin converting enzyme (ACE) 2 levels, and an increase in ACE level in cerebrospinal fluid from patients with multiple sclerosis. These results suggest that RAS is also involved in neurodegenerative disease. Therefore, regulation of RAS might be a new therapeutic target to protect neurons from neural diseases.

Telmisartan directly ameliorates the neuronal inflammatory response to IL-1β partly through the JNK/c-Jun and NADPH oxidase pathways

Background

Blockade of angiotensin II type 1 (AT₁) receptors ameliorates brain inflammation, and reduces excessive brain interleukin-1 beta (IL-1β) production and release from cortical microglia. The aim of this study was to determine whether, in addition, AT₁ receptor blockade directly attenuates IL-1β-induced inflammatory responses in neuronal cultures.

Methods

SK-N-SH human neuroblasts and primary rat cortical neurons were pretreated with telmisartan followed by exposure to IL-1β. Gene expression was determined by reverse transcriptase (RT)-PCR, protein expression and kinase activation by western blotting, NADPH oxidase activity by the lucigenin method, prostaglandin E₂ (PGE₂) release by enzyme immunoassay, reactive oxygen species (ROS) generation by the dichlorodihydrofluorescein diacetate fluorescent probe assay, and peroxisome proliferator-activated receptor gamma (PPARγ) involvement was assessed with the antagonists GW9662 and T0070907, the agonist pioglitazone and the expression of PPARγ target genes ABCG1 and CD36.

Results

We found that SK-N-SH neuroblasts expressed AT₁ but not AT₂ receptor mRNA. Telmisartan reduced IL-1β-induced cyclooxygenase-2 (COX-2) expression and PGE₂ release more potently than did candesartan and losartan. Telmisartan reduced the IL-1β-induced increase in IL-1R1 receptor and NADPH oxidase-4 (NOX-4) mRNA expression, NADPH oxidase activity, and ROS generation, and reduced hydrogen peroxide-induced COX-2 gene expression. Telmisartan did not modify IL-1β-induced ERK1/2 and p38 mitogen-activated protein kinase (MAPK) phosphorylation or nuclear factor-κB activation but significantly decreased IL-1β-induced c-Jun N-terminal kinase (JNK) and c-Jun activation. The JNK inhibitor SP600125 decreased IL-1β-induced PGE₂ release with a potency similar to that of telmisartan. The PPARγ agonist pioglitazone reduced IL-1β-induced inflammatory reaction, whereas telmisartan did not activate PPARγ, as shown by its failure to enhance the expression of the PPARγ target genes ABCG1 and CD36, and the inability of the PPARγ antagonists GW9662 and T0070907 to modify the effect of telmisartan on COX-2 induction. The effect of telmisartan on IL-1β-stimulated COX-2 and IL-1R1 mRNA expression and ROS production was replicated in primary rat cortical neurons.

Conclusions

Telmisartan directly ameliorates IL-1β-induced neuronal inflammatory response by inhibition of oxidative stress and the JNK/c-Jun pathway. Our results support the hypothesis that AT₁ receptor blockers are directly neuroprotective, and should be considered for the treatment of inflammatory conditions of the brain.

Fetal stress and programming of hypoxic/ischemic-sensitive phenotype in the neonatal brain: mechanisms and possible interventions

Growing evidence of epidemiological, clinical and experimental studies has clearly shown a close link between adverse in utero environment and the increased risk of neurological, psychological and psychiatric disorders in later life. Fetal stresses, such as hypoxia, malnutrition, and fetal exposure to nicotine, alcohol, cocaine and glucocorticoids may directly or indirectly act at cellular and molecular levels to alter the brain development and result in programming of heightened brain vulnerability to hypoxic-ischemic encephalopathy and the development of neurological diseases in the postnatal life. The underlying mechanisms are not well understood. However, glucocorticoids may play a crucial role in epigenetic programming of neurological disorders of fetal origins. This review summarizes the recent studies about the effects of fetal stress on the abnormal brain development, focusing on the cellular, molecular and epigenetic mechanisms and highlighting the central effects of glucocorticoids on programming of hypoxic-ischemic-sensitive phenotype in the neonatal brain, which may enhance the understanding of brain pathophysiology resulting from fetal stress and help explore potential targets of timely diagnosis, prevention and intervention in neonatal hypoxic-ischemic encephalopathy and other brain disorders.

The renin–angiotensin system, bone marrow and progenitor cells

Modulation of the RAS (renin–angiotensin system), in particular of the function of the hormones AngII (angiotensin II) and Ang-(1–7) [angiotensin-(1–7)], is an important target for pharmacotherapy in the cardiovascular system. In the classical view, such modulation affects cardiovascular cells to decrease hypertrophy, fibrosis and endothelial dysfunction, and improves diuresis. In this view, excessive stimulation of AT1 receptors (AngII type 1 receptors) fulfils a detrimental role, as it promotes cardiovascular pathogenesis, and this is opposed by stimulation of the AT2 receptor (angiotensin II type 2 receptor) and the Ang-(1–7) receptor encoded by the Mas proto-oncogene. In recent years, this view has been broadened with the observation that the RAS regulates bone marrow stromal cells and stem cells, thus involving haematopoiesis and tissue regeneration by progenitor cells. This change of paradigm has enlarged the field of perspectives for therapeutic application of existing as well as newly developed medicines that alter angiotensin signalling, which now stretches beyond cardiovascular therapy. In the present article, we review the role of AngII and Ang-(1–7) and their respective receptors in haematopoietic and mesenchymal stem cells, and discuss possible pharmacotherapeutical implications.

The regenerative potential of angiotensin AT2 receptor in cardiac repair

Angiotensin II, the main effector peptide of the renin–angiotensin system, interferes with cardiac remodeling and repair through its receptors, including AT1 and AT2 receptor (R). The functional relevance of the previously neglected AT2R is currently intensively studied. Pharmacological therapies with AT1R blockers have improved outcomes in patients with ischemic heart injury, probably involving an indirect stimulation of AT2R. Previous experimental studies have clearly shown a protective action of AT2R in tissue repair and regeneration. We have recently identified the c-kit+AT2R+ progenitor cell population in rat heart and bone marrow, which increases after induction of myocardial infarction. Further experimental evidence demonstrates that AT2R mediates cardiac homing and repair process of the c-kit+ progenitor cells. AT2R stimulation through AT1R blockers or directly by AT2R agonist or both in combination may potentially offer the translational options to improve the regenerative potentials of stem/progenitor cells derived from patients with cardiovascular disease.

Direct stimulation of angiotensin II type 2 receptor enhances spatial memory

We examined the possibility that direct stimulation of the angiotensin II type 2 (AT(2)) receptor by a newly generated direct AT(2) receptor agonist, Compound 21 (C21), enhances cognitive function. Treatment with C21 intraperitoneal injection for 2 weeks significantly enhanced cognitive function evaluated by the Morris water maze test in C57BL6 mice, but this effect was not observed in AT(2) receptor-deficient mice. However, C21-induced cognitive enhancement in C57BL6 mice was attenuated by coadministration of icatibant, a bradykinin B(2) receptor antagonist. Administration of C21 dose dependently increased cerebral blood flow assessed by laser speckle flowmetry and hippocampal field-excitatory postsynaptic potential (f-EPSP) determined by electrophysiological techniques in C57BL6 mice. Furthermore, activation of the AT(2) receptor by C21 promoted neurite outgrowth of cultured hippocampal neurons prepared from fetal transgenic mice expressing green fluorescent protein. Finally, we investigated the pathologic relevance of C21 for spatial learning using an Alzheimer's disease mouse model with intracerebroventricular injection of amyloid-β (1 to 40). We observed that treatment with C21 prevented cognitive decline in this model. These results suggest that a direct AT(2) receptor agonist, C21, enhances cognitive function at least owing to an increase in CBF, enhancement of f-EPSP, and neurite outgrowth in hippocampal neurons.

How does angiotensin AT(2) receptor activation help neuronal differentiation and improve neuronal pathological situations?

The angiotensin type 2 (AT(2)) receptor of angiotensin II has long been thought to be limited to few tissues, with the primary effect of counteracting the angiotensin type 1 (AT(1)) receptor. Functional studies in neuronal cells have demonstrated AT(2) receptor capability to modulate neuronal excitability, neurite elongation, and neuronal migration, suggesting that it may be an important regulator of brain functions. The observation that the AT(2) receptor was expressed in brain areas implicated in learning and memory led to the hypothesis that it may also be implicated in cognitive functions. However, linking signaling pathways to physiological effects has always proven challenging since information relative to its physiological functions has mainly emerged from indirect observations, either from the blockade of the AT(1) receptor or through the use of transgenic animals. From a mechanistic standpoint, the main intracellular pathways linked to AT(2) receptor stimulation include modulation of phosphorylation by activation of kinases and phosphatases or the production of nitric oxide and cGMP, some of which are associated with the Gi-coupling protein. The receptor can also interact with other receptors, either G protein-coupled such as bradykinin, or growth factor receptors such as nerve growth factor or platelet-derived growth factor receptors. More recently, new advances have also led to identification of various partner proteins, thus providing new insights into this receptor's mechanism of action. This review summarizes the recent advances regarding the signaling pathways induced by the AT(2) receptor in neuronal cells, and discussed the potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT(2) receptor activation by non-peptide and selective agonists could represent new pharmacological tools that may help to improve impaired cognitive performance in Alzheimer's disease and other neurological cognitive disorders.