The brain renin-angiotensin system contributes to the hypertension in mice containing both the human renin and human angiotensinogen transgenes

New Approaches Targeting the Renin-Angiotensin System: Inhibition of Brain Aminopeptidase A, ACE2 Ubiquitination, and Angiotensinogen

Despite the availability of various therapeutic classes of antihypertensive drugs, hypertension remains poorly controlled, in part because of poor adherence. Hence, there is a need for the development of antihypertensive drugs acting on new targets to improve control of blood pressure. This review discusses novel insights (including the data of recent clinical trials) with regard to interference with the renin-angiotensin system, focusing on the enzymes aminopeptidase A and angiotensin-converting enzyme 2 (ACE2) in the brain, as well as the substrate of renin- angiotensinogen-in the liver. It raises the possibility that centrally acting amino peptidase A inhibitors (eg, firibastat), preventing the conversion of angiotensin II to angiotensin III in the brain, might be particularly useful in African Americans and patients with obesity. Firibastat additionally upregulates brain ACE2, allowing the conversion of angiotensin II to its protective metabolite angiotensin-(1-7). Furthermore, antisense oligonucleotides or small interfering ribonucleic acids suppress hepatic angiotensinogen for weeks to months after 1 injection and thus could potentially overcome adherence issues. Finally, interference with ACE2 ubiquitination is emerging as a future option for the treatment of neurogenic hypertension, given that ubiquitination resistance might upregulate ACE2 activity.

Exercise training improves cardiovascular control in sinoaortic denervated SHR by reducing the elevated angiotensin II and augmenting angiotensin-(1-7) availability within autonomic and neuroendocrine PVN nuclei

Previous studies have shown that baroreceptors- and chemoreceptors-denervated SHR exhibit impaired central autonomic circuitry and worsening of the cardiovascular function. It was also known that exercise training (T) ameliorates the autonomic control of the circulation. In the present study we sought to investigate whether sinoaortic denervation (SAD) is able to modify the expression/activity of the renin-angiotensin system (RAS) within brain autonomic areas and the effects induced by T. SHR submitted to SAD or SHAM surgery were trained or kept sedentary (S) for 8 weeks. Femoral artery and vein were chronically cannulated for hemodynamic/autonomic recordings and baroreflex testing (phenylephrine and sodium nitroprusside, i.v). Ang II and Ang (1-7) protein expression (immunofluorescence assays) were quantified within autonomic and neuroendocrine nuclei of the hypothalamic paraventricular nucleus (PVN). SAD-S vs. SHAM-S exhibited large increase in Ang II availability into the ventromedial, dorsal cap and magnocellular PVN nuclei, which are accompanied by augmented sympathetic activity, elevated arterial pressure variability and higher MAP. There was no change in Ang-(1-7) content within these nuclei. In contrast, T largely augmented Ang-(1-7) immunofluorescence in all nuclei, reduced and normalized Ang II availability and ameliorated the autonomic control of the circulation in SAD rats, but did not reduce MAP levels. Data showed that tonic baroreceptors and chemoreceptors' activity is essential to maintain lower Ang II levels within PVN nuclei. In the absence of afferent signaling, exercise training is still efficient to alter Ang II/Ang-(1-7) balance thus improving cardiovascular control even in the presence of high-pressure levels.

Inflammation: A Mediator Between Hypertension and Neurodegenerative Diseases

Hypertension is the most prevalent and modifiable risk factor for stroke, vascular cognitive impairment, and Alzheimer's disease. However, the mechanistic link between hypertension and neurodegenerative diseases remains to be understood. Recent evidence indicates that inflammation is a common pathophysiological trait for both hypertension and neurodegenerative diseases. Low-grade chronic inflammation at the systemic and central nervous system levels is now recognized to contribute to the physiopathology of hypertension. This review speculates that inflammation represents a mediator between hypertension and neurodegenerative diseases, either by a decrease in cerebral blood flow or a disruption of the blood-brain barrier which will, in turn, let inflammatory cells and neurotoxic molecules enter the brain parenchyma. This may impact brain functions including cognition and contribute to neurodegenerative diseases. This review will thus discuss the relationship between hypertension, systemic inflammation, cerebrovascular functions, neuroinflammation, and brain dysfunctions. The potential clinical future of immunotherapies against hypertension and associated cerebrovascular risks will also be presented.

Cardiovascular Neuroendocrinology: Emerging Role for Neurohypophyseal Hormones in Pathophysiology

Arginine vasopressin (AVP) and oxytocin (OXY) are released by magnocellular neurosecretory cells that project to the posterior pituitary. While AVP and OXY currently receive more attention for their contributions to affiliative behavior, this mini-review discusses their roles in cardiovascular function broadly defined to include indirect effects that influence cardiovascular function. The traditional view is that neither AVP nor OXY contributes to basal cardiovascular function, although some recent studies suggest that this position might be re-evaluated. More evidence indicates that adaptations and neuroplasticity of AVP and OXY neurons contribute to cardiovascular pathophysiology.

Angiotensin-(1-7) and Mas receptor in the brain

The renin-angiotensin system (RAS) is a key regulator of blood pressure and electrolyte homeostasis. Besides its importance as regulator of the cardiovascular function, the RAS has also been associated to the modulation of higher brain functions, including cognition, memory, depression and anxiety. For many years, angiotensin II (Ang II) has been considered the major bioactive component of the RAS. However, the existence of many other biologically active RAS components has currently been recognized, with similar, opposite, or distinct effects to those exerted by Ang II. Today, it is considered that the RAS is primarily constituted by two opposite arms. The pressor arm is composed by Ang II and the Ang II type 1 (AT1) receptor (AT1R), which mediates the vasoconstrictor, proliferative, hypertensive, oxidative and pro-inflammatory effects of the RAS. The depressor arm is mainly composed by Ang-(1-7), its Mas receptor (MasR) which mediates the depressor, vasodilatory, antiproliferative, antioxidant and anti-inflammatory effects of Ang-(1-7) and the AT2 receptor (AT2R), which opposes to the effects mediated by AT1R activation. Central Ang-(1-7) is implicated in the control of the cardiovascular function, thus participating in the regulation of blood pressure. Ang-(1-7) also exerts neuroprotective actions through MasR activation by opposing to the harmful effects of the Ang II/AT1R axis. This review is focused on the expression and regulation of the Ang-(1-7)/MasR axis in the brain, its main neuroprotective effects and the evidence regarding its involvement in the pathophysiology of several diseases at cardiovascular and neurological level.

Firibastat: A Novel Brain Aminopeptidase Inhibitor - A New Era of Antihypertensive therapy

Global incidence and prevalence of hypertension continues to increase and remains a significant challenge. The ever-increasing number of cases are due to comorbid conditions such as obesity and diabetes, as well as lifestyle indiscretions such as excessive salt intake. Hypertension, congestive heart failure, and kidney disease are all conditions resulting from abnormal Renin-Angiotensin-Aldosterone activation and adverse remodeling. Firibastat, a novel Brain Aminopeptidase inhibitor, may be able to help achieve blood pressure control in those with resistant hypertension. In this review article, we will discuss the biochemical pathway of firibastat and various trials assessing drug efficacy in animals and humans. This drug has the potential to curb the risk of uncontrolled hypertension and help improve long term cardiovascular morbidity and mortality.

Firibastat, the first-in-class brain aminopeptidase a inhibitor, in the management of hypertension: A review of clinical trials

An unfortunate subset of hypertensive patients develops resistant hypertension in which optimal doses of three or more first-line antihypertensive drugs fail to sufficiently control blood pressure. Patients with resistant hypertension represent a high-risk and difficult-to-treat group, and such patients are at amplified jeopardies for substantial hypertension-related multi-organ failure, morbidity, and mortality. Thus, there is a pressing requirement to better improve blood pressure control through the pharmaceutical generation of novel classes of antihypertensive drugs that act on newer and alternative therapeutic targets. The hyperactivity of the brain renin-angiotensin system (RAS) has been shown to play a role in the pathogenesis of hypertension in various experimental and genetic hypertensive animal models. In the brain, angiotensin-II is metabolized to angiotensin-III by aminopeptidase A (APA), a membrane-bound zinc metalloprotease enzyme. A large body of evidence has previously established that angiotensin-III is one of the main effector peptides of the brain RAS. Angiotensin-III exerts central stimulatory regulation over blood pressure through several proposed mechanisms. Accumulating evidence from preclinical studies demonstrated that the centrally acting APA inhibitor prodrugs (firibastat and NI956) are very safe and effective at reducing blood pressure in various hypertensive animal models. The primary purpose of this study is to narratively review the published phase I-II literature on the safety and efficacy of APA inhibitors in the management of patients with hypertension. Moreover, a summary of ongoing clinical trials and future perspectives are presented.

Firibastat: An Oral First-in-Class Brain Aminopeptidase A Inhibitor for Systemic Hypertension

Systemic hypertension is the leading cause of death and disability worldwide. The management of hypertension is challenging in the high-risk patient population with high salt-sensitivity and low serum renin levels. The renin-angiotensin system (RAS) plays a central role in blood pressure (BP) regulation. While we have effective medications to act on peripheral RAS, our understanding of brain RAS and its effect on BP regulation is still in an evolving stage. Brain RAS hyperactivity is associated with the development and maintenance of hypertension. In comparison to peripheral RAS, where angiotensin II is the most crucial component responsible for BP regulation, angiotensin III is likely the main active peptide in the brain RAS. Angiotensin II is metabolized by aminopeptidase A into angiotensin III in the brain. EC33 is a potent inhibitor of brain aminopeptidase A tested in animal models. The use of EC33 in conscious spontaneously hypertensive rats, hypertensive deoxycorticosterone acetate-salt rats, and conscious normotensive rat models leads to a reduction in BP. In order to facilitate the passage of EC33 through the blood-brain barrier, the 2 molecules of EC33 were linked by a disulfide bridge to form a prodrug called RB150. RB150, later renamed as QGC001 or firibastat, was found to be effective in animal models and well-tolerated when used in healthy participants. Firibastat was found to be safe and effective in phase 2 trials, and is now planned to undergo a phase 3 trial. Firibastat has the potential to be groundbreaking in the management of resistant hypertension.

Overexpression of Central ACE2 (Angiotensin-Converting Enzyme 2) Attenuates the Pressor Response to Chronic Central Infusion of Ang II (Angiotensin II): A Potential Role for Nrf2 (Nuclear Factor [Erythroid-Derived 2]-Like 2)

We investigated the mechanism by which ACE2 (angiotensin-converting enzyme 2) overexpression alters neurohumoral outflow and central oxidative stress. Nrf2 (nuclear factor [erythroid-derived 2]-like 2) is a master antioxidant transcription factor that regulates cytoprotective and antioxidant genes. We hypothesized that upregulation of central ACE2 inhibits the pressor response to Ang II (angiotensin II) by reducing reactive oxygen species through a Nrf2/antioxidant enzyme-mediated mechanism in the rostral ventrolateral medulla. Synapsin human Angiotensin Converting Enzyme 2 positive (SynhACE2^+/+) mice and their littermate controls synhACE2^-/- were used to evaluate the consequence of intracerebroventricular infusion of Ang II. In control mice, Ang II infusion evoked a significant increase in blood pressure and norepinephrine excretion, along with polydipsia and polyuria. The pressor effect of central Ang II was completely blocked in synhACE2^+/+ mice. Polydipsia, norepinephrine excretion, and markers of oxidative stress in response to central Ang II were also reduced in synhACE2^+/+ mice. The MasR (Mas receptor) agonist Ang 1-7 and blocker A779 had no effects on blood pressure. synhACE2^+/+ mice showed enhanced expression of Nrf2 in the rostral ventrolateral medulla which was blunted following Ang II infusion. Ang II evoked nuclear translocation of Nrf2 in cultured Neuro 2A (N2A) cells. In synhACE2^-/- mice, the central Ang II pressor response was attenuated by simultaneous intracerebroventricular infusion of the Nrf2 activator sulforaphane; blood pressure was enhanced by knockdown of Nrf2 in the rostral ventrolateral medulla in Nrf2 floxed (Nrf2^f/f) mice. These data suggest that the hypertensive effects of intracerebroventricular Ang II are attenuated by selective overexpression of brain synhACE2 and may be mediated by Nrf2-upregulated antioxidant enzymes in the rostral ventrolateral medulla.

Targeting Brain Aminopeptidase A: A New Strategy for the Treatment of Hypertension and Heart Failure

The pathophysiology of heart failure (HF) and hypertension are thought to involve brain renin-angiotensin system (RAS) hyperactivity. Angiotensin III, a key effector peptide in the brain RAS, provides tonic stimulatory control over blood pressure (BP) in hypertensive rats. Aminopeptidase A (APA), the enzyme responsible for generating brain angiotensin III, constitutes a potential therapeutic target for hypertension treatment. We focus here on studies of RB150/firibastat, the first prodrug of the specific and selective APA inhibitor EC33 able to cross the blood-brain barrier. We consider its development from therapeutic target discovery to clinical trials of the prodrug. After oral administration, firibastat crosses the gastrointestinal and blood-brain barriers. On arrival in the brain, it is cleaved to generate EC33, which inhibits brain APA activity, lowering BP in various experimental models of hypertension. Firibastat was clinically and biologically well tolerated, even at high doses, in phase I trials conducted in healthy human subjects. It was then shown to decrease BP effectively in patients of various ethnic origins with hypertension in phase II trials. Brain RAS hyperactivity leads to excessive sympathetic activity, which can contribute to HF after myocardial infarction (MI). Chronic treatment with oral firibastat (4 or 8 weeks after MI) has been shown to normalize brain APA activity in mice. This effect is accompanied by a normalization of brain RAS and sympathetic activities, reducing cardiac fibrosis and hypertrophy and preventing cardiac dysfunction. Firibastat may therefore represent a novel therapeutic advance in the clinical management of patients with hypertension and potentially with HF after MI.

Novel Therapeutic Approaches Targeting the Renin-Angiotensin System and Associated Peptides in Hypertension and Heart Failure

Despite the success of renin-angiotensin system (RAS) blockade by angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 receptor (AT1R) blockers, current therapies for hypertension and related cardiovascular diseases are still inadequate. Identification of additional components of the RAS and associated vasoactive pathways, as well as new structural and functional insights into established targets, have led to novel therapeutic approaches with the potential to provide improved cardiovascular protection and better blood pressure control and/or reduced adverse side effects. The simultaneous modulation of several neurohumoral mediators in key interconnected blood pressure-regulating pathways has been an attractive approach to improve treatment efficacy, and several novel approaches involve combination therapy or dual-acting agents. In addition, increased understanding of the complexity of the RAS has led to novel approaches aimed at upregulating the ACE2/angiotensin-(1-7)/Mas axis to counter-regulate the harmful effects of the ACE/angiotensin II/angiotensin III/AT1R axis. These advances have opened new avenues for the development of novel drugs targeting the RAS to better treat hypertension and heart failure. Here we focus on new therapies in preclinical and early clinical stages of development, including novel small molecule inhibitors and receptor agonists/antagonists, less conventional strategies such as gene therapy to suppress angiotensinogen at the RNA level, recombinant ACE2 protein, and novel bispecific designer peptides.

Central antihypertensive effects of chronic treatment with RB150: an orally active aminopeptidase A inhibitor in deoxycorticosterone acetate-salt rats

BACKGROUND AND OBJECTIVE

Hyperactivity of the brain renin-angiotensin (Ang) system has been implicated in the development and maintenance of hypertension. AngIII, one of the main effector peptides of the brain renin-Ang system, exerts a tonic stimulatory control over blood pressure (BP) in hypertensive rats. Aminopeptidase A (APA), the enzyme generating brain AngIII, represents a new therapeutic target for the treatment of hypertension. We developed RB150, a prodrug of the specific and selective APA inhibitor, EC33. When given orally in acute treatment in hypertensive rats, RB150 crosses the gastrointestinal and blood-brain barriers, enters the brain, inhibits brain APA activity and decreases BP. We investigate, here, the antihypertensive effects of chronic oral RB150 (50 mg/kg per day) treatment over 24 days in alert hypertensive deoxycorticosterone acetate-salt rats.

METHODS

We measured variations in Brain APA enzymatic activity, SBP, plasma arginine vasopressin levels and metabolic parameters after RB150 chronic administration.

RESULTS

This resulted in a significant decrease in SBP over the 24-day treatment period showing that no tolerance to the antihypertensive RB150 effect was observed throughout the treatment period. Chronic RB150 treatment also significantly decreased plasma arginine vasopressin levels and increased diuresis, which participate to BP decrease by reducing the size of fluid compartment. Interestingly, we observed an increased natriuresis without modifying both plasma sodium and potassium levels.

CONCLUSION

Our results strengthen the interest of developing RB150 as a novel central-acting antihypertensive agent and evaluating its efficacy in salt-sensitive hypertension.

Revisiting the Brain Renin-Angiotensin System-Focus on Novel Therapies

Purpose of Review

Although an independent brain renin-angiotensin system is often assumed to exist, evidence for this concept is weak. Most importantly, renin is lacking in the brain, and both brain angiotensinogen and angiotensin (Ang) II levels are exceptionally low. In fact, brain Ang II levels may well represent uptake of circulating Ang II via Ang II type 1 (AT₁) receptors.

Recent Findings

Nevertheless, novel drugs are now aimed at the brain RAS, i.e., aminopeptidase A inhibitors should block Ang III formation from Ang II, and hence diminish AT₁ receptor stimulation by Ang III, while AT₂ and Mas receptor agonists are reported to induce neuroprotection after stroke. The endogenous agonists of these receptors and their origin remain unknown.

Summary

This review addresses the questions whether independent angiotensin generation truly occurs in the brain, what its relationship with the kidney is, and how centrally acting RAS blockers/agonists might work.

Angiotensin generation in the brain: a re-evaluation

The existence of a so-called brain renin-angiotensin system (RAS) is controversial. Given the presence of the blood-brain barrier, angiotensin generation in the brain, if occurring, should depend on local synthesis of renin and angiotensinogen. Yet, although initially brain-selective expression of intracellular renin was reported, data in intracellular renin knockout animals argue against a role for this renin in angiotensin generation. Moreover, renin levels in brain tissue at most represented renin in trapped blood. Additionally, in neurogenic hypertension brain prorenin up-regulation has been claimed, which would generate angiotensin following its binding to the (pro)renin receptor. However, recent studies reported no evidence for prorenin expression in the brain, nor for its selective up-regulation in neurogenic hypertension, and the (pro)renin receptor rather displays RAS-unrelated functions. Finally, although angiotensinogen mRNA is detectable in the brain, brain angiotensinogen protein levels are low, and even these low levels might be an overestimation due to assay artefacts. Taken together, independent angiotensin generation in the brain is unlikely. Indeed, brain angiotensin levels are extremely low, with angiotensin (Ang) I levels corresponding to the small amounts of Ang I in trapped blood plasma, and Ang II levels at most representing Ang II bound to (vascular) brain Ang II type 1 receptors. This review concludes with a unifying concept proposing the blood origin of angiotensin in the brain, possibly resulting in increased levels following blood-brain barrier disruption (e.g. due to hypertension), and suggesting that interfering with either intracellular renin or the (pro)renin receptor has consequences in an RAS-independent manner.

Aging exacerbates cognitive and anxiety alterations induced by an intracerebroventricular injection of amyloid-β1-42 peptide in mice

An increasing body of evidence indicates that the activation of indoleamine-2,3-dyoxigenase (IDO), a first and rate-limiting enzyme in the kynurenine (KYN) pathway, is involved in Aβ_1-42-neurotoxicity and AD pathogenesis. We have reported for the first time that brain IDO activation is related to Aβ_1-42 exposure in young mice. Because aging is characterized by a brain dyshomeostasis and because it remains the most dominant risk factor for AD, the purpose of this study was to determine whether aging is associated with a higher sensitivity to behavioural and neurochemical alterations elicited by an intracerebroventricular (i.c.v.) injection of Aβ_1-42 (400 pmol/mice), and whether KYN pathway is involved in these effects. We confirmed that aged mice displayed higher cognitive deficit in the object recognition test and higher anxiety-like behaviour in the elevated plus-maze and open field tests after the Aβ_1-42 administration. Aged mice also responded to Aβ_1-42 with a higher deficiency of brain-derived neurotrophic factor, glutathione levels and total radical-trapping antioxidant capacity, a higher IDO activity, and a higher KYN and KYN/tryptophan ratio in the prefrontal cortex and hippocampus. These effects of Aβ_1-42 were associated with a higher proinflammatory status, as measured by higher levels of interleukin-6, lower levels of interleukin-10 and higher expression of glial fibrillary acidic protein (GFAP) and allograft inflammatory factor 1 (Iba1) in the brain of aged mice. These results represent primary evidence suggesting that age-associated inflammatory signature and down-regulation of neuroprotectants in the brain render aged mice more vulnerable to Aβ_1-42-induced memory loss, anxiety symptoms and KYN pathway dysregulation.

Hypothalamic and inflammatory basis of hypertension

Hypertension is a major health problem with great consequences for public health. Despite its role as the primary cause of significant morbidity and mortality associated with cardiovascular disease, the pathogenesis of essential hypertension remains largely unknown. The central nervous system (CNS) in general, and the hypothalamus in particular, are intricately involved in the development and maintenance of hypertension. Over the last several decades, the understanding of the brain's role in the development of hypertension has dramatically increased. This brief review is to summarize the neural mechanisms of hypertension with a focus on neuroendocrine and neurotransmitter involvement, highlighting recent findings that suggest that hypothalamic inflammation disrupts key signalling pathways to affect the central control of blood pressure, and therefore suggesting future development of interventional strategies that exploit recent findings pertaining to the hypothalamic control of blood pressure as well as the inflammatory-sympathetic mechanisms involved in hypertension.

Increased mitochondrial superoxide in the brain, but not periphery, sensitizes mice to angiotensin II-mediated hypertension

Angiotensin II (AngII) elicits the production of superoxide (O₂^•−) from mitochondria in numerous cell types within peripheral organs and in the brain suggesting a role for mitochondrial-produced O₂^•− in the pathogenesis of hypertension. However, it remains unclear if mitochondrial O₂^•− is causal in the development of AngII-induced hypertension, or if mitochondrial O₂^•− in the absence of elevated AngII is sufficient to increase blood pressure. Further, the tissue specific (i.e. central versus peripheral) redox regulation of AngII hypertension remains elusive. Herein, we hypothesized that increased mitochondrial O₂^•− in the absence of pro-hypertensive stimuli, such as AngII, elevates baseline systemic mean arterial pressure (MAP), and that AngII-mediated hypertension is exacerbated in animals with increased mitochondrial O₂^•− levels. To address this hypothesis, we generated novel inducible knock-down mouse models of manganese superoxide dismutase (MnSOD), the O₂^•− scavenging antioxidant enzyme specifically localized to mitochondria, targeted to either the brain subfornical organ (SFO) or peripheral tissues. Contrary to our hypothesis, knock-down of MnSOD either in the SFO or in peripheral tissues was not sufficient to alter baseline systemic MAP. Interestingly, when mice were challenged with chronic, peripheral infusion of AngII, only the MnSOD knock-down confined to the SFO, and not the periphery, demonstrated an increased sensitization and potentiated hypertension. In complementary experiments, over-expressing MnSOD in the SFO significantly decreased blood pressure in response to chronic AngII. Overall, these studies indicate that mitochondrial O₂^•− in the brain SFO works in concert with other AngII-dependent factors to drive an increase in MAP, as elevated mitochondrial O₂^•− alone, either in the SFO or peripheral tissues, failed to raise baseline blood pressure.

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Mitochondrial O₂^•− has been implicated as a primary contributor to hypertension.

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Novel models with altered MnSOD expression utilized to influence mitochondrial O₂^•−.

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Knock-down of MnSOD alone is not sufficient to alter systemic hemodynamics.

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Knock-down of MnSOD in the brain SFO, but not periphery, exacerbates hypertension.

A Disintegrin and Metalloprotease 17 in the Cardiovascular and Central Nervous Systems

ADAM17 is a metalloprotease and disintegrin that lodges in the plasmatic membrane of several cell types and is able to cleave a wide variety of cell surface proteins. It is somatically expressed in mammalian organisms and its proteolytic action influences several physiological and pathological processes. This review focuses on the structure of ADAM17, its signaling in the cardiovascular system and its participation in certain disorders involving the heart, blood vessels, and neural regulation of autonomic and cardiovascular modulation.

Selective Deletion of the Brain-Specific Isoform of Renin Causes Neurogenic Hypertension

The renin-angiotensin system (RAS) in the brain is a critical determinant of blood pressure, but the mechanisms regulating RAS activity in the brain remain unclear. Expression of brain renin (renin-b) occurs from an alternative promoter-first exon. The predicted translation product is a nonsecreted enzymatically active renin whose function is unknown. We generated a unique mouse model by selectively ablating the brain-specific isoform of renin (renin-b) while preserving the expression and function of the classical isoform expressed in the kidney (renin-a). Preservation of renal renin was confirmed by measurements of renin gene expression and immunohistochemistry. Surprisingly, renin-b-deficient mice exhibited hypertension, increased sympathetic nerve activity to the kidney and heart, and impaired baroreflex sensitivity. Whereas these mice displayed decreased circulating RAS activity, there was a paradoxical increase in brain RAS activity. Physiologically, renin-b-deficient mice exhibited an exaggerated depressor response to intracerebroventricular administration of losartan, captopril, or aliskiren. At the molecular level, renin-b-deficient mice exhibited increased expression of angiotensin-II type 1 receptor in the paraventricular nucleus, which correlated with an increased renal sympathetic nerve response to leptin, which was dependent on angiotensin-II type 1 receptor activity. Interestingly, despite an ablation of renin-b expression, expression of renin-a was significantly increased in rostral ventrolateral medulla. These data support a new paradigm for the genetic control of RAS activity in the brain by a coordinated regulation of the renin isoforms, with expression of renin-b tonically inhibiting expression of renin-a under baseline conditions. Impairment of this control mechanism causes neurogenic hypertension.

Indoleamine-2,3-dioxygenase mediates neurobehavioral alterations induced by an intracerebroventricular injection of amyloid-β1-42 peptide in mice

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterized by a progressive cognitive decline along with various neuropsychiatric symptoms, including depression and anxiety. Increasing evidence has been proposed the activation of the tryptophan-degrading indoleamine-2,3-dyoxigenase (IDO), the rate-limiting enzyme of kynurerine pathway (KP), as a pathogenic factor of amyloid-beta (Aβ)-related inflammation in AD. In the current study, the effects of an intracerebroventricular (i.c.v.) injection of Aβ1-42 peptide (400pmol/mice; 3μl/site) on the regulation of KP biomarkers (IDO activity, tryptophan and kynurerine levels) and the impact of Aβ1-42 on neurotrophic factors levels were investigated as potential mechanisms linking neuroinflammation to cognitive/emotional disturbances in mice. Our results demonstrated that Aβ1-42 induced memory impairment in the object recognition test. Aβ1-42 also induced emotional alterations, such as depressive and anxiety-like behaviors, as evaluated in the tail suspension and elevated-plus maze tests, respectively. We observed an increase in levels of proinflammatory cytokines in the Aβ1-42-treated mice, which led to an increase in IDO activity in the prefrontal cortex (PFC) and the hippocampus (HC). The IDO activation subsequently increased kynurerine production and the kynurenine/tryptophan ratio and decreased the levels of neurotrophic factors in the PFC and HC, which contributed to Aβ-associated behavioral disturbances. The inhibition of IDO activation by IDO inhibitor 1-methyltryptophan (1-MT), prevented the development of behavioral and neurochemical alterations. These data demonstrate that brain IDO activation plays a key role in mediating the memory and emotional disturbances in an experimental model based on Aβ-induced neuroinflammation.

FGF21 Mediates Endocrine Control of Simple Sugar Intake and Sweet Taste Preference by the Liver

The liver is an important integrator of nutrient metabolism, yet no liver-derived factors regulating nutrient preference or carbohydrate appetite have been identified. Here we show that the liver regulates carbohydrate intake through production of the hepatokine fibroblast growth factor 21 (FGF21), which markedly suppresses consumption of simple sugars, but not complex carbohydrates, proteins, or lipids. Genetic loss of FGF21 in mice increases sucrose consumption, whereas acute administration or overexpression of FGF21 suppresses the intake of both sugar and non-caloric sweeteners. FGF21 does not affect chorda tympani nerve responses to sweet tastants, instead reducing sweet-seeking behavior and meal size via neurons in the hypothalamus. This liver-to-brain hormonal axis likely represents a negative feedback loop as hepatic FGF21 production is elevated by sucrose ingestion. We conclude that the liver functions to regulate macronutrient-specific intake by producing an endocrine satiety signal that acts centrally to suppress the intake of "sweets."

Angiotensin II regulates brain (pro)renin receptor expression through activation of cAMP response element-binding protein

We reported that brain (pro)renin receptor (PRR) expression levels are elevated in DOCA-salt-induced hypertension; however, the underlying mechanism remained unknown. To address whether ANG II type 1 receptor (AT1R) signaling is involved in this regulation, we implanted a DOCA pellet and supplied 0.9% saline as the drinking solution to C57BL/6J mice. Sham pellet-implanted mice that were provided regular drinking water served as controls. Concurrently, mice were intracerebroventricularly infused with the AT1R blocker losartan, angiotensin-converting-enzyme inhibitor captopril, or artificial cerebrospinal fluid for 3 wk. Intracerebroventricular infusion of losartan or captopril attenuated DOCA-salt-induced PRR mRNA elevation in the paraventricular nucleus of the hypothalamus, suggesting a role for ANG II/AT1R signaling in regulating PRR expression during DOCA-salt hypertension. To test which ANG II/AT1R downstream transcription factors were involved in PRR regulation, we treated Neuro-2A cells with ANG II with or without CREB (cAMP response element-binding protein) or AP-1 (activator protein-1) inhibitors, or CREB siRNA. CREB and AP-1 inhibitors, as well as CREB knockdown abolished ANG II-induced increases in PRR levels. ANG II also induced PRR upregulation in primary cultured neurons. Chromatin immunoprecipitation assays revealed that ANG II treatment increased CREB binding to the endogenous PRR promoter in both cultured neurons and hypothalamic tissues of DOCA-salt hypertensive mice. This increase in CREB activity was reversed by AT1R blockade. Collectively, these findings indicate that ANG II acts via AT1R to upregulate PRR expression both in cultured cells and in DOCA-salt hypertensive mice by increasing CREB binding to the PRR promoter.

Vascular Type 1A Angiotensin II Receptors Control BP by Regulating Renal Blood Flow and Urinary Sodium Excretion

Inappropriate activation of the type 1A angiotensin (AT1A) receptor contributes to the pathogenesis of hypertension and its associated complications. To define the role for actions of vascular AT1A receptors in BP regulation and hypertension pathogenesis, we generated mice with cell-specific deletion of AT1A receptors in smooth muscle cells (SMKO mice) using Loxp technology and Cre transgenes with robust expression in both conductance and resistance arteries. We found that elimination of AT1A receptors from vascular smooth muscle cells (VSMCs) caused a modest (approximately 7 mmHg) yet significant reduction in baseline BP and exaggerated sodium sensitivity in mice. Additionally, the severity of angiotensin II (Ang II)-dependent hypertension was dramatically attenuated in SMKO mice, and this protection against hypertension was associated with enhanced urinary excretion of sodium. Despite the lower BP, acute vasoconstrictor responses to Ang II in the systemic vasculature were largely preserved (approximately 80% of control levels) in SMKO mice because of exaggerated activity of the sympathetic nervous system rather than residual actions of AT1B receptors. In contrast, Ang II-dependent responses in the renal circulation were almost completely eliminated in SMKO mice (approximately 5%-10% of control levels). These findings suggest that direct actions of AT1A receptors in VSMCs are essential for regulation of renal blood flow by Ang II and highlight the capacity of Ang II-dependent vascular responses in the kidney to effect natriuresis and BP control.

Rapid metabolism of exogenous angiotensin II by catecholaminergic neuronal cells in culture media

Angiotensin II (AngII) acts on central neurons to increase neuronal firing and induce sympathoexcitation, which contribute to the pathogenesis of cardiovascular diseases including hypertension and heart failure. Numerous studies have examined the precise AngII-induced intraneuronal signaling mechanism in an attempt to identify new therapeutic targets for these diseases. Considering the technical challenges in studying specific intraneuronal signaling pathways in vivo, especially in the cardiovascular control brain regions, most studies have relied on neuronal cell culture models. However, there are numerous limitations in using cell culture models to study AngII intraneuronal signaling, including the lack of evidence indicating the stability of AngII in culture media. Herein, we tested the hypothesis that exogenous AngII is rapidly metabolized in neuronal cell culture media. Using liquid chromatography-tandem mass spectrometry, we measured levels of AngII and its metabolites, Ang III, Ang IV, and Ang-1-7, in neuronal cell culture media after administration of exogenous AngII (100 nmol/L) to a neuronal cell culture model (CATH.a neurons). AngII levels rapidly declined in the media, returning to near baseline levels within 3 h of administration. Additionally, levels of Ang III and Ang-1-7 acutely increased, while levels of Ang IV remained unchanged. Replenishing the media with exogenous AngII every 3 h for 24 h resulted in a consistent and significant increase in AngII levels for the duration of the treatment period. These data indicate that AngII is rapidly metabolized in neuronal cell culture media, and replenishing the media at least every 3 h is needed to sustain chronically elevated levels.

[Orally active aminopeptidase A inhibitors reduce blood pressure: a new strategy for treating hypertension]

The hyperactivity of the brain renin-angiotensin system (RAS) has been implicated in the development and maintenance of hypertension in several types of experimental and genetic hypertension animal models. Among the main bioactive peptides of the brain RAS, angiotensin (Ang) II and Ang III display the same affinity for type 1 and type 2 Ang II receptors. Both peptides, injected intracerebroventricularly, similarly increase arginine vasopressin release and blood pressure (BP); however, because Ang II is converted in vivo to Ang III, the identity of the true effector is unknown. We first identified the enzymes involved in the metabolism of brain angiotensins and developed specific and selective inhibitors. Here we review new insights into the predominant role of brain Ang III in the control of BP, underlining the fact that brain aminopeptidase A (APA), the enzyme generating brain Ang III, may therefore be an interesting candidate target for the treatment of hypertension. This justifies the development of potent systemically active APA inhibitors, such as RB150, as prototypes of a new class of antihypertensive agents for the treatment of certain forms of hypertension.

Intracerebroventricular infusion of the (Pro)renin receptor antagonist PRO20 attenuates deoxycorticosterone acetate-salt-induced hypertension

We previously reported that binding of prorenin to the (pro)renin receptor (PRR) plays a major role in brain angiotensin II formation and the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Here, we designed and developed an antagonistic peptide, PRO20, to block prorenin binding to the PRR. Fluorescently labeled PRO20 bound to both mouse and human brain tissues with dissociation constants of 4.4 and 1.8 nmol/L, respectively. This binding was blocked by coincubation with prorenin and was diminished in brains of neuron-specific PRR-knockout mice, indicating specificity of PRO20 for PRR. In cultured human neuroblastoma cells, PRO20 blocked prorenin-induced calcium influx in a concentration- and AT(1) receptor-dependent manner. Intracerebroventricular infusion of PRO20 dose-dependently inhibited prorenin-induced hypertension in C57Bl6/J mice. Furthermore, acute intracerebroventricular infusion of PRO20 reduced blood pressure in both DOCA-salt and genetically hypertensive mice. Chronic intracerebroventricular infusion of PRO20 attenuated the development of hypertension and the increase in brain hypothalamic angiotensin II levels induced by DOCA-salt. In addition, chronic intracerebroventricular infusion of PRO20 improved autonomic function and spontaneous baroreflex sensitivity in mice treated with DOCA-salt. In summary, PRO20 binds to both mouse and human PRRs and decreases angiotensin II formation and hypertension induced by either prorenin or DOCA-salt. Our findings highlight the value of the novel PRR antagonist, PRO20, as a lead compound for a novel class of antihypertensive agents and as a research tool to establish the validity of brain PRR antagonism as a strategy for treating hypertension.

Constitutive activation of the renin-angiotensin system reduces visceral fat and improves glucose tolerance in mice

INTRODUCTION

The renin-angiotensin system (RAS), and particularly angiotensin II, is involved in the control of energy balance, glucose homeostasis and kidney functions. The integrated impact of the RAS on glucose homeostasis is still a matter of debate.

MATERIALS AND METHODS

We used a model of constitutive RAS activation in double transgenic mice (dTGM) carrying both human angiotensinogen and human renin genes. We evaluated energy balance, measured renal functions, performed glucose and insulin tolerance tests, and used ramipril to inhibit the angiotensin-converting enzyme.

RESULTS

dTGM had a lower physical activity and an increased food intake without change in body weight. Renal impairment was characterized by low-grade albuminuria. High urinary output secondary to polydipsia was associated with proximal tubule dysfunction. Compared to controls, dTGM had a lower hyperglycemia induced by an intraperitoneal glucose administration. This decrease was not due to changes in insulin sensitivity and/or secretion. dTGM had an increased creatinine production and a lower epididymal fat mass. Acute inhibition of angiotensin-converting enzyme with ramipril did not suppress this improved glucose tolerance profile.

CONCLUSION

Chronic RAS activation is not sufficient to cause insulin resistance in mice. Moreover, adaptation to constitutive RAS activation in mice results in a better glucose tolerance.

Activation of the renin-angiotensin system, specifically in the subfornical organ is sufficient to induce fluid intake

Increased activity of the renin-angiotensin system within the brain elevates fluid intake, blood pressure, and resting metabolic rate. Renin and angiotensinogen are coexpressed within the same cells of the subfornical organ, and the production and action of ANG II through the ANG II type 1 receptor in the subfornical organ (SFO) are necessary for fluid intake due to increased activity of the brain renin-angiotensin system. We generated an inducible model of ANG II production by breeding transgenic mice expressing human renin in neurons controlled by the synapsin promoter with transgenic mice containing a Cre-recombinase-inducible human angiotensinogen construct. Adenoviral delivery of Cre-recombinase causes SFO-selective induction of human angiotensinogen expression. Selective production of ANG II in the SFO results in increased water intake but did not change blood pressure or resting metabolic rate. The increase in water intake was ANG II type 1 receptor-dependent. When given a choice between water and 0.15 M NaCl, these mice increased total fluid and sodium, but not water, because of an increased preference for NaCl. When provided a choice between water and 0.3 M NaCl, the mice exhibited increased fluid, water, and sodium intake, but no change in preference for NaCl. The increase in fluid intake was blocked by an inhibitor of PKC, but not ERK, and was correlated with increased phosphorylated cyclic AMP response element binding protein in the subfornical organ. Thus, increased production and action of ANG II specifically in the subfornical organ are sufficient on their own to mediate an increase in drinking through PKC.

A new strategy for treating hypertension by blocking the activity of the brain renin-angiotensin system with aminopeptidase A inhibitors

Hypertension affects one-third of the adult population and is a growing problem due to the increasing incidence of obesity and diabetes. Brain RAS (renin-angiotensin system) hyperactivity has been implicated in the development and maintenance of hypertension in several types of experimental and genetic hypertension animal models. We have identified in the brain RAS that APA (aminopeptidase A) and APN (aminopeptidase N), two membrane-bound zinc metalloproteases, are involved in the metabolism of AngII (angiotensin II) and AngIII (angiotensin III) respectively. The present review summarizes the main findings suggesting that AngIII plays a predominant role in the brain RAS in the control of BP (blood pressure). We first explored the organization of the APA active site by site-directed mutagenesis and molecular modelling. The development and the use in vivo of specific and selective APA and APN inhibitors EC33 and PC18 respectively, has allowed the demonstration that brain AngIII generated by APA is one of the main effector peptides of the brain RAS, exerting a tonic stimulatory control over BP in conscious hypertensive rats. This identified brain APA as a potential therapeutic target for the treatment of hypertension, which has led to the development of potent orally active APA inhibitors, such as RB150. RB150 administered orally in hypertensive DOCA (deoxycorticosteroneacetate)-salt rats or SHRs (spontaneously hypertensive rats) crosses the intestinal, hepatic and blood-brain barriers, enters the brain, generates two active molecules of EC33 which inhibit brain APA activity, block the formation of brain AngIII and normalize BP for several hours. The decrease in BP involves two different mechanisms: a decrease in vasopressin release into the bloodstream, which in turn increases diuresis resulting in a blood volume reduction that participates in the decrease in BP and/or a decrease in sympathetic tone, decreasing vascular resistance. RB150 constitutes the prototype of a new class of centrally acting antihypertensive agents and is currently being evaluated in a Phase Ib clinical trial.

Activity of protein kinase C-α within the subfornical organ is necessary for fluid intake in response to brain angiotensin

Angiotensin-II production in the subfornical organ acting through angiotensin-II type-1 receptors is necessary for polydipsia, resulting from elevated renin-angiotensin system activity. Protein kinase C and mitogen-activated protein kinase pathways have been shown to mediate effects of angiotensin-II in the brain. We investigated mechanisms that mediate brain angiotensin-II-induced polydipsia. We used double-transgenic sRA mice, consisting of human renin controlled by the neuron-specific synapsin promoter crossed with human angiotensinogen controlled by its endogenous promoter, which results in brain-specific overexpression of angiotensin-II, particularly in the subfornical organ. We also used the deoxycorticosterone acetate-salt model of hypertension, which exhibits polydipsia. Inhibition of protein kinase C, but not extracellular signal-regulated kinases, protein kinase A, or vasopressin V₁A and V₂ receptors, corrected the elevated water intake of sRA mice. Using an isoform selective inhibitor and an adenovirus expressing dominant negative protein kinase C-α revealed that protein kinase C-α in the subfornical organ was necessary to mediate elevated fluid and sodium intake in sRA mice. Inhibition of protein kinase C activity also attenuated polydipsia in the deoxycorticosterone acetate-salt model. We provide evidence that inducing protein kinase C activity centrally is sufficient to induce water intake in water-replete wild-type mice, and that cell surface localization of protein kinase C-α can be induced in cultured cells from the subfornical organ. These experimental findings demonstrate a role for central protein kinase C activity in fluid balance, and further mechanistically demonstrate the importance of protein kinase C-α signaling in the subfornical organ in fluid intake stimulated by angiotensin-II in the brain.

Neuron-specific (pro)renin receptor knockout prevents the development of salt-sensitive hypertension

The (pro)renin receptor (PRR), which binds both renin and prorenin, is a newly discovered component of the renin-angiotensin system that is highly expressed in the central nervous system. The significance of brain PRRs in mediating local angiotensin II formation and regulating blood pressure remains unclear. The current study was performed to test the hypothesis that PRR-mediated, nonproteolytic activation of prorenin is the main source of angiotensin II in the brain. Thus, PRR knockout in the brain is expected to prevent angiotensin II formation and development of deoxycorticosterone acetate-salt-induced hypertension. A neuron-specific PRR (ATP6AP2) knockout mouse model was generated using the Cre-LoxP system. Physiological parameters were recorded by telemetry. PRR expression, detected by immunostaining and reverse transcription-polymerase chain reaction, was significantly decreased in the brains of knockout mice compared with wild-type mice. Intracerebroventricular infusion of mouse prorenin increased blood pressure and angiotensin II formation in wild-type mice. This hypertensive response was abolished in PRR-knockout mice in association with a reduction in angiotensin II levels. Deoxycorticosterone acetate-salt increased PRR expression and angiotensin II formation in the brains of wild-type mice, an effect that was attenuated in PRR-knockout mice. PRR knockout in neurons prevented the development of deoxycorticosterone acetate-salt-induced hypertension as well as activation of cardiac and vasomotor sympathetic tone. In conclusion, nonproteolytic activation of prorenin through binding to the PRR mediates angiotensin II formation in the brain. Neuron-specific PRR knockout prevents the development of deoxycorticosterone acetate-salt-induced hypertension, possibly through diminished angiotensin II formation.

Brain angiotensin-converting enzyme type 2 shedding contributes to the development of neurogenic hypertension

RATIONALE

Overactivity of the brain renin-angiotensin system is a major contributor to neurogenic hypertension. Although overexpression of angiotensin-converting enzyme type 2 (ACE2) has been shown to be beneficial in reducing hypertension by transforming angiotensin II into angiotensin-(1-7), several groups have reported decreased brain ACE2 expression and activity during the development of hypertension.

OBJECTIVE

We hypothesized that ADAM17-mediated ACE2 shedding results in decreased membrane-bound ACE2 in the brain, thus promoting the development of neurogenic hypertension.

METHODS AND RESULTS

To test this hypothesis, we used the deoxycorticosterone acetate-salt model of neurogenic hypertension in nontransgenic and syn-hACE2 mice overexpressing ACE2 in neurons. Deoxycorticosterone acetate-salt treatment in nontransgenic mice led to significant increases in blood pressure, hypothalamic angiotensin II levels, inflammation, impaired baroreflex sensitivity, and autonomic dysfunction, as well as decreased hypothalamic ACE2 activity and expression, although these changes were blunted or prevented in syn-hACE2 mice. In addition, reduction of ACE2 expression and activity in the brain paralleled an increase in ACE2 activity in the cerebrospinal fluid of nontransgenic mice after deoxycorticosterone acetate-salt treatment and were accompanied by enhanced ADAM17 expression and activity in the hypothalamus. Chronic knockdown of ADAM17 in the brain blunted the development of hypertension and restored ACE2 activity and baroreflex function.

CONCLUSIONS

Our data provide the first evidence that ADAM17-mediated shedding impairs brain ACE2 compensatory activity, thus contributing to the development of neurogenic hypertension.

Selective leptin resistance revisited

In addition to effects on appetite and metabolism, leptin influences many neuroendocrine and physiological systems, including the sympathetic nervous system. Building on my Carl Ludwig Lecture of the American Physiological Society, I review the sympathetic and cardiovascular actions of leptin. The review focuses on a critical analysis of the concept of selective leptin resistance (SLR) and the role of leptin in the pathogenesis of obesity-induced hypertension in both experimental animals and humans. We introduced the concept of SLR in 2002 to explain how leptin might increase blood pressure (BP) in obese states, such as diet-induced obesity (DIO), that are accompanied by partial leptin resistance. This concept, analogous to selective insulin resistance in the metabolic syndrome, holds that in several genetic and acquired models of obesity, there is preservation of the renal sympathetic and pressor actions of leptin despite attenuation of the appetite and weight-reducing actions. Two potential overlapping mechanisms of SLR are reviewed: 1) differential leptin molecular signaling pathways that mediate selective as opposed to universal leptin action and 2) brain site-specific leptin action and resistance. Although the phenomenon of SLR in DIO has so far focused on preservation of sympathetic and BP actions of leptin, consideration should be given to the possibility that this concept may extend to preservation of other actions of leptin. Finally, I review perplexing data on the effects of leptin on sympathetic activity and BP in humans and its role in human obesity-induced hypertension.

Centrally mediated erectile dysfunction in rats with type 1 diabetes: role of angiotensin II and superoxide

INTRODUCTION

Erectile dysfunction is a serious complication of diabetes mellitus. Apart from the peripheral actions, central mechanisms are also responsible for penile erection.

AIM

This study aims to determine the contribution of angiotensin (ANG) II in the dysfunction of central N-methyl-D-aspartic acid (NMDA)- and nitric oxide (NO)-induced erectile responses in streptozotocin-induced type 1 diabetic (T1D) rats.

METHODS

Three weeks after streptozotocin injections, rats were randomly treated with the angiotensin-converting enzyme inhibitor-enalapril, or the ANG II type 1 receptor blocker, losartan, or the superoxide dismutase mimetic, tempol, or vehicle via chronic intracerebroventricular infusion by osmotic mini-pump for 2 weeks.

MAIN OUTCOME MEASURE

Central NMDA receptor stimulation or the administration of the NO donor, sodium nitroprusside (SNP)-induced penile erectile responses and concurrent behavioral responses were monitored in conscious rats.

RESULTS

Two weeks of enalapril, losartan, or tempol treatment significantly improved the erectile responses to central microinjection of both NMDA and SNP in the paraventricular nucleus (PVN) of conscious T1D rats (NMDA responses-T1D+enalapril: 1.7 ± 0.6, T1D+losartan: 2.0 ± 0.3, T1D+tempol: 2.0 ± 0.6 vs. T1D+vehicle: 0.6 ± 0.3 penile erections/rat in the first 20 minutes, P < 0.05; SNP responses-T1D+enalapril: 0.9 ± 0.3, T1D+losartan: 1.3 ± 0.3, T1D+tempol: 1.4 ± 0.4 vs. T1D+vehicle: 0.4 ± 0.2 penile erections/rat in the first 20 minutes, P < 0.05). Concurrent behavioral responses including yawning and stretching, induced by central NMDA and SNP microinjections, were also significantly increased in T1D rats after enalapril, losartan, or tempol treatments. Neuronal NO synthase expression within the PVN was also significantly increased, and superoxide production was reduced in T1D rats after these treatments.

CONCLUSIONS

These data strongly support the contention that enhanced ANG II mechanism/s within the PVN of T1D rats contributes to the dysfunction of central NMDA-induced erectile responses in T1D rats via stimulation of superoxide.

Hypertension in mice with transgenic activation of the brain renin-angiotensin system is vasopressin dependent

An indispensable role for the brain renin-angiotensin system (RAS) has been documented in most experimental animal models of hypertension. To identify the specific efferent pathway activated by the brain RAS that mediates hypertension, we examined the hypothesis that elevated arginine vasopressin (AVP) release is necessary for hypertension in a double-transgenic model of brain-specific RAS hyperactivity (the "sRA" mouse model). sRA mice experience elevated brain RAS activity due to human angiotensinogen expression plus neuron-specific human renin expression. Total daily loss of the 4-kDa AVP prosegment (copeptin) into urine was grossly elevated (≥8-fold). Immunohistochemical staining for AVP was increased in the supraoptic nucleus of sRA mice (~2-fold), but no quantitative difference in the paraventricular nucleus was observed. Chronic subcutaneous infusion of a nonselective AVP receptor antagonist conivaptan (YM-087, Vaprisol, 22 ng/h) or the V(2)-selective antagonist tolvaptan (OPC-41061, 22 ng/h) resulted in normalization of the baseline (~15 mmHg) hypertension in sRA mice. Abdominal aortas and second-order mesenteric arteries displayed AVP-specific desensitization, with minor or no changes in responses to phenylephrine and endothelin-1. Mesenteric arteries exhibited substantial reductions in V(1A) receptor mRNA, but no significant changes in V(2) receptor expression in kidney were observed. Chronic tolvaptan infusion also normalized the (5 mmol/l) hyponatremia of sRA mice. Together, these data support a major role for vasopressin in the hypertension of mice with brain-specific hyperactivity of the RAS and suggest a primary role of V(2) receptors.

Central antihypertensive effects of orally active aminopeptidase A inhibitors in spontaneously hypertensive rats

Brain renin-angiotensin system hyperactivity has been implicated in the development and maintenance of hypertension. We reported previously in the brain that aminopeptidase A and aminopeptidase N are involved in the metabolism of angiotensin II and angiotensin III, respectively. By using in vivo specific and selective aminopeptidase A and aminopeptidase N inhibitors, we showed that angiotensin III is one of the main effector peptides of the brain renin-angiotensin system, exerting a tonic stimulatory control more than blood pressure in hypertensive rats. Aminopeptidase A, the enzyme generating brain angiotensin III, thus represents a potential target for the treatment of hypertension. We demonstrated here the antihypertensive effects of RB150, a prodrug of the specific and selective aminopeptidase A inhibitor, EC33, in spontaneously hypertensive rats, a model of human essential hypertension. Oral administration of RB150 in conscious spontaneously hypertensive rats inhibited brain aminopeptidase A activity, demonstrating the central bioavailability of RB150 and its ability to generate EC33 into the brain. Oral RB150 treatment dose-dependently reduced blood pressure in spontaneously hypertensive rats with an ED(50) of 30 mg/kg, lasting for several hours. This decrease in blood pressure is partly attributed to a decrease in sympathetic tone, reducing vascular resistance. This treatment did not modify systemic renin-angiotensin system activity. Concomitant oral administration of RB150 with a systemic renin-angiotensin system blocker, enalapril, potentiated the RB150-induced blood pressure decrease achieved in <2 hours. Thus, RB150 may be the prototype of a new class of centrally active antihypertensive agents that might be used in combination with classic systemic renin-angiotensin system blockers to improve blood pressure control.

A brain leptin-renin angiotensin system interaction in the regulation of sympathetic nerve activity

The sympathetic nervous system, leptin, and renin-angiotensin system (RAS) have been implicated in obesity-associated hypertension. There is increasing evidence for the presence of both leptin and angiotensin II receptors in several key brain cardiovascular and metabolic control regions. We tested the hypothesis that the brain RAS plays a facilitatory role in the sympathetic nerve responses to leptin. In rats, intracerebroventricular (ICV) administration of losartan (5 μg) selectively inhibited increases in renal and brown adipose tissue (BAT) sympathetic nerve activity (SNA) produced by leptin (10 μg ICV) but did not reduce the SNA responses to corticotrophin-releasing factor (CRF) or the melanocortin receptor agonist MTII. In mice with deletion of angiotensin II type-1a receptors (AT(1a)R(-/-)), increases in renal and BAT SNA induced by leptin (2 μg ICV) were impaired whereas SNA responses to MTII were preserved. Decreases in food intake and body weight with ICV leptin did not differ in AT(1a)R(-/-) vs. AT(1a)R(+/+) mice. ICV leptin in rats increased AT(1a)R and angiotensin-converting enzyme (ACE) mRNA in the subfornical organ and AT(1a)R mRNA in the arcuate nucleus, suggesting leptin-induced upregulation of the brain RAS in specific brain regions. To evaluate the role of de novo production of brain angiotensin II in SNA responses to leptin, we treated rats with captopril (12.5 μg ICV). Captopril attenuated leptin effects on renal and BAT SNA. In conclusion, these studies provide evidence that the brain RAS selectively facilitates renal and BAT sympathetic nerve responses to leptin while sparing effects on food intake.

Brain-targeted (pro)renin receptor knockdown attenuates angiotensin II-dependent hypertension

The (pro)renin receptor is a newly discovered member of the brain renin-angiotensin system. To investigate the role of brain (pro)renin receptor in hypertension, adeno-associated virus-mediated (pro)renin receptor short hairpin RNA was used to knockdown (pro)renin receptor expression in the brain of nontransgenic normotensive and human renin-angiotensinogen double-transgenic hypertensive mice. Blood pressure was monitored using implanted telemetric probes in conscious animals. Real-time PCR and immunostaining were performed to determine (pro)renin receptor, angiotensin II type 1 receptor, and vasopressin mRNA levels. Plasma vasopressin levels were determined by ELISA. Double-transgenic mice exhibited higher blood pressure, elevated cardiac and vascular sympathetic tone, and impaired spontaneous baroreflex sensitivity. Intracerebroventricular delivery of (pro)renin receptor short-hairpin RNA significantly reduced blood pressure, cardiac and vasomotor sympathetic tone, and improved baroreflex sensitivity compared with the control virus treatment in double-transgenic mice. (Pro)renin receptor knockdown significantly reduced angiotensin II type 1 receptor and vasopressin levels in double-transgenic mice. These data indicate that (pro)renin receptor knockdown in the brain attenuates angiotensin II-dependent hypertension and is associated with a decrease in sympathetic tone and an improvement of the baroreflex sensitivity. In addition, brain-targeted (pro)renin receptor knockdown is associated with downregulation of angiotensin II type 1 receptor and vasopressin levels. We conclude that central (pro)renin receptor contributes to the pathogenesis of hypertension in human renin-angiotensinogen transgenic mice.

Role of angiotensin II receptor subtype activation in cognitive function and ischaemic brain damage

Recent clinical studies have demonstrated that angiotensin II type 1 (AT(1) ) receptor blockers (ARBs) reduce the onset of stroke, stroke severity and the incidence and progression of Alzheimer's disease and dementia. We can expect that ARBs exert these effects by both AT(1) receptor blockade and angiotensin II type 2 (AT(2) ) receptor stimulation. Moreover, recent experimental results support the notion that AT(2) receptor stimulation with AT(1) receptor blockade could contribute to protection against ischaemic brain damage at least partly due to an increase in cerebral blood flow and decrease in oxidative stress, and prevent cognitive decline. Cellular therapy has been focused on as a new therapeutic approach to restore injured neurons. In this context, it has been reported that AT(2) receptor stimulation enhances neurite outgrowth and decreases neural damage, thereby enhancing neurogenesis. Moreover, additional beneficial effects of ARBs with an AT(1) receptor blocking action with a partial peroxisome proliferator-activated receptor (PPAR)-γ agonistic effect have been reported, and interaction of AT(2) receptor activation and PPAR-γ might be involved in these ARBs' effects. This article reviews the effects of regulation of activation of angiotensin II receptor subtypes on ischaemic brain damage and cognitive function, focusing on the effects of AT(2) receptor stimulation.

Divergent mechanism regulating fluid intake and metabolism by the brain renin-angiotensin system

The purpose of this review is two-fold. First, I will highlight recent advances in our understanding of the mechanisms regulating angiotensin II (ANG II) synthesis in the brain, focusing on evidence that renin is expressed in the brain and is expressed in two forms: a secreted form, which may catalyze extracellular ANG I generation from glial or neuronal angiotensinogen (AGT), and an intracellular form, which may generate intracellular ANG in neurons that may act as a neurotransmitter. Second, I will discuss recent studies that advance the concept that the renin-angiotensin system (RAS) in the brain not only is a potent regulator of blood pressure and fluid intake but may also regulate metabolism. The efferent pathways regulating the blood pressure/dipsogenic effects and the metabolic effects of elevated central RAS activity appear different, with the former being dependent upon the hypothalamic-pituitary-adrenal axis, and the latter being dependent upon an interaction between the brain and the systemic (or adipose) RAS.

The role of the brain renin-angiotensin system in hypertension: implications for new treatment

Hypertension affects 26% of adults and is in constant progress related to increased incidence of obesity and diabetes. One-third of hypertensive patients may be successfully treated with one antihypertensive agent, one-third may require two agents and in the remaining patients will need three agents for effective blood pressure (BP) control. The development of new classes of antihypertensive agents with different mechanisms of action therefore remains an important goal. Brain renin-angiotensin system (RAS) hyperactivity has been implicated in hypertension development and maintenance in several types of experimental and genetic hypertension animal models. Among the main bioactive peptides of the brain RAS, angiotensin (Ang) II and Ang III have similar affinities for type 1 (AT1) and type 2 (AT2) Ang II receptors. Following intracerebroventricular (i.c.v.) injection, Ang II and Ang III similarly increase arginine-vasopressin (AVP) release and BP. Blocking the brain RAS may be advantageous as it simultaneously (1) decreases sympathetic tone and consequently vascular resistance, (2) decreases AVP release, reducing blood volume and vascular resistance and (3) blocks angiotensin-induced baroreflex inhibition, decreasing both vascular resistance and cardiac output. However, as Ang II is converted to Ang III in vivo, the exact nature of the active peptide is not precisely determined. We summarize here the main findings identifying AngIII as one of the major effector peptides of the brain RAS in the control of AVP release and BP. Brain AngIII exerts a tonic stimulatory effect on BP in hypertensive rats, identifying brain aminopeptidase A (APA), the enzyme generating brain Ang III, as a potentially candidate target for hypertension treatment. This has led to the development of potent orally active APA inhibitors, such as RB150--the prototype of a new class of centrally acting antihypertensive agents.

Systemic candesartan reduces brain angiotensin II via downregulation of brain renin-angiotensin system

The renin-angiotensin system has an important function in the regulation of blood pressure as well as in pathophysiological processes in the central nervous system. We examined the effects of the angiotensin receptor blocker candesartan (10 mg kg(-1) per day, p.o.) on brain angiotensin II levels in angiotensin II-infused hypertensive rats. Angiotensin II or vehicle was infused subcutaneously for 14 days in Sprague-Dawley rats. Angiotensin II infusion resulted in increased blood pressure, an effect that was blocked by candesartan treatment. There was no effect of the angiotensin II infusion on Angiotensin II levels in the brain or on blood-brain barrier permeability. Brain tissue angiotensinogen and angiotensin converting enzyme mRNA levels were not changed by angiotensin II infusion but were decreased by candesartan treatment. At 2 weeks of treatment, CV11974, an active form of candesartan, was detectable in the plasma but was not detectable in brain tissue. These data suggest that treatment with candesartan decreases brain angiotensin II by decreasing brain angiotensinogen and angiotensin converting enzyme gene expression.

The angiotensin II type 2 receptor in the brain

Recent clinical studies indicate that blockade of the renin-angiotensin system is important to prevent stroke, and accumulating results of basic research also indicate the possible involvement of the central renin-angiotensin system in ischaemic brain damage and cognition. When the angiotensin II type 1 receptor is blocked by an angiotensin type 1 receptor blocker, unbound angiotensin II acts preferentially on the angiotensin II type 2 (AT(2)) receptor. These results suggest the pathophysiological importance of the AT(2) receptor in the clinical use of angiotensin type 1 receptor blockers, which are widely used in patients with hypertension with the expectation of a decrease in the onset of cardiovascular and cerebrovascular disease. We review here the possible roles of AT(2) receptor activation in the brain, focusing on ischaemic stroke, cognitive function and neurogenesis, and potential effects of specific AT(2) receptor agonists.

Neurohormonal regulation of the sympathetic nervous system: new insights into central mechanisms of action

To regulate blood pressure, the brain controls circulating hormones, which influence the brain by binding to brain neurons that lie outside the blood-brain barrier. Recent work has demonstrated that "cardiovascular" hormones are synthesized and released in the brain as neurotransmitters/neuromodulators and can, in some cases, signal through the blood-brain barrier. The renin-angiotensin system is a prototype for these newly appreciated mechanisms. The brain's intrinsic renin-angiotensin system plays an important role in blood pressure control. Angiotensin II in brain neurons affects other neurons both through activation of angiotensin receptors and via generation of nitric oxide and reactive oxygen molecules. Similarly, angiotensin in blood vessels activates endothelial nitric oxide, which can diffuse across the blood-brain barrier and thereby alter neuronal activity in cardiovascular control nuclei. The relative importance of these mechanisms to blood pressure control remains to be fully elucidated.

Angiotensin II type 1 receptor-mediated reduction of angiotensin-converting enzyme 2 activity in the brain impairs baroreflex function in hypertensive mice

Angiotensin-converting enzyme 2 (ACE2), a new component of the brain renin-angiotensin system, has been suggested to participate in the central regulation of blood pressure (BP). To clarify the relationship between ACE2 and other brain renin-angiotensin system components, we hypothesized that central angiotensin II type 1 receptors reduce ACE2 expression/activity in hypertensive mice, thereby impairing baroreflex function and promoting hypertension. To test this hypothesis, chronically hypertensive mice (RA) with elevated angiotensin II levels were treated with losartan (angiotensin II type 1 receptor blocker) or PD123319 (angiotensin II type 2 antagonist; 10 mg/kg per day, SC) for 2 weeks. Baseline spontaneous baroreflex sensitivity and brain ACE2 activity were dramatically decreased in RA compared with nontransgenic mice, whereas peripheral ACE2 activity/expression remained unaffected. Losartan, but not PD123319, increased central ACE2 activity, spontaneous baroreflex sensitivity, and normalized BP in RA mice. To confirm the critical role of central ACE2 in BP regulation, we generated a triple-transgenic model with brain ACE2 overexpression on a hypertensive RA background. Triple-transgenic-model mice exhibit lower BP and blunted water intake versus RA, suggesting lower brain angiotensin II levels. Moreover, the impaired spontaneous baroreflex sensitivity, parasympathetic tone, and increased sympathetic drive, observed in RA, were normalized in triple-transgenic-model mice. These data suggest that angiotensin II type 1 receptors inhibit ACE2 activity in RA mice brain, thus contributing to the maintenance of hypertension. In addition, overexpression of ACE2 in the brain reduces hypertension by improving arterial baroreflex and autonomic function. Together, our data suggest that angiotensin II type 1 receptor-mediated ACE2 inhibition impairs baroreflex function and support a critical role for ACE2 in the central regulation of BP and the development of hypertension.