Aminopeptidase A: distribution in rat brain nuclei and increased activity in spontaneously hypertensive rats

Brain ACE2 activation following brain aminopeptidase A blockade by firibastat in salt-dependent hypertension

In the brain, aminopeptidase A (APA), a membrane-bound zinc metalloprotease, generates angiotensin III from angiotensin II. Brain angiotensin III exerts a tonic stimulatory effect on the control of blood pressure (BP) in hypertensive rats and increases vasopressin release. Blocking brain angiotensin III formation by the APA inhibitor prodrug RB150/firibastat normalizes arterial BP in hypertensive deoxycorticosterone acetate (DOCA)-salt rats without inducing angiotensin II accumulation. We therefore hypothesized that another metabolic pathway of brain angiotensin II, such as the conversion of angiotensin II into angiotensin 1-7 (Ang 1-7) by angiotensin-converting enzyme 2 (ACE2) might be activated following brain APA inhibition. We found that the intracerebroventricular (icv) administration of RB150/firibastat in conscious DOCA-salt rats both inhibited brain APA activity and induced an increase in brain ACE2 activity. Then, we showed that the decreases in BP and vasopressin release resulting from brain APA inhibition with RB150/firibastat were reduced if ACE2 was concomitantly inhibited by MLN4760, a potent ACE2 inhibitor, or if the Mas receptor (MasR) was blocked by A779, a MasR antagonist. Our findings suggest that in the brain, the increase in ACE2 activity resulting from APA inhibition by RB150/firibastat treatment, subsequently increasing Ang 1-7 and activating the MasR while blocking angiotensin III formation, contributes to the antihypertensive effect and the decrease in vasopressin release induced by RB150/firibastat. RB150/firibastat treatment constitutes an interesting therapeutic approach to improve BP control in hypertensive patients by inducing in the brain renin-angiotensin system, hyperactivity of the beneficial ACE2/Ang 1-7/MasR axis while decreasing that of the deleterious APA/Ang II/Ang III/ATI receptor axis.

Structural insight into the catalytic mechanism and inhibitor binding of aminopeptidase A

Aminopeptidase A (APA) is a membrane-bound monozinc aminopeptidase. In the brain, APA generates angiotensin III which exerts a tonic stimulatory effect on the control of blood pressure (BP) in hypertensive animals. The oral administration of RB150 renamed firibastat by WHO, an APA inhibitor prodrug, targeting only the S1 subsite, decreases BP in hypertensive patients from various ethnic origins. To identify new families of potent and selective APA inhibitors, we explored the organization of the APA active site, especially the S2' subsite. By molecular modeling, docking, molecular dynamics simulations and site-directed mutagenesis, we revealed that Arg368 and Arg386, in the S2' subsite of human APA established various types of interactions in major part with the P2' residue but also with the P1' residue of APA inhibitors, required for their nanomolar inhibitory potency. We also demonstrated an important role for Arg368 in APA catalysis, in maintaining the structural integrity of the GAMEN motif, a conserved sequence involved in exopeptidase specificity and optimal positioning of the substrate in monozinc aminopeptidases. This arginine together with the GAMEN motif are key players for the catalytic mechanism of these enzymes.

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.

Evolution of a New Class of Antihypertensive Drugs: Targeting the Brain Renin-Angiotensin System

In addition to the circulating renin-angiotensin system, activation of the brain renin-angiotensin system plays an important role in the pathophysiology of hypertension. One of the major components of the brain renin-angiotensin system implicated in the development of hypertension is Ang III (angiotensin III). Brain Ang III, produced from Ang II (angiotensin II) by APA (aminopeptidase A), exerts a tonic stimulatory control over blood pressure in hypertensive rats. Targeting Ang III by inhibiting brain APA is now considered a potentially important target in the management of hypertension. This has led to development of RB150, an orally active prodrug of the specific and selective APA inhibitor, EC33. Orally administered RB150 crosses the gastrointestinal and blood-brain barriers, enters the brain where it generates 2 active molecules of EC33 that block brain APA activity. This results in decreased brain Ang III formation and reduced blood pressure in hypertensive rats. The RB150-induced blood pressure decrease is due to a reduced vasopressin release, which increases diuresis, reducing extracellular volume, a decrease in sympathetic tone, leading to a reduction of vascular resistances, and the improvement of the baroreflex function. RB150 was renamed firibastat by the World Health Organization. Phase Ia/Ib clinical trials showed that firibastat is clinically and biologically well tolerated in healthy volunteers. Clinical efficacy of firibastat in hypertensive patients was, therefore, demonstrated in 2 phase II studies. Accordingly, firibastat could represent the first drug of a novel class of antihypertensive drugs targeting the brain renin-angiotensin system.

NI956/QGC006, a Potent Orally Active, Brain-Penetrating Aminopeptidase A Inhibitor for Treating Hypertension

Brain renin-angiotensin system hyperactivity has been implicated in the development and maintenance of hypertension. We have shown that aminopeptidase A is involved in the formation of brain angiotensin III, which exerts tonic stimulatory control over blood pressure in hypertensive deoxycorticosterone acetate-salt rats and spontaneously hypertensive rats. We have also shown that injection of the specific and selective aminopeptidase A inhibitor, (3S)-3-amino-4-sulfanyl-butane-1-sulfonic acid (EC33), by central route or its prodrug, RB150/firibastat, by oral route inhibited brain aminopeptidase A activity and blocked the formation of brain angiotensin III, normalizing blood pressure in hypertensive rats. These findings identified brain aminopeptidase A as a potential new therapeutic target for hypertension. We report here the development of a new aminopeptidase A inhibitor prodrug, NI956/QGC006, obtained by the disulfide bridge-mediated dimerization of NI929. NI929 is 10× more efficient than EC33 at inhibiting recombinant mouse aminopeptidase A activity in vitro. After oral administration at a dose of 4 mg/kg in conscious deoxycorticosterone acetate-salt rats, NI956/QGC006 normalized brain aminopeptidase A activity and induced a marked decrease in blood pressure of -44±13 mm Hg 4 hours after treatment ( P<0.001), sustained over 10 hours (-21±12 mm Hg; P<0.05). Moreover, NI956/QGC006 decreased plasma arginine-vasopressin levels, and increased diuresis and natriuresis, that may participate to the blood pressure decrease. Finally, NI956/QGC006 did not affect plasma sodium and potassium concentrations. This study shows that NI956/QGC006 is a best-in-class central-acting aminopeptidase A inhibitor prodrug. Our results support the development of hypertension treatments targeting brain aminopeptidase A.

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.

Angiotensin II, dopamine and nitric oxide. An asymmetrical neurovisceral interaction between brain and plasma to regulate blood pressure

Vital functions, such as blood pressure, are regulated within a framework of neurovisceral integration in which various factors are involved under normal conditions maintaining a delicate balance. Imbalance of any of these factors can lead to various pathologies. Blood pressure control is the result of the balanced action of central and peripheral factors that increase or decrease. Special attention for blood pressure control was put on the neurovisceral interaction between Angiotensin II and the enzymes that regulate its activity as well as on nitric oxide and dopamine. Several studies have shown that such interaction is asymmetrically organized. These studies suggest that the neuronal activity related to the production of nitric oxide in plasma is also lateralized and, consequently, changes in plasma nitric oxide influence neuronal function. This observation provides a new aspect revealing the complexity of the blood pressure regulation and, undoubtedly, makes such study more motivating as it may affect the approach for treatment.

Orally Active Aminopeptidase A Inhibitor Prodrugs: Current State and Future Directions

PURPOSE OF REVIEW

To review the data supporting the use of aminopeptidase A (APA) inhibitor prodrugs as centrally acting antihypertensive agents.

RECENT FINDINGS

Brain renin-angiotensin system (RAS) hyperactivity has been implicated in the development and maintenance of hypertension. Angiotensin III, generated by APA, one of the main effector peptides of the brain RAS, exerts a tonic stimulatory control over blood pressure in hypertensive rats. This identified brain APA as a potential therapeutic target for the treatment of hypertension, leading to the development of RB150/firibastat, an orally active prodrug of the specific and selective APA inhibitor, EC33. When given orally, RB150/firibastat crosses the gastrointestinal and blood-brain barriers, enters the brain, and generates two active molecules of EC33 which inhibit brain APA activity, blocking brain angiotensin III formation, and decrease blood pressure for several hours in hypertensive rats. Orally active APA inhibitor prodrugs, by blocking brain RAS activity, represent promising novel strategy for treating hypertension.

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.

Brain renin-angiotensin system blockade with orally active aminopeptidase A inhibitor prevents cardiac dysfunction after myocardial infarction in mice

Brain renin-angiotensin system (RAS) hyperactivity has been implicated in sympathetic hyperactivity and progressive left ventricular (LV) dysfunction after myocardial infarction (MI). Angiotensin III, generated by aminopeptidase A (APA), is one of the main effector peptides of the brain RAS in the control of cardiac function. We hypothesized that orally administered firibastat (previously named RB150), an APA inhibitor prodrug, would attenuate heart failure (HF) development after MI in mice, by blocking brain RAS hyperactivity. Two days after MI, adult male CD1 mice were randomized to three groups, for four to eight weeks of oral treatment with vehicle (MI + vehicle), firibastat (150 mg/kg; MI + firibastat) or the angiotensin I converting enzyme inhibitor enalapril (1 mg/kg; MI + enalapril) as a positive control. From one to four weeks post-MI, brain APA hyperactivity occurred, contributing to brain RAS hyperactivity. Firibastat treatment normalized brain APA hyperactivity, with a return to the control values measured in sham group two weeks after MI. Four and six weeks after MI, MI + firibastat mice had a significant lower LV end-diastolic pressure, LV end-systolic diameter and volume, and a higher LV ejection fraction than MI + vehicle mice. Moreover, the mRNA levels of biomarkers of HF (Myh7, Bnp and Anf) were significantly lower following firibastat treatment. For a similar infarct size, the peri-infarct area of MI + firibastat mice displayed lower levels of mRNA for Ctgf and collagen types I and III (markers of fibrosis) than MI + vehicle mice. Thus, chronic oral firibastat administration after MI in mice prevents cardiac dysfunction by normalizing brain APA hyperactivity, and attenuates cardiac hypertrophy and fibrosis.

Neuropeptidome of the Hypothalamus and Pituitary Gland of Indicine × Taurine Heifers: Evidence of Differential Neuropeptide Processing in the Pituitary Gland before and after Puberty

Puberty in cattle is regulated by an endocrine axis, which includes a complex milieu of neuropeptides in the hypothalamus and pituitary gland. The neuropeptidome of hypothalamic-pituitary gland tissue of pre- (PRE) and postpubertal (POST) Bos indicus-influenced heifers was characterized, followed by quantitative analysis of 51 fertility-related neuropeptides in these tissues. Comparison of peptide abundances with gene expression levels allowed assessment of post-transcriptional peptide processing. On the basis of classical cleavage, 124 mature neuropeptides from 35 precursor proteins were detected in hypothalamus and pituitary gland tissues of three PRE and three POST Brangus heifers. An additional 19 peptides (cerebellins, PEN peptides) previously reported as neuropeptides that did not follow classical cleavage were also identified. In the pre-pubertal hypothalamus, a greater diversity of neuropeptides (25.8%) was identified relative to post-pubertal heifers, while in the pituitary gland, 38.6% more neuropeptides were detected in the post-pubertal heifers. Neuro-tissues of PRE and POST heifers revealed abundance differences ( p < 0.05) in peptides from protein precursors involved in packaging and processing (e.g., the granin family and ProSAAS) or neuron stimulation (PENK, CART, POMC, cerebellins). On their own, the transcriptome data of the precursor genes could not predict the neuropeptide profile in the exact same tissues in several cases. This provides further evidence of the importance of differential processing of the neuropeptide precursors in the pituitary before and after puberty.

Update on angiotensin II: new endocrine connections between the brain, adrenal glands and the cardiovascular system

In the brain, angiotensinergic pathways play a major role in chronic regulation of cardiovascular and electrolyte homeostasis. Increases in plasma angiotensin II (Ang II), aldosterone, [Na⁺] and cytokines can directly activate these pathways. Chronically, these stimuli also activate a slow neuromodulatory pathway involving local aldosterone, mineralocorticoid receptors (MRs), epithelial sodium channels and endogenous ouabain (EO). This pathway increases AT₁R and NADPH oxidase subunits and maintains/further increases the activity of angiotensinergic pathways. These brain pathways not only increase the setpoint of sympathetic activity per se, but also enhance its effectiveness by increasing plasma EO and EO-dependent reprogramming of arterial and cardiac function. Blockade of any step in this slow pathway or of AT₁R prevents Ang II-, aldosterone- or salt and renal injury-induced forms of hypertension. MR/AT₁R activation in the CNS also contributes to the activation of sympathetic activity, the circulatory and cardiac RAAS and increase in circulating cytokines in HF post MI. Chronic central infusion of an aldosterone synthase inhibitor, MR blocker or AT₁R blocker prevents a major part of the structural remodeling of the heart and the decrease in LV function post MI, indicating that MR activation in the CNS post MI depends on aldosterone, locally produced in the CNS. Thus, Ang II, aldosterone and EO are not simply circulating hormones that act on the CNS but rather they are also paracrine neurohormones, locally produced in the CNS, that exert powerful effects in key CNS pathways involved in the long-term control of sympathetic and neuro-endocrine function and cardiovascular homeostasis.

Involvement of arginine 878 together with Ca2+ in mouse aminopeptidase A substrate specificity for N-terminal acidic amino-acid residues

Aminopeptidase A (APA) is a membrane-bound zinc metalloprotease cleaving, in the brain, the N-terminal aspartyl residue of angiotensin II to generate angiotensin III, which exerts a tonic stimulatory effect on the control of blood pressure in hypertensive animals. Using a refined APA structure derived from the human APA crystal structure, we docked the specific and selective APA inhibitor, EC33 in the presence of Ca2+. We report the presence in the S1 subsite of Arg-887 (Arg-878 in mouse APA), the guanidinium moiety of which established an interaction with the electronegative sulfonate group of EC33. Mutagenic replacement of Arg-878 with an alanine or a lysine residue decreased the affinity of the recombinant enzymes for the acidic substrate, α-L-glutamyl-β-naphthylamide, with a slight decrease in substrate hydrolysis velocity either with or without Ca2+. In the absence of Ca2+, the mutations modified the substrate specificity of APA for the acidic substrate, the mutated enzymes hydrolyzing more efficiently basic and neutral substrates, although the addition of Ca2+ partially restored the acidic substrate specificity. The analysis of the 3D models of the Arg-878 mutated APAs revealed a change in the volume of the S1 subsite, which may impair the binding and/or the optimal positioning of the substrate in the active site as well as its hydrolysis. These findings demonstrate the key role of Arg-878 together with Ca2 + in APA substrate specificity for N-terminal acidic amino acid residues by ensuring the optimal positioning of acidic substrates during catalysis.

Brain renin-angiotensin system in the pathophysiology of cardiovascular diseases

Cardiovascular diseases (CVD) are among the main causes of death globally and in this context hypertension represents one of the key risk factors for developing a CVD. It is well established that the peripheral renin-angiotensin system (RAS) plays an important role in regulating blood pressure (BP). All components of the classic RAS can also be found in the brain but, in contrast to the peripheral RAS, how the endogenous RAS is involved in modulating cardiovascular effects in the brain is not fully understood yet. It is a complex system that may work differently in diverse areas of the brain and is linked to the peripheral system by the circumventricular organs (CVO), which do not have a blood brain barrier (BBB). In this review, we focus on the brain angiotensin peptides, their interactions with each other, and the consequences in the central nervous system (CNS) concerning cardiovascular control. Additionally, we present potential drug targets in the brain RAS for the treatment of hypertension.

The vasoprotective axes of the renin-angiotensin system: Physiological relevance and therapeutic implications in cardiovascular, hypertensive and kidney diseases

The renin-angiotensin system (RAS) is undisputedly one of the most prominent endocrine (tissue-to-tissue), paracrine (cell-to-cell) and intracrine (intracellular/nuclear) vasoactive systems in the physiological regulation of neural, cardiovascular, blood pressure, and kidney function. The importance of the RAS in the development and pathogenesis of cardiovascular, hypertensive and kidney diseases has now been firmly established in clinical trials and practice using renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, type 1 (AT₁) angiotensin II (ANG II) receptor blockers (ARBs), or aldosterone receptor antagonists as major therapeutic drugs. The major mechanisms of actions for these RAS inhibitors or receptor blockers are mediated primarily by blocking the detrimental effects of the classic angiotensinogen/renin/ACE/ANG II/AT₁/aldosterone axis. However, the RAS has expanded from this classic axis to include several other complex biochemical and physiological axes, which are derived from the metabolism of this classic axis. Currently, at least five axes of the RAS have been described, with each having its key substrate, enzyme, effector peptide, receptor, and/or downstream signaling pathways. These include the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor, the ANG II/APA/ANG III/AT₂/NO/cGMP, the ANG I/ANG II/ACE2/ANG (1-7)/Mas receptor, the prorenin/renin/prorenin receptor (PRR or Atp6ap2)/MAP kinases ERK1/2/V-ATPase, and the ANG III/APN/ANG IV/IRAP/AT₄ receptor axes. Since the roles and therapeutic implications of the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor axis have been extensively reviewed, this article will focus primarily on reviewing the roles and therapeutic implications of the vasoprotective axes of the RAS in cardiovascular, hypertensive and kidney diseases.

Excess of Aminopeptidase A in the Brain Elevates Blood Pressure via the Angiotensin II Type 1 and Bradykinin B2 Receptors without Dipsogenic Effect

Aminopeptidase A (APA) cleaves angiotensin (Ang) II, kallidin, and other related peptides. In the brain, it activates the renin angiotensin system and causes hypertension. Limited data are available on the dipsogenic effect of APA and pressor effect of degraded peptides of APA such as bradykinin. Wistar-Kyoto rats received intracerebroventricular (icv) APA in a conscious, unrestrained state after pretreatment with (i) vehicle, (ii) 80 μg of telmisartan, an Ang II type-1 (AT1) receptor blocker, (iii) 800 nmol of amastatin, an aminopeptidase inhibitor, and (iv) 1 nmol of HOE-140, a bradykinin B2 receptor blocker. Icv administration of 400 and 800 ng of APA increased blood pressure by 12.6 ± 3.0 and 19.0 ± 3.1 mmHg, respectively. APA did not evoke drinking behavior. Pressor response to APA was attenuated on pretreatment with telmisartan (vehicle: 22.1 ± 2.2 mmHg versus telmisartan: 10.4 ± 3.2 mmHg). Pressor response to APA was also attenuated with amastatin and HOE-140 (vehicle: 26.5 ± 1.1 mmHg, amastatin: 14.4 ± 4.2 mmHg, HOE-140: 16.4 ± 2.2 mmHg). In conclusion, APA increase in the brain evokes a pressor response via enzymatic activity without dipsogenic effect. AT1 receptors and B2 receptors in the brain may contribute to the APA-induced pressor response.

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.

Novel role of aminopeptidase-A in angiotensin-(1-7) metabolism post myocardial infarction

Aminopeptidase-A (APA) is a less well-studied enzyme of the renin-angiotensin system. We propose that it is involved in cardiac angiotensin (ANG) metabolism and its pathologies. ANG-(1-7) can ameliorate remodeling after myocardial injury. The aims of this study are to (1) develop mass spectrometric (MS) approaches for the assessment of ANG processing by APA within the myocardium; and (2) investigate the role of APA in cardiac ANG-(1-7) metabolism after myocardial infarction (MI) using sensitive MS techniques. MI was induced in C57Bl/6 male mice by ligating the left anterior descending (LAD) artery. Frozen mouse heart sections (in situ assay) or myocardial homogenates (in vitro assay) were incubated with the endogenous APA substrate, ANG II. Results showed concentration- and time-dependent cardiac formation of ANG III from ANG II, which was inhibited by the specific APA inhibitor, 4-amino-4-phosphonobutyric acid. Myocardial APA activity was significantly increased 24 h after LAD ligation (0.82 ± 0.02 vs. 0.32 ± 0.02 ρmol·min(-1)·μg(-1), MI vs. sham, P < 0.01). Both MS enzyme assays identified the presence of a new peptide, ANG-(2-7), m/z 784, which accumulated in the MI (146.45 ± 6.4 vs. 72.96 ± 7.0%, MI vs. sham, P < 0.05). Use of recombinant APA enzyme revealed that APA is responsible for ANG-(2-7) formation from ANG-(1-7). APA exhibited similar substrate affinity for ANG-(1-7) compared with ANG II {Km (ANG II) = 14.67 ± 1.6 vs. Km [ANG-(1-7)] = 6.07 ± 1.12 μmol/l, P < 0.05}. Results demonstrate a novel role of APA in ANG-(1-7) metabolism and suggest that the upregulation of APA, which occurs after MI, may deprive the heart of cardioprotective ANG-(1-7). Thus APA may serve as a potentially novel therapeutic target for management of tissue remodeling after MI.

Structural insights into central hypertension regulation by human aminopeptidase A

Hypertension is regulated through both the central and systemic renin-angiotensin systems. In the central renin-angiotensin system, zinc-dependent aminopeptidase A (APA) up-regulates blood pressure by specifically cleaving the N-terminal aspartate, but not the adjacent arginine, from angiotensin II, a process facilitated by calcium. Here, we determined the crystal structures of human APA and its complexes with different ligands and identified a calcium-binding site in the S1 pocket of APA. Without calcium, the S1 pocket can bind both acidic and basic residues through formation of salt bridges with the charged side chains. In the presence of calcium, the binding of acidic residues is enhanced as they ligate the cation, whereas the binding of basic residues is no longer favorable due to charge repulsion. Of the peptidomimetic inhibitors of APA, amastatin has higher potency than bestatin by fitting better in the S1 pocket and interacting additionally with the S3' subsite. These results explain the calcium-modulated substrate specificity of APA in central hypertension regulation and can guide the design and development of brain-targeting antihypertensive APA inhibitors.

Central angiotensinergic mechanisms associated with hypertension

Following its generation by both systemic and tissue-based renin-angiotensin systems, angiotensin II interacts with specific, G-protein coupled receptors to modulate multiple physiological systems, including the cardiovascular system. Genetic models in which the different components of the renin-angiotensin system have been deleted show large changes in resting blood pressure. Interruption of the generation of angiotensin II, or its interaction with these receptors, decreases blood pressure in hypertensive humans and experimental animal models of hypertension. Whilst the interaction of angiotensin II with the kidney is pivotal in this modulation of blood pressure, an involvement of the system in other tissues is important. Both systemic angiotensins, acting via the blood-brain barrier deficient circumventricular organs, and centrally-generated angiotensin modulate cardiovascular control by regulating fluid and electrolyte ingestion, autonomic activity and neuroendocrine function. This review discusses the pathways in the brain that are involved in this regulation of blood pressure as well as examining the sites in which altered angiotensin function might contribute to the development and maintenance of high blood pressure.

Inhibition of brain angiotensin III attenuates sympathetic hyperactivity and cardiac dysfunction in rats post-myocardial infarction

AIMS

In rats post-myocardial infarction (MI), activation of angiotensinergic pathways in the brain contributes to sympathetic hyperactivity and progressive left ventricle (LV) dysfunction. The present study examined whether angiotensin III (Ang III) is one of the main effector peptides of the brain renin-angiotensin system controlling these effects.

METHODS AND RESULTS

After coronary artery ligation, Wistar rats were infused intracerebroventricularly for 4 weeks via minipumps with vehicle, the aminopeptidase A (APA) inhibitor RB150 (0.3 mg/day), which blocks the formation of brain Ang III, or losartan (0.25 mg/day). Blood pressure (BP), heart rate, and renal sympathetic nerve activity in response to air stress and acute changes in BP were measured, and LV function was evaluated by echocardiography and Millar catheter. At 4 weeks post-MI, brain APA activity was increased, sympatho-excitatory and pressor responses to air stress enhanced, and arterial baroreflex function impaired. LV end-diastolic pressure (LVEDP) was increased and ejection fraction (EF) and maximal first derivative of change in pressure over time (dP/dt(max)) were decreased. Central infusion of RB150 during 4 weeks post-MI normalized brain APA activity and responses to stress and baroreflex function, and improved LVEDP, EF, and dP/dt(max). Central infusion of losartan had similar effects but was somewhat less effective, and had no effect on brain APA activity.

CONCLUSION

These results indicate that brain APA and Ang III appear to play a pivotal role in the sympathetic hyperactivity and LV dysfunction in rats post-MI. RB150 may be a potential candidate for central nervous system-targeted therapy post-MI.

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.

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.

The hypothalamic arcuate nucleus: a new site of cardiovascular action of angiotensin-(1-12) and angiotensin II

The hypothalamic arcuate nucleus (ARCN) has been reported to play a significant role in cardiovascular regulation. It has been hypothesized that the ARCN may be one of the sites of cardiovascular actions of angiotensins (ANGs). Experiments were carried out in urethane-anesthetized, artificially ventilated, adult male Wistar rats. The ARCN was identified by microinjections of N-methyl-d-aspartic acid (NMDA; 10 mM). Microinjections (50 nl) of ANG-(1-12) (1 mM) into the ARCN elicited increases in mean arterial pressure (MAP), heart rate (HR), and greater splanchnic nerve activity (GSNA). The tachycardic responses to ANG-(1-12) were attenuated by bilateral vagotomy. The cardiovascular responses elicited by ANG-(1-12) were attenuated by microinjections of ANG II type 1 receptor (AT(1)R) antagonists but not ANG type 2 receptor (AT(2)R) antagonist. Combined inhibition of ANG-converting enzyme (ACE) and chymase in the ARCN abolished ANG-(1-12)-induced responses. Microinjections of ANG II (1 mM) into the ARCN also increased MAP and HR. Inhibition of ARCN by microinjections of muscimol (1 mM) attenuated the pressor and tachycardic responses to intravenously administered ANG-(1-12) and ANG II (300 pmol/kg each). These results indicated that 1) microinjections of ANG-(1-12) into the ARCN elicited increases in MAP, HR, and GSNA; 2) HR responses were mediated via both sympathetic and vagus nerves; 3) AT(1)Rs, but not AT(2)Rs, in the ARCN mediated ANG-(1-12)-induced responses; 4) both ACE and chymase were needed to convert ANG-(1-12) to ANG II in the ARCN; and 5) ARCN plays a role in mediating the cardiovascular responses to circulating ANGs.

Identification of threonine 348 as a residue involved in aminopeptidase A substrate specificity

Aminopeptidase A (APA; EC 3.4.11.7) is a membrane-bound zinc metalloprotease cleaving in the brain the N-terminal aspartyl residue of angiotensin II to generate angiotensin III, which exerts a tonic stimulatory effect on the central control of blood pressure in hypertensive animals. We docked the specific APA inhibitor, glutamate phosphonate, in the three-dimensional model of the mouse APA ectodomain in the presence of Ca(2+). In the S1 subsite of this model, the Ca(2+) atom was coordinated with Asp-213, Asp-218,y and Glu-215 and three water molecules, one of which formed a hydrogen bond with the carboxylate side chain of the inhibitor. We report here that the carboxylate side chain of glutamate phosphonate also formed a hydrogen bond with the alcohol side chain of Thr-348. Mutagenic replacement of Thr-348 with an aspartate, tyrosine, or serine residue led to a modification of the hydrolysis velocity, with no change in the affinity of the recombinant enzymes for the substrate GluNA, either in the absence or presence of Ca(2+). In the absence of Ca(2+), the mutations modified the substrate specificity of APA, which was nevertheless restored by the addition of Ca(2+). An analysis of three-dimensional models of the corresponding Thr-348 mutants revealed that the interaction between this residue and the inhibitor was abolished or disturbed, leading to a change in the position of the inhibitor in the active site. These findings demonstrate a key role of Thr-348 in substrate specificity of APA for N-terminal acidic amino acids by insuring the optimal positioning of the substrate during catalysis.

Asp218 participates with Asp213 to bind a Ca2+ atom into the S1 subsite of aminopeptidase A: a key element for substrate specificity

APA (aminopeptidase A; EC 3.4.11.7) is a membrane-bound zinc metallopeptidase, also activated by Ca(2+), involved in the formation of brain angiotensin III, which exerts a tonic stimulatory action on the central control of blood pressure in hypertensive animals. In the present study, in the three-dimensional model of the ectodomain of mouse APA, we docked the specific APA inhibitor glutamate phosphonate, in the presence of Ca(2+). The model showed the presence of one Ca(2+) atom in an hydrophilic pocket corresponding to the S1 subsite in which the lateral chain of the inhibitor is pointing. In this pocket, the Ca(2+) atom was hexaco-ordinated with the acidic side chains of Asp(213) and Asp(218), the carbonyl group of Glu(215) and three water molecules, one of them being engaged in a hydrogen bond with the negatively charged carboxylate side chain of the inhibitor. Mutagenic replacement of Asp(213) and Asp(218) with a conservative residue maintained the ability of mutated APAs to be activated by Ca(2+). However, the replacement by a non-conservative residue abolished this property, demonstrating the crucial role of these residues in Ca(2+) binding. We also showed the involvement of these residues in the strict specificity of APA in the presence of Ca(2+) for N-terminal acidic residues from substrates or inhibitors, since mutagenic replacement of Asp(213) and Asp(218) induced a decrease of the inhibitory potencies of inhibitors homologous with acidic residues. Finally, this led to the rational design of a new potent APA inhibitor, NI926 (K(i)=70 nM), which allowed us to precisely localize Asp(213) at the entrance and Asp(218) at the bottom of the S1 subsite. Taken together, these data provide new insight into the organization and functional role of the APA S1 subsite and will allow the design of pharmacophore of the inhibitor, helpful for the development of a new generation of APA inhibitors as central-acting antihypertensive agents.

Human brain aminopeptidase A: biochemical properties and distribution in brain nuclei

Aminopeptidase A (APA) generated brain angiotensin III, one of the main effector peptides of the brain renin angiotensin system, exerting a tonic stimulatory effect on the control of blood pressure in hypertensive rats. The distribution of APA in human brain has not been yet studied. We first biochemically characterized human brain APA (apparent molecular mass of 165 and 130 kDa) and we showed that the human enzyme exhibited similar enzymatic characteristics to recombinant mouse APA. Both enzymes had similar sensitivity to Ca(2+). Kinetic studies showed that the K(m) (190 mumol/L) of the human enzyme for the synthetic substrate-l-glutamyl-beta-naphthylamide was close from that of the mouse enzyme (256 mumol/L). Moreover, various classes of inhibitors including the specific and selective APA inhibitor, (S)-3-amino-4-mercapto-butyl sulfonic acid, had similar inhibitory potencies toward both enzymes. Using (S)-3-amino-4-mercapto-butyl sulfonic acid, we then specifically measured the activity of APA in 40 microdissected areas of the adult human brain. Significant heterogeneity was found in the activity of APA in the various analyzed regions. The highest activity was measured in the choroids plexus and the pineal gland. High activity was also detected in the dorsomedial medulla oblongata, in the septum, the prefrontal cortex, the olfactory bulb, the nucleus accumbens, and the hypothalamus, especially in the paraventricular and supraoptic nuclei. Immunostaining of human brain sections at the level of the medulla oblongata strengthened these data, showing for the first time a high density of immunoreactive neuronal cell bodies and fibers in the motor hypoglossal nucleus, the dorsal motor nucleus of the vagus, the nucleus of the solitary tract, the Roller nucleus, the ambiguus nucleus, the inferior olivary complex, and in the external cuneate nucleus. APA immunoreactivity was also visualized in vessels and capillaries in the dorsal motor nucleus of the vagus and the inferior olivary complex. The presence of APA in several human brain nuclei sensitive to angiotensins and involved in blood pressure regulation suggests that APA in humans is an integral component of the brain renin angiotensin system and strengthens the idea that APA inhibitors could be clinically tested as an additional therapy for the treatment of certain forms of hypertension.

Role of central and peripheral aminopeptidase activities in the control of blood pressure: a working hypothesis

Although there is a large body of knowledge on protein synthesis, the available data on protein catabolism, although quite substantial, are still inadequate. This is due to the marked differences in the activity of proteolytic enzymes, compounded by different substrate specificities and multiple environmental factors. Understanding enzyme behavior under physiological and pathological conditions requires the identification of specific proteolytic activities, such as aminopeptidases, as able to degrade certain peptidergic hormones or neuropeptides. Another requirement is the isolation, purification and characterization of the enzymes involved. In addition, systematic studies are needed to determine each enzyme's subcellular location, tissue distribution, and the influence of environmental factors such as diurnal rhythm, age, gender, diet, cholesterol, or steroids. Central and peripheral aminopeptidases may play a role in the control of blood pressure by coordinating the effect of the different peptides of the renin-angiotensin system cascade, acting through the AT(1), AT(2), and AT(4) receptors. Our review of the available data suggests the hypothesis that cholesterol or steroids, particularly testosterone, significantly influence aminopeptidase activities, their substrate availability and consequently their functions. These observations may have relevant clinical implications for a better understanding of the pathophysiology of cardiovascular diseases, and thus for their treatment with aminopeptidase inhibitors.

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

Overactivity of the brain renin-angiotensin system has been implicated in the development and maintenance of hypertension. We reported previously that angiotensin II is converted to angiotensin III by aminopeptidase A in the mouse brain. We then used specific and selective aminopeptidase A inhibitors to show that angiotensin III is one of the main effector peptides of the brain renin-angiotensin system, exerting tonic stimulatory control over blood pressure in hypertensive rats. Aminopeptidase A, the enzyme generating brain angiotensin III, thus represents a potential candidate central nervous system target for the treatment of hypertension. Given this possible clinical use of aminopeptidase A inhibitors, it was, therefore, important to investigate their pharmacological activity after oral administration. We investigated RB150, a dimer of the selective aminopeptidase A inhibitor, EC33, generated by creating a disulfide bond. This chemical modification allows prodrug to cross the blood-brain barrier when administered by systemic route. Oral administration of RB150 in conscious DOCA-salt rats inhibited brain aminopeptidase A activity, resulting in values similar to those obtained with the brains of normotensive rats, demonstrating the central bioavailability of RB150. Oral RB150 treatment resulted in a marked dose-dependent reduction in blood pressure in DOCA-salt but not in normotensive rats, with an ED(50) in the 1-mg/kg range, achieved in <2 hours and lasting for several hours. This treatment also significantly decreased plasma arginine-vasopressin levels and increased diuresis, which may participate to the blood pressure decrease by reducing the size of fluid compartment. Thus, RB150 may be the prototype of a new class of centrally active antihypertensive agents.

Jacques Benoit lecture: the neuroendocrine view of the angiotensin and apelin systems

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 (AVP) 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 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. We also searched for a putative angiotensin receptor subtype specific for Ang III and isolated a seven transmembrane-domain G protein-coupled receptor corresponding to the receptor for apelin, a newly-discovered peptide isolated from bovine stomach. Apelin and its receptor are expressed in magnocellular vasopressinergic neurones in the hypothalamus. The central injection of apelin in lactating rats decreases the phasic electrical activity of vasopressinergic neurones and the systemic secretion of AVP, inducing water diuresis. Apelin is therefore a natural inhibitor of the antidiuretic effect of AVP. In addition, systemic administration of apelin decreases BP, improves cardiac contractility and reduces cardiac loading. The development of nonpeptide agonists of the apelin receptor may provide new therapeutic tools for treating water retention, hyponatraemia and cardiovascular diseases. Angiotensins and apelin thus exert opposing but complementary effects, and are thereby determinant for the maintenance of body fluid homeostasis and cardiovascular functions.

Aminopeptidase A inhibitors as centrally acting antihypertensive agents

Among the main bioactive peptides of the brain renin-angiotensin system, angiotensin (Ang) II and AngIII exhibit the same affinity for the type 1 and type 2 Ang receptors. Both peptides, injected intracerebroventricularly, cause similar increase in blood pressure (BP). Because AngII is converted in vivo to AngIII, the identity of the true effector is unknown. This review summarized recent insights into the predominant role of brain AngIII in the central control of BP underlining the fact that brain aminopeptidase A (APA), the enzyme forming central AngIII, could constitute a putative central therapeutic target for the treatment of hypertension. This led to the development of potent, systematically active APA inhibitors, such as RB150, as a prototype of a new class of centrally acting antihypertensive agents for the treatment of certain forms of hypertension.

Aspartyl, arginyl and alanyl aminopeptidase activities in the hippocampus and hypothalamus of streptozotocin-induced diabetic rats

Acid (aspartyl), basic (arginyl) and neutral (alanyl) aminopeptidases degrade angiotensins, vasopressin, oxytocin, bradykinin and enkephalins. These peptides regulate memory, energy homeostasis, water-salt balance and blood pressure, functions that are mainly exerted in the hippocampus and hypothalamus, and that can be affected by diabetes mellitus. To evaluate the relationship between the diabetes mellitus and processing and inactivation roles of these representative aminopeptidases, we measured their activities in both brain structures of control and streptozotocin-diabetic rats. Hypothalamic soluble aspartyl and arginyl aminopeptidases presented significant decreased activity levels in diabetic rats, which were mitigated by insulin therapy. In addition to membrane-bound puromycin sensitive and insensitive alanyl aminopeptidases, its soluble puromycin sensitive form did not differ between diabetic and control rats in both brain structures. Glucose and/or insulin did not seem to alter in vitro the hypothalamic activities of soluble aspartyl and arginyl aminopeptidases. The implied hypothalamic control of regulatory peptide activity by aspartyl and arginyl aminopeptidases supports the hypothesis that the hydrolytic ability of these enzyme types could be a common link for the disruptions of water-salt balance, blood pressure and energy homeostasis in diabetes mellitus.

Brain aminopeptidases and hypertension

The brain aminopeptidases that participate in the enzymatic cascade of the renin-angiotensin system play a major role in blood pressure (BP) control, and their study offers new perspectives for the understanding of central BP control and the treatment of hypertension. In this system, angiotensin II is converted to angiotensin III (Ang III) by glutamyl aminopeptidase (GluAP) and Ang III is further metabolised to angiotensin IV by alanyl aminopeptidase or arginine-aminopeptidase. It is now clear that Ang III is the key active form of the central angiotensins, exerting tonic stimulatory control over BP. Therefore, the development of GluAP inhibitors as potential antihypertensive agents offers new perspectives for therapy. Brain aspartyl aminopeptidase, which converts angiotensin I to angiotensin 2-10, is also a possible target for antihypertensive therapy because of its potential role in BP control. Finally, since changes in BP levels, that paralleled changes in brain and plasma aminopeptidase activities, were observed after unilateral lesions of the nigrostriatal system, brain asymmetry, aminopeptidase activities and BP control appear to be related, resulting their interplay in an asymmetrical neuroendocrine response that differentially affect BP control. The study of this interaction may contribute to our understanding of how the brain controls BP.

Cystyl aminopeptidase activity in the plasma, viscera and brain of the snake Bothrops jararaca

The relationship between plasma osmolality and cystyl aminopeptidase was characterized in the snake Bothrops jararaca and comparisons were made with the emerging picture of this relationship in rats. The profile of cystyl aminopeptidase activity under basal conditions was determined in the soluble and membrane-bound forms in visceral organs and in the central nervous system in comparison with that of alanyl aminopeptidase. The regional localization of cystyl and alanyl aminopeptidase activities was studied in the central nervous system. The basal level of plasma cystyl aminopeptidase, four- to six-fold higher than in rats, suggests its importance to help regulate circulating levels of neurohypophysial peptides in B. jararaca snake. The osmotic sensitivity of this plasma enzyme, undetectable in male, but about three-fold higher in female snakes than in rats, reveals a sexual dimorphism. In marked contrast to those observed in rats, low levels of soluble and particulate forms in the kidney indicate that cystyl aminopeptidase plays a minor metabolizing role at this anatomical location in B. jararaca. Despite of the regional-specific divergence between the levels of rat and snake enzymes, the bilaterally symmetric pattern of the diencephalic distribution of alanyl aminopeptidase reflects functional homologies between these two distantly related species.

The C-terminal Domain of Aminopeptidase A Is an Intramolecular Chaperone Required for the Correct Folding, Cell Surface Expression, and Activity of This Monozinc Aminopeptidase

Aminopeptidase A (APA, EC 3.4.11.7) is a type II integral membrane glycoprotein responsible for the conversion of angiotensin II to angiotensin III in the brain. Previous site-directed mutagenesis studies and the recent molecular modeling of the APA zinc metallopeptidase domain have shown that all the amino acids involved in catalysis are located between residues 200 and 500. The APA ectodomain is cleaved in the kidney into an N-terminal fragment corresponding to the zinc metallopeptidase domain, and a C-terminal fragment of unknown function. We investigated the function of this C-terminal domain, by expressing truncated APAs in Chinese hamster ovary and AtT-20 cells. Deletion of the C-terminal domain abolished the maturation and enzymatic activity of the N-terminal domain, which was retained in the endoplasmic reticulum as an unfolded protein bound to calnexin. Expression in trans of the C-terminal domain resulted in association of the N- and C-terminal domains soon after biosynthesis, allowing folding rescue, maturation, cell surface expression, and activity of the N-terminal zinc metallopeptidase domain. We also show that the C-terminal domain is not required for the catalytic activity of APA but is essential for its activation. Moreover, we show that the C-terminal domain of aminopeptidase N (EC 3.4.11.2, APN) also promotes maturation and cell surface expression of the N-terminal domain of APN, suggesting a common role of the C-terminal domain in the monozinc aminopeptidase family. Our data provide the first demonstration that the C-terminal domain of an eukaryotic exopeptidase acts as an intramolecular chaperone.

Brain renin-angiotensin system blockade by systemically active aminopeptidase A inhibitors: a potential treatment of salt-dependent 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. We previously reported that in the murine brain, aminopeptidase A (APA) is involved in the conversion of angiotensin II (AngII) to AngIII and that AngIII is one of the main effector peptides of the brain RAS in the control of vasopressin release. Here we report that brain AngIII exerts a tonic stimulatory effect on blood pressure in a model of salt-dependent hypertension, the DOCA-salt rat, characterized by a depressed systemic but a hyperactive brain RAS. Similar high blood pressure accompanied by a low systemic renin state was described in some patients, especially in hypertensive African Americans who are resistant to treatment by blockers of the systemic RAS. We developed RB150, a prodrug of the specific and selective APA inhibitor, EC33. RB150 given i.v. is able to cross the blood-brain barrier, to inhibit brain APA, and to block the formation of central AngIII. A single dose of systemic RB150 (15 mg/kg, i.v.) in conscious DOCA-salt rats inhibited brain APA activity and markedly reduced blood pressure for up to 24 h. These results demonstrate the crucial role of brain APA as a candidate target for the treatment of hypertension and suggest that RB150, a potent systemically active APA inhibitor, could be the prototype of a new class of antihypertensive agents for the treatment of certain forms of hypertension.

Contribution of molecular modeling and site-directed mutagenesis to the identification of two structural residues, Arg-220 and Asp-227, in aminopeptidase A

Aminopeptidase A is a zinc metalloenzyme involved in the formation of brain angiotensin III, which exerts a tonic stimulatory action on the central control of blood pressure. Thus, central inhibitors of aminopeptidase A constitute putative central antihypertensive agents. Mutagenic studies have been performed to investigate organization of the aminopeptidase A active site, with a view to designing such inhibitors. The structure of one monozinc aminopeptidase (leukotriene A(4) hydrolase) was recently resolved and used to construct a three-dimensional model of the aminopeptidase A ectodomain. This new model, highly consistent with the results of mutagenic studies, showed a critical structural interaction between two conserved residues, Arg-220 and Asp-227. Mutagenic replacement of either of these two residues disrupted maturation and subcellular localization and abolished the enzymatic activity of aminopeptidase A, confirming the critical structural role of these residues. In this study, we generated the first three-dimensional model of a strict aminopeptidase, aminopeptidase A. This model constitutes a new tool to probe further the active site of aminopeptidase A and to design new inhibitors of this enzyme.

The luminal membrane of rat thick limb expresses AT1 receptor and aminopeptidase activities

BACKGROUND

Endogenous intratubular angiotensin II (Ang II) supports an autocrine tonic stimulation of NaCl absorption in the proximal tubule, and its production may be regulated independently of circulating Ang II. In addition, endogenous Ang II activity may be regulated at the brush border membrane (BBM), by the rate of aminopeptidase A and N (APA and APN) activities and the rate of Ca2+-independent phospholipase A2 (PLA2-dependent endocytosis and recycling of the complex Ang II subtype 1 (AT1) receptor (AT1-R). The aim of the present study was to look for subcellular localization of AT1-R, and APA and APN activities in the medullary thick ascending limb of Henle (mTAL), as well as search for an asymmetric coupling of AT1-R to signal transduction pathways.

METHODS

Preparations of isolated basolateral membrane (BLMV) and luminal (LMV) membrane vesicles from rat mTAL were used to localize first, AT1-R by 125I-[Sar1, Ile8] Ang II binding studies and immunoblot experiments with a specific AT1-R antibody, and second, APA and APN activities. Microfluorometric monitoring of cytosolic Ca2+ with a Fura-2 probe was performed in mTAL microperfused in vitro, after apical or basolateral application of Ang II.

RESULTS

AT1-R were present in both LMV and BLMV, with a similar Kd (nmol/L range) and Bmax. Accordingly, BLMV and LMV preparations similarly stained specific AT1-R antibody. APA and APN activities were selectively localized in LMV, although to a lesser extent than those measured in BBM. In the in vitro microperfused mTAL, basolateral but not apical Ang II induced a transient increase in cytosolic [Ca2+].

CONCLUSIONS

Besides the presence of basolateral AT1-R in mTAL coupled to the classical Ca2+-dependent transduction pathways, AT1-R are present in LMV, not coupled with Ca2+ signaling, and co-localized with APA and APN activities. Thus, apical APA and APN may play an important role in modulating endogenous Ang II activity on NaCl reabsorption in mTAL.

Pituitary aminopeptidase activities involved in blood-pressure regulation are modified by dietary cholesterol: sex differences

Given that the existence of a local renin-angiotensin system (RAS) in the pituitary and its participation in the regulation of blood pressure and other biological functions are widely accepted, the aim of this work is to analyze the influence of dietary cholesterol on the activity of the enzymes involved in the metabolism of the effector peptides of the renin-angiotensin system (angiotensin II and III) and vasopressin, in the pituitary of male and female mice fed on a cholesterol-enriched diet (1% cholesterol and 0.5% cholic acid). Soluble and membrane-bound pituitary aminopeptidase A (aspartyl- and glutamyl-aminopeptidase), aminopeptidase M (alanyl-aminopeptidase), aminopeptidase B (arginyl-aminopeptidase) and cystinyl-aminopeptidase activities were fluorimetrically measured. In female mice, cholesterol-enriched diet produced a significant increase in soluble aspartyl- and membrane-bound aspartyl- and glutamyl-aminopeptidase activities, and a significant decrease in membrane-bound alanyl-, arginyl- and cystinyl-aminopeptidase activities. In male mice, after feeding the diet, a significant increase in soluble glutamyl- and membrane-bound arginyl-aminopeptidase activities was observed. Our results indicate differential effects of dietary cholesterol on the metabolism of angiotensin II and III and vasopressin in the pituitary of male and female mice.

Angiotensin III: a central regulator of vasopressin release and blood pressure

Among the main bioactive peptides of the brain renin-angiotensin system, angiotensin (Ang) II and AngIII exhibit the same affinity for type 1 and type 2 AngII receptors. Both peptides, injected intracerebroventricularly, cause similar increases in vasopressin release and blood pressure. Because AngII is converted in vivo to AngIII, the identity of the true effector is unknown. This review summarizes new insights into the predominant role of brain AngIII in the control of vasopressin release and blood pressure and underlines the fact that brain aminopeptidase A, the enzyme forming central AngIII, could constitute a putative central therapeutic target for the treatment of hypertension.

Hormonal and neurotransmitter roles for angiotensin in the regulation of central autonomic function

In this review we present the case for both hormonal and neurotransmitter actions of angiotensin II (ANG) in the control of neuronal excitability in a simple neural pathway involved in central autonomic regulation. We will present both single-cell and whole-animal data highlighting hormonal roles for ANG in controlling the excitability of subfornical organ (SFO) neurons. More controversially we will also present the case for a neurotransmitter role for ANG in SFO neurons in controlling the excitability of identified neurons in the paraventricular nucleus (PVN) of the hypothalamus. In this review we highlight the similarities between the actions of ANG on these two populations of neurons in an attempt to emphasize that whether we call such actions "hormonal" or "neurotransmitter" is largely semantic. In fact such definitions only refer to the method of delivery of the chemical messenger, in this case ANG, to its cellular site of action, in this case the AT1 receptor. We also described in this review some novel concepts that may underlie synthesis, metabolic processing, and co-transmitter actions of ANG in this pathway. We hope that such suggestions may lead ultimately to the development of broader guiding principles to enhance our understanding of the multiplicity of physiological uses for single chemical messengers.

Placental aminopeptidase A as a possible barrier of angiotensin II between mother and fetus

Aminopeptidase A (AP-A EC 3.4.11.7), which is a membrane-bound zinc metalloprotease, is present in the placenta. AP-A selectively hydrolyzes N-terminal glutamyl and aspartyl residues and cleaves angiotensin II to form angiotensin III. To determine the role of placental aminopeptidase A under physiological and pathological conditions, we evaluated its immunolocalization and enzymatic activities in the placenta. AP-A was localized in the basal zone of the syncytiotrophoblast, in the membranes of the cytotrophoblast, and in fetal arterioles and venules within the stem villi. AP-A activity in the microsomal fraction of placental villi seemed to be remained essentially constant throughout gestation. The renin-angiotensin system is considered to be accelerated in pre-eclampsia. This AP-A activity was higher in pre-eclampsia (2.86+/-0.30 nmol beta NA/mg protein/h) than in uncomplicated pregnancy from 28 to 41 weeks of gestation (2.08+/-0.18 nmol beta NA/mg protein/h). Angiotensin II evoked AP-A activity in first trimester trophoblast, and Losartan and PD 123177 in combination significantly inhibited this induction of AP-A activity. The results of immunohistochemical evaluation and enzymatic activity suggested that placental aminopeptidase A may play a role as a component of the barrier of angiotensin II between mother and fetus.

Regulation of aminopeptidase A in human brain tumor vasculature: evidence for a role of transforming growth factor-beta

Angiotensin peptides are potent vasoconstrictors, cell growth factors, and neuromodulators in normal and pathological situations. To assess the potential role of the angiotensins in brain tumor-associated vessels, the expression of the enzymes of the angiotensin cascade were evaluated in these tumors. The production of these bioactive peptides is dependent on the activities of exopeptidases, including several aminopeptidases and carboxypeptidases, producing angiotensin (Ang) I, II, III, IV and Ang 1-7. Human cerebral parenchymal and glioblastoma cells expressed renin, and tumor vasculature, but not glioblastoma cells, expressed angiotensin-converting enzyme. High aminopeptidase A (APA) activity, but no aminopeptidase N/B activity, was observed in human brain tumor vasculature, suggesting a predominant production of Ang III. Grafting of rat glioma cells in rat brains yielded tumors with high APA and low aminopeptidase N/B activities in tumor vessels, confirming human results. Tumor growth and APA activity in tumor vessels were not affected by chronic angiotensin-converting enzyme inhibition. The brain-derived EC219 endothelial cells expressed high APA activity, which was not involved in endothelial cell proliferation, but was down-regulated by exposure of cells to transforming growth factor-beta (TGF beta) or to TGF beta-secreting tumor cells, suggesting a role for this peptide in the control of APA activity in cerebral vasculature. Thus, APA is a potential marker of chronic dysfunction, involving loss of TGF beta function, of the metabolic blood-brain barrier, but not of neovascularization.

Non-cardiac nucleic acid composition and protein synthesis rates in hypertension: studies on the spontaneously hypertensive rat (SHR) model

Various studies have shown the involvement of extracardiac tissues in hypertension, including the hepato-intestinal tract, musculo-skeletal system, skin, and the kidney. It was our hypothesis that these perturbations in non-cardiac tissues would also include alterations in protein metabolism. Thus, the reported differences in soleus contractile protein composition may be related to changes in muscle protein synthesis or reduced protein synthetic efficiencies. The aim of the present study was to characterise tissue composition of nucleic acids and rates of protein synthesis in non-cardiac tissues, such as liver, skeletal muscle (i.e., the Type I fibre-predominant soleus and Type II fibre-predominant plantaris), kidney, bone (tibia), skin and the gastrointestinal tract in a genetic model of hypertension (i.e., spontaneously hypertensive rats (SHRs), 15 weeks old) compared to their genetic aged-matched counterparts, i.e., normotensive Wistar-Kyoto (WKY) controls. Rates of protein synthesis were measured in vivo after injection with a flooding dose of L-[4-(3)H]phenylalanine. The results showed changed tissue wet weights (g per organ) for plantaris (+10%, P<0.05), liver (+25%, P<0.01), brain (-9%, P<0.01), jejunum (+39%, P<0.001) and tibia (+17%, P<0.001) in SHRs compared to WKY controls. Protein content (g or mg per organ) was increased in the liver (+32%, P<0. 01) and tibia (+37%, P<0.05). RNA contents (mg per organ) were increased in plantaris (+17%, P<0.01), liver (+22%, P<0.01) and jejunum (+11%, P<0.05). DNA (mg per organ) was increased in plantaris (+16%, P<0.025) and jejunum (+12%, P<0.025). The protein synthetic capacities (i.e., C(s), mg RNA/g protein) were higher in soleus (+41%, P<0.01) and plantaris (+6%, P<0.05) muscles of SHRs compared to WKYs, whereas values were lower in liver (-11%, P<0.01) and kidney (-6%, P<0.01) of SHRs compared to WKYs. The fractional rate of protein synthesis (i.e., k(s), the percentage of the protein pool renewed each day) was not significantly different for any of the tissues, though the rate of protein synthesis per unit RNA (i.e., k(RNA), mg protein/day per mg RNA) was reduced in the soleus (-24%, P<0.05) and the synthesis rate per unit DNA, i.e., k(DNA) (mg protein/day per mg DNA) was increased in the tibia (+31%, P<0.025). This is the first report of significant differences between indices of protein metabolism in extracardiac tissues in hypertension, which may reflect endocrine factors and/or the systemic influence of hypertension per se.

Aminopeptidase A inhibitors as potential central antihypertensive agents

Overactivity of the brain renin-angiotensin system (RAS) has been implicated in the development and maintenance of hypertension in several experimental models, such as spontaneously hypertensive rats and transgenic mice expressing both human renin and human angiotensinogen transgenes. We recently reported that, in the murine brain, angiotensin II (AngII) is converted to angiotensin III (AngIII) by aminopeptidase A (APA), whereas AngIII is inactivated by aminopeptidase N (APN). If injected into cerebral ventricles (ICV), AngII and AngIII cause similar pressor responses. Because AngII is metabolized in vivo into AngIII, the exact nature of the active peptide is not precisely determined. Here we report that, in rats, ICV injection of the selective APA inhibitor EC33 [(S)-3-amino-4-mercaptobutyl sulfonic acid] blocked the pressor response of exogenous AngII, suggesting that the conversion of AngII to AngIII is required to increase blood pressure (BP). Furthermore, ICV injection, but not i.v. injection, of EC33 alone caused a dose-dependent decrease in BP by blocking the formation of brain but not systemic AngIII. This is corroborated by the fact that the selective APN inhibitor, PC18 (2-amino-4-methylsulfonyl butane thiol), administered alone via the ICV route, increases BP. This pressor response was blocked by prior treatment with the angiotensin type 1 (AT(1)) receptor antagonist, losartan, showing that blocking the action of APN on AngIII metabolism leads to an increase in endogenous AngIII levels, resulting in BP increase, through interaction with AT(1) receptors. These data demonstrate that AngIII is a major effector peptide of the brain RAS, exerting tonic stimulatory control over BP. Thus, APA, the enzyme responsible for the formation of brain AngIII, represents a potential central therapeutic target that justifies the development of APA inhibitors as central antihypertensive agents.