Discovery of a distinct binding site for angiotensin II (3-8), a putative angiotensin IV receptor

The Discovery of New Inhibitors of Insulin-Regulated Aminopeptidase by a High-Throughput Screening of 400,000 Drug-like Compounds

With the ambition to identify novel chemical starting points that can be further optimized into small drug-like inhibitors of insulin-regulated aminopeptidase (IRAP) and serve as potential future cognitive enhancers in the clinic, we conducted an ultra-high-throughput screening campaign of a chemically diverse compound library of approximately 400,000 drug-like small molecules. Three biochemical and one biophysical assays were developed to enable large-scale screening and hit triaging. The screening funnel, designed to be compatible with high-density microplates, was established with two enzyme inhibition assays employing either fluorescent or absorbance readouts. As IRAP is a zinc-dependent enzyme, the remaining active compounds were further evaluated in the primary assay, albeit with the addition of zinc ions. Rescreening with zinc confirmed the inhibitory activity for most compounds, emphasizing a zinc-independent mechanism of action. Additionally, target engagement was confirmed using a complementary biophysical thermal shift assay where compounds causing positive/negative thermal shifts were considered genuine binders. Triaging based on biochemical activity, target engagement, and drug-likeness resulted in the selection of 50 qualified hits, of which the IC50 of 32 compounds was below 3.5 µM. Despite hydroxamic acid dominance, diverse chemotypes with biochemical activity and target engagement were discovered, including non-hydroxamic acid compounds. The most potent compound (QHL1) was resynthesized with a confirmed inhibitory IC50 of 320 nM. Amongst these compounds, 20 new compound structure classes were identified, providing many new starting points for the development of unique IRAP inhibitors. Detailed characterization and optimization of lead compounds, considering both hydroxamic acids and other diverse structures, are in progress for further exploration.

Intrathecal injections of angiotensin IV and oxytocin conjugates induce antihyperalgesia and antiallodynia in both sexes of rats

Our previous studies have established that intrathecal oxytocin (OT) and angiotensin IV (Ang IV) injections induce antihyperalgesia and antiallodynia in rodents. Ang IV, a renin-angiotensin system hexapeptide, acts as an endogenous inhibitor that inhibits the oxytocin-degrading enzyme insulin-regulated aminopeptidase (IRAP). The pain inhibitory effects by Ang IV were found to be through its inhibition on IRAP to potentiate the effect of OT. However, these effects were found to be with a significant sex difference, which could be partially due to the higher expression of IRAP at the spinal cords of female. Therefore, we synthesized Ang IV and OT conjugates connected with a peptide bond and tested for their effects on hyperalgesia and allodynia. Carrageenan-induced hyperalgesia and partial sciatic nerve ligation (PSNL) were performed using rat models. Conjugates Ang IV-OT (Ang IV at the N-terminal) and OT-Ang IV (OT at the N-terminal) were synthesized and intrathecally injected into male and female rats. Our results showed that Ang IV-OT exhibited prominent antihyperalgesia in male rats, particularly during hyperalgesia recovery, whereas OT-Ang IV was more effective during development stage. Ang IV-OT showed clear antihyperalgesia in female rats, but OT-Ang IV had no significant effect. Notably, both conjugates alleviated neuropathic allodynia in male rats; however, OT-Ang IV had no effect in female rats, whereas Ang IV-OT induced significant antiallodynia. In conclusion, Ang IV-OT has greater therapeutic potential for treating hyperalgesia and allodynia than OT-Ang IV. Its effects were not affected by sex, unlike those of OT and OT-Ang IV, extending its possible clinical applications.

What do we know about the renin angiotensin system and inflammatory bowel disease?

INTRODUCTION

The renin-angiotensin system (RAS) is an important homeostatic pathway, with emerging evidence for the impact of its components on inflammation and fibrosis in gastrointestinal tissues. This review aims to review current knowledge of the physiological mechanism of RAS in inflammatory bowel disease (IBD), and potential therapeutic implications.

AREAS COVERED

An extensive online literature review including Pubmed, Medline, and Google Scholar was undertaken. Discussion on the components of the RAS, localization, and physiological functions in the gastrointestinal tract, preclinical, and clinical data in IBD, and the relation with SARS-Cov-2 are covered in this review.

EXPERT OPINION

RAS inhibition may have a role as anti-fibrotic adjunct therapy. Targeting the local gastrointestinal RAS with novel modes of delivery may be a target for future therapeutics for IBD, given the widespread availability and safety of current options as utilized in other diseases. Further insight into the mechanism and downstream effects of gastrointestinal ACE2 may lead to a better understanding of the pathogenesis of IBD.

Angiotensin-(1-7) as a Potential Therapeutic Strategy for Delayed Cerebral Ischemia in Subarachnoid Hemorrhage

Aneurysmal subarachnoid hemorrhage (SAH) is a substantial cause of mortality and morbidity worldwide. Moreover, survivors after the initial bleeding are often subject to secondary brain injuries and delayed cerebral ischemia, further increasing the risk of a poor outcome. In recent years, the renin-angiotensin system (RAS) has been proposed as a target pathway for therapeutic interventions after brain injury. The RAS is a complex system of biochemical reactions critical for several systemic functions, namely, inflammation, vascular tone, endothelial activation, water balance, fibrosis, and apoptosis. The RAS system is classically divided into a pro-inflammatory axis, mediated by angiotensin (Ang)-II and its specific receptor AT₁R, and a counterbalancing system, presented in humans as Ang-(1-7) and its receptor, MasR. Experimental data suggest that upregulation of the Ang-(1-7)/MasR axis might be neuroprotective in numerous pathological conditions, namely, ischemic stroke, cognitive disorders, Parkinson's disease, and depression. In the presence of SAH, Ang-(1-7)/MasR neuroprotective and modulating properties could help reduce brain damage by acting on neuroinflammation, and through direct vascular and anti-thrombotic effects. Here we review the role of RAS in brain ischemia, with specific focus on SAH and the therapeutic potential of Ang-(1-7).

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

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

Breast cancer and the renin-angiotensin system (RAS): Therapeutic approaches and related metabolic diseases

The Renin-Angiotensin System (RAS) is classically recognized for regulating blood pressure and fluid balance. Recently, this role has extended to other areas including inflammation, obesity, diabetes, as well as breast cancer. RAS components are expressed in normal and cancerous breast tissues, and downregulation of RAS inhibits metastasis, proliferation, angiogenesis, and desmoplasia in the tumor microenvironment. Therefore, RAS inhibitors (Angiotensin receptor blockers, ARBs, or angiotensin converting enzyme inhibitors, ACE-I) may be beneficial as preventive adjuvant therapies to thwart breast cancer development and improve outcomes, respectively. Given the beneficial effects of RAS inhibitors in metabolic diseases, which often co-exist in breast cancer patients, combining RAS inhibitors with other breast cancer therapies may enhance the effectiveness of current treatments. This review scrutinizes above associations, to advance our understanding of the role of RAS in breast cancer and its potential for repurposing of RAS inhibitors to improve the therapeutic approach for breast cancer patients.

Metabolism of angiotensin peptides by angiotensin converting enzyme 2 (ACE2) and analysis of the effect of excess zinc on ACE2 enzymatic activity

After decades of notoriety for its adverse cardiovascular, proinflammatory and profibrotic actions, the renin-angiotensin system (RAS) began to be cast in a more favorable light with the discovery of angiotensin-converting enzyme-2 (ACE2) in 2000. This monocarboxypeptidase, best known for its ability to metabolize angiotensin (Ang) II to Ang 1-7, counteracts the adverse effects of Ang II mediated by the AT₁ Ang II receptor. Ang peptides are classically considered to be metabolized by aminopeptidases, by which the nomenclature Ang III (des-Asp¹Ang II, 2-8 heptapeptide) and Ang IV (des-Asp¹des-Arg²Ang II, 3-8 hexapeptide) are derived. This report compares the ability of recombinant human ACE2 (rhACE2) to metabolize Ang III, Ang IV and Ang V, (4-8 pentapeptide) relative to Ang II to form corresponding des-omega-Phe metabolites. rhACE2 has highest affinity (lowest K_m) for Ang III, followed by Ang II ∼ Ang V, followed by Ang IV. However, rhACE2 has the highest Kcat for metabolising Ang IV followed by Ang V, Ang III and Ang II. The enzymatic efficiency (Kcat/Km) is highest for Ang V and Ang III followed by Ang IV and is lowest for Ang II. As a gluzincin metallopeptidase, ACE2 requires a zinc molecule at its active site for catalysis. This report also documents inhibition of ACE2 activity by concentrations of zinc exceeding 10 μM. These observations extend the functional significance of ACE2 to include the metabolic inactivation of Ang III, Ang IV and Ang V, reemphasizing the importance of monitoring zinc intake to maintain metabolic homeostasis.

From Angiotensin IV to Small Peptidemimetics Inhibiting Insulin-Regulated Aminopeptidase

It was reported three decades ago that intracerebroventricular injection of angiotensin IV (Ang IV, Val-Tyr-Ile-His-Pro-Phe) improved memory and learning in the rat. There are several explanations for these positive effects of the hexapeptide and related analogues on cognition available in the literature. In 2001, it was proposed that the insulin-regulated aminopeptidase (IRAP) is a main target for Ang IV and that Ang IV serves as an inhibitor of the enzyme. The focus of this review is the efforts to stepwise transform the hexapeptide into more drug-like Ang IV peptidemimetics serving as IRAP inhibitors. Moreover, the discovery of IRAP inhibitors by virtual and substance library screening and direct design applying knowledge of the structure of IRAP and of related enzymes is briefly presented.

Is There an Interplay Between the Functional Domains of IRAP?

As a member of the M1 family of aminopeptidases, insulin regulated aminopeptidase (IRAP) is characterized by distinct binding motifs at the active site in the C-terminal domain that mediate the catalysis of peptide substrates. However, what makes IRAP unique in this family of enzymes is that it also possesses trafficking motifs at the N-terminal domain which regulate the movement of IRAP within different intracellular compartments. Research on the role of IRAP has focused predominantly on the C-terminus catalytic domain in different physiological and pathophysiological states ranging from pregnancy to memory loss. Many of these studies have utilized IRAP inhibitors, that bind competitively to the active site of IRAP, to explore the functional significance of its catalytic activity. However, it is unknown whether these inhibitors are able to access intracellular sites where IRAP is predominantly located in a basal state as the enzyme may need to be at the cell surface for the inhibitors to mediate their effects. This property of IRAP has often been overlooked. Interestingly, in some pathophysiological states, the distribution of IRAP is altered. This, together with the fact that IRAP possesses trafficking motifs, suggest the localization of IRAP may play an important role in defining its physiological or pathological functions and provide insights into the interplay between the two functional domains of the protein.

Hepatocyte Growth Factor and Macrophage-stimulating Protein "Hinge" Analogs to Treat Pancreatic Cancer

Pancreatic cancer (PC) ranks twelfth in frequency of diagnosis but is the fourth leading cause of cancer related deaths with a 5 year survival rate of less than 7 percent. This poor prognosis occurs because the early stages of PC are often asymptomatic. Over-expression of several growth factors, most notably vascular endothelial growth factor (VEGF), has been implicated in PC resulting in dysfunctional signal transduction pathways and the facilitation of tumor growth, invasion and metastasis. Hepatocyte growth factor (HGF) acts via the Met receptor and has also received research attention with ongoing efforts to develop treatments to block the Met receptor and its signal transduction pathways. Macrophage-stimulating protein (MSP), and its receptor Ron, is also recognized as important in the etiology of PC but is less well studied. Although the angiotensin II (AngII)/AT1 receptor system is best known for mediating blood pressure and body water/electrolyte balance, it also facilitates tumor vascularization and growth by stimulating the expression of VEGF. A metabolite of AngII, angiotensin IV (AngIV) has sequence homology with the "hinge regions" of HGF and MSP, key structures in the growth factor dimerization processes necessary for Met and Ron receptor activation. We have developed AngIV-based analogs designed to block dimerization of HGF and MSP and thus receptor activation. Norleual has shown promise as tested utilizing PC cell cultures. Results indicate that cell migration, invasion, and pro-survival functions were suppressed by this analog and tumor growth was significantly inhibited in an orthotopic PC mouse model.

Renin-Angiotensin System in Lung Tumor and Microenvironment Interactions

The mechanistic involvement of the renin-angiotensin system (RAS) reaches beyond cardiovascular physiopathology. Recent knowledge pinpoints a pleiotropic role for this system, particularly in the lung, and mainly through locally regulated alternative molecules and secondary pathways. Angiotensin peptides play a role in cell proliferation, immunoinflammatory response, hypoxia and angiogenesis, which are critical biological processes in lung cancer. This manuscript reviews the literature supporting a role for the renin-angiotensin system in the lung tumor microenvironment and discusses whether blockade of this pathway in clinical settings may serve as an adjuvant therapy in lung cancer.

Contributions by the Brain Renin-Angiotensin System to Memory, Cognition, and Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive neuron losses in memory-associated brain structures that rob patients of their dignity and quality of life. Five drugs have been approved by the FDA to treat AD but none modify or significantly slow disease progression. New therapies are needed to delay the course of this disease with the ultimate goal of preventing neuron losses and preserving memory functioning. In this review we describe the renin-angiotensin II (AngII) system (RAS) with specific regard to its deleterious contributions to hypertension, facilitation of neuroinflammation and oxidative stress, reduced cerebral blood flow, tissue remodeling, and disruption of memory consolidation and retrieval. There is evidence that components of the RAS, AngIV and Ang(1-7), are positioned to counter such damaging influences and these systems are detailed with the goal of drawing attention to their importance as drug development targets. Ang(1-7) binds at the Mas receptor, while AngIV binds at the AT4 receptor subtype, and these receptor numbers are significantly decreased in AD patients, accompanied by declines in brain aminopeptidases A and N, enzymes essential for the synthesis of AngIV. Potent analogs may be useful to counter these changes and facilitate neuronal functioning and reduce apoptosis in memory associated brain structures of AD patients.

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Neuropeptides: metabolism to bioactive fragments and the pharmacology of their receptors

The proteolytic processing of neuropeptides has an important regulatory function and the peptide fragments resulting from the enzymatic degradation often exert essential physiological roles. The proteolytic processing generates, not only biologically inactive fragments, but also bioactive fragments that modulate or even counteract the response of their parent peptides. Frequently, these peptide fragments interact with receptors that are not recognized by the parent peptides. This review discusses tachykinins, opioid peptides, angiotensins, bradykinins, and neuropeptide Y that are present in the central nervous system and their processing to bioactive degradation products. These well-known neuropeptide systems have been selected since they provide illustrative examples that proteolytic degradation of parent peptides can lead to bioactive metabolites with different biological activities as compared to their parent peptides. For example, substance P, dynorphin A, angiotensin I and II, bradykinin, and neuropeptide Y are all degraded to bioactive fragments with pharmacological profiles that differ considerably from those of the parent peptides. The review discusses a selection of the large number of drug-like molecules that act as agonists or antagonists at receptors of neuropeptides. It focuses in particular on the efforts to identify selective drug-like agonists and antagonists mimicking the effects of the endogenous peptide fragments formed. As exemplified in this review, many common neuropeptides are degraded to a variety of smaller fragments but many of the fragments generated have not yet been examined in detail with regard to their potential biological activities. Since these bioactive fragments contain a small number of amino acid residues, they provide an ideal starting point for the development of drug-like substances with ability to mimic the effects of the degradation products. Thus, these substances could provide a rich source of new pharmaceuticals. However, as discussed herein relatively few examples have so far been disclosed of successful attempts to create bioavailable, drug-like agonists or antagonists, starting from the structure of endogenous peptide fragments and applying procedures relying on stepwise manipulations and simplifications of the peptide structures.

Pharmacodynamic effects of C-domain-specific ACE inhibitors on the renin-angiotensin system in myocardial infarcted rats

INTRODUCTION

The renin-angiotensin system (RAS) is a dynamic network that plays a critical role in blood pressure regulation and fluid and electrolyte homeostasis. Modulators of the RAS, such as angiotensin-converting enzyme (ACE) inhibitors, are widely used to treat hypertension, heart failure and myocardial infarction.

METHODS

The effect of ACE inhibitors (lisinopril and C-domain-selective LisW-S) on the constituent peptides of the RAS following myocardial infarction was examined in rats. Ten angiotensin peptides were analysed using a sensitive LC-MS/MS-based assay to examine both the circulating and equilibrium levels of these peptides.

RESULTS

Administration of lisinopril or LisW-S caused a significant decrease in Ang 1-8/Ang 1-10 ratios as determined by circulating and equilibrium peptide level analysis. Furthermore, Ang 1-7 levels were elevated by both ACE inhibitors, but only lisinopril decreased the Ang 1-5/Ang 1-7 ratio. This indicates LisW-S C-domain specificity as Ang 1-5 is generated by hydrolysis of Ang 1-7 by the N-domain. Further corroboration of LisW-S C-domain specificity is that only lisinopril increased the circulating levels of the N-domain ACE substrate Ac-SDKP.

CONCLUSION

LisW-S is able to effectively block ACE in vivo by C-domain-selective inhibition. The LC-MS/MS-based assay allows the evaluation of the pharmacologic impact of RAS inhibitors in different pathophysiological conditions.

Inhibition of Insulin-Regulated Aminopeptidase (IRAP) by Arylsulfonamides

The inhibition of insulin-regulated aminopeptidase (IRAP, EC 3.4.11.3) by angiotenesin IV is known to improve memory and learning in rats. Screening 10 500 low-molecular-weight compounds in an enzyme inhibition assay with IRAP from Chinese Hamster Ovary (CHO) cells provided an arylsulfonamide (N-(3-(1H-tetrazol-5-yl)phenyl)-4-bromo-5-chlorothiophene-2-sulfonamide), comprising a tetrazole in the meta position of the aromatic ring, as a hit. Analogues of this hit were synthesized, and their inhibitory capacities were determined. A small structure-activity relationship study revealed that the sulfonamide function and the tetrazole ring are crucial for IRAP inhibition. The inhibitors exhibited a moderate inhibitory potency with an IC50=1.1±0.5 μm for the best inhibitor in the series. Further optimization of this new class of IRAP inhibitors is required to make them attractive as research tools and as potential cognitive enhancers.

The development of small molecule angiotensin IV analogs to treat Alzheimer's and Parkinson's diseases

Alzheimer's (AD) and Parkinson's (PD) diseases are neurodegenerative diseases presently without effective drug treatments. AD is characterized by general cognitive impairment, difficulties with memory consolidation and retrieval, and with advanced stages episodes of agitation and anger. AD is increasing in frequency as life expectancy increases. Present FDA approved medications do little to slow disease progression and none address the underlying progressive loss of synaptic connections and neurons. New drug design approaches are needed beyond cholinesterase inhibitors and N-methyl-d-aspartate receptor antagonists. Patients with PD experience the symptomatic triad of bradykinesis, tremor-at-rest, and rigidity with the possibility of additional non-motor symptoms including sleep disturbances, depression, dementia, and autonomic nervous system failure. This review summarizes available information regarding the role of the brain renin-angiotensin system (RAS) in learning and memory and motor functions, with particular emphasis on research results suggesting a link between angiotensin IV (AngIV) interacting with the AT4 receptor subtype. Currently there is controversy over the identity of this AT4 receptor protein. Albiston and colleagues have offered convincing evidence that it is the insulin-regulated aminopeptidase (IRAP). Recently members of our laboratory have presented evidence that the brain AngIV/AT4 receptor system coincides with the brain hepatocyte growth factor/c-Met receptor system. In an effort to resolve this issue we have synthesized a number of small molecule AngIV-based compounds that are metabolically stable, penetrate the blood-brain barrier, and facilitate compromised memory and motor systems. These research efforts are described along with details concerning a recently synthesized molecule, Dihexa that shows promise in overcoming memory and motor dysfunctions by augmenting synaptic connectivity via the formation of new functional synapses.

Angiotensin IV is induced in experimental autoimmune encephalomyelitis but fails to influence the disease

In multiple sclerosis (MS) and its corresponding animal models, over-activity of the renin-angiotensin system (RAS) has been reported and pharmacological RAS blockade exerts beneficial effects. The RAS generates a number of bioactive angiotensins, thereby primarily regulating the body's sodium homeostasis and blood pressure. In this regard, angiotensin IV (AngIV), a metabolite of the RAS has been shown to modulate inflammatory responses. Here we studied potential implications of AngIV signalling in myelin oligodendrocyte glycoprotein (MOG) peptide induced murine experimental autoimmune encephalomyelitis (EAE), a close-to-MS animal model. Mass spectrometry revealed elevated plasma levels of AngIV in EAE. Expression of cognate AT4 receptors was detected in macrophages and T cells as major drivers of pathology in EAE. Yet, AngIV did not modulate macrophage or T cell functions in vitro or displayed detectable effects on neuroantigen specific immune responses in vivo. The data argue against a major contribution of AngIV signalling in the immunopathogenesis of MOG-EAE.

Renin Angiotensin System in Cognitive Function and Dementia

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

Discovery of inhibitors of insulin-regulated aminopeptidase as cognitive enhancers

The hexapeptide angiotensin IV (Ang IV) is a metabolite of angiotensin II (Ang II) and plays a central role in the brain. It was reported more than two decades ago that intracerebroventricular injection of Ang IV improved memory and learning in the rat. Several hypotheses have been put forward to explain the positive effects of Ang IV and related analogues on cognition. It has been proposed that the insulin-regulated aminopeptidase (IRAP) is the main target of Ang IV. This paper discusses progress in the discovery of inhibitors of IRAP as potential enhancers of cognitive functions. Very potent inhibitors of the protease have been synthesised, but pharmacokinetic issues (including problems associated with crossing the blood-brain barrier) remain to be solved. The paper also briefly presents an overview of the status in the discovery of inhibitors of ACE and renin, and of AT1R antagonists and AT2R agonists, in order to enable other discovery processes within the RAS system to be compared. The paper focuses on the relationship between binding affinities/inhibition capacity and the structures of the ligands that interact with the target proteins.

Oxidative stress-mediated effects of angiotensin II in the cardiovascular system

Angiotensin II (Ang II), an endogenous peptide hormone, plays critical roles in the pathophysiological modulation of cardiovascular functions. Ang II is the principle effector of the renin-angiotensin system for maintaining homeostasis in the cardiovascular system, as well as a potent stimulator of NAD(P)H oxidase, which is the major source and primary trigger for reactive oxygen species (ROS) generation in various tissues. Recent accumulating evidence has demonstrated the importance of oxidative stress in Ang II-induced heart diseases. Here, we review the recent progress in the study on oxidative stress-mediated effects of Ang II in the cardiovascular system. In particular, the involvement of Ang II-induced ROS generation in arrhythmias, cell death/heart failure, ischemia/reperfusion injury, cardiac hypertrophy and hypertension are discussed. Ca²⁺/calmodulin-dependent protein kinase II is an important molecule linking Ang II, ROS and cardiovascular pathological conditions.

Focus on Brain Angiotensin III and Aminopeptidase A in the Control of Hypertension

The classic renin-angiotensin system (RAS) was initially described as a hormone system designed to mediate cardiovascular and body water regulation. The discovery of a brain RAS composed of the necessary functional components (angiotensinogen, peptidases, angiotensins, and specific receptor proteins) independent of the peripheral system significantly expanded the possible physiological and pharmacological functions of this system. This paper first describes the enzymatic pathways resulting in active angiotensin ligands and their interaction with AT₁, AT₂, and mas receptor subtypes. Recent evidence points to important contributions by brain angiotensin III (AngIII) and aminopeptidases A (APA) and N (APN) in sustaining hypertension. Next, we discuss current approaches to the treatment of hypertension followed by novel strategies that focus on limiting the binding of AngII and AngIII to the AT₁ receptor subtype by influencing the activity of APA and APN. We conclude with thoughts concerning future treatment approaches to controlling hypertension and hypotension.

Alternative pathways for angiotensin II generation in the cardiovascular system

The classical renin-angiotensin system (RAS) consists of enzymes and peptides that regulate blood pressure and electrolyte and fluid homeostasis. Angiotensin II (Ang II) is one of the most important and extensively studied components of the RAS. The beneficial effects of angiotensin converting enzyme (ACE) inhibitors in the treatment of hypertension and heart failure, among other diseases, are well known. However, it has been reported that patients chronically treated with effective doses of these inhibitors do not show suppression of Ang II formation, suggesting the involvement of pathways alternative to ACE in the generation of Ang II. Moreover, the finding that the concentration of Ang II is preserved in the kidney, heart and lungs of mice with an ACE deletion indicates the important role of alternative pathways under basal conditions to maintain the levels of Ang II. Our group has characterized the serine protease elastase-2 as an alternative pathway for Ang II generation from Ang I in rats. A role for elastase-2 in the cardiovascular system was suggested by studies performed in heart and conductance and resistance vessels of normotensive and spontaneously hypertensive rats. This mini-review will highlight the pharmacological aspects of the RAS, emphasizing the role of elastase-2, an alternative pathway for Ang II generation.

New insights and perspectives on intrarenal renin-angiotensin system: focus on intracrine/intracellular angiotensin II

Although renin, the rate-limiting enzyme of the renin-angiotensin system (RAS), was first discovered by Robert Tigerstedt and Bergman more than a century ago, the research on the RAS still remains stronger than ever. The RAS, once considered to be an endocrine system, is now widely recognized as dual (circulating and local/tissue) or multiple hormonal systems (endocrine, paracrine and intracrine). In addition to the classical renin/angiotensin I-converting enzyme (ACE)/angiotensin II (Ang II)/Ang II receptor (AT₁/AT₂) axis, the prorenin/(Pro)renin receptor (PRR)/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, and the Ang IV/AT₄/insulin-regulated aminopeptidase (IRAP) axis have recently been discovered. Furthermore, the roles of the evolving RAS have been extended far beyond blood pressure control, aldosterone synthesis, and body fluid and electrolyte homeostasis. Indeed, novel actions and underlying signaling mechanisms for each member of the RAS in physiology and diseases are continuously uncovered. However, many challenges still remain in the RAS research field despite of more than one century's research effort. It is expected that the research on the expanded RAS will continue to play a prominent role in cardiovascular, renal and hypertension research. The purpose of this article is to review the progress recently being made in the RAS research, with special emphasis on the local RAS in the kidney and the newly discovered prorenin/PRR/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, the Ang IV/AT₄/IRAP axis, and intracrine/intracellular Ang II. The improved knowledge of the expanded RAS will help us better understand how the classical renin/ACE/Ang II/AT₁ receptor axis, extracellular and/or intracellular origin, interacts with other novel RAS axes to regulate blood pressure and cardiovascular and kidney function in both physiological and diseased states.

Blood pressure and renal hemodynamic effects of angiotensin fragments

Angiotensin (Ang) II, the main effector peptide of the renin-Ang system, increases arterial blood pressure through Ang II type 1A (AT(1a)) receptor-dependent arterial vasoconstriction and by decreasing renal salt and water excretion through extrarenal and intrarenal mechanisms. AT(2) receptors are assumed to oppose these responses mediated by AT(1) receptors, thereby attenuating the pressor effects of Ang II. Nevertheless, a possible role of AT(2) receptors in the regulation of renal hemodynamics and sodium homeostasis remains to be unclear. Several other Ang fragments such as Ang III, Ang IV, Ang-(1-7) and Ang A have also been shown to display biological activity. In this review, we focus on the effects of these Ang on blood pressure, renal hemodynamics and sodium water handling, and discuss the receptors involved in these actions.

Renal vasoconstrictor and pressor responses to angiotensin IV in mice are AT1a-receptor mediated

OBJECTIVES

Angiotensin (Ang) IV was reported to induce renal vasoconstriction or vasodilation in rats via AT1 or AT4 receptors, respectively, whereby the latter one has been identified to be the insulin-regulated aminopeptidase (IRAP). We investigated the effects of Ang IV on mean arterial pressure (MAP) and renal cortical blood flow (CBF) in AT1a, AT1b, AT2 receptor and IRAP knockout (-/-) mice and their corresponding wild-type littermates. Ang II, known as a renal vasoconstrictor in mice, was used as a reference.

METHODS

MAP was recorded via a femoral catheter and CBF was measured using a light amplification by stimulated emission of radiation (LASER) Doppler probe; cortical vascular resistance (CVR) was calculated as MAP divided by CBF.

RESULTS

Baseline MAP, CBF and CVR in AT1a (-/-) mice were significantly lower than wild-type mice. AT2 (-/-) mice had a significantly higher baseline MAP, but similar CBF. In wild-type mice, Ang IV and Ang II induced dose-dependent pressor and renal vasoconstrictor responses, which were antagonized by the AT1 receptor blocker candesartan. These responses were almost completely absent in AT1a (-/-) mice, but were enhanced in AT2 (-/-) mice; responses in AT1b (-/-) and IRAP (-/-) mice were comparable to those in corresponding wild-type mice.

CONCLUSION

Ang IV mediates pressure and renal vasoconstrictor effects in mice via AT1a receptors, whereas IRAP/AT4 is not involved.

Angiotensin IV elevates oxytocin levels in the rat amygdala and produces anxiolytic-like activity through subsequent oxytocin receptor activation

INTRODUCTION

The effects of angiotensin (Ang) IV result from binding to a constitutively active metallopeptidase known as the AT(4) receptor (or oxytocinase/insulin-regulated membrane aminopeptidase). While in vitro evidence indicates that Ang IV inhibits the peptidase activity of AT(4) receptors, leading to increases in the concentrations of several peptides, including oxytocin, the consequence of inhibiting AT(4) peptidase activity in vivo remains unresolved.

DISCUSSION

Microdialysis coupled to immunoassay techniques revealed that systemic and intra-amygdala injection of Nle-Ang IV, a metabolically stable derivative of Ang IV, significantly elevated extracellular levels of oxytocin in the rat amygdala. Based on earlier reports describing the anxiolytic-like effects of oxytocin, we investigated whether disrupting AT(4) peptidase activity would yield similar responses. In the mouse four-plate test, acute treatment with either Nle-Ang IV or LVV-hemorphin-7, a related AT(4) receptor ligand, elicited significant increases in the number of punished crossings. These behavioral responses were comparable to the anxiolytic-like effects of oxytocin and to the standard anxiolytic agent, chlordiazepoxide. Cotreatment with either the AT(4) receptor antagonist, divalinal, or the selective oxytocin receptor antagonist, WAY-162720, reversed the anxiolytic-like effects of Nle-Ang IV, while combining ineffective doses of Nle-Ang IV and oxytocin increased the number of punished crossings in this assay. Conversely, Nle-Ang IV and LVV-hemorphin-7 were inactive in the mouse tail suspension test of antidepressant activity. These findings represent the first in vivo demonstration of the peptidase activity of AT(4) receptors, confirm the anxiolytic-like properties of Ang IV, and reveal a unique and previously uncharacterized relationship between AT(4) and oxytocin receptor systems.

The angiotensin IV analog Nle-Tyr-Leu-psi-(CH2-NH2)3-4-His-Pro-Phe (norleual) can act as a hepatocyte growth factor/c-Met inhibitor

The angiotensin (Ang) IV analog norleual [Nle-Tyr-Leu-psi-(CH2-NH2)(3-4)-His-Pro-Phe] exhibits structural homology with the hinge (linker) region of hepatocyte growth factor (HGF) and is hypothesized to act as a hinge region mimic. Norleual competitively inhibited the binding of HGF to its receptor c-Met in mouse liver membranes, with an IC(50) value of 3 pM. Predictably, norleual was able to inhibit HGF-dependent signaling, proliferation, migration, and invasion in multiple cell types at concentrations in the picomolar range. Ex vivo studies demonstrated that norleual exhibited potent antiangiogenic activity, an attribute that would be predicted for a HGF/c-Met antagonist. Furthermore, norleual suppressed pulmonary colonization by B16-F10 murine melanoma cells, which are characterized by an overactive HGF/c-Met system. Together, these data suggest that AngIV analogs exert at least some of their biological activity through interference with the HGF/c-Met system and may have utility as therapeutic agents in disorders that are dependent on an intact HGF/c-Met system. Finally, the ability of norleual to induce marked biological responses in human embryonic kidney cells, which do not express insulin-responsive aminopeptidase (IRAP), coupled with the observed effects of norleual on the HGF/c-Met system, casts doubt on the physiological significance of AngIV-dependent inhibition of IRAP. [Corrected]

Involvement of the AT1 receptor subtype in the effects of angiotensin IV and LVV-haemorphin 7 on hippocampal neurotransmitter levels and spatial working memory

Intracerebroventricular (i.c.v.) administration of angiotensin IV (Ang IV) or Leu-Val-Val-haemorphin 7 (LVV-H7) improves memory performance in normal rats and reverses memory deficits in rat models for cognitive impairment. These memory effects were believed to be mediated via the putative 'AT4 receptor'. However, this binding site was identified as insulin-regulated aminopeptidase (IRAP). Correspondingly, Ang IV and LVV-H7 were characterised as IRAP inhibitors. This study investigates whether and how IRAP may be involved in the central effects of Ang IV and LVV-H7. We determined the effects of i.c.v. administration of Ang IV or LVV-H7 on hippocampal neurotransmitter levels using microdialysis in rats. We observed that Ang IV modulates hippocampal acetylcholine levels, whereas LVV-H7 does not. This discrepancy was reflected in the observation that Ang IV binds with micromolar affinity to the AT1 receptor whereas no binding affinity was observed for LVV-H7. Correspondingly, we demonstrated that the AT1 receptor is involved in the effects of Ang IV on hippocampal neurotransmitter levels and on spatial working memory in a plus maze spontaneous alternation task. However, the AT1 receptor was not involved in the spatial memory facilitating effect of LVV-H7. Finally, we demonstrated that Ang IV did not diffuse to the hippocampus following i.c.v. injection, suggesting an extrahippocampal site of action. We propose that AT1 receptors are implicated in the neurochemical and cognitive effects of Ang IV, whereas LVV-H7 may mediate its effects via IRAP.

The brain RAS and Alzheimer's disease

Alzheimer's disease (AD) has become a major world-wide health problem with ever rising costs associated with the treatment and care of afflicted individuals. As life expectancy has increased the occurrence of dementia has also increased. Hypertension during middle adulthood is correlated with a significantly elevated risk of cognitive impairment later in life. Treatment with antihypertensive drugs, particularly angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), has been reported to reduce the likelihood and slow the progression of AD; however, the use of ACE inhibitors may be accompanied by an increase in amyloid beta protein(1-42) accumulation. This review summarizes available information regarding the brain renin-angiotensin system (RAS), and specifically the efficacy of ACE inhibitors as anti-dementia agents, and considers the recently discovered AT(4) receptor and associated agonist drugs as potential new therapeutic targets to treat memory impairments associated with AD. We conclude with a description of recent efforts by members of our laboratory to develop blood-brain barrier penetrant angiotensin IV analogue drugs that facilitate cognition in animal models of AD. These efforts have resulted in a small molecule with desirable hydrophobicity characteristics that shows promise with respect to memory facilitation when peripherally administered.

The angiotensin AT4 receptor subtype as a target for the treatment of memory dysfunction associated with Alzheimer's disease

Over recent years antihypertensive drugs, particularly angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), have been reported to have beneficial effects upon cognitive impairment. Such findings suggest that pharmacological manipulation of angiotensin ligands may be of clinical importance in slowing or halting the cognitive deterioration seen in vascular dementia and Alzheimer's disease. The mechanism(s) underlying these improvements in cognitive function remains unclear; however, important leads are emerging. The angiotensin AT4 receptor subtype, discovered by our laboratory in 1992, influences several important behaviours and physiologies, including learning and memory, and may play a role in this cognitive improvement. This review initially describes the therapeutic drugs approved by the Federal Drug Administration and new approaches presently being developed to treat Alzheimer's disease-induced cognitive impairment. Next, the biologically-active angiotensin ligands and their respective receptor subtypes are discussed, followed by the roles of angiotensin II, angiotensin IV, ACE inhibitors and ARBs in cognitive function. We conclude with a working hypothesis concerning the importance of the AT4 receptor subtype as a new potential drug target for the treatment of Alzheimer's disease-associated memory loss.

Vascular and metabolic effects of angiotensin II receptor blockers

Angiotensin II receptor blockers (ARBs) are widely used in patients with hypertension, heart failure and type 2 diabetes mellitus (T2DM). Several large clinical trials have demonstrated that these agents are effective in reducing cardiovascular mortality and morbidity. These benefits are partly independent of the degree of blood pressure reduction and most likely related to ARBs' anti-inflammatory, metabolic and vascular effects. Clinical studies showed that the anti-inflammatory effect of ARBs could be related to the dosage and/or the length of the treatment. In large clinical trials, ARBs have inconsistently reduced the risk of new-onset T2DM. Among ARBs, only losartan significantly reduced serum uric acid levels. Moreover, it has been demonstrated that ARBs improve endothelial dysfunction in patients with hypertension and/or coronary artery disease (CAD), while all but one of the studies proved that these agents could usually, after 6-12 months of therapy, induce regression of vascular hypertrophy in hypertensive patients. These positive effects could be relevant to vascular protection and, together with the blood pressure reduction, constitute the background of the improved outcome observed in clinical studies on mortality and/or morbidity in hypertensive, high-risk and CAD patients. The clinical significance of the different potency of ARBs needs to be investigated further in specific and adequately powered trials.

From angiotensin IV binding site to AT4 receptor

One of the fragments of the cardiovascular hormone Angiotensin II incited the interest of several research groups. This 3-8 fragment, denoted as Angiotensin IV (Ang IV) causes a number of distinct biological effects (see Introduction), unlikely to be explained by its weak binding to AT(1) and/or AT(2) receptors. Moreover the discovery of high affinity [(125)I]-Ang IV binding sites and their particular tissue distribution led to the concept of the AT(4) receptor. An important breakthrough was achieved by defining the AT(4) receptor as the membrane-bound insulin-regulated aminopeptidase (IRAP). Crucial for the definition as a receptor the binding of the endogenous ligand(s) should be linked to particular cellular and/or biochemical processes. With this respect, cultured cells offer the possibility to study the presence of binding sites in conjunction with ligand induced signaling. This link is discussed for the AT(4) receptor by providing an overview of the cellular effects by AT(4) ligands.

Ang II and Ang IV: unraveling the mechanism of action on synaptic plasticity, memory, and epilepsy

The central angiotensin system plays a crucial role in cardiovascular regulation. More recently, angiotensin peptides have been implicated in stress, anxiety, depression, cognition, and epilepsy. Angiotensin II (Ang II) exerts its actions through AT(1) and AT(2) receptors, while most actions of its metabolite Ang IV were believed to be independent of AT(1) or AT(2) receptor activation. A specific binding site with high affinity for Ang IV was discovered and denominated "AT(4) receptor". The beneficiary effects of AT(4) ligands in animal models for cognitive impairment and epileptic seizures initiated the search for their mechanism of action. This proved to be a challenging task, and after 20 years of research, the nature of the "AT(4) receptor" remains controversial. Insulin-regulated aminopeptidase (IRAP) was first identified as the high-affinity binding site for AT(4) ligands. Recently, the hepatocyte growth factor receptor c-MET was also proposed as a receptor for AT(4) ligands. The present review focuses on the effects of Ang II and Ang IV on synaptic transmission and plasticity, learning, memory, and epileptic seizure activity. Possible interactions of Ang IV with the classical AT(1) and AT(2) receptor subtypes are evaluated, and other potential mechanisms by which AT(4) ligands may exert their effects are discussed. Identification of these mechanisms may provide a valuable target in the development in novel drugs for the treatment of cognitive disorders and epilepsy.

Cognitive-enhancing effects of angiotensin IV

Angiotensin IV is a derivative of the potent vasoconstrictor angiotensin II and it has been shown to enhance acquisition, consolidation and recall in animal models of learning and memory when administered centrally or peripherally. Whether changes in angiotensin IV activity underlie the cognitive effects of those cardiovascular drugs designed to disrupt the peripheral renin-angiotensin system in humans remains undetermined, but angiotensin IV appears to be a worthy candidate for consideration in drug development programmes. The mechanism of action of angiotensin IV is still debated, although its AT₄ receptor has been convincingly identified as being insulin-regulated amino peptidase, which is also known as oxytocinase and placental leucine aminopeptidase. It is speculated that angiotensin IV may interact with insulin-regulated amino peptidase to enhance neuronal glucose uptake, prevent metabolism of other neuroactive peptides, induce changes in extracellular matrix molecules, or induce release of acetylcholine and/or dopamine. All of these things may be responsible for the beneficial effects on cognition, but none of them are yet proven. Importantly, strain differences in murine responses to angiotensin IV suggest that some individuals may benefit from drugs targeted to the AT₄ receptor whilst others may be refractory. At present it thus appears that those individuals with the poorest baseline cognition may receive greatest benefit, but possible genetic differences in responses to angiotensin IV cannot be ruled-out.

AT2 receptors: functional relevance in cardiovascular disease

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