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

The Renin Angiotensin System as a Therapeutic Target in Traumatic Brain Injury

Traumatic brain injury (TBI) is a major public health problem, with limited pharmacological options available beyond symptomatic relief. The renin angiotensin system (RAS) is primarily known as a systemic endocrine regulatory system, with major roles controlling blood pressure and fluid homeostasis. Drugs that target the RAS are used to treat hypertension, heart failure and kidney disorders. They have now been used chronically by millions of people and have a favorable safety profile. In addition to the systemic RAS, it is now appreciated that many different organ systems, including the brain, have their own local RAS. The major ligand of the classic RAS, Angiotensin II (Ang II) acts predominantly through the Ang II Type 1 receptor (AT1R), leading to vasoconstriction, inflammation, and heightened oxidative stress. These processes can exacerbate brain injuries. Ang II receptor blockers (ARBs) are AT1R antagonists. They have been shown in several preclinical studies to enhance recovery from TBI in rodents through improvements in molecular, cellular and behavioral correlates of injury. ARBs are now under consideration for clinical trials in TBI. Several different RAS peptides that signal through receptors distinct from the AT1R, are also potential therapeutic targets for TBI. The counter regulatory RAS pathway has actions that oppose those stimulated by AT1R signaling. This alternative pathway has many beneficial effects on cells in the central nervous system, bringing about vasodilation, and having anti-inflammatory and anti-oxidative stress actions. Stimulation of this pathway also has potential therapeutic value for the treatment of TBI. This comprehensive review will provide an overview of the various components of the RAS, with a focus on their direct relevance to TBI pathology. It will explore different therapeutic agents that modulate this system and assess their potential efficacy in treating TBI patients.

Potential use of renin-angiotensin-aldosterone system inhibitors to reduce COVID-19 severity

SARS-CoV-2 infection and its clinical manifestations (COVID-19) quickly evolved to a pandemic and a global public health emergency. The limited effectivity of available treatments aimed at reducing virus replication and the lessons learned from other coronavirus infections (SARS-CoV-1 or NL63) that share the internalization process of SARS-CoV-2, led us to revisit the COVID-19 pathogenesis and potential treatments. Virus protein S binds to the angiotensin-converting enzyme 2 (ACE2) initiating the internalization process. Endosome formation removes ACE2 from the cellular membrane preventing its counter-regulative effect mediated by the metabolism of angiotensin II to angiotensin (1-7). Internalized virus-ACE2 complexes have been identified for these coronaviruses. SARS-CoV-2 presents the highest affinity for ACE2 and produces the most severe symptoms. Assuming ACE2 internalization is the trigger for COVID-19 pathogenesis, accumulation of angiotensin II can be viewed as the potential cause of symptoms. Angiotensin II is a strong vasoconstrictor, but has also important roles in hypertrophy, inflammation, remodeling, and apoptosis. Higher levels of ACE2 in the lungs explain the acute respiratory distress syndrome as primary symptoms. Most of the described findings and clinical manifestations of COVID-19, including increased interleukin levels, endothelial inflammation, hypercoagulability, myocarditis, dysgeusia, inflammatory neuropathies, epileptic seizures and memory disorders can be explained by excessive angiotensin II levels. Several meta-analyses have demonstrated that previous use of angiotensin-converting enzyme inhibitors or angiotensin receptor blockers were associated with better prognosis for COVID-19. Therefore, pragmatic trials to assess the potential therapeutic benefits of renin-angiotensin-aldosterone system inhibitors should be urgently promoted by health authorities to widen the therapeutic options for COVID-19.

Role of Chrononutrition in the Antihypertensive Effects of Natural Bioactive Compounds

Hypertension (HTN) is one of the main cardiovascular risk factors and is considered a major public health problem. Numerous approaches have been developed to lower blood pressure (BP) in hypertensive patients, most of them involving pharmacological treatments. Within this context, natural bioactive compounds have emerged as a promising alternative to drugs in HTN prevention. This work reviews not only the mechanisms of BP regulation by these antihypertensive compounds, but also their efficacy depending on consumption time. Although a plethora of studies has investigated food-derived compounds, such as phenolic compounds or peptides and their impact on BP, only a few addressed the relevance of time consumption. However, it is known that BP and its main regulatory mechanisms show a 24-h oscillation. Moreover, evidence shows that phenolic compounds can interact with clock genes, which regulate the biological rhythm followed by many physiological processes. Therefore, further research might be carried out to completely elucidate the interactions along the time–nutrition–hypertension axis within the framework of chrononutrition.

Targeting brain Renin-Angiotensin System for the prevention and treatment of Alzheimer's disease: Past, present and future

Alzheimer's disease (AD) is a well-known neurodegenerative disease characterized by the presence of two main hallmarks - Tau hyperphosphorylation and Aβ deposits. Notwithstanding, in the last few years the scientific evidence about the drivers of AD have been changing and nowadays age-related vascular alterations and several cardiovascular risk factors have been shown to trigger the development of AD. In this context, drugs targeting the Renin Angiotensin System (RAS), commonly used for the treatment of hypertension, are evidencing a high potential to delay AD development due to their action on brain RAS. Indeed, the ACE 1/Ang II/AT1R axis is believed to be upregulated in AD and to be responsible for deleterious effects such as increased oxidative stress, neuroinflammation, blood-brain barrier (BBB) hyperpermeability, astrocytes dysfunction and a decrease in cerebral blood flow. In contrast, the alternative axis - ACE 1/Ang II/AT2R; ACE 2/Ang (1-7)/MasR; Ang IV/ AT4R(IRAP) - seems to counterbalance the deleterious effects of the principal axis and to exert beneficial effects on memory and cognition. Accordingly, retrospective studies demonstrate a reduced risk of developing AD among people taking RAS medication as well as several in vitro and in vivo pre-clinical studies as it is herein critically reviewed. In this review, we first revise, at a glance, the pathophysiology of AD focused on its classic hallmarks. Secondly, an overview about the impact of the RAS on the pathophysiology of AD is also provided, focused on their four essential axes ACE 1/Ang II/AT2R; ACE 2/Ang (1-7)/MasR; Ang IV/ AT4R(IRAP) and ACE 1/Ang II/AT1R. Finally, the therapeutic potential of available drugs targeting RAS on AD, namely angiotensin II receptor blockers (ARBs) and angiotensin converting enzyme inhibitors (ACEIs), is highlighted and data supporting this hope will be presented, from in vitro and in vivo pre-clinical to clinical studies.

Impact of drug treatment and drug interactions in post-stroke epilepsy

Stroke is a huge burden on our society and this is expected to grow in the future due to the aging population and the associated co-morbidities. The improvement of acute stroke care has increased the survival rate of stroke patients, and many patients are left with permanent disability, which makes stroke the main cause of adult disability. Unfortunately, many patients face other severe complications such as post-stroke seizures and epilepsy. Acute seizures (ASS) occur within 1 week after the stroke while later occurring unprovoked seizures are diagnosed as post-stroke epilepsy (PSE). Both are associated with a poor prognosis of a functional recovery. The underlying neurobiological mechanisms are complex and poorly understood. There are no universal guidelines on the management of PSE. There is increasing evidence for several risk factors for ASS/PSE, however, the impacts of recanalization, drugs used for secondary prevention of stroke, treatment of stroke co-morbidities and antiseizure medication are currently poorly understood. This review focuses on the common medications that stroke patients are prescribed and potential drug interactions possibly complicating the management of ASS/PSE.

Possible repair mechanisms of renin-angiotensin system inhibitors, matrix metalloproteinase-9 inhibitors and protein hormones on methamphetamine-induced neurotoxicity

Methamphetamine is a highly addictive central stimulant with extensive and strong neurotoxicity. The neurotoxicity of methamphetamine is closely related to the imbalance of dopamine levels and the destruction of the blood-brain barrier. An increase in dopamine may induce adverse effects such as behavioral sensitization and excessive locomotion. Damage to the blood-brain barrier can cause toxic or harmful substances to leak to the central nervous system, leading to neurotoxicity. The renin-angiotensin system is essential for the regulation of dopamine levels in the brain. Matrix metalloproteinase-9 causes reward effects and behavioral sensitization by inducing dopamine release. Prolactin has been shown to be involved in the regulation of tight junction proteins and the integrity of the blood-brain barrier. At present, the treatment of methamphetamine detoxification is still based on psychotherapy, and there is no specific medicine. With the rapid increase in global seizures of methamphetamine, the treatment of its toxicity has attracted more and more attention. This review intends to summarize the therapeutic mechanisms of renin-angiotensin inhibitors, matrix metalloproteinase-9 inhibitors and protein hormones (prolactin) on methamphetamine neurotoxicity. The repair effects of these three on methamphetamine may be related to the maintenance of brain dopamine balance and the integrity of the blood-brain barrier. This review is expected to provide the new therapeutic strategy of methamphetamine toxicity.

Interactions of antiepileptic drugs with drugs approved for the treatment of indications other than epilepsy

Introduction: Comorbidities of epilepsy may significantly interfere with its treatment as diseases in the general population are also encountered in epilepsy patients and some of them even more frequently (for instance, depression, anxiety, or heart disease). Obviously, some drugs approved for other than epilepsy indications can modify the anticonvulsant activity of antiepileptics. Areas covered: This review highlights the drug-drug interactions between antiepileptics and aminophylline, some antidepressant, antiarrhythmic (class I-IV), selected antihypertensive drugs and non-barbiturate injectable anesthetics (ketamine, propofol, etomidate, and alphaxalone). The data were reviewed mainly from experimental models of seizures. Whenever possible, clinical data were provided. PUBMED data base was the main search source.Expert opinion: Aminophylline generally reduced the protective activity of antiepileptics, which, to a certain degree, was consistent with scarce clinical data on methylxanthine derivatives and worse seizure control. The only antiarrhythmic with this profile of action was mexiletine when co-administered with VPA. Among antidepressants and non-barbiturate injectable anesthetics, trazodone, mianserin and etomidate or alphaxalone, respectively, negatively affected the anticonvulsant action of some antiepileptic drugs. Clinical data indicate that only amoxapine, bupropion, clomipramine and maprotiline should be used with caution. Possibly, drugs reducing the anticonvulsant potential of antiepileptics should be avoided in epilepsy patients.

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.

A review of clinically significant drug-drug interactions involving angiotensin II receptor antagonists and antiepileptic drugs

INTRODUCTION

Angiotensin II receptor blockers are widely used for the treatment of arterial hypertension and heart failure. However, recent studies on animal models of seizures showed that in the brain, the renin-angiotensin-aldosterone system might be involved in neuroinflammation; therefore, the administration of angiotensin II receptor blockers that cross the blood/brain barrier, reduces not only blood pressure but reduces neuroinflammation-induced neuronal injury. Apart from this neuroprotective effect, these drugs exhibit anticonvulsant activity in animal models of seizures, and losartan is associated with a probable anti-epileptogenic activity.

AREAS COVERED

In this review, we intended to highlight the role of drug-drug interactions involving angiotensin II receptor antagonists with antiepileptic drugs accompanied by a brief characteristic of the role of RAS in neuroinflammation.

EXPERT OPINION

Some combinations of antiepileptic drugs (lamotrigine or valproate) with sartans are particularly effective in terms of enhanced seizure control. Considering a possible anti-epileptogenic activity of losartan, its combinations with antiepileptic drugs may prove especially beneficial in epileptogenesis inhibition.

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.

Effects of angiotensin-II on brain endothelial cell permeability via PPARalpha regulation of para- and trans-cellular pathways

Angiotensin-II (Ang-II) is a key factor in hypertension, diabetes and aging, which are all primary risk factors for CNS disease. Furthermore, Ang-II may play under-appreciated roles in neurogenesis, angiogenesis and CNS remodeling. Therefore, any contemplated attempts for neurorestorative therapies in the CNS should consider the context of Ang-II signaling. Here, we investigate how Ang-II may regulate cerebral endothelial permeability, a key functional feature of the neurovascular unit. Exposure of human brain endothelial cell cultures to Ang-II increased its permeability to BSA-Alexa488 tracer. Immunocytochemistry and pulse-chase experiments suggested that both para-cellular as well as trans-cellular pathways were involved. Candesartan but not PD123319 blocked Ang-II permeability effects, suggesting that Ang-II effects may be mediated via type 1 receptor. Immunocytochemistry and western blots showed that Ang-II disrupted the membrane distributions of ZO-1 and VE-Cad, decreased total levels of JAM-A and Mfsd2a, and increased Cav1. These effects of Ang-II were accompanied by dephosphorylation of PPARalpha. Finally, Ang-II-induced increases in endothelial permeability were ameliorated by PPARalpha agonists. Taken together, these studies suggest that Ang-II may disrupt both para- and trans-cellular permeability in cerebral endothelium, and PPARalpha-related pathways may offer potential therapeutic targets for ameliorating these effects in cell-based regenerative medicine.

The Potential Therapeutic Capacity of Inhibiting the Brain Renin-Angiotensin System in the Treatment of Co-Morbid Conditions in Epilepsy

Epilepsy is one of the most prevalent neurological diseases and although numerous novel anticonvulsants have been approved, the proportion of patients who are refractory to medical treatment of seizures and have progressive co-morbidities such as cognitive impairment and depression remains at about 20-30%. In the last decade, extensive research has identified a therapeutic capacity of the components of the brain renin-angiotensin system (RAS) in seizure- and epilepsy-related phenomena. Alleviating the activity of RAS in the central nervous system is considered to be a potential adjuvant strategy for the treatment of numerous detrimental consequences of epileptogenesis. One of the main advantages of RAS is associated with its modulatory influence on different neurotransmitter systems, thereby exerting a fine-tuning control mechanism for brain excitability. The most recent scientific findings regarding the involvement of the components of brain RAS show that angiotensin II (Ang II), angiotensin-converting enzyme (ACE), Ang II type 1 (AT₁) and type 2 (AT₂) receptors are involved in the control of epilepsy and its accompanying complications, and therefore they are currently of therapeutic interest in the treatment of this disease. However, data on the role of different components of brain RAS on co-morbid conditions in epilepsy, including hypertension, are insufficient. Experimental and clinical findings related to the involvement of Ang II, ACE, AT₁, and AT₂ receptors in the control of epilepsy and accompanying complications may point to new therapeutic opportunities and adjuvants for the treatment of common co-morbid conditions of epilepsy.

Epilepsy and hypertension: The possible link for sudden unexpected death in epilepsy?

Epilepsy affects about 50 million people worldwide. Sudden unexpected death in epilepsy (SUDEP) is the main cause of death in epilepsy accounting for up to 17% of all deaths in epileptic patients, and therefore remains a major public health problem. SUDEP likely arises from a combination and interaction of multiple risk factors (such as being male, drug resistance, frequent generalized tonic-clonic seizures) making risk prediction and mitigation challenging. While there is a general understanding of the physiopathology of SUDEP, mechanistic hypotheses linking risk factors with a risk of SUDEP are still lacking. Identifying cross-talk between biological systems implicated in SUDEP may facilitate the development of improved models for SUDEP risk assessment, treatment and clinical management. In this review, the aim was to explore an overlap between the pathophysiology of hypertension, cardiovascular disease and epilepsy, and discuss its implication for SUDEP. Presented herein, evidence in literature in support of a cross-talk between the renin-angiotensin system (RAS) and sympathetic nervous system, both known to be involved in the development of hypertension and cardiovascular disease, and as one of the underlying mechanisms of SUDEP. This article also provides a brief description of local RAS in brain neuroinflammation and the role of centrally acting RAS inhibitors in epileptic seizure alleviation.

Long non-coding RNA and mRNA analysis of Ang II-induced neuronal dysfunction

The sustained activation of Angiotensin II (Ang II) induces the remodelling of neurovascular units, inflammation and oxidative stress reactions in the brain. Long non-coding RNAs (lncRNAs) play a crucial regulatory role in the pathogenesis of hypertensive neuronal damage. The present study aimed to substantially extend the list of potential candidate genes involved in Ang II-related neuronal damage. This study assessed apoptosis and energy metabolism with Annexin V/PI staining and a Seahorse assay after Ang II exposure in SH-SY5Y cells. The expression of mRNA and lncRNA was investigated by transcriptome sequencing. The integrated analysis of mRNA and lncRNAs and the molecular mechanism of Ang II on neuronal injury was analysed by bioinformatics. Ang II increased the apoptosis rate and reduced the energy metabolism of SH-SY5Y cells. The data showed that 702 mRNAs and 821 lncRNAs were differentially expressed in response to Ang II exposure (244 mRNAs and 432 lncRNAs were upregulated, 458 mRNAs and 389 lncRNAs were downregulated) (fold change ≥ 1.5, P < 0.05). GO and KEGG analyses showed that both DE mRNA and DE lncRNA were enriched in the metabolism, differentiation, apoptosis and repair of nerve cells. This is the first report of the lncRNA-mRNA integrated profile of SH-SY5Y cells induced by Ang II. The novel targets revealed that the metabolism of the vitamin B group, the synthesis of unsaturated fatty acids and glycosphingolipids are involved in the Ang II-related cognitive impairment. Sphingolipid metabolism, the Hedgehog signalling pathway and vasopressin-regulated water reabsorption play important roles in nerve damage.

Role of the Angiotensin Pathway and its Target Therapy in Epilepsy Management

Despite extensive research on epileptogenesis, there is still a need to investigate new pathways and targeted therapeutic approaches in this complex process. Inflammation, oxidative stress, neurotoxicity, neural cell death, gliosis, and blood–brain barrier (BBB) dysfunction are the most common causes of epileptogenesis. Moreover, the renin–angiotensin system (RAS) affects the brain’s physiological and pathological conditions, including epilepsy and its consequences. While there are a variety of available pharmacotherapeutic approaches, information on new pathways is in high demand and the achievement of treatment goals is greatly desired. Therefore, targeting the RAS presents an interesting opportunity to better understand this process. This has been supported by preclinical studies, primarily based on RAS enzyme, receptor-inhibition, and selective agonists, which are characterized by pleiotropic properties. Although there are some antiepileptic drugs (AEDs) that interfere with RAS, the main targeted therapy of this pathway contributes in synergy with AEDs. However, the RAS-targeted treatment alone, or in combination with AEDs, requires clinical studies to contribute to, and clarify, the evidence on epilepsy management. There is also a genetic association between RAS and epilepsy, and an involvement of pharmacogenetics in RAS, so there are possibilities for the development of new diagnostic and personalized treatments for epilepsy.

Identification of insulin-regulated aminopeptidase (IRAP) in the rat pineal gland and the modulation of melatonin synthesis by angiotensin IV

A local renin-angiotensin system (RAS) has been postulated in the pineal gland. In addition to angiotensin II (Ang II), other active metabolites have been described. In this study, we aimed to investigate a role for Ang IV in melatonin synthesis and the presence of its proposed (IRAP)/AT₄ receptor (insulin-regulated aminopeptidase) in the pineal gland. The effect of Ang IV on melatonin synthesis was investigated in vitro using isolated pinealocytes. IRAP protein expression and activity were evaluated by Western blot and fluorimetry using Leu-4Me-β-naphthylamide as a substrate. Melatonin was analyzed by HPLC, calcium content by confocal microscopy and cAMP by immunoassay. Ang IV significantly augmented the NE-induced melatonin synthesis to a similar degree as that achieved by Ang II. This Ang IV effect in pinealocytes appears to be mediated by an increase in the intracellular calcium content but not by cAMP. The (IRAP)/AT₄ expression and activity were identified in the pineal gland, which were significantly higher in membrane fractions than in soluble fractions. Ang IV significantly reduced IRAP activity in the pineal membrane fractions. The main findings of the present study are as follows: (1) Ang IV potentiates NE-stimulated melatonin production in pinealocytes, (2) the (IRAP)/AT₄ receptor is present in the rat pineal gland, and (3) Ang IV inhibits IRAP activity and increases pinealocytes [Ca²⁺]_i. We conclude that Ang IV is an important component of RAS and modulates melatonin synthesis in the rat pineal gland.

Brain Renin-Angiotensin System Blockade Attenuates Methamphetamine-Induced Hyperlocomotion and Neurotoxicity

Methamphetamine (METH) abuse has become a major public health concern worldwide without approved pharmacotherapies. The brain renin-angiotensin system (RAS) is involved in the regulation of neuronal function as well as neurological disorders. Angiotensin II (Ang II), which interacts with Ang II type 1 receptor (AT1-R) in the brain, plays an important role as a neuromodulator in dopaminergic transmission. However, the role of brain RAS in METH-induced behavior is largely unknown. Here, we revealed that repeated METH administration significantly upregulated the expression of AT1-R in the striatum of mice, but downregulated dopamine D3 receptor (D3R) expression. A specific AT1-R blocker telmisartan, which can penetrate the brain-blood barrier (BBB), or genetic deletion of AT1-R was sufficient to attenuate METH-triggered hyperlocomotion in mice. However, intraperitoneal injection of AT1-R blocker losartan, which cannot penetrate BBB, failed to attenuate METH-induced behavior. Moreover, intra-striatum re-expression of AT1 with lentiviral virus expressing AT1 reversed the weakened locomotor activity of AT1-/- mice treated with METH. Losartan alleviated METH-induced cytotoxicity in SH-SY5Y cells in vitro, which was accompanied by upregulated expressions of D3R and dopamine transporter. In addition, intraperitoneal injection of perindopril, which is a specific ACE inhibitor and can penetrate BBB, significantly attenuated METH-induced hyperlocomotor activity. Collectively, our results show that blockade of brain RAS attenuates METH-induced hyperlocomotion and neurotoxicity possibly through modulation of D3R expression. Our findings reveal a novel role of Ang II-AT1-R in METH-induced hyperlocomotion.

Renin-angiotensin system as a potential target for new therapeutic approaches in Parkinson's disease

INTRODUCTION

Currently, available therapies for Parkinson's disease (PD) are symptomatic. Therefore, the search for neuroprotective drugs remains a top priority. Areas covered: In this review, the potential symptomatic or disease-modifying effect of drugs targeting the Renin-Angiotensin System (RAS) in PD will be explored. Expert opinion: The importance of nigrostriatal local RAS has only begun to be unraveled in the last decades. On one hand, there is a complex feedback cycle between RAS and dopamine (DA). On the other hand, RAS affects dopaminergic neurons vulnerability. Neuroprotective effects in animal PD models have been shown for the angiotensin-converting enzyme (ACE) inhibitors captopril and perindopril, and the AT1 receptor antagonists losartan, candesartan and telmisartan. These effects appear to be mediated by a reduction in the overproduction of reactive oxygen species. In a proof-of-concept, randomized, double-blind, crossover study in PD patients, perindopril enhanced the effect of levodopa without inducing dyskinesias. There has not been any clinical trial exploring the neuroprotective effect of RAS drugs, but one cohort study in hypertensive patients suggested a protective effect of ACE inhibitors on PD risk. RAS is a promising target for symptomatic and neuroprotective therapies in PD. Further studies in PD animal models and patients are warranted.

Renin-angiotensin system as a potential therapeutic target in stroke and retinopathy: experimental and clinical evidence

As our knowledge expands, it is now clear that the renin-angiotensin (Ang) system (RAS) mediates functions other than regulating blood pressure (BP). The RAS plays a central role in the pathophysiology of different neurovascular unit disorders including stroke and retinopathy. Moreover, the beneficial actions of RAS modulation in brain and retina have been documented in experimental research, but not yet exploited clinically. The RAS is a complex system with distinct yet interconnected components. Understanding the different RAS components and their functions under brain and retinal pathological conditions is crucial to reap their benefits. The aim of the present review is to provide an experimental and clinical update on the role of RAS in the pathophysiology and treatment of stroke and retinopathy. Combining the evidence from both these disorders allows a unique opportunity to move both fields forward.

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.

Cerebroprotective effects of RAS inhibitors: Beyond their cardio-renal actions

Work on the brain renin-angiotensin system has been explored by various researchers and has led to elucidation of its basic physiologies and behavior, including its role in reabsorption and uptake of body fluid, blood pressure maintenance with angiotensin II being its prominent effector. Currently, this system has been implicated for its newly established effects, which are far beyond its cardio-renal effects accounting for maintenance of cerebral blood flow and cerebroprotection, seizure, in the etiology of Alzheimer's disease, Parkinson's disease, multiple sclerosis, and bipolar disorder. In this review, we have discussed the distribution of angiotensin receptor subtypes in the central nervous system (CNS) together with enzymatic pathways leading to active angiotensin ligands and its interaction with angiotensin receptor 2 (AT2) and Mas receptors. Secondly, the use of angiotensin analogues (angiotensin converting enzyme inhibitors and AT1 and/or AT2 receptor blockers) in the treatment and management of the CNS disorders mentioned above has been discussed.

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.

Angiotensin II and its receptor in activated microglia enhanced neuronal loss and cognitive impairment following pilocarpine-induced status epilepticus

Neuroinflammation plays a role in the pathology of epilepsy and in cognitive impairment. Angiotensin II (AII) and the angiotensin receptor type 1 (AT1) have been shown to regulate seizure susceptibility in different models of epilepsy. Inhibition of AT1 attenuates neuroinflammatory responses in different neurological diseases. In the present study, we showed that the protein expression of AII and AT1 was increased in activated microglia following lithium pilocarpine-induced status epilepticus (SE) in rats. Furthermore, the AT1 receptor antagonist, losartan, significantly inhibited SE-induced cognitive impairment and microglia-mediated inflammation. Losartan also prevented SE induced neuronal loss in the hippocampus and exerted neuroprotection. These data suggest that losartan improves SE-induced cognitive impairment by suppressing microglia mediated inflammatory responses and attenuating hippocampal neuronal loss. Overall, our findings provide a possible therapeutic strategy for the treatment of cognitive impairment in epilepsy.

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.

Neuropeptides as targets for the development of anticonvulsant drugs

Epilepsy is a common neurological disorder characterized by recurrent seizures. These seizures are due to abnormal excessive and synchronous neuronal activity in the brain caused by a disruption of the delicate balance between excitation and inhibition. Neuropeptides can contribute to such misbalance by modulating the effect of classical excitatory and inhibitory neurotransmitters. In this review, we discuss 21 different neuropeptides that have been linked to seizure disorders. These neuropeptides show an aberrant expression and/or release in animal seizure models and/or epilepsy patients. Many of these endogenous peptides, like adrenocorticotropic hormone, angiotensin, cholecystokinin, cortistatin, dynorphin, galanin, ghrelin, neuropeptide Y, neurotensin, somatostatin, and thyrotropin-releasing hormone, are able to suppress seizures in the brain. Other neuropeptides, such as arginine-vasopressine peptide, corticotropin-releasing hormone, enkephalin, β-endorphin, pituitary adenylate cyclase-activating polypeptide, and tachykinins have proconvulsive properties. For oxytocin and melanin-concentrating hormone both pro- and anticonvulsive effects have been reported, and this seems to be dose or time dependent. All these neuropeptides and their receptors are interesting targets for the development of new antiepileptic drugs. Other neuropeptides such as nesfatin-1 and vasoactive intestinal peptide have been less studied in this field; however, as nesfatin-1 levels change over the course of epilepsy, this can be considered as an interesting marker to diagnose patients who have suffered a recent epileptic seizure.

Central administration of angiotensin IV rapidly enhances novel object recognition among mice

Angiotensin IV (Val(1)-Tyr(2)-Ile(3)-His(4)-Pro(5)-Phe(6)) has demonstrated potential cognitive-enhancing effects. The present investigation assessed and characterized: (1) dose-dependency of angiotensin IV's cognitive enhancement in a C57BL/6J mouse model of novel object recognition, (2) the time-course for these effects, (3) the identity of residues in the hexapeptide important to these effects and (4) the necessity of actions at angiotensin IV receptors for procognitive activity. Assessment of C57BL/6J mice in a novel object recognition task demonstrated that prior administration of angiotensin IV (0.1, 1.0, or 10.0, but not 0.01 nmol, i.c.v.) significantly enhanced novel object recognition in a dose-dependent manner. These effects were time dependent, with improved novel object recognition observed when angiotensin IV (0.1 nmol, i.c.v.) was administered 10 or 20, but not 30 min prior to the onset of the novel object recognition testing. An alanine scan of the angiotensin IV peptide revealed that replacement of the Val(1), Ile(3), His(4), or Phe(6) residues with Ala attenuated peptide-induced improvements in novel object recognition, whereas Tyr(2) or Pro(5) replacement did not significantly affect performance. Administration of the angiotensin IV receptor antagonist, divalinal-Ang IV (20 nmol, i.c.v.), reduced (but did not abolish) novel object recognition; however, this antagonist completely blocked the procognitive effects of angiotensin IV (0.1 nmol, i.c.v.) in this task. Rotorod testing demonstrated no locomotor effects with any angiotensin IV or divalinal-Ang IV dose tested. These data demonstrate that angiotensin IV produces a rapid enhancement of associative learning and memory performance in a mouse model that was dependent on the angiotensin IV receptor.

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

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

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.

Importance of the brain Angiotensin system in Parkinson's disease

Parkinson's disease (PD) has become a major health problem affecting 1.5% of the world's population over 65 years of age. As life expectancy has increased so has the occurrence of PD. The primary direct consequence of this disease is the loss of dopaminergic (DA) neurons in the substantia nigra and striatum. As the intensity of motor dysfunction increases, the symptomatic triad of bradykinesia, tremors-at-rest, and rigidity occur. Progressive neurodegeneration may also impact non-DA neurotransmitter systems including cholinergic, noradrenergic, and serotonergic, often leading to the development of depression, sleep disturbances, dementia, and autonomic nervous system failure. L-DOPA is the most efficacious oral delivery treatment for controlling motor symptoms; however, this approach is ineffective regarding nonmotor symptoms. New treatment strategies are needed designed to provide neuroprotection and encourage neurogenesis and synaptogenesis to slow or reverse this disease process. The hepatocyte growth factor (HGF)/c-Met receptor system is a member of the growth factor family and has been shown to protect against degeneration of DA neurons in animal models. Recently, small angiotensin-based blood-brain barrier penetrant mimetics have been developed that activate this HGF/c-Met system. These compounds may offer a new and novel approach to the treatment of Parkinson's disease.

Evaluation of metabolically stabilized angiotensin IV analogs as procognitive/antidementia agents

Angiotensin IV (AngIV: VYIHPF)-related peptides have long been recognized as procognitive agents with potential as antidementia therapeutics. Their development as useful therapeutics, however, has been limited by physiochemical properties that make them susceptible to metabolic degradation and impermeable to gut and blood-brain barriers. A previous study demonstrated that the core structural information required to impart the procognitive activity of the AngIV analog, norleucine(1)-angiotensin IV, resides in its three N-terminal amino acids, Nle-Tyr-Ile. The goal of this project was to chemically modify this tripeptide in such a way to enhance its metabolic stability and barrier permeability to produce a drug candidate with potential clinical utility. Initial results demonstrated that several N- and C-terminal modifications lead to dramatically improved stability while maintaining the capability to reverse scopolamine-induced deficits in Morris water maze performance and augment hippocampal synaptogenesis. Subsequent chemical modifications, which were designed to increase hydrophobicity and decrease hydrogen bonding, yielded an orally active, blood-barrier permeant, metabolically stabilized analog, N-hexanoic-Tyr-Ile-(6) aminohexanoic amide (dihexa), that exhibits excellent antidementia activity in the scopolamine and aged rat models and marked synaptogenic activity. These data suggest that dihexa may have therapeutic potential as a treatment of disorders, such as Alzheimer's disease, where augmented synaptic connectivity may be beneficial.

AGTR1 gene variation: association with depression and frontotemporal morphology

The renin-angiotensin system (RAS) is implicated in the response to physiological and psychosocial stressors, but its role in stress-related psychiatric disorders is poorly understood. We examined if variation in AGTR1, the gene coding for the type 1 angiotensin II receptor (AT(1)R), is associated with a diagnosis of depression and differences in white matter hyperintensities and frontotemporal brain volumes. Participants comprised 257 depressed and 116 nondepressed elderly Caucasian subjects who completed clinical assessments and provided blood samples for genotyping. We used a haplotype-tagging single nucleotide polymorphism (htSNP) analysis to test for variation in AGTR1. For measurement of hyperintense lesions, 1.5 Tesla magnetic resonance imaging (MRI) data were available on 33 subjects. For measurements of the hippocampus and dorsolateral prefrontal cortex (dlPFC), 3 Tesla MRI data were available on 70 subjects. Two htSNPs exhibited statistically significant frequency differences between diagnostic cohorts: rs10935724 and rs12721331. Although hyperintense lesion volume did not significantly differ by any htSNP, dlPFC and hippocampus volume differed significantly for several htSNPs. Intriguingly, for those htSNPs differing significantly for both dlPFC and hippocampus volume, the variant associated with smaller dlPFC volume was associated with larger hippocampal volume. This supports the idea that genetic variation in AGTR1 is associated with depression and differences in frontotemporal morphology.

The role of insulin-regulated aminopeptidase in MHC class I antigen presentation

Production of MHC-I ligands from antigenic proteins generally requires multiple proteolytic events. While the proteolytic steps required for antigen processing in the endogenous pathway are clearly established, persisting gaps of knowledge regarding putative cross-presentation compartments have made it difficult to map the precise proteolytic events required for generation of cross-presented antigens. It is only in the past decade that the importance of aminoterminal trimming as the final step in the endogenous presentation pathway has been recognized and that the corresponding enzymes have been described. This review focuses on the aminoterminal trimming of exogenous cross-presented peptides, with particular emphasis on the identification of insulin responsive aminopeptidase (IRAP) as the principal trimming aminopeptidase in endosomes and phagosomes.

Facilitation of hippocampal synaptogenesis and spatial memory by C-terminal truncated Nle1-angiotensin IV analogs

Angiotensin IV (AngIV; Val(1)-Tyr(2)-Ile(3)-His(4)-Pro(5)-Phe(6))-related peptides have emerged as potential antidementia agents. However, their development as practical therapeutic agents has been impeded by a combination of metabolic instability, poor blood-brain barrier permeability, and an incomplete understanding of their mechanism of action. This study establishes the core structure contained within norleucine(1)-angiotensin IV (Nle(1)-AngIV) that is required for its procognitive activity. Results indicated that Nle(1)-AngIV-derived peptides as small as tetra- and tripeptides are capable of reversing scopolamine-induced deficits in Morris water maze performance. This identification of the active core structure contained within Nle(1)-AngIV represents an initial step in the development of AngIV-based procognitive drugs. The second objective of the study was to clarify the general mechanism of action of these peptides by assessing their ability to affect changes in dendritic spines. A correlation was observed between a peptide's procognitive activity and its capacity to increase spine numbers and enlarge spine head size. These data suggest that the procognitive activity of these molecules is attributable to their ability to augment synaptic connectivity.

Strain differences in the effects of angiotensin IV on mouse cognition

Angiotensin IV has been shown to improve learning and memory in rodents. Strain dependent variation in murine behaviour, aminopeptidase activity and inhibitory effect of Angiotensin IV, structural variation in insulin regulated aminopeptidase (IRAP) and aminopeptidase N (ApN) and expression of the encoding genes were explored. Strain differences in the behavioural response to Angiotensin IV were observed, where CD mice were refractory. All strains showed inhibition of aminopeptidase activity by Angiotensin IV but CD mice displayed reduced endogenous aminopeptidase activity. No differences in the coding sequence of IRAP or ApN were found. RT-PCR analysis showed no difference in IRAP expression between strains but an increased expression of ApN was observed in CD mice. The lack of cognitive response of CD mice to Angiotensin IV cannot be explained through variation within IRAP sequence nor expression but the results highlight a potential role for ApN in the effects of Angiotensin IV.

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.

Participation of D 1-4 dopamine receptors in the pro-cognitive effects of angiotensin IV and des-Phe 6 angiotensin IV

Angiotensin IV (Ang IV) and des-Phe(6)Ang IV are naturally occurring neuroactive peptides of the renin-angiotensin system (RAS) involved in memory processing. However, the relevant mechanisms are poorly understood. In this review it is proposed that the pro-cognitive effects of these peptides are, at least partly, mediated by dopamine (DA). Recent studies demonstrated that the improvement of several memory aspects; recall of appetitively and aversively motivated behaviors and learning of spatial tasks by Ang IV and des-Phe(6)Ang IV was abolished, or significantly diminished by behaviorally inactive per se doses of the D(1) and D(2) receptor blockers SCH 23390 (R-[+]-7-chloro-8-hydroxy-3 methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and remoxipride, respectively. The D(3) receptor inhibition with nafadotride was almost ineffective but again, the D(4) receptor blockade by L745,870 hydrochloride (3-{[4-(4-chlorophenyl)piperazin-1-yl]methyl}-1H-pyrrolo[2,3-b]pyridine hydrochloride) diminished all, except for spatial memory, improving actions of the peptides. These results suggest that Ang IV and des-Phe(6)Ang IV enhance memory in a brain region-specific manner, dependent on local DA receptor subpopulations and the memory aspects controlled by them. The data reviewed here, demonstrating DA-Ang IV and des-Phe(6)Ang IV interactions in brain, strongly suggest probability of clinically relevant effects of concomitant use of antipsychotic and RAS affecting drugs.