Direct angiotensin AT2 receptor stimulation using a novel AT2 receptor agonist, compound 21, evokes neuroprotection in conscious hypertensive rats

Angiotensin II Type 2 Receptor Agonism Alleviates Progressive Post-stroke Cognitive Impairment in Aged Spontaneously Hypertensive Rats

Hypertension and aging are leading risk factors for stroke and vascular contributions to cognitive impairment and dementia (VCID). Most animal models fail to capture the complex interplay between these pathophysiological processes. In the current study, we examined the development of cognitive impairment in 18-month-old spontaneously hypertensive rats (SHR) before and following ischemic stroke. Sixty SHRs were housed for 18 months with cognitive assessments every 6 months and post-surgery. MRI scans were performed at baseline and throughout the study. On day 3 post-stroke, rats were randomized to receive either angiotensin II type 2 receptor (AT2R) agonist Compound 21 (C21) or plain water for 8 weeks. SHRs demonstrated a progressive cognitive decline and significant MRI abnormalities before stroke. Perioperative mortality within 72 h of stroke was low. Stroke resulted in significant acute brain swelling, chronic brain atrophy, and sustained sensorimotor and behavioral deficits. There was no evidence of anhedonia at week 8. C21 enhanced sensorimotor recovery and ischemic lesion resolution at week 8. SHRs represent a clinically relevant animal model to study aging and stroke-associated VCID. This study underscores the importance of translational disease modeling and provides evidence that modulation of the AT2R signaling via C21 may be a useful therapeutic option to improve sensorimotor and cognitive outcomes even in aged animals.

Inhibition of insulin-regulated aminopeptidase confers neuroprotection in a conscious model of ischemic stroke

Stroke is a leading cause of mortality and morbidity with a paucity of effective pharmacological treatments. We have previously identified insulin-regulated aminopeptidase (IRAP) as a potential target for the development of a new class of drugs for the treatment of stroke, as global deletion of this gene in mice significantly protected against ischemic damage. In the current study, we demonstrate that small molecular weight IRAP inhibitors reduce infarct volume and improve neurological outcome in a hypertensive animal model of ischemic stroke. The effects of two structurally distinct IRAP inhibitors (HFI419 or SJM164) were investigated in a model of stroke where the middle cerebral artery was transiently occluded with endothelin-1 in the conscious spontaneously hypertensive rat. IRAP inhibitor was administered into the lateral ventricle at 2 or 6 h after stroke, with subsequent doses delivered at 24, 48 and 70 h post-stroke. Functional outcomes were assessed prior to drug treatment, and on day 1 and 3 post-stroke. Histological analyses and neuroinflammatory cytokine profiling were conducted at 72 and 24 h post-stroke respectively. IRAP inhibitor treatment following stroke significantly reduced infarct volume and improved neurological and motor deficits. These protective effects were maintained even when the therapeutic window was extended to 6 h. Examination of the cellular architecture at 72 h post-stroke demonstrated that IRAP expression was upregulated in CD11b positive cells and activated astrocytes. Furthermore, IRAP inhibitor treatment significantly increased gene expression for interleukin 6 and C-C motif chemokine ligand 2 in the ischemic core. This study provides proof-of-principle that selective inhibition of IRAP activity with two structurally distinct IRAP inhibitors reduces infarct volume and improves functional outcome even when the first dose is administered 6 h post-stroke. This is the first direct evidence that IRAP inhibitors are a class of drug with potential use in the treatment of ischemic stroke.

Macrophage angiotensin AT2 receptor activation is protective against early phases of LPS-induced acute kidney injury

Lipopolysaccharide (LPS)-induced acute kidney injury (AKI) is characterized by inflammation and infiltration of immune cells, mainly neutrophils and macrophages, and results in sudden renal dysfunction. Previously, we have reported the anti-inflammatory and renoprotective role of the angiotensin II type 2 receptor (AT2R), expressed on kidney tubular cells and immune cells, in LPS-induced AKI. Moreover, in vitro studies revealed macrophage AT2R activation shifts the cells to the anti-inflammatory M2 subtype. However, the protective role of the macrophage AT2R in a model of AKI is unknown. The present study addressed this question by adoptive transfer of bone marrow-derived macrophages (BMDMs) in systemic macrophage-depleted mice. We acquired significant systemic macrophage depletion by two doses of liposomal clodronate (CLD), and the mice were repopulated with BMDMs (CD11b+F4/80+, double positive) primed with AT2R agonist C21 (CLD + MacC21 + LPS) or vehicle (CLD + Mac + LPS) in vitro for 60 min, followed by LPS (5 mg/kg body wt ip) challenge. We observed a gradual increase in the CD11b+ cells at 2 and 24 h after the LPS challenge. However, kidney CD11b+ cells in the CLD + Mac + LPS group were elevated compared with the CLD + MacC21 + LPS group at 2 h after the LPS challenge. The level of inflammatory cytokine (tumor necrosis factor-α) was elevated at 2 h, which was reduced significantly in CLD + MacC21 + LPS-treated animals. Also, CLD + MacC21 + LPS-treated animals had elevated plasma and renal IL-10, indicating an anti-inflammatory role of C21-treated BMDMs. Renal functional injury in CLD + MacC21 + LPS-treated animals was partially improved. Collectively, the data demonstrate that BMDM AT2R stimulation results in anti-inflammation and partial renoprotection against early stages of LPS-induced AKI.NEW & NOTEWORTHY Endotoxin such as lipopolysaccharide (LPS) induces acute kidney injury (AKI), which is a risk factor for and often leads to chronic kidney diseases. The present study revealed that bone marrow-derived macrophage activation of the angiotensin II type 2 receptor (AT2R) contributes to the anti-inflammation and partial renoprotection against early stages of LPS-induced AKI. Since AT2R is an emerging anti-inflammatory and organ-protective target, this study advances our understanding of AT2R's anti-inflammatory mechanisms associated with renoprotection.

Circulating renin-angiotensin systems mediated feedback controls over the mean-arterial pressure

The renin-angiotensin systems play pivotal role in cardiovascular physiology through its effects on regulating blood pressure and electrolyte homeostasis. Components of circulating RAS (cRAS) that include precursor angiotensinogen, two critical enzymes (renin and angiotensin-converting enzyme, ACE), their bioactive products, angiotensin- I, II together with its receptors (AT1R and AT2R) essentially determine this homeostasis. Most classical studies, however, showed the deleterious role of cRAS in elevating the blood pressure. Contemporary discovery of non-canonical components of the RAS has challenged this classic hypothesis that it can only exert deleterious effects on the cardiovascular systems. Using the classic cRAS model, we have designed in-silico experiments to test the hypothesis that AT2R-mediated feedback effects play pivotal role for maintaining the normal variation of the mean-arterial pressure (MAP).Beside the AT2R-mediation of downstream singling pathways consisting of several non-canonical RAS components, this study first time illustrated AT2R mediated feedback controls over the blood pressure regulation: one that impedes AT1R activity, and the other that downregulates renin. It has been shown that relatively stronger suppression of renin activity significantly contributes in maintaining the normal MAP and that tight AT2R-mediated regulation is relaxed in hyper-and hypotension. This control mechanism is noted to be robustly maintained with the MAP variations through an established linear steady-state relationship among renin, angiotensin I and angiotensin II. This examination suggests that AT2R-mediated downregulation of renin activities potentially counteracts the AT1R-mediated deleterious actions of Ang II. This study, therefore, provides a solid ground for considering different AT2 receptor adaptor protein and direct agonism at AT2R that can cause greater effects along with contemporary approaches of blocking AT1R mediation to attenuate hypertension or other cardiovascular disorders.

The angiotensin AT2-receptor agonist compound 21 is an antagonist for the thromboxane TP-receptor - Implications for preclinical studies and future clinical use

Since the AT₂-receptor (AT₂R) agonist C21 has structural similarity to the AT₁-receptor antagonists Irbesartan and Losartan, which are antagonists not only at the AT₁R, but also at thromboxane TP-receptors, we tested the hypothesis that C21 has TP-receptor antagonistic properties as well. Isolated mouse mesenteric arteries from C57BL/6 J and AT₂R-knockout mice (AT₂R^-/y) were mounted in wire myographs, contracted with either phenylephrine or the thromboxane A₂ (TXA₂) analogue U46619, and the relaxing effect of C21 (0.1 nM - 10 µM) was investigated. The effect of C21 on U46619-induced platelet aggregation was measured by an impedance aggregometer. Direct interaction of C21 with TP-receptors was determined by an β-arrestin biosensor assay. C21 caused significant, concentration-dependent relaxations in phenylephrine- and U46619-contracted mesenteric arteries from C57BL/6 J mice. The relaxing effect of C21 was absent in phenylephrine-contracted arteries from AT₂R^-/y mice, whereas it was unchanged in U46619-contracted arteries from AT₂R^-/y mice. C21 inhibited U46619-stimulated aggregation of human platelets, which was not inhibited by the AT₂R-antagonist PD123319. C21 reduced U46619-induced recruitment of β-arrestin to human thromboxane TP-receptors with a calculated K_i of 3.74 µM. We conclude that in addition to AT₂R-agonistic properties, C21 also acts as low-affinity TP-receptor antagonist, and that - depending on the constrictor - both mechanisms can be responsible for C21-induced vasorelaxation. Furthermore, by acting as a TP-receptor antagonist, C21 inhibits platelet aggregation. These findings are important for understanding potential off-target effects of C21 in the preclinical and clinical context and for the interpretation of C21-related myography data in assays with TXA₂-analogues as constrictor.

A review of literature on Compound 21-loaded gelatin nanoparticle: a promising nose-to-brain therapy for multi-infarct dementia

Multi-infarct dementia (MID) is described as a chronic progressive decline in cortical cognitive function due to the occurrence of multiple infarcts in the cerebral vascularization throughout the gray and white matter. Current therapies of MID mostly focus only on slowing down MID progression and symptomatic medications. A novel therapy which is able to provide both preventive and curative properties for MID is of high interest. The purpose of this review is to identify the potential of Compound 21 (C21) gelatin nanoparticle through the nose-to-brain route as therapy for MID. C21, an angiotensin II type 2 receptor (AT2R) agonist, has shown to reduce the size of cerebral infarct in rodent models, resulting in the preservation and improvement of overall cognitive function and prevention of secondary neurodegenerative effects. It is also shown that C21 decreases neuronal apoptosis, improves damaged axons, and encourage synapse development. The challenge remains in preventing systemic AT2R activation and increasing its low oral bioavailability which can be overcome through nose-to-brain administration of C21. Nose-to-brain drug delivery of C21 significantly increases drug efficiency and limits C21 exposure in order to specifically target the multiple infarcts located in the cerebral cortex. Adhering C21 onto gelatin nanoparticles may enable longer contact time with the olfactory and the trigeminal nerve endings, increasing the potency of C21. In summary, treatment of C21 gelatin nanoparticle through nose-to-brain delivery shows high potential as therapy for vascular dementia. However, clinical trials must be further studied in order to test the safety and efficacy of C21.

Microglia: The breakthrough to treat neovascularization and repair blood-retinal barrier in retinopathy

Microglia are the primary resident retinal macrophages that monitor neuronal activity in real-time and facilitate angiogenesis during retinal development. In certain retinal diseases, the activated microglia promote retinal angiogenesis in hypoxia stress through neurovascular coupling and guide neovascularization to avascular areas (e.g., the outer nuclear layer and macula lutea). Furthermore, continuously activated microglia secrete inflammatory factors and expedite the loss of the blood-retinal barrier which causes irreversible damage to the secondary death of neurons. In this review, we support microglia can be a potential cellular therapeutic target in retinopathy. We briefly describe the relevance of microglia to the retinal vasculature and blood-retinal barrier. Then we discuss the signaling pathway related to how microglia move to their destinations and regulate vascular regeneration. We summarize the properties of microglia in different retinal disease models and propose that reducing the number of pro-inflammatory microglial death and conversing microglial phenotypes from pro-inflammatory to anti-inflammatory are feasible for treating retinal neovascularization and the damaged blood-retinal barrier (BRB). Finally, we suppose that the unique properties of microglia may aid in the vascularization of retinal organoids.

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

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

Contralesional angiotensin type 2 receptor activation contributes to recovery in experimental stroke

We and others have previously shown that angiotensin II receptor type 2 receptor (AT2R) is upregulated in the contralesional hemisphere after stroke in normoglycemic Wistar rats. In this study, we examined the expression of AT2R in type 2 diabetic Goto-Kakizaki (GK) rats and control Wistars after stroke. We also tested the contribution of the contralesional AT2R in recovery after stroke through a specific knockdown of the AT2R in this hemisphere only. Two experiments were conducted. In the first experiment, GK rats were subjected to middle cerebral artery occlusion (MCAO) and treated with the angiotensin II receptor type 1 receptor (AT1R) blocker candesartan or saline at reperfusion. Stroke outcomes, as well as AT2R expression, were examined and compared to control Wistars at 24 h. In the second experiment, localized AT2R knockdown was achieved through intrastriatal injection of short hairpin RNA (shRNA) lentiviral particles or non-targeting control into the left-brain hemisphere of Wistar rats. After 14 days, rats were subjected to right MCAO and treated with the AT2R agonist, Compound 21 (C21), or saline for 7 days. Behavioral outcomes were assessed for up to 10 days. In the first experiment, stroke reduced the expression of AT2R in GK rats. Candesartan treatment failed to improve the neurobehavioral outcomes, preserve vascular integrity or reduce oxidative/nitrative stress or apoptotic markers at 24 h post stroke in these animals. In the second experiment, contralesional AT2R knockdown reduced the C21-mediated functional recovery after stroke. In conclusion, contralesional AT2R upregulation after stroke is blunted in diabetic rats which show reduced sensitivity to post-stroke candesartan treatment. Contralesional AT2R could be involved in C21-mediated functional recovery after stroke.

Angiotensin-(1-9) in hypertension

Angiotensin-(1-9) [Ang-(1-9)] is a peptide of the non-canonical renin-angiotensin system (RAS) synthesized from angiotensin I by the monopeptidase angiotensin-converting enzyme type 2 (ACE2). Using osmotic minipumps, infusion of Ang-(1-9) consistently reduces blood pressure in several rat hypertension models. In these animals, hypertension-induced end-organ damage is also decreased. Several pieces of evidence suggest that Ang-(1-9) is the endogenous ligand that binds and activates the type-2 angiotensin II receptor (AT2R). Activation of AT2R triggers different tissue-specific signaling pathways. This phenomenon could be explained by the ability of AT2R to form different heterodimers with other G protein-coupled receptors. Because of the antihypertensive and protective effects of AT2R activation by Ang-(1-9), associated with a short half-life of RAS peptides, several synthetic AT2R agonists have been synthesized and assayed. Some of them, particularly CGP42112, C21 and novokinin, have demonstrated antihypertensive properties. Only two synthetic AT2R agonists, C21 and LP2-3, have been tested in clinical trials, but none of them like an antihypertensive. Therefore, Ang-(1-9) is a promising antihypertensive drug that reduces hypertension-induced end-organ damage. However, further research is required to translate this finding successfully to the clinic.

Direct AT2R Stimulation Slows Post-stroke Cognitive Decline in the 5XFAD Alzheimer's Disease Mice

Alzheimer's disease (AD), currently the single leading cause of death still on the rise, almost always coexists alongside vascular cognitive impairment (VCI). In fact, the ischemic disease affects up to 90% of AD patients, with strokes and major infarctions representing over a third of vascular lesions. Studies also confirmed that amyloid plaques, typical of AD, are much more likely to cause dementia if strokes or cerebrovascular damage also exist, leading to the term "mixed pathology" cognitive impairment. Although its incidence is expected to grow, there are no satisfactory treatments. There is hence an urgent need for safe and effective therapies that preserve cognition, maintain function, and prevent the clinical deterioration that results from the progression of this irreversible, neurodegenerative disease. To our knowledge, this is the first study to investigate the effects of long-term treatment with C21, a novel angiotensin II type 2 receptor (AT2R) agonist, on the development of "mixed pathology" cognitive impairment. This was accomplished using a unique model that employs the fundamental elements of both AD and VCI. Treatment with C21/vehicle was started 1 h post-stroke and continued for 5 weeks in mice with concurrent AD pathology. Efficacy was established through a series of functional tests assessing various aspects of cognition, including spatial learning, short-term/working memory, long-term/reference memory, and cognitive flexibility, in addition to the molecular markers characteristic of AD. Our findings demonstrate that C21 treatment preserves cognitive function, maintains cerebral blood flow, and reduces Aβ accumulation and toxic tau phosphorylation in AD animals post-stroke.

Neuroprotection in Stroke-Focus on the Renin-Angiotensin System: A Systematic Review

Stroke is the primary cause of disability in the adult population. Hypertension represents the leading risk factor being present in almost half the patients. The renin-angiotensin system is involved in the physiopathology of stroke and has an essential impact on hypertension as a risk factor. This article targeted the role of the renin-angiotensin system in stroke neuroprotection by reviewing the current literature available. The mechanism of action of the renin-angiotensin system was observed through the effects on AT1, AT2, and Mas receptors. The neuroprotective properties ascertained by angiotensin in stroke seem to be independent of the blood pressure reduction mechanism, and include neuroregeneration, angiogenesis, and increased neuronal resistance to hypoxia. The future relationship of stroke and the renin-angiotensin system is full of possibilities, as new agonist molecules emerge as potential candidates to restrict the impairment caused by stroke.

Stimulation of angiotensin II receptor 2 preserves cognitive function and is associated with an enhanced cerebral vascular density after stroke

Angiotensin signaling is known to be sexually dimorphic. Although it is a well-studied target for intervention in stroke and cognitive impairment, female studies are rare. With females suffering a disproportionately greater negative impact of stroke and dementia vs. males, effective interventions are of utmost urgency. The aim of the current study was to determine the impact of activation of the angiotensin II type 2 receptor (AT2R) with the agonist compound 21 (C21) on the development of post-stroke cognitive impairment, after experimental ischemic stroke. Ovariectomized (OVX) spontaneously hypertensive rats (SHRs) were subjected to 1 h of middle cerebral artery occlusion (MCAO). At 24 h, rats with a significant neurologic deficit were randomized to receive either saline or C21 (0.03 mg/kg/day) intraperitoneally (IP) for 5 days, then orally (0.12 mg/kg/day) for a total of 6 weeks. Cognitive function, brain structure by MRI and vascular architecture by microCT angiography were measured. C21 preserved cognitive function, specifically spatial memory, and improved vascular density in the ischemic hemisphere at 6 weeks, reflecting both arteriogenesis and angiogenesis. In conclusion, C21 prevented cognitive impairment after stroke, likely through a mechanism involving vascular protection and restoration.

The Protective Effects of AT2R Agonist, CGP42112A, Against Angiotensin II-Induced Oxidative Stress and Inflammatory Response in Astrocytes: Role of AT2R/PP2A/NFκB/ROS Signaling

Angiotensin II receptor type 2 (AT2R) agonists have been known to promote neuroprotection by limiting ischemic insult, neuronal proliferation, and differentiation. Further, AT2R agonists have also been associated with the suppression of neuroinflammation and neurodegeneration. Of note, brain astrocytes play a critical role in these neuroinflammatory and neurodegenerative processes. However, the role of AT2R in astrocytic activation remains elusive. Therefore, this study evaluated the role and molecular mechanism of AT2R agonist CGP42112A (CGP) against Angiotensin II (Ang II)-induced astrocytic activation in primary astrocytes, and in a rat model of hypertension. Here, we demonstrated that AT2R activation by CGP abrogated Ang II-induced astrocytic activation, by mitigating the ROS production, mitochondrial dysfunction, IκB-α degradation, NFκB nuclear translocation, and release of TNF-α in astrocytes. However, AT2R-mediated anti-inflammatory effects were reversed by AT2R antagonist, PD123319 (PD), in both in vitro and in vivo conditions. Mechanistically, AT2R via protein phosphatase-2A (PP2A) abrogated the Ang II-induced NFκB activation, ROS generation, and subsequent astrocytic activation. Importantly, PP2A antagonist, okadaic acid, reversed the anti-inflammatory effects of AT2R in Ang II-stimulated primary astrocytes and in the cortex of hypertensive rats. Thus, the present study suggests that AT2R by activating PP2A inhibits oxidative stress and NFκB activation, thereby preventing the astrocytic pro-inflammatory activation. Therefore, AT2R might be advantageous therapeutic target for neuroinflammatory/neurodegenerative diseases perpetuated by astrocytic activation.

An overview of ongoing clinical trials assessing pharmacological therapeutic strategies to manage chemotherapy-induced peripheral neuropathy, based on preclinical studies in rodent models

Chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting side effect induced by a variety of chemotherapeutic agents. Symptoms are mainly sensory: pain, tingling, numbness, and temperature sensitivity. They may require the tapering of chemotherapy regimens or even their cessation; thus, the prevention/treatment of CIPN is critical to increase effectiveness of cancer treatment. However, CIPN management is mainly based on conventional neuropathic pain treatments, with poor clinical efficacy. Therefore, significant effort is made to identify new pharmacological targets to prevent/treat CIPN. Animal modeling is a key component in predicting human response to drugs and in understanding the pathophysiological mechanisms underlying CIPN. In fact, studies performed in rodents highlighted several pharmacological targets to treat/prevent CIPN. This review provides updated information about ongoing clinical trials testing drugs for the management of CIPN and presents some of their proof-of-concept studies conducted in rodent models. The presented drugs target oxidative stress, renin-angiotensin system, glutamatergic neurotransmission, sphingolipid metabolism, neuronal uptake transporters, nicotinamide adenine dinucleotide metabolism, endocannabinoid system, transient receptor potential channels, and serotoninergic receptors. As some clinical trials focus on the effect of the drugs on pain, others evaluate their efficacy by assessing general neuropathy. Moreover, based on studies conducted in rodent models, it remains unclear if some of the tested drugs act in an antinociceptive fashion or have neuroprotective properties. Thus, further investigations are needed to understand their mechanism of action, as well as a global standardization of the methods used to assess efficacy of new therapeutic strategies in the treatment of CIPN.

The counter regulatory axis of the renin angiotensin system in the brain and ischaemic stroke: Insight from preclinical stroke studies and therapeutic potential

Stroke is the 2nd leading cause of death worldwide and the leading cause of physical disability and cognitive issues. Although we have made progress in certain aspects of stroke treatment, the consequences remain substantial and new treatments are needed. Hypertension has long been recognised as a major risk factor for stroke, both haemorrhagic and ischaemic. The renin angiotensin system (RAS) plays a key role in blood pressure regulation and this, plus local expression and signalling of RAS in the brain, both support the potential for targeting this axis therapeutically in the setting of stroke. While historically, focus has been on suppressing classical RAS signalling through the angiotensin type 1 receptor (AT1R), the identification of a counter-regulatory axis of the RAS signalling via the angiotensin type 2 receptor (AT2R) and Mas receptor has renewed interest in targeting the RAS. This review describes RAS signalling in the brain and the potential of targeting the Mas receptor and AT2R in preclinical models of ischaemic stroke. The animal and experimental models, and the route and timing of intervention, are considered from a translational perspective.

Correcting the imbalanced protective RAS in COVID-19 with angiotensin AT2-receptor agonists

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that is responsible for the global corona virus disease 2019 (COVID-19) pandemic enters host cells via a mechanism that includes binding to angiotensin converting enzyme (ACE) 2 (ACE2). Membrane-bound ACE2 is depleted as a result of this entry mechanism. The consequence is that the protective renin-angiotensin system (RAS), of which ACE2 is an essential component, is compromised through lack of production of the protective peptides angiotensin-(1-7) and angiotensin-(1-9), and therefore decreased stimulation of Mas (receptor Mas) and angiotensin AT2-receptors (AT2Rs), while angiotensin AT1-receptors (AT1Rs) are overstimulated due to less degradation of angiotensin II (Ang II) by ACE2. The protective RAS has numerous beneficial actions, including anti-inflammatory, anti-coagulative, anti-fibrotic effects along with endothelial and neural protection; opposite to the deleterious effects caused by heightened stimulation of angiotensin AT1R. Given that patients with severe COVID-19 exhibit an excessive immune response, endothelial dysfunction, increased clotting, thromboses and stroke, enhancing the activity of the protective RAS is likely beneficial. In this article, we discuss the evidence for a dysfunctional protective RAS in COVID and develop a rationale that the protective RAS imbalance in COVID-19 may be corrected by using AT2R agonists. We further review preclinical studies with AT2R agonists which suggest that AT2R stimulation may be therapeutically effective to treat COVID-19-induced disorders of various organ systems such as lung, vasculature, or the brain. Finally, we provide information on the design of a clinical trial in which patients with COVID-19 were treated with the AT2R agonist Compound 21 (C21). This trial has been completed, but results have not yet been reported.

Emerging Role of Angiotensin AT2 Receptor in Anti-Inflammation: An Update

The hyperactive RAS and inflammation are closely associated. The angiotensin-II/AT1R axis of the RAS has been explored extensively for its role in inflammation and a plethora of pathological conditions. Understanding the role of AT2R in inflammation is an emerging area of research. The AT2R is expressed on a variety of immune and non-immune cells, which upon activation triggers the release of a host of cytokines and has multiple effects that coalesce to anti-inflammation and prevents maladaptive repair. The anti-inflammatory outcomes of AT2R activation are linked to its well-established signaling pathways involving formation of nitric oxide and activation of phosphatases. Collectively, these effects promote cell survival and tissue function. The consideration of AT2R as a therapeutic target requires further investigations.

Targeting the renin angiotensin system for the treatment of anxiety and depression

Emotional disorders like anxiety and depression are responsible for considerable morbidity and mortality all over the world. Several antidepressant and anxiolytic medications are available for the treatment of anxiety and depression. However, a significant number of patients either do not respond to these medications or respond inadequately. Hence, there is a need to identify novel targets for the treatment of anxiety and depression. In this review we focus on the renin angiotensin system (RAS) as a potential target for the treatment of these disorders. We review work that has evaluated the effects of various compounds targeting the RAS on anxiety- and depression-like behaviors. Further, we suggest future work that must be carried out to fully exploit the RAS for the treatment of anxiety and depression. The RAS provides an attractive target for both the identification of novel anxiolytic and antidepressant medications and/or for enhancing the efficacy of currently available medications used for the treatment of anxiety and depression.

The role of the brain renin-angiotensin system (RAS) in mild traumatic brain injury (TBI)

There is considerable interest in traumatic brain injury (TBI) induced by repeated concussions suffered by athletes in sports, military personnel from combat-and non-combat related activities, and civilian populations who suffer head injuries from accidents and domestic violence. Although the renin-angiotensin system (RAS) is primarily a systemic cardiovascular regulatory system that, when dysregulated, causes hypertension and cardiovascular pathology, the brain contains a local RAS that plays a critical role in the pathophysiology of several neurodegenerative diseases. This local RAS includes receptors for angiotensin (Ang) II within the brain parenchyma, as well as on circumventricular organs outside the blood-brain-barrier. The brain RAS acts primarily via the type 1 Ang II receptor (AT₁R), exacerbating insults and pathology. With TBI, the brain RAS may contribute to permanent brain damage, especially when a second TBI occurs before the brain recovers from an initial injury. Agents are needed that minimize the extent of injury from an acute TBI, reducing TBI-mediated permanent brain damage. This review discusses how activation of the brain RAS following TBI contributes to this damage, and how drugs that counteract activation of the AT₁R including AT₁R blockers (ARBs), renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, and agonists at type 2 Ang II receptors (AT₂) and at Ang (1-7) receptors (Mas) can potentially ameliorate TBI-induced brain damage.

Angiotensin AT1 and AT2 receptor heteromer expression in the hemilesioned rat model of Parkinson's disease that increases with levodopa-induced dyskinesia

BACKGROUND/AIMS

The renin-angiotensin system (RAS) is altered in Parkinson's disease (PD), a disease due to substantia nigra neurodegeneration and whose dopamine-replacement therapy, using the precursor levodopa, leads to dyskinesias as the main side effect. Angiotensin AT₁ and AT₂ receptors, mainly known for their role in regulating water homeostasis and blood pressure and able to form heterodimers (AT_1/2Hets), are present in the central nervous system. We assessed the functionality and expression of AT_1/2Hets in Parkinson disease (PD).

METHODS

Immunocytochemistry was used to analyze the colocalization between angiotensin receptors; bioluminescence resonance energy transfer was used to detect AT_1/2Hets. Calcium and cAMP determination, MAPK activation, and label-free assays were performed to characterize signaling in homologous and heterologous systems. Proximity ligation assays were used to quantify receptor expression in mouse primary cultures and in rat striatal sections.

RESULTS

We confirmed that AT₁ and AT₂ receptors form AT_1/2Hets that are expressed in cells of the central nervous system. AT_1/2Hets are novel functional units with particular signaling properties. Importantly, the coactivation of the two receptors in the heteromer reduces the signaling output of angiotensin. Remarkably, AT_1/2Hets that are expressed in both striatal neurons and microglia make possible that candesartan, the antagonist of AT₁, increases the effect of AT₂ receptor agonists. In addition, the level of striatal expression increased in the unilateral 6-OH-dopamine lesioned rat PD model and was markedly higher in parkinsonian-like animals that did not become dyskinetic upon levodopa chronic administration if compared with expression in those that became dyskinetic.

CONCLUSION

The results indicate that boosting the action of neuroprotective AT₂ receptors using an AT₁ receptor antagonist constitutes a promising therapeutic strategy in PD.

The Brain AT2R-a Potential Target for Therapy in Alzheimer's Disease and Vascular Cognitive Impairment: a Comprehensive Review of Clinical and Experimental Therapeutics

Dementia is a potentially avertable tragedy, currently considered among the top 10 greatest global health challenges of the twenty-first century. Dementia not only robs individuals of their dignity and independence, it also has a ripple effect that starts with the inflicted individual's family and projects to the society as a whole. The constantly growing number of cases, along with the lack of effective treatments and socioeconomic impact, poses a serious threat to the sustainability of our health care system. Hence, there is a worldwide effort to identify new targets for the treatment of Alzheimer's disease (AD), the leading cause of dementia. Due to its multifactorial etiology and the recent clinical failure of several novel amyloid-β (Aβ) targeting therapies, a comprehensive "multitarget" approach may be most appropriate for managing this condition. Interestingly, renin angiotensin system (RAS) modulators were shown to positively impact all the factors involved in the pathophysiology of dementia including vascular dysfunction, Aβ accumulation, and associated cholinergic deficiency, in addition to tau hyperphosphorylation and insulin derangements. Furthermore, for many of these drugs, the preclinical evidence is also supported by epidemiological data and/or preliminary clinical trials. The purpose of this review is to provide a comprehensive update on the major causes of dementia including the risk factors, current diagnostic criteria, pathophysiology, and contemporary treatment strategies. Moreover, we highlight the angiotensin II receptor type 2 (AT2R) as an effective drug target and present ample evidence supporting its potential role and clinical applications in cognitive impairment to encourage further investigation in the clinical setting.

Brain Angiotensin Type-1 and Type-2 Receptors in Physiological and Hypertensive Conditions: Focus on Neuroinflammation

PURPOSE OF REVIEW

To review recent data that suggest opposing effects of brain angiotensin type-1 (AT1R) and type-2 (AT2R) receptors on blood pressure (BP). Here, we discuss recent studies that suggest pro-hypertensive and pro-inflammatory actions of AT1R and anti-hypertensive and anti-inflammatory actions of AT2R. Further, we propose mechanisms for the interplay between brain angiotensin receptors and neuroinflammation in hypertension.

RECENT FINDINGS

The renin-angiotensin system (RAS) plays an important role in regulating cardiovascular physiology. This includes brain AT1R and AT2R, both of which are expressed in or adjacent to brain regions that control BP. Activation of AT1R within those brain regions mediate increases in BP and cause neuroinflammation, which augments the BP increase in hypertension. The fact that AT1R and AT2R have opposing actions on BP suggests that AT1R and AT2R may have similar opposing actions on neuroinflammation. However, the mechanisms by which brain AT1R and AT2R mediate neuroinflammatory responses remain unclear. The interplay between brain angiotensin receptor subtypes and neuroinflammation exacerbates or protects against hypertension.

Effect of renin-angiotensin system on senescence

The renin-angiotensin system (RAS) plays crucial roles in the control of blood pressure and sodium homeostasis. Moreover, RAS also acts as a key player in cell and organ senescence, mainly by activation of the classical axis of angiotensin (Ang) converting enzyme (ACE)/Ang II/Ang II type 1 receptor via overproduction of reactive oxygen species. Overactivation of the classical RAS axis induces organ dysfunction in the vasculature, brain, kidney and skeletal muscle, resulting in atherosclerosis, stroke, chronic kidney disease and sarcopenia. Moreover, RAS has been shown to regulate lifespan, using gene-modification models. Recently, mice lacking the Ang II type 1 receptor were shown to exhibit an increase in lifespan compared with control mice. Here, the effect of RAS on age-related tissue dysfunction in several organs is reviewed, including not only the classical axis but also protective functions of RAS such as the ACE2/Ang (1-7)/Mas axis. Geriatr Gerontol Int 2020; ••: ••-••.

Deterioration of cognitive function after transient cerebral ischemia with amyloid-β infusion-possible amelioration of cognitive function by AT2 receptor activation

Background

To promote understanding of the pathogenesis of cognitive impairment or dementia, we explored the potential interaction between transient cerebral ischemia and amyloid-β (Aβ) infusion in mediating cognitive decline and examined the possible ameliorative effect of angiotensin II type 2 (AT₂) receptor activation in vascular smooth muscle cells (VSMC) on this cognitive deficit.

Methods

Adult male wild-type mice (WT) and mice with VSMC-specific AT₂ receptor overexpression (smAT₂) were subjected to intracerebroventricular (ICV) injection of Aβ1-40. Transient cerebral ischemia was induced by 15 min of bilateral common carotid artery occlusion (BCCAO) 24 h after Aβ injection.

Results

Aβ injection in WT induced a cognitive decline, whereas BCCAO did not cause a significant cognitive deficit. In contrast, WT with BCCAO following Aβ injection exhibited more marked cognitive decline compared to Aβ injection alone, in concert with increases in superoxide anion production, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity, and expression of p22phox, p40phox, monocyte chemoattractant protein (MCP)-1 and interleukin (IL)-1β in the hippocampus, and upregulation of RAGE (receptor for advanced glycation end product), an Aβ transporter. BCCAO following Aβ injection further enhanced neuronal pyknosis in the hippocampus, compared with BCCAO or Aβ injection alone. In contrast, smAT₂ did not show a cognitive decline, increase in oxidative stress, inflammation, and RAGE level or neuronal pyknosis, which were induced by BCCAO with/without Aβ injection in WT.

Conclusions

Transient cerebral ischemia might worsen Aβ infusion-mediated cognitive decline and vice versa, with possible involvement of amplified oxidative stress and inflammation and impairment of the RAGE-mediated Aβ clearance system, contributing to exaggerated neuronal degeneration. AT₂ receptor activation in VSMC could play an inhibitory role in this cognitive deficit.

The Other Angiotensin II Receptor: AT2R as a Therapeutic Target

The active hormone of the renin-angiotensin system (RAS), angiotensin II (Ang II), is involved in several human diseases, driving the development and clinical use of several therapeutic drugs, mostly angiotensin I converting enzyme (ACE) inhibitors and angiotensin receptor type I (AT₁R) antagonists. However, angiotensin peptides can also bind to receptors different from AT₁R, in particular, angiotensin receptor type II (AT₂R), resulting in biological and physiological effects different, and sometimes antagonistic, of their binding to AT₁R. In the present Perspective, the components of the RAS and the therapeutic tools developed to control it will be reviewed. In particular, the characteristics of AT₂R and tools to modulate its functions will be discussed. Agonists or antagonists to AT₂R are potential therapeutics in cardiovascular diseases, for agonists, and in the control of pain, for antagonists, respectively. However, controlling their binding properties and their targeting to the target tissues must be optimized.

Depression as an Immunometabolic Disorder: Exploring Shared Pharmacotherapeutics with Cardiovascular Disease

Modern times have seen depression and cardiovascular disease (CVD) become notorious public health concerns, corresponding to alarming proportions of morbidity, mortality, decreased quality of life, and economic costs. Expanding comprehension of the pathogenesis of depression as an immunometabolic disorder has identified numerous pathophysiologic phenomena in common with CVD, including chronic inflammation, insulin resistance, and oxidative stress. These shared components could be exploited to offer improved alternatives in the joint management of these conditions. Abundant preclinical and clinical data on the impact of established treatments for CVD in the management of depression have allowed for potential candidates to be proposed for the joint management of depression and CVD as immunometabolic disorders. However, a large proportion of the clinical investigation currently available exhibits marked methodological flaws which preclude the formulation of concrete recommendations in many cases. This situation may be a reflection of pervasive problems present in clinical research in psychiatry, especially pertaining to study homogeneity. Therefore, further high-quality research is essential in the future in this regard.

Angiotensin II Type 2 Receptor-Expressing Neurons in the Central Amygdala Influence Fear-Related Behavior

BACKGROUND

The renin-angiotensin system has been implicated in posttraumatic stress disorder; however, the mechanisms responsible for this connection and the therapeutic potential of targeting the renin-angiotensin system in posttraumatic stress disorder remain unknown. Using an angiotensin receptor bacterial artificial chromosome (BAC) and enhanced green fluorescent protein (eGFP) reporter mouse, combined with neuroanatomical, pharmacological, and behavioral approaches, we examined the role of angiotensin II type 2 receptor (AT₂R) in fear-related behavior.

METHODS

Dual immunohistochemistry with retrograde labeling was used to characterize AT₂R-eGFP⁺ cells in the amygdala of the AT₂R-eGFP-BAC reporter mouse. Pavlovian fear conditioning and behavioral pharmacological analyses were used to demonstrate the effects of AT₂R activation on fear memory in male C57BL/6 mice.

RESULTS

AT₂R-eGFP⁺ neurons in the amygdala were predominantly expressed in the medial amygdala and the medial division of the central amygdala (CeM), with little AT₂R-eGFP expression in the basolateral amygdala or lateral division of the central amygdala. Characterization of AT₂R-eGFP⁺ neurons in the CeM demonstrated distinct localization to gamma-aminobutyric acidergic projection neurons. Mice receiving acute intra-central amygdala injections of the selective AT₂R agonist compound 21 prior to tests for cued or contextual fear expression displayed less freezing. Retrograde labeling of AT₂R-eGFP⁺ neurons projecting to the periaqueductal gray revealed AT₂R-eGFP⁺ neuronal projections from the CeM to the periaqueductal gray, a key brain structure mediating fear-related freezing.

CONCLUSIONS

These findings suggest that CeM AT₂R-expressing neurons can modulate central amygdala outputs that play a role in fear expression, providing new evidence for a novel angiotensinergic circuit in the regulation of fear.

Dose-response, therapeutic time-window and tPA-combinatorial efficacy of compound 21: A randomized, blinded preclinical trial in a rat model of thromboembolic stroke

The aim of this translational, randomized, controlled, blinded preclinical trial was to determine the effect of compound 21 (C21) in embolic stroke. Rats were subjected to embolic-middle cerebral artery occlusion (eMCAO). They received C21 (0.01, 0.03 and 0.06 mg/kg/d) or saline (orally) for five days, with the first-dose given IV at 3 h post-eMCAO. For the time-window study, the optimal-dose of C21 was initiated at 3, 6 or 24 h post-eMCAO and continued for five days. For the combinatorial study, animals received IV-tissue plasminogen activator (tPA) at either 2 or 4 h, with IV-C21 (0.01 mg/kg) or saline at 3 h post-eMCAO and daily thereafter for five days. After performing the behavior tests, brains were collected for analyses. The dose-response study showed significant motor improvements with the lowest-dose (0.01 mg/kg) of C21. In the time-window study, this same dose resulted in improvements when given 6 h and 24 h post-eMCAO. Moreover, C21-treated animals performed better on the novel object recognition test. Neither the single treatment with C21 or tPA (4 h) nor the combination therapy was effective in reducing the hemorrhage or infarct size, although C21 alone lowered sensorimotor deficit scores post-eMCAO. Future studies should focus on the long-term cognitive benefits of C21, rather than acute neuroprotection.

Angiotensin II type 2 receptor stimulation with compound 21 improves neurological function after stroke in female rats: a pilot study

The angiotensin II type 2 receptor (AT2R) agonist, compound 21 (C21), has been shown to be neurovascularly protective after ischemic stroke in male rats. In the current study, we aim to study the impact of C21 treatment on female rats. Young female Wistar rats were subjected to different durations of middle cerebral artery occlusion (MCAO) (3 h, 2 h, and 1 h) using a silicone-coated monofilament, treated at reperfusion with 0.03 mg/kg ip of C21 and followed up for different times (1, 3, and 14 days) after stroke. Behavioral tests were performed (Bederson, paw grasp, beam walk, and rotarod), and animals were euthanized for infarct size analysis and Western blot analysis. In vitro, primary male and female brain microvascular endothelial cells (ECs) were grown in culture, and the expression of the AT2R was compared between males and females. At 1 day, C21 treatment resulted in an improvement in Bederson scores. However, at 3 days and 14 days, the impact of C21 on stroke outcomes was less robust. In vitro, the expression of the AT2R was significantly higher in female ECs compared with male ECs. In conclusion, C21 improves Bederson scores after stroke in female rats when administered early at reperfusion. The ability of C21 to exert its neuroprotective effects might be affected by fluctuating levels of female hormones. NEW & NOTEWORTHY The present study shows the neuroprotective impact of C21 on ischemic stroke in female rats and how the protective effects of C21 can be influenced by the hormonal status of female rodents.

Neuroprotective effect of angiotensin II type 2 receptor stimulation in vincristine-induced mechanical allodynia

Peripheral neuropathy is the major dose-limiting side effect of many currently used chemotherapies, such as vincristine (VCR). We recently demonstrated that candesartan, an angiotensin II type 1 receptor antagonist, was neuroprotective against resiniferatoxin-induced sensory neuropathy, and that this effect is mediated by stimulation of the angiotensin II type 2 receptor (AT2R). Thus, we evaluated the effect of preventive treatment with candesartan and a specific AT2R agonist, C21, on a mouse model of VCR-induced neuropathy. Vincristine was administered daily for 7 days to male Swiss mice. Treatment with candesartan and C21 was started on day 1, before VCR treatment, and continued until day 7. We evaluated the development of VCR-induced neuropathy and the effect of treatment by functional tests, immunohistochemical analyses of intraepidermal nerve fibers and dorsal root ganglia neurons, and ultrastructural analysis of the sciatic nerve. Mice treated with VCR showed high mechanical allodynia but no modifications of motor performance or mechanical/thermal nociception. Treatment with candesartan and C21 completely restored normal tactile sensitivity of VCR-treated mice. Both drugs prevented VCR-induced nonpeptidergic intraepidermal nerve fiber loss. Only C21 displayed neuroprotective effects against VCR-induced loss and enlargement of myelinated nerve fibers in the sciatic nerve. Our finding that candesartan and C21 are protective against VCR-induced neuropathic pain through AT2R stimulation favors evaluation of its therapeutic potential in patients receiving chemotherapy.

Fiend and friend in the renin angiotensin system: An insight on acute kidney injury

Besides assisting the maintenance of blood pressure and sodium homeostasis, the renin-angiotensin system (RAS) plays a pivotal role in pathogenesis of acute kidney injury (AKI). The RAS is equipped with two arms i) the pressor arm composed of Angiotensin II (Ang II)/Angiotensin converting enzyme (ACE)/Angiotensin II type 1 receptor (AT1R) also called conventional RAS, and ii) the depressor arm consisting of Angiotensin (1-7) (Ang 1-7)/Angiotensin converting enzyme 2 (ACE2)/MasR known as non-conventional RAS. Activation of conventional RAS triggers oxidative stress, inflammatory, hypertrophic, apoptotic, and pro-fibrotic signaling cascades which promote AKI. The preclinical and clinical studies have reported beneficial as well as deleterious effects of RAS blockage either by angiotensin receptor blocker or ACE inhibitor in AKI. On the contrary, the depressor arm opposes the conventional RAS, has beneficial effects on the kidney but has been less explored in pathogenesis of AKI. This review focuses on significance of RAS in pathogenesis of AKI and provides better understanding of novel and possible therapeutic approaches to combat AKI.

Cell-Specific Protective Signaling Induced by the Novel AT2R-Agonist NP-6A4 on Human Endothelial and Smooth Muscle Cells

Cardiovascular disease incidence continues to rise and new treatment paradigms are warranted. We reported previously that activation of Angiotensin II receptor (encoded by the X-linked Agtr2 gene) by a new peptide agonist, NP-6A4, was more effective in protecting mouse cardiomyocyte HL-1 cells and human coronary artery vascular smooth muscle cells (hCAVSMCs) from acute nutrient deficiency than other drugs tested. To elucidate further the protective effects of NP-6A4 in human cells, we studied the effects of NP-6A4 treatment on functions of human coronary artery endothelial cells (hCAECs), and hCAVSMCs. In hCAVSMCs, NP-6A4 (1 μM) increased Agtr2 mRNA (sixfold, p < 0.05) after 12-h exposure, whereas in hCAECs, significant increase in Agtr2 mRNA (hCAECs: eightfold) was observed after prolonged exposure. Interestingly, NP-6A4 treatment (1 μM, 12 h) increased AT2R protein levels in all human cells tested. Pre-treatment with AT2R-antagonist PD123319 (20 μM) and anti-AT2R siRNA (1 μM) suppressed this effect. Thus, NP-6A4 activates a positive feedback loop for AT2R expression and signaling in hCAVSMCs and hCAECs. NP-6A4 (1–20 μM) increased cell index (CI) of hCAVSMCs as determined by real time cell analyzer (RTCA), indicating that high concentrations of NP-6A4 were not cytotoxic for hCAVSMCs, rather promoting better cell attachment and growth. Seahorse Extracellular Flux Assay revealed that NP-6A4 (1 μM) treatment for 7 days increased whole cell-based mitochondrial parameters of hCAVSMCs, specifically maximal respiration (p < 0.05), spare respiratory capacity (p < 0.05) and ATP production (p < 0.05). NP-6A4 (1 μM; 7 days) also suppressed Reactive Oxygen Species (ROS) in hCAVSMCs. Exposure to Doxorubicin (DOXO) (1 μM) increased ROS in hCAVSMCs and this effect was suppressed by NP-6A4 (1 μM). In hCAECs grown in complete medium, NP-6A4 (1 μM) and Ang II (1 μM) exerted similar changes in CI. Additionally, NP-6A4 (5 μM: 12 h) increased expression of eNOS (sixfold, p < 0.05) and generation of nitric oxide (1.3-fold, p < 0.05) in hCAECs and pre-treatment with PD123319 (20 μM) suppressed this effect partially (65%). Finally, NP-6A4 decreased phosphorylation of Jun-N-terminal kinase, implicated in apoptosis of ECs in atherosclerotic sites. Taken together, NP-6A4, through its ability to increase AT2R expression and signaling, exerts different cell-specific protective effects in human VSMCs and ECs.

Neuroprotection via AT2 receptor agonists in ischemic stroke

Stroke is a devastating disease that afflicts millions of people each year worldwide. Ischemic stroke, which accounts for ~88% of cases, occurs when blood supply to the brain is decreased, often because of thromboembolism or atherosclerotic occlusion. This deprives the brain of oxygen and nutrients, causing immediate, irreversible necrosis within the core of the ischemic area, but more delayed and potentially reversible neuronal damage in the surrounding brain tissue, the penumbra. The only currently approved therapies for ischemic stroke, the thrombolytic agent recombinant tissue plasminogen activator (rtPA) and the endovascular clot retrieval/destruction processes, are aimed at restoring blood flow to the infarcted area, but are only available for a minority of patients and are not able in most cases to completely restore neurological deficits. Consequently, there remains a need for agents that will protect neurones against death following ischemic stroke. Here, we evaluate angiotensin II (Ang II) type 2 (AT2) receptor agonists as a possible therapeutic target for this disease. We first provide an overview of stroke epidemiology, pathophysiology, and currently approved therapies. We next review the large amount of preclinical evidence, accumulated over the past decade and a half, which indicates that AT2 receptor agonists exert significant neuroprotective effects in various animal models, and discuss the potential mechanisms involved. Finally, after discussing the challenges of delivering blood–brain barrier (BBB) impermeable AT2 receptor agonists to the infarcted areas of the brain, we summarize the evidence for and against the development of these agents as a promising therapeutic strategy for ischemic stroke.

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

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

Within the Brain: The Renin Angiotensin System

For many years, modulators of the renin angiotensin system (RAS) have been trusted by clinicians for the control of essential hypertension. It was recently demonstrated that these modulators have other pleiotropic properties independent of their hypotensive effects, such as enhancement of cognition. Within the brain, different components of the RAS have been extensively studied in the context of neuroprotection and cognition. Interestingly, a crosstalk between the RAS and other systems such as cholinergic, dopaminergic and adrenergic systems have been demonstrated. In this review, the preclinical and clinical evidence for the impact of RAS modulators on cognitive impairment of multiple etiologies will be discussed. In addition, the expression and function of different receptor subtypes within the RAS such as: Angiotensin II type I receptor (AT1R), Angiotensin II type II receptor (AT2R), Angiotensin IV receptor (AT4R), Mas receptor (MasR), and Mas-related-G protein-coupled receptor (MrgD), on different cell types within the brain will be presented. We aim to direct the attention of the scientific community to the plethora of evidence on the importance of the RAS on cognition and to the different disease conditions in which these agents can be beneficial.

Protective effects of the angiotensin II AT2 receptor agonist compound 21 in ischemic stroke: a nose-to-brain delivery approach

Significant neuroprotective effects of angiotensin II type 2 (AT₂) receptor (AT₂ receptor) agonists in ischemic stroke have been previously demonstrated in multiple studies. However, the routes of agonist application used in these pre-clinical studies, direct intracerebroventricular (ICV) and systemic administration, are unsuitable for translation into humans; in the latter case because AT₂ receptor agonists are blood-brain barrier (BBB) impermeable. To circumvent this problem, in the current study we utilized the nose-to-brain (N2B) route of administration to bypass the BBB and deliver the selective AT₂ receptor agonist Compound 21 (C21) to naïve rats or rats that had undergone endothelin 1 (ET-1)-induced ischemic stroke. The results obtained from the present study indicated that C21 applied N2B entered the cerebral cortex and striatum within 30 min in amounts that are therapeutically relevant (8.4-9 nM), regardless of whether BBB was intact or disintegrated. C21 was first applied N2B at 1.5 h after stroke indeed provided neuroprotection, as evidenced by a highly significant, 57% reduction in cerebral infarct size and significant improvements in Bederson and Garcia neurological scores. N2B-administered C21 did not affect blood pressure or heart rate. Thus, these data provide proof-of-principle for the idea that N2B application of an AT₂ receptor agonist can exert neuroprotective actions when administered following ischemic stroke. Since N2B delivery of other agents has been shown to be effective in certain human central nervous system diseases, the N2B application of AT₂ receptor agonists may become a viable mode of delivering these neuroprotective agents for human ischemic stroke patients.

Angiotensin II AT2 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

Contractile Responses in Spontaneously Hypertensive Rats after Transient Middle Cerebral Artery Occlusion

Stroke is one of the leading causes of mortality and morbidity worldwide, and few therapeutic treatments have shown beneficial effect clinically. One reason for this could be the lack of risk factors incorporated into the preclinical stroke research. We have previously demonstrated phenotypic receptor changes to be one of the injurious mechanisms occurring after stroke but mostly in healthy rats. The aim of this study was to investigate if hypertension has an effect on vasoconstrictive receptor responses to endothelin 1, sarafotoxin 6c and angiotensin II after stroke by inducing transient middle cerebral artery occlusion in spontaneously hypertensive rats and Wistar-Kyoto rats using the wire-myograph. We demonstrated an increased contractile response to endothelin 1 and extracellular potassium as well as an increased carbachol-induced dilator response in the middle cerebral arteries from hypertensive rats after stroke. This study demonstrates the importance of including risk factors in experimental stroke research.

The renin-angiotensin system: a possible new target for depression

Depression remains a debilitating condition with an uncertain aetiology. Recently, attention has been given to the renin-angiotensin system. In the central nervous system, angiotensin II may be important in multiple pathways related to neurodevelopment and regulation of the stress response. Studies of drugs targeting the renin-angiotensin system have yielded promising results. Here, we review the potential beneficial effects of angiotensin blockers in depression and their mechanisms of action. Drugs blocking the angiotensin system have efficacy in several animal models of depression. While no randomised clinical trials were found, case reports and observational studies showed that angiotensin-converting enzyme inhibitors or angiotensin receptor blockers had positive effects on depression, whereas other antihypertensive agents did not. Drugs targeting the renin-angiotensin system act on inflammatory pathways implicated in depression. Both preclinical and clinical data suggest that these drugs possess antidepressant properties. In light of these results, angiotensin system-blocking agents offer new horizons in mood disorder treatment.

Post-stroke angiotensin II type 2 receptor activation provides long-term neuroprotection in aged rats

Activation of the angiotensin II type 2 receptor (AT2R) by administration of Compound 21 (C21), a selective AT2R agonist, induces neuroprotection in models of ischemic stroke in young adult animals. The mechanisms of this neuroprotective action are varied, and may include direct and indirect effects of AT2R activation. Our objectives were to assess the long-term protective effects of post-stroke C21 treatments in a clinically-relevant model of stroke in aged rats and to characterize the cellular localization of AT2Rs in the mouse brain of transgenic reporter mice following stroke. Intraperitoneal injections of C21 (0.03mg/kg) after ischemic stroke induced by transient monofilament middle cerebral artery occlusion resulted in protective effects that were sustained for up to at least 3-weeks post-stroke. These included improved neurological function across multiple assessments and a significant reduction in infarct volume as assessed by magnetic resonance imaging. We also found AT2R expression to be on neurons, not astrocytes or microglia, in normal female and male mouse brains. Stroke did not induce altered cellular localization of AT2R when assessed at 7 and 14 days post-stroke. These findings demonstrate that the neuroprotection previously characterized only during earlier time points using stroke models in young animals is sustained long-term in aged rats, implying even greater clinical relevance for the study of AT2R agonists for the acute treatment of ischemic stroke in human disease. Further, it appears that this sustained neuroprotection is likely due to a mix of both direct and indirect effects stemming from selective activation of AT2Rs on neurons or other cells besides astrocytes and microglia.

Compound 21 and Telmisartan combination mitigates type 2 diabetic nephropathy through amelioration of caspase mediated apoptosis

The current study aimed to understand the role of novel, highly selective, orally active, non-peptide Angiotensin II type 2 receptor (AT2R) agonist, Compound 21 and its potential additive effect with Telmisartan on apoptosis and underlying posttranslational modifications in a non-genetic murine model for type 2 diabetic nephropathy (T2DN). An experimental model for T2DN was developed by administering low dose Streptozotocin in high fat diet fed male Wistar rats, followed by their treatment with Telmisartan, C21 or their combination. Our results demonstrated that C21 and Telmisartan combination attenuated metabolic and renal dysfunction, renal morphological and micro-architectural aberrations and hemodynamic disturbances in type 2 diabetic rats. The anti-apoptotic and anti-inflammatory effects of Telmisartan were significantly accentuated by C21 indicated by expression of apoptotic markers (Parp1, Caspase 8, Caspase 7, cleaved PARP and cleaved Caspase 3) and NF-κB mediated inflammatory molecules like interleukin 6, tumour necrosis factor alpha; monocyte chemoattractant protein 1 and vascular cell adhesion molecule 1. C21 was found to improve Telmisartan mediated reversal of histone H3 acetylation at lysine 14 and 27 and expression of histone acetyl transferase, p300/CBP-associated factor also known to regulate NF-κB activity and DNA damage response. C21 in combination with Telmisartan markedly mitigates caspase mediated apoptosis and NF-κB signalling in T2D kidney, which could be partially attributed to its influence on PCAF mediated histone H3 acetylation. Hence further research should be done to develop this combination to treat T2DN.

Angiotensin II Type 2 Receptor Activation With Compound 21 Augments Islet Function and Regeneration in Streptozotocin-Induced Neonatal Rats and Human Pancreatic Progenitor Cells

OBJECTIVES

We investigated the effects of compound 21 (C21), a nonpeptide angiotensin II type 2 receptor agonist, on islet cell function and survival in streptozotocin (STZ)-treated neonatal rats and human pancreatic progenitor cells.

METHODS

Neonatal rats were randomized into 5 groups, including a control, an STZ (100 mg/kg, intraperitoneally), and 3 STZ + C21 (0.25, 0.5, and 1 mg/kg per day for 7 days, intraperitoneally) groups. Body weight and blood glucose were monitored daily. On the last experimental day, serum insulin levels and glucose tolerance were assessed, and the rat pups' pancreata were extracted for examination of islet cell function/mass and involvement of signaling pathways.

RESULTS

The C21-treated STZ rats, particularly in the 0.5- and 1 mg/kg-dosage groups, had significantly decreased blood glucose, increased serum insulin concentrations, higher glucose-stimulated insulin secretion activity, and greater islet-cell mass and up-regulated expression of insulin and Ngn3 in the pancreas than did the control groups; these rats also demonstrated increased β-cell proliferation, lower superoxide levels and enhanced SOD1 expression, and up-regulated phospho-AKT expression; consistently, similar results were also observed in human pancreatic progenitor cells.

CONCLUSIONS

These data suggest that C21 has a beneficial effect on islet cell function and regeneration, probably via proliferative and antioxidative pathways.

Role of interleukin-10 in the neuroprotective effect of the Angiotensin Type 2 Receptor agonist, compound 21, after ischemia/reperfusion injury

INTRODUCTION

We and others have shown that the angiotensin type 2 (AT2) receptor agonist, compound 21 (C21), provides neuroprotection and enhances recovery in rodent stroke models yet the mechanism involved is not known. Moreover, C21 treatment is associated with an anti-inflammatory response. Here we tested the hypothesis that C21 mediates neuroprotection by upregulating the neuroprotective and anti-inflammatory cytokine, interleukin (IL)-10.

METHODS

Wistar rats were subjected to 3h-middle cerebral artery suture occlusion and treated at reperfusion with C21 (0.03mg/kg)±IL-10 neutralizing antibody (0.1mg/kg) both given i.p. Infarct size, behavioral outcomes, and molecular analysis were performed at 24h post-injury. Primary rat neurons were used to test the direct neuroprotective effect of C21 in vitro.

RESULTS

C21 treatment reduced infarct size, improved functional outcome and decreased the pro-inflammatory cytokine, tumor necrosis factor alpha (TNF-α) in the ischemic hemisphere compared to saline. Anti-IL-10 co-treatment blocked the C21-induced reduction in infarct size and inflammation, and the improvement in behavioral outcome. In vitro, C21 treatment increased neuron survival and reduced cell apoptosis after oxygen glucose deprivation (OGD) and OGD/reoxygenation. These effects were mediated through AT2R stimulation.

CONCLUSION

C21 provides direct neuroprotection as well as indirect protection through IL-10.

Use of Antihypertensive Drugs and Ischemic Stroke Severity - Is There a Role for Angiotensin-II?

Background

The increase in angiotensin II (Ang II) formation by selected antihypertensive drugs is said to exhibit neuroprotective properties, but this translation into improvement in clinical outcomes has been inconclusive. We undertook a study to investigate the relationship between types of antihypertensive drugs used prior to a stroke event and ischemic stroke severity. We hypothesized that use of antihypertensive drugs that increase Ang II formation (Ang II increasers) would reduce ischemic stroke severity when compared to antihypertensive drugs that suppress Ang II formation (Ang II suppressors).

Methods

From the Malaysian National Neurology Registry, we included hypertensive patients with first ischemic stroke who presented within 48 hours from ictus. Antihypertensive drugs were divided into Ang II increasers (angiotensin-I receptor blockers (ARBs), calcium channel blockers (CCBs) and diuretics) and Ang II suppressors (angiotensin-converting-enzyme inhibitors (ACEIs) and beta blockers). We evaluated stroke severity during admission with the National Institute of Health Stroke Scale (NIHSS). We performed a multivariable logistic regression with the score being dichotomized at 15. Scores of less than 15 were categorized as less severe stroke.

Results

A total of 710 patients were included. ACEIs was the most commonly prescribed antihypertensive drug in patients using Ang II suppressors (74%) and CCBs, in patients prescribed with Ang II increasers at 77%. There was no significant difference in the severity of ischemic stroke between patients who were using Ang II increasers in comparison to patients with Ang II suppressors (OR: 1.32, 95%CI: 0.83–2.10, p = 0.24).

Conclusion

In our study, we found that use of antihypertensive drugs that increase Ang II formation was not associated with less severe ischemic stroke as compared to use of antihypertensive drugs that suppress Ang II formation.

Therapeutic potential of the renin angiotensin system in ischaemic stroke

The renin angiotensin system (RAS) consists of the systemic hormone system, critically involved in regulation and homeostasis of normal physiological functions [i.e. blood pressure (BP), blood volume regulation], and an independent brain RAS, which is involved in the regulation of many functions such as memory, central control of BP and metabolic functions. In general terms, the RAS consists of two opposing axes; the ‘classical axis’ mediated primarily by Angiotensin II (Ang II), and the ‘alternative axis’ mediated mainly by Angiotensin-(1–7) (Ang-(1–7)). An imbalance of these two opposing axes is thought to exist between genders and is thought to contribute to the pathology of cardiovascular conditions such as hypertension, a stroke co-morbidity. Ischaemic stroke pathophysiology has been shown to be influenced by components of the RAS with specific RAS receptor antagonists and agonists improving outcome in experimental models of stroke. Manipulation of the two opposing axes following acute ischaemic stroke may provide an opportunity for protection of the neurovascular unit, particularly in the presence of pre-existing co-morbidities where the balance may be shifted. In the present review we will give an overview of the experimental stroke studies that have investigated pharmacological interventions of the RAS.

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.