Douglas M. Bennion

The angiotensin type 2 receptor agonist Compound 21 elicits cerebroprotection in endothelin-1 induced ischemic stroke

Evidence indicates that angiotensin II type 2 receptors (AT2R) exert cerebroprotective actions during stroke. A selective non-peptide AT2R agonist, Compound 21 (C21), has been shown to exert beneficial effects in models of cardiac and renal disease, as well as hemorrhagic stroke. Here, we hypothesize that C21 may exert beneficial effects against cerebral damage and neurological deficits produced by ischemic stroke. We determined the effects of central and peripheral administration of C21 on the cerebral damage and neurological deficits in rats elicited by endothelin-1 induced middle cerebral artery occlusion (MCAO), a model of cerebral ischemia. Rats infused centrally (intracerebroventricular) with C21 before endothelin-1 induced MCAO exhibited significant reductions in cerebral infarct size and the neurological deficits produced by cerebral ischemia. Similar cerebroprotection was obtained in rats injected systemically (intraperitoneal) with C21 either before or after endothelin-1 induced MCAO. The protective effects of C21 were reversed by central administration of an AT2R inhibitor, PD123319. While C21 did not alter cerebral blood flow at the doses used here, peripheral post-stroke administration of this agent significantly attenuated the MCAO-induced increases in inducible nitric oxide synthase, chemokine (C-C) motif ligand 2 and C-C chemokine receptor type 2 mRNAs in the cerebral cortex, indicating that the cerebroprotective action is associated with an anti-inflammatory effect. These results strengthen the view that AT2R agonists may have potential therapeutic value in ischemic stroke, and provide the first evidence of cerebroprotection induced by systemic post stroke administration of a selective AT2R agonist.

Activation of the Neuroprotective Angiotensin-Converting Enzyme 2 in Rat Ischemic Stroke

The angiotensin-converting enzyme 2/angiotensin-(1–7)/Mas axis represents a promising target for inducing stroke neuroprotection. Here, we explored stroke-induced changes in expression and activity of endogenous angiotensin-converting enzyme 2 and other system components in Sprague–Dawley rats. To evaluate the clinical feasibility of treatments that target this axis and that may act in synergy with stroke-induced changes, we also tested the neuroprotective effects of diminazene aceturate, an angiotensin-converting enzyme 2 activator, administered systemically post stroke. Among rats that underwent experimental endothelin-1–induced ischemic stroke, angiotensin-converting enzyme 2 activity in the cerebral cortex and striatum increased in the 24 hours after stroke. Serum angiotensin-converting enzyme 2 activity was decreased within 4 hours post stroke, but rebounded to reach higher than baseline levels 3 days post stroke. Treatment after stroke with systemically applied diminazene resulted in decreased infarct volume and improved neurological function without apparent increases in cerebral blood flow. Central infusion of A-779, a Mas receptor antagonist, resulted in larger infarct volumes in diminazene-treated rats, and central infusion of the angiotensin-converting enzyme 2 inhibitor MLN-4760 alone worsened neurological function. The dynamic alterations of the protective angiotensin-converting enzyme 2 pathway after stroke suggest that it may be a favorable therapeutic target. Indeed, significant neuroprotection resulted from poststroke angiotensin-converting enzyme 2 activation, likely via Mas signaling in a blood flow–independent manner. Our findings suggest that stroke therapeutics that target the angiotensin-converting enzyme 2/angiotensin-(1–7)/Mas axis may interact cooperatively with endogenous stroke-induced changes, lending promise to their further study as neuroprotective agents.

Neuroprotective Mechanisms of the ACE2–Angiotensin-(1-7)–Mas Axis in Stroke

The discovery of beneficial neuroprotective effects of the angiotensin converting enzyme 2–angiotensin-(1-7)–Mas axis [ACE2–Ang-(1-7)–Mas] in ischemic and hemorrhagic stroke has spurred interest in a more complete characterization of its mechanisms of action. Here, we summarize findings that describe the protective role of the ACE2–Ang-(1-7)–Mas axis in stroke, along with a focused discussion on the potential mechanisms of neuroprotective effects of Ang-(1-7) in stroke. The latter incorporates evidence describing the actions of Ang-(1-7) to counter the deleterious effects of angiotensin II (AngII) via its type 1 receptor, including anti-inflammatory, anti-oxidant, vasodilatory, and angiogenic effects, and the role of altered kinase–phosphatase signaling. Interactions of Mas with other receptors, including bradykinin receptors and AngII type 2 receptors are also considered. A more complete understanding of the mechanisms of action of Ang-(1-7) to elicit neuroprotection will serve as an essential step toward research into potential targeted therapeutics in the clinical setting.

Serum activity of angiotensin converting enzyme 2 is decreased in patients with acute ischemic stroke.

Levels of angiotensin converting enzyme 2 (ACE2), a cardio and neuro-protective carboxypeptidase, are dynamically altered after stroke in preclinical models. We sought to characterize the previously unexplored changes in serum ACE2 activity of stroke patients and the mechanism of these changes. Serum samples were obtained from patients during acute ischemic stroke (n=39), conditions mimicking stroke (stroke-alert, n=23), or from control participants (n=20). Enzyme activity levels were analyzed by fluorometric assay and correlated with clinical variables by regression analyses. Serum ACE2 activity was significantly lower in acute ischemic stroke as compared to both control and stroke-alert patients, followed by an increase to control levels at three days. Serum ACE2 activity significantly correlated with the presence of ischemic stroke after controlling for other factors (P=0.01). Additional associations with ACE2 activity included a positive correlation with systolic blood pressure at presentation in stroke-alert (R2=0.24, P=0.03), while stroke levels showed no correlation (R2=0.01, P=0.50). ACE2 sheddase activity was unchanged between groups. These dynamic changes in serum ACE2 activity in stroke, which concur with preclinical studies, are not likely to be driven primarily by acute changes in blood pressure or sheddase activity. These findings provide new insight for developing therapies targeting this protective system in ischemic stroke.

Neuroprotection by post‐stroke administration of an oral formulation of angiotensin‐(1–7) in ischaemic stroke

What is the central question of this study? Angiotensin‐(1–7) decreases cerebral infarct volume and improves neurological function when delivered centrally before and during ischaemic stroke. Here, we assessed the neuroprotective effects of angiotensin‐(1–7) when delivered orally post‐stroke. What is the main finding and its importance? We show that oral delivery of angiotensin‐(1–7) attenuates cerebral damage induced by middle cerebral artery occlusion in rats, without affecting blood pressure or cerebral blood flow. Importantly, these treatments begin post‐stroke at times coincident with the treatment window for tissue plasminogen activator, providing supporting evidence for clinical translation of this new therapeutic strategy.

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.

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.

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.

OS 32-03 ANGIOTENSIN II TYPE 2 RECEPTOR AGONIST EXERTS SUSTAINED NEUROPROTECTIVE EFFECTS IN AGED RATS.

Objective: The renin angiotensin system is a promising target for stroke neuroprotection and therapy through activation of angiotensin type II receptors (AT2R). The selective non-peptide AT2R agonist, Compound 21 (C21), has been shown to exhibit neuroprotection and improve stroke outcomes in preclinical studies, effects that likely involve neurotropic actions. However, these beneficial actions of C21 have not been demonstrated to occur beyond 1 week post stroke. The objective of this study was to determine if systemic administration of C21 would exert sustained neuroprotective effects in aged rats. Design and Method: Aged adult male Sprague Dawley rats (18–20 months) underwent ischemic stroke by monofilament middle cerebral artery occlusion (MCAO) and were randomly divided into two groups that received intraperitoneal (IP) injections of either 0.9% NaCl or 0.03 mg/kg C21 at reperfusion (90 min), 24 h, and 48 h after stroke. Infarct size was assessed by magnetic resonance imaging at 21 days post MCAO. Animals received blinded neurological exams at 4 h, 24 h, 72 h, 7d, 14d, and 21d post-MCAO. Results: Systemic treatment with C21 after stroke significantly improved neurological function, as evidenced by neurological testing using Rotarod and somatosensory dysfunction exams. At 7d and 14d after stroke, C21-treated rats had significantly increased Rotarod times versus saline-treated rats, and at 21d, the somatosensory function was significantly improved as measured by time to removal of paw adhesive. Infarct volume tended to be non-significantly decreased by C21 treatment at 21d post-stroke. Conclusions: These findings indicate that targeting the renin-angiotensin system, by stimulation of AT2Rs with C21, improves neurological function in aged rats with stroke over a sustained period of 21 days. These findings encourage further research into AT2R agonists and stroke, and offer hope for effective therapeutics for treating stroke.

Mas and Neuroprotection in Stroke

In this chapter, we explore the beneficial actions in stroke elicited by the activation of the angiotensin-converting enzyme 2/angiotensin-(1-7)/receptor Mas axis of the renin–angiotensin system. The neuroprotective efficacy in stroke of targeted manipulation of this axis is becoming well-established, thanks to findings from an increasing number of studies, which use a variety of stroke models and activating compounds. As part of a discussion of the effects of angiotensin-(1-7) in the brain and other tissues, we also describe several possible mechanisms by which the receptor Mas exerts its neuroprotective effects. Finally, we identify important steps to the clinical translation of these promising findings from the bench to the bedside.