Gene transfer of angiotensin-converting enzyme 2 in the nucleus tractus solitarius improves baroreceptor heart rate reflex in spontaneously hypertensive rats

Targeting the Renin-Angiotensin System (RAS) for Neuropsychiatric Disorders

BACKGROUND

Neuropsychiatric disorders, such as mood disorders, schizophrenia, and Alzheimer's disease (AD) and related dementias, are associated to significant morbidity and mortality worldwide. The pathophysiological mechanisms of neuropsychiatric disorders remain to be fully elucidated, which has hampered the development of effective therapies. The Renin Angiotensin System (RAS) is classically viewed as a key regulator of cardiovascular and renal homeostasis. The discovery that RAS components are expressed in the brain pointed out a potential role for this system in central nervous system (CNS) pathologies. The understanding of RAS involvement in the pathogenesis of neuropsychiatric disorders may contribute to identifying novel therapeutic targets.

AIMS

We aim to report current experimental and clinical evidence on the role of RAS in physiology and pathophysiology of mood disorders, schizophrenia, AD and related dementias. We also aim to discuss bottlenecks and future perspectives that can foster the development of new related therapeutic strategies.

CONCLUSION

The available evidence supports positive therapeutic effects for neuropsychiatric disorders with the inhibition/antagonism of the ACE/Ang II/AT1 receptor axis or the activation of the ACE2/Ang-(1-7)/Mas receptor axis. Most of this evidence comes from pre-clinical studies and clinical studies lag much behind, hampering a potential translation into clinical practice.

Neurological manifestations of coronavirus infections, before and after COVID-19: a review of animal studies

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This virus, which was first identified in December 2019 in China, has resulted in a yet ongoing viral pandemic. Coronaviridae could potentially cause several disorders in a wide range of hosts such as birds and mammals. Although infections caused by this family of viruses are predominantly limited to the respiratory tract, Betacoronaviruses are potentially able to invade the central nervous system (CNS) as well as many other organs, thereby inducing neurological damage ranging from mild to lethal in both animals and humans. Over the past two decades, three novel CoVs, SARS-CoV-1, MERS-CoV, and SARS-CoV-2, emerging from animal reservoirs have exhibited neurotropic properties causing severe and even fatal neurological diseases. The pathobiology of these neuroinvasive viruses has yet to be fully known. Both clinical features of the previous CoV epidemics (SARS-CoV-1 and MERS-CoV) and lessons from animal models used in studying neurotropic CoVs, especially SARS and MERS, constitute beneficial tools in comprehending the exact mechanisms of virus implantation and in illustrating pathogenesis and virus dissemination pathways in the CNS. Here, we review the animal research which assessed CNS infections with previous more studied neurotropic CoVs to demonstrate how experimental studies with appliable animal models can provide scientists with a roadmap in the CNS impacts of SARS-CoV-2. Indeed, animal studies can finally help us discover the underlying mechanisms of damage to the nervous system in COVID-19 patients and find novel therapeutic agents in order to reduce mortality and morbidity associated with neurological complications of SARS-CoV-2 infection.

Angiotensin II and the Cardiac Parasympathetic Nervous System in Hypertension

The renin-angiotensin-aldosterone system (RAAS) impacts cardiovascular homeostasis via direct actions on peripheral blood vessels and via modulation of the autonomic nervous system. To date, research has primarily focused on the actions of the RAAS on the sympathetic nervous system. Here, we review the critical role of the RAAS on parasympathetic nerve function during normal physiology and its role in cardiovascular disease, focusing on hypertension. Angiotensin (Ang) II receptors are present throughout the parasympathetic nerves and can modulate vagal activity via actions at the level of the nerve endings as well as via the circumventricular organs and as a neuromodulator acting within brain regions. There is tonic inhibition of cardiac vagal tone by endogenous Ang II. We review the actions of Ang II via peripheral nerve endings as well as via central actions on brain regions. We review the evidence that Ang II modulates arterial baroreflex function and examine the pathways via which Ang II can modulate baroreflex control of cardiac vagal drive. Although there is evidence that Ang II can modulate parasympathetic activity and has the potential to contribute to impaired baseline levels and impaired baroreflex control during hypertension, the exact central regions where Ang II acts need further investigation. The beneficial actions of angiotensin receptor blockers in hypertension may be mediated in part via actions on the parasympathetic nervous system. We highlight important unknown questions about the interaction between the RAAS and the parasympathetic nervous system and conclude that this remains an important area where future research is needed.

Multifunctional angiotensin converting enzyme 2, the SARS-CoV-2 entry receptor, and critical appraisal of its role in acute lung injury

The recent emergence of coronavirus disease-2019 (COVID-19) as a pandemic affecting millions of individuals has raised great concern throughout the world, and the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) was identified as the causative agent for COVID-19. The multifunctional protein angiotensin converting enzyme 2 (ACE2) is accepted as its primary target for entry into host cells. In its enzymatic function, ACE2, like its homologue ACE, regulates the renin-angiotensin system (RAS) critical for cardiovascular and renal homeostasis in mammals. Unlike ACE, however, ACE2 drives an alternative RAS pathway by degrading Ang-II and thus operates to balance RAS homeostasis in the context of hypertension, heart failure, and cardiovascular as well as renal complications of diabetes. Outside the RAS, ACE2 hydrolyzes key peptides, such as amyloid-β, apelin, and [des-Arg9]-bradykinin. In addition to its enzymatic functions, ACE2 is found to regulate intestinal amino acid homeostasis and the gut microbiome. Although the non-enzymatic function of ACE2 as the entry receptor for SARS-CoV-2 has been well established, the contribution of enzymatic functions of ACE2 to the pathogenesis of COVID-19-related lung injury has been a matter of debate. A complete understanding of this central enzyme may begin to explain the various symptoms and pathologies seen in SARS-CoV-2 infected individuals, and may aid in the development of novel treatments for COVID-19.

The Vagal Autonomic Pathway of COVID-19 at the Crossroad of Alzheimer's Disease and Aging: A Review of Knowledge

Coronavirus Disease 2019 (COVID-19) pandemic-triggered mortality is significantly higher in older than in younger populations worldwide. Alzheimer's disease (AD) is related to aging and was recently reported to be among the major risk factors for COVID-19 mortality in older people. The symptomatology of COVID-19 indicates that lethal outcomes of infection rely on neurogenic mechanisms. The present review compiles the available knowledge pointing to the convergence of COVID-19 complications with the mechanisms of autonomic dysfunctions in AD and aging. The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is prone to neuroinvasion from the lung along the vagus nerve up to the brainstem autonomic nervous centers involved in the coupling of cardiovascular and respiratory rhythms. The brainstem autonomic network allows SARS-CoV-2 to trigger a neurogenic switch to hypertension and hypoventilation, which may act in synergy with aging- and AD-induced dysautonomias, along with an inflammatory "storm". The lethal outcomes of COVID-19, like in AD and unhealthy aging, likely rely on a critical hypoactivity of the efferent vagus nerve cholinergic pathway, which is involved in lowering cardiovascular pressure and systemic inflammation tone. We further discuss the emerging evidence supporting the use of 1) the non-invasive stimulation of vagus nerve as an additional therapeutic approach for severe COVID-19, and 2) the demonstrated vagal tone index, i.e., heart rate variability, via smartphone-based applications as a non-serological low-cost diagnostic of COVID-19. These two well-known medical approaches are already available and now deserve large-scale testing on human cohorts in the context of both AD and COVID-19.

Impact of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in the Nervous System: Implications of COVID-19 in Neurodegeneration

Coronavirus Disease 2019 (COVID-19), caused by the Severe Acute Respiratory Syndrome coronavirus-2 (SARS-CoV-2), began in December 2019, in Wuhan, China and was promptly declared as a pandemic by the World Health Organization (WHO). As an acute respiratory disease, COVID-19 uses the angiotensin-converting enzyme 2 (ACE2) receptor, which is the same receptor used by its predecessor, SARS-CoV, to enter and spread through the respiratory tract. Common symptoms of COVID-19 include fever, cough, fatigue and in a small population of patients, SARS-CoV-2 can cause several neurological symptoms. Neurological malaise may include severe manifestations, such as acute cerebrovascular disease and meningitis/encephalitis. Although there is evidence showing that coronaviruses can invade the central nervous system (CNS), studies are needed to address the invasion of SARS-CoV-2 in the CNS and to decipher the underlying neurotropic mechanisms used by SARS-CoV-2. This review summarizes current reports on the neurological manifestations of COVID-19 and addresses potential routes used by SARS-CoV-2 to invade the CNS.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

Development of obesity can be prevented in rats by chronic icv infusions of AngII but less by Ang(1-7)

Considering that obesity is one of the leading risks for death worldwide, it should be noted that a brain-related mechanism is involved in AngII-induced and AT₁-receptor-dependent weight loss. It is moreover established that activation of the Ang(1-7)/ACE2/Mas axis reduces weight, but it remains unclear whether this Ang(1-7) effect is also mediated via a brain-related mechanism. Additionally to Sprague Dawley (SD) rats, we used TGR(ASrAOGEN) selectively lacking brain angiotensinogen, the precursor to AngII, as we speculated that effects are more pronounced in a model with low brain RAS activity. Rats were fed with high-calorie cafeteria diet. We investigated weight regulation, food behavior, and energy balance in response to chronic icv.-infusions of AngII (200 ng•h^-1), or Ang(1-7) (200/600 ng•h^-1) or artificial cerebrospinal fluid. High- but not low-dose Ang(1-7) slightly decreased weight gain and energy intake in SD rats. AngII showed an anti-obese efficacy in SD rats by decreasing energy intake and increasing energy expenditure and also improved glucose control. TGR(ASrAOGEN) were protected from developing obesity. However, Ang(1-7) did not reveal any effects in TGR(ASrAOGEN) and those of AngII were minor compared to SD rats. Our results emphasize that brain AngII is a key contributor for regulating energy homeostasis and weight in obesity by serving as a negative brain-related feedback signal to alleviate weight gain. Brain-related anti-obese potency of Ang(1-7) is lower than AngII but must be further investigated by using other transgenic models as TGR(ASrAOGEN) proved to be less valuable for answering this question.

Brain renin-angiotensin system in the pathophysiology of cardiovascular diseases

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

Overexpression of AT2R in the solitary-vagal complex improves baroreflex in the spontaneously hypertensive rat

The aim of this study was to investigate the physiological effects of increased angiotensin II type 2 receptor (AT2R) expression in the solitary-vagal complex (nucleus of the solitary tract/dorsal motor nucleus of the vagus; NTS/DVM) on baroreflex function in non-anaesthetised normotensive (NT) and spontaneously hypertensive rats (SHR). Ten week old NT Holtzman and SHR were microinjected with either an adeno-associated virus expressing AT2R (AAV2-CBA-AT2R) or enhanced green fluorescent protein (control; AAV2-CBA-eGFP) into the NTS/DVM. Baroreflex and telemetry recordings were performed on four experimental groups: 1) NTeGFP, 2) NTAT2R, 3) SHReGFP and 4) SHRAT2R (n=4-7/group). Following in-vivo experimental procedures, brains were harvested for gene expression analysis. Impaired bradycardia in SHReGFP was restored in SHR rats overexpressing AT2R in the NTS/DMV. mRNA levels of angiotensin converting enzyme decreased and angiotensin converting enzyme 2 increased in the NTS/DMV of SHRAT2R compared to SHReGFP. Increased levels of pro-inflammatory cytokine mRNA levels in the SHReGFP group also decreased in the SHRAT2R group. AT2R overexpression did not elicit any significant change in mean arterial pressure (MAP) in all groups from baseline to 4weeks post viral transfection. Both SHReGFP and SHRAT2R showed a significant elevation in MAP compared to the NTeGFP and NTAT2R groups. Increased AT2R expression within the NTS/DMV of SHR was effective at improving baroreflex function but not MAP. We propose possible mediators involved in improving baroreflex are in the ANG II/ACE2 axis, suggesting a potential beneficial modulatory effect of AT2R overexpression in the NTS/DMV of neurogenic hypertensive rats.

ACE2 and Microbiota: Emerging Targets for Cardiopulmonary Disease Therapy

The health of the cardiovascular and pulmonary systems is inextricably linked to the renin-angiotensin system (RAS). Physiologically speaking, a balance between the vasodeleterious (Angiotensin-converting enzyme [ACE]/Angiotensin II [Ang II]/Ang II type 1 receptor [AT1R]) and vasoprotective (Angiotensin-converting enzyme 2 [ACE2]/Angiotensin-(1-7) [Ang-(1-7)]/Mas receptor [MasR]) components of the RAS is critical for cardiopulmonary homeostasis. Upregulation of the ACE/Ang II/AT1R axis shifts the system toward vasoconstriction, proliferation, hypertrophy, inflammation, and fibrosis, all factors that contribute to the development and progression of cardiopulmonary diseases. Conversely, stimulation of the vasoprotective ACE2/Ang-(1-7)/MasR axis produces a counter-regulatory response that promotes cardiovascular health. Current research is investigating novel strategies to augment actions of the vasoprotective RAS components, particularly ACE2, in order to treat various pathologies. Although multiple approaches to increase the activity of ACE2 have displayed beneficial effects against experimental disease models, the mechanisms behind its protective actions remain incompletely understood. Recent work demonstrating a non-catalytic role for ACE2 in amino acid transport in the gut has led us to speculate that the therapeutic effects of ACE2 can be mediated, in part, by its actions on the gastrointestinal tract and/or gut microbiome. This is consistent with emerging data which suggest that dysbiosis of the gut and lung microbiomes is associated with cardiopulmonary disease. This review highlights new developments in the protective actions of ACE2 against cardiopulmonary disorders, discusses innovative approaches to targeting ACE2 for therapy, and explores an evolving role for gut and lung microbiota in cardiopulmonary health.

Role of neurons and glia in the CNS actions of the renin-angiotensin system in cardiovascular control

Despite tremendous research efforts, hypertension remains an epidemic health concern, leading often to the development of cardiovascular disease. It is well established that in many instances, the brain plays an important role in the onset and progression of hypertension via activation of the sympathetic nervous system. Further, the activity of the renin-angiotensin system (RAS) and of glial cell-mediated proinflammatory processes have independently been linked to this neural control and are, as a consequence, both attractive targets for the development of antihypertensive therapeutics. Although it is clear that the predominant effector peptide of the RAS, ANG II, activates its type-1 receptor on neurons to mediate some of its hypertensive actions, additional nuances of this brain RAS control of blood pressure are constantly being uncovered. One of these complexities is that the RAS is now thought to impact cardiovascular control, in part, via facilitating a glial cell-dependent proinflammatory milieu within cardiovascular control centers. Another complexity is that the newly characterized antihypertensive limbs of the RAS are now recognized to, in many cases, antagonize the prohypertensive ANG II type 1 receptor (AT1R)-mediated effects. That being said, the mechanism by which the RAS, glia, and neurons interact to regulate blood pressure is an active area of ongoing research. Here, we review the current understanding of these interactions and present a hypothetical model of how these exchanges may ultimately regulate cardiovascular function.

Brain ACE2 overexpression reduces DOCA-salt hypertension independently of endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress was previously reported to contribute to neurogenic hypertension while neuronal angiotensin-converting enzyme type 2 (ACE2) overexpression blunts the disease. To assess which brain regions are important for ACE2 beneficial effects and the contribution of ER stress to neurogenic hypertension, we first used transgenic mice harboring a floxed neuronal hACE2 transgene (SL) and tested the impact of hACE2 knockdown in the subfornical organ (SFO) and paraventricular nucleus (PVN) on deoxycorticosterone acetate (DOCA)-salt hypertension. SL and nontransgenic (NT) mice underwent DOCA-salt or sham treatment while infected with an adenoassociated virus (AAV) encoding Cre recombinase (AAV-Cre) or a control virus (AAV-green fluorescent protein) to the SFO or PVN. DOCA-salt-induced hypertension was reduced in SL mice, with hACE2 overexpression in the brain. This reduction was only partially blunted by knockdown of hACE2 in the SFO or PVN, suggesting that both regions are involved but not essential for ACE2 regulation of blood pressure (BP). DOCA-salt treatment did not increase the protein levels of ER stress and autophagy markers in NT mice, despite a significant increase in BP. In addition, these markers were not affected by hACE2 overexpression in the brain, despite a significant reduction of hypertension in SL mice. To further assess the role of ER stress in neurogenic hypertension, NT mice were infused intracerebroventricularlly with tauroursodeoxycholic acid (TUDCA), an ER stress inhibitor, during DOCA-salt treatment. However, TUDCA infusion failed to blunt the development of hypertension in NT mice. Our data suggest that brain ER stress does not contribute to DOCA-salt hypertension and that ACE2 blunts neurogenic hypertension independently of ER stress.

Protective axis of the renin-angiotensin system in the brain

The RAS (renin-angiotensin system) is composed of two arms: the pressor arm containing AngII (angiotensin II)/ACE (angiotensin-converting enzyme)/AT1Rs (AngII type 1 receptors), and the depressor arm represented by Ang-(1-7) [angiotensin-(1-7)]/ACE2/Mas receptors. All of the components of the RAS are present in the brain. Within the brain, Ang-(1-7) contributes to the regulation of BP (blood pressure) by acting at regions that control cardiovascular function such that, when Ang-(1-7) is injected into the nucleus of the solitary tract, caudal ventrolateral medulla, paraventricular nucleus or anterior hypothalamic area, a reduction in BP occurs; however, when injected into the rostral ventrolateral medulla, Ang-(1-7) stimulates an increase in BP. In contrast with AngII, Ang-(1-7) improves baroreflex sensitivity and has an inhibitory neuromodulatory role in hypothalamic noradrenergic neurotransmission. Ang-(1-7) not only exerts effects related to BP regulation, but also acts as a cerebroprotective component of the RAS by reducing cerebral infarct size and neuronal apoptosis. In the present review, we provide an overview of effects elicited by Ang-(1-7) in the brain, which suggest a potential role for Ang-(1-7) in controlling the central development of hypertension.

Neuroimmune communication in hypertension and obesity: a new therapeutic angle?

Hypertension is an epidemic health concern and a major risk factor for the development of cardiovascular disease. Although there are available treatment strategies for hypertension, numerous hypertensive patients do not have their clinical symptoms under control and it is imperative that new avenues to treat or prevent high blood pressure in these patients are developed. It is well established that increases in sympathetic nervous system (SNS) outflow and enhanced renin-angiotensin system (RAS) activity are common features of hypertension and various pathological conditions that predispose individuals to hypertension. More recently, hypertension has also become recognized as an immune condition and accumulating evidence suggests that interactions between the RAS, SNS and immune systems play a role in blood pressure regulation. This review summarizes what is known about the interconnections between the RAS, SNS and immune systems in the neural regulation of blood pressure. Based on the reviewed studies, a model for RAS/neuroimmune interactions during hypertension is proposed and the therapeutic potential of targeting RAS/neuroimmune interactions in hypertensive patients is discussed. Special emphasis is placed on the applicability of the proposed model to obesity-related hypertension.

Angiotensin-(1-7) in paraventricular nucleus modulates sympathetic activity and cardiac sympathetic afferent reflex in renovascular hypertensive rats

Background

Excessive sympathetic activity contributes to the pathogenesis and progression of hypertension. Enhanced cardiac sympathetic afferent reflex (CSAR) is involved in sympathetic activation. This study was designed to determine the roles of angiotensin (Ang)-(1–7) in paraventricular nucleus (PVN) in modulating sympathetic activity and CSAR and its signal pathway in renovascular hypertension.

Methodology/Principal Findings

Renovascular hypertension was induced with two-kidney, one-clip method. Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded in sinoaortic-denervated and cervical-vagotomized rats with anesthesia. CSAR was evaluated with the RSNA and MAP responses to epicardial application of capsaicin. PVN microinjection of Ang-(1–7) and cAMP analogue db-cAMP caused greater increases in RSNA and MAP, and enhancement in CSAR in hypertensive rats than in sham-operated rats, while Mas receptor antagonist A-779 produced opposite effects. There was no significant difference in the angiotensin-converting enzyme 2 (ACE2) activity and Ang-(1–7) level in the PVN between sham-operated rats and hypertensive rats, but the Mas receptor protein expression in the PVN was increased in hypertensive rats. The effects of Ang-(1–7) were abolished by A-779, adenylyl cyclase inhibitor SQ22536 or protein kinase A (PKA) inhibitor Rp-cAMP. SQ22536 or Rp-cAMP reduced RSNA and MAP in hypertensive rats, and attenuated the CSAR in both sham-operated and hypertensive rats.

Conclusions

Ang-(1–7) in the PVN increases RSNA and MAP and enhances the CSAR, which is mediated by Mas receptors. Endogenous Ang-(1–7) and Mas receptors contribute to the enhanced sympathetic outflow and CSAR in renovascular hypertension. A cAMP-PKA pathway is involved in the effects of Ang-(1–7) in the PVN.

Three key proteases--angiotensin-I-converting enzyme (ACE), ACE2 and renin--within and beyond the renin-angiotensin system

The discovery of angiotensin-I-converting enzyme 2 (ACE2) and a (pro)renin receptor has renewed interest in the physiology of the renin-angiotensin system (RAS). Through the ACE2/angiotensin-(1–7)/Mas counter-regulatory axis, ACE2 balances the vasoconstrictive, proliferative, fibrotic and proinflammatory effects of the ACE/angiotensin II/AT1 axis. The (pro)renin receptor system shows an angiotensin-dependent function related to increased generation of angiotensin I, and an angiotensin-independent aspect related to intracellular signalling. Activation of ACE2 and inhibition of ACE and renin have been at the core of the RAS regulation. The aim of this review is to discuss the biochemistry and biological functions of ACE, ACE2 and renin within and beyond the RAS, and thus provide a perspective for future bioactives from natural plant and/or food resources related to the three proteases.

Chronic activation of endogenous angiotensin-converting enzyme 2 protects diabetic rats from cardiovascular autonomic dysfunction

In this study, we evaluated whether the activation of endogenous angiotensin-converting enzyme 2 (ACE2) would improve the cardiovascular autonomic dysfunction of diabetic rats. Ten days after induction of type 1 diabetes (streptozotocin, 50 mg kg(-1) i.v.), the rats were treated orally with 1-[(2-dimethylamino)ethylamino]-4-(hydroxymethyl)-7-[(4-methylphenyl) sulfonyl oxy]-9H-xanthene-9-one (XNT), a newly discovered ACE2 activator (1 mg kg(-1) day(-1)), or saline (equivalent volume) for 30 days. Autonomic cardiovascular parameters were evaluated in conscious animals, and an isolated heart preparation was used to analyse cardiac function. Diabetes induced a significant decrease in the baroreflex bradycardia sensitivity, as well as in the chemoreflex chronotropic response and parasympathetic tone. The XNT treatment improved these parameters by ≈ 76% [0.82 ± 0.09 versus 1.44 ± 0.17 Ratio between changes in pulse interval and changes in mean arterial pressure (ΔPI/ΔmmHg)], ∼85% (-57 ± 9 versus -105 ± 10 beats min(-1)) and ≈ 205% (22 ± 2 versus 66 ± 12 beats min(-1)), respectively. Also, XNT administration enhanced the bradycardia induced by the chemoreflex activation by v 74% in non-diabetic animals (-98 ± 16 versus -170 ± 9 Δbeats min(-1)). No significant changes were observed in the mean arterial pressure, baroreflex tachycardia sensitivity, chemoreflex pressor response and sympathetic tone among any of the groups. Furthermore, chronic XNT treatment ameliorated the cardiac function of diabetic animals. However, the coronary vasoconstriction observed in diabetic rats was unchanged by ACE2 activation. These findings indicate that XNT protects against the autonomic and cardiac dysfunction induced by diabetes. Thus, our results provide evidence for the viability and effectiveness of oral administration of an ACE2 activator for the treatment of the cardiovascular autonomic dysfunction caused by diabetes.

Angiotensin-converting enzyme 2: the first decade

The renin-angiotensin system (RAS) is a critical regulator of hypertension, primarily through the actions of the vasoactive peptide Ang II, which is generated by the action of angiotensin-converting enzyme (ACE) mediating an increase in blood pressure. The discovery of ACE2, which primarily metabolises Ang II into the vasodilatory Ang-(1-7), has added a new dimension to the traditional RAS. As a result there has been huge interest in ACE2 over the past decade as a potential therapeutic for lowering blood pressure, especially elevation resulting from excess Ang II. Studies focusing on ACE2 have helped to reveal other actions of Ang-(1-7), outside vasodilation, such as antifibrotic and antiproliferative effects. Moreover, investigations focusing on ACE2 have revealed a variety of roles not just catalytic but also as a viral receptor and amino acid transporter. This paper focuses on what is known about ACE2 and its biological roles, paying particular attention to the regulation of ACE2 expression. In light of the entrance of human recombinant ACE2 into clinical trials, we discuss the potential use of ACE2 as a therapeutic and highlight some pertinent questions that still remain unanswered about ACE2.