Angiotensin generation in the brain: a re-evaluation

The Contribution of the Renin-Angiotensin System to Alzheimer's Disease

The renin-angiotensin system (RAS) is becoming increasingly recognised as a biochemical pathway relevant to the development and progression of Alzheimer's disease (AD). RAS involvement in AD was initially linked to AD via numerous genetic association studies and more recent Genome-Wide Association Studies (GWAS), and in some cases in relation to classical hallmarks of AD pathology. Since these initial findings, which will be summarised here, several complementary areas of research are converging in support of what has been proposed as the Angiotensin Hypothesis for Alzheimer's disease. This hypothesis proposes how the RAS and disease-associated changes to the normal balance between opposing regulatory pathways within RAS warrant careful consideration in the pathogenesis of AD and its pathology. We discuss some of these in relation to RAS-targeting therapeutics, originally developed for the treatment of cardiovascular conditions, and how they might be repurposed as interventions for AD.

Endothelin-1- and acetylcholine-mediated effects in human and rat vessels: impact of perivascular adipose tissue, diabetes, angiotensin II, and chemerin

OBJECTIVE

To study the role of perivascular adipose tissue (PVAT) in the reactivity of rat and human vessels.

METHODS

Iliac and mesenteric arteries were obtained from normotensive Sprague-Dawley rats, hypertensive transgenic (mRen2)27 rats overexpressing mouse renin, and (mRen2)27 rats made diabetic with streptozotocin. Human coronary arteries were obtained from donors. Concentration-response curves were constructed to endothelin-1 and acetylcholine with and without PVAT. The contribution of NO and endothelium-dependent hyperpolarization (EDH) were determined making use of the NO synthase inhibitor L-NAME and the EDH inhibitors apamin + TRAM-34. The endothelin type A and type B (ETA, ETB) receptor blockers BQ123 and BQ788, the chemerin inhibitors α-NETA and pravastatin, and the angiotensin receptor blocker losartan were also used.

RESULTS

In rat iliac arteries, PVAT diminished endothelin-induced constriction, while the opposite was true in human coronaries. Coronary effects were unaltered by α-NETA, pravastatin, or losartan. ETB receptor-mediated relaxation in iliac arteries occurred only with PVAT, and BQ123 blocked endothelin-1-induced constriction. Diabetes upregulated the anticontractile effects of PVAT. In rat mesenteric arteries, acetylcholine-induced relaxation with PVAT relied on NO, and on NO + EDH without PVAT. Diabetes upregulated the EDH component exclusively with PVAT.

CONCLUSION

PVAT modulates ET-1-induced constriction in a vessel type-dependent manner. Its enhancing effects in coronaries involved neither chemerin nor angiotensin II. Its anticontractile effects in rat iliac arteries involved ETB receptor-mediated relaxation. Diabetes upregulated PVAT's anticontractile effects. In mesenteric arteries, PVAT counterbalanced the EDH component of the relaxant effect of acetylcholine. Diabetes reversed this effect by upregulating the EDH component.

Spontaneous and evoked angiotensin II sniffer cell activity in the lamina terminalis in vitro

Angiotensin II (ANG II) has been shown to have central nervous system effects. Although tissue renin-angiotensin systems (RAS) have been demonstrated in multiple tissues, the existence of a brain RAS is still a matter of debate. These studies test for angiotensin release from brain slices prepared from adult male Sprague-Dawley rats and male and female renin knock-out rats using Chinese hamster ovary cells modified to express both the angiotensin II type 1 receptor and a fluorescent calcium indicator. Sniffer cells were placed on the slices and calcium transients were measured from those located on or adjacent to the median preoptic nucleus with and without stimulation of the subfornical organ. Bath application of tetrodotoxin (1 µM) significantly attenuated spontaneous events while abolishing evoked sniffer cell activity. Bath application of dl-AP4 (10 µM, glutamatergic antagonist) did not affect either spontaneous or evoked release. Incubating the slices with fluorocitrate to inactive astrocytes did not influence sniffer cell activity in the MnPO. Pharmacological experiments indicate that ANG II release is largely both renin (aliskiren 10 µM) and ACE-1 (captopril 100 µM) dependent. However, experiments with brain slices prepared from male and female Renin knock-out rats suggest that alternative synthetic pathways may exist. Finally, these studies demonstrate that increases in ANG II release are observed following 7 days of chronic intermittent hypoxia. These studies suggest the existence of a tissue-specific RAS in the brain that involves canonical and alternative ANG II synthetic pathways and is upregulated in an animal model of sleep apnea.NEW & NOTEWORTHY These studies used Chinese hamster ovary cells that were cloned to express an angiotensin receptor (At1ra) and a calcium indicator (R-GECO) to detect the release of angiotensin from brain slices containing the lamina terminalis of rats. Some of the experiments use tissue from renin knockout rats. The results support the existence of an angiotensin system in the brain that may involve alternative synthetic pathways and is upregulated by intermittent hypoxia.

Control of fluid intake in dehydrated rats and evolution of sodium appetite

The objective of the present work is to examine from a new perspective the existence of causal factors not predicted by the classical theory that thirst and sodium appetite are two distinct motivations. For example, we ask why water deprivation induces sodium appetite, thirst is not "water appetite", and intracellular dehydration potentially causes sodium appetite. Contrary to the classical theory, we suggest that thirst first, and sodium appetite second, designate a temporal sequence underlying the same motivation. The single motivation becomes an "intervenient variable" a concept borrowed from the literature, fully explained in the text, between causes of dehydration (extracellular, intracellular, or both together), and respective behavioral responses subserved by hindbrain-dependent inhibition (e.g., lateral parabrachial nucleus) and forebrain facilitation (e.g., angiotensin II). A corollary is homology between rat sodium appetite and marine teleost thirst-like motivation that we name "protodipsia". The homology argument rests on similarities between behavior (salty water intake) and respective neuroanatomical as well as functional mechanisms. Tetrapod origin in a marine environment provides additional support for the homology. The single motivation hypothesis is also consistent with ingestive behaviors in nature given similarities (e.g., thirst producing brackish water intake) between the behavior of the laboratory rat and wild animals, rodents included. The hypotheses of single motivation and homology might explain why hyperosmotic rats, or eventually any other hyperosmotic tetrapod, shows paradoxical signs of sodium appetite. They might also explain how ingestive behaviors determined by dehydration and subserved by hindbrain inhibitory mechanisms contributed to tetrapod transition from sea to land.

Potential molecular pathways of angiotensin receptor blockers in the brain toward cognitive improvement in dementia

The alarming rise of cognitive impairment and memory decline and limited effective solutions present a worldwide concern for dementia patients. The multivariant role of the renin-angiotensin system (RAS) in the brain offers strong evidence of a role for angiotensin receptor blockers (ARBs) in the management of memory impairment by modifying glutamate excitotoxicity, downregulating inflammatory cytokines such as interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)α, inhibiting kynurenine aminotransferase (KAT)-II, nucleotide-binding domain, leucine-rich-containing family and pyrin-domain-containing-3 (NLRP3) inflammasomes, boosting cholinergic activity, activating peroxisome proliferator-activated receptor (PPAR)-γ, countering cyclooxygenase (COX) and mitigating the hypoxic condition. The present work focuses on the intricate molecular mechanisms involved in brain-RAS, highlighting the role of ARBs, connecting links between evidence-based unexplored pathways and investigating probable biomarkers involved in dementia through supported preclinical and clinical literature.

The contribution of the AT1 receptor to erythropoiesis

The renin-angiotensin system (RAS) comprises a broad set of functional peptides and receptors that play a role in cardiovascular homeostasis and contribute to cardiovascular pathologies. Angiotensin II (Ang II) is the most potent peptide hormone produced by the RAS due to its high abundance and its strong and pleiotropic impact on the cardiovascular system. Formation of Ang II takes place in the bloodstream and additionally in tissues in the so-called local RAS. Of the two Ang II receptors (AT1 and AT2) that Ang II binds to, AT1 is the most expressed throughout the mammalian body. AT1 expression is not restricted to cells of the cardiovascular system but in fact AT1 protein is found in nearly all organs, hence, Ang II takes part in several modulatory physiological processes one of which is erythropoiesis. In this review, we present multiple evidence supporting that Ang II modulates physiological and pathological erythropoiesis processes trough the AT1 receptor. Cumulative evidence indicates that Ang II by three distinct mechanisms influences erythropoiesis: 1) stimulation of renal erythropoietin synthesis; 2) direct action on bone marrow precursor cells; and 3) modulation of sympathetic nerve activity to the bone marrow. The text highlights clinical and preclinical evidence focusing on mechanistic studies using rodent models.

Neuronal progenitors of the dentate gyrus express the SARS-CoV-2 cell receptor during migration in the developing human hippocampus

The COVID-19 pandemic spread around the world is due to the enormous capacity of the SARS-CoV-2 coronavirus to be transmitted between humans, causing a threat to global public health. It has been shown that the entry of this virus into cells is highly facilitated by the presence of angiotensin-converting enzyme 2 (ACE2) in the cell membrane. Currently, we have no precise knowledge of how this receptor expresses in the brain of human fetus and, as a consequence, we do not know how susceptible the neural cells in the developing brain are to being infected through the vertical transmission of this virus, from mother to fetus. In this work, we describe the expression of ACE2 in the human brain at 20 weeks of gestation. This stage corresponds to the period of neuronal generation, migration, and differentiation in the cerebral cortex. We describe the specific expression of ACE2 in neuronal precursors and migratory neuroblasts of the dentate gyrus in the hippocampus. This finding implies that SARS-CoV-2 infection during the fetal period may affect neuronal progenitor cells and alter the normal development of the brain region where memory engrams are generated. Thus, although vertical transmission of SARS-CoV-2 infection was reported in few cases, the massive infection rate of young people in terms of the new variants leads to the possibility of increasing the ratio of congenital infections and originating cognitive alterations, as well as neuronal circuit anomalies that may represent vulnerability to mental problems throughout life.

Fifty years of research on the brain renin-angiotensin system: what have we learned?

Although the existence of a brain renin-angiotensin system (RAS) had been proposed five decades ago, we still struggle to understand how it functions. The main reason for this is the virtual lack of renin at brain tissue sites. Moreover, although renin's substrate, angiotensinogen, appears to be synthesized locally in the brain, brain angiotensin (Ang) II disappeared after selective silencing of hepatic angiotensinogen. This implies that brain Ang generation depends on hepatic angiotensinogen after all. Rodrigues et al. (Clin Sci (Lond) (2021) 135:1353-1367) generated a transgenic mouse model overexpressing full-length rat angiotensinogen in astrocytes, and observed massively elevated brain Ang II levels, increased sympathetic nervous activity and vasopressin, and up-regulated erythropoiesis. Yet, blood pressure and kidney function remained unaltered, and surprisingly no other Ang metabolites occurred in the brain. Circulating renin was suppressed. This commentary critically discusses these findings, concluding that apparently in the brain, overexpressed angiotensinogen can be cleaved by an unidentified non-renin enzyme, yielding Ang II directly, which then binds to Ang receptors, allowing no metabolism by angiotensinases like ACE2 and aminopeptidase A. Future studies should now unravel the identity of this non-renin enzyme, and determine whether it also contributes to Ang II generation at brain tissue sites in wildtype animals. Such studies should also re-evaluate the concept that Ang-(1-7) and Ang III, generated by ACE2 and aminopeptidase A, respectively, have important functions in the brain.

Angiotensin-(1-7) and Mas receptor in the brain

The renin-angiotensin system (RAS) is a key regulator of blood pressure and electrolyte homeostasis. Besides its importance as regulator of the cardiovascular function, the RAS has also been associated to the modulation of higher brain functions, including cognition, memory, depression and anxiety. For many years, angiotensin II (Ang II) has been considered the major bioactive component of the RAS. However, the existence of many other biologically active RAS components has currently been recognized, with similar, opposite, or distinct effects to those exerted by Ang II. Today, it is considered that the RAS is primarily constituted by two opposite arms. The pressor arm is composed by Ang II and the Ang II type 1 (AT1) receptor (AT1R), which mediates the vasoconstrictor, proliferative, hypertensive, oxidative and pro-inflammatory effects of the RAS. The depressor arm is mainly composed by Ang-(1-7), its Mas receptor (MasR) which mediates the depressor, vasodilatory, antiproliferative, antioxidant and anti-inflammatory effects of Ang-(1-7) and the AT2 receptor (AT2R), which opposes to the effects mediated by AT1R activation. Central Ang-(1-7) is implicated in the control of the cardiovascular function, thus participating in the regulation of blood pressure. Ang-(1-7) also exerts neuroprotective actions through MasR activation by opposing to the harmful effects of the Ang II/AT1R axis. This review is focused on the expression and regulation of the Ang-(1-7)/MasR axis in the brain, its main neuroprotective effects and the evidence regarding its involvement in the pathophysiology of several diseases at cardiovascular and neurological level.

Increased angiotensin II formation in the brain modulates cardiovascular homeostasis and erythropoiesis

In spite of the fact that the modulatory effects of angiotensin II (Ang II) on the sympathetic nerve activity to targeted organs involved in blood pressure (BP) regulation is well acknowledged, the local production of this peptide in the brain and the consequences of enhanced central Ang II beyond the cardiovascular system are not yet well comprehended. In the present study, we generated and validated a new transgenic mouse line overexpressing the rat full-length angiotensinogen (Agt) protein specifically in the brain (Agt-Tg). Adult Agt-Tg mice presented overall increased gene expression of total Agt in the brain including brainstem and hypothalamus. In addition, the excess of Agt led to abundantly detectable brain Ang II levels as well as increased circulating copeptin levels. Agt-Tg displayed raised BP in acute recordings, while long-term telemetrically measured basal BP was indistinguishable from wild-types. Agt-Tg has altered peripheral renin–angiotensin system and vasomotor sympathetic tone homeostasis because renal gene expression analysis, plasma Ang II measurements and ganglionic blockade experiments revealed suppressed renin expression and reduced Ang II and higher neurogenic pressure response, respectively. Plasma and urine screens revealed apparently normal fluid and electrolyte handling in Agt-Tg. Interestingly, hematological analyses showed increased hematocrit in Agt-Tg caused by enhanced erythropoiesis, which was reverted by submitting the transgenic mice to a long-term peripheral sympathectomy protocol. Collectively, our findings suggest that Agt-Tg is a valuable tool to study not only brain Ang II formation and its modulatory effects on cardiovascular homeostasis but also its role in erythropoiesis control via autonomic modulation.

Angiotensin Receptor Blockers Are Not Just for Hypertension Anymore

Beyond blood pressure control, angiotensin receptor blockers reduce common injury mechanisms, decreasing excessive inflammation and protecting endothelial and mitochondrial function, insulin sensitivity, the coagulation cascade, immune responses, cerebrovascular flow, and cognition, properties useful to treat inflammatory, age-related, neurodegenerative, and metabolic disorders of many organs including brain and lung.

Immunohistochemistry for (Pro)renin Receptor in Humans

The (pro)renin receptor is a multifunctional protein with roles in angiotensin-II-dependent and -independent intracellular cell signaling and roles as an intracellular accessory protein for the vacuolar H⁺-ATPase, including hormone secretion. While (pro)renin receptor mRNA is widely expressed in various human tissues, localization of (pro)renin receptor protein expression has not yet been systemically determined. Therefore, this study localized (pro)renin receptor protein expression in human organs. Systemic immunohistochemical examination of (pro)renin receptor expression was performed in whole body organs of autopsy cases. (Pro)renin receptor immunostaining was observed in the cytoplasm of cells in almost all human organs. It was observed in thyroid follicular epithelial cells, hepatic cells, pancreatic duct epithelial cells, zona glomerulosa and zona reticularis of the cortex and medulla of the adrenal gland, proximal and distal tubules and collecting ducts of the kidney, cardiomyocytes, and skeletal muscle cells. In the brain, (pro)renin receptor staining was detected in neurons throughout all areas, especially in the medulla oblongata, paraventricular nucleus and supraoptic nucleus of the hypothalamus, cerebrum, granular layer of the hippocampus, Purkinje cell layer of the cerebellum, and the pituitary anterior and posterior lobes. In the anterior lobe of the pituitary gland, all types of anterior pituitary hormone-positive cells showed double staining with (pro)renin receptor. These data showed that (pro)renin receptor protein was expressed in almost all organs of the human body. Its expression pattern was not uniform, and cell-specific expression pattern was observed, supporting the notion that (pro)renin receptor plays numerous physiological roles in each human organ.

Renin-Angiotensin System and Alzheimer's Disease Pathophysiology: From the Potential Interactions to Therapeutic Perspectives

New roles of the Renin-Angiotensin System (RAS), apart from fluid homeostasis and Blood Pressure (BP) regulation, are being progressively unveiled, since the discoveries of RAS alternative axes and local RAS in different tissues, including the brain. Brain RAS is reported to interact with pathophysiological mechanisms of many neurological and psychiatric diseases, including Alzheimer's Disease (AD). Even though AD is the most common cause of dementia worldwide, its pathophysiology is far from elucidated. Currently, no treatment can halt the disease course. Successive failures of amyloid-targeting drugs have challenged the amyloid hypothesis and increased the interest in the inflammatory and vascular aspects of AD. RAS compounds, both centrally and peripherally, potentially interact with neuroinflammation and cerebrovascular regulation. This narrative review discusses the AD pathophysiology and its possible interaction with RAS, looking forward to potential therapeutic approaches. RAS molecules affect BP, cerebral blood flow, neuroinflammation, and oxidative stress. Angiotensin (Ang) II, via angiotensin type 1 receptors may promote brain tissue damage, while Ang-(1-7) seems to elicit neuroprotection. Several studies dosed RAS molecules in AD patients' biological material, with heterogeneous results. The link between AD and clinical conditions related to classical RAS axis overactivation (hypertension, heart failure, and chronic kidney disease) supports the hypothesized role of this system in AD. Additionally, RAStargeting drugs as Angiotensin Converting Enzyme inhibitors (ACEis) and Angiotensin Receptor Blockers (ARBs) seem to exert beneficial effects on AD. Results of randomized controlled trials testing ACEi or ARBs in AD are awaited to elucidate whether AD-RAS interaction has implications on AD therapeutics.

Use of dual-flow bioreactor to develop a simplified model of nervous-cardiovascular systems crosstalk: A preliminary assessment

Chronic conditions requiring long-term rehabilitation therapies, such as hypertension, stroke, or cancer, involve complex interactions between various systems/organs of the body and mutual influences, thus implicating a multiorgan approach. The dual-flow IVTech LiveBox2 bioreactor is a recently developed inter-connected dynamic cell culture model able to mimic organ crosstalk, since cells belonging to different organs can be connected and grown under flow conditions in a more physiological environment. This study aims to setup for the first time a 2-way connected culture of human neuroblastoma cells, SH-SY5Y, and Human Coronary Artery Smooth Muscle Cells, HCASMC through a dual-flow IVTech LiveBox2 bioreactor, in order to represent a simplified model of nervous-cardiovascular systems crosstalk, possibly relevant for the above-mentioned diseases. The system was tested by treating the cells with 10nM angiotensin II (AngII) inducing PKCβII/HuR/VEGF pathway activation, since AngII and PKCβII/HuR/VEGF pathway are relevant in cardiovascular and neuroscience research. Three different conditions were applied: 1- HCASMC and SH-SY5Y separately seeded in petri dishes (static condition); 2- the two cell lines separately seeded under flow (dynamic condition); 3- the two lines, seeded in dynamic conditions, connected, each maintaining its own medium, with a membrane as interface for biohumoral changes between the two mediums, and then treated. We detected that only in condition 3 there was a synergic AngII-dependent VEGF production in SH-SY5Y cells coupled to an AngII-dependent PKCβII/HuR/VEGF pathway activation in HCASMC, consistent with the observed physiological response in vivo. HCASMC response to AngII seems therefore to be generated by/derived from the reciprocal cell crosstalk under the dynamic inter-connection ensured by the dual flow LiveBox 2 bioreactor. This system can represent a useful tool for studying the crosstalk between organs, helpful for instance in rehabilitation research or when investigating chronic diseases; further, it offers the advantageous opportunity of cultivating each cell line in its own medium, thus mimicking, at least in part, distinct tissue milieu.

Cerebrovascular function in hypertension: Does high blood pressure make you old?

The majority of individuals over an age of 60 have hypertension. Elevated blood pressure and older age are associated with very similar changes in brain structure and function. We review the parallel brain changes associated with increasing age and blood pressure. This review focuses on joint associations of aging and elevated blood pressure with neuropsychological function, regional cerebral blood flow responses to cognitive and metabolic challenges, white matter disruptions, grey matter volume, cortical thinning, and neurovascular coupling. Treatment of hypertension ameliorates many of these changes but fails to reverse them. Treatment of hypertension itself appears more successful with better initial brain function. We show evidence that sympathetic and renal influences known to increase blood pressure also impact brain integrity. Possible central mechanisms contributing to the course of hypertension and aging are then suggested. An emphasis is placed on psychologically relevant factors: stress, cardiovascular reactions to stress, and diet/obesity. The contribution of some of these factors to biological aging remains unclear and may provide a starting point for defining the independent and interacting effects of aging and increasing blood pressure on the brain.

Regulatory peptides and systems biology: A new era of translational and reverse-translational neuroendocrinology

Recently, there has been a resurgence in regulatory peptide science as a result of three converging trends. The first is the increasing population of the drug pipeline with peptide-based therapeutics, mainly in, but not restricted to, incretin-like molecules for treatment of metabolic disorders such as diabetes. The second is the development of genetic and optogenetic tools enabling new insights into how peptides actually function within brain and peripheral circuits to accomplish homeostatic and allostatic regulation. The third is the explosion in defined structures of the G-protein coupled receptors to which most regulatory peptides bind and exert their actions. These trends have closely wedded basic systems biology to drug discovery and development, creating a "two-way street" on which translational advances travel from basic research to the clinic, and, equally importantly, "reverse-translational" information is gathered, about the molecular, cellular and circuit-level mechanisms of action of regulatory peptides, comprising information required for the fine-tuning of drug development through testing in animal models. This review focuses on a small group of 'influential' peptides, including oxytocin, vasopressin, pituitary adenylate cyclase-activating polypeptide, ghrelin, relaxin-3 and glucagon-like peptide-1, and how basic discoveries and their application to therapeutics have intertwined over the past decade.

The (pro)renin receptor in health and disease

The (pro)renin receptor ((P)RR) was first identified as a single-transmembrane receptor in human kidneys and initially attracted attention owing to its potential role as a regulator of the tissue renin-angiotensin system (RAS). Subsequent studies found that the (P)RR is widely distributed in organs throughout the body, including the kidneys, heart, brain, eyes, placenta and the immune system, and has multifaceted functions in vivo. The (P)RR has roles in various physiological processes, such as the cell cycle, autophagy, acid-base balance, energy metabolism, embryonic development, T cell homeostasis, water balance, blood pressure regulation, cardiac remodelling and maintenance of podocyte structure. These roles of the (P)RR are mediated by its effects on important biological systems and pathways including the tissue RAS, vacuolar H⁺-ATPase, Wnt, partitioning defective homologue (Par) and tyrosine phosphorylation. In addition, the (P)RR has been reported to contribute to the pathogenesis of diseases such as fibrosis, hypertension, pre-eclampsia, diabetic microangiopathy, acute kidney injury, cardiovascular disease, cancer and obesity. Current evidence suggests that the (P)RR has key roles in the normal development and maintenance of vital organs and that dysfunction of the (P)RR is associated with diseases that are characterized by a disruption of the homeostasis of physiological functions.

Role of brain renin angiotensin system in neurodegeneration: An update

Renin angiotensin system (RAS) is an endocrine system widely known for its physiological roles in electrolyte homeostasis, body fluid volume regulation and cardiovascular control in peripheral circulation. However, brain RAS is an independent form of RAS expressed locally in the brain, which is known to be involved in brain functions and disorders. There is strong evidence for a major involvement of excessive brain angiotensin converting enzyme (ACE)/Angiotensin II (Ang II)/Angiotensin type-1 receptor (AT-1R) axis in increased activation of oxidative stress, apoptosis and neuroinflammation causing neurodegeneration in several brain disorders. Numerous studies have demonstrated strong neuroprotective effects by blocking AT1R in these brain disorders. Additionally, the angiotensin converting enzyme 2 (ACE2)/Angiotensin (1–7)/Mas receptor (MASR), is another axis of brain RAS which counteracts the damaging effects of ACE/Ang II/AT1R axis on neurons in the brain. Thus, angiotensin II receptor blockers (ARBs) and activation of ACE2/Angiotensin (1–7)/MASR axis may serve as an exciting and novel method for neuroprotection in several neurodegenerative diseases. Here in this review article, we discuss the expression of RAS in the brain and highlight how altered RAS level may cause neurodegeneration. Understanding the pathophysiology of RAS and their links to neurodegeneration has enormous potential to identify potentially effective pharmacological tools to treat neurodegenerative diseases in the brain.

Angiotensin-(1-7): Translational Avenues in Cardiovascular Control

Despite decades of research and numerous treatment approaches, hypertension and cardiovascular disease remain leading global public health problems. A major contributor to regulation of blood pressure, and the development of hypertension, is the renin-angiotensin system. Of particular concern, uncontrolled activation of angiotensin II contributes to hypertension and associated cardiovascular risk, with antihypertensive therapies currently available to block the formation and deleterious actions of this hormone. More recently, angiotensin-(1-7) has emerged as a biologically active intermediate of the vasodilatory arm of the renin-angiotensin system. This hormone antagonizes angiotensin II actions as well as offers antihypertensive, antihypertrophic, antiatherogenic, antiarrhythmogenic, antifibrotic and antithrombotic properties. Angiotensin-(1-7) elicits beneficial cardiovascular actions through mas G protein-coupled receptors, which are found in numerous tissues pivotal to control of blood pressure including the brain, heart, kidneys, and vasculature. Despite accumulating evidence for favorable effects of angiotensin-(1-7) in animal models, there is a paucity of clinical studies and pharmacokinetic limitations, thus limiting the development of therapeutic agents to better understand cardiovascular actions of this vasodilatory peptide hormone in humans. This review highlights current knowledge on the role of angiotensin-(1-7) in cardiovascular control, with an emphasis on significant animal, human, and therapeutic research efforts.

Novel Therapeutic Approaches Targeting the Renin-Angiotensin System and Associated Peptides in Hypertension and Heart Failure

Despite the success of renin-angiotensin system (RAS) blockade by angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 receptor (AT1R) blockers, current therapies for hypertension and related cardiovascular diseases are still inadequate. Identification of additional components of the RAS and associated vasoactive pathways, as well as new structural and functional insights into established targets, have led to novel therapeutic approaches with the potential to provide improved cardiovascular protection and better blood pressure control and/or reduced adverse side effects. The simultaneous modulation of several neurohumoral mediators in key interconnected blood pressure-regulating pathways has been an attractive approach to improve treatment efficacy, and several novel approaches involve combination therapy or dual-acting agents. In addition, increased understanding of the complexity of the RAS has led to novel approaches aimed at upregulating the ACE2/angiotensin-(1-7)/Mas axis to counter-regulate the harmful effects of the ACE/angiotensin II/angiotensin III/AT1R axis. These advances have opened new avenues for the development of novel drugs targeting the RAS to better treat hypertension and heart failure. Here we focus on new therapies in preclinical and early clinical stages of development, including novel small molecule inhibitors and receptor agonists/antagonists, less conventional strategies such as gene therapy to suppress angiotensinogen at the RNA level, recombinant ACE2 protein, and novel bispecific designer peptides.

Sniffer cells for the detection of neural Angiotensin II in vitro

Neuropeptide release in the brain has traditionally been difficult to observe. Existing methods lack temporal and spatial resolution that is consistent with the function and size of neurons. We use cultured "sniffer cells" to improve the temporal and spatial resolution of observing neuropeptide release. Sniffer cells were created by stably transfecting Chinese Hamster Ovary (CHO) cells with plasmids encoding the rat angiotensin type 1a receptor and a genetically encoded Ca2+ sensor. Isolated, cultured sniffer cells showed dose-dependent increases in fluorescence in response to exogenously applied angiotensin II and III, but not other common neurotransmitters. Sniffer cells placed on the median preoptic nucleus (a presumptive site of angiotensin release) displayed spontaneous activity and evoked responses to either electrical or optogenetic stimulation of the subfornical organ. Stable sniffer cell lines could be a viable method for detecting neuropeptide release in vitro, while still being able to distinguish differences in neuropeptide concentration.

The Possible Role of the Angiotensin System in the Pathophysiology of Schizophrenia: Implications for Pharmacotherapy

A growing body of literature has elucidated the involvement of the central renin-angiotensin system (RAS) in various neuropsychiatric diseases. While consensus on the exact mechanism of the central RAS in schizophrenia pathophysiology does not currently exist, increasing evidence reveals promise in harnessing the therapeutic potential of RAS modulation in the treatment of schizophrenia. In this review, we examine how the central RAS affects inflammation, glutamate, dopamine, gamma-aminobutyric acid (GABA), and peroxisome proliferator-activated receptor (PPAR)-γ, all of which are associated with schizophrenia etiology. In addition, a recent study has demonstrated the therapeutic potential of RAS modulators, especially angiotensin II type 1 receptor blockers (ARBs), as adjunctive therapy to the currently available antipsychotic medications for schizophrenia treatment. With a greater understanding of how RAS inhibition directly modulates neurotransmitter balance in the brain, it is possible that compounds with RAS-inhibiting properties could be used to optimize physiological levels of glutamate, dopamine, and GABA, and the balance among the three neurotransmitters, analogously to how antipsychotic medications mediate the dopaminergic pathways. It can be hoped that a novel approach based on this concept, such as adjunctive telmisartan therapy, may offer practical interventional strategies to address currently unmet therapeutic needs in patients with schizophrenia, especially those with treatment-resistant schizophrenia.

Losartan and isoproterenol promote alterations in the local renin-angiotensin system of rat salivary glands

Renin-angiotensin system (RAS) systemically or locally collaborates with tissue homeostasis, growth and development, which has been extensively studied for its pharmacological implications. This study was primarily aimed at finding and characterizing local RAS in rat parotid, sublingual and submandibular glands. It was also hypothesized that vasoactive drugs could affect the expression of RAS targets, as well as saliva flow and its composition. Therefore, another objective of this study was to compare the effects of losartan (angiotensin II receptor blocker) and isoproterenol (β-adrenergic receptor agonist). Forty-one Wistar rats were divided into three groups and administered a daily intraperitoneal dose of saline, losartan or isoproterenol solutions for one week. The following RAS targets were studied using qPCR: renin (REN), angiotensinogen (AGT), angiotensin converting enzyme (ACE), ACE-2, elastase-2 (ELA-2), AT1-a and MAS receptors, using RPL-13 as a reference gene. Morphology of glands was analyzed by immunohistochemistry using REN, ACE, ACE-2, AT1, AT2 and MAS antibodies. The volume and total protein content of saliva were measured. Our results revealed that ACE, ACE-2, AT1-a, AT2 and MAS receptors were expressed in all salivary gland samples, but REN and ELA-2 were absent. Losartan decreased mRNA expression of RAS targets in parotid (MAS) and submandibular glands (ACE and both AT receptors), without affecting morphological alterations, and significantly decreased saliva and total protein secretions. Isoproterenol treatment affected gene expression profiles in parotid (ACE, ACE-2, AT1-a, MAS, AGT), and submandibular (ACE, AT2, AGT) glands, thus promoting acinar hypertrophy in serous acini, without significant changes in salivary flow or total protein content. These drugs affected mainly acini, followed by duct systems and myoepithelial cells, whereas blood vessels were not affected. In conclusion, there is a local RAS in major rat salivary glands and losartan, an angiotensin II receptor blocker, affected not only the RAS-target gene expression but also decreased salivary flow and total protein content.

A microanalytical capillary electrophoresis mass spectrometry assay for quantifying angiotensin peptides in the brain

The renin-angiotensin system (RAS) of the brain produces a series of biologically active angiotensinogen-derived peptides involved in physiological homeostasis and pathophysiology of disease. Despite significant research efforts to date, a comprehensive understanding of brain RAS physiology is lacking. A significant challenge has been the limited set of bioanalytical assays capable of detecting angiotensin (Ang) peptides at physiologically low concentrations (2-15 fmol/g of wet tissue) and sufficient chemical specificity for unambiguous molecular identifications. Additionally, a complex brain anatomy calls for microanalysis of specific tissue regions, thus further taxing sensitivity requirements for identification and quantification in studies of the RAS. To fill this technology gap, we here developed a microanalytical assay by coupling a laboratory-built capillary electrophoresis (CE) nano-electrospray ionization (nano-ESI) platform to a high-resolution mass spectrometer (HRMS). Using parallel reaction monitoring, we demonstrated that this technology achieved confident identification and quantification of the Ang peptides at approx. 5 amol to 300 zmol sensitivity. This microanalytical assay revealed differential Ang peptide profiles between tissues that were micro-sampled from the subfornical organ and the paraventricular nucleus of the hypothalamus, important brain regions involved in thirst and water homeostasis and neuroendocrine regulation to stress. Microanalytical CE-nano-ESI-HRMS extends the analytical toolbox of neuroscience to help better understand the RAS.

The Involvement of Renin-Angiotensin System in Lipopolysaccharide-Induced Behavioral Changes, Neuroinflammation, and Disturbed Insulin Signaling

Brain insulin signaling is accounted for the development of a variety of neuropsychiatric disorders, such as anxiety and depression, whereas both inflammation and the activated renin-angiotensin system (RAS) are two major contributors to insulin resistance. Intriguingly, inflammation and RAS can activate each other, forming a positive feedback loop that would result in exacerbated unwanted tissue damage. To further examine the interrelationship among insulin signaling, neuroinflammation and RAS in the brain, the effect of repeated lipopolysaccharide (LPS) exposure and co-treatment with the angiotensin II (Ang II) receptor type 1 (AT1) blocker, candesartan (Cand), on anxiety and depression-like behaviors, RAS, neuroinflammation and insulin signaling was explored. Our results demonstrated that prolonged LPS challenge successfully induced the rats into anxiety and depression-like state, accompanied with significant neural apoptosis and neuroinflammation. LPS also activated RAS as evidenced by the enhanced angiotensin converting enzyme (ACE) expression, Ang II generation and AT1 expression. However, blocking the activated RAS with Cand co-treatment conferred neurobehavioral protective properties. The AT1 blocker markedly ameliorated the microglial activation, the enhanced gene expression of the proinflammatory cytokines and the overactivated NF-κB signaling. In addition, Cand also mitigated the LPS-induced disturbance of insulin signaling with the normalized phosphorylation of serine 307 and tyrosine 896 of insulin receptor substrate-1 (IRS-1). Collectively, the present study, for the first time, provided the direct evidence indicating that the inflammatory condition may interact with RAS to impede brain insulin pathway, resulting in neurobehavioral damage, and inhibiting RAS seems to be a promising strategy to block the cross-talk and cut off the vicious cycle between RAS and immune system.

Intratubular and intracellular renin-angiotensin system in the kidney: a unifying perspective in blood pressure control

The renin-angiotensin system (RAS) is widely recognized as one of the most important vasoactive hormonal systems in the physiological regulation of blood pressure and the development of hypertension. This recognition is derived from, and supported by, extensive molecular, cellular, genetic, and pharmacological studies on the circulating (tissue-to-tissue), paracrine (cell-to-cell), and intracrine (intracellular, mitochondrial, nuclear) RAS during last several decades. Now, it is widely accepted that circulating and local RAS may act independently or interactively, to regulate sympathetic activity, systemic and renal hemodynamics, body salt and fluid balance, and blood pressure homeostasis. However, there remains continuous debate with respect to the specific sources of intratubular and intracellular RAS in the kidney and other tissues, the relative contributions of the circulating RAS to intratubular and intracellular RAS, and the roles of intratubular compared with intracellular RAS to the normal control of blood pressure or the development of angiotensin II (ANG II)-dependent hypertension. Based on a lecture given at the recent XI International Symposium on Vasoactive Peptides held in Horizonte, Brazil, this article reviews recent studies using mouse models with global, kidney- or proximal tubule-specific overexpression (knockin) or deletion (knockout) of components of the RAS or its receptors. Although much knowledge has been gained from cell- and tissue-specific transgenic or knockout models, a unifying and integrative approach is now required to better understand how the circulating and local intratubular/intracellular RAS act independently, or with other vasoactive systems, to regulate blood pressure, cardiovascular and kidney function.