Angiotensin II attenuates endothelium-dependent responses in the cerebral microcirculation through nox-2-derived radicals

Atherosclerotic burden and cerebral small vessel disease: exploring the link through microvascular aging and cerebral microhemorrhages

Cerebral microhemorrhages (CMHs, also known as cerebral microbleeds) are a critical but frequently underestimated aspect of cerebral small vessel disease (CSVD), bearing substantial clinical consequences. Detectable through sensitive neuroimaging techniques, CMHs reveal an extensive pathological landscape. They are prevalent in the aging population, with multiple CMHs often being observed in a given individual. CMHs are closely associated with accelerated cognitive decline and are increasingly recognized as key contributors to the pathogenesis of vascular cognitive impairment and dementia (VCID) and Alzheimer's disease (AD). This review paper delves into the hypothesis that atherosclerosis, a prevalent age-related large vessel disease, extends its pathological influence into the cerebral microcirculation, thereby contributing to the development and progression of CSVD, with a specific focus on CMHs. We explore the concept of vascular aging as a continuum, bridging macrovascular pathologies like atherosclerosis with microvascular abnormalities characteristic of CSVD. We posit that the same risk factors precipitating accelerated aging in large vessels (i.e., atherogenesis), primarily through oxidative stress and inflammatory pathways, similarly instigate accelerated microvascular aging. Accelerated microvascular aging leads to increased microvascular fragility, which in turn predisposes to the formation of CMHs. The presence of hypertension and amyloid pathology further intensifies this process. We comprehensively overview the current body of evidence supporting this interconnected vascular hypothesis. Our review includes an examination of epidemiological data, which provides insights into the prevalence and impact of CMHs in the context of atherosclerosis and CSVD. Furthermore, we explore the shared mechanisms between large vessel aging, atherogenesis, microvascular aging, and CSVD, particularly focusing on how these intertwined processes contribute to the genesis of CMHs. By highlighting the role of vascular aging in the pathophysiology of CMHs, this review seeks to enhance the understanding of CSVD and its links to systemic vascular disorders. Our aim is to provide insights that could inform future therapeutic approaches and research directions in the realm of neurovascular health.

Neurovascular coupling impairment as a mechanism for cognitive deficits in COVID-19

Components that comprise our brain parenchymal and cerebrovascular structures provide a homeostatic environment for proper neuronal function to ensure normal cognition. Cerebral insults (e.g. ischaemia, microbleeds and infection) alter cellular structures and physiologic processes within the neurovascular unit and contribute to cognitive dysfunction. COVID-19 has posed significant complications during acute and convalescent stages in multiple organ systems, including the brain. Cognitive impairment is a prevalent complication in COVID-19 patients, irrespective of severity of acute SARS-CoV-2 infection. Moreover, overwhelming evidence from in vitro, preclinical and clinical studies has reported SARS-CoV-2-induced pathologies in components of the neurovascular unit that are associated with cognitive impairment. Neurovascular unit disruption alters the neurovascular coupling response, a critical mechanism that regulates cerebromicrovascular blood flow to meet the energetic demands of locally active neurons. Normal cognitive processing is achieved through the neurovascular coupling response and involves the coordinated action of brain parenchymal cells (i.e. neurons and glia) and cerebrovascular cell types (i.e. endothelia, smooth muscle cells and pericytes). However, current work on COVID-19-induced cognitive impairment has yet to investigate disruption of neurovascular coupling as a causal factor. Hence, in this review, we aim to describe SARS-CoV-2's effects on the neurovascular unit and how they can impact neurovascular coupling and contribute to cognitive decline in acute and convalescent stages of the disease. Additionally, we explore potential therapeutic interventions to mitigate COVID-19-induced cognitive impairment. Given the great impact of cognitive impairment associated with COVID-19 on both individuals and public health, the necessity for a coordinated effort from fundamental scientific research to clinical application becomes imperative. This integrated endeavour is crucial for mitigating the cognitive deficits induced by COVID-19 and its subsequent burden in this especially vulnerable population.

Superoxide dismutase 2 deficiency is associated with enhanced central chemoreception in mice: Implications for breathing regulation

AIMS

In mammals, central chemoreception plays a crucial role in the regulation of breathing function in both health and disease conditions. Recently, a correlation between high levels of superoxide anion (O2.-) in the Retrotrapezoid nucleus (RTN), a main brain chemoreceptor area, and enhanced central chemoreception has been found in rodents. Interestingly, deficiency in superoxide dismutase 2 (SOD2) expression, a pivotal antioxidant enzyme, has been linked to the development/progression of several diseases. Despite, the contribution of SOD2 on O2.- regulation on central chemoreceptor function is unknown. Accordingly, we sought to determine the impact of partial deletion of SOD2 expression on i) O2.-accumulation in the RTN, ii) central ventilatory chemoreflex function, and iii) disordered-breathing. Finally, we study cellular localization of SOD2 in the RTN of healthy mice.

METHODS

Central chemoreflex drive and breathing function were assessed in freely moving heterozygous SOD2 knockout mice (SOD2+/- mice) and age-matched control wild type (WT) mice by whole-body plethysmography. O2.- levels were determined in RTN brainstem sections and brain isolated mitochondria, while SOD2 protein expression and tissue localization were determined by immunoblot, RNAseq and immunofluorescent staining, respectively.

RESULTS

Our results showed that SOD2+/- mice displayed reductions in SOD2 levels and high O2.- formation and mitochondrial dysfunction within the RTN compared to WT. Additionally, SOD2+/- mice displayed a heightened ventilatory response to hypercapnia and exhibited overt signs of altered breathing patterns. Both, RNAseq analysis and immunofluorescence co-localization studies showed that SOD2 expression was confined to RTN astrocytes but not to RTN chemoreceptor neurons. Finally, we found that SOD2+/- mice displayed alterations in RTN astrocyte morphology compared to RTN astrocytes from WT mice.

INNOVATION & CONCLUSION

These findings provide first evidence of the role of SOD2 in the regulation of O2.- levels in the RTN and its potential contribution on the regulation of central chemoreflex function. Our results suggest that reductions in the expression of SOD2 in the brain may contribute to increase O2.- levels in the RTN being the outcome a chronic surge in central chemoreflex drive and the development/maintenance of altered breathing patterns. Overall, dysregulation of SOD2 and the resulting increase in O2.- levels in brainstem respiratory areas can disrupt normal respiratory control mechanisms and contribute to breathing dysfunction seen in certain disease conditions characterized by high oxidative stress.

Female mice are protected from impaired parenchymal arteriolar TRPV4 function and impaired cognition in hypertension

Hypertension is a leading modifiable risk factor for cerebral small vessel disease. Our laboratory has shown that endothelium-dependent dilation in cerebral parenchymal arterioles (PAs) is dependent on transient receptor potential vanilloid 4 (TRPV4) activation, and this pathway is impaired in hypertension. This impaired dilation is associated with cognitive deficits and neuroinflammation. Epidemiological evidence suggests that women with midlife hypertension have an increased dementia risk that does not exist in age-matched men, though the mechanisms responsible for this are unclear. This study aimed to determine the sex differences in young, hypertensive mice to serve as a foundation for future determination of sex differences at midlife. We tested the hypothesis that young hypertensive female mice would be protected from the impaired TRPV4-mediated PA dilation and cognitive dysfunction observed in male mice. Angiotensin II (ANG II)-filled osmotic minipumps (800 ng/kg/min, 4 wk) were implanted in 16- to 19-wk-old male C56BL/6 mice. Age-matched female mice received either 800 ng/kg/min or 1,200 ng/kg/min ANG II. Sham-operated mice served as controls. Systolic blood pressure was elevated in ANG II-treated male mice and in 1,200 ng ANG II-treated female mice versus sex-matched shams. PA dilation in response to the TRPV4 agonist GSK1016790A (10-9-10-5 M) was impaired in hypertensive male mice, which was associated with cognitive dysfunction and neuroinflammation, reproducing our previous findings. Hypertensive female mice exhibited normal TRPV4-mediated PA dilation and were cognitively intact. Female mice also showed fewer signs of neuroinflammation than male mice. Determining the sex differences in cerebrovascular health in hypertension is critical for developing effective therapeutic strategies for women.NEW & NOTEWORTHY Vascular dementia is a significant public health concern, and the effect of biological sex on dementia development is not well understood. TRPV4 channels are essential regulators of cerebral parenchymal arteriolar function and cognition. Hypertension impairs TRPV4-mediated dilation and memory in male rodents. Data presented here suggest female sex protects against impaired TRPV4 dilation and cognitive dysfunction during hypertension. These data advance our understanding of the influence of biological sex on cerebrovascular health in hypertension.

Neurovascular Coupling in Hypertension Is Impaired by IL-17A through Oxidative Stress

Hypertension, a multifactorial chronic inflammatory condition, is an important risk factor for neurovascular and neurodegenerative diseases, including stroke and Alzheimer's disease. These diseases have been associated with higher concentrations of circulating interleukin (IL)-17A. However, the possible role that IL-17A plays in linking hypertension with neurodegenerative diseases remains to be established. Cerebral blood flow regulation may be the crossroads of these conditions because regulating mechanisms may be altered in hypertension, including neurovascular coupling (NVC), known to participate in the pathogenesis of stroke and Alzheimer's disease. In the present study, the role of IL-17A on NVC impairment induced by angiotensin (Ang) II in the context of hypertension was examined. Neutralization of IL-17A or specific inhibition of its receptor prevents the NVC impairment (p < 0.05) and cerebral superoxide anion production (p < 0.05) induced by Ang II. Chronic administration of IL-17A impairs NVC (p < 0.05) and increases superoxide anion production. Both effects were prevented with Tempol and NADPH oxidase 2 gene deletion. These findings suggest that IL-17A, through superoxide anion production, is an important mediator of cerebrovascular dysregulation induced by Ang II. This pathway is thus a putative therapeutic target to restore cerebrovascular regulation in hypertension.

Discovering the Effects of Fisetin on NF-κB/NLRP-3/NRF-2 Molecular Pathways in a Mouse Model of Vascular Dementia Induced by Repeated Bilateral Carotid Occlusion

Vascular dementia (VaD) is the second leading cause of dementia. The majority of VaD patients have cognitive abnormalities, which are caused by cerebral hypoperfusion-induced ischemia, endothelial dysfunction, oxidative stress, and neuroinflammation. Natural products are receiving increasing attention for the treatment of neuroinflammatory diseases. The aim of this study was to investigate the molecular pathways underlying the protective effects of fisetin, a flavonoid present in many fruits and vegetables, in a mouse model of VaD induced by repeated ischemia-reperfusion (IR) of the total bilateral carotid artery. Here, we found that VaD caused brain injury, lipid peroxidation, and neuronal death in the hippocampus, as well as astrocyte and microglial activation, and reduced BDNF neurotrophic factor expression together with behavioral alterations. In addition, VaD induced the activation of inflammasome components (NLRP-3, ASC, and caspase 1), and their downstream products (IL-1β and IL-18) release and promote activation of apoptotic cell death. Fisetin attenuated histological injury, malondialdehyde levels, inflammasome pathway activation, apoptosis, as well as increased BDNF expression, reduced astrocyte, microglial activation, and cognitive deficits. In conclusion, the protective effects of fisetin could be due to the inhibition of the ROS-induced activation of NF-κB/NLRP3 inflammasome together with the activation of antioxidant Nrf2/HO-1, suggesting a possible crosstalk between these molecular pathways.

RGS5 Protein Deficiency Differentially Influences Blood Pressure, Vascular and Behavioral Effects in Aged Male Mice

ABSTRACT

Aging and elevated activity of the renin-angiotensin-system (RAS) are associated with hypertension, vascular and emotional behavioral abnormalities, like anxiety and depression. Many actions of the main effector hormone of the RAS, angiotensin II (Ang II), are mediated by Ang II type 1 receptor whose activity is modulated by the regulator of G-protein signaling 5 (RGS5) protein. We assessed the role of RGS5 on blood pressure, vascular and emotional behavioral outcomes in aged male mice in the presence and absence of chronically elevated Ang II levels. We used aged (∼21 month old) male RGS5-deficient (RGS5-/-) and wild-type (RGS5+/+) mice treated with vehicle (saline) or Ang II (1 mg/kg/d for 21 days). RGS5 deficiency increased baseline systolic blood pressure (SBP), and cerebral vascular superoxide levels in the presence of chronically elevated Ang II levels, suggesting that RGS5 deficiency leads to elevated blood pressure and deleterious cerebral vascular outcomes in aged mice. RGS5 deletion had no effect on Ang II-induced increases in SBP. Chronically elevated Ang II levels increased spontaneous locomotor activity in RGS5+/+, but not RGS5-/- mice. RGS5 deficiency and Ang II treatment had no effect on anxiety- and depression-like behavior. This is the first study to assess the effects of deficiency of an RGS protein in the vasculature or on emotional behavioral outcomes in aged mice. We report that RGS5 has protective effects on blood pressure and the cerebral vasculature in aged mice. Clinically, these data suggest that RAS blockers may significantly reduce cerebrovascular disease risk in aged males lacking RGS5.

Effects of Natural Polyphenols on Oxidative Stress-Mediated Blood–Brain Barrier Dysfunction

The blood-brain barrier (BBB), which consists mainly of brain microvascular endothelial cells and astrocytes connected by tight junctions (TJs) and adhesion molecules (AMs), maintains the homeostatic balance between brain parenchyma and extracellular fluid. Accumulating evidence shows that BBB dysfunction is a common feature of neurodegenerative diseases, including stroke, traumatic brain injury, and Alzheimer’s disease. Among the various pathological pathways of BBB dysfunction, reactive oxygen species (ROS) are known to play a key role in inducing BBB disruption mediated via TJ modification, AM induction, cytoskeletal reorganization, and matrix metalloproteinase activation. Thus, antioxidants have been suggested to exert beneficial effects on BBB dysfunction-associated brain diseases. In this review, we summarized the sources of ROS production in multiple cells that constitute or surround the BBB, such as BBB endothelial cells, astrocytes, microglia, and neutrophils. We also reviewed various pathological mechanisms by which BBB disruption is caused by ROS in these cells. Finally, we summarized the effects of various natural polyphenols on BBB dysfunction to suggest a therapeutic strategy for BBB disruption-related brain diseases.

Inflammation: A Mediator Between Hypertension and Neurodegenerative Diseases

Hypertension is the most prevalent and modifiable risk factor for stroke, vascular cognitive impairment, and Alzheimer's disease. However, the mechanistic link between hypertension and neurodegenerative diseases remains to be understood. Recent evidence indicates that inflammation is a common pathophysiological trait for both hypertension and neurodegenerative diseases. Low-grade chronic inflammation at the systemic and central nervous system levels is now recognized to contribute to the physiopathology of hypertension. This review speculates that inflammation represents a mediator between hypertension and neurodegenerative diseases, either by a decrease in cerebral blood flow or a disruption of the blood-brain barrier which will, in turn, let inflammatory cells and neurotoxic molecules enter the brain parenchyma. This may impact brain functions including cognition and contribute to neurodegenerative diseases. This review will thus discuss the relationship between hypertension, systemic inflammation, cerebrovascular functions, neuroinflammation, and brain dysfunctions. The potential clinical future of immunotherapies against hypertension and associated cerebrovascular risks will also be presented.

NADPH Oxidases in the Central Nervous System: Regional and Cellular Localization and the Possible Link to Brain Diseases

Significance: The significant role of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) in signal transduction is mediated by the production of reactive oxygen species (ROS), especially in the central nervous system (CNS). The pathogenesis of some neurologic and psychiatric diseases is regulated by ROS, acting as a second messenger or pathogen. Recent Advances: In the CNS, the involvement of Nox-derived ROS has been implicated in the regulation of multiple signals, including cell survival/apoptosis, neuroinflammation, migration, differentiation, proliferation, and synaptic plasticity, as well as the integrity of the blood/brain barrier. In these processes, the intracellular signals mediated by the members of the Nox family vary among different tissues. The present review illuminates the regions and cellular, subcellular localization of Nox isoforms in the brain, the signal transduction, and the role of NOX enzymes in pathophysiology, respectively. Critical Issues: Different signal transduction cascades are coupled to ROS derived from various Nox homologues with varying degrees. Therefore, a critical issue worth noting is the varied role of the homologues of NOX enzymes in different signaling pathways and also they mediate different phenotypes in the diverse pathophysiological condition. This substantiates the effectiveness of selective Nox inhibitors in the CNS. Future Directions: Further investigation to elucidate the role of various homologues of NOX enzymes in acute and chronic brain diseases and signaling mechanisms, and the development of more specific NOX inhibitors for the treatment of CNS disease are urgently needed. Antioxid. Redox Signal. 35, 951-973.

Hypertension-induced cognitive impairment: from pathophysiology to public health

Hypertension affects two-thirds of people aged >60 years and significantly increases the risk of both vascular cognitive impairment and Alzheimer's disease. Hypertension compromises the structural and functional integrity of the cerebral microcirculation, promoting microvascular rarefaction, cerebromicrovascular endothelial dysfunction and neurovascular uncoupling, which impair cerebral blood supply. In addition, hypertension disrupts the blood-brain barrier, promoting neuroinflammation and exacerbation of amyloid pathologies. Ageing is characterized by multifaceted homeostatic dysfunction and impaired cellular stress resilience, which exacerbate the deleterious cerebromicrovascular effects of hypertension. Neuroradiological markers of hypertension-induced cerebral small vessel disease include white matter hyperintensities, lacunar infarcts and microhaemorrhages, all of which are associated with cognitive decline. Use of pharmaceutical and lifestyle interventions that reduce blood pressure, in combination with treatments that promote microvascular health, have the potential to prevent or delay the pathogenesis of vascular cognitive impairment and Alzheimer's disease in patients with hypertension.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Cardiovascular co-morbidities, inflammation and cerebral small vessel disease

Cerebral small vessel disease (cSVD) is the most common cause of vascular cognitive impairment and affects all levels of the brain's vasculature. Features include diverse structural and functional changes affecting small arteries and capillaries that lead to a decline in cerebral perfusion. Due to an aging population, incidence of cerebral small vessel disease (cSVD) is continually rising. Despite its prevalence and its ability to cause multiple debilitating illnesses, such as stroke and dementia, there are currently no therapeutic strategies for the treatment of cSVD. In the healthy brain, interactions between neuronal, vascular and inflammatory cells are required for normal functioning. When these interactions are disturbed, chronic pathological inflammation can ensue. The interplay between cSVD and inflammation has attracted much recent interest and this review discusses chronic cardiovascular diseases, particularly hypertension, and explores how the associated inflammation may impact on the structure and function of the small arteries of the brain in cSVD. Molecular approaches in animal studies are linked to clinical outcomes in patients and novel hypotheses regarding inflammation and cSVD are proposed that will hopefully stimulate further discussion and study in this important area.

Angiotensin II Disrupts Neurovascular Coupling by Potentiating Calcium Increases in Astrocytic Endfeet

Background Angiotensin II (Ang II), a critical mediator of hypertension, impairs neurovascular coupling. Since astrocytes are key regulators of neurovascular coupling, we sought to investigate whether Ang II impairs neurovascular coupling through modulation of astrocytic Ca2+ signaling. Methods and Results Using laser Doppler flowmetry, we found that Ang II attenuates cerebral blood flow elevations induced by whisker stimulation or the metabotropic glutamate receptors agonist, 1S, 3R-1-aminocyclopentane-trans-1,3-dicarboxylic acid (P<0.01). In acute brain slices, Ang II shifted the vascular response induced by 1S, 3R-1-aminocyclopentane-trans-1,3-dicarboxylic acid towards vasoconstriction (P<0.05). The resting and 1S, 3R-1-aminocyclopentane-trans-1,3-dicarboxylic acid-induced Ca2+ levels in the astrocytic endfeet were more elevated in the presence of Ang II (P<0.01). Both effects were reversed by the AT1 receptor antagonist, candesartan (P<0.01 for diameter and P<0.05 for calcium levels). Using photolysis of caged Ca2+ in astrocytic endfeet or pre-incubation of 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis (acetoxymethyl ester), we demonstrated the link between potentiated Ca2+ elevation and impaired vascular response in the presence of Ang II (P<0.001 and P<0.05, respectively). Both intracellular Ca2+ mobilization and Ca2+ influx through transient receptor potential vanilloid 4 mediated Ang II-induced astrocytic Ca2+ elevation, since blockade of these pathways significantly prevented the intracellular Ca2+ in response to 1S, 3R-1-aminocyclopentane-trans-1,3-dicarboxylic acid (P<0.05). Conclusions These results suggest that Ang II through its AT1 receptor potentiates the astrocytic Ca2+ responses to a level that promotes vasoconstriction over vasodilation, thus altering cerebral blood flow increases in response to neuronal activity.

Increases in hypertension-induced cerebral microhemorrhages exacerbate gait dysfunction in a mouse model of Alzheimer's disease

Clinical studies show that cerebral amyloid angiopathy (CAA) associated with Alzheimer's disease (AD) and arterial hypertension are independent risk factors for cerebral microhemorrhages (CMHs). To test the hypothesis that amyloid pathology and hypertension interact to promote the development of CMHs, we induced hypertension in the Tg2576 mouse model of AD and respective controls by treatment with angiotensin II (Ang II) and the NO synthesis inhibitor L-NAME. The number, size, localization, and neurological consequences (gait alterations) of CMHs were compared. We found that compared to control mice, in TG2576 mice, the same level of hypertension led to significantly increased CMH burden and exacerbation of CMH-related gait alterations. In hypertensive TG2576 mice, CMHs were predominantly located in the cerebral cortex at the cortical-subcortical boundary, mimicking the clinical picture seen in patients with CAA. Collectively, amyloid pathologies exacerbate the effects of hypertension, promoting the genesis of CMHs, which likely contribute to their deleterious effects on cognitive function. Therapeutic strategies for prevention of CMHs that reduce blood pressure and preserve microvascular integrity are expected to exert neuroprotective effects in high-risk elderly AD patients.

Brain Renin-Angiotensin System at the Intersect of Physical and Cognitive Frailty

The renin–angiotensin system (RAS) was initially considered to be part of the endocrine system regulating water and electrolyte balance, systemic vascular resistance, blood pressure, and cardiovascular homeostasis. It was later discovered that intracrine and local forms of RAS exist in the brain apart from the endocrine RAS. This brain-specific RAS plays essential roles in brain homeostasis by acting mainly through four angiotensin receptor subtypes; AT₁R, AT₂R, MasR, and AT₄R. These receptors have opposing effects; AT₁R promotes vasoconstriction, proliferation, inflammation, and oxidative stress while AT₂R and MasR counteract the effects of AT₁R. AT₄R is critical for dopamine and acetylcholine release and mediates learning and memory consolidation. Consequently, aging-associated dysregulation of the angiotensin receptor subtypes may lead to adverse clinical outcomes such as Alzheimer’s disease and frailty via excessive oxidative stress, neuroinflammation, endothelial dysfunction, microglial polarization, and alterations in neurotransmitter secretion. In this article, we review the brain RAS from this standpoint. After discussing the functions of individual brain RAS components and their intracellular and intracranial locations, we focus on the relationships among brain RAS, aging, frailty, and specific neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and vascular cognitive impairment, through oxidative stress, neuroinflammation, and vascular dysfunction. Finally, we discuss the effects of RAS-modulating drugs on the brain RAS and their use in novel treatment approaches.

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.

Oxidative Stress-Mediated Blood-Brain Barrier (BBB) Disruption in Neurological Diseases

The blood-brain barrier (BBB), as a crucial gate of brain-blood molecular exchange, is involved in the pathogenesis of multiple neurological diseases. Oxidative stress is caused by an imbalance between the production of reactive oxygen species (ROS) and the scavenger system. Since oxidative stress plays a significant role in the production and maintenance of the BBB, the cerebrovascular system is especially vulnerable to it. The pathways that initiate BBB dysfunction include, but are not limited to, mitochondrial dysfunction, excitotoxicity, iron metabolism, cytokines, pyroptosis, and necroptosis, all converging on the generation of ROS. Interestingly, ROS also provide common triggers that directly regulate BBB damage, parameters including tight junction (TJ) modifications, transporters, matrix metalloproteinase (MMP) activation, inflammatory responses, and autophagy. We will discuss the role of oxidative stress-mediated BBB disruption in neurological diseases, such as hemorrhagic stroke, ischemic stroke (IS), Alzheimer’s disease (AD), Parkinson’s disease (PD), traumatic brain injury (TBI), amyotrophic lateral sclerosis (ALS), and cerebral small vessel disease (CSVD). This review will also discuss the latest clinical evidence of potential biomarkers and antioxidant drugs towards oxidative stress in neurological diseases. A deeper understanding of how oxidative stress damages BBB may open up more therapeutic options for the treatment of neurological diseases.

Impact of the Renin-Angiotensin System on the Endothelium in Vascular Dementia: Unresolved Issues and Future Perspectives

The effects of the renin-angiotensin system (RAS) surpass the renal and cardiovascular systems to encompass other body tissues and organs, including the brain. Angiotensin II (Ang II), the most potent mediator of RAS in the brain, contributes to vascular dementia via different mechanisms, including neuronal homeostasis disruption, vascular remodeling, and endothelial dysfunction caused by increased inflammation and oxidative stress. Other RAS components of emerging significance at the level of the blood-brain barrier include angiotensin-converting enzyme 2 (ACE2), Ang(1-7), and the AT2, Mas, and AT4 receptors. The various angiotensin hormones perform complex actions on brain endothelial cells and pericytes through specific receptors that have either detrimental or beneficial actions. Increasing evidence indicates that the ACE2/Ang(1-7)/Mas axis constitutes a protective arm of RAS on the blood-brain barrier. This review provides an update of studies assessing the different effects of angiotensins on cerebral endothelial cells. The involved signaling pathways are presented and help highlight the potential pharmacological targets for the management of cognitive and behavioral dysfunctions associated with vascular dementia.

ACE (Angiotensin-Converting Enzyme) Inhibition Reverses Vasoconstriction and Impaired Dilation of Pial Collaterals in Chronic Hypertension

Leptomeningeal anastomoses (LMAs) are pial collaterals that perfuse the penumbra and important for stroke outcome. We previously showed LMAs from SHRs (spontaneously hypertensive rats) were vasoconstricted compared with normotensive Wistar rats. Here, we investigated mechanisms by which hypertension causes LMA vasoconstriction. SHRs were treated with the ACE (angiotensin-converting enzyme) inhibitor captopril, an Ang II (angiotensin II)-independent antihypertensive agent hydralazine, or vehicle for 5 weeks in drinking water (n=8/group). A group of Wistar rats (n=8) had regular drinking water served as controls. Blood pressure was measured twice weekly by tail-cuff. LMAs were isolated and studied under pressurized conditions. Vasoreactivity of LMAs, including myogenic responses, reactivity to Rho-kinase inhibitor Y-27632, and nitric oxide were measured. Both captopril and hydralazine lowered blood pressure in SHRs similar to Wistar. However, only captopril normalized LMA increased tone compared with untreated SHRs (15±2% versus 50±3%; P<0.01) that was similar to Wistar (16±2%) but not hydralazine (38±6%; P>0.05). Vasodilatory response of LMAs to Y-27632 was impaired in SHRs compared with Wistar (28±3% versus 81±4%; P<0.01) that was restored by captopril (84±5%; P<0.01) and partially hydralazine (59±4%). LMAs from all groups constricted similarly to NOS (NO synthase) inhibition; however, the vasodilatory response of LMAs to the nitric oxide donor sodium nitroprusside was impaired in SHRs compared with Wistar rats (29±4% versus 80±2%; P<0.01) that was restored by captopril (84±4%; P<0.01), not hydralazine (38±8%; P>0.05). These results suggest that ACE inhibition during chronic hypertension reversed vascular dysfunction and hyperconstriction of LMAs that could improve stroke outcome by increasing collateral perfusion.

Prospects for Diminishing the Impact of Nonamyloid Small-Vessel Diseases of the Brain

Small-vessel diseases (SVDs) of the brain are involved in about one-fourth of ischemic strokes and a vast majority of intracerebral hemorrhages and are responsible for nearly half of dementia cases in the elderly. SVDs are a heavy burden for society, a burden that is expected to increase further in the absence of significant therapeutic advances, given the aging population. Here, we provide a critical appraisal of currently available therapeutic approaches for nonamyloid sporadic SVDs that are largely based on targeting modifiable risk factors. We review what is known about the pathogenic mechanisms of vascular risk factor-related SVDs and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), the most frequent hereditary SVD, and elaborate on two mechanism-based therapeutic approaches worth exploring in sporadic SVD and CADASIL. We conclude by discussing opportunities and challenges that need to be tackled if efforts to achieve significant therapeutic advances for these diseases are to be successful.

CD4+ Regulatory T Lymphocytes Prevent Impaired Cerebral Blood Flow in Angiotensin II-Induced Hypertension

Background

Immune cells are key regulators of the vascular inflammatory response characteristic of hypertension. In hypertensive rodents, regulatory T lymphocytes (Treg, CD4⁺CD25⁺) prevented vascular injury, cardiac damage, and endothelial dysfunction of mesenteric arteries. Whether Treg modulate the cerebrovascular damage induced by hypertension is unknown.

Methods and Results

C57BL/6 mice were perfused with angiotensin II (Ang II; 1000 ng/kg per minute) for 14 days and adoptive transfer of 3×10⁵CD4⁺CD25⁺ T cells was performed via 2 intravenous injections. Control mice received a sham surgery and PBS. Treg prevented Ang II‐induced neurovascular uncoupling (P<0.05) and endothelial impairment (P<0.05), evaluated by laser Doppler flowmetry in the somatosensory cortex. The neuroprotective effect of Treg was abolished when they were isolated from mice deficient in interleukin‐10. Administration of interleukin‐10 (60 ng/d) to hypertensive mice prevented Ang II‐induced neurovascular uncoupling (P<0.05). Treg adoptive transfer also diminished systemic inflammation induced by Ang II (P<0.05), examined with a peripheral blood cytokine array. Mice receiving Ang II + Treg exhibited reduced numbers of Iba‐1+ cells in the brain cortex (P<0.05) and hippocampus (P<0.001) compared with mice infused only with Ang II. Treg prevented the increase in cerebral superoxide radicals. Overall, these effects did not appear to be directly modulated by Treg accumulating in the brain parenchyma, because only a nonsignificant number of Treg were detected in brain. Instead, Treg penetrated peripheral tissues such as the kidney, inguinal lymph nodes, and the spleen.

Conclusions

Treg prevent impaired cerebrovascular responses in Ang II‐induced hypertension. The neuroprotective effects of Treg involve the modulation of inflammation in the brain and periphery.

The angiotensin II type I receptor contributes to impaired cerebral blood flow autoregulation caused by placental ischemia in pregnant rats

BACKGROUND

Placental ischemia and hypertension, characteristic features of preeclampsia, are associated with impaired cerebral blood flow (CBF) autoregulation and cerebral edema. However, the factors that contribute to these cerebral abnormalities are not clear. Several lines of evidence suggest that angiotensin II can impact cerebrovascular function; however, the role of the renin angiotensin system in cerebrovascular function during placental ischemia has not been examined. We tested whether the angiotensin type 1 (AT1) receptor contributes to impaired CBF autoregulation in pregnant rats with placental ischemia caused by surgically reducing uterine perfusion pressure.

METHODS

Placental ischemic or sham operated rats were treated with vehicle or losartan from gestational day (GD) 14 to 19 in the drinking water. On GD 19, we assessed CBF autoregulation in anesthetized rats using laser Doppler flowmetry.

RESULTS

Placental ischemic rats had impaired CBF autoregulation that was attenuated by treatment with losartan. In addition, we examined whether an agonistic autoantibody to the AT1 receptor (AT1-AA), reported to be present in preeclamptic women, contributes to impaired CBF autoregulation. Purified rat AT1-AA or vehicle was infused into pregnant rats from GD 12 to 19 via mini-osmotic pumps after which CBF autoregulation was assessed. AT1-AA infusion impaired CBF autoregulation but did not affect brain water content.

CONCLUSIONS

These results suggest that the impaired CBF autoregulation associated with placental ischemia is due, at least in part, to activation of the AT1 receptor and that the RAS may interact with other placental factors to promote cerebrovascular changes common to preeclampsia.

Contributions of Aging to Cerebral Small Vessel Disease

Cerebral small vessel disease (SVD) is characterized by changes in the pial and parenchymal microcirculations. SVD produces reductions in cerebral blood flow and impaired blood-brain barrier function, which are leading contributors to age-related reductions in brain health. End-organ effects are diverse, resulting in both cognitive and noncognitive deficits. Underlying phenotypes and mechanisms are multifactorial, with no specific treatments at this time. Despite consequences that are already considerable, the impact of SVD is predicted to increase substantially with the growing aging population. In the face of this health challenge, the basic biology, pathogenesis, and determinants of SVD are poorly defined. This review summarizes recent progress and concepts in this area, highlighting key findings and some major unanswered questions. We focus on phenotypes and mechanisms that underlie microvascular aging, the greatest risk factor for cerebrovascular disease and its subsequent effects.

Effects of senescence and angiotensin II on expression and processing of amyloid precursor protein in human cerebral microvascular endothelial cells

The present study was designed to determine the effects of senescence and angiotensin II (Ang II) on expression and processing of amyloid precursor protein (APP) in human brain microvascular endothelial cells (BMECs). Senescence caused a decrease in APP expression thereby resulting in reduced secretion of soluble APPα (sAPPα). In contrast, β-site APP cleaving enzyme (BACE1) expression and production of amyloid β (Aβ)40 were increased in senescent endothelium. Importantly, in senescent human BMECs, treatment with BACE1 inhibitor IV inhibited Aβ generation and increased sAPPα production by enhancing a disintegrin and metalloprotease (ADAM)10 expression. Furthermore, Ang II impaired expression of ADAM10 and significantly reduced generation of sAPPα in senescent human BMECs. This inhibitory effect of Ang II was prevented by treatment with BACE1 inhibitor IV. Our results suggest that impairment of α-processing and shift to amyloidogenic pathway of APP contribute to endothelial dysfunction induced by senescence. Loss of sAPPα in senescent cells treated with Ang II exacerbates detrimental effects of senescence on APP processing. Notably, inhibition of BACE1 has beneficial effects on senescence induced endothelial dysfunction. Reported findings may help to explain contributions of senescent cerebral microvascular endothelium to development of cerebral amyloid angiopathy and Alzheimer’s disease (AD) pathology.

Emerging molecular mechanisms of vascular dementia

PURPOSE OF REVIEW

Microvascular ischemic disease of the brain is a common cause of cognitive impairment and dementia, particularly in the context of preexisting cardiovascular risk factors and aging. This review summarizes our current understanding of the emerging molecular themes that underlie progressive and irreparable vascular disease leading to neuronal tissue injury and dementia.

RECENT FINDINGS

Cardiometabolic risk factors including diabetes and hypertension are known to contribute to vascular disease. Currently, the impact of these risk factors on the integrity and function of the brain vasculature has been target of intense investigation. Molecularly, the consequences associated with these risk factors indicate that reactive oxygen species are strong contributors to cerebrovascular dysfunction and injury. In addition, genetic linkage analyses have identified penetrant monogenic causes of vascular dementia. Finally, recent reports begun to uncover a large number of polymorphisms associated with a higher risk for cerebrovascular disease.

SUMMARY

A comprehensive picture of key risk factors and genetic predispositions that contribute to brain microvascular disease and result in vascular dementia is starting to emerge. Understanding their relationships and cross-interactions will significantly aid in the development of preventive and intervention strategies for this devastating condition.

Apocynin alters redox signaling in conductance and resistance vessels of spontaneously hypertensive rats

Chronic treatment with apocynin reduces blood pressure and prevents endothelial dysfunction development in spontaneously hypertensive rats (SHR). Mechanisms underlying apocynin effects on SHR remain unclear. Compared to diapocynin and other drugs, apocynin is a weak antioxidant, which suggests that its effects on SHR are associated with other mechanisms besides its antioxidant capacity. Angiotensin (Ang) II regulates NOX, the major reactive oxygen species (ROS) source in the cardiovascular system. We hypothesized that, by inhibiting NOX, apocynin could alter Ang II pressor and vasoconstrictor effects on SHR. We analyzed how Ang II affects blood pressure and vascular reactivity in aorta and mesenteric resistance arteries and evaluated plasma antioxidant capacity, NOX isoforms and subunits, NOS isoforms, AT₁ and AT₂ receptors expression, ROS production, and NOS activity in apocynin-treated SHR blood vessels (30 mg/Kg/day, p.o.). In SHR, apocynin reduced Ang II pressor effects, increased plasmatic antioxidant capacity, and blunted aortic and mesenteric NOX-dependent oxidants production and NOX2 and p47phox overexpression, which demonstrated that apocynin inhibits NOX in SHR blood vessels. Moreover, apocynin raised plasmatic and aortic nitrate/nitrite levels, maintained NOS activity and eNOS, p-eNOS, nNOS, iNOS, sGC-α, and sGC-β expression in mesenteric bed, diminished AT₁ expression in aorta and mesenteric bed, and elevated AT₂ expression in SHR aorta. Apocynin increased Ang II vasoconstriction endothelial modulation in SHR resistance arteries. All these results showed that in vivo treatment with apocynin alters several mechanisms that reduce Ang II pressor and vasoconstrictor effects on SHR. Such apocynin effects involve other mechanisms besides vascular ROS modulation, which improves NO availability in SHR vascular cells. These integrated data could help us to understand the promising apocynin activity as an antihypertensive drug that acts differently from the drugs that are currently being used in the clinical setting.

Neurovascular and Cognitive Dysfunction in Hypertension

Hypertension has emerged as a leading cause of age-related cognitive impairment. Long known to be associated with dementia caused by vascular factors, hypertension has more recently been linked also to Alzheimer disease-the major cause of dementia in older people. Thus, although midlife hypertension is a risk factor for late-life dementia, hypertension may also promote the neurodegenerative pathology underlying Alzheimer disease. The mechanistic bases of these harmful effects remain to be established. Hypertension is well known to alter in the structure and function of cerebral blood vessels, but how these cerebrovascular effects lead to cognitive impairment and promote Alzheimer disease pathology is not well understood. Furthermore, critical questions also concern whether treatment of hypertension prevents cognitive impairment, the blood pressure threshold for treatment, and the antihypertensive agents to be used. Recent advances in neurovascular biology, epidemiology, brain imaging, and biomarker development have started to provide new insights into these critical issues. In this review, we will examine the progress made to date, and, after a critical evaluation of the evidence, we will highlight questions still outstanding and seek to provide a path forward for future studies.

Contributions of sex to cerebrovascular function and pathology

Sex differences exist in how cerebral blood vessels function under both physiological and pathological conditions, contributing to observed sex differences in risk and outcomes of cerebrovascular diseases (CBVDs), such as vascular contributions to cognitive impairment and dementia (VCID) and stroke. Throughout most of the lifespan, women are protected from CBVDs; however, risk increases following menopause, suggesting sex hormones may play a significant role in this protection. The cerebrovasculature is a target for sex hormones, including estrogens, progestins, and androgens, where they can influence numerous vascular functions and pathologies. While there is a plethora of information on estrogen, the effects of progestins and androgens on the cerebrovasculature are less well-defined. Estrogen decreases cerebral tone and increases cerebral blood flow, while androgens increase tone. Both estrogens and androgens enhance angiogenesis/cerebrovascular remodeling. While both estrogens and androgens attenuate cerebrovascular inflammation, pro-inflammatory effects of androgens under physiological conditions have also been demonstrated. Sex hormones exert additional neuroprotective effects by attenuating oxidative stress and maintaining integrity and function of the blood brain barrier. Most animal studies utilize young, healthy, gonadectomized animals, which do not mimic the clinical conditions of aging individuals likely to get CBVDs. This is also concerning, as sex hormones appear to mediate cerebrovascular function differently based on age and disease state (e.g. metabolic syndrome). Through this review, we hope to inspire others to consider sex as a key biological variable in cerebrovascular research, as greater understanding of sex differences in cerebrovascular function will assist in developing personalized approaches to prevent and treat CBVDs.

Hypertension, dietary salt and cognitive impairment

Dementia is growing at an alarming rate worldwide. Although Alzheimer disease is the leading cause, over 50% of individuals diagnosed with Alzheimer disease have vascular lesions at autopsy. There has been an increasing appreciation of the pathogenic role of vascular risk factors in cognitive impairment caused by neurodegeneration. Midlife hypertension is a leading risk factor for late-life dementia. Hypertension alters cerebrovascular structure, impairs the major factors regulating the cerebral microcirculation, and promotes Alzheimer pathology. Experimental studies have identified brain perivascular macrophages as the major free radical source mediating neurovascular dysfunction of hypertension. Recent evidence indicates that high dietary salt may also induce cognitive impairment. Contrary to previous belief, the effect is not necessarily associated with hypertension and is mediated by a deficit in endothelial nitric oxide. Collectively, the evidence suggests a remarkable cellular diversity of the impact of vascular risk factors on the cerebral vasculature and cognition. Whereas long-term longitudinal epidemiological studies are needed to resolve the temporal relationships between vascular risk factors and cognitive dysfunction, single-cell molecular studies of the vasculature in animal models will provide a fuller mechanistic understanding. This knowledge is critical for developing new preventive, diagnostic, and therapeutic approaches for these devastating diseases of the mind.

Oxidative Stress: A Major Player in Cerebrovascular Alterations Associated to Neurodegenerative Events

The brain is one of the most exquisite organs in the body with high metabolic demands, and requires a tight regulation of the surrounding environment. This tight control is exerted by the neurovascular unit (NVU) comprising different cell types, where endothelial cells play the commander-in-chief role. Thus, it is assumable that even slight perturbations in NVU might affect, in some cases irreversibly, brain homeostasis and health. In this line, recent findings support the two-hit vascular hypothesis for neurodegenerative conditions, where vascular dysfunction underlies the development of neurodegenerative diseases, such as Alzheimer’s disease (AD). Knowing that endothelial cells are rich in mitochondria and nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, two major reactive oxygen species (ROS) sources, this review aims to gather information on how oxidative stress is in the front line of vascular alterations observed in brain aging and neurodegenerative conditions, particularly AD. Also, a brief discussion about the therapeutic strategies aimed to protect against cerebrovascular diseases is included.

Stepwise occlusion of the carotid arteries of the rat: MRI assessment of the effect of donepezil and hypoperfusion-induced brain atrophy and white matter microstructural changes

Bilateral common carotid artery occlusion (BCCAo) in the rat is a widely used animal model of vascular dementia and a valuable tool for preclinical pharmacological drug testing, although the varying degrees of acute focal ischemic lesions it induces could interfere with its translational value. Recently, a modification to the BCCAo model, the stepwise occlusion of the two carotid arteries, has been introduced. To acquire objective translatable measures, we used longitudinal multimodal magnetic resonance imaging (MRI) to assess the effects of semi-chronic (8 days) donepezil treatment in this model, with half of the Wistar rats receiving the treatment one week after the stepwise BCCAo. With an ultrahigh field MRI, we measured high-resolution anatomy, diffusion tensor imaging, cerebral blood flow measurements and functional MRI in response to whisker stimulation, to evaluate both the structural and functional effects of the donepezil treatment and stepwise BCCAo up to 5 weeks post-occlusion. While no large ischemic lesions were detected, atrophy in the striatum and in the neocortex, along with widespread white matter microstructural changes, were found. Donepezil ameliorated the transient drop in the somatosensory BOLD response in distant cortical areas, as detected 2 weeks after the occlusion but the drug had no effect on the long term structural changes. Our results demonstrate a measurable functional MRI effect of the donepezil treatment and the importance of diffusion MRI and voxel based morphometry (VBM) analysis in the translational evaluation of the rat BCCAo model.

Oxidative stress in cerebral small vessel disease. Role of reactive species

Cerebral small vessel disease (CSVD) is a wide term describing the condition affecting perforating arterial branches as well as arterioles, venules, and capillaries. Cerebral vascular net is one of the main targets of localised oxidative stress processes causing damage to vasculature, changes in the blood flow and blood-brain barrier and, in consequence, promoting neurodegenerative alterations in the brain tissue. Numerous studies report the fact of oxidation to proteins, sugars, lipids and nucleic acids, occurring in most neurodegenerative diseases mainly in the earliest stages and correlations with the development of cognitive and motor disturbances. The dysfunction of endothelium can be caused by oxidative stress and inflammatory mechanisms as a result of reactions and processes generating extensive reactive oxygen species (ROS) production such as high blood pressure, oxidised low density lipoproteins (oxLDL), very low density lipoproteins (vLDL), diabetes, homocysteinaemia, smoking, and infections. Several animal studies show positive aspects of ROS, especially within cerebral vasculature.

A Mini-Review of the NADPH oxidases in Vascular Dementia: Correlation with NOXs and Risk Factors for VaD

Oxidative stress (OS) is one of the factors that cause dementia conditions such as Alzheimer’s disease and vascular dementia (VaD). In the pathogenesis of VaD, OS is associated with risk factors that include increased age, hypertension, and stroke. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a molecular source of reactive oxygen species (ROS). According to recent studies, inhibition of NOX activity can reduce cognitive impairment in animal models of VaD. In this article, we review the evidence linking cognitive impairment with NOX-dependent OS, including the vascular NOX and non-vascular NOX systems, in VaD.

Hypertension impairs neurovascular coupling and promotes microvascular injury: role in exacerbation of Alzheimer's disease

Hypertension in the elderly substantially increases both the risk of vascular cognitive impairment (VCI) and Alzheimer's disease (AD); however, the underlying mechanisms are not completely understood. This review discusses the effects of hypertension on structural and functional integrity of cerebral microcirculation, including hypertension-induced alterations in neurovascular coupling responses, cellular and molecular mechanisms involved in microvascular damage (capillary rarefaction, blood-brain barrier disruption), and the genesis of cerebral microhemorrhages and their potential role in exacerbation of cognitive decline associated with AD. Understanding and targeting the hypertension-induced cerebromicrovascular alterations that are involved in the onset and progression of AD and contribute to cognitive impairment are expected to have a major role in preserving brain health in high-risk older individuals.

Microvascular NADPH oxidase in health and disease

The systemic and cerebral microcirculation contribute critically to regulation of local and global blood flow and perfusion pressure. Microvascular dysfunction, commonly seen in numerous cardiovascular pathologies, is associated with alterations in the oxidative environment including potentiated production of reactive oxygen species (ROS) and subsequent activation of redox signaling pathways. NADPH oxidases (Noxs) are a primary source of ROS in the vascular system and play a central role in cardiovascular health and disease. In this review, we focus on the roles of Noxs in ROS generation in resistance arterioles and capillaries, and summarize their contributions to microvascular physiology and pathophysiology in both systemic and cerebral microcirculation. In light of the accumulating evidence that Noxs are pivotal players in vascular dysfunction of resistance arterioles, selectively targeting Nox isozymes could emerge as a novel and effective therapeutic strategy for preventing and treating microvascular diseases.

Cerebral blood flow regulation and neurovascular dysfunction in Alzheimer disease

Cerebral blood flow (CBF) regulation is essential for normal brain function. The mammalian brain has evolved a unique mechanism for CBF control known as neurovascular coupling. This mechanism ensures a rapid increase in the rate of CBF and oxygen delivery to activated brain structures. The neurovascular unit is composed of astrocytes, mural vascular smooth muscle cells and pericytes, and endothelia, and regulates neurovascular coupling. This Review article examines the cellular and molecular mechanisms within the neurovascular unit that contribute to CBF control, and neurovascular dysfunction in neurodegenerative disorders such as Alzheimer disease.

Hypertension-induced synapse loss and impairment in synaptic plasticity in the mouse hippocampus mimics the aging phenotype: implications for the pathogenesis of vascular cognitive impairment

Strong epidemiological and experimental evidence indicates that hypertension has detrimental effects on the cerebral microcirculation and thereby promotes accelerated brain aging. Hypertension is an independent risk factor for both vascular cognitive impairment (VCI) and Alzheimer's disease (AD). However, the pathophysiological link between hypertension-induced cerebromicrovascular injury (e.g., blood-brain barrier disruption, increased microvascular oxidative stress, and inflammation) and cognitive decline remains elusive. The present study was designed to characterize neuronal functional and morphological alterations induced by chronic hypertension and compare them to those induced by aging. To achieve that goal, we induced hypertension in young C57BL/6 mice by chronic (4 weeks) infusion of angiotensin II. We found that long-term potentiation (LTP) of performant path synapses following high-frequency stimulation of afferent fibers was decreased in hippocampal slices obtained from hypertensive mice, mimicking the aging phenotype. Hypertension and advanced age were associated with comparable decline in synaptic density in the stratum radiatum of the mouse hippocampus. Hypertension, similar to aging, was associated with changes in mRNA expression of several genes involved in regulation of neuronal function, including down-regulation of Bdnf, Homer1, and Dlg4, which may have a role in impaired synaptic plasticity. Collectively, hypertension impairs synaptic plasticity, reduces synaptic density, and promotes dysregulation of genes involved in synaptic function in the mouse hippocampus mimicking the aging phenotype. These hypertension-induced neuronal alterations may impair establishment of memories in the hippocampus and contribute to the pathogenesis and clinical manifestation of both vascular cognitive impairment (VCI) and Alzheimer's disease (AD).

Hypertension alters the endothelial-dependent biphasic response of bradykinin in isolated Microminipig basilar artery

Angiotensin (Ang) II is known to promote vascular disease and hypertension, partly through its effect on vascular endothelium. Bradykinin (BK) is an endothelium-dependent agonist that induces relaxation followed by contraction of the porcine basilar artery through release of NO and PGF_2α, respectively. In this study, we evaluated the effect of Ang II-induced hypertension on basilar artery responsiveness to BK in the Microminipig (MMPig). Ang II (200ng/kg/min) or vehicle was infused into MMPigs for 14days using an osmotic mini-pump and blood pressure was monitored regularly. The responsiveness of subsequently isolated basilar arteries was then measured using a micro organ bath system. MMPig basilar artery endothelial cells were cultured and stimulated with Ang II or vehicle for 48h. Mean blood pressure was significantly (P<0.05; n=5) higher in Ang II-infused MMPigs than in vehicle-infused MMPigs. In vitro, BK-induced endothelium-dependent dilation of isolated basilar artery specimens was abolished and BK-induced contraction was significantly increased (E_max: 15.85±2.42% and 56.54±2.71% of 60mM KCl in control and Ang II group respectively at 10^-7M concentration of BK; P<0.01; n=5) in Ang II-infused MMPigs. Ang II stimulation of the endothelial cells significantly decreased (54.15% at 24h; P<0.05; n=three independent experiment performed in triplicate) the amount of BK-elicited NO and increased (44.27% at 24h; P<0.05; n=three independent experiment performed in triplicate) the amount of BK-elicited PGF_2α. These results suggest that the decrease of NO and increase of PGF_2α production from endothelial cells are responsible for cerebrovascular dysfunction in hypertension, possibly causing cerebrovascular contraction and thus increasing the risk of brain infarction.

Cardiac Cell Senescence and Redox Signaling

Aging is characterized by a progressive loss of the ability of the organism to cope with stressors and to repair tissue damage. As a result, chronic diseases, including cardiovascular disease, increase their prevalence with aging, underlining the existence of common mechanisms that lead to frailty and age-related diseases. In this frame, the progressive decline of the homeostatic and reparative function of primitive cells has been hypothesized to play a major role in the evolution of cardiac pathology to heart failure. Although initially it was believed that reactive oxygen species (ROS) were produced in an unregulated manner as a byproduct of cellular metabolism, causing macromolecular damage and aging, accumulating evidence indicate the major role played by redox signaling in physiology. Aim of this review is to critically revise evidence linking ROS to cell senescence and aging and to provide evidence of the primary role played by redox signaling, with a particular emphasis on the multifunctional protein APE1/Ref in stem cell biology. Finally, we will discuss evidence supporting the role of redox signaling in cardiovascular cells.

The Pathophysiological Role of NOX2 in Hypertension and Organ Damage

NADPH oxidases (NOXs) represent one of the major sources of reactive oxygen species in the vascular district. Reactive oxygen species are responsible for vascular damage that leads to several cardiovascular pathological conditions. Among NOX isoforms, NOX2 is widely expressed in many cells types, such as cardiomyocytes, endothelial cells, and vascular smooth muscle cells, confirming its pivotal role in vascular pathophysiology. Studies in mice models with systemic deletion of NOX2, as well as in transgenic mice overexpressing NOX2, have demonstrated the undeniable involvement of NOX2 in the development of hypertension, atherosclerosis, diabetes mellitus, cardiac hypertrophy, platelet aggregation, and aging. Of note, the inhibition of NOX2 has been found to be protective for cardiovascular homeostasis. Here, we review the evidence demonstrating that the modulation of NOX2 activity is able to improve vascular physiology, suggesting that NOX2 may be a potential target for therapeutic applications.

COPD and stroke: are systemic inflammation and oxidative stress the missing links?

Chronic obstructive pulmonary disease (COPD) is characterized by progressive airflow limitation and loss of lung function, and is currently the third largest cause of death in the world. It is now well established that cardiovascular-related comorbidities such as stroke contribute to morbidity and mortality in COPD. The mechanisms linking COPD and stroke remain to be fully defined but are likely to be interconnected. The association between COPD and stroke may be largely dependent on shared risk factors such as aging and smoking, or the association of COPD with traditional stroke risk factors. In addition, we propose that COPD-related systemic inflammation and oxidative stress may play important roles by promoting cerebral vascular dysfunction and platelet hyperactivity. In this review, we briefly discuss the pathogenesis of COPD, acute exacerbations of COPD (AECOPD) and cardiovascular comorbidities associated with COPD, in particular stroke. We also highlight and discuss the potential mechanisms underpinning the link between COPD and stroke, with a particular focus on the roles of systemic inflammation and oxidative stress.

Cerebral Vascular Disease and Neurovascular Injury in Ischemic Stroke

The consequences of cerebrovascular disease are among the leading health issues worldwide. Large and small cerebral vessel disease can trigger stroke and contribute to the vascular component of other forms of neurological dysfunction and degeneration. Both forms of vascular disease are driven by diverse risk factors, with hypertension as the leading contributor. Despite the importance of neurovascular disease and subsequent injury after ischemic events, fundamental knowledge in these areas lag behind our current understanding of neuroprotection and vascular biology in general. The goal of this review is to address select key structural and functional changes in the vasculature that promote hypoperfusion and ischemia, while also affecting the extent of injury and effectiveness of therapy. In addition, as damage to the blood-brain barrier is one of the major consequences of ischemia, we discuss cellular and molecular mechanisms underlying ischemia-induced changes in blood-brain barrier integrity and function, including alterations in endothelial cells and the contribution of pericytes, immune cells, and matrix metalloproteinases. Identification of cell types, pathways, and molecules that control vascular changes before and after ischemia may result in novel approaches to slow the progression of cerebrovascular disease and lessen both the frequency and impact of ischemic events.

Pulse pressure-dependent cerebrovascular eNOS regulation in mice

Arterial blood pressure is oscillatory; whether pulse pressure (PP) regulates cerebral artery myogenic tone (MT) and endothelial function is currently unknown. To test the impact of PP on MT and dilation to flow (FMD) or to acetylcholine (Ach), isolated pressurized mouse posterior cerebral arteries were subjected to either static pressure (SP) or a physiological PP (amplitude: 30 mm Hg; frequency: 550 bpm). Under PP, MT was significantly higher than in SP conditions ( p < 0.05) and was not affected by eNOS inhibition. In contrast, under SP, eNOS inhibition increased ( p < 0.05) MT to levels observed under PP, suggesting that PP may inhibit eNOS. At a shear stress of 20 dyn/cm², FMD was lower ( p < 0.05) under SP than PP. Under SP, eNOS-dependent [Formula: see text] production contributed to FMD, while under PP, eNOS-dependent NO was responsible for FMD, indicating that PP favours eNOS coupling. Differences in FMD between pressure conditions were abolished after NOX2 inhibition. In contrast to FMD, Ach-induced dilations were higher ( p < 0.05) under SP than PP. Reactive oxygen species scavenging reduced ( p < 0.05) Ach-dependent dilations under SP, but increased ( p < 0.05) them under PP; hence, under PP, Ach promotes ROS production and limits eNOS-derived NO activity. In conclusion, PP finely regulates eNOS, controlling cerebral artery reactivity.

The Polarization States of Microglia in TBI: A New Paradigm for Pharmacological Intervention

Traumatic brain injury (TBI) is a serious medical and social problem worldwide. Because of the complex pathophysiological mechanisms of TBI, effective pharmacotherapy is still lacking. The microglial cells are resident tissue macrophages located in the brain and have two major polarization states, M1 phenotype and M2 phenotype, when activated. The M1 phenotype is related to the release of proinflammatory cytokines and secondary brain injury, while the M2 phenotype has been proved to be responsible for the release of anti-inflammation cytokines and for central nervous system (CNS) repair. In animal models, pharmacological strategies inhibiting the M1 phenotype and promoting the M2 phenotype of microglial cells could alleviate cerebral damage and improve neurological function recovery after TBI. In this review, we aimed to summarize the current knowledge about the pathological significance of microglial M1/M2 polarization in the pathophysiology of TBI. In addition, we reviewed several drugs that have provided neuroprotective effects against brain injury following TBI by altering the polarization states of the microglia. We emphasized that future investigation of the regulation mechanisms of microglial M1/M2 polarization in TBI is anticipated, which could contribute to the development of new targets of pharmacological intervention in TBI.

Perivascular macrophages mediate the neurovascular and cognitive dysfunction associated with hypertension

Hypertension is a leading risk factor for dementia, but the mechanisms underlying its damaging effects on the brain are poorly understood. Due to a lack of energy reserves, the brain relies on continuous delivery of blood flow to its active regions in accordance with their dynamic metabolic needs. Hypertension disrupts these vital regulatory mechanisms, leading to the neuronal dysfunction and damage underlying cognitive impairment. Elucidating the cellular bases of these impairments is essential for developing new therapies. Perivascular macrophages (PVMs) represent a distinct population of resident brain macrophages that serves key homeostatic roles but also has the potential to generate large amounts of reactive oxygen species (ROS). Here, we report that PVMs are critical in driving the alterations in neurovascular regulation and attendant cognitive impairment in mouse models of hypertension. This effect was mediated by an increase in blood-brain barrier permeability that allowed angiotensin II to enter the perivascular space and activate angiotensin type 1 receptors in PVMs, leading to production of ROS through the superoxide-producing enzyme NOX2. These findings unveil a pathogenic role of PVMs in the neurovascular and cognitive dysfunction associated with hypertension and identify these cells as a putative therapeutic target for diseases associated with cerebrovascular oxidative stress.

Mechanistic insights into a TIMP3-sensitive pathway constitutively engaged in the regulation of cerebral hemodynamics

Cerebral small vessel disease (SVD) is a leading cause of stroke and dementia. CADASIL, an inherited SVD, alters cerebral artery function, compromising blood flow to the working brain. TIMP3 (tissue inhibitor of metalloproteinase 3) accumulation in the vascular extracellular matrix in CADASIL is a key contributor to cerebrovascular dysfunction. However, the linkage between elevated TIMP3 and compromised cerebral blood flow (CBF) remains unknown. Here, we show that TIMP3 acts through inhibition of the metalloprotease ADAM17 and HB-EGF to regulate cerebral arterial tone and blood flow responses. In a clinically relevant CADASIL mouse model, we show that exogenous ADAM17 or HB-EGF restores cerebral arterial tone and blood flow responses, and identify upregulated voltage-dependent potassium channel (K_V) number in cerebral arterial myocytes as a heretofore-unrecognized downstream effector of TIMP3-induced deficits. These results support the concept that the balance of TIMP3 and ADAM17 activity modulates CBF through regulation of myocyte K_V channel number.

DOI:http://dx.doi.org/10.7554/eLife.17536.001

There are currently no effective treatments or cures for small blood vessel diseases of the brain, which lead to strokes and subsequent decreases in mental abilities. Normally, smooth muscle cells that surround the vessels relax or contract to regulate blood flow and ensure the right amount of oxygen and nutrients reaches the different regions of the brain. In a syndrome called CADASIL, which is the most common form of inherited small vessel disease, a genetic mutation causes the smooth muscle cells to weaken over time.

The accumulation of several proteins – including one called TIMP3 – around the smooth muscle cells plays a key role in the smooth muscle cell weakening seen in CADASIL. Capone et al. have now studied mice that display the symptoms of CADASIL to investigate how TIMP3 decreases blood flow through blood vessels in the brain. This revealed that TIMP3 inactivates another protein called ADAM17. The latter protein is normally responsible for starting a signaling pathway that helps smooth muscle cells to regulate blood flow according to the needs of the brain cells. Artificially adding more ADAM17 to the brains of the CADASIL mice reduced their symptoms of small vessel disease.

Using smooth muscle cells freshly isolated from the brains of CADASIL mice, Capone et al. also demonstrated that abnormal TIMP3-ADAM17 signaling increases the number of voltage-dependent potassium channels in the membrane of the muscle cells. Having too many of these channels impairs the flow of blood through vessels in the brain.

Further experiments are needed to investigate whether correcting TIMP3-ADAM17 signaling could prevent strokes in people with inherited CADASIL. It also remains to be seen whether similar signaling mechanisms are at play in other small vessel diseases.

DOI:http://dx.doi.org/10.7554/eLife.17536.002

Cerebral Small Vessel Disease: Targeting Oxidative Stress as a Novel Therapeutic Strategy?

Cerebral small vessel disease (SVD) is a major contributor to stroke, and a leading cause of cognitive impairment and dementia. Despite the devastating effects of cerebral SVD, the pathogenesis of cerebral SVD is still not completely understood. Moreover, there are no specific pharmacological strategies for its prevention or treatment. Cerebral SVD is characterized by marked functional and structural abnormalities of the cerebral microcirculation. The clinical manifestations of these pathological changes include lacunar infarcts, white matter hyperintensities, and cerebral microbleeds. The main purpose of this review is to discuss evidence implicating oxidative stress in the arteriopathy of both non-amyloid and amyloid (cerebral amyloid angiopathy) forms of cerebral SVD and its most important risk factors (hypertension and aging), as well as its contribution to cerebral SVD-related brain injury and cognitive impairment. We also highlight current evidence of the involvement of the NADPH oxidases in the development of oxidative stress, enzymes that are a major source of reactive oxygen species in the cerebral vasculature. Lastly, we discuss potential pharmacological strategies for oxidative stress in cerebral SVD, including some of the historical and emerging NADPH oxidase inhibitors.