Superoxide mediates angiotensin II-induced influx of extracellular calcium in neural cells

Reactive oxygen species in hypertension

Hypertension is a leading risk factor for stroke, heart disease and chronic kidney disease. Multiple interacting factors and organ systems increase blood pressure and cause target-organ damage. Among the many molecular elements involved in the development of hypertension are reactive oxygen species (ROS), which influence cellular processes in systems that contribute to blood pressure elevation (such as the cardiovascular, renal, immune and central nervous systems, or the renin-angiotensin-aldosterone system). Dysregulated ROS production (oxidative stress) is a hallmark of hypertension in humans and experimental models. Of the many ROS-generating enzymes, NADPH oxidases are the most important in the development of hypertension. At the cellular level, ROS influence signalling pathways that define cell fate and function. Oxidative stress promotes aberrant redox signalling and cell injury, causing endothelial dysfunction, vascular damage, cardiovascular remodelling, inflammation and renal injury, which are all important in both the causes and consequences of hypertension. ROS scavengers reduce blood pressure in almost all experimental models of hypertension; however, clinical trials of antioxidants have yielded mixed results. In this Review, we highlight the latest advances in the understanding of the role and the clinical implications of ROS in hypertension. We focus on cellular sources of ROS, molecular mechanisms of oxidative stress and alterations in redox signalling in organ systems, and their contributions to hypertension.

The organum vasculosum of the lamina terminalis and subfornical organ: regulation of thirst

Thirst and water intake are regulated by the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), located around the anteroventral third ventricle, which plays a critical role in sensing dynamic changes in sodium and water balance in body fluids. Meanwhile, neural circuits involved in thirst regulation and intracellular mechanisms underlying the osmosensitive function of OVLT and SFO are reviewed. Having specific Nax channels in the glial cells and other channels (such as TRPV1 and TRPV4), the OVLT and SFO detect the increased Na+ concentration or hyperosmolality to orchestrate osmotic stimuli to the insular and cingulate cortex to evoke thirst. Meanwhile, the osmotic stimuli are relayed to the supraoptic nucleus (SON) and paraventricular nucleus of the hypothalamus (PVN) via direct neural projections or the median preoptic nucleus (MnPO) to promote the secretion of vasopressin which plays a vital role in the regulation of body fluid homeostasis. Importantly, the vital role of OVLT in sleep-arousal regulation is discussed, where vasopressin is proposed as the mediator in the regulation when OVLT senses osmotic stimuli.

Nrf2 and autonomic dysregulation in chronic heart failure and hypertension

Redox imbalance plays essential role in the pathogenesis of cardiovascular diseases. Chronic heart failure (CHF) and hypertension are associated with central oxidative stress, which is partly mediated by the downregulation of antioxidant enzymes in the central autonomic neurons that regulate sympathetic outflow, resulting in sympathoexcitation. Antioxidant proteins are partially regulated by the transcriptional factor nuclear factor erythroid 2-related factor 2 (Nrf2). Downregulation of Nrf2 is key to disrupting central redox homeostasis and mediating sympathetic nerve activity in the setting of Chronic heart failure and hypertension. Nrf2, in turn, is regulated by various mechanisms, such as extracellular vesicle-enriched microRNAs derived from several cell types, including heart and skeletal muscle. In this review, we discuss the role of Nrf2 in regulating oxidative stress in the brain and its impact on sympathoexcitation in Chronic heart failure and hypertension. Importantly, we also discuss interorgan communication via extracellular vesicle pathways that mediate central redox imbalance through Nrf2 signaling.

Homeostasis of carbohydrates and reactive oxygen species is critically changed in the brain of middle-aged mice: molecular mechanisms and functional reasons

The brain is an organ that consumes a lot of energy. In the brain, energy is required for synaptic transmission, numerous biosynthetic processes and axonal transport in neurons, and for many supportive functions of glial cells. The main source of energy in the brain is glucose and to a lesser extent lactate and ketone bodies. ATP is formed at glucose catabolism via glycolysis and oxidative phosphorylation in mitochondrial electron transport chain (ETC) within mitochondria being the main source of ATP. With age, brain's energy metabolism is disturbed, involving a decrease in glycolysis and mitochondrial dysfunction. The latter is accompanied by intensified generation of reactive oxygen species (ROS) in ETC leading to oxidative stress. Recently, we have found that crucial changes in energy metabolism and intensity of oxidative stress in the mouse brain occur in middle age with minor progression in old age. In this review, we analyze the metabolic changes and functional causes that lead to these changes in the aging brain.

Angiotensin II-Induced Signal Transduction Mechanisms for Cardiac Hypertrophy

Although acute exposure of the heart to angiotensin (Ang II) produces physiological cardiac hypertrophy and chronic exposure results in pathological hypertrophy, the signal transduction mechanisms for these effects are of complex nature. It is now evident that the hypertrophic response is mediated by the activation of Ang type 1 receptors (AT₁R), whereas the activation of Ang type 2 receptors (AT₂R) by Ang II and Mas receptors by Ang-(1-7) exerts antihypertrophic effects. Furthermore, AT₁R-induced activation of phospholipase C for stimulating protein kinase C, influx of Ca²⁺ through sarcolemmal Ca²⁺- channels, release of Ca²⁺ from the sarcoplasmic reticulum, and activation of sarcolemmal NADPH oxidase 2 for altering cardiomyocytes redox status may be involved in physiological hypertrophy. On the other hand, reduction in the expression of AT₂R and Mas receptors, the release of growth factors from fibroblasts for the occurrence of fibrosis, and the development of oxidative stress due to activation of mitochondria NADPH oxidase 4 as well as the depression of nuclear factor erythroid-2 activity for the occurrence of Ca²⁺-overload and activation of calcineurin may be involved in inducing pathological cardiac hypertrophy. These observations support the view that inhibition of AT₁R or activation of AT₂R and Mas receptors as well as depression of oxidative stress may prevent or reverse the Ang II-induced cardiac hypertrophy.

Reactive Oxygen Species: Angels and Demons in the Life of a Neuron

Reactive oxygen species (ROS) have emerged as regulators of key processes supporting neuronal growth, function, and plasticity across lifespan. At normal physiological levels, ROS perform important roles as secondary messengers in diverse molecular processes such as regulating neuronal differentiation, polarization, synapse maturation, and neurotransmission. In contrast, high levels of ROS are toxic and can ultimately lead to cell death. Excitable cells, such as neurons, often require high levels of metabolic activity to perform their functions. As a consequence, these cells are more likely to produce high levels of ROS, potentially enhancing their susceptibility to oxidative damage. In addition, because neurons are generally post-mitotic, they may be subject to accumulating oxidative damage. Thus, maintaining tight control over ROS concentration in the nervous system is essential for proper neuronal development and function. We are developing a more complete understanding of the cellular and molecular mechanisms for control of ROS in these processes. This review focuses on ROS regulation of the developmental and functional properties of neurons, highlighting recent in vivo studies. We also discuss the current evidence linking oxidative damage to pathological conditions associated with neurodevelopmental and neurodegenerative disorders.

Another controller system for arterial pressure. AngII-vasopressin neural network of the parvocellular paraventricular nucleus may regulate arterial pressure during hypotension

Angiotensin II (AngII) immunoreactive cells, fibers and receptors, were found in the parvocelluar region of paraventricular nucleus (PVNp) and AngII receptors are present on vasopressinergic neurons. However, the mechanism by which vasopressin (AVP) and AngII may interact to regulate arterial pressure is not known. Thus, we tested the cardiovascular effects of blockade of the AngII receptors on AVP neurons and blockade of vasopressin V1a receptors on AngII neurons. We also explored whether the PVNp vasopressin plays a regulatory role during hypotension in anesthetized rat or not. Hypovolemic-hypotension was induced by gradual bleeding from femoral venous catheter. Either AngII or AVP injected into the PVNp produced pressor and tachycardia responses. The responses to AngII were blocked by V1a receptor antagonist. The responses to AVP were partially attenuated by AT1 antagonist and greatly attenuated by AT2 antagonist. Hemorrhage augmented the pressor response to AVP, indicating that during hemorrhage, sensitivity of PVNp to vasopressin was increased. By hemorrhagic-hypotension and bilateral blockade of V1a receptors of the PVNp, we found that vasopressinergic neurons of the PVNp regulate arterial pressure towards normal during hypotension. Taken together these findings and our previous findings about angII (Khanmoradi and Nasimi, 2017a) for the first time, we found that a mutual cooperative system of angiotensinergic and vasopressinergic neurons in the PVNp is a major regulatory controller of the cardiovascular system during hypotension.

Sources and triggers of oxidative damage in neurodegeneration

Neurodegeneration describes a group of more than 300 neurological diseases, characterised by neuronal loss and intra- or extracellular protein depositions, as key neuropathological features. Multiple factors play role in the pathogenesis of these group of disorders: mitochondrial dysfunction, membrane damage, calcium dyshomeostasis, metallostasis, defect clearance and renewal mechanisms, to name a few. All these factors, without exceptions, have in common the involvement of immensely increased generation of free radicals and occurrence of oxidative stress, and as a result - exhaustion of the scavenging potency of the cellular redox defence mechanisms. Besides genetic predisposition and environmental exposure to toxins, the main risk factor for developing neurodegeneration is age. And although the "Free radical theory of ageing" was declared dead, it is undisputable that accumulation of damage occurs with age, especially in systems that are regulated by free radical messengers and those that oppose oxidative stress, protein oxidation and the accuracy in protein synthesis and degradation machinery has difficulties to be maintained. This brief review provides a comprehensive summary on the main sources of free radical damage, occurring in the setting of neurodegeneration.

Angiotensin-II Modulates GABAergic Neurotransmission in the Mouse Substantia Nigra

GABAergic projections neurons of the substantia nigra reticulata (SNr), through an extensive network of dendritic arbors and axon collaterals, provide robust inhibitory input to neighboring dopaminergic neurons in the substantia nigra compacta (SNc). Angiotensin-II (Ang-II) receptor signaling increases SNc dopaminergic neuronal sensitivity to insult, thus rendering these cells susceptible to dysfunction and destruction. However, the mechanisms by which Ang-II regulates SNc dopaminergic neuronal activity are unclear. Given the complex relationship between SN dopaminergic and GABAergic neurons, we hypothesized that Ang-II could regulate SNc dopaminergic neuronal activity directly and indirectly by modulating SNr GABAergic neurotransmission. Here, using transgenic mice, slice electrophysiology, and optogenetics, we provide evidence of an AT₁ receptor-mediated signaling mechanism in SNr GABAergic neurons where Ang-II suppresses electrically-evoked neuronal output by facilitating postsynaptic GABA_A receptors (GABA_ARs) and prolonging the action potential (AP) duration. Unexpectedly, Ang-II had no discernable effects on the electrical properties of SNc dopaminergic neurons. Also, and indicating a nonlinear relationship between electrical activity and neuronal output, following phasic photoactivation of SNr GABAergic neurons, Ang-II paradoxically enhanced the feedforward inhibitory input to SNc dopaminergic neurons. In sum, our observations describe an increasingly complex and heterogeneous response of the SN to Ang-II by revealing cell-specific responses and nonlinear effects on intranigral GABAergic neurotransmission. Our data further implicate the renin-angiotensin-system (RAS) as a functionally relevant neuromodulator in the substantia nigra, thus underscoring a need for additional inquiry.

Oxidative Stress and Hypertension

A link between oxidative stress and hypertension has been firmly established in multiple animal models of hypertension but remains elusive in humans. While initial studies focused on inactivation of nitric oxide by superoxide, our understanding of relevant reactive oxygen species (superoxide, hydrogen peroxide, and peroxynitrite) and how they modify complex signaling pathways to promote hypertension has expanded significantly. In this review, we summarize recent advances in delineating the primary and secondary sources of reactive oxygen species (nicotinamide adenine dinucleotide phosphate oxidases, uncoupled endothelial nitric oxide synthase, endoplasmic reticulum, and mitochondria), the posttranslational oxidative modifications they induce on protein targets important for redox signaling, their interplay with endogenous antioxidant systems, and the role of inflammasome activation and endoplasmic reticular stress in the development of hypertension. We highlight how oxidative stress in different organ systems contributes to hypertension, describe new animal models that have clarified the importance of specific proteins, and discuss clinical studies that shed light on how these processes and pathways are altered in human hypertension. Finally, we focus on the promise of redox proteomics and systems biology to help us fully understand the relationship between ROS and hypertension and their potential for designing and evaluating novel antihypertensive therapies.

Conventional cardiovascular risk factors in Transient Global Amnesia: Systematic review and proposition of a novel hypothesis

Transient Global Amnesia (TGA) is an enigmatic amnestic syndrome. We conducted a systematic review to investigate the relationship between the conventional cardiovascular risk factors and TGA. MEDLINE, CENTRAL, EMBASE and PsycINFO were comprehensively searched and 23 controlled observational studies were retrieved. The prevalence of hypertension, diabetes mellitus, dyslipidemia and smoking was lower among patients with TGA compared to Transient Ischemic Attack. Regarding the comparison of TGA with healthy individuals, there was strong evidence suggesting a protective effect of diabetes mellitus on TGA and weaker evidence for a protective effect of smoking. Hypertension was associated with TGA only in more severe stages, while dyslipidemia was not related. In view of these findings, a novel pathophysiological hypothesis is proposed, in which the functional interactions of Angiotensin-II type-1 and N-methyl-D-aspartate receptors are of pivotal importance. The whole body of clinical evidence (nature of precipitating events, associations with migraine, gender-based association patterns) was integrated.

Strategies to expand the therapeutic potential of superoxide dismutase by exploiting delivery approaches

The overproduction of free radicals can cause oxidative-stress damage to a range of biomolecules, and thus potentially contribute to several pathologies, from neurodegenerative disorders to cardiovascular diseases and metabolic disorders. Endogenous antioxidant enzymes, such as superoxide dismutase (SOD), play an important role in diminishing oxidative stress. SOD supplementation could therefore be an effective preventive strategy to reduce the risk of free-radical overproduction. However, the efficacy of SOD administration is hampered by its rapid clearance. Several different approaches to improve the bioavailability of SOD have been explored in recent decades. This review intends to describe the rationale that underlie the various approaches and chemical strategies that have led to the most recent advances in SOD delivery. This critical description includes SOD conjugates, SOD loaded into particulate carriers (micelles, liposomes, nanoparticles, microparticles) and the most promising and suitable formulations for oral delivery, with a particular emphasis on reports of preclinical/clinical results. Likely future directions are also considered and reported.

Overexpression of Central ACE2 (Angiotensin-Converting Enzyme 2) Attenuates the Pressor Response to Chronic Central Infusion of Ang II (Angiotensin II): A Potential Role for Nrf2 (Nuclear Factor [Erythroid-Derived 2]-Like 2)

We investigated the mechanism by which ACE2 (angiotensin-converting enzyme 2) overexpression alters neurohumoral outflow and central oxidative stress. Nrf2 (nuclear factor [erythroid-derived 2]-like 2) is a master antioxidant transcription factor that regulates cytoprotective and antioxidant genes. We hypothesized that upregulation of central ACE2 inhibits the pressor response to Ang II (angiotensin II) by reducing reactive oxygen species through a Nrf2/antioxidant enzyme-mediated mechanism in the rostral ventrolateral medulla. Synapsin human Angiotensin Converting Enzyme 2 positive (SynhACE2^+/+) mice and their littermate controls synhACE2^-/- were used to evaluate the consequence of intracerebroventricular infusion of Ang II. In control mice, Ang II infusion evoked a significant increase in blood pressure and norepinephrine excretion, along with polydipsia and polyuria. The pressor effect of central Ang II was completely blocked in synhACE2^+/+ mice. Polydipsia, norepinephrine excretion, and markers of oxidative stress in response to central Ang II were also reduced in synhACE2^+/+ mice. The MasR (Mas receptor) agonist Ang 1-7 and blocker A779 had no effects on blood pressure. synhACE2^+/+ mice showed enhanced expression of Nrf2 in the rostral ventrolateral medulla which was blunted following Ang II infusion. Ang II evoked nuclear translocation of Nrf2 in cultured Neuro 2A (N2A) cells. In synhACE2^-/- mice, the central Ang II pressor response was attenuated by simultaneous intracerebroventricular infusion of the Nrf2 activator sulforaphane; blood pressure was enhanced by knockdown of Nrf2 in the rostral ventrolateral medulla in Nrf2 floxed (Nrf2^f/f) mice. These data suggest that the hypertensive effects of intracerebroventricular Ang II are attenuated by selective overexpression of brain synhACE2 and may be mediated by Nrf2-upregulated antioxidant enzymes in the rostral ventrolateral medulla.

Redox-sensitive calcium/calmodulin-dependent protein kinase IIα in angiotensin II intra-neuronal signaling and hypertension

Dysregulation of brain angiotensin II (AngII) signaling results in modulation of neuronal ion channel activity, an increase in neuronal firing, enhanced sympathoexcitation, and subsequently elevated blood pressure. Studies over the past two decades have shown that these AngII responses are mediated, in part, by reactive oxygen species (ROS). However, the redox-sensitive target(s) that are directly acted upon by these ROS to execute the AngII pathophysiological responses in neurons remain unclear. Calcium/calmodulin-dependent protein kinase II (CaMKII) is an AngII-activated intra-neuronal signaling protein, which has been suggested to be redox sensitive as overexpressing the antioxidant enzyme superoxide dismutase attenuates AngII-induced activation of CaMKII. Herein, we hypothesized that the neuronal isoform of CaMKII, CaMKII-alpha (CaMKIIα), is a redox-sensitive target of AngII, and that mutation of potentially redox-sensitive amino acids in CaMKIIα influences AngII-mediated intra-neuronal signaling and hypertension. Adenoviral vectors expressing wild-type mouse CaMKIIα (Ad.wtCaMKIIα) or mutant CaMKIIα (Ad.mutCaMKIIα) with C280A and M281V mutations were generated to overexpress either CaMKIIα isoform in mouse catecholaminergic cultured neurons (CATH.a) or in the brain subfornical organ (SFO) of hypertensive mice. Overexpressing wtCaMKIIα exacerbated AngII pathophysiological responses as observed by a potentiation of AngII-induced inhibition of voltage-gated K+ current, enhanced in vivo pressor response following intracerebroventricular injection of AngII, and sensitization to chronic peripheral infusion of AngII resulting in a more rapid increase in blood pressure. In contrast, expressing the mutant CaMKIIα in CATH.a neurons or the SFO failed to intensify these AngII responses. Taken together, these data identify neuronal CaMKIIα as a redox-sensitive signaling protein that contributes to AngII-induced neuronal activation and hypertension.

Exercise training upregulates Nrf2 protein in the rostral ventrolateral medulla of mice with heart failure

Chronic heart failure (CHF) is associated with global oxidative stress, which contributes to sympathoexcitation. Increased reactive oxygen species in the brain accumulate within neurons and lead to enhanced neuronal excitability. Exercise training (ExT) is associated with a reduction of oxidative stress by upregulation of antioxidant enzymes. The link between ExT and antioxidant enzyme expression in the brain of animals with CHF is not clear. We hypothesized that ExT enhances transcription and translation of the nuclear factor erythroid 2-related factor 2 (Nrf2) gene, a master transcription factor that modulates antioxidant enzyme gene expression, in the rostral ventrolateral medulla (RVLM) of mice with CHF. Mice were divided into the following groups: Sham sedentary (Sham-Sed), Sham-ExT, CHF-Sed, and CHF-ExT. After 8 wk of ExT, we measured Nrf2 and NAD(P)H dehydrogenase [quinone] 1 (NQO-1) message and protein expression along with maximal exercise tolerance and urinary norepinephrine (NE) excretion. We found that Nrf2 and NQO-1 mRNA and protein expression in the RVLM were lower in CHF-Sed mice compared with Sham-Sed. ExT attenuated the CHF-induced reduction of Nrf2 and NQO-1 mRNA and protein expression in the RVLM. NE excretion was higher in CHF-Sed mice compared with Sham-Sed (666.8 ± 79.3 ng/24 h, n = 6 vs. 397.8 ± 43.7 ng/24 h, P = 0.04). CHF-ExT mice exhibited reduced urinary NE excretion compared with CHF-Sed (360.7 ± 41.7 ng, n = 4 vs. 666.8 ± 79.3 ng, n = 6; P = 0.03). We conclude that ExT-induced upregulation of Nrf2 in the RVLM contributes to the beneficial effects of ExT on sympathetic function in the heart failure state.NEW & NOTEWORTHY This study provide evidence for an important role for exercise training in the modulation of antioxidant enzyme production in the rostral ventrolateral medulla (RVLM) in the heart failure state. We show here a correlation between exercise training and the expression of the antioxidant transcription factor Nrf2 in the RVLM. Exercise training reduced sympathetic function (norepinephrine excretion) and upregulated both Nrf2 and the antioxidant enzyme NQO-1. We conclude that Nrf2 in the RVLM may be an important target for controlling sympathetic outflow in heart failure.

Examining the relationship between astrocyte dysfunction and neurodegeneration in ALS using hiPSCs

Amyotrophic lateral sclerosis (ALS) is a complex and fatal neurodegenerative disease for which the causes of disease onset and progression remain unclear. Recent advances in human induced pluripotent stem cell (hiPSC)-based models permit the study of the genetic factors associated with ALS in patient-derived neural cell types, including motor neurons and glia. While astrocyte dysfunction has traditionally been thought to exacerbate disease progression, astrocytic dysfunction may play a more direct role in disease initiation and progression. Such non-cell autonomous mechanisms expand the potential targets of therapeutic intervention, but only a handful of ALS risk-associated genes have been examined for their impact on astrocyte dysfunction and neurodegeneration. This review summarizes what is currently known about astrocyte function in ALS and suggests ways in which hiPSC-based models can be used to more effectively study the role of astrocytes in neurodegenerative disease.

Differential impacts of brain stem oxidative stress and nitrosative stress on sympathetic vasomotor tone

Based on work-done in the rostral ventrolateral medulla (RVLM), this review presents four lessons learnt from studying the differential impacts of oxidative stress and nitrosative stress on sympathetic vasomotor tone and their clinical and therapeutic implications. The first lesson is that an increase in sympathetic vasomotor tone because of augmented oxidative stress in the RVLM is responsible for the generation of neurogenic hypertension. On the other hand, a shift from oxidative stress to nitrosative stress in the RVLM underpins the succession of increase to decrease in sympathetic vasomotor tone during the progression towards brain stem death. The second lesson is that, by having different cellular sources, regulatory mechanisms on synthesis and degradation, kinetics of chemical reactions, and downstream signaling pathways, reactive oxygen species and reactive nitrogen species should not be regarded as a singular moiety. The third lesson is that well-defined differential roles of oxidative stress and nitrosative stress with distinct regulatory mechanisms in the RVLM during neurogenic hypertension and brain stem death clearly denote that they are not interchangeable phenomena with unified cellular actions. Special attention must be paid to their beneficial or detrimental roles under a specific disease or a particular time-window of that disease. The fourth lesson is that, to be successful, future antioxidant therapies against neurogenic hypertension must take into consideration the much more complicated picture than that presented in this review on the generation, maintenance, regulation or modulation of the sympathetic vasomotor tone. The identification that the progression towards brain stem death entails a shift from oxidative stress to nitrosative stress in the RVLM may open a new vista for therapeutic intervention to slow down this transition.

Glucose-dependent trans-plasma membrane electron transport and p70S6k phosphorylation in skeletal muscle cells

The reduction of extracellular oxidants by intracellular electrons is known as trans-plasma membrane electron transport (tPMET). The goal of this study was to characterize a role of tPMET in the sensing of glucose as a physiological signal. tPMET from C2C12 myotubes was monitored using a cell-impermeable extracellular electron acceptor, water-soluble tetrazolium salt-1 (WST-1). Superoxide dismutase in the incubation medium or exposure to an NADPH oxidase (NOX) isoform 1/4 inhibitor suppressed WST-1 reduction by 70%, suggesting a role of NOXs in tPMET. There was a positive correlation between medium glucose concentration and WST-1 reduction, suggesting that tPMET is a glucose-sensing process. WST-1 reduction was also decreased by an inhibitor of the pentose phosphate pathway, dehydroepiandrosterone. In contrast, glycolytic inhibitors, 3PO and sodium fluoride, did not affect WST-1 reduction. Thus, it appears that glucose uptake and processing in the pentose phosphate pathway drives NOX-dependent tPMET. Western blot analysis demonstrated that p70^S6k phosphorylation is glucose-dependent, while the phosphorylation of AKT and MAPK did not differ in the presence or absence of glucose. Further, phosphorylation of p70^S6k was dependent upon NOX enzymes. Finally, glucose was required for full stimulation of p70^S6k by insulin, again in a fashion prevented by NOX inhibition. Taken together, the data suggest that muscle cells have a novel glucose-sensing mechanism dependent on NADPH production and NOX activity, culminating in increased p70^S6k phosphorylation.

Sex-dependent expression of brain medullary MAP and PI3 kinases in adult sheep with antenatal betamethasone exposure

Antenatal betamethasone (BM) therapy for women in jeopardy of premature delivery accelerates the lung development in preterm infants and reduces infant mortality rates, but may induce early programming events with chronic cardiovascular consequences. In a sheep model of fetal programming, in utero BM-exposed (BMX) offspring as adults exhibit elevated mean arterial pressure (MAP), decreased baroreflex sensitivity (BRS) for the control of heart rate and insulin resistance accompanied by dysregulation of the brain renin-angiotensin (Ang) system (RAS). However, the status of signaling mechanisms in the brain dorsomedial medulla (DMM) of the BMX sheep that comprise both the mitogen activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) cellular pathways is unknown. Given the importance of these signaling pathways in the actions of Ang peptides as well as baroreflex function and autonomic integration, we applied both a kinase signaling array and associated individual immunoblots of the dorsomedial brain medulla from adult female and male sheep with antenatal BMX. MAPK and PI3K pathways were altered significantly in the BMX sheep in a sex-dependent manner. A protein array for kinases (PathScan® Intracellular Signaling Array Kit, Cell Signaling) and subsequent verification by immunoblot revealed that within the DMM, female BMX sheep exhibit lower expression of proteins in the PI3K pathway, while male BMX sheep show greater expression of p-MAPK pathway proteins extracellular signal regulated kinase (ERK) 1/2. We conclude that maladaptive changes in these two kinase pathways in the DMM may contribute to the chronic dysregulation of blood pressure in this model of fetal programming.

Excessive Glutamate Stimulation Impairs ACE2 Activity Through ADAM17-Mediated Shedding in Cultured Cortical Neurons

The excitotoxicity of glutamate plays an important role in the progression of various neurological disorders via participating in inflammation and neuronal damage. In this study, we identified the role of excessive glutamate stimulation in the modulation of angiotensin-converting enzyme type 2 (ACE2), a critical component in the compensatory axis of the renin-angiotensin system (RAS). In primary cultured cortical neurons, high concentration of glutamate (100 µM) significantly reduced the enzymatic activity of ACE2. The elevated activity of ADAM17, a member of the 'A Disintegrin And Metalloprotease' (ADAM) family, was found to contribute to this glutamate-induced ACE2 down-regulation. The decrease of ACE2 activity could be prevented by pre-treatment with antagonists targeting ionotropic glutamate receptors. In addition, the glutamate-induced decrease in ACE2 activity was significantly attenuated when the neurons were co-treated with MitoTEMPOL or blockers that target oxidative stress-mediated signaling pathway. In summary, our study reveals a strong relationship between excessive glutamate stimulation and ADAM17-mediated impairment in ACE2 activity, suggesting a possible cross-talk between glutamate-induced excitotoxicity and dysregulated RAS.

Hypoxia-Inducible Factor-1α Mediates Increased Sympathoexcitation via Glutamatergic N-Methyl-d-Aspartate Receptors in the Paraventricular Nucleus of Rats With Chronic Heart Failure

BACKGROUND

Increased sympathetic outflow is a major contributor to the progression of chronic heart failure (CHF). Potentiation of glutamatergic tone has been causally related to the sympathoexcitation in CHF. Specifically, an increase in the N-methyl-d-aspartate-type 1 receptor (NMDA-NR₁) expression within the paraventricular nucleus (PVN) is critically linked to the increased sympathoexcitation during CHF. However, the molecular mechanism(s) for the upregulation of NMDA-NR₁ remains unexplored. We hypothesized that hypoxia via hypoxia-inducible factor 1α (HIF-1α) might contribute to the augmentation of the NMDA-NR₁-mediated sympathoexcitatory responses from the PVN in CHF.

METHODS AND RESULTS

Immunohistochemistry staining, mRNA, and protein for hypoxia-inducible factor 1α were upregulated within the PVN of left coronary artery-ligated CHF rats. In neuronal cell line (NG108-15) in vitro, hypoxia caused a significant increase in mRNA and protein for HIF-1α (2-fold) with the concomitant increase in NMDA-NR₁ mRNA, protein levels, and glutamate-induced Ca⁺ influx. Chromatin immunoprecipitation assay identified HIF-1α binding to NMDA-NR₁ promoter during hypoxia. Silencing of HIF-1α in NG108 cells leads to a significant decrease in expression of NMDA-NR₁, suggesting that expression of HIF-1α is necessary for the upregulation of NMDA-NR₁. Consistent with these observations, HIF-1α silencing within the PVN abrogated the increased basal sympathetic tone and sympathoexcitatory responses to microinjection of NMDA in the PVN of rats with CHF.

CONCLUSIONS

These results uncover a critical role for HIF-1 in the upregulation of NMDA-NR₁ to mediate sympathoexcitation in CHF. We conclude that subtle hypoxia within the PVN may act as a metabolic cue to modulate sympathoexcitation during CHF.

Angiotensin II Peptide Vaccine Protects Ischemic Brain Through Reducing Oxidative Stress

Background and Purpose—

Medication nonadherence is one of major risk factors for the poor outcome in ischemic stroke. Vaccination is expected to solve such a problem because of its long-lasting effects, but its effect on ischemic brain damage is still unknown. Here, we focused on vaccination for renin–angiotensin system and examined the effects of angiotensin II (Ang II) peptide vaccine in permanent middle cerebral artery occlusion model in rats.

Methods—

Male Wistar rats were exposed to permanent middle cerebral artery occlusion after 3× injections of Ang II peptide vaccine, and the serum or brain level of anti–Ang II antibody was examined. The effects of the vaccine were evaluated by differences in infarction volume, brain renin–angiotensin system components, and markers for neurodegeneration and oxidative stress.

Results—

Ang II vaccination successfully produced anti–Ang II antibodies in serum without concomitant change in blood pressure. Sufficient production of serum anti–Ang II antibody led to reduction of infarct volume and induced the penetration of anti–Ang II antibody in ischemic hemisphere, with suppressed expression of Ang II type 1 receptor mRNA. Vaccinated rats with sufficient antibody production showed the reduction of Fluoro-Jade B–positive cells, spectrin fragmentation, 4-hydroxynonenal-positive cells, and Nox 2 mRNA expression.

Conclusions—

Our findings indicate that Ang II vaccination exerts neuroprotective and antioxidative effects in cerebral ischemia, with renin–angiotensin system blockade by penetration of anti–Ang II antibodies into ischemic brain lesion. Ang II peptide vaccination could be a promising approach to treat ischemic stroke.

H2O2 augments cytosolic calcium in nucleus tractus solitarii neurons via multiple voltage-gated calcium channels

Reactive oxygen species (ROS) play a profound role in cardiorespiratory function under normal physiological conditions and disease states. ROS can influence neuronal activity by altering various ion channels and transporters. Within the nucleus tractus solitarii (nTS), a vital brainstem area for cardiorespiratory control, hydrogen peroxide (H₂O₂) induces sustained hyperexcitability following an initial depression of neuronal activity. The mechanism(s) associated with the delayed hyperexcitability are unknown. Here we evaluate the effect(s) of H₂O₂ on cytosolic Ca²⁺ (via fura-2 imaging) and voltage-dependent calcium currents in dissociated rat nTS neurons. H₂O₂ perfusion (200 µM; 1 min) induced a delayed, slow, and moderate increase (~27%) in intracellular Ca²⁺ concentration ([Ca²⁺]_i). The H₂O₂-mediated increase in [Ca²⁺]_i prevailed during thapsigargin, excluding the endoplasmic reticulum as a Ca²⁺ source. The effect, however, was abolished by removal of extracellular Ca²⁺ or the addition of cadmium to the bath solution, suggesting voltage-gated Ca²⁺ channels (VGCCs) as targets for H₂O₂ modulation. Recording of the total voltage-dependent Ca²⁺ current confirmed H₂O₂ enhanced Ca²⁺ entry. Blocking VGCC L, N, and P/Q subtypes decreased the number of cells and their calcium currents that respond to H₂O₂ The number of responder cells to H₂O₂ also decreased in the presence of dithiothreitol, suggesting the actions of H₂O₂ were dependent on sulfhydryl oxidation. In summary, here, we have shown that H₂O₂ increases [Ca²⁺]_i and its Ca²⁺ currents, which is dependent on multiple VGCCs likely by oxidation of sulfhydryl groups. These processes presumably contribute to the previously observed delayed hyperexcitability of nTS neurons in in vitro brainstem slices.

Paraventricular Nucleus Infusion of Epigallocatechin-3-O-Gallate Improves Renovascular Hypertension

Oxidative stress plays an important role in the pathogenesis of hypertension. Epigallocatechin-3-O-gallate (EGCG) is the main polyphenol present in green tea and is known for its potent antioxidant and anti-inflammatory properties. In the present study, we hypothesize that EGCG attenuates oxidative stress in the paraventricular nucleus of hypothalamus (PVN), thereby decreasing the blood pressure and sympathetic activity in renovascular hypertensive rats. After renovascular hypertension was induced in male Sprague-Dawley rats by the two-kidney one-clip (2K-1C) method, the rats received bilateral PVN infusion of EGCG (20 μg/h) or vehicle via osmotic minipump for 4 weeks. Our results were shown as follows: (1) Hypertension induced by 2K-1C was associated with the production of reactive oxygen species in the PVN; (2) chronic infusion of EGCG in the PVN decreased stress-related NAD(P)H oxidase subunit gp91(phox) and NOX-4 and increased the activity of antioxidant enzymes (SOD-1), also balanced the content of cytokines (IL-1β, IL-6, IL-10 and MCP-1) in the PVN, and attenuated the level of norepinephrine in plasma of 2K-1C rats. Our findings provide strong evidence that PVN infusion of EGCG inhibited renovascular hypertension progression through its potent anti-oxidative and anti-inflammatory activity in the PVN.

Oxidative Stress and Hypertensive Diseases

It has become clear that reactive oxygen species (ROS) contribute to the development of hypertension via myriad effects. ROS are essential for normal cell function; however, they mediate pathologic changes in the brain, the kidney, and blood vessels that contribute to the genesis of chronic hypertension. There is also emerging evidence that ROS contribute to immune activation in hypertension. This article discusses these events and how they coordinate to contribute to hypertension and its consequent end-organ damage.

Central Angiotensin-II Increases Blood Pressure and Sympathetic Outflow via Rho Kinase Activation in Conscious Rabbits

Elevated sympathetic tone and activation of the renin-angiotensin system are pathophysiologic hallmarks of hypertension, and the interactions between these systems are particularly deleterious. The importance of Rho kinase as a mediator of the effects of angiotensin-II (AngII) in the periphery is clear, but the role of Rho kinase in sympathoexcitation caused by central AngII is not well established. We hypothesized that AngII mediates its effects in the brain by the activation of the RhoA/Rho kinase pathway. Chronically instrumented, conscious rabbits received the following intracerebroventricular infusion treatments for 2 weeks via osmotic minipump: AngII, Rho kinase inhibitor Fasudil, AngII plus Fasudil, or a vehicle control. AngII increased mean arterial pressure over the course of the infusion, and this effect was prevented by the coadministration of Fasudil. AngII increased cardiac and vascular sympathetic outflow as quantified by the heart rate response to metoprolol and the depressor effect of hexamethonium; coadministration of Fasudil abolished both of these effects. AngII increased baseline renal sympathetic nerve activity in conscious animals and impaired baroreflex control of sympathetic nerve activity; again Fasudil coinfusion prevented these effects. Each of these end points showed a statistically significant interaction between AngII and Fasudil. Quantitative immunofluorescence of brain slices confirmed that Rho kinase activity was increased by AngII and decreased by Fasudil. Taken together, these data indicate that hypertension, elevated sympathetic outflow, and baroreflex dysfunction caused by central AngII are mediated by Rho kinase activation and suggest that Rho kinase inhibition may be an important therapeutic target in sympathoexcitatory cardiovascular diseases.

Angiotensin II-superoxide-NFκB signaling and aortic baroreceptor dysfunction in chronic heart failure

Chronic heart failure (CHF) affects approximately 5.7 million people in the United States. Increasing evidence from both clinical and experimental studies indicates that the sensitivity of arterial baroreflex is blunted in the CHF state, which is a predictive risk factor for sudden cardiac death. Normally, the arterial baroreflex regulates blood pressure and heart rate through sensing mechanical alteration of arterial vascular walls by baroreceptor terminals in the aortic arch and carotid sinus. There are aortic baroreceptor neurons in the nodose ganglion (NG), which serve as the main afferent component of the arterial baroreflex. Functional changes of baroreceptor neurons are involved in the arterial baroreflex dysfunction in CHF. In the CHF state, circulating angiotensin II (Ang II) and local Ang II concentration in the NG are elevated, and AT1R mRNA and protein are overexpressed in the NG. Additionally, Ang II-superoxide-NFκB signaling pathway regulates the neuronal excitability of aortic baroreceptors through influencing the expression and activation of Na_v channels in aortic baroreceptors, and subsequently causes the impairment of the arterial baroreflex in CHF. These new findings provide a basis for potential pharmacological interventions for the improvement of the arterial baroreflex sensitivity in the CHF state. This review summarizes the mechanisms responsible for the arterial baroreflex dysfunction in CHF.

Modulation of angiotensin II signaling following exercise training in heart failure

Sympathetic activation is a consistent finding in the chronic heart failure (CHF) state. Current therapy for CHF targets the renin-angiotensin II (ANG II) and adrenergic systems. Angiotensin converting enzyme (ACE) inhibitors and ANG II receptor blockers are standard treatments along with β-adrenergic blockade. However, the mortality and morbidity of this disease is still extremely high, even with good medical management. Exercise training (ExT) is currently being used in many centers as an adjunctive therapy for CHF. Clinical studies have shown that ExT is a safe, effective, and inexpensive way to improve quality of life, work capacity, and longevity in patients with CHF. This review discusses the potential neural interactions between ANG II and sympatho-excitation in CHF and the modulation of this interaction by ExT. We briefly review the current understanding of the modulation of the angiotensin type 1 receptor in sympatho-excitatory areas of the brain and in the periphery (i.e., in the carotid body and skeletal muscle). We discuss possible cellular mechanisms by which ExT may impact the sympatho-excitatory process by reducing oxidative stress, increasing nitric oxide. and reducing ANG II. We also discuss the potential role of ACE2 and Ang 1-7 in the sympathetic response to ExT. Fruitful areas of further investigation are the role and mechanisms by which pre-sympathetic neuronal metabolic activity in response to individual bouts of exercise regulate redox mechanisms and discharge at rest in CHF and other sympatho-excitatory states.

Endoplasmic reticulum and oxidant stress mediate nuclear factor-κB activation in the subfornical organ during angiotensin II hypertension

Endoplasmic reticulum (ER) stress and reactive oxygen species (ROS) generation in the brain circumventricular subfornical organ (SFO) mediate the central hypertensive actions of Angiotensin II (ANG II). However, the downstream signaling events remain unclear. Here we tested the hypothesis that angiotensin type 1a receptors (AT1aR), ER stress, and ROS induce activation of the transcription factor nuclear factor-κB (NF-κB) during ANG II-dependent hypertension. To spatiotemporally track NF-κB activity in the SFO throughout the development of ANG II-dependent hypertension, we used SFO-targeted adenoviral delivery and longitudinal bioluminescence imaging in mice. During low-dose infusion of ANG II, bioluminescence imaging revealed a prehypertensive surge in NF-κB activity in the SFO at a time point prior to a significant rise in arterial blood pressure. SFO-targeted ablation of AT1aR, inhibition of ER stress, or adenoviral scavenging of ROS in the SFO prevented the ANG II-induced increase in SFO NF-κB. These findings highlight the utility of bioluminescence imaging to longitudinally track transcription factor activation during the development of ANG II-dependent hypertension and reveal an AT1aR-, ER stress-, and ROS-dependent prehypertensive surge in NF-κB activity in the SFO. Furthermore, the increase in NF-κB activity before a rise in arterial blood pressure suggests a causal role for SFO NF-κB in the development of ANG II-dependent hypertension.

Rapid metabolism of exogenous angiotensin II by catecholaminergic neuronal cells in culture media

Angiotensin II (AngII) acts on central neurons to increase neuronal firing and induce sympathoexcitation, which contribute to the pathogenesis of cardiovascular diseases including hypertension and heart failure. Numerous studies have examined the precise AngII-induced intraneuronal signaling mechanism in an attempt to identify new therapeutic targets for these diseases. Considering the technical challenges in studying specific intraneuronal signaling pathways in vivo, especially in the cardiovascular control brain regions, most studies have relied on neuronal cell culture models. However, there are numerous limitations in using cell culture models to study AngII intraneuronal signaling, including the lack of evidence indicating the stability of AngII in culture media. Herein, we tested the hypothesis that exogenous AngII is rapidly metabolized in neuronal cell culture media. Using liquid chromatography-tandem mass spectrometry, we measured levels of AngII and its metabolites, Ang III, Ang IV, and Ang-1-7, in neuronal cell culture media after administration of exogenous AngII (100 nmol/L) to a neuronal cell culture model (CATH.a neurons). AngII levels rapidly declined in the media, returning to near baseline levels within 3 h of administration. Additionally, levels of Ang III and Ang-1-7 acutely increased, while levels of Ang IV remained unchanged. Replenishing the media with exogenous AngII every 3 h for 24 h resulted in a consistent and significant increase in AngII levels for the duration of the treatment period. These data indicate that AngII is rapidly metabolized in neuronal cell culture media, and replenishing the media at least every 3 h is needed to sustain chronically elevated levels.

Angiotensin II type 1 receptor expression in astrocytes is upregulated leading to increased mortality in mice with myocardial infarction-induced heart failure

Enhanced central sympathetic outflow worsens left ventricular (LV) remodeling and prognosis in heart failure after myocardial infarction (MI). Previous studies suggested that activation of brain angiotensin II type 1 receptors (AT1R) in the brain stem leads to sympathoexcitation due to neuronal AT1R upregulation. Recent studies, however, revealed the importance of astrocytes for modulating neuronal activity, but whether changes in astrocytes influence central sympathetic outflow in heart failure is unknown. In the normal state, AT1R are only weakly expressed in astrocytes. We hypothesized that AT1R in astrocytes are upregulated in heart failure and modulate the activity of adjacent neurons, leading to enhanced sympathetic outflow. In the present study, by targeting deletion of astrocyte-specific AT1R, we investigated whether AT1R in astrocytes have a key role in enhancing central sympathetic outflow, and thereby influencing LV remodeling process and the prognosis of MI-induced heart failure. Using the Cre-LoxP system, we generated glial fibrillary acidic protein (GFAP)-specific AT1R knockout (GFAP/AT1RKO) mice. Urinary norepinephrine excretion for 24 h, as an indicator of sympathoexcitation, was significantly lower in GFAP/AT1RKO-MI mice than in control-MI mice. LV size and heart weight after MI were significantly smaller in GFAP/AT1RKO mice than in control mice. Prognosis was significantly improved in GFAP/AT1RKO-MI mice compared with control-MI mice. Our findings indicated that AT1R expression was upregulated in brain stem astrocytes in MI-induced heart failure, which worsened LV remodeling and prognosis via sympathoexcitation. Thus, in addition to neuronal AT1R, AT1R in astrocytes appear to have a key role in enhancing central sympathetic outflow in heart failure.

Disruption of cardiovascular circadian rhythms in mice post myocardial infarction: relationship with central angiotensin II receptor expression

Angiotensin II (Ang II) is well known to participate in the abnormal autonomic cardiovascular control that occurs during the development of chronic heart failure (CHF). Disrupted cardiovascular circadian rhythm in CHF is also well accepted; however, the mechanisms underlying and the role of central Ang II type 1 receptors (AT1R) and oxidative stress in mediating such changes are not clear. In a post myocardial infarction (MI) CHF mouse model we investigated the circadian rhythm for mean arterial pressure (MAP), heart rate (HR), and baroreflex sensitivity (BRS) following MI. The cardiovascular parameters represent the middle 6‐h averages during daytime (6:00–18:00) and nighttime (18:00–6:00). HR increased with the severity of CHF reaching its maximum by 12 weeks post‐MI; loss of circadian HR and BRS rhythms were observed as early as 4 weeks post‐MI in conjunction with a significant blunting of the BRS and an upregulation in the AT1R and gp91^phox proteins in the brainstem. Loss of MAP circadian rhythm was observed 8 weeks post‐MI. Circadian AT1R expression was demonstrated in sham animals but was lost 8 weeks following MI. Losartan reduced AT1R expression in daytime (1.18 ± 0.1 vs. 0.85 ± 0.1; P < 0.05) with a trend toward a reduction in the AT1R mRNA expression in the nighttime (1.2 ± 0.1 vs. 1.0 ± 0.1; P > 0.05) but failed to restore circadian variability. The disruption of circadian rhythm for HR, MAP and BRS along with the upregulation of AT1 and gp91^phox suggests a possible role for central oxidative stress as a mediator of circadian cardiovascular parameters in the post‐MI state.

Increases in central angiotenisn II signaling provide a driving force for sympatho‐excitation in heart failure. In this study, we show a loss of circadian variability in angiotensin type 1 receptor expression in the brainstem of mice post myocardial infarction. These changes correlate with a loss of cardiovascular circadian variability. These data suggest that sympatho‐ excitation may be increased in the post‐MI state at times when sympathetic outflow is normally reduced.

Neuronal uptake of nanoformulated superoxide dismutase and attenuation of angiotensin II-dependent hypertension after central administration

Excessive production of superoxide (O2(-)) in the central nervous system has been widely implicated in the pathogenesis of cardiovascular diseases, including chronic heart failure and hypertension. In an attempt to overcome the failed therapeutic impact of currently available antioxidants in cardiovascular disease, we developed a nanomedicine-based delivery system for the O2(-)-scavenging enzyme copper/zinc superoxide dismutase (CuZnSOD), in which CuZnSOD protein is electrostatically bound to a poly-l-lysine (PLL50)-polyethylene glycol (PEG) block copolymer to form a CuZnSOD nanozyme. Various formulations of CuZnSOD nanozyme are covalently stabilized by either reducible or nonreducible crosslinked bonds between the PLL50-PEG polymers. Herein, we tested the hypothesis that PLL50-PEG CuZnSOD nanozyme delivers active CuZnSOD protein to neurons and decreases blood pressure in a mouse model of angiotensin II (AngII)-dependent hypertension. As determined by electron paramagnetic resonance spectroscopy, nanozymes retain full SOD enzymatic activity compared to native CuZnSOD protein. Nonreducible CuZnSOD nanozyme delivers active CuZnSOD protein to central neurons in culture (CATH.a neurons) without inducing significant neuronal toxicity. Furthermore, in vivo studies conducted in adult male C57BL/6 mice demonstrate that hypertension established by chronic subcutaneous infusion of AngII is significantly attenuated for up to 7 days after a single intracerebroventricular injection of nonreducible nanozyme. These data indicate the efficacy of nonreducible PLL50-PEG CuZnSOD nanozyme in counteracting excessive O2(-) and decreasing blood pressure in AngII-dependent hypertensive mice after central administration. Additionally, this study supports the further development of PLL50-PEG CuZnSOD nanozyme as an antioxidant-based therapeutic option for hypertension.

Angiotensin II induces membrane trafficking of natively expressed transient receptor potential vanilloid type 4 channels in hypothalamic 4B cells

Transient receptor potential vanilloid family type 4 (TRPV4) channels are expressed in central neuroendocrine neurons and have been shown to be polymodal in other systems. We previously reported that in the rodent, a model of dilutional hyponatremia associated with hepatic cirrhosis, TRPV4 expression is increased in lipid rafts from the hypothalamus and that this effect may be angiotensin dependent. In this study, we utilized the immortalized neuroendocrine rat hypothalamic 4B cell line to more directly test the effects of angiotensin II (ANG II) on TRPV4 expression and function. Our results demonstrate the expression of corticotropin-releasing factor (CRF) transcripts, for sex-determining region Y (SRY) (male genotype), arginine vasopressin (AVP), TRPV4, and ANG II type 1a and 1b receptor in 4B cells. After a 1-h incubation in ANG II (100 nM), 4B cells showed increased TRPV4 abundance in the plasma membrane fraction, and this effect was prevented by the ANG II type 1 receptor antagonist losartan (1 μM) and by a Src kinase inhibitor PP2 (10 μM). Ratiometric calcium imaging experiments demonstrated that ANG II incubation potentiated TRPV4 agonist (GSK 1016790A, 20 nM)-induced calcium influx (control 18.4 ± 2.8% n = 5 and ANG II 80.5 ± 2.4% n = 5). This ANG II-induced increase in calcium influx was also blocked by 1 μM losartan and 10 μM PP2 (losartan 26.4 ± 3.8% n = 5 and PP2 19.7 ± 3.9% n = 5). Our data suggests that ANG II can increase TRPV4 channel membrane expression in 4B cells through its action on AT1R involving a Src kinase pathway.

Central command dysfunction in rats with heart failure is mediated by brain oxidative stress and normalized by exercise training

Sympathoexcitation elicited by central command, a parallel activation of the motor and autonomic neural circuits in the brain, has been shown to become exaggerated in chronic heart failure (CHF). The present study tested the hypotheses that oxidative stress in the medulla in CHF plays a role in exaggerating central command-elicited sympathoexcitation, and that exercise training in CHF suppresses central command-elicited sympathoexcitation through its antioxidant effects in the medulla. In decerebrate rats, central command was activated by electrically stimulating the mesencephalic locomotor region (MLR) after neuromuscular blockade. The MLR stimulation at a current intensity greater than locomotion threshold in rats with CHF after myocardial infarction (MI) evoked larger (P < 0.05) increases in renal sympathetic nerve activity and arterial pressure than in sham-operated healthy rats (Sham) and rats with CHF that had completed longterm (8–12 weeks) exercise training (MI + TR). In the Sham and MI + TR rats, bilateral microinjection of a superoxide dismutase (SOD) mimetic Tempol into the rostral ventrolateral medulla (RVLM) had no effects on MLR stimulation-elicited responses. By contrast, in MI rats, Tempol treatment significantly reduced MLR stimulation-elicited responses. In a subset of MI rats, treatment with Tiron, another SOD mimetic, within the RVLM also reduced responses. Superoxide generation in the RVLM, as evaluated by dihydroethidium staining, was enhanced in MI rats compared with that in Sham and MI + TR rats. Collectively, these results support the study hypotheses. We suggest that oxidative stress in the medulla in CHF mediates central command dysfunction, and that exercise training in CHF is capable of normalizing central command dysfunction through its antioxidant effects in the medulla.

Reactive oxygen species, vascular Noxs, and hypertension: focus on translational and clinical research

SIGNIFICANCE

Reactive oxygen species (ROS) are signaling molecules that are important in physiological processes, including host defense, aging, and cellular homeostasis. Increased ROS bioavailability and altered redox signaling (oxidative stress) have been implicated in the onset and/or progression of chronic diseases, including hypertension.

RECENT ADVANCES

Although oxidative stress may not be the only cause of hypertension, it amplifies blood pressure elevation in the presence of other pro-hypertensive factors, such as salt loading, activation of the renin-angiotensin-aldosterone system, and sympathetic hyperactivity, at least in experimental models. A major source for ROS in the cardiovascular-renal system is a family of nicotinamide adenine dinucleotide phosphate oxidases (Noxs), including the prototypic Nox2-based Nox, and Nox family members: Nox1, Nox4, and Nox5.

CRITICAL ISSUES

Although extensive experimental data support a role for increased ROS levels and altered redox signaling in the pathogenesis of hypertension, the role in clinical hypertension is unclear, as a direct causative role of ROS in blood pressure elevation has yet to be demonstrated in humans. Nevertheless, what is becoming increasingly evident is that abnormal ROS regulation and aberrant signaling through redox-sensitive pathways are important in the pathophysiological processes which is associated with vascular injury and target-organ damage in hypertension.

FUTURE DIRECTIONS

There is a paucity of clinical information related to the mechanisms of oxidative stress and blood pressure elevation, and a few assays accurately measure ROS directly in patients. Such further ROS research is needed in humans and in the development of adequately validated analytical methods to accurately assess oxidative stress in the clinic.

Brain stem NOS and ROS in neural mechanisms of hypertension

SIGNIFICANCE

There is now compelling evidence to substantiate the notion that by depressing baroreflex regulation of blood pressure and augmenting central sympathetic outflow through their actions on the nucleus tractus solitarii (NTS) and rostral ventrolateral medulla (RVLM), brain stem nitric oxide synthase (NOS) and reactive oxygen species (ROS) are important contributing factors to neural mechanisms of hypertension. This review summarizes our contemporary views on the impact of NOS and ROS in the NTS and RVLM on neurogenic hypertension, and presents potential antihypertensive strategies that target brain stem NOS/ROS signaling.

RECENT ADVANCES

NO signaling in the brain stem may be pro- or antihypertensive depending on the NOS isoform that generates this gaseous moiety and the site of action. Elevation of the ROS level when its production overbalances its degradation in the NTS and RVLM underlies neurogenic hypertension. Interventional strategies with emphases on alleviating the adverse actions of these molecules on blood pressure regulation have been investigated.

CRITICAL ISSUES

The pathological roles of NOS in the RVLM and NTS in neural mechanisms of hypertension are highly complex. Likewise, multiple signaling pathways underlie the deleterious roles of brain-stem ROS in neurogenic hypertension. There are recent indications that interactions between brain stem ROS and NOS may play a contributory role.

FUTURE DIRECTIONS

Given the complicity of action mechanisms of brain-stem NOS and ROS in neural mechanisms of hypertension, additional studies are needed to identify the most crucial therapeutic target that is applicable not only in animal models but also in patients suffering from neurogenic hypertension.

Over-expressed copper/zinc superoxide dismutase localizes to mitochondria in neurons inhibiting the angiotensin II-mediated increase in mitochondrial superoxide

Angiotensin II (AngII) is the main effector peptide of the renin–angiotensin system (RAS), and contributes to the pathogenesis of cardiovascular disease by exerting its effects on an array of different cell types, including central neurons. AngII intra-neuronal signaling is mediated, at least in part, by reactive oxygen species, particularly superoxide (O₂^•−). Recently, it has been discovered that mitochondria are a major subcellular source of AngII-induced O₂^•−. We have previously reported that over-expression of manganese superoxide dismutase (MnSOD), a mitochondrial matrix-localized O₂^•− scavenging enzyme, inhibits AngII intra-neuronal signaling. Interestingly, over-expression of copper/zinc superoxide dismutase (CuZnSOD), which is believed to be primarily localized to the cytoplasm, similarly inhibits AngII intra-neuronal signaling and provides protection against AngII-mediated neurogenic hypertension. Herein, we tested the hypothesis that CuZnSOD over-expression in central neurons localizes to mitochondria and inhibits AngII intra-neuronal signaling by scavenging mitochondrial O₂^•−. Using a neuronal cell culture model (CATH.a neurons), we demonstrate that both endogenous and adenovirus-mediated over-expressed CuZnSOD (AdCuZnSOD) are present in mitochondria. Furthermore, we show that over-expression of CuZnSOD attenuates the AngII-mediated increase in mitochondrial O₂^•− levels and the AngII-induced inhibition of neuronal potassium current. Taken together, these data clearly show that over-expressed CuZnSOD in neurons localizes in mitochondria, scavenges AngII-induced mitochondrial O₂^•−, and inhibits AngII intra-neuronal signaling.

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Endogenous CuZnSOD is localized to mitochondria of AngII-sensitive neurons.

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Adenovirus-mediated over-expressed CuZnSOD is localized to neuron mitochondria.

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AngII-induced mitochondrial O₂^•− flux is attenuated by CuZnSOD over-expression.

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Over-expressed CuZnSOD reduces AngII-mediated inhibition of neuronal K⁺ current.

Angiotensin II-induced production of mitochondrial reactive oxygen species: potential mechanisms and relevance for cardiovascular disease

SIGNIFICANCE

The role of reactive oxygen species (ROS) in angiotensin II (AngII) induced endothelial dysfunction, cardiovascular and renal remodeling, inflammation, and fibrosis has been well documented. The molecular mechanisms of AngII pathophysiological activity involve the stimulation of NADPH oxidases, which produce superoxide and hydrogen peroxide. AngII also increases the production of mitochondrial ROS, while the inhibition of AngII improves mitochondrial function; however, the specific molecular mechanisms of the stimulation of mitochondrial ROS is not clear.

RECENT ADVANCES

Interestingly, the overexpression of mitochondrial thioredoxin 2 or mitochondrial superoxide dismutase attenuates AngII-induced hypertension, which demonstrates the importance of mitochondrial ROS in AngII-mediated cardiovascular diseases.

CRITICAL ISSUES

Although mitochondrial ROS plays an important role in normal physiological cell signaling, AngII, high glucose, high fat, or hypoxia may cause the overproduction of mitochondrial ROS, leading to the feed-forward redox stimulation of NADPH oxidases. This vicious cycle may contribute to the development of pathological conditions and facilitate organ damage in hypertension, atherosclerosis, and diabetes.

FUTURE DIRECTIONS

The development of antioxidant strategies specifically targeting mitochondria could be therapeutically beneficial in these disease conditions.

Angiotensin-generated reactive oxygen species in brain and pathogenesis of cardiovascular diseases

SIGNIFICANCE

Overproduction of angiotensin II (Ang II) in brain contributes to the pathogenesis of cardiovascular diseases. One of the most promising theses that emerged during the last decade is that production of reactive oxygen species (ROS) and activation of redox-dependent signaling cascades underlie those Ang II actions. This review summarizes our status of understanding on the roles of ROS and redox-sensitive signaling in brain Ang II-dependent cardiovascular diseases, using hypertension and heart failure as illustrative examples.

RECENT ADVANCES

ROS generated by NADPH oxidase, mitochondrial electron transport chain, and proinflammatory cytokines activates mitogen-activated protein kinases and transcription factors, which in turn modulate ion channel functions and ultimately increase neuronal activity and sympathetic outflow in brain Ang II-dependent cardiovascular diseases. Antioxidants targeting ROS have been demonstrated to be beneficial to Ang II-induced hypertension and heart failure via protection from oxidative stress in brain regions that subserve cardiovascular regulation.

CRITICAL ISSUES

Intra-neuronal signaling and the downstream redox-sensitive proteins involved in controlling the neuronal discharge rate, the sympathetic outflow, and the pathogenesis of cardiovascular diseases need to be identified. The cross talk between Ang II-induced oxidative stress and neuroinflammation in neural mechanisms of cardiovascular diseases also warrants further elucidation.

FUTURE DIRECTIONS

Future studies are needed to identify new redox-based therapeutics that work not only in animal models, but also in patients suffering from the prevalent diseases. Upregulation of endogenous antioxidants in the regulation of ROS homeostasis is a potential therapeutic target, as are small molecule- or nanoformulated conjugate-based antioxidant therapy.

COX-1-derived PGE2 and PGE2 type 1 receptors are vital for angiotensin II-induced formation of reactive oxygen species and Ca(2+) influx in the subfornical organ

Regulation of blood pressure by angiotensin II (ANG II) is a process that involves the reactive oxygen species (ROS) and calcium. We have shown that ANG-II type 1 receptor (AT1R) and prostaglandin E2 (PGE2) type 1 receptors (EP1R) are required in the subfornical organ (SFO) for ROS-mediated hypertension induced by slow-pressor ANG-II infusion. However, the signaling pathway associated with this process remains unclear. We sought to determine mechanisms underlying the ANG II-induced ROS and calcium influx in mouse SFO cells. Ultrastructural studies showed that cyclooxygenase 1 (COX-1) codistributes with AT1R in the SFO, indicating spatial proximity. Functional studies using SFO cells revealed that ANG II potentiated PGE2 release, an effect dependent on AT1R, phospholipase A2 (PLA2) and COX-1. Furthermore, both ANG II and PGE2 increased ROS formation. While the increase in ROS initiated by ANG II, but not PGE2, required the activation of the AT1R/PLA2/COX-1 pathway, both ANG II and PGE2 were dependent on EP1R and Nox2 as downstream effectors. Finally, ANG II potentiated voltage-gated L-type Ca(2+) currents in SFO neurons via the same signaling pathway required for PGE2 production. Blockade of EP1R and Nox2-derived ROS inhibited ANG II and PGE2-mediated Ca(2+) currents. We propose a mechanism whereby ANG II increases COX-1-derived PGE2 through the AT1R/PLA2 pathway, which promotes ROS production by EP1R/Nox2 signaling in the SFO. ANG II-induced ROS are coupled with Ca(2+) influx in SFO neurons, which may influence SFO-mediated sympathoexcitation. Our findings provide the first evidence of a spatial and functional framework that underlies ANG-II signaling in the SFO and reveal novel targets for antihypertensive therapies.

Sex differences in angiotensin II- and aldosterone-induced hypertension: the central protective effects of estrogen

Premenopausal women have lower blood pressure and a reduced incidence of cardiovascular disease compared with age-matched men. Similar sex differences have been seen across species and in multiple animal models of hypertension. While important progress over the last decade has been made in elucidating some of the mechanisms underlying these differences, there are still significant gaps in our knowledge. Understanding the cellular and molecular mechanisms responsible for sex differences in hypertension will be important for developing sex-specific therapies targeted toward the prevention and treatment of hypertension. Female sex hormones, especially estrogen, have been demonstrated to modulate the renin-angiotensin-aldosterone system (RAAS) and to have beneficial effects on cardiovascular function through actions not only on the kidney, heart, and vasculature, but also on the central nervous system (CNS). This review primarily focuses on the central regulatory actions of estrogen on brain nuclei involved in blood pressure regulation and the interactions between estrogen and the RAAS in the CNS by which estrogen plays an important protective role against the development of hypertension.

Membrane trafficking of NADPH oxidase p47(phox) in paraventricular hypothalamic neurons parallels local free radical production in angiotensin II slow-pressor hypertension

NADPH oxidase-generated reactive oxygen species (ROS) are highly implicated in the development of angiotensin II (AngII)-dependent hypertension mediated in part through the hypothalamic paraventricular nucleus (PVN). This region contains vasopressin and non-vasopressin neurons that are responsive to cardiovascular dysregulation, but it is not known whether ROS is generated by one or both cell types in response to "slow-pressor" infusion of AngII. We addressed this question using ROS imaging and electron microscopic dual labeling for vasopressin and p47(phox), a cytoplasmic NADPH oxidase subunit requiring mobilization to membranes for the initiation of ROS production. C57BL/6 mice or vasopressin-enhanced green fluorescent protein (VP-eGFP) mice were infused systemically with saline or AngII (600 ng · kg(-1) · min(-1), s.c.) for 2 weeks, during which they slowly developed hypertension. Ultrastructural analysis of the PVN demonstrated p47(phox) immunolabeling in many glial and neuronal profiles, most of which were postsynaptic dendrites. Compared with saline, AngII recipient mice had a significant increase in p47(phox) immunolabeling on endomembranes just beneath the plasmalemmal surface (+42.1 ± 11.3%; p < 0.05) in non-vasopressin dendrites. In contrast, AngII infusion decreased p47(phox) immunolabeling on the plasma membrane (-35.5 ± 16.5%; p < 0.05) in vasopressin dendrites. Isolated non-VP-eGFP neurons from the PVN of AngII-infused mice also showed an increase in baseline ROS production not seen in VP-eGFP neurons. Our results suggest that chronic low-dose AngII may offset the homeostatic control of blood pressure by differentially affecting membrane assembly of NADPH oxidase and ROS production in vasopressin and non-vasopressin neurons located within the PVN.

Mitochondrial-localized NADPH oxidase 4 is a source of superoxide in angiotensin II-stimulated neurons

Angiotensin II (ANG II) plays an important role in the central regulation of systemic cardiovascular function. ANG II-mediated intraneuronal signaling has been shown to be predicated by an increase in mitochondrial superoxide (O₂∙-), yet the source of this reactive oxygen species (ROS) production remains unclear. NADPH oxidase 4 (Nox4), a member of the NADPH oxidase family, has been reported to be localized in mitochondria of various cell types and has been implicated in brain angiotensinergic signaling. However, the subcellular localization and function of Nox4 in neurons has not been fully elucidated. In this study, we hypothesized that Nox4 is expressed in neuron mitochondria and is involved in ANG II-dependent O₂∙--mediated intraneuronal signaling. To query this, Nox4 immunofluorescent staining and mitochondrial enrichment were performed in a mouse catecholaminergic neuronal cell model (CATH.a). Nox4 was shown to be present in neuron mitochondria as evidenced by colocalization with both the mitochondrial-localized protein manganese superoxide dismutase (MnSOD) and dye MitoTracker Red. Moreover, Nox4 expression was significantly increased in enriched mitochondrial fractions compared with whole cell lysates. Additionally, adenoviral-encoded small interfering RNA for Nox4 (AdsiNox4) caused a robust knockdown in Nox4 mRNA and protein levels, which led to the attenuation of ANG II-induced mitochondrial O₂∙- production. Finally, in the subfornical organ (SFO) of the brain, Nox4 not only demonstrated mitochondrial localization but was induced by chronic, peripheral infusion of ANG II. Collectively, these data suggest that Nox4 is a source of O₂∙- in neuron mitochondria that contributes to ANG II intraneuronal signaling.

Which NADPH oxidase isoform is relevant for ischemic stroke? The case for nox 2

Significance and Recent Advances: Ischemic stroke is the leading cause of disability and third in mortality in industrialized nations. Immediate restoration of cerebral blood flow is crucial to salvage brain tissue, but only few patients are eligible for recanalization therapy. Thus, the need for alternative neuroprotective strategies is huge, and antioxidant interventions have long been studied in this context. Reactive oxygen species (ROS) physiologically serve as signaling molecules, but excessive amounts of ROS, as generated during ischemia/reperfusion (I/R), contribute to tissue injury.

CRITICAL ISSUES

Nevertheless and despite a strong rational of ROS being a pharmacological target, all antioxidant interventions failed to improve functional outcome in human clinical trials. Antioxidants may interfere with physiological functions of ROS or do not reach the crucial target structures of ROS-induced injury effectively.

FUTURE DIRECTIONS

Thus, a potentially more promising approach is the inhibition of the source of disease-promoting ROS. Within recent years, NADPH oxidases (Nox) of the Nox family have been identified as mediators of neuronal pathology. As, however, several Nox homologs are expressed in neuronal tissue, and as many of the pharmacological inhibitors employed are rather unspecific, the concept of Nox as mediators of brain damage is far from being settled. In this review, we will discuss the contribution of Nox homologs to I/R injury at large as well as to neuronal damage in particular. We will illustrate that the current data provide evidence for Nox2 as the most important NADPH oxidase mediating cerebral injury.

Antioxidant-based therapies for angiotensin II-associated cardiovascular diseases

Cardiovascular diseases, including hypertension and heart failure, are associated with activation of the renin-angiotensin system (RAS) and increased circulating and tissue levels of ANG II, a primary effector peptide of the RAS. Through its actions on various cell types and organ systems, ANG II contributes to the pathogenesis of cardiovascular diseases by inducing cardiac and vascular hypertrophy, vasoconstriction, sodium and water reabsorption in kidneys, sympathoexcitation, and activation of the immune system. Cardiovascular research over the past 15-20 years has clearly implicated an important role for elevated levels of reactive oxygen species (ROS) in mediating these pathophysiological actions of ANG II. As such, the use of antioxidants, to reduce the elevated levels of ROS, as potential therapies for various ANG II-associated cardiovascular diseases has been intensely investigated. Although some antioxidant-based therapies have shown therapeutic impact in animal models of cardiovascular disease and in human patients, others have failed. In this review, we discuss the benefits and limitations of recent strategies, including gene therapy, dietary sources, low-molecular-weight free radical scavengers, polyethylene glycol conjugation, and nanomedicine-based technologies, which are designed to deliver antioxidants for the improved treatment of cardiovascular diseases. Although much work has been completed, additional research focusing on developing specific antioxidant molecules or proteins and identifying the ideal in vivo delivery system for such antioxidants is necessary before the use of antioxidant-based therapies for cardiovascular diseases become a clinical reality.

Reactive oxygen species, Nox and angiotensin II in angiogenesis: implications for retinopathy

Pathological angiogenesis is a key feature of many diseases including retinopathies such as ROP (retinopathy of prematurity) and DR (diabetic retinopathy). There is considerable evidence that increased production of ROS (reactive oxygen species) in the retina participates in retinal angiogenesis, although the mechanisms by which this occurs are not fully understood. ROS is produced by a number of pathways, including the mitochondrial electron transport chain, cytochrome P450, xanthine oxidase and uncoupled nitric oxide synthase. The family of NADPH oxidase (Nox) enzymes are likely to be important given that their primary function is to produce ROS. Seven isoforms of Nox have been identified named Nox1-5, Duox (dual oxidase) 1 and Duox2. Nox1, Nox2 and Nox4 have been most extensively studied and are implicated in the development of conditions such as hypertension, cardiovascular disease and diabetic nephropathy. In recent years, evidence has accumulated to suggest that Nox1, Nox2 and Nox4 participate in pathological angiogenesis; however, there is no clear consensus about which Nox isoform is primarily responsible. In terms of retinopathy, there is growing evidence that Nox contribute to vascular injury. The RAAS (renin-angiotensin-aldosterone system), and particularly AngII (angiotensin II), is a key stimulator of Nox. It is known that a local RAAS exists in the retina and that blockade of AngII and aldosterone attenuate pathological angiogenesis in the retina. Whether the RAAS influences the production of ROS derived from Nox in retinopathy is yet to be fully determined. These topics will be reviewed with a particular emphasis on ROP and DR.

Conjugates of superoxide dismutase 1 with amphiphilic poly(2-oxazoline) block copolymers for enhanced brain delivery: synthesis, characterization and evaluation in vitro and in vivo

Superoxide dismutase 1 (SOD1) efficiently catalyzes dismutation of superoxide, but its poor delivery to the target sites in the body, such as brain, hinders its use as a therapeutic agent for superoxide-associated disorders. Here to enhance the delivery of SOD1 across the blood-brain barrier (BBB) and in neurons the enzyme was conjugated with poly(2-oxazoline) (POx) block copolymers, P(MeOx-b-BuOx) or P(EtOx-b-BuOx), composed of (1) hydrophilic 2-methyl-2-oxazoline (MeOx) or 2-ethyl-2-oxazoline (EtOx) and (2) hydrophobic 2-butyl-2-oxazoline (BuOx) repeating units. The conjugates contained from 2 to 3 POx chains joining the protein amino groups via cleavable -(ss)- or noncleavable -(cc)- linkers at the BuOx block terminus. They retained 30% to 50% of initial SOD1 activity, were conformationally and thermally stable, and assembled in 8 or 20 nm aggregates in aqueous solution. They had little if any toxicity to CATH.a neurons and displayed enhanced uptake in these neurons as compared to native or PEGylated SOD1. Of the two conjugates, SOD1-(cc)-P(MeOx-b-BuOx) and SOD1-(cc)-P(EtOx-b-BuOx), compared, the latter was entering cells 4 to 7 times faster and at 6 h colocalized predominantly with endoplasmic reticulum (41 ± 3%) and mitochondria (21 ± 2%). Colocalization with endocytosis markers and pathway inhibition assays suggested that it was internalized through lipid raft/caveolae, also employed by the P(EtOx-b-BuOx) copolymer. The SOD activity in cell lysates and ability to attenuate angiotensin II (Ang II)-induced superoxide in live cells were increased for this conjugate compared to SOD1 and PEG-SOD1. Studies in mice showed that SOD1-POx had ca. 1.75 times longer half-life in blood than native SOD1 (28.4 vs 15.9 min) and after iv administration penetrated the BBB significantly faster than albumin to accumulate in brain parenchyma. The conjugate maintained high stability both in serum and in brain (77% vs 84% at 1 h postinjection). Its amount taken up by the brain reached a maximum value of 0.08% ID/g (percent of the injected dose taken up per gram of brain) 4 h postinjection. The entry of SOD1-(cc)-P(EtOx-b-BuOx) to the brain was mediated by a nonsaturable mechanism. Altogether, SOD1-POx conjugates are promising candidates as macromolecular antioxidant therapies for superoxide-associated diseases such as Ang II-induced neurocardiovascular diseases.