Uncoupling of endothelial nitric oxide synthase in cerebral vasculature of Tg2576 mice

Protein arginine methyltransferase 4 modulates nitric oxide synthase uncoupling and cerebral blood flow in Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of mortality, disability, and long-term care burden in the United States, with women comprising the majority of AD diagnoses. While AD-related dementia is associated with tau and amyloid beta accumulation, concurrent derangements in cerebral blood flow have been observed alongside these proteinopathies in humans and rodent models. The homeostatic production of nitric oxide synthases (NOS) becomes uncoupled in AD which leads to decreased NO-mediated vasodilation and oxidative stress via the production of peroxynitrite (ONOO-∙) superoxide species. Here, we investigate the role of the novel protein arginine methyltransferase 4 (PRMT4) enzyme function and its downstream product asymmetric dimethyl arginine (ADMA) as it relates to NOS dysregulation and cerebral blood flow in AD. ADMA (type-1 PRMT product) has been shown to bind NOS as a noncanonic ligand causing enzymatic dysfunction. Our results from RT-qPCR and protein analyses suggest that aged (9-12 months) female mice bearing tau- and amyloid beta-producing transgenic mutations (3xTg-AD) express higher levels of PRMT4 in the hippocampus when compared to age- and sex-matched C57BL6/J mice. In addition, we performed studies to quantify the expression and activity of different NOS isoforms. Furthermore, laser speckle contrast imaging analysis was indicative that 3xTg-AD mice have dysfunctional NOS activity, resulting in reduced production of NO metabolites, enhanced production of free-radical ONOO-, and decreased cerebral blood flow. Notably, the aforementioned phenomena can be reversed via pharmacologic PRMT4 inhibition. Together, these findings implicate the potential importance of PRMT4 signaling in the pathogenesis of Alzheimer's-related cerebrovascular derangement.

Constitutive NOS Production Is Modulated by Alzheimer's Disease Pathology Depending on APOE Genotype

Both the endothelial (eNOS) and the neuronal (nNOS) isoforms of constitutive Nitric Oxide Synthase have been implicated in vascular dysfunctions in Alzheimer's disease (AD). We aimed to explore the relationship between amyloid pathology and NO dynamics by comparing the cerebrospinal fluid (CSF) levels of nNOS and eNOS of 8 healthy controls (HC) and 27 patients with a clinical diagnosis of Alzheimer's disease and isolated CSF amyloid changes, stratified according to APOE ε genotype (APOE ε3 = 13, APOE ε4 = 14). Moreover, we explored the associations between NOS isoforms, CSF AD biomarkers, age, sex, cognitive decline, and blood-brain barrier permeability. In our cohort, both eNOS and nNOS levels were increased in APOE ε3 with respect to HC and APOE ε4. CSF eNOS inversely correlated with CSF Amyloid-β42 selectively in carriers of APOE ε3; CSF nNOS was negatively associated with age and CSF p-tau only in the APOE ε4 subgroup. Increased eNOS could represent compensative vasodilation to face progressive Aβ-induced vasoconstriction in APOE ε3, while nNOS could represent the activation of NO-mediated plasticity strategies in the same group. Our results confirm previous findings that the APOE genotype is linked with different vascular responses to AD pathology.

The Impact of LY487379 or CDPPB on eNOS Expression in the Mouse Brain and the Effect of Joint Administration of Compounds with NO• Releasers on MK-801- or Scopolamine-Driven Cognitive Dysfunction in Mice

The role of endothelial nitric oxide synthase (eNOS) in the regulation of a variety of biological processes is well established, and its dysfunction contributes to brain pathologies, including schizophrenia or Alzheimer's disease (AD). Positive allosteric modulators (PAMs) of metabotropic glutamate (mGlu) receptors were shown to be effective procognitive compounds, but little is known about their impact on eNOS expression and stability. Here, we investigated the influence of the acute and chronic administration of LY487379 or CDPPB (mGlu2 and mGlu5 PAMs), on eNOS expression in the mouse brain and the effect of the joint administration of the ligands with nitric oxide (NO) releasers, spermineNONOate or DETANONOate, in different combinations of doses, on MK-801- or scopolamine-induced amnesia in the novel object recognition (NOR) test. Our results indicate that both compounds provoked eNOS monomer formation, and CDPPB at a dose of 5 mg/kg exaggerated the effect of MK-801 or scopolamine. The coadministration of spermineNONOate or DETANONOate enhanced the antiamnesic effect of CDPPB or LY487379. The best activity was observed for ineffective or moderate dose combinations. The results indicate that treatment with mGluR2 and mGluR5 PAMs may be burdened with the risk of promoting eNOS uncoupling through the induction of dimer dissociation. Administration of the lowest possible doses of the compounds with NO• donors, which themselves have procognitive efficacy, may be proposed for the treatment of schizophrenia or AD.

Xanthine oxidase mediates chronic stress-induced cerebrovascular dysfunction and cognitive impairment

Xanthine oxidase (XO) mediates vascular function. Chronic stress impairs cerebrovascular function and increases the risk of stroke and cognitive decline. Our study determined the role of XO on stress-induced cerebrovascular dysfunction and cognitive decline. We measured middle cerebral artery (MCA) function, free radical formation, and working memory in 6-month-old C57BL/6 mice who underwent 8 weeks of control conditions or unpredictable chronic mild stress (UCMS) with or without febuxostat (50 mg/L), a XO inhibitor. UCMS mice had an impaired MCA dilation to acetylcholine vs. controls (p < 0.0001), and increased total free radical formation, XOR protein levels, and hydrogen peroxide production in the liver compared to controls. UCMS increased hydrogen peroxide production in the brain and cerebrovasculature compared to controls. Working memory, using the y-maze test, was impaired (p < 0.05) in UCMS mice compared to control mice. However, blocking XO using febuxostat prevented the UCMS-induced impaired MCA response, while free radical production and hydrogen peroxide levels were similar to controls in the liver and brain of UCMS mice treated with febuxostat. Further, UCMS + Feb mice did not have a significant reduction in working memory. These data suggest that the cerebrovascular dysfunction associated with chronic stress may be driven by XO, which leads to a reduction in working memory.

Soluble pathogenic tau enters brain vascular endothelial cells and drives cellular senescence and brain microvascular dysfunction in a mouse model of tauopathy

Vascular mechanisms of Alzheimer's disease (AD) may constitute a therapeutically addressable biological pathway underlying dementia. We previously demonstrated that soluble pathogenic forms of tau (tau oligomers) accumulate in brain microvasculature of AD and other tauopathies, including prominently in microvascular endothelial cells. Here we show that soluble pathogenic tau accumulates in brain microvascular endothelial cells of P301S(PS19) mice modeling tauopathy and drives AD-like brain microvascular deficits. Microvascular impairments in P301S(PS19) mice were partially negated by selective removal of pathogenic soluble tau aggregates from brain. We found that similar to trans-neuronal transmission of pathogenic forms of tau, soluble tau aggregates are internalized by brain microvascular endothelial cells in a heparin-sensitive manner and induce microtubule destabilization, block endothelial nitric oxide synthase (eNOS) activation, and potently induce endothelial cell senescence that was recapitulated in vivo in microvasculature of P301S(PS19) mice. Our studies suggest that soluble pathogenic tau aggregates mediate AD-like brain microvascular deficits in a mouse model of tauopathy, which may arise from endothelial cell senescence and eNOS dysfunction triggered by internalization of soluble tau aggregates.

Dual role of nitric oxide in Alzheimer's Disease

Nitric oxide (NO), an enzymatic product of nitric oxide synthase (NOS), has been associated with a variety of neurological diseases such as Alzheimer's disease (AD). NO has long been thought to contribute to neurotoxic insults caused by neuroinflammation in AD. This perception shifts as more attention is paid to the early stages before cognitive problems manifest. However, it has revealed a compensatory neuroprotective role for NO that protects synapses by increasing neuronal excitability. NO can positively affect neurons by inducing neuroplasticity, neuroprotection, and myelination, as well as having cytolytic activity to reduce inflammation. NO can also induce long-term potentiation (LTP), a process by which synaptic connections among neurons become more potent. Not to mention that such functions give rise to AD protection. Notably, it is unquestionably necessary to conduct more research to clarify NO pathways in neurodegenerative dementias because doing so could help us better understand their pathophysiology and develop more effective treatment options. All these findings bring us to the prevailing notion that NO can be used either as a therapeutic agent in patients afflicted with AD and other memory impairment disorders or as a contributor to the neurotoxic and aggressive factor in AD. In this review, after presenting a general background on AD and NO, various factors that have a pivotal role in both protecting and exacerbating AD and their correlation with NO will be elucidated. Following this, both the neuroprotective and neurotoxic effects of NO on neurons and glial cells among AD cases will be discussed in detail.

Emerging Roles of Endothelial Nitric Oxide in Preservation of Cognitive Health

eNOS (endothelial nitric oxide synthase) is critically important enzyme responsible for regulation of cardiovascular homeostasis. Under physiological conditions, constitutive eNOS activity and production of endothelial nitric oxide (NO) exert essential neurovascular protective functions. In this review, we first discuss the roles of endothelial NO in prevention of neuronal amyloid accumulation and formation of neurofibrillary tangles, hallmarks of Alzheimer disease pathology. Next, we review existing evidence suggesting that NO released from endothelium prevents activation of microglia, stimulates glycolysis in astrocytes, and increases biogenesis of mitochondria. We also address major risk factors for cognitive impairment including aging and ApoE4 (apolipoprotein 4) genotype with focus on their detrimental effects on eNOS/NO signaling. Relevant to this review, recent studies suggested that aged eNOS heterozygous mice are unique model of spontaneous cerebral small vessel disease. In this regard, we review contribution of dysfunctional eNOS to deposition of Aβ (amyloid-β) into blood vessel wall leading to development of cerebral amyloid angiopathy. We conclude that endothelial dysfunction manifested by the loss of neurovascular protective functions of NO may significantly contribute to development of cognitive impairment.

Targeting hydrogen sulfide and nitric oxide to repair cardiovascular injury after trauma

The systemic cardiovascular effects of major trauma, especially neurotrauma, contribute to death and permanent disability in trauma patients and treatments are needed to improve outcomes. In some trauma patients, dysfunction of the autonomic nervous system produces a state of adrenergic overstimulation, causing either a sustained elevation in catecholamines (sympathetic storm) or oscillating bursts of paroxysmal sympathetic hyperactivity. Trauma can also activate innate immune responses that release cytokines and damage-associated molecular patterns into the circulation. This combination of altered autonomic nervous system function and widespread systemic inflammation produces secondary cardiovascular injury, including hypertension, damage to cardiac tissue, vascular endothelial dysfunction, coagulopathy and multiorgan failure. The gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) are small gaseous molecules with potent effects on vascular tone regulation. Exogenous NO (inhaled) has potential therapeutic benefit in cardio-cerebrovascular diseases, but limited data suggests potential efficacy in traumatic brain injury (TBI). H2S is a modulator of NO signaling and autonomic nervous system function that has also been used as a drug for cardio-cerebrovascular diseases. The inhaled gases NO and H2S are potential treatments to restore cardio-cerebrovascular function in the post-trauma period.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Association of cerebral microvascular dysfunction and white matter injury in Alzheimer's disease

Patients with Alzheimer's disease (AD) often have cerebral white matter (WM) hyperintensities on MRI and microinfarcts of presumed microvascular origin pathologically. Here, we determined if vasodilator dysfunction of WM-penetrating arterioles is associated with pathologically defined WM injury and disturbances in quantitative MRI-defined WM integrity in patients with mixed microvascular and AD pathology. We analyzed tissues from 28 serially collected human brains from research donors diagnosed with varying degrees of AD neuropathologic change (ADNC) with or without cerebral microinfarcts (mVBI). WM-penetrating and pial surface arteriolar responses to the endothelium-dependent agonist bradykinin were quantified ex vivo with videomicroscopy. Vascular endothelial nitric oxide synthase (eNOS) and NAD(P)H-oxidase (Nox1, 2 and 4 isoforms) expression were measured with quantitative PCR. Glial fibrillary acidic protein (GFAP)-labeled astrocytes were quantified by unbiased stereological approaches in regions adjacent to the sites of WM-penetrating vessel collection. Post-mortem diffusion tensor imaging (DTI) was used to measure mean apparent diffusion coefficient (ADC) and fractional anisotropy (FA), quantitative indices of WM integrity. In contrast to pial surface arterioles, white matter-penetrating arterioles from donors diagnosed with high ADNC and mVBI exhibited a significantly reduced dilation in response to bradykinin when compared to the other groups. Expression of eNOS was reduced, whereas Nox1 expression was increased in WM arterioles in AD and mVBI cases. WM astrocyte density was increased in AD and mVBI, which correlated with a reduced vasodilation in WM arterioles. Moreover, in cases with low ADNC, bradykinin-induced WM arteriole dilation correlated with lower ADC and higher FA values. Comorbid ADNC and mVBI appear to synergistically interact to selectively impair bradykinin-induced vasodilation in WM-penetrating arterioles, which may be related to reduced nitric oxide- and excess reactive oxygen species-mediated vascular endothelial dysfunction. WM arteriole vasodilator dysfunction is associated with WM injury, as supported by reactive astrogliosis and MRI-defined disrupted WM microstructural integrity.

Nitric Oxide-Dependent Mechanisms Underlying MK-801- or Scopolamine-Induced Memory Dysfunction in Animals: Mechanistic Studies

MK-801, an NMDA receptor antagonist, and scopolamine, a cholinergic receptor blocker, are widely used as tool compounds to induce learning and memory deficits in animal models to study schizophrenia or Alzheimer-type dementia (AD), respectively. Memory impairments are observed after either acute or chronic administration of either compound. The present experiments were performed to study the nitric oxide (NO)-related mechanisms underlying memory dysfunction induced by acute or chronic (14 days) administration of MK-801 (0.3 mg/kg, i.p.) or scopolamine (1 mg/kg, i.p.). The levels of L-arginine and its derivatives, L-citrulline, L-glutamate, L-glutamine and L-ornithine, were measured. The expression of constitutive nitric oxide synthases (cNOS), dimethylaminohydrolase (DDAH1) and protein arginine N-methyltransferases (PMRTs) 1 and 5 was evaluated, and the impact of the studied tool compounds on cGMP production and NMDA receptors was measured. The studies were performed in both the cortex and hippocampus of mice. S-nitrosylation of selected proteins, such as GLT-1, APP and tau, was also investigated. Our results indicate that the availability of L-arginine decreased after chronic administration of MK-801 or scopolamine, as both the amino acid itself as well as its level in proportion to its derivatives (SDMA and NMMA) were decreased. Additionally, among all three methylamines, SDMA was the most abundant in the brain (~70%). Administration of either compound impaired eNOS-derived NO production, increasing the monomer levels, and had no significant impact on nNOS. Both compounds elevated DDAH1 expression, and slight decreases in PMRT1 and PMRT5 in the cortex after scopolamine (acute) and MK-801 (chronic) administration were observed in the PFC, respectively. Administration of MK-801 induced a decrease in the cGMP level in the hippocampus, accompanied by decreased NMDA expression, while increased cGMP production and decreased NMDA receptor expression were observed after scopolamine administration. Chronic MK-801 and scopolamine administration affected S-nitrosylation of GLT-1 transport protein. Our results indicate that the analyzed tool compounds used in pharmacological models of schizophrenia or AD induce changes in NO-related pathways in the brain structures involved in cognition. To some extent, the changes resemble those observed in human samples.

A Fundamental Role for Oxidants and Intracellular Calcium Signals in Alzheimer's Pathogenesis-And How a Comprehensive Antioxidant Strategy May Aid Prevention of This Disorder

Oxidative stress and increased cytoplasmic calcium are key mediators of the detrimental effects on neuronal function and survival in Alzheimer's disease (AD). Pathways whereby these perturbations arise, and then prevent dendritic spine formation, promote tau hyperphosphorylation, further amplify amyloid β generation, and induce neuronal apoptosis, are described. A comprehensive program of nutraceutical supplementation, comprised of the NADPH oxidase inhibitor phycocyanobilin, phase two inducers, the mitochondrial antioxidant astaxanthin, and the glutathione precursor N-acetylcysteine, may have important potential for antagonizing the toxic effects of amyloid β on neurons and thereby aiding prevention of AD. Moreover, nutraceutical antioxidant strategies may oppose the adverse impact of amyloid β oligomers on astrocyte clearance of glutamate, and on the ability of brain capillaries to export amyloid β monomers/oligomers from the brain. Antioxidants, docosahexaenoic acid (DHA), and vitamin D, have potential for suppressing microglial production of interleukin-1β, which potentiates the neurotoxicity of amyloid β. Epidemiology suggests that a health-promoting lifestyle, incorporating a prudent diet, regular vigorous exercise, and other feasible measures, can cut the high risk for AD among the elderly by up to 60%. Conceivably, complementing such lifestyle measures with long-term adherence to the sort of nutraceutical regimen outlined here may drive down risk for AD even further.

Cerebrovascular damage after midlife transient hypertension in non-transgenic and Alzheimer's disease rats

Hypertension, including transient events, is a major risk factor for developing late-onset dementia and Alzheimer's disease (AD). Anti-hypertensive drugs facilitate restoration of normotension without amelioration of increased dementia risk suggesting that transient hypertensive insults cause irreversible damage. This study characterized the contribution of transient hypertension to sustained brain damage as a function of normal aging and AD. To model transient hypertension, we treated F344TgAD and non-transgenic littermate rats with L-NG-Nitroarginine methyl ester (L-NAME) for one month, ceased treatment and allowed for a month of normotensive recovery. We then examined the changes in the structure and function of the cerebrovasculature, integrity of white matter, and progression of AD pathology. As independent factors, both transient hypertension and AD compromised structural and functional integrity across the vascular bed, while combined effects of hypertension and AD yielded the largest deficits. Combined effects of transient hypertension and AD genotype resulted in loss of cortical myelin particularly in the cingulate cortex which is crucial for cognitive function. Increased cerebral amyloid angiopathy, a prominent pathology of AD, was detected after transient hypertension as were up- and down-regulation of proteins associated with cerebrovascular remodeling - osteopontin, ROCK1 and ROCK2, in F344TgAD rats even 30 days after restoration of normotension. In conclusion, transient hypertension caused permanent cerebrovasculature and brain parenchymal damage in both normal aging and AD. Our results corroborate human studies that have found close correlation between transient hypertension in midlife and white matter lesions later in life outlining vascular pathologies as pathological links to increased risk of dementia.

Tetrahydrobioterin (BH4) Pathway: From Metabolism to Neuropsychiatry

Tetrahydrobipterin (BH4) is a pivotal enzymatic cofactor required for the synthesis of serotonin, dopamine and nitric oxide. BH4 is essential for numerous physiological processes at periphery and central levels, such as vascularization, inflammation, glucose homeostasis, regulation of oxidative stress and neurotransmission. BH4 de novo synthesis involves the sequential activation of three enzymes, the major controlling point being GTP cyclohydrolase I (GCH1). Complementary salvage and recycling pathways ensure that BH4 levels are tightly kept within a physiological range in the body. Even if the way of transport of BH4 and its ability to enter the brain after peripheral administration is still controversial, data showed increased levels in the brain after BH4 treatment. Available evidence shows that GCH1 expression and BH4 synthesis are stimulated by immunological factors, notably pro-inflammatory cytokines. Once produced, BH4 can act as an anti- inflammatory molecule and scavenger of free radicals protecting against oxidative stress. At the same time, BH4 is prone to autoxidation, leading to the release of superoxide radicals contributing to inflammatory processes, and to the production of BH2, an inactive form of BH4, reducing its bioavailability. Alterations in BH4 levels have been documented in many pathological situations, including Alzheimer's disease, Parkinson's disease and depression, in which increased oxidative stress, inflammation and alterations in monoaminergic function are described. This review aims at providing an update of the knowledge about metabolism and the role of BH4 in brain function, from preclinical to clinical studies, addressing some therapeutic implications.

Tetrahydrobiopterin Improves Recognition Memory in the Triple-Transgenic Mouse Model of Alzheimer's Disease, Without Altering Amyloid-β and Tau Pathologies

BACKGROUND

Alzheimer's disease (AD) is a multifactorial disease, implying that multi-target treatments may be necessary to effectively cure AD. Tetrahydrobiopterin (BH4) is an enzymatic cofactor required for the synthesis of monoamines and nitric oxide that also exerts antioxidant and anti-inflammatory effects. Despite its crucial role in the CNS, the potential of BH4 as a treatment in AD has never been scrutinized.

OBJECTIVE

Here, we investigated whether BH4 peripheral administration improves cognitive symptoms and AD neuropathology in the triple-transgenic mouse model of AD (3xTg-AD), a model of age-related tau and amyloid-β (Aβ) neuropathologies associated with behavior impairment.

METHODS

Non-transgenic (NonTg) and 3xTg-AD mice were subjected to a control diet (5% fat - CD) or to a high-fat diet (35% fat - HFD) from 6 to 13 months to exacerbate metabolic disorders. Then, mice received either BH4 (15 mg/kg/day, i.p.) or vehicle for ten consecutive days.

RESULTS

This sub-chronic administration of BH4 rescued memory impairment in 13-month-old 3xTg-AD mice, as determined using the novel object recognition test. Moreover, the HFD-induced glucose intolerance was completely reversed by the BH4 treatment in 3xTg-AD mice. However, the HFD or BH4 treatment had no significant impact on Aβ and tau neuropathologies.

CONCLUSION

Overall, our data suggest a potential benefit from BH4 administration against AD cognitive and metabolic deficits accentuated by HFD consumption in 3xTg-AD mice, without altering classical neuropathology. Therefore, BH4 should be considered as a candidate for drug repurposing, at least in subtypes of cognitively impaired patients experiencing metabolic disorders.

Antifibrotic Effects of Amyloid-Beta and Its Loss in Cirrhotic Liver

The function and regulation of amyloid-beta (Aβ) in healthy and diseased liver remains unexplored. Because Aβ reduces the integrity of the blood-brain barrier we have examined its potential role in regulating the sinusoidal permeability of normal and cirrhotic liver. Aβ and key proteins that generate (beta-secretase 1 and presenilin-1) and degrade it (neprilysin and myelin basic protein) were decreased in human cirrhotic liver. In culture, activated hepatic stellate cells (HSC) internalized Aβ more efficiently than astrocytes and HSC degraded Aβ leading to suppressed expression of α-smooth muscle actin (α-SMA), collagen 1 and transforming growth factor β (TGFβ). Aβ also upregulated sinusoidal permeability marker endothelial NO synthase (eNOS) and decreased TGFβ in cultured human liver sinusoidal endothelial cells (hLSEC). Liver Aβ levels also correlate with the expression of eNOS in transgenic Alzheimer’s disease mice and in human and rodent cirrhosis/fibrosis. These findings suggest a previously unexplored role of Aβ in the maintenance of liver sinusoidal permeability and in protection against cirrhosis/fibrosis via attenuation of HSC activation.

Differential Sensitivity of Two Endothelial Cell Lines to Hydrogen Peroxide Toxicity: Relevance for In Vitro Studies of the Blood-Brain Barrier

Oxidative stress (OS) has been linked to blood–brain barrier (BBB) dysfunction which in turn has been implicated in the initiation and propagation of some neurological diseases. In this study, we profiled, for the first time, two endothelioma cell lines of mouse brain origin, commonly used as in vitro models of the blood–brain barrier, for their resistance against oxidative stress using viability measures and glutathione contents as markers. OS was induced by exposing cultured cells to varying concentrations of hydrogen peroxide and fluorescence microscopy/spectrometry was used to detect and estimate cellular glutathione contents. A colorimetric viability assay was used to determine changes in the viability of OS-exposed cells. Both the b.End5 and bEnd.3 cell lines investigated showed demonstrable content of glutathione with a statistically insignificant difference in glutathione quantity per unit cell, but with a statistically significant higher capacity for the b.End5 cell line for de novo glutathione synthesis. Furthermore, the b.End5 cells demonstrated greater oxidant buffering capacity to higher concentrations of hydrogen peroxide than the bEnd.3 cells. We concluded that mouse brain endothelial cells, derived from different types of cell lines, differ enormously in their antioxidant characteristics. We hereby recommend caution in making comparisons across BBB models utilizing distinctly different cell lines and require further prerequisites to ensure that in vitro BBB models involving these cell lines are reliable and reproducible.

Dysregulation of stress systems and nitric oxide signaling underlies neuronal dysfunction in Alzheimer's disease

Stress is a multimodal response involving the coordination of numerous body systems in order to maximize the chance of survival. However, long term activation of the stress response results in neuronal oxidative stress via reactive oxygen and nitrogen species generation, contributing to the development of depression. Stress-induced depression shares a high comorbidity with other neurological conditions including Alzheimer's disease (AD) and dementia, often appearing as one of the earliest observable symptoms in these diseases. Furthermore, stress and/or depression appear to exacerbate cognitive impairment in the context of AD associated with dysfunctional catecholaminergic signaling. Given there are a number of homologous pathways involved in the pathophysiology of depression and AD, this article will highlight the mechanisms by which stress-induced perturbations in oxidative stress, and particularly NO signaling, contribute to neurodegeneration.

Endothelium-specific amyloid precursor protein deficiency causes endothelial dysfunction in cerebral arteries

The exact physiological function of amyloid-β precursor protein (APP) in endothelial cells is unknown. Endothelium-specific APP-deficient (eAPP^-/-) mice were created to gain new insights into the role of APP in the control of vascular endothelial function. Endothelium-dependent relaxations to acetylcholine were significantly impaired in basilar arteries of global APP knockout (APP^-/-) and eAPP^-/- mice ( P < 0.05). In contrast, endothelium-independent relaxations to nitric oxide (NO)-donor diethylamine-NONOate were unchanged. Western blot analysis revealed that protein expression of endothelial nitric oxide synthase (eNOS) was significantly downregulated in large cerebral arteries of APP^-/- mice and eAPP^-/- mice as compared to respective wild-type littermates ( P < 0.05). Furthermore, basal levels of cyclic guanosine monophosphate (cGMP) were also significantly reduced in large cerebral arteries of APP-deficient mice ( P < 0.05). In contrast, protein expression of prostacyclin synthase as well as levels of cyclic adenosine monophosphate (cAMP) was not affected by genetic inactivation of APP in endothelial cells. By using siRNA to knockdown APP in cultured human brain microvascular endothelial cells we also found a significant downregulation of eNOS mRNA and protein expressions in APP-deficient endothelium ( P < 0.05). These findings indicate that under physiological conditions, expression of APP in cerebral vascular endothelium plays an important protective function by maintaining constitutive expression of eNOS .

Computational analysis of interactions of oxidative stress and tetrahydrobiopterin reveals instability in eNOS coupling

In cardiovascular and neurovascular diseases, an increase in oxidative stress and endothelial dysfunction has been reported. There is a reduction in tetrahydrobiopterin (BH4), which is a cofactor for the endothelial nitric oxide synthase (eNOS), resulting in eNOS uncoupling. Studies of the enhancement of BH4 availability have reported mixed results for improvement in endothelial dysfunction. Our understanding of the complex interactions of eNOS uncoupling, oxidative stress and BH4 availability is not complete and a quantitative understanding of these interactions is required. In the present study, we developed a computational model for eNOS uncoupling that considers the temporal changes in biopterin ratio in the oxidative stress conditions. Using the model, we studied the effects of cellular oxidative stress (Qsupcell) representing the non-eNOS based oxidative stress sources and BH4 synthesis (QBH4) on eNOS NO production and biopterin ratio (BH4/total biopterins (TBP)). Model results showed that oxidative stress levels from 0.01 to 1nM·s-1 did not affect eNOS NO production and eNOS remained in coupled state. When the Qsupcell increased above 1nM·s-1, the eNOS coupling and NO production transitioned to an oscillatory state. Oxidative stress levels dynamically changed the biopterin ratio. When Qsupcell increased from 1 to 100nM·s-1, the endothelial cell NO production, TBP levels and biopterin ratio reduced significantly from 26.5 to 2nM·s-1, 3.75 to 0.002μM and 0.99 to 0.25, respectively. For an increase in BH4 synthesis, the improvement in NO production rate and BH4 levels were dependent on the extent of cellular oxidative stress. However, a 10-fold increase in QBH4 at higher oxidative stresses did not restore the NO-production rate and the biopterin ratio. Our mechanistic analysis reveals that a combination of enhancing tetrahydrobiopterin level with a reduction in cellular oxidative stress may result in significant improvement in endothelial dysfunction.

Neuroprotective potential of high-dose biotin

A recent controlled trial has established that high-dose biotin supplementation - 100 mg, three times daily - has a stabilizing effect on progression of multiple sclerosis (MS). Although this effect has been attributed to an optimization of biotin's essential cofactor role in the brain, a case can be made that direct stimulation of soluble guanylate cyclase (sGC) by pharmacological concentrations of biotin plays a key role in this regard. The utility of high-dose biotin in MS might reflect an anti-inflammatory effect of cGMP on the cerebral microvasculature, as well on oligodendrocyte differentiation and on Schwann cell production of neurotrophic factors thought to have potential for managing MS. But biotin's ability to boost cGMP synthesis in the brain may have broader neuroprotective potential. In many types of neurons and neural cells, cGMP exerts neurotrophic-mimetic effects - entailing activation of the PI3K-Akt and Ras-ERK pathways - that promote neuron survival and plasticity. Hippocampal long term potentiation requires nitric oxide synthesis, which in turn promotes an activating phosphorylation of CREB via a pathway involving cGMP and protein kinase G (PKG). In Alzheimer's disease (AD), amyloid beta suppresses this mechanism by inhibiting sGC activity; agents which exert a countervailing effect by boosting cGMP levels tend to restore effective long-term potentiation in rodent models of AD. Moreover, NO/cGMP suppresses amyloid beta production within the brain by inhibiting expression of amyloid precursor protein and BACE1. In conjunction with cGMP's ability to oppose neuron apoptosis, these effects suggest that high-dose biotin might have potential for the prevention and management of AD. cGMP also promotes neurogenesis, and may lessen stroke risk by impeding atherogenesis and hypertrophic remodeling in the cerebral vasculature. The neuroprotective potential of high-dose biotin likely could be boosted by concurrent administration of brain-permeable phosphodiesterase-5 inhibitors.

Expression and Processing of Amyloid Precursor Protein in Vascular Endothelium

Amyloid precursor protein (APP) is evolutionary conserved protein expressed in endothelial cells of cerebral and peripheral arteries. In this review, we discuss mechanisms responsible for expression and proteolytic cleavage of APP in endothelial cells. We focus on physiological and pathological implications of APP expression in vascular endothelium.

The Positive Regulation of eNOS Signaling by PPAR Agonists in Cardiovascular Diseases

Increasing evidence shows that activation of peroxisome proliferator-activated receptors (PPARs) plays an essential role in the regulation of vascular endothelial function through a range of mechanisms, including non-metabolic. Among these, the PPAR-mediated activation of endothelial nitric oxide synthase (eNOS) appears to be of considerable importance. The regulated and sustained bioavailability of nitric oxide (NO) in the endothelium is essential to avoid the development of cardiovascular diseases such as hypertension or atherosclerosis. Therefore, a deeper understanding of the different effects of specific PPAR ligands on NO bioavailability could be useful in the development of novel or multi-targeted PPAR agonists. In this review, we report the most meaningful and up-to-date in vitro and in vivo studies of the regulation of NO production performed by different PPAR agonists. Insights into the molecular mechanisms of PPAR-mediated eNOS activation are also provided. Although findings from animal studies in which the activation of PPARα, PPARβ/δ, or PPARγ have provided clear vasoprotective effects have been promising, several benefits from PPAR agonists are offset by unwanted outcomes. Therefore, new insights could be useful in the development of tissue-targeted PPAR agonists with more tolerable side effects to improve treatment options for cardiovascular diseases.

Asymmetric Dimethylarginine Is a Well Established Mediating Risk Factor for Cardiovascular Morbidity and Mortality-Should Patients with Elevated Levels Be Supplemented with Citrulline?

The arginine metabolite asymmetric dimethylarginine (ADMA) is a competitive inhibitor and uncoupler of endothelial nitric oxide synthase (eNOS), an enzyme that acts in multifarious ways to promote cardiovascular health. This phenomenon likely explains, at least in part, why elevated ADMA has been established as an independent risk factor for cardiovascular events, ventricular hypertrophy, and cardiovascular mortality. Fortunately, the suppressive impact of ADMA on eNOS activity can be offset by increasing intracellular arginine levels with supplemental citrulline. Although the long-term impact of supplemental citrulline on cardiovascular health in patients with elevated ADMA has not yet been studied, shorter-term clinical studies of citrulline administration demonstrate effects suggestive of increased NO synthesis, such as reductions in blood pressure and arterial stiffness, improved endothelium-dependent vasodilation, increased erection hardness, and increased ejection fractions in patients with heart failure. Supplemental citrulline could be a practical option for primary or secondary prevention of cardiovascular events and mortality, as it is inexpensive, has a mild flavor, and is well tolerated in doses (3-6 g daily) that can influence eNOS activity. Large and long-term clinical trials, targeting patients at high risk for cardiovascular events in whom ADMA is elevated, are needed to evaluate citrulline's potential for aiding cardiovascular health.

A multifunctional therapeutic approach to disease modification in multiple familial mouse models and a novel sporadic model of Alzheimer's disease

Background

Clinical failures singularly targeting amyloid-β pathology indicate a critical need for alternative Alzheimer’s disease (AD) therapeutic strategies. The mixed pathology reported in a large population of AD patients demands a multifunctional drug approach. Since activation of cAMP response element binding protein (CREB) plays a crucial role in synaptic strengthening and memory formation, we retooled a clinical drug with known neuroprotective and anti-inflammatory activity to activate CREB, and validated this novel multifunctional drug, NMZ, in 4 different mouse models of AD.

Results

NMZ was tested in three mouse models of familial AD and one model of sporadic AD. In 3 × Tg hippocampal slices, NMZ restored LTP. In vivo, memory was improved with NMZ in all animal models with robust cognitive deficits. NMZ treatment lowered neurotoxic forms of Aβ in both APP/PS1 and 3 × Tg transgenic mice while also restoring neuronal plasticity biomarkers in the 3 × Tg mice. In EFAD mice, incorporation of the major genetic AD risk factor, hAPOE4, did not mute the beneficial drug effects. In a novel sporadic mouse model that manifests AD-like pathology caused by accelerated oxidative stress in the absence of any familial AD mutation, oral administration of NMZ attenuated hallmark AD pathology and restored biomarkers of synaptic and neuronal function.

Conclusions

The multifunctional approach, embodied by NMZ, was successful in mouse models of AD incorporating Aβ pathology (APP/PS1), tau pathology (3xTg), and APOE4, the major human genetic risk factor for AD (EFAD). The efficacy observed in a novel model of sporadic AD (Aldh2^−/−) demonstrates that the therapeutic approach is not limited to rare, familial AD genetic mutations. The multifunctional drug, NMZ, was not designed directly to target Aβ and tau pathology; however, the attenuation of this hallmark pathology suggests the approach to be a highly promising, disease-modifying strategy for AD and mixed pathology dementia.

Getting to NO Alzheimer's Disease: Neuroprotection versus Neurotoxicity Mediated by Nitric Oxide

Alzheimer's disease (AD) is a neurodegenerative disorder involving the loss of neurons in the brain which leads to progressive memory loss and behavioral changes. To date, there are only limited medications for AD and no known cure. Nitric oxide (NO) has long been considered part of the neurotoxic insult caused by neuroinflammation in the Alzheimer's brain. However, focusing on early developments, prior to the appearance of cognitive symptoms, is changing that perception. This has highlighted a compensatory, neuroprotective role for NO that protects synapses by increasing neuronal excitability. A potential mechanism for augmentation of excitability by NO is via modulation of voltage-gated potassium channel activity (Kv7 and Kv2). Identification of the ionic mechanisms and signaling pathways that mediate this protection is an important next step for the field. Harnessing the protective role of NO and related signaling pathways could provide a therapeutic avenue that prevents synapse loss early in disease.

Characterization of cerebral microvasculature in transgenic mice with endothelium targeted over-expression of GTP-cyclohydrolase I

Tetrahydrobiopterin (BH4) is a critical determinant of nitric oxide (NO) production by nitric oxide synthase (NOS) in the vascular endothelium and its biosynthesis is regulated by the enzymatic activity of GTP-cyclohydrolase I (GTPCH I). The present study was designed to determine the effects of endothelium-targeted overexpression of GTPCH I (eGCH-Tg) on murine cerebral vascular function. Endothelium targeted over-expression of GTPCH I was associated with a significant increase in levels of BH4, as well as its oxidized product, 7,8-dihydrobiopterin (7,8-BH2) in cerebral microvessels. Importantly, ratio of BH4 to 7,8-BH2, indicative of BH4 available for eNOS activation, was significantly increased in eGCH-Tg mice. However, expression of endothelial NOS, levels of nitrate/nitrite--indicative of NO production--remained unchanged between cerebral microvessels of wild-type and eGCH-Tg mice. Furthermore, increased BH4 biosynthesis neither affected production of superoxide anion nor expression of antioxidant proteins. Moreover, endothelium-specific GTPCH I overexpression did not alter intracellular levels of cGMP, reflective of NO signaling in cerebral microvessels. The obtained results suggest that, despite a significant increase in BH4 bioavailability, generation of endothelial NO in cerebral microvessels remained unchanged in eGCH-Tg mice. We conclude that under physiological conditions the levels of BH4 in the cerebral microvessels are optimal for activation of endothelial NOS and NO/cGMP signaling.