A beta-peptides enhance vasoconstriction in cerebral circulation

The changing landscape of voltage-gated calcium channels in neurovascular disorders and in neurodegenerative diseases

It is a common belief that voltage-gated calcium channels (VGCC) cannot carry toxic amounts of Ca²⁺ in neurons. Also, some of them as L-type channels are essential for Ca²⁺-dependent regulation of prosurvival gene-programs. However, a wealth of data show a beneficial effect of drugs acting on VGCCs in several neurodegenerative and neurovascular diseases. In the present review, we explore several mechanisms by which the “harmless” VGCCs may become “toxic” for neurons. These mechanisms could explain how, though usually required for neuronal survival, VGCCs may take part in neurodegeneration. We will present evidence showing that VGCCs can carry toxic Ca²⁺ when: a) their density or activity increases because of aging, chronic hypoxia or exposure to β-amyloid peptides or b) Ca²⁺-dependent action potentials carry high Ca²⁺ loads in pacemaker neurons. Besides, we will examine conditions in which VGCCs promote neuronal cell death without carrying excess Ca²⁺. This can happen, for instance, when they carry metal ions into the neuronal cytoplasm or when a pathological decrease in their activity weakens Ca²⁺-dependent prosurvival gene programs. Finally, we will explore the role of VGCCs in the control of nonneuronal cells that take part to neurodegeneration like those of the neurovascular unit or of microglia.

Cerebrovascular reactivity to carbon dioxide in Alzheimer's disease

There is growing evidence that cerebrovascular reactivity to carbon dioxide (CVRCO2) is impaired in Alzheimer's disease (AD). Preclinical and animal studies suggest chronic hypercontractility in brain vessels in AD. We review (a) preclinical studies of mechanisms for impaired CVRCO2 in AD; (b) clinical studies of cerebrovascular function in subjects with AD dementia, mild cognitive impairment (MCI), and normal cognition. Although results of clinical studies are inconclusive, an increasing number of reports reveal an impairment of vascular reactivity to carbon dioxide in subjects with AD, and possibly also in MCI. Thus, CVRCO2 may be an attractive means to detect an early vascular dysfunction in subjects at risk.

Magnetic resonance angiography and vascular corrosion casting as tools in biomedical research: application to transgenic mice modeling Alzheimer's disease

In vivo imaging technologies are presently receiving considerable attention in the biomedical and pharmaceutical research areas. One of the principal imaging modalities is magnetic resonance imaging (MRI). The multiparametric nature of MRI enables anatomical, functional and even molecular information to be obtained non-invasively from intact organisms at high spatial resolution. Here we describe the use of one MRI modality, namely angiography (MRA), to non-invasively study the arterial vascular architecture of APP23 transgenic mice modeling Alzheimer's disease. Because the spatial resolution of the technique is limited, the in vivo studies are complemented by a powerful analysis of the vasculature using vascular corrosion casting. Both techniques revealed age-dependent blood flow alterations and cerebrovascular abnormalities in these mice. Our experience suggests that MRA complemented by cast analysis are important tools to describe vascular alterations and test new therapy concepts in animal models of AD. Furthermore, being non-invasive, MRA can also be applied to studies in patients suffering from this disease.

Aging enhances vascular dysfunction induced by the Alzheimer's peptide β-amyloid

Aging is a major risk factor for Alzheimer's disease and the evidence suggests a role for cerebrovascular pathology in cognitive dysfunction. The hypothesis in this study is that aging is a significant risk factor in the effect of the Alzheimer peptide beta-amyloid on endothelium-dependent function of cerebral and peripheral vessels. The diameter response to acetylcholine, an endothelium-dependent vasodilator, was recorded in pressurized segments of rat posterior cerebral vessels from mature (3 months) and aged (20 months) rats. The threshold concentration of beta-amyloid for a significant decrease in the response to acetylcholine was lower in vessels from aged rats (10(-9) M) than in vessels from mature rats (10(-8) M). The threshold concentration of beta-amyloid for a significant decrease in the sensitivity to acetylcholine was lower for ring segments of aorta from aged rats (10(-10) M) than for aorta from mature rats (10(-8) M). Structural changes of the endothelium were first observed in electron micrographs of aorta from aged rats when the concentration of beta-amyloid was 10(-8) M, whereas structural changes in aorta from mature rats did not occur until the concentration of beta-amyloid was increased to 10(-7) M. The results suggest that aging increases the susceptibility of cerebral and peripheral blood vessels to beta-amyloid related dysfunction and that functional change precedes structural change.

Pioglitazone improves reversal learning and exerts mixed cerebrovascular effects in a mouse model of Alzheimer's disease with combined amyloid-β and cerebrovascular pathology

Animal models of Alzheimer's disease (AD) are invaluable in dissecting the pathogenic mechanisms and assessing the efficacy of potential new therapies. Here, we used the peroxisome proliferator-activated receptor gamma agonist pioglitazone in an attempt to rescue the pathogenic phenotype in adult (12 months) and aged (>18 months) bitransgenic A/T mice that overexpress a mutated human amyloid precursor protein (APPSwe,Ind) and a constitutively active form of transforming growth factor-β1 (TGF-β1). A/T mice recapitulate the AD-related cognitive deficits, amyloid beta (Aβ) and cerebrovascular pathologies, as well as the altered metabolic and vascular coupling responses to increased neuronal activity. Pioglitazone normalized neurometabolic and neurovascular coupling responses to sensory stimulation, and reduced cortical astroglial and hippocampal microglial activation in both age groups. Spatial learning and memory deficits in the Morris water maze were not rescued by pioglitazone, but reversal learning was improved in the adult cohort notwithstanding a progressing Aβ pathology. While pioglitazone preserved the constitutive nitric oxide synthesis in the vessel wall, it unexpectedly failed to restore cerebrovascular reactivity in A/T mice and even exacerbated the dilatory deficits. These data demonstrate pioglitazone's efficacy on selective AD hallmarks in a complex AD mouse model of comorbid amyloidosis and cerebrovascular pathology. They further suggest a potential benefit of pioglitazone in managing neuroinflammation, cerebral perfusion and glucose metabolism in AD patients devoid of cerebrovascular pathology.

Assessing white matter ischemic damage in dementia patients by measurement of myelin proteins

White matter ischemia is difficult to quantify histologically. Myelin-associated glycoprotein (MAG) is highly susceptible to ischemia, being expressed only adaxonally, far from the oligodendrocyte cell body. Myelin-basic protein (MBP) and proteolipid protein (PLP) are expressed throughout the myelin sheath. We compared MAG, MBP, and PLP levels in parietal white matter homogenates from 17 vascular dementia (VaD), 49 Alzheimer's disease (AD), and 33 control brains, after assessing the post-mortem stability of these proteins. Small vessel disease (SVD) and cerebral amyloid angiopathy (CAA) severity had been assessed in paraffin sections. The concentration of MAG remained stable post-mortem, declined with increasing SVD, and was significantly lower in VaD than controls. The concentration of MBP fell progressively post-mortem, limiting its diagnostic utility in this context. Proteolipid protein was stable post-mortem and increased significantly with SVD severity. The MAG/PLP ratio declined significantly with SVD and CAA severity. The MAG and PLP levels and MAG/PLP did not differ significantly between AD and control brains. We validated the utility of MAG and MAG/PLP measurements on analysis of 74 frontal white matter samples from an Oxford cohort in which SVD had previously been scored. MAG concentration and the MAG/PLP ratio are useful post-mortem measures of ante-mortem white matter ischemia.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

The capillary dysfunction hypothesis of Alzheimer's disease

It is widely accepted that hypoperfusion and changes in capillary morphology are involved in the etiopathogenesis of Alzheimer's disease (AD). This is difficult to reconcile with the hyperperfusion observed in young high-risk subjects. Differences in the way cerebral blood flow (CBF) is coupled with the local metabolic needs during different phases of the disease can explain this apparent paradox. This review describes this coupling in terms of a model of cerebral oxygen availability that takes into consideration the heterogeneity of capillary blood flow patterns. The model predicts that moderate increases in heterogeneity requires elevated CBF in order to maintain adequate oxygenation. However, with progressive increases in heterogeneity, the resulting low tissue oxygen tension will require a suppression of CBF in order to maintain tissue metabolism. The observed biphasic nature of CBF responses in preclinical AD and AD is therefore consistent with progressive disturbances of capillary flow patterns. Salient features of the model are discussed in the context of AD pathology along with potential sources of increased capillary flow heterogeneity.

Cerebral amyloid angiopathy impact on endothelium

Cerebral amyloid angiopathy (CAA) is an age-associated disease characterized by amyloid deposition in cerebral and meningeal vessel walls. CAA is detected in the majority of the individuals with dementia and also in a large number of non-demented elderly individuals. In addition, CAA is strongly associated with Alzheimer's disease (AD) pathology. Mechanical consequences including intra-cerebral or subarachnoid hemorrhage remains CAA most feared complication, but only a small fraction of CAA results in severe bleeding. On the hand the non-mechanical consequences in cerebrovascular regulation are prevalent and may be even more deleterious. Studies of animal models have provided strong evidence linking the vasoactive Aβ 1-40, the main species found in CAA, to disturbances in endothelial-dependent factors, disrupting cerebrovascular regulation Here, we aimed to review experimental findings regarding the non-mechanical consequences of CAA for cerebrovascular regulation and discuss the implications of these results to clinical practice.

Role of endothelial nitric oxide in cerebrovascular regulation

Endothelial nitric oxide (NO) plays important roles in the vascular system. Animal models that show vascular dysfunction demonstrate the protective role of endothelial NO dependent pathways. This review focuses on the role of endothelial NO in the regulation of cerebral blood flow and vascular tone. We will discuss the importance of NO in cerebrovascular function using animal models with altered endothelial NO production under normal, ischemic and reperfusion conditions, as well as in hyperoxia. Pharmacological and genetic manipulations of the endothelial NO system demonstrate the essential roles of endothelial NO synthase in maintenance of vascular tone and cerebral perfusion under normal and pathological conditions.

Neuronal life span versus health span: principles of natural selection at work in the degenerating brain

Impaired nutrient delivery to the brain due to decreased blood flow contributes to cognitive decline and dementia in Alzheimer's disease (AD). Considering this, many studies have suggested that neuroprotective agents like those used in stroke could prevent AD onset or progression by promoting cell survival. However, research in the past decade suggests that the culprit behind the cognitive loss in AD models is actually the soluble tau accumulating inside of surviving neurons. In fact, tau reductions improve cognition in mouse models of AD, even those that only deposit amyloid plaques. There is emerging evidence that neuroprotection alone in these AD models may be insufficient to restore neuron function and cognition. Only when soluble tau is reduced on a neuroprotective background could memory be rescued. Thus, once a neuron begins to accumulate tau, it may survive in a malfunctioning capacity, leading to impaired electrical signaling and memory formation in the brain. These data imply that multiple drugs may be necessary to ameliorate the different disease components. In fact, strategies to preserve neurons without affecting the soluble protein burden within neurons may accelerate the disease course.

Role of astrocytes in neurovascular coupling

Neural activity is intimately tied to blood flow in the brain. This coupling is specific enough in space and time that modern imaging methods use local hemodynamics as a measure of brain activity. In this review, we discuss recent evidence indicating that neuronal activity is coupled to local blood flow changes through an intermediary, the astrocyte. We highlight unresolved issues regarding the role of astrocytes and propose ways to address them using novel techniques. Our focus is on cellular level analysis in vivo, but we also relate mechanistic insights gained from ex vivo experiments to native tissue. We also review some strategies to harness advances in optical and genetic methods to study neurovascular coupling in the intact brain.

Genetic associations of autopsy-confirmed vascular dementia subtypes

BACKGROUND/AIMS

Genetic risk factors have not been clearly established for vascular dementias (VaD) related to stroke and cerebrovascular disease.

METHODS

Samples were genotyped for APOE, MTHFR and ICAM. Aβ levels and choline acetyltransferase (ChAT) activities were assayed in controls and individuals with VaD.

RESULTS

Associations were found between the APOE-ε4 allele and mixed dementia, infarct/stroke dementia and subcortical ischemic vascular dementia (SIVD), and higher Aβ1-42 levels and decreased ChAT activity. MTHFR was more associated with SIVD, mixed dementia, and lower ChAT activity.

CONCLUSIONS

The study demonstrates important differences in the genetic associations of VaD and begins to clarify the genetic basis of key pathological substrates.

Estrogen prevents β-amyloid inhibition of sympathetic α7-nAChR-mediated nitrergic neurogenic dilation in porcine basilar arteries

AIM

β-amyloid peptides (Aβs) have been shown to block cerebral nitrergic neurogenic vasodilation by blocking sympathetic α7-nAChRs, and that oestrogen prevents Aβ-induced neurotoxicity. We examined whether Aβ-inhibition of α7-nAChR-mediated cerebral nitrergic vasodilation was prevented by oestrogen.

METHODS

Effects of Aβ and 17β-oestradiol on neurogenic nitrergic vasodilation in isolated porcine basilar arteries were examined using wire-myography. Drug effects on nicotine- and choline-induced calcium influx and inward currents in porcine cultured superior cervical ganglion (SCG) were investigated using confocal microscopy and patch-clamp techniques respectively.

RESULTS

Precontracted endothelium-denuded basilar arteries relaxed exclusively upon transmural nerve stimulation (TNS, 8 Hz), and applications of nicotine (100 μm) or choline (1 mm), which was sensitive to nitro-L-arginine (L-NNA, 30 μm) and tetrodotoxin (0.3 μm). The relaxation induced by nicotine and choline but not that by TNS was blocked reversibly by Aβ(1-40) in a concentration-dependent manner. Aβ(1-40) also reversibly blocked nicotine- and choline-induced increase of calcium influx and inward currents in the SCG neurons. Aβ inhibition of nicotine- and choline-induced α7-nAChR-mediated nitrergic vasodilation and inward currents was prevented by 17β-oestradiol (10 μm), but not by α-oestradiol (10 μm) or testosterone (10 μm).

CONCLUSION

These results provide further evidence supporting that Aβ is an antagonist for the α7-nAChR found on post-ganglionic sympathetic adrenergic nerve terminals originating in the SCG. Aβ can cause constriction of cerebral arteries with possible decreased regional cerebral blood flow by blocking sympathetic nerve-mediated release of nitric oxide from the perivascular nitrergic nerves. This effect of Aβ can be prevented by endogenous oestrogen but not testosterone.

Vascular contributions to cognitive impairment and dementia: a statement for healthcare professionals from the american heart association/american stroke association

BACKGROUND AND PURPOSE

This scientific statement provides an overview of the evidence on vascular contributions to cognitive impairment and dementia. Vascular contributions to cognitive impairment and dementia of later life are common. Definitions of vascular cognitive impairment (VCI), neuropathology, basic science and pathophysiological aspects, role of neuroimaging and vascular and other associated risk factors, and potential opportunities for prevention and treatment are reviewed. This statement serves as an overall guide for practitioners to gain a better understanding of VCI and dementia, prevention, and treatment.

METHODS

Writing group members were nominated by the writing group co-chairs on the basis of their previous work in relevant topic areas and were approved by the American Heart Association Stroke Council Scientific Statement Oversight Committee, the Council on Epidemiology and Prevention, and the Manuscript Oversight Committee. The writing group used systematic literature reviews (primarily covering publications from 1990 to May 1, 2010), previously published guidelines, personal files, and expert opinion to summarize existing evidence, indicate gaps in current knowledge, and, when appropriate, formulate recommendations using standard American Heart Association criteria. All members of the writing group had the opportunity to comment on the recommendations and approved the final version of this document. After peer review by the American Heart Association, as well as review by the Stroke Council leadership, Council on Epidemiology and Prevention Council, and Scientific Statements Oversight Committee, the statement was approved by the American Heart Association Science Advisory and Coordinating Committee.

RESULTS

The construct of VCI has been introduced to capture the entire spectrum of cognitive disorders associated with all forms of cerebral vascular brain injury-not solely stroke-ranging from mild cognitive impairment through fully developed dementia. Dysfunction of the neurovascular unit and mechanisms regulating cerebral blood flow are likely to be important components of the pathophysiological processes underlying VCI. Cerebral amyloid angiopathy is emerging as an important marker of risk for Alzheimer disease, microinfarction, microhemorrhage and macrohemorrhage of the brain, and VCI. The neuropathology of cognitive impairment in later life is often a mixture of Alzheimer disease and microvascular brain damage, which may overlap and synergize to heighten the risk of cognitive impairment. In this regard, magnetic resonance imaging and other neuroimaging techniques play an important role in the definition and detection of VCI and provide evidence that subcortical forms of VCI with white matter hyperintensities and small deep infarcts are common. In many cases, risk markers for VCI are the same as traditional risk factors for stroke. These risks may include but are not limited to atrial fibrillation, hypertension, diabetes mellitus, and hypercholesterolemia. Furthermore, these same vascular risk factors may be risk markers for Alzheimer disease. Carotid intimal-medial thickness and arterial stiffness are emerging as markers of arterial aging and may serve as risk markers for VCI. Currently, no specific treatments for VCI have been approved by the US Food and Drug Administration. However, detection and control of the traditional risk factors for stroke and cardiovascular disease may be effective in the prevention of VCI, even in older people.

CONCLUSIONS

Vascular contributions to cognitive impairment and dementia are important. Understanding of VCI has evolved substantially in recent years, based on preclinical, neuropathologic, neuroimaging, physiological, and epidemiological studies. Transdisciplinary, translational, and transactional approaches are recommended to further our understanding of this entity and to better characterize its neuropsychological profile. There is a need for prospective, quantitative, clinical-pathological-neuroimaging studies to improve knowledge of the pathological basis of neuroimaging change and the complex interplay between vascular and Alzheimer disease pathologies in the evolution of clinical VCI and Alzheimer disease. Long-term vascular risk marker interventional studies beginning as early as midlife may be required to prevent or postpone the onset of VCI and Alzheimer disease. Studies of intensive reduction of vascular risk factors in high-risk groups are another important avenue of research.

Differential effect of amyloid β on the cytochrome P450 epoxygenase activity in rat brain

One of the prominent features of Alzheimer's disease is the excessive accumulation of the protein amyloid beta (Aβ) in certain areas of the brain leading to neurodegeneration. Aβ is cytotoxic and disrupts several cytoprotective pathways. Recent literature has demonstrated that certain cytochrome P450 (CYP) products are neuroprotective, including epoxide metabolites of arachidonic acid (AA), epoxyeicosatrienoic acids (EETs). The action of Aβ with respect to regionally produced EETs in the brain has yet to be defined. Epoxygenases metabolize AA into four regioisomers of EETs (14,15-, 11,12-, 8,9- and 5,6-EET). EETs are rapidly degraded into dihydroxyeicosatrienoic acids (DiHETEs) by soluble epoxide hydrolase (sEH). To determine the effect of Aβ on the epoxygenase activity in different regions of the brain, microsomes were prepared from the cerebrum and cerebellum of adult Sprague-Dawley rats and incubated with 1 and 10 μM Aβ for 30 min after which epoxygenase activity assay was performed. Mass spectrometry indicated that incubation with Aβ reduced 14,15-EET production by 30% as compared to vehicle in the cerebrum, but not in the cerebellum. When we separated the cerebrum into cortex and hippocampus, significant decrease in the production of total EETs and DiHETEs were seen in presence of Aβ (81% and 74%) in the cortex. Moreover, 11,12-EET production was decreased to ∼70% of vehicle in both cortex and hippocampus. Epoxygenase activity in the cultured astrocytes and neurons also showed reduction in total EET and DiHETE production (to 80% and ∼70% of vehicle respectively) in presence of Aβ. Altogether, our data suggest that Aβ reduces epoxygenase activity differentially in a region-specific and cell-specific manner. The reduction of cytoprotective EETs by Aβ in the cerebrum may make it more prone to degeneration than the cerebellum. Further understanding of these interactions will improve our ability to protect against the pathology of Alzheimer's disease.

Hypertension is a potential risk factor for vascular dementia: systematic review

OBJECTIVE

The aim of the study was to conduct a meta-analysis of epidemiological and case control studies to determine whether arterial hypertension is specifically associated with an increased risk of vascular dementia (VaD).

DESIGN

Longitudinal and cross-sectional prospective studies using operationalised criteria to define VaD and hypertension, with a normal control comparison group were systematically reviewed. Cochrane Library, Embase, Medline, and PsycInfo data sources were searched along with reference lists of included articles and reviews. Original, prevalence or incidence studies were included if operationalised criteria for hypertension and VaD as well as number of cases with and without hypertension in VaD and non-demented groups were provided. Intervention studies and post-stroke and CADASIL studies were excluded.

RESULTS

Eleven studies recruiting either volunteers or clinical patients, or which were population-based, examined a total of 768 people with VaD and 9857 control cases. A meta-analysis of the six longitudinal studies showed that hypertension was significantly associated with increased risk of incident VaD (odds ratio, OR: 1.59, CI: 1.29-1.95, p < 0.0001). A similar association between hypertension and the risk of prevalent VaD was found in the five cross-sectional studies (OR: 4.84, CI: 3.52-6.67, p < 0.00001).

CONCLUSIONS

Hypertension significantly increases the risk of vascular dementia. The current meta-analysis highlights the potential importance of rigorous treatment of hypertension as a key measure to help prevent the development of VaD.

Functional magnetic resonance imaging as a dynamic candidate biomarker for Alzheimer's disease

During the last two decades, imaging of neural activation has become an invaluable tool for assessing the functional organization of the human brain in vivo. Due to its widespread application in neuroscience, functional neuroimaging has raised the interest of clinical researchers in its possible use as a diagnostic biomarker. A hallmark feature of many neurodegenerative diseases is their chronic non-linear dynamic and highly complex preclinical course. Neurodegenerative diseases unfold over years to decades through clinically silent and asymptomatic stages of early adaptive, compensatory to pathophysiological (i.e. actively neurodegenerative) and decompensatory mechanisms in the brain - phases that are increasingly being considered as critical for primary and secondary preventive and therapeutic measures. Emerging evidence supports the concept of a potentially fully reversible functional phase that may precede the onset of micro- and macrostructural and cognitive decline, a potentially late-stage "neurodegenerative" phase of a primary neurodegenerative disorder. Alzheimer's disease serves as an ideal model to test this hypothesis supported by the neural network model of the healthy and diseased brain. Being highly dynamic in nature, brain activation and neuronal network functional connectivity represent not only candidate diagnostic but also candidate surrogate markers for interventional trials. Potential caveats of functional imaging are critically reviewed with focus on confound variables such as altered neurovascular coupling as well as parameters related to task- and study design.

Nutrition and Alzheimer's disease: the detrimental role of a high carbohydrate diet

Alzheimer's disease is a devastating disease whose recent increase in incidence rates has broad implications for rising health care costs. Huge amounts of research money are currently being invested in seeking the underlying cause, with corresponding progress in understanding the disease progression. In this paper, we highlight how an excess of dietary carbohydrates, particularly fructose, alongside a relative deficiency in dietary fats and cholesterol, may lead to the development of Alzheimer's disease. A first step in the pathophysiology of the disease is represented by advanced glycation end-products in crucial plasma proteins concerned with fat, cholesterol, and oxygen transport. This leads to cholesterol deficiency in neurons, which significantly impairs their ability to function. Over time, a cascade response leads to impaired glutamate signaling, increased oxidative damage, mitochondrial and lysosomal dysfunction, increased risk to microbial infection, and, ultimately, apoptosis. Other neurodegenerative diseases share many properties with Alzheimer's disease, and may also be due in large part to this same underlying cause.

Nitric oxide signaling in brain function, dysfunction, and dementia

Nitric oxide (NO) is an important signaling molecule that is widely used in the nervous system. With recognition of its roles in synaptic plasticity (long-term potentiation, LTP; long-term depression, LTD) and elucidation of calcium-dependent, NMDAR-mediated activation of neuronal nitric oxide synthase (nNOS), numerous molecular and pharmacological tools have been used to explore the physiology and pathological consequences for nitrergic signaling. In this review, the authors summarize the current understanding of this subtle signaling pathway, discuss the evidence for nitrergic modulation of ion channels and homeostatic modulation of intrinsic excitability, and speculate about the pathological consequences of spillover between different nitrergic compartments in contributing to aberrant signaling in neurodegenerative disorders. Accumulating evidence points to various ion channels and particularly voltage-gated potassium channels as signaling targets, whereby NO mediates activity-dependent control of intrinsic neuronal excitability; such changes could underlie broader mechanisms of synaptic plasticity across neuronal networks. In addition, the inability to constrain NO diffusion suggests that spillover from endothelium (eNOS) and/or immune compartments (iNOS) into the nervous system provides potential pathological sources of NO and where control failure in these other systems could have broader neurological implications. Abnormal NO signaling could therefore contribute to a variety of neurodegenerative pathologies such as stroke/excitotoxicity, Alzheimer's disease, multiple sclerosis, and Parkinson's disease.

Endothelin receptor antagonists: potential in Alzheimer's disease

Alzheimer's disease (AD) is believed to be initiated by the accumulation of neurotoxic forms of Aβ peptide within the brain. AD patients show reduction of cerebral blood flow (CBF), the extent of the reduction correlating with the impairment of cognition. There is evidence that cerebral hypoperfusion precedes and may even trigger the onset of dementia in AD. Cerebral hypoperfusion impairs neuronal function, reduces the clearance of Aβ peptide and other toxic metabolites from the brain, and upregulates Aβ production. Studies in animal models of AD have shown the reduction in CBF to be more than would be expected for the reduction in neuronal metabolic activity. Aβ may contribute to the reduction in CBF in AD, as both Aβ₁₋₄₀ and Aβ₁₋₄₂ induce cerebrovascular dysfunction. Aβ₁₋₄₀ acts directly on cerebral arteries to cause cerebral smooth muscle cell contraction. Aβ₁₋₄₂ causes increased neuronal production and release of endothelin-1 (ET-1), a potent vasoconstrictor, and upregulation of endothelin-converting enzyme-2 (ECE-2), the enzyme which cleaves ET-1 from its inactive precursor. ET-1 and ECE-2 are also elevated in AD, making it likely that upregulation of the ECE-2-ET-1 axis by Aβ₁₋₄₂ contributes to the chronic reduction of CBF in AD. At present, only a few symptomatic treatment options exist for AD. The involvement of ET-1 in the pathogenesis of endothelial dysfunction associated with elevated Aβ indicates the potential for endothelin receptor antagonists in the treatment of AD. It has already been demonstrated that the endothelin receptor antagonist bosentan, preserves aortic and carotid endothelial function in Tg2576 mice, and our findings suggest that endothelin receptor antagonists may be beneficial in maintaining CBF in AD.

Amyloid-β inhibits No-cGMP signaling in a CD36- and CD47-dependent manner

Amyloid-β interacts with two cell surface receptors, CD36 and CD47, through which the matricellular protein thrombospondin-1 inhibits soluble guanylate cyclase activation. Here we examine whether amyloid-β shares this inhibitory activity. Amyloid-β inhibited both drug and nitric oxide-mediated activation of soluble guanylate cyclase in several cell types. Known cGMP-dependent functional responses to nitric oxide in platelets and vascular smooth muscle cells were correspondingly inhibited by amyloid-β. Functional interaction of amyloid-β with the scavenger receptor CD36 was indicated by inhibition of free fatty acid uptake via this receptor. Both soluble oligomer and fibrillar forms of amyloid-β were active. In contrast, amyloid-β did not compete with the known ligand SIRPα for binding to CD47. However, both receptors were necessary for amyloid-β to inhibit cGMP accumulation. These data suggest that amyloid-β interaction with CD36 induces a CD47-dependent signal that inhibits soluble guanylate cyclase activation. Combined with the pleiotropic effects of inhibiting free fatty acid transport via CD36, these data provides a molecular mechanism through which amyloid-β can contribute to the nitric oxide signaling deficiencies associated with Alzheimer's disease.

Phospholipase A2 and arachidonic acid in Alzheimer's disease

Essential fatty acids (EFA) play a critical role in the brain and regulate many of the processes altered in Alzheimer's disease (AD). Technical advances are allowing for the dissection of complex lipid pathways in normal and diseased states. Arachidonic acid (AA) and specific isoforms of phospholipase A(2) (PLA(2)) appear to be critical mediators in amyloid-beta (Abeta)-induced pathogenesis, leading to learning, memory, and behavioral impairments in mouse models of AD. These findings and ongoing research into lipid biology in AD and related disorders promise to reveal new pharmacological targets that may lead to better treatments for these devastating conditions.

Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease

Because regional blood flow increases in association with the increased metabolic demand generated by localized increases in neural activity, functional imaging researchers often assume that changes in blood flow are an accurate read-out of changes in underlying neural activity. An understanding of the mechanisms that link changes in neural activity to changes in blood flow is crucial for assessing the validity of this assumption, and for understanding the processes that can go wrong during disease states such as ischaemic stroke. Many studies have investigated the mechanisms of neurovascular regulation in arterioles but other evidence suggests that blood flow regulation can also occur in capillaries, because of the presence of contractile cells, pericytes, on the capillary wall. Here we review the evidence that pericytes can modulate capillary diameter in response to neuronal activity and assess the likely importance of neurovascular regulation at the capillary level for functional imaging experiments. We also discuss evidence suggesting that pericytes are particularly sensitive to damage during pathological insults such as ischaemia, Alzheimer's disease and diabetic retinopathy, and consider the potential impact that pericyte dysfunction might have on the development of therapeutic interventions and on the interpretation of functional imaging data in these disorders.

Soluble amyloid-beta, effect on cerebral arteriolar regulation and vascular cells

Background

Evidence indicates that soluble forms of amyloid-β (Aβ) are vasoactive, which may contribute to cerebrovascular dysfunction noted in patients with Alzheimer's Disease and cerebral amyloid angiopathy. The effects of soluble Aβ on penetrating cerebral arterioles - the vessels most responsible for controlling cerebrovascular resistance - have not been studied.

Results

Freshly dissolved Aβ_1-40 and Aβ_1-42, but not the reverse peptide Aβ_40-1 constricted isolated rat penetrating arterioles and diminished dilation to adenosine tri-phosphate (ATP). Aβ_1-42 also enhanced ATP-induced vessel constriction. Aβ_1-40 diminished arteriolar myogenic response, and an anti-Aβ antibody reduced Aβ_1-40 induced arteriolar constriction. Prolonged Aβ exposure in vessels of Tg2576 mice resulted in a marked age-dependent effect on ATP-induced vascular responses. Vessels from 6 month old Tg2576 mice had reduced vascular responses whereas these were absent from 12 month old animals. Aβ_1-40 and Aβ_1-42 acutely increased production of reactive oxygen species (ROS) in cultured rat cerebro-microvascular cells. The radical scavenger MnTBAP attenuated this Aβ-induced oxidative stress and Aβ_1-40-induced constriction in rat arterioles.

Conclusions

Our results suggest that soluble Aβ_1-40 and Aβ_1-42 directly affect the vasomotor regulation of isolated rodent penetrating arterioles, and that ROS partially mediate these effects. Once insoluble Aβ deposits are present, arteriolar reactivity is greatly diminished.

Use of angiotensin receptor blockers and risk of dementia in a predominantly male population: prospective cohort analysis

OBJECTIVE

To investigate whether angiotensin receptor blockers protect against Alzheimer's disease and dementia or reduce the progression of both diseases.

DESIGN

Prospective cohort analysis.

SETTING

Administrative database of the US Veteran Affairs, 2002-6. Population 819 491 predominantly male participants (98%) aged 65 or more with cardiovascular disease.

MAIN OUTCOME MEASURES

Time to incident Alzheimer's disease or dementia in three cohorts (angiotensin receptor blockers, lisinopril, and other cardiovascular drugs, the "cardiovascular comparator") over a four year period (fiscal years 2003-6) using Cox proportional hazard models with adjustments for age, diabetes, stroke, and cardiovascular disease. Disease progression was the time to admission to a nursing home or death among participants with pre-existing Alzheimer's disease or dementia.

RESULTS

Hazard rates for incident dementia in the angiotensin receptor blocker group were 0.76 (95% confidence interval 0.69 to 0.84) compared with the cardiovascular comparator and 0.81 (0.73 to 0.90) compared with the lisinopril group. Compared with the cardiovascular comparator, angiotensin receptor blockers in patients with pre-existing Alzheimer's disease were associated with a significantly lower risk of admission to a nursing home (0.51, 0.36 to 0.72) and death (0.83, 0.71 to 0.97). Angiotensin receptor blockers exhibited a dose-response as well as additive effects in combination with angiotensin converting enzyme inhibitors. This combination compared with angiotensin converting enzyme inhibitors alone was associated with a reduced risk of incident dementia (0.54, 0.51 to 0.57) and admission to a nursing home (0.33, 0.22 to 0.49). Minor differences were shown in mean systolic and diastolic blood pressures between the groups. Similar results were observed for Alzheimer's disease.

CONCLUSIONS

Angiotensin receptor blockers are associated with a significant reduction in the incidence and progression of Alzheimer's disease and dementia compared with angiotensin converting enzyme inhibitors or other cardiovascular drugs in a predominantly male population.

Hypertension and cognitive function

Cumulative evidence implicates hypertension in the pathogenesis of Alzheimer disease. Although it may not presently be possible to completely differentiate the effects of treatment and control of hypertension itself from those of the medication used to achieve such treatment goals, efforts directed at the treatment and control of hypertension can have significant public health impact.

Endothelin-converting enzyme-2 is increased in Alzheimer's disease and up-regulated by Abeta

Alzheimer's disease (AD) is thought to be caused by the accumulation of amyloid beta (Abeta) peptide within the brain. Endothelin-converting enzyme-2 (ECE-2), which is expressed in neural tissues, cleaves 'big endothelin' to produce the vasoconstrictor endothelin-1. ECE-2 also degrades Abeta. We have examined ECE-2 expression in the temporal cortex of brain tissue from patients with AD, vascular dementia, and controls. Immunohistochemistry with specific antibodies showed ECE-2 to be abundant within pyramidal neurons in both the hippocampus and neocortex, but also to be present in certain astrocytes and microglia, particularly in AD brains. Quantitative real-time PCR showed ECE-2 mRNA to be markedly elevated in AD but not in vascular dementia. ECE-2 protein concentration, measured by sandwich enzyme-linked immunosorbent assay, was also significantly elevated in AD but not in vascular dementia. Exposure of SH-SY5Y human neuroblastoma cells to monomeric or oligomeric Abeta(1-42) caused an initial decrease in ECE-2 mRNA at 4 hours, but a marked increase by 24 hours. Our findings indicate that Abeta accumulation in AD is unlikely to be caused by ECE-2 deficiency. However, ECE-2 expression is up-regulated, perhaps to minimize Abeta accumulation, but this may also be a mechanism through which endothelin-1 production is increased and cerebral blood flow is reduced in AD. Our findings suggest that endothelin-1 receptor antagonists, already licensed for treating other diseases, could be of benefit in AD therapies.

DHA and cholesterol containing diets influence Alzheimer-like pathology, cognition and cerebral vasculature in APPswe/PS1dE9 mice

Cholesterol and docosahexenoic acid (DHA) may affect degenerative processes in Alzheimer's Disease (AD) by influencing Abeta metabolism indirectly via the vasculature. We investigated whether DHA-enriched diets or cholesterol-containing Typical Western Diets (TWD) alter behavior and cognition, cerebral hemodynamics (relative cerebral blood volume (rCBV)) and Abeta deposition in 8- and 15-month-old APP(swe)/PS1(dE9) mice. In addition we investigated whether changes in rCBV precede changes in Abeta deposition or vice versa. Mice were fed regular rodent chow, a TWD-, or a DHA-containing diet. Behavior, learning and memory were investigated, and rCBV was measured using contrast-enhanced MRI. The Abeta load was visualized immunohistochemically. We demonstrate that DHA altered rCBV in 8-month-old APP/PS1 and wild type mice[AU1]. In 15-month-old APP/PS1 mice DHA supplementation improved spatial memory, decreased Abeta deposition and slightly increased rCBV, indicating that a DHA-enriched diet can diminish AD-like pathology. In contrast, TWD diets decreased rCBV in 15-month-old mice. The present data indicate that long-term dietary interventions change AD-like pathology in APP/PS1 mice. Additionally, effects of the tested diets on vascular parameters were observed before effects on Abeta load were noted. These data underline the importance of vascular factors in the APP/PS1 mouse model of AD pathology.

Arterial stiffness and cognition in elderly persons with impaired glucose tolerance and microalbuminuria

BACKGROUND

Cognitive decline that occurs frequently in impaired glucose tolerance (IGT) may be largely due to endothelial dysfunction. We assessed: (i) the relationships between impact of urinary albumin excretion rate (UAER), as marker of generalized endothelial dysfunction, and cognition; (ii) if cognitive decline could be explained by arterial stiffening using pulse wave velocity (PWV).

METHODS

One hundred forty older patients (age range 70-85 years) with IGT and no dementia were selected. Patients were classified according to 24-hour UAER: normoalbuminuric (NA) (UAER<20 microg/min) or microalbuminuric (MA) (UAER between 20 and 199 microg/min). Cognitive abilities were assessed by the Mini-Mental State Examination (MMSE) and a composite score of executive and attention functioning (CCS) at baseline and after 12 months of follow-up.

RESULTS

In MA patients (n=80), increased UAERs correlated with intimal media thickness (IMT) (r=0.268; p=02) and PWV (r=0.310; p=004). In the same group, increased UAERs were correlated with MMSE and CCS even after adjusting for age and mean arterial blood pressure (MABP). After adding PWV, the associations among UAERs, MMSE, and CCS were no longer significant. In MA patients, PWV correlated with IMT, MMSE, and CCS. In NA patients, no significant correlations were found among UAERs, MMSE, and CCS. At follow-up, baseline UAERs predicted an approximately 20% risk of poor cognition (according to MMSE and CCS) after adjusting for confounders. After adding PWV, UAERs no longer predicted cognitive performance.

CONCLUSIONS

MA older persons with IGT showed a decline in cognition performance that may be partially explained by arterial stiffness.

Soluble amyloid-beta peptides potently disrupt hippocampal synaptic plasticity in the absence of cerebrovascular dysfunction in vivo

Long before the onset of clinical Alzheimer's disease non-fibrillar, soluble assembly states of amyloid-beta (Abeta) peptides are believed to cause cognitive problems by disrupting synaptic function in the absence of significant neurodegeneration. Since many of the risk factors for Alzheimer's disease are vascular, impairment of cerebral blood flow by soluble Abeta has been proposed to be critical in triggering these early changes. However, it is not known if soluble Abeta can affect cerebrovascular function at the concentrations required to cause inhibition of synaptic plasticity mechanisms believed to underlie the early cognitive deficits of Alzheimer's disease. Here we developed a new method to simultaneously assess the ability of soluble Abeta to impair plasticity at synapses and to affect resting and activity-dependent local blood flow in the rat hippocampus in vivo. Intracerebroventricular injection of soluble synthetic Abeta(40) dimers rapidly inhibited plasticity of excitatory synaptic transmission at doses (10-42 pmol) comparable to natural Abeta, but failed to affect vascular function measured using laser-Doppler flowmetry (LDF). Like wild-type Abeta(40), the more vasculotropic Abeta produced by people with familial hemorrhagic stroke of the Dutch type (Abeta(40)E22Q), impaired hippocampal plasticity without causing a significant change in local blood flow. Furthermore, neither resting nor activation-evoked hippocampal perfusion was affected by soluble Abeta(42), even at a concentration that markedly (25%) reduced baseline synaptic transmission. These findings demonstrate that the putative synaptotoxic soluble Abeta species of early Alzheimer's disease cause synaptic dysfunction in the absence of detectible changes in local blood flow. This strongly indicates that early cognitive deficits can be caused by soluble Abeta independently of deleterious effects on cerebrovascular dynamics.

Nox2-derived radicals contribute to neurovascular and behavioral dysfunction in mice overexpressing the amyloid precursor protein

Alterations in cerebrovascular regulation related to vascular oxidative stress have been implicated in the mechanisms of Alzheimer's disease (AD), but their role in the amyloid deposition and cognitive impairment associated with AD remains unclear. We used mice overexpressing the Swedish mutation of the amyloid precursor protein (Tg2576) as a model of AD to examine the role of reactive oxygen species produced by NADPH oxidase in the cerebrovascular alterations, amyloid deposition, and behavioral deficits observed in these mice. We found that 12- to 15-month-old Tg2576 mice lacking the catalytic subunit Nox2 of NADPH oxidase do not develop oxidative stress, cerebrovascular dysfunction, or behavioral deficits. These improvements occurred without reductions in brain amyloid-beta peptide (Abeta) levels or amyloid plaques. The findings unveil a previously unrecognized role of Nox2-derived radicals in the behavioral deficits of Tg2576 mice and provide a link between the neurovascular dysfunction and cognitive decline associated with amyloid pathology.

Favourable effects of nilvadipine on cognitive function and regional cerebral blood flow on SPECT in hypertensive patients with mild cognitive impairment

OBJECTIVE

We compared the effects of nilvadipine and amlodipine on the cognitive function and regional cerebral blood flow (rCBF) in patients with mild cognitive impairment (MCI) and hypertension.

METHODS

Twelve patients with MCI and hypertension were randomly assigned to receive nilvadipine or amlodipine for 12-16 weeks. Before and after treatment all patients underwent neuropsychological evaluation and single photon emission computed tomography (SPECT) studies with N-isopropyl-p-[123I]iodoamphetamine.

RESULTS

Both nilvadipine (n=6) and amlodipine (n=6) groups had similar significant reduction in systolic and diastolic blood pressure after treatment. The Logical Memory subscore of the Wechsler Memory Scale-Revised increased significantly in the nilvadipine group, but not in the amlodipine group after treatment. Although SPECT demonstrated no significant differences in rCBF deficits at baseline between the two groups, the nilvadipine group showed an improvement of rCBF in the left frontal lobe, while the amlodipine group showed a decrease of rCBF in the left temporal lobe.

CONCLUSION

Our results suggest that nilvadipine, a highly lipophilic agent that easily penetrates the central nervous system, may have additional benefits and be potentially useful for the treatment of patients with MCI and hypertension.

Novel isoforms of NADPH-oxidase in cerebral vascular control

Reactive oxygen species (ROS) are thought to play an important role in the initiation and progression of a variety of vascular diseases. Furthermore, accumulating evidence indicates that ROS may also serve as important cell signalling molecules for the regulation of normal vascular function. Recently, a novel family of proteins (Nox1, 2 and 4) that act as the catalytic subunit of the superoxide (O2-) producing enzyme NADPH-oxidase has been discovered in vascular cells. There is now preliminary evidence suggesting that NADPH-oxidase-derived ROS may serve as a physiological vasodilator mechanism in the cerebral circulation. Moreover, the activity of NADPH-oxidase is profoundly greater in cerebral versus systemic arteries. Studies have shown that Nox1, Nox2 (also known as gp91phox) and Nox4 are all expressed in cerebral arteries, suggesting that multiple isoforms of NADPH-oxidase may be important for ROS production by cerebral arteries. Enhanced NADPH-oxidase activity is associated with several vascular-related diseases, including hypertension, stroke, subarachnoid haemorrhage and Alzheimer's dementia; however, the consequences of this for cerebral vascular function are controversial. For example, there is some evidence suggesting that NADPH-oxidase-derived O2- may play a role in endothelial dysfunction of cerebral arteries and a subsequent rise in cerebral vascular tone, associated with hypertension. However, activation of NADPH-oxidase elicits cerebral vasodilatation in vivo, and this mechanism is enhanced in chronic hypertension. While further supportive evidence is needed, it is an intriguing possibility that NADPH-oxidase-derived ROS may play a protective role in regulating cerebral vascular tone during disease.

Selective cerebral vascular dysfunction in Mn-SOD-deficient mice

We tested the hypothesis that the mitochondrial form of superoxide dismutase [manganese superoxide dismutase (Mn-SOD)] protects the cerebral vasculature. Basilar arteries (baseline diameter ∼140 μm) from mice were isolated, cannulated, and pressurized to measure vessel diameter. In arteries from C57BL/6 mice preconstricted with U-46619, acetylcholine (ACh; an endothelium-dependent vasodilator) produced dilation that was similar in male and female mice and abolished by an inhibitor of nitric oxide synthase. Vasodilation to ACh was not altered in heterozygous male or female Mn-SOD-deficient (Mn-SOD+/−) mice compared with wild-type littermate controls (Mn-SOD+/+). Constriction of the basilar artery to arginine vasopressin, but not KCl or U-46619, was increased in Mn-SOD+/− mice ( P < 0.05), and this effect was prevented by tempol, a scavenger of superoxide. We also examined responses of cerebral (pial) arterioles (branches of the middle cerebral artery, control diameter ∼30 μm) to ACh in anesthetized mice using a cranial window. Responses to ACh, but not nitroprusside (an endothelium-independent agonist), were reduced ( P < 0.05) in cerebral arterioles in Mn-SOD+/− mice, and this effect was prevented by tempol. Thus these are the first data on the role of Mn-SOD in cerebral circulation. In the basilar artery, ACh produced nitric oxide-mediated dilation that was similar in male and female mice. Under normal conditions in cerebral arteries, responses to ACh were not altered but constrictor responses were selectively enhanced in Mn-SOD+/− mice. In the cerebral microcirculation, there was superoxide-mediated impairment of responses to ACh.

Vascular remodeling versus amyloid beta-induced oxidative stress in the cerebrovascular dysfunctions associated with Alzheimer's disease

The roles of oxidative stress and structural alterations in the cerebrovascular dysfunctions associated with Alzheimer's disease (AD) were investigated in transgenic mice overexpressing amyloid precusor protein (APP+) or transforming growth factor-beta1 (TGF+). Age-related impairments and their in vitro reversibility were evaluated, and underlying pathogenic mechanisms were assessed and compared with those seen in AD brains. Vasoconstrictions to 5-HT and endothelin-1 were preserved, except in the oldest (18-21 months of age) TGF+ mice. Despite unaltered relaxations to sodium nitroprusside, acetylcholine (ACh) and calcitonin gene-related peptide-mediated dilatations were impaired, and there was an age-related deficit in the basal availability of nitric oxide (NO) that progressed more gradually in TGF+ mice. The expression and progression of these deficits were unrelated to the onset or extent of thioflavin-S-positive vessels. Manganese superoxide dismutase (SOD2) was upregulated in pial vessels and around brain microvessels of APP+ mice, pointing to a role of superoxide in the dysfunctions elicited by amyloidosis. In contrast, vascular wall remodeling associated with decreased levels of endothelial NO synthase and cyclooxygenase-2 and increased contents of vascular endothelial growth factor and collagen-I and -IV characterized TGF+ mice. Exogenous SOD or catalase normalized ACh dilatations and NO availability in vessels from aged APP+ mice but had no effect in those of TGF+ mice. Increased perivascular oxidative stress was not evidenced in AD brains, but vascular wall alterations compared well with those seen in TGF+ mice. We conclude that brain vessel remodeling and associated alterations in levels of vasoactive signaling molecules are key contributors to AD cerebrovascular dysfunctions.

Reactive oxygen species: influence on cerebral vascular tone

Reactive oxygen species have multiple effects on vascular cells. Defining the sources and the impact of the various reactive oxygen species within the vessel wall has emerged as a major area of study in vascular biology. This review will focus on recent findings related to effects of reactive oxygen species on cerebral vascular tone. Effects of superoxide radical, hydrogen peroxide, and the reactive nitrogen species peroxynitrite are summarized. Although higher concentrations may be important for cerebral vascular biology in disease, relatively low concentrations of reactive oxygen species may function as signaling molecules involved with normal regulation of cerebral vascular tone. The mechanisms by which reactive oxygen species affect vascular tone may be quite complex, and our understanding of these processes is increasing. Additionally, the role of reactive oxygen species as mediators of endothelium-dependent relaxation is addressed. Finally, the consequences of the molecular interactions of superoxide with nitric oxide and arachidonic acid are discussed.

Neurovascular coupling in the normal brain and in hypertension, stroke, and Alzheimer disease

The brain is critically dependent on a continuous supply of blood to function. Therefore, the cerebral vasculature is endowed with neurovascular control mechanisms that assure that the blood supply of the brain is commensurate to the energy needs of its cellular constituents. The regulation of cerebral blood flow (CBF) during brain activity involves the coordinated interaction of neurons, glia, and vascular cells. Thus, whereas neurons and glia generate the signals initiating the vasodilation, endothelial cells, pericytes, and smooth muscle cells act in concert to transduce these signals into carefully orchestrated vascular changes that lead to CBF increases focused to the activated area and temporally linked to the period of activation. Neurovascular coupling is disrupted in pathological conditions, such as hypertension, Alzheimer disease, and ischemic stroke. Consequently, CBF is no longer matched to the metabolic requirements of the tissue. This cerebrovascular dysregulation is mediated in large part by the deleterious action of reactive oxygen species on cerebral blood vessels. A major source of cerebral vascular radicals in models of hypertension and Alzheimer disease is the enzyme NADPH oxidase. These findings, collectively, highlight the importance of neurovascular coupling to the health of the normal brain and suggest a therapeutic target for improving brain function in pathologies associated with cerebrovascular dysfunction.

Statin therapy in the treatment of Alzheimer disease: what is the rationale?

Alzheimer disease (AD) is a chronic neurodegenerative disorder that is manifested by cognitive decline, neuropsychiatric symptoms, and diffuse structural abnormalities in the brain. Its prevalence is predicted to rise 4-fold in the next 50 years. AD is characterized pathologically by deposition of extracellular beta-amyloid and accumulation of neurofibrillary tangles. Neuronal death and specific neurotransmitter deficits also are part of the pathologic picture. Strategies to delay symptom progression have focused on addressing the neurotransmitter deficits. Strategies to delay the onset or biologic progression of AD largely have targeted the plaques formed by the deposition of beta-amyloid. AD and cardiovascular disease share common risk factors, notably hypercholesterolemia, and occur together more often than expected by chance. Therapy with the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) is the first-line treatment option for hypercholesterolemia, and observational studies have suggested that the risk of AD is reduced in patients who receive statin therapy in midlife. This reduction in risk of AD observed with statin therapy may be due to statins reducing beta-amyloid formation and deposition or to their known anti-inflammatory effects. Two randomized double-blind statin trials in patients with AD to assess the potential for statins to slow disease progression are currently under way. If successful, statin AD primary prevention trials may be developed.

Statins prevent beta-amyloid inhibition of sympathetic alpha7-nAChR-mediated nitrergic neurogenic dilation in porcine basilar arteries

The exact mechanism underlying regional cerebral hypoperfusion in the early phase of Alzheimer's disease (AD) is not understood. We have shown in isolated porcine cerebral arteries that stimulation of sympathetic alpha7-nicotinic acetylcholine receptors (alpha7-nAChRs) causes release of nitric oxide in parasympathetic nitrergic nerves and vasodilation. We therefore examined if beta-amyloid peptides (Abetas), which play a key role in pathogenesis of AD, blocked sympathetic alpha7-nAChRs leading to reduced neurogenic nitrergic dilation in isolated porcine basilar arteries, using in vitro tissue bath, calcium image, and patch clamping techniques. The results indicated that Abeta(1-40), but not Abeta(40-1), blocked relaxation of endothelium-denuded basilar arterial rings induced by nicotine (100 micromol/L) and choline (1 mmol/L) without affecting that induced by sodium nitroprusside or isoproterenol. In cultured superior cervical ganglion (SCG) cells, Abeta(1-40), but not Abeta(40-1), blocked choline- and nicotine-induced calcium influx and inward currents. The Abeta blockade of the nitrergic vasodilation and inward currents, but not that of calcium influx, was prevented by acute pretreatment with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors mevastatin and lovastatin. These results suggest that Abeta(1-40) blocks cerebral perivascular sympathetic alpha7-nAChRs, resulting in the attenuation of cerebral nitrergic neurogenic vasodilation. This effect of Abeta may be responsible in part for cerebral hypoperfusion occurred in the early phase of the AD, which may be prevented by statins most likely because of their effects independent of cholesterol lowering. Statins may offer an alternative strategy in the prevention and treatment of AD.

NADPH-oxidase-derived reactive oxygen species mediate the cerebrovascular dysfunction induced by the amyloid beta peptide

Overproduction of the amyloid beta (Abeta) peptide is a key factor in the pathogenesis of Alzheimer's disease (AD), but the mechanisms of its pathogenic effects have not been defined. Patients with AD have cerebrovascular alterations attributable to the deleterious effects of Abeta on cerebral blood vessels. We report here that NADPH oxidase, the major source of free radicals in blood vessels, is responsible for the cerebrovascular dysregulation induced by Abeta. Thus, the free-radical production and the associated alterations in vasoregulation induced by Abeta are abrogated by the NADPH oxidase peptide inhibitor gp91ds-tat and are not observed in mice lacking the catalytic subunit of NADPH oxidase (gp91phox). Furthermore, oxidative stress and cerebrovascular dysfunction do not occur in transgenic mice overexpressing the amyloid precursor protein but lacking gp91phox. The mechanisms by which NADPH oxidase-derived radicals mediate the cerebrovascular dysfunction involve reduced bioavailability of nitric oxide. Thus, a gp91phox-containing NADPH oxidase is the critical link between Abeta and cerebrovascular dysfunction, which may underlie the alteration in cerebral blood flow regulation observed in AD patients.

Alzheimer's disease and the 'ABSENT' hypothesis: mechanism for amyloid beta endothelial and neuronal toxicity

Alzheimer's disease [AD] is the most common cause of dementia among people age 65 and older. One of the biggest stumbling blocks in developing effective drug therapy for Alzheimer's disease has been the lack of a comprehensive hypothesis that explains the mechanism behind all of the histopathological changes seen in patients suffering from Alzheimer's disease. An overview of the currently popular 'amyloid' and 'vascular' hypotheses for AD demonstrates that neither hypothesis by itself can explain all the known histopathological and biochemical lesions seen in Alzheimer's disease. The paper presents a hypothesis that tries to explain the mechanism behind almost all the histopathological changes, and varying clinical manifestations seen in both diagnosed AD and Vascular Dementia [VaD]. The new hypothesis is based on the known dual toxicity of beta amyloid to both vascular and neuronal tissues, their synergy and the resultant net effect on the onset and progression of AD. The new hypothesis therefore will be known as the Amyloid Beta Synergistic Endothelial and Neuronal Toxicity [ABSENT] hypothesis. The ABSENT hypothesis will try to show the common chemical mechanism behind almost all of the pathological changes seen in AD. According to the ABSENT hypothesis, beta amyloid itself generates all the free radicals that cause both vascular dysfunction and the neuronal damage seen in AD. The chemical mechanism proposed is based on evidence from physical chemistry experiments, calculations as well as in vitro/in vivo experiments. The ABSENT hypothesis does not favor one mode of beta amyloid-induced brain damage over the other, rather it considers the net effects of the neuronal stress/damage caused by both the cerebrovascular dysfunction and direct neurotoxicity caused by beta amyloid. The hypothesis states that each patient has a different balance of predisposing factors that modulate the extent of neurotoxicity and cerebrovascular dysfunction caused by beta amyloid and thereby explains the wide range and mixed nature of damage and dysfunction seen in the studies done on patients diagnosed with AD, VaD or 'mixed dementias'. According to the hypothesis, beta amyloid peptides are necessary if not sufficient to cause AD, VaD and mixed senile dementias. The hypothesis, therefore, proposes the term Beta Amyloid Dementias [BAD] to describe the conditions currently covered by the diagnoses of 'AD', 'VaD' and 'Mixed [senile] Dementias'. Finally, the ABSENT hypothesis tries to put forth a direct chemical mechanism behind the apparent synergy and increased association between old age, pre- and coexisting vascular disease, diabetes and AD.

Neutral sphingomyelinase activation in endothelial and glial cell death induced by amyloid beta-peptide

We have explored the molecular mechanism underlying amyloid beta-peptide (Abeta)-mediated cytotoxicity in vitro. Exposure of murine cerebral endothelial cells (CECs) or C6 glioma cells to Abeta25-35 resulted in dose-dependent cell death. Ceramide is a pro-apoptotic lipid mediator. Forced elevation of cellular ceramide levels, either by application of an exogenous C2 ceramide analogue or bacterial sphingomyelinase that induces endogenous ceramide release from sphingomyelin, mimicked Abeta25-35 cytotoxicity in both CECs and C6 glioma cells. Abeta25-35-induced synthesis of ceramide was selectively mediated by activation of neutral sphingomyelinase (nSMase), but not acidic sphingomyelinase (aSMase) or ceramide synthase. Both 3-O-Me-SM and N-acetyl-L-cysteine, the selective and nonselective pharmacological inhibitors of nSMase, respectively, suppressed nSMase activation, ceramide production, and cytotoxic action induced by Abeta25-35 in CECs. Furthermore, genetic knockdown of nSMase by an antisense strategy rendered C6 glioma cells specifically resistant to Abeta25-35 cytotoxicity without affecting their vulnerability to serum deprivation. Together, nSMase activation with subsequent ceramide production may contribute, at least partially, to Abeta25-35 cytotoxicity in cell types with cerebral endothelial and glial lineage.

Catalytic antioxidants: a radical approach to new therapeutics

In humans, several pathologies involve the overproduction of reactive oxygen species. Metal-containing catalytic antioxidants have emerged as a novel class of potential therapeutic agents that scavenge a wide range of reactive oxygen species. There are three structural classes of manganese-containing catalytic antioxidants that have efficacy in several oxidative stress models of human disease. The classes are divided based on their in vitro selectivity towards the scavenging of superoxide. The selective catalytic antioxidants include the macrocyclics, whereas the non-selective catalytic antioxidants include the salens and porphyrins. Cardiovascular, neurodegenerative and inflammatory lung disorders are all potentially important targets for catalytic antioxidant therapy.