A beta-peptides enhance vasoconstriction in cerebral circulation

Nilvadipine antagonizes both Abeta vasoactivity in isolated arteries, and the reduced cerebral blood flow in APPsw transgenic mice

The development of Alzheimer's disease (AD) is generally thought to correlate with cerebral accumulation of Abeta. It has previously been shown that Abeta peptides enhance vasoconstriction in isolated arteries and oppose certain vasorelaxants. Moreover, exogenous application of Abeta peptides causes cerebral vasoconstriction in rodents and in transgenic mouse models of AD that overexpress Abeta there is reduced cerebral blood flow. In the present study, we investigated the effect of nilvadipine, a dihydropyridine-type calcium channel blocker, on Abeta induced vasoconstriction in isolated arteries and in vivo on cerebral blood flow (CBF) of an AD transgenic mouse model overexpressing Abeta (Tg APPsw line 2576). Nilvadipine completely inhibited the vasoactivity elicited by Abeta in rat aortae and in human middle cerebral arteries. The effect of a short treatment duration (2 weeks) with nilvadipine on regional CBF was investigated in 13-month-old Tg APPsw mice and control littermates using a laser Doppler imager. Additionally, CBF was also measured in 20-month-old Tg APPsw mice and control littermates that were chronically treated with nilvadipine for 7 months. Untreated Tg APPsw mice showed a reduction of regional CBF compared to their untreated control littermates. Nilvadipine restored cortical perfusion levels in Tg APPsw to values similar to those observed in control littermates without notably affecting the CBF of control mice. All together, these data suggest that nilvadipine might be useful for the treatment of oligemia associated with AD.

Cerebrovascular effects of amyloid-beta peptides: mechanisms and implications for Alzheimer's dementia

1. The amyloid B-peptide (AB) is involved in the mechanisms of Alzheimer dementia. This paper reviews experimental evidence indicating that AB exerts profound effects on the regulation of the cerebral circulation. 2. Thus, AB compromises the ability of cerebral endothelial cells to produce vascular relaxing factors, impairs the ability of cerebral blood vessels to maintain adequate flow during hypotension, and attenuates the increases in CBF evoked by enhanced brain activity. 3. Studies in transgenic mice overexpressing the amyloid precursor protein suggest that these cerebrovascular alterations disrupt the delicate balance between the brain's energy requirements and cerebral blood supply, rendering the brain more vulnerable to ischemic injury. 4. The findings support the recently emerged notion that vascular factors play a pathogenic role in the early stages of Alzheimer dementia.

Abeta-induced vascular oxidative stress and attenuation of functional hyperemia in mouse somatosensory cortex

We investigated the role of vascular oxidative stress in the mechanisms of the impairment in cerebrovascular regulation produced by the amyloid-beta peptide (Abeta). In particular, we sought to provide evidence of vascular oxidative stress in mice overexpressing the amyloid precursor protein (APP) and to determine whether the Abeta-induced attenuation in functional hyperemia is mediated by free radical overproduction. Oxidative/nitrosative stress was assessed by 3-nitrotyrosine immunoreactivity, while free radical production was determined in cerebral microvessels by hydroethidine microfluorography. To study functional hyperemia the somatosensory cortex was activated by whisker stimulation while local blood flow was monitored by laser-Doppler flowmetry. It was found that APP mice show signs of oxidative/nitrosative stress in pial and intracerebral blood vessels well before they develop oxidative stress in neurons and glia or amyloid plaques. Treatment of cerebral microvessels isolated from wild-type mice with Abeta (1 microM) increased free radical production as assessed by the hydroethidine technique. The Abeta-induced attenuation of the increase in somatosensory cortex blood flow produced by whisker stimulation was prevented by treatment with the free radical scavengers MnTBAP or tiron. These data provide evidence that in APP mice vascular oxidative stress precedes the development of parenchymal oxidative stress, and that Abeta-produced vascular reactive oxygen species are involved in the attendant attenuation in functional hyperemia. Thus, vascular oxidative stress is an early event in the course of the brain dysfunction produced by APP overexpression and Abeta, and, as such, could be the target of early therapeutic interventions based on antioxidants.

Age-dependent cerebrovascular abnormalities and blood flow disturbances in APP23 mice modeling Alzheimer's disease

Neuropathological changes associated with Alzheimer's disease (AD) such as amyloidplaques, cerebral amyloid angiopathy, and related pathologies are reproduced in APP23 transgenic mice overexpressing amyloid precursor protein (APP) with the Swedish mutation. Magnetic resonance angiography (MRA) was applied to probe, in vivo, the cerebral arterial hemodynamics of these mice. Flow voids were detected at the internal carotid artery of 11-month-old APP23 mice. At the age of 20 months, additional flow disturbances were observed in large arteries at the circle of Willis. Vascular corrosion casts obtained from the same mice revealed that vessel elimination, deformation, or both had taken place at the sites where flow voids were detected by MRA. The detailed three-dimensional architecture of the vasculature visible in the casts assisted the identification of smaller vessels most likely formed as substitution or anastomosis within the circle of Willis. Angiograms and corrosion casts from nontransgenic, age-matched mice manifested no major abnormalities in the cerebrovascular arterial flow pattern. Because no transgene overexpression has been found in the cerebrovasculature of APP23 mice and no deposits of amyloid-beta (Abeta) were observed in large arteries in the region of the circle of Willis, the present results suggest that soluble Abeta may exert deleterious effects on the vasculature. Our findings support the idea that cerebral circulatory abnormalities evolving progressively could contribute to AD pathogenesis. The study also shows the power of MRA to identify changes of vascular function in genetically engineered mice. MRA as a noninvasive technique could be applied to test new therapeutic concepts in animal models of AD and in humans.

Superoxide dismutases in the lung and human lung diseases

The lungs are directly exposed to higher oxygen concentrations than most other tissues. Increased oxidative stress is a significant part of the pathogenesis of obstructive lung diseases such as asthma and chronic obstructive pulmonary disease, parenchymal lung diseases (e.g., idiopathic pulmonary fibrosis and lung granulomatous diseases), and lung malignancies. Lung tissue is protected against these oxidants by a variety of antioxidant mechanisms among which the superoxide dismutases (SODs) are the only ones converting superoxide radicals to hydrogen peroxide. There are three SODs: cytosolic copper-zinc, mitochondrial manganese, and extracellular SODs. These enzymes have specific distributions and functions. Their importance in protecting lung tissue has been confirmed in transgenic and knockout animal studies. Relatively few studies have been conducted on these enzymes in the normal human lung or in human lung diseases. Most human studies suggest that there is induction of manganese SOD and, possibly, extracellular SOD during inflammatory, but not fibrotic, phases of parenchymal lung diseases and that both copper-zinc SOD and manganese SOD may be downregulated in asthmatic airways. Many previous antioxidant therapies have been disappointing, but newly characterized SOD mimetics are being shown to protect against oxidant-related lung disorders in animal models.

Proinflammatory and vasoactive effects of Abeta in the cerebrovasculature

Abeta peptides are thought to be critical molecules in the pathophysiology of Alzheimer's disease (AD) and are the major protein constituents of senile plaques. In most AD cases, Abeta peptides also form some deposits in the cerebrovasculature, leading to cerebral amyloid angiopathy and hemorrhagic stroke. Regional cerebral hypoperfusion is one of the earlier clinical manifestations in both the sporadic and familial forms of AD. In addition, a variety of vascular risk factors of different etiologies (for instance, diabetes, hypertension, high cholesterol level, atherosclerosis, and smoking) constitute risk factors for AD as well, suggesting that functional vascular abnormalities may contribute to AD pathology. We studied the effect of Abeta on constrictor responses elicited by endothelin-1 in isolated human cerebral arteries collected following rapid autopsies. We report that freshly solubilized Abeta potentiates endothelin-1-induced vasoconstriction in isolated human middle cerebral and basilar arteries. The vasoconstriction elicited by Abeta in these large human cerebral arteries appears to be completely antagonized by NS-398, a selective cyclooxygenase-2 inhibitor, or by SB202190, a specific p38 mitogen-activated protein kinase inhibitor, suggesting that Abeta vasoactivity is mediated via the stimulation of a proinflammatory pathway. In addition, a similar proinflammatory response appears to be mediated by Abeta in isolated human brain microvessels, resulting in an increased production of prostaglandin E(2) and F(2alpha). Using a scanner laser Doppler imager, we show a progressive decline with aging in cortical perfusion level in transgenic APPsw mice (line 2576) compared with age-matched control littermates. The relation between the acute proinflammatory and vasoactive properties of Abeta and the chronic progressive hypoperfusion seen in AD (and transgenic models thereof) is yet to be elucidated.

Cerebrovascular autoregulation is profoundly impaired in mice overexpressing amyloid precursor protein

The amyloid-beta (A beta) peptide, which is derived from the amyloid precursor protein (APP), is involved in the pathogenesis of Alzheimer's dementia and impairs endothelium-dependent vasodilation in cerebral vessels. We investigated whether cerebrovascular autoregulation, i.e., the ability of the cerebral circulation to maintain flow in the face of changes in mean arterial pressure (MAP), is impaired in transgenic mice that overexpress APP and A beta. Neocortical cerebral blood flow (CBF) was monitored by laser-Doppler flowmetry in anesthetized APP(+) and APP(-) mice. MAP was elevated by intravenous infusion of phenylephrine and reduced by controlled exsanguination. In APP(-) mice, autoregulation was preserved. However, in APP(+) mice, autoregulation was markedly disrupted. The magnitude of the disruption was linearly related to brain A beta concentration. The failure of autoregulation was paralleled by impairment of the CBF response to endothelium-dependent vasodilators. Thus A beta disrupts a critical homeostatic mechanism of the cerebral circulation and renders CBF highly dependent on MAP. The resulting alterations in cerebral perfusion may play a role in the brain dysfunction and periventricular white-matter changes associated with Alzheimer's dementia.

Alzheimer disease as a vascular disorder: nosological evidence

BACKGROUND

The main stumbling block in the clinical management and in the search for a cure of Alzheimer disease (AD) is that the cause of this disorder has remained uncertain until now.

SUMMARY OF REVIEW

Evidence that sporadic (nongenetic) AD is primarily a vascular rather than a neurodegenerative disorder is reviewed. This conclusion is based on the following evidence: (1) epidemiological studies showing that practically all risk factors for AD reported thus far have a vascular component that reduces cerebral perfusion; (2) risk factor association between AD and vascular dementia (VaD); (3) improvement of cerebral perfusion obtained from most pharmacotherapy used to reduce the symptoms or progression of AD; (4) detection of regional cerebral hypoperfusion with the use of neuroimaging techniques to preclinically identify AD candidates; (5) presence of regional brain microvascular abnormalities before cognitive and neurodegenerative changes; (6) common overlap of clinical AD and VaD cognitive symptoms; (7) similarity of cerebrovascular lesions present in most AD and VaD patients; (8) presence of cerebral hypoperfusion preceding hypometabolism, cognitive decline, and neurodegeneration in AD; and (9) confirmation of the heterogeneous and multifactorial nature of AD, likely resulting from the diverse presence of vascular risk factors or indicators of vascular disease.

CONCLUSIONS

Since the value of scientific evidence generally revolves around probability and chance, it is concluded that the data presented here pose a powerful argument in support of the proposal that AD should be classified as a vascular disorder. According to elementary statistics, the probability or chance that all these findings are due to an indirect pathological effect or to coincidental circumstances related to the disease process of AD seems highly unlikely. The collective data presented in this review strongly support the concept that sporadic AD is a vascular disorder. It is recommended that current clinical management of patients, treatment targets, research designs, and disease prevention efforts need to be critically reassessed and placed in perspective in light of these important findings.