Degeneration of vascular muscle cells in cerebral amyloid angiopathy of Alzheimer disease

Cerebral amyloid angiopathy: amyloid beta accumulates in putative interstitial fluid drainage pathways in Alzheimer's disease

Cerebral amyloid angiopathy in Alzheimer's disease is characterized by deposition of amyloid beta (Abeta) in cortical and leptomeningeal vessel walls. Although it has been suggested that Abeta is derived from vascular smooth muscle, deposition of Abeta is not seen in larger cerebral vessel walls nor in extracranial vessels. In the present study, we examine evidence for the hypothesis that Abeta is deposited in periarterial interstitial fluid drainage pathways of the brain in Alzheimer's disease and that this contributes significantly to cerebral amyloid angiopathy. There is firm evidence in animals for drainage of interstitial fluid from the brain to cervical lymph nodes along periarterial spaces; similar periarterial channels exist in humans. Biochemical study of 6 brains without Alzheimer's disease revealed a pool of soluble Abeta in the cortex. Histology and immunocytochemistry of 17 brains with Alzheimer's disease showed that Abeta accumulates five times more frequently around arteries than around veins, with selective involvement of smaller arteries. Initial deposits of Abeta occur at the periphery of arteries at the site of the putative interstitial fluid drainage pathways. These observations support the hypothesis that Abeta is deposited in periarterial interstitial fluid drainage pathways of the brain and contributes significantly to cerebral amyloid angiopathy in Alzheimer's disease.

Neuropathology of white matter changes in Alzheimer's disease and vascular dementia

Morphological white matter changes were investigated in clinically and neuropathologically diagnosed cases of Alzheimer's disease (AD; 60 cases) and vascular dementia (VaD; 40 cases). In 33 of 60 AD cases, a white matter disease (WMD) characterized by tissue rarefaction, mild gliosis and a non-amyloid small-vessel sclerosis occurred in the central, preferentially frontal deep white matter. The mean vessel density was significantly lower than in normal control case frontal white matter. The presence of WMD did not parallel the severity of grey matter Alzheimer encephalopathy. In 25 of 60 AD cases, white matter degeneration with signs of both condensation and rarefaction of tissue elements was seen subjacent to advanced cortical degeneration in the temporal lobes. It concurred with WMD in only 13 cases and was judged to be of anterograde, Wallerian type and not related to angiopathy. In VaD, similar changes occurred, accompanied by several types of focal and topographically related lesions. For diffuse white matter pathology of similar appearance in vascular and neurodegenerative disease with dementia, there may be various at least partly contrasting aetiologies, which can be differentiated by the presence of even minor focal lesions in some cases. For the recognition of such subtle variations in the spectrum of WMD, modern imaging techniques are crucial for detailed clinical diagnosis and attempts at therapeutic intervention.

Possible link between lipid metabolism and cerebral amyloid angiopathy in Alzheimer's disease: A role for high-density lipoproteins?

Although apolipoprotein E4 (ApoE4) is a well-established risk factor for the development of Alzheimer's disease (AD), it is unclear how ApoE affects the progression of the disease. beta-amyloid (Abeta) is a major constituent of cerebrovascular amyloid deposits in brains of subjects with Alzheimer's disease. In cerebrospinal fluid and in plasma, Abeta is normally present in association with high density lipoproteins (HDL). These lipoproteins may play a role in the removal of excess cholesterol from the brain through interaction with ApoE and heparan sulphate proteoglycans (HSPG) in the subendothelial space of cerebral microvessels. At the same time, HDL may have a role in maintaining Abeta soluble and in mediating its clearance. Therefore, similar factors, e.g. HDL, ApoE and HSPG, may be involved in the regulation of reverse cholesterol transport in the brain and in the processing of Abeta. Alterations in the process of cholesterol secretion from the brain may contribute to the deposition of Abeta in the vascular wall.

Cerebrovascular muscle atrophy is a feature of Alzheimer's disease

We examined vascular amyloid-beta deposition and other abnormalities in the posterior cerebral artery of consecutive cases of Alzheimer's disease (AD) compared to controls. Smooth muscle atrophy was a consistent feature in the cases of AD examined (p<0.01) and was surprisingly independent of adjacent amyloid-beta deposition. These findings suggest that vascular abnormalities are a consistent feature in AD and may be an important contributor to the pathogenesis and complications of AD.

Characteristics of the in vitro vasoactivity of beta-amyloid peptides

The beta-amyloid (A beta 1-40) peptide has previously been shown to enhance phenylephrine contraction of aortic rings in vitro. We have employed a novel observation, that A beta peptides enhance endothelin-1 (ET-1) contraction, to examine the relationship between vasoactivity and potential amyloidogenicity of A beta peptides, the role played by free radicals and calcium in the vasoactive mechanism, and the requirement of an intact endothelial layer for enhancement of vasoactivity. Rings of rat aortae were constricted with ET-1 before and after addition of amyloid peptide and/or other compounds, and a comparison was made between post- and pre-treatment contractions. In this system, vessel constriction is consistently dramatically enhanced by A beta 1-40, is enhanced less so by A beta 1-42, and is not enhanced by A beta 25-35. The endothelium is not required for A beta vasoactivity, and calcium channel blockers have a greater effect than antioxidants in blocking enhancement of vasoconstriction by A beta peptides. In contrast to A beta-induced cytotoxicity, A beta-induced vasoactivity is immediate, occurs in response to low doses of freshly solubilized peptide, and appears to be inversely related to the amyloidogenic potential of the A beta peptides. We conclude that the mechanism of A beta vasoactivity is distinct from that of A beta cytotoxicity. Although free radicals appear to modulate the vasoactive effects, the lack of requirement for endothelium suggests that loss of the free radical balance (between NO and O2-) may be a secondary influence on A beta enhancement of vasoconstriction. These effects of A beta on isolated vessels, and reported effects of A beta in cells of the vasculature, suggest that A beta-induced disruption of vascular tone may be a factor in the pathogenesis of cerebral amyloid angiopathy and Alzheimer's disease. Although the mechanism of enhanced vasoconstriction is unknown, it is reasonable to propose that in vivo contact of A beta peptides with small cerebral vessels may increase their tendency to constrict and oppose their tendency to relax. The subclinical ischemia resulting from this would be expected to up-regulate beta APP production in and around the vasculature with further increase in A beta formation and deposition. The disruptive and degenerative effects of such a cycle would lead to the complete destruction of cerebral vessels and consequently neuronal degeneration in the affected areas.

Magnetic resonance imaging relaxation times and gadolinium-DTPA relaxivity values in human cerebrospinal fluid

RATIONALE AND OBJECTIVES

This study was conducted to prove the feasibility of using cerebrospinal fluid (CSF) T1 and T2 measurements to assess the blood-brain barrier integrity in disease states not noted for focal blood-brain barrier disruption, such as Alzheimer's disease.

METHODS

T1 and T2 of human CSF samples were measured with and without gadolinium Gd-DTPA over a concentration range of 1.98 x 10(-3) to 6.32 mM, in a GE 1.5-T Signa scanner.

RESULTS

T1 and T2 of human CSF without Gd-DTPA were measured as 2.39 and 0.23 s. K1 and K2 were calculated as 6.25 and 6.74 mM(-1) s(-1). The lowest Gd-DTPA concentration with measurable T1 and T2 was 1.98 x 10(-3) mM. There is no statistically significant difference in T2 and K2 at different repetition times.

CONCLUSIONS

This work demonstrates that a single measurement of relaxation times after contrast-enhanced magnetic resonance imaging could be used to determine the Gd-DTPA concentration in CSF. It may thus be feasible, using this technique, to measure intersubject and intraregional variability in the quantity of Gd-DTPA transferred across the blood-brain barrier after intravenous injection of contrast agent.

Cerebral vessels in ageing and Alzheimer's disease

The integrity of the cerebral vasculature is crucial to the maintenance of cognitive functions during ageing. Prevailing evidence suggests that cerebrovascular functions decline during normal ageing, with pronounced effects in Alzheimer's disease (AD). The causes of these changes largely remain unknown. While previous studies recorded ageing-related impairments, such as atherosclerosis and loss of innervation in basal surface arteries of the brain, it only recently has been realized that a number of subtle alterations in both the intracranial resistance vessels and the smaller capillaries is apparent in both ageing animals and humans. The dominant changes include alterations in composition of connective tissues and smooth muscle of large vessel walls, thickening of the vascular basement membrane, thinning of the endothelium in some species, loss of endothelial mitochondria and increased pericytes. Some of these attributes appear more affected in AD. Other abnormalities entail profound irregularities in the course of microvessels, unexplained inclusions in the basement membrane and changes in unique proteins and membrane lipids associated with the blood-brain barrier. Brain imaging and permeability studies show no clear functional evidence to support the structural and biochemical anomalies, but it is plausible that focal and transient breach of the blood-brain barrier in ageing, and more notably in AD, occurs. Thus, circumscribed neuronal populations in certain brain regions could become vulnerable. Furthermore, the characteristic deposition of amyloid in vessels in AD may exacerbate the decline in vascular function and promote chronic hypoperfusion. Although not explicit from current studies, it is likely that the brain vasculature is continually modified by growth and repair mechanisms in attempts to maintain perfusion during ageing and disease.

Animal models of cerebral beta-amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is a significant risk factor for hemorrhagic stroke in the elderly, and occurs as a sporadic disorder, as a frequent component of Alzheimer's disease, and in several rare, hereditary conditions. The most common type of amyloid found in the vasculature of the brain is beta-amyloid (A beta), the same peptide that occurs in senile plaques. A paucity of animal models has hindered the experimental analysis of CAA. Several transgenic mouse models of cerebral beta-amyloidosis have now been reported, but only one appears to develop significant cerebrovascular amyloid. However, well-characterized models of naturally occurring CAA, particularly aged dogs and non-human primates, have contributed unique insights into the biology of vascular amyloid in recent years. Some non-human primate species have a predilection for developing CAA; the squirrel monkey (Saimiri sciureus), for example, is particularly likely to manifest beta-amyloid deposition in the cerebral blood vessels with age, whereas the rhesus monkey (Macaca mulatta) develops more abundant parenchymal amyloid. These animals have been used to test in vivo beta-amyloid labeling strategies with monoclonal antibodies and radiolabeled A beta. Species-differences in the predominant site of A beta deposition also can be exploited to evaluate factors that direct amyloid selectively to a particular tissue compartment of the brain. For example, the cysteine protease inhibitor, cystatin C, in squirrel monkeys has an amino acid substitution that is similar to the mutant substitution found in some humans with a hereditary form of cystatin C amyloid angiopathy, possibly explaining the predisposition of squirrel monkeys to CAA. The existing animal models have shown considerable utility in deciphering the pathobiology of CAA, and in testing strategies that could be used to diagnose and treat this disorder in humans.

Amyloid-beta protein angiopathies masquerading as Alzheimer's disease?

Current evidence from genetic and epidemiological studies supports the view that Alzheimer's disease (AD) is a heterogeneous disorder. While the disease is pathologically defined by the presence of specified lesions in form of amyloid plaques and neurofibrillary tangles within the parenchyma, other features of pathology are often either neglected or considered coincidental. Our studies suggest that cerebrovascular pathology is inherently part of the disorder, which could be an important factor in a cause or effect manner. We have recently identified subjects having died with severe amyloid beta (A beta) protein cerebral amyloid angiopathy (CAA) in the absence of a profound Alzheimer pathology. These subjects, diagnosed with dementia had a late onset disease and were found at autopsy to exhibit severe CAA but paucity of typical AD changes. Immunocytochemical studies showed numerous microvascular abnormalities as well as characteristic degeneration of the vascular smooth muscle in both surface and intracortical vessels. The pathology was also characterized by occasional intracerebral hemorrhages and multiple infarcts. Further assessment of the abnormalities and amyloid infiltrated cerebral vessels with antibodies to the carboxyl terminus of A beta indicated that the longer, more pathogenic form of A beta(1-42) was found to be highly associated with intracerebral hemorrhages. Our observations suggest that these mild AD cases with a predominantly vascular pathology are variants of AD and bear resemblance to the familial Dutch and Flemish versions of cerebral amyloidosis. We propose that AD is a group of diseases with a variable pathology analogous to the prion diseases, in which a vascular variant also exists.

Misery perfusion with preserved vascular reactivity in Alzheimer's disease

To elucidate the hemodynamic pathophysiology underlying Alzheimer's disease (AD), cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2) and oxygen extraction fraction (OEF) were measured with positron emission tomography in 10 patients with probable AD and in 20 age-matched normal volunteers. By the 15O intravenous bolus injection method, CBF was measured during resting state, CO2 inhalation (hypercapnia) and hyperventilation (hypocapnia), and the vascular reactivity (VR) was estimated by comparing the CBF changes (delta CBF%/PaCO2 mmHg) in the hyper- or hypocapnic to the resting state. By the 15O2 single-breath method or 15O steady-state method, CMRO2 and OEF were measured during resting state. Based on 26 regions of interest, local CBF, CMRO2 and OEF were compared statistically between the two groups. As compared with the control group, the mean CBF and CMRO2 decreased to as low as 77.0% and 88.4% of the normal values, respectively, while the mean OEF increased by 12.1% (p < 0.05) in AD patients. These changes were most pronounced in the supramarginal and superior temporal gyri. There was no focal change in VR in the AD group, and no significant difference was seen in VR to either hyper- or hypocapnia between AD and control groups. The results may suggest a vascular involvement, possibly at the capillary level, that might cause a relative misery perfusion syndrome accompanied by preserved vascular reactivity in AD.

Cerebrovascular degeneration is related to amyloid-beta protein deposition in Alzheimer's disease

Current evidence is not inconsistent with the suggestion that cerebrovascular functions decline during normal aging with pronounced effects in both sporadic and familial Alzheimer's disease (AD). The primary causes of these changes remain unknown. It is possible that amyloid beta (A beta) protein is involved in the degeneration of both the larger penetrating vessels as well as the cerebral capillaries that represent the blood-brain barrier (BBB). A beta-induced endothelial changes could also alter muscular tone, resulting not only in increased expression of vascular amyloid precursor protein (APP) and production of A beta, but also in oxidative injury. We used immunochemical methods to examine the status of the perfusing cerebral vessels and the microvascular endothelium in relation to deposition of A beta in AD and non-AD aging control subjects. Double-immunostaining with antibodies to vascular markers revealed marked loss of smooth muscle in larger vessels and absence or attenuation of the endothelium in capillary profiles that still appeared to retain their basement membranes. These vascular changes were predominantly restricted to neocortical regions abundant in A beta deposits. Quantitative studies showed that the microvascular abnormalities were correlated to A beta deposition rather than neurofibrillary tangles or neuronal numbers. Our studies suggest that A beta, irrespective of its origin within vascular myocytes or brain parenchyma, is responsible not only for cerebral amyloid angiopathy, but also for the degeneration of the cerebral microvasculature, which may profoundly affect brain perfusion and BBB functions.

The role of amyloid in the pathogenesis of Alzheimer's disease

Since the identification in 1984 of the amyloid beta protein (Abeta) as the major component of senile plaques and cerebrovascular amyloid in Alzheimer's disease (AD) brains, it is well accepted that the production of this protein is a crucial factor in the pathogenesis of AD. Abeta is produced by cleavage from the amyloid precursor protein (APP) and can form fibrils in vivo and in vitro. The formation of these fibrils is influenced by proteins that are found in association with Abeta-containing lesions in the AD brain. Several of these proteins arise by an inflammatory response of the brain to Abeta production. The distribution of different isoforms of Abeta, varying at the C-terminus of the peptide, varies among the Abeta-containing lesions in AD brains. Such variations may have consequences for the pathogenesis of AD because the various Abeta isoforms differ in their capacity to form fibrils, and they have different toxic effects on neurons and vascular cells, respectively. The experimental data indicate that the pathogenesis of senile plaques is different from the generation of cerebrovascular amyloidosis. Summarizing models for either type of AD pathology are presented.

Microvascular degeneration in hereditary cystatin C amyloid angiopathy of the brain

Hereditary cystatin C amyloid angiopathy (HCCAA), an autosomal dominant form of cerebral amyloid angiopathy (CAA) occurring primarily in Iceland, is characterized by a variant cystatin C amyloid deposition in the walls of cerebral parenchymal and leptomeningeal vessels. Cystatin C is also found to colocalize with amyloid beta/A4 protein in cerebral vessel walls of patients with Alzheimer's disease (AD), sporadic CAA, and hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D). The abundance of cystatin C deposition in cerebral blood vessel walls suggests that cellular elements of the vessel wall itself may play a role in its deposition. Microvascular changes in the brains of HCCAA patients were investigated by single- and double-label immunohistochemistry. We found that cystatin C amyloid immunoreactivity was present not only in cerebral cortical and leptomeningeal vessels, but also in white matter parenchymal vessels. Cystatin C deposition was more prominent in the media of parenchymal vessels and in the adventitia of leptomeningeal vessels. Smooth muscle (sm) cells were few or could not be identified within vessel walls showing extensive cystatin C deposition, suggesting progressive loss of these cells as cystatin C accumulates. However, in less severely affected vessels, cystatin C was present in cells that also had the phenotype of sm, suggesting that sm cells synthesize or process cystatin C. Cystatin C immunoreactivity was in addition, detected in some neuronal cell bodies throughout the cortex in patients with HCCAA and AD-related CAA. Our results indicate that cellular components of the vessel walls may play an important role in cystatin C deposition, as they do in beta/A4 deposition in AD-related CAA. Cystatin C deposition within the vascular media and adventitia, with associated vessel wall injury as manifested by sm cell loss, represents microvascular degeneration that leads to cerebral hemorrhage.

Amyloid beta protein in plasma from patients with sporadic Alzheimer's disease

Fibrillar amyloid beta protein (A beta) deposition is increased in the brains of patients with Alzheimer's disease (AD), and is manifested as senile plaques (SPs) and congophilic angiopathy (CA). A beta 40 and A beta 42(43), two chief species of A beta, are documented in SPs and CA, as well as in cerebrospinal fluid (CSF) and cell culture media. A beta 42(43) is the major component of diffuse plaques, the earliest form of SPs. Thus, we hypothesized that determination of the amount of A beta 42(43) in CSF or plasma might provide a diagnostic laboratory test for AD. We measured amounts of different A beta species in plasma from 28 patients with sporadic probable AD, 40 age-matched neurologic patients without dementia and 25 age-matched normal controls using enzyme-linked immunosorbent assays (ELISAs). Plasma concentrations of A beta 1-40 and A beta 1-42(43) did not significantly differ among these groups. These findings suggest the unlikelihood that plasma A beta assays would be useful as a diagnostic tool for AD.

Amyloid beta-peptide and oxidative cellular injury in Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disorder that affects primarily learning and memory functions. There is significant neuronal loss and impairment of metabolic functioning in the temporal lobe, an area believed to be crucial for learning and memory tasks. Aggregated deposits of amyloid beta-peptide may have a causative role in the development and progression of AD. We review the cellular actions of A beta and how they can contribute to the cytotoxicity observed in AD. A beta causes plasma membrane lipid peroxidation, impairment of ion-motive ATPases, glutamate uptake, uncoupling of a G-protein linked receptor, and generation of reactive oxygen species. These effects contribute to the loss of intracellular calcium homeostasis reported in cultured neurons. Many cell types other than neurons show alterations in the Alzheimer's brain. The effects of A beta on these cell types is also reviewed.

Enhanced pathologic properties of Dutch-type mutant amyloid beta-protein

Cerebrovascular amyloid beta-protein (Abeta) deposition is a pathological feature of several related disorders including Alzheimer disease and hereditary cerebral hemorrhage with amyloidosis Dutch-type (HCHWA-D). HCHWA-D is caused by a point mutation in the gene that encodes the Abeta precursor and results in a Glu --> Gln substitution at position 22 of Abeta. In comparison to Alzheimer disease, the cerebrovascular Abeta deposition in HCHWA-D is generally more severe, often resulting in intracerebral hemorrhage when patients reach 50 years of age. We recently reported that Abeta(1-42), but not the shorter Abeta(1-40) induces pathologic responses in cultured human leptomeningeal smooth muscle cells including cellular degeneration that is accompanied by a marked increase in the levels of cellular Abeta precursor and soluble Abeta peptide. In the present study, we show that the HCHWA-D mutation converts the normally nonpathologic Abeta(1-40) into a highly pathologic form of the peptide for cultured human leptomeningeal smooth muscle cells. These findings suggest that these altered functional properties of HCHWA-D mutated Abeta may contribute to the early and often severe cerebrovascular pathology that is the hallmark of this disorder.

Hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D): II--A review of histopathological aspects

Cerebral amyloid-beta (A beta) angiopathy is the histopathological hallmark of hereditary cerebral hemorrhage with amyloidosis (Dutch) (HCHWA-D). A beta deposits are found mainly in the cerebral and cerebellar meningocortical blood vessels and as plaques throughout the cerebrocortical gray matter. A beta deposition in arteries and arterioles starts at the junction of media and adventitia and proceeds to involve the media causing degeneration of the vascular smooth muscle cells. Cerebrocortical arterioles often show one or two layers of radial A beta around a layer of homogenous A beta that replaces the media. Degenerating neurites, reactive astrocytes and microglial cells may surround cerebrocortical angiopathic arterioles and capillaries, probably in reaction to invasion of the perivascular neuropil by A beta fibrils. Furthermore, clusters of coarse extracellular matrix deposits may be found near A beta-laden cerebrocortical arterioles. The amyloid-associated proteins, cystatin C, and beta PP colocalize diffusely with Dutch vascular A beta, whereas HLA-DR immunoreactivity is found only in the periphery of the diseased vessel wall. The latter phenomenon may be related to the presence of perivascular cells. Angiopathic blood vessels frequently show structural changes. The relation of the described pathology to the development of hemorrhage, infarction and leukoencephalopathy needs further elucidation.

Brain parenchymal and microvascular amyloid in Alzheimer's disease

Brains of patients with Alzheimer disease/senile dementia of Alzheimer type (AD/SDAT) develop a progressive accumulation of amyloid, which deposits primarily in the form of characteristic parenchymal 'plaques' (senile or neuritic plaques/SP's) and as mural deposits in the walls of capillaries and arterioles (cerebral amyloid angiopathy /CAA). A major component of this amyloid is a small and unique peptide composed of 39-43 amino acids, beta/A4, which is cleaved from a much larger precursor protein (APP) that has several isoforms. Brain amyloid can be detected in autopsy or biopsy brain tissue by classical, immunohistochemical and ultrastructural (including immuno-electron microscopic) methods of varying sensitivity and specificity. Beta/A4 amyloid deposition is remarkably variable (e.g. predominantly parenchymal or vascular, or a mixture of parenchymal and vascular) among patients with AD/SDAT. Despite its abundance in the brains of AD/SDAT patients, the precise role of beta/A4 in the pathogenesis of the neurological deficit, neocortical atrophy and progressive synapse loss associated with AD/SDAT has yet to be determined. However, mutations in the gene that encodes APP are clearly associated with familial AD syndromes in which there is significant brain amyloid deposition. CAA, in addition to its association with AD/SDAT, can result in hemorrhagic and (possibly) ischemic forms of stroke. Work with recently developed transgenic mice which express large amounts of beta/A4 in the central nervous system is likely to elucidate mechanisms by which the protein is selectively or deposited in the brain in a parenchymal or microvascular form, and how it contributes to the pathogenesis of neurodegeneration.

Water-soluble Abeta (N-40, N-42) oligomers in normal and Alzheimer disease brains

Ultracentrifugation and graded molecular sieving, as well as a sensitive sandwich enzyme-linked immunosorbent assay were used to isolate and quantitate the amounts of water-soluble oligomers of beta amyloid (Abeta) peptides N-40 and N-42 in cerebral cortex of normal and Alzheimer disease (AD) brains. AD brains contained 6-fold more water-soluble Abeta (wsAbeta) than control brains. The majority of water-soluble peptides in most AD cases was A beta N-42, representing 12 times the amount found in control brains. The wsAbeta was present in the form of monomers and oligomers ranging from less than 10 kDa to greater than 100 kDa. The amount of wsAbeta N-42 in AD brains is about 50 times greater than the level of soluble Abeta N-42 found in the CSF of AD patients. This disparity may be due to the rapid association of wsAbeta N-42 into fibrillar deposits and/or to the integrity of the anatomical barriers which separate the two extracellular spaces. In this paper, we consider soluble any form of Abeta which has not yet polymerized into its insoluble, filamentous form. This includes both the newly synthesized forms of Abeta and those peptides which may be loosely attached to insoluble filaments but which can, nevertheless, still be considered soluble. It has been previously shown that, once it has aggregated into its filamentous form, the Abeta peptides are resistant to disaggregation and degradation by a number of denaturing agents and aqueous buffers containing proteolytic enzymes. Therefore, it is likely that the water-soluble Abeta peptides we quantified are precursors to its insoluble, filamentous form. Consequently, reducing the levels of soluble Abeta in AD brains could have profound effects on AD pathophysiology.

Apolipoprotein E alleles and brain vascular pathology in Alzheimer's disease

The presence of apolipoprotein E (ApoE)-E4 allele has been implicated as a risk factor for Alzheimer's disease (AD). We examined the occurrence of ApoE 4 alleles in AD associated with cerebral amyloid angiopathy and other vascular lesions. We found significantly high frequency of the ApoE 4 allele in AD with moderate to severe CAA. The frequency of the allele was also high in AD cases with other vascular lesions such as multiple infarcts and lacunes. As previously reported, we confirm a greater frequency of the ApoE 4 allele in the diffuse Lewy body variant of AD. Our results suggest ApoE 4 allele to be a significant factor in the development of CAA in AD. While this may be related to increased brain amyloid load as a consequence of ApoE genotype, the possibility exists that ApoE may be a specific factor in vascular abnormalities associated with AD.

Altered expression of amyloid beta precursor mRNAs in cerebral vessels, meninges, and choroid plexus in Alzheimer's disease

Altered tissue-specific processing or production of the amyloid precursor protein (APP) is thought to be central to amyloid deposition in cerebrovascular and the neocortical tissues in Alzheimer's disease (AD). We demonstrate that A beta peptide(s) is readily detectable and increased in cerebral vessels, meninges and choroid plexus obtained at autopsy from AD subjects compared to age-matched controls. Using the reverse transcription (RT)-polymerase chain reaction (PCR), we further found that A beta transcripts encoding the A beta sequence in all forms of APP containing exons 16 and 17 (of APP770) were significantly increased in vessel samples in AD subjects. This was also evident in the neocortical samples and not related to pre-mortem factors or postmortem interval. It is possible that the increased A beta mRNAs reflect enhanced expression of the L-APP isoform (APP770 without exon 15) expressed in leukocytes and glia alike. We also found evidence for changed proportions of APP 770, 756 and 695 mRNAs in cerebral vessel samples from AD subjects compared to controls. Whereas APP770 and APP751, the predominant forms, were significantly decreased, APP695 transcript was increased in vessel samples from AD subjects. Such changes were not evident in neocortical samples from the same subjects. These observations suggest tissue-specific changes in expression of APP isoforms implicating one of the mechanisms for increased accumulation of A beta in cerebrovascular tissues in AD.

Amyloid beta-protein induces the cerebrovascular cellular pathology of Alzheimer's disease and related disorders

One of the hallmark pathologic characteristics of Alzheimer's disease (AD) and related disorders is deposition of the 39-42 amino acid amyloid beta-protein (A beta) in the walls of cerebral blood vessels. The cerebrovascular A beta deposits in these disorders are associated with degenerating smooth muscle cells in the vessel wall which have been implicated in the expression of the amyloid beta-protein precursor (A beta PP) and formation of A beta. We have established primary cultures of human cerebrovascular smooth muscle cells as a model for investigating the cellular pathologic processes involved in the cerebral amyloid angiopathy of AD and related disorders. Recently, we have shown that A beta 1-42, the predominant pathologic cerebrovascular form of A beta, causes extensive cellular degeneration that is accompanied by a striking increase in the levels of cellular A beta PP, potentially amyloidogenic carboxyl terminal A beta PP fragments, and soluble A beta peptide in the cultured human cerebrovascular smooth muscle cells. Together, these studies provide evidence that A beta contributes to the onset and progression of the cerebrovascular pathology associated with AD and related disorders and suggests the mechanism involves a molecular cascade with a novel product-precursor relationship that results in the adverse production and accumulation of A beta.

Production and increased detection of amyloid beta protein and amyloidogenic fragments in brain microvessels, meningeal vessels and choroid plexus in Alzheimer's disease

Recent advances indicate soluble amyloid beta (A beta) protein is produced constitutively during normal metabolism of the amyloid precursor protein (APP). This has not been directly examined in human brain vascular tissues. Using a panel of well-characterized antibodies, here we show that increased amounts of soluble A beta were found in isolated vascular tissues from AD subjects compared to age-matched controls without significant Alzheimer pathology. Immunocytochemical analyses of isolated vessel preparations showed characteristic transverse patterns of A beta deposits in large vessels with smooth muscle, however, fine A beta deposits were apparent even in capillaries. A proportion of such A beta protein and potentially amyloidogenic carboxyl terminal fragments were released by solubilization and disruption of the vascular basement membrane by collagenase treatments. We further demonstrated by in vitro metabolic labelling that soluble A beta or an A beta-like peptide is associated and produced by cerebral microvessels, meningeal vessels and the choroid plexus isolated postmortem from human as well as rat brain. Compared to those from young rats, cerebral microvessels from aging rats showed increased release of carboxyl terminal fragments of APP and A beta-like peptide. Our observations provide the first direct demonstration that human vascular tissues produce soluble A beta, a product of the secretory pathway in APP processing. Our findings also suggest that aging associated alterations in the basement membranes are a factor in A beta accumulation that results in vascular amyloid deposition, the principal feature of cerebral amyloid angiopathy.

The origin of amyloid in cerebral vessels of aged dogs

Our morphometric study of 30 dogs, mongrels, from 6.5 to 26.5 years of age, shows amyloid angiopathy in cortical and leptomeningeal vessels of all dogs older than 13.2 years of age, and the increase in the numerical density of amyloid-positive vessels correlated with age. Cluster analysis distinguished the group of six dogs (25%) to be relatively less affected, a large group of 13 animals (54%) to have moderate pathology, and five dogs (21%) to have severe amyloid angiopathy. Amyloid accumulation starts in large vessels, particularly in the tunica media of large arteries. Amyloid deposition appears to be associated with smooth muscle cells. Ultrastructural studies of samples from nine dogs are in agreement with in vitro studies suggesting that smooth muscle cells are the source of soluble amyloid beta. beta-protein polymerizes in the basal lamina of the tunica media. Muscle cells in the area of amyloid-beta accumulation degenerate and die. Thioflavin-positivity of only 24% of cortical and 66% of leptomeningeal beta-protein-positive vessels suggests that thioflavin-negative deposits contain soluble, not yet fibrillized protein and/or partially degraded and depolymerized amyloid.

Increased collagen content of cerebral microvessels in Alzheimer's disease

Based on previous evidence suggesting abnormalities in the brain microvasculature, we examined basement membrane collagen in isolated cerebral microvessels (CMV) from subjects with Alzheimer's disease (AD) and age-matched controls. Concentrations of hydroxyproline, the principal constituent of collagen IV, were significantly increased by 55% in CMV from AD subjects compared to controls. This result was corroborated by the finding of 60% increased total collagen content in CMV as evident by the selective binding of Sirius red dye. Hydroxyproline and collagen concentrations in samples of cerebral cortex assayed in parallel were 6-20 times smaller than in CMV and were not changed between controls and AD subjects. To further differentiate AD and control samples, fractions of CMV were solubilized and the pepsin digested collagen proteins resolved by SDS-PAGE. Upon immunoblotting, AD samples with increased collagen revealed proportionally greater specific immunoreactivities detected by antibodies to collagen IV. Our observations suggest altered collagen IV content of cerebral vessels in subjects with AD that may affect brain microvascular functions.

Amyloid beta-protein aggregation nullifies its pathologic properties in cultured cerebrovascular smooth muscle cells

Alzheimer's disease and related disorders are characterized by deposition of aggregated amyloid beta-protein (A beta) and accompanying pathologic changes in the neuropil and in the walls of cerebral blood vessels. A beta induces neurotoxicity in vitro, and this effect is markedly enhanced when the peptide is preaggregated. Recently, we reported that freshly solubilized A beta 1-42 can induce cellular degeneration and a striking increase in the levels of cellular amyloid beta-protein precursor and soluble A beta peptide in cultured cerebrovascular smooth muscle cells (Davis-Salinas, J., Saporito-Irwin, S. M., Cotman, C. W., and Van Nostrand, W. E. (1995) J. Neurochem. 65, 931-934). In the present study, we show that preaggregation of A beta 1-42 abolishes the ability of the peptide to induce these cellular pathologic responses in these cells in vitro. These findings suggest that distinct mechanisms for A beta-induced cytotoxicity exist for cultured neurons and cerebrovascular smooth muscle cells, supporting that different processes may be involved in the parenchymal and cerebrovascular pathology of Alzheimer's disease and related disorders.

Amyloid beta-proteins 1-40 and 1-42(43) in the soluble fraction of extra- and intracranial blood vessels

To investigate the process of amyloid beta-protein (A beta) accumulation in cerebral amyloid angiopathy (CAA), the levels of A beta were determined in the soluble fraction of extra- and intracranial blood vessels and leptomeninges obtained at autopsy. Two enzyme immunoassays were employed that are known to sensitively and specifically quantify two A beta species, A beta 1-40 and 1-42(43). A beta was detectable in the intracranial blood vessels and leptomeninges with the latter containing the highest levels, while it was undetectable in the extracranial blood vessels. Thus the levels of soluble A beta correlated well with the predilection sites for CAA. Among individuals aged 20 to 90, the A beta levels in the leptomeninges increased sharply in those aged 50 to 70 and thereafter tended to decline. However, only slight degrees of CAA were detected by immunocytochemistry, even when those leptomeninges contained high levels of A beta comparable with those in Alzheimer's disease. The level of A beta 1-42 was almost always severalfold that of A beta 1-40 in the soluble fraction of leptomeninges. This is in good agreement with the immunocytochemical result showing the presence of A beta 40-negative, A beta 42(43)-positive meningeal vessels. These results indicate that A beta 1-42 is the initially deposited species in CAA and that the disruption of A beta homeostasis precedes A beta deposition in the meningeal vessels.

Beta-amyloid precursor protein in human digital skin

The occurrence and distribution of beta-amyloid precursor protein (beta APP) and of beta-amyloid peptide (beta/A4) was investigated using immunoblotting and immunohistochemical techniques in the digital skin of healthy adult subjects. beta APP-like proteic bands with apparent molecular masses between 55-60 kDa, 100-125 kDa (corresponding to the full-length beta APP isoforms), 145-150 kDa, and 200 kDa were found in pellets and supernatants of whole skin and dermis. The same proteins, except that of approximately 200 kDa, were also found in pellets from the epidermis, whereas epidermic supernatants were unreactive. beta/A4 was not found by immunoblotting. Light microscope immunohistochemistry showed beta APP immunoreactivity (IR) in: (a) dermal nerves; (b) lamellar cells of Meissner, as well as inner-core, outer-core and capsule of Pacinian corpuscles; and (c) dermal blood vessels, sweat glands and, occasionally, epidermis. The distribution of beta/A4 IR matched that of beta APP, and no evidence of extracellular beta/A4 IR was encountered. Present results demonstrate that beta APP, but not beta/A4, is normally present in human glabrous (digital) skin. The potential clinical relevance of these findings is discussed.

Tau protein directly interacts with the amyloid beta-protein precursor: implications for Alzheimer's disease

The simultaneous presence of intracellular neurofibrillary tangles (NFT) and extracellular senile plaques in Alzheimer's disease (AD) suggests that the two lesions could be synergistically interrelated. However, although the main protein components of NFT and senile plaques, tau (tau) and amyloid beta-protein, respectively, are well characterized, the molecular mechanisms responsible for their deposition in lesions are unknown. We demonstrate, using four independent techniques, that tau directly interacts with a conformation-dependent domain of the amyloid beta-protein precursor (beta PP) encompassing residues beta PP714-723. The putative tau-binding domain includes beta PP717 mutation sites that are associated with familial forms of AD. Our findings strongly suggest that NFT and senile plaques, often thought of as independent structures, may play a role in each other's formation during the pathogenesis of AD.

beta-Amyloid precursor protein (beta APP) in human gut with special reference to the enteric nervous system

The distribution of the beta-amyloid precursor protein (betaAPP) in the human gastrointestinal tract, from esophagus trough rectum, was studied using immunoblotting, as well as combined immunohistochemical and image analysis (optic microdensitometry) techniques. The study was focused on the enteric nervous system. betaAPP was detected by means of a monoclonal antibody (22C11), which recognizes all betaAPP isoforms as well as betaAPP-like proteins. Immunoblotting revealed two main protein bands, one corresponding to full-length betaAPPs (estimated molecular masses of approximately 97-115 kDa); the other corresponded to a protein with estimated molecular masses of 55 kDa. Specific betaAPP immunoreactivity (IR) was found in the submucous and myenteric plexuses localized in the supporting glial cells rather than in neurons. Differences were encountered neither in the localization nor in the intensity of immunostaining among different segments of the gastrointestinal tract. Moreover, no age-dependent changes were found. betaAPP IR was also regularly observed in blood vessels, primarily labelling endothelial cells. Our results provide evidence for the occurrence of betaAPP in human gastrointestinal tract of healthy people in both neuronal and nonneuronal tissues. Whether or not these findings have functional or clinical relevance remains to be clarified in future studies.

Nicotinic cholinergic receptors associated with mammalian cerebral vessels

Current evidence suggests that the cerebral vasculature may be modulated by cholinergic nerves. We used ligand binding methods to examine the presence of nicotinic cholinergic receptors in brain vasculature. We found carbachol-displaceable [3H]acetylcholine (ACh) and [3H]nicotine (NIC) binding sites in preparations of intraparenchymal cerebral microvessels (CMV) and larger pial vessels from human and pig brains. Specific binding sites for [3H]ACh and [3H]NIC in cerebral microvessels were saturable and comparable in density to those in cerebral cortex. The Kds for the two ligands ranged 3-18 nM whereas the Bmaxs were 25-45 fmol/mg protein. In contrast, the binding of [3H]pirenzipine or [3H]quinuclidinyl benzilate, index for muscarinic receptors, was low (9-15% of cortex) in microvessels compared to the cerebral cortex. Our observations suggest the association of cholinergic nicotinic receptors with cerebral microvessels, which may be involved in the modulation of the cerebral circulation by cholinergic neurons.