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

How Does SARS-CoV-2 Affect the Central Nervous System? A Working Hypothesis

Interstitial pneumonia was the first manifestation to be recognized as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); however, in just a few weeks, it became clear that the coronavirus disease-2019 (COVID-19) overrun tissues and more body organs than just the lungs, so much so that it could be considered a systemic pathology. Several studies reported the involvement of the conjunctiva, the gut, the heart and its pace, and vascular injuries such as thromboembolic complications and Kawasaki disease in children and toddlers were also described. More recently, it was reported that in a sample of 214 SARS-CoV-2 positive patients, 36.4% complained of neurological symptoms ranging from non-specific manifestations (dizziness, headache, and seizures), to more specific symptoms such hyposmia or hypogeusia, and stroke. Older individuals, especially males with comorbidities, appear to be at the highest risk of developing such severe complications related to the Central Nervous System (CNS) involvement. Neuropsychiatric manifestations in COVID-19 appear to develop in patients with and without pre-existing neurological disorders. Growing evidence suggests that SARS-CoV-2 binds to the human Angiotensin-Converting Enzyme 2 (ACE2) for the attachment and entrance inside host cells. By describing ACE2 and the whole Renin Angiotensin Aldosterone System (RAAS) we may better understand whether specific cell types may be affected by SARS-CoV-2 and whether their functioning can be disrupted in case of an infection. Since clear evidences of neurological interest have already been shown, by clarifying the topographical distribution and density of ACE2, we will be able to speculate how SARS-CoV-2 may affect the CNS and what is the pathogenetic mechanism by which it contributes to the specific clinical manifestations of the disease. Based on such evidences, we finally hypothesize the process of SARS-CoV-2 invasion of the CNS and provide a possible explanation for the onset or the exacerbation of some common neuropsychiatric disorders in the elderly including cognitive impairment and Alzheimer disease.

Accumulation of Amyloid Beta (Aβ) Peptide on Blood Vessel Walls in the Damaged Brain after Transient Middle Cerebral Artery Occlusion

It is well known that amyloid beta (Aβ) peptides are generated in blood vessels, released into the brain during thrombosis, and temporarily accumulate in this organ after injury. Here we demonstrate that 24 h after transient middle cerebral artery occlusion (tMCAO), one of the standard models of focal ischemic stroke, Aβ peptide accumulates in the brain, concentrating on the blood vessel walls. Because Aβ oligomers are known to induce significant damage to brain cells, they act as an additional damaging factor during ischemic stroke. Considering that they have been shown to form ion channels in cells, affecting osmotic balance, we used an Aβ peptide channel blocker, tromethamine (2-amino-2-(hydroxymethyl) propane-1,3-diol), to prevent this additional injury. Tromethamine injected 0.1 g/100 g body weight intraperitoneally at 5 min before tMCAO decreased water content in the damaged hemisphere, as measured by dry brain weight. Congo red staining, which binds only to Aβ oligomer plaques (amyloid), showed that there was no significant presence of plaques. Therefore, we suggest that Aβ peptide oligomers are responsible for some of the brain damage during stroke and that blockage of the ion channels that they form could be beneficial in treating this complex neurological syndrome.

Preparation and Culturing of Human Primary Vascular Cells

Cerebral amyloid angiopathy (CAA) results from amyloid accumulation within arteries of the cerebral cortex and leptomeninges. This condition is age-related, especially prevalent in Alzheimer's disease (AD), and the main feature of certain hereditary disorders (i.e., HCHWA-I). The vascular smooth muscle cells (VSMCs) appear to play a vital role in the development of CAA, which makes them well suited as an experimental model to study the disease and screen for possible remedies. We describe two different methods for isolating and culturing human VSMCs: First, using the human umbilical cord as an easy source of robust cells, and secondly, using brain tissue that provides the proper cerebral VSMCs, but is more problematic to work with. The umbilical cord also provides human umbilical vascular endothelial cells (HUVEC), useful primary cells for vascular research. Finally, the maintenance, preservation, and characterization of the isolated vascular cells are described.

A Role for the Brain RAS in Alzheimer's and Parkinson's Diseases

The brain renin-angiotensin system (RAS) has available the necessary functional components to produce the active ligands angiotensins II (AngII), angiotensin III, angiotensins (IV), angiotensin (1-7), and angiotensin (3-7). These ligands interact with several receptor proteins including AT1, AT2, AT4, and Mas distributed within the central and peripheral nervous systems as well as local RASs in several organs. This review first describes the enzymatic pathways in place to synthesize these ligands and the binding characteristics of these angiotensin receptor subtypes. We next discuss current hypotheses to explain the disorders of Alzheimer's disease (AD) and Parkinson's disease (PD), as well as research efforts focused on the use of angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), in their treatment. ACE inhibitors and ARBs are showing promise in the treatment of several neurodegenerative pathologies; however, there is a need for the development of analogs capable of penetrating the blood-brain barrier and acting as agonists or antagonists at these receptor sites. AngII and AngIV have been shown to play opposing roles regarding memory acquisition and consolidation in animal models. We discuss the development of efficacious AngIV analogs in the treatment of animal models of AD and PD. These AngIV analogs act via the AT4 receptor subtype which may coincide with the hepatocyte growth factor/c-Met receptor system. Finally, future research directions are described concerning new approaches to the treatment of these two neurological diseases.

Roles of Brain Angiotensin II in Cognitive Function and Dementia

The brain renin-angiotensin system (RAS) has been highlighted as having a pathological role in stroke, dementia, and neurodegenerative disease. Particularly, in dementia, epidemiological studies indicate a preventive effect of RAS blockade on cognitive impairment in Alzheimer disease (AD). Moreover, basic experiments suggest a role of brain angiotensin II in neural injury, neuroinflammation, and cognitive function and that RAS blockade attenuates cognitive impairment in rodent dementia models of AD. Therefore, RAS regulation is expected to have therapeutic potential for AD. Here, we discuss the role of angiotensin II in cognitive impairment and AD. Angiotensin II binds to the type 2 receptor (AT₂) and works mainly by binding with the type 1 receptor (AT₁). AT₂ receptor signaling plays a role in protection against multiple-organ damage. A direct AT₂ receptor agonist is now available and is expected to reduce inflammation and oxidative stress and enhance cell differentiation. We and other groups reported that AT₂ receptor activation enhances neuronal differentiation and neurite outgrowth in the brain. Here, we also review the effect of the AT₂ receptor on cognitive function. RAS modulation may be a new therapeutic option for dementia including AD in the future.

Neuroprotection and neurodegeneration in Alzheimer's disease: role of cardiovascular disease risk factors, implications for dementia rates, and prevention with aerobic exercise in african americans

Prevalence of Alzheimer's disease (AD) will reach epidemic proportions in the United States and worldwide in the coming decades, and with substantially higher rates in African Americans (AAs) than in Whites. Older age, family history, low levels of education, and ɛ4 allele of the apolipoprotein E (APOE) gene are recognized risk factors for the neurodegeneration in AD and related disorders. In AAs, the contributions of APOE gene to AD risk continue to engender a considerable debate. In addition to the established role of cardiovascular disease (CVD) risk in vascular dementia, it is now believed that CVD risk and its endophenotype may directly comediate AD phenotype. Given the pleiotropic effects of APOE on CVD and AD risks, the higher rates of CVD risks in AAs than in Whites, it is likely that CVD risks contribute to the disproportionately higher rates of AD in AAs. Though the advantageous effects of aerobic exercise on cognition is increasingly recognized, this evidence is hardly definitive, and data on AAs is lacking. In this paper, we will discuss the roles of CVD risk factors in the development of AD and related dementias, the susceptibility of these risk factors to physiologic adaptation, and fitness-related improvements in cognitive function. Its relevance to AD prevention in AAs is emphasized.

Preparation of cultured human vascular cells

Cerebral amyloid angiopathy (CAA) results from amyloid accumulation within arteries of the cerebral cortex and leptomeninges. This condition is age-related, especially prevalent in Alzheimer's disease (AD), and the main feature of certain hereditary disorders (i.e., HCHWA-I). The vascular smooth muscle cells (VSMCs) appear to play a vital role in the development of CAA, which makes them well suited as an experimental model to study the disease and screen for possible remedies. We describe two different methods for isolating and culturing human VSMCs. First, using the human umbilical cord as an easy source of robust cells, and secondly, using brain tissue that provides the proper cerebral VSMCs, but is more problematic to work with. The umbilical cord also provides human umbilical vascular endothelial cells (HUVECs), useful primary cells for vascular research. Finally, the maintenance, preservation, and characterization of the isolated vascular cells are described.

Aβ-degrading enzymes: potential for treatment of Alzheimer disease

There is increasing evidence that deficient clearance of β-amyloid (Aβ) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aβ-degrading enzymes, including neprilysin (NEP), insulin-degrading enzyme, and endothelin-converting enzyme reduce Aβ levels and protect against cognitive impairment in mouse models of AD. The activity of several Aβ-degrading enzymes rises with age and increases still further in AD, perhaps as a physiological response to minimize the buildup of Aβ. The age- and disease-related changes in expression of more recently recognized Aβ-degrading enzymes (e.g. NEP-2 and cathepsin B) remain to be investigated, and there is strong evidence that reduced NEP activity contributes to the development of cerebral amyloid angiopathy. Regardless of the role of Aβ-degrading enzymes in the development of AD, experimental data indicate that increasing the activity of these enzymes (NEP in particular) has therapeutic potential in AD, although targeting their delivery to the brain remains a major challenge. The most promising current approaches include the peripheral administration of agents that enhance the activity of Aβ-degrading enzymes and the direct intracerebral delivery of NEP by convection-enhanced delivery. In the longer term, genetic approaches to increasing the intracerebral expression of NEP or other Aβ-degrading enzymes may offer advantages.

Neprilysin protects against cerebral amyloid angiopathy and Aβ-induced degeneration of cerebrovascular smooth muscle cells

Neprilysin (NEP), which degrades amyloid-β (Aβ), is expressed by neurons and cerebrovascular smooth muscle cells (CVSMCs). NEP immunolabeling is reduced within cerebral blood vessels of Alzheimer's disease (AD) patients with cerebral amyloid angiopathy (CAA). We have now measured NEP enzyme activity in leptomeningeal and purified cerebral cortical blood vessel preparations from control and AD patients with and without CAA. Measurements were adjusted for smooth muscle actin (SMA) to control for variations in CVSMC content. NEP activity was reduced in CAA, in both controls and AD. In leptomeningeal vessels, NEP activity was related to APOE genotype, being highest in ε2-positive and lowest in ε4-positive brains. To assess the role of NEP in protecting CVSMCs from Aβ toxicity, we measured cell death in primary human adult CVSMCs exposed to Aβ(1-40) , Aβ(1-42) or Aβ(1-40(Dutch variant)) . Aβ(1-42) was most cytotoxic to CVSMCs. Aβ(1-42) -mediated cell death was increased following siRNA-mediated knockdown or thiorphan-mediated inhibition of NEP activity; conversely Aβ(1-42) -mediated cytotoxicity was reduced by the addition of somatostatin and NEP over-expression following transfection with NEP cDNA. Our findings suggest that NEP protects CVSMCs from Aβ toxicity and protects cerebral blood vessels from the development and complications of CAA.

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.

Cerebral amyloid angiopathy: progressive disruption of the neurovascular unit

Cellular elements of the neurovascular unit are essential for the physiological functioning of brain vessels. If any of these vascular elements are disturbed the consequences can be dire. Cerebral amyloid angiopathy (CAA), a disorder caused by the accumulation of amyloid in cerebral vessels, provides a case study of progressive neurovascular unit dysfunction leading to failure of vascular reactivity, smooth muscle cell loss, and eventual frank breakdown of vessel integrity resulting in recurrent and sometimes fatal intracerebral hemorrhage.

Solubilized cystatin C amyloid is cytotoxic to cultured human cerebrovascular smooth muscle cells

Cerebral amyloid angiopathy (CAA) is characterized by the accumulation of amyloid within arteries of the cerebral cortex and leptomeninges. This condition is age related, especially prevalent in Alzheimer's disease (AD) and the main feature of certain hereditary disorders. The vascular smooth muscle cells (VSMC) appear to play a vital role in the development of CAA and have been found to produce the amyloid beta precursor protein (AbetaPP) and process it to Abeta the major component of most CAA amyloid. Moreover, synthesized Abeta has proven to be toxic to cerebral VSMC in culture possibly explaining the disintegration and disappearance of the muscle cells from affected cerebral blood vessels seen in CAA. An aggressive and extremely rare form of CAA, known as Hereditary Cerebral Hemorrhage With Amyloidosis-Icelandic Type (HCHWA-I), exhibits this withdrawal of VSMC as amyloid accumulates in the vessel wall. However, the amyloid in HCHWA-I is made from a variant of cystatin C (L68Q) instead of the more common Abeta. To evaluate possible cytotoxicity in this condition solubilized cystatin C amyloid extracted from HCHWA-I leptomeninges was applied to cerebral smooth muscle cells in culture and was found to kill the cells.

Effects of anoxia and hypoxia on amyloid precursor protein processing in cerebral microvascular smooth muscle cells

Cerebral amyloid angiopathy (CAA) is characterized by the degeneration of cerebral microvascular smooth muscle cells (MV-SMC) and the replacement of normal vessel wall components by beta-amyloid (Abeta) protein. Little is known regarding the mechanisms of SMC degeneration in CAA. The effects of anoxia on the metabolism of the amyloid precursor protein (APP) were studied to investigate the MV-SMC response to anoxic stress and its possible role in the pathogenesis of CAA. MV-SMC exposed to chronic anoxia (24-48 hours) showed a decrease in expression of the 2 putative alpha-secretase enzymes, mature TACE (TNFalpha-converting enzyme) and ADAM10 (a disintegrin and metalloprotease). A concomitant decrease in the alpha-secretase cleavage products sAPPalpha and C83 was observed. Investigation of mRNA expression showed an increase in TACE and a sharp decrease in ADAM10 at 24 hours. Exposing MV-SMC to hypoxia (1% O2) revealed a different pattern of expression with no significant change in TACE protein, but an increase in TACE mRNA occurring at a later time point (48 hours). There was no change in ADAM10 mRNA expression, but a reduction in mature ADAM10 with a parallel increase in immature ADAM10 protein. These results demonstrate a requirement for oxygen in the regulation of the alpha-secretase pathway during APP metabolism.

Mechanism of cerebral beta-amyloid angiopathy: murine and cellular models

Cerebral amyloid angiopathy of the beta-amyloid type (Abeta-CAA) is a risk factor for hemorrhagic stroke and independently is believed to contribute to dementia. Naturally occurring animal models of Abeta-CAA are scarce and not well suited for the laboratory. To this end, a variety of transgenic mouse models have been developed that, similar to cerebral Abeta-amyloidosis in humans, develop either Abeta-CAA only or both Abeta-CAA and parenchymal amyloid, or primarily parenchymal amyloid with only scarce Abeta-CAA. The lessons learned from these mouse models are: i) Abeta-CAA alone is sufficient to induce cerebral hemorrhage and associate pathologies including neuroinflammation, ii) the origin of vascular amyloid is mainly neuronal, iii) Abeta-CAA results largely from impaired Abeta clearance, iv) a high ratio Abeta40:42 favors vascular over parenchymal amyloidosis, and v) genetic risk factors such as ApoE modulate Abeta-CAA and CAA-induced hemorrhages. Therapeutic strategies to inhibit Abeta-CAA are poor at the present time. Once Abeta-CAA is present current Abeta immunotherapy strategies have failed to clear vascular amyloid and even run the risk of serious side effects. Despite this progress in deciphering the pathomechanism of Abeta-CAA, with these first generation mouse models of Abeta-CAA, refining these models is needed and will help to understand the emerging importance of Abeta-CAA for dementia and to develop biomarkers and therapeutic strategies.

Cerebral amyloid angiopathy with severe secondary vascular pathology: a histopathological study

Cerebral amyloid angiopathy (CAA) is a common neuropathological finding and is characterized by deposition of fibrillar amyloid in cortical and leptomeningeal vessels. In this study we describe the macroscopic and microscopic neuropathological findings of 5 patients with severe CAA-associated secondary vascular changes, including smooth muscle cell degeneration, hyalinization, 'double-barreling' phenomenon, macrophage infiltration, and aneurysmal dilatation of the vessel wall. In 3 of the 5 patients these vascular changes were associated with multiple small hemorrhages, whereas in 2 patients areas of ischemic necrosis were observed. However, none of these patients suffered from large (lobar) hemorrhagic accidents. Nevertheless, severe CAA, particularly when associated with secondary vascular pathology, may lead to vascular dementia-like ischemic changes. Hence, the distinction between patients with severe CAA and secondary vascular abnormalities from those suffering from vascular dementia can be difficult. We speculate that CAA, particularly when associated with secondary vascular pathology, although not resulting in large hemorrhages, may contribute to cognitive decline. The functional impact of CAA and CAA-related secondary vascular changes on cognitive performance warrants further exploration.

Vascular smooth muscle actin is reduced in Alzheimer disease brain: a quantitative analysis

We analyzed smooth muscle actin (SMA) immunoreactivity in brain blood vessels of 10 ApoE 4,4 Alzheimer disease (AD) patients and 10 ApoE 3,3 AD patients matched for age, sex, and duration of dementia. We also examined 10 cognitively and neuropathologically normal controls matched for age and sex. Vascular SMA immunoreactivity in the arachnoid, grey matter, and white matter was quantified by image analysis. There was less SMA immunoreactivity in blood vessels of all AD patients when compared to cognitively and neuropathologically normal controls (p < 0.001). In addition, arachnoidal vessels of ApoE 4,4 AD patients had less SMA immunoreactivity than ApoE 3,3 AD patients (p < 0.05). There is decreased vascular SMA density in arachnoid, grey matter, and white matter blood vessels in patients with AD when compared to age matched, cognitively and neuropathologically normal controls. The severity of the loss of SMA within the AD group may depend on ApoE type.

Cerebral amyloid angiopathy and thrombolysis-related intracerebral haemorrhage

Intracerebral haemorrhage is a complication of thrombolytic therapy for acute myocardial infarction, pulmonary embolism, and ischaemic stroke. There is increasing evidence that cerebral amyloid angiopathy (CAA), which itself can cause haemorrhage (CAAH), may be a risk factor for thrombolysis-related intracerebral haemorrhage. CAAH and thrombolysis-related intracerebral haemorrhage share some clinical features, such as predisposition to lobar or superficial regions of the brain, multiple haemorrhages, increasing frequency with age, and an association with dementia. In vitro work showed that accumulation of amyloid-beta peptide causes degeneration of cells in the walls of blood vessels, affects vasoactivity, and improves proteolytic mechanisms, such as fibrinolysis, anticoagulation, and degradation of the extracellular matrix. In a mouse model of CAA there is a low haemorrhagic threshold after thrombolytic therapy compared with that in wild-type mice. To date only a small number of anecdotal clinicopathological relations have been reported; neuroimaging advances and further study of the frequency and role of CAA in patients with thrombolysis-related intracerebral haemorrhage are required.

Pathogenesis of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is the result of the deposition of an amyloidogenic protein in cortical and leptomeningeal vessels. The most common type of CAA is caused by amyloid beta-protein (Abeta), which is particularly associated with Alzheimer's disease (AD). Excessive Abeta-CAA formation can be caused by several mutations in the Abeta precursor protein and presenilin genes. The origin of Abeta in CAA is likely to be neuronal, although cerebrovascular cells or the circulation cannot be excluded as a source. Despite the apparent similarity, the pathogenesis of CAA appears to differ from that of senile plaques in several aspects, including the mechanism of Abeta-induced cellular toxicity, the extent of inflammatory reaction and the role of oxidative stress. Therefore, therapeutic strategies for AD should, at least in part, also target CAA. Moreover, CAA and cerebrovascular disease (CVD) may set a lower threshold for AD-like changes to cause dementia and may even cause dementia on its own, since patients with AD and CAA and/or CVD appear to be more cognitively impaired than patients with only AD. In conclusion, the precise impact of CAA on AD or dementia remains unclear, however, its role may have been underestimated in the past, and more extensive studies of in vitro and in vivo models for CAA will be needed to elucidate the importance of CAA-specific approaches in designing intervention strategies for AD.

Amyloid beta-protein stimulates the expression of urokinase-type plasminogen activator (uPA) and its receptor (uPAR) in human cerebrovascular smooth muscle cells

The accumulation of fibrillar amyloid-beta protein (A beta) in cerebral blood vessels, a condition known as cerebral amyloid angiopathy (CAA), is a key pathological feature of Alzheimer's disease and certain related disorders and is intimately associated with cerebrovascular cell death both in vivo and in vitro. Moreover, severe CAA leads to loss of vessel wall integrity and cerebral hemorrhage. Although the basis for these latter pathological consequences in CAA remains unresolved alterations in local proteolytic mechanisms may be involved. Here we show that pathogenic forms of A beta stimulate the expression of plasminogen activator activity in cultured human cerebrovascular smooth muscle (HCSM) cells, an in vitro model of CAA. RNase protection assays and plasminogen zymography showed that urokinase-type plasminogen activator (uPA) was responsible for this activity. There was preferential accumulation of uPA on the HCSM cell surface that was mediated through a concomitant increase in expression of the uPA receptor. In the presence of plasminogen there was robust degradation of A beta that was added to the HCSM cells resulting in restoration of cell viability. This suggests that increased expression of uPA may initially serve as a protective mechanism leading to localized degradation and clearance of the pathogenic stimulus A beta. On the other hand, chronic expression of uPA and plasminogen activation led to a profound loss of HCSM cell attachment. This suggests that a similar prolonged effect in vivo in the cerebral vessel wall may contribute to loss of integrity and cerebral hemorrhage in CAA.

Localization of a fibrillar amyloid beta-protein binding domain on its precursor

Deposition of fibrillar amyloid-beta protein (Abeta) in senile plaques and in the walls of cerebral blood vessels is a key pathological feature of Alzheimer's disease and certain related disorders. Fibrillar Abeta deposition is intimately associated with neuronal and cerebrovascular cell death both in vivo and in vitro. Similarly, accumulation of the Abeta protein precursor (AbetaPP) is also observed at sites of fibrillar Abeta deposition. Recently, we reported that fibrillar Abeta, but not unassembled Abeta, promotes the specific binding of AbetaPP through its cysteine-rich, amino-terminal region (Melchor, J. P., and Van Nostrand, W. E. (2000) J. Biol. Chem. 275, 9782-9791). In the present study we sought to determine the precise site on AbetaPP that facilitates its binding to fibrillar Abeta. A series of synthesized overlapping peptides spanning the cysteine-rich, amino-terminal region of AbetaPP were used as competitors for AbetaPP binding to fibrillar Abeta. A peptide spanning residues 105-119 of AbetaPP competitively inhibited AbetaPP binding to fibrillar Abeta in a solid-phase binding assay and on the surface of cultured human cerebrovascular smooth muscle cells. Alanine-scanning mutagenesis of residues 105-117 within glutathione S-transferase (GST)-AbetaPP-(18-119) revealed that His(110), Val(112), and Ile(113) are key residues that facilitate AbetaPP binding to fibrillar Abeta. These specific residues belong to a common beta-strand within this region of AbetaPP. Wild-type GST-AbetaPP-(18-119) protected cultured human cerebrovascular smooth muscle cells from Abeta-induced toxicity whereas H110A mutant GST-AbetaPP-(18-119) did not. Wild-type GST-AbetaPP-(18-119) bound to different isoforms of fibrillar Abeta and fibrillar amylin peptides whereas H110A mutant and I113A mutant GST-AbetaPP-(18-119) were substantially less efficient binding to each fibrillar peptide. We conclude that His(110), Val(112), and Ile(113), residing in a common beta-strand region within AbetaPP-(18-119), comprise a domain that mediates the binding of AbetaPP to fibrillar peptides.

Dense-core senile plaques in the Flemish variant of Alzheimer's disease are vasocentric

Alzheimer's disease (AD) is characterized by deposition of beta-amyloid (Abeta) in diffuse and senile plaques, and variably in vessels. Mutations in the Abeta-encoding region of the amyloid precursor protein (APP) gene are frequently associated with very severe forms of vascular Abeta deposition, sometimes also accompanied by AD pathology. We earlier described a Flemish APP (A692G) mutation causing a form of early-onset AD with a prominent cerebral amyloid angiopathy and unusually large senile plaque cores. The pathogenic basis of Flemish AD is unknown. By image and mass spectrometric Abeta analyses, we demonstrated that in contrast to other familial AD cases with predominant brain Abeta42, Flemish AD patients predominantly deposit Abeta40. On serial histological section analysis we further showed that the neuritic senile plaques in APP692 brains were centered on vessels. Of a total of 2400 senile plaque cores studied from various brain regions from three patients, 68% enclosed a vessel, whereas the remainder were associated with vascular walls. These observations were confirmed by electron microscopy coupled with examination of serial semi-thin plastic sections, as well as three-dimensional observations by confocal microscopy. Diffuse plaques did not associate with vessels, or with neuritic or inflammatory pathology. Together with earlier in vitro data on APP692, our analyses suggest that the altered biological properties of the Flemish APP and Abeta facilitate progressive Abeta deposition in vascular walls that in addition to causing strokes, initiates formation of dense-core senile plaques in the Flemish variant of AD.

Beta amyloid fragments derived from activated platelets deposit in cerebrovascular endothelium: usage of a novel blood brain barrier endothelial cell model system

Amyloid precursor protein (A betaPP) processing results in generation of amyloid beta peptide (A beta) which deposits in the brain parenchyma and cerebrovasculature of patients with Alzheimer's disease (AD). Evidence that the vascular deposits derive in part from A betaPP fragments originating from activated platelets includes findings that individuals who have had multiple small strokes have a higher prevalence of AD compared to individuals who have taken anti-platelet drugs. Thus, determination of whether platelet A betaPP fragments are capable of traversing the blood-brain barrier (BBB) is critical. We have established that activated platelets from patients with AD retain more surface transmembrane-bound A betaPP (mA betaPP) than control platelets. We report here that this mA betaPP can be cleaved to A beta-containing fragments which pass through a novel BBB model system. This model utilizes human BBB endothelial cells (BEC) isolated from brains of patients with AD. These BEC, after exposure to activated platelets which have been surface-labeled with fluorescein and express surface-retained mA betaPP, cleave fluorescein-tagged surface proteins, including mA betaPP, resulting in passage to the BEC layer The data confirm that BEC contribute to processing of platelet-derived mA betaPP and show that the processing yields A beta containing fragments which could potentially contribute to cerebrovascular A beta deposition.

The genetics of the amyloidoses

The amyloidoses are diseases in which abnormalities in the secondary structure of precursor proteins result in decreased solubility under physiologic conditions, with subsequent organ compromise. A total of 18 proteins have been definitively identified as amyloid precursors associated with human disease. Mutations in the genes that encode some of these proteins produce autosomal dominant disease in mid to late adult life. Until recently, the late onset has obscured the familial nature of some of the disorders. This is especially true in the apparently sporadic disease-producing deposits found even later in life. In many instances, these deposits are derived from precursors encoded by wild-type genes (perhaps influenced by alleles that are polymorphic in the normal population); in other cases, they represent autosomal dominant disease with age-dependent penetrance. The genetic aspects of amyloid diseases produced by the deposition of four different proteins have been investigated in detail and provide insights into the particular diseases and amyloidogenesis in general.

Cerebrovascular smooth muscle cell surface fibrillar A beta. Alteration of the proteolytic environment in the cerebral vessel wall

Cerebrovascular deposition of the amyloid beta-protein (A beta) is a common pathologic event in patients with Alzheimer's disease (AD) and certain related disorders including hereditary cerebral hemorrhage with amyloidosis Dutch-type (HCHWA-D). A beta deposition occurs primarily in the medial layer of the cerebral vessel wall in an assembled fibrillar state. These deposits are associated with several pathological responses including degeneration of the smooth muscle cells in the cerebral vessel wall. Severe cases of cerebrovascular A beta deposition are also accompanied by loss of vessel wall integrity and hemorrhagic stroke. Although the reasons for this pathological consequence are unclear, altered proteolytic mechanisms within the cerebral vessel wall may be involved. Recent studies from our laboratory have shown that cell-surface assembly of A beta into fibrillar structures causes cellular degeneration via an apoptotic pathway and creates an altered proteolytic microenvironment on the cell surface of human cerebrovascular smooth muscle cells (HCSM cells). For example, HCSM cell-surface A beta fibrils serve as a site for tight binding of cell-secreted amyloid beta-precursor protein (A beta PP). Since A beta PP is a potent inhibitor of key proteinases of coagulation cascade, its enhanced localization on the A beta fibrils would provide an strong anticoagulant environment. In addition, HCSM cell-surface A beta fibrils are potent stimulators of tissue plasminogen activator (tPA) creating a profibrinolytic milieu. Our findings indicate that A beta fibril assembly on the HCSM cell surface causes cellular degeneration and results in both a strong anticoagulant and fibrinolytic environment. Together, these altered proteolytic events could create a setting that is conducive to loss of vessel wall integrity and hemorrhagic stroke.

Role of aberrant nitric oxide synthase-3 expression in cerebrovascular degeneration and vascular-mediated injury in Alzheimer's disease

Nitric oxide (NO) is an important signaling molecule that is generated through the catalytic activity of nitric oxide synthase (NOS). In the brain, NO mediates neuronal survival, synaptic plasticity, vascular smooth muscle relaxation, and endothelial cell permeability. Previous studies demonstrated aberrant expression of the NOS-III gene in neurons and glial cells in brains with Alzheimer's disease (AD). Since NOS-III is also expressed in vascular cells, and cerebrovascular disease (CVD) frequently complicates the pathology of AD, we investigated the role of NOS-III in relation to CVD in AD. Vasculopathy in AD + CVD was characterized by thickening and hyalinization of the media of small and medium-size vessels, variable degrees of beta-amyloid (A beta) deposition, and increased apoptosis of vascular smooth muscle and endothelial cells, particularly involving white matter vessels. These abnormalities were correlated with reduced levels of NOS-III expression in cerebral vessels. Double-labeling studies demonstrated that the low levels of cerebrovascular NOS-III were associated with increased levels of the pro-apoptosis gene product, p53 in smooth muscle and endothelial cells, suggesting a role for altered NOS-III expression in AD-associated vascular degeneration. Constitutively reduced cerebrovascular NOS-III expression and NO production could also lead to cerebral hypoperfusion due to impaired vasodilation responses, and diminished capacity to remove respiratory waste products and toxins from the extracellular space due to reduced capillary permeability. The role for phosphodiesterases as modulators of NOS activity is discussed, as these molecules represent potential therapeutic targets given their cell type and cyclic nucleotide specificities of action.

Fibrillar amyloid beta-protein mediates the pathologic accumulation of its secreted precursor in human cerebrovascular smooth muscle cells

Cerebrovascular deposition of the amyloid beta-protein (Abeta) is a key pathologic lesion seen in patients with Alzheimer's disease and certain related disorders, including hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D). The deposition of Abeta has pronounced deleterious effects on smooth muscle cells within the cerebral vessel wall. We have previously shown that Abeta(1-40) possessing the E22Q HCHWA-D mutation extensively assembles into fibrils on the surface of cultured human cerebrovascular smooth muscle (HCSM) cells. This cell-surface Abeta fibril formation induces a series of pathologic responses in cultured HCSM cells, including a marked increase in the levels of cell-associated amyloid beta-protein precursor (AbetaPP) and cell death. In the present study, we investigated the relationship between HCSM cell-surface Abeta fibril formation and the striking increase in cell-associated AbetaPP. Time course studies showed that cell-surface HCHWA-D Abeta(1-40) fibril formation occurred rapidly, whereas both the increase in cell-associated AbetaPP and loss of cell viability were delayed responses. Domain analysis using site-specific antibodies indicated that the vast majority of the increase in cell-associated AbetaPP was secreted AbetaPP (sAbetaPP). Localization studies showed that the sAbetaPP was present on the HCSM cell surface. This result raised the possibility that sAbetaPP may bind back to HCSM cell-surface fibrils formed by HCHWA-D Abeta(1-40). Indeed, binding of biotinylated sAbetaPP to fibrillar HCHWA-D Abeta(1-40) was demonstrated by transmission electron microscopy. Furthermore, solid-phase binding assays showed that biotinylated sAbetaPP exhibited dose-dependent, saturable binding to fibrillar (but not soluble) HCHWA-D Abeta(1-40) with k(d) approximately 28 nM. Exon deletion experiments further defined a fragment of sAbetaPP (AbetaPP(18-119)), encoded by AbetaPP exons 2 and 3, to contain the fibrillar Abeta-binding domain. In addition, AbetaPP(18-119) effectively blocked the cell-surface accumulation of sAbetaPP and subsequent cell death in HCSM cells treated with pathogenic Abeta. Together, these findings could explain the accumulation of AbetaPP in cerebrovascular Abeta deposits observed both in vitro and in vivo and may contribute to the pathologic responses evoked by pathogenic forms of Abeta in HCSM cells.

Aberrant expression of nitric oxide synthase III in Alzheimer's disease: relevance to cerebral vasculopathy and neurodegeneration

Alzheimer's disease (AD) has heterogeneous pathology, in part due to the large subset of cases (AD+CVD) with superimposed vascular lesions that are sufficient in number and distribution to accelerate the clinical course of dementia. Brains with AD+CVD have lower densities of neurofibrillary tangles and A beta-amyloid diffuse plaques, and increased numbers of cerebral vessels exhibiting p53-associated apoptosis relative to brains with uncomplicated AD. AD and AD+CVD both exhibit altered expression of the nitric oxide synthase 3 (NOS-III) gene; however, in AD+CVD, reduced NOS-III expression in cerebral vessels is associated with an increased frequency of vascular lesions, vascular smooth muscle cell apoptosis, and A beta-amyloid plaques. In contrast, experimental and spontaneous focal acute and subacute cerebral infarcts are associated with increased NOS-III expression in perifocal neurons, glial cells, cerebrovascular smooth muscle and endothelial cells, and diffuse A beta-amyloid plaque formation. This suggests that ischemic injury and oxidative stress can precipitate NOS-III-mediated cell loss and neurodegeneration. A role for aging-associated impaired mitochondrial function as a contributing factor in AD and CVD is suggested by the reduced levels of mitochondrial protein observed in AD and AD+CVD cortical neurons and vascular smooth muscle and endothelial cells. The aggregate findings suggest that cell loss and neurodegeneration may be mediated by somewhat distinct but overlapping mechanisms in AD and AD+CVD.

Brain endothelial cell enzymes cleave platelet-retained amyloid precursor protein

We have previously demonstrated that thrombin-activated platelets from patients with advanced Alzheimer's disease (AD) retain significantly more surface membrane-bound amyloid precursor protein (mAPP) than platelets from non-demented age-matched individuals (AM). We have studied interactions between these platelets and the cerebrovascular endothelium to which activated platelets adhere in a model system, investigating their involvement in the formation of amyloid beta peptide (Abeta) deposits in AD patients. We report here that there appear to be alpha and beta secretase-like activities in primary human blood brain barrier endothelial cell (BEC) cultures from both AD patients and AM control subjects (AD-BEC and AM-BEC, respectively) as well as a gamma secretase-like activity that appears only in AD-BEC. No such activities were observed in human umbilical vein endothelial cells (HUVECs). Furthermore, there is more penetration of the platelet-released products platelet factor 4 and soluble APP through the BEC layer grown from AD patients than that grown from AM individuals, whereas none penetrate through a HUVEC layer. Thus the interaction between platelets, the APP they have retained or released, and cerebral vascular endothelial cells may be at least partially responsible for amyloidogenic deposits around the cerebral vasculature of AD patients.

Blood brain barrier endothelial cells express candidate amyloid precursor protein-cleaving secretases

Proteolytic cleavage of the amyloid precursor protein (A beta PP) results in the generation of the amyloidogenic fragment known as amyloid beta peptide (A beta). Deposition of A beta in the brain parenchyma and cerebrovasculature is a feature of Alzheimer's disease (AD). To date, the process whereby A beta is generated and deposited remains unclear. We have previously established that activated platelets from AD patients retain more A beta PP on their surface than control platelets. We report here that an endothelial cell-derived enzyme can cleave this surface platelet A beta PP. Human blood brain barrier endothelial cells from brains of AD patients were assayed for potential A beta PP-cleaving enzymes using synthetic peptide substrates encompassing the A beta N-terminus cleavage site. A protease activity capable of cleaving A beta PP on the surface of AD platelets was noted. The A beta PP cleavage is partially inhibited by EDTA, by ZincOV, as well as by a specific inhibitor of the Zn metalloprotease E.C.3.4.24.15. Furthermore, the protease is recognized by an antibody directed against it, using immunohistochemistry, Western blot analysis and flow cytometry. The protease is not secreted, but rather resides intracellularly as well as on the surface of the endothelial cells. The data suggest that E.C.3.4.24.15 synthesized by brain endothelial cells may process the platelet-derived A beta PP, yielding fragments which could contribute to cerebrovascular A beta deposits.

Aging and cerebrovascular disease

Aging influences cerebrovascular disease expression by a variety of mechanisms. Age-related changes in cerebral autoregulation, cellular metabolism, the blood-brain barrier, and autonomic function may leave the cerebrovascular system vulnerable to injury. Certain cerebrovascular disease, such as atrial fibrillation, watershed infarctions, carotid artery atherosclerosis, cerebral hemorrhages, subdural hematomas, and transient global amnesia manifest in the elderly. Vascular dementia and white matter disease are better understood with newer neuroimaging studies, careful neuropsychological and histopathologic examinations. Atherosclerosis and cerebral amyloid angiopathy may have larger roles than previously understood in Alzheimer's disease.

Brain microvascular changes in Alzheimer's disease and other dementias

Vasculopathy in Alzheimer's disease (AD) may represent an important pathogenetic factor of this disorder. In the present study, microvasculature was studied by immunohistochemistry using a monoclonal antibody against a vascular heparan sulfate proteoglycan. Vascular changes were consistently observed in AD and included decrease in vascular density, presence of atrophic and coiling vessels, and glomerular loop formations. The laminar and regional distribution of these vascular alterations was correlated with the presence of neurofibrillary tangles. However, vascular changes may also follow neuronal loss. Vascular density may be related to a decrease in brain metabolism. Furthermore, one of the main features of AD is the presence of amyloid deposits within brain parenchyma and blood vessel walls. It is not yet clear whether amyloid components are derived from the blood or the central nervous system. Because AD is clearly heterogeneous, based on clinical and genetic data, evidence for either a brain or peripheral origin is discussed. Microvasculature was also analyzed in other neurodegenerative disorders devoid of amyloid deposits including amyotrophic lateral sclerosis/parkinsonism-dementia complex of Guam and Pick's disease. In conclusion, if vasculopathy in neurodegenerative disorders is not directly involved in pathogenesis, it may act synergistically with other pathogenetic mechanisms including genetic and environmental factors. This aspect of pathology is particularly interesting in view of its accessibility to therapeutic interventions.

Chloroquine administration in mice increases beta-amyloid immunoreactivity and attenuates kainate-induced blood-brain barrier dysfunction

The anti-malarial drug chloroquine (CHL) has been reported to cause the accumulation of beta-amyloid peptide containing fragments (fA beta) of the amyloid precursor protein within lysosomes in vitro. However, the significance of this finding with regards to the development of Alzheimer's disease (AD) pathology in vivo is not known. Hence, we investigated the effects of chronic CHL administration in the mouse. Systemically administered CHL caused an astrocytic response and an increase in intracellular A beta immunoreactivity throughout the brain, but no plaque-like pathology. Pharmacological challenge with the excitotoxin kainic acid (KA) revealed a mild proconvulsant effect of CHL pretreatment (P < 0.06). Interestingly, CHL protected the blood-brain barrier from characteristic KA-induced dysfunction. Given the hypothesized involvement of both excitotoxic processes and the vascular system in AD, the observed interactions may assist in elucidating the pathogenesis of AD.