Analysis of cerebral amyloid angiopathy in a transgenic mouse model of Alzheimer disease using in vivo multiphoton microscopy

Loss of spontaneous vasomotion precedes impaired cerebrovascular reactivity and microbleeds in a mouse model of cerebral amyloid angiopathy

Background

Cerebral amyloid angiopathy (CAA) is a cerebral small vessel disease in which amyloid-β accumulates in vessel walls. CAA is a leading cause of symptomatic lobar intracerebral hemorrhage and an important contributor to age-related cognitive decline. Recent work has suggested that vascular dysfunction may precede symptomatic stages of CAA, and that spontaneous slow oscillations in arteriolar diameter (termed vasomotion), important for amyloid-β clearance, may be impaired in CAA.

Methods

To systematically study the progression of vascular dysfunction in CAA, we used the APP23 mouse model of amyloidosis, which is known to develop spontaneous cerebral microbleeds mimicking human CAA. Using in vivo 2-photon microscopy, we longitudinally imaged unanesthetized APP23 transgenic mice and wildtype littermates from 7 to 14 months of age, tracking amyloid-β accumulation and vasomotion in individual pial arterioles over time. MRI was used in separate groups of 12-, 18-, and 24-month-old APP23 transgenic mice and wildtype littermates to detect microbleeds and to assess cerebral blood flow and cerebrovascular reactivity with pseudo-continuous arterial spin labeling.

Results

We observed a significant decline in vasomotion with age in APP23 mice, while vasomotion remained unchanged in wildtype mice with age. This decline corresponded in timing to initial vascular amyloid-β deposition (∼8-10 months of age), although was more strongly correlated with age than with vascular amyloid-β burden in individual arterioles. Declines in vasomotion preceded the development of MRI-visible microbleeds and the loss of smooth muscle actin in arterioles, both of which were observed in APP23 mice by 18 months of age. Additionally, evoked cerebrovascular reactivity was intact in APP23 mice at 12 months of age, but significantly lower in APP23 mice by 24 months of age.

Conclusions

Our findings suggest that a decline in spontaneous vasomotion is an early, potentially pre-symptomatic, manifestation of CAA and vascular dysfunction, and a possible future treatment target.

Diffuse white matter loss in a transgenic rat model of cerebral amyloid angiopathy

Diffuse white matter (WM) disease is highly prevalent in elderly with cerebral small vessel disease (cSVD). In humans, cSVD such as cerebral amyloid angiopathy (CAA) often coexists with Alzheimer's disease imposing a significant impediment for characterizing their distinct effects on WM. Here we studied the burden of age-related CAA pathology on WM disease in a novel transgenic rat model of CAA type 1 (rTg-DI). A cohort of rTg-DI and wild-type rats was scanned longitudinally using MRI for characterization of morphometry, cerebral microbleeds (CMB) and WM integrity. In rTg-DI rats, a distinct pattern of WM loss was observed at 9 M and 11 M. MRI also revealed manifestation of small CMB in thalamus at 6 M, which preceded WM loss and progressively enlarged until the moribund disease stage. Histology revealed myelin loss in the corpus callosum and thalamic CMB in all rTg-DI rats, the latter of which manifested in close proximity to occluded and calcified microvessels. The quantitation of CAA load in rTg-DI rats revealed that the most extensive microvascular Aβ deposition occurred in the thalamus. For the first time using in vivo MRI, we show that CAA type 1 pathology alone is associated with a distinct pattern of WM loss.

Idiopathic degenerative thoracic aneurysms are associated with increased aortic medial amyloid

Objective: To explore the relationship of aortic medial amyloid with biochemical and micromechanical properties of the aortic wall in aneurysm patients. Methods: Human aortic tissues removed during aneurysm surgery from tricuspid (idiopathic degenerative aneurysm, DA) and bicuspid valve (BAV) patients were subjected to oscillatory nanoindentation experiments to determine localised mechanical properties of the tissue (shear storage modulus, G´ and shear loss modulus, G˝). Collagen, elastin, matrix metalloproteinase 2 and glycosaminoglycans concentrations were determined, along with relative levels of aortic medial amyloid-related factors (medin, milk fat globule-EGF factor 8, oligomers and fibrils). Measurements were combined with clinical data and statistical analyses performed. Results: The DA cohort can be divided based on their phenotype. One group shared similar characteristics with BAV patients, termed bicuspid like phenotype-tricuspid valve. The second group had high amyloid oligomer species present with a significantly lower G´ (p = .01), indicative of reduced elastic response of the tissue, termed amyloid-rich. Conclusions: We identified a group of DA patients with high amyloid oligomers and altered micromechanical and structural properties of the vessel wall. We propose these findings as a cause for aneurysm formation in these patients. Amyloid is not found in BAV patients, suggesting at least two distinct mechanisms for pathogenesis.

Animal models of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA), due to vascular amyloid β (Aβ) deposition, is a risk factor for intracerebral haemorrhage and dementia. CAA can occur in sporadic or rare hereditary forms, and is almost invariably associated with Alzheimer's disease (AD). Experimental (animal) models are of great interest in studying mechanisms and potential treatments for CAA. Naturally occurring animal models of CAA exist, including cats, dogs and non-human primates, which can be used for longitudinal studies. However, due to ethical considerations and low throughput of these models, other animal models are more favourable for research. In the past two decades, a variety of transgenic mouse models expressing the human Aβ precursor protein (APP) has been developed. Many of these mouse models develop CAA in addition to senile plaques, whereas some of these models were generated specifically to study CAA. In addition, other animal models make use of a second stimulus, such as hypoperfusion or hyperhomocysteinemia (HHcy), to accelerate CAA. In this manuscript, we provide a comprehensive review of existing animal models for CAA, which can aid in understanding the pathophysiology of CAA and explore the response to potential therapies.

Ischemic brain injury in cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is a common form of cerebral small vessel disease and an important risk factor for intracerebral hemorrhage and cognitive impairment. While the majority of research has focused on the hemorrhagic manifestation of CAA, its ischemic manifestations appear to have substantial clinical relevance as well. Findings from imaging and pathologic studies indicate that ischemic lesions are common in CAA, including white-matter hyperintensities, microinfarcts, and microstructural tissue abnormalities as detected with diffusion tensor imaging. Furthermore, imaging markers of ischemic disease show a robust association with cognition, independent of age, hemorrhagic lesions, and traditional vascular risk factors. Widespread ischemic tissue injury may affect cognition by disrupting white-matter connectivity, thereby hampering communication between brain regions. Challenges are to identify imaging markers that are able to capture widespread microvascular lesion burden in vivo and to further unravel the etiology of ischemic tissue injury by linking structural magnetic resonance imaging (MRI) abnormalities to their underlying pathophysiology and histopathology. A better understanding of the underlying mechanisms of ischemic brain injury in CAA will be a key step toward new interventions to improve long-term cognitive outcomes for patients with CAA.

Matrix metalloproteinase 9-mediated intracerebral hemorrhage induced by cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA), the deposition of amyloid-β in cerebrovascular walls, is the most common cause of lobar hemorrhagic stroke. Previous studies show that cerebrovascular amyloid-β induces expression and activation of matrix metalloproteinase 9 (MMP-9) in cerebral vessels of amyloid precursor protein transgenic mice. Here, we extended these findings and evaluated MMP-9 expression in postmortem brain tissues of human CAA cases. MMP-9 colocalized with CAA, correlated with the severity of the vascular pathology, and was detected in proximity to microbleeds. We characterized a novel assay using longitudinal multiphoton microscopy and a novel tracer to visualize and quantify the magnitude and kinetics of hemorrhages in three dimensions in living mouse brains. We demonstrated that topical application of recombinant MMP-9 resulted in a time- and dose-dependent cerebral hemorrhage. Amyloid precursor protein mice with significant CAA developed more extensive hemorrhages which also appeared sooner after exposure to MMP-9. Our data suggest an important role for MMP-9 in development of hemorrhages in the setting of CAA. Inhibition of MMP-9 may present a preventive strategy for CAA-associated hemorrhage.

Translating basic science research to clinical application: models and strategies for intracerebral hemorrhage

Preclinical stroke models provide insights into mechanisms of cellular injury and potential therapeutic targets. Renewed efforts to standardize preclinical practices and adopt more rigorous approaches reflect the assumption that a better class of compounds will translate into clinical efficacy. While the need for novel therapeutics is clear, it is also critical that diagnostics be improved to allow for more rapid treatment upon hospital admission. Advances in imaging techniques have aided in the diagnosis of stroke, yet current limitations and expenses demonstrate the need for new and complementary approaches. Intracerebral hemorrhage (ICH) exhibits the highest mortality rate, displays unique pathology and requires specialized treatment strategies relative to other forms of stroke. The aggressive nature and severe consequences of ICH underscore the need for novel therapeutic approaches as well as accurate and expeditious diagnostic tools. The use of experimental models will continue to aid in addressing these important issues as the field attempts to translate basic science findings into the clinical setting. Several preclinical models of ICH have been developed and are widely used to recapitulate human pathology. Because each model has limitations, the burden lies with the investigator to clearly define the question being asked and select the model system that is most relevant to that question. It may also be necessary to optimize and refine pre-existing paradigms, or generate new paradigms, as the future success of translational research is dependent upon the ability to mimic human sequelae and assess clinically relevant outcome measures as means to evaluate therapeutic efficacy.

APP transgenic mice for modelling behavioural and psychological symptoms of dementia (BPSD)

The discovery of gene mutations responsible for autosomal dominant Alzheimer's disease has enabled researchers to reproduce in transgenic mice several hallmarks of this disorder, notably Aβ accumulation, though in most cases without neurofibrillary tangles. Mice expressing mutated and wild-type APP as well as C-terminal fragments of APP exhibit variations in exploratory activity reminiscent of behavioural and psychological symptoms of Alzheimer dementia (BPSD). In particular, open-field, spontaneous alternation, and elevated plus-maze tasks as well as aggression are modified in several APP transgenic mice relative to non-transgenic controls. However, depending on the precise murine models, changes in open-field and elevated plus-maze exploration occur in either direction, either increased or decreased relative to controls. It remains to be determined which neurotransmitter changes are responsible for this variability, in particular with respect to GABA, 5HT, and dopamine.

Cerebral amyloid angiopathy in the elderly

Cerebral amyloid angiopathy (CAA) results from deposition of β-amyloid in the media and adventitia of small arteries and capillaries of the leptomeninges and cerebral cortex and is a major cause of lobar intracerebral hemorrhage and cognitive impairment in the elderly. CAA is associated with a high prevalence of magnetic resonance imaging markers of small vessel disease, including cerebral microbleeds and white matter hyperintensities. Although advanced CAA is present in approximately ¼ of brains with Alzheimer disease (AD), fewer than half of CAA cases meet pathologic criteria for AD. This review will discuss the pathophysiology of CAA and focus on new imaging modalities and laboratory biomarkers that may aid in the clinical diagnosis of individuals with the disease.

Review: sporadic cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) may result from focal to widespread amyloid-β protein (Aβ) deposition within leptomeningeal and intracortical cerebral blood vessels. In addition, pericapillary Aβ refers to Aβ depositions in the glia limitans and adjacent neuropil, whereas in capillary CAA Aβ depositions are present in the capillary wall. CAA may cause lobar intracerebral haemorrhages and microbleeds. Hypoperfusion and reduced vascular autoregulation due to CAA might cause infarcts and white matter lesions. CAA thus causes vascular lesions that potentially lead to (vascular) dementia and may further contribute to dementia by impeding the clearance of solutes out of the brain and transport of nutrients across the blood brain barrier. Severe CAA is an independent risk factor for cognitive decline. The clinical diagnosis of CAA is based on the assessment of associated cerebrovascular lesions. In addition, perivascular spaces in the white matter and reduced concentrations of both Aβ(40) and Aβ(42) in cerebrospinal fluid may prove to be suggestive for CAA. Transgenic mouse models that overexpress human Aβ precursor protein show parenchymal Aβ and CAA, thus corroborating the current concept of CAA pathogenesis: neuronal Aβ enters the perivascular drainage pathway and may accumulate in vessel walls due to increased amounts and/or decreased clearance of Aβ, respectively. We suggest that pericapillary Aβ represents early impairment of the perivascular drainage pathway while capillary CAA is associated with decreased transendothelial clearance of Aβ. CAA plays an important role in the multimorbid condition of the ageing brain but its contribution to neurodegeneration remains to be elucidated.

Cocaine-induced intracerebral hemorrhage in a patient with cerebral amyloid angiopathy

Intracerebral hemorrhage (ICH) is a well-recognized complication of recreational cocaine use. The precise mechanism of the cocaine-induced hemorrhagic event is unclear, although multiple factors have been implicated. We report a case of a 62-year-old woman who suffered left parieto-occipital ICH with herniation and death, following a cocaine binge. Microscopic examination also revealed extensive cerebral amyloid angiopathy (CAA) in the vicinity of the hemorrhage. We additionally studied brain tissue in eight subjects between ages of 60 and 80 who were positive for cocaine metabolites at autopsy; of these, none had vascular amyloid-β deposits by immunohistochemistry. Whereas we found no evidence that chronic cocaine use is a risk factor for CAA, given the age-associated nature of CAA and the aging population using cocaine, CAA-induced hemorrhage in the setting of cocaine use may be more common than recognized. This is the first reported case of CAA-associated ICH precipitated by cocaine.

Aging and cerebrovascular dysfunction: contribution of hypertension, cerebral amyloid angiopathy, and immunotherapy

Age-related cerebrovascular dysfunction contributes to ischemic stroke, intracerebral hemorrhages (ICHs), microbleeds, cerebral amyloid angiopathy (CAA), and cognitive decline. Importantly, there is increasing recognition that this dysfunction plays a critical secondary role in many neurodegenerative diseases, including Alzheimer's disease (AD). Atherosclerosis, hypertension, and CAA are the most common causes of blood-brain barrier (BBB) lesions. The accumulation of amyloid beta (Aβ) in the cerebrovascular system is a significant risk factor for ICH and has been linked to endothelial transport failure and blockage of perivascular drainage. Moreover, recent anti-Aβ immunotherapy clinical trials demonstrated efficient clearance of parenchymal amyloid deposits but have been plagued by CAA-associated adverse events. Although management of hypertension and atherosclerosis can reduce the incidence of ICH, there are currently no approved therapies for attenuating CAA. Thus, there is a critical need for new strategies that improve BBB function and limit the development of β-amyloidosis in the cerebral vasculature.

Genetics and molecular pathogenesis of sporadic and hereditary cerebral amyloid angiopathies

In cerebral amyloid angiopathy (CAA), amyloid fibrils deposit in walls of arteries, arterioles and less frequently in veins and capillaries of the central nervous system, often resulting in secondary degenerative vascular changes. Although the amyloid-beta peptide is by far the commonest amyloid subunit implicated in sporadic and rarely in hereditary forms of CAA, a number of other proteins may also be involved in rare familial diseases in which CAA is also a characteristic morphological feature. These latter proteins include the ABri and ADan subunits in familial British dementia and familial Danish dementia, respectively, which are also known under the umbrella term BRI2 gene-related dementias, variant cystatin C in hereditary cerebral haemorrhage with amyloidosis of Icelandic-type, variant transthyretins in meningo-vascular amyloidosis, disease-associated prion protein (PrP(Sc)) in hereditary prion disease with premature stop codon mutations and mutated gelsolin (AGel) in familial amyloidosis of Finnish type. In this review, the characteristic morphological features of the different CAAs is described and the implication of the biochemical, genetic and transgenic animal data for the pathogenesis of CAA is discussed.

Functional studies in living animals using multiphoton microscopy

In vivo microscopy is a powerful method for studying fundamental issues of physiology and pathophysiology. The recent development of multiphoton fluorescence microscopy has extended the reach of in vivo microscopy, supporting high-resolution imaging deep into the tissues and organs of living animals. As compared with other in vivo imaging techniques, multiphoton microscopy is uniquely capable of providing a window into cellular and subcellular processes in the context of the intact, functioning animal. In addition, the ability to collect multiple colors of fluorescence from the same sample makes in vivo microscopy uniquely capable of characterizing up to three parameters from the same volume, supporting powerful correlative analyses. Since its invention in 1990, multiphoton microscopy has been increasingly applied to numerous areas of medical investigation, providing invaluable insights into cell physiology and pathology. However, researchers have only begun to realize the true potential of this powerful technology as it has proliferated beyond the laboratories of a relatively few pioneers. In this article we present an overview of the advantages and limitations of multiphoton microscopy as applied to in vivo imaging. We also review specific examples of the application of in vivo multiphoton microscopy to studies of physiology and pathology in a variety of organs including the brain, skin, skeletal muscle, tumors, immune cells, and visceral organs.

Small molecule inhibitors of Abeta assembly

The Abeta peptide assembles into a variety of distinct types of structures in vitro and in the brain which have different biological consequences. Differential effects of inhibitory small molecules suggest that a sequential monomer - oligomer - fibril mechanism is overly simplistic and that soluble toxic oligomers and fibrils can be formed in common or separate pathways depending on the local environment. As a result, the effects of inhibitors are often assay-dependent because multiple pathways are operating. This review discusses strategies for teasing apart the intricate protein-protein interactions that result in Abeta assembly.

Kinetics of cerebral amyloid angiopathy progression in a transgenic mouse model of Alzheimer disease

Cerebral amyloid angiopathy (CAA), the deposition of cerebrovascular beta-amyloid (Abeta) in the walls of arterial vessels, has been implicated in hemorrhagic stroke and is present in most cases of Alzheimer disease. Previous studies of the progression of CAA in humans and animal models have been limited to the comparison of pathological tissue from different brains at single time points. Our objective was to visualize in real time the initiation and progression of CAA in Tg2576 mice by multiphoton microscopy through cranial windows. Affected vessels were labeled by methoxy-X04, a fluorescent dye that selectively binds cerebrovascular beta-amyloid and plaques. With serial imaging sessions spaced at weekly intervals, we were able to observe the earliest appearance of CAA in leptomeningeal arteries as multifocal deposits of band-like Abeta. Over subsequent imaging sessions, we were able to identify growth of these deposits (propagation), as well as appearance of new bands (additional initiation events). Statistical modeling of the data suggested that as the extent of CAA progressed in this vascular bed, there was increased prevalence of propagation over initiation. During the early phases of CAA development, the overall pathology burden progressed at a rate of 0.35% of total available vessel area per day (95% confidence interval, 0.3-0.4%). The consistent rate of disease progression implies that this model is amenable to investigations of therapeutic interventions.

Progression of Cerebral Amyloid Angiopathy in Transgenic Mouse Models of Alzheimer Disease

Cerebral amyloid angiopathy (CAA), the deposition of beta-amyloid (Abeta3) in cerebral vessels, has been implicated as a common cause of hemorrhagic stroke and other forms of vascular disease. CAA is also a frequent concomitant of Alzheimer disease (AD). While the longterm consequences of CAA are well recognized from clinical and pathologic studies, numerous questions remain unanswered regarding the progression of the disease. Examination of CAA in traditional histologic sections does not easily allow for characterization of CAA, particularly in leptomeningeal vessels. In order to approach this topic, we used low magnification imaging of intact, postmortem brains from transgenic mouse models of AD-like pathology to define the spatial and temporal characteristics of CAA in leptomeningeal vessels. Imaging of brains from 10- to 26-month-old animals demonstrated a stereotypical pattern to the development of CAA, with vessels over the dorsal surface of the brain showing an anterior-to-posterior and large-to-small vessel gradient of involvement. High magnification imaging revealed that CAA deposition began with a banding pattern determined by the organization of the vascular smooth muscle cells. Further analysis of the pattern of amyloid deposits showed shrinkage and disappearance of the gaps between clusters of amyloid bands, gradually reaching a confluent pattern. These data led to a classification system to describe the severity of CAA deposition and demonstrate the potential of using intact brains to generate maps defining the progression and kinetics of CAA. This approach should lead to more informed analysis of the consequences of evolving therapeutic options for AD on this related form of vascular pathology.

Anti-Abeta antibody treatment promotes the rapid recovery of amyloid-associated neuritic dystrophy in PDAPP transgenic mice

Neuritic plaques are a defining feature of Alzheimer disease (AD) pathology. These structures are composed of extracellular accumulations of amyloid-beta peptide (Abeta) and other plaque-associated proteins, surrounded by large, swollen axons and dendrites (dystrophic neurites) and activated glia. Dystrophic neurites are thought to disrupt neuronal function, but whether this damage is static, dynamic, or reversible is unknown. To address this, we monitored neuritic plaques in the brains of living PDAPP;Thy-1:YFP transgenic mice, a model that develops AD-like pathology and also stably expresses yellow fluorescent protein (YFP) in a subset of neurons in the brain. Using multiphoton microscopy, we observed and monitored amyloid through cranial windows in PDAPP;Thy-1:YFP double-transgenic mice using the in vivo amyloid-imaging fluorophore methoxy-X04, and individual YFP-labeled dystrophic neurites by their inherent fluorescence. In vivo studies using this system suggest that amyloid-associated dystrophic neurites are relatively stable structures in PDAPP;Thy-1:YFP transgenic mice over several days. However, a significant reduction in the number and size of dystrophic neurites was seen 3 days after Abeta deposits were cleared by anti-Abeta antibody treatment. This analysis suggests that ongoing axonal and dendritic damage is secondary to Abeta and is, in part, rapidly reversible.

Amyloid Angiopathy-Related Vascular Cognitive Impairment

We review accumulating evidence that cerebrovascular amyloid deposition (cerebral amyloid angiopathy [CAA]) is an independent risk factor for cognitive dysfunction. The two population-based autopsy studies that have analyzed cognitive status during life as a function of CAA have each suggested deleterious effects of CAA on cognition even after controlling for age and Alzheimer disease pathology. We also review data from patients with CAA-related intracerebral hemorrhage (the one form of CAA that can be noninvasively recognized) suggesting associations of CAA with radiographic white matter abnormalities and cognitive impairment. These data highlight the importance of elucidating the effects of vascular amyloid on cerebrovascular function and of developing therapeutic strategies for this potentially widespread form of microvascular cognitive impairment.

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.

Cerebral amyloid angiopathy: major contributor or decorative response to Alzheimer’s disease pathogenesis

Amyloid deposition within cerebral vessels, or cerebral amyloid angiopathy (CAA), is common in advanced age and even more common in Alzheimer's disease. CAA may be complicated by lobar intracerebral hemorrhage, while rare kindreds of autosomal dominant CAA also show propensity for intracerebral hemorrhage, due to germline mutations in specific amyloidogenic precursor proteins and apparent compromise of structural integrity of the blood vessel wall due to marked amyloid deposition. The relationship between cerebral amyloid angiopathy and cognitive dysfunction, however, is less clear. While cognitive dysfunction in familial CAA is likely related to prodigious amyloid deposits and vascular luminal compromise (e.g., hereditary cerebral hemorrhage with angiopathy-Dutch type (HCHWA-D)), cerebral amyloid angiopathy with intracerebral hemorrhage often presents sporadically in cognitively intact elderly patients. Moreover, while about 80% of subjects with Alzheimer's disease have demonstrable amyloid beta within blood vessel walls at autopsy, the vast majority of these fail to suffer clinically relevant intracerebral hemorrhage during life. The remaining 20% manage to progress and die of their disease with virtual no amyloid within blood vessels. Thus, the role of amyloid beta deposits in cerebral vessels as regards cognitive function on the one hand, and tendency for hemorrhage on the other, remain to be resolved for sporadic late onset Alzheimer's disease and CAA. Recent studies on transgenic APP23 mice suggest a relationship between passive immunization and amyloid angiopathy-associated cerebral hemorrhage, although the mechanism of hemorrhage was unclear from the data presented. We suggest that amyloid accumulation represents a response to chronic stress, and that the neurodegenerative process occurs at the neuronal level, encompassing oxidative stress and aberrant cell cycle activation. As such, CAA represents tissue homeostasis, such that an abrupt perturbation of this balance (e.g., amyloid beta immunization) is deleterious.

Cerebral Amyloid Angiopathies: A Pathologic, Biochemical, and Genetic View

Amyloid deposition can take place in the walls of arteries, arterioles, and, less often, capillaries and veins of the central nervous system, a phenomenon known as cerebral amyloid angiopathy (CAA). The major clinicopathological manifestations of CAA include cerebral hemorrhage, ischemic lesions, and dementia. CAA may be classified according to the amyloid protein deposited. In the most common form, sporadic CAA, and in CAA related to sporadic Alzheimer disease (AD). A beta deposition is characteristic. CAA can also be severe in variants of familial AD caused by mutations of the amyloid-beta precursor protein or presenilin-1 genes in which deposition of A beta variants and/or wild-type A beta occurs. Other amyloid proteins involved in familial CAAs include 1) the mutant cystatin C (ACys) in hereditary cerebral hemorrhage with amyloidosis of Icelandic type, 2) variant transthyretins (ATTR) in meningo-vascular amyloidoses, 3) mutated gelsolin (AGel) in familial amyloidosis of Finnish type, 4) disease-associated prion protein (PrP(Sc)) in a variant of the Gerstmann-Sträussler-Scheinker syndrome, and 5) ABri and ADan in CAAs observed in the recently described BRI2 gene-related dementias, familial British dementia and familial Danish dementia, respectively. This review addresses issues related to the correlation between morphology, biochemistry, and genetics, and briefly discusses both the pathogenesis and animal models of CAAs.

In vivo multiphoton imaging of a transgenic mouse model of Alzheimer disease reveals marked thioflavine-S-associated alterations in neurite trajectories

Postmortem analyses of senile plaques reveal numerous dystrophic processes in their vicinity. We used in vivo multiphoton microscopy of a transgenic model of Alzheimer disease (AD) to simultaneously image senile plaques and nearby neuronal processes. Plaques were labeled by immunofluorescent staining or thioflavine-S and neuronal processes were labeled with a fluorescent dextran conjugate. Imaging of 3-dimensional volumes in the vicinity of plaques revealed subtle changes in neurite geometry in or near diffuse plaques. By contrast, disruptions in neurite morphology, including dystrophic neurites immediately surrounding plaques as well as major alterations in neurite trajectories, were seen in association with thioflavine-S-positive plaques. Nearly half of all labeled processes that came within 50 microm of a thioflavine-S-positive plaque were altered, suggesting a fairly large "halo" of neuropil alterations that extend beyond the discrete border of a thioflavine-S plaque. These results support the hypothesis that compact thioflavine-S-positive plaques disrupt the neuropil in AD.

Vascular changes in Iowa-type hereditary cerebral amyloid angiopathy

Vascular dysfunction due to cerebral amyloid angiopathy (CAA) may contribute to cognitive impairment. The Iowa D694N amyloid precursor protein mutation, a recently identified cause of hereditary CAA with dementia, offers an opportunity to explore the anatomic basis of CAA-related vascular dysfunction. Examination by immunolabeling and confocal microscopy demonstrated extensive loss of smooth muscle cells in affected segments as well as a perivascular inflammatory reaction of astrocytes and microglia. On 3-D reconstruction, vessels appeared tortuous with twiglike projections that may represent areas of vascular degeneration. The observed changes in the Iowa brain suggest pathophysiologic mechanisms for vascular dysfunction in CAA and possible approaches to treatment of CAA-related cognitive impairment.

Imaging Abeta plaques in living transgenic mice with multiphoton microscopy and methoxy-X04, a systemically administered Congo red derivative

The identification of amyloid deposits in living Alzheimer disease (AD) patients is important for both early diagnosis and for monitoring the efficacy of newly developed anti-amyloid therapies. Methoxy-X04 is a derivative of Congo red and Chrysamine-G that contains no acid groups and is therefore smaller and much more lipophilic than Congo red or Chrysamine-G. Methoxy-X04 retains in vitro binding affinity for amyloid beta (Abeta) fibrils (Ki = 26.8 nM) very similar to that of Chrysamine-G (Ki = 25.3 nM). Methoxy-X04 is fluorescent and stains plaques, tangles, and cerebrovascular amyloid in postmortem sections of AD brain with good specificity. Using multiphoton microscopy to obtain high-resolution (1 microm) fluorescent images from the brains of living PSI/APP mice, individual plaques could be distinguished within 30 to 60 min after a single i.v. injection of 5 to 10 mg/kg methoxy-X04. A single i.p. injection of 10 mg/kg methoxy-X04 also produced high contrast images of plaques and cerebrovascular amyloid in PSI/APP mouse brain. Complementary quantitative studies using tracer doses of carbon- 11-labeled methoxy-X04 show that it enters rat brain in amounts that suggest it is a viable candidate as a positron emission tomography (PET) amyloid-imaging agent for in vivo human studies.

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.