Negative association between amyloid plaques and cerebral amyloid angiopathy in Alzheimer's disease

Towards a better diagnosis and treatment of dementia: Identifying common and distinct neuropathological mechanisms in Alzheimer's and vascular dementia

Alzheimer's disease (AD) and vascular dementia (VaD) together contribute to almost 90 % of all dementia cases leading to major health challenges of our time with a substantial global socioeconomic burden. While in AD, the improved understanding of Amyloid beta (Aß) mismetabolism and tau hyperphosphorylation as pathophysiological hallmarks has led to significant clinical breakthroughs, similar advances in VaD are lacking. After comparing the clinical presentation, including risk factors, disease patterns, course of diseases and further diagnostic parameters for both forms of dementia, we highlight the importance of shared pathomechanisms found in AD and VaD: Endothelial damage, blood brain barrier (BBB) breakdown and hypoperfusion inducing oxidative stress and inflammation and thus trophic uncoupling in the neurovascular unit. A dysfunctional endothelium and BBB lead to the accumulation of neurotoxic molecules and Aß through impaired clearance, which in turn leads to neurodegeneration. In this context we discuss possible neuropathological parameters, which might serve as biomarkers and thus improve diagnostic accuracy or reveal targets for novel therapeutic strategies for both forms of dementia.

Traumatic Brain Injury and Chronic Traumatic Encephalopathy: Not Only Trigger for Neurodegeneration but Also for Cerebral Amyloid Angiopathy?

Traumatic brain injury (TBI) has been linked to the development of neurodegenerative diseases, particularly Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). This review critically assesses the relationship between TBI and cerebral amyloid angiopathy (CAA), highlighting the complexities of diagnosing CAA in the context of prior head trauma. While TBI has been shown to facilitate the accumulation of amyloid plaques and tau pathology, the interplay between neurodegenerative processes and vascular contributions remains underexplored. Epidemiological studies indicate that TBI increases the risk of various dementias, not solely AD, emphasizing the need for a comprehensive understanding of TBI-related neurodegeneration as a polypathological condition. This review further delineates the mechanisms by which TBI can lead to CAA, particularly focusing on the vascular changes that occur post-injury. It discusses the challenges associated with diagnosing CAA after TBI, particularly due to the overlapping symptoms and pathologies that complicate clinical evaluations. Notably, this review includes a clinical case that exemplifies the diagnostic challenges posed by TBI in patients with subsequent cognitive decline and vascular pathology. By synthesizing current research on TBI, CAA, and associated neurodegenerative conditions, this review aims to foster a more nuanced understanding of how these conditions interact and contribute to long-term cognitive outcomes. The findings underscore the importance of developing standardized diagnostic criteria and imaging techniques to better elucidate the relationship between TBI and vascular pathology, which could enhance clinical interventions and inform therapeutic strategies for affected individuals.

Prevalence, distribution, and severity of cerebral amyloid angiopathy differ between Lewy body diseases and Alzheimer's disease

Dementia with Lewy bodies (DLB), Parkinson's disease dementia (PDD), and Parkinson's disease (PD) collectively known as Lewy body diseases (LBDs) are neuropathologically characterised by α-synuclein deposits (Lewy bodies and Lewy neurites). However, LBDs also exhibit pathology associated with Alzheimer's disease (AD) (i.e. hyperphosphorylated tau and amyloid β (Aβ). Aβ can be deposited in the walls of blood vessels in the brains of individuals with AD, termed cerebral amyloid angiopathy (CAA). The aim of this study was to investigate the type and distribution of CAA in DLB, PDD, and PD and determine if this differs from AD. CAA type, severity, and topographical distribution was assessed in 94 AD, 30 DLB, 17 PDD, and 11 PD cases, and APOE genotype evaluated in a subset of cases where available. 96.3% AD cases, 70% DLB cases and 82.4% PDD cases exhibited CAA (type 1 or type 2). However only 45.5% PD cases had CAA. Type 1 CAA accounted for 37.2% of AD cases, 10% of DLB cases, and 5.9% of PDD cases, and was not observed in PD cases. There was a hierarchical topographical distribution in regions affected by CAA where AD and DLB displayed the same distribution pattern that differed from PDD and PD. APOE ε4 was associated with severity of CAA in AD cases. Topographical patterns and severity of CAA in DLB more closely resembled AD rather than PDD, and as type 1 CAA is associated with clinical dementia in AD, further investigations are warranted into whether the increased presence of type 1 CAA in DLB compared to PDD are related to the onset of cognitive symptoms and is a distinguishing factor between LBDs. Possible alignment of the the topographical distribution of CAA and microbleeds in DLB warrants further investigation. CAA in DLB more closely resembles AD rather than PDD or PD, and should be taken into consideration when stratifying patients for clinical trials or designing disease modifying therapies.

Loss of cholinergic innervation differentially affects eNOS-mediated blood flow, drainage of Aβ and cerebral amyloid angiopathy in the cortex and hippocampus of adult mice

Vascular dysregulation and cholinergic basal forebrain degeneration are both early pathological events in the development of Alzheimer’s disease (AD). Acetylcholine contributes to localised arterial dilatation and increased cerebral blood flow (CBF) during neurovascular coupling via activation of endothelial nitric oxide synthase (eNOS). Decreased vascular reactivity is suggested to contribute to impaired clearance of β-amyloid (Aβ) along intramural periarterial drainage (IPAD) pathways of the brain, leading to the development of cerebral amyloid angiopathy (CAA). However, the possible relationship between loss of cholinergic innervation, impaired vasoreactivity and reduced clearance of Aβ from the brain has not been previously investigated. In the present study, intracerebroventricular administration of mu-saporin resulted in significant death of cholinergic neurons and fibres in the medial septum, cortex and hippocampus of C57BL/6 mice. Arterial spin labelling MRI revealed a loss of CBF response to stimulation of eNOS by the Rho-kinase inhibitor fasudil hydrochloride in the cortex of denervated mice. By contrast, the hippocampus remained responsive to drug treatment, in association with altered eNOS expression. Fasudil hydrochloride significantly increased IPAD in the hippocampus of both control and saporin-treated mice, while increased clearance from the cortex was only observed in control animals. Administration of mu-saporin in the TetOAPPSweInd mouse model of AD was associated with a significant and selective increase in Aβ40-positive CAA. These findings support the importance of the interrelationship between cholinergic innervation and vascular function in the aetiology and/or progression of CAA and suggest that combined eNOS/cholinergic therapies may improve the efficiency of Aβ removal from the brain and reduce its deposition as CAA.

Clinical Profiles of Arteriolosclerosis and Alzheimer Disease at Mild Cognitive Impairment and Mild Dementia in a National Neuropathology Cohort

OBJECTIVE

We sought to evaluate early clinical differences between cerebral arteriolosclerosis (pARTE), Alzheimer disease (pAD), and AD with arteriolosclerosis (ADARTE).

METHODS

Using National Alzheimer's Coordinating Center neuropathology diagnoses, we defined pARTE (n=21), pAD (n=203), and ADARTE (n=158) groups. We compared demographics, medical history, psychometrics, neuropsychiatric symptoms, and apolipoprotein E (APOE) allele variants across neuropathology groups. Retrospective timepoints were first evaluation with Global Clinical Dementia Rating (CDR) score of 0.5 and 1.0, via the CDR Dementia Staging Instrument, corresponding to mild cognitive impairment (MCI) and mild dementia, respectively.

RESULTS

In MCI, clinical differences were minimal but pARTE subjects were older, had later onset cognitive decline, and progressed less severely than pAD. In mild dementia, pAD subjects were younger and had earlier onset of decline. Neuropsychiatric (depression) and psychometric (Logical Memory Delayed Recall, Trails B) differences also emerged between the groups. In MCI, APOE4 associated with worse Logical Memory Delayed Recall in pAD and ADARTE. In mild dementia, APOE4 associated with better animal fluency in pAD, but with better Trails A performance and more neuropsychiatric symptoms (Neuropsychiatric Inventory Questionnaire) in ADARTE.

CONCLUSIONS

Differences between pARTE, pAD, and ADARTE emerge at mild dementia rather than MCI. APOE4 has varied cognitive and psychiatric impact dependent on neuropathology group and stage.

Chronic Traumatic Encephalopathy and Neuropathological Comorbidities

With age, the presence of multiple neuropathologies in a single individual becomes increasingly common. Given that traumatic brain injury and the repetitive head impacts (RHIs) that occur in contact sports have been associated with the development of many neurodegenerative diseases, including chronic traumatic encephalopathy (CTE), Alzheimer's disease, Lewy body disease, and amyotrophic lateral sclerosis, it is becoming critical to understand the relationship and interactions between these pathologies. In fact, comorbid pathology is common in CTE and likely influenced by both age and the severity and type of exposure to RHI as well as underlying genetic predisposition. Here, we review the major comorbid pathologies seen with CTE and in former contact sports athletes and discuss what is known about the associations between RHI, age, and the development of neuropathologies. In addition, we examine the distinction between CTE and age-related pathology including primary age-related tauopathy and age-related tau astrogliopathy.

Amyloid-β plaques may be reduced in advanced stages of cerebral amyloid angiopathy in the elderly

We examined 29 cases in which cerebral amyloid angiopathy (CAA) was detected among routine aged autopsies. Most cases with severe CAA had many amyloid-β (Aβ) plaques in the occipital cortex. Nonetheless, two cases had few Aβ plaques with many small vessels and capillaries with CAA. In the two cases, severe CAA was widely distributed, except in the frontal lobes. Aβ deposits in capillaries often showed the characteristic pattern of dysphoric amyloid angiopathy. A few naked plaques were present. Although Aβ plaques were sparse near small vessels with CAA, there were many Aβ plaques distant from small vessels with CAA. Some of the remaining plaques had a moth-eaten appearance. Based on Aβ-positive star-like appearance and results of double immunohistochemistry for glial fibrillary acidic protein and Aβ_1-42 , some astrocytes appeared to contain Aβ. Ionized calcium-binding adapter molecule 1 (Iba1)-positive microglia were scattered within the neuropil, with some present around small vessels with CAA. Iba1-positive microglia also seemed to phagocytose Aβ in several senile plaques by double immunostaining. Neurons and neurites identified with a monoclonal antibody against phosphorylated tau (clone AT8) were occasionally detected in sparse plaque areas, with AT8-identified dot-like structures present around capillaries with CAA. Accumulation of T lymphocytes was detected around vessels in the subarachnoid space in one case. The morphological changes detected in our two cases were similar to those of morphological markers of plaque clearance after Aβ immunotherapy. Nonetheless, our cases did not receive Aβ immunotherapy, but similar pathologies were observed. Overall, advanced CAA cases, including our two cases, may be examples of plaque clearance without Aβ immunotherapy. Further studies are needed to resolve the mechanism of Aβ plaque clearance using these cases.

The Amyloid-Tau-Neuroinflammation Axis in the Context of Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA) is typified by the cerebrovascular deposition of amyloid. Currently, there is no clear understanding of the mechanisms underlying the contribution of CAA to neurodegeneration. Despite the fact that CAA is highly associated with the accumulation of Aβ, other types of amyloids have been shown to associate with the vasculature. Interestingly, in many cases, vascular amyloidosis has been associated with an active immune response and perivascular deposition of hyperphosphorylated tau. Despite the fact that in Alzheimer’s disease (AD) a major focus of research has been the understanding of the connection between parenchymal amyloid plaques, tau aggregates in the form of neurofibrillary tangles (NFTs), and immune activation, the contribution of tau and neuroinflammation to neurodegeneration associated with CAA remains understudied. In this review, we discussed the existing evidence regarding the amyloid diversity in CAA and its relation to tau pathology and immune response, as well as the possible contribution of molecular and cellular mechanisms, previously associated with parenchymal amyloid in AD and AD-related dementias, to the pathogenesis of CAA. The detailed understanding of the “amyloid-tau-neuroinflammation” axis in the context of CAA could open the opportunity to develop therapeutic interventions for dementias associated with CAA that are currently being proposed for AD and AD-related dementias.

Contact sport participation and chronic traumatic encephalopathy are associated with altered severity and distribution of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) consists of beta-amyloid deposition in the walls of the cerebrovasculature and is commonly associated with Alzheimer's disease (AD). However, the association of CAA with repetitive head impacts (RHI) and with chronic traumatic encephalopathy (CTE) is unknown. We evaluated the relationship between RHI from contact sport participation, CTE, and CAA within a group of deceased contact sport athletes (n = 357), a community-based cohort (n = 209), and an AD cohort from Boston University AD Center (n = 241). Unsupervised hierarchal cluster analysis demonstrated a unique cluster (n = 11) with increased CAA in the leptomeningeal vessels compared to the intracortical vessels (p < 0.001) comprised of participants with significantly greater frequencies of CTE (7/11) and history of RHI. Overall, participants with CTE (n = 251) had more prevalent (p < 0.001) and severe (p = 0.010) CAA within the frontal leptomeningeal vessels compared to intracortical vessels. Compared to those with AD, participants with CTE had more severe CAA in frontal than parietal lobes (p < 0.001) and more severe CAA in leptomeningeal than intracortical vessels (p = 0.002). The overall frequency of CAA in participants with CTE was low, and there was no significant association between contact sport participation and the presence of CAA. However, in those with CAA, a history of contact sports was associated with increased CAA severity in the frontal leptomeningeal vessels (OR = 4.01, 95% CI 2.52-6.38, p < 0.001) adjusting for AD, APOE ε4 status, and age. Participants with CAA had increased levels of sulcal tau pathology and decreased levels of the synaptic marker PSD-95 (p's < 0.05), and CAA was a predictor of dementia (OR = 1.75, 95% CI 1.02-2.99, p = 0.043) adjusting for age, sex, and comorbid pathology. Overall, contact sport participation and CTE were associated with more severe frontal and leptomeningeal CAA, and CAA was independently associated with worse pathological and clinical outcomes.

Blood-brain barrier and innate immunity in the pathogenesis of Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is only partly understood. This is the probable reason why significant efforts to treat or prevent AD have been unsuccessful. In fact, as of April 2019, there have been 2094 studies registered for AD on the clinicaltrials.gov U.S. National Library of Science web page, of which only a few are still ongoing. In AD, abnormal accumulation of amyloid and tau proteins in the brain are thought to begin 10-20 years before the onset of overt symptoms, suggesting that interventions designed to prevent pathological amyloid and tau accumulation may be more effective than attempting to reverse a pathology once it is established. However, to be successful, such early interventions need to be selectively administered to individuals who will likely develop the disease long before the symptoms occur. Therefore, it is critical to identify early biomarkers that are strongly predictive of AD. Currently, patients are diagnosed on the basis of a variety of clinical scales, neuropsychological tests, imaging and laboratory modalities, but definitive diagnosis can be made only by postmortem assessment of underlying neuropathology. People suffering from AD thus may be misdiagnosed clinically with other primary causes of dementia, and vice versa, thereby also reducing the power of clinical trials. The amyloid cascade hypothesis fits well for the familial cases of AD with known mutations, but is not sufficient to explain sporadic, late-onset AD (LOAD) that accounts for over 95% of all cases. Since the earliest descriptions of AD there have been neuropathological features described other than amyloid plaques (AP) and neurofibrillary tangles (NFT), most notably gliosis and neuroinflammation. However, it is only recently that genetic and experimental studies have implicated microglial dysfunction as a causal factor for AD, as opposed to a merely biological response of its accumulation around AP. Additionally, many studies have suggested the importance of changes in blood-brain barrier (BBB) permeability in the pathogenesis of AD. Here we suggest how these less investigated aspects of the disease that have gained increased attention in recent years may contribute mechanistically to the development of lesions and symptoms of AD.

Computational modeling and biomarker studies of pharmacological treatment of Alzheimer's disease (Review)

Alzheimer's disease (AD) is a complex and multifactorial disease. In order to understand the genetic influence in the progression of AD, and to identify novel pharmaceutical agents and their associated targets, the present study discusses computational modeling and biomarker evaluation approaches. Based on mechanistic signaling pathway approaches, various computational models, including biochemical and morphological models, are discussed to explore the strategies that may be used to target AD treatment. Different biomarkers are interpreted on the basis of morphological and functional features of amyloid β plaques and unstable microtubule‑associated tau protein, which is involved in neurodegeneration. Furthermore, imaging and cerebrospinal fluids are also considered to be key methods in the identification of novel markers for AD. In conclusion, the present study reviews various biochemical and morphological computational models and biomarkers to interpret novel targets and agonists for the treatment of AD. This review also highlights several therapeutic targets and their associated signaling pathways in AD, which may have potential to be used in the development of novel pharmacological agents for the treatment of patients with AD. Computational modeling approaches may aid the quest for the development of AD treatments with enhanced therapeutic efficacy and reduced toxicity.

The effects of cerebral amyloid angiopathy on integrity of the blood-brain barrier

Cerebral amyloid angiopathy (CAA), in which amyloid accumulates predominantly in the walls of arterioles and capillaries, is seen in most patients with Alzheimer disease (AD) and may contribute to compromise of blood-brain barrier (BBB) function seen in AD. We investigated the effects of CAA on BBB integrity by examining the expression of the endothelial marker CD31, basement membrane protein collagen IV (COL4), tight junction protein claudin-5, and fibrinogen, a marker of BBB leakage, by immunohistochemistry in the occipital cortex of autopsy brains with AD and capillary CAA (CAA type 1; n = 8), AD with noncapillary CAA (CAA type 2; n = 10), and AD without CAA (n = 7) compared with elderly controls (n = 10). Given the difference in pathogenesis of capillary and noncapillary CAA, we hypothesize that features of BBB breakdown are observed only in capillary CAA. We found decreased expression of CD31 in AD subjects with CAA types 1 and 2 compared with AD without CAA and an increase in COL4 in AD without CAA compared with controls. Furthermore, there was increased immunoreactivity for fibrinogen in AD with CAA type 1 compared with controls. These findings suggest that capillary CAA is associated with morphologic and possibly physiologic alterations of the neurovascular unit and increased BBB permeability in AD.

Dogs with Cognitive Dysfunction as a Spontaneous Model for Early Alzheimer's Disease: A Translational Study of Neuropathological and Inflammatory Markers

Aged companion dogs with canine cognitive dysfunction (CCD) spontaneously develop varying degrees of progressive cognitive decline and particular neuropathological features correspondent to the changes associated with Alzheimer's disease (AD) in humans. The aim of the present study was to characterize certain aspects of neuropathology and inflammatory markers related to aging and CCD in dogs in comparison with human AD. Fifteen brains from aged dogs with normal cognitive function, mild cognitive impairment, or CCD were investigated and compared with two control brains from young dogs and brain sections from human AD subjects. The neuropathological investigations included evaluation of amyloid-β (Aβ) plaque deposition (N-terminally truncated and pyroglutamyl-modified Aβ included), tau pathology, and inflammatory markers in prefrontal cortex. Cortical Aβ deposition was found to be only of the diffuse subtype as no dense-core or neuritic plaques were found. The Aβ deposition followed a progressive pattern in four maturation stages. Accumulation of the Aβ peptide was also observed in the vessel walls. Both immunohistochemically and biochemically measured levels of Aβ pathology in prefrontal cortex showed a consistent positive correlation to age but not to cognitive deficit severity. No evidence of neurofibrillary tau pathology was found. The level of pro-inflammatory cytokines was generally low and showed no significant association to cognitive status. The findings of the present study support the senescent dog with spontaneous cognitive dysfunction as a valuable non-transgenic model for further investigations of the molecular events involved in the neurodegenerative processes associated with aging and early stage AD, especially the Aβ-related pathology.

Attenuation of functional hyperemia to visual stimulation in mild Alzheimer's disease and its sensitivity to cholinesterase inhibition

Despite the growing recognition of the significance of cerebrovascular impairment in the etiology and progression of Alzheimer's disease (AD), the early stage brain vascular dysfunction and its sensitivity to pharmacological interventions is still not fully characterized. Due to the early and aggressive treatment of probable AD with cholinesterase inhibitors (ChEI), which in and of themselves have direct effects on brain vasculature, the vast majority of hemodynamic measurements in early AD subjects reported hitherto have consequently been made only after the start of treatment, complicating the disentanglement of disease- vs. treatment-related effects on the cerebral vasculature. To address this gap, we used pseudo continuous arterial spin labeling MRI to measure resting perfusion and visual stimulation elicited changes in cerebral blood flow (CBF) and blood oxygenation dependent (BOLD) fMRI signal in a cohort of mild AD patients immediately prior to, 6months post, and 12months post commencement of open label cholinesterase inhibitor treatment. Although patients exhibited no gray matter atrophy prior to treatment and their resting perfusion was not distinguishable from that in age, education and gender-matched controls, the patients' visual stimulation-elicited changes in BOLD fMRI and blood flow were decreased by 10±4% (BOLD) and 23±2% (CBF), relative to those in controls. Induction of cholinesterase inhibition treatment was associated with a further, 7±2% reduction in patients' CBF response to visual stimulation, but it stabilized, at this new lower level, over the follow-up period. Likewise, MMSE scores remained stable during the treatment; furthermore, higher MMSE scores were associated with higher perfusion responses to visual stimulation. This study represents the initial step in disentangling the effects of AD pathology from those of the first line treatment with cholinesterase inhibitors on cerebral hemodynamics and supports the use of arterial spin labeling MRI for quantitative evaluation of the brain vascular function in mild Alzheimer's disease. The findings provide evidence of a pronounced deficit in the visual cortex hyperemia despite the relative sparing of visual function in early stage AD, its reduction with ChEI treatment induction, and its stabilization in the first year of cholinesterase inhibition treatment. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Patterns of cerebral amyloid angiopathy define histopathological phenotypes in Alzheimer's disease

AIMS

Pathological heterogeneity of Aβ deposition in senile plaques (SP) and cerebral amyloid angiopathy (CAA) in Alzheimer's disease (AD) has been long noted. The aim of this study was to classify cases of AD according to their pattern of Aβ deposition, and to seek factors which might predict, or predispose towards, this heterogeneity.

METHODS

The form, distribution and severity of Aβ deposition (as SP and/or CAA) was assessed semiquantitatively in immunostained sections of frontal, temporal and occipital cortex from 134 pathologically confirmed cases of AD.

RESULTS

Four patterns of Aβ deposition were defined. Type 1 describes cases predominantly with SP, with or without CAA within leptomeningeal vessels alone. Type 2 describes cases where, along with many SP, CAA is present in both leptomeningeal and deeper penetrating arteries. Type 3 describes cases where capillary CAA is present along with SP and arterial CAA. Type 4 describes a predominantly vascular phenotype, where Aβ deposition is much more prevalent in and around blood vessels, than as SP. As would be anticipated from the group definitions, there were significant differences in the distribution and degree of CAA across the phenotype groups, although Aβ deposition as SP did not vary. There were no significant differences between phenotype groups with regard to age of onset, age at death, disease duration and brain weight, or disease presentation. Women were over-represented in the type 1 phenotype and men in type 2. Genetically, type 3 (capillary subtype) cases were strongly associated with possession of the APOE ε4 allele.

CONCLUSIONS

This study offers an alternative method of pathologically classifying cases of AD. Further studies may derive additional genetic, environmental or clinical factors which associate with, or may be responsible for, these varying pathological presentations of AD.

The relationship between cerebral amyloid angiopathy and cortical microinfarcts in brain ageing and Alzheimer's disease

AIMS

Cerebral amyloid angiopathy (CAA) represents the deposition of amyloid β protein (Aβ) in the meningeal and intracerebral vessels. It is often observed as an accompanying lesion of Alzheimer's disease (AD) or in the brain of elderly individuals even in the absence of dementia. CAA is largely age-dependent. In subjects with severe CAA a higher frequency of vascular lesions has been reported. The goal of our study was to define the frequency and distribution of CAA in a 1-year autopsy population (91 cases) from the Department of Internal Medicine, Rehabilitation, and Geriatrics, Geneva.

MATERIALS AND METHODS

Five brain regions were examined, including the hippocampus, and the inferior temporal, frontal, parietal and occipital cortex, using an antibody against Aβ, and simultaneously assessing the severity of AD-type pathology with Braak stages for neurofibrillary tangles identified with an anti-tau antibody. In parallel, the relationships of CAA with vascular brain lesions were established.

RESULTS

CAA was present in 53.8% of the studied population, even in cases without AD (50.6%). The strongest correlation was seen between CAA and age, followed by the severity of amyloid plaques deposition. Microinfarcts were more frequent in cases with CAA; however, our results did not confirm a correlation between these parameters.

CONCLUSION

The present data show that CAA plays a role in the development of microvascular lesions in the ageing brain, but cannot be considered as the most important factor in this vascular pathology, suggesting that other mechanisms also contribute importantly to the pathogenesis of microvascular changes.

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.

Cerebral amyloid angiopathy in East and West

Cerebral amyloid angiopathy, a vasculopathy characterised by the deposition of amyloid fibrils in the arteries and arterioles in the cerebral cortex and meninges, has been reported to be associated with intracerebral haemorrhage and cognitive impairment in the elderly. Advances in neuroimaging and validation of the clinical diagnostic criteria aid in making a correct clinical diagnosis. Associations with Alzheimer's disease, asymptomatic microbleeds and white matter changes on neuroimaging have an influence on the clinical treatment for patients with probable cerebral amyloid angiopathy. Reviewing the reports from Asian countries, we found that patients with cerebral amyloid angiopathy have a strong age-related prevalence and a consistent association with dementia, but a weaker correlation with intracerebral haemorrhage, most likely due to a higher incidence of hypertensive intracerebral haemorrhage. Involvement of the occipital lobe arteries by CAA is common in all races and ethnicities, while frontal lobe arteries may be more frequently involved in the East compared to the West. The clinical impact of cerebral amyloid angiopathy on intracerebral haemorrhage and cognitive impairment could be increasingly obvious in Asian countries with ageing populations, especially with improving control of hypertension, the leading cause of intracerebral haemorrhage.

Microvasculature changes and cerebral amyloid angiopathy in Alzheimer's disease and their potential impact on therapy

The introduction of immunotherapy and its ultimate success will require re-evaluation of the pathogenesis of Alzheimer's disease particularly with regard to the role of the ageing microvasculature and the effects of APOE genotype. Arteries in the brain have two major functions (a) delivery of blood and (b) elimination of interstitial fluid and solutes, including amyloid-beta (Abeta), along perivascular pathways (lymphatic drainage). Both these functions fail with age and particularly severely in Alzheimer's disease and vascular dementia. Accumulation of Abeta as plaques in brain parenchyma and artery walls as cerebral amyloid angiopathy (CAA) is associated with failure of perivascular elimination of Abeta from the brain in the elderly and in Alzheimer's disease. High levels of soluble Abeta in the brain correlate with cognitive decline in Alzheimer's disease and reflect the failure of perivascular drainage of solutes from the brain and loss of homeostasis of the neuronal environment. Clinically and pathologically, there is a spectrum of disease related to functional failure of the ageing microvasculature with "pure" Alzheimer's disease at one end of the spectrum and vascular dementia at the other end. Changes in the cerebral microvasculature with age have a potential impact on therapy with cholinesterase inhibitors and especially on immunotherapy that removes Abeta from plaques in the brain, but results in an increase in severity of CAA and no clear improvement in cognition. Drainage of Abeta along perivascular pathways in ageing artery walls may need to be improved to maximise the potential for improvement of cognitive function with immunotherapy.

Population studies of sporadic cerebral amyloid angiopathy and dementia: a systematic review

BACKGROUND

Deposition of amyloid-beta (Abeta) in vessel walls of the brain as cerebral amyloid angiopathy (CAA) could be a major factor in the pathogenesis of dementia. Here we investigate the relationship between dementia and the prevalence of CAA in older populations. We searched the literature for prospective population-based epidemiological clinicopathological studies, free of the biases of other sampling techniques, which were used as a comparison.

METHODS

To identify population-based studies assessing CAA and dementia, a previous systematic review of population-based clinicopathological studies of ageing and dementia was employed. To identify selected-sample studies, PsychInfo (1806-April Week 3 2008), OVID MEDLINE (1950-April Week 2 2008) and Pubmed (searched 21 April 2008) databases were searched using the term "amyloid angiopathy". These databases were also employed to search for any population-based studies not included in the previous systematic review. Studies were included if they reported the prevalence of CAA relative to a dementia classification (clinical or neuropathological).

RESULTS

Four population-based studies were identified. They showed that on average 55-59% of those with dementia displayed CAA (of any severity) compared to 28-38% of the non-demented. 37-43% of the demented displayed severe CAA in contrast to 7-24% of the non-demented. There was no overlap in the range of these averages and they were less variable and lower than those reported in 38 selected sample studies (demented v non-demented: 32-100 v 0-77% regardless of severity; 0-50 v 0-11% for severe only).

CONCLUSION

CAA prevalence in populations is consistently higher in the demented as compared to the non-demented. This supports a significant role for CAA in the pathogenesis of dementia.

Deletion of tumor necrosis factor death receptor inhibits amyloid beta generation and prevents learning and memory deficits in Alzheimer's mice

The tumor necrosis factor type 1 death receptor (TNFR1) contributes to apoptosis. TNFR1, a subgroup of the TNFR superfamily, contains a cytoplasmic death domain. We recently demonstrated that the TNFR1 cascade is required for amyloid beta protein (Abeta)-induced neuronal death. However, the function of TNFR1 in Abeta plaque pathology and amyloid precursor protein (APP) processing in Alzheimer's disease (AD) remains unclear. We report that the deletion of the TNFR1 gene in APP23 transgenic mice (APP23/TNFR1(-/-)) inhibits Abeta generation and diminishes Abeta plaque formation in the brain. Genetic deletion of TNFR1 leads to reduced beta-secretase 1 (BACE1) levels and activity. TNFR1 regulates BACE1 promoter activity via the nuclear factor-kappaB pathway, and the deletion of TNFR1 in APP23 transgenic mice prevents learning and memory deficits. These findings suggest that TNFR1 not only contributes to neurodegeneration but also that it is involved in APP processing and Abeta plaque formation. Thus, TNFR1 is a novel therapeutic target for AD.

Topographical distribution of cerebral amyloid angiopathy and its effect on cognitive decline are influenced by Alzheimer disease pathology

Cerebral amyloid angiopathy (CAA) is defined by beta-amyloid peptide (Abeta) depositions in cerebral vessels and is associated with Alzheimer disease (AD). It has been suggested that severe CAA is an independent risk factor for cognitive decline. 171 autopsy brains underwent standardized neuropathological assessment, the patients age ranged from 54 to 104 years (mean age: 83.9 years, +/-9.2, 59.6% female, 56.1% clinically demented). Using immunohistochemistry, the severity of Abeta depositions in vessels was assessed semiquantitatively in the frontal, frontobasal, hippocampal, and occipital region, respectively. CAA was present in 117 cases (68.4%), with the occipital region being affected significantly stronger than other regions. The overall incidence of CAA was significantly higher in cases with high grade neuritic AD pathology (ADP) compared to those with low grade or no ADP. The severity of CAA significantly increased with increasing ADP, with CAA in the occipital region increasing significantly stronger than that in other regions. The association of CAA and clinical dementia failed to remain statistically significant when adjusting for concomitant ADP. However, in cases devoid of any ADP CAA was significantly associated with the presence of clinical dementia. These results indicate a strong association of AD with CAA, but do not unequivocally support reports suggesting CAA to be an independent risk factor for cognitive decline, except for a subgroup of demented patients lacking any ADP.

Syndecans in wound healing, inflammation and vascular biology

Syndecans are heparan sulphate proteoglycans consisting of a type I transmembrane core protein modified by heparan sulphate and sometimes chondroitin sulphate chains. They are major proteoglycans of many organs including the vasculature, along with glypicans and matrix proteoglycans. Heparan sulphate chains have potential to interact with a wide array of ligands, including many growth factors, cytokines, chemokines and extracellular matrix molecules relevant to growth regulation in vascular repair, hypoxia, angiogenesis and immune cell function. This is consistent with the phenotypes of syndecan knock-out mice, which while viable and fertile, show deficits in tissue repair. Furthermore, there are potentially important changes in syndecan distribution and function described in a variety of human vascular diseases. The purpose of this review is to describe syndecan structure and function, consider the role of syndecan core proteins in transmembrane signalling and also their roles as co-receptors with other major classes of cell surface molecules. Current debates include potential redundancy between syndecan family members, the significance of multiple heparan sulphate interactions, regulation of the cytoskeleton and cell behaviour and the switch between promoter and inhibitor of important cell functions, resulting from protease-mediated shedding of syndecan ectodomains.

Relationships in Alzheimer's disease between the extent of Abeta deposition in cerebral blood vessel walls, as cerebral amyloid angiopathy, and the amount of cerebrovascular smooth muscle cells and collagen

The relationship between degree of cerebral amyloid angiopathy (CAA) and the amount of smooth muscle cells (SMCs) and deposition of collagen IV fibres (COL IV) was investigated in the frontal and occipital cortex of 70 patients with autopsy confirmed Alzheimer's disease (AD). The extent of CAA was significantly greater in occipital than in frontal cortex, although SMC loss was greater in frontal than in occipital cortex. COL IV staining was significantly higher in occipital than in frontal cortex. The degree of SMC loss correlated with CAA, as Abeta40 but not as Abeta42 or total Abeta, in frontal cortex, but not in occipital cortex. Leptomeningeal arteries within occipital cortex showed significantly greater external diameter, greater wall thickness and greater luminal area than those in frontal cortex. The degree of CAA correlated with thickness of blood vessel wall and external diameter in frontal cortex, whereas extent of SMC loss correlated with thickness of blood vessel wall in occipital cortex. There were significant negative correlations between duration of disease and thickness of vessel wall, external diameter and luminal area. In patients with disease durations exceeding 10 years, external vessel diameter and thickness of the vessel wall were both halved compared with patients with durations less than 5 years; luminal area was reduced by about 75%. Blood vessels in AD undergo degenerative changes involving deposition of Abeta and COL IV with loss of SMC. SMC loss may relate to increasing Abeta deposition in early stages of disease, but this relationship may be lost with disease progression.

Relative paucity of tau accumulation in the small areas with abundant Abeta42-positive capillary amyloid angiopathy within a given cortical region in the brain of patients with Alzheimer pathology

Cerebral amyloid angiopathy (CAA) is a manifestation of amyloid beta-protein (Abeta) accumulation in the elderly as well as in patients with Alzheimer's disease (AD). Two types of CAA have been noted, based on the type of vasculature in which Abeta is deposited: cerebral capillary amyloid angiopathy (capCAA) and non-capCAA. Non-capCAA is a common form of CAA that consists of Abeta deposited in arteries and arterioles. Recent information on Abeta metabolism in the brain suggests that non-capCAA is associated with Abeta secretion into the cerebrospinal fluid via the perivascular space, whereas capCAA is associated with Abeta removal to blood plasma via the capillary endothelium. Abeta40, a major and relatively soluble Abeta isoform, is deposited predominantly in non-capCAA, and Abeta42, which is insoluble and associated more closely than Abeta40 with AD, is deposited predominantly in capCAA. Studying small areas of microscopic size within a given cortical region, we found an inverse association of capCAA and senile plaques. We also found a relative paucity of tau pathology in the small areas with abundant capCAA compared with the small areas with abundant senile plaques within a cortical region with the same cytoarchitecture. We suppose that both capCAA and senile plaques indicate high Abeta42 in the neuropil but that only Abeta42 in the form of insoluble deposits in senile plaques promotes tau abnormality.

Differential deposition of amyloid beta peptides in cerebral amyloid angiopathy associated with Alzheimer's disease and vascular dementia

Cerebral amyloid angiopathy (CAA) caused by deposition of amyloid beta (Abeta) peptides in the cerebrovasculature, involves degeneration of normal vascular components and increases the risk of infarction and cerebral hemorrhage. Accumulating evidence suggests that sporadic CAA is also a significant contributor to cognitive decline and dementia in the elderly. However, the mechanisms by which CAA arises are poorly understood. While neuronal sources of Abeta peptides are sufficient to cause CAA in transgenic mice overexpressing the amyloid precursor protein, there is reason to believe that in aging man, vascular disease modulates the disease process. To better understand CAA mechanisms in dementia, we assessed the frontal cortex of 62 consecutive cases of Alzheimer's disease (AD), vascular dementia (VaD), and mixed dementia (MD) using immunohistochemistry with antibodies to Abeta, smooth muscle actin and the carboxyl-terminal peptides to detect Abeta(40) and Abeta(42). While vascular Abeta deposition was invariably associated with smooth muscle degeneration as indicated by absence of smooth muscle cell actin reactivity, VaD/MD cases exhibited markedly more vascular Abeta(42) deposits and smooth muscle actin loss compared to AD cases with similar degrees of CAA and Abeta(40) deposition. This suggests that distinct mechanisms are responsible for the differential deposition of Abeta in CAA associated with AD and that associated with ischemic/cerebrovascular disease. It is plausible that experimental studies on the effects of cerebrovascular disease on Abeta production and elimination will yield important clues on the pathogenesis of CAA.

[Alzheimer's disease: cardiovascular risk factors must be assessed]

BACKGROUND

Dementia is nowadays of major importance in public health. Alzheimer's disease and vascular cognitive impairments are its main aetiology in the elderly. The cause of Alzheimer's disease remains unknown. The factor initiating the physiopathology of this neurodegenerative disease is source of controversy.

CURRENT KNOWLEDGE AND KEY POINTS

The theory of a neurotoxicity initiated by amyloid deposition is questioned. A growing number of data suggest a central role of cardiovascular risk factors and alteration of arterial walls, inducing chronic brain hypoperfusion, as the primary trigger in the physiopathology of the disease. These data are based on epidemiological, physiopathological, neuroimaging, neuropathological and pharmacological studies. However, the exact link between arteriosclerosis, vascular cognitive impairment and Alzheimer's disease remains controversial.

FUTURE PROSPECTS AND PROJECTS

These debates point out the crucial importance of the assessment of cardiovascular risk factors, as a preventable cause, either of cognitive decline, morbidity and mortality. In this aim, major targets could be different when primary or secondary prevention are at stake. These controversies also suggest new research directions towards Alzheimer's disease physiopathology, and for pharmacological interventions aimed on the prevention of cognitive decline or the curative treatment for this disease.

A 3'-UTR polymorphism in the oxidized LDL receptor 1 gene increases Abeta40 load as cerebral amyloid angiopathy in Alzheimer's disease

It is presently unclear whether polymorphic variations in the oxidized low-density lipoprotein receptor 1 (OLR1), or low-density lipoprotein receptor-related protein 1 (LRP1), genes act as risk factors for Alzheimer's disease (AD). In the present study, we have investigated the extent of amyloid beta protein (Abeta) deposition as cerebral amyloid angiopathy (CAA) or senile plaques (SP) in relationship to OLR1 +1071 and +1073 polymorphisms and LRP1 C766T polymorphism in patients with AD There was an increased Abeta40 load as CAA, but not as SP, in frontal cortex of AD patients carrying OLR1+1073 CC genotype, compared to those with CT, TT or CT+TT genotypes, but only in those individuals without apolipoprotein (APOE) epsilon4 allele. No differences in total Abeta or Abeta42 load as CAA or SP between OLR1+1073 genotypes was seen, nor were there any differences between OLR1+1071 and LRP1 genotypes for any measure of Abeta. Present data suggests that homozygosity for the C allele for OLR1+1073 polymorphism, selectively in individuals without APOE epsilon4 allele, may impair clearance of Abeta, and particularly Abeta40, from the brain across the blood-brain barrier, leading to its 'diversion' into perivascular drainage channels, thereby increasing the severity of CAA in such persons.

Cerebral amyloid angiopathy in a 95+ cohort: complement activation and apolipoprotein E (ApoE) genotype

There is growing evidence that in Alzheimer's disease (AD) amyloid beta-protein (Abeta) triggers a chronic inflammatory reaction in cerebral amyloid plaques, including complement proteins. Abeta also accumulates cerebrovascularly in age- and AD-associated cerebral amyloid angiopathy (CAA). We investigated complement proteins in CAA in a population-based series using histological and immunohistochemical staining methods. The 74 subjects, aged 95 years or more, had undergone clinical neurological examination and apolipoprotein E (ApoE) genotyping. The brains had been studied for AD post-mortem, allowing us to relate the histopathological findings to clinical and genetic conditions. CAA with congophilic amyloid was found in 36/74 individuals (48.6%). The vascular amyloid deposits immunoreacted with antibodies to Abeta and complements 3d (C3d) and 9 (C9). The positivity in complement stains increased with growing severity of CAA (P = 0.001). The presence of CAA associated with ApoE epsilon4 (P = 0.0005) and overrepresentation of epsilon4 among those with moderate or severe vs. mild CAA (P = 0.03) was demonstrated. The presence of CAA associated with dementia (P = 0.01), which was contributed by both epsilon4+ (P = 0.02) and epsilon4- (P = 0.06) subjects. Our study shows that complement proteins are deposited in the affected vessels in Abeta-associated CAA. They may solely represent the cerebral Abeta- burden associated to inflammatory stimuli, or signal a contribution in the clearance of cerebral Abeta, thereby contributing to the events associated with evolution of clinical dementia. Our results demonstrate a strong association between CAA and ApoE epsilon4 as well as dementia and suggest that the contribution of CAA to dementia is largely independent of ApoE epsilon4.

Sporadic cerebral amyloid angiopathy: pathology, clinical implications, and possible pathomechanisms

Cerebral amyloid angiopathy (CAA) was observed for the first time nearly 100 years ago and systematically described in 1938. It is a common finding in elderly individuals, defined by beta-amyloid peptide (Abeta) depositions in cerebral blood vessels, and associated with Alzheimer's disease (AD). A variety of genetic mutations cause hereditary forms of CAA; in this review, however, only the sporadic variant of CAA is considered. In CAA, Abeta depositions primarily occur in the abluminal portion of the tunica media, and with increasing severity all layers of the blood vessel wall are infiltrated and an additional spread of Abeta into the surrounding neuropil may be seen (i.e., dyshoric changes). CAA is most pronounced in the occipital lobe and its distribution is usually patchy. The relationship between CAA and AD is poorly understood; however, low positive correlations between the severity of both CAA and AD pathology have been observed. CAA is a frequent cause of (warfarin-associated) intracerebral hemorrhage, and the diagnosis of probable CAA-related hemorrhage can be made during life with high accuracy. Both APOE-epsilon4 and APOE-epsilon2 are risk factors for CAA, while only APOE-epsilon2 increases the risk for hemorrhage in CAA. Although the role of CAA as an independent risk factor for cognitive decline is unclear, severe CAA is likely to lower the threshold for clinically overt dementia in neurodegenerative diseases. As for the origin of Abeta in CAA, it may be both produced by smooth muscle cells (vessel wall) and derived from neurons in the course of perivascular drainage.

Alzheimer's disease pathology influences severity and topographical distribution of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is defined by beta-amyloid peptide (Abeta) depositions in cerebral vessels and is associated with Alzheimer's disease (AD). The relationship between sporadic CAA and AD, and the origin of Abeta in CAA are poorly understood. The aim of our study was to investigate the relationship between CAA and AD. Autopsy brains (n=113, 61.1% female, 55.8% clinically demented, age range 54-102 years, mean +/- SE 83.5+/-0.93 years) underwent standardized neuropathological assessment. CAA was evaluated in frontal, frontobasal, hippocampal, and occipital regions. Using immunohistochemistry, the severity of Abeta deposition in vessels was assessed semiquantitatively for each region separately. Evaluation of APOE genotype in 53 cases using real-time PCR showed significant correlations with severe AD pathology and CAA. CAA was present in 77 cases (68.1%), with the occipital region being affected significantly more often and more severely than other regions (P<0.01). Of brains without AD pathology 23.5% revealed CAA, whereas 24% with AD pathology showed no CAA. In concordance with other studies, the severity of both AD pathology and CAA showed a low, but significant correlation. This correlation, however, was only caused by the significant increase of occipital CAA with increasing AD pathology (P<0.01), and was independent of APOE genotype. Our results suggest that progressing AD pathology not only increases the severity of CAA, but also shifts its topographical distribution towards the occipital cortex.

Transgenic mouse models for APP processing and Alzheimer's disease: early and late defects

Transgenic mice with neuronal expression of human AD-mutant APP[V7171] in their brain recapitulate robustly the amyloid pathology as seen in Alzheimer's disease (AD) patients. The AD related pathological phenotype consisting of amyloid plaques and vascular amyloid pathology, develop progressively and relative late in ageing APP transgenic mice, between 10 and 15 months of age. In contrast to the late - and clinically irrelevant - amyloid plaque-pathology, the early cognitive defects and behavioural features are clinically more interesting. This review discusses the generation and in depth phenotypic characterization of both aspects of the APP[V7171] transgenic mice. Attention is focussed on the relation of biochemical data of the different APP fragments and amyloid peptides to the formation of the typical early defects and the late parenchymal and vascular amyloid depositions. The APP[V7171] transgenic mice are a perfect model to characterize and investigate early biochemical and cognitive aspects and a potential resource to define pathological interactions of different factors known to be involved in AD. Finally, any therapeutic intervention can be directly tested and explored in these transgenic mice as excellent pre-clinical models.

beta-site amyloid precursor protein cleaving enzyme 1 increases amyloid deposition in brain parenchyma but reduces cerebrovascular amyloid angiopathy in aging BACE x APP[V717I] double-transgenic mice

The generation of amyloid peptides (Abeta) from the amyloid precursor protein (APP) is initiated by beta-secretase (BACE), whereas subsequent gamma-secretase cleavage mediated by presenilin-1, produces Abeta peptides mainly of 40 or 42 amino acids long. In addition, alternative beta'-cleavage of APP at position 11 of the amyloid sequence results in N-truncated Abeta(11-40/42) peptides, but the functional significance or pathological impact is unknown. Here we demonstrate that in the brain of BACE x APP[V717I] double-transgenic mice, amyloidogenic processing at both Asp1 and Glu11 is increased resulting in more and different Abeta species and APP C-terminal fragments. Pathologically, BACE significantly increased the number of diffuse and senile amyloid plaques in old double-transgenic mice. Unexpectedly, vascular amyloid deposition was dramatically lower in the same BACE x APP[V717I] double-transgenic mice, relative to sex- and age-matched APP[V717I] single-transgenic mice in the same genetic background. The tight inverse relation of vascular amyloid to the levels of the less soluble N-terminally truncated Abeta peptides is consistent with the hypothesis that vascular amyloid deposition depends on drainage of excess tissue Abeta. This provides biochemical evidence in vivo for the preferential contribution of N-truncated Abeta to parenchymal amyloid deposition in contrast to vascular amyloid pathology.

The nature and effects of cortical microvascular pathology in aging and Alzheimer's disease

Age-related and amyloid-induced pathology of the cerebral microvasculature have been implicated as potential contributing factors to the pathogenesis of Alzheimer's disease (AD). The microvasculature plays a crucial role in maintaining brain homeostasis and deterioration of its integrity may have deleterious effects on brain function in AD, possibly leading to neurofibrillary degeneration, plaque formation, and cell loss. Brain vessels possess peculiar anatomical and physiological properties owing to their role in the exchange processes of various substances between blood and brain, which are highly regulated for the maintenance of ionic homeostasis of the neuronal environment. Here we review neuropathological aspects of cortical microvessels in aging and AD in relationship to known cardiovascular risk factors and their possible impact on the cognitive decline seen in late-onset dementia.

Modelling neuroinflammatory phenotypes in vivo

Inflammation of the central nervous system is an important but poorly understood part of neurological disease. After acute brain injury or infection there is a complex inflammatory response that involves activation of microglia and astrocytes and increased production of cytokines, chemokines, acute phase proteins, and complement factors. Antibodies and T lymphocytes may be involved in the response as well. In neurodegenerative disease, where injury is more subtle but consistent, the inflammatory response is continuous. The purpose of this prolonged response is unclear, but it is likely that some of its components are beneficial and others are harmful. Animal models of neurological disease can be used to dissect the specific role of individual mediators of the inflammatory response and assess their potential benefit. To illustrate this approach, we discuss how mutant mice expressing different levels of the cytokine transforming growth factor beta-1 (TGF-beta1), a major modulator of inflammation, produce important neuroinflammatory phenotypes. We then demonstrate how crosses of TGF-beta1 mutant mice with mouse models of Alzheimer's disease (AD) produced important new information on the role of inflammation in AD and on the expression of different neuropathological phenotypes that characterize this disease.