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

Robust EEG Based Biomarkers to Detect Alzheimer's Disease

Biomarkers to detect Alzheimer's disease (AD) would enable patients to gain access to appropriate services and may facilitate the development of new therapies. Given the large numbers of people affected by AD, there is a need for a low-cost, easy to use method to detect AD patients. Potentially, the electroencephalogram (EEG) can play a valuable role in this, but at present no single EEG biomarker is robust enough for use in practice. This study aims to provide a methodological framework for the development of robust EEG biomarkers to detect AD with a clinically acceptable performance by exploiting the combined strengths of key biomarkers. A large number of existing and novel EEG biomarkers associated with slowing of EEG, reduction in EEG complexity and decrease in EEG connectivity were investigated. Support vector machine and linear discriminate analysis methods were used to find the best combination of the EEG biomarkers to detect AD with significant performance. A total of 325,567 EEG biomarkers were investigated, and a panel of six biomarkers was identified and used to create a diagnostic model with high performance (≥85% for sensitivity and 100% for specificity).

Neurovascular Ageing and Age-Related Diseases

Proper functioning of the brain is dependent on integrity of the cerebral vasculature. During ageing, a number of factors including aortic or arterial stiffness, autonomic dysregulation, neurovascular uncoupling and blood-brain barrier (BBB) damage will define the dynamics of brain blood flow and local perfusion. The nature and extent of ageing-related cerebrovascular changes, the degree of involvement of the heart and extracranial vessels and the consequent location of tissue pathology may vary considerably. Atheromatous disease retarding flow is a common vascular insult, which increases exponentially with increasing age. Arteriolosclerosis characterized as a prominent feature of small vessel disease is one of the first changes to occur during the natural history of cerebrovascular pathology. At the capillary level, the cerebral endothelium, which forms the BBB undergoes changes including reduced cytoplasm, fewer mitochondria, loss of tight junctions and thickened basement membranes with collagenosis. Astrocyte end-feet protecting the BBB retract as part of the clasmatodendrotic response whereas pericyte coverage is altered. The consequences of these microvascular changes are lacunar infarcts, cortical and subcortical microinfarcts, microbleeds and diffuse white matter disease, which involves myelin loss and axonal abnormalities. The deeper structures are particularly vulnerable because of the relatively reduced density of the microvascular network formed by perforating and penetrating end arteries. Ultimately, the integrity of both the neurovascular and gliovascular units is compromised such that there is an overall synergistic effect reflecting on ageing associated cerebral perfusion and permeability. More than one protagonist appears to be involved in ageing-related cognitive dysfunction characteristically associated with the neurocognitive disorders.

Stroke, cerebrovascular diseases and vascular cognitive impairment in Africa

With increased numbers of older people a higher burden of neurological disorders worldwide is predicted. Stroke and other cerebrovascular diseases do not necessarily present with different phenotypes in Africa but their incidence is rising in tandem with the demographic change in the population. Age remains the strongest irreversible risk factor for stroke and cognitive impairment. Modifiable factors relating to vascular disease risk, diet, lifestyle, physical activity and psychosocial status play a key role in shaping the current spate of stroke related diseases in Africa. Hypertension is the strongest modifiable risk factor for stroke but is also likely associated with co-inheritance of genetic traits among Africans. Somewhat different from high-income countries, strokes attributed to cerebral small vessel disease (SVD) are higher >30% among sub-Saharan Africans. Raised blood pressure may explain most of the incidence of SVD-related strokes but there are likely other contributing factors including dyslipidaemia and diabetes in some sectors of Africa. However, atherosclerotic and cardioembolic diseases combined also appear to be common subtypes as causes of strokes. Significant proportions of cerebrovascular diseases are ascribed to various forms of infectious disease including complications of human immunodeficiency virus. Cerebral SVD leads to several clinical manifestations including gait disturbance, autonomic dysfunction and depression. Pathological processes are characterized by arteriolosclerosis, lacunar infarcts, perivascular spaces, microinfarcts and diffuse white matter changes, which can now all be detected on neuroimaging. Except for isolated cases of cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy or CADASIL, hereditary arteriopathies have so far not been reported in Africa. Prevalence estimates of vascular dementia (2-3%), delayed dementia after stroke (10-20%) and vascular cognitive impairment (30-40%) do not appear to be vastly different from those in other parts of the world. However, given the current demographic transition in both urban and rural settings these figures will likely rise. Wider application of neuroimaging modalities and implementation of stroke care in Africa will enable better estimates of SVD and other subtypes of stroke. Stroke survivors with SVD type pathology are likely to have low mortality and therefore portend increased incidence of dementia.

The pathology and pathophysiology of vascular dementia

Vascular dementia (VaD) is widely recognised as the second most common type of dementia. Consensus and accurate diagnosis of clinically suspected VaD relies on wide-ranging clinical, neuropsychological and neuroimaging measures in life but more importantly pathological confirmation. Factors defining subtypes of VaD include the nature and extent of vascular pathologies, degree of involvement of extra and intracranial vessels and the anatomical location of tissue changes as well as time after the initial vascular event. Atherosclerotic and cardioembolic diseases combined appear the most common subtypes of vascular brain injury. In recent years, cerebral small vessel disease (SVD) has gained prominence worldwide as an important substrate of cognitive impairment. SVD is characterised by arteriolosclerosis, lacunar infarcts and cortical and subcortical microinfarcts and diffuse white matter changes, which involve myelin loss and axonal abnormalities. Global brain atrophy and focal degeneration of the cerebrum including medial temporal lobe atrophy are also features of VaD similar to Alzheimer's disease. Hereditary arteriopathies have provided insights into the mechanisms of dementia particularly how arteriolosclerosis, a major contributor of SVD promotes cognitive impairment. Recently developed and validated neuropathology guidelines indicated that the best predictors of vascular cognitive impairment were small or lacunar infarcts, microinfarcts, perivascular space dilation, myelin loss, arteriolosclerosis and leptomeningeal cerebral amyloid angiopathy. While these substrates do not suggest high specificity, VaD is likely defined by key neuronal and dendro-synaptic changes resulting in executive dysfunction and related cognitive deficits. Greater understanding of the molecular pathology is needed to clearly define microvascular disease and vascular substrates of dementia. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Update on Vascular Dementia

Vascular dementia (VaD) is a major contributor to the dementia syndrome and is described as having problems with reasoning, planning, judgment, and memory caused by impaired blood flow to the brain and damage to the blood vessels resulting from events such as stroke. There are a variety of etiologies that contribute to the development of vascular cognitive impairment and VaD, and these are often associated with other dementia-related pathologies such as Alzheimer disease. The diagnosis of VaD is difficult due to the number and types of lesions and their locations in the brain. Factors that increase the risk of vascular diseases such as stroke, high blood pressure, high cholesterol, and smoking also raise the risk of VaD. Therefore, controlling these risk factors can help lower the chances of developing VaD. This update describes the subtypes of VaD, with details of their complex presentation, associated pathological lesions, and issues with diagnosis, prevention, and treatment.

Neuropathological diagnosis of vascular cognitive impairment and vascular dementia with implications for Alzheimer's disease

Vascular dementia (VaD) is recognised as a neurocognitive disorder, which is explained by numerous vascular causes in the general absence of other pathologies. The heterogeneity of cerebrovascular disease makes it challenging to elucidate the neuropathological substrates and mechanisms of VaD as well as vascular cognitive impairment (VCI). Consensus and accurate diagnosis of VaD relies on wide-ranging clinical, neuropsychometric and neuroimaging measures with subsequent pathological confirmation. Pathological diagnosis of suspected clinical VaD requires adequate postmortem brain sampling and rigorous assessment methods to identify important substrates. Factors that define the subtypes of VaD include the nature and extent of vascular pathologies, degree of involvement of extra and intracranial vessels and the anatomical location of tissue changes. Atherosclerotic and cardioembolic diseases appear the most common substrates of vascular brain injury or infarction. Small vessel disease characterised by arteriolosclerosis and lacunar infarcts also causes cortical and subcortical microinfarcts, which appear to be the most robust substrates of cognitive impairment. Diffuse WM changes with loss of myelin and axonal abnormalities are common to almost all subtypes of VaD. Medial temporal lobe and hippocampal atrophy accompanied by variable hippocampal sclerosis are also features of VaD as they are of Alzheimer’s disease. Recent observations suggest that there is a vascular basis for neuronal atrophy in both the temporal and frontal lobes in VaD that is entirely independent of any Alzheimer pathology. Further knowledge on specific neuronal and dendro-synaptic changes in key regions resulting in executive dysfunction and other cognitive deficits, which define VCI and VaD, needs to be gathered. Hereditary arteriopathies such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy or CADASIL have provided insights into the mechanisms of dementia associated with cerebral small vessel disease. Greater understanding of the neurochemical and molecular investigations is needed to better define microvascular disease and vascular substrates of dementia. The investigation of relevant animal models would be valuable in exploring the pathogenesis as well as prevention of the vascular causes of cognitive impairment.

Structural polymorphism of Alzheimer Abeta and other amyloid fibrils

Deposits of amyloid fibrils characterize a diverse group of human diseases that includes Alzheimer's disease, Creutzfeldt-Jakob disease and type II diabetes. Amyloid fibrils formed from different polypeptides contain a common cross-beta spine. Nevertheless, amyloid fibrils formed from the same polypeptide can occur in a range of structurally different morphologies. The heterogeneity of amyloid fibrils reflects different types of polymorphism: (i) variations in the protofilament number, (ii) variations in the protofilament arrangement and (iii) different polypeptide conformations. Amyloid fibril polymorphism implies that fibril formation can lead, for the same polypeptide sequence, to many different patterns of inter- or intra-residue interactions. This property differs significantly from native, monomeric protein folding reactions that produce, for one protein sequence, only one ordered conformation and only one set of inter-residue interactions.

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.

Cerebral amyloid angiopathy and cortical microinfarcts as putative substrates of vascular dementia

BACKGROUND AND PURPOSE

Vascular dementia (VaD) has occasionally been associated with cerebral amyloid angiopathy (CAA), but the prevalence and significance of this counterintuitive relationship are poorly known. Therefore, we investigated the presence and characteristics of CAA in brains of VaD cases.

METHODS

We examined temporal and parietal regions of the cerebral cortex of 26 consecutive VaD cases from the Lund Longitudinal Dementia Study. We carried out immunohistochemistry and routine stainings, determined Apolipoprotein E (ApoE) genotypes, and obtained clinical characteristics on the studied group for retrospective analysis.

RESULTS

CAA was marked in eight out of 26 cases, and correlated strongly with the presence of cortical microinfarcts, both in the temporal lobe and in the parietal lobe. Based on comparisons with eight age-matched VaD cases without CAA, the clinical records suggested that VaD cases with CAA as a group exhibited less pronounced neurological symptoms. A clear contribution of the ApoE genotype could not be identified.

CONCLUSIONS

Based on a combination of the clinical and pathological data, we suggest that microinfarcts in the cerebral cortex associated with severe CAA may be the primary pathological substrate in a significant proportion of VaD cases. Future studies should be undertaken to confirm or dismiss the hypothesis that these cases exhibit a different symptom profile than VaD cases without CAA.

Severe cerebral amyloid angiopathy characterizes an underestimated variant of vascular dementia

Cerebral amyloid angiopathy (CAA) is a frequent finding on neuropathological examination of patients with Alzheimer's disease (AD). A recent study from our laboratory showed that CAA also frequently occurred in vascular dementia with additional mild Alzheimer encephalopathy (VaD-ae, i.e. Alzheimer pathology that does not fulfill criteria for AD). Because CAA is associated with cerebral hemorrhages and infarctions, it is of significant interest to confirm or dismiss the hypothesis that CAA contributes clinically in the many patients that present with VaD-ae. Therefore, we examined entire temporal lobes of 11 VaD-ae cases and 11 age-matched AD cases with Abeta immunohistochemistry. Six of 11 VaD-ae cases had severe CAA, more extensive than in any AD case. There was a trend toward more cortical infarctions in this group, indicating that CAA in VaD may be of clinical importance and an underestimated cause of dementia.

Towards defining the neuropathological substrates of vascular dementia

Cerebrovascular disease is highly heterogeneous but can culminate in vascular cognitive impairment or vascular dementia (VaD). As much as the clinical diagnosis warrants scrutiny, the neuropathological substrates of VaD also need to be better defined. Atherosclerosis and small vessel disease are the main causes of brain infarction. Lacunar infarcts or multiple microinfarcts in the basal ganglia, thalamus, brainstem and white matter are associated with more than half of VaD cases consistent with subcortical ischaemic VaD. White matter changes including regions of incomplete infarction are usually widespread in VaD, but their contribution to impairment is not explicit. Other pathologies including hippocampal injury and Alzheimer type of lesions may also modify the course of dementia. Similar to other common dementias consensus criteria for VaD need unambiguous definition to impact on preventative and treatment strategies and are critical for selective recruitment to clinical trials.

Dementia in hereditary cerebral hemorrhage with amyloidosis-Dutch type is associated with cerebral amyloid angiopathy but is independent of plaques and neurofibrillary tangles

Cerebral amyloid angiopathy is frequently found in demented and nondemented elderly persons, but its contribution to the causation of dementia is unknown. Therefore, we investigated the relation between the amount of cerebral amyloid angiopathy and the presence of dementia in 19 patients with hereditary cerebral hemorrhage with amyloidosis-Dutch type. The advantage of studying hereditary cerebral hemorrhage in amyloidosis-Dutch type is that patients with this disease consistently have severe cerebral amyloid angiopathy with minimal neurofibrillary pathology. The amount of cerebral amyloid angiopathy, as quantified by computerized morphometry, was strongly associated with the presence of dementia independent of neurofibrillary pathology, plaque density, or age. The number of cortical amyloid beta-laden severely stenotic vessels, vessel-within-vessel configurations, and cerebral amyloid angiopathy-associated microvasculopathies was associated with the amount of cerebral amyloid angiopathy and dementia. A semiquantitative score, based on the number of amyloid beta-laden severely stenotic vessels, completely separated demented from nondemented patients. These results suggest that extensive (more than 15 amyloid beta-laden severely stenotic vessels in five frontal cortical sections) cerebral amyloid angiopathy alone is sufficient to cause dementia in hereditary cerebral hemorrhage with amyloidosis-Dutch type. This may have implications for clinicopathological correlations in Alzheimer's disease and other dementias with cerebral amyloid angiopathy.

CD40 signaling and Alzheimer's disease pathogenesis

The interaction between CD40 and its cognate ligand, CD40 ligand, is a primary regulator of the peripheral immune response, including modulation of T lymphocyte activation, B lymphocyte differentiation and antibody secretion, and innate immune cell activation, maturation, and survival. Recently, we and others have identified CD40 expression on a variety of CNS cells, including endothelial cells, smooth muscle cells, astroglia and microglia, and have found that, on many of these cells, CD40 expression is enhanced by pro-inflammatory stimuli. Importantly, the CD40-CD40 ligand interaction on microglia triggers a series of intracellular signaling events that are discussed, beginning with Src-family kinase activation and culminating in microglial activation as evidenced by tumor necrosis factor-alpha secretion. Based on the involvement of microglial activation and brain inflammation in Alzheimer's disease pathogenesis, we have investigated co-stimulation of microglia, smooth muscle, and endothelial cells with CD40 ligand in the presence of low doses of freshly solubilized amyloid-beta peptides. Data reviewed herein show that CD40 ligand and amyloid-beta act synergistically to promote pro-inflammatory responses by these cells, including secretion of interleukin-1 beta by endothelial cells and tumor necrosis factor-alpha by microglia. As these cytokines have been implicated in neuronal injury, a comprehensive model of pro-inflammatory CD40 ligand and amyloid-beta initiated Alzheimer's disease pathogenesis (mediated by multiple CNS cells) is proposed.

Advances in molecular genetics and pathology of cerebrovascular disorders

Progress in molecular genetics has enabled the dissection of several autosomal dominantly inherited forms of cerebrovascular disorders. Mutations in diverse genes might induce pathological changes in intracranial vessels, resulting in cerebral haemorrhages and ischaemic strokes. Such pathologies, however, might also result from systemic vascular disease caused by mutations or polymorphisms in genes that regulate cardiovascular physiology, blood coagulation, lipid metabolism and metabolic functions. Interestingly, several mutations that directly affect CNS vasculature involve genes that control inter- or intracellular signalling functions. Although highly variable phenotypes make it difficult to pinpoint the genotypes, genetic characterization of cerebrovascular disorders is valuable for understanding the pathogenesis and management of sporadic disease.

Cerebral amyloid angiopathy: accumulation of A beta in interstitial fluid drainage pathways in Alzheimer's disease

Cerebral amyloid angiopathy (CAA) is characterized by the accumulation of beta-amyloid (A beta) peptides in the walls of arteries both in the cortex and meninges. Here, we test the hypothesis that CAA results from the progressive accumulation of A beta in the perivascular interstitial fluid drainage pathways of the brain. Experimental studies have shown that interstitial fluid (ISF) from the rat brain flows along periarterial spaces to join the cerebrospinal fluid (CSF) to drain to cervical lymph nodes. Such lymphatic drainage plays a key role in B-cell and T-cell mediated immunity of the brain. Anatomical studies have defined periarterial ISF drainage pathways in the human brain that are homologous with the lymphatic pathways in the rat brain but are largely separate from the CSF. Periarterial channels in the brain in man are in continuity with those of leptomeningeal arteries and can be traced from the brain to the extracranial portions of the internal carotid arteries related to deep cervical lymph nodes. The pattern of deposition of A beta in senile plaques and in CAA suggests that A beta accumulates in pericapillary and periarterial ISF drainage pathways. A beta could accumulate in CAA due to either (i) increased production of A beta, (ii) reduced solubility of A beta peptides, or (iii) impedance of drainage of A beta along periarterial ISF drainage pathways within the brain and leptomeninges due to aging factors in cerebral arteries. Elucidation of factors that reduce elimination of A beta via perivascular drainage pathways may lead to their rectification and to new strategies for treatment of Alzheimer's disease.

Clinical and radiological determinants of prestroke cognitive decline in a stroke cohort

OBJECTIVES

Stroke seems to be related to dementia more often than previously assumed and vascular factors are also related to Alzheimer's disease. The pathophysiology of poststroke dementia includes ischaemic changes in the brain, a combination of degenerative and vascular changes, and changes only related to Alzheimer's disease. Some cognitive decline recognised after a stroke may be due to pre-existing cognitive decline. The aim of this study was to determine the clinical and radiological determinants of prestroke cognitive decline.

METHODS

The study group comprised 337 of 486 consecutive patients aged 55 to 85 years who 3 months after ischaemic stroke completed a comprehensive neuropsychological test battery; structured medical, neurological, and mental status examination; interview of a knowledgeable informant containing structured questions on abnormality in the cognitive functions; assessment of social functions before the index stroke; and MRI.

RESULTS

Frequency of prestroke cognitive decline including that of dementia was 9.2% (31/337). The patients with prestroke cognitive decline were older, more often had less than 6 years of education, and had history of previous stroke. Vascular risk factors did not differ significantly between these two groups. White matter changes (p=0.004), cortical entorhinal, hippocampal, and medial temporal atrophy (p<0.001), cortical frontal atrophy (p=0.008); and any central atrophy (p<0.01), but not the frequencies or volumes of old, silent, or all infarcts on MRI differentiated those with and without prestroke cognitive decline. The correlates of prestroke cognitive decline in logistic regression analysis were medial temporal cortical atrophy (odds ratio (OR) 7.5, 95% confidence interval (95%CI) 3.2-18.2), history of previous ischaemic stroke (OR 4.4, 95% CI 1.8-10.6), and education (OR 0.9, 95% CI 0.8-0.9).

CONCLUSIONS

History of previous stroke, but not volumes or frequencies was found to correlate with prestroke cognitive decline. Other associating factors were rather those usually associated with degenerative dementia: white matter changes and cerebral atrophy; and in multiple models medial temporal cortical atrophy and education. The possible overlap between two or more underlying diseases must be remembered in diagnosis and treatment of patients with vascular cognitive impairment.

Alzheimer's beta-amyloid peptides induce inflammatory cascade in human vascular cells: the roles of cytokines and CD40

Accumulating evidence suggests that beta-amyloid (Abeta)-induced inflammatory reactions may partially drive the pathogenesis of Alzheimer's disease (AD). Recent data also implicate similar inflammatory processes in cerebral amyloid angiopathy (CAA). To evaluate the roles of Abeta in the inflammatory processes in vascular tissues, we have tested the ability of Abeta to trigger inflammatory responses in cultured human vascular cells. We found that stimulation with Abeta dose-dependently increased the expression of CD40, and secretion of interferon-gamma (IFN-gamma) and interleukin-1beta (IL-1beta) in endothelial cells. Abeta also induced expression of IFN-gamma receptor (IFN-gammaR) both in endothelial and smooth muscle cells. Characterization of the Abeta-induced inflammatory responses in the vascular cells showed that the ligation of CD40 further increased cytokine production and/or the expression of IFN-gammaR. Moreover, IL-1beta and IFN-gamma synergistically increased the Abeta-induced expression of CD40 and IFN-gammaR. We have recently found that Abeta induces expression of adhesion molecules, and that cytokine production and interaction of CD40-CD40 ligand (CD40L) further increase the Abeta-induced expression of adhesion molecules in these same cells. These results suggest that Abeta can function as an inflammatory stimulator to activate vascular cells and induces an auto-amplified inflammatory molecular cascade, through interactions among adhesion molecules, CD40-CD40L and cytokines. Additionally, Abeta1-42, the more pathologic form of Abeta, induces much stronger effects in endothelial cells than in smooth muscle cells, while the reverse is true for Abeta1-40. Collectively, these findings support the hypothesis that the Abeta-induced inflammatory responses in vascular cells may play a significant role in the pathogenesis of CAA and AD.