Apolipoprotein E co-localizes with newly formed amyloid beta-protein (Abeta) deposits lacking immunoreactivity against N-terminal epitopes of Abeta in a genotype-dependent manner

Synergism of ApoE4 and systemic infectious burden is mediated by the APOE-NLRP3 axis in Alzheimer's disease

BACKGROUND

Systemic infections are associated with the development of AD, especially in individuals carrying the APOE4 genotype. However, the detailed mechanism through which APOE4 affects microglia inflammatory response remains unclear.

METHODS

We obtained human snRNA-seq data from the Synapse AD Knowledge Portal and assessed the DEGs between APOE3 and APOE4 isoforms in microglia. To verify the interaction between ApoE and infectious products, we used ApoE to stimulate in vitro and in vivo models in the presence or absence of LPS (or ATP). The NLRP3 gene knockout experiment was performed to demonstrate whether the APOE-NLRP3 axis was indispensable for microglia to regulate inflammation and mitochondrial autophagy. Results were evaluated by biochemical analyses and fluorescence imaging.

RESULTS

Compared with APOE3, up-regulated genes in APOE4 gene carriers were involved in pro-inflammatory responses. ApoE4-stimulation significantly increased the levels of NLRP3 inflammasomes and ROS in microglia. Moreover, compared with ApoE4 alone, the co-incubation of ApoE4 with LPS (or ATP) markedly promoted pyroptosis. Both NF-κB activation and mitochondrial autophagy dysfunction were contributed by the increased level of NLRP3 inflammasomes induced by ApoE4. Furthermore, the pathological impairment induced by ApoE4 could be reversed by NLRP3 KO.

CONCLUSIONS

Our study highlights the importance of NLRP3 inflammasomes in linking ApoE4 with microglia innate immune function. These findings not only provide a molecular basis for APOE4-mediated neuroinflammatory but also reveal the potential reason for the increased risk of AD in APOE4 gene carriers after contracting infectious diseases.

Current understanding of the interactions between metal ions and Apolipoprotein E in Alzheimer's disease

Alzheimer's disease (AD), the most common type of dementia in the elderly, is a chronic and progressive neurodegenerative disorder with no effective disease-modifying treatments to date. Studies have shown that an imbalance in brain metal ions, such as zinc, copper, and iron, is closely related to the onset and progression of AD. Many efforts have been made to understand metal-related mechanisms and therapeutic strategies for AD. Emerging evidence suggests that interactions of brain metal ions and apolipoprotein E (ApoE), which is the strongest genetic risk factor for late-onset AD, may be one of the mechanisms for neurodegeneration. Here, we summarize the key points regarding how metal ions and ApoE contribute to the pathogenesis of AD. We further describe the interactions between metal ions and ApoE in the brain and propose that their interactions play an important role in neuropathological alterations and cognitive decline in AD.

Lipid Peroxidation Induced ApoE Receptor-Ligand Disruption as a Unifying Hypothesis Underlying Sporadic Alzheimer's Disease in Humans

BACKGROUND

Sporadic Alzheimer's disease (sAD) lacks a unifying hypothesis that can account for the lipid peroxidation observed early in the disease, enrichment of ApoE in the core of neuritic plaques, hallmark plaques and tangles, and selective vulnerability of entorhinal-hippocampal structures.

OBJECTIVE

We hypothesized that 1) high expression of ApoER2 (receptor for ApoE and Reelin) helps explain this anatomical vulnerability; 2) lipid peroxidation of ApoE and ApoER2 contributes to sAD pathogenesis, by disrupting neuronal ApoE delivery and Reelin-ApoER2-Dab1 signaling cascades.

METHODS

In vitro biochemical experiments; Single-marker and multiplex fluorescence-immunohistochemistry (IHC) in postmortem specimens from 26 individuals who died cognitively normal, with mild cognitive impairment or with sAD.

RESULTS

ApoE and ApoER2 peptides and proteins were susceptible to attack by reactive lipid aldehydes, generating lipid-protein adducts and crosslinked ApoE-ApoER2 complexes. Using in situ hybridization alongside IHC, we observed that: 1) ApoER2 is strongly expressed in terminal zones of the entorhinal-hippocampal 'perforant path' projections that underlie memory; 2) ApoE, lipid aldehyde-modified ApoE, Reelin, ApoER2, and the downstream Reelin-ApoER2 cascade components Dab1 and Thr19-phosphorylated PSD95 accumulated in the vicinity of neuritic plaques in perforant path terminal zones in sAD cases; 3) several ApoE/Reelin-ApoER2-Dab1 pathway markers were higher in sAD cases and positively correlated with histological progression and cognitive deficits.

CONCLUSION

Results demonstrate derangements in multiple ApoE/Reelin-ApoER2-Dab1 axis components in perforant path terminal zones in sAD and provide proof-of-concept that ApoE and ApoER2 are vulnerable to aldehyde-induced adduction and crosslinking. Findings provide the foundation for a unifying hypothesis implicating lipid peroxidation of ApoE and ApoE receptors in sAD.

Apolipoprotein E Proteinopathy Is a Major Dementia-Associated Pathologic Biomarker in Individuals with or without the APOE Epsilon 4 Allele

The amygdala is vulnerable to multiple or "mixed" mis-aggregated proteins associated with neurodegenerative conditions that can manifest clinically with amnestic dementia; the amygdala region is often affected even at earliest disease stages. With the original intent of identifying novel dementia-associated proteins, the detergent-insoluble proteome was characterized from the amygdalae of 40 participants from the University of Kentucky Alzheimer's Disease Center autopsy cohort. These individuals encompassed a spectrum of clinical conditions (cognitively normal to severe amnestic dementia). Polypeptides from the detergent-insoluble fraction were interrogated using liquid chromatography-electrospray ionization-tandem mass spectrometry. As anticipated, portions of peptides previously associated with neurologic diseases were enriched from subjects with dementia. Among all detected peptides, Apolipoprotein E (ApoE) stood out: even more than the expected Tau, APP/Aβ, and α-Synuclein peptides, ApoE peptides were strongly enriched in dementia cases, including from individuals lacking the APOE ε4 genotype. The amount of ApoE protein detected in detergent-insoluble fractions was robustly associated with levels of complement proteins C3 and C4. Immunohistochemical staining of APOE ε3/ε3 subjects' amygdalae confirmed ApoE co-localization with C4 in amyloid plaques. Thus, analyses of human amygdala proteomics indicate that rather than being only an "upstream" genetic risk factor, ApoE is an aberrantly aggregated protein in its own right, and show that the ApoE protein may play active disease-driving mechanistic roles in persons lacking the APOE ε4 allele.

Apolipoprotein E and Alzheimer's Disease: Findings, Hypotheses, and Potential Mechanisms

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder that involves dysregulation of many cellular and molecular processes. It is notoriously difficult to develop therapeutics for AD due to its complex nature. Nevertheless, recent advancements in imaging technology and the development of innovative experimental techniques have allowed researchers to perform in-depth analyses to uncover the pathogenic mechanisms of AD. An important consideration when studying late-onset AD is its major genetic risk factor, apolipoprotein E4 (apoE4). Although the exact mechanisms underlying apoE4 effects on AD initiation and progression are not fully understood, recent studies have revealed critical insights into the apoE4-induced deficits that occur in AD. In this review, we highlight notable studies that detail apoE4 effects on prominent AD pathologies, including amyloid-β, tau pathology, neuroinflammation, and neural network dysfunction. We also discuss evidence that defines the physiological functions of apoE and outlines how these functions are disrupted in apoE4-related AD. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease, Volume 17 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Brain injury-induced dysfunction of the blood brain barrier as a risk for dementia

The blood-brain barrier (BBB) is a complex and dynamic physiological interface between brain parenchyma and cerebral vasculature. It is composed of closely interacting cells and signaling molecules that regulate movement of solutes, ions, nutrients, macromolecules, and immune cells into the brain and removal of products of normal and abnormal brain cell metabolism. Dysfunction of multiple components of the BBB occurs in aging, inflammatory diseases, traumatic brain injury (TBI, severe or mild repetitive), and in chronic degenerative dementing disorders for which aging, inflammation, and TBI are considered risk factors. BBB permeability changes after TBI result in leakage of serum proteins, influx of immune cells, perivascular inflammation, as well as impairment of efflux transporter systems and accumulation of aggregation-prone molecules involved in hallmark pathologies of neurodegenerative diseases with dementia. In addition, cerebral vascular dysfunction with persistent alterations in cerebral blood flow and neurovascular coupling contribute to brain ischemia, neuronal degeneration, and synaptic dysfunction. While the idea of TBI as a risk factor for dementia is supported by many shared pathological features, it remains a hypothesis that needs further testing in experimental models and in human studies. The current review focusses on pathological mechanisms shared between TBI and neurodegenerative disorders characterized by accumulation of pathological protein aggregates, such as Alzheimer's disease and chronic traumatic encephalopathy. We discuss critical knowledge gaps in the field that need to be explored to clarify the relationship between TBI and risk for dementia and emphasize the need for longitudinal in vivo studies using imaging and biomarkers of BBB dysfunction in people with single or multiple TBI.

Side-chain moieties from the N-terminal region of Aβ are Involved in an oligomer-stabilizing network of interactions

Oligomeric forms of the Aβ peptide represent the most probable neurotoxic agent in Alzheimer’s disease. The dynamic and heterogeneous character of these oligomers makes their structural characterization by classic methods difficult. Native mass spectrometry, when supported by additional gas phase techniques, like ion mobility separation and hydrogen-deuterium exchange (IM-HDX-MS), enable analysis of different oligomers coexisting in the sample and may provide species-specific structural information for each oligomeric form populated in the gas phase. Here, we have combined these three techniques to obtain insight into the structural properties of oligomers of Aβ1–40 and two variants with scrambled sequences. Gas-phase HDX-MS revealed a sequence-specific engagement of the side-chains of residues located at the N-terminal part of the peptide in a network of oligomer-stabilizing interactions. Oligomer-specific interactions were no longer observed in the case of the fully scrambled sequence. Also, the ability to form alternative structures, observed for WT Aβ peptide, was lost upon scrambling. Our data underscore a role for the N-terminal residues in shaping the equilibria of oligomeric forms. Although the peptide lacking the N-terminal 1–16 residues (p3 peptide) is thought to be benign, the role of the N-terminus has not been sufficiently characterized yet. We speculate that the interaction networks revealed here may be crucial for enabling structural transitions necessary to obtain mature parallel cross-β structures from smaller antiparallel oligomers. We provide a hypothetical molecular model of the trajectory that allows a gradual conversion from antiparallel to parallel oligomers without decomposition of oligomers. Oligomer-defining interactions involving the Aβ peptide N-terminus may be important in production of the neurotoxic forms and thus should not be neglected.

Modified amyloid variants in pathological subgroups of β-amyloidosis

Objective

Amyloid β (Aβ) depositions in plaques and cerebral amyloid angiopathy (CAA) represent common features of Alzheimer's disease (AD). Sequential deposition of post-translationally modified Aβ in plaques characterizes distinct biochemical stages of Aβ maturation. However, the molecular composition of vascular Aβ deposits in CAA and its relation to plaques remain enigmatic.

Methods

Vascular and parenchymal deposits were immunohistochemically analyzed for pyroglutaminated and phosphorylated Aβ in the medial temporal and occipital lobe of 24 controls, 27 pathologically-defined preclinical AD, and 20 symptomatic AD cases.

Results

Sequential deposition of Aβ in CAA resembled Aβ maturation in plaques and enabled the distinction of three biochemical stages of CAA. B-CAA stage 1 was characterized by deposition of Aβ in the absence of pyroglutaminated Aβ_N3pE and phosphorylated Aβ_pS8. B-CAA stage 2 showed additional Aβ_N3pE and B-CAA stage 3 additional Aβ_pS8. Based on the Aβ maturation staging in CAA and plaques, three case groups for Aβ pathology could be distinguished: group 1 with advanced Aβ maturation in CAA; group 2 with equal Aβ maturation in CAA and plaques; group 3 with advanced Aβ maturation in plaques. All symptomatic AD cases presented with end-stage plaque maturation, whereas CAA could exhibit immature Aβ deposits. Notably, Aβ pathology group 1 was associated with arterial hypertension, and group 2 with the development of dementia.

Interpretation

Balance of Aβ maturation in CAA and plaques defines distinct pathological subgroups of β-amyloidosis. The association of CAA-related Aβ maturation with cognitive decline, the individual contribution of CAA and plaque pathology to the development of dementia within the defined Aβ pathology subgroups, and the subgroup-related association with arterial hypertension should be considered for differential diagnosis and therapeutic intervention.

ApoE4 Accelerates Early Seeding of Amyloid Pathology

Accumulation and aggregation of amyloid-β (Aβ) in the brain is an initiating step in the pathogenesis of Alzheimer's disease (AD). The ε4 allele of apolipoprotein E (apoE) gene is the strongest genetic risk factor for late-onset AD. Although there is strong evidence showing that apoE4 enhances amyloid pathology, it is not clear what the critical stage(s) is during amyloid development in which apoE4 has the strongest impact. Using apoE inducible mouse models, we show that increased expression of astrocytic apoE4, but not apoE3, during the seeding stage of amyloid development enhanced amyloid deposition and neuritic dystrophy in amyloid model mice. ApoE4, but not apoE3, significantly increased brain Aβ half-life measured by in vivo microdialysis. Furthermore, apoE4 expression increased whereas apoE3 reduced amyloid-related gliosis in the mouse brains. Together, our results demonstrate that apoE4 has the greatest impact on amyloid during the seeding stage, likely by perturbing Aβ clearance and enhancing Aβ aggregation.

Distinct spatiotemporal accumulation of N-truncated and full-length amyloid-β42 in Alzheimer's disease

Accumulation of amyloid-β peptides is a dominant feature in the pathogenesis of Alzheimer's disease; however, it is not clear how individual amyloid-β species accumulate and affect other neuropathological and clinical features in the disease. Thus, we compared the accumulation of N-terminally truncated amyloid-β and full-length amyloid-β, depending on disease stage as well as brain area, and determined how these amyloid-β species respectively correlate with clinicopathological features of Alzheimer's disease. To this end, the amounts of amyloid-β species and other proteins related to amyloid-β metabolism or Alzheimer's disease were quantified by enzyme-linked immunosorbent assays (ELISA) or theoretically calculated in 12 brain regions, including neocortical, limbic and subcortical areas from Alzheimer's disease cases (n = 19), neurologically normal elderly without amyloid-β accumulation (normal ageing, n = 13), and neurologically normal elderly with cortical amyloid-β accumulation (pathological ageing, n = 15). We observed that N-terminally truncated amyloid-β42 and full-length amyloid-β42 accumulations distributed differently across disease stages and brain areas, while N-terminally truncated amyloid-β40 and full-length amyloid-β40 accumulation showed an almost identical distribution pattern. Cortical N-terminally truncated amyloid-β42 accumulation was increased in Alzheimer's disease compared to pathological ageing, whereas cortical full-length amyloid-β42 accumulation was comparable between Alzheimer's disease and pathological ageing. Moreover, N-terminally truncated amyloid-β42 were more likely to accumulate more in specific brain areas, especially some limbic areas, while full-length amyloid-β42 tended to accumulate more in several neocortical areas, including frontal cortices. Immunoprecipitation followed by mass spectrometry analysis showed that several N-terminally truncated amyloid-β42 species, represented by pyroglutamylated amyloid-β11-42, were enriched in these areas, consistent with ELISA results. N-terminally truncated amyloid-β42 accumulation showed significant regional association with BACE1 and neprilysin, but not PSD95 that regionally associated with full-length amyloid-β42 accumulation. Interestingly, accumulations of tau and to a greater extent apolipoprotein E (apoE, encoded by APOE) were more strongly correlated with N-terminally truncated amyloid-β42 accumulation than those of other amyloid-β species across brain areas and disease stages. Consistently, immunohistochemical staining and in vitro binding assays showed that apoE co-localized and bound more strongly with pyroglutamylated amyloid-β11-x fibrils than full-length amyloid-β fibrils. Retrospective review of clinical records showed that accumulation of N-terminally truncated amyloid-β42 in cortical areas was associated with disease onset, duration and cognitive scores. Collectively, N-terminally truncated amyloid-β42 species have spatiotemporal accumulation patterns distinct from full-length amyloid-β42, likely due to different mechanisms governing their accumulations in the brain. These truncated amyloid-β species could play critical roles in the disease by linking other clinicopathological features of Alzheimer's disease.

The domestic cat as a natural animal model of Alzheimer's disease

Introduction

Alzheimer’s disease (AD) is the most dominant neurodegenerative disorder that causes dementia, and no effective treatments are available. To study its pathogenesis and develop therapeutics, animal models representing its pathologies are needed. Although many animal species develop senile plaques (SP) composed of amyloid-β (Aβ) proteins that are identical to those found in humans, none of them exhibit neurofibrillary tangles (NFT) and subsequent neurodegeneration, which are integral parts of the pathology of AD.

Results

The present study shows that Aβ accumulation, NFT formation, and significant neuronal loss all emerge naturally in the hippocampi of aged domestic cats. The NFT that form in the cat brain are identical to those seen in human AD in terms of their spatial distribution, the cells they affect, and the tau isoforms that comprise them. Interestingly, aged cats do not develop mature argyrophilic SP, but instead accumulate intraneuronal Aβ oligomers in their hippocampal pyramidal cells, which might be due to the amino acid sequence of felid Aβ.

Conclusions

These results suggest that Aβ oligomers are more important than SP for NFT formation and the subsequent neurodegeneration. The domestic cat is a unique animal species that naturally replicates various AD pathologies, especially Aβ oligomer accumulation, NFT formation, and neuronal loss.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0258-3) contains supplementary material, which is available to authorized users.

Amyloid-β peptides act as allosteric modulators of cholinergic signalling through formation of soluble BAβACs

Amyloid-β peptides, through highly sophisticated enzymatic machinery, are universally produced and released in an action potential synchronized manner into the interstitial fluids in the brain. Yet no native functions are attributed to amyloid-β. The amyloid-β hypothesis ascribes just neurotoxicity properties through build-up of soluble homomeric amyloid-β oligomers or fibrillar deposits. Apolipoprotein-ε4 (APOE4) allele is the only confirmed genetic risk factor of sporadic Alzheimer’s disease; once more it is unclear how it increases the risk of Alzheimer’s disease. Similarly, central cholinergic signalling is affected selectively and early in the Alzheimer’s disease brain, again why cholinergic neurons show this sensitivity is still unclear. However, the three main known Alzheimer’s disease risk factors, advancing age, female gender and APOE4, have been linked to a high apolipoprotein-E and accumulation of the acetylcholine degrading enzyme, butyrylcholinesterase in cerebrospinal fluids of patients. Furthermore, numerous reports indicate that amyloid-β interacts with butyrylcholinesterase and apolipoprotein-E. We have proposed that this interaction leads to formation of soluble ultrareactive acetylcholine-hydrolyzing complexes termed BAβACs, to adjust at demand both synaptic and extracellular acetylcholine signalling. This hypothesis predicted presence of acetylcholine-synthesizing enzyme, choline acetyltransferase in extracellular fluids to allow maintenance of equilibrium between breakdown and synthesis of acetylcholine through continuous in situ syntheses. A recent proof-of-concept study led to the discovery of this enzyme in the human extracellular fluids. We report here that apolipoprotein-E, in particular ε4 isoprotein acts as one of the strongest endogenous anti-amyloid-β fibrillization agents reported in the literature. At biological concentrations, apolipoprotein-E prevented amyloid-β fibrillization for at least 65 h. We show that amyloid-β interacts readily in an apolipoprotein-facilitated manner with butyrylcholinesterase, forming highly stable and soluble complexes, BAβACs, which can be separated in their native states by sucrose density gradient technique. Enzymological analyses further evinced that amyloid-β concentration dependently increased the acetylcholine-hydrolyzing capacity of cholinesterases. In silico biomolecular analysis further deciphered the allosteric amino acid fingerprint of the amyloid-β-cholinesterase molecular interaction in formation of BAβACs. In the case of butyrylcholinesterase, the results indicated that amyloid-β interacts with a putative activation site at the mouth of its catalytic tunnel, most likely leading to increased acetylcholine influx into the catalytic site, and thereby increasing the intrinsic catalytic rate of butyrylcholinesterase. In conclusion, at least one of the native physiological functions of amyloid-β is allosteric modulation of the intrinsic catalytic efficiency of cholinesterases, and thereby regulation of synaptic and extrasynaptic cholinergic signalling. High apolipoprotein-E may pathologically alter the biodynamics of this amyloid-β function.

The multifaceted nature of amyloid precursor protein and its proteolytic fragments: friends and foes

The amyloid precursor protein (APP) has occupied a central position in Alzheimer's disease (AD) pathophysiology, in large part due to the seminal role of amyloid-β peptide (Aβ), a proteolytic fragment derived from APP. Although the contribution of Aβ to AD pathogenesis is accepted by many in the research community, recent studies have unveiled a more complicated picture of APP's involvement in neurodegeneration in that other APP-derived fragments have been shown to exert pathological influences on neuronal function. However, not all APP-derived peptides are neurotoxic, and some even harbor neuroprotective effects. In this review, we will explore this complex picture by first discussing the pleiotropic effects of the major APP-derived peptides cleaved by multiple proteases, including soluble APP peptides (sAPPα, sAPPβ), various C- and N-terminal fragments, p3, and APP intracellular domain fragments. In addition, we will highlight two interesting sequences within APP that likely contribute to this duality in APP function. First, it has been found that caspase-mediated cleavage of APP in the cytosolic region may release a cytotoxic peptide, C31, which plays a role in synapse loss and neuronal death. Second, recent studies have implicated the -YENPTY- motif in the cytoplasmic region as a domain that modulates several APP activities through phosphorylation and dephosphorylation of the first tyrosine residue. Thus, this review summarizes the current understanding of various APP proteolytic products and the interplay among them to gain deeper insights into the possible mechanisms underlying neurodegeneration and AD pathophysiology.

Anti-ApoE antibody given after plaque onset decreases Aβ accumulation and improves brain function in a mouse model of Aβ amyloidosis

Apolipoprotein E (apoE) is the strongest known genetic risk factor for late onset Alzheimer's disease (AD). It influences amyloid-β (Aβ) clearance and aggregation, which likely contributes in large part to its role in AD pathogenesis. We recently found that HJ6.3, a monoclonal antibody against apoE, significantly reduced Aβ plaque load when given to APPswe/PS1ΔE9 (APP/PS1) mice starting before the onset of plaque deposition. To determine whether the anti-apoE antibody HJ6.3 affects Aβ plaques, neuronal network function, and behavior in APP/PS1 mice after plaque onset, we administered HJ6.3 (10 mg/kg/week) or PBS intraperitoneally to 7-month-old APP/PS1 mice for 21 weeks. HJ6.3 mildly improved spatial learning performance in the water maze, restored resting-state functional connectivity, and modestly reduced brain Aβ plaque load. There was no effect of HJ6.3 on total plasma cholesterol or cerebral amyloid angiopathy. To investigate the underlying mechanisms of anti-apoE immunotherapy, HJ6.3 was applied to the brain cortical surface and amyloid deposition was followed over 2 weeks using in vivo imaging. Acute exposure to HJ6.3 affected the course of amyloid deposition in that it prevented the formation of new amyloid deposits, limited their growth, and was associated with occasional clearance of plaques, a process likely associated with direct binding to amyloid aggregates. Topical application of HJ6.3 for only 14 d also decreased the density of amyloid plaques assessed postmortem. Collectively, these studies suggest that anti-apoE antibodies have therapeutic potential when given before or after the onset of Aβ pathology.

Extracellular proteolysis of apolipoprotein E (apoE) by secreted serine neuronal protease

Under normal conditions, brain apolipoprotein E (apoE) is secreted and lipidated by astrocytes, then taken up by neurons via receptor mediated endocytosis. Free apoE is either degraded in intraneuronal lysosomal compartments or released. Here we identified a novel way by which apoE undergoes proteolysis in the extracellular space via a secreted neuronal protease. We show that apoE is cleaved in neuronal conditioned media by a secreted serine protease. This apoE cleavage was inhibited by PMSF and α1-antichymotrypsin, but not neuroserpin-1 or inhibitors of thrombin and cathepsin G, supporting its identity as a chymotrypsin like protease. In addition, apoE incubation with purified chymotrypsin produced a similar pattern of apoE fragments. Analysis of apoE fragments by mass spectrometry showed cleavages occurring at the C-terminal side of apoE tryptophan residues, further supporting our identification of cleavage by chymotrypsin like protease. Hippocampal neurons were more efficient in mediating this apoE cleavage than cortical neurons. Proteolysis of apoE4 generated higher levels of low molecular weight fragments compared to apoE3. Primary glial cultures released an inhibitor of this proteolytic activity. Together, these studies reveal novel mechanism by which apoE can be regulated and therefore could be useful in designing apoE directed AD therapeutic approaches.

Epilepsy: neuroinflammation, neurodegeneration, and APOE genotype

Background

Precocious development of Alzheimer-type neuropathological changes in epilepsy patients, especially in APOE ϵ4,4 carriers is well known, but not the ways in which other APOE allelic combinations influence this outcome. Frozen and paraffin-embedded tissue samples resected from superior temporal lobes of 92 patients undergoing temporal lobectomies as a treatment for medication-resistant temporal lobe epilepsy were used in this study. To determine if epilepsy-related changes reflect those in another neurological condition, analogous tissue samples harvested from 10 autopsy-verified Alzheimer brains, and from 10 neurologically and neuropathologically normal control patients were analyzed using immunofluorescence histochemistry, western immunoblot, and real-time PCR to determine genotype effects on neuronal number and size, neuronal and glial expressions of amyloid β (Aβ) precursor protein (βAPP), Aβ, apolipoprotein E (ApoE), S100B, interleukin-1α and β, and α and β secretases; and on markers of neuronal stress, including DNA/RNA damage and caspase 3 expression.

Results

Allelic combinations of APOE influenced each epilepsy-related neuronal and glial response measured as well as neuropathological change. APOE ϵ3,3 conferred greatest neuronal resilience denoted as greatest production of the acute phase proteins and low neuronal stress as assessed by DNA/RNA damage and caspase-3 expression. Among patients having an APOE ϵ2 allele, none had Aβ plaques; their neuronal sizes, like those with APOE ϵ3,3 genotype were larger than those with other genotypes. APOE ϵ4,4 conferred the weakest neuronal resilience in epilepsy as well as in Alzheimer patients, but there were no APOE genotype-dependent differences in these parameters in neurologically normal patients.

Conclusions

Our findings provide evidence that the strength of the neuronal stress response is more related to patient APOE genotype than to either the etiology of the stress or to the age of the patient, suggesting that APOE genotyping may be a useful tool in treatment decisions.

Neurochemical profile of dementia pugilistica

Dementia pugilistica (DP), a suite of neuropathological and cognitive function declines after chronic traumatic brain injury (TBI), is present in approximately 20% of retired boxers. Epidemiological studies indicate TBI is a risk factor for neurodegenerative disorders including Alzheimer disease (AD) and Parkinson disease (PD). Some biochemical alterations observed in AD and PD may be recapitulated in DP and other TBI persons. In this report, we investigate long-term biochemical changes in the brains of former boxers with neuropathologically confirmed DP. Our experiments revealed biochemical and cellular alterations in DP that are complementary to and extend information already provided by histological methods. ELISA and one-dimensional and two dimensional Western blot techniques revealed differential expression of select molecules between three patients with DP and three age-matched non-demented control (NDC) persons without a history of TBI. Structural changes such as disturbances in the expression and processing of glial fibrillary acidic protein, tau, and α-synuclein were evident. The levels of the Aβ-degrading enzyme neprilysin were reduced in the patients with DP. Amyloid-β levels were elevated in the DP participant with the concomitant diagnosis of AD. In addition, the levels of brain-derived neurotrophic factor and the axonal transport proteins kinesin and dynein were substantially decreased in DP relative to NDC participants. Traumatic brain injury is a risk factor for dementia development, and our findings are consistent with permanent structural and functional damage in the cerebral cortex and white matter of boxers. Understanding the precise threshold of damage needed for the induction of pathology in DP and TBI is vital.

Frontal cortex neuropathology in dementia pugilistica

Dementia pugilistica (DP) is associated with chronic traumatic brain injury (CTBI), and leads to a "punch drunk" syndrome characterized by impairments in memory and executive function, behavioral changes, and motor signs. Microscopic features include the accumulation of neurofibrillary tangles (NFTs), beta-amyloid (Aβ), and TAR DNA binding protein 43 (TDP-43) pathology. Here we describe detailed clinical and neuropathological data about a 55-year-old retired boxer (ApoE3/4), who presented with executive dysfunction and behavioral impairments. At autopsy, significant Aβ pathology was seen, primarily in the form of diffuse plaques. Tau pathology was extensive and was determined to be of Braak and Braak stage VI. Frontal white matter showed evidence of glial tau inclusions (astrocytes and oligodendroglia). Cerebrovascular pathology was minimal with patchy amyloid angiopathy. Inflammation was another key feature, including microglial activation and significant C1q labeling of neurons, along with NFTs. TDP-43-positive pathology was also observed. Inflammation may be a key inciting as well as propagating feature of DP neuropathology.

Apolipoprotein E level and cholesterol are associated with reduced synaptic amyloid beta in Alzheimer's disease and apoE TR mouse cortex

The apolipoprotein E4 allele (APOE4) contributes to Alzheimer's disease (AD) risk and APOE2 is protective, but the relevant cellular mechanisms are unknown. We have used flow cytometry analysis to measure apolipoprotein E (apoE) and amyloid beta peptide (Aβ) levels in large populations of synaptic terminals from AD and aged cognitively normal controls, and demonstrate that modest but significant increases in soluble apoE levels accompany elevated Aβ in AD cortical synapses and in an APP/PS1 rat model of AD. Dual labeling experiments document co-localization of apoE and Aβ in individual synapses with concentration of Aβ in a small population of apoE-positive synapses in both AD and controls. Consistent with a clearance role, the apoE level was higher in Aβ-positive synapses in control cases. In aged targeted replacement mice expressing human apoE, apoE2/4 synaptic terminals demonstrated the highest level of apoE and the lowest level of Aβ compared to apoE3/3 and apoE4/4 lines. In apoE2/4 terminals, the pattern of immunolabeling for apoE and Aβ closely resembled the pattern in human control cases, and elevated apoE was accompanied by elevated free cholesterol in apoE2/4 synaptic terminals. These results are consistent with a role for APOE in Aβ clearance in AD synapses, and suggest that optimal lipidation of apoE2 compared to E3 and E4 makes an important contribution to Aβ clearance and synaptic function.

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.

Amyloid-β protein modulates the perivascular clearance of neuronal apolipoprotein E in mouse models of Alzheimer's disease

The deposition of amyloid-β protein (Aβ) in the brain is a hallmark of Alzheimer's disease (AD). Apolipoprotein E (apoE) is involved in the clearance of Aβ from brain and the APOE ε4 allele is a major risk factor for sporadic AD. We have recently shown that apoE is drained into the perivascular space (PVS), where it co-localizes with Aβ. To further clarify the role of apoE in perivascular clearance of Aβ, we studied apoE-transgenic mice over-expressing human apoE4 either in astrocytes (GE4) or in neurons (TE4). These animals were crossbred with amyloid precursor protein (APP)-transgenic mice and with APP-presenilin-1 (APP-PS1) double transgenic mice. Using an antibody that specifically detects human apoE (h-apoE), we observed that astroglial expression of h-apoE in GE4 mice leads to its perivascular drainage, whereas neuronal expression in TE4 mice does not, indicating that neuron-derived apoE is usually not the subject of perivascular drainage. However, h-apoE was observed not only in the PVS of APP-GE4 and APP-PS1-GE4 mice, but also in that of APP-TE4 and APP-PS1-TE4 mice. In all these mouse lines, we found co-localization of neuron-derived h-apoE and Aβ in the PVS. Aβ and h-apoE were also found in the cytoplasm of perivascular astrocytes indicating that astrocytes take up the neuron-derived apoE bound to Aβ, presumably prior to its clearance into the PVS. The uptake of apoE-Aβ complexes into glial cells was further investigated in glioblastoma cells. It was mediated by α(2)macroglobulin receptor/low density lipoprotein receptor-related protein (LRP-1) and inhibited by adding receptor-associated protein (RAP). It results in endosomal Aβ accumulation within these cells. These results suggest that neuronal apoE-Aβ complexes, but not neuronal apoE alone, are substrates for LRP-1-mediated astroglial uptake, transcytosis, and subsequent perivascular drainage. Thus, the production of Aβ and its interaction with apoE lead to the pathological perivascular drainage of neuronal apoE and provide insight into the pathological interactions of Aβ with neuronal apoE metabolism.

Capillary CAA and perivascular Aβ-deposition: two distinct features of Alzheimer's disease pathology

Cerebral amyloid angiopathy (CAA) is frequently seen in Alzheimer's disease (AD) cases and represents one of its histopathological hallmarks. CAA is characterized by amyloid β-protein (Aβ) deposits within vessel walls. In addition to arteries and veins capillaries can also be affected. Aβ deposition into the capillary wall is, thereby, known as capillary CAA (capCAA) and strongly associated with the apolipoprotein E APOEε4 allele as a risk factor. Aβ deposits along the pericapillary glia limitans are described as pericapillary Aβ (pericapAβ: synonymous with pericapillary CAA in other studies). Here, we studied the relationship between pericapAβ and capCAA in 58 human autopsy cases. Although pericapAβ and capCAA were more frequently found in AD cases compared to controls and although they exhibited a correlation to one another, detailed analysis revealed that there is a significant number of cases with pericapAβ lacking capCAA and vice versa. Moreover, single capillaries show either both pathologies or pericapAβ or capCAA only. There was no local association between these pathologies when analyzing multiple capillaries in each given case. Moreover, pericapAβ predominantly exhibited Aβ(42) whereas capCAA contained both Aβ(42) and Aβ(40). These differences as well as differences in the related astroglial reaction indicate that pericapAβ and capCAA are not directly linked. PericapAβ appears to represent initial Aβ accumulation along the glia limitans that is involved in the perivascular drainage of apoE and Aβ regardless of the APOE genotype whereas capCAA could be explained by a limited transendothelial clearance of apoE4-Aβ complexes compared to apoE2/3-Aβ complexes.

Effect of ApoE epsilon4 allele on visual evoked potentials and resultant flow coupling in patients with Alzheimer

The apolipoprotein E epsilon4 (ApoE epsilon4) allele is a strong susceptibility factor for Alzheimer disease, which promotes neurodegeneration and cerebrovascular dysfunction. To address this issue in more detail, we simultaneously obtained visual evoked potentials and resultant hemodynamic responses in newly diagnosed Alzheimer patients without signs of vascular lesions on a cerebral magnetic resonance imaging (MRI) scan. Patients were grouped according to ApoE genotype (n = 19 ApoE epsilon4 carrier and n = 12 noncarrier). ApoE epsilon4 carrier had significantly longer peak latencies and a trend to higher interpeak latencies of late potential components. Potential amplitudes and hemodynamic responses were similar in both groups. At the incidental stage of disease process, it appears that the ApoE epsilon4 allele mainly promotes neuronal dysfunction rather than aggravates neurovascular dysfunction. Studies with larger patient samples are warranted to corroborate the first findings.

Differential levels of apolipoprotein E and butyrylcholinesterase show strong association with pathological signs of Alzheimer's disease in the brain in vivo

Recently, we reported that 3 of the known risk factors of Alzheimer's disease (AD), i.e., advanced age, apolipoprotein E (ApoE) ε4, and female gender, are associated with differential levels of ApoE proteins and butyrylcholinesterase (BuChE) in the cerebrospinal fluid (CSF) of AD patients. The ApoE ε4 allele and certain BuChE polymorphisms synergistically affect the conversion rate of mild cognitive impairment (MCI) to AD. Here, we investigated interrelationships between ApoE and BuChE levels, and pathological markers of AD in vivo. CSF from patients with probable AD, assessed for cerebral glucose metabolism (CMRglc; n = 50) and Pittsburgh compound B (PIB) retention (β-amyloid [Aβ] load, n = 29) by positron emission tomography (PET), was used for measurement of BuChE, ApoE, Aβ, tau, phosphorylated tau (P-tau) and interleukin-1β (IL-1β) levels. Levels of ApoE and BuChE strongly correlated with CMRglc (fluorodeoxyglucose [FDG]-PET, r = 0.54, p < 0.0001, n = 50), cerebral Aβ load (PIB retention, r = 0.73, p < 0.0001, n = 29), and CSF P-tau (r = 0.73, p < 0.0001, n = 33). High ApoE protein was tied to low CMRglc and high PIB retention and P-tau. BuChE levels had opposite relationships. Other CSF covariates were levels of interleukin-1β and Aβ(42) peptide. The pattern of the patients' cognitive Z-scores strongly supported these observations. High ApoE protein was also linked to changes in 3 of the biodynamic properties of BuChE. In vitro analysis indicated that high ApoE protein levels were related to an increased pool of dormant BuChE molecules with an abnormally high intrinsic catalytic rate in CSF, which was "turned on" by excess Aβ peptides. The findings suggest that abnormally high levels of ApoE may play a causative role in the pathological events of AD, particularly those involving the early cholinergic deficit in the AD brain, through modulation of cholinesterases activities, hence disturbing the acetylcholine-dependent activity of neurons and nonexcitable cells such as glial cells.

A megalin polymorphism associated with promoter activity and Alzheimer's disease risk

Elevated cerebral levels of amyloid beta-protein (Abeta) occur in Alzheimer's disease (AD), yet only a few patients show evidence of increased Abeta production. This observation suggests that many, perhaps most, cases of AD are caused by faulty clearance of Abeta. Megalin, which plays an important role in mediating Abeta clearance, is an attractive candidate gene for genetic association with AD. To investigate this hypothesis, we analyzed the megalin gene in a population of 2,183 subjects. Genetic analysis indicated that the rs3755166 (G/A) polymorphism located in the megalin promoter associated with risk for AD, dependently of apolipoprotein E genotype. The rs3755166 AA genotype frequency was significantly greater in AD patients than in control subjects. Furthermore, the luciferase reporter assay indicated that the rs3755166 A variant has 20% less transcriptional activity than the rs3755166 G variant. This study provides strong evidence that this megalin polymorphism confers a greater risk for AD, and supports a biological role for megalin in the neurodegenerative processes involved in AD.

Alzheimer's disease: old problem, new views from transgenic and viral models

Alzheimer's dementia is developing ever more as a complex syndrome with various unknown genetic and epigenetic contributions. These are compounded on and exacerbating the underlying amyloid and tau pathology that remain the basis of the pathological definition of Alzheimer's disease. Here, we present a selection of aspects of recent bigenic and virus-based mouse strains, developed as pre-clinical models for Alzheimer's disease. We discuss newer features in the context of the characteristics defined in previously validated transgenic models. We focus on specific aspects of single and multiple transgenic mouse models for Alzheimer's disease and for tauopathies, rather than providing an exhaustive list of all available models. We concentrate on the content of information related to neurodegeneration and disease mechanisms. We pay attention to aspects and defects that are predicted by the models and can be tested in humans. We discuss implications that help translate the fundamental knowledge into clinical, diagnostic and therapeutic applications. We elaborate on the increasing knowledge extracted from transgenic models and from newer adeno-associated viral models. We advocate this combination as a valuable strategy to study molecular, cellular and system-related pathogenic mechanisms in AD and tauopathies. We believe that innovative animal models remain needed to critically test current views, to identify and validate therapeutic targets, to allow testing of compounds, to help understand and eventually treat tauopathies, including Alzheimer's disease.

Classification and basic pathology of Alzheimer disease

The lesions of Alzheimer disease include accumulation of proteins, losses of neurons and synapses, and alterations related to reactive processes. Extracellular Abeta accumulation occurs in the parenchyma as diffuse, focal or stellate deposits. It may involve the vessel walls of arteries, veins and capillaries. The cases in which the capillary vessel walls are affected have a higher probability of having one or two apoepsilon 4 alleles. Parenchymal as well as vascular Abeta deposition follows a stepwise progression. Tau accumulation, probably the best histopathological correlate of the clinical symptoms, takes three aspects: in the cell body of the neuron as neurofibrillary tangle, in the dendrites as neuropil threads, and in the axons forming the senile plaque neuritic corona. The progression of tau pathology is stepwise and stereotyped from the entorhinal cortex, through the hippocampus, to the isocortex. The neuronal loss is heterogeneous and area-specific. Its mechanism is still discussed. The timing of the synaptic loss, probably linked to Abeta peptide itself, maybe as oligomers, is also controversial. Various clinico-pathological types of Alzheimer disease have been described, according to the type of the lesions (plaque only and tangle predominant), the type of onset (focal onset), the cause (genetic or sporadic) and the associated lesions (Lewy bodies, vascular lesions, hippocampal sclerosis, TDP-43 inclusions and argyrophilic grain disease).

[Morphologic and molecular neuropathology of Alzheimer's disease]

Alzheimer disease lesions include the abnormal accumulation of two proteins normally present in neurons: tau protein and Abeta peptide. Tau protein aggregates into fibrils in the cell body of neurons (neurofibrillary tangles), in dendrites (neuropil threads) and in degenerating axons that constitute the corona of the senile plaque. Tau pathology progresses in the brain areas in a stereotyped manner and in parallel with the clinical symptoms. Abeta extracellular deposits may be diffuse or focal. The Abeta focal deposit constitutes the core of the senile plaque. Progression of the Abeta lesions, which initially affect the isocortex, then the hippocampus, basal ganglia, various brainstem nuclei and cerebellum, is not directly correlated with symptoms. Mutations involving the genes implicated in Abeta peptide metabolism are responsible for familial Alzheimer disease. Mutations of the tau gene are not associated with Alzheimer disease but with frontotemporal dementia. The link between altered Abeta peptide metabolism and tau pathology has not been fully elucidated. Animal models mimic several aspects of the disease and have contributed to a better understanding of the mechanisms of the lesions.

Loss of gamma-secretase function impairs endocytosis of lipoprotein particles and membrane cholesterol homeostasis

Presenilins (PSs) are components of the gamma-secretase complex that mediates intramembranous cleavage of type I membrane proteins. We show that gamma-secretase is involved in the regulation of cellular lipoprotein uptake. Loss of gamma-secretase function decreased endocytosis of low-density lipoprotein (LDL) receptor. The decreased uptake of lipoproteins led to upregulation of cellular cholesterol biosynthesis by increased expression of CYP51 and enhanced metabolism of lanosterol. Genetic deletion of PS1 or transgenic expression of PS1 mutants that cause early-onset Alzheimer's disease led to accumulation of gamma-secretase substrates and mistargeting of adaptor proteins that regulate endocytosis of the LDL receptor. Consistent with decreased endocytosis of these receptors, PS1 mutant mice have elevated levels of apolipoprotein E in the brain. Thus, these data demonstrate a functional link between two major genetic factors that cause early-onset and late-onset Alzheimer's disease.

Relationships between expression of apolipoprotein E and beta-amyloid precursor protein are altered in proximity to Alzheimer beta-amyloid plaques: potential explanations from cell culture studies

Theories regarding the initiation and progression of Alzheimer disease (AD) often consider potential roles played by elevations of beta-amyloid precursor protein (betaAPP). Because it is the source of amyloid beta-peptide, betaAPP may simply contribute more pathogenic stimulus when elevated; some analyses have, however, reported a decline in betaAPP in AD. We found a progressive increase in neuronal betaAPP expression with increasing age in the brains of nondemented individuals, whereas in AD patient samples, betaAPP antigenicity decreased in neuronal somata in a manner that correlated with accumulation of mature amyloid beta-peptide plaques. In contrast, apolipoprotein E (ApoE) expression correlated with accumulation of plaques, and even greater amounts of ApoE were detected in plaques. Induction of betaAPP by glutamate in neuronal cell cultures was found to depend upon ApoE levels or activity. Thus, elevations in expression of ApoE and betaAPP by cellular stresses are likely normally linked in vivo, and uncoupling of this link, or other pathologic events in AD initiation, may leave neurons with diminished betaAPP expression, which might in turn reduce their resistance to stressors.

Parenchymal and vascular Abeta-deposition and its effects on the degeneration of neurons and cognition in Alzheimer's disease

The deposition of the amyloid beta-protein (Abeta) is one of the pathological hallmarks of Alzheimer's disease (AD). Abeta-deposits show the morphology of senile plaques and cerebral amyloid angiopathy (CAA). Senile plaques and vascular Abeta-deposits occur first in neocorti-cal areas. Then, they expand hierarchically into further brain regions. The distribution of Abeta plaques throughout the entire brain, thereby correlates with the clinical status of the patients. Imaging techniques for Abeta make use of the hierarchical distribution of Abeta to distinguish AD patients from non-AD patients. However, pathology seen in AD patients represents a late stage of a pathological process starting 10-30 years earlier in cognitively normal individuals. In addition to the fibrillar amyloid of senile plaques, oligomeric and monomeric Abeta is found in the brain. Recent studies revealed that oligomeric Abeta is presumably the most toxic Abeta-aggregate, which interacts with glutamatergic synapses. In doing so, dendrites are presumed to be the primary target for Abeta-toxicity. In addition, vascular Abeta-deposits can lead to capillary occlusion and blood flow disturbances presumably contributing to the alteration of neurons in addition to the direct neurotoxic effects of Abeta. All these findings point to an important role of Abeta and its aggregates in the neurodegenerative process of AD. Since there is already significant neuron loss in AD patients, treatment strategies aimed at reducing the amyloid load will presumably not cure the symptoms of dementia but they may stop disease progression. Therefore, it seems to be necessary to protect the brain from Abeta-toxicity already in stages of the disease with minor neuron loss before the onset of cognitive symptoms.

Cerebral amyloid angiopathy and its relationship to Alzheimer's disease

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of the amyloid beta-protein (A beta) within cerebral vessels. The involvement of different brain areas in CAA follows a hierarchical sequence similar to that of Alzheimer-related senile plaques. Alzheimer's disease patients frequently exhibit CAA. The expansion of CAA in AD often shows the pattern of full-blown CAA. The deposition of A beta within capillaries distinguishes two types of CAA. One with capillary A beta-deposition is characterized by a strong association with the apolipoprotein E (APOE) epsilon 4 allele and by its frequent occurrence in Alzheimer's disease cases whereas the other one lacking capillary A beta-deposits is not associated with APOE epsilon 4. Capillary CAA can be seen in every stage of CAA or AD-related A beta-deposition. AD cases with capillary CAA show more widespread capillary A beta-deposition than non-demented cases as well as capillary occlusion. In a mouse model of CAA, capillary CAA was associated with capillary occlusion and cerebral blood flow disturbances. Thus, blood flow alterations with subsequent hypoperfusion induced by CAA-related capillary occlusion presumably point to a second mechanism in which A beta adversely affects the brain in AD in addition to its direct neurotoxic effects.

Mechanisms of amyloid plaque pathogenesis

The first ultrastructural investigations of Alzheimer's disease noted the prominence of degenerating mitochondria in the dystrophic neurites of amyloid plaques, and speculated that this degeneration might be a major contributor to plaque pathogenesis. However, the fate of these organelles has received scant consideration in the intervening decades. A number of hypotheses for the formation and progression of amyloid plaques have since been suggested, including glial secretion of amyloid, somal and synaptic secretion of amyloid-beta protein from neurons, and endosomal-lysosomal aggregation of amyloid-beta protein in the cell bodies of neurons, but none of these hypotheses fully account for the focal accumulation of amyloid in plaques. In addition to Alzheimer's disease, amyloid plaques occur in a variety of conditions, and these conditions are all accompanied by dystrophic neurites characteristic of disrupted axonal transport. The disruption of axonal transport results in the autophagocytosis of mitochondria without normal lysosomal degradation, and recent evidence from aging, traumatic injury, Alzheimer's disease and transgenic mice models of Alzheimer's disease, suggests that the degeneration of these autophagosomes may lead to amyloid production within dystrophic neurites. The theory of amyloid plaque pathogenesis has thus come full circle, back to the intuitions of the very first researchers in the field.

High sensitivity analysis of amyloid-beta peptide composition in amyloid deposits from human and PS2APP mouse brain

Cortical amyloid-beta (Abeta) deposition is considered essential in Alzheimer's disease (AD) and is also detectable in nondemented individuals with pathologic aging (PA). The present work presents a detailed analysis of the Abeta composition in various plaque types from human AD and PA cases, compared with plaque Abeta isolated from PS2APP mice. To determine minute amounts of Abeta from 30 to 50 laser-dissected amyloid deposits, we used a highly sensitive mass spectrometry procedure after restriction protease lysyl endopeptidase (Lys-C) digestion. This approach allowed the analysis of the amino-terminus and, including a novel ionization modifier, for the first time the carboxy-terminus of Abeta at a detection limit of approximately 200 fmol. In addition, full length Abeta 40/42 and pyroglutamate 3-42 were analyzed using a highly sensitive urea-based Western blot procedure. Generally, Abeta fragments were less accessible in human deposits, indicative of more posttranslational modifications. Thioflavine S positive cored plaques in AD were found to contain predominantly Abeta 42, whereas thioflavine S positive compact plaques and vascular amyloid consist mostly of Abeta 40. Diffuse plaques from AD and PA, as well as from PS2APP mice are composed predominantly of Abeta 1-42. Despite biochemical similarities in human and PS2APP mice, immuno-electron microscopy revealed an extensive extracellular matrix associated with Abeta fibrils in AD, specifically in diffuse plaques. Amino-terminal truncations of Abeta, especially pyroglutamate 3-40/42, are more frequently found in human plaques. In cored plaques we measured an increase of N-terminal truncations of approximately 20% between Braak stages IV to VI. In contrast, diffuse plaques of AD and PA cases, show consistently only low levels of amino-terminal truncations. Our data support the concept that diffuse plaques represent initial Abeta deposits but indicate a structural difference for Abeta depositions in human AD compared with PS2APP mice already at the stage of diffuse plaque formation.

Better memory and neural efficiency in young apolipoprotein E epsilon4 carriers

The apolipoprotein E (APOE) epsilon4 allele is the major genetic risk factor for Alzheimer's disease, but an APOE effect on memory performance and memory-related neurophysiology in young, healthy subjects is unknown. We found an association of APOE epsilon4 with better episodic memory compared with APOE epsilon2 and epsilon3 in 340 young, healthy persons. Neuroimaging was performed in a subset of 34 memory-matched individuals to study genetic effects on memory-related brain activity independently of differential performance. E4 carriers decreased brain activity over 3 learning runs, whereas epsilon2 and epsilon3 carriers increased activity. This smaller neural investment of epsilon4 carriers into learning reappeared during retrieval: epsilon4 carriers exhibited reduced retrieval-related activity with equal retrieval performance. APOE isoforms had no differential effects on cognitive measures other than memory, brain volumes, and brain activity related to working memory. We suggest that APOE epsilon4 is associated with good episodic memory and an economic use of memory-related neural resources in young, healthy humans.

Occurrence and co-localization of amyloid beta-protein and apolipoprotein E in perivascular drainage channels of wild-type and APP-transgenic mice

The deposition of the amyloid beta-protein (Abeta) is a hallmark of Alzheimer's disease (AD). One reason for Abeta-accumulation and deposition in the brain may be an altered drainage along perivascular channels. Extracellular fluid is drained from the brain towards the cervical lymph nodes via perivascular channels. The perivascular space around cerebral arteries is the morphological correlative of these drainage channels. Here, we show that Abeta is immunohistochemically detectable within the perivascular space of 25 months old wild-type and amyloid precursor protein (APP)-transgenic mice harboring the Swedish double mutation driven by a neuron specific promoter. Only small amounts of Abeta can be detected immunohistochemically in the perivascular space of wild-type mice. Cerebrovascular and parenchymal Abeta-deposits were absent. In APP-transgenic mice, large amounts of Abeta were found in the perivascular drainage channels accompanied with cerebrovascular and parenchymal Abeta-deposition. The apolipoprotein E (apoE) immunostaining within the perivascular channels did not vary between wild-type and APP-transgenic mice. Almost 100% of the area that represents the perivascular space was stained with an antibody directed against apoE. Here, Abeta co-localized with apoE indicating an involvement of apoE in the perivascular clearance of Abeta. Fibrillar congophilic amyloid was not seen in wild-type mice. In APP-transgenic animals, congophilic fibrillar amyloid material was seen in the wall of cerebral blood vessels but not in the perivascular space. In conclusion, our results suggest that non-fibrillar forms of Abeta are drained along perivascular channels and that apoE is presumably involved in this clearance mechanism. Overloading such a clearance mechanism in APP-transgenic mice appears to result in insufficient Abeta-clearance, increased Abeta-levels in the brain and the perivascular drainage channels, and finally in Abeta-deposition. In so doing, our results strengthen the hypothesis that an alteration of perivascular drainage supports Abeta-deposition and the development of AD.

The development of amyloid beta protein deposits in the aged brain

The deposition of amyloid beta protein (Abeta) in the human brain and the generation of neurofibrillary tangles are the histopathological hallmarks of Alzheimer's disease. Accumulation of Abeta takes place in senile plaques and in cerebrovascular deposits as a result of an imbalance between Abeta production and clearance. This Review describes the different types of Abeta deposits, which can be distinguished by their morphology and by the hierarchical involvement of distinct areas of the brain in Abeta deposition. The role of intracellular Abeta in Abeta deposition and the mechanism of Abeta toxicity are also discussed.