APOE polymorphism and clinical duration determine regional neuropathology in Swedish APP(670, 671) mutation carriers: implications for late-onset Alzheimer's disease

Clinical characteristics and genotype-phenotype correlation analysis of familial Alzheimer’s disease patients with pathogenic/likely pathogenic amyloid protein precursor mutations

Alzheimer’s disease (AD) is a progressive neurodegenerative disease associated with aging, environmental, and genetic factors. Amyloid protein precursor (APP) is a known pathogenic gene for familial Alzheimer’s disease (FAD), and now more than 70 APP mutations have been reported, but the genotype-phenotype correlation remains unclear. In this study, we collected clinical data from patients carrying APP mutations defined as pathogenic/likely pathogenic according to the American college of medical genetics and genomics (ACMG) guidelines. Then, we reanalyzed the clinical characteristics and identified genotype-phenotype correlations in APP mutations. Our results indicated that the clinical phenotypes of APP mutations are generally consistent with typical AD despite the fact that they show more non-demented symptoms and neurological symptoms. We also performed genotype-phenotype analysis according to the difference in APP processing caused by the mutations, and we found that there were indeed differences in onset age, behavioral and psychological disorders of dementia (BPSD) and myoclonus.

Processing of Mutant β-Amyloid Precursor Protein and the Clinicopathological Features of Familial Alzheimer's Disease

Alzheimer's disease (AD) is a complex, multifactorial disease involving many pathological mechanisms. Nonetheless, single pathogenic mutations in amyloid precursor protein (APP) or presenilin 1 or 2 can cause AD with almost all of the clinical and neuropathological features, and therefore, we believe an important mechanism of pathogenesis in AD could be revealed from examining pathogenic APP missense mutations. A comprehensive review of the literature, including clinical, neuropathological, cellular and animal model data, was conducted through PubMed and the databases of Alzforum mutations, HGMD, UniProt, and AD&FTDMDB. Pearson correlation analysis combining the clinical and neuropathological data and aspects of mutant APP processing in cellular models was performed. We find that an increase in Aβ42 has a significant positive correlation with the appearance of neurofibrillary tangles (NFTs) and tends to cause an earlier age of AD onset, while an increase in Aβ40 significantly increases the age at death. The increase in the α-carboxyl terminal fragment (CTF) has a significantly negative correlation with the age of AD onset, and β-CTF has a similar effect without statistical significance. Animal models show that intracellular Aβ is critical for memory defects. Based on these results and the fact that amyloid plaque burden correlates much less well with cognitive impairment than do NFT counts, we propose a "snowball hypothesis": the accumulation of intraneuronal NFTs caused by extracellular Aβ42 and the increase in intraneuronal APP proteolytic products (CTFs and Aβs) could cause cellular organelle stress that leads to neurodegeneration in AD, which then resembles the formation of abnormal protein "snowballs" both inside and outside of neurons.

Amyloid tracers binding sites in autosomal dominant and sporadic Alzheimer's disease

INTRODUCTION

Amyloid imaging has been integrated into diagnostic criteria for Alzheimer's disease (AD). How amyloid tracers binding differ for different tracer structures and amyloid-β aggregates in autosomal dominant AD (ADAD) and sporadic AD is unclear.

METHODS

Binding properties of different amyloid tracers were examined in brain homogenates from six ADAD with APPswe, PS1 M146V, and PS1 EΔ9 mutations, 13 sporadic AD, and 14 control cases.

RESULTS

³H-PIB, ³H-florbetaben, ³H-AZD2184, and BTA-1 shared a high- and a varying low-affinity binding site in the frontal cortex of sporadic AD. AZD2184 detected another binding site (affinity 33 nM) in the frontal cortex of ADAD. The ³H-AZD2184 and ³H-PIB binding were significantly higher in the striatum of ADAD compared to sporadic AD and control. Polyphenol resveratrol showed strongest inhibition on ³H-AZD84 binding followed by ³H-florbetaben and minimal on ³H-PIB.

DISCUSSION

This study implies amyloid tracers of different structures detect different sites on amyloid-β fibrils or conformations.

Apolipoprotein E isoform-specific effects on lipoprotein receptor processing

Recent findings indicate an isoform-specific role for apolipoprotein E (apoE) in the elimination of beta-amyloid (Aβ) from the brain. ApoE is closely associated with various lipoprotein receptors, which contribute to Aβ brain removal via metabolic clearance or transit across the blood–brain barrier (BBB). These receptors are subject to ectodomain shedding at the cell surface, which alters endocytic transport and mitigates Aβ elimination. To further understand the manner in which apoE influences Aβ brain clearance, these studies investigated the effect of apoE on lipoprotein receptor shedding. Consistent with prior reports, we observed an increased shedding of the low-density lipoprotein receptor (LDLR) and the LDLR-related protein 1 (LRP1) following Aβ exposure in human brain endothelial cells. When Aβ was co-treated with each apoE isoform, there was a reduction in Aβ-induced shedding with apoE2 and apoE3, while lipoprotein receptor shedding in the presence of apoE4 remained increased. Likewise, intracranial administration of Aβ to apoE-targeted replacement mice (expressing the human apoE isoforms) resulted in an isoform-dependent effect on lipoprotein receptor shedding in the brain (apoE4 > apoE3 > apoE2). Moreover, these results show a strong inverse correlation with our prior work in apoE transgenic mice in which apoE4 animals showed reduced Aβ clearance across the BBB compared to apoE3 animals. Based on these results, apoE4 appears less efficient than other apoE isoforms in regulating lipoprotein receptor shedding, which may explain the differential effects of these isoforms in removing Aβ from the brain.

HFE gene variants, iron, and lipids: a novel connection in Alzheimer's disease

Iron accumulation and associated oxidative stress in the brain have been consistently found in several neurodegenerative diseases. Multiple genetic studies have been undertaken to try to identify a cause of neurodegenerative diseases but direct connections have been rare. In the iron field, variants in the HFE gene that give rise to a protein involved in cellular iron regulation, are associated with iron accumulation in multiple organs including the brain. There is also substantial epidemiological, genetic, and molecular evidence of disruption of cholesterol homeostasis in several neurodegenerative diseases, in particular Alzheimer's disease (AD). Despite the efforts that have been made to identify factors that can trigger the pathological events associated with neurodegenerative diseases they remain mostly unknown. Because molecular phenotypes such as oxidative stress, synaptic failure, neuronal loss, and cognitive decline, characteristics associated with AD, have been shown to result from disruption of a number of pathways, one can easily argue that the phenotype seen may not arise from a linear sequence of events. Therefore, a multi-targeted approach is needed to understand a complex disorder like AD. This can be achieved only when knowledge about interactions between the different pathways and the potential influence of environmental factors on them becomes available. Toward this end, this review discusses what is known about the roles and interactions of iron and cholesterol in neurodegenerative diseases. It highlights the effects of gene variants of HFE (H63D- and C282Y-HFE) on iron and cholesterol metabolism and how they may contribute to understanding the etiology of complex neurodegenerative diseases.

A multifaceted role for apoE in the clearance of beta-amyloid across the blood-brain barrier

BACKGROUND

While apolipoprotein E4 (apoE4) is highly correlated with the development of Alzheimer's disease (AD), its role in AD pathology and, in particular, beta-amyloid (Aβ) removal from the brain, is not clearly defined.

OBJECTIVE

To elucidate the influence of apoE on the clearance of Aβ across the blood-brain barrier (BBB).

METHODS

Aβ(1-42) was intracerebrally administered to transgenic mice expressing human apoE isoforms and examined in the periphery.

RESULTS

apoE3 and apoE4 mice had 5 times and 2 times, respectively, more Aβ(1-42) appearing in the plasma than wild-type or apoE knockout mice, indicating an enhanced clearance of Aβ from the brain to the periphery. In vitro, unbound basolateral apoE3 (i.e., not bound to Aβ), and to a lesser extent unbound apoE4, at concentrations ≤10 nM facilitated basolateral-to-apical fluorescein-Aβ(1-42) transcytosis across a BBB model, while apoE isoforms bound to Aβ significantly disrupted Aβ transcytosis. Additionally, following apical exposure to the BBB model, we found that apoE4 bound to Aβ is able to penetrate the BBB more readily than apoE3 bound to Aβ and does so via the RAGE (receptor for advanced glycation end products) transporter.

CONCLUSION

These studies indicate a multifaceted, isoform-dependent role for apoE in the exchange of Aβ across the BBB and may partially explain the association of apoE4 and Aβ brain accumulation in AD.

Semantic dementia with lower motor neuron disease showing FTLD-TDP type 3 pathology (sensu Mackenzie)

We describe a case of frontotemporal lobar degeneration with semantic dementia and lower motor neuron disease. A 63-year-old man presented with the full clinical picture of semantic dementia, including semantic anomia, surface alexia, lexical agraphia, associative agnosia, prosopagnosia and phonagnosia. Flaccid dysarthria, bulbar dysphagia and fasciculations developed 7 years after onset, followed by death within a year. The neuropathological examination showed heavy neuronal loss in the anterior temporal lobe cortex, dorsal vagal and hypoglossal nuclei and anterior horns of the spinal cord. Ubiquitin- and TDP-43-positive cytoplasmic inclusions were abundant in layer II of affected cortices and in granular cells of the hippocampal dentate gyrus, whereas dystrophic neurites were sparse and intranuclear inclusions absent. It is concluded that FTLD-TDP type 3 can be associated with semantic dementia and lower motor neuron disease in combination.

Apolipoprotein E: from lipid transport to neurobiology

Apolipoprotein (apo) E has a storied history as a lipid transport protein. The integral association between cholesterol homeostasis and lipoprotein clearance from circulation are intimately related to apoE's function as a ligand for cell-surface receptors of the low-density lipoprotein receptor family. The receptor binding properties of apoE are strongly influenced by isoform specific amino acid differences as well as the lipidation state of the protein. As understanding of apoE as a structural component of circulating plasma lipoproteins has evolved, exciting developments in neurobiology have revitalized interest in apoE. The strong and enduring correlation between the apoE4 isoform and age of onset and increased risk of Alzheimer's disease has catapulted apoE to the forefront of neurobiology. Using genetic tools generated for study of apoE lipoprotein metabolism, transgenic "knock-in" and gene-disrupted mice are now favored models for study of its role in a variety of neurodegenerative diseases. Key structural knowledge of apoE and isoform-specific differences is driving research activity designed to elucidate how a single amino acid change can manifest such profoundly significant pathological consequences. This review describes apoE through a lens of structure-based knowledge that leads to hypotheses that attempt to explain the functions of apoE and isoform-specific effects relating to disease mechanism.

Mass spectrometric characterization of brain amyloid beta isoform signatures in familial and sporadic Alzheimer's disease

A proposed key event in the pathogenesis of Alzheimer's disease (AD) is the formation of neurotoxic amyloid beta (Abeta) oligomers and amyloid plaques in specific brain regions that are affected by the disease. The main plaque component is the 42 amino acid isoform of Alphabeta (Abeta1-42), which is thought to initiate plaque formation and AD pathogenesis. Numerous isoforms of Abeta, e.g., Abeta1-42, Abeta1-40 and the 3-pyroglutamate derivate of Abeta3-42 (pGluAbeta3-42), have been detected in the brains of sporadic AD (SAD) and familial AD (FAD) subjects. However, the relative importance of these isoforms in the pathogenesis of AD is not fully understood. Here, we report a detailed study using immunoprecipitation in combination with mass spectrometric analysis to determine the Abeta isoform pattern in the cerebellum, cortex and hippocampus in AD, including subjects with a mutation in the presenilin (M146V) or amyloid precursor protein (KM670/671NL) genes, SAD subjects and non-demented controls. We show that the dominating Abeta isoforms in the three different brain regions analyzed from control, SAD, and FAD are Abeta1-42, pGluAbeta3-42, Abeta4-42 and Abeta1-40 of which Abeta1-42 and Abeta4-42 are the dominant isoforms in the hippocampus and the cortex in all groups analyzed, controls included. No prominent differences in Abeta isoform patterns between FAD and SAD patients were seen, underscoring the similarity in the amyloid pathology of these two disease entities.

Comparison of Abeta levels in the brain of familial and sporadic Alzheimer's disease

Mutations in presenilin (PS) and amyloid precursor protein (APP) genes are a predominant cause for early-onset familial Alzheimer disease (AD). Although these mutations are rare, they have in the past decades advanced our understanding of the underlying molecular mechanisms of AD. In the present study, Abeta levels were measured in cortical regions of APPsw and PS1 (M146V) mutation carriers, sporadic AD (SAD) and age-matched non-demented individuals. We found similar levels of soluble Abeta42, insoluble and soluble Abeta40 in both APPsw mutation carriers and SAD. However, lower levels of insoluble Abeta42 were detected in the frontal and temporal cortex of APPsw brain. In PS1 brain, insoluble Abeta40 and Abeta42 levels were significantly lower in all four cortical regions compared with SAD, whilst levels of Abeta40 were lower in frontal and occipital cortex compared with APPsw brain. The insoluble Abeta42/40 ratio was similar in SAD and APPsw but significantly higher in PS1 mutation carriers. Our results indicate that the pattern of Abeta deposition in PS1 mutation carriers differs from that in both APPsw and SAD, whereas the pattern in APPsw mutation carriers is more similar to that in SAD.

Apolipoprotein E and its receptors in Alzheimer's disease: pathways, pathogenesis and therapy

The vast majority of Alzheimer's disease (AD) cases are late-onset and their development is probably influenced by both genetic and environmental risk factors. A strong genetic risk factor for late-onset AD is the presence of the epsilon4 allele of the apolipoprotein E (APOE) gene, which encodes a protein with crucial roles in cholesterol metabolism. There is mounting evidence that APOE4 contributes to AD pathogenesis by modulating the metabolism and aggregation of amyloid-beta peptide and by directly regulating brain lipid metabolism and synaptic functions through APOE receptors. Emerging knowledge of the contribution of APOE to the pathophysiology of AD presents new opportunities for AD therapy.

Environmental enrichment alters dentate granule cell morphology in oldest-old rat

The hippocampus of aged rats shows marked age-related morphological changes that could cause memory deficits. Experimental evidence has established that environmental enrichment attenuates memory deficits in aged rats. We therefore studied whether environmental enrichment produces morphological changes on the dentate granule cells of aged rats. Fifteen male Sprague-Dawley rats, 24 months of age, were randomly distributed in two groups that were housed under standard (n = 7) or enriched (n = 8) environmental conditions for 26 days. Quantitative data of dendritic morphology from dentate gyrus granule cells were obtained on Golgi-Cox stained sections. Environmental enrichment significantly increased the complexity and size of dendritic tree (total number of segments increased by 61% and length by 116%), and spine density (88% increase). There were large interindividual differences within the enriched group, indicating differential individual responses to environmental stimulation. Previous studies in young animals have shown changes produced by environmental enrichment in the morphology of dentate gyrus granule cells. The results of the present study show that environmental enrichment can also produce changes in dentate granule cell morphology in the senescent brain. In conclusion, the hippocampus retains its neuroplastic capacity during aging, and enriched environmental housing conditions can attenuate age-related dendritic regression and synaptic loss, thus preserving memory functions.

Amyloid beta-peptide levels in laser capture microdissected cornu ammonis 1 pyramidal neurons of Alzheimer's brain

Deposition of the amyloid beta-peptide (Abeta) is a pathophysiological event associated with Alzheimer's disease. Although much is known about the molecular composition of extracellular Abeta deposits, the role of the intracellular pool of Abeta is not fully understood. We investigated whether Abeta levels are increased in cornu ammonis 1 pyramidal neurons of Alzheimer's disease hippocampus, using laser capture microdissection to isolate the neurons and enzyme-linked immunosorbent assay for quantification. Our results showed increased Abeta42 levels and an elevated Abeta42/Abeta40 ratio in neurons from sporadic as well as from familial cases of Alzheimer's disease, whereas Abeta40 levels remain unchanged between the cases and controls. We speculate that intracellular accumulation of Abeta42 increase vulnerability of cornu ammonis 1 pyramidal neurons in Alzheimer's disease.

Apolipoprotein E and Alzheimer's disease: molecular mechanisms and therapeutic opportunities

Multiple genetic and environmental factors are likely to contribute to the development of Alzheimer's disease (AD). The most important known risk factor for AD is presence of the E4 isoform of apolipoprotein E (apoE). Epidemiological studies demonstrated that apoE4 carriers have a higher risk and develop the disease and an early onset. Moreover, apoE4 is the only molecule that has been associated with all the biochemical disturbances characteristic of the disease: amyloid-beta (Abeta) deposition, tangle formation, oxidative stress, lipid homeostasis deregulation, synaptic plasticity loss and cholinergic dysfunction. This large body of evidence suggest that apoE is a key player in the pathogenesis of AD. This short review examines the current facts and hypotheses of the association between apoE4 and AD, as well as the therapeutic possibilities that apoE might offer for the treatment of this disease.

Alzheimer dementia caused by a novel mutation located in the APP C-terminal intracytosolic fragment

Since the first report showing that Alzheimer disease (AD) might be caused by mutations in the amyloid precursor protein gene (APP), 20 different missense mutations have been reported. The majority of early-onset AD mutations alter processing of APP increasing relative levels of Abeta42 peptide, either by increasing Abeta42 or decreasing Abeta40 peptide levels or both. In a diagnostic setting using direct sequence analysis, we identified in one patient with familial early-onset AD a novel mutation in APP (c.2172G>C), predicting a K724N substitution in the intracytosolic fragment. The mutation is located downstream of the epsilon-cleavage site of APP and is the furthermost C-terminal mutation reported to date. In vitro expression of APP K724N cDNA showed an increase in Abeta42 and a decrease in Abeta40 levels resulting in a near three-fold increase of the Abeta42/Abeta40 ratio. Further, in vivo amyloid positron emission tomography (PET) imaging revealed significantly increased cortical amyloid deposits, supporting that in human this novel APP mutation is likely causing disease.

Stimulation of G-proteins in human control and Alzheimer's disease brain by FAD mutants of APP(714-723): implication of oxidative mechanisms

We report the effects of amyloid precursor protein (APP) fragment 714-723 (APP(714-723); peptide P1) and its V717F and V717G mutants (peptides P2 and P3, respectively) on G-protein activity ([35S]GTPgammaS binding) in membranes from postmortem human control and Alzheimer's disease (AD) brains. The peptides P1, P2, and P3 revealed a significant stimulatory effect on [35S]GTPgammaS binding in control temporal cortex. The most potent stimulator, P3, at 10 microM concentration enhanced [35S]GTPgammaS binding by 500%. The effect was threefold stronger than that for wild-type P1 and twofold stronger than that for P2. In sporadic AD, the stimulatory effect of P1, P2, and P3 on G-proteins was reduced significantly whereas in Swedish familial AD (SFAD), only P1 elicited marked stimulation (at 10 microM by 50%). In control sensory postcentral cortex, the stimulation of G-proteins by P3 was 1.5-fold lower than that in control temporal cortex, whereas in AD and SFAD the effect showed no remarkable regional difference. Treatment of membranes with H2O2 produced 1.5-fold higher stimulation in [35S]GTPgammaS binding to temporal cortex than that in binding to sensory postcentral cortex. In AD and SFAD, the stimulation by H2O2 revealed no significant regional difference. Glutathione, desferrioxamine (DFO), and 17beta-estradiol markedly decreased the strong stimulatory effect by P3 on [35S]GTPgammaS binding to control temporal cortex, with the protective effect by DFO being most potent. The G(alphaO)-protein levels were not changed in AD or SFAD brain membranes as compared to levels in control membranes. We suggest that strong G-protein stimulation by P3 in the human brain implies the specific (per)oxidation mechanism that might be affected by regional content of peroxidizing substrates and antioxidants.

High selective expression of alpha7 nicotinic receptors on astrocytes in the brains of patients with sporadic Alzheimer's disease and patients carrying Swedish APP 670/671 mutation: a possible association with neuritic plaques

In the present study, we have investigated the expression of nicotinic acetylcholine receptors (nAChRs) on astrocytes and neurons in the hippocampus and temporal cortex of subjects carrying the Swedish amyloid precursor protein (APP) 670/671 mutation (APPswe), patients with sporadic Alzheimer's disease (AD), and age-matched control subjects. Significant increases in the total numbers of astrocytes and of astrocytes expressing the alpha7 nAChR subunit, along with significant decreases in the levels of alpha7 and alpha4 nAChR subunits on neurons, were observed in the hippocampus and temporal cortex of both APPswe and sporadic AD brains. Both of these phenomena were more pronounced in APPswe than sporadic AD cases. Furthermore, the number of [(125)I]alpha-BTX binding sites (alpha7 nAChR) in the temporal cortex of the APPswe brain was significant lower than in the younger control group, reflecting the lower neuronal level of alpha7 nAChR. The increase in the level of expression of alpha7 nAChR on astrocytes was positively correlated with the extent of neuropathological alternations, especially the number of neuritic plaques, in the AD brain. The elevated expression of alpha7 nAChR on astrocytes might participate in Abeta cascade and formation of neuritic plaques, thereby playing an important role in the pathogenesis of AD.

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

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