Intraneuronal Abeta immunoreactivity is not a predictor of brain amyloidosis-beta or neurofibrillary degeneration

Regional AT-8 reactive tau species correlate with intracellular Aβ levels in cases of low AD neuropathologic change

The amyloid cascade hypothesis states that Aβ aggregates induce pathological changes in tau, leading to neurofibrillary tangles (NFTs) and cell death. A caveat with this hypothesis is the spatio-temporal divide between plaques and NFTs. This has been addressed by the inclusion of soluble Aβ and tau species in the revised amyloid cascade hypothesis. Nevertheless, despite the potential for non-plaque Aβ to contribute to tau pathology, few studies have examined relative correlative strengths between total Aβ, plaque Aβ and intracellular Aβ with tau pathology within a single tissue cohort. Employing frozen and fixed frontal cortex grey and white matter tissue from non-AD controls (Con; n = 39) and Alzheimer's disease (AD) cases (n = 21), biochemical and immunohistochemical (IHC) measures of Aβ and AT-8 phosphorylated tau were assessed. Biochemical native-state dot blots from crude tissue lysates demonstrated robust correlations between total Aβ and AT-8 tau, when considered as a combined cohort (Con and AD) and when as Con and AD cases, separately. In contrast, no associations between Aβ plaques and AT-8 were reported when using IHC measurements in either Con or AD cases. However, when intracellular Aβ was measured via the Aβ specific antibody MOAB-2, a correlative relationship with AT-8 tau was reported in non-AD controls but not in AD cases. Collectively the data suggests that accumulating intracellular Aβ may influence AT-8 pathology, early in AD-related neuropathological change. Despite the lower levels of phospho-tau and Aβ in controls, the robust correlative relationships observed suggest a physiological association of Aβ production and tau phosphorylation, which may be modified during disease. This study is supportive of a revised amyloid cascade hypothesis and demonstrates regional associative relationships between tau pathology and intracellular Aβ, but not extracellular Aβ plaques.

Intraneuronal accumulation of amyloid-β peptides as the pathomechanism linking autism and its co-morbidities: epilepsy and self-injurious behavior - the hypothesis

Autism spectrum disorder (ASD) is associated with enhanced processing of amyloid-β precursor protein (APP) by secretase-α, higher blood levels of sAPPα and intraneuronal accumulation of N-terminally truncated Aβ peptides in the brain cortex - mainly in the GABAergic neurons expressing parvalbumin - and subcortical structures. Brain Aβ accumulation has been also described in epilepsy-the frequent ASD co-morbidity. Furthermore, Aβ peptides have been shown to induce electroconvulsive episodes. Enhanced production and altered processing of APP, as well as accumulation of Aβ in the brain are also frequent consequences of traumatic brain injuries which result from self-injurious behaviors, another ASD co-morbidity. We discuss distinct consequences of accumulation of Aβ in the neurons and synapses depending on the Aβ species, their posttranslational modifications, concentration, level of aggregation and oligomerization, as well as brain structures, cell types and subcellular structures where it occurs. The biological effects of Aβ species which are discussed in the context of the pathomechanisms of ASD, epilepsy, and self-injurious behavior include modulation of transcription-both activation and repression; induction of oxidative stress; activation and alteration of membrane receptors' signaling; formation of calcium channels causing hyper-activation of neurons; reduction of GABAergic signaling - all of which lead to disruption of functions of synapses and neuronal networks. We conclude that ASD, epilepsy, and self-injurious behaviors all contribute to the enhanced production and accumulation of Aβ peptides which in turn cause and enhance dysfunctions of the neuronal networks that manifest as autism clinical symptoms, epilepsy, and self-injurious behaviors.

Endoplasmic reticulum stress induces Alzheimer's disease-like phenotypes in the neuron derived from the induced pluripotent stem cell with D678H mutation on amyloid precursor protein

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is mainly caused by the interaction of genetic and environmental factors. The impact of environmental factors on the genetic mutation in the amyloid precursor protein (APP) is not well characterized. We hypothesized that Endoplasmic Reticulum (ER) stress would promote disease for the patient carrying the APP D678H mutation. Therefore, we analyzed the impact of a familial AD mutation on amyloid precursor protein (APP D678H) under ER stress. Induced pluripotent stem cell (iPSC) from APP D678H mutant carrier was differentiated into neurons, which were then analyzed for AD-like changes. Immunocytochemistry and whole-cell patch-clamp recording revealed that the derived neurons on day 28 after differentiation showed neuronal markers and electrophysiological properties similar to those of mature neurons. However, the APP D678H mutant neurons did not have significant alterations in the levels of amyloid-β (Aβ) and phosphorylated tau (pTau) compared to its isogenic wild-type neuron. Only under ER stress, the neurons with the APP D678H mutation had more Aβ and pTau via immune detection assays. The higher level of Aβ in the APP D678H mutant neurons was probably due to the increased level of β-site APP cleaving enzyme (BACE1) and decreased level of Aβ degrading enzymes under ER stress. Increased Aβ and pTau under ER stress reduced the N-methyl-D-aspartate receptor (NMDAR) in Western blot analysis and altered electrophysiological properties in the mutant neurons. Our study provides evidence that the interaction between genetic mutation and ER stress would induce AD-like changes.

Regulation of phase I and phase II neurosteroid enzymes in the hippocampus of an Alzheimer's disease rat model: A focus on sulphotransferases and UDP-glucuronosyltransferases

Neurosteroids have been associated with neurodegenerative diseases because they are involved in the modulation of neurotransmitter, neurotropic and neuroprotective actions. Emerging evidence suggests that the enzymes responsible for the synthesis of neurosteroids change during the progression of Alzheimer's disease (AD). The present study aimed to assess the changes in phase I and II enzymes involved in the metabolism of neurosteroids of the progestogen, androgenic and estrogenic steroidogenic pathways and the possibility that the neurosteroids are actively converted into the most abundant metabolites (i.e. glucuronides and sulphates). The gene expression for the phase I and II neurosteroid biosynthetic enzymes were studied in the hippocampus of streptozotocin AD rat model. Male Sprague-Dawley rats were randomly divided into control, sham (saline injected into the hippocampus) and 3 and 12 weeks post-STZ administration (STZ-G3w and STZ-G12w, respectively) groups. Behavioral assessments showed memory impairment in both STZ-injected groups, whereas the formation of amyloid-beta was more pronounced in the STZ-12w group. Gene expression of the hippocampus revealed that glucuronidation and sulphation enzymes transcript of the phase I metabolites were upregulated at the late stage of the disease progression (Hsd17b10, Hsd3b1, Akr1c3 and Cyp19a1) except for Sts. The phase II Sult and Ugt enzymes were mostly upregulated in the STZ-G12w rats (Sult1a1, Sult1e1, Ugt1a1, Ugt1a7c, Ugt1a6, Ugt2b35 and Ugt2b17) and normally expressed in the STZ-G3w group (Sult2a2, Sult2a6, Sult2b1, Ugt2b7, Sult4a1 and Ugt1a7c). In conclusion, changes occur in the phase I and II enzymes transcript of the progestogen, androgenic and estrogenic steroidogenic pathways during the progression of AD.

Neuropathology and cholinesterase expression in the brains of octogenarians and older

A subset of octogenarians and older maintain normal cognitive function (CNOO) despite high prevalence and incidence of cognitive decline attributed to neurodegeneration or aging in the population. The rostral prefrontal cortex (rPFC) and hippocampal formation are brain regions integral to cognition, namely attention and memory, facilitated in part by cholinergic innervation. We hypothesized that preserved cholinergic neurotransmission in these regions contributes to intact cognition in the CNOO. To test this, we evaluated the burden of neuropathological and cholinesterase-associated protein aggregates in the rPFC and hippocampal formation. Tissues from age- and sex-matched CNOO and Alzheimer's disease (AD) rPFC and hippocampal formation were stained for β-amyloid (Aβ), tau, α-synuclein, phosphorylated TAR DNA-binding protein 43 (pTDP-43), acetylcholinesterase (AChE), and butyrylcholinesterase (BChE). The relative abundance of neuropathological aggregates was semi-quantitatively scored. Deposition of Aβ plaques, tau neurofibrillary tangles (NFT) and pTDP-43 inclusions were comparable between CNOO and AD cases. Intraneuronal Aβ and tau-positive thorny astrocytes consistent with aging-related tau astrogliopathy, were also noted in the rPFC. Abundance of BChE-positive plaque pathology was significantly higher in AD than in CNOO cases in most regions of interest, followed closely by abundance of AChE-positive plaque pathology. BChE- and AChE-activities were also associated with varied NFT morphologies. CNOO cases maintained cognition despite a high neuropathological burden in the rPFC and hippocampal formation. BChE-positive and, to a lesser extent, AChE-positive pathologies were significantly lower in most regions in the CNOO compared to AD. This suggests a specificity of cholinesterase-associated neuropathology with AD. We conclude that while CNOO have cholinesterase-associated neuropathology in the rPFC and hippocampal formation, abundance in this population is significantly lower compared to AD which may contribute to their intact cognition.

Aging induces abnormal accumulation of Aβ in extracellular vesicle and/or intraluminal membrane vesicle-rich fractions in nonhuman primate brain

Aβ metabolism in the brain is mediated by endocytosis, one part of the intracellular membrane trafficking system. We previously showed that aging attenuates the interaction of dynein with dynactin, which disrupts the endosomal/lysosomal trafficking pathway involved in Aβ metabolism, resulting in intracellular accumulation of Aβ. Several studies have shown that in Alzheimer's disease (AD), intraneuronal accumulation of Aβ precedes extracellular Aβ depositions. However, it is unclear what accounts for this transition from intracellular to extracellular depositions. Accumulating evidence suggest that autophagy has an important role in AD pathology, and we observed that autophagy-related protein levels began to decrease before amyloid plaque formation in cynomolgus monkey brains. Surprisingly, experimental induction of autophagosome formation in Neuro2a cells significantly increased intracellular Aβ and decreased extracellular release of Aβ, accompanied by the prominent reduction of extracellular vesicle (EV) secretion. RNAi study confirmed that EV secretion affected intracellular and extracellular Aβ levels, and siRNA-induced downregulation of autophagosome formation enhanced EV secretion to ameliorate intracellular Aβ accumulation induced by dynein knockdown. In aged cynomolgus monkeys, Aβ levels in EV/intraluminal membrane vesicle (ILV)-rich fractions isolated from temporal lobe parenchyma were drastically increased. Moreover, EV/ILV marker proteins overlapped spatially with amyloid plaques. These findings suggest that EV would be an important carrier of Aβ in brain and abnormal accumulation of Aβ in EVs/ILVs may be involved in the transition of age-dependent Aβ pathology.

Evidence for the Role of Mitochondrial DNA Release in the Inflammatory Response in Neurological Disorders

Mitochondria are regarded as the metabolic centers of cells and are integral in many other cell processes, including the immune response. Each mitochondrion contains numerous copies of mitochondrial DNA (mtDNA), a small, circular, and bacterial-like DNA. In response to cellular damage or stress, mtDNA can be released from the mitochondrion and trigger immune and inflammatory responses. mtDNA release into the cytosol or bloodstream can occur as a response to hypoxia, sepsis, traumatic injury, excitatory cytotoxicity, or drastic mitochondrial membrane potential changes, some of which are hallmarks of neurodegenerative and mood disorders. Released mtDNA can mediate inflammatory responses observed in many neurological and mood disorders by driving the expression of inflammatory cytokines and the interferon response system. The current understanding of the role of mtDNA release in affective mood disorders and neurodegenerative diseases will be discussed.

Therapeutic potential of mangiferin in the treatment of various neuropsychiatric and neurodegenerative disorders

Xanthones are important chemical class of bioactive products that confers therapeutic benefits. Of several xanthones, mangiferin is known to be distributed widely across several fruits, vegetables and medicinal plants. Mangiferin has been shown to exert neuroprotective effects in both in-vitro and in-vivo models. Mangiferin attenuates cerebral infarction, cerebral edema, lipid peroxidation (MDA), neuronal damage, etc. Mangiferin further potentiate levels of endogenous antioxidants to confer protection against the oxidative stress inside the neurons. Mangiferin is involved in the regulation of various signaling pathways that influences the production and levels of proinflammatory cytokines in brain. Mangiferin cosunteracted the neurotoxic effect of amyloid-beta, MPTP, rotenone, 6-OHDA etc and confer protection to neurons. These evidence suggested that the mangiferin may be a potential therapeutic strategy for the treatment of various neurological disorders. The present review demonstrated the pharmacodynamics-pharmacokinetics of mangiferin and neurotherapeutic potential in several neurological disorders with underlying mechanisms.

Down syndrome

Trisomy 21, the presence of a supernumerary chromosome 21, results in a collection of clinical features commonly known as Down syndrome (DS). DS is among the most genetically complex of the conditions that are compatible with human survival post-term, and the most frequent survivable autosomal aneuploidy. Mouse models of DS, involving trisomy of all or part of human chromosome 21 or orthologous mouse genomic regions, are providing valuable insights into the contribution of triplicated genes or groups of genes to the many clinical manifestations in DS. This endeavour is challenging, as there are >200 protein-coding genes on chromosome 21 and they can have direct and indirect effects on homeostasis in cells, tissues, organs and systems. Although this complexity poses formidable challenges to understanding the underlying molecular basis for each of the many clinical features of DS, it also provides opportunities for improving understanding of genetic mechanisms underlying the development and function of many cell types, tissues, organs and systems. Since the first description of trisomy 21, we have learned much about intellectual disability and genetic risk factors for congenital heart disease. The lower occurrence of solid tumours in individuals with DS supports the identification of chromosome 21 genes that protect against cancer when overexpressed. The universal occurrence of the histopathology of Alzheimer disease and the high prevalence of dementia in DS are providing insights into the pathology and treatment of Alzheimer disease. Clinical trials to ameliorate intellectual disability in DS signal a new era in which therapeutic interventions based on knowledge of the molecular pathophysiology of DS can now be explored; these efforts provide reasonable hope for the future.

Blast-Mediated Traumatic Brain Injury Exacerbates Retinal Damage and Amyloidosis in the APPswePSENd19e Mouse Model of Alzheimer's Disease

Purpose

Traumatic brain injury (TBI) is a risk factor for developing chronic neurodegenerative conditions including Alzheimer's disease (AD). The purpose of this study was to examine chronic effects of blast TBI on retinal ganglion cells (RGC), optic nerve, and brain amyloid load in a mouse model of AD amyloidosis.

Methods

Transgenic (TG) double-mutant APPswePSENd19e (APP/PS1) mice and nontransgenic (Non-TG) littermates were exposed to a single blast TBI (20 psi) at age 2 to 3 months. RGC cell structure and function was evaluated 2 months later (average age at endpoint = 4.5 months) using pattern electroretinogram (PERG), optical coherence tomography (OCT), and the chromatic pupil light reflex (cPLR), followed by histologic analysis of retina, optic nerve, and brain amyloid pathology.

Results

APP/PS1 mice exposed to blast TBI (TG-Blast) had significantly lower PERG and cPLR responses 2 months after injury compared to preblast values and compared to sham groups of APP/PS1 (TG-Sham) and nontransgenic (Non-TG-Sham) mice as well as nontransgenic blast-exposed mice (Non-TG-Blast). The TG-Blast group also had significantly thinner RGC complex and more optic nerve damage compared to all groups. No amyloid-β (Aβ) deposits were detected in retinas of APP/PS1 mice; however, increased amyloid precursor protein (APP)/Aβ-immunoreactivity was seen in TG-Blast compared to TG-Sham mice, particularly near blood vessels. TG-Blast and TG-Sham groups exhibited high variability in pathology severity, with a strong, but not statistically significant, trend for greater cerebral cortical Aβ plaque load in the TG-Blast compared to TG-Sham group.

Conclusions

When combined with a genetic susceptibility for developing amyloidosis of AD, blast TBI exposure leads to earlier RGC and optic nerve damage associated with modest but detectable increase in cerebral cortical Aβ pathology. These findings suggest that genetic risk factors for AD may increase the sensitivity of the retina to blast-mediated damage.

Effects of in vivo conditions on amyloid aggregation

One of the grand challenges of biophysical chemistry is to understand the principles that govern protein misfolding and aggregation, which is a highly complex process that is sensitive to initial conditions, operates on a huge range of length- and timescales, and has products that range from protein dimers to macroscopic amyloid fibrils. Aberrant aggregation is associated with more than 25 diseases, which include Alzheimer's, Parkinson's, Huntington's, and type II diabetes. Amyloid aggregation has been extensively studied in the test tube, therefore under conditions that are far from physiological relevance. Hence, there is dire need to extend these investigations to in vivo conditions where amyloid formation is affected by a myriad of biochemical interactions. As a hallmark of neurodegenerative diseases, these interactions need to be understood in detail to develop novel therapeutic interventions, as millions of people globally suffer from neurodegenerative disorders and type II diabetes. The aim of this review is to document the progress in the research on amyloid formation from a physicochemical perspective with a special focus on the physiological factors influencing the aggregation of the amyloid-β peptide, the islet amyloid polypeptide, α-synuclein, and the hungingtin protein.

Expression of the Alzheimer's Disease Mutations AβPP695sw and PSEN1M146I in Double-Transgenic Göttingen Minipigs

Mutations in the amyloid-β protein precursor gene (AβPP), the presenilin 1 gene (PSEN1) or the presenilin 2 gene (PSEN2) that increase production of the AβPP-derived peptide Aβ42 cause early-onset Alzheimer's disease. Rodent models of the disease show that further increase in Aβ42 production and earlier brain pathology can be obtained by coexpressing AβPP and PSEN1 mutations. To generate such elevated Aβ42 level in a large animal model, we produced Göttingen minipigs carrying in their genome one copy of a human PSEN1 cDNA with the Met146Ile (PSEN1M146I) mutation and three copies of a human AβPP695 cDNA with the Lys670Asn/Met671Leu (AβPPsw) double-mutation. Both transgenes were expressed in fibroblasts and in the brain, and their respective proteins were processed normally. Immunohistochemical staining with Aβ42-specific antibodies detected intraneuronal accumulation of Aβ42 in brains from a 10- and an 18-month-old pig. Such accumulation may represent an early event in the pathogenesis of Alzheimer's disease.

Understanding the roles of mutations in the amyloid precursor protein in Alzheimer disease

Many models of disease progression in Alzheimer's disease (AD) have been proposed to help guide experimental design and aid the interpretation of results. Models focussing on the genetic evidence include the amyloid cascade (ACH) and presenilin (PSH) hypotheses and the amyloid precursor protein (APP) matrix approach (AMA), of which the ACH has held a dominant position for over two decades. However, the ACH has never been fully accepted and has not yet delivered on its therapeutic promise. We review the ACH, PSH and AMA in relation to levels of APP proteolytic fragments reported from AD-associated mutations in APP. Different APP mutations have diverse effects on the levels of APP proteolytic fragments. This evidence is consistent with at least three disease pathways that can differ between familial and sporadic AD and two pathways associated with cerebral amyloid angiopathy. We cannot fully evaluate the ACH, PSH and AMA in relation to the effects of mutations in APP as the APP proteolytic system has not been investigated systematically. The confounding effects of sequence homology, complexity of competing cleavages and antibody cross reactivities all illustrate limitations in our understanding of the roles these fragments and the APP proteolytic system as a whole in normal aging and disease play. Current experimental design should be refined to generate clearer evidence, addressing both aging and complex disorders with standardised reporting formats. A more flexible theoretical framework capable of accommodating the complexity of the APP proteolytic system is required to integrate available evidence.

Generation and Partial Characterization of Rabbit Monoclonal Antibody to Amyloid-β Peptide 1-37 (Aβ37)

Secreted soluble amyloid-β 1-37 (Aβ37) peptide is one of the prominent Aβ forms next to Aβ40, and is found in cerebrospinal fluid (CSF) and blood. Recent studies have shown the importance of quantitation of CSF Aβ37 levels in combination with Aβ38, Aβ40, and Aβ42 to support the diagnosis of patients with probable Alzheimer's disease (AD), and the value of antibody to Aβ37 to facilitate drug discovery studies. However, the availability of reliable and specific monoclonal antibody to Aβ37 is very limited. Our aims were: 1) to generate and partially characterize rabbit monoclonal antibody (RabmAb) to Aβ37, and 2) to determine whether the antibody detects changes in Aβ37 levels produced by a γ-secretase modulator (GSM). Our generated RabmAb to Aβ37 was found to be specific to Aβ37, since it did not react with Aβ36, Aβ38, Aβ39, Aβ40, and Aβ42 in an ELISA or immunoblotting. The epitope of the antibody was contained in the seven C-terminal residues of Aβ37. The antibody was sensitive enough to measure CSF and plasma Aβ37 levels in ELISA. Immunohistological studies showed the presence of Aβ37-positive deposits in the brain of AD, and Down syndrome persons diagnosed with AD. Our studies also showed that the antibody detected Aβ37 increases in CSF and brains of rodents following treatment with a GSM. Thus, our antibody can be widely applied to AD research, and in a panel based approach it may have potential to support the diagnosis of probable AD, and in testing the effect of GSMs to target AD.

Characterization of Amyloid-β Deposits in Bovine Brains

Amyloid-β (Aβ) deposits are seen in aged individuals of many mammalian species that possess the same aminoacid sequence as humans. This study describes Aβ deposition in 102 clinically characterized cattle brains from animals aged 0 to 20 years. Extracellular and intracellular Aβ deposition was detected with 4G8 antibody in the cortex, hippocampus, and cerebellum. X-34 staining failed to stain Aβ deposits, indicating the non β-pleated nature of these deposits. Western blot analysis and surface-enhanced laser desorption/ionization time-of-flight (SELDI-TOF) mass spectrometry revealed in Tris, Triton, and formic acid fractions the presence of different Aβ peptides, characterized mainly by C-terminally truncated forms. Exploration of the genetic variability of APOE, PSEN1, and PSEN2 genes involved in Alzheimer’s disease pathogenesis revealed several previously unreported polymorphisms. This study demonstrates certain similarities between Aβ deposition patterns exhibited in cattle brains and those in the human brain in early stages of aging. Furthermore, the identification of the same Aβ peptides reported in humans, but unable to form aggregates, supports the hypothesis that cattle may be protected against amyloid plaque formation.

Amyloid-β dimers in the absence of plaque pathology impair learning and synaptic plasticity

Despite amyloid plaques, consisting of insoluble, aggregated amyloid-β peptides, being a defining feature of Alzheimer’s disease, their significance has been challenged due to controversial findings regarding the correlation of cognitive impairment in Alzheimer’s disease with plaque load. The amyloid cascade hypothesis defines soluble amyloid-β oligomers, consisting of multiple amyloid-β monomers, as precursors of insoluble amyloid-β plaques. Dissecting the biological effects of single amyloid-β oligomers, for example of amyloid-β dimers, an abundant amyloid-β oligomer associated with clinical progression of Alzheimer’s disease, has been difficult due to the inability to control the kinetics of amyloid-β multimerization. For investigating the biological effects of amyloid-β dimers, we stabilized amyloid-β dimers by an intermolecular disulphide bridge via a cysteine mutation in the amyloid-β peptide (Aβ-S8C) of the amyloid precursor protein. This construct was expressed as a recombinant protein in cells and in a novel transgenic mouse, termed tgDimer mouse. This mouse formed constant levels of highly synaptotoxic soluble amyloid-β dimers, but not monomers, amyloid-β plaques or insoluble amyloid-β during its lifespan. Accordingly, neither signs of neuroinflammation, tau hyperphosphorylation or cell death were observed. Nevertheless, these tgDimer mice did exhibit deficits in hippocampal long-term potentiation and age-related impairments in learning and memory, similar to what was observed in classical Alzheimer’s disease mouse models. Although the amyloid-β dimers were unable to initiate the formation of insoluble amyloid-β aggregates in tgDimer mice, after crossbreeding tgDimer mice with the CRND8 mouse, an amyloid-β plaque generating mouse model, Aβ-S8C dimers were sequestered into amyloid-β plaques, suggesting that amyloid-β plaques incorporate neurotoxic amyloid-β dimers that by themselves are unable to self-assemble. Our results suggest that within the fine interplay between different amyloid-β species, amyloid-β dimer neurotoxic signalling, in the absence of amyloid-β plaque pathology, may be involved in causing early deficits in synaptic plasticity, learning and memory that accompany Alzheimer’s disease.

10.1093/brain/awv355_video_abstract awv355_video_abstract

The keystone of Alzheimer pathogenesis might be sought in Aβ physiology

For several years Amyloid-beta peptide (Aβ) has been considered the main pathogenetic factor of Alzheimer's disease (AD). According to the so called Amyloid Cascade Hypothesis the increase of Aβ triggers a series of events leading to synaptic dysfunction and memory loss as well as to the structural brain damage in the later stage of the disease. However, several evidences suggest that this hypothesis is not sufficient to explain AD pathogenesis, especially considering that most of the clinical trials aimed to decrease Aβ levels have been unsuccessful. Moreover, Aβ is physiologically produced in the healthy brain during neuronal activity and it is needed for synaptic plasticity and memory. Here we propose a model interpreting AD pathogenesis as an alteration of the negative feedback loop between Aβ and its physiological receptors, focusing on alpha7 nicotinic acetylcholine receptors (α7-nAchRs). According to this vision, when Aβ cannot exert its physiological function a negative feedback mechanism would induce a compensatory increase of its production leading to an abnormal accumulation that reduces α7-nAchR function, leading to synaptic dysfunction and memory loss. In this perspective, the indiscriminate Aβ removal might worsen neuronal homeostasis, causing a further impoverishment of learning and memory. Even if further studies are needed to better understand and validate these mechanisms, we believe that to deepen the role of Aβ in physiological conditions might represent the keystone to elucidate important aspects of AD pathogenesis.

GATA1-mediated transcriptional regulation of the γ-secretase activating protein increases Aβ formation in Down syndrome

Because of an extra copy of the Aβ precursor protein gene on chromosome 21, Down syndrome (DS) individuals develop high levels of Aβ peptides and Alzheimer disease-like brain amyloidosis early in life. Here we show that the γ-secretase activating protein (GSAP), a key enzyme in amyloidogenesis, is increased in DS brains and specifically regulated at the transcriptional level by GATA1 transcription factor. The discovery of this novel pathway has translational implications for DS, because pharmacological inhibition of GSAP is an attractive and viable Aβ-lowering therapeutic strategy for this disorder.

A genetic cause of Alzheimer disease: mechanistic insights from Down syndrome

Down syndrome, which arises in individuals carrying an extra copy of chromosome 21, is associated with a greatly increased risk of early-onset Alzheimer disease. It is thought that this risk is conferred by the presence of three copies of the gene encoding amyloid precursor protein (APP)--an Alzheimer disease risk factor--although the possession of extra copies of other chromosome 21 genes may also play a part. Further study of the mechanisms underlying the development of Alzheimer disease in people with Down syndrome could provide insights into the mechanisms that cause dementia in the general population.

Intracellular oligomeric amyloid-beta rapidly regulates GluA1 subunit of AMPA receptor in the hippocampus

The acute neurotoxicity of oligomeric forms of amyloid-β 1-42 (Aβ) is implicated in the pathogenesis of Alzheimer’s disease (AD). However, how these oligomers might first impair neuronal function at the onset of pathology is poorly understood. Here we have examined the underlying toxic effects caused by an increase in levels of intracellular Aβ, an event that could be important during the early stages of the disease. We show that oligomerised Aβ induces a rapid enhancement of AMPA receptor-mediated synaptic transmission (EPSC_A) when applied intracellularly. This effect is dependent on postsynaptic Ca²⁺ and PKA. Knockdown of GluA1, but not GluA2, prevents the effect, as does expression of a S845-phosphomutant of GluA1. Significantly, an inhibitor of Ca²⁺-permeable AMPARs (CP-AMPARs), IEM 1460, reverses the increase in the amplitude of EPSC_A. These results suggest that a primary neuronal response to intracellular Aβ oligomers is the rapid synaptic insertion of CP-AMPARs.

Neuronal nucleus and cytoplasm volume deficit in children with autism and volume increase in adolescents and adults

Introduction

Characterization of the type and topography of structural changes and their alterations throughout the lifespan of individuals with autism is essential for understanding the mechanisms contributing to the autistic phenotype. The aim of this stereological study of neurons in 16 brain structures of 14 autistic and 14 control subjects from 4 to 64 years of age was to establish the course of neuronal nuclear and cytoplasmic volume changes throughout the lifespan of individuals with autism.

Results

Our data indicate that a deficit of neuronal soma volume in children with autism is associated with deficits in the volume of the neuronal nucleus and cytoplasm. The significant deficits of neuronal nuclear and cytoplasmic volumes in 13 of 16 examined subcortical structures, archicortex, cerebellum, and brainstem in 4- to 8-year-old autistic children suggest a global nature of brain developmental abnormalities, but with region-specific differences in the severity of neuronal pathology. The observed increase in nuclear volumes in 8 of 16 structures in the autistic teenagers/young adults and decrease in nuclear volumes in 14 of 16 regions in the age-matched control subjects reveal opposite trajectories throughout the lifespan. The deficit in neuronal nuclear volumes, ranging from 7% to 42% in the 16 examined regions in children with autism, and in neuronal cytoplasmic volumes from 1% to 31%, as well as the broader range of interindividual differences for the nuclear than the cytoplasmic volume deficits, suggest a partial distinction between nuclear and cytoplasmic pathology.

Conclusions

The most severe deficit of both neuronal nucleus and cytoplasm volume in 4-to 8-year-old autistic children appears to be a reflection of early developmental alterations that may have a major contribution to the autistic phenotype. The broad range of functions of the affected structures implies that their developmental and age-associated abnormalities contribute not only to the diagnostic features of autism but also to the broad spectrum of clinical alterations associated with autism. Lack of clinical improvement in autistic teenagers and adults indicates that the observed increase in neuron nucleus and cytoplasm volume close to control level does not normalize brain function.

Can BACE1 inhibition mitigate early axonal pathology in neurological diseases?

β-Secretase-1 (BACE1) is the rate-limiting enzyme for the genesis of amyloid-β (Aβ) peptides, the main constituents of the amyloid plaques in the brains of Alzheimer's disease (AD) patients. BACE1 is being evaluated as an anti-Aβ target for AD therapy. Recent studies indicate that BACE1 elevation is associated with axonal and presynaptic pathology during plaque development. Evidence also points to a biological role for BACE1 in axonal outgrowth and synapse formation during development. Axonal, including presynaptic, pathology exists in AD as well as many other neurological disorders such as Parkinson's disease, epilepsy, stroke, and trauma. In this review, we discuss pharmaceutical BACE1 inhibition as a therapeutic option for axonal pathogenesis, in addition to amyloid pathology. We first introduce the amyloidogenic processing of amyloid-β protein precursor and describe the normal expression pattern of the amyloidogenic proteins in the brain, with an emphasis on BACE1. We then address BACE1 elevation relative to amyloid plaque development, followed by updating recent understanding of a neurotrophic role of BACE1 in axon and synapse development. We further elaborate the occurrence of axonal pathology in some other neurological conditions. Finally, we propose pharmacological inhibition of excessive BACE1 activity as an option to mitigate early axonal pathology occurring in AD and other neurological disorders.

Nuclear translocation uncovers the amyloid peptide Aβ42 as a regulator of gene transcription

Although soluble species of the amyloid-β peptide Aβ42 correlate with disease symptoms in Alzheimer disease, little is known about the biological activities of amyloid-β (Aβ). Here, we show that Aβ peptides varying in lengths from 38 to 43 amino acids are internalized by cultured neuroblastoma cells and can be found in the nucleus. By three independent methods, we demonstrate direct detection of nuclear Aβ42 as follows: (i) biochemical analysis of nuclear fractions; (ii) detection of biotin-labeled Aβ in living cells by confocal laser scanning microscopy; and (iii) transmission electron microscopy of Aβ in cultured cells, as well as brain tissue of wild-type and transgenic APPPS1 mice (overexpression of amyloid precursor protein and presenilin 1 with Swedish and L166P mutations, respectively). Also, this study details a novel role for Aβ42 in nuclear signaling, distinct from the amyloid precursor protein intracellular domain. Chromatin immunoprecipitation showed that Aβ42 specifically interacts as a repressor of gene transcription with LRP1 and KAI1 promoters. By quantitative RT-PCR, we confirmed that mRNA levels of the examined candidate genes were exclusively decreased by the potentially neurotoxic Aβ42 wild-type peptide. Shorter peptides (Aβ38 or Aβ40) and other longer peptides (nontoxic Aβ42 G33A substitution or Aβ43) did not affect mRNA levels. Overall, our data indicate that the nuclear translocation of Aβ42 impacts gene regulation, and deleterious effects of Aβ42 in Alzheimer disease pathogenesis may be influenced by altering the expression profiles of disease-modifying genes.

Linking pathways in the developing and aging brain with neurodegeneration

The molecular and cellular mechanisms, which coordinate the critical stages of brain development to reach a normal structural organization with appropriate networks, are progressively being elucidated. Experimental and clinical studies provide evidence of the occurrence of developmental alterations induced by genetic or environmental factors leading to the formation of aberrant networks associated with learning disabilities. Moreover, evidence is accumulating that suggests that also late-onset neurological disorders, even Alzheimer's disease, might be considered disorders of aberrant neural development with pathological changes that are set up at early stages of development before the appearance of the symptoms. Thus, evaluating proteins and pathways that are important in age-related neurodegeneration in the developing brain together with the characterization of mechanisms important during brain development with relevance to brain aging are of crucial importance. In the present review we focus on (1) aspects of neurogenesis with relevance to aging; (2) neurodegenerative disease (NDD)-associated proteins/pathways in the developing brain; and (3) further pathways of the developing or neurodegenerating brains that show commonalities. Elucidation of complex pathogenetic routes characterizing the earliest stage of the detrimental processes that result in pathological aging represents an essential first step toward a therapeutic intervention which is able to reverse these pathological processes and prevent the onset of the disease. Based on the shared features between pathways, we conclude that prevention of NDDs of the elderly might begin during the fetal and childhood life by providing the mothers and their children a healthy environment for the fetal and childhood development.

Accumulation of amyloid-like Aβ1-42 in AEL (autophagy-endosomal-lysosomal) vesicles: potential implications for plaque biogenesis

Abnormal accumulation of Aβ (amyloid β) within AEL (autophagy-endosomal-lysosomal) vesicles is a prominent neuropathological feature of AD (Alzheimer's disease), but the mechanism of accumulation within vesicles is not clear. We express secretory forms of human Aβ1-40 or Aβ1-42 in Drosophila neurons and observe preferential localization of Aβ1-42 within AEL vesicles. In young animals, Aβ1-42 appears to associate with plasma membrane, whereas Aβ1-40 does not, suggesting that recycling endocytosis may underlie its routing to AEL vesicles. Aβ1-40, in contrast, appears to partially localize in extracellular spaces in whole brain and is preferentially secreted by cultured neurons. As animals become older, AEL vesicles become dysfunctional, enlarge and their turnover appears delayed. Genetic inhibition of AEL function results in decreased Aβ1-42 accumulation. In samples from older animals, Aβ1-42 is broadly distributed within neurons, but only the Aβ1-42 within dysfunctional AEL vesicles appears to be in an amyloid-like state. Moreover, the Aβ1-42-containing AEL vesicles share properties with AD-like extracellular plaques. They appear to be able to relocate to extracellular spaces either as a consequence of age-dependent neurodegeneration or a non-neurodegenerative separation from host neurons by plasma membrane infolding. We propose that dysfunctional AEL vesicles may thus be the source of amyloid-like plaque accumulation in Aβ1-42-expressing Drosophila with potential relevance for AD.

A generic method for design of oligomer-specific antibodies

Antibodies that preferentially and specifically target pathological oligomeric protein and peptide assemblies, as opposed to their monomeric and amyloid counterparts, provide therapeutic and diagnostic opportunities for protein misfolding diseases. Unfortunately, the molecular properties associated with oligomer-specific antibodies are not well understood, and this limits targeted design and development. We present here a generic method that enables the design and optimisation of oligomer-specific antibodies. The method takes a two-step approach where discrimination between oligomers and fibrils is first accomplished through identification of cryptic epitopes exclusively buried within the structure of the fibrillar form. The second step discriminates between monomers and oligomers based on differences in avidity. We show here that a simple divalent mode of interaction, as within e.g. the IgG isotype, can increase the binding strength of the antibody up to 1500 times compared to its monovalent counterpart. We expose how the ability to bind oligomers is affected by the monovalent affinity and the turnover rate of the binding and, importantly, also how oligomer specificity is only valid within a specific concentration range. We provide an example of the method by creating and characterising a spectrum of different monoclonal antibodies against both the Aβ peptide and α-synuclein that are associated with Alzheimer's and Parkinson's diseases, respectively. The approach is however generic, does not require identification of oligomer-specific architectures, and is, in essence, applicable to all polypeptides that form oligomeric and fibrillar assemblies.

Early intervention in the 3xTg-AD mice with an amyloid β-antibody fragment ameliorates first hallmarks of Alzheimer disease

The single-chain variable fragment, scFv-h3D6, has been shown to prevent in vitro toxicity induced by the amyloid β (Aβ) peptide in neuroblastoma cell cultures by withdrawing Aβ oligomers from the amyloid pathway. Present study examined the in vivo effects of scFv-h3D6 in the triple-transgenic 3xTg-AD mouse model of Alzheimer disease. Prior to the treatment, five-month-old female animals, corresponding to early stages of the disease, showed the first behavioral and psychological symptoms of dementia -like behaviors. Cognitive deficits included long- and short-term learning and memory deficits and high swimming navigation speed. After a single intraperitoneal dose of scFv-h3D6, the swimming speed was reversed to normal levels and the learning and memory deficits were ameliorated. Brain tissues of these animals revealed a global decrease of Aβ oligomers in the cortex and olfactory bulb after treatment, but this was not seen in the hippocampus and cerebellum. In the untreated 3xTg-AD animals, we observed an increase of both apoJ and apoE concentrations in the cortex, as well as an increase of apoE in the hippocampus. Treatment significantly recovered the non-pathological levels of these apolipoproteins. Our results suggest that the benefit of scFv-h3D6 occurs at both behavioral and molecular levels.

Extracellular chaperones prevent Aβ42-induced toxicity in rat brains

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by cognitive decline, formation of the extracellular amyloid β (Aβ42) plaques, neuronal and synapse loss, and activated microglia and astrocytes. Extracellular chaperones, which are known to inhibit amyloid fibril formation and promote clearance of misfolded aggregates, have recently been shown to reduce efficiently the toxicity of HypF-N misfolded oligomers to immortalised cell lines, by binding and clustering them into large species. However, the role of extracellular chaperones on Aβ oligomer toxicity remains unclear, with reports often appearing contradictory. In this study we microinjected into the hippocampus of rat brains Aβ42 oligomers pre-incubated for 1h with two extracellular chaperones, namely clusterin and α2-macroglobulin. The chaperones were found to prevent Aβ42-induced learning and memory impairments, as assessed by the Morris Water Maze test, and reduce Aβ42-induced glia inflammation and neuronal degeneration in rat brains, as probed by fluorescent immunohistochemical analyses. Moreover, the chaperones were able to prevent Aβ42 colocalisation with PSD-95 at post-synaptic terminals of rat primary neurons, suppressing oligomer cytotoxicity. All such effects were not effective by adding pre-formed oligomers and chaperones without preincubation. Molecular chaperones have therefore the potential to prevent the early symptoms of AD, not just by inhibiting Aβ42 aggregation, as previously demonstrated, but also by suppressing the toxicity of Aβ42 oligomers after they are formed. These findings elect them as novel neuroprotectors against amyloid-induced injury and excellent candidates for the design of therapeutic strategies against AD.

Amyloid plaque pathogenesis in 5XFAD mouse spinal cord: retrograde transneuronal modulation after peripheral nerve injury

The spinal cord is composed of distinct neuronal groups with well-defined anatomic connections. In some transgenic (Tg) models of Alzheimer's disease (AD), amyloid plaques develop in this structure, although the underlying cellular mechanism remains elusive. We attempted to explore the origin, evolution, and modulation of spinal β-amyloid (Aβ) deposition using Tg mice harboring five familiar AD-related mutations (5XFAD) as an experiential model. Dystrophic neuritic elements with enhanced β-secretase-1 (BACE1) immunoreactivity (IR) appeared as early as 2 months of age, and increased with age up to 12 months examined in this study, mostly over the ventral horn (VH). Extracellular Aβ IR emerged and developed during this same period, site-specifically co-existing with BACE1-labeled neurites often in the vicinity of large VH neurons that expressed the mutant human APP. The BACE1-labeled neurites almost invariably colocalized with β-amyloid precursor protein (APP) and synaptophysin, and frequently with the vesicular glutamate transporter-1 (VGLUT). Reduced IR for the neuronal-specific nuclear antigen (NeuN) occurred in the VH by 12 months of age. In 8-month-old animals surviving 6 months after a unilateral sciatic nerve transection, there were significant increases of Aβ, BACE1, and VGLUT IR in the VN of the ipsilateral relative to contralateral lumbar spinal segments. These results suggest that extracellular Aβ deposition in 5XFAD mouse spinal cord relates to a progressive and amyloidogenic synaptic pathology largely involving presynaptic axon terminals from projection neurons in the brain. Spinal neuritic plaque formation is enhanced after peripheral axotomy, suggesting a retrograde transneuronal modulation on pathogenesis.

Therapeutic interventions targeting Beta amyloid pathogenesis in an aging dog model

Aged dogs and humans share complex cognitive and pathological responses to aging. Specifically, dogs develop Alzheimer's Disease (AD) like beta-amyloid (Aβ) that are associated with cognitive deficits. Currently, therapeutic approaches to prevent AD are targeted towards reduced production, aggregation and increased clearance of Aβ. The current review discusses cognition and neuropathology of the aging canine model and how it has and continues to be useful in further understanding the safety and efficacy of potential AD prevention therapies targeting Aβ.

Endoplasmic reticulum enrollment in Alzheimer's disease

Alzheimer's disease (AD) poses a huge challenge for society and health care worldwide as molecular pathogenesis of the disease is poorly understood and curative treatment does not exist. The mechanisms leading to accelerated neuronal cell death in AD are still largely unknown, but accumulation of misfolded disease-specific proteins has been identified as potentially involved. In the present review, we describe the essential role of endoplasmic reticulum (ER) in AD. Despite the function that mitochondria may play as the central major player in the apoptotic process, accumulating evidence highlights ER as a critical organelle in AD. Stress that impairs ER physiology leads to accumulation of unfolded or misfolded proteins, such as amyloid β (Aβ) peptide, the major component of amyloid plaques. In an attempt to ameliorate the accumulation of unfolded proteins, ER stress triggers a protective cellular mechanism, which includes the unfolded protein response (UPR). However, when activation of the UPR is severe or prolonged enough, the final cellular outcome is pathologic apoptotic cell death. Distinct pathways can be activated in this process, involving stress sensors such as the JNK pathway or ER chaperones such as Bip/GRP94, stress modulators such as Bcl-2 family proteins, or even stress effectors such as caspase-12. Here, we detail the involvement of the ER and associated stress pathways in AD and discuss potential therapeutic strategies targeting ER stress.

The toxicity of amyloid β oligomers

In this review, we elucidate the mechanisms of Aβ oligomer toxicity which may contribute to Alzheimer's disease (AD). In particular, we discuss on the interaction of Aβ oligomers with the membrane through the process of adsorption and insertion. Such interaction gives rises to phase transitions in the sub-structures of the Aβ peptide from α-helical to β-sheet structure. By means of a coarse-grained model, we exhibit the tendency of β-sheet structures to aggregate, thus providing further insights to the process of membrane induced aggregation. We show that the aggregated oligomer causes membrane invagination, which is a precursor to the formation of pore structures and ion channels. Other pathological progressions to AD due to Aβ oligomers are also covered, such as their interaction with the membrane receptors, and their direct versus indirect effects on oxidative stress and intraneuronal accumulation. We further illustrate that the molecule curcumin is a potential Aβ toxicity inhibitor as a β-sheet breaker by having a high propensity to interact with certain Aβ residues without binding to them. The comprehensive understanding gained from these current researches on the various toxicity mechanisms show promises in the provision of better therapeutics and treatment strategies in the near future.

Abnormal intracellular accumulation and extracellular Aβ deposition in idiopathic and Dup15q11.2-q13 autism spectrum disorders

BACKGROUND

It has been shown that amyloid ß (Aβ), a product of proteolytic cleavage of the amyloid β precursor protein (APP), accumulates in neuronal cytoplasm in non-affected individuals in a cell type-specific amount.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, we found that the percentage of amyloid-positive neurons increases in subjects diagnosed with idiopathic autism and subjects diagnosed with duplication 15q11.2-q13 (dup15) and autism spectrum disorder (ASD). In spite of interindividual differences within each examined group, levels of intraneuronal Aβ load were significantly greater in the dup(15) autism group than in either the control or the idiopathic autism group in 11 of 12 examined regions (p<0.0001 for all comparisons; Kruskall-Wallis test). In eight regions, intraneuronal Aβ load differed significantly between idiopathic autism and control groups (p<0.0001). The intraneuronal Aβ was mainly N-terminally truncated. Increased intraneuronal accumulation of Aβ(17-40/42) in children and adults suggests a life-long enhancement of APP processing with α-secretase in autistic subjects. Aβ accumulation in neuronal endosomes, autophagic vacuoles, Lamp1-positive lysosomes and lipofuscin, as revealed by confocal microscopy, indicates that products of enhanced α-secretase processing accumulate in organelles involved in proteolysis and storage of metabolic remnants. Diffuse plaques containing Aβ(1-40/42) detected in three subjects with ASD, 39 to 52 years of age, suggest that there is an age-associated risk of alterations of APP processing with an intraneuronal accumulation of a short form of Aβ and an extracellular deposition of full-length Aβ in nonfibrillar plaques.

CONCLUSIONS/SIGNIFICANCE

The higher prevalence of excessive Aβ accumulation in neurons in individuals with early onset of intractable seizures, and with a high risk of sudden unexpected death in epilepsy in autistic subjects with dup(15) compared to subjects with idiopathic ASD, supports the concept of mechanistic and functional links between autism, epilepsy and alterations of APP processing leading to neuronal and astrocytic Aβ accumulation and diffuse plaque formation.

Intraneuronal Aβ detection in 5xFAD mice by a new Aβ-specific antibody

Background

The form(s) of amyloid-β peptide (Aβ) associated with the pathology characteristic of Alzheimer's disease (AD) remains unclear. In particular, the neurotoxicity of intraneuronal Aβ accumulation is an issue of considerable controversy; even the existence of Aβ deposits within neurons has recently been challenged by Winton and co-workers. These authors purport that it is actually intraneuronal APP that is being detected by antibodies thought to be specific for Aβ. To further address this issue, an anti-Aβ antibody was developed (MOAB-2) that specifically detects Aβ, but not APP. This antibody allows for the further evaluation of the early accumulation of intraneuronal Aβ in transgenic mice with increased levels of human Aβ in 5xFAD and 3xTg mice.

Results

MOAB-2 (mouse IgG_2b) is a pan-specific, high-titer antibody to Aβ residues 1-4 as demonstrated by biochemical and immunohistochemical analyses (IHC), particularly compared to 6E10 (a commonly used commercial antibody to Aβ residues 3-8). MOAB-2 did not detect APP or APP-CTFs in cell culture media/lysates (HEK-APP_Swe or HEK-APP_Swe/BACE1) or in brain homogenates from transgenic mice expressing 5 familial AD (FAD) mutation (5xFAD mice). Using IHC on 5xFAD brain tissue, MOAB-2 immunoreactivity co-localized with C-terminal antibodies specific for Aβ40 and Aβ42. MOAB-2 did not co-localize with either N- or C-terminal antibodies to APP. In addition, no MOAB-2-immunreactivity was observed in the brains of 5xFAD/BACE^-/- mice, although significant amounts of APP were detected by N- and C-terminal antibodies to APP, as well as by 6E10. In both 5xFAD and 3xTg mouse brain tissue, MOAB-2 co-localized with cathepsin-D, a marker for acidic organelles, further evidence for intraneuronal Aβ, distinct from Aβ associated with the cell membrane. MOAB-2 demonstrated strong intraneuronal and extra-cellular immunoreactivity in 5xFAD and 3xTg mouse brain tissues.

Conclusions

Both intraneuronal Aβ accumulation and extracellular Aβ deposition was demonstrated in 5xFAD mice and 3xTg mice with MOAB-2, an antibody that will help differentiate intracellular Aβ from APP. However, further investigation is required to determine whether a molecular mechanism links the presence of intraneuronal Aβ with neurotoxicity. As well, understanding the relevance of these observations to human AD patients is critical.

Intraneuronal Aβ accumulation and neurodegeneration: lessons from transgenic models

AIMS

In the present review we summarize current knowledge on the concept of intraneuronal Aβ as a determinant for neuron loss and other pathological alterations in transgenic models for Alzheimer disease.

MAIN METHODS

We discuss the use of transgenic mouse and non-vertebrate transgenic models accumulating intracellular Aβ peptides and their impact on the ongoing discussion.

KEY FINDINGS

Intraneuronal Aβ accumulation in transgenic models is intimately linked to pathological alterations including neuron loss. One of the technical caveats for visualizing intraneuronal Aβ is the antibody used to unequivocally demonstrate its presence. Very often antibodies were used that recognize both Aβ and APP, leading to false positive results due to misinterpretation.

SIGNIFICANCE

Whereas a clear relationship between intraneuronal Aβ accumulation and neuron loss is evident in transgenic mouse models it remains an unresolved issue whether the concept of intraneuronal Aβ can be integrated into the human pathology as well.

Intraneuronal APP, not free Aβ peptides in 3xTg-AD mice: implications for tau versus Aβ-mediated Alzheimer neurodegeneration

Alzheimer's disease (AD) is characterized by the accumulation of intraneuronal tau and extracellular amyloid-β (Aβ) peptide. A triple transgenic (Tg) mouse (3xTg-AD) was reported to develop Aβ plaques and tau inclusions as well as remarkable accumulations of intracellular Aβ that were suggested to be the initiators of AD pathogenesis. However, it was unclear whether the anti-Aβ antibodies were able to distinguish Aβ peptide from the same Aβ epitopes within the amyloid precursor protein (APP). To further elucidate the identity of the immunoreactive intraneuronal material in 3xTg-AD mice, we conducted immunohistochemical, biochemical, and ultrastructural studies using a well characterized panel of antibodies that distinguish Aβ within APP from cleaved Aβ peptides. We found that the intraneuronal material shared epitopes with full-length APP but not free Aβ. To demonstrate unequivocally that this intraneuronal material was not free Aβ peptide, we generated 3xTg-AD mice deficient for β-secretase (BACE), the protease required for Aβ generation from APP. In the absence of Aβ production, robust intraneuronal APP immunostaining was detected in the 3xTg-AD/BACE(-/-) mice. Finally, we found that the formation of tau lesions was not different between 3xTg-AD versus 3xTg-AD/BACE(-/-) mice, thereby demonstrating that tau pathology forms independently from Aβ peptide generation in this mouse model. Although we cannot corroborate the presence of intraneuronal Aβ peptide in 3xTg-AD mice, our findings warrant further study as to the role of aberrant APP accumulation in this unique model of AD.

BACE1 elevation is involved in amyloid plaque development in the triple transgenic model of Alzheimer's disease: differential Aβ antibody labeling of early-onset axon terminal pathology

β-amyloid precursor protein (APP) and presenilins mutations cause early-onset familial Alzheimer's disease (FAD). Some FAD-based mouse models produce amyloid plaques, others do not. β-Amyloid (Aβ) deposition can manifest as compact and diffuse plaques; it is unclear why the same Aβ molecules aggregate in different patterns. Is there a basic cellular process governing Aβ plaque pathogenesis? We showed in some FAD mouse models that compact plaque formation is associated with a progressive axonal pathology inherent with increased expression of β-secretase (BACE1), the enzyme initiating the amyloidogenic processing of APP. A monoclonal Aβ antibody, 3D6, visualized distinct axon terminal labeling before plaque onset. The present study was set to understand BACE1 and axonal changes relative to diffuse plaque development and to further characterize the novel axonal Aβ antibody immunoreactivity (IR), using triple transgenic AD (3xTg-AD) mice as experimental model. Diffuse-like plaques existed in the forebrain in aged transgenics and were regionally associated with increased BACE1 labeled swollen/sprouting axon terminals. Increased BACE1/3D6 IR at axon terminals occurred in young animals before plaque onset. These axonal elements were also co-labeled by other antibodies targeting the N-terminal and mid-region of Aβ domain and the C-terminal of APP, but not co-labeled by antibodies against the Aβ C-terminal and APP N-terminal. The results suggest that amyloidogenic axonal pathology precedes diffuse plaque formation in the 3xTg-AD mice, and that the early-onset axonal Aβ antibody IR in transgenic models of AD might relate to a cross-reactivity of putative APP β-carboxyl terminal fragments.

Modes of Aβ toxicity in Alzheimer's disease

Alzheimer’s disease (AD) is reaching epidemic proportions, yet a cure is not yet available. While the genetic causes of the rare familial inherited forms of AD are understood, the causes of the sporadic forms of the disease are not. Histopathologically, these two forms of AD are indistinguishable: they are characterized by amyloid-β (Aβ) peptide-containing amyloid plaques and tau-containing neurofibrillary tangles. In this review we compare AD to frontotemporal dementia (FTD), a subset of which is characterized by tau deposition in the absence of overt plaques. A host of transgenic animal AD models have been established through the expression of human proteins with pathogenic mutations previously identified in familial AD and FTD. Determining how these mutant proteins cause disease in vivo should contribute to an understanding of the causes of the more frequent sporadic forms. We discuss the insight transgenic animal models have provided into Aβ and tau toxicity, also with regards to mitochondrial function and the crucial role tau plays in mediating Aβ toxicity. We also discuss the role of miRNAs in mediating the toxic effects of the Aβ peptide.

Aβ internalization by neurons and glia

In the brain, the amyloid β peptide (Aβ) exists extracellularly and inside neurons. The intracellular accumulation of Aβ in Alzheimer's disease brain has been questioned for a long time. However, there is now sufficient strong evidence indicating that accumulation of Aβ inside neurons plays an important role in the pathogenesis of Alzheimer's disease. Intraneuronal Aβ originates from intracellular cleavage of APP and from Aβ internalization from the extracellular milieu. We discuss here the different molecular mechanisms that are responsible for Aβ internalization in neurons and the links between Aβ internalization and neuronal dysfunction and death. A brief description of Aβ uptake by glia is also presented.

Mechanisms of amyloid-Beta Peptide uptake by neurons: the role of lipid rafts and lipid raft-associated proteins

A hallmark pathological feature of Alzheimer's disease (AD) is the accumulation of extracellular plaques composed of the amyloid-beta (Aβ) peptide. Thus, classically experiments were designed to examine Aβ toxicities within the central nervous system (CNS) from the extracellular space. However, a significant amount of evidence now suggests that intraneuronal accumulation of Aβ is neurotoxic and may play an important role in the disease progression of AD. One of the means by which neurons accumulate intracellular Aβ is through uptake of extracellular Aβ peptides, and this process may be a potential link between Aβ generation, synaptic dysfunction, and AD pathology. Recent studies have found that neuronal internalization of Aβ involves lipid rafts and various lipid raft-associated receptor proteins. Uptake mechanisms independent of lipid rafts have also been implicated. The aim of this paper is to summarize these findings and discuss their significance in the pathogenesis of AD.

The effects of cholesterol on learning and memory

Cholesterol is vital to normal brain function including learning and memory but that involvement is as complex as the synthesis, metabolism and excretion of cholesterol itself. Dietary cholesterol influences learning tasks from water maze to fear conditioning even though cholesterol does not cross the blood brain barrier. Excess cholesterol has many consequences including peripheral pathology that can signal brain via cholesterol metabolites, pro-inflammatory mediators and antioxidant processes. Manipulations of cholesterol within the central nervous system through genetic, pharmacological, or metabolic means circumvent the blood brain barrier and affect learning and memory but often in animals already otherwise compromised. The human literature is no less complex. Cholesterol reduction using statins improves memory in some cases but not others. There is also controversy over statin use to alleviate memory problems in Alzheimer's disease. Correlations of cholesterol and cognitive function are mixed and association studies find some genetic polymorphisms are related to cognitive function but others are not. In sum, the field is in flux with a number of seemingly contradictory results and many complexities. Nevertheless, understanding cholesterol effects on learning and memory is too important to ignore.

An overview of APP processing enzymes and products

The generation of amyloid beta-peptide (A beta) by enzymatic cleavages of the beta-amyloid precursor protein (APP) has been at the center of Alzheimer's disease (AD) research. While the basic process of beta- and gamma-secretase-mediated generation of A beta is text book knowledge, new aspects of A beta and other cleavage products have emerged in recent years. Also our understanding of the enzymes involved in APP proteolysis has increased dramatically. All of these discoveries contribute to a more complete understanding of APP processing and the physiologic and pathologic roles of its secreted and intracellular protein products. Understanding APP processing is important for any therapeutic strategy aimed at reducing A beta levels in AD. In this review, we provide a concise description of the current state of understanding the enzymes involved in APP processing, the cleavage products generated by different processing patterns, and the potential functions of those cleavage products.

Accumulation of intraneuronal Abeta correlates with ApoE4 genotype

In contrast to extracellular plaque and intracellular tangle pathology, the presence and relevance of intraneuronal Aβ in Alzheimer’s disease (AD) is still a matter of debate. Human brain tissue offers technical challenges such as post-mortem delay and uneven or prolonged tissue fixation that might affect immunohistochemical staining. In addition, previous studies on intracellular Aβ accumulation in human brain often used antibodies targeting the C-terminus of Aβ and differed strongly in the pretreatments used. To overcome these inconsistencies, we performed extensive parametrical testing using a highly specific N-terminal Aβ antibody detecting the aspartate at position 1, before developing an optimal staining protocol for intraneuronal Aβ detection in paraffin-embedded sections from AD patients. To rule out that this antibody also detects the β-cleaved APP C-terminal fragment (β-CTF, C99) bearing the same epitope, paraffin-sections of transgenic mice overexpressing the C99-fragment were stained without any evidence for cross-reactivity in our staining protocol. The staining intensity of intraneuronal Aβ in cortex and hippocampal tissue of 10 controls and 20 sporadic AD cases was then correlated to patient data including sex, Braak stage, plaque load, and apolipoprotein E (ApoE) genotype. In particular, the presence of one or two ApoE4 alleles strongly correlated with an increased accumulation of intraneuronal Aβ peptides. Given that ApoE4 is a major genetic risk factor for AD and is involved in neuronal cholesterol transport, it is tempting to speculate that perturbed intracellular trafficking is involved in the increased intraneuronal Aβ aggregation in AD.

Intracellular accumulation of amyloid-Beta - a predictor for synaptic dysfunction and neuron loss in Alzheimer's disease

Despite of long-standing evidence that beta-amyloid (Abeta) peptides have detrimental effects on synaptic function, the relationship between Abeta, synaptic and neuron loss is largely unclear. During the last years there is growing evidence that early intraneuronal accumulation of Abeta peptides is one of the key events leading to synaptic and neuronal dysfunction. Many studies have been carried out using transgenic mouse models of Alzheimer's disease (AD) which have been proven to be valuable model systems in modern AD research. The present review discusses the impact of intraneuronal Abeta accumulation on synaptic impairment and neuron loss and provides an overview of currently available AD mouse models showing these pathological alterations.

Pyroglutamate Abeta pathology in APP/PS1KI mice, sporadic and familial Alzheimer's disease cases

The presence of Abeta(pE3) (N-terminal truncated Abeta starting with pyroglutamate) in Alzheimer's disease (AD) has received considerable attention since the discovery that this peptide represents a dominant fraction of Abeta peptides in senile plaques of AD brains. This was later confirmed by other reports investigating AD and Down's syndrome postmortem brain tissue. Importantly, Abeta(pE3) has a higher aggregation propensity, and stability, and shows an increased toxicity compared to full-length Abeta. We have recently shown that intraneuronal accumulation of Abeta(pE3) peptides induces a severe neuron loss and an associated neurological phenotype in the TBA2 mouse model for AD. Given the increasing interest in Abeta(pE3), we have generated two novel monoclonal antibodies which were characterized as highly specific for Abeta(pE3) peptides and herein used to analyze plaque deposition in APP/PS1KI mice, an AD model with severe neuron loss and learning deficits. This was compared with the plaque pattern present in brain tissue from sporadic and familial AD cases. Abundant plaques positive for Abeta(pE3) were present in patients with sporadic AD and familial AD including those carrying mutations in APP (arctic and Swedish) and PS1. Interestingly, in APP/PS1KI mice we observed a continuous increase in Abeta(pE3) plaque load with increasing age, while the density for Abeta(1-x ) plaques declined with aging. We therefore assume that, in particular, the peptides starting with position 1 of Abeta are N-truncated as disease progresses, and that, Abeta(pE3) positive plaques are resistant to age-dependent degradation likely due to their high stability and propensity to aggregate.

Intraneuronal amyloid beta accumulation and oxidative damage to nucleic acids in Alzheimer disease

In an analysis of amyloid pathology in Alzheimer disease, we used an in situ approach to identify amyloid-beta (Abeta) accumulation and oxidative damage to nucleic acids in postmortem brain tissue of the hippocampal formation from subjects with Alzheimer disease. When carboxyl-terminal-specific antibodies directed against Abeta40 and Abeta42 were used for immunocytochemical analyses, Abeta42 was especially apparent within the neuronal cytoplasm, at sites not detected by the antibody specific to Abeta-oligomer. In comparison to the Abeta42-positive neurons, neurons bearing oxidative damage to nucleic acids were more widely distributed in the hippocampus. Comparative density measurements of the immunoreactivity revealed that levels of intraneuronal Abeta42 were inversely correlated with levels of intraneuronal 8-hydroxyguanosine, an oxidized nucleoside (r=- 0.61, p<0.02). Together with recent evidence that the Abeta peptide can act as an antioxidant, these results suggest that intraneuronal accumulation of non-oligomeric Abeta may be a compensatory response in neurons to oxidative stress in Alzheimer disease.

Formic acid is essential for immunohistochemical detection of aggregated intraneuronal Abeta peptides in mouse models of Alzheimer's disease

The staining protocols so far applied to study intracellular Abeta accumulation in human tissue have been inconsistent with varying use of heat and formic acid (FA) for antigen retrieval. Microwave heat treatment has been reported to enhance the staining of intraneuronal Abeta as compared to no or enzymatic pretreatment. FA is widely used to increase the staining of plaque pathology in AD, yet the effect of FA on intraneuronal Abeta staining has been reported to be low and similar to the effect of heat or even to counteract the enhancing effect of heat pretreatment on intraneuronal Abeta immunohistochemical detection. To overcome these inconsistencies, there is a need for optimization of the staining protocol for intraneuronal Abeta detection and more knowledge is required concerning the effects of the different antigen retrieval methods. In the present work, we optimized the staining protocol for intraneuronal Abeta in paraffin-embedded sections in relation to heat and FA using four different mouse models known to accumulate intraneuronal Abeta peptides. It was found that FA is essential for the staining of highly aggregated intraneuronal Abeta peptides in AD transgenic mouse tissue.