The Arctic Alzheimer mutation favors intracellular amyloid-beta production by making amyloid precursor protein less available to alpha-secretase

Aggregation of Disordered Proteins Associated with Neurodegeneration

Cellular deposition of protein aggregates, one of the hallmarks of neurodegeneration, disrupts cellular functions and leads to neuronal death. Mutations, posttranslational modifications, and truncations are common molecular underpinnings in the formation of aberrant protein conformations that seed aggregation. The major proteins involved in neurodegeneration include amyloid beta (Aβ) and tau in Alzheimer's disease, α-synuclein in Parkinson's disease, and TAR DNA-binding protein (TDP-43) in amyotrophic lateral sclerosis (ALS). These proteins are described as intrinsically disordered and possess enhanced ability to partition into biomolecular condensates. In this review, we discuss the role of protein misfolding and aggregation in neurodegenerative diseases, specifically highlighting implications of changes to the primary/secondary (mutations, posttranslational modifications, and truncations) and the quaternary/supramolecular (oligomerization and condensation) structural landscapes for the four aforementioned proteins. Understanding these aggregation mechanisms provides insights into neurodegenerative diseases and their common underlying molecular pathology.

Using stable isotope labeling to advance our understanding of Alzheimer's disease etiology and pathology

Stable isotope labeling with mass spectrometry (MS)-based proteomic analysis has become a powerful strategy to assess protein steady-state levels, protein turnover, and protein localization. Applying these analyses platforms to neurodegenerative disorders may uncover new aspects of the etiology of these devastating diseases. Recently, stable isotopes-MS has been used to investigate early pathological mechanisms of Alzheimer's disease (AD) with mouse models of AD-like pathology. In this review, we summarize these stable isotope-MS experimental designs and the recent application in the context of AD pathology. We also describe our current efforts aimed at using nuclear magnetic resonance (NMR) analysis of stable isotope-labeled amyloid fibrils from AD mouse model brains. Collectively, these methodologies offer new opportunities to study proteome changes in AD and other neurodegenerative diseases by elucidating mechanisms to target for treatment and prevention.

The Amyloid-β Pathway in Alzheimer's Disease

Breakthroughs in molecular medicine have positioned the amyloid-β (Aβ) pathway at the center of Alzheimer's disease (AD) pathophysiology. While the detailed molecular mechanisms of the pathway and the spatial-temporal dynamics leading to synaptic failure, neurodegeneration, and clinical onset are still under intense investigation, the established biochemical alterations of the Aβ cycle remain the core biological hallmark of AD and are promising targets for the development of disease-modifying therapies. Here, we systematically review and update the vast state-of-the-art literature of Aβ science with evidence from basic research studies to human genetic and multi-modal biomarker investigations, which supports a crucial role of Aβ pathway dyshomeostasis in AD pathophysiological dynamics. We discuss the evidence highlighting a differentiated interaction of distinct Aβ species with other AD-related biological mechanisms, such as tau-mediated, neuroimmune and inflammatory changes, as well as a neurochemical imbalance. Through the lens of the latest development of multimodal in vivo biomarkers of AD, this cross-disciplinary review examines the compelling hypothesis- and data-driven rationale for Aβ-targeting therapeutic strategies in development for the early treatment of AD.

Amyloid Beta Hypothesis in Alzheimer's Disease: Major Culprits and Recent Therapeutic Strategies

Alzheimer's disease (AD) is one of the most common forms of dementia and has been a global concern for several years. Due to the multi-factorial nature of the disease, AD has become irreversible, fatal and imposes a tremendous socio-economic burden. Even though experimental medicines suggested moderate benefits, AD still lacks an effective treatment strategy for the management of symptoms or cure. Among the various hypotheses that describe development and progression of AD, the amyloid hypothesis has been a long-term adherent to the AD due to the involvement of various forms of Amyloid beta (Aβ) peptides in the impairment of neuronal and cognitive functions. Hence, majority of the drug discovery approaches in the past have focused on the prevention of the accumulation of Aβ peptides. Currently, there are several agents in the phase III clinical trials that target Aβ or the various macromolecules triggering Aβ deposition. In this review, we present the state of the art knowledge on the functional aspects of the key players involved in the amyloid hypothesis. Furthermore, we also discuss anti-amyloid agents present in the Phase III clinical trials.

Aß40 displays amyloidogenic properties in the non-transgenic mouse brain but does not exacerbate Aß42 toxicity in Drosophila

BACKGROUND

Self-assembly of the amyloid-β (Aβ) peptide into aggregates, from small oligomers to amyloid fibrils, is fundamentally linked with Alzheimer's disease (AD). However, it is clear that not all forms of Aβ are equally harmful and that linking a specific aggregate to toxicity also depends on the assays and model systems used (Haass et al., J Biol. Chem 269:17741-17748, 1994; Borchelt et al., Neuron 17:1005-1013, 1996). Though a central postulate of the amyloid cascade hypothesis, there remain many gaps in our understanding regarding the links between Aβ deposition and neurodegeneration.

METHODS

In this study, we examined familial mutations of Aβ that increase aggregation and oligomerization, E22G and ΔE22, and induce cerebral amyloid angiopathy, E22Q and D23N. We also investigated synthetic mutations that stabilize dimerization, S26C, and a phospho-mimetic, S8E, and non-phospho-mimetic, S8A. To that end, we utilized BRI2-Aβ fusion technology and rAAV2/1-based somatic brain transgenesis in mice to selectively express individual mutant Aβ species in vivo. In parallel, we generated PhiC31-based transgenic Drosophila melanogaster expressing wild-type (WT) and Aβ40 and Aβ42 mutants, fused to the Argos signal peptide to assess the extent of Aβ42-induced toxicity as well as to interrogate the combined effect of different Aβ40 and Aβ42 species.

RESULTS

When expressed in the mouse brain for 6 months, Aβ42 E22G, Aβ42 E22Q/D23N, and Aβ42WT formed amyloid aggregates consisting of some diffuse material as well as cored plaques, whereas other mutants formed predominantly diffuse amyloid deposits. Moreover, while Aβ40WT showed no distinctive phenotype, Aβ40 E22G and E22Q/D23N formed unique aggregates that accumulated in mouse brains. This is the first evidence that mutant Aβ40 overexpression leads to deposition under certain conditions. Interestingly, we found that mutant Aβ42 E22G, E22Q, and S26C, but not Aβ40, were toxic to the eye of Drosophila. In contrast, flies expressing a copy of Aβ40 (WT or mutants), in addition to Aβ42WT, showed improved phenotypes, suggesting possible protective qualities for Aβ40.

CONCLUSIONS

These studies suggest that while some Aβ40 mutants form unique amyloid aggregates in mouse brains, they do not exacerbate Aβ42 toxicity in Drosophila, which highlights the significance of using different systems for a better understanding of AD pathogenicity and more accurate screening for new potential therapies.

Behavioral and neural network abnormalities in human APP transgenic mice resemble those of App knock-in mice and are modulated by familial Alzheimer's disease mutations but not by inhibition of BACE1

BACKGROUND

Alzheimer's disease (AD) is the most frequent and costly neurodegenerative disorder. Although diverse lines of evidence suggest that the amyloid precursor protein (APP) is involved in its causation, the precise mechanisms remain unknown and no treatments are available to prevent or halt the disease. A favorite hypothesis has been that APP contributes to AD pathogenesis through the cerebral accumulation of the amyloid-β peptide (Aβ), which is derived from APP through sequential proteolytic cleavage by BACE1 and γ-secretase. However, inhibitors of these enzymes have failed in clinical trials despite clear evidence for target engagement.

METHODS

To further elucidate the roles of APP and its metabolites in AD pathogenesis, we analyzed transgenic mice overexpressing wildtype human APP (hAPP) or hAPP carrying mutations that cause autosomal dominant familial AD (FAD), as well as App knock-in mice that do not overexpress hAPP but have two mouse App alleles with FAD mutations and a humanized Aβ sequence.

RESULTS

Although these lines of mice had marked differences in cortical and hippocampal levels of APP, APP C-terminal fragments, soluble Aβ, Aβ oligomers and age-dependent amyloid deposition, they all developed cognitive deficits as well as non-convulsive epileptiform activity, a type of network dysfunction that also occurs in a substantive proportion of humans with AD. Pharmacological inhibition of BACE1 effectively reduced levels of amyloidogenic APP C-terminal fragments (C99), soluble Aβ, Aβ oligomers, and amyloid deposits in transgenic mice expressing FAD-mutant hAPP, but did not improve their network dysfunction and behavioral abnormalities, even when initiated at early stages before amyloid deposits were detectable.

CONCLUSIONS

hAPP transgenic and App knock-in mice develop similar pathophysiological alterations. APP and its metabolites contribute to AD-related functional alterations through complex combinatorial mechanisms that may be difficult to block with BACE inhibitors and, possibly, also with other anti-Aβ treatments.

Impact of late-onset Alzheimer's genetic risk factors on beta-amyloid endocytic production

The increased production of the 42 aminoacids long beta-amyloid (Aβ42) peptide has been established as a causal mechanism of the familial early onset Alzheimer's disease (AD). In contrast, the causal mechanisms of the late-onset AD (LOAD), that affects most AD patients, remain to be established. Indeed, Aβ42 accumulation has been detected more than 30 years before diagnosis. Thus, the mechanisms that control Aβ accumulation in LOAD likely go awry long before pathogenesis becomes detectable. Early on, APOE4 was identified as the biggest genetic risk factor for LOAD. However, since APOE4 is not present in all LOAD patients, genome-wide association studies of thousands of LOAD patients were undertaken to identify other genetic variants that could explain the development of LOAD. PICALM, BIN1, CD2AP, SORL1, and PLD3 are now with APOE4 among the identified genes at highest risk in LOAD that have been implicated in Aβ42 production. Recent evidence indicates that the regulation of the endocytic trafficking of the amyloid precursor protein (APP) and/or its secretases to and from sorting endosomes is determinant for Aβ42 production. Thus, here, we will review the described mechanisms, whereby these genetic risk factors can contribute to the enhanced endocytic production of Aβ42. Dissecting causal LOAD mechanisms of Aβ42 accumulation, underlying the contribution of each genetic risk factor, will be required to identify therapeutic targets for novel personalized preventive strategies.

Istradefylline reduces memory deficits in aging mice with amyloid pathology

Adenosine A2A receptors are putative therapeutic targets for neurological disorders. The adenosine A2A receptor antagonist istradefylline is approved in Japan for Parkinson's disease and is being tested in clinical trials for this condition elsewhere. A2A receptors on neurons and astrocytes may contribute to Alzheimer's disease (AD) by impairing memory. However, it is not known whether istradefylline enhances cognitive function in aging animals with AD-like amyloid plaque pathology. Here, we show that elevated levels of Aβ, C-terminal fragments of the amyloid precursor protein (APP), or amyloid plaques, but not overexpression of APP per se, increase astrocytic A2A receptor levels in the hippocampus and neocortex of aging mice. Moreover, in amyloid plaque-bearing mice, low-dose istradefylline treatment enhanced spatial memory and habituation, supporting the conclusion that, within a well-defined dose range, A2A receptor blockers might help counteract memory problems in patients with Alzheimer's disease.

Genetics of Alzheimer's disease: From pathogenesis to clinical usage

Alzheimer's disease (AD) is the most common type of dementia and has caused a major global health concern. Understanding the etiology of AD can be beneficial for the diagnosis and intervention of this disease. Genetics plays a vital role in the pathogenesis of AD. Research methods in genetics such as the linkage analysis, study of candidate genes, genome-wide association study (GWAS), and next-generation sequencing (NGS) technology help us map the genetic information in AD, which can not only provide a new insight into the pathogenesis of AD but also be beneficial for early targeted intervention of AD. This review summarizes the pathogenesis as well as the diagnostic and therapeutic value of genetics in AD.

Alzheimer's disease as oligomeropathy

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder and is characterized by pathological aggregates of amyloid β-protein (Aβ) and tau protein. On the basis of genetic evidence, biochemical data, and animal models, Aβ has been suggested to be responsible for the pathogenesis of AD (the amyloid hypothesis). Aβ molecules tend to aggregate to form oligomers, protofibrils, and mature fibrils. Although mature fibrils in the final stage have been thought to be the cause of AD pathogenesis, recent studies using synthetic Aβ peptides, a cell culture model, Aβ precursor protein transgenic mice models, and human samples, such as cerebrospinal fluids and postmortem brains of AD patients, suggest that pre-fibrillar forms (oligomers of Aβ) are more deleterious than are extracellular fibril forms. Based on this recent evidence showing that oligomers have a central role in the pathogenesis of AD, the term "oligomeropathy" could be used to define AD and other protein-misfolding diseases. In this review, I discuss recent developments in the "oligomer hypothesis" including our research findings regarding the pathogenesis of AD.

Rapid amyloid-β oligomer and protofibril accumulation in traumatic brain injury

Deposition of amyloid-β (Aβ) is central to Alzheimer's disease (AD) pathogenesis and associated with progressive neurodegeneration in traumatic brain injury (TBI). We analyzed predisposing factors for Aβ deposition including monomeric Aβ40, Aβ42 and Aβ oligomers/protofibrils, Aβ species with pronounced neurotoxic properties, following human TBI. Highly selective ELISAs were used to analyze N-terminally intact and truncated Aβ40 and Aβ42, as well as Aβ oligomers/protofibrils, in human brain tissue, surgically resected from severe TBI patients (n = 12; mean age 49.5 ± 19 years) due to life-threatening brain swelling/hemorrhage within one week post-injury. The TBI tissues were compared to post-mortem AD brains (n = 5), to post-mortem tissue of neurologically intact (NI) subjects (n = 4) and to cortical biopsies obtained at surgery for idiopathic normal pressure hydrocephalus patients (iNPH; n = 4). The levels of Aβ40 and Aβ42 were not elevated by TBI. The levels of Aβ oligomers/protofibrils in TBI were similar to those in the significantly older AD patients and increased compared to NI and iNPH controls (P < 0.05). Moreover, TBI patients carrying the AD risk genotype Apolipoprotein E epsilon3/4 (APOE ε3/4; n = 4) had increased levels of Aβ oligomers/protofibrils (P < 0.05) and of both N-terminally intact and truncated Aβ42 (P < 0.05) compared to APOE ε3/4-negative TBI patients (n = 8). Neuropathological analysis showed insoluble Aβ aggregates (commonly referred to as Aβ plaques) in three TBI patients, all of whom were APOE ε3/4 carriers. We conclude that soluble intermediary Aβ aggregates form rapidly after TBI, especially among APOE ε3/4 carriers. Further research is needed to determine whether these aggregates aggravate the clinical short- and long-term outcome in TBI.

BACE1 Cleavage Site Selection Critical for Amyloidogenesis and Alzheimer's Pathogenesis

Mutations in amyloid β precursor protein (APP) gene alter APP processing, either causing familial Alzheimer's disease (AD) or protecting against dementia. Under normal conditions, β-site APP cleaving enzyme 1 (BACE1) cleaves APP at minor Asp¹ site to generate C99 for amyloid β protein (Aβ) production, and predominantly at major Glu¹¹ site to generate C89, resulting in truncated Aβ production. We discovered that A673V mutation, the only recessive AD-associated APP mutation, shifted the preferential β-cleavage site of BACE1 in APP from the Glu¹¹ site to the Asp¹ site both in male and female transgenic mice in vivo and in cell lines and primary neuronal culture derived from timed pregnant rats in vitro, resulting in a much higher C99 level and C99/C89 ratio. All other mutations at this site, including the protective Icelandic A673T mutation, reduced C99 generation, and decreased the C99/C89 ratio. Furthermore, A673V mutation caused stronger dimerization between mutant and wild-type APP, enhanced the lysosomal degradation of the mutant APP, and inhibited γ-secretase cleavage of the mutant C99 to generate Aβ, leading to recessively inherited AD. The results demonstrate that APP673 regulates APP processing and the BACE1 cleavage site selection is critical for amyloidogenesis in AD pathogenesis, and implicate a pharmaceutical potential for targeting the APP673 site for AD drug development.SIGNIFICANCE STATEMENT β-site APP cleaving enzyme 1 (BACE1) is essential for amyloid β protein production. We discovered that A673V mutation shifted the BACE1 cleavage site from the Glu¹¹ to the Asp¹ site, resulting in much higher C99 level and C99/C89 ratio. All other mutations at this site of amyloid β precursor protein (APP) reduced C99 generation and decreased the C99/C89 ratio. Furthermore, A673V mutation resulted in stronger dimerization between mutant and wild-type APP, enhanced the lysosomal degradation of the mutant APP, and inhibited γ-secretase cleavage of the mutant C99 to generate amyloid β protein, leading to recessively inherited Alzheimer's disease (AD). The results demonstrate that APP673 regulates APP processing, and the BACE1 cleavage site selection is critical for amyloidogenesis in AD pathogenesis, and implicate a pharmaceutical potential for targeting the APP673 site for AD drug development.

Comparison of Efficacy of Preventive and Therapeutic Vaccines Targeting the N Terminus of β-Amyloid in an Animal Model of Alzheimer's Disease

Previously, we reported that Alzheimer's disease (AD) epitope vaccines (EVs) composed of N-terminal β-amyloid (Aβ42) B cell epitope fused with universal foreign T helper (Th) epitope(s) were immunogenic, potent, and safe in different amyloid precursor protein (APP) transgenic mice with early AD-like pathology. However, developing an effective therapeutic vaccine is much more challenging, especially when a self-antigen such as Aβ42 is a target. Here, we directly compare the efficacy of anti-Aβ42 antibodies in Tg2576 mice with low or high levels of AD-like pathology at the start of immunizations: 6-6.5 months for preventive vaccinations and 16-19 months for therapeutic vaccinations. EV in a preventive setting induced high levels of anti-Aβ antibodies, significantly reducing pathologic forms of Aβ in the brains of Tg2576 mice. When used therapeutically for immunesenescent Tg2576 mice, EV induced low levels of antibodies not sufficient for clearing of AD-like pathology. Separately, we demonstrated that EV was also not effective in 11-11.5-month-old Tg2576 mice with moderate AD-like pathology. However, we augmented the titers of anti-Aβ antibodies in transgenic (Tg) mice of the same age possessing the pre-existing memory Th cells and detected a significant decrease in diffuse and core plaques in cortical regions compared to control animals along with improved novel object recognition performance.

Visualizing APP and BACE-1 approximation in neurons yields insight into the amyloidogenic pathway

Cleavage of APP (amyloid precursor protein) by BACE-1 (β-site APP cleaving enzyme-1) is the rate-limiting step in amyloid-beta (Aβ) production and a neuropathologic hallmark of Alzheimer's disease (AD); thus physical approximation of this substrate-enzyme pair is a critical event with broad biological and therapeutic implications. Despite much research, neuronal locales of APP/BACE-1 convergence and APP-cleavage remain unclear. Here we report an optical assay – based on fluorescence complementation – to visualize in-cellulo APP/BACE-1 interactions as a simple on/off signal. Combined with other assays tracking the fate of internalized APP in hippocampal neurons, we found that APP/BACE-1 interact in both biosynthetic and endocytic compartments; particularly along recycling-microdomains such as dendritic spines and presynaptic boutons. In axons, APP and BACE-1 are co-transported, and also interact during transit. Finally, our assay reveals that the AD-protective “Icelandic” mutation greatly attenuates APP/BACE-1 interactions, suggesting a mechanistic basis for protection. Collectively, the data challenge canonical models and provide concrete insights into long-standing controversies in the field.

Targeting protein aggregation for the treatment of degenerative diseases

The aggregation of specific proteins is hypothesized to underlie several degenerative diseases, which are collectively known as amyloid disorders. However, the mechanistic connection between the process of protein aggregation and tissue degeneration is not yet fully understood. Here, we review current and emerging strategies to ameliorate aggregation-associated degenerative disorders, with a focus on disease-modifying strategies that prevent the formation of and/or eliminate protein aggregates. Persuasive pharmacological and genetic evidence now supports protein aggregation as the cause of postmitotic tissue dysfunction or loss. However, a more detailed understanding of the factors that trigger and sustain aggregate formation and of the structure-activity relationships underlying proteotoxicity is needed to develop future disease-modifying therapies.

Early etiology of Alzheimer's disease: tipping the balance toward autophagy or endosomal dysfunction?

Alzheimer’s disease (AD) is the most common form of dementia in the elderly. This brain neuropathology is characterized by a progressive synaptic dysfunction and neuronal loss, which lead to decline in memory and other cognitive functions. Histopathologically, AD manifests via synaptic abnormalities, neuronal degeneration as well as the deposition of extracellular amyloid plaques and intraneuronal neurofibrillary tangles. While the exact pathogenic contribution of these two AD hallmarks and their abundant constituents [aggregation-prone amyloid β (Aβ) peptide species and hyperphosphorylated tau protein, respectively] remain debated, a growing body of evidence suggests that their development may be paralleled or even preceded by the alterations/dysfunctions in the endolysosomal and the autophagic system. In AD-affected neurons, abnormalities in these cellular pathways are readily observed already at early stages of disease development, and even though many studies agree that defective lysosomal degradation may relate to or even underlie some of these deficits, specific upstream molecular defects are still deliberated. In this review we summarize various pathogenic events that may lead to these cellular abnormalities, in light of our current understanding of molecular mechanisms that govern AD progression. In addition, we also highlight the increasing evidence supporting mutual functional dependence of the endolysosomal trafficking and autophagy, in particular focusing on those molecules and processes which may be of significance to AD.

Lack of evidence of the interaction of the Aβ peptide with the Wnt signaling cascade in Drosophila models of Alzheimer's disease

Background

Alzheimer’s disease (AD) is the leading form of dementia worldwide. The Aβ-peptide is believed to be the major pathogenic compound of the disease. Since several years it is hypothesized that Aβ impacts the Wnt signaling cascade and therefore activation of this signaling pathway is proposed to rescue the neurotoxic effect of Aβ.

Findings

Expression of the human Aβ42 in the Drosophila nervous system leads to a drastically shortened life span. We found that the action of Aβ42 specifically in the glutamatergic motoneurons is responsible for the reduced survival. However, we find that the morphology of the glutamatergic larval neuromuscular junctions, which are widely used as the model for mammalian central nervous system synapses, is not affected by Aβ42 expression. We furthermore demonstrate that genetic activation of the Wnt signal transduction pathway in the nervous system is not able to rescue the shortened life span or a rough eye phenotype in Drosophila.

Conclusions

Our data confirm that the life span is a useful readout of Aβ42 induced neurotoxicity in Drosophila; the neuromuscular junction seems however not to be an appropriate model to study AD in flies. Additionally, our results challenge the hypothesis that Wnt signaling might be implicated in Aβ42 toxicity and might serve as a drug target against AD.

Perspectives on future Alzheimer therapies: amyloid-β protofibrils - a new target for immunotherapy with BAN2401 in Alzheimer's disease

The symptomatic drugs currently on the market for Alzheimer's disease (AD) have no effect on disease progression, and this creates a large unmet medical need. The type of drug that has developed most rapidly in the last decade is immunotherapy: vaccines and, especially, passive vaccination with monoclonal antibodies. Antibodies are attractive drugs as they can be made highly specific for their target and often with few side effects. Data from recent clinical AD trials indicate that a treatment effect by immunotherapy is possible, providing hope for a new generation of drugs. The first anti-amyloid-beta (anti-Aβ) vaccine developed by Elan, AN1792, was halted in phase 2 because of aseptic meningoencephalitis. However, in a follow-up study, patients with antibody response to the vaccine demonstrated reduced cognitive decline, supporting the hypothesis that Aβ immunotherapy may have clinically relevant effects. Bapineuzumab (Elan/Pfizer Inc./Johnson & Johnson), a monoclonal antibody targeting fibrillar Aβ, was stopped because the desired clinical effect was not seen. Solanezumab (Eli Lilly and Company) was developed to target soluble, monomeric Aβ. In two phase 3 studies, Solanezumab did not meet primary endpoints. When data from the two studies were pooled, a positive pattern emerged, revealing a significant slowing of cognitive decline in the subgroup of mild AD. The Arctic mutation has been shown to specifically increase the formation of soluble Aβ protofibrils, an Aβ species shown to be toxic to neurons and likely to be present in all cases of AD. A monoclonal antibody, mAb158, was developed to target Aβ protofibrils with high selectivity. It has at least a 1,000-fold higher selectivity for protofibrils as compared with monomers of Aβ, thus targeting the toxic species of the peptide. A humanized version of mAb158, BAN2401, has now entered a clinical phase 2b trial in a collaboration between BioArctic Neuroscience and Eisai without the safety concerns seen in previous phase 1 and 2a trials. Experiences from the field indicate the importance of initiating treatment early in the course of the disease and of enriching the trial population by improving the diagnostic accuracy. BAN2401 is a promising candidate for Aβ immunotherapy in early AD. Other encouraging efforts in immunotherapy as well as in the small-molecule field offer hope for new innovative therapies for AD in the future.

Amyloid-ß-directed immunotherapy for Alzheimer's disease

Current treatment options for Alzheimer's disease (AD) are limited to medications that reduce dementia symptoms. Given the rapidly ageing populations in most areas of the world, new therapeutic interventions for AD are urgently needed. In recent years, a number of drug candidates targeting the amyloid-ß (Aß) peptide have advanced into clinical trials; however, most have failed because of safety issues or lack of efficacy. The Aß peptide is central to the pathogenesis, and immunotherapy against Aß has attracted considerable interest. It offers the possibility to reach the target with highly specific drugs. Active immunization and passive immunization have been the most widely studied approaches to immunotherapy of AD. A favourable aspect of active immunization is the capacity for a small number of vaccinations to generate a prolonged antibody response. A potential disadvantage is the variability in the antibody response across patients. The potential advantages of passive immunotherapy include the reproducible delivery of a known amount of therapeutic antibodies to the patient and rapid clearance of those antibodies if side effects develop. A disadvantage is the requirement for repeated infusions of antibodies over time. After more than a decade of research, anti-amyloid immunotherapy remains one of the most promising emerging strategies for developing disease-modifying treatments for AD. In this review, we examine the presently ongoing Aß-directed immunotherapies that have passed clinical development Phase IIa.

Stability of transmembrane amyloid β-peptide and membrane integrity tested by molecular modeling of site-specific Aβ42 mutations

Interactions of the amyloid β-protein (Aβ) with neuronal cell membranes, leading to the disruption of membrane integrity, are considered to play a key role in the development of Alzheimer’s disease. Natural mutations in Aβ₄₂, such as the Arctic mutation (E22G) have been shown to increase Aβ₄₂ aggregation and neurotoxicity, leading to the early-onset of Alzheimer’s disease. A correlation between the propensity of Aβ₄₂ to form protofibrils and its effect on neuronal dysfunction and degeneration has been established. Using rational mutagenesis of the Aβ₄₂ peptide it was further revealed that the aggregation of different Aβ₄₂ mutants in lipid membranes results in a variety of polymorphic aggregates in a mutation dependent manner. The mutant peptides also have a variable ability to disrupt bilayer integrity. To further test the connection between Aβ₄₂ mutation and peptide–membrane interactions, we perform molecular dynamics simulations of membrane-inserted Aβ₄₂ variants (wild-type and E22G, D23G, E22G/D23G, K16M/K28M and K16M/E22G/D23G/K28M mutants) as β-sheet monomers and tetramers. The effects of charged residues on transmembrane Aβ₄₂ stability and membrane integrity are analyzed at atomistic level. We observe an increased stability for the E22G Aβ₄₂ peptide and a decreased stability for D23G compared to wild-type Aβ₄₂, while D23G has the largest membrane-disruptive effect. These results support the experimental observation that the altered toxicity arising from mutations in Aβ is not only a result of the altered aggregation propensity, but also originates from modified Aβ interactions with neuronal membranes.

The Arctic AβPP mutation leads to Alzheimer's disease pathology with highly variable topographic deposition of differentially truncated Aβ

BACKGROUND

The Arctic mutation (p.E693G/p.E22G)fs within the β-amyloid (Aβ) region of the β-amyloid precursor protein gene causes an autosomal dominant disease with clinical picture of typical Alzheimer's disease. Here we report the special character of Arctic AD neuropathology in four deceased patients.

RESULTS

Aβ deposition in the brains was wide-spread (Thal phase 5) and profuse. Virtually all parenchymal deposits were composed of non-fibrillar, Congo red negative Aβ aggregates. Congo red only stained angiopathic vessels. Mass spectrometric analyses showed that Aβ deposits contained variably truncated and modified wild type and mutated Aβ species. In three of four Arctic AD brains, most cerebral cortical plaques appeared targetoid with centres containing C-terminally (beyond aa 40) and variably N-terminally truncated Aβ surrounded by coronas immunopositive for Aβx-42. In the fourth patient plaque centres contained almost no Aβ making the plaques ring-shaped. The architectural pattern of plaques also varied between different anatomic regions. Tau pathology corresponded to Braak stage VI, and appeared mainly as delicate neuropil threads (NT) enriched within Aβ plaques. Dystrophic neurites were scarce, while neurofibrillary tangles were relatively common. Neuronal perikarya within the Aβ plaques appeared relatively intact.

CONCLUSIONS

In Arctic AD brain differentially truncated abundant Aβ is deposited in plaques of variable numbers and shapes in different regions of the brain (including exceptional targetoid plaques in neocortex). The extracellular non-fibrillar Aβ does not seem to cause overt damage to adjacent neurons or to induce formation of neurofibrillary tangles, supporting the view that intracellular Aβ oligomers are more neurotoxic than extracellular Aβ deposits. However, the enrichment of NTs within plaques suggests some degree of intra-plaque axonal damage including accumulation of hp-tau, which may impair axoplasmic transport, and thereby contribute to synaptic loss. Finally, similarly as the cotton wool plaques in AD resulting from exon 9 deletion in the presenilin-1 gene, the Arctic plaques induced only modest glial and inflammatory tissue reaction.

Genetics of Alzheimer's disease

Alzheimer's disease is the most common form of dementia and is the only top 10 cause of death in the United States that lacks disease-altering treatments. It is a complex disorder with environmental and genetic components. There are two major types of Alzheimer's disease, early onset and the more common late onset. The genetics of early-onset Alzheimer's disease are largely understood with variants in three different genes leading to disease. In contrast, while several common alleles associated with late-onset Alzheimer's disease, including APOE, have been identified using association studies, the genetics of late-onset Alzheimer's disease are not fully understood. Here we review the known genetics of early- and late-onset Alzheimer's disease.

Early accumulation of intracellular fibrillar oligomers and late congophilic amyloid angiopathy in mice expressing the Osaka intra-Aβ APP mutation

Pathogenic amyloid-β peptide precursor (APP) mutations clustered around position 693 of APP-position 22 of the Aβ sequence--are commonly associated with congophilic amyloid angiopathy (CAA) and intracerebral hemorrhages. In contrast, the Osaka (E693Δ) intra-Aβ APP mutation shows a recessive pattern of inheritance that leads to AD-like dementia despite low brain amyloid on in vivo positron emission tomography imaging. Here, we investigated the effects of the Osaka APP mutation on Aβ accumulation and deposition in vivo using a newly generated APP transgenic mouse model (E22ΔAβ) expressing the Osaka mutation together with the Swedish (K670N/M671L) double mutation. E22ΔAβ mice exhibited reduced α-processing of APP and early accumulation of intraneuronal fibrillar Aβ oligomers associated with cognitive deficits. In line with our in vitro findings that recombinant E22Δ-mutated Aβ peptides form amyloid fibrils, aged E22ΔAβ mice showed extracellular CAA deposits in leptomeningeal cerebellar and cortical vessels. In vitro results from thioflavin T aggregation assays with recombinant Aβ peptides revealed a yet unknown antiamyloidogenic property of the E693Δ mutation in the heterozygous state and an inhibitory effect of E22Δ Aβ42 on E22Δ Aβ40 fibrillogenesis. Moreover, E22Δ Aβ42 showed a unique aggregation kinetics lacking exponential fibril growth and poor seeding effects on wild-type Aβ aggregation. These results provide a possible explanation for the recessive trait of inheritance of the Osaka APP mutation and the apparent lack of amyloid deposition in E693Δ mutation carriers.

Novel APP/Aβ mutation K16N produces highly toxic heteromeric Aβ oligomers

Here, we describe a novel missense mutation in the amyloid precursor protein (APP) causing a lysine-to-asparagine substitution at position 687 (APP770; herein, referred to as K16N according to amyloid-β (Aβ) numbering) resulting in an early onset dementia with an autosomal dominant inheritance pattern. The K16N mutation is located exactly at the α-secretase cleavage site and influences both APP and Aβ. First, due to the K16N mutation APP secretion is affected and a higher amount of Aβ peptides is being produced. Second, Aβ peptides carrying the K16N mutation are unique in that the peptide itself is not harmful to neuronal cells. Severe toxicity, however, is evident upon equimolar mixture of wt and mutant peptides, mimicking the heterozygous state of the subject. Furthermore, Aβ42 K16N inhibits fibril formation of Aβ42 wild-type. Even more, Aβ42 K16N peptides are protected against clearance activity by the major Aβ-degrading enzyme neprilysin. Thus the mutation characterized here harbours a combination of risk factors that synergistically may contribute to the development of early onset Alzheimer disease.

Aβ toxicity in Alzheimer's disease

Alzheimer's Disease (AD), the most common age-related neurodegenerative disorder, is characterized by progressive cognitive decline, synaptic loss, the formation of extracellular β-amyloid plaques and intracellular neurofibrillary tangles, and neuronal cell death. Despite the massive neuronal loss in the 'late stage' of disease, dendritic spine loss represents the best pathological correlate to the cognitive impairment in AD patients. The 'amyloid hypothesis' of AD recognizes the Aβ peptide as the principal player in the pathological process. Many lines of evidence point out to the neurotoxicity of Aβ, highlighting the correlation between soluble Aβ oligomer accumulation, rather than insoluble Aβ fibrils and disease progression. Pathological increase of Aβ in AD brains, resulting from an imbalance between its production, aggregation and clearance, might target mitochondrial function promoting a progressive synaptic impairment. The knowledge of the exact mechanisms by which Aβ peptide impairs neuronal function will help us to design new pharmacological tools for preventing AD neurodegeneration.

Aβ peptide fibrillar architectures controlled by conformational constraints of the monomer

Anomalous self-assembly of the Aβ peptide into fibrillar amyloid deposits is strongly correlated with the development of Alzheimer's disease. Aβ fibril extension follows a template guided “dock and lock” mechanism where polymerisation is catalysed by the fibrillar ends. Using surface plasmon resonance (SPR) and quenched hydrogen-deuterium exchange NMR (H/D-exchange NMR), we have analysed the fibrillar structure and polymerisation properties of both the highly aggregation prone Aβ1–40 Glu22Gly (Aβ^40Arc) and wild type Aβ1–40 (Aβ^40WT). The solvent protection patterns from H/D exchange experiments suggest very similar structures of the fibrillar forms. However, through cross-seeding experiments monitored by SPR, we found that the monomeric form of Aβ^40WT is significantly impaired to acquire the fibrillar architecture of Aβ^40Arc. A detailed characterisation demonstrated that Aβ^40WT has a restricted ability to dock and isomerise with high binding affinity onto Aβ^40Arc fibrils. These results have general implications for the process of fibril assembly, where the rate of polymerisation, and consequently the architecture of the formed fibrils, is restricted by conformational constraints of the monomers. Interestingly, we also found that the kinetic rate of fibril formation rather than the thermodynamically lowest energy state determines the overall fibrillar structure.

Observations in APP bitransgenic mice suggest that diffuse and compact plaques form via independent processes in Alzheimer's disease

Studies of familial Alzheimer's disease suggest that misfolding and aggregation of amyloid-β (Aβ) peptides initiate the pathogenesis. The Arctic mutation of Aβ precursor protein (APP) results in AD, and Arctic Aβ is more prone to form Aβ protofibrils and extracellular deposits. Herein is demonstrated that the burden of diffuse Aβ deposits but not compact plaques is increased when tg-Swe mice are crossed with tg-ArcSwe mice synthesizing low levels of Arctic Aβ. The diffuse deposits in bitransgenic mice, which contain primarily wild-type Aβ42, accumulate in regions both with and without transgene expression. However, APP processing, when compared with tg-Swe, remains unchanged in young bitransgenic mice, whereas wild-type Aβ42 aggregation is accelerated and fibril architecture is altered in vitro and in vivo when a low level of Arctic Aβ42 is introduced. Thus, the increased number of diffuse deposits is likely due to physical interactions between Arctic Aβ and wild-type Aβ42. The selective increase of a single type of parenchymal Aβ deposit suggests that different pathways lead to formation of diffuse and compact plaques. These findings could have general implications for Alzheimer's disease pathogenesis and particular relevance to patients heterozygous for the Arctic APP mutation. Moreover, it further illustrates how Aβ neuropathologic features can be manipulated in vivo by mechanisms similar to those originally conceptualized in prion research.

Mutant A53T alpha-synuclein induces neuronal death by increasing mitochondrial autophagy

Parkinson disease is characterized by the accumulation of aggregated α-synuclein as the major component of the Lewy bodies. α-Synuclein accumulation in turn leads to compensatory effects that may include the up-regulation of autophagy. Another common feature of Parkinson disease (PD) is mitochondrial dysfunction. Here, we provide evidence that the overactivation of autophagy may be a link that connects the intracellular accumulation of α-synuclein with mitochondrial dysfunction. We found that the activation of macroautophagy in primary cortical neurons that overexpress mutant A53T α-synuclein leads to massive mitochondrial destruction and loss, which is associated with a bioenergetic deficit and neuronal degeneration. No mitochondrial removal or net loss was observed when we suppressed the targeting of mitochondria to autophagosomes by silencing Parkin, overexpressing wild-type Mitofusin 2 and dominant negative Dynamin-related protein 1 or blocking autophagy by silencing autophagy-related genes. The inhibition of targeting mitochondria to autophagosomes or autophagy was also partially protective against mutant A53T α-synuclein-induced neuronal cell death. These data suggest that overactivated mitochondrial removal could be one of the contributing factors that leads to the mitochondrial loss observed in PD models.

Insights into Alzheimer disease pathogenesis from studies in transgenic animal models

Alzheimer disease is the most common cause of dementia among the elderly, accounting for ~60-70% of all cases of dementia. The neuropathological hallmarks of Alzheimer disease are senile plaques (mainly containing p-amyloid peptide derived from amyloid precursor protein) and neurofibrillary tangles (containing hyperphosphorylated Tau protein), along with neuronal loss. At present there is no effective treatment for Alzheimer disease. Given the prevalence and poor prognosis of the disease, the development of animal models has been a research priority to understand pathogenic mechanisms and to test therapeutic strategies. Most cases of Alzheimer disease occur sporadically in people over 65 years old, and are not genetically inherited. Roughly 5% of patients with Alzheimer disease have familial Alzheimer disease--that is, related to a genetic predisposition, including mutations in the amyloid precursor protein, presenilin 1, and presenilin 2 genes. The discovery of genes for familial Alzheimer disease has allowed transgenic models to be generated through the overexpression of the amyloid precursor protein and/or presenilins harboring one or several mutations found in familial Alzheimer disease. Although none of these models fully replicates the human disease, they have provided valuable insights into disease mechanisms as well as opportunities to test therapeutic approaches. This review describes the main transgenic mouse models of Alzheimer disease which have been adopted in Alzheimer disease research, and discusses the insights into Alzheimer disease pathogenesis from studies in such models. In summary, the Alzheimer disease mouse models have been the key to understanding the roles of soluble b-amyloid oligomers in disease pathogenesis, as well as of the relationship between p-amyloid and Tau pathologies.

Translating research on brain aging into public health: a new type of immunotherapy for Alzheimer's disease

The identification of disease-causing mutations in Alzheimer's disease has contributed greatly to the understanding of the pathogenesis of this disease. The amyloid-β (Aβ) peptide has come into focus and is believed to be central to the pathogenesis of Alzheimer's disease. With only symptomatic treatment available, efforts to develop new therapeutics aimed at lowering the amount of Aβ peptides in the affected brain have intensified. In particular, immunotherapy against Aβ peptides has attracted considerable interest, as it offers the possibility to generate highly specific molecules targeting highly specific moieties. Due to intense research efforts and massive investments at universities and in the pharmaceutical industry, the outlook for patients and their relatives has never been brighter.

Genetic dissection of the amyloid precursor protein in developmental function and amyloid pathogenesis

Proteolytic processing of the amyloid precursor protein (APP) generates large soluble APP derivatives, β-amyloid (Aβ) peptides, and APP intracellular domain. Expression of the extracellular sequences of APP or its Caenorhabditis elegans counterpart has been shown to be sufficient in partially rescuing the CNS phenotypes of the APP-deficient mice and the lethality of the apl-1 null C. elegans, respectively, leaving open the question as what is the role of the highly conserved APP intracellular domain? To address this question, we created an APP knock-in allele in which the mouse Aβ sequence was replaced by the human Aβ. A frameshift mutation was introduced that replaced the last 39 residues of the APP sequence. We demonstrate that the C-terminal mutation does not overtly affect APP processing and amyloid pathology. In contrast, crossing the mutant allele with APP-like protein 2 (APLP2)-null mice results in similar neuromuscular synapse defects and early postnatal lethality as compared with mice doubly deficient in APP and APLP2, demonstrating an indispensable role of the APP C-terminal domain in these development activities. Our results establish an essential function of the conserved APP intracellular domain in developmental regulation, and this activity can be genetically uncoupled from APP processing and Aβ pathogenesis.

Animal models of amyloid-beta-related pathologies in Alzheimer's disease

In the early 1990s, breakthrough discoveries on the genetics of Alzheimer's disease led to the identification of missense mutations in the amyloid-beta precursor protein gene. Research findings quickly followed, giving insights into molecular pathogenesis and possibilities for the development of new types of animal models. The complete toolbox of transgenic techniques, including pronuclear oocyte injection and homologous recombination, has been applied in the Alzheimer's disease field, to produce overexpressors, knockouts, knockins and regulatable transgenics. Transgenic models have dramatically advanced our understanding of pathogenic mechanisms and allowed therapeutic approaches to be tested. Following a brief introduction to Alzheimer's disease, various nontransgenic and transgenic animal models are described in terms of their values and limitations with respect to pathogenic, therapeutic and functional understandings of the human disease.

An amyloid-beta protofibril-selective antibody prevents amyloid formation in a mouse model of Alzheimer's disease

Human genetics link Alzheimer's disease pathogenesis to excessive accumulation of amyloid-beta (Abeta) in brain, but the symptoms do not correlate with senile plaque burden. Since soluble Abeta aggregates can cause synaptic dysfunctions and memory deficits, these species could contribute to neuronal dysfunction and dementia. Here we explored selective targeting of large soluble aggregates, Abeta protofibrils, as a new immunotherapeutic strategy. The highly protofibril-selective monoclonal antibody mAb158 inhibited in vitro fibril formation and protected cells from Abeta protofibril-induced toxicity. When the mAb158 antibody was administered for 4 months to plaque-bearing transgenic mice with both the Arctic and Swedish mutations (tg-ArcSwe), Abeta protofibril levels were lowered while measures of insoluble Abeta were unaffected. In contrast, when treatment began before the appearance of senile plaques, amyloid deposition was prevented and Abeta protofibril levels diminished. Therapeutic intervention with mAb158 was however not proven functionally beneficial, since place learning depended neither on treatment nor transgenicity. Our findings suggest that Abeta protofibrils can be selectively cleared with immunotherapy in an animal model that display highly insoluble Abeta deposits, similar to those of Alzheimer's disease brain.

Progressive neuropathology and cognitive decline in a single Arctic APP transgenic mouse model

The Arctic APP mutation (E693G) leads to dementia with clinical features similar to Alzheimer disease (AD), but little is known about the pathogenic mechanism of this mutation. To address this question, we have generated a transgenic mouse model, TgAPParc, with neuron-specific expression of human APP with the Arctic mutation (hAPParc). Heterozygous mice from two separate founder lines with different levels of expression of hAPParc were analyzed with respect to brain morphology and behavior every 3 months until the age of 18 months. Standard histological stainings and immunohistochemistry using a panel of Aβ antibodies showed an age- and dose-dependant progression of amyloid deposition in the brain, starting in the subiculum and spreading to the thalamus. Cognitive behavioral testing revealed deficits in hippocampus-dependent spatial learning and memory in the Barnes maze test. This study demonstrates that the Arctic APP mutation is sufficient to cause amyloid deposition and cognitive dysfunction, and thus the TgAPParc mouse model provides a valuable tool to study the effect of the Arctic mutation in vivo without possible confounding effect of other APP mutations.

The E693Delta mutation in amyloid precursor protein increases intracellular accumulation of amyloid beta oligomers and causes endoplasmic reticulum stress-induced apoptosis in cultured cells

The E693Delta mutation within the amyloid precursor protein (APP) has been suggested to cause dementia via the enhanced formation of synaptotoxic amyloid beta (Abeta) oligomers. However, this mutation markedly decreases Abeta secretion, implying the existence of an additional mechanism of neuronal dysfunction that is independent of extracellular Abeta. We therefore examined the effects of this mutation on both APP processing to produce Abeta as well as subcellular localization and accumulation of Abeta in transfected HEK293 and COS-7 cells. Both beta- and gamma-cleavage of mutant APP increased, indicating a lack of inhibition in Abeta production. Instead, this mutation promoted Abeta accumulation within cells, including the endoplasmic reticulum (ER), Golgi apparatus, early and late endosomes, lysosomes, and autophagosomes, all of which have been proposed as intracellular sites of Abeta generation and/or degradation, suggesting impairment of APP/Abeta trafficking. Notably, the intracellular mutant Abeta was found to predominantly form oligomers. Concomitant with this accumulation, the ER stress markers Grp78 and phosphorylated eIF2alpha were both strongly induced. Furthermore, the activation of caspase-4 and -3 as well as DNA fragmentation were detected in these cells. These results suggest that mutant Abeta induces alteration of Abeta trafficking and subsequent ER stress-induced apoptosis via enhancement of its intracellular oligomerization. Our findings suggest that Abeta oligomers exhibit toxicity in the extracellular space and within the cells themselves.

The beta amyloid peptide can act as a modular aggregation domain

Although there is compelling evidence that the beta amyloid peptide (Abeta) can be centrally involved in Alzheimer's disease, the natural role (if any) of this peptide remains unclear. Here we use green fluorescent protein (GFP) fusions to demonstrate that the Abeta sequence, like prion domains, can act as a modular aggregation domain when terminally appended to a normally soluble protein. We find that a single amino acid substitution (Leu(17) to Pro) in the beta peptide sequence can abolish this cis capacity to induce aggregation. Introduction of this substitution into full-length APP (i.e., a Leu(613)Pro substitution in APP695) alters the processing of APP leading to the accumulation of the C99 C-terminal fragment (CTF). We suggest that in at least some aggregation disease-related proteins the presence of an aggregation domain is not "accidental", but reflects a selected role of these domains in modulating the trafficking or metabolism of the parental protein.

The Arctic mutation alters helix length and type in the 11-28 beta-amyloid peptide monomer-CD, NMR and MD studies in an SDS micelle

The beta-amyloid (Abeta) is the major peptide constituent of neuritic plaques in Alzheimer's disease, and its aggregation is believed to play a central role in the pathogenesis of the disease. Naturally occurring mutations resulting in changes in the Abeta sequence (pos. 21-23) are associated with familial Alzheimer's-like diseases with extensive cerebrovascular pathology. It has been demonstrated that such mutations alter the aggregation ability of Abeta and its neurotoxicity. Among the five mutations at positions 21-23 there is one with distinct clinical characteristics and a potentially distinct pathogenic mechanism-the Arctic (E22G) mutation. We have examined the structures of fragment 11-28 of the native peptide and its E22G variant. This fragment was chosen because it has been shown to be a good model for conformational and aggregation studies as it contains the hydrophobic core responsible for aggregation and the residues critical to alpha-secretase cleavage of APP. The detailed structure of the two peptides was determined using CD, 2D NMR and molecular dynamics techniques under water-SDS micelle conditions. Our studies indicated the existence of partially alpha- and 3(10)-helical conformations in the native and mutated peptide, respectively.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

Docosahexaenoic acid stimulates non-amyloidogenic APP processing resulting in reduced Abeta levels in cellular models of Alzheimer's disease

Epidemiological studies suggest that a high intake of polyunsaturated fatty acids, such as docosahexaenoic acid (DHA), is associated with a reduced risk of Alzheimer's disease. Here, we examined the effects of DHA on amyloid precursor protein (APP) processing in cellular models of Alzheimer's disease by analysing levels of different APP fragments, including amyloid-beta (Abeta). DHA administration stimulated non-amyloidogenic APP processing and reduced levels of Abeta, providing a mechanism for the reported beneficial effects of DHA in vivo. However, an increased level of APP intracellular domain was also observed, highlighting the need to increase our knowledge about the relevance of this fragment in Alzheimer's disease pathogenesis. In conclusion, our results suggest that the proposed protective role of DHA in Alzheimer's disease pathogenesis might be mediated by altered APP processing and Abeta production.

Imaging distinct conformational states of amyloid-beta fibrils in Alzheimer's disease using novel luminescent probes

Using luminescent conjugated polyelectrolyte probes (LCPs), we demonstrate the possibility to distinguish amyloid-beta 1-42 peptide (Abeta1-42) fibril conformations, by analyzing in vitro generated amyloid fibrils of Abeta1-42 formed under quiescent and agitated conditions. LCPs were then shown to resolve such conformational heterogeneity of amyloid deposits in vivo. A diversity of amyloid deposits depending upon morphology and anatomic location was illustrated with LCPs in frozen ex vivo brain sections from a transgenic mouse model (tg-APP swe) of Alzheimer's disease. Comparative LCP fluorescence showed that compact-core plaques of amyloid beta precursor protein transgenic mice were composed of rigid dense amyloid. A more abundant form of amyloid plaque displayed morphology of a compact center with a protruding diffuse exterior. Surprisingly, the compact center of these plaques showed disordered conformations of the fibrils, and the exterior was composed of rigid amyloid protruding from the disordered center. This type of plaque appears to grow from more loosely assembled regions toward solidified amyloid tentacles. This work demonstrates how application of LCPs can prove helpful to monitor aggregate structure of in vivo formed amyloid deposits such as architecture, maturity, and origin.