Charge alterations of E22 enhance the pathogenic properties of the amyloid beta-protein

Aβ40 Fibril Assembly on Human Cerebral Smooth Muscle Cells Impairs Cell Viability

Cerebral vascular deposition of the amyloid-β (Aβ) peptide, a condition known as cerebral amyloid angiopathy (CAA), is associated with intracerebral hemorrhaging and contributes to disease progression in Alzheimer's disease (AD) and vascular cognitive impairment and dementia (VCID). Familial mutations at positions 22 and 23 within the Aβ peptide lead to early onset and severe CAA pathology. Here, we evaluate the effects of fibrillar Aβ peptides on the viability of primary-cultured human cerebral smooth muscle (HCSM) cells, which are the major site of amyloid deposition in cerebral blood vessel walls. Comparisons are made of the familial E22Q (Dutch) mutant of Aβ40 with wild-type Aβ40 and Aβ42. In agreement with previous studies, we find that there is a significant reduction in cell viability when Aβ40-Dutch or Aβ42-WT peptides are added to HCSM cell cultures as monomeric Aβ, whereas Aβ40-WT is relatively nontoxic. The binding of Aβ fibrils derived from sporadic CAA or familial Dutch-type CAA brain tissue to the membrane surface of HCSM cells does not result in a significant loss of cell viability. In contrast, when Aβ40-WT monomers and sporadic CAA fibrils are coincubated in HCSM cell cultures, there is a significant reduction in HCSM cell viability that is accompanied by an increase in cell surface fibril formation. Lastly, intrathecal administration of Aβ40-Dutch fibrillar seeds promotes fibrillar amyloid accumulation in the smooth muscle of meningeal vessels in the rTg-D transgenic rat model of CAA. Together, the present findings suggest that fibrillar Aβ seeds propagate the expansion of new amyloid fibrils on cerebral vascular smooth muscle, leading to membrane disruption and cell death.

Cerebral Proteomic Changes in the rTg-D Rat Model of Cerebral Amyloid Angiopathy Type-2 With Cortical Microhemorrhages and Cognitive Impairments

Cerebral amyloid angiopathy (CAA) is a common disorder of the elderly, a prominent comorbidity of Alzheimer's disease, and causes vascular cognitive impairment and dementia. Previously, we generated a novel transgenic rat model (rTg-D) that produces human familial CAA Dutch E22Q mutant amyloid β-protein (Aβ) in brain and develops arteriolar CAA type-2. Here, we show that deposition of fibrillar Aβ promotes arteriolar smooth muscle cell loss and cerebral microhemorrhages that can be detected by magnetic resonance imaging and confirmed by histopathology. Aged rTg-D rats also present with cognitive deficits. Cerebral proteomic analyses revealed 241 proteins that were significantly elevated with an increase of >50% in rTg-D rats presenting with CAA compared to wild-type rats. Fewer proteins were significantly decreased in rTg-D rats. Of note, high temperature requirement peptidase A (HTRA1), a proteinase linked to transforming growth factor beta 1 (TGF-β1) signaling, was elevated and found to accumulate in cerebral vessels harboring amyloid deposits. Pathway analysis indicated elevation of the TGF-β1 pathway and increased TGF-β1 levels were detected in rTg-D rats. In conclusion, the present findings provide new molecular insights into the pathogenesis of CAA and suggest a role for interactions between HTRA1 and TGF-β1 in the disease process.

Alzheimer's Disease and Its Possible Evolutionary Origin: Hypothesis

The enormous, 2-3-million-year evolutionary expansion of hominin neocortices to the current enormity enabled humans to take over the planet. However, there appears to have been a glitch, and it occurred without a compensatory expansion of the entorhinal cortical (EC) gateway to the hippocampal memory-encoding system needed to manage the processing of the increasing volume of neocortical data converging on it. The resulting age-dependent connectopathic glitch was unnoticed by the early short-lived populations. It has now surfaced as Alzheimer's disease (AD) in today's long-lived populations. With advancing age, processing of the converging neocortical data by the neurons of the relatively small lateral entorhinal cortex (LEC) inflicts persistent strain and high energy costs on these cells. This may result in their hyper-release of harmless Aβ1-42 monomers into the interstitial fluid, where they seed the formation of toxic amyloid-β oligomers (AβOs) that initiate AD. At the core of connectopathic AD are the postsynaptic cellular prion protein (PrPC). Electrostatic binding of the negatively charged AβOs to the positively charged N-terminus of PrPC induces hyperphosphorylation of tau that destroys synapses. The spread of these accumulating AβOs from ground zero is supported by Aβ's own production mediated by target cells' Ca2+-sensing receptors (CaSRs). These data suggest that an early administration of a strongly positively charged, AβOs-interacting peptide or protein, plus an inhibitor of CaSR, might be an effective AD-arresting therapeutic combination.

Evaluation of Copper Chelation Therapy in a Transgenic Rat Model of Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA) is characterized by the accumulation of the amyloid β (Aβ) protein in blood vessels and leads to hemorrhages, strokes, and dementia in elderly individuals. Recent reports have shown elevated copper levels colocalized with vascular amyloid in human CAA and Alzheimer's disease patients, which have been suggested to contribute to cytotoxicity through the formation of reactive oxygen species. Here, we treated a transgenic rat model of CAA (rTg-DI) with the copper-specific chelator, tetrathiomolybdate (TTM), via intraperitoneal (IP) administration for 6 months to determine if it could lower copper content in vascular amyloid deposits and modify CAA pathology. Results showed that TTM treatment led to elevated Aβ load in the hippocampus of the rTg-DI rats and increased microbleeds in the wild type (WT) animals. X-ray fluorescence microscopy was performed to image the distribution of copper and revealed a surprising increase in copper colocalized with Aβ aggregates in TTM-treated rTg-DI rats. Unexpectedly, we also found an increase in the copper content in unaffected vessels of both rTg-DI and WT animals. These results show that IP administration of TTM was ineffective in removing copper from vascular Aβ aggregates in vivo and increased the development of disease pathology in CAA.

Amyloid beta accumulations and enhanced neuronal differentiation in cerebral organoids of Dutch-type cerebral amyloid angiopathy patients

Introduction

ADutch-type cerebral amyloid angiopathy (D-CAA) is a hereditary brain disorder caused by a point mutation in the amyloid precursor protein (APP) gene. The mutation is located within the amyloid beta (Aβ) domain of APP and leads to Aβ peptide accumulation in and around the cerebral vasculature. There lack of disease models to study the cellular and molecular pathological mechanisms of D-CAA together with the absence of a disease phenotype in vitro in overexpression cell models, as well as the limited availability of D-CAA animal models indicates the need for a D-CAA patient-derived model.

Methods

We generated cerebral organoids from four D-CAA patients and four controls, cultured them up to 110 days and performed immunofluorescent and targeted gene expression analyses at two time points (D52 and D110).

Results

D-CAA cerebral organoids exhibited Aβ accumulations, showed enhanced neuronal and astrocytic gene expression and TGFβ pathway de-regulation.

Conclusions

These results illustrate the potential of cerebral organoids as in vitro disease model of D-CAA that can be used to understand disease mechanisms of D-CAA and can serve as therapeutic intervention platform for various Aβ-related disorders.

Cross-seeding of WT amyloid-β with Arctic but not Italian familial mutants accelerates fibril formation in Alzheimer's disease

Alzheimer’s disease (AD) involves the neurotoxic self-assembly of a 40 and 42 residue peptide, Amyloid-β (Aβ). Inherited early-onset AD can be caused by single point mutations within the Aβ sequence, including Arctic (E22G) and Italian (E22K) familial mutants. These mutations are heterozygous, resulting in an equal proportion of the WT and mutant Aβ isoform expression. It is therefore important to understand how these mixtures of Aβ isoforms interact with each other and influence the kinetics and morphology of their assembly into oligomers and fibrils. Using small amounts of nucleating fibril seeds, here, we systematically monitored the kinetics of fibril formation, comparing self-seeding with cross-seeding behavior of a range of isoform mixtures of Aβ42 and Aβ40. We confirm that Aβ40(WT) does not readily cross-seed Aβ42(WT) fibril formation. In contrast, fibril formation of Aβ40(Arctic) is hugely accelerated by Aβ42(WT) fibrils, causing an eight-fold reduction in the lag-time to fibrillization. We propose that cross-seeding between the more abundant Aβ40(Arctic) and Aβ42(WT) may be important for driving early-onset AD and will propagate fibril morphology as indicated by fibril twist periodicity. This kinetic behavior is not emulated by the Italian mutant, where minimal cross-seeding is observed. In addition, we studied the cross-seeding behavior of a C-terminal-amidated Aβ42 analog to probe the coulombic charge interplay between Glu22/Asp23/Lys28 and the C-terminal carboxylate. Overall, these studies highlight the role of cross-seeding between WT and mutant Aβ40/42 isoforms, which can impact the rate and structure of fibril assembly.

Macrocyclic Peptides Derived from Familial Alzheimer's Disease Mutants Show Charge-Dependent Oligomeric Assembly and Toxicity

This work probes the role of charge in the oligomeric assembly, toxicity, and membrane destabilization of a series of peptides derived from Aβ and the E22Q and E22K familial mutants. In the mutant Aβ peptides, an acidic residue (E) is replaced with either a neutral or basic residue (Q or K), thus altering the net charge of the peptide. Acetylation at peripheral positions permits modulation of charge of the peptides and allows investigation of the role of charge in their oligomeric assembly, cytotoxicity, and membrane disruption. Peptides with the same net charge generally behave similarly even if the amino acid residue at position 22 differs. As the net charge of the peptide decreases, so does the extent of assembly, cytotoxicity, and membrane destabilization, which were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, lactate dehydrogenase (LDH)-release assays with SH-SY5Y cells, and dye leakage assays using liposomes. These findings suggest that the charge of the amino acid side chain, rather than its size or hydrophobicity, accounts for the differences in the oligomeric assembly and toxicity of the E22 familial mutants of Aβ.

Impact of Non-pharmacological Chronic Hypertension on a Transgenic Rat Model of Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA), a common comorbidity of Alzheimer’s disease (AD), is a cerebral small vessel disease (CSVD) characterized by deposition of fibrillar amyloid β (Aβ) in blood vessels of the brain and promotes neuroinflammation and vascular cognitive impairment and dementia (VCID). Hypertension, a prominent non-amyloidal CSVD, has been found to increase risk of dementia, but clinical data regarding its effects in CAA patients is controversial. To understand the effects of hypertension on CAA, we bred rTg-DI transgenic rats, a model of CAA, with spontaneously hypertensive, stroke prone (SHR-SP) rats producing bigenic rTg-DI/SHR-SP and non-transgenic SHR-SP littermates. At 7 months (M) of age, cohorts of both rTg-DI/SHR-SP and SHR-SP littermates exhibit elevated systolic blood pressures. However, transgene human amyloid β-protein (Aβ) precursor and Aβ peptide levels, as well as behavioral testing showed no changes between bigenic rTg-DI/SHR-SP and rTg-DI rats. Subsequent cohorts of rats were aged further to 10 M where bigenic rTg-DI/SHR-SP and SHR-SP littermates exhibit elevated systolic and diastolic blood pressures. Vascular amyloid load in hippocampus and thalamus was significantly decreased, whereas pial surface vessel amyloid increased, in bigenic rTg-DI/SHR-SP rats compared to rTg-DI rats suggesting a redistribution of vascular amyloid in bigenic animals. There was activation of both astrocytes and microglia in rTg-DI rats and bigenic rTg-DI/SHR-SP rats not observed in SHR-SP rats indicating that glial activation was likely in response to the presence of vascular amyloid. Thalamic microbleeds were present in both rTg-DI rats and bigenic rTg-DI/SHR-SP rats. Although the number of thalamic small vessel occlusions were not different between rTg-DI and bigenic rTg-DI/SHR-SP rats, a significant difference in occlusion size and distribution in the thalamus was found. Proteomic analysis of cortical tissue indicated that bigenic rTg-DI/SHR-SP rats largely adopt features of the rTg-DI rats with enhancement of certain changes. Our findings indicate that at 10 M of age non-pharmacological hypertension in rTg-DI rats causes a redistribution of vascular amyloid and significantly alters the size and distribution of thalamic occluded vessels. In addition, our findings indicate that bigenic rTg-DI/SHR-SP rats provide a non-pharmacological model to further study hypertension and CAA as co-morbidities for CSVD and VCID.

Amyloid beta acts synergistically as a pro-inflammatory cytokine

The amyloid beta (Aβ) peptide is believed to play a central role in Alzheimer's disease (AD), the most common age-related neurodegenerative disorder. However, the natural, evolutionarily selected functions of Aβ are incompletely understood. Here, we report that nanomolar concentrations of Aβ act synergistically with known cytokines to promote pro-inflammatory activation in primary human astrocytes (a cell type increasingly implicated in brain aging and AD). Using transcriptomics (RNA-seq), we show that Aβ can directly substitute for the complement component C1q in a cytokine cocktail previously shown to induce astrocyte immune activation. Furthermore, we show that astrocytes synergistically activated by Aβ have a transcriptional signature similar to neurotoxic "A1" astrocytes known to accumulate with age and in AD. Interestingly, we find that this biological action of Aβ at low concentrations is distinct from the transcriptome changes induced by the high/supraphysiological doses of Aβ often used in in vitro studies. Collectively, our results suggest an important, cytokine-like function for Aβ and a novel mechanism by which it may directly contribute to the neuroinflammation associated with brain aging and AD.

An evaluation of the self-assembly enhancing properties of cell-derived hexameric amyloid-β

A key hallmark of Alzheimer’s disease is the extracellular deposition of amyloid plaques composed primarily of the amyloidogenic amyloid-β (Aβ) peptide. The Aβ peptide is a product of sequential cleavage of the Amyloid Precursor Protein, the first step of which gives rise to a C-terminal Fragment (C99). Cleavage of C99 by γ-secretase activity releases Aβ of several lengths and the Aβ42 isoform in particular has been identified as being neurotoxic. The misfolding of Aβ leads to subsequent amyloid fibril formation by nucleated polymerisation. This requires an initial and critical nucleus for self-assembly. Here, we identify and characterise the composition and self-assembly properties of cell-derived hexameric Aβ42 and show its assembly enhancing properties which are dependent on the Aβ monomer availability. Identification of nucleating assemblies that contribute to self-assembly in this way may serve as therapeutic targets to prevent the formation of toxic oligomers.

Conjugated Linoleic Acid and Brain Metabolism: A Possible Anti-Neuroinflammatory Role Mediated by PPARα Activation

Fatty acids play a crucial role in the brain as specific receptor ligands and as precursors of bioactive metabolites. Conjugated linoleic acid (CLA), a group of positional and geometric isomers of linoleic acid (LA, 18:2 n-6) present in meat and dairy products of ruminants and synthesized endogenously in non-ruminants and humans, has been shown to possess different nutritional properties associated with health benefits. Its ability to bind to peroxisome proliferator-activated receptor (PPAR) α, a nuclear receptor key regulator of fatty acid metabolism and inflammatory responses, partly mediates these beneficial effects. CLA is incorporated and metabolized into brain tissue where induces the biosynthesis of endogenous PPARα ligands palmitoylethanolamide (PEA) and oleoylethanolamide (OEA), likely through a positive feedback mechanism where PPARα activation sustains its own cellular effects through ligand biosynthesis. In addition to PPARα, PEA and OEA may as well bind to other receptors such as TRPV1, further extending CLA own anti-neuroinflammatory actions. Future studies are needed to investigate whether dietary CLA may exert anti-inflammatory activity, particularly in the setting of neurodegenerative diseases and neuropsychiatric disorders with a neuroinflammatory basis.

Copper stabilizes antiparallel β-sheet fibrils of the amyloid β40 (Aβ40)-Iowa variant

Cerebral amyloid angiopathy (CAA) is a vascular disorder that primarily involves deposition of the 40-residue-long β-amyloid peptide (Aβ40) in and along small blood vessels of the brain. CAA is often associated with Alzheimer's disease (AD), which is characterized by amyloid plaques in the brain parenchyma enriched in the Aβ42 peptide. Several recent studies have suggested a structural origin that underlies the differences between the vascular amyloid deposits in CAA and the parenchymal plaques in AD. We previously have found that amyloid fibrils in vascular amyloid contain antiparallel β-sheet, whereas previous studies by other researchers have reported parallel β-sheet in fibrils from parenchymal amyloid. Using X-ray fluorescence microscopy, here we found that copper strongly co-localizes with vascular amyloid in human sporadic CAA and familial Iowa-type CAA brains compared with control brain blood vessels lacking amyloid deposits. We show that binding of Cu(II) ions to antiparallel fibrils can block the conversion of these fibrils to the more stable parallel, in-register conformation and enhances their ability to serve as templates for seeded growth. These results provide an explanation for how thermodynamically less stable antiparallel fibrils may form amyloid in or on cerebral vessels by using Cu(II) as a structural cofactor.

Non-Equilibrium Polymerization of Cross-β Amyloid Peptides for Temporal Control of Electronic Properties

Hydrophobic collapse plays crucial roles in protein functions, from accessing the complex three-dimensional structures of native enzymes to the dynamic polymerization of non-equilibrium microtubules. However, hydrophobic collapse can also lead to the thermodynamically downhill aggregation of aberrant proteins, which has interestingly led to the development of a unique class of soft nanomaterials. There remain critical gaps in the understanding of the mechanisms of how hydrophobic collapse can regulate such aggregation. Demonstrated herein is a methodology for non-equilibrium amyloid polymerization through mutations of the core sequence of Aβ peptides by a thermodynamically activated moiety. An out of equilibrium state is realized because of the negative feedback from the transiently formed cross-β amyloid networks. Such non-equilibrium amyloid nanostructures were utilized to access temporal control over its electronic properties.

Understanding the Pathophysiology of Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA), one of the main types of cerebral small vessel disease, is a major cause of spontaneous intracerebral haemorrhage and an important contributor to cognitive decline in elderly patients. Despite the number of experimental in vitro studies and animal models, the pathophysiology of CAA is still largely unknown. Although several pathogenic mechanisms including an unbalance between production and clearance of amyloid beta (Aβ) protein as well as ‘the prion hypothesis’ have been invoked as possible disease triggers, they do not explain completely the disease pathogenesis. This incomplete disease knowledge limits the implementation of treatments able to prevent or halt the clinical progression. The continuous increase of CAA patients makes imperative the development of suitable experimental in vitro or animal models to identify disease biomarkers and new pharmacological treatments that could be administered in the early disease stages to prevent irreversible changes and disease progression.

Cerebral amyloid angiopathy and Alzheimer disease - one peptide, two pathways

The shared role of amyloid-β (Aβ) deposition in cerebral amyloid angiopathy (CAA) and Alzheimer disease (AD) is arguably the clearest instance of crosstalk between neurodegenerative and cerebrovascular processes. The pathogenic pathways of CAA and AD intersect at the levels of Aβ generation, its circulation within the interstitial fluid and perivascular drainage pathways and its brain clearance, but diverge in their mechanisms of brain injury and disease presentation. Here, we review the evidence for and the pathogenic implications of interactions between CAA and AD. Both pathologies seem to be driven by impaired Aβ clearance, creating conditions for a self-reinforcing cycle of increased vascular Aβ, reduced perivascular clearance and further CAA and AD progression. Despite the close relationship between vascular and plaque Aβ deposition, several factors favour one or the other, such as the carboxy-terminal site of the peptide and specific co-deposited proteins. Amyloid-related imaging abnormalities that have been seen in trials of anti-Aβ immunotherapy are another probable intersection between CAA and AD, representing overload of perivascular clearance pathways and the effects of removing Aβ from CAA-positive vessels. The intersections between CAA and AD point to a crucial role for improving vascular function in the treatment of both diseases and indicate the next steps necessary for identifying therapies.

Interaction between amyloidogenic proteins and biomembranes in protein misfolding diseases: Mechanisms, contributors, and therapy

The toxic deposition of misfolded amyloidogenic proteins is associated with more than fifty protein misfolding diseases (PMDs), including Alzheimer's disease, Parkinson's disease and type 2 diabetes mellitus. Protein deposition is a multi-step process modulated by a variety of factors, in particular by membrane-protein interaction. The interaction results in permeabilization of biomembranes contributing to the cytotoxicity that leads to PMDs. Different biological and physiochemical factors, such as protein sequence, lipid composition, and chaperones, are known to affect the membrane-protein interaction. Here, we provide a comprehensive review of the mechanisms and contributing factors of the interaction between biomembranes and amyloidogenic proteins, and a summary of the therapeutic approaches to PMDs that target this interaction. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

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.

Cerebral vascular amyloid seeds drive amyloid β-protein fibril assembly with a distinct anti-parallel structure

Cerebrovascular accumulation of amyloid β-protein (Aβ), a condition known as cerebral amyloid angiopathy (CAA), is a common pathological feature of patients with Alzheimer's disease. Familial Aβ mutations, such as Dutch-E22Q and Iowa-D23N, can cause severe cerebrovascular accumulation of amyloid that serves as a potent driver of vascular cognitive impairment and dementia. The distinctive features of vascular amyloid that underlie its unique pathological properties remain unknown. Here, we use transgenic mouse models producing CAA mutants (Tg-SwDI) or overproducing human wild-type Aβ (Tg2576) to demonstrate that CAA-mutant vascular amyloid influences wild-type Aβ deposition in brain. We also show isolated microvascular amyloid seeds from Tg-SwDI mice drive assembly of human wild-type Aβ into distinct anti-parallel β-sheet fibrils. These findings indicate that cerebrovascular amyloid can serve as an effective scaffold to promote rapid assembly and strong deposition of Aβ into a unique structure that likely contributes to its distinctive pathology.

Induced Dipole-Dipole Interactions Influence the Unfolding Pathways of Wild-Type and Mutant Amyloid β-Peptides

Amyloid-forming proteins undergo a structural transition from α-helical to disordered conformations and, ultimately, cross-β fibrils. The unfolding and aggregation of the amyloid β-peptide (Aβ) have been implicated in the development and progression of Alzheimer’s disease (AD) and cerebral amyloid angiopathy (CAA). However, the events underlying the initial structural transition leading to the disease state remain unclear. Although most cases are sporadic, several genetic variants exist that alter the electrostatic properties of Aβ and lead to more rapid unfolding and more severe phenotypes. In the present study, the enhanced unfolding is shown to be due to the mutated side chains altering the local peptide-bond dipole moments leading to local destabilization of the α-helix, as determined from polarizable molecular dynamics (MD) simulations of wild-type (WT) Aβ fragments and several common mutations. The local perturbation of the helix then leads to progressive unwinding of the α-helix in a cooperative fashion due to decreases in adjacent (i ± 1) and hydrogen-bonded (i + 4) peptide-bond dipole moments. Side-chain dynamics, subsequent variations in dipole moments, and ultimately the response in the peptide-bond dipole moments are all modulated by solvent dielectric properties based on simulations in water versus ethanol. The polarizable simulation results, along with simulations using the additive CHARMM36 force field, further indicate that cooperativity due to the alignment of peptide bonds leading to enhanced dipole moments is a fundamental force in stabilizing α-helices.

The influence of the amyloid ß-protein and its precursor in modulating cerebral hemostasis

Ischemic and hemorrhagic strokes are a significant cause of brain injury leading to vascular cognitive impairment and dementia (VCID). These deleterious events largely result from disruption of cerebral hemostasis, a well-controlled and delicate balance between thrombotic and fibrinolytic pathways in cerebral blood vessels and surrounding brain tissue. Ischemia and hemorrhage are both commonly associated with cerebrovascular deposition of amyloid ß-protein (Aß). In this regard, Aß directly and indirectly modulates cerebral thrombosis and fibrinolysis. Further, major isoforms of the Aß precursor protein (AßPP) function as a potent inhibitor of pro-thrombotic proteinases. The purpose of this review article is to summarize recent research on how cerebral vascular Aß and AßPP influence cerebral hemostasis. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia, edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Two distinct β-sheet structures in Italian-mutant amyloid-beta fibrils: a potential link to different clinical phenotypes

Most Alzheimer’s disease (AD) cases are late-onset and characterized by the aggregation and deposition of the amyloid-beta (Aβ) peptide in extracellular plaques in the brain. However, a few rare and hereditary Aβ mutations, such as the Italian Glu₂₂-to-Lys (E22K) mutation, guarantee the development of early-onset familial AD. This type of AD is associated with a younger age at disease onset, increased β-amyloid accumulation, and Aβ deposition in cerebral blood vessel walls, giving rise to cerebral amyloid angiopathy (CAA). It remains largely unknown how the Italian mutation results in the clinical phenotype that is characteristic of CAA. We therefore investigated how this single point mutation may affect the aggregation of Aβ_1–42 in vitro and structurally characterized the resulting fibrils using a biophysical approach. This paper reports that wild-type and Italian-mutant Aβ both form fibrils characterized by the cross-β architecture, but with distinct β-sheet organizations, resulting in differences in thioflavin T fluorescence and solvent accessibility. E22K Aβ_1–42 oligomers and fibrils both display an antiparallel β-sheet structure, in comparison with the parallel β-sheet structure of wild-type fibrils, characteristic of most amyloid fibrils described in the literature. Moreover, we demonstrate structural plasticity for Italian-mutant Aβ fibrils in a pH-dependent manner, in terms of their underlying β-sheet arrangement. These findings are of interest in the ongoing debate that (1) antiparallel β-sheet structure might represent a signature for toxicity, which could explain the higher toxicity reported for the Italian mutant, and that (2) fibril polymorphism might underlie differences in disease pathology and clinical manifestation.

Crucial role of nonspecific interactions in amyloid nucleation

Protein oligomers have been implicated as toxic agents in a wide range of amyloid-related diseases. However, it has remained unsolved whether the oligomers are a necessary step in the formation of amyloid fibrils or just a dangerous byproduct. Analogously, it has not been resolved if the amyloid nucleation process is a classical one-step nucleation process or a two-step process involving prenucleation clusters. We use coarse-grained computer simulations to study the effect of nonspecific attractions between peptides on the primary nucleation process underlying amyloid fibrillization. We find that, for peptides that do not attract, the classical one-step nucleation mechanism is possible but only at nonphysiologically high peptide concentrations. At low peptide concentrations, which mimic the physiologically relevant regime, attractive interpeptide interactions are essential for fibril formation. Nucleation then inevitably takes place through a two-step mechanism involving prefibrillar oligomers. We show that oligomers not only help peptides meet each other but also, create an environment that facilitates the conversion of monomers into the β-sheet-rich form characteristic of fibrils. Nucleation typically does not proceed through the most prevalent oligomers but through an oligomer size that is only observed in rare fluctuations, which is why such aggregates might be hard to capture experimentally. Finally, we find that the nucleation of amyloid fibrils cannot be described by classical nucleation theory: in the two-step mechanism, the critical nucleus size increases with increases in both concentration and interpeptide interactions, which is in direct contrast with predictions from classical nucleation theory.

Cerebral microvascular rather than parenchymal amyloid-β protein pathology promotes early cognitive impairment in transgenic mice

Alzheimer's disease (AD) is an age-dependent neurodegenerative condition that causes a progressive decline in cognitive function. Accumulation of amyloid β-protein (Aβ) in the brain is a prominent feature of AD and related disorders. However, the levels of Aβ accumulation alone are not a reliable predictor of cognitive deficits. Aβ accumulates in AD brain in the form of parenchymal amyloid plaques and cerebral vascular deposits. Although both types of lesions can contribute to cognitive decline, their temporal impact remains unclear. Moreover, cerebral microvascular pathology is identified as an early driver of cognitive impairment. Here for the first time, we compared two transgenic mouse strains, Tg-5xFAD and Tg-SwDI, which exhibit similar onset and anatomical accumulation of Aβ, but with distinct parenchymal and microvascular compartmental deposition, respectively, to assess their impact on cognitive impairment. Cohorts of each line were tested at 3 and 6 months of age to assess the relationship between spatial working memory performance and quantitative pathology. At 3 months of age, Tg-SwDI mice with onset of cerebral microvascular amyloid were behaviorally impaired, while the Tg-5xFAD, which had disproportionately higher levels of total Aβ, soluble oligomeric Aβ, and parenchymal amyloid were not. However, at 6 months of age, behavioral deficits for both groups of transgenic mice were evident, as the levels of Aβ pathologies in the Tg-5xFAD accumulated to extremely high amounts. The present findings suggest early-onset cerebral microvascular amyloid deposition, that precedes high parenchymal levels of Aβ, may be an important early factor in the development of cognitive deficits.

Early-onset formation of parenchymal plaque amyloid abrogates cerebral microvascular amyloid accumulation in transgenic mice

The fibrillar assembly and deposition of amyloid β (Aβ) protein, a key pathology of Alzheimer disease, can occur in the form of parenchymal amyloid plaques and cerebral amyloid angiopathy (CAA). Familial forms of CAA exist in the absence of appreciable parenchymal amyloid pathology. The molecular interplay between parenchymal amyloid plaques and CAA is unclear. Here we investigated how early-onset parenchymal amyloid plaques impact the development of microvascular amyloid in transgenic mice. Tg-5xFAD mice, which produce non-mutated human Aβ and develop early-onset parenchymal amyloid plaques, were bred to Tg-SwDI mice, which produce familial CAA mutant human Aβ and develop cerebral microvascular amyloid. The bigenic mice presented with an elevated accumulation of Aβ and fibrillar amyloid in the brain compared with either single transgenic line. Tg-SwDI/Tg-5xFAD mice were devoid of microvascular amyloid, the prominent pathology of Tg-SwDI mice, but exhibited larger parenchymal amyloid plaques compared with Tg-5xFAD mice. The larger parenchymal amyloid deposits were associated with a higher loss of cortical neurons and elevated activated microglia in the bigenic Tg-SwDI/Tg-5xFAD mice. The periphery of parenchymal amyloid plaques was largely composed of CAA mutant Aβ. Non-mutated Aβ fibril seeds promoted CAA mutant Aβ fibril formation in vitro. Further, intrahippocampal administration of biotin-labeled CAA mutant Aβ peptide accumulated on and adjacent to pre-existing parenchymal amyloid plaques in Tg-5xFAD mice. These findings indicate that early-onset parenchymal amyloid plaques can serve as a scaffold to capture CAA mutant Aβ peptides and prevent their accumulation in cerebral microvessels.

Fine mapping of the amyloid β-protein binding site on myelin basic protein

The assembly and deposition of amyloid β-protein (Aβ) in brain is a key pathological feature of Alzheimer's disease and related disorders. Factors have been identified that can either promote or inhibit Aβ assembly in brain. We previously reported that myelin basic protein (MBP) is a potent inhibitor of Aβ fibrillar assembly [Hoos, M. D., et al. (2007) J. Biol. Chem. 282, 9952-9961; Hoos, M. D., et al. (2009) Biochemistry 48, 4720-4727]. Moreover, the region on MBP responsible for this activity was localized to the 64 N-terminal amino acids (MBP1-64) [Liao, M. C., et al. (2010) J. Biol. Chem. 285, 35590-35598]. In the study presented here, we sought to further define the site on MBP1-64 involved in this activity. Deletion mapping studies showed that the C-terminal region (residues 54-64) is required for the ability of MBP1-64 to bind Aβ and inhibit fibril assembly. Alanine scanning mutagenesis revealed that amino acids K54, R55, G56, and K59 within MBP1-64 are important for both Aβ binding and inhibition of fibril assembly as assessed by solid phase binding, thioflavin T binding and fluorescence, and transmission electron microscopy studies. Strong spectral shifts are observed by solution nuclear magnetic resonance spectroscopy of specific N-terminal residues (E3, R5, D7, E11, and Q15) of Aβ42 upon the interaction with MBP1-64. Although the C-terminal region of MBP1-64 is required for interactions with Aβ, a synthetic MBP50-64 peptide was itself devoid of activity. These studies identify key residues in MBP and Aβ involved in their interactions and provide structural insight into how MBP regulates Aβ fibrillar assembly.

Cell "vision": complementary factor of protein corona in nanotoxicology

Engineered nanoparticles are increasingly being considered for use as biosensors, imaging agents and drug delivery vehicles. Their versatility in design and applications make them an attractive proposition for new biological and biomedical approaches. Despite the remarkable speed of development in nanoscience, relatively little is known about the interaction of nanoscale objects with living systems. In a biological fluid, proteins associate with nanoparticles, and the amount and the presentation of the proteins on their surface could lead to a different in vivo response than an uncoated particle. Here, in addition to protein adsorption, we are going to introduce concept of cell "vision", which would be recognized as another crucial factor that should be considered for the safe design of any type of nanoparticles that will be used in specific biomedical applications. The impact of exactly the same nanoparticles on various cells is significantly different and could not be assumed for other cells; the possible mechanisms that justify this cellular response relate to the numerous detoxification strategies that any particular cell can utilize in response to nanoparticles. The uptake and defence mechanism could be considerably different according to the cell type. Thus, what the cell "sees", when it is faced with nanoparticles, is most likely dependent on the cell type.

β-Amyloid carrying the Dutch mutation has diverse effects on calpain-mediated toxicity in hippocampal neurons

Hereditary cerebral hemorrhage with amyloidosis-Dutch type is a disorder associated with a missense mutation (E693Q) in the β-amyloid (Aβ)-coding region of the amyloid precursor protein (APP). This familial disease is characterized by cognitive deficits secondary to intracerebral hemorrhage and, in some cases, progressive Alzheimer's disease (AD)-like dementia. Although this mutation was the first ever reported in the human APP gene, little is known about the molecular mechanisms underlying the direct toxic effects of this mutated Aβ on central neurons. In the present study, we assessed the role of calpain-mediated toxicity in such effects using an AD primary culture model system. Our results showed that Dutch mutant Aβ (E22Q) induced calpain-mediated cleavage of dynamin 1 and a significant decrease in synaptic contacts in mature hippocampal cultures. These synaptic deficits were similar to those induced by wild-type (WT) Aβ. In contrast, calpain-mediated tau cleavage leading to the generation of a 17-kDa neurotoxic fragment, as well as neuronal death, were significantly reduced in E22Q Aβ-treated neurons when compared with WT Aβ-treated ones. This complex regulation of the calpain-mediated toxicity pathway by E22Q Aβ could have some bearing in the pathobiology of this familial AD form.

Genetic animal models of cerebral vasculopathies

Cerebral amyloid angiopathy (CAA) and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) are genetic cerebrovasculopathies associated with neurodegeneration and vascular cognitive impairment. Linked to autosomal dominant mutations in diverse genes that encode cell-surface receptors (i.e., amyloid precursor protein in CAA and NOTCH3 in CADASIL), both diseases are associated with accumulation of abnormal material around cerebral vessels, such as amyloid in CAA or granular osmiophilic material in CADASIL. Both CAA and CADASIL share clinical features of white matter degeneration and infarcts, and vascular dementia in the human adult; microbleeds occur in both CADASIL and CAA, but large intracerebral hemorrhages are more characteristic for the latter. While the mechanisms are poorly understood, wall thickening, luminal narrowing, and eventual loss of vascular smooth muscle cells are overlapping pathologies involving leptomeningeal, and pial or penetrating small arteries and arterioles in CAA and CADASIL. Dysregulation of cerebral blood flow and eventual hypoperfusion are believed to be the key pathophysiological steps in neurodegeneration and cognitive impairment. Although animal models expressing CAA or CADASIL mutations have partially reproduced the human pathology, there has been marked heterogeneity in the phenotypic spectrum, possibly due to genetic background differences among mouse models, and obvious species differences between mouse and man. Here, we provide an overview of animal models of CAA and CADASIL and the insight on molecular and physiological mechanisms of disease gained from these models.

The blood-brain barrier in Alzheimer's disease: novel therapeutic targets and nanodrug delivery

Treatment strategies for Alzheimer's disease (AD) are still elusive. Thus, new strategies are needed to understand the pathogenesis of AD in order to provide suitable therapeutic measures. Available evidences suggest that in AD, passage across the blood-brain barrier (BBB) and transport exchanges for amyloid-β-peptide (ABP) between blood and the central nervous system (CNS) compartments play an important regulatory role for the deposition of brain ABP. New evidences suggest that BBB is altered in AD. Studies favoring transport theory clearly show that ABP putative receptors at the BBB control the level of soluble isoform of ABP in brain. This is achieved by regulating influx of circulating ABP into brain via specific receptor for advanced glycation end products (RAGE) and gp330/megalin-mediated transcytosis. On the other hand, the efflux of brain-derived ABP into the circulation across the vascular system via BBB is accomplished by low-density receptor-related protein-1 (LRP1). Furthermore, an increased BBB permeability in AD is also likely since structural damage of endothelial cells is quite frequent in AD brain. Thus, enhanced drug delivery in AD is needed to induce neuroprotection and therapeutic success. For this purpose, nanodrug delivery could be one of the available options that require active consideration for novel therapeutic strategies to treat AD cases. This review is focused on these aspects and provides new data showing that BBB plays an important role in AD-induced neurodegeneration and neurorepair.

Structural dynamics of the ΔE22 (Osaka) familial Alzheimer's disease-linked amyloid β-protein

A familial form of Alzheimer disease recently was described in a kindred in Osaka, Japan. This kindred possesses an amyloid β-protein (Aβ) precursor mutation within the Aβ coding region that results in the deletion of Glu22 (ΔE22). We report here results of studies of [ΔE22]Aβ40 and [ΔE22]Aβ42 that sought to elucidate the conformational dynamics, oligomerization behavior, fibril formation kinetics, fibril morphology, and fibril stability of these mutant peptides. Both [ΔE22]Aβ peptides had extraordinary β-sheet formation propensities. The [ΔE22]Aβ40 mutant formed β-sheet secondary structure elements ≈400-fold faster. Studies of β-sheet stability in the presence of fluorinated alcohol cosolvents or high pH revealed that the ΔE22 mutation substantially increased stability, producing a rank order of [ΔE22]Aβ42 >>Aβ42 > [ΔE22]Aβ40 > Aβ40. The mutation facilitated formation of oligomers by [ΔE22]Aβ42 (dodecamers and octadecamers) that were not observed with Aβ42. Both Aβ40 and Aβ42 peptides formed nebulous globular and small string-like structures immediately upon solvation from lyophilizates, whereas short protofibrillar and fibrillar structures were evident immediately in the ΔE22 samples. Determination of the critical concentration for fibril formation for the [ΔE22]Aβ peptides showed it to be ≈1/2 that of the wild type homologues, demonstrating that the mutations causes a modest increase in fibril stability. The magnitude of this increase, when considered in the context of the extraordinary increase in β-sheet propensity for the ΔE22 peptides, suggests that the primary biophysical effect of the mutation is to accelerate conformational changes in the peptide monomer that facilitate oligomerization and higher-order assembly.

Structural evolution of Iowa mutant β-amyloid fibrils from polymorphic to homogeneous states under repeated seeded growth

Structural variations in β-amyloid fibrils are potentially important to the toxicity of these fibrils in Alzheimer's disease (AD). We describe a repeated seeding protocol that selects a homogeneous fibril structure from a polymorphic initial state in the case of 40-residue β-amyloid fibrils with the Asp23-to-Asn, or Iowa, mutation (D23N-Aβ(1-40)). We use thioflavin T (ThT) fluorescence, transmission electron microscopy (TEM), and solid-state nuclear magnetic resonance (NMR) to track the evolution of fibril structure through multiple generations under this protocol. The data show that (i) repeated seeding selectively amplifies a single D23N-Aβ(1-40) fibril structure that can be a minor component of the initial polymorphic state; (ii) the final structure is highly sensitive to growth conditions, including pH, temperature, and agitation; (iii) although the initial state can include fibrils that contain both antiparallel and parallel β-sheets, the final structures contain only parallel β-sheets, suggesting that antiparallel β-sheet structures are thermodynamically and kinetically metastable. Additionally, our data demonstrate that ThT fluorescence enhancements, which are commonly used to monitor amyloid fibril formation, vary strongly with structural variations, even among fibrils comprised of the same polypeptide. Finally, we present a simple mathematical model that describes the structural evolution of fibril samples under repeated seeding.

The Osaka FAD mutation E22Δ leads to the formation of a previously unknown type of amyloid β fibrils and modulates Aβ neurotoxicity

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cerebral deposition of amyloid fibrils formed by the amyloid β (Aβ) peptide. Aβ has a length of 39-43 amino acid residues; the predominant Aβ isoforms are Aβ1-40 and Aβ1-42. While the majority of AD cases occur spontaneously, a subset of early-onset familial AD cases is caused by mutations in the genes encoding the Aβ precursor protein or presenilin 1/presenilin 2. Recently, a deletion of glutamic acid at position 22 within the Aβ sequence (E22Δ) was identified in Japanese patients with familial dementia, but the aggregation properties of the deletion variant of Aβ are not well understood. We investigated the aggregation characteristics and neurotoxicity of recombinantly expressed Aβ isoforms 1-40 and 1-42 with and without the E22Δ mutation. We show that the E22Δ mutation strongly accelerates the fibril formation of Aβ1-42 E22Δ compared to Aβ1-42 wild type (wt). In addition, we demonstrate that fibrils of Aβ1-40 E22Δ form a unique quaternary structure characterized by a strong tendency to form fibrillar bundles and a strongly increased thioflavin T binding capacity. Aβ1-40 E22Δ was neurotoxic in rat primary neuron cultures as compared to nontoxic Aβ1-40 wt. Aβ1-42 E22Δ was less toxic than Aβ1-42 wt, but it significantly decreased neurite outgrowth per cell in neuronal primary cultures. Because Aβ1-40 is the major Aβ form in vivo, the gain of toxic function caused by the E22 deletion may explain the development of familial AD in mutation carriers.

Lipid-induced conformational transition of amyloid beta peptide fragments

Conformational transition of soluble monomeric amyloid beta-peptide (Abeta) into oligomeric and protofibrillar aggregates plays a key role in the pathogenesis of Alzheimer's disease (AD). One of the central questions surrounding the molecular pathophysiology of AD is how the soluble Abeta is converted into its aggregated toxic form. A more detailed understanding of the conformational transitions involved in the self-assembly of Abeta may facilitate the design of inhibitors of aggregation. In this study, we evaluated the wild-type (WT) Abeta 16-28 peptide (KLVFFAEDVGSNK) and its associated mutants, including A21G (Flemish), E22K (Italian), E22Q (Dutch), and E22G (Arctic) mutants, by examining, in particular, their aggregation kinetics in the presence and in the absence of negatively charged and zwitterionic lipids. Circular dichroic and thioflavin T fluorescence studies indicated that the WT peptide undergoes a rapid conformational transition into beta-sheet structure in solution, whereas the Arctic and Dutch variants show a markedly rapid transition into beta-sheet structure in the presence of negatively charged lipids. These results provide strong evidence suggesting that the reduction in net charge, with a concurrent increase in the net hydrophobicity of the peptide alone or when complexed with lipid in solution, determines the rate of aggregate formation.

Up-and-down topological mode of amyloid beta-peptide lying on hydrophilic/hydrophobic interface of ganglioside clusters

Growing evidence has indicated that GM1 ganglioside specifically interacts with Amyloid beta-peptide (Abeta) and thereby promotes Alzheimer's disease-associated Abeta assembly. To characterize the conformation of Abeta bound to the ganglioside, we performed 920 MHz ultra-high field NMR analyses using isotopically labeled Abeta(1-40) in association with GM1 and lyso-GM1 micelles. Our NMR data revealed that (1) Abeta(1-40) forms discontinuous alpha-helices at the segments His(14)-Val(24) and Ile(31)-Val(36) upon binding to the gangliosidic micelles, leaving the remaining regions disordered, and (2) Abeta(1-40) lies on hydrophobic/hydrophilic interface of the ganglioside cluster exhibiting an up-and-down topological mode in which the two alpha-helices and the C-terminal dipeptide segment are in contact with the hydrophobic interior, whereas the remaining regions are exposed to the aqueous environment. These findings suggest that the ganglioside clusters serve as a unique platform for binding coupled with conformational transition of Abeta molecules, rendering their spatial rearrangements restricted to promote specific intermolecular interactions.

Novel semisynthetic method for generating full length beta-amyloid peptides

Bacterial expression of full length beta-amyloid (Abeta) is problematic because of toxicity and poor solubility of the expressed protein, and a strong tendency of Met35 to become oxidized in inclusion bodies. We have developed a semisynthetic method in which Abeta1-29 is expressed in bacteria as part of a fusion protein with a C-terminal intein and Chitin-Binding Domain (CBD). There is also a single residue, N-terminal Met extension. The protein, Met-Abeta1-29-Intein-CBD, is well expressed and highly water-soluble. After binding of the expressed protein to Chitin beads, treatment with sodium 2-mercapto-ethane sulfonate (MESNA) yields Met-Abeta1-29-MESNA, with a C-terminal thioester suitable for native chemical ligation. Met-Abeta1-29-MESNA is first subjected to CNBr cleavage, which removes the N-terminal Met residue, but leaves the thioester intact. We synthesized NH2-A30C-Abeta30-40, which has an N-terminal Cys residue and is the partner for native chemical ligation with Met-Abeta1-29-MESNA. Native chemical ligation proceeds rapidly and efficiently (>90% yield) to give A30C-Abeta1-40. The final step is selective desulfurization using Raney-Ni, which also proceeds rapidly and efficiently (>90% yield) to give native sequence Abeta1-40. Overall, this system is highly efficient, and can yield approximately 8-10 mg of pure Abeta1-40 from one liter of bacterial culture medium. This procedure is adaptable for producing other Abeta peptides. We have also expressed an Abeta construct bearing a point mutation associated with one type of familial Alzheimer's Disease, the Iowa mutation, i.e., Met-D23N-Abeta1-29-Intein-CBD. Since expression of the intein-containing fusion protein is robust in minimal media as well as standard enriched media, this procedure also can be readily modified for incorporating 15N or 13C labels for NMR. Future work will also include extending this system to longer Abeta peptides, such as Abeta1-42.

Impact of phospholipid bilayer saturation on amyloid-beta protein aggregation intermediate growth: a quartz crystal microbalance analysis

Evidence that membrane-associated amyloid aggregate growth can impart membrane damage represents one possible mechanism for the neurodegeneration associated with deposited amyloid-beta protein (Abeta) aggregates in the brains of Alzheimer's disease (AD) patients. This potential pathogenic event necessitates an understanding of the impact that cellular membrane composition may have on Abeta aggregate growth. In the current study, a quartz crystal microbalance (QCM) was employed to examine the growth of Abeta(1-40) aggregation intermediates on supported phospholipid bilayers (SPBs) assembled at the crystal surface. These surface-specific measurements illustrate that zwitterionic SPBs selectively bind aggregated but not monomeric protein, and these bound aggregates are capable of supporting nonsaturable reversible growth via monomer addition. Growth-capable Abeta(1-40) aggregation intermediates more readily bind SPBs composed of phospholipids with a greater degree of carbon saturation. Furthermore, kinetic analysis afforded by the quantitative real-time QCM measurements reveals that SPBs with greater saturation also better support the growth of bound Abeta(1-40) aggregation intermediates as a result of the slower dissociation of bound monomer rather than more efficient recognition between aggregate and monomeric protein. These findings correlate with epidemiological and experimental evidence that links increased dietary intake of polyunsaturated fatty acids to a reduced risk of AD.

Influence of residue 22 on the folding, aggregation profile, and toxicity of the Alzheimer's amyloid beta peptide

Several biophysical techniques have been used to determine differences in the aggregation profile (i.e., the secondary structure, aggregation propensity, dynamics, and morphology of amyloid structures) and the effects on cell viability of three variants of the amyloid beta peptide involved in Alzheimer's disease. We focused our study on the Glu22 residue, comparing the effects of freshly prepared samples and samples aged for at least 20 days. In the aged samples, a high propensity for aggregation and beta-sheet secondary structure appears when residue 22 is capable of establishing polar (Glu22 in wild-type) or hydrophobic (Val22 in E22V) interactions. The Arctic variant (E22G) presents a mixture of mostly disordered and alpha-helix structures (with low beta-sheet contribution). Analysis of transmission electron micrographs and atomic force microscopy images of the peptide variants after aging showed significant quantitative and qualitative differences in the morphology of the formed aggregates. The effect on human neuroblastoma cells of these Abeta(12-28) variants does not correlate with the amount of beta-sheet of the aggregates. In samples allowed to age, the native sequence was found to have an insignificant effect on cell viability, whereas the Arctic variant (E22G), the E22V variant, and the slightly-aggregating control (F19G-F20G) had more prominent effects.

Myelin basic protein binds to and inhibits the fibrillar assembly of Abeta42 in vitro

The deposition of amyloid beta-protein (Abeta) fibrils into plaques within the brain parenchyma and along cerebral blood vessels is a hallmark of Alzheimer's disease. Abeta peptides are produced through the successive cleavage of the Abeta precursor protein by beta- and gamma-secretase, producing peptides between 39 and 43 amino acids in length. The most common of these are Abeta40 (the most abundant) and Abeta42. Abeta42 is more fibrillogenic than Abeta40 and has been implicated in early Abeta plaque deposition. Our previous studies determined that myelin basic protein (MBP) was capable of inhibiting fibril formation of a highly fibrillogenic Abeta peptide containing both E22Q (Dutch) and D23N (Iowa) mutations associated with familial forms of cerebral amyloid angiopathy [Hoos, M. D., et al. (2007) J. Biol. Chem. 282, 9952-9961]. In this study, we show through a combination of biochemical and ultrastructural techniques that MBP is also capable of inhibiting the beta-sheet fibrillar assembly of the normal Abeta42 peptide. These findings suggest that MBP may play a role in regulating the deposition of Abeta42 and thereby also may regulate the early formation of amyloid plaques in Alzheimer's disease.

Evidence for novel beta-sheet structures in Iowa mutant beta-amyloid fibrils

Asp23-to-Asn mutation within the coding sequence of beta-amyloid, called the Iowa mutation, is associated with early onset, familial Alzheimer's disease and cerebral amyloid angiopathy, in which patients develop neuritic plaques and massive vascular deposition predominantly of the mutant peptide. We examined the mutant peptide, D23N-Abeta40, by electron microscopy, X-ray diffraction, and solid-state NMR spectroscopy. D23N-Abeta40 forms fibrils considerably faster than the wild-type peptide (k = 3.77 x 10(-3) min(-1) and 1.07 x 10(-4) min(-1) for D23N-Abeta40 and the wild-type peptide WT-Abeta40, respectively) and without a lag phase. Electron microscopy shows that D23N-Abeta40 forms fibrils with multiple morphologies. X-ray fiber diffraction shows a cross-beta pattern, with a sharp reflection at 4.7 A and a broad reflection at 9.4 A, which is notably smaller than the value for WT-Abeta40 fibrils (10.4 A). Solid-state NMR measurements indicate molecular level polymorphism of the fibrils, with only a minority of D23N-Abeta40 fibrils containing the in-register, parallel beta-sheet structure commonly found in WT-Abeta40 fibrils and most other amyloid fibrils. Antiparallel beta-sheet structures in the majority of fibrils are indicated by measurements of intermolecular distances through (13)C-(13)C and (15)N-(13)C dipole-dipole couplings. An intriguing possibility exists that there is a relationship between the aberrant structure of D23N-Abeta40 fibrils and the unusual vasculotropic clinical picture in these patients.

Hereditary and sporadic forms of abeta-cerebrovascular amyloidosis and relevant transgenic mouse models

Cerebral amyloid angiopathy (CAA) refers to the specific deposition of amyloid fibrils in the leptomeningeal and cerebral blood vessel walls, often causing secondary vascular degenerative changes. Although many kinds of peptides are known to be deposited as vascular amyloid, amyloid-beta (Abeta)-CAA is the most common type associated with normal aging, sporadic CAA, Alzheimer's disease (AD) and Down's syndrome. Moreover, Abeta-CAA is also associated with rare hereditary cerebrovascular amyloidosis due to mutations within the Abeta domain of the amyloid precursor protein (APP) such as Dutch and Flemish APP mutations. Genetics and clinicopathological studies on these familial diseases as well as sporadic conditions have already shown that CAA not only causes haemorrhagic and ischemic strokes, but also leads to progressive dementia. Transgenic mouse models based on familial AD mutations have also successfully reproduced many of the features found in human disease, providing us with important insights into the pathogenesis of CAA. Importantly, such studies have pointed out that specific vastopic Abeta variants or an unaltered Abeta42/Abeta40 ratio favor vascular Abeta deposition over parenchymal plaques, but higher than critical levels of Abeta40 are also observed to be anti-amyloidogenic. These data would be important in the development of therapies targeting amyloid in vessels.