Annular protofibrils are a structurally and functionally distinct type of amyloid oligomer

Spectroscopic detection of β -sheet structure in nascent Aβ oligomers

Deep-UV resonance Raman (UVRR) spectroscopy and circular dichroism (CD) were employed to study the secondary structure of Aβ(1-42) in fresh samples with increasing fractions of oligomeric peptide. A feature with a minimum at ~217 nm appeared in CD spectra of samples containing oligomeric Aβ(1-42). UVRR spectra more closely resembled those of disordered proteins. The primary difference between UVRR spectra was the ratio of the 1236 cm(-1) to 1260 cm(-1) amide III peak intensities, which shifted in favor of the 1236 cm(-1) band as the fraction of oligomeric peptide increased.

Ligand binding to distinct states diverts aggregation of an amyloid-forming protein

Although small molecules that modulate amyloid formation in vitro have been identified, significant challenges remain in determining precisely how these species act. Here we describe the identification of rifamycin SV as a potent inhibitor of β(2) microglobulin (β(2)m) fibrillogenesis when added during the lag time of assembly or early during fibril elongation. Biochemical experiments demonstrate that the small molecule does not act by a colloidal mechanism. Exploiting the ability of electrospray ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) to resolve intermediates of amyloid assembly, we show instead that rifamycin SV inhibits β(2)m fibrillation by binding distinct monomeric conformers, disfavoring oligomer formation and diverting the course of assembly to the formation of spherical aggregates. The results demonstrate the power of ESI-IMS-MS to identify specific protein conformers as targets for intervention in fibrillogenesis using small molecules and reveal a mechanism of action in which ligand binding diverts unfolded protein monomers toward alternative assembly pathways.

Astrocytes contain amyloid-β annular protofibrils in Alzheimer's disease brains

Annular protofibrils (APFs) represent a newly described and distinct class of amyloid structures formed by disease-associated proteins. In vitro, these pore-like structures have been implicated in membrane permeabilization and ion homeostasis via pore formation. Still, their formation and relevance in vivo are poorly understood. Herein, we report that APFs are in human Alzheimer's disease brain samples and that amyloid-β APFs were associated with activated astrocytes. Moreover, we show that amyloid-β APFs in astrocytes adopt a conformation in which the N-terminal region is buried inside the wall of the pore. Our results together with previous studies suggest that the formation of amyloid-β APFs in astrocytes could be a relevant event in the pathogenesis of Alzheimer's disease and validate this amyloidogenic structure as a target for the prevention of the disease.

Structural, morphological, and kinetic studies of β-amyloid peptide aggregation on self-assembled monolayers

The misfolding and aggregation of β-amyloid peptides (Aβ) into amyloid fibrils, a process that has been pathologically linked to the onset of Alzheimer's disease, is dependent on the presence of a heterogeneous surface (e.g., cell membrane). Understanding of the kinetics of amyloid fibril formation and associated structural transition from monomers to intermediates and eventually to fibrils is critical for the development of viable therapeutic agents. In this work, using circular dichroism (CD), atomic force microscopy (AFM), surface plasmon resonance (SPR), and molecular dynamics (MD) simulations, we studied the adsorption, aggregation, and conformational changes of Aβ(1-42) from fresh monomers to fully grown fibrils on four model self-assembled monolayers (SAMs): hydrophobic CH(3)-terminated SAM, hydrophilic OH-terminated SAM, negatively charged COOH-terminated SAMs, and positively charged NH(2)-terminated SAM. The seeding effect of Aβ(1-42) on the kinetics of Aβ aggregation on different SAMs is also examined. The CD, AFM, and SPR data show that all of these SAMs greatly accelerate the formation of β-sheets and amyloid fibrils through surface-enhanced interactions, but Aβ(1-42) peptides preferentially adsorb on a hydrophobic CH(3)-SAM and a positively charged NH(2)-SAM with much stronger interactions than on a hydrophilic OH-SAM and a negatively charged COOH-SAM. MD simulations further reveal that hydrophobic interactions present a general driving force for Aβ adsorption on all SAMs. As Aβ aggregates grow into larger species by packing hydrophobic C-terminals to form a hydrophobic core while exposing hydrophilic and negatively charged N-terminals to solution, electrostatic interactions become more strengthened when they interact with the SAMs especially for the COOH-SAM and the NH(2)-SAM. Thus, both hydrophobic and electrostatic interactions contribute differently to different Aβ-SAM systems and to different aggregation stages. A postulated mechanism is proposed to describe the structure and kinetics of Aβ aggregation from aqueous solution to the SAMs, providing valuable insights into Aβ aggregation on biological cell membranes.

In vivo and in vitro analyses of toxic mutants of HET-s: FTIR antiparallel signature correlates with amyloid toxicity

The folding and interactions of amyloid proteins are at the heart of the debate as to how these proteins may or may not become toxic to their host. Although little is known about this issue, the structure seems to be clearly involved with effects on molecular events. To understand how an amyloid may be toxic, we previously generated a yeast toxic amyloid (mutant 8) from the nontoxic HET-s((218-289)) prion domain of Podospora anserina. Here, we performed a comprehensive structure-toxicity study by mutating individually each of the 10 mutations found in mutant 8. The study of the library of new mutants generated allowed us to establish a clear link between Fourier transform infrared antiparallel signature and amyloid toxicity. All of the mutants that form parallel β-sheets are not toxic. Double mutations may be sufficient to shift a parallel structure to antiparallel amyloids, which are toxic to yeast. Our findings also suggest that the toxicity of antiparallel structured mutants may be linked to interaction with membranes.

Polymorphic structures of Alzheimer's β-amyloid globulomers

Background

Misfolding and self-assembly of Amyloid-β (Aβ) peptides into amyloid fibrils is pathologically linked to the development of Alzheimer's disease. Polymorphic Aβ structures derived from monomers to intermediate oligomers, protofilaments, and mature fibrils have been often observed in solution. Some aggregates are on-pathway species to amyloid fibrils, while the others are off-pathway species that do not evolve into amyloid fibrils. Both on-pathway and off-pathway species could be biologically relevant species. But, the lack of atomic-level structural information for these Aβ species leads to the difficulty in the understanding of their biological roles in amyloid toxicity and amyloid formation.

Methods and Findings

Here, we model a series of molecular structures of Aβ globulomers assembled by monomer and dimer building blocks using our peptide-packing program and explicit-solvent molecular dynamics (MD) simulations. Structural and energetic analysis shows that although Aβ globulomers could adopt different energetically favorable but structurally heterogeneous conformations in a rugged energy landscape, they are still preferentially organized by dynamic dimeric subunits with a hydrophobic core formed by the C-terminal residues independence of initial peptide packing and organization. Such structural organizations offer high structural stability by maximizing peptide-peptide association and optimizing peptide-water solvation. Moreover, curved surface, compact size, and less populated β-structure in Aβ globulomers make them difficult to convert into other high-order Aβ aggregates and fibrils with dominant β-structure, suggesting that they are likely to be off-pathway species to amyloid fibrils. These Aβ globulomers are compatible with experimental data in overall size, subunit organization, and molecular weight from AFM images and H/D amide exchange NMR.

Conclusions

Our computationally modeled Aβ globulomers provide useful insights into structure, dynamics, and polymorphic nature of Aβ globulomers which are completely different from Aβ fibrils, suggesting that these globulomers are likely off-pathway species and explaining the independence of the aggregation kinetics between Aβ globulomers and fibrils.

Suppression of amyloid beta A11 antibody immunoreactivity by vitamin C: possible role of heparan sulfate oligosaccharides derived from glypican-1 by ascorbate-induced, nitric oxide (NO)-catalyzed degradation

Amyloid β (Aβ) is generated from the copper- and heparan sulfate (HS)-binding amyloid precursor protein (APP) by proteolytic processing. APP supports S-nitrosylation of the HS proteoglycan glypican-1 (Gpc-1). In the presence of ascorbate, there is NO-catalyzed release of anhydromannose (anMan)-containing oligosaccharides from Gpc-1-nitrosothiol. We investigated whether these oligosaccharides interact with Aβ during APP processing and plaque formation. anMan immunoreactivity was detected in amyloid plaques of Alzheimer (AD) and APP transgenic (Tg2576) mouse brains by immunofluorescence microscopy. APP/APP degradation products detected by antibodies to the C terminus of APP, but not Aβ oligomers detected by the anti-Aβ A11 antibody, colocalized with anMan immunoreactivity in Tg2576 fibroblasts. A 50-55-kDa anionic, sodium dodecyl sulfate-stable, anMan- and Aβ-immunoreactive species was obtained from Tg2576 fibroblasts using immunoprecipitation with anti-APP (C terminus). anMan-containing HS oligo- and disaccharide preparations modulated or suppressed A11 immunoreactivity and oligomerization of Aβ42 peptide in an in vitro assay. A11 immunoreactivity increased in Tg2576 fibroblasts when Gpc-1 autoprocessing was inhibited by 3-β[2(diethylamino)ethoxy]androst-5-en-17-one (U18666A) and decreased when Gpc-1 autoprocessing was stimulated by ascorbate. Neither overexpression of Gpc-1 in Tg2576 fibroblasts nor addition of copper ion and NO donor to hippocampal slices from 3xTg-AD mice affected A11 immunoreactivity levels. However, A11 immunoreactivity was greatly suppressed by the subsequent addition of ascorbate. We speculate that temporary interaction between the Aβ domain and small, anMan-containing oligosaccharides may preclude formation of toxic Aβ oligomers. A portion of the oligosaccharides are co-secreted with the Aβ peptides and deposited in plaques. These results support the notion that an inadequate supply of vitamin C could contribute to late onset AD in humans.

Polymorphism of amyloid β peptide in different environments: implications for membrane insertion and pore formation

Amyloid-β (Aβ) peptides are thought to be involved in neurodegenerative diseases such as Alzheimer's disease and Down's syndrome. They form a large number of polymorphic structures, including heterogeneous ionic pores in membranes as well as different types of fibrillar and globular structures on surfaces and in solution. Understanding the origin of these structures and the factors that influence their occurrence is of great biomedical interest because of the possible relationship between structure and pathogenicity. Here, we use atomic force microscopy (AFM) and molecular dynamics (MD) simulations to demonstrate that at room temperature a truncated Aβ peptide which is generated in vivo and shown to be toxic in vitro forms fibrillar structures on hydrophobic graphite surfaces, but not on hydrophilic mica or lipid bilayers. Our results suggest that the toxic pores and fibrillar polymorphic organizations can be explained in terms of the U-shaped β-strand-turn-β-strand structural motif observed for full length Aβ and other amyloids, as well as the physicochemical properties at the interfaces. The interactions of the hydrophobic, truncated Aβ with its environment illustrate that the universal amyloid motif can provide a link between the pore and fibrillar structures and indicate that surfaces with different physicochemical properties can shift the polymorphic landscape toward other conformational states.

Alzheimers disease: review of emerging treatment role for intravenous immunoglobulins

Alzheimer’s disease (AD) is the most common neurodegenerative disorder. Currently available therapies are symptomatic but do not alter underlying disease progression. Immunotherapeutic approaches such as anti Aβ peptide active vaccination trials have had limited success to date. Intravenous immunoblobulin (IVIg) is widely used in immune-mediated neurological disorders such myasthenia gravis and Guillain-Barre syndrome. These preparations have been obtained from the pooled plasma of healthy human donors and contain natural anti-amyloid antibodies and are well tolerated. A small pilot study of passive immunotherapy using IVIg has suggested cognitive improvement. A multicenter phase III trial is ongoing and will determine whether or not this treatment can ameliorate cognitive deficits in mild-to-moderate AD. Here, we briefly review the pathogenic role of amyloid and tau in AD, as well as immunotherapeutic efforts to date. We also summarize what is known about naturally occurring anti-Aβ and tau antibodies in IVIg with a view toward explaining potential mechanisms underlying their therapeutic effects.

Characteristics of amyloid-related oligomers revealed by crystal structures of macrocyclic β-sheet mimics

Protein amyloid oligomers have been strongly linked to amyloid diseases and can be intermediates to amyloid fibers. β-Sheets have been identified in amyloid oligomers. However, because of their transient and highly polymorphic properties, the details of their self-association remain elusive. Here we explore oligomer structure using a model system: macrocyclic peptides. Key amyloidogenic sequences from Aβ and tau were incorporated into macrocycles, thereby restraining them to β-strands, but limiting the growth of the oligomers so they may crystallize and cannot fibrillate. We determined the atomic structures for four such oligomers, and all four reveal tetrameric interfaces in which β-sheet dimers pair together by highly complementary, dry interfaces, analogous to steric zippers found in fibers, suggesting a common structure for amyloid oligomers and fibers. In amyloid fibers, the axes of the paired sheets are either parallel or antiparallel, whereas the oligomeric interfaces display a variety of sheet-to-sheet pairing angles, offering a structural explanation for the heterogeneity of amyloid oligomers.

Amyloid-β annular protofibrils evade fibrillar fate in Alzheimer disease brain

Annular protofibrils (APFs) represent a new and distinct class of amyloid structures formed by disease-associated proteins. In vitro, these pore-like structures have been implicated in membrane permeabilization and ion homeostasis via pore formation. Still, evidence for their formation and relevance in vivo is lacking. Herein, we report that APFs are in a distinct pathway from fibril formation in vitro and in vivo. In human Alzheimer disease brain samples, amyloid-β APFs were associated with diffuse plaques, but not compact plaques; moreover, we show the formation of intracellular APFs. Our results together with previous studies suggest that the prevention of amyloid-β annular protofibril formation could be a relevant target for the prevention of amyloid-β toxicity in Alzheimer disease.

Comparative studies of nontoxic and toxic amyloids interacting with membrane models at the air-water interface

Many in vitro studies have pointed out the interaction between amyloids and membranes, and their potential involvement in amyloid toxicity. In a previous study, we generated a yeast toxic mutant (M8) of the harmless model amyloid protein HET-s((218-289)). In this study, we compared the self-assembling process of the nontoxic wild-type (WT) and toxic (M8) protein at the air-water interface and in interaction with various phospholipid monolayers (DOPE, DOPC, DOPI, DOPS and DOPG). We first demonstrate using ellipsometry measurements and polarization-modulated infrared reflection absorption spectroscopy (PMIRRAS) that the air-water interface promotes and modifies the assembly of WT since an amyloid-like film was instantaneously formed at the interface with an antiparallel β-sheet structuration instead of the parallel β-sheet commonly observed for amyloid fibers generated in solution. The toxic mutant (M8) behaves in a similar manner at the air-water interface or in bulk, with a fast self-assembling and an antiparallel β-sheet organization. The transmission electron microscopy (TEM) images established the fibrillous morphology of the protein films formed at the air-water interface. Second, we demonstrate for the first time that the main driving force between this particular fungus amyloid and membrane interaction is based on electrostatic interactions with negatively charged phospholipids (DOPG, DOPI, DOPS). Interestingly, the toxic mutant (M8) clearly induces perturbations of the negatively charged phospholipid monolayers, leading to a massive surface aggregation, whereas the nontoxic (WT) exhibits a slight effect on the membrane models. This study allows concluding that the toxicity of the M8 mutant could be due to its high propensity to interact with membranes.

Membrane Incorporation, Channel Formation, and Disruption of Calcium Homeostasis by Alzheimer's β-Amyloid Protein

Oligomerization, conformational changes, and the consequent neurodegeneration of Alzheimer's β-amyloid protein (AβP) play crucial roles in the pathogenesis of Alzheimer's disease (AD). Mounting evidence suggests that oligomeric AβPs cause the disruption of calcium homeostasis, eventually leading to neuronal death. We have demonstrated that oligomeric AβPs directly incorporate into neuronal membranes, form cation-sensitive ion channels (“amyloid channels”), and cause the disruption of calcium homeostasis via the amyloid channels. Other disease-related amyloidogenic proteins, such as prion protein in prion diseases or α-synuclein in dementia with Lewy bodies, exhibit similarities in the incorporation into membranes and the formation of calcium-permeable channels. Here, based on our experimental results and those of numerous other studies, we review the current understanding of the direct binding of AβP into membrane surfaces and the formation of calcium-permeable channels. The implication of composition of membrane lipids and the possible development of new drugs by influencing membrane properties and attenuating amyloid channels for the treatment and prevention of AD is also discussed.

Intraneuronal amyloid β oligomers cause cell death via endoplasmic reticulum stress, endosomal/lysosomal leakage, and mitochondrial dysfunction in vivo

Intraneuronal accumulation of amyloid β (Aβ) is an early pathological change in Alzheimer's disease. Previously, we showed that the E693Δ mutation (referred to as the "Osaka" mutation) of amyloid precursor protein (APP) caused intracellular accumulation of Aβ oligomers and apoptosis in transfected COS-7 cells. We also showed that transgenic mice expressing APP(E693Δ) (APP(OSK) ) displayed both an age-dependent accumulation of intraneuronal Aβ oligomers from 8 months of age and apparent neuronal loss in the hippocampus at 24 months of age. These findings indicate that intraneuronal Aβ oligomers cause cell death, but the mechanism of this process remains unclear. Accordingly, here we investigated the subcellular localization and toxicity of intraneuronal Aβ oligomers in APP(OSK) -transgenic mice. We found Aβ oligomer accumulation in the endoplasmic reticulum (ER), endosomes/lysosomes, and mitochondria in hippocampal neurons of 22-month-old mice. We also detected up-regulation of Grp78 and HRD1 (an E3 ubiquitin ligase), leakage of cathepsin D from endosomes/lysosomes into cytoplasm, cytochrome c release from mitochondria, and activation of caspase-3 in the hippocampi of 18-month-old mice. Collectively, our findings suggest that intraneuronal Aβ oligomers cause cell death by inducing ER stress, endosomal/lysosomal leakage, and mitochondrial dysfunction in vivo. © 2011 Wiley-Liss, Inc.

Transformation of amyloid β(1-40) oligomers into fibrils is characterized by a major change in secondary structure

Alzheimer's disease (AD) is a neurodegenerative disorder occurring in the elderly. It is widely accepted that the amyloid beta peptide (Aβ) aggregation and especially the oligomeric states rather than fibrils are involved in AD onset. We used infrared spectroscopy to provide structural information on the entire aggregation pathway of Aβ(1-40), starting from monomeric Aβ to the end of the process, fibrils. Our structural study suggests that conversion of oligomers into fibrils results from a transition from antiparallel to parallel β-sheet. These structural changes are described in terms of H-bonding rupture/formation, β-strands reorientation and β-sheet elongation. As antiparallel β-sheet structure is also observed for other amyloidogenic proteins forming oligomers, reorganization of the β-sheet implicating a reorientation of β-strands could be a generic mechanism determining the kinetics of protein misfolding. Elucidation of the process driving aggregation, including structural transitions, could be essential in a search for therapies inhibiting aggregation or disrupting aggregates.

Dissociation of β-amyloid from lipoprotein in cerebrospinal fluid from Alzheimer's disease accelerates β-amyloid-42 assembly

Monoclonal 2C3 specific to β-amyloid (Aβ) oligomers (AβOs) enabled us to test our hypothesis that the alteration of lipoprotein-Aβ interaction in the central nervous system (CNS) initiates and/or accelerates the cascade favoring Aβ assembly. Immunoprecipitation of frontal cortex employing 2C3 unequivocally detected soluble 4-, 8-, and 12-mers in Alzheimer's disease (AD) brains. Immunoblot analysis of the entorhinal cortex employing 2C3 revealed that the accumulation of soluble 12-mers precedes the appearance of neuronal loss or cognitive impairment and is enhanced as the Braak neurofibrially tangle (NFT) stages progress. The dissociation of soluble Aβ from lipoprotein particles occurs in cerebrospinal fluid (CSF), and the presence of lipoprotein-free oligomeric 2C3 conformers (4- to 35-mers) was evident, which mimic CNS environments. Such CNS environments may strongly affect conformation of soluble Aβ peptides, resulting in the conversion of soluble Aβ(42) monomers into soluble Aβ(42) assembly. The findings suggest that functionally declined lipoproteins may accelerate the generation of metabolic conditions leading to higher levels of soluble Aβ(42) assembly in the CNS.

Molecular dynamic simulation of wild type and mutants of the polymorphic amyloid NNQNTF segments of elk prion: structural stability and thermodynamic of association

A hexapeptide with amino acid sequence NNQNTF from the elk prion protein forms amyloid fibrils. Here we use molecular dynamic simulations of the oligomers and their single point glycine mutants to study their stability. In an effort to probe the structural stability and association thermodynamic in a realistic environment, all wildtype of NNQNTF polymorphic forms with different size and their corresponding double layer 5 strands single point glycine mutants were subjected to a total of 500 ns of explicit-solvent molecular dynamics (MD) simulation. Our results show that the structural stability of the NNQNTF oligomers increases with increasing the number of β-strands for double layers. Our results also demonstrated that hydrophobic interaction is the principle driving force to stabilize the adjacent β-strands while the steric zipper is responsible for holding the neighboring β-sheet layers together. We used MM-PBSA approach free energy calculations to determine the role of nonpolar effects, electrostatics and entropy in binding. Nonpolar effects remained consistently more favorable in wild type and mutants reinforcing the importance of hydrophobic effects in protein-protein binding. While entropy systematically opposed binding in all cases, there was no observed trend in the entropy difference between wildtype and glycine mutant. Free energy decomposition shows residues situated at the interface were found to make favorable contributions to the peptide-peptide association. The study of the wild type and mutants in an explicit solvent may provide valuable insight for amyloid aggregation inhibitor design efforts.

Amyloid oligomer neurotoxicity, calcium dysregulation, and lipid rafts

Amyloid proteins constitute a chemically heterogeneous group of proteins, which share some biophysical and biological characteristics, the principal of which are the high propensity to acquire an incorrect folding and the tendency to aggregate. A number of diseases are associated with misfolding and aggregation of proteins, although only in some of them—most notably Alzheimer's disease (AD) and transmissible spongiform encephalopathies (TSEs)—a pathogenetic link with misfolded proteins is now widely recognized. Lipid rafts (LRs) have been involved in the pathophysiology of diseases associated with protein misfolding at several levels, including aggregation of misfolded proteins, amyloidogenic processing, and neurotoxicity. Among the pathogenic misfolded proteins, the AD-related protein amyloid β (Aβ) is by far the most studied protein, and a large body of evidence has been gathered on the role played by LRs in Aβ pathogenicity. However, significant amount of data has also been collected for several other amyloid proteins, so that their ability to interact with LRs can be considered an additional, shared feature characterizing the amyloid protein family. In this paper, we will review the evidence on the role of LRs in the neurotoxicity of huntingtin, α-synuclein, prion protein, and calcitonin.

Amyloidogenic protein-membrane interactions: mechanistic insight from model systems

The toxicity of amyloid-forming proteins is correlated with their interactions with cell membranes. Binding events between amyloidogenic proteins and membranes result in mutually disruptive structural perturbations, which are associated with toxicity. Membrane surfaces promote the conversion of amyloid-forming proteins into toxic aggregates, and amyloidogenic proteins, in turn, compromise the structural integrity of the cell membrane. Recent studies with artificial model membranes have highlighted the striking resemblance of the mechanisms of membrane permeabilization of amyloid-forming proteins to those of pore-forming toxins and antimicrobial peptides.

Distinct effects of Zn2+, Cu2+, Fe3+, and Al3+ on amyloid-beta stability, oligomerization, and aggregation: amyloid-beta destabilization promotes annular protofibril formation

Abnormally high concentrations of Zn(2+), Cu(2+), and Fe(3+) are present along with amyloid-β (Aβ) in the senile plaques in Alzheimer disease, where Al(3+) is also detected. Aβ aggregation is the key pathogenic event in Alzheimer disease, where Aβ oligomers are the major culprits. The fundamental mechanism of these metal ions on Aβ remains elusive. Here, we employ 4,4'-Bis(1-anilinonaphthalene 8-sulfonate) and tyrosine fluorescence, CD, stopped flow fluorescence, guanidine hydrochloride denaturation, and photo-induced cross-linking to elucidate the effect of Zn(2+), Cu(2+), Fe(3+), and Al(3+) on Aβ at the early stage of the aggregation. Furthermore, thioflavin T assay, dot blotting, and transmission electron microscopy are utilized to examine Aβ aggregation. Our results show that Al(3+) and Zn(2+), but not Cu(2+) and Fe(3+), induce larger hydrophobic exposures of Aβ conformation, resulting in its significant destabilization at the early stage. The metal ion binding induces Aβ conformational changes with micromolar binding affinities and millisecond binding kinetics. Cu(2+) and Zn(2+) induce similar assembly of transiently appearing Aβ oligomers at the early state. During the aggregation, we found that Zn(2+) exclusively promotes the annular protofibril formation without undergoing a nucleation process, whereas Cu(2+) and Fe(3+) inhibit fibril formation by prolonging the nucleation phases. Al(3+) also inhibits fibril formation; however, the annular oligomers co-exist in the aggregation pathway. In conclusion, Zn(2+), Cu(2+), Fe(3+), and Al(3+) adopt distinct folding and aggregation mechanisms to affect Aβ, where Aβ destabilization promotes annular protofibril formation. Our study facilitates the understanding of annular Aβ oligomer formation upon metal ion binding.

Measurement of the attachment and assembly of small amyloid-β oligomers on live cell membranes at physiological concentrations using single-molecule tools

It is thought that the pathological cascade in Alzheimer's disease is initiated by the formation of amyloid-β (Aβ) peptide complexes on cell membranes. However, there is considerable debate about the nature of these complexes and the type of solution-phase Aβ aggregates that may contribute to their formation. Also, it is yet to be shown that Aβ attaches strongly to living cell membranes, and that this can happen at low, physiologically relevant Aβ concentrations. Here, we simultaneously measure the aggregate size and fluorescence lifetime of fluorescently labeled Aβ(1-40) on and above the membrane of cultured PC12 cells at near-physiological concentrations. We find that at 350 nM Aβ concentration, large (>>10 nm average hydrodynamic radius) assemblies of codiffusing, membrane-attached Aβ molecules appear on the cell membrane together with a near-monomeric species. When the extracellular concentration is 150 nM, the membrane contains only the smaller species, but with a similar degree of attachment. At both concentrations, the extracellular solution contains only small (∼2.3 nm average hydrodynamic radius) Aβ oligomers or monomers. We conclude that at near-physiological concentrations only the small oligomeric Aβ species are relevant, they are capable of attaching to the cell membrane, and they assemble in situ to form much larger complexes.

Lipid Rafts: Linking Alzheimer's Amyloid-β Production, Aggregation, and Toxicity at Neuronal Membranes

Lipid rafts are membrane microdomains, enriched in cholesterol and sphingolipids, into which specific subsets of proteins and lipids partition, creating cell-signalling platforms that are vital for neuronal functions. Lipid rafts play at least three crucial roles in Alzheimer's Disease (AD), namely, in promoting the generation of the amyloid-β (Aβ) peptide, facilitating its aggregation upon neuronal membranes to form toxic oligomers and hosting specific neuronal receptors through which the AD-related neurotoxicity and memory impairments of the Aβ oligomers are transduced. Recent evidence suggests that Aβ oligomers may exert their deleterious effects through binding to, and causing the aberrant clustering of, lipid raft proteins including the cellular prion protein and glutamate receptors. The formation of these pathogenic lipid raft-based platforms may be critical for the toxic signalling mechanisms that underlie synaptic dysfunction and neuropathology in AD.

Can molecular dynamics simulations assist in design of specific inhibitors and imaging agents of amyloid aggregation? Structure, stability and free energy predictions for amyloid oligomers of VQIVYK, MVGGVV and LYQLEN

The aggregation modes of hexapeptide fragments of Tau, Insulin and Aβ peptide (VQIVYK, MVGGVV and LYQLEN) were found from their microcrystalline structures that had been recently resolved by X-ray analysis. The atomic structures reveal a dry self-complementary interface between the neighboring β-sheet layers, termed "steric zipper". In this study we perform several all-atom molecular dynamics simulations with explicit water to analyze stability of the crystalline fragments of 2-10 hexapeptides each and their analogs with single glycine replacement mutations to investigate the structural stability, aggregation behavior and thermodynamic of the amyloid oligomers. Upon comparing single and double layer models, our results reveal that additional strands contribute significantly to the structural stability of the peptide oligomers for double layer model, while in the case of single layer model the stability decreases (or remains the same in the case of LYQLEN). This is in agreement with the previous studies performed on different types of amyloid models. We also replaced the side-chains participating in the steric zipper interfaces with glycine. None of the mutants were structurally stable compared to the respective wild type model, except for mutants V2G and V6G in MVGGVV2 case. The exception can be explained by structural features of this particular polymorph. The double layer decamer and dodecamer aggregates of the wild type hexapeptides appear to be stable at 300K, which is confirmed by the conservation of high anti-parallel β-sheet content throughout the whole simulation time. Deletions of the side chains resulted in decline of secondary structure content compared to corresponding wild type indicating that the role of the replaced amino acid in stabilizing the structure. Detailed analysis of the binding energy reveals that stability of these peptide aggregates is determined mainly by the van der Waals and hydrophobic forces that can serve as quantitative measure of shape complementarities between the side chains. This observation implies that interactions among side chains forming the dehydrated steric zipper, rather than among those exposed to water, are the major structural determinant. The electrostatic repulsion destabilizes the studied double layer aggregates in two cases, while stabilizes the other two. Negative total binding free energy indicates that both wild type and mutants complex formation is favorable. However, the mutants complexation is less favorable than the wild type's. The present study provides the atomic level understanding of the aggregation behavior and the driving force for the amyloid aggregates, and could be useful for rational design of amyloid inhibitors and amyloid-specific biomarkers for diagnostic purposes.

β-Barrel topology of Alzheimer's β-amyloid ion channels

Emerging evidence supports the ion channel mechanism for Alzheimer's disease pathophysiology wherein small β-amyloid (Aβ) oligomers insert into the cell membrane, forming toxic ion channels and destabilizing the cellular ionic homeostasis. Solid-state NMR-based data of amyloid oligomers in solution indicate that they consist of a double-layered β-sheets where each monomer folds into β-strand-turn-β-strand and the monomers are stacked atop each other. In the membrane, Aβ peptides are proposed to be β-type structures. Experimental structural data available from atomic force microscopy (AFM) imaging of Aβ oligomers in membranes reveal heterogeneous channel morphologies. Previously, we modeled the channels in a non-tilted organization, parallel with the cross-membrane normal. Here, we modeled a β-barrel-like organization. β-Barrels are common in transmembrane toxin pores, typically consisting of a monomeric chain forming a pore, organized in a single-layered β-sheet with antiparallel β-strands and a right-handed twist. Our explicit solvent molecular dynamics simulations of a range of channel sizes and polymorphic turns and comparisons of these with AFM image dimensions support a β-barrel channel organization. Different from the transmembrane β-barrels where the monomers are folded into a circular β-sheet with antiparallel β-strands stabilized by the connecting loops, these Aβ barrels consist of multimeric chains forming double β-sheets with parallel β-strands, where the strands of each monomer are connected by a turn. Although the Aβ barrels adopt the right-handed β-sheet twist, the barrels still break into heterogeneous, loosely attached subunits, in good agreement with AFM images and previous modeling. The subunits appear mobile, allowing unregulated, hence toxic, ion flux.

Abeta42 neurotoxicity is mediated by ongoing nucleated polymerization process rather than by discrete Abeta42 species

The identification of toxic Aβ species and/or the process of their formation is crucial for understanding the mechanism(s) of Aβ neurotoxicity in Alzheimer disease and also for the development of effective diagnostic and therapeutic interventions. To elucidate the structural basis of Aβ toxicity, we developed different procedures to isolate Aβ species of defined size and morphology distribution, and we investigated their toxicity in different cell lines and primary neurons. We observed that crude Aβ42 preparations, containing a monomeric and heterogeneous mixture of Aβ42 oligomers, were more toxic than purified monomeric, protofibrillar fractions, or fibrils. The toxicity of protofibrils was directly linked to their interactions with monomeric Aβ42 and strongly dependent on their ability to convert into amyloid fibrils. Subfractionation of protofibrils diminished their fibrillization and toxicity, whereas reintroduction of monomeric Aβ42 into purified protofibril fractions restored amyloid formation and enhanced their toxicity. Selective removal of monomeric Aβ42 from these preparations, using insulin-degrading enzyme, reversed the toxicity of Aβ42 protofibrils. Together, our findings demonstrate that Aβ42 toxicity is not linked to specific prefibrillar aggregate(s) but rather to the ability of these species to grow and undergo fibril formation, which depends on the presence of monomeric Aβ42. These findings contribute significantly to the understanding of amyloid formation and toxicity in Alzheimer disease, provide novel insight into mechanisms of Aβ protofibril toxicity, and important implications for designing anti-amyloid therapies.

Conformation dependent monoclonal antibodies distinguish different replicating strains or conformers of prefibrillar Aβ oligomers

Background

Age-related neurodegenerative diseases share a number of important pathological features, such as accumulation of misfolded proteins as amyloid oligomers and fibrils. Recent evidence suggests that soluble amyloid oligomers and not the insoluble amyloid fibrils may represent the primary pathological species of protein aggregates.

Results

We have produced several monoclonal antibodies that specifically recognize prefibrillar oligomers and do not recognize amyloid fibrils, monomer or natively folded proteins. Like the polyclonal antisera, the individual monoclonals recognize generic epitopes that do not depend on a specific linear amino acid sequence, but they display distinct preferences for different subsets of prefibrillar oligomers. Immunological analysis of a number of different prefibrillar Aβ oligomer preparations show that structural polymorphisms exist in Aβ prefibrillar oligomers that can be distinguished on the basis of their reactivity with monoclonal antibodies. Western blot analysis demonstrates that the conformers defined by the monoclonal antibodies have distinct size distributions, indicating that oligomer structure varies with size. The different conformational types of Aβ prefibrillar oligomers can serve as they serve as templates for monomer addition, indicating that they seed the conversion of Aβ monomer into more prefibrillar oligomers of the same type.

Conclusions

These results indicate that distinct structural variants or conformers of prefibrillar Aβ oligomers exist that are capable of seeding their own replication. These conformers may be analogous to different strains of prions.

A yeast toxic mutant of HET-s((218-289)) prion displays alternative intermediates of amyloidogenesis

Amyloids are thought to be involved in various types of neurodegenerative disorders. Several kinds of intermediates, differing in morphology, size, and toxicity, have been identified in the multistep amyloidogenesis process. However, the mechanisms explaining amyloid toxicity remain unclear. We previously generated a toxic mutant of the nontoxic HET-s((218-289)) amyloid in yeast. Here we report that toxic and nontoxic amyloids differ not only in their structures but also in their assembling process. We used multiple and complementary methods to investigate the intermediates formed by these two amyloids. With the methods used, no intermediates were observed for the nontoxic amyloid; however, under the same experimental conditions, the toxic mutant displayed visible oligomeric and fibrillar intermediates.

Beta-barrel models of soluble amyloid beta oligomers and annular protofibrils

Both soluble and membrane-bound prefibrillar assemblies of Abeta (Aβ) peptides have been associated with Alzheimer's disease (AD). The size and nature of these assemblies vary greatly and are affected by many factors. Here, we present models of soluble hexameric assemblies of Aβ42 and suggest how they can lead to larger assemblies and eventually to fibrils. The common element in most of these assemblies is a six-stranded β-barrel formed by the last third of Aβ42, which is composed of hydrophobic residues and glycines. The hydrophobic core β-barrels of the hexameric models are shielded from water by the N-terminus and central segments. These more hydrophilic segments were modeled to have either predominantly β or predominantly α secondary structure. Molecular dynamics simulations were performed to analyze stabilities of the models. The hexameric models were used as starting points from which larger soluble assemblies of 12 and 36 subunits were modeled. These models were developed to be consistent with numerous experimental results.

Aromatic small molecules remodel toxic soluble oligomers of amyloid beta through three independent pathways

In protein conformational disorders ranging from Alzheimer to Parkinson disease, proteins of unrelated sequence misfold into a similar array of aggregated conformers ranging from small oligomers to large amyloid fibrils. Substantial evidence suggests that small, prefibrillar oligomers are the most toxic species, yet to what extent they can be selectively targeted and remodeled into non-toxic conformers using small molecules is poorly understood. We have evaluated the conformational specificity and remodeling pathways of a diverse panel of aromatic small molecules against mature soluble oligomers of the Aβ42 peptide associated with Alzheimer disease. We find that small molecule antagonists can be grouped into three classes, which we herein define as Class I, II, and III molecules, based on the distinct pathways they utilize to remodel soluble oligomers into multiple conformers with reduced toxicity. Class I molecules remodel soluble oligomers into large, off-pathway aggregates that are non-toxic. Moreover, Class IA molecules also remodel amyloid fibrils into the same off-pathway structures, whereas Class IB molecules fail to remodel fibrils but accelerate aggregation of freshly disaggregated Aβ. In contrast, a Class II molecule converts soluble Aβ oligomers into fibrils, but is inactive against disaggregated and fibrillar Aβ. Class III molecules disassemble soluble oligomers (as well as fibrils) into low molecular weight species that are non-toxic. Strikingly, Aβ non-toxic oligomers (which are morphologically indistinguishable from toxic soluble oligomers) are significantly more resistant to being remodeled than Aβ soluble oligomers or amyloid fibrils. Our findings reveal that relatively subtle differences in small molecule structure encipher surprisingly large differences in the pathways they employ to remodel Aβ soluble oligomers and related aggregated conformers.

Structural determination of Abeta25-35 micelles by molecular dynamics simulations

Amyloid-beta (Abeta) peptides and other amyloidogenic proteins can form a wide range of soluble oligomers of varied morphologies at the early aggregation stage, and some of these oligomers are biologically relevant to the pathogenesis of Alzheimer's disease. Spherical micelle-like oligomers have been often observed for many different types of amyloids. Here, we report a hybrid computational approach to systematically construct, search, optimize, and rank soluble micelle-like Abeta25-35 structures with different side-chain packings at the atomic level. Simulations reveal for the first time, to our knowledge, that two Abeta micelles with antiparallel peptide organization and distinct surface hydrophobicity display high structural stability. Stable micelles experience a slow secondary structural transition from turn to alpha-helix. Energetic analysis coupled with computational mutagenesis reveals that van der Waals and solvation energies play a more pronounced role in stabilizing the micelles, whereas the electrostatic energies present a stable but minor energetic contribution to peptide assemblies. Modeled Abeta micelles with shapes and dimensions similar to those of experimentally derived spherical structures also provide detailed information about the roles of structural dynamics and transition in the formation of amyloid fibrils. The strong binding affinity of our micelles to antibodies implies that micelles may be a biologically relevant species.

Inhibition of mitochondrial fusion by α-synuclein is rescued by PINK1, Parkin and DJ-1

Aggregation of α-synuclein (αS) is involved in the pathogenesis of Parkinson's disease (PD) and a variety of related neurodegenerative disorders. The physiological function of αS is largely unknown. We demonstrate with in vitro vesicle fusion experiments that αS has an inhibitory function on membrane fusion. Upon increased expression in cultured cells and in Caenorhabditis elegans, αS binds to mitochondria and leads to mitochondrial fragmentation. In C. elegans age-dependent fragmentation of mitochondria is enhanced and shifted to an earlier time point upon expression of exogenous αS. In contrast, siRNA-mediated downregulation of αS results in elongated mitochondria in cell culture. αS can act independently of mitochondrial fusion and fission proteins in shifting the dynamic morphologic equilibrium of mitochondria towards reduced fusion. Upon cellular fusion, αS prevents fusion of differently labelled mitochondrial populations. Thus, αS inhibits fusion due to its unique membrane interaction. Finally, mitochondrial fragmentation induced by expression of αS is rescued by coexpression of PINK1, parkin or DJ-1 but not the PD-associated mutations PINK1 G309D and parkin Δ1-79 or by DJ-1 C106A.

Human cytomegalovirus UL97 kinase prevents the deposition of mutant protein aggregates in cellular models of Huntington's disease and ataxia

The presence of aggregates of abnormally expanded polyglutamine (polyQ)-containing proteins are a pathological hallmark of a number of neurodegenerative diseases including Huntington's disease (HD) and spinocerebellar ataxia-3 (SCA3). Previous studies in cellular, Drosophila, and mouse models of HD and SCA have shown that neurodegeneration can be prevented by manipulations that inhibit polyQ aggregation. We have shown that the UL97 kinase of the human cytomegalovirus (HCMV) prevents aggregation of the pp71 and pp65 viral tegument proteins. To explore whether UL97 may act as a general antiaggregation factor, we examined whether UL97 prevents aggregation of cellular non-polyQ and polyQ proteins. We report that UL97 prevents the deposition of aggregates of two non-polyQ proteins: a protein chimera (GFP170*) composed of the green fluorescent protein and a fragment of the Golgi Complex protein (GCP-170) and a chimera composed of the red fluorescent protein (RFP) fused to the Werner syndrome protein (WRN), a RecQ helicase and exonuclease involved in DNA repair. Furthermore, we show that UL97 inhibits aggregate deposition in cellular models of HD and SCA3. UL97 prevents the deposition of aggregates of the mutant huntingtin exon 1 containing 82 glutamine repeats (HttExon1-Q82) or full length ataxin-3 containing a 72 polyQ track (AT3-72Q). The kinase activity of UL97 appears critical, as the kinase-dead UL97 mutant (K335M) fails to prevent aggregate formation. We further show that UL97 disrupts nuclear PML bodies and decreases p53-mediated transcription. The universality of the antiaggregation effect of UL97 suggests that UL97 targets a key cellular factor that regulates cellular aggregation mechanisms. Our results identify UL97 as a novel means to modulate polyQ aggregation and suggest that UL97 can serve as a novel tool to probe the cellular mechanisms that contribute to the formation of aggregates in polyglutamine disorders.

Gut microbiota in health and disease

Gut microbiota is an assortment of microorganisms inhabiting the length and width of the mammalian gastrointestinal tract. The composition of this microbial community is host specific, evolving throughout an individual's lifetime and susceptible to both exogenous and endogenous modifications. Recent renewed interest in the structure and function of this "organ" has illuminated its central position in health and disease. The microbiota is intimately involved in numerous aspects of normal host physiology, from nutritional status to behavior and stress response. Additionally, they can be a central or a contributing cause of many diseases, affecting both near and far organ systems. The overall balance in the composition of the gut microbial community, as well as the presence or absence of key species capable of effecting specific responses, is important in ensuring homeostasis or lack thereof at the intestinal mucosa and beyond. The mechanisms through which microbiota exerts its beneficial or detrimental influences remain largely undefined, but include elaboration of signaling molecules and recognition of bacterial epitopes by both intestinal epithelial and mucosal immune cells. The advances in modeling and analysis of gut microbiota will further our knowledge of their role in health and disease, allowing customization of existing and future therapeutic and prophylactic modalities.

The culprit behind amyloid beta peptide related neurotoxicity in Alzheimer's disease: oligomer size or conformation?

Since the reformulation of the amyloid cascade hypothesis to focus on oligomeric aggregates of amyloid beta as the prime toxic species causing Alzheimer's disease, many researchers refocused on detecting a specific molecular assembly of defined size thatis the main trigger of Alzheimer's disease. The result has been the identification of a host of molecular assemblies containing from two up to a hundred molecules of the amyloid beta peptide, which were all found to impair memory formation in mice. This clearly demonstrates that size is insufficient to define toxicity and peptide conformation has to be taken into account. In this review we discuss the interplay between oligomer size and peptide conformation as the key determinants of the neurotoxicity of the amyloid beta peptide.

Amyloid-beta-induced neuronal dysfunction in Alzheimer's disease: from synapses toward neural networks

Alzheimer's disease is the most frequent neurodegenerative disorder and the most common cause of dementia in the elderly. Diverse lines of evidence suggest that amyloid-beta (Abeta) peptides have a causal role in its pathogenesis, but the underlying mechanisms remain uncertain. Here we discuss recent evidence that Abeta may be part of a mechanism controlling synaptic activity, acting as a positive regulator presynaptically and a negative regulator postsynaptically. The pathological accumulation of oligomeric Abeta assemblies depresses excitatory transmission at the synaptic level, but also triggers aberrant patterns of neuronal circuit activity and epileptiform discharges at the network level. Abeta-induced dysfunction of inhibitory interneurons likely increases synchrony among excitatory principal cells and contributes to the destabilization of neuronal networks. Strategies that block these Abeta effects may prevent cognitive decline in Alzheimer's disease. Potential obstacles and next steps toward this goal are discussed.

Preparation and characterization of toxic Abeta aggregates for structural and functional studies in Alzheimer's disease research

The amyloid cascade hypothesis, supported by strong evidence from genetics, pathology and studies using animal models, implicates amyloid-beta (Abeta) oligomerization and fibrillogenesis as central causative events in the pathogenesis of Alzheimer's disease (AD). Today, significant efforts in academia, biotechnology and the pharmaceutical industry are devoted to identifying the mechanisms by which the process of Abeta aggregation contributes to neurodegeneration in AD and to the identity of the toxic Abeta species. In this paper, we describe methods and detailed protocols for reproducibly preparing Abeta aggregates of defined size distribution and morphology, including monomers, protofibrils and fibrils, using size exclusion chromatography. In addition, we describe detailed biophysical procedures for elucidating the structural features, aggregation kinetics and toxic properties of the different Abeta aggregation states, with special emphasis on protofibrillar intermediates. The information provided by this approach allows for consistent correlation between the properties of the aggregates and their toxicity toward primary neurons and/or cell lines. A better understanding of the molecular and structural basis of Abeta aggregation and toxicity is crucial for the development of effective strategies aimed at prevention and/or treatment of AD. Furthermore, the identification of specific aggregation states, which correlate with neurodegeneration in AD, could lead to the development of diagnostic tools to detect and monitor disease progression. The procedures described can be performed in as little as 1 day, or may take longer, depending on the exact toxicity assays used.

Effects of Congo red on aβ(1-40) fibril formation process and morphology

Alzheimer's disease (AD), an age-related neurodegenerative disorder, is the most common form of dementia, and the seventh-leading cause of death in the United States. Current treatments offer only symptomatic relief; thus, there is a great need for new treatments with disease-modifying potential. One pathological hallmark of AD is so-called senile plaques, mainly made up of β-sheet-rich assemblies of 40- or 42-residue amyloid β-peptides (Aβ). Hence, inhibition of Aβ aggregation is actively explored as an option to prevent or treat AD. Congo red (CR) has been widely used as a model antiamyloid agent to prevent Aβ aggregation. Herein, we report detailed morphological studies on the effect of CR as an antiamyloid agent, by circular dichroism spectroscopy, photo-induced cross-linking reactions, and atomic force microscopy. We also demonstrate the effect of CR on a preaggregated sample of Aβ(1-40). Our result suggests that Aβ(1-40) follows a different path for aggregation in the presence of CR.

Molecular modeling of two distinct triangular oligomers in amyloid beta-protein

Amyloid-beta (Abeta) peptides exhibit many distinct structural morphology at the early aggregate stage, some of which are biological relevant to the pathogenesis of Alzheimer's disease (AD). Atomic-resolution structures of the early Abeta aggregates and their conformational changes in amyloid aggregation remain elusive. Here, we perform all-atom molecular modeling and dynamics simulations to obtain two stable triangular-like Abeta structures with the lowest packing energy, one corresponding to the Tycko's model (Paravastu, A.; Leapman, R.; Yau, W.; Tycko, R. Proc. Nat. Acad. Soc. U.S.A. 2008, 105, 18349-18354) (referred to C-WT model) and the other corresponding to computational model (N-WT model). Both models have the same 3-fold symmetry but distinct beta-sheet organizations in which three Abeta hexamers pack together via either C-terminal beta-strand residues or N-terminal beta-strand residues forming distinct hydrophobic cross section. Structural and energetic comparisons of two 3-fold Abeta oligomers, coupled with structural changes upon the mutations occurring at the interacting interfaces, reveal that although hydrophobic interactions are still dominant forces, electrostatic interactions are more favorable in the N-WT model due to the formation of more and stable intersheet salt bridges, while solvation energy is more favorable in the C-WT model due to more exposed hydrophilic residues to solvent. Both models display many common features similar to other amyloid oligomers and therefore are likely to be biologically relevant.

The transcellular spread of cytosolic amyloids, prions, and prionoids

Recent reports indicate that a growing number of intracellular proteins are not only prone to pathological aggregation but can also be released and "infect" neighboring cells. Therefore, many complex diseases may obey a simple model of propagation where the penetration of seeds into hosts determines spatial spread and disease progression. We term these proteins prionoids, as they appear to infect their neighbors just like prions--but how can bulky protein aggregates be released from cells and how do they access other cells? The widespread existence of such prionoids raises unexpected issues that question our understanding of basic cell biology.

The functional neurophysiology of the amyloid precursor protein (APP) processing pathway

Amyloid beta (Abeta) peptides derived from proteolytic cleavage of amyloid precursor protein (APP) are thought to be a pivotal toxic species in the pathogenesis of Alzheimer's disease (AD). Furthermore, evidence has been accumulating that components of APP processing pathway are involved in non-pathological normal function of the CNS. In this review we aim to cover the extensive body of research aimed at understanding how components of this pathway contribute to neurophysiological function of the CNS in health and disease. We briefly outline changes to clinical neurophysiology seen in AD patients before discussing functional changes in mouse models of AD which range from changes to basal synaptic transmission and synaptic plasticity through to abnormal synchronous network activity. We then describe the various neurophysiological actions that are produced by application of exogenous Abeta in various forms, and finally discuss a number or other neurophysiological aspects of the APP pathway, including functional activities of components of secretase complexes other than Abeta production.

Oligomeric structure of brain abundant proteins GAP-43 and BASP1

Brain abundant proteins GAP-43 and BASP1 participate in the regulation of actin cytoskeleton dynamics in neuronal axon terminals. The proposed mechanism suggests that the proteins sequester phosphatidylinositol-4,5-diphosphate (PIP(2)) in the inner leaflet of the plasma membrane. We found that model anionic phospholipid membranes in the form of liposomes induce rapid oligomerization of GAP-43 and BASP1 proteins. Multiply charged phosphoinositides produced the most potent effect. Anionic detergent sodium dodecyl sulfate (SDS) at submicellar concentration stimulated formation of similar oligomers in solution. BASP1, but not GAP-43, also formed oligomers at sufficiently high concentration in the absence of lipids and SDS. Electron microscopy study demonstrated that the oligomers have disk-shaped or annular structure of 10-30nm in diameter. BASP1 also formed higher aggregates of linear rod-like structure, with average length of about 100nm. In outward appearance, the oligomers and linear aggregates are reminiscent of oligomers and protofibrils of amyloid proteins. Both the synthetic N-terminal peptide GAP-43(1-40) and the brain-derived fragment GAP-43-3 preserved the ability to oligomerize under the action of acidic phospholipids and SDS. On the contrary, BASP1 fragment truncated by the short N-terminal myristoylated peptide was unable to form oligomers. GAP-43 and BASP1 oligomerization can be regulated by calmodulin, which disrupts the oligomers and displaces the proteins from the membrane. We suggest that in vivo, the role of membrane-bound GAP-43 and BASP1 oligomers consists in accumulation of PIP(2) in functional clusters, which become accessible for other PIP(2)-binding proteins after dissociation of the oligomers.