Human serum albumin inhibits Abeta fibrillization through a "monomer-competitor" mechanism

Modulation of linoleic acid-binding properties of human serum albumin by divalent metal cations

Human serum albumin (HSA) is an abundant multiligand carrier protein, linked to progression of Alzheimer's disease (AD). Blood HSA serves as a depot of amyloid β (Aβ) peptide. Aβ peptide-buffering properties of HSA depend on interaction with its ligands. Some of the ligands, namely, linoleic acid (LA), zinc and copper ions are involved into AD progression. To clarify the interplay between LA and metal ion binding to HSA, the dependence of LA binding to HSA on Zn²⁺, Cu²⁺, Mg²⁺ and Ca²⁺ levels and structural consequences of these interactions have been explored. Seven LA molecules are bound per HSA molecule in the absence of the metal ions. Zn²⁺ binding to HSA causes a loss of one bound LA molecule, while the other metals studied exert an opposite effect (1-2 extra LA molecules are bound). In most cases, the observed effects are not related to the metal-induced changes in HSA quaternary structure. However, the Zn²⁺-induced decline in LA capacity of HSA could be due to accumulation of multimeric HSA forms. Opposite to Ca²⁺/Mg²⁺-binding, Zn²⁺ or Cu²⁺ association with HSA induces marked changes in its hydrophobic surface. Overall, the divalent metal ions modulate LA capacity and affinity of HSA to a different extent. LA- and Ca²⁺-binding to HSA synergistically support each other. Zn²⁺ and Cu²⁺ induce more pronounced changes in hydrophobic surface and quaternary structure of HSA and its LA capacity. A misbalanced metabolism of these ions in AD could modify interactions of HSA with LA, other fatty acids and hydrophobic substances, associated with AD.

Inhibition of Insulin Amyloid Fibrillation by a Novel Amphipathic Heptapeptide: MECHANISTIC DETAILS STUDIED BY SPECTROSCOPY IN COMBINATION WITH MICROSCOPY

The aggregation of insulin into amyloid fibers has been a limiting factor in the development of fast acting insulin analogues, creating a demand for excipients that limit aggregation. Despite the potential demand, inhibitors specifically targeting insulin have been few in number. Here we report a non-toxic and serum stable-designed heptapeptide, KR7 (KPWWPRR-NH2), that differs significantly from the primarily hydrophobic sequences that have been previously used to interfere with insulin amyloid fibrillation. Thioflavin T fluorescence assays, circular dichroism spectroscopy, and one-dimensional proton NMR experiments suggest KR7 primarily targets the fiber elongation step with little effect on the early oligomerization steps in the lag time period. From confocal fluorescence and atomic force microscopy experiments, the net result appears to be the arrest of aggregation in an early, non-fibrillar aggregation stage. This mechanism is noticeably different from previous peptide-based inhibitors, which have primarily shifted the lag time with little effect on later stages of aggregation. As insulin is an important model system for understanding protein aggregation, the new peptide may be an important tool for understanding peptide-based inhibition of amyloid formation.

Investigation of the Biological Impact of Charge Distribution on a NTR1-Targeted Peptide

The neurotensin receptor 1 (NTR1) has been shown to be a promising target, due to its increased level of expression relative to normal tissue, for pancreatic and colon cancers. This has prompted the development of a variety of NTR1-targeted radiopharmaceuticals, based on the neurotensin (NT) peptide, for diagnostic and radiotherapeutic applications. A major obstacle for the clinical translation of NTR1-targeted radiotherapeutics would likely be nephrotoxicity due to the high levels of kidney retention. It is well-known that for many peptide-based agents, renal uptake is influenced by the overall molecular charge. Herein, we investigated the effect of charge distribution on receptor binding and kidney retention. Using the [(N-α-Me)Arg⁸,Dmt¹¹,Tle¹²]NT(6-13) targeting vector, three peptides (¹⁷⁷Lu-K2, ¹⁷⁷Lu-K4, and ¹⁷⁷Lu-K6), with the Lys moved closer (K6) or further away (K2) from the pharmacophore, were synthesized. In vitro competitive binding, internalization and efflux, and confocal microscopy studies were conducted using the NTR1-positive HT-29, human colon cancer cell line. The ^177/natLu-K6 demonstrated the highest binding affinity (21.8 ± 1.2 nM) and the highest level of internalization (4.06% ± 0.20% of the total added amount). In vivo biodistribution, autoradiography, and metabolic studies of ¹⁷⁷Lu-radiolabeled K2, K4, and K6 were examined using CF-1 mice. ¹⁷⁷Lu-K4 and ¹⁷⁷Lu-K6 gave the highest levels of in vivo uptake in NTR1-positive tissues, whereas ¹⁷⁷Lu-K2 yielded nearly 2-fold higher renal uptake relative to the other radioconjugates. In conclusion, the position of the Lys (positively charged amino acid) influences the receptor binding, internalization, in vivo NTR1-targeting efficacy, and kidney retention profile of the radioconjugates. In addition, we have found that hydrophobicity likely play a role in the unique biodistribution profiles of these agents.

Multifunctional Effect of Human Serum Albumin Reduces Alzheimer's Disease Related Pathologies in the 3xTg Mouse Model

Alzheimer's disease (AD), the prevalent dementia in the elderly, involves many related and interdependent pathologies that manifests simultaneously, eventually leading to cognitive impairment and death. No treatment is currently available; however, an agent addressing several key pathologies simultaneously has a better therapeutic potential. Human serum albumin (HSA) is a highly versatile protein, harboring multifunctional properties that are relevant to key pathologies underlying AD. This study provides insight into the mechanism for HSA's therapeutic effect. In vivo, a myriad of beneficial effects were observed by pumps infusing HSA intracerebroventricularly, for the first time in an AD 3xTg mice model. A significant effect on amyloid-β (Aβ) pathology was observed. Aβ1-42, soluble oligomers, and total plaque area were reduced. Neuroblastoma SHSY5Y cell line confirmed that the reduction in Aβ1-42 toxicity was due to direct binding rather than other properties of HSA. Total and hyperphosphorylated tau were reduced along with an increase in tubulin, suggesting increased microtubule stability. HSA treatment also reduced brain inflammation, affecting both astrocytes and microglia markers. Finally, evidence for blood-brain barrier and myelin integrity repair was observed. These multidimensional beneficial effects of intracranial administrated HSA, together or individually, contributed to an improvement in cognitive tests, suggesting a non-immune or Aβ efflux dependent means for treating AD.

Serum Levels of Albumin-β-Amyloid Complex in Patients with Depression

OBJECTIVE

Epidemiologic studies have demonstrated that suffering from depression may be a risk for Alzheimer disease (AD). As a possible biologic mechanism underlying the transition from depression to AD, it has been speculated that pathologic changes in β-amyloid (Aβ) metabolism are involved. To further understand the peripheral kinetics of amyloid in patients with depression, we investigated serum levels of free Aβ and albumin-bound Aβ.

METHODS

Seventy inpatients with DSM-IV major depressive disorder (MDD) and 81 healthy individuals (the comparison group) were recruited between June 2012 and February 2014. Serum Aβ40 and Aβ42 levels, Aβ40/Aβ42 ratio, and serum levels of albumin-Aβ complexes (SLAACs) were compared between the comparison group and patients in two age groups comprising younger (<60 years) and elderly (≥60 years) people.

RESULTS

SLAAC was decreased in older patients with MDD but not in younger patients. The serum-free Aβ40/Aβ42 ratio was higher in patients with depression, even in younger patients.

CONCLUSION

Our findings suggest that free Aβ and the albumin-bound Aβ reflect a different serum amyloid kinetics in depression. We speculate that serum-free Aβ reflects changes in amyloid metabolism in patients suffering from depression and albumin-bound Aβ reflects AD pathology and may be a potential predictor of the prodromal stage of AD.

HSA targets multiple Aβ42 species and inhibits the seeding-mediated aggregation and cytotoxicity of Aβ42 aggregates

HSA inhibits Aβ42 fibrillation and cytotoxicity through interfering with different stages of Aβ42 fibrillation and targeting different Aβ42 intermediate aggregates.

How Epigallocatechin-3-gallate and Tetracycline Interact with the Josephin Domain of Ataxin-3 and Alter Its Aggregation Mode

Epigallocatechin-3-gallate (EGCG) and tetracycline are two known inhibitors of amyloid aggregation able to counteract the fibrillation of most of the proteins involved in neurodegenerative diseases. We have recently investigated their effect on ataxin-3 (AT3), the polyglutamine-containing protein responsible for spinocerebellar ataxia type 3. We previously showed that EGCG and tetracycline can contrast the aggregation process and toxicity of expanded AT3, although by different mechanisms. Here, we have performed further experiments by using the sole Josephin domain (JD) to further elucidate the mechanism of action of the two compounds. By protein solubility assays and FTIR spectroscopy we have first observed that EGCG and tetracycline affect the JD aggregation essentially in the same way displayed when acting on the full-length expanded AT3. Then, by saturation transfer difference (STD) NMR experiments, we have shown that EGCG binds both the monomeric and the oligomeric JD form, whereas tetracycline can only interact with the oligomeric one. Surface plasmon resonance (SPR) analysis has confirmed the capability of the sole EGCG to bind monomeric JD, although with a KD value suggestive for a non-specific interaction. Our investigations provide new details on the JD interaction with EGCG and tetracycline, which could explain the different mechanisms by which the two compounds reduce the toxicity of AT3.

Inhibitory effect of human serum albumin on Cu-induced Aβ(40) aggregation and toxicity

It has been suggested that the aggregation and cytotoxicity of amyloid-β (Aβ) peptide with transition-metal ions in neuronal cells is involved in the development and progression of Alzheimer's disease (AD). As the most abundant protein in blood plasma and in cerebrospinal fluid, human serum albumin (HSA) can bind Aβ in vivo and subsequently inhibit Aβ fibril growth. However, the roles of albumin in Cu-induced Aβ aggregation and toxicity, and its potential biological relevance to AD therapy, were not stressed enough. Here, we showed that HSA was capable of binding Cu (I) with much higher affinity than Aβ, competitively inhibiting the interaction of Aβ and Cu ions. In the presence of biological reducing agent ascorbate, HSA inhibited Cu (II)/Cu (I)-mediated Aβ40 aggregation, reactive oxygen species production, and neurotoxicity. However, in the absence of Cu (II)/Cu (I), HSA could not effectively inhibit Aβ40 aggregation and neurotoxicity at 24 h (or less) incubation time, but decreased Aβ40 aggregation at much longer incubation (120 h). Our data suggested that through competitively decreasing Cu-Aβ interaction, HSA could effectively inhibit Cu (II)/Cu (I)-induced Aβ40 aggregation and neurotoxicity, and play important roles in regulating redox balance as well as metal homeostasis in AD prevention and therapy.

Fluorescent filter-trap assay for amyloid fibril formation kinetics in complex solutions

Amyloid fibrils are the most distinct components of the plaques associated with various neurodegenerative diseases. Kinetic studies of amyloid fibril formation shed light on the microscopic mechanisms that underlie this process as well as the contributions of internal and external factors to the interplay between different mechanistic steps. Thioflavin T is a widely used noncovalent fluorescent probe for monitoring amyloid fibril formation; however, it may suffer from limitations due to the unspecific interactions between the dye and the additives. Here, we present the results of a filter-trap assay combined with the detection of fluorescently labeled amyloid β (Aβ) peptide. The filter-trap assay separates formed aggregates based on size, and the fluorescent label attached to Aβ allows for their detection. The times of half completion of the process (t1/2) obtained by the filter-trap assay are comparable to values from the ThT assay. High concentrations of human serum albumin (HSA) and carboxyl-modified polystyrene nanoparticles lead to an elevated ThT signal, masking a possible fibril formation event. The filter-trap assay allows fibril formation to be studied in the presence of those substances and shows that Aβ fibril formation is kinetically inhibited by HSA and that the amount of fibrils formed are reduced. In contrast, nanoparticles exhibit a dual-behavior governed by their concentration.

Insulin-degrading enzyme prevents α-synuclein fibril formation in a nonproteolytical manner

The insulin-degrading enzyme (IDE) degrades amyloidogenic proteins such as Amyloid β (Αβ) and Islet Amyloid Polypeptide (IAPP), i.e. peptides associated with Alzheimer’s disease and type 2 diabetes, respectively. In addition to the protease activity normally associated with IDE function an additional activity involving the formation of stable, irreversible complexes with both Αβ and α-synuclein, an amyloidogenic protein involved in Parkinson’s disease, was recently proposed. Here, we have investigated the functional consequences of IDE-α-synuclein interactions in vitro. We demonstrate that IDE in a nonproteolytic manner and at sub-stoichiometric ratios efficiently inhibits α-synuclein fibril formation by binding to α-synuclein oligomers making them inert to amyloid formation. Moreover, we show that, within a defined range of α-synuclein concentrations, interaction with α-synuclein oligomers increases IDE’s proteolytic activity on a fluorogenic substrate. We propose that the outcomes of IDE-α-synuclein interactions, i.e. protection against α-synuclein amyloid formation and stimulated IDE protease activity, may be protective in vivo.

Amyloid-β sequester proteins as blood-based biomarkers of cognitive decline

Introduction

There are no blood-based biomarkers for cognitive decline in aging, or mild cognitive impairment (MCI) and Alzheimer's disease (AD). Cumulative evidence suggests that apolipoproteins, complement system, and transthyretin are involved in AD pathogenesis by sequestration of amyloid β. However, there is no clinical study to assess the utility of “sequester proteins” in risk assessment and/or diagnosis of MCI and AD.

Methods

Serum levels of sequester proteins and their clinical potential in cognitive decline assessment were analyzed by longitudinal and cross-sectional studies using independent cohorts and were confirmed by a prospective study.

Results

A combination of apolipoprotein A1, complement C3, and transthyretin achieved an area under the curve of 0.89 (sensitivity 91% and specificity 80%) in MCI versus healthy controls and also discriminated individuals with mild cognitive decline from healthy controls.

Discussion

A set of sequester proteins could be blood-based biomarkers for assessment of early stages of cognitive decline.

Structure based aggregation studies reveal the presence of helix-rich intermediate during α-Synuclein aggregation

Mechanistic understanding of nucleation dependent polymerization by α-synuclein (α-Syn) into toxic oligomers and amyloids is important for the drug development against Parkinson's disease. However the structural and morphological characterization during nucleation and subsequent fibrillation process of α-Syn is not clearly understood. Using a variety of complementary biophysical techniques monitoring entire pathway of nine different synucleins, we found that transition of unstructured conformation into β-sheet rich fibril formation involves helix-rich intermediates. These intermediates are common for all aggregating synucleins, contain high solvent-exposed hydrophobic surfaces, are cytotoxic to SHSY-5Y cells and accelerate α-Syn aggregation efficiently. A multidimensional NMR study characterizing the intermediate accompanied with site-specific fluorescence study suggests that the N-terminal and central portions mainly participate in the helix-rich intermediate formation while the C-terminus remained in an extended conformation. However, significant conformational transitions occur at the middle and at the C-terminus during helix to β-sheet transition as evident from Trp fluorescence study. Since partial helix-rich intermediates were also observed for other amyloidogenic proteins such as Aβ and IAPP, we hypothesize that this class of intermediates may be one of the important intermediates for amyloid formation pathway by many natively unstructured protein/peptides and represent a potential target for drug development against amyloid diseases.

Molecular basis for nonspecificity of nonsteroidal anti-inflammatory drugs (NSAIDs)

Inhibition of the production of inflammatory mediators by the action of nonsteroidal anti-inflammatory drugs (NSAIDs) is highly accredited to their recognition of cyclooxygenase enzymes. Along with inflammation relief, however, NSAIDs also cause adverse effects. Although NSAIDs strongly inhibit enzymes of the prostaglandin synthesis pathways, several other proteins also serve as fairly potent targets for these drugs. Based on their recognition pattern, these receptors are categorised as enzymes modifying NSAIDs, noncatalytic proteins binding to NSAIDs and enzymes with catalytic functions that are inhibited by NSAIDs. The extensive binding of NSAIDs is responsible for their limited in vivo efficacy as well as the large spectrum of their effects. The biochemical nature of drugs binding to multiple protein targets and its implications on physiology are discussed.

A Microtus fortis protein, serum albumin, is a novel inhibitor of Schistosoma japonicum schistosomula

Schistosomiasis is an endemic parasite disease and praziquantel is the only drug currently in use to control this disease. Experimental and epidemiological evidence strongly suggests that Microtus fortis ( Mf ) is a naturally resistant vertebrate host of Schistosoma japonicum . In the present study, we found that Mf serum albumin ( Mf -albumin) and the conditioned medium of pcDNA3.1- Mf -albumin caused 46.2% and 38.7% schistosomula death rates in 96 h, respectively, which were significantly higher than that of the negative control (p < 0.05). We also found that mice injected with Mf -albumin had a 43.5% reduction in worm burden and a 48.1% reduction in liver eggs per gram (p < 0.05) in comparison to the control animals. To characterise the mechanisms involved in clearance, schistosomula were incubated with fluorescein isothiocyanate-labelled Mf -albumin and fluorescent enrichment effects were found in the gut lumen of schistosomula after 48 h of incubation. Next, digestive tract excretions from schistosomula were collected and the sensitivity of Mf -albumin to digestive tract excretions was evaluated. The results indicated that schistosomula digestive tract excretions showed indigestibility of Mf -albumin. The death of schistosomula could be partially attributed to the lack of digestion of Mf -albumin by digestive tract excretions during the development of the schistosomula stage. Therefore, these data indicate the potential of Mf -albumin as one of the major selective forces for schistosomiasis.

Probing the sources of the apparent irreproducibility of amyloid formation: drastic changes in kinetics and a switch in mechanism due to micellelike oligomer formation at critical concentrations of IAPP

The aggregation of amyloidogenic proteins is infamous for being highly chaotic, with small variations in conditions sometimes leading to large changes in aggregation rates. Using the amyloidogenic protein IAPP (islet amyloid polypeptide protein, also known as amylin) as an example, we show that a part of this phenomenon may be related to the formation of micellelike oligomers at specific critical concentrations and temperatures. We show that pyrene fluorescence can sensitively detect micellelike oligomer formation by IAPP and discriminate between micellelike oligomers from fibers and monomers, making pyrene one of the few chemical probes specific to a prefibrillar oligomer. We further show that oligomers of this type reversibly form at critical concentrations in the low micromolar range and at specific critical temperatures. Micellelike oligomer formation has several consequences for amyloid formation by IAPP. First, the kinetics of fiber formation increase substantially as the critical concentration is approached but are nearly independent of concentration below it, suggesting a direct role for the oligomers in fiber formation. Second, the critical concentration is strongly correlated with the propensity to form amyloid: higher critical concentrations are observed for both IAPP variants with lower amyloidogenicity and for native IAPP at acidic pH in which aggregation is greatly slowed. Furthermore, using the DEST NMR technique, we show that the pathway of amyloid formation switches as the critical point is approached, with self-interactions primarily near the N-terminus below the critical temperature and near the central region above the critical temperature, reconciling two apparently conflicting views of the initiation of IAPP aggregation.

Peptide amyloid surface display

Homomeric self-assembly of peptides into amyloid fibers is a feature of many diseases. A central role has been suggested for the lateral fiber surface affecting gains of toxic function. To investigate this, a protein scaffold that presents a discrete, parallel β-sheet surface for amyloid subdomains up to eight residues in length has been designed. Scaffolds that present the fiber surface of islet amyloid polypeptide (IAPP) were prepared. The designs show sequence-specific surface effects apparent in that they gain the capacity to attenuate rates of IAPP self-assembly in solution and affect IAPP-induced toxicity in insulin-secreting cells.

Unfolded protein response-induced ERdj3 secretion links ER stress to extracellular proteostasis

The Unfolded Protein Response (UPR) indirectly regulates extracellular proteostasis through transcriptional remodeling of endoplasmic reticulum (ER) proteostasis pathways. This remodeling attenuates secretion of misfolded, aggregation-prone proteins during ER stress. Through these activities, the UPR has a critical role in preventing the extracellular protein aggregation associated with numerous human diseases. Here, we demonstrate that UPR activation also directly influences extracellular proteostasis through the upregulation and secretion of the ER HSP40 ERdj3/DNAJB11. Secreted ERdj3 binds misfolded proteins in the extracellular space, substoichiometrically inhibits protein aggregation, and attenuates proteotoxicity of disease-associated toxic prion protein. Moreover, ERdj3 can co-secrete with destabilized, aggregation-prone proteins in a stable complex under conditions where ER chaperoning capacity is overwhelmed, preemptively providing extracellular chaperoning of proteotoxic misfolded proteins that evade ER quality control. This regulated co-secretion of ERdj3 with misfolded clients directly links ER and extracellular proteostasis during conditions of ER stress. ERdj3 is, to our knowledge, the first metazoan chaperone whose secretion into the extracellular space is regulated by the UPR, revealing a new mechanism by which UPR activation regulates extracellular proteostasis.

Determination of supplier-to-supplier and lot-to-lot variability in glycation of recombinant human serum albumin expressed in Oryza sativa

The use of different expression systems to produce the same recombinant human protein can result in expression-dependent chemical modifications (CMs) leading to variability of structure, stability and immunogenicity. Of particular interest are recombinant human proteins expressed in plant-based systems, which have shown particularly high CM variability. In studies presented here, recombinant human serum albumins (rHSA) produced in Oryza sativa (Asian rice) (OsrHSA) from a number of suppliers have been extensively characterized and compared to plasma-derived HSA (pHSA) and rHSA expressed in yeast (Pichia pastoris and Saccharomyces cerevisiae). The heterogeneity of each sample was evaluated using size exclusion chromatography (SEC), reversed-phase high-performance liquid chromatography (RP-HPLC) and capillary electrophoresis (CE). Modifications of the samples were identified by liquid chromatography-mass spectrometry (LC-MS). The secondary and tertiary structure of the albumin samples were assessed with far U/V circular dichroism spectropolarimetry (far U/V CD) and fluorescence spectroscopy, respectively. Far U/V CD and fluorescence analyses were also used to assess thermal stability and drug binding. High molecular weight aggregates in OsrHSA samples were detected with SEC and supplier-to-supplier variability and, more critically, lot-to-lot variability in one manufactures supplied products were identified. LC-MS analysis identified a greater number of hexose-glycated arginine and lysine residues on OsrHSA compared to pHSA or rHSA expressed in yeast. This analysis also showed supplier-to-supplier and lot-to-lot variability in the degree of glycation at specific lysine and arginine residues for OsrHSA. Both the number of glycated residues and the degree of glycation correlated positively with the quantity of non-monomeric species and the chromatographic profiles of the samples. Tertiary structural changes were observed for most OsrHSA samples which correlated well with the degree of arginine/lysine glycation. The extensive glycation of OsrHSA from multiple suppliers may have further implications for the use of OsrHSA as a therapeutic product.

Insight into the inhibition effect of acidulated serum albumin on amyloid β-protein fibrillogenesis and cytotoxicity

Alzheimer's disease (AD) is the most prevalent form of dementia, and aggregation of amyloid β-proteins (Aβ) into soluble oligomers and fibrils has been implicated in the pathogenesis of AD. Herein we developed acidulated serum albumin for the inhibition of Aβ42 fibrillogenesis. Bovine serum albumin (BSA) was modified with diglycolic anhydride, leading to the coupling of 14.5 more negative charges (carboxyl groups) on average on each protein surface. The acidulated BSA (A-BSA) was characterized and confirmed to keep the tertiary structure and stability of BSA. Extensive biophysical and biological analyses showed that A-BSA significantly inhibited Aβ42 fibrillogenesis and mitigated amyloid cytotoxicity. As compared to the Aβ42-treated group (cell viability, 50%), the cell viability increased to 88% by the addition of equimolar A-BSA. The inhibitory effect was remarkably higher than that of BSA at the same concentration. On the basis of the experimental findings, a mechanistic model was proposed. The model considers that Aβ42 is bound to the A-BSA surface by hydrophobic interactions, but the widely distributed negative charges on the A-BSA surface give rise to electrostatic repulsions to the bound Aβ42 that is also negatively charged. The two well-balanced opposite forces make Aβ42 adopt extended conformations instead of the β-sheet structure that is necessary for the on-pathway fibrillogenesis, even when the protein is released off the surface. Thus, A-BSA greatly slows down the fibrillation and changes the fibrillogenesis pathway, leading to the formation of less toxic aggregates. The findings and the mechanistic model offer new insights into the development of more potent inhibitors of Aβ fibrillogenesis and cytotoxicity.

Non-chaperone proteins can inhibit aggregation and cytotoxicity of Alzheimer amyloid β peptide

Many factors are known to influence the oligomerization, fibrillation, and amyloid formation of the Aβ peptide that is associated with Alzheimer disease. Other proteins that are present when Aβ peptides deposit in vivo are likely to have an effect on these aggregation processes. To separate specific versus broad spectrum effects of proteins on Aβ aggregation, we tested a series of proteins not reported to have chaperone activity: catalase, pyruvate kinase, albumin, lysozyme, α-lactalbumin, and β-lactoglobulin. All tested proteins suppressed the fibrillation of Alzheimer Aβ(1-40) peptide at substoichiometric ratios, albeit some more effectively than others. All proteins bound non-specifically to Aβ, stabilized its random coils, and reduced its cytotoxicity. Surprisingly, pyruvate kinase and catalase were at least as effective as known chaperones in inhibiting Aβ aggregation. We propose general mechanisms for the broad-spectrum inhibition Aβ fibrillation by proteins. The mechanisms we discuss are significant for prognostics and perhaps even for prevention and treatment of Alzheimer disease.

Mapping the interactions between the Alzheimer's Aβ-peptide and human serum albumin beyond domain resolution

Human serum albumin (HSA) is a potent inhibitor of Aβ self-association and this novel, to our knowledge, function of HSA is of potential therapeutic interest for the treatment of Alzheimer's disease. It is known that HSA interacts with Aβ oligomers through binding sites evenly partitioned across the three albumin domains and with comparable affinities. However, as of this writing, no information is available on the HSA-Aβ interactions beyond domain resolution. Here, we map the HSA-Aβ interactions at subdomain and peptide resolution. We show that each separate subdomain of HSA domain 3 inhibits Aβ self-association. We also show that fatty acids (FAs) compete with Aβ oligomers for binding to domain 3, but the determinant of the HSA/Aβ oligomer interactions are markedly distinct from those of FAs. Although salt bridges with the FA carboxylate determine the FA binding affinities, hydrophobic contacts are pivotal for Aβ oligomer recognition. Specifically, we identified a site of Aβ oligomer recognition that spans the HSA (494-515) region and aligns with the central hydrophobic core of Aβ. The HSA (495-515) segment includes residues affected by FA binding and this segment is prone to self-associate into β-amyloids, suggesting that sites involved in fibrilization may provide a lead to develop inhibitors of Aβ self-association.

Aβ association inhibition by transferrin

The iron-transport glycoprotein transferrin has recently been shown to serve as a potent inhibitor of Aβ self-association. Although this novel, to our knowledge, inhibitory function of transferrin is of potential therapeutic interest for the treatment of Alzheimer's disease, the underlying mechanism is still not fully understood. Although it has been shown that the Fe(III) sequestration by transferrin reduces oxidative damage and Aβ aggregation, it is not clear whether transferrin is also able to inhibit Aβ self-association through direct binding of Aβ. Here, using saturation transfer and off-resonance relaxation NMR spectroscopy, we show that transferrin inhibits Aβ aggregation also by preferentially binding Aβ oligomers and outcompeting Aβ monomers that would otherwise cause the growth of the Aβ oligomers into larger assemblies. This inhibitory mechanism is different from the iron-sequestration model, but it is qualitatively similar to a mechanism previously proposed for the inhibition of Aβ self-association by another plasma and cerebrospinal fluid protein, i.e., human serum albumin. These results suggest that Aβ monomer competition through direct Aβ oligomer binding might be a general strategy adopted by proteins in plasma and cerebrospinal fluid to prevent Aβ aggregation.

Surface-bound basement membrane components accelerate amyloid-β peptide nucleation in air-free wells: an in vitro model of cerebral amyloid angiopathy

Cerebral amyloid angiopathy is caused by deposition of the amyloid β-peptide which consists of mainly 39-40 residues to the cortical and leptomeningeal vessel walls. There are no definite in vitro systems to support the hypothesis that the vascular basement membrane may act as a scaffold of amyloid β-peptide carried by perivascular drainage flow and accelerate its amyloid fibril formation in vivo. We previously reported the critical roles of interfaces and agitation on the nucleation of amyloid fibrils at low concentrations of amyloid β-peptide monomers. Here, we reproduced the perivascular drainage flow in vitro by using N-hydroxysuccinimide-Sepharose 4 Fast flow beads as an inert stirrer in air-free wells rotated at 1rpm. We then reproduced the basement membranes in the media of cerebral arteries in vitro by conjugating Matrigel and other proteins on the surface of Sepharose beads. These beads were incubated with 5μM amyloid β(1-40) at 37°C without air, where amyloid β(1-40) alone does not form amyloid fibrils. Using the initiation time of fibril growth kinetics (i.e., the lag time of fibril growth during which nuclei, on-pathway oligomers and protofibrils are successively formed) as a parameter of the efficiency of biological molecules to induce amyloid fibril formation, we found that basement membrane components including Matrigel, laminin, fibronectin, collagen type IV and fibrinogen accelerate the initiation of amyloid β-peptide fibril growth in vitro. These data support the essential role of vascular basement membranes in the development of cerebral amyloid angiopathy.

Human serum albumin can regulate amyloid-β peptide fiber growth in the brain interstitium: implications for Alzheimer disease

Alzheimer disease is a neurodegenerative disorder characterized by extracellular accumulation of amyloid-β peptide (Aβ) in the brain interstitium. Human serum albumin (HSA) binds 95% of Aβ in blood plasma and is thought to inhibit plaque formation in peripheral tissue. However, the role of albumin in binding Aβ in the cerebrospinal fluid has been largely overlooked. Here we investigate the effect of HSA on both Aβ(1-40) and Aβ(1-42) fibril growth. We show that at micromolar cerebrospinal fluid levels, HSA inhibits the kinetics of Aβ fibrillization, significantly increasing the lag time and decreasing the total amount of fibrils produced. Furthermore, we show that the amount of amyloid fibers generated directly correlates to the proportion of Aβ not competitively bound to albumin. Our observations suggest a significant role for HSA regulating Aβ fibril growth in the brain interstitium.

Phenolic compounds prevent amyloid β-protein oligomerization and synaptic dysfunction by site-specific binding

Cerebral deposition of amyloid β protein (Aβ) is an invariant feature of Alzheimer disease (AD), and epidemiological evidence suggests that moderate consumption of foods enriched with phenolic compounds reduce the incidence of AD. We reported previously that the phenolic compounds myricetin (Myr) and rosmarinic acid (RA) inhibited Aβ aggregation in vitro and in vivo. To elucidate a mechanistic basis for these results, we analyzed the effects of five phenolic compounds in the Aβ aggregation process and in oligomer-induced synaptic toxicities. We now report that the phenolic compounds blocked Aβ oligomerization, and Myr promoted significant NMR chemical shift changes of monomeric Aβ. Both Myr and RA reduced cellular toxicity and synaptic dysfunction of the Aβ oligomers. These results suggest that Myr and RA may play key roles in blocking the toxicity and early assembly processes associated with Aβ through different binding.

Specific disruption of transthyretin(105-115) fibrilization using "stabilizing" inhibitors of transthyretin amyloidogenesis

Transthyretin (TTR) is a cerebrospinal fluid and serum protein that undergoes ordered aggregation (amyloidogenesis) in familial amyloidotic polyneuropathy (FAP) and senile systemic amyloidosis (SSA). It is now widely accepted that dissociation of the native TTR tetramer is a precondition for amyloidogenesis; thus, molecules that stabilize the tetramer have received much attention as potential TTR amyloidosis inhibitors. Many of these inhibitors bind to the thyroxine (T(4)) binding pocket and interact specifically with a section of the TTR sequence, corresponding to residues 105-115, that is implicated in amyloidogenic propensity. In this work, we study the effects of "stabilizing" inhibitors on ordered aggregation of TTR(105-115) peptide. We show that molecules known to bind full-length TTR at the T(4) site are potent, specific inhibitors of ordered aggregation, while molecules that do not interact with TTR exhibit milder, nonspecific disruption through a "hyperbundling" effect. Our results suggest that, in addition to annealing the native tetramer, "stabilizing" inhibitors may also directly disrupt amyloidogenic aggregation of TTR monomers through specific interactions with the exposed TTR(105-115) sequence.

Stoichiometry and affinity of the human serum albumin-Alzheimer's Aβ peptide interactions

A promising strategy to control the aggregation of the Alzheimer's Aβ peptide in the brain is the clearance of Aβ from the central nervous system into the peripheral blood plasma. Among plasma proteins, human serum albumin plays a critical role in the Aβ clearance to the peripheral sink by binding to Aβ oligomers and preventing further growth into fibrils. However, the stoichiometry and the affinities of the albumin-Aβ oligomer interactions are still to be fully characterized. For this purpose, here we investigate the Aβ oligomer-albumin complexes through a novel and generally applicable experimental strategy combining saturation transfer and off-resonance relaxation NMR experiments with ultrafiltration, domain deletions, and dynamic light scattering. Our results show that the Aβ oligomers are recognized by albumin through sites that are evenly partitioned across the three albumin domains and that bind the Aβ oligomers with similar dissociation constants in the 1-100 nM range, as assessed based on a Scatchard-like model of the albumin inhibition isotherms. Our data not only explain why albumin is able to inhibit amyloid formation at physiological nM Aβ concentrations, but are also consistent with the presence of a single high affinity albumin-binding site per Aβ protofibril, which avoids the formation of extended insoluble aggregates.