Apolipoprotein E includes a binding site which is recognized by several amyloidogenic polypeptides

Prion Efficiently Replicates in α-Synuclein Knockout Mice

Prion diseases are a group of neurodegenerative disorders associated with the conformational conversion of the cellular prion protein (PrP^C) into an abnormal misfolded form named PrP^Sc. Other than accumulating in the brain, PrP^Sc can bind PrP^C and force it to change conformation to PrP^Sc. The exact mechanism which underlies the process of PrP^C/PrP^Sc conversion still needs to be defined and many molecules or cofactors might be involved. Several studies have documented an important role of PrP^C to act as receptor for abnormally folded forms of α-synuclein which are responsible of a group of diseases known as synucleinopathies. The presence of PrP^C was required to promote efficient internalization and spreading of abnormal α-synuclein between cells. In this work, we have assessed whether α-synuclein exerts any role in PrP^Sc conversion and propagation either in vitro or in vivo. Indeed, understanding the mechanism of PrP^C/PrP^Sc conversion and the identification of cofactors involved in this process is crucial for developing new therapeutic strategies. Our results showed that PrP^Sc was able to efficiently propagate in the brain of animals even in the absence of α-synuclein thus suggesting that this protein did not act as key modulator of prion propagation. Thus, α-synuclein might take part in this process but is not specifically required for sustaining prion conversion and propagation.

How our bodies fight amyloidosis: effects of physiological factors on pathogenic aggregation of amyloidogenic proteins

The process of protein aggregation from soluble amyloidogenic proteins to insoluble amyloid fibrils plays significant roles in the onset of over 30 human amyloidogenic diseases, such as Prion disease, Alzheimer's disease and type 2 diabetes mellitus. Amyloid deposits are commonly found in patients suffered from amyloidosis; however, such deposits are rarely seen in healthy individuals, which may be largely attributed to the self-regulation in vivo. A vast number of physiological factors have been demonstrated to directly affect the process of amyloid formation in vivo. In this review, physiological factors that influence amyloidosis, including biological factors (chaperones, natural antibodies, enzymes, lipids and saccharides) and physicochemical factors (metal ions, pH environment, crowding and pressure, etc.), together with the mechanisms underlying these proteostasis effects, are summarized.

Exploring the potential of the platelet membrane proteome as a source of peripheral biomarkers for Alzheimer's disease

Introduction

Peripheral biomarkers to diagnose Alzheimer's disease (AD) have not been established. Given parallels between neuron and platelet biology, we hypothesized platelet membrane-associated protein changes may differentiate patients clinically defined with probable AD from noncognitive impaired controls.

Methods

Purified platelets, confirmed by flow cytometry were obtained from individuals before fractionation by ultracentrifugation. Following a comparison of individual membrane fractions by SDS-PAGE for general proteome uniformity, equal protein weight from the membrane fractions for five representative samples from AD and five samples from controls were pooled. AD and control protein pools were further divided into molecular weight regions by one-dimensional SDS-PAGE, prior to digestion in gel. Tryptic peptides were analyzed by reverse-phase liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). Ionized peptide intensities were averaged for each identified protein in the two pools, thereby measuring relative protein abundance between the two membrane protein pools. Log₂-transformed ratio (AD/control) of protein abundances fit a normal distribution, thereby permitting determination of significantly changed protein abundances in the AD pool.

Results

We report a comparative analysis of the membrane-enriched platelet proteome between patients with mild to moderate AD and cognitively normal, healthy subjects. A total of 144 proteins were determined significantly altered in the platelet membrane proteome from patients with probable AD. In particular, secretory (alpha) granule proteins were dramatically reduced in AD. Of these, we confirmed significant reduction of thrombospondin-1 (THBS1) in the AD platelet membrane proteome by immunoblotting. There was a high protein-protein connectivity of proteins in other pathways implicated by proteomic changes to the proteins that define secretory granules.

Conclusions

Depletion of secretory granule proteins is consistent with a preponderance of post-activated platelets in circulation in AD. Significantly changed pathways implicate additional AD-related defects in platelet glycoprotein synthesis, lipid homeostasis, amyloidogenic proteins, and regulators of protease activity, many of which may be useful plasma membrane-expressed markers for AD. This study highlights the utility of LC-MS/MS to quantify human platelet membrane proteins and suggests that platelets may serve as a source of blood-based biomarkers in neurodegenerative disease.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

Hereditary gelsolin amyloidosis

Hereditary gelsolin amyloidosis (HGA) is an autosomally dominantly inherited form of systemic amyloidosis, characterized mainly by cranial and sensory peripheral neuropathy, corneal lattice dystrophy, and cutis laxa. HGA, originally reported from Finland and now increasingly from other countries in Europe, North and South America, and Asia, may still be underdiagnosed worldwide. It is the first and so-far only known disorder caused by a gelsolin gene defect, namely a G654A or G654T mutation. Gelsolin is a principal actin-modulating protein, implicated in multiple biological processes, also in the nervous system, e.g. axonal transport, myelination, neurite outgrowth, and neuroprotection. The gelsolin gene defect causes expression of variant gelsolin, followed by systemic deposition of gelsolin amyloid (AGel) in HGA patients and even other consequences on the metabolism and function of gelsolin. In HGA, specific therapy is not yet available but correct diagnosis enables adequate symptomatic treatment which decisively improves the quality of life in these patients. A transgenic murine model of HGA expressing AGel is available, in anticipation of new treatment options targeted toward this slowly progressive but devastating amyloidosis. Present and future lessons learned from HGA may be applicable even in diagnosis and treatment of other hereditary and sporadic amyloidoses.

Identification and removal of proteins that co-purify with infectious prion protein improves the analysis of its secondary structure

Prion diseases are neurodegenerative disorders associated with the accumulation of an abnormal isoform of the mammalian prion protein (PrP). Fourier transform infrared spectroscopy (FTIR) has previously been used to show that the conformation of aggregated, infectious PrP (PrP(Sc) ) varies between prion strains and these unique conformations may determine strain-specific disease phenotypes. However, the relative amounts of α-helix, β-sheet and other secondary structures have not always been consistent between studies, suggesting that other proteins might be confounding the analysis of PrP(Sc) secondary structure. We have used FTIR and LC-MS/MS to analyze enriched PrP(Sc) from mouse and hamster prion strains both before and after the removal of protein contaminants that commonly co-purify with PrP(Sc) . Our data show that non-PrP proteins do contribute to absorbances that have been associated with α-helical, loop, turn and β-sheet structures attributed to PrP(Sc) . The major contaminant, the α-helical protein ferritin, absorbs strongly at 1652 cm(-1) in the FTIR spectrum associated with PrP(Sc) . However, even the removal of more than 99% of the ferritin from PrP(Sc) did not completely abolish absorbance at 1652 cm(-1) . Our results show that contaminating proteins alter the FTIR spectrum attributed to PrP(Sc) and suggest that the α-helical, loop/turn and β-sheet secondary structure that remains following their removal are derived from PrP(Sc) itself.

High ability of apolipoprotein E4 to stabilize amyloid-β peptide oligomers, the pathological entities responsible for Alzheimer's disease

Nowadays, the emerging role of amyloid-β peptide (Aβ) oligomers in Alzheimer's disease (AD) is widely accepted, putting aside the old idea that fibrils are the primary entities responsible for the onset of the disease. Besides, carrying the E4 isoform of apolipoprotein E (apoE) represents the highest risk of developing AD. Nevertheless, the involvement of apoE4 in AD remains confusing. The goal of this study was to bring new insights into the role of apoE4 in Aβ aggregation. We used infrared spectroscopy, thioflavin T fluorescence, and Western blots to evaluate the influence of apoE isoforms on Aβ aggregation in vitro. Comparing Aβ controls with Aβ incubated either with the apoE3 or apoE4 isoform, we report a 30% reduction of the Aβ fibrillar content, whereas the oligomeric content is 2 times higher on incubation with the pathological isoform apoE4. ApoE4 would bind and block Aβ in its oligomeric conformation, inhibiting further formation of less toxic fibrillar forms of Aβ. While previous studies mostly correlated E4 with fibrils, our report underlines a link between apoE4 and Aβ oligomers and therefore reconciles apoE4 with the new amyloid cascade hypothesis. Our observations suggest that apoE4 strongly stabilizes Aβ oligomers, the pathological species responsible for Alzheimer's disease.

Abeta20-29 peptide blocking apoE/Abeta interaction reduces full-length Abeta42/40 fibril formation and cytotoxicity in vitro

A key event in the pathogenesis of Alzheimer's disease (AD) is the conversion of the peptide beta-amyloid (Abeta) from its soluble monomeric form into various aggregated morphologies in the brain. Apolipoprotein E (apoE) is known to act as a pathological chaperone of Abeta in this process, promoting its fibril formation from soluble Abeta by binding interaction between carboxy-terminal domain of apoE and residues 12-28 of full-length Abeta. Therefore, blocking apoE/Abeta interaction is being actively pursued as a primary therapeutic strategy for AD. Abeta20-29, a short peptide, contains the residues to competitively bind to apoE and may potentially block the interaction between apoE and full-length Abeta. However, little is known whether Abeta20-29 could block apoE/Abeta interaction to play an effective role in reducing full-length Abeta fibrillization and cytotoxicity. Utilizing fluorescence spectroscopic analysis with thioflavin T and electron microscopic study, we show here that Abeta20-29 alone was non-fibrillogenic, and had no direct effects on Abeta1-42 or Abeta1-40 aggregation. Moreover, apoE can directly promote both Abeta1-42 and Abeta1-40 aggregation and fibril formation, while this promoting effect was inhibited when adding Abeta20-29, with a dose-dependent manner. In the series of cell culture experiments, Abeta20-29 alone shows no cytotoxicity to PC12 cells as demonstrated by MTT assay, while co-incubation apoE isoforms and Abeta1-42 or Abeta1-40 shows stronger cytotoxicity as compared to Abeta1-42 or Abeta1-40 alone. When incubated with Abeta20-29, whereas such strong cytotoxic effect was concentration-dependently reduced. Taken together, we demonstrate for the first time that Abeta20-29 has no direct effect on full-length Abeta aggregation, and may competitively block the binding of full-length Abeta to apoE, resulting in an inhibitory effect on apoE's promoting full-length Abeta fibrillogenesis and Abeta-induced cytotoxicity. Our results raise the possibility that Abeta20-29 peptide blocking the interaction between full-length Abeta and apoE isoforms may be effective as a therapeutic agent for AD.

Sialic acid moiety of apolipoprotein E3 at Thr(194) affects its interaction with beta-amyloid(1-42) peptides

BACKGROUND

The interaction between apolipoprotein (apo) E and beta-amyloid (Abeta) is associated with the development of Alzheimer's disease (AD); however, the details remain unknown. ApoE in cerebrospinal fluid is extensively sialylated, and sialylation of certain proteins are known to modulate biological function. We investigated the effects of a sialic acid moiety of apoE on the apoE-Abeta interaction.

METHODS

We prepared normal apoE3 and its mutant (Thr(194) --> Ala) and analyzed their interactions with Abeta(1-42) by using the surface plasmon resonance (SPR) assay. In addition, we performed the SPR assay by using apoE-containing lipoproteins treated with neuraminidase. We also assessed the effect of the mutation on the interaction of apoE3 with liposomes.

RESULTS

The binding avidity of the mutant apoE3(#) was approximately 50% that of normal apoE3 (p<0.0001). The binding avidity of the apoE-containing lipoproteins for Abeta(1-42) reduced after neuraminidase treatment.

CONCLUSIONS

We suggest that AD development is controlled not only by the apoE isoforms but also by the posttranslational modifications in apoE, such as those in the sialic acid moieties, which are abundant in apoE derived from the brain.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

Surface plasmon resonance for the analysis of beta-amyloid interactions and fibril formation in Alzheimer's disease research

Alzheimer's disease (AD) is a neurodegenerative disorder characterised by the accumulation of amyloid deposits, the major component of which is a 4 kDa polypeptide known as beta-amyloid protein (ABeta). Identifying the mechanism underlying the formation of Abeta and the pathways that lead to its toxicity is crucial to understanding the mechanism of AD and addressing the urgent need for new and effective treatments for AD. The accumulation of ABeta is the result of a complex interplay between genetic and environmental factors that affect the generation, clearance and aggregation of the peptide. Because of its propensity to aggregate, ABeta builds up in the brain and assembles into amyloid fibrils, ultimately creating amyloid plaques (APs) and cerebral amyloid angiopathy (CAA). Abeta has been shown to interact with a number of intracellular and extracellular molecules, but the relative contribution of these interactions to the toxicity of Abeta is not well understood. A critical step in characterising the importance of these interactions is the ability to measure both the affinity and kinetics of these interactions. Surface plasmon resonance (SPR) spectroscopy has become a widely used technique to study molecular interactions such as antibody-antigen, DNA-DNA, DNA-protein, protein-protein, receptor-ligand and peptide- and protein-membrane interactions. This article reviews the application of SPR to the study of the molecular interactions associated with AD and how this information enhances our molecular understanding of ABeta -mediated toxicity.

[BIG-H3 protein: mutation of codon 124 and corneal amyloidosis]

In 1997, a group of hereditary corneal dystrophies was related to mutations in the TGFBI (BIGH3) gene. Within this group, some corneal dystrophies present particular biochemical features in that they are characterized by corneal amyloid deposition. Contrary to clinical and genetic knowledge, the biochemical characteristics of the encoded protein (Big-h3) and the mechanisms of its amyloid conversion remain unclear. We review the current knowledge on the Big-h3 protein and focus on the behavior of the codon 124 region. We discuss this protein's mechanisms of amyloid conversion from our results and previous reports as well as from other types of amyloidosis. These data provide a better understanding of the putative processes leading to the phenotypic variations linked with their respective codon 124 mutation.

Tau-associated neuropathology in ganglion cell tumours increases with patient age but appears unrelated to ApoE genotype

Ganglion cell tumours, including gangliogliomas and gangliocytomas, are low grade neoplasms with a mature neuronal component. Ganglion cells within these lesions occasionally exhibit neurodegenerative changes including neurofibrillary tangles (NFT) similar to those in Alzheimer's disease. The frequency and spectrum of degenerative pathology in these lesions have not been defined, nor has their relation to patient age or factors such as apolipoprotein E (ApoE) genotype that predispose to Alzheimer's disease. We studied 72 ganglion cell tumours (61 gangliogliomas, 11 gangliocytomas) from patients 7 months to 72-years-old. Haematoxylin and eosin (H&E), silver stains (Hirano method) and immunohistochemistry for tau, alpha-synuclein and beta-amyloid were performed on formalin-fixed, paraffin-embedded tissue from surgical specimens. Tau-and silver-positive NFT and neuropil threads (NPT) were present in four of 26 ganglion cell tumours from patients over 30-years-old (ages 31, 38, 50, and 58 years). Neuronal granulovacuolar degeneration (GVD) was noted in five of 26 tumours from patients over 30-years-old (mean, 48 years). NFT, NPT, and GVD were not seen in ganglion cell tumours from patients under 30-years-old[0/46]. Cytoplasmic argentophilic bodies distinct from NFT were present in five of 26 tumours from patients over 30-years-old and in two of 46 under 30 years. Neither alpha-synuclein positive neuronal inclusions nor beta-amyloid immunoreactivity was noted in ganglion cell tumours from any age group. The distribution of ApoE genotypes was similar among those tumours that contained tau-associated neuropathology and those that did not. Neurodegenerative changes are uncommon in ganglion cell tumours, but increase in frequency with patient age. GVD, tau-positive NFT and NPT, and argentophilic bodies occur more often in ganglion cell tumours from patients over 30-yrs-old, but do not appear to be associated with a specific ApoE genotype.