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Amyloid-β peptide is a substrate of the human 20S proteasome.

ABSTRACT: Intraneuronal accumulation of ubiquitin conjugates is a pathological feature of neurodegenerative disorders such as Alzheimer's disease (AD). Previous reports propose that accumulation of ubiquitinated species in AD is a result of inhibition of proteasomal activity by amyloid-β (Aβ) peptides, which leads to blocking of ubiquitin-dependent protein degradation by the proteasome. Here, we provide additional insight into proteasomal dysfunction by Aβ peptides by revealing that aggregated forms of Aβ(1-42) peptides (especially small oligomers) are, in fact, competitive substrates for the chymotrypsin-like activity of the human 20S (h20S) proteasome. In addition to examining the kinetics of the h20S proteasome activity in the presence or absence of Aβ peptides, we use gel electrophoresis, LC-MS, and TOF-MS/MS analyses to examine the degradation of Aβ(1-42) by the h20S proteasome. The observed peptide fragments resulting from proteolytic cleavage of Aβ were consistent with predicted cleavage sites from proteasome degradation. These results support that the interaction of Aβ peptides with the proteasome may play a mechanistic role in proteasomal dysfunction in AD pathology. These results may also reveal a previously unknown natural pathway for clearance of Aβ in normal or diseased cells.

SUBMITTER: Zhao X

PROVIDER: S-EPMC2992454 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The amyloid beta (Aβ) peptide represents a 37 to 49 amino acids endoproteolytic fragment of the amyloid precursor protein. The cellular biology that governs the formation and clearance of Aβ has been understood to play a critical role in Alzheimer’s disease (AD). The primary objective of this study was to generate an in-depth inventory of human brain proteins that oligomeric preparations of Aβ1-42 can bind to using an unbiased in vitro discovery approach. Synthetic Aβ1-42 peptides and a brain extract generated from adult human frontal lobe tissue served in these studies as baits and biological source material, respectively. oAβ1-42 was prepared by aggregating the peptide at 4 ºC for 24 h, using previously described procedures known to generate amyloid-β-derived diffusible ligands (ADDLs). Because the interaction with a given binding partner may rely on a binding epitope that comprises N- or C-terminal residues of Aβ1-42, two separate experiments (I and II) were conducted, which differed in the orientation the oAβ1-42 bait was tethered to the affinity matrix. To facilitate meaningful comparisons across experiments, the method of Aβ1-42 capture was not based on immunoaffinity reagents. Instead, alternative Aβ1-42 baits were equipped with biotin moieties attached to the N- or C-terminus by a 6-carbon linker chain, enabling their consistent affinity-capture on streptavidin agarose matrices. Biotin-saturated streptavidin agarose matrices served as negative controls and three biological replicates of samples and controls were generated for each of the three separate interactome datasets by reproducing the affinity-capture step side-by-side on three separate streptavidin agarose affinity matrices that had been saturated with the biotinylated baits. To identify differences in protein-protein interactions of monomeric versus oligomeric Aβ1-42, a 3rd interactome experiment was conducted in which oAβ1-42-biotin or mAβ1-42-biotin served as baits. Digitonin-solubilized brain extracts, which are known to comprise extracellular and most cellular proteins (except for nuclear proteins) served as biological starting material, consistent with the main subcellular areas previously reported to harbor Aβ. Following extensive washes of affinity matrices in their protein-bound state, binders to the bait peptides were eluted by rapid acidification, fully denatured in 9 M urea, and trypsinized. To avoid notorious confounders related to variances in the subsequent handling and analysis of samples, individual peptide mixtures were labeled with distinct isobaric tandem mass tags (TMT) in a six-plex format, then combined and concomitantly subjected to ZipTip-based pre-analysis clean-up by strong cation exchange (SCX) and reversed phase (RP) separation. Four-hour split-free reversed phase nanospray separations were online coupled to an Orbitrap Fusion Tribrid mass spectrometer, which was configured to run an MS3 analysis method.

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Amyloid-β peptide is a substrate of the human 20S proteasome.

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