SFN:Antimicrobial activity of soluble Aβ oligomers is mediated by the VHHQKL heparin-binding domain

 The β-amyloid protein (Aβ) plays a key role in the pathogenesis of Alzheimer's disease. The Aβ peptide sequence includes a VHHQKL heparin-binding domain. Although pathological activities mediated by Aβ’s VHHQKL domain have been extensively investigated, a normal physiological function has not been proposed for the peptide’s apparent affinity for sugars. We recently presented data consistent with a role for Aβ as an antimicrobial peptide (AMP). Our most recent data show picomolar concentrations of soluble Aβ oligomers have potent in vitro antimicrobial activities that appear to be mediated by the heparin-binding VHHQKL domain. Previous studies demonstrate binding of microbial cell-wall carbohydrates plays a key role in the antimicrobial function the archetypal AMP LL-37, which shares the same XBBXBX heparin-binding motif as Aβ. Here we present data characterizing the role of Aβ’s heparin-binding domain in context of the peptide’s AMP activities. 
METHODS: Experiments first characterized the binding of monomeric and oligomeric Aβ to host and microbial carbohydrates using ELISA and chromatographic assays. Aβ’s anti-adhesion and agglutination activities against pathogenic forms of Candida albicans were then characterized in a cell culture monolayer infection model. Yeast aggregates were also analyzed for markers of β-amyloid fibrils. Experiments included the heparin-binding AMP LL-37 as a control. 
RESULTS: Aβ oligomers bound microbial polysaccharides in in vitro binding assays. Affinity of soluble, cell-secreted oligomeric Aβ was two-to-three orders of magnitude higher than synthetic monomeric peptide. Deglycosylation of Candida cells attenuated Aβ binding. Microbial sugars that specifically block the actions of heparin-binding AMPs ablated Aβ’s anti-adhesion and agglutination activities in cell culture. Analysis ofCandida aggregates suggests agglutination involves entrapment of yeast cells by β-amyloid fibrils.
CONCLUSIONS: Data is consistent with protective antimicrobial activity for Aβ oligomers mediated by the heparin-binding VHHQKL domain. Findings suggest the normal target sugars for Aβ are not host structural carbohydrates but rather microbial cell-wall polysaccharides. Based on our new data, we propose a three-part mechanism for Aβ antimicrobial activity. Aβ oligomers first bind to microbial cell walls via the heparin-binding VHHQKL domain. Bound Aβ oligomers interfere with the normal adhesion of microbial cells to host tissues. Lastly, resulting planktonic microbial cells are agglutinated by β-amyloid fibrils that are seeded by bound peptide and grow through recruitment of free Aβ oligomers.

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