Yu, Peter H2009-12-142013-01-042011-01-132013-01-042009-102009-10October 20http://hdl.handle.net/10388/etd-12142009-170726Beta-amyloid (Aβ) remains to be the focus of research interest of the pathogenesis of Alzheimer’s disease (AD). Aβ is subject to oligomerization and its polymers are cytotoxic. Advanced aggregation leads to formation of senile plaques. Depositions of Aβ surrounding the cerebral vasculature, i.e. cerebral amyloid angiopathy (CAA), occur in most AD patients. The occurrence of Aβ aggregation in AD brains is not due to over-expression of amyloid precursor protein in most cases of AD. Factors influencing Aβ polymerization are yet to be established. Aldehydes are highly reactive. They can cause protein crosslinkage. It is interesting to study whether endogenous aldehydes may be involved in Aβ polymerization process. In order to investigate the potential interaction of endogenous aldehydes with Aβ and their effects on its aggregation, various techniques including thioflavin T fluometry, dynamic light scattering, circular dichroism and atomic force microscopy were employed to assess Aβ aggregation at different stages. Formaldehyde, methylglyoxal, malondialdehyde and 4-hydroxyl-nonenal were found to enhance Aβ β-sheets formation, oligomerization and fibrillogenesis in vitro. The sizes of the oligomers are increased after interaction with the aldehydes. Lysine residues of Aβ were identified to be the primary site of interaction with aldehydes by forming Schiff bases, which may subsequently lead to intra- and inter-molecular crosslinkage. Aldehydes can also crosslink Aβ with other proteins such as apolipoprotein E and α2-macroglobulin (α2M), to form large complexes. Results suggest that aldehydes substantially increase the rate of Aβ oligomerization at each stage of fibrillogenesis. The native and formaldehyde-modified Aβ oligomers were isolated by size exclusion chromatography and their cytotoxic effects towards SH-SY5Y neuroblastoma cells were assessed using MTT, LDH and caspase-3 activity assays. The aldehyde-modified oligomers are slightly but significantly more cytotoxic compared to the native oligomers. Since aldehydes significantly increase the production of Aβ oligomers, an increase in aldehydes would enhance the total cytotoxicity, suggesting that aldehydes may potentially exacerbate neurovascular damage and neurodegeneration caused by Aβ. Low-density lipoprotein receptor related protein-1 (LRP-1) plays a crucial role in Aβ clearance via the cerebral vasculature. Semicarbazide-sensitive amine oxidase (SSAO) and LRP-1 are both richly expressed on the vascular smooth muscle cells (VSMCs). We demonstrated that SSAO-mediated deamination affects LRP-1 function using isolated VSMCs. Formaldehyde at low concentrations decreases LRP-1-mediated uptake of α2M, a substrate of LRP-1 and a carrier for Aβ. Methylamine, an SSAO substrate that is converted to formaldehyde, also inactivates LRP-1 function, but not in the presence of an SSAO inhibitor. Increased SSAO-mediated deamination can potentially impair Aβ clearance via LRP-1. In conclusion, aldehydes derived from oxidative stress and SSAO-mediated deamination induce Aβ aggregation, enhance Aβ cytotoxicity and impair Aβ clearance. The exclusive localization of SSAO on the cerebral vasculature may be responsible for the perivascular deposition of Aβ, i.e. CAA, which is associated both with vascular dementia and with AD. Vascular surface SSAO may be a novel pharmacological target for the treatment of AD.en-USbeta-amyloidaggregationAlzheimer's diseaseendogenous aldehydessemicarbazide-sensitive amine oxidasetext