Proteoglycan-mediated Inhibition of Aβ Proteolysis

Abstract

Senile plaques of Alzheimer's disease brain contain, in addition to beta amyloid peptide (Aβ), multiple proteoglycans. Systemic amyloidotic deposits also routinely contain proteoglycan, suggesting that these glycoconjugates are generally involved in amyloid plaque formation and/or persistence. We demonstrate that heparan sulfate proteoglycan (HSPG) and chondroitin sulfate proteoglycan (CSPG) inhibit the proteolytic degradation of fibrillar, but not non-fibrillar, Aβ at physiological pH. In accordance with the proteolysis studies, high affinity binding of proteoglycans to fibrillar Aβ(1-40) and Aβ(1-42) is observed from pH 4 to 9, whereas appreciable binding of HSPG or CSPG to non-fibrillar peptide is only seen at pH < 6. This differing pH dependence of binding suggests that a lysine residue is involved in proteoglycan association with fibrillar Aβ, whereas a protonated histidine appears to be needed for binding of the glycoconjugates to non-fibrillar peptide. Scatchard analysis of fibrillar Aβ association with proteoglycans indicates a single affinity interaction, and the binding of both HSPG and CSPG to fibrillar Aβ is completely inhibited by free glycosaminoglycan chains. This implies that these sulfated carbohydrate moieties are primarily responsible for proteoglycan·Aβ interaction. The ability of proteoglycans to bind fibrillar Aβ and inhibit its proteolytic degradation suggests a possible mechanism of senile plaque accumulation and persistence in Alzheimer's disease.