Interactions of Alzheimer amyloid-beta peptides with glycosaminoglycans - Effects on fibril nucleation and growth

Proteoglycans and their constituent glycosaminoglycans are associated with all amyloid deposits and may be involved in the amyloidogenic pathway. In A lzheimer's disease, plaques are composed of the amyloid-beta peptide and ar e associated with at least four different proteoglycans. Using CD spectrosc opy, fluorescence spectroscopy and electron microscopy, we examined glycosa minoglycan interaction with the amyloid-beta peptides 1-40 (A beta 40) and 1-42 (A beta 42) to determine the effects on peptide conformation and fibri l formation. Monomeric amyloid-beta peptides in trifluoroethanol, when dilu ted in aqueous buffer, undergo a slow random to amyloidogenic beta sheet tr ansition. In the presence of heparin, heparan sulfate, keratan sulfate or c hondroitin sulfates, this transition was accelerated with A beta 42 rapidly adopting a beta-sheet conformation. This was accompanied by the appearance of well-defined amyloid fibrils indicating an enhanced nucleation of A bet a 42. Incubation of preformed A beta 42 fibrils with glycosaminoglycans res ulted in extensive lateral aggregation and precipitation of the fibrils. Th e glycosaminoglycans differed in their relative activities with the chondro itin sulfates producing the most pronounced effects. The less amyloidogenic A beta 40 isoform did not show an immediate structural transition that was dependent upon the shielding effect by the phosphate counter ion. Removal or substitution of phosphate resulted in similar glycosaminoglycan-induced conformational and aggregation changes. These findings clearly demonstrate that glycosaminoglycans act at the earliest stage of fibril formation, name ly amyloid-beta nucleation, and are not simply involved in the lateral aggr egation of preformed fibrils or nonspecific adhesion to plaques. The identi fication of a structure-activity relationship between amyloid-beta and the different glycosaminoglycans, as well as the condition dependence for glyco saminoglycan binding, are important for the successful development and eval uation of glycosaminoglycan-specific therapeutic interventions.