Polysaccharides and glycosaminoglycans (GAGs), particularly heparin, have been shown to directly affect fibrillation phenomena and the biological activities of amyloid proteins. We present a systematic analysis of the impact of heparin upon fibrillation of the amyloidogenic determinant of the prion protein PrP(106-126). Experimental data, including thioflavin T fluorescence, transmission electron microscopy, and circular dichroism, demonstrate that heparin induced dramatically diverging aggregation pathways of PrP(106-126). Specifically, enhanced β-sheet formation of the prion fragment leading to fibril assemblies occurred in solutions containing low heparin/prion mole ratios, while mixtures containing a greater abundance of heparin showed almost complete inhibition of PrP(106-126) fibril formation. Based upon the experimental data we have proposed a unified model accounting for the interplay between the roles of heparin as a scaffold for nucleation and fibril growth on the one hand and as a disruptor of fibrillation through electrostatic affinity with the monomeric peptide units on the other. This study clarifies previous conflicting studies, and concludes that GAGs inhibit fibrillation and amyloid toxicity in some cases, and promote amyloidogenesis in others.

The peptide fragment 106-126 of prion protein [PrP(106-126)] is a prominent amyloidogenic determinant. We present analysis of PrP(106-126) fibrillation at the air/water interface and, in particular, the relationship between the fibrillation process and interactions of the peptide with phospholipid monolayers. We find that lipid monolayers deposited at the air/water interface induce rapid formation of remarkably highly ordered fibrils by PrP(106-126), and that the extent of fibrillation and fiber organization were dependent upon the presence of negatively charged and unsaturated phospholipids in the monolayers. We also observe that fibrillation was enhanced when PrP(106-126) was injected underneath preassembled phospholipid monolayers, compared to deposition and subsequent compression of mixed monolayers of the peptide and phospholipids. In a broader context, this study demonstrates that Langmuir systems constitute a useful platform for studying lipid interactions of amyloidogenic peptides and lipid-induced fibrillation phenomena.

Glycosaminoglycans (GAGs), particularly heparin, are known to reduce the toxicities of various amyloidogenic proteins. The molecular factors underlying the antitoxic effects of GAGs, however, are still not fully understood. Because interactions of amyloidogenic proteins and their aggregates with membranes are believed to play major roles in affecting amyloid pathogenesis, our objective in this study was to elucidate the effect of heparin on membrane interactions of the 21-residue amyloidogenic determinant of the prion protein [PrP(106-126)]. Indeed, the experimental results indicate that heparin significantly interferes in membrane interactions of the prion peptide. Specifically, we show that there is direct competition for binding of PrP(106-126) between heparin on the one hand and negatively charged phospholipids on the other hand. The data reveal that heparin, even in very low molar concentrations, exhibited high affinity towards PrP(106-126) and consequently suppressed interactions of the peptide with lipid vesicles. Interestingly, whereas heparin significantly inhibited lipid-induced PrP(106-126) fibrillation, it still promoted fibril formation in aqueous solutions independently of the lipid vesicles present. Our results strongly suggest that the primary effects of GAGs in attenuating amyloid toxicities are due to blocking of membrane interactions of the amyloidogenic proteins rather than modulation of their fibrillation properties.