Effects of a lipid environment on the fibrillogenic pathway of the N-terminal polypeptide of human apolipoprotein A-I, responsible for in vivo amyloid fibril formation

In amyloidosis associated to Apolipoprotein A-I, heart amyloid deposits are mainly constituted by the 93-residue N-terminal region of the protein. The recombinant polypeptide, named [1-93]ApoA-I, is natively unfolded at neutral pH and able to undergo fibrillogenesis under acidic conditions, as its natural counterpart. Using a multidisciplinary approach, including CD, fluorescence, electrophoretic and atomic force microscopy analyses, we investigated the effects of the detergent Triton X-100 on the conformational state and aggregation propensity of [1-93]ApoA-I. In the absence of the detergent, the polypeptide is predominantly in a random coil state at pH 8.0. Acidification to pH 4.0 switches on the fibrillogenic process, by inducing a conformational transition towards a helical/molten globule transient state, that leads to the formation of aggregates. We also demonstrated that fibrillogenesis occurs in physiologic-like conditions. At pH 6.4, [1-93]ApoA-I was found to assume predominantly an alpha-helical state, which undergoes aggregation at 37°C overtime, although at a lower rate than at pH 4.0. After 7 days incubation at pH 6.4 at 37°C, protofibrils were observed by AFM. By contrast, in the presence of Triton X-100, the polypeptide was found to be in a helical state both at pH 8.0 and 6.4, with no conformational transition occurring upon acidification. Conformational analyses revealed that in the hydrophobic environment the helical conformers are stable and do not generate aggregated species. No fibrillar-like structures were observed by AFM analysis after 21 days incubation. Thus, Triton X-100 is able to strongly influence the aggregation propensity of [1-93]ApoA-I by inducing and stabilizing helical conformers.