Amyloid plaques in brain tissue are a hallmark of Alzheimer's disease. Primary components of these plaques are 40- and 42-residue peptides, denoted Aβ(1-40) and Aβ(1-42), that are derived by proteolysis of cellular amyloid precursor protein. Synthetic Aβ(1-40) and Aβ(1-42) form amyloid fibrils in vitro that share many features with the amyloid in plaques. Soluble intermediates in Aβ fibrillogenesis, termed protofibrils, have been identified previously, and here we describe the in vitro formation and isolation of Aβ(1-40) protofibrils by size exclusion chromatography. In some experiments, the Aβ(1-40) was radiomethylated to better quantify various Aβ species. Mechanistic studies clarified two separate modes of protofibril growth, elongation by monomer deposition and protofibril-protofibril association, that could be resolved by varying the NaCl concentration. Small isolated protofibrils in dilute Tris-HCl buffers were directed along the elongation pathway by addition of Aβ(1-40) monomer or along the association pathway by addition of NaCl. Multi-angle light scattering analysis revealed that protofibrils with initial molecular masses Mw of (7-30) × 103 kDa grew to Mw values of up to 250 × 103 kDa by these two growth processes. However, the mass per unit length of the associated protofibrils was about 2-3 times that of the elongated protofibrils. Rate constants for further elongation by monomer deposition with the elongated, associated, and initial protofibril pools were identical when equal number concentrations of original protofibrils were compared, indicating that the original number of protofibril ends had not been altered by the elongation or association processes. Atomic force microscopy revealed heterogeneous initial protofibrils that became more rodlike following the elongation reaction. Our data indicate that protofibril elongation in the absence of NaCl results from monomer deposition only at the ends of protofibrils and proceeds without an increase in protofibril diameter. In contrast, protofibril association occurs in the absence of monomer when NaCl is introduced, but this association involves lateral interactions that result in a relatively disordered fibril structure.
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