Familial Alzheimer's Disease Mutations within the Amyloid Precursor Protein Alter the Aggregation and Conformation of the Amyloid-β Peptide

Most cases of Alzheimer's disease (AD) are sporadic, but a small percentage of AD cases, called familial AD (FAD), are associated with mutations in presenilin 1, presenilin 2, or the amyloid precursor protein. Amyloid precursor protein mutations falling within the amyloid- (A) sequence lead to a wide range of disease phenotypes. There is increasing evidence that distinct amyloid structures distinguished by amyloid conformation-dependent monoclonal antibodies have similarly distinct roles in pathology. It is possible that this phenotypic diversity of FAD associated with mutations within the A sequence is due to differences in the conformations adopted by mutant A peptides, but the effects of FAD mutations on aggregation kinetics and conformational and morphological changes of the A peptide are poorly defined. To gain more insight into this possibility, we therefore investigated the effects of 11 FAD mutations on the aggregation kinetics of A, as well as its ability to form distinct conformations recognized by a panel of amyloid conformation-specific monoclonal antibodies. We found that most FAD mutations increased the rate of aggregation of A. The FAD mutations also led to the adoption of alternative amyloid conformations distinguished by monoclonal antibodies and resulted in the formation of distinct aggregate morphologies as determined by transmission electron microscopy. In addition, several of the mutant peptides displayed a large reduction in thioflavin T fluorescence, despite forming abundant fibrils indicating that thioflavin T is a probe of conformational polymorphisms rather than a reliable indicator of fibrillization. Taken together, these results indicate that FAD mutations falling within the A sequence lead to dramatic changes in aggregation kinetics and influence the ability of A to form immunologically and morphologically distinct amyloid structures.