Abstract
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-β (Aβ) 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 Aβ sequence is due to differences in the conformations adopted by mutant Aβ peptides, but the effects of FAD mutations on aggregation kinetics and conformational and morphological changes of the Aβ 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 Aβ, 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 Aβ. 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 Aβ sequence lead to dramatic changes in aggregation kinetics and influence the ability of Aβ to form immunologically and morphologically distinct amyloid structures.
Highlights
ResultsMost of the FAD mutations cause a decrease in the lag time and an increase in the rate of aggregation, including E22G, L34V, D7N, E22K, A2V, and H6R
Unchanged total A levels, accelerated fibril formation with no increase in protofibril levels, enhanced elongation phase with no effects on nucleatio
Reduced secreted A in favor of intracellular aggregate formation, accelerated aggregation
Summary
Most of the FAD mutations cause a decrease in the lag time and an increase in the rate of aggregation, including E22G, L34V, D7N, E22K, A2V, and H6R. This effect is not restricted to mutations falling within a specific region of the A sequence, as the mutations leading to increases in the rate of aggregation may be N- or C-terminal, or fall within the mid-region of A. There was some variation in the maximum ThT fluorescence within this highly specific fluorescence group, with the E22G and L34V mutants exhibiting higher ThT fluorescence values than the E22K, D23N, and D7N peptides. Summary of FAD mutations studied and their reported pathological effects and disease phenotypes
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