Lin Liu, PhD 2007

Thesis Title: Role of transthyretin in modulating beta-amyloid aggregation and toxicity

Deposition of β-amyloid (Aβ) fibrils is an early event in the neurodegenerative processes associated with Alzheimer's disease. According to the "amyloid cascade" hypothesis, Aβ aggregation and its subsequent deposition as fibrils, is the underlying cause of disease. Thus, there is considerable interest in identifying compounds that interfere with Aβ aggregation. Although once it was assumed that mature insoluble fibrils were the toxic species, it has become increasingly accepted that soluble Aβ oligomers, intermediates on the fibrillogenesis pathway, may be the most neurotoxic.

We found that the peptides selected for binding to monomeric Aβ did not perturb aggregation, but those selected for binding to highly aggregated Aβ increase the rate of aggregation drastically. This result suggests that Aβ aggregates are the proper target for altering aggregation. Recent studies suggest that transthyretin (TTR), a homotetrameric serum protein, interacts directly with Aβ to inhibit its toxicity. We report results from biophysical analysis of Aβ aggregation kinetics in the presence of plasma-derived TTR (pTTR). We hypothesized that pTTR also targets aggregated Aβ and arrests further growth of the aggregates.

Next we produced TTR recombinantly (rTTR). We compared the effect of pTTR and rTTR on Aβ aggregation and toxicity. pTTR slowed Aβ aggregation but failed to protect primary cortical neurons from Aβ toxicity. In contrast, rTTR accelerated Aβ aggregation and effectively protected neurons from Aβ toxicity. This inverse correlation between Aβ aggregation kinetics and toxicity is consistent with the hypothesis that soluble oligomers rather than insoluble fibrils are the most toxic Aβ species. We found that at acidic pH, pTTR was more resistant than rTTR to dissociation into monomers and to aggregation. The more amyloidogenic rTTR effectively protects cells against Aβ toxicity while pTTR, the more stable structural variant, does not. This result hints of the existence of a network of interacting amyloidogenic proteins that modulate each others' stability, aggregation, and toxicity. Compounds that moderately de-stabilize TTR tetramers might form the basis for a novel therapeutic strategy for Alzheimer's treatment.