Jiali Du, PhD 2011

Thesis Title: Characterization of the specific interaction between transthyretin and beta-amyloid

β-Amyloid (Aβ) is the main protein component of the amyloid plaques associated with Alzheimer's disease. Transthyretin (TTR) is a homotetramer that circulates in both blood and cerebrospinal fluid. Several recent studies provide support for the hypothesis that these two amyloidogenic proteins interact, and that this interaction is biologically relevant. We examined the interaction of Aβ with wild type (wt) TTR as well as two mutants: F87M/L110M and T119M. On the basis of enzyme-linked immunoassays (ELISA) as well as cross-linking experiments, we conclude that Aβ monomers bind more to TTR monomers than to TTR tetramers. The data further suggest that TTR tetramers interact preferably with Aβ aggregates rather than Aβ monomers. Through tandem mass spectrometry analysis of cross-linked TTR--Aβ fragments, we identified the A strand, in the inner β-sheet of TTR, as well as the EF helix, as regions of TTR that are involved with Aβ association. Light scattering and electron microscopy studies demonstrate that the outcome of the TTR--Aβ interaction strongly depends on TTR quaternary structure. While TTR tetramers may modestly enhance aggregation, TTR monomers decidedly arrest Aβ aggregate growth.

Next we identified two specific Aβ-binding sequences of TTR, strand G and the region including both strand E and EF helix, by using SPOT peptide array analysis. Three peptides with sequences corresponding to these putative binding regions were synthesized: peptide G, peptide E-EF, and peptide E. ELISA of Aβ binding to TTR in the presence of each peptide were conducted. The results support the hypothesis that strand G and the strand E-EF helix region on TTR are involved with binding of Aβ. These data provide important new insights into the nature of TTR--Aβ interactions. Such interactions may regulate TTR-mediated protection against Aβ toxicity.

TTR has one Cys10 residue that is extensively modified through mixed disulfide bonds with a number of different compounds in human serum. Detailed structural comparison of human plasma-derived TTR (pTTR) with wt TTR showed no differences. However, pTTR differed from wt TTR in the extent and nature of modification at Cys10. We hypothesize that differential modifications at Cys10 affect the interaction between TTR and Aβ.