Abstract
The protective effect of transthyretin (TTR) on cellular toxicity of β-amyloid (Aβ) has been previously reported. TTR is a tetrameric carrier of thyroxine in blood and cerebrospinal fluid, the pathogenic aggregation of which causes systemic amyloidosis. However, studies have documented a protective effect of TTR against cellular toxicity of pathogenic Aβ, a protein associated with Alzheimer's disease. TTR binds Aβ, alters its aggregation, and inhibits its toxicity both in vitro and in vivo In this study, we investigate whether the amyloidogenic ability of TTR and its antiamyloid inhibitory effect are associated. Using protein aggregation and cytotoxicity assays, we found that the dissociation of the TTR tetramer, required for its amyloid pathogenesis, is also necessary to prevent cellular toxicity from Aβ oligomers. These findings suggest that the Aβ-binding site of TTR may be hidden in its tetrameric form. Aided by computational docking and peptide screening, we identified a TTR segment that is capable of altering Aβ aggregation and toxicity, mimicking TTR cellular protection. EM, immune detection analysis, and assessment of aggregation and cytotoxicity revealed that the TTR segment inhibits Aβ oligomer formation and also promotes the formation of nontoxic, nonamyloid amorphous aggregates, which are more sensitive to protease digestion. Finally, this segment also inhibits seeding of Aβ catalyzed by Aβ fibrils extracted from the brain of an Alzheimer's patient. Together, these findings suggest that mimicking the inhibitory effect of TTR with peptide-based therapeutics represents an additional avenue to explore for the treatment of Alzheimer's disease.
Highlights
The physiological importance of transthyretin in Alzheimer’s disease was first reported by Schwarzman et al in 1994 [1]
We show the identification and characterization of a segment of TTR that binds amyloid-b peptide 1–42 (Ab42) triggering the formation of nonamyloid amorphous aggregates that are more sensitive to protease digestion, mimicking TTR inhibitory effect over Ab42
We found that monomeric TTR variant (M-TTR), a monomeric-engineered form of transthyretin that is soluble at physiological pH [14], shows notable aggregation after 1 day of incubation at pH 4.3 (Fig. 1, A–C)
Summary
The physiological importance of transthyretin in Alzheimer’s disease was first reported by Schwarzman et al in 1994 [1]. We incubated Ab42 with the samples obtained from Fig. 1A for 16 h and evaluated its cytotoxicity by following MTT reduction. We first analyzed the amyloidogenicity of TTR sequence by ZipperDB, which measures the propensity of every six-residue segment to form amyloid fibrils [19, 20] (Fig. S2A).
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