Transthyretin Suppresses Amyloid-β Secretion by Interfering with Processing of the Amyloid-β Protein Precursor

In Alzheimer's disease (AD), most hippocampal and cortical neurons show increased staining with anti-transthyretin (TTR) antibodies. Genetically programmed overexpression of wild type human TTR suppressed the neuropathologic and behavioral abnormalities in APP23 AD model mice and TTR-A beta complexes have been isolated from some human AD brains and those of APP23 transgenic mice. In the present study, in vitro NMR analysis showed interaction between the hydrophobic thyroxine binding pocket of TTR and the cytoplasmic loop of the C99 fragment released by beta-secretase cleavage of A beta PP, with K-d = 86 +/- 9 mu M. In cultured cells expressing both proteins, the interaction reduced phosphorylation of C99 (at T668) and suppressed its cleavage by gamma-secretase, significantly, decreasing A beta secretion. Coupled with its previously demonstrated capacity to inhibit A beta aggregation (with the resultant cytotoxicity in tissue culture) and its regulation by HSF1, these findings indicate that TTR can behave as a stress responsive multimodal suppressor of AD pathogenesis.