Impact of Amyloid Precursor Protein Hydrophilic Transmembrane Residues on Amyloid-Beta Generation

Amyloid-beta (A beta) peptides are likely the molecular cause of neurodegeneration observed in Alzheimer's disease. In the brain, A beta 42 and A beta 40 are toxic and the most important proteolytic fragments generated through sequential processing of the amyloid precursor protein (APP) by beta- and gamma-secretases. Impeding the generation of A beta 42 and A beta 40 is thus considered as a promising strategy to prevent Alzheimer's disease. We therefore wanted to determine key parameters of the APP transmembrane sequence enabling production of these A beta species. Here we show that the hydrophilicity of amino acid residues G33, T43, and T48 critically determines the generation of A beta 42 and A beta 40 peptides (amino acid numbering according to A beta nomenclature starting with aspartic acid 1). First, we performed a comprehensive mutational analysis of glycine residue G33 positioned within the N-terminal half of the APP transmembrane sequence by exchanging it against the 19 other amino acids. We found that hydrophilicity of the residue at position 33 positively correlated with A beta 42 and A beta 40 generation. Second, we analyzed two threonine residues at positions T43 and T48 in the C-terminal half of the APP-transmembrane sequence. Replacement of single threonine residues by hydrophobic valines inversely affected A beta 42 and A beta 40 generation. We observed that threonine mutants affected the initial gamma-secretase cut, which is associated with levels of A beta 42 or A beta 40. Overall, hydrophilic residues of the APP transmembrane sequence decide on the exact initial gamma-cut and the amounts of A beta 42 and A beta 40.