The Protease Inhibitor, MG132, Blocks Maturation of the Amyloid Precursor Protein Swedish Mutant Preventing Cleavage by β-Secretase

Amyloid (Aβ) peptides found aggregated into plaques in Alzheimer's disease are derived from the sequential cleavage of the amyloid precursor protein (APP) first by β- and then by γ-secretases. Peptide aldehydes, which inhibit cysteine proteases and proteasomes, reportedly block Aβ peptide secretion by interfering with γ-secretase cleavage. Using a novel, specific, and sensitive enzyme-linked immunosorbent assay for the β-secretase-cleaved fragment of the Swedish mutant of APP (APPSw), we determined that the peptide aldehyde, MG132, prevented β-secretase cleavage. This block in β-secretase cleavage was not observed with clasto-lactacystin β-lactone and thus, cannot be attributed to proteasomal inhibition. MG132 inhibition of γ-secretase cleavage was compared with the serine protease inhibitor, 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF). AEBSF inhibition of β-secretase cleavage was immediate and did not affect a-secretase cleavage. With MG132, inhibition was delayed and it decreased secretion of α-cleaved APPSw as well. Furthermore, MG132 treatment impaired maturation of full-length APPSw. Both inhibited intracellular formation of the β-cleaved product. These results suggest that peptide aldehydes such as MG132 have multiple effects on the maturation and processing of APP. We conclude that the MG132-induced decrease in β-secretase cleavage of APPSw is due to a block in maturation. This is sufficient to explain the previously reported peptide aldehyde-induced decrease in Aβ peptide secretion.