The protease inhibitor, MG132, blocks maturation of the amyloid precursor protein Swedish mutant preventing cleavage by β-secretase

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