Metalloendopeptidase EC 3.4.24.15 is necessary for Alzheimer's amyloid-β peptide degradation

We have investigated the functional relationship between metalloendopeptidase EC 3.4.24.15 (MP24.15) and the amyloid precursor protein involved in Alzheimer's disease (AD) and discovered that the enzyme promotes A beta degradation. We show here that conditioned medium (CM) of MP24.15 antisense-transfected SKNMC neuroblastoma has significantly higher levels of A beta, Furthermore, synthetic-A beta degradation was increased or decreased following incubation with CM of sense or antisense-transfected cells, respectively, Soluble A beta 1-42 was degraded more slowly than soluble A beta 1-40, while aggregated A beta 1-42 showed almost no degradation. Pretreatment of CM with serine proteinase inhibitors 4-(2-aminoethyl)benzenesulfonyl fluoride and diisopropyl fluorophosphate completely inhibited A beta degradation. Additionally, alpha(1)-antichymotrypsin (ACT), a serpin family inhibitor tightly associated with plaques and elevated in AD brain, blocked up to 60% of A beta degradation. Interestingly, incubation of CM of MP24.15-overexpressing cells with ACT formed an SDS-resistant ACT complex, suggesting an ACT-serine proteinase interaction. Recombinant MP24.15 alone did not degrade A beta, C-14-Diisopropyl fluorophosphate-radiolabeled CM from MP24.15-overexpressing cells contained increased levels of several active serine proteinases, suggesting that MP24.15 activates one or more A beta-degrading serine proteases. Thus, ACT may cause A beta accumulation by inhibiting an A beta-degrading enzyme or by direct binding to A beta, rendering it degradation-resistant. Identification of the A beta-degrading enzyme and MP24.15's role in its activation is underway. Pharmacological modulation of either enzyme may provide a means of regulating A beta in the brain.