α1-Antichymotrypsin Inhibits Aβ Degradation in Vitro and in Vivo

The neuropathology of Alzheimer's disease (AD) is characterized by extensive deposition of the toxic amyloid beta peptide (A beta) in selected regions of the brain and brain vasculature (Selkoe, 1999). Thus, lowering the levels of A beta may be beneficial for AD patients. A beta is a proteolytic fragment derived from the amyloid precursor protein (APP). The mechanisms of A beta formation from its precursor have been studied extensively; however, considerably less effort has been invested into studying A beta clearance. We find that the degradation of A beta in our system is dependent upon the presence of a metallopeptidase E.C.3.4.24.15 (MP24.15) (Yamin et al., 1999). We have previously purified MP24.15 to homogeneity from AD brain and identified it as an APP-processing protease in vitro (Papastoitsis, 1994). To confirm its role in cell culture, we transfected SKNMC neuroblastoma cells with sense and antisense cDNAs of MP24.15 and with a mock construct. Compared to mock conditioned media (CM), CM of MP24.15-overexpressing cells had very high A beta-degrading activity. Conversely, CM of antisense-expressing cells lacked A beta-degrading activity. These results suggested that MP24.15 is involved in A beta degradation. Characterization of the proteolytic activity directly responsible for A beta degradation using a spectrum of protease inhibitors revealed that only serine protease inhibitors completely blocked A beta degradation. Therefore, MP24.15 appears to activate a serine protease, which then cleaves A beta. Interestingly, alpha 1-antichymotrypsin (ACT) which we discovered to be highly elevated in AD brain (Abraham, et al., 1988) also inhibited A beta degradation. To our delight, ACT proved to be an inhibitor of A beta degradation in vivo as well. When we crossed transgenic mice expressing human ACT with plaque-producing mice expressing human APP, the doubly transgenic mice had twice as many plaques at 20 months of age as the APP mice (Mucke et al., 2000). Successful completion of this study could lead to the design of reagents that would reduce the amyloid load in AD patients.