Matrix metalloproteinase-9 degrades amyloid-β fibrils in vitro and compact plaques in situ

The pathological hallmark of Alzheimer disease is the senile plaque principally composed of tightly aggregated amyloid-beta fibrils (fA beta), which are thought to be resistant to degradation and clearance. In this study, we explored whether proteases capable of degrading soluble A beta (sA beta) could degrade fA beta as well. We demonstrate that matrix metalloproteinase-9 (MMP-9) can degrade fA beta and that this ability is not shared by other sA beta-degrading enzymes examined, including endothelin-converting enzyme, insulin-degrading enzyme, and neprilysin. fA beta was decreased in samples incubated with MMP-9compared with other proteases, assessed using thioflavin-T. Furthermore, fA beta breakdown with MMP-9 but not with other proteases was demonstrated by transmission electron microscopy. Proteolytic digests of purified fA beta were analyzed with matrix-assisted laser desorption ionization time-of-flight mass spectrometry to identify sites of A beta that are cleaved during its degradation. Only MMP-9 digests contained fragments (A beta(1-20) and A beta(1-30)) from fA beta(1-42) substrate; the corresponding cleavage sites are thought to be important for beta-pleated sheet formation. To determine whether MMP-9 can degrade plaques formed in vivo, fresh brain slices from aged APP/PS1 mice were incubated with proteases. MMP-9 digestion resulted in a decrease in thioflavin-S (ThS) staining. Consistent with a role for endogenous MMP-9 in this process in vivo, MMP-9 immunoreactivity was detected in astrocytes surrounding amyloid plaques in the brains of aged APP/PS1 and APPsw mice, and increased MMP activity was selectively observed in compact ThS-positive plaques. These findings suggest that MMP-9 can degrade fA beta and may contribute to ongoing clearance of plaques from amyloid-laden brains.