Plasmin cleavage of the amyloid β-protein:: Alteration of secondary structure and stimulation of tissue plasminogen activator activity

Cerebrovascular amyloid beta-protein (A beta) deposition, a key pathological feature of Alzheimer's disease and hereditary cerebral hemorrhage with amyloidosis Dutch-type, can lead to intracerebral hemorrhage; however, the mechanism for this remains unclear. Assembled A beta is a potent stimulator of tissue-type plasminogen activator (tPA) in vitro. Herein, we investigated the stimulation of tPA by freshly solubilized A beta(1-40). The rate of tPA stimulation by A beta(1-40) increased dramatically over time, suggesting that A beta may be altered during the course of the reaction, SDS-PAGE analysis showed that A beta(1-40) was cleaved during the course of the reaction. Subsequent studies showed that it was plasmin, the product of tPA activation of plasminogen, that specifically cleaved A beta(1-40) in the amino terminal region between Arg(5) and His(6). Plasmin effectively cleaved a chromogenic substrate corresponding to this cleavage sire in A beta. Circular dichroism spectral analysis showed that A beta(6-40) adopted a strong beta-sheet secondary structure. This truncated A beta(6-40) peptide was a potent stimulator of tPA in vitro. Our results indicate that beta-sheet secondary structure of A beta, which can be promoted by plasmin cleavage, stimulates tPA activity. These findings suggest that pathologic interactions between A beta, tPA, and plasmin in the cerebral vessel wall could result in excessive proteolysis contributing to intracerebral hemorrhages.