COAGULATION-FACTOR XIA CLEAVES THE RHDS SEQUENCE AND ABOLISHES THE CELL ADHESIVE PROPERTIES OF THE AMYLOID BETA-PROTEIN

Amyloid beta-protein (A beta) is the major constituent of senile plaques and cerebrovascular amyloid deposits in Alzheimer's disease and is proteolytically derived from its transmembrane parent protein the amyloid beta-protein precursor (A beta PP). Although the physiological role(s) of secreted A beta PPs are not fully understood, several potential functions have been described including the regulation of hemostatic enzymes factors XIa and Ma and a role in cell adhesion. In the present study, we investigated the proteolytic processing of A beta PP by factor XIa (FXIa). Incubation of the human glioblastoma cell line U138 stably transfected to overexpress the 695 isoform of A beta PP with FXIa (2.55 nM) resulted in proteolytic cleavage of secreted A beta PP. Higher concentrations of FXIa (>25 nM) resulted in loss in cell adherence. Coin cubation of FXIa with purified, recombinant Kunitz protease inhibitor domain of A beta PP blocked both the proteolytic processing of A beta PP and the loss of cell adhesion. The RHDS cell adhesion site of A beta PP resides within residues 5-8 of the A beta domain. Incubation of synthetic AP(1-40) peptide with increasing concentrations of FXIa resulted in cleavage of A beta between Arg(5) and His(6) within the cell adhesion domain of the peptide. FXIa-digested A beta(1-40) or A beta PP695 lost their abilities to serve as cell adhesion substrates consistent with cleavage through this cell adhesion site, Together, these results suggest a new potential biological function for FXIa in the modulation of cell adhesion. In addition, we have shown that FXIa can proteolytically alter A beta and therefore possibly modify its physiological and perhaps pathological properties.