Fibrillar Amyloid β-Protein Binds Protease Nexin-2/Amyloid β-Protein Precursor:  Stimulation of Its Inhibition of Coagulation Factor XIa

Cerebrovascular deposition of fibrillar 39−42 amino acid amyloid β-protein (Aβ), a condition known as cerebral amyloid angiopathy (CAA), is a key pathological feature of Alzheimer's disease and related disorders including hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D). Severe cases of CAA, particularly in HCHWA-D, lead to recurrent and often fatal hemorrhagic strokes. Although the reasons for this pathological consequence remain unclear, alterations in proteolytic hemostasis mechanisms have been implicated. For example, the Aβ parent molecule protease nexin-2/amyloid β-protein precursor (PN-2/AβPP), which is elevated in HCHWA-D cerebral vessels with Aβ deposits, is a potent inhibitor of coagulation factor XIa (FXIa). Here we show that fibrillar HCHWA-D Aβ binds PN-2/AβPP, but not its isolated Kunitz-type proteinase inhibitor (KPI) domain, in a saturable, dose-dependent manner with a K d of ≈28 nM. Neither PN-2/AβPP nor its KPI domain bound to nonfibrillar HCHWA-D Aβ. The fibrillar Aβ binding domain on PN-2/AβPP was localized to residues 18−119. PN-2/AβPP that bound to fibrillar HCHWA-D Aβ immobilized either in plastic wells or on the surface of cultured cerebrovascular smooth muscle cells was active in inhibiting FXIa. Quantitative kinetic measurements revealed that fibrillar HCHWA-D Aβ caused a >5-fold enhancement of FXIa inhibition by PN-2/AβPP. Similar stimulatory effects on FXIa inhibition by PN-2/AβPP were also observed with fibrillar wild-type Aβ. However, fibrillar Aβ had no effect on the inhibition of trypsin by PN-2/AβPP. These findings suggest that fibrillar Aβ deposits in cerebral vessels can effectively localize and enhance the anticoagulant functions of PN-2/AβPP, thereby contributing to a microenvironment conducive to hemorrhaging.