Platelets and DAMI megakaryocytes possess beta-secretase-like activity

We report here the discovery of two novel human platelet and megakaryocytic DAMI cell enzymes that have beta-secretase-like activity. These activities could potentially effect cleavage of the amyloid precursor protein (APP) a t the beta-amyloid peptide N-terminus, by an EC 3.4.24.15-like metalloprote ase, and the N terminus-1 position, by a serine protease. Thus both enzymes may generate the amyloidogenic beta-peptide. Studies of intact and Triton X-100-lysed DAMI cells, as well as intact versus subcellular fractions of p latelets, demonstrate the presence of these proteolytic activities. The res ting platelet has (1) a surface serine protease, demonstrated by its abilit y to cleave a beta-secretase substrate and by its inhibitor sensitivity; an d (2) a metalloprotease, recognized by an antibody to EC 3.4.24.15, which r esides intracellularly in the alpha-granule membrane, is translocated to th e surface on activation, and shows beta-secretase-like activity by cleaving the same substrate, This metalloprotease can also cleave recombinant APP t o a potentially amyloidogenic fragment. Surface metalloprotease was identif ied in DAMI cells by flow cytometry and Western blotting with a specific an ti-EC 3.4.24.15 monoclonal antibody, while activity was identified by using two beta-secretase substrates. This article is the first to document two p reviously unknown endoproteinases with beta-secretase-like activity in plat elets and DAMI cells. These proteases are capable of effecting cleavage of APP and could therefore contribute to A beta deposition in the cerebrovascu lature.