PROTEOLYTIC PROCESSING OF ALZHEIMERS-DISEASE BETA-A4 AMYLOID PRECURSOR PROTEIN IN HUMAN PLATELETS

The processing of amyloid precursor protein (APP) and production of be ta A4 amyloid are events likely to influence the development and progr ession of Alzheimer's disease, since beta A4 is the major constituent of amyloid deposited in this disorder. Our previous studies showed tha t human platelets contain full-length APP (APP(FL)) and are a suitable substrate to study normal APP processing, In the present study, we sh ow that a 22-kDa beta A4-containing carboxyl-terminal fragment (22-CTF ) of APP is present in unstimulated platelets. Both APP(FL) and 22-CTF are proteolytically degraded when platelets are activated with thromb in, collagen, or calcium ionophore A23187. Complete cleavage of APP(FL ) and 22-CTF require the presence of extracellular calcium, Following stimulation in the presence of calcium, a new CTF of 17 kDa is generat ed, and the NH2-terminal epitope of beta A4 amyloid is lost. Preincuba tion of platelets with the cell-permeable cysteine protease inhibitors calpeptin, -trans-epoxysuccinyl-L-leucyl-amido-3-methylbutane ethyl e ster (E64d), Na-alpha-p-tosyl-L-lysine chloromethyl ketone, or calcium chelator EGTA before platelet stimulation inhibits the degradation of both APP(FL) and 22-CTF. Divalent metal ions including zinc, copper, and cobalt inhibit the degradation of APP(FL) and 22-CTF. This study s uggests that a calcium-dependent neutral cysteine protease is involved in the proteolytic processing of an amyloidogenic species of APP in h uman platelets.