DETECTION OF DISTINCT ISOFORM PATTERNS OF THE BETA-AMYLOID PRECURSOR PROTEIN IN HUMAN PLATELETS AND LYMPHOCYTES

Cerebral deposition of the amyloid-beta-protein (A-beta-P), a approximately 40 residue fragment of the integral membrane protein, beta-amyloid precursor protein (beta-APP), has been implicated as the probable cause of some cases of familial Alzheimer's disease (AD). The parallels between A-beta-P deposition in AD and the deposition of certain plasma proteins in systemic amyloid diseases has heightened interest in the analysis of beta-APP in circulating cells and plasma. Here, we describe distinct isoform patterns of beta-APP in peripheral platelets and lymphocytes. PCR-mediated amplification of mRNA from purified platelets demonstrated the expression of all three major beta-APP transcripts (beta-APP770,751,695). The full-length, approximately 140 kDa form of beta-APP751,770 was detected in membranes of resting and activated platelets but very little immature, approximately 122 kDa beta-APP751,770 was found, suggesting a different processing of beta-APP in platelets than that described in a variety of cultured cells and tissues. Platelets stimulated with thrombin, calcium ionophore. or collagen released the soluble, carboxyl-truncated form of beta-APP (protease nexin-II), but no evidence for the shedding of full-length-beta-APP associated with platelet microparticles was found, in contrast to previous reports. As a positive control marker for microparticles, the fibrinogen receptor subunit, GPIIIa, was readily detected in platelet releasates. Resting and activated platelets contained similar amounts of the approximately 10 kDa carboxyl terminal-beta-APP fragment that is retained in platelet membranes following the constitutive cleavage of protease nexin-II. Nonstimulated peripheral B and T lymphocytes contained small amounts of membrane-associated mature and immature-beta-APP751,770. The potentially amyloidogenic full-length-beta-APP molecules present in circulating platelets and lymphocytes but not in microparticles could serve as a source of the microvascular A-beta-P deposited during aging and particularly in AD.