Detection of distinct isoform patterns of the β-amyloid precursor protein in human platelets and lymphocytes

Abstract

—Cerebral deposition of the amyloid β-protein (AβP), a ∼40 residue fragment of the integral membrane protein, β-amyloid precursor protein (βAPP), has been implicated as the probable cause of some cases of familial Alzheimer's disease (AD). The parallels between AβP deposition in AD and the deposition of certain plasma proteins in systemic amyloid diseases has heightened interest in the analysis of βAPP in circulating cells and plasma. Here, we describe distinct isoform patterns of βAPP in peripheral platelets and lymphocytes. PCR-mediated amplification of mRNA from purified platelets demonstrated the expression of all three major βAPP transcripts (βAPP 770,751,695). The full-length, ∼140 kDa form of βAPP 751,770 was detected in membranes of resting and activated platelets but very little immature, ∼122 kDa βAPP 751,770 was found, suggesting a different processing of βAPP 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 βAPP (protease nexin-II), but no evidence for the shedding of full-length βAPP 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 ∼10 kDa carboxyl terminal βAPP 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 βAPP 751,770. The potentially amyloidogenic full-length βAPP molecules present in circulating platelets and lymphocytes but not in microparticles could serve as a source of the microvascular AβP deposited during aging and particularly in AD.