PRP27-30 is a normal soluble prion protein fragment released by human platelets

Prion diseases are neurodegenerative disorders characterized by the accumulation of abnormal isoforms of prion protein (PrP(Sc)) in the central nervous system. PrP(Sc) isoforms differ from their normal homologue (PrP(C)), in that they possess increased β-sheet conformation, are partially protease resistant and may be associated with amyloid deposition. Amyloid proteins are thought to derive from soluble precursors or fragments thereof, present in biological fluids, which in the disease state undergo conformational change lending to aggregation and deposition in target tissues. We report here that platelets carry PrP mRNA and release PrP(C), a sialoglycoprotein bound to the cell surface by a glycosylphosphatidylinositol (GPI) anchor. Soluble PrP(C) and a N-terminal truncated PrP(C) isoform starting at position 90 are secreted by resting and agonist-stimulated platelets and are detectable after partial deglycosylation of releasates. N-terminal sequence analysis of the soluble 27-30 kDa isoform, GQGGGTHSQ(W)NKP, revealed homology to scrapie PrP_27-30, the protease resistant core derived from PrP(Sc). These findings indicate that in addition to PrP(C), platelets process a soluble PrP_27-30 isoform. Whether this isoform can be converted into scrapie PrP_27-30 remains to be determined.