Endoproteolysis of cellular prion protein by plasmin hinders propagation of prions

Many questions surround the underlying mechanism for the differential metabolic processing observed for the prion protein (PrP) in healthy and prion-infected mammals. Foremost, the physiological alpha-cleavage of PrP interrupts a region critical for both toxicity and conversion of cellular PrP (PrPC) into its misfolded pathogenic isoform (PrPSc) by generating a glycosylphosphatidylinositol (GPI)-anchored C1 fragment. During prion diseases, alternative beta-cleavage of PrP becomes prominent, producing a GPI-anchored C2 fragment with this particular region intact. It remains unexplored whether physical up-regulation of alpha-cleavage can inhibit disease progression. Furthermore, several pieces of evidence indicate that a disintegrin and metalloproteinase (ADAM) 10 and ADAM17 play a much smaller role in the alpha-cleavage of PrPC than originally believed, thus presenting the need to identify the primary protease(s) responsible. For this purpose, we characterized the ability of plasmin to perform PrP alpha-cleavage. Then, we conducted functional assays using protein misfolding cyclic amplification (PMCA) and prion-infected cell lines to clarify the role of plasmin-mediated alpha-cleavage during prion propagation. Here, we demonstrated an inhibitory role of plasmin for PrPSc formation through PrP alpha-cleavage that increased C1 fragments resulting in reduced prion conversion compared with non-treated PMCA and cell cultures. The reduction of prion infectious titer in the bioassay of plasmin-treated PMCA material also supported the inhibitory role of plasmin on PrPSc replication. Our results suggest that plasmin-mediated endoproteolytic cleavage of PrP may be an important event to prevent prion propagation.