Beta-endoproteolysis of the cellular prion protein by dipeptidyl peptidase-4 and fibroblast activation protein

Cellular prion protein (PrPC) converts to alternatively folded pathogenic conformations (PrPSc) in prion infections and binds neurotoxic oligomers created by amyloid-ß a-synuclein, and tau. ß-Endoproteolysis, which splits PrPC into N- and C-terminal fragments (N2 and C2, correspondingly), wil attract just because a protease-resistant, C2-sized fragment (C2Sc) builds up within the brain during prion infections, apparently comprising nearly all PrPSc at disease endpoint in rodents. However, candidates for that underlying proteolytic mechanism(s) remain unconfirmed in vivo. Here, a cell-based screen of protease inhibitors suddenly linked type II membrane proteins from the S9B serine peptidase subfamily to PrPC ß-cleavage. Overexpression experiments in cells and assays with recombinant proteins confirmed that fibroblast activation protein (FAP) and it is paralog, dipeptidyl peptidase-4 (DPP4), cleave directly at multiple sites within PrPC’s N-terminal domain. For wild-type mouse and human PrPC substrates expressed in cells, the rank orders of activity were human FAP ~ mouse FAP > mouse DPP4 > human DPP4 and human FAP > mouse FAP > mouse DPP4 >> human DPP4, correspondingly. C2 levels in accordance with total PrPC were reduced in a number of tissues from FAP-null rodents, and, while knockout of DPP4 lacked an similar effect, the combined DPP4/FAP inhibitor linagliptin, although not the FAP-specific inhibitor SP-13786, reduced C2Sc and total PrPSc levels in 2 murine cell-based types of prion infections. Thus, the internet activity from the S9B peptidases FAP and DPP4 as well as their cognate inhibitors/modulators modify the physiology and pathogenic potential of PrPC.