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

by Selleckchem | Sep 28 2023

The cellular prion protein (PrPC) converts to alternatively folded pathogenic conformations (PrPSc) in prion infections and binds neurotoxic oligomers formed by amyloid-β α-synuclein, and tau. β-Endoproteolysis, which splits PrPC into N- and C-terminal fragments (N2 and C2, respectively), is of interest because a protease-resistant, C2-sized fragment (C2Sc) accumulates in the brain during prion infections, seemingly comprising the majority of PrPSc at disease endpoint in mice. However, candidates for the underlying proteolytic mechanism(s) remain unconfirmed in vivo. Here, a cell-based screen of protease inhibitors unexpectedly linked type II membrane proteins of the S9B serine peptidase subfamily to PrPC β-cleavage. Overexpression experiments in cells and assays with recombinant proteins confirmed that fibroblast activation protein (FAP) and its 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, respectively. C2 levels relative to total PrPC were reduced in several tissues from FAP-null mice, and, while knockout of DPP4 lacked an analogous effect, the combined DPP4/FAP inhibitor linagliptin, but not the FAP-specific inhibitor SP-13786, reduced C2Sc and total PrPSc levels in two murine cell-based models of prion infections. Thus, the net activity of the S9B peptidases FAP and DPP4 and their cognate inhibitors/modulators affect the physiology and pathogenic potential of PrPC.

Comments:

The passage you provided describes a study conducted to investigate the proteolytic cleavage of the cellular prion protein (PrPC) and its potential role in prion infections and neurodegenerative diseases. PrPC can convert into pathogenic conformations known as PrPSc, which are associated with prion infections and can bind to toxic oligomers formed by amyloid-β, α-synuclein, and tau.

The researchers aimed to identify the protease responsible for cleaving PrPC into N-terminal (N2) and C-terminal (C2) fragments, particularly focusing on the accumulation of the protease-resistant C2 fragment (C2Sc) during prion infections. They conducted a cell-based screen of protease inhibitors and unexpectedly discovered a link between the S9B serine peptidase subfamily, which includes type II membrane proteins, and PrPC β-cleavage.

Further experiments involving the overexpression of fibroblast activation protein (FAP) and its paralog, dipeptidyl peptidase-4 (DPP4), confirmed that these enzymes directly cleave PrPC at multiple sites within its N-terminal domain. The activity of FAP and DPP4 was evaluated using wild-type mouse and human PrPC substrates, and the results showed differences in cleavage efficiency between species.

The study also investigated the impact of FAP and DPP4 on the accumulation of C2Sc and total PrPSc levels in various tissues. Tissue samples from FAP-null mice showed reduced C2Sc levels relative to total PrPC, suggesting the involvement of FAP in C2Sc generation. On the other hand, knockout of DPP4 did not have a similar effect. However, when a combined DPP4/FAP inhibitor called linagliptin was used, C2Sc and total PrPSc levels were reduced in cell-based models of prion infections.

In summary, this research identified FAP and DPP4, members of the S9B serine peptidase subfamily, as proteases involved in PrPC cleavage. The study suggests that the activity of these peptidases, along with their inhibitors/modulators, can influence the physiological and pathogenic properties of PrPC. These findings contribute to our understanding of prion infections and may have implications for the development of therapeutic strategies targeting PrPSc accumulation in neurodegenerative diseases.