Abstract
In infectious and familial prion disorders, neurodegeneration is often seen without obvious deposits of the scrapie prion protein (PrP(Sc)), the principal cause of neuronal death in prion disorders. In such cases, neurotoxicity must be mediated by alternative pathways of cell death. One such pathway is through a transmembrane form of PrP. We have investigated the relationship between intracellular accumulation of prion protein aggregates and the consequent up-regulation of transmembrane prion protein in a cell model. Here, we report that exposure of neuroblastoma cells to the prion peptide 106-126 catalyzes the aggregation of cellular prion protein to a weakly proteinase K-resistant form and induces the synthesis of transmembrane prion protein, the proposed mediator of neurotoxicity in certain prion disorders. The N terminus of newly synthesized transmembrane prion protein is cleaved spontaneously on the cytosolic face of the endoplasmic reticulum, and the truncated C-terminal fragment accumulates on the cell surface. Our results suggest that neurotoxicity in prion disorders is mediated by a complex pathway involving transmembrane prion protein and not by deposits of aggregated and proteinase K-resistant PrP alone.
Highlights
Is partially resistant to limited digestion by proteinase K
We report that exposure of neuroblastoma cells to the prion peptide 106 –126 catalyzes the aggregation of cellular prion protein to a weakly proteinase K-resistant form and induces the synthesis of transmembrane prion protein, the proposed mediator of neurotoxicity in certain prion disorders
Our results suggest that neurotoxicity in prion disorders is mediated by a complex pathway involving transmembrane prion protein and not by deposits of aggregated and proteinase K-resistant PrP alone
Summary
PrPC, normal cell-associated prion protein; PrP106–126, PrP peptide including residues 106 –126 biotin-tagged at the N terminus; PrPScr106126, PrP peptide with a scrambled 106 –126 sequence biotin-tagged at the N terminus; PrPSc, conformationally transformed scrapie form of PrP; A, -amyloid peptide of amyloid precursor protein; PK, proteinase K; PI-PLC, phosphatidylinositol-specific phospholipase C; ER, endoplasmic reticulum; DAB, 3,3Ј-diaminobenzidine; GPI, glycosylphosphatidylinositol; PNGase-F, N-glycosidase F; CtmPrP, transmembrane PrP; L, lysosomes; N, nucleus; E, endosomes. Because neurodegenerative changes typical of prion disorders are often seen without detectable PrPSc, alternative mechanisms of neuronal death besides PrPSc deposition have been suggested (4 – 6) One such mechanism is through the preferred synthesis of CtmPrP, a transmembrane form of PrP that spans the endoplasmic reticulum (ER) membrane at residues 113–135 with its N terminus in the cytosol, rather than the normal glycolipid-linked PrPC that is translocated co-translationally into the ER lumen. Mice carrying the mutant PrP transgene A117V that has an increased predilection for CtmPrP synthesis show spontaneous neurodegeneration without detectable PrPSc and, when challenged with infectious prions, show neurodegeneration earlier and with smaller amounts of accumulated PrPSc than the corresponding animals with a deleted transmembrane domain. Our findings show a direct correlation between intracellular PrPC aggregation and CtmPrP up-regulation, indicating that prion-related neuropathology is mediated by complex cellular pathway(s) involving CtmPrP and not by deposits of PrPSc
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