Toxicity of novel C-terminal prion protein fragments and peptides harbouring disease-related C-terminal mutations

Mice expressing a C-terminal fragment of the prion protein instead of wild- type prion protein die from massive neuronal degeneration within weeks of b irth. The C-terminal region of PrPc (PrP121-231) expressed in these mice ha s an intrinsic neurotoxicity to cultured neurones. Unlike PrPSc, which is n ot neurotoxic to neurones lacking PrPc expression, PrP121-231 was more neur otoxic to PrPc-deficient cells. Human mutations E200K and F198S were found to enhance toxicity of PrP121-231 to PrP-knockout neurones and E200K enhanc ed toxicity to wild-type neurones. The normal metabolic cleavage point of P rPc is approximately aminoacid residue 113. A fragment of PrPc correspondin g to the whole C-terminus of PrPc (PrP113-231), which is eight amino acids longer than PrP121-231, lacked any toxicity. This suggests the first eight amino residues of PrPl13-121 suppress toxicity of the toxic domain in PrP12 1-231. Addition to cultures of a peptide (PrP112-125) corresponding to this region, in parallel with PrP121-231, suppressed the toxicity of PrP121-231 . These results suggest that the prion protein contains two domains that ar e toxic on their own but which neutralize each other's toxicity in the inta ct protein. Point mutations in the inherited forms of disease might have th eir effects by diminishing this inhibition.