Figure 4. Schematic presentation of the mirror effect hypothesis.

The activated cellular and/or tissue response in the presence of PrPSc (right side) is a negative image of that observed in the absence of PrP (left side). It suggests a mechanism of prion disease associated neurodegeneration in which the over activation or subversion (Harris and True, 2006) of PrP by PrPSc could lead to neurotoxicity. In Prnp knockout mice, the presence of the protein shadoo seems to be essential for an efficient compensatory mechanism and survival. In Prnp-knockout, Sprn-knockdown mice, an embryonic lethality was observed, this lethality could be caused by a gastrulation defect characterized by placentation and hematopoesis defaults.