Minor misfolding events probably lead to continual ejection of PrP from the ER into the cytosol for degradation by proteasomes. The team's inhibition of cytosolic proteasomes preferentially killed cells expressing PrP, and expression of PrP lacking ER translocation signals was highly toxic to neural cells both in vitro and in vivo. Transgenic mice producing cytosolic PrP suffered from unsteady gait and massive neuronal loss due to degeneration rather than problems in development. Thus, a cytosolic PrP rather than extracellular PrPSc aggregate appears to be the toxic species.
“This represents a real breakthrough in the prion field,” says Peter Lansbury (Harvard Medical School, Boston, MA). At the same time, he says, “I'm not at all surprised. It's very consistent with all the other neurodegenerative diseases.”
Fibrillar forms of β-amyloid (in Alzheimer's disease) and α-synuclein (in Parkinson's disease) initially dominated theories of pathogenesis in these neurodegenerative conditions. But others have demonstrated extraordinary toxicity of microinjected β-amyloid protein, and Lansbury has pointed to cytoplasmic protofilaments—a non-plaque form that resemble pore-forming bacterial toxins—as the toxic species.
The prion field, however, has assumed that the infectious aggregates of PrPSc are also the toxic form. “The whole field has been driven by neuropathology,” says Lansbury. “That's why it's so good to have people from different backgrounds working on this. [Lindquist] isn't subject to the years and years of focus on amyloid plaques.”
The Lindquist team also showed that proteasome inhibition could eventually lead to formation of PrPSc. Lindquist suggests that the toxicity of cytosolic PrP may be a way of sabotaging this event. In this theory, if folding problems lead to excessive ejection of PrP from the ER to the cytosol, the cell commits suicide rather than risk making the infectious version, PrPSc. Cytosolic PrP, says Lindquist, “is so extremely toxic that it may represent a defense mechanism.” ▪