The anti-prion activity of Congo red. Putative mechanism

PrP(Sc), an abnormal conformational isoform of the normal prion protein, PrP(C), is the only known component of the prion, a proteinacious agent that causes fatal neurodegenerative disorders in humans and other animals. The hallmark properties of PrP(Sc) are its insolubility in nondenaturing detergents and its resistance to digestion by proteases. Anions such as Congo red (CR) have been shown to reduce the accumulation of PrP(Sc) in a neuroblastoma cell line permanently infected with prions as well as to delay disease onset in rodents when administrated prophylactically. The mechanism by which such anti-prion agents operate is unknown. We show here that in vitro incubation with CR renders native PrP(Sc) resistant to denaturation by boiling SDS. This resulted from PrP(Sc) conformation, since neither the properties of PrP(C) nor those of predenatured PrP(Sc) were changed by the addition of CR. CR-PrP(Sc) could only be denatured by the addition of acidic 3 M guanidine thiocyanate. Since in vitro conversion experiments have suggested that partial denaturation may be required for PrP(Sc) to serve as template in the PrP(C) → PrP(Sc) conversion, we propose that CR inhibits prion propagation by overstabilizing the conformation of PrP(Sc) molecules.