Branched Polyamines Cure Prion-Infected Neuroblastoma Cells

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Abstract
Branched polyamines, including polyamidoamine and polypropyleneimine (PPI) dendrimers, are able to purge PrPSc, the disease-causing isoform of the prion protein, from scrapie-infected neuroblastoma (ScN2a) cells in culture (S. Supattapone, H.-O. B. Nguyen, F. E. Cohen, S. B. Prusiner, and M. R. Scott, Proc. Natl. Acad. Sci. USA 96:14529–14534, 1999). We now demonstrate that exposure of ScN2a cells to 3 μg of PPI generation 4.0/ml for 4 weeks not only reduced PrPSc to a level undetectable by Western blot but also eradicated prion infectivity as determined by a bioassay in mice. Exposure of purified RML prions to branched polyamines in vitro disaggregated the prion rods, reduced the β-sheet content of PrP 27-30, and rendered PrP 27-30 susceptible to proteolysis. The susceptibility of PrPSc to proteolytic digestion induced by branched polyamines in vitro was strain dependent. Notably, PrPSc from bovine spongiform encephalopathy-infected brain was susceptible to PPI-mediated denaturation in vitro, whereas PrPSc from natural sheep scrapie-infected brain was resistant. Fluorescein-labeled PPI accumulated specifically in lysosomes, suggesting that branched polyamines act within this acidic compartment to mediate PrPSc clearance. Branched polyamines are the first class of compounds shown to cure prion infection in living cells and may prove useful as therapeutic, disinfecting, and strain-typing reagents for prion diseases.