Multiple biochemical similarities between infectious and non-infectious aggregates of a prion protein carrying an octapeptide insertion.

A nine-octapeptide insertion in the prion protein (PrP) gene is associated with an inherited form of human prion disease. Transgenic (Tg) mice that express the mouse homolog of this mutation (designated PG14) spontaneously accumulate in their brains an insoluble and weakly protease-resistant form of the mutant protein. This form (designated PG14Spon) is highly neurotoxic, but is not infectious in animal bioassays. In contrast, when Tg(PG14) mice are inoculated with the Rocky Mountain Laboratory (RML) strain of prions, they accumulate a different form of PG14 PrP (designated PG14RML) that is highly protease resistant and infectious in animal transmission experiments. We have been interested in characterizing the molecular properties of PG14Spon and PG14RML, with a view to identifying features that determine two, apparently distinct properties of PrP aggregates: their infectivity and their pathogenicity. In this paper, we have subjected PG14Spon and PG14RML to a panel of assays commonly used to distinguish infectious PrP (PrPSc) from cellular PrP (PrPC), including immobilized metal affinity chromatography, precipitation with sodium phosphotungstate, and immunoprecipitation with PrPC- and PrPSc-specific antibodies. Surprisingly, we found that aggregates of PG14Spon and PG14RML behave identically to each other, and to authentic PrPSc, in each of these biochemical assays. PG14Spon however, in contrast to PG14RML and PrPSc, was unable to seed the misfolding of PrPC in an in vitro protein misfolding cyclic amplification reaction. Collectively, these results suggest that infectious and non-infectious aggregates of PrP share common structural features accounting for their toxicity, and that self-propagation of PrP involves more subtle molecular differences. [ABSTRACT FROM AUTHOR]