Prion Infectivity Plateaus and Conversion to Symptomatic Disease Originate from Falling Precursor Levels and Increased Levels of Oligomeric PrPSc Species

In lethal prion neurodegenerative diseases, misfolded prion proteins (PrP Sc ) replicate by redirecting the folding of the cellular prion glycoprotein (PrP C ). Infections of different durations can have a subclinical phase with constant levels of infectious particles, but the mechanisms underlying this plateau and a subsequent exit to overt clinical disease are unknown. Using tandem biophysical techniques, we show that attenuated accumulation of infectious particles in presymptomatic disease is preceded by a progressive fall in PrP C level, which constricts replication rate and thereby causes the plateau effect. Furthermore, disease symptoms occurred at the threshold associated with increasing levels of small, relatively less protease-resistant oligomeric prion particles (oPrP Sc ). Although a hypothetical lethal isoform of PrP cannot be excluded, our data argue that diminishing residual PrP C levels and continuously increasing levels of oPrP Sc are crucial determinants in the transition from presymptomatic to symptomatic prion disease. IMPORTANCE Prions are infectious agents that cause lethal brain diseases; they arise from misfolding of a cell surface protein, PrP C to a form called PrP Sc . Prion infections can have long latencies even though there is no protective immune response. Accumulation of infectious prion particles has been suggested to always reach the same plateau in the brain during latent periods, with clinical disease only occurring when hypothetical toxic forms (called PrP L or TPrP) begin to accumulate. We show here that infectivity plateaus arise because PrP C precursor levels become downregulated and that the duration of latent periods can be accounted for by the level of residual PrP C , which transduces a toxic effect, along with the amount of oligomeric forms of PrP Sc .