Credit: DIGITAL VISION

The host cellular prion protein (PrPC) can be converted into the misfolded and infectious scrapie form (PrPSc) when the two forms come into contact, causing prion diseases. By developing a new cell system, this study offers novel insights into how PrPC is converted into the misfolded protein.

To study prion conversion in more detail, the authors developed PrPC constructs expressing a MYC tag at the carboxyl terminus. These were transfected into a prion-susceptible mouse cell line in which endogenous PrPC had been silenced (PrP-224AlaMYC cells). The MYC-tagged constructs were expressed at wild-type levels and localized normally, and exposure to prions led to robust conversion into MYC-tagged PrPSc. To confirm that the cells were producing infectious prions, extracts were used to infect mice, which subsequently developed neuropathology typical of prion disease.

Previous studies had suggested that de novo-synthesized PrPSc appears within 72 hours (or longer) of exposure to infectious material. Interestingly, the authors show that some PrP-224AlaMYC cells accumulated PrPSc in under 2 hours, and in fact only 1–2 minutes of exposure was enough to lead to conversion. Therefore, conversion is much faster than previously thought.

initial prion conversion seems to first occur at the plasma membrane

So where is PrPSc synthesized? The plasma membrane had been suggested as the site of prion conversion, and indeed, in cells exposed to infectious material for just 1 minute, PrPSc was present only on the plasma membrane. However, intracellular PrPSc was observed after 2 minutes of exposure, suggesting that it is rapidly endocytosed and trafficked into the cell. Indeed, the location of PrPSc in the cell was found to depend on the length of exposure, with PrPSc being located exclusively on the plasma membrane at the earliest time-point and distributed inside the cell later on.

To confirm that prion conversion can occur on the plasma membrane, the authors blocked endocytosis; this did not prevent PrPSc formation. Importantly, PrPSc levels were significantly reduced in PrP-224AlaMYC cells that had been treated with phosphoinositide-specific phospholipase C or with agents that disrupt lipid raft integrity. Therefore, initial prion conversion seems to occur first at the plasma membrane, perhaps in lipid rafts. However, intracellular compartments may also have a role in this, possibly by accelerating conversion after PrPSc has been endocytosed.

Together, these findings provide new insights into prion conversion by revealing the kinetics and location of PrPSc formation, and may partly explain why prion infections spread throughout the nervous system so quickly.