Date Published: July 10, 2019
Publisher: Public Library of Science
Author(s): Anne Ward, Jason R. Hollister, Young Pyo Choi, Brent Race, Katie Williams, Daniel W. Shoup, Roger A. Moore, Suzette A. Priola, Anthony E. Kincaid.
Prion protein (PrPC) is a protease-sensitive and soluble cell surface glycoprotein expressed in almost all mammalian cell types. PrPSc, a protease-resistant and insoluble form of PrPC, is the causative agent of prion diseases, fatal and transmissible neurogenerative diseases of mammals. Prion infection is initiated via either ingestion or inoculation of PrPSc or when host PrPC stochastically refolds into PrPSc. In either instance, the early events that occur during prion infection remain poorly understood. We have used transgenic mice expressing mouse PrPC tagged with a unique antibody epitope to monitor the response of host PrPC to prion inoculation. Following intracranial inoculation of either prion-infected or uninfected brain homogenate, we show that host PrPC can accumulate both intra-axonally and within the myelin membrane of axons suggesting that it may play a role in axonal loss following brain injury. Moreover, in response to the inoculation host PrPC exhibits an increased insolubility and protease resistance similar to that of PrPSc, even in the absence of infectious prions. Thus, our results raise the possibility that damage to the brain may be one trigger by which PrPC stochastically refolds into pathogenic PrPSc leading to productive prion infection.
Prion diseases are rare, transmissible and inevitably fatal neurodegenerative diseases characterized by the accumulation of an infectious, aggregated and insoluble protease-resistant isoform (PrPSc) of the normally soluble and protease-sensitive host prion protein (PrPC). They have been identified in multiple mammalian species including sheep (scrapie), deer (chronic wasting disease), cattle (bovine spongiform encephalopathy) and humans (Creutzfeldt-Jakob disease). Prion diseases are distinguished neuropathologically by significant gliosis and cellular loss in the brain which leads to the spongiform change that is a hallmark of these diseases. The causative agent of prion diseases, known as the prion, is composed primarily of PrPSc. Prions replicate via a mechanism known as seeded polymerization whereby aggregates of PrPSc bind to host PrPC and induce its conformational conversion into new PrPSc [1, 2]. Over a period of months to years, persistent prion replication leads to the accumulation of PrPSc in the brain and eventually clinical prion disease. Interestingly, prion diseases can be initiated either via exposure to exogenous prions or by the spontaneous refolding of the host PrPC molecule into PrPSc. It is unclear what may trigger host PrPC to spontaneously refold into PrPSc but the end result is sporadic Creutzfeldt-Jakob disease (sCJD), the most common form of prion disease in humans . Regardless of the origin of infection, there are no clinically proven preventative or therapeutic treatments for prion diseases and they are inevitably fatal.
In this study, we used transgenic mice expressing a uniquely epitope-tagged PrP molecule (PrP-3F4) to monitor host PrPC and the de novo formation of host-derived PrPSc following IC inoculation of prions. In some mice, we detected increased levels of protease-resistant host PrP by immunoblot as early 72 hrs post-inoculation, a result consistent with a previous study  and suggestive of de novo PrPSc formation. However, unlike the previous study , we also detected similar levels of aggregated and protease-resistant PrP in our NBH controls at 72 hours (Fig 4C). Thus, due to changes in both the cellular distribution and biochemical properties of host PrPC as a result of the inoculation itself, we were unable to definitively distinguish host derived PrPSc from host PrPC at 72 hours. At one and two weeks post-infection we did find some evidence of PrPSc propagation, but that was only in a single mouse at each timepoint. Our study highlights the difficulties in distinguishing newly formed PrPScin vivo, even when using uniquely tagged PrPC molecules, and suggests that the response of host PrPC to intracranial inoculation may confound the biochemical identification of bona fide PrPSc during early prion infection.