Abstract
Prion diseases are infectious neurodegenerative disorders that affect humans and other mammals and are inevitably fatal. The infectious agent in prion disease (PrPSc) is an abnormal isoform of an endogenous host protein (PrPC). Prion conversion involves a conformational change of PrPC into PrPSc, and this process may involve host polyanions such as ribonucleic acid (RNA). Conformational alterations in PrPSc are hypothesized to encode different prion strains, and the type of prion strain affects the disease incubation period, clinical signs, and neuropathology. Prions can exist as a mixture of strains in a host, and one implication of prion mixtures is that prion strains can interfere with one another. In prion strain interference, a blocking strain (typically a long incubation period strain) interferes with another strain by completely inhibiting the emergence of the second strain or by extending its incubation period. Strain interference is likely involved in prion transmission and adaptation, so it is important to elucidate the mechanism of prion strain interference. We hypothesize that in prion strain interference, the blocking strain sequesters host factors necessary for prion conversion, preventing their use by the superinfecting strain. |To investigate the capacity of long incubation period strains to serve as blocking strains in prion strain interference, we performed experiments utilizing the long incubation period hamster strain 139H. Following intrasciatic nerve (i.sc.) inoculation, we determined that 139H PrPSc is retrogradely transported throughout the central nervous system (CNS) using the same four descending motor tracts as the short incubation period strain hyper (HY) transmissible mink encephalopathy (TME). Co-infection of hamsters with 139H and HY TME led to strain interference when the interval between inoculations was 75 days, indicating that 139H can serve as a blocking strain. Using a modified conformational stability assay, we found evidence of mixtures of HY and 139H PrPSc in the brains of animals that did not show signs of strain interference. This suggests that although clinical signs and incubation period might not have been affected, strain interference was likely still occurring in the form of 139H PrPSc interfering with HY PrPSc conversion.|Ribonucleic acid is hypothesized to be a prion conversion cofactor, and we investigated the potential role of RNA in prion strain interference. Conversion of HY and drowsy (DY) PrPSc in vitro was inhibited when RNA was depleted from the conversion substrate, indicating that these strains require RNA. Synthetic poly(A) RNA rescued PrPSc conversion, leading us to use synthetic RNA to supplement our in vitro strain interference model. The addition of RNA to the conversion substrate did not enable HY PrPSc to overcome the blocking effect of DY PrPSc, indicating that RNA is not likely the host factor involved in prion strain interference.|Another host factor hypothesized to be involved in prion strain interference is PrPC. To investigate the role of PrPC in strain interference, we utilized rabbit kidney epithelial cells that express hamster PrPC that is detectable using multiple antibodies. The lysate of these cells supported conversion of HY and DY PrPSc, and we added the cells to our in vitro strain interference model to increase the PrPC content of the conversion substrate. Depending on the antibody used to detect the PrPSc in the interference samples, we observed either no change to the strain interference effect or an earlier emergence of HY PrPSc. Passage conditions between the rounds of the in vitro strain interference model were also altered as a means to investigate the role of PrPC in strain interference. The change in passage conditions led to an earlier emergence of HY PrPSc. Our results indicate that PrPC might be involved in prion strain interference, but further studies are necessary to support this conclusion.