The role of prion protein glycosylation in prion propagation

Abstract

Transmissible Spongiform Encephalopathies (TSEs) are a group of neurodegenerative diseases that affect humans and animals alike. TSEs are caused by the accumulation of a disease producing isoform referred to as PrPSc that results from the misfolding of the normal cellular prion protein PrPC. The pathological outcomes of TSEs include amyloid plaque build-up and spongiform degeneration in the brain of infected hosts. Clinical signs of prion disease can vary between TSEs, but often include neurologic impairment that is subtle in onset and tends to progress slowly. Prion diseases are relatively recently discovered and have sparked much controversy due to the scientific findings that directly challenge some of the most well established scientific dogmas. Among these is that the infectious agent responsible for the transmission of TSEs is proteinacious in nature and devoid of the nucleic acids present in pathogens like viruses and bacteria. As a result of this hypothesis, both PrPC and PrPSc share the same amino acid sequence in the host. Therefore, central to our understanding of the prion hypothesis is to recognize the structural differences between PrPC and PrPSc. PrPC has been proven to include three α-helices and two, short β-pleated sheets whereas PrPSc consists of high β-sheet content, aggregates in the presence of detergents, and is resistant to protease treatment. These characteristics of PrPSc have inhibited researchers to successfully examine the abnormal isoform in high-resolution structural studies. Therefore, an alternative means of distinguishing PrPC and PrPSc is necessary. Since then, several groups have created monoclonal antibodies (mAbs) that differentiate between infectious prion protein (PrP) aggregates. Two such mAbs, PRC5 and PRC7 were the first mAbs discovered in which the involvement of individual residues in functional, discontinuous, and conformationaly dependent epitopes was studied. Of these antibodies, PRC7, is dependent on N-linked glycosylation at mono-1 of the prion protein and specifically binds to the infected isoform of PrP. Therefore, we hypothesized that an underglycosylated form of PrP is preferentially generated during prion replication in the infected host. In this body of work, we have systemically ablated mono-1, one of the two N-linked glycan attachment sites on the murine prion protein to address the role of underglycosylation in prion propagation at N180 and at S/T182 of the consensus sequence by mutating N or S/T to each of the other 19 amino acids individually. Here we present novel evidence showing the effects of underglycosylation in prion propagation of prion isolates RML, 22L, 139A, and mCWD. These preliminary data demonstrate the importance of post-translational differences between PrPC and PrPSc which represent a fundamental, unresolved aspect of the prion hypothesis.