Prion replication vs. neurotoxicity

Read with caution!

This post was written during early stages of trying to understand a complex scientific problem, and we didn't get everything right. The original author no longer endorses the content of this post. It is being left online for historical reasons, but read at your own risk.

“The role of GPI-anchored PrP(c) in mediating the neurotoxic effect of scrapie prions in neurons,” Radford and Mallucci, Curr Issues Mol Biol (2010)

http://www.ncbi.nlm.nih.gov/pubmed/19767655

There are two central phenomena in prion disease: prion replication and prion neurotoxicity. Underlying them both is the conversion of a host-encoded ubiquitously expressed protein, prion protein (PrP(C)), into a partially-protease resistant isoform, PrP(Sc), which accumulates in the brain. PrP(Sc) is associated with both pathology and infectivity. In the absence of PrP(C), PrP(Sc) cannot be generated and PrP-null mice do not propagate infectivity or develop pathology on infection with scrapie. However, while PrP(C) expression is fundamental to both prion infectivity and neurodegeneration, the uncoupling of these processes is a growing concept in the field. This dissociation is evident in subclinical states of prion infection, where PrP(Sc) levels are high in the absence of disease, and in transgenic mice experiments, where, despite extra-neuronal PrP(Sc) accumulation, even in very high amounts, there is no neurotoxicity. Both these models have further implications. Thus depleting PrP(C) from neurons (but not glia) of prion-infected mice prevents clinical disease, and detaching it from the surface of cells by removing its anchor does the same. The uncoupling toxicity and infectivity has implications for the nature of the neurotoxic species; these mouse models suggest that the site for the generation of this species is intra-neuronal. This review considers the role of neuronal surface-expressed PrP(C) in mediating toxicity in prion infection, and the dissociation of this from the deposition of PrP(Sc).

“Dissociation of infectivity from seeding ability in prions with alternate docking mechanism,” Miller et. al., PLoS Pathogens (2011)

http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1002128

Previous studies identified two mammalian prion protein (PrP) polybasic domains that bind the disease-associated conformer PrP^Sc, suggesting that these domains of cellular prion protein (PrP^C) serve as docking sites for PrP^Sc during prion propagation. To examine the role of polybasic domains in the context of full-length PrP^C, we used prion proteins lacking one or both polybasic domains expressed from Chinese hamster ovary (CHO) cells as substrates in serial protein misfolding cyclic amplification (sPMCA) reactions. After ~5 rounds of sPMCA, PrP^Sc molecules lacking the central polybasic domain (ΔC) were formed. Surprisingly, in contrast to wild-type prions, ΔC-PrP^Sc prions could bind to and induce quantitative conversion of all the polybasic domain mutant substrates into PrP^Sc molecules. Remarkably, ΔC-PrP^Sc and other polybasic domain PrP^Sc molecules displayed diminished or absent biological infectivity relative to wild-type PrP^Sc, despite their ability to seed sPMCA reactions of normal mouse brain homogenate. Thus, ΔC-PrP^Sc prions interact with PrP^C molecules through a novel interaction mechanism, yielding an expanded substrate range and highly efficient PrP^Sc propagation. Furthermore, polybasic domain deficient PrP^Sc molecules provide the first example of dissociation between normal brain homogenate sPMCA seeding ability from biological prion infectivity. These results suggest that the propagation of PrP^Sc molecules may not depend on a single stereotypic mechanism, but that normal PrP^C/PrP^Sc interaction through polybasic domains may be required to generate prion infectivity.

Previous studies have suggested that prion infectivity depends upon the ability of a sample to change the shape of a normal brain protein called the prion protein (PrP) into a disease-associated shape. Other studies have identified a pair of positively charged domains within the structure of PrP that appear to be important for the interaction between the normal and disease-associated shapes of the prion protein. In this report, we show that the shape of normal PrP can change into the disease-associated form through a novel mechanism that does not involve positively charged domains. However, it appears that interaction through the positively charged domains is required to produce infectious prions efficiently. Our results show for the first time that the ability to change the shape of normal PrP into its disease-associated state is not the sole determinant of prion infectivity.

While wild-type PrP^Sc propagates by binding to substrate polybasic domains, ΔPBD mutant PrP^Sc molecules appear to utilize a different mechanism to bind PrP^C, as outlined by the model in Figure 6. This indicates that prion docking, and perhaps other events in conversion, do not necessarily follow a rigidly conserved mechanism.

To our knowledge, this report is the first demonstration of absent or minimal infectivity in samples that successfully seed propagation of wild-type brain homogenate during sPMCA, suggesting that the normal route of PrP^C/PrP^Sc interaction through polybasic domains may be required for generating infectious prions. Why might appropriate PBD-mediated interaction be required for infectivity? One possibility is that PBD-deficient PrP^Sc molecules may be more susceptible to existing host mechanisms for prion clearance [50], [51], [52], [53], perhaps by exposure of a neoepitope on PrP^Sc that serves as a clearance signal. If this explanation were correct, then all three PBD-deficient PrP^Sc molecules must be preferential targets for the clearance mechanism.

A more plausible explanation is that the non-PBD mediated interaction mechanisms used by ΔPBD-PrP^Sc molecules to propagate in vitro lead to the production of alternative PrP^Sc conformations that are intrinsically non-infectious or have reduced infectivity. Consistent with this explanation, we observed different PrP^Sc glycosylation patterns in animals inoculated with ΔC-PrP^Sc compared to animals inoculated with wild-type PrP^Sc. Such a distinction could be caused by an altered ability of mutant PrP^Sc to interact with non-PrP host molecules during in vivo propagation. For example, the in vitro detergent micelle environment differs from the membrane environments where propagation occurs in vivo. On the other hand, high titers of strain-preserved prion infectivity are propagated in vitro in detergent micelles [8], [11], [32], suggesting that in vitro propagation recapitulates native events fairly well.