Scientific Report 2005

Immunology

Prion Diseases: Insights Into the Biology of an Infectious Protein

L. Solforosi, M. Schaller, A. Bellon, G. Moroncini, E. Ollmann Saphire, G. Abalos, J. Cruite, E. Wiseman, R.A. Williamson

The prion diseases, or transmissible spongiform encephalopathies, are diseases of protein conformation that cause profound neurodegeneration and death. They include bovine spongiform encephalopathy, also known as mad cow disease; scrapie in sheep; and chronic wasting disease, which is spreading rapidly in deer and elk within the United States. Human consumption of foodstuffs contaminated with the prions that cause bovine spongiform encephalopathy led to the emergence of a variant of Creutzfeldt-Jakob disease (vCJD). To date, more than 180 cases of vCJD have occurred, primarily in the United Kingdom. More recently, transmission of vCJD via blood products obtained from apparently healthy donors in whom vCJD later developed has been documented. These events have reignited concern about the widespread dissemination of prion diseases in humans. Because no diagnostic test for early-stage prion infections exists, the risks to public health cannot be accurately quantified.

Uniquely, the infectious agent in transmissible spongiform encephalopathies, the prion, is thought to be composed largely of PrP^Sc, an abnormally shaped version of the cellular prion protein PrP^C, a molecule of unknown function that is found in all healthy individuals. Once established within an infected host, prions replicate by converting the normal PrP^C form of the protein into additional molecules of the disease-associated form, PrP^Sc, through a templating-type mechanism that is poorly understood. Over time, PrP^Sc accumulates in the CNS, and its appearance is closely associated with profound neuropathologic changes.

We have developed antibody reagents that specifically recognize different components of either the normal PrP^C or the abnormal PrP^Sc conformers of the prion protein. We have used these immunologic reagents to gain insights into several aspects of prion biology that remain poorly understood. For example, the mechanisms through which the accumulation of PrP^Sc within the CNS leads to the destruction of brain tissues are undetermined, although the presence of PrP^Sc in and of itself appears to be insufficient to promote damage in the absence of PrP^C.We hypothesized that PrP^C may contribute directly to the prion-induced neurodegenerative cascade, perhaps through an unknown signaling pathway. To test this possibility experimentally, we introduced recombinant monoclonal IgG antibodies that recognize PrP^C into the brain in mice. Upon binding to and effectively cross-linking PrP^C on the surface of neuronal cells, the antibodies rapidly triggered extensive neuronal death by apoptosis. These findings indicate that PrP^C may be co-opted twice in prion diseases, once as a substrate for conformational conversion into nascent PrP^Sc molecules and additionally as a signaling vehicle that promotes neuronal injury and death, perhaps after cross-linking by oligomeric forms of PrP^Sc.

In additional experiments, we are using PrP^Sc-specific antibodies to map the association between PrP^C and PrP^Sc, a key event in the formation of the prion replicative complex. Elucidating how these different PrP conformers interact will enhance the prospect of efficiently inhibiting their association and thereby halting prion replication and disease. Finally, our comprehension of prion disease would be greatly enhanced if a detailed molecular structure of PrP^Sc were solved. We are using the PrP^Sc-reactive antibodies in an effort to cocrystalize the disease-associated form of PrP. In the longer term, lessons learned in the study of prion disease most likely will increase our understanding of other more common neurodegenerative conditions that are intimately linked to abnormally folded proteins, such as Alzheimer’s and Parkinson’s diseases.

Publications

Deleault, N.R., Geoghegan, J.C., Nishina, K., Kascsak, R., Williamson, R.A., Supattapone, S. Protease-resistant prion protein amplification reconstituted with partially purified substrates and synthetic polyanions. J. Biol. Chem. 280:26873, 2005.

Yadavalli, R., Guttmann, R.P., Seward, T., Centers, A.P., Williamson, R.A., Telling, G.C. Calpain-dependent endoproteolytic cleavage of PrP^Sc modulates scrapie prion propagation. J. Biol. Chem. 279:21948, 2004.