N-terminus (Y145STOP) fragment of the prion protein plays a role in prion misfolding

Abstract

Prion diseases are transmissible protein misfolding disorders in which misfolding of a host-encoded prion protein (PrP) occurs. They comprise of a group of distinct diseases in animals and humans, which show similar clinical and neuropathological changes. Human prion diseases can arise sporadically, be hereditary or be acquired. Sporadic human prion diseases include Cruetzfeldt-Jacob disease (CJD) and fatal insomnia. Genetic or familial prion diseases are caused by autosomal dominantly inherited mutations in the gene encoding for PrPC and include familial or genetic CJD, fatal familial insomnia and Gerstmann-Sträussler-Scheinker syndrome. Acquired human prion diseases account for only 5% of cases of human prion disease. They include Kuru, iatrogenic CJD and a new variant form of CJD that is transmitted to humans from affected cattle via meat consumption. Despite considerable effort in understanding the structure of the prion protein, the precise role of different prion protein domains that may be important in basic misfolding is poorly understood. The linkage between Prnp mutations and hereditary prion disease provide support for the central role of PrP in pathogenesis, because the genetic disease can be propagated in an infectious way. One of the most intriguing disease-related mutations is the tyrosine to stop codon substitution at position 145; Y145Stop variant of prion protein. Y145Stop variant; PrP23-144, is the only truncated prion molecule that is linked to an autosomal dominant inherited genetic TSE, Gerstmann-Sträussler-Scheinker syndrome. This disease related mutation indicates an essential role of the N-terminus fragment of the prion protein in seeding and misfolding of the mammalian prions. Therefore, it was hypothesized that the N-terminus fragment of the prion protein plays a central role in the mechanism of prion conversion. Studies were designed to test the hypothesis using a Protein Misfolding Cyclic Amplification (PMCA) assay in the presence or absence of preexisting prions. It was identified that recombinant Y145Stop had the propensity for spontaneous conversion to protease resistant isoforms. Systematic molecular investigations established that Y145Stop was able to induce PrPSc formation in cell culture in comparison to other recombinant PrP fragments and prions extracted from infected brain tissues. Misfolded Y145Stop showed similar kinetics as naturally occurring PrPSc in cell culture infectivity. Lastly, it was demonstrated that the toxicity of Y145Stop in the cell culture was correlated with apoptotic cell death. The toxic activity of the peptide was dependent on the activation of caspase and p38 MAP kinase pathway. These experiments established a critical role for Y145Stop molecule in prion conversion of recombinant and mammalian prions. Experimental evidence findings lead to propose a two-step phenomenon in prion misfolding whereby, the N-terminus first interacts with metals or polyanions leading to its misfolding that then catalyzes the conformational conversion of the structured C-terminus of the molecule. Taken together, data generated from these studies will provide better understanding of the prion conversion mechanism. Elucidating the role of the N-terminus in seeding and misfolding of mammalian prions has important implications for designing diagnostics and therapeutics of prion diseases, as well as for understanding pathogenic mechanisms operative in interspecies transmission.