Aggresome formation by mutant prion proteins: The unfolding role of proteasomes in familial prion disorders

Although familial prion disorders are a direct consequence of mutations in the prion protein gene, the underlying mechanisms leading to neurodegeneration remain unclear. Potential pathogenic mechanisms include abnormal cellular metabolism of the mutant prion protein (PrPM), or destabilization of PrPM structure inducing a change in its conformation to the pathogenic PrP-scrapie (PrPSc) form. To further clarify these mechanisms, we investigated the biogenesis of mutant PrP V203I and E211Q associated with Creutzfeldt-Jakob disease, and PrP Q212P associated with Gerstmann-Straussler-Scheinker syndrome in neuroblastoma cells. We report that all three PrPM forms accumulate similarly in the cytosol in response to proteasomal inhibition, and finally assemble as classical aggresomes. Since the three PrPM forms tested in this report are distinct, we propose that sequestration of misfolded PrPM into aggresomes is likely a general response of the cellular quality control that is not specific to a particular mutation in PrP. Moreover, since PrP has the remarkable ability to refold into PrPSc that can subsequently replicate, PrPM sequestered in aggresomes may cause neurotoxicity by both direct and indirect pathways; directly through PrPSc aggregates, and indirectly by depleting normal PrP, through induction of a cellular stress response, or by other undefined pathways. On the other hand, sequestered PrPM may be relatively inert, and cellular toxicity may be mediated by early intermediates in aggresome formation. Taken together, these observations demonstrate the role of proteasomes in the pathogenesis of familial prion disorders, and argue for further explanation of its mechanistic details.