Mutant Prion Protein-induced Ion Channel Activity

Prion diseases, or transmissible spongiform encephalopathies, comprise a group of rapidly progressive and invariably fatal neurodegenerative disorders for which there are no effective treatments or cures. Much is known about prion infection and propagation, which is mediated by the conversion of the cellular prion protein (PrPC) to an aggregated isoform (PrPSc) through a self-templating mechanism. However, the pathways underlying prion-mediated neurotoxicity are poorly understood, and progress has been hampered by the lack of robust cellular assays. Deletion of conserved residues in the central region of PrP has been shown to produce a highly toxic molecule (ΔCR PrP) that causes death within a week of birth in mice lacking wild-type PrP. To further investigate the mechanism of ΔCR PrP toxicity, I performed whole-cell patch clamping both on cultured cells and on neurons from transgenic mice. In every cell type examined, I observed large, spontaneous inward currents, resulting from the activity of a non-selective cation channel. I observed similar currents for PrP molecules containing point mutations in the central region that are associated with familial prion diseases in humans, indicative of a common mechanism of action. Pharmacological and biophysical characterizations of the currents failed to identify a previously described ion channel capable of generating the observed currents. Taken together with the wide variety of cell types in which the currents have been observed, this data indicates that mutant PrP molecules may themselves form pores or channels in the membrane. Through an extensive structure-function study, I determined that the N-terminal polybasic domain of PrP, as well as attachment to the outer plasma membrane, is necessary for ion channel activity. This suggests that mutant PrP may function as a tethered protein transduction domain, similar to the HIV-1 TAT protein, allowing sporadic entry of cations into the cytoplasm. Such pores are likely to cause membrane depolarization and increase susceptibility of neurons to glutamate-induced excitotoxicity. My studies have raised the possibility that PrP-induced ion channel activity could be the pathogenic mechanism underlying prion toxicity, and we are now using inhibition of these currents as a novel method to identify anti-prion compounds