Role of the Lysosome in ER Associated Degradation of PrP

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Prion diseases, such as BSE (i.e. mad cow disease), are transmissible neurodegnerative diseases related to the misfolded isoform (PrP-Sc) of the normal cellular prion protein (PrP-C). Recent work suggests that misfolded PrP undergoes ER associated degradation (ERAD), whereby it is retrotranslocated to the cytosol for degradation by the proteasome. Proteasome dysfunction is hypothesized to induce prion disease. Accumulation of PrP in the cytosol (cyPrP) following proteasome inhibition or directed expression into the cytosol of cultured cells produce insoluble, Proteinase-K (PK) resistant PrP, but not in transgenic mice expressing cyPrP (Tg1D4), nor is it known if these mice produce infectious prions. Our preliminary work provides evidence that misfolded PrP within the ER is delivered to the lysosome, in addition to the proteasome. However, rather than undergoing effective degradation, we find that this pathway enhances the formation of PK-resistant PrP, a marker of PrP-Sc. In addition, we show that PrP-Sc is released from secretory lysosomes, providing a new mechanism for prion spread. The primary goal of this work is to better define this pathway and its role in prion generation. Specifically, we will; 1.) Test the hypothesis that early delivery of PrP to the lysosome is a feature of ER quality control, 2.) Study the functional consequences of PrP that follows this pathway, and 3.) Define the specific signals and nature of delivery to the lysosome. Using confocal immunohistofluorescence microscopy, immunoelectron microscopy, lysosome fractionation, cyPrP expressing (Tg1D4) mice, [GFP-tagged light chain MAP kinase 3 expressing transgenic (TgGFP-LC3) mice, and knockout LC3-fibroblasts], in addition to a variety of other systems, we will assess the importance of this pathway,in comparison with the proteasome pathway, in the de novo generation and spread of prions. These studies will likely provide new concepts about ERAD, the cellular handling of misfolded proteins, and define new avenues for treatment of these enigmatic and highly controversial diseases.