Strain-Dependent Effect of Macroautophagy on Abnormally Folded Prion Protein Degradation in Infected Neuronal Cells

Daisuke Ishibashi,Noriyuki Nishida,Takehiro Nakagaki,Ryuichiro Atarashi,Hanae Takatsuki,Takayuki Fuse,Takujiro Homma,Kazunori Sano,Jiyan Ma

doi:10.1371/journal.pone.0137958

Abstract

Prion diseases are neurodegenerative disorders caused by the accumulation of abnormal prion protein (PrPSc) in the central nervous system. With the aim of elucidating the mechanism underlying the accumulation and degradation of PrPSc, we investigated the role of autophagy in its degradation, using cultured cells stably infected with distinct prion strains. The effects of pharmacological compounds that inhibit or stimulate the cellular signal transduction pathways that mediate autophagy during PrPSc degradation were evaluated. The accumulation of PrPSc in cells persistently infected with the prion strain Fukuoka-1 (FK), derived from a patient with Gerstmann–Sträussler–Scheinker syndrome, was significantly increased in cultures treated with the macroautophagy inhibitor 3-methyladenine (3MA) but substantially reduced in those treated with the macroautophagy inducer rapamycin. The decrease in FK-derived PrPSc levels was mediated, at least in part, by the phosphatidylinositol 3-kinase/MEK signalling pathway. By contrast, neither rapamycin nor 3MA had any apparently effect on PrPSc from either the 22L or the Chandler strain, indicating that the degradation of PrPSc in host cells might be strain-dependent.

Highlights

Transmissible spongiform encephalopathies, so-called prion diseases, are fatal neurodegenerative disorders that include Creutzfeldt–Jakob disease in humans, bovine spongiform encephalopathy in cattle and scrapie in sheep
When the cells were treated with 0.1 to 10 mM NH4Cl, PrPSc degradation was inhibited dose-dependently and significantly (Fig 1A and 1B). These results demonstrate the PrPSc clearance in N2a-FK cells may relate to the autophagy-lysosomal pathway because it is known that NH4Cl will block the late stage of the autophagy-lysosomal pathway in lysosomal system
Rapamycin had no effect on PrPC in N2a-58 cells (S2B Fig), indicating that autophagy might be one of the degradation system involved in PrPSc but not in PrPC degradation

Summary

Introduction

Transmissible spongiform encephalopathies, so-called prion diseases, are fatal neurodegenerative disorders that include Creutzfeldt–Jakob disease in humans, bovine spongiform encephalopathy in cattle and scrapie in sheep. They are transmitted by prions, unconventional infectious agents that mainly consist of proteinase-resistant and β-sheet-rich amyloidogenic isoforms (PrPSc) of the normal host protein PrP (referred to as the conformational isoform, PrPC) [1, 2]. The degradation of cellular organelles and cytoplasmic proteins is carried out by a process referred to as autophagy, of which there are three types: macroautophagy, microautophagy and chaperone-mediated autophagy (CMA). Cytoplasmic proteins or PLOS ONE | DOI:10.1371/journal.pone.0137958. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

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Conclusion