Prion conversion occurs in the MVB

Conversion of the correctly folded cellular prion protein (PrPc) to an amyloid-prone, insoluble PrPsc is thought to be the crucial defining event in the pathogenesis of transmissible spongiform encephalopathies. Although there is a consensus that this process occurs in the endocytic pathway, evidence as to its exact localisation has so far remained inconclusive. Now, Lois Greene and colleagues (p. 1434) combine live cell imaging with super-resolution microscopy and biochemical techniques to investigate which specific endosomal organelle is the main internal site of prion conversion. The authors examined two chronically prion-infected cell lines, SMB and ScN2a, in response to calpain inhibitors, which – owing to an incompletely understood mechanism – decreased PrPsc levels. They found that, before being cleared from the cells, PrPsc localised to enlarged multivesicular bodies (MVBs). Interestingly, the same outcome was observed when MVB maturation was blocked by knocking down Rab7, Tsg101 or Hrs, or when trafficking from the early endosome was inhibited by overexpressing Rab5 or Rab22. By contrast, PrPsc levels increased when trafficking out of the MVB was blocked by disabling the retromer complex through knockdown of Vps26 or SNX2. Crucially, the authors eliminated enhanced degradation or perturbations in intracellular cholesterol as processes contributing to PrPsc clearance, which indicates that a reduction in PrPsc levels is due to its decreased conversion from PrPc, thus demonstrating that the main internal site of prion conversion is the MVB.