ABSTRACT
Transmissible spongiform encephalopathies are fatal neurodegenerative diseases caused by the conversion of prion protein (PrP(C)) into an infectious isoform (PrP(Sc)). How this event leads to pathology is not fully understood. Here we demonstrate that protein synthesis in neurons is enhanced via PrP(C) interaction with stress-inducible protein 1 (STI1). We also show that neuroprotection and neuritogenesis mediated by PrP(C)-STI1 engagement are dependent upon the increased protein synthesis mediated by PI3K-mTOR signaling. Strikingly, the translational stimulation mediated by PrP(C)-STI1 binding is corrupted in neuronal cell lines persistently infected with PrP(Sc), as well as in primary cultured hippocampal neurons acutely exposed to PrP(Sc). Consistent with this, high levels of eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation were found in PrP(Sc)-infected cells and in neurons acutely exposed to PrP(Sc). These data indicate that modulation of protein synthesis is critical for PrP(C)-STI1 neurotrophic functions, and point to the impairment of this process during PrP(Sc) infection as a possible contributor to neurodegeneration.
Subject(s)
Heat-Shock Proteins/metabolism , Intracellular Signaling Peptides and Proteins/metabolism , Neurons/metabolism , Prions/metabolism , Protein Biosynthesis , Protein Serine-Threonine Kinases/metabolism , Animals , Cell Line , Cytoprotection , Eukaryotic Initiation Factor-2/metabolism , Extracellular Signal-Regulated MAP Kinases/metabolism , MAP Kinase Signaling System , Mice , Neurites/enzymology , Neurons/cytology , Neurons/enzymology , Phosphatidylinositol 3-Kinases/metabolism , Phosphorylation , PrPSc Proteins/metabolism , Protein Binding , TOR Serine-Threonine Kinases , Up-RegulationABSTRACT
The secreted cochaperone STI1 triggers activation of protein kinase A (PKA) and ERK1/2 signaling by interacting with the cellular prion (PrP(C)) at the cell surface, resulting in neuroprotection and increased neuritogenesis. Here, we investigated whether STI1 triggers PrP(C) trafficking and tested whether this process controls PrP(C)-dependent signaling. We found that STI1, but not a STI1 mutant unable to bind PrP(C), induced PrP(C) endocytosis. STI1-induced signaling did not occur in cells devoid of endogenous PrP(C); however, heterologous expression of PrP(C) reconstituted both PKA and ERK1/2 activation. In contrast, a PrP(C) mutant lacking endocytic activity was unable to promote ERK1/2 activation induced by STI1, whereas it reconstituted PKA activity in the same condition, suggesting a key role of endocytosis in the former process. The activation of ERK1/2 by STI1 was transient and appeared to depend on the interaction of the two proteins at the cell surface or shortly after internalization. Moreover, inhibition of dynamin activity by expression of a dominant-negative mutant caused the accumulation and colocalization of these proteins at the plasma membrane, suggesting that both proteins use a dynamin-dependent internalization pathway. These results show that PrP(C) endocytosis is a necessary step to modulate STI1-dependent ERK1/2 signaling involved in neuritogenesis.