The Bone Morphogenetic Pathway Controls the Uptake of Infectious Prions

Prion diseases are transmissible neurodegenerative disorders caused by the spread of misfolded prion protein between cells and individuals, yet the paths by which prions colonize cells are undefined. Here we map the determinants of prion uptake with a genome-wide quadruple-guide CRISPR activation screen. Uptake of prions was measured by flow cytometry in PRNP-ablated human SHSY-5Y cells exposed to synthetic ovine prions. We identified 43 genes modulating prion uptake, 6 of which belonged to the core components of the Bone Morphogenetic Protein (BMP) signaling axis. Prion internalization was increased by overexpression of BMP receptors (BMPR1B, BMPR2, ACVRL1) and the Small Mothers Against Decapentaplegic (SMAD1 and SMAD5) intracellular BMP effectors, whereas the inhibitory SMAD6 reduced it. The internalization of other proteopathic seeds (-synuclein, tau K18, amyloid-{beta}) and other endocytic probes (transferrin, dextran, E. coli bioparticles) was unaffected. Transcriptional profiles of a panel of persistently prion-infected cell lines showed broad dysregulation of BMP-related signaling. Hence, BMP signaling is a gatekeeper of prion entry mechanistically distinct from bulk endocytosis and amyloid uptake. Chronic prion propagation induced SMAD1/5 phosphorylation, suggesting a positive-feedback loop during prion infection. The connection of prion uptake to a developmentally conserved morphogen pathway with rich pharmacology suggests that BMP signaling nodes may serve as tractable levers to modulate early events in prion transmission.