Prion protein localizes at the ciliary base during neural and cardiovascular development, and its depletion affects α-tubulin post-translational modifications

Prions are proteinaceous infectious agents responsible for a broad range of fatal neurodegenerative diseases in animals and humans. They are primarily composed of macromolecular assemblies of PrPSc, a misfolded isoform of the host-encoded PrPC. Whether prion toxicity in the central nervous system (CNS) is linked to the generation of toxic PrPSc (sub)species, to PrPC gain of toxic functions or to the activation of generic toxic pathways1 remains a fiercely debated issue, substantially due to the elusive physiological functions of PrPC,2. Unravelling the functions of PrPC may have a broader significance given its increasingly apparent role in mediating toxic signalling associated with more common neurodegenerative diseases such as Alzheimer’s disease3. Moreover, PrPC possesses highly conserved primary and tertiary structures among mammals, and the presence of genes homologous to Prnp (the gene encoding PrPC) in birds, reptiles, amphibians and fish lends support for evolutionarily conserved functions2. PrPC is a ubiquitously expressed, glycosylphosphatidylinositol-anchored cell surface sialoglycoprotein that is present in specific membrane domains termed lipid rafts, which are critical to the biology of the cell4. PrPC is involved in a variety of cytoprotective cellular functions and signal transduction pathways5. In the last decade, studies of cell systems depleted in or overexpressing PrPC have linked PrPC to the self-renewal of haematopoietic6 and embryonic7,8 stem cells and to the proliferation and/or differentiation of embryonic stem cells9 and neural progenitors10. However, our knowledge of the underlying cellular mechanisms remains limited. Further complicating the issue is the absence of any drastic phenotype in adult mice upon embryonic11 or post-natal12 inactivation of Prnp, although transient alterations in cell differentiation were later identified in these animals10,13. The upregulation of Prnp expression can be detected as early as embryonic day (E) 6.5 in extra-embryonic regions and E8 in the embryo proper14,15,16, thus supporting a developmental role for PrPC. However, the embryonic expression pattern of PrPC is poorly documented; its presence in neural progenitors can be questioned, with some studies reporting initial expression during differentiation10,16. To characterize how the PrPC expression pattern is specified relative to stem/progenitor cell fate in vivo, we examined the spatiotemporal distribution of PrPC in early developing wild-type mouse embryos. From E8.25 onwards, we found that PrPC was enriched at the base of the primary cilium in stem cells and progenitors of the CNS, heart and forming blood vessels. In the mouse developing neural tube and even more markedly in a neuroepithelial cell line displaying neuroectodermal progenitor hallmarks, the depletion of PrPC altered key cilium-dependent processes and pathways, including α-tubulin post-translational modifications (PTMs) and the Sonic hedgehog signalling pathway. Collectively, these data reveal a new link between PrPC and microtubule dynamics as well as primary cilium functions during development.