Ciliogenesis and Hedgehog signalling are suppressed downstream of KRAS during acinar-ductal metaplasia in mouse.

Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related deaths worldwide, but has a 5-year survival rate of only 7% primarily due to late diagnosis and ineffective therapies. To treat or even prevent PDAC, it is vital that we understand the initiating events that lead to tumour onset. PDAC develops from preneoplastic lesions, most commonly pancreatic intraepithelial neoplasias (PanINs), driven by constitutive activation of KRAS. In patients, PanINs are associated with regions of acinar-to-ductal metaplasia (ADM) where, in response to inflammation, acini dedifferentiate to a pancreatic progenitor-like fate. In healthy tissue this process is reversible leading to regeneration of the pancreas; however, in the presence of oncogenic KRAS, regeneration is blocked and ADM can give rise to PanIN lesions. Here, we used a 3D mouse acinar culture that recapitulates ADM in vitro to explore how KRAS prevents regeneration. Regeneration is regulated by Hedgehog (Hh) signalling, which is transduced via the primary cilium. In wild-type acini, cilia assemble upon ADM and Hh target gene expression is upregulated; however, ciliogenesis and Hh signalling are suppressed during ADM in cells expressing oncogenic KRAS. We show that ciliogenesis fails due to ectopic activation of the cilium disassembly pathway, which is mediated by AurkA, a direct transcriptional target of KRAS. Inhibition of AurkA is able to rescue primary cilia and restore Hh signalling. We suggest that this could be used as a mechanism to prevent the formation of early lesions and thereby prevent progression to PDAC.

Lineage specificity of primary cilia in the mouse embryo.

Primary cilia are required for vertebrate cells to respond to specific intercellular signals. Here we define when and where primary cilia appear in the mouse embryo using a transgenic line that expresses ARL13B-mCherry in cilia and Centrin 2-GFP in centrosomes. Primary cilia first appear on cells of the epiblast at E6.0 and are subsequently present on all derivatives of the epiblast. In contrast, extraembryonic cells of the visceral endoderm and trophectoderm lineages have centrosomes but no cilia. Stem cell lines derived from embryonic lineages recapitulate the in vivo pattern: epiblast stem cells are ciliated, whereas trophoblast stem cells and extraembryonic endoderm (XEN) stem cells lack cilia. Basal bodies in XEN cells are mature and can form cilia when the AURKA-HDAC6 cilium disassembly pathway is inhibited. The lineage-dependent distribution of cilia is stable throughout much of gestation, defining which cells in the placenta and yolk sac are able to respond to Hedgehog ligands.

The chicken talpid3 gene encodes a novel protein essential for Hedgehog signaling.

Talpid3 is a classical chicken mutant with abnormal limb patterning and malformations in other regions of the embryo known to depend on Hedgehog signaling. We combined the ease of manipulating chicken embryos with emerging knowledge of the chicken genome to reveal directly the basis of defective Hedgehog signal transduction in talpid3 embryos and to identify the talpid3 gene. We show in several regions of the embryo that the talpid3 phenotype is completely ligand independent and demonstrate for the first time that talpid3 is absolutely required for the function of both Gli repressor and activator in the intracellular Hedgehog pathway. We map the talpid3 locus to chromosome 5 and find a frameshift mutation in a KIAA0586 ortholog (ENSGALG00000012025), a gene not previously attributed with any known function. We show a direct causal link between KIAA0586 and the mutant phenotype by rescue experiments. KIAA0586 encodes a novel protein, apparently specific to vertebrates, that localizes to the cytoplasm. We show that Gli3 processing is abnormal in talpid3 mutant cells but that Gli3 can still translocate to the nucleus. These results suggest that the talpid3 protein operates in the cytoplasm to regulate the activity of both Gli repressor and activator proteins.

A robust system for RNA interference in the chicken using a modified microRNA operon.

RNA interference (RNAi) provides an effective method to silence gene expression and investigate gene function. However, RNAi tools for the chicken embryo have largely been adapted from vectors designed for mammalian cells. Here we present plasmid and retroviral RNAi vectors specifically designed for optimal gene silencing in chicken cells. The vectors use a chicken U6 promoter to express RNAs modelled on microRNA30, which are embedded within chicken microRNA operon sequences to ensure optimal Drosha and Dicer processing of transcripts. The chicken U6 promoter works significantly better than promoters of mammalian origin and in combination with a microRNA operon expression cassette (MOEC), achieves up to 90% silencing of target genes. By using a MOEC, we show that it is also possible to simultaneously silence two genes with a single vector. The vectors express either RFP or GFP markers, allowing simple in vivo tracking of vector delivery. Using these plasmids, we demonstrate effective silencing of Pax3, Pax6, Nkx2.1, Nkx2.2, Notch1 and Shh in discrete regions of the chicken embryonic nervous system. The efficiency and ease of use of this RNAi system paves the way for large-scale genetic screens in the chicken embryo.