William E. Dowdle

Kif3a constrains β-catenin-dependent Wnt signalling through dual ciliary and non-ciliary mechanisms

Primary cilia are microtubule-based organelles involved in signal transduction and project from the surface of most vertebrate cells. Proteins that can localize to the cilium, for example, Inversin and Bardet-Biedl syndrome (BBS) proteins, are implicated in both β-catenin-dependent and -independent Wnt signalling. Given that Inversin and BBS proteins are found both at the cilium and elsewhere in the cell, the role of the cilium itself in Wnt signalling is not clear. Using three separate mutations that disrupt ciliogenesis (affecting Kif3a, Ift88 and Ofd1), we show in this study that the primary cilium restricts the activity of the canonical Wnt pathway in mouse embryos, primary fibroblasts, and embryonic stem cells. Interestingly, unciliated cells activate transcription only in response to Wnt stimulation, but do so much more robustly than ciliated cells. Loss of Kif3a, but not other ciliogenic genes, causes constitutive phosphorylation of Dishevelled (Dvl). Blocking the activity of casein kinase I (CKI) reverses this constitutive Dvl phosphorylation and abrogates pathway hyper-responsiveness. These results suggest that Kif3a restrains canonical Wnt signalling both by restricting the CKI-dependent phosphorylation of Dvl and through a separate ciliary mechanism. More generally, these findings reveal that, in contrast to its role in promoting Hedgehog (Hh) signalling, the cilium restrains canonical Wnt signalling.

Disruption of a ciliary B9 protein complex causes Meckel syndrome.

Nearly every ciliated organism possesses three B9 domain-containing proteins: MKS1, B9D1, and B9D2. Mutations in human MKS1 cause Meckel syndrome (MKS), a severe ciliopathy characterized by occipital encephalocele, liver ductal plate malformations, polydactyly, and kidney cysts. Mouse mutations in either Mks1 or B9d2 compromise ciliogenesis and result in phenotypes similar to those of MKS. Given the importance of these two B9 proteins to ciliogenesis, we examined the role of the third B9 protein, B9d1. Mice lacking B9d1 displayed polydactyly, kidney cysts, ductal plate malformations, and abnormal patterning of the neural tube, concomitant with compromised ciliogenesis, ciliary protein localization, and Hedgehog (Hh) signal transduction. These data prompted us to screen MKS patients for mutations in B9D1 and B9D2. We identified a homozygous c.301A>C (p.Ser101Arg) B9D2 mutation that segregates with MKS, affects an evolutionarily conserved residue, and is absent from controls. Unlike wild-type B9D2 mRNA, the p.Ser101Arg mutation failed to rescue zebrafish phenotypes induced by the suppression of b9d2. With coimmunoprecipitation and mass spectrometric analyses, we found that Mks1, B9d1, and B9d2 interact physically, but that the p.Ser101Arg mutation abrogates the ability of B9d2 to interact with Mks1, further suggesting that the mutation compromises B9d2 function. Our data indicate that B9d1 is required for normal Hh signaling, ciliogenesis, and ciliary protein localization and that B9d1 and B9d2 are essential components of a B9 protein complex, disruption of which causes MKS.

TMEM231, mutated in orofaciodigital and Meckel syndromes, organizes the ciliary transition zone

TMEM231, a functional component of the MKS complex at the ciliary transition zone, is mutated in orofaciodigital syndrome type 3 and Meckel syndrome.

A high-fat diet regulates gastrin and acid secretion through primary cilia

The role of primary cilia in the gastrointestinal tract has not been examined. Here we report the presence of primary cilia on gastric endocrine cells producing gastrin, ghrelin, and somatostatin (Sst), hormones regulated by food intake. During eating, cilia in the gastric antrum decreased, whereas gastric acid and circulating gastrin increased. Mice fed high‐fat chow showed a delayed decrease in antral cilia, increased plasma gastrin, and gastric acidity. Mice fed high‐fat chow for 3 wk showed lower cilia numbers and acid but higher gastrin levels than mice fed a standard diet, suggesting that fat affects gastric physiology. Ex vivo experiments showed that cilia in the corpus responded to acid and distension, whereas cilia in the antrum responded to food. To analyze the role of gastric cilia, we conditionally deleted the intraflagellar transport protein Ift88 (Ift88−/fl). In fed Ift88−/fl mice, gastrin levels were higher, and gastric acidity was lower. Moreover, gastrin and Sst gene expression did not change in response to food as in controls. At 8 mo, Ift88−/fl mice developed foveolar hyperplasia, hypergastrinemia, and hypochlorhydria associated with endocrine dysfunction. Our results show that components of food (fat) are sensed by antral cilia on endocrine cells, which modulates gastrin secretion and gastric acidity.—Saqui‐Salces, M., Dowdle, W. E., Reiter, J. F., Merchant, J. L. A high‐fat diet regulates gastrin and acid secretion through primary cilia. FASEB J. 26, 3127–3139 (2012). www.fasebj.org

A transition zone complex of ciliopathy proteins regulates ciliary composition

We have identified a complex of proteins that form part of the transition zone, a region at the base of the cilium. This complex includes the three members of the Tectonic family, extracytosolic glycoproteins that interact with transmembrane components of the transition zone such as Tmem67, Tmem216, and Tmem231. These transmembrane proteins connect to an intracellular transition zone complex comprised of many known Joubert- and Meckel-associated proteins including Cc2d2a, B9d1, B9d2, Mks1. Loss of components of this transition zone complex in mice compromise ciliogenesis in some tissues, and deregulate ciliary protein composition in others. In particular, the ciliary localization of Smoothened (Smo), a central component of the Hedgehog pathway, depends on this complex. As Smo functions at the cilium, many mouse transition zone mutants show deregulation of Hh signaling, resulting in ventralization of the neural tube and polydactyly. Defining the components of the transition zone has led to the identification of additional genes underlying Joubert and Meckel syndromes including Tctn1, Tctn2 and B9d2. We hypothesize that Joubert and Meckel syndromes are caused by transition zone dysfunction that disrupts intercellular signaling, leading to developmental defects.

Disruption of Primary Cilia Induces Gastric Mucosal Hyperplasia

BACKGROUND: Primary cilia are necessary for Hedgehog (Hh) signaling and other pathways. Anterograde ciliary transport is mediated by kinesin-II, a heterotrimeric protein composed of two motor subunits, KIF3A, KIF3B, and a non-motor subunit KAP3. Intraflagellar transport protein 88 (IFT88) participates in protein transport along the cilia. Conventional null mice for KIF3A and IFT88 are not viable. Sonic hedgehog (Shh) is highly expressed in gastric epithelial cells of the corpus compared to the antrum. Therefore we examined the stomach of mice conditionally null for KIF3A and IFT88 to define the role of primary cilia in this organ. METHODS: KIF3A and IFT88 mice harboring one null allele and loxP sites on the other allele on a ROSA26 reporter background were crossed to Shh-Cre mice (KIF3A-/ FL and IFT88-/FL respectively.) KIF3A+/FL, IFT88+/FL, and wild type (WT) littermates were analyzed at 6 and 8 months. Cell lineages were identified by immunohistochemistry in paraffin-sections, and by β-galactosidase (β-gal) staining for the Cre-expressing cells. Acetylated-α-tubulin was used as the marker for cilia. RESULTS: Epithelial cells in the antrum and corpus expressed primary cilia. β-gal staining confirmed variegated glandular Cre expression in all epithelial cells. Gastric epithelial cells did not express cilia in the IFT88-/FL mice, while KIF3A-/FL mice showed reduced numbers of cilia in the corpus versus controls (0.59+0.04 cilia/gland, N=8 vs. 0.72+0.06 cilia/gland, N=14), but no changes in the antrum (0.94+0.05 vs. 1.08+0.09 cilia/gland.) KIF3A-/FL and IFT88-/FL mice exhibited different phenotypes. By 6 months, KIF3A-/FL mice showed mucous pit hyperplasia in the corpus. Mucous neck and zymogenic lineages were not expanded, but parietal cells were still present. The KIF3A-/FL mice phenotype resembled the mucous pit hyperplasia observed in the Shh conditionally null mice generated with H+K+ATPase-Cre. In contrast, the phenotype of the IFT88-/FL mice showed parietal cell atrophy, and proximal corpus hyperplasia due to expansion of the mucous neck cell compartment assessed by TFF2 expression (SPEM) by 4 months of age. We have previously observed that gastrinand ghrelin-secreting cells exhibit primary cilia, but none of the mouse models showed differences in the number of these endocrine cells.CONCLUSION:The IFT88-/FLmice exhibited amore severe phenotype than the KIF3A-/FL mice even though both molecules affect primary cilia function. The phenotype of these two mouse models suggests that Hh signaling is necessary for the maintenance of the corpus epithelium. Also, gastric epithelial cilia might be important for the function but not differentiation of gastrinand ghrelin-secreting cells.