Fiona Bangs

The Talpid3 gene (KIAA0586) encodes a centrosomal protein that is essential for primary cilia formation

The chicken talpid3 mutant, with polydactyly and defects in other embryonic regions that depend on hedgehog (Hh) signalling (e.g. the neural tube), has a mutation in KIAA0568. Similar phenotypes are seen in mice and in human syndromes with mutations in genes that encode centrosomal or intraflagella transport proteins. Such mutations lead to defects in primary cilia, sites where Hh signalling occurs. Here, we show that cells of talpid3 mutant embryos lack primary cilia and that primary cilia can be rescued with constructs encoding Talpid3. talpid3 mutant embryos also develop polycystic kidneys, consistent with widespread failure of ciliogenesis. Ultrastructural studies of talpid3 mutant neural tube show that basal bodies mature but fail to dock with the apical cell membrane, are misorientated and almost completely lack ciliary axonemes. We also detected marked changes in actin organisation in talpid3 mutant cells, which may explain misorientation of basal bodies. KIAA0586 was identified in the human centrosomal proteome and, using an antibody against chicken Talpid3, we detected Talpid3 in the centrosome of wild-type chicken cells but not in mutant cells. Cloning and bioinformatic analysis of the Talpid3 homolog from the sea anemone Nematostella vectensis identified a highly conserved region in the Talpid3 protein, including a predicted coiled-coil domain. We show that this region is required to rescue primary cilia formation and neural tube patterning in talpid3 mutant embryos, and is sufficient for centrosomal localisation. Thus, Talpid3 is one of a growing number of centrosomal proteins that affect both ciliogenesis and Hh signalling.

Foxj1 regulates floor plate cilia architecture and modifies the response of cells to sonic hedgehog signalling

Sonic hedgehog signalling is essential for the embryonic development of many tissues including the central nervous system, where it controls the pattern of cellular differentiation. A genome-wide screen of neural progenitor cells to evaluate the Shh signalling-regulated transcriptome identified the forkhead transcription factor Foxj1. In both chick and mouse Foxj1 is expressed in the ventral midline of the neural tube in cells that make up the floor plate. Consistent with the role of Foxj1 in the formation of long motile cilia, floor plate cells produce cilia that are longer than the primary cilia found elsewhere in the neural tube, and forced expression of Foxj1 in neuroepithelial cells is sufficient to increase cilia length. In addition, the expression of Foxj1 in the neural tube and in an Shh-responsive cell line attenuates intracellular signalling by decreasing the activity of Gli proteins, the transcriptional mediators of Shh signalling. We show that this function of Foxj1 depends on cilia. Nevertheless, floor plate identity and ciliogenesis are unaffected in mouse embryos lacking Foxj1 and we provide evidence that additional transcription factors expressed in the floor plate share overlapping functions with Foxj1. Together, these findings identify a novel mechanism that modifies the cellular response to Shh signalling and reveal morphological and functional features of the amniote floor plate that distinguish these cells from the rest of the neuroepithelium.

Generation of mice with functional inactivation of talpid3, a gene first identified in chicken

Specification of digit number and identity is central to digit pattern in vertebrate limbs. The classical talpid3 chicken mutant has many unpatterned digits together with defects in other regions, depending on hedgehog (Hh) signalling, and exhibits embryonic lethality. The talpid3 chicken has a mutation in KIAA0586, which encodes a centrosomal protein required for the formation of primary cilia, which are sites of vertebrate Hh signalling. The highly conserved exons 11 and 12 of KIAA0586 are essential to rescue cilia in talpid3 chicken mutants. We constitutively deleted these two exons to make a talpid3–/– mouse. Mutant mouse embryos lack primary cilia and, like talpid3 chicken embryos, have face and neural tube defects but also defects in left/right asymmetry. Conditional deletion in mouse limb mesenchyme results in polydactyly and in brachydactyly and a failure of subperisoteal bone formation, defects that are attributable to abnormal sonic hedgehog and Indian hedgehog signalling, respectively. Like talpid3 chicken limbs, the mutant mouse limbs are syndactylous with uneven digit spacing as reflected in altered Raldh2 expression, which is normally associated with interdigital mesenchyme. Both mouse and chicken mutant limb buds are broad and short. talpid3–/– mouse cells migrate more slowly than wild-type mouse cells, a change in cell behaviour that possibly contributes to altered limb bud morphogenesis. This genetic mouse model will facilitate further conditional approaches, epistatic experiments and open up investigation into the function of the novel talpid3 gene using the many resources available for mice.

Enhancer elements upstream of the SHOX gene are active in the developing limb

Léri-Weill Dyschondrosteosis (LWD) is a dominant skeletal disorder characterized by short stature and distinct bone anomalies. SHOX gene mutations and deletions of regulatory elements downstream of SHOX resulting in haploinsufficiency have been found in patients with LWD. SHOX encodes a homeodomain transcription factor and is known to be expressed in the developing limb. We have now analyzed the regulatory significance of the region upstream of the SHOX gene. By comparative genomic analyses, we identified several conserved non-coding elements, which subsequently were tested in an in ovo enhancer assay in both chicken limb bud and cornea, where SHOX is also expressed. In this assay, we found three enhancers to be active in the developing chicken limb, but none were functional in the developing cornea. A screening of 60 LWD patients with an intact SHOX coding and downstream region did not yield any deletion of the upstream enhancer region. Thus, we speculate that SHOX upstream deletions occur at a lower frequency because of the structural organization of this genomic region and/or that SHOX upstream deletions may cause a phenotype that differs from the one observed in LWD.

Human intronic enhancers control distinct sub-domains of Gli3 expression during mouse CNS and limb development

BackgroundThe zinc-finger transcription factor GLI3 is an important mediator of Sonic hedgehog signaling and crucial for patterning of many aspects of the vertebrate body plan. In vertebrates, the mechanism of SHH signal transduction and its action on target genes by means of activating or repressing forms of GLI3 have been studied most extensively during limb development and the specification of the central nervous system. From these studies it has emerged, that Gli3 expression must be subject to a tight spatiotemporal regulation. However, the genetic mechanisms and the cis-acting elements controlling the expression of Gli3 remained largely unknown.ResultsHere, we demonstrate in chicken and mouse transgenic embryos that human GLI3-intronic conserved non-coding sequence elements (CNEs) autonomously control individual aspects of Gli3 expression. Their combined action shows many aspects of a Gli3-specific pattern of transcriptional activity. In the mouse limb bud, different CNEs enhance Gli3-specific expression in evolutionary ancient stylopod and zeugopod versus modern skeletal structures of the autopod. Limb bud specificity is also found in chicken but had not been detected in zebrafish embryos. Three of these elements govern central nervous system specific gene expression during mouse embryogenesis, each targeting a subset of endogenous Gli3 transcription sites. Even though fish, birds, and mammals share an ancient repertoire of gene regulatory elements within Gli3, the functions of individual enhancers from this catalog have diverged significantly. During evolution, ancient broad-range regulatory elements within Gli3 attained higher specificity, critical for patterning of more specialized structures, by abolishing the potential for redundant expression control.ConclusionThese results not only demonstrate the high level of complexity in the genetic mechanisms controlling Gli3 expression, but also reveal the evolutionary significance of cis-acting regulatory networks of early developmental regulators in vertebrates.

Identification of genes downstream of the Shh signalling in the developing chick wing and syn-expressed with Hoxd13 using microarray and 3D computational analysis

Sonic hedgehog (Shh) signalling by the polarizing region at the posterior margin of the chick wing bud is pivotal in patterning the digits but apart from a few key downstream genes, such as Hoxd13, which is expressed in the posterior region of the wing that gives rise to the digits, the genes that mediate the response to Shh signalling are not known. To find genes that are co-expressed with Hoxd13 in the posterior of chick wing buds and regulated in the same way, we used microarrays to compare gene expression between anterior and posterior thirds of wing buds from normal chick embryos and from polydactylous talpid³ mutant chick embryos, which have defective Shh signalling due to lack of primary cilia. We identified 1070 differentially expressed gene transcripts, which were then clustered. Two clusters contained genes predominantly expressed in posterior thirds of normal wing buds; in one cluster, genes including Hoxd13, were expressed at high levels in anterior and posterior thirds in talpid³ wing buds, in the other cluster, genes including Ptc1, were expressed at low levels in anterior and posterior thirds in talpid³ wing buds. Expression patterns of genes in these two clusters were validated in normal and talpid³ mutant wing buds by in situ hybridisation and demonstrated to be responsive to application of Shh. Expression of several genes in the Hoxd13 cluster was also shown to be responsive to manipulation of protein kinase A (PKA) activity, thus demonstrating regulation by Gli repression. Genes in the Hoxd13 cluster were then sub-clustered by computational comparison of 3D expression patterns in normal wing buds to produce syn-expression groups. Hoxd13 and Sall1 are syn-expressed in the posterior region of early chick wing buds together with 6 novel genes which are likely to be functionally related and represent secondary targets of Shh signalling. Other groups of syn-expressed genes were also identified, including a group of genes involved in vascularisation.

3D expression patterns of cell cycle genes in the developing chick wing and comparison with expression patterns of genes implicated in digit specification

Sonic hedgehog (Shh) signalling controls integrated specification of digit pattern and growth in the chick wing but downstream gene networks remain to be unravelled. We analysed 3D expression patterns of genes encoding cell cycle regulators using Optical Projection Tomography. Hierarchical clustering of spatial matrices of gene expression revealed a dorsal layer of the wing bud, in which almost all genes were expressed, and that genes encoding positive cell cycle regulators had similar expression patterns while those of N‐myc and CyclinD2 were distinct but closely related. We compared these patterns computationally with those of genes implicated in digit specification and Ptch1, 50 genes in total. Nineteen genes have similar posterior expression to Ptch1, including Hoxd13, Sall1, Hoxd11, and Bmp2, all likely Gli targets in mouse limb, and cell cycle genes, N‐myc, CyclinD2. We suggest that these genes contribute to a network integrating digit specification and growth in response to Shh. Developmental Dynamics 240:1278–1288, 2011.

A new mouse ciliopathy with a deletion in the Talpid3 gene, first identified in chicken

Genomic imprinting provides an exception to the Mendelian rule of inheritance, as imprinted genes are preferentially expressed in a parent-of-origin specific manner. They play important roles in the development of embryonic and extra-embryonic lineages and postnatally in the maintenance of correct metabolic homeostasis as well as regulation of adult behaviour. The parental conflict theory predicts that maternally expressed genes act as growth suppressors, limiting the usage of maternal resources, and that paternally expressed genes function in an opposite manner to promote growth at the expense of maternal resources. Growth factor bound protein 10 (Grb10) is an imprinted gene encoding an intracellular adaptor protein that can interact with several receptor tyrosine kinases and downstream signalling molecules. Recently, our lab has identified Grb10 as a unique imprinted gene capable of influencing fetal growth, postnatal energy metabolism and adult behaviour depending on functions of each of the parental alleles in distinct tissues. Grb10 predominantly expressed from the maternal allele during embryogenesis affects fetal and placental growth along with postnatal glucose homeostasis, whereas paternal Grb10 expression within the CNS influences social behaviour. Delta-like homologue 1 is (Dlk1) a paternally expressed imprinted gene coding for a protein belonging to the Notch/Delta family that acts as a membrane-associated or a soluble protein known to regulate differentiation of various cell types, notably adipocytes. In vivo Dlk1 has been associated with perinatal survival, regulation of normal growth and development and maintenance of the correct course of adipogenesis. Here a hypothesis is proposed that Grb10, as a predominantly maternally expressed growth inhibitor and Dlk1, a paternally expressed growth promoter, act antagonistically in a common genetic pathway. To test this hypothesis, we have generated Grb10m/+/Dlk1+/p double knockout mice and performed a phenotypic characterisation in comparison with wild type as well as the respective single knockout animals. Results obtained from allometric and metabolic analyses, together with histological studies, reveal strong similarities between the phenotypes of Grb10m/+and Grb10m/+/Dlk1+/p knockout mice. We found that overgrowth of Grb10m/+/Dlk1+/p embryos and placentae resemble the phenotype seen in Grb10m/+ mutants and that tissue overgrowth most likely results from higher proliferation rates of Grb10m/+and Grb10m/+/Dlk1+/p cells. Furthermore, Grb10m/+and Grb10m/+/Dlk1+/p knockout mice each exhibit improved glucose clearance and share an unusual characteristic accumulation of lipid in neonatal liver. These results are consistent with the proposed hypothesis and indicate that the Dlk1 and Grb10 genes might be involved in the same genetic pathway. Moreover, the data suggest Dlk1 is an inhibitor of Grb10 which is in turn acting as a growth suppressor.

18-P015 3D analysis of gene expression during limb development in the Chick

involves a major collaboration between groups in Edinburgh, Dublin, Bath and London. This database will be based on EMAGE and cross-referenced to the mouse through orthologous gene pairs (http://www.emouseatlas.org/testemage/home.php). Throughout this project, the data and framework will be used to identify groups of genes that are co-expressed in important signalling regions. Conservation of these genes will be examined in the chick and mouse. This database will be made publicly available (http://www.echickatlas.org/) and will be a valuable resource to the developmental community.

The developmental mutant talpid 3 lacks primary cilia

Marc A. Willaredt, Kerstin Hasenpusch-Theil, Humphrey Gardner, Igor Kitanovic, Vera Hirschfeld-Warneken, Christian Gojak, Karin Gorgas, C. Lulu Bradford, Joachim Spatz, Stefan Woelfl, Thomas Theil, Kerry L. Tucker 1 Interdisciplinary Center for Neurosciences, University of Heidelberg, Heidelberg, Germany 2 Dept. of Anatomy, University of Heidelberg, Heidelberg, Germany 3 Centres for Neuroscience Research and Integrative Physiology, University of Edinburgh, Edinburgh, United Kingdom 4 Novartis Institutes forBioMedicalResearch, Cambridge, MA, United States 5 Institute of Pharmacy and Molecular Biotechnology, University of Heidelberg, Heidelberg, Germany 6 Institute for Physical Chemistry, Dept. of Biophysical Chemistry, University of Heidelberg, Heidelberg, Germany 7 Max Planck Institute for Metals Research, Stuttgart, Germany