The surface ectoderm is essential for nephric duct formation in intermediate mesoderm.
The nephric duct is the first epithelial tubule to differentiate from intermediate mesoderm that is essential for all further urogenital development. In this study we identify the domain of intermediate mesoderm that gives rise to the nephric duct and demonstrate that the surface ectoderm is required for its differentiation. Removal of the surface ectoderm resulted in decreased levels of Sim-1 and Pax-2 mRNA expression in mesenchymal nephric duct progenitors, and caused inhibition of nephric duct formation and subsequent kidney development. The surface ectoderm expresses BMP-4 and we show that it is required for the maintenance of high-level BMP-4 expression in lateral plate mesoderm. Addition of a BMP-4-coated bead to embryos lacking the surface ectoderm restored normal levels of Sim-1 and Pax-2 mRNA expression in nephric duct progenitors, nephric duct formation and the initiation of nephrogenesis. Thus, BMP-4 signaling can substitute for the surface ectoderm in supporting nephric duct morphogenesis. Collectively, these data suggest that inductive interactions between the surface ectoderm, lateral mesoderm and intermediate mesoderm are essential for nephric duct formation and the initiation of urogenital development. (+info
Stromal cells mediate retinoid-dependent functions essential for renal development.
The essential role of vitamin A and its metabolites, retinoids, in kidney development has been demonstrated in vitamin A deficiency and gene targeting studies. Retinoids signal via nuclear transcription factors belonging to the retinoic acid receptor (RAR) and retinoid X receptor (RXR) families. Inactivation of RARaplpha and RARbeta2 receptors together, but not singly, resulted in renal malformations, suggesting that within a given renal cell type, their concerted function is required for renal morphogenesis. At birth, RARalpha beta2(-) mutants displayed small kidneys, containing few ureteric bud branches, reduced numbers of nephrons and lacking the nephrogenic zone where new nephrons are continuously added. These observations have prompted us to investigate the role of RARalpha and RARbeta2 in renal development in detail. We have found that within the embryonic kidney, RARalpha and RARbeta2 are colocalized in stromal cells, but not in other renal cell types, suggesting that stromal cells mediate retinoid-dependent functions essential for renal development. Analysis of RARalpha beta2(-) mutant kidneys at embryonic stages revealed that nephrons were formed and revealed no changes in the intensity or distribution of molecular markers specific for different metanephric mesenchymal cell types. In contrast the development of the collecting duct system was greatly impaired in RARalpha beta2(-) mutant kidneys. Fewer ureteric bud branches were present, and ureteric bud ends were positioned abnormally, at a distance from the renal capsule. Analysis of genes important for ureteric bud morphogenesis revealed that the proto-oncogene c-ret was downregulated. Our results suggest that RARalpha and RARbeta2 are required for generating stromal cell signals that maintain c-ret expression in the embryonic kidney. Since c-ret signaling is required for ureteric bud morphogenesis, loss of c-ret expression is a likely cause of impaired ureteric bud branching in RARalpha beta2(-) mutants. (+info
FGF8 induces formation of an ectopic isthmic organizer and isthmocerebellar development via a repressive effect on Otx2 expression.
Beads containing recombinant FGF8 (FGF8-beads) were implanted in the prospective caudal diencephalon or midbrain of chick embryos at stages 9-12. This induced the neuroepithelium rostral and caudal to the FGF8-bead to form two ectopic, mirror-image midbrains. Furthermore, cells in direct contact with the bead formed an outgrowth that protruded laterally from the neural tube. Tissue within such lateral outgrowths developed proximally into isthmic nuclei and distally into a cerebellum-like structure. These morphogenetic effects were apparently due to FGF8-mediated changes in gene expression in the vicinity of the bead, including a repressive effect on Otx2 and an inductive effect on En1, Fgf8 and Wnt1 expression. The ectopic Fgf8 and Wnt1 expression domains formed nearly complete concentric rings around the FGF8-bead, with the Wnt1 ring outermost. These observations suggest that FGF8 induces the formation of a ring-like ectopic signaling center (organizer) in the lateral wall of the brain, similar to the one that normally encircles the neural tube at the isthmic constriction, which is located at the boundary between the prospective midbrain and hindbrain. This ectopic isthmic organizer apparently sends long-range patterning signals both rostrally and caudally, resulting in the development of the two ectopic midbrains. Interestingly, our data suggest that these inductive signals spread readily in a caudal direction, but are inhibited from spreading rostrally across diencephalic neuromere boundaries. These results provide insights into the mechanism by which FGF8 induces an ectopic organizer and suggest that a negative feedback loop between Fgf8 and Otx2 plays a key role in patterning the midbrain and anterior hindbrain. (+info
Reverse genetic analysis of Caenorhabditis elegans presenilins reveals redundant but unequal roles for sel-12 and hop-1 in Notch-pathway signaling.
Mutations in the human presenilin genes PS1 and PS2 cause early-onset Alzheimer's disease. Studies in Caenorhabditis elegans and in mice indicate that one function of presenilin genes is to facilitate Notch-pathway signaling. Notably, mutations in the C. elegans presenilin gene sel-12 reduce signaling through an activated version of the Notch receptor LIN-12. To investigate the function of a second C. elegans presenilin gene hop-1 and to examine possible genetic interactions between hop-1 and sel-12, we used a reverse genetic strategy to isolate deletion alleles of both loci. Animals bearing both hop-1 and sel-12 deletions displayed new phenotypes not observed in animals bearing either single deletion. These new phenotypes-germ-line proliferation defects, maternal-effect embryonic lethality, and somatic gonad defects-resemble those resulting from a reduction in signaling through the C. elegans Notch receptors GLP-1 and LIN-12. Thus SEL-12 and HOP-1 appear to function redundantly in promoting Notch-pathway signaling. Phenotypic analyses of hop-1 and sel-12 single and double mutant animals suggest that sel-12 provides more presenilin function than does hop-1. (+info
The head inducer Cerberus is a multifunctional antagonist of Nodal, BMP and Wnt signals.
Embryological and genetic evidence indicates that the vertebrate head is induced by a different set of signals from those that organize trunk-tail development. The gene cerberus encodes a secreted protein that is expressed in anterior endoderm and has the unique property of inducing ectopic heads in the absence of trunk structures. Here we show that the cerberus protein functions as a multivalent growth-factor antagonist in the extracellular space: it binds to Nodal, BMP and Wnt proteins via independent sites. The expression of cerberus during gastrulation is activated by earlier nodal-related signals in endoderm and by Spemann-organizer factors that repress signalling by BMP and Wnt. In order for the head territory to form, we propose that signals involved in trunk development, such as those involving BMP, Wnt and Nodal proteins, must be inhibited in rostral regions. (+info
Conserved function of mSpry-2, a murine homolog of Drosophila sprouty, which negatively modulates respiratory organogenesis.
In Drosophila embryos, the loss of sprouty gene function enhances branching of the respiratory system. Three human sprouty homologues (h-Spry1-3) have been cloned recently, but their function is as yet unknown . Here, we show that a murine sprouty gene (mSpry-2), the product of which shares 97% homology with the respective human protein, is expressed in the embryonic murine lung. We used an antisense oligonucleotide strategy to reduce expression of mSpry-2 by 96%, as measured by competitive reverse transcriptase PCR, in E11. 5 murine embryonic lungs cultured for 4 days . Morphologically, the decrease in mSpry-2 expression resulted in a 72% increase in embryonic murine lung branching morphogenesis as well as a significant increase in expression of the lung epithelial marker genes SP-C, SP-B and SP-A. These results support a striking conservation of function between the Drosophila and mammalian sprouty gene families to negatively modulate respiratory organogenesis. (+info
Sonic Hedgehog-induced activation of the Gli1 promoter is mediated by GLI3.
Drosophila transcription factor cubitus interruptus (Ci) and its co-activator CRE (cAMP response element)-binding protein (CBP) activate a group of target genes on the anterior-posterior border in response to hedgehog protein (Hh) signaling. In the anterior region, in contrast, the carboxyl-truncated form of Ci generated by protein processing represses Hh expression. In vertebrates, three Ci-related transcription factors (glioblastoma gene products (GLIs) 1, 2, and 3) were identified, but their functional difference in Hh signal transduction is unknown. Here, we report distinct roles for GLI1 and GLI3 in Sonic hedgehog (Shh) signaling. GLI3 containing both repression and activation domains acts both as an activator and a repressor, as does Ci, whereas GLI1 contains only the activation domain. Consistent with this, GLI3, but not GLI1, is processed to generate the repressor form. Transcriptional co-activator CBP binds to GLI3, but not to GLI1. The trans-activating capacity of GLI3 is positively and negatively regulated by Shh and cAMP-dependent protein kinase, respectively, through a specific region of GLI3, which contains the CBP-binding domain and the phosphorylation sites of cAMP-dependent protein kinase. GLI3 directly binds to the Gli1 promoter and induces Gli1 transcription in response to Shh. Thus, GLI3 may act as a mediator of Shh signaling in the activation of the target gene Gli1. (+info
A Meis family protein caudalizes neural cell fates in Xenopus.
A homologue of the Drosophila homothorax (hth) gene, Xenopus Meis3 (XMeis3), was cloned from Xenopus laevis. XMeis3 is expressed in a single stripe of cells in the early neural plate stage. By late neurula, the gene is expressed predominantly in rhombomeres two, three and four, and in the anterior spinal cord. Ectopic expression of RNA encoding XMeis3 protein causes anterior neural truncations with a concomitant expansion of hindbrain and spinal cord. Ectopic XMeis3 expression inhibits anterior neural induction in neuralized animal cap ectoderm explants without perturbing induction of pan-neural markers. In naive animal cap ectoderm, ectopic XMeis3 expression activates transcription of the posteriorly expressed neural markers, but not pan-neural markers. These results suggest that caudalizing proteins, such as XMeis3, can alter A-P patterning in the nervous system in the absence of neural induction. Regionally expressed proteins like XMeis3 could be required to overcome anterior signals and to specify posterior cell fates along the A-P axis. (+info