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)
A Wnt5a pathway underlies outgrowth of multiple structures in the vertebrate embryo.
Morphogenesis depends on the precise control of basic cellular processes such as cell proliferation and differentiation. Wnt5a may regulate these processes since it is expressed in a gradient at the caudal end of the growing embryo during gastrulation, and later in the distal-most aspect of several structures that extend from the body. A loss-of-function mutation of Wnt5a leads to an inability to extend the A-P axis due to a progressive reduction in the size of caudal structures. In the limbs, truncation of the proximal skeleton and absence of distal digits correlates with reduced proliferation of putative progenitor cells within the progress zone. However, expression of progress zone markers, and several genes implicated in distal outgrowth and patterning including Distalless, Hoxd and Fgf family members was not altered. Taken together with the outgrowth defects observed in the developing face, ears and genitals, our data indicates that Wnt5a regulates a pathway common to many structures whose development requires extension from the primary body axis. The reduced number of proliferating cells in both the progress zone and the primitive streak mesoderm suggests that one function of Wnt5a is to regulate the proliferation of progenitor cells. (+info)
Alzheimer's disease: clues from flies and worms.
Presenilin mutations give rise to familial Alzheimer's disease and result in elevated production of amyloid beta peptide. Recent evidence that presenilins act in developmental signalling pathways may be the key to understanding how senile plaques, neurofibrillary tangles and apoptosis are all biochemically linked. (+info)
Axin prevents Wnt-3a-induced accumulation of beta-catenin.
When Axin, a negative regulator of the Wnt signaling pathway, was expressed in COS cells, it coeluted with glycogen synthase kinase-3beta (GSK-3beta), beta-catenin, and adenomatous polyposis coli protein (APC) in a high molecular weight fraction on gel filtration column chromatography. In this fraction, GSK-3beta, beta-catenin, and APC were co-precipitated with Axin. Although beta-catenin was detected in the high molecular weight fraction in L cells on gel filtration column chromatography, addition of conditioned medium expressing Wnt-3a to the cells increased beta-catenin in the low molecular weight fraction. However, Wnt-3a-dependent accumulation of beta-catenin was greatly inhibited in L cells stably expressing Axin. Axin also suppressed Wnt-3a-dependent activation of Tcf-4 which binds to beta-catenin and acts as a transcription factor. These results suggest that Axin forms a complex with GSK-3beta, beta-catenin, and APC, resulting in the stimulation of the degradation of beta-catenin and that Wnt-3a induces the dissociation of beta-catenin from the Axin complex and accumulates beta-catenin. (+info)
Regulation of neurotrophin-3 expression by epithelial-mesenchymal interactions: the role of Wnt factors.
Neurotrophins regulate survival, axonal growth, and target innervation of sensory and other neurons. Neurotrophin-3 (NT-3) is expressed specifically in cells adjacent to extending axons of dorsal root ganglia neurons, and its absence results in loss of most of these neurons before their axons reach their targets. However, axons are not required for NT-3 expression in limbs; instead, local signals from ectoderm induce NT-3 expression in adjacent mesenchyme. Wnt factors expressed in limb ectoderm induce NT-3 in the underlying mesenchyme. Thus, epithelial-mesenchymal interactions mediated by Wnt factors control NT-3 expression and may regulate axonal growth and guidance. (+info)
Expression of CD44 in Apc and Tcf mutant mice implies regulation by the WNT pathway.
Overexpression of cell surface glycoproteins of the CD44 family is an early event in the colorectal adenoma-carcinoma sequence. This suggests a link with disruption of APC tumor suppressor protein-mediated regulation of beta-catenin/Tcf-4 signaling, which is crucial in initiating tumorigenesis. To explore this hypothesis, we analyzed CD44 expression in the intestinal mucosa of mice and humans with genetic defects in either APC or Tcf-4, leading to constitutive activation or blockade of the beta-catenin/Tcf-4 pathway, respectively. We show that CD44 expression in the non-neoplastic intestinal mucosa of Apc mutant mice is confined to the crypt epithelium but that CD44 is strongly overexpressed in adenomas as well as in invasive carcinomas. This overexpression includes the standard part of the CD44 (CD44s) as well as variant exons (CD44v). Interestingly, deregulated CD44 expression is already present in aberrant crypt foci with dysplasia (ACFs), the earliest detectable lesions of colorectal neoplasia. Like ACFs of Apc-mutant mice, ACFs of familial adenomatous polyposis (FAP) patients also overexpress CD44. In sharp contrast, Tcf-4 mutant mice show a complete absence of CD44 in the epithelium of the small intestine. This loss of CD44 concurs with loss of stem cell characteristics, shared with adenoma cells. Our results indicate that CD44 expression is part of a genetic program controlled by the beta-catenin/Tcf-4 signaling pathway and suggest a role for CD44 in the generation and turnover of epithelial cells. (+info)
WNT signaling in the control of hair growth and structure.
Characterization of the molecular pathways controlling differentiation and proliferation in mammalian hair follicles is central to our understanding of the regulation of normal hair growth, the basis of hereditary hair loss diseases, and the origin of follicle-based tumors. We demonstrate that the proto-oncogene Wnt3, which encodes a secreted paracrine signaling molecule, is expressed in developing and mature hair follicles and that its overexpression in transgenic mouse skin causes a short-hair phenotype due to altered differentiation of hair shaft precursor cells, and cyclical balding resulting from hair shaft structural defects and associated with an abnormal profile of protein expression in the hair shaft. A putative effector molecule for WNT3 signaling, the cytoplasmic protein Dishevelled 2 (DVL2), is normally present at high levels in a subset of cells in the outer root sheath and in precursor cells of the hair shaft cortex and cuticle which lie immediately adjacent to Wnt3-expressing cells. Overexpression of Dvl2 in the outer root sheath mimics the short-hair phenotype produced by overexpression of Wnt3, supporting the hypothesis that Wnt3 and Dvl2 have the potential to act in the same pathway in the regulation of hair growth. These experiments demonstrate a previously unrecognized role for WNT signaling in the control of hair growth and structure, as well as presenting the first example of a mammalian phenotype resulting from overexpression of a Dvl gene and providing an accessible in vivo system for analysis of mammalian WNT signaling pathways. (+info)
BMP7 acts in murine lens placode development.
Targeted inactivation of the Bmp7 gene in mouse leads to eye defects with late onset and variable penetrance (A. T. Dudley et al., 1995, Genes Dev. 9, 2795-2807; G. Luo et al., 1995, Genes Dev. 9, 2808-2820). Here we report that the expressivity of the Bmp7 mutant phenotype markedly increases in a C3H/He genetic background and that the phenotype implicates Bmp7 in the early stages of lens development. Immunolocalization experiments show that BMP7 protein is present in the head ectoderm at the time of lens placode induction. Using an in vitro culture system, we demonstrate that addition of BMP7 antagonists during the period of lens placode induction inhibits lens formation, indicating a role for BMP7 in lens placode development. Next, to integrate Bmp7 into a developmental pathway controlling formation of the lens placode, we examined the expression of several early lens placode-specific markers in Bmp7 mutant embryos. In these embryos, Pax6 head ectoderm expression is lost just prior to the time when the lens placode should appear, while in Pax6-deficient (Sey/Sey) embryos, Bmp7 expression is maintained. These results could suggest a simple linear pathway in placode induction in which Bmp7 functions upstream of Pax6 and regulates lens placode induction. At odds with this interpretation, however, is the finding that expression of secreted Frizzled Related Protein-2 (sFRP-2), a component of the Wnt signaling pathway which is expressed in prospective lens placode, is absent in Sey/Sey embryos but initially present in Bmp7 mutants. This suggests a different model in which Bmp7 function is required to maintain Pax6 expression after induction, during a preplacodal stage of lens development. We conclude that Bmp7 is a critical component of the genetic mechanism(s) controlling lens placode formation. (+info)