FGF8 induces formation of an ectopic isthmic organizer and isthmocerebellar development via a repressive effect on Otx2 expression.
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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)
Increased expression of fibroblast growth factor 8 in human breast cancer.
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Fibroblast growth factor 8 (FGF8) is an important developmental protein which is oncogenic and able to cooperate with wnt-1 to produce mouse mammary carcinoma. The level of expression of FGF8 mRNA was measured in 68 breast cancers and 24 non-malignant breast tissues. Elevated levels of FGF8 mRNA were found in malignant compared to non-malignant breast tissues with significantly more malignant tissues expressing FGF8 (P=0.019) at significantly higher levels (P=0.031). In situ hybridization of breast cancer tissues and analysis of purified populations of normal epithelial cells and breast cancer cell lines showed that malignant epithelial cells expressed FGF8 mRNA at high levels compared to non-malignant epithelial and myoepithelial cells and fibroblasts. Although two of the receptors which FGF8 binds to (FGFR2-IIIc, FGFR3-IIIc) are not expressed in breast cancer cells, an autocrine activation loop is possible since expression of fibroblast growth factor receptor (FGFR) 4 and FGFR1 are retained in malignant epithelial cells. This is the first member of the FGF family to have increased expression in breast cancer and a potential autocrine role in its progression. (+info)
Fibroblast growth factor-8 expression is regulated by intronic engrailed and Pbx1-binding sites.
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Fibroblast growth factor-8 (FGF8) plays a critical role in vertebrate development and is expressed normally in temporally and spatially restricted regions of the vertebrate embryo. We now report on the identification of regions of Fgf8 important for its transcriptional regulation in murine ES cell-derived embryoid bodies. Stable transfection of ES cells, using a human growth hormone reporter gene, was employed to identify regions of the Fgf8 gene with promoter/enhancer activity. A 2-kilobase 5' region of Fgf8 was shown to contain promoter activity. A 0.8-kilobase fragment derived from the large intron of Fgf8 was found to enhance human growth hormone expressed from the Fgf8 promoter 3-4-fold in an orientation dependent manner. The intronic fragment contains DNA-binding sites for the AP2, Pbx1, and Engrailed transcription factors. Gel shift and Western blot experiments documented the presence of these transcription factors in nuclear extracts from ES cell embryoid bodies. In vitro mutagenesis of the Engrailed or Pbx1 site demonstrated that these sites modulate the activity of the intronic fragment. In addition, in vitro mutagenesis of both Engrailed and Pbx1 sites indicated that other unidentified sites are responsible for the transcriptional enhancement observed with the intronic fragment. (+info)
FGF8 functions in the specification of the right body side of the chick.
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Left-right asymmetry in vertebrate embryos is first recognisable using molecular markers that encode secreted proteins or transcription factors. The asymmetry becomes morphologically obvious in the turning of the embryo and in the development of the heart, the gut and other visceral organs. In the chick embryo, a signalling pathway for the specification of the left body side was demonstrated. Here, Sonic hedgehog (Shh) protein is the first asymmetric signal identified in the node [1] [2]. Further downstream in this pathway are the left-specific genes nodal, lefty-1, lefty-2 and Pitx2 [1] [3] [4] [5]. On the right body side, a function of the activin pathway is indicated by the right-sided expression of cActRIIa [1] [6]. We detected that another key molecule in vertebrate development, fibroblast growth factor 8 (FGF8) [7] [8], is expressed asymmetrically on the right side of the posterior node. We demonstrate that transcription of FGF8 is induced by activin and the FGF8 protein inhibits the expression of nodal and Pitx2 and leads to expression of the chicken snail related gene (cSnR) [9]. Left-sided application of FGF8 randomises the direction of heart looping. (+info)
The flat-top gene is required for the expansion and regionalization of the telencephalic primordium.
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The telencephalic vesicles form in the mouse embryo by the expansion of precursor regions in the anterior neural tube. Once the vesicles have formed, discrete dorsal and ventral territories can be recognized that later give rise to cortical and subcortical structures, respectively. To investigate the mechanisms that regulate the expansion and regionalization of the telencephalon, we have carried out a screen to identify recessive mutations that disrupt these events. We isolated a mouse mutant in which an early and critical step in development of the telencephalic vesicles is disrupted. Telencephalic primordia are present in flat-top embryos but they fail to progress to form the telencephalic vesicles. An increased rate of proliferation in the forebrain neurectoderm that accompanies telencephalic expansion in wild-type embryos fails to occur in flat-top embryos. Regionalization events that would normally take place during expansion of the primordia also fail to occur. Thus the phenotype of the flat-top mouse reveals that outgrowth of the telencephalic vesicles and their regionalization are coupled processes. (+info)
Zebrafish aussicht mutant embryos exhibit widespread overexpression of ace (fgf8) and coincident defects in CNS development.
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During the development of the zebrafish nervous system both noi, a zebrafish pax2 homolog, and ace, a zebrafish fgf8 homolog, are required for development of the midbrain and cerebellum. Here we describe a dominant mutation, aussicht (aus), in which the expression of noi and ace is upregulated. In aus mutant embryos, ace is upregulated at many sites in the embryo, while noi expression is only upregulated in regions of the forebrain and midbrain which also express ace. Subsequent to the alterations in noi and ace expression, aus mutants exhibit defects in the differentiation of the forebrain, midbrain and eyes. Within the forebrain, the formation of the anterior and postoptic commissures is delayed and the expression of markers within the pretectal area is reduced. Within the midbrain, En and wnt1 expression is expanded. In heterozygous aus embryos, there is ectopic outgrowth of neural retina in the temporal half of the eyes, whereas in putative homozygous aus embryos, the ventral retina is reduced and the pigmented retinal epithelium is expanded towards the midline. The observation that aus mutant embryos exhibit widespread upregulation of ace raised the possibility that aus might represent an allele of the ace gene itself. However, by crossing carriers for both aus and ace, we were able to generate homozygous ace mutant embryos that also exhibited the aus phenotype. This indicated that aus is not tightly linked to ace and is unlikely to be a mutation directly affecting the ace locus. However, increased Ace activity may underly many aspects of the aus phenotype and we show that the upregulation of noi in the forebrain of aus mutants is partially dependent upon functional Ace activity. Conversely, increased ace expression in the forebrain of aus mutants is not dependent upon functional Noi activity. We conclude that aus represents a mutation involving a locus normally required for the regulation of ace expression during embryogenesis. (+info)
Expression of chick Barx-1 and its differential regulation by FGF-8 and BMP signaling in the maxillary primordia.
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The vertebrate face develops from a series of primordia surrounding the primitive mouth and is thought to be patterned by the differential expression of homeobox-containing genes. Here we describe the isolation of the chick homologue of the homeobox-containing gene, Barx-1, and show its expression in the developing facial primordia, stomach, and appendicular skeleton. In the maxillary primordia, mesenchymal expression of Barx-1 is complementary to that of Msx-1, which correlate with overlying epithelial expression of Fgf-8 and Bmp-4, respectively. We show that epithelial signals are required to maintain Barx-1 expression and that FGF-8 can substitute for the epithelium. By contrast, BMPs reduce Barx-1 expression and can antagonize FGF-8 signaling. This suggests that in vivo, FGF-8/BMP signaling may regulate Barx-1 gene expression. This provides evidence that the differential expression of FGF-8 and BMPs may determine homeobox-containing gene expression and hence patterning of the facial primordia. (+info)
Fine mapping of the split-hand/split-foot locus (SHFM3) at 10q24: evidence for anticipation and segregation distortion.
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Split-hand/split-foot malformation (SHFM, ectrodactyly, or lobster-claw deformity) is a human limb malformation characterized by aberrant development of central digital rays with absence of fingers and toes, a deep median cleft, and fusion of remaining digits. SHFM is clinically heterogeneous, presenting both in an isolated form and in combination with additional abnormalities affecting the tibia and/or other organ systems, including the genitourinary, craniofacial, and ectodermal structures. Three SHFM disease loci have been genetically mapped to chromosomes 7q21 (SHFM1), Xq26 (SHFM2), and 10q24 (SHFM3). We mapped data from a large Turkish family with isolated SHFM to chromosome 10q24 and have narrowed the SHFM3 region from 9 cM to an approximately 2-cM critical interval between genetic markers D10S1147 and D10S1240. In several instances we found evidence for a more severe phenotype in offspring of a mildly affected parent, suggesting anticipation. Finally, data from this family, combined with those from six other pedigrees, mapped to 10q24, demonstrate biased transmission of SHFM3 alleles from affected fathers to offspring. The degree of this segregation distortion is obvious in male offspring and is possibly of the same magnitude for female offspring. (+info)