Mechanical induction of Twist in the Drosophila foregut/stomodeal primordium.
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BACKGROUND: Morphogenetic movements are closely regulated by the expression of developmental genes. Here I examine whether developmental gene expression can in turn be mechanically regulated by morphogenetic movements. I have analyzed the effects of mechanical stress on the expression of Twist, which is normally expressed only in the most ventral cells of the cellular blastoderm embryo under the control of the Dorsal morphogen gradient. At embryogenesis gastrulation (stage 7), Twist is also expressed in the anterior foregut and stomodeal primordia. RESULTS: Submitting the early Drosophila embryo to a transient 10% uniaxial lateral deformation induces the ectopic expression of Twist around the entire dorsal-ventral axis and results in the ventralization of the embryo. This induction is independent of the Dorsal gradient and is triggered by mechanically induced Armadillo nuclear translocation. I also show that Twist is not expressed in the anterior foregut and stomodeal primordia at stage 7 in mutants that block the morphogenetic movement of germ-band extension. Because I can rescue the mutants with gentle compression of these cells, my interpretation is that the stomodeal-cell compression normally caused by the germ-band extension induces the expression of Twist. Correspondingly, laser ablation of dorsal cells in wild-type embryos relaxes stomodeal cell compression and reduces Twist expression in the stomodeal primordium. I also demonstrate that the induction of Twist in these cells depends on the nuclear translocation of Armadillo. CONCLUSIONS: I propose that anterior-gut formation is mechanically induced by the movement of germ-band extension through the induction of Twist expression in stomodeal cells. (+info)
Msx2 and Twist cooperatively control the development of the neural crest-derived skeletogenic mesenchyme of the murine skull vault.
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The flat bones of the vertebrate skull vault develop from two migratory mesenchymal cell populations, the cranial neural crest and paraxial mesoderm. At the onset of skull vault development, these mesenchymal cells emigrate from their sites of origin to positions between the ectoderm and the developing cerebral hemispheres. There they combine, proliferate and differentiate along an osteogenic pathway. Anomalies in skull vault development are relatively common in humans. One such anomaly is familial calvarial foramina, persistent unossified areas within the skull vault. Mutations in MSX2 and TWIST are known to cause calvarial foramina in humans. Little is known of the cellular and developmental processes underlying this defect. Neither is it known whether MSX2 and TWIST function in the same or distinct pathways. We trace the origin of the calvarial foramen defect in Msx2 mutant mice to a group of skeletogenic mesenchyme cells that compose the frontal bone rudiment. We show that this cell population is reduced not because of apoptosis or deficient migration of neural crest-derived precursor cells, but because of defects in its differentiation and proliferation. We demonstrate, in addition, that heterozygous loss of Twist function causes a foramen in the skull vault similar to that caused by loss of Msx2 function. Both the quantity and proliferation of the frontal bone skeletogenic mesenchyme are reduced in Msx2-Twist double mutants compared with individual mutants. Thus Msx2 and Twist cooperate in the control of the differentiation and proliferation of skeletogenic mesenchyme. Molecular epistasis analysis suggests that Msx2 and Twist do not act in tandem to control osteoblast differentiation, but function at the same epistatic level. (+info)
A comprehensive screen of genes implicated in craniosynostosis.
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Recent advances in human molecular genetics have identified mutations in the TWIST, FGFR-1, FGFR-2 and FGFR-3 genes to be important causes of craniosynostosis. Despite this, however, mutations cannot be identified in the majority of patients. This study reports the first comprehensive screen of mutations in TWIST, FGFR-1, FGFR-2 and FGFR-3 genes in a cohort of patients with craniosynostosis. This has led to the identification of Saethre-Chotzen syndrome to be a new microdeletion disorder and reports the first example of a gene-environment interaction leading to craniosynostosis. In addition, investigation of the expression patterns of the Fgfr and Twist genes in the normal developing mouse coronal suture has identified the TWIST protein to be important in cranial suture initiation and biogenesis. These findings have significant clinical implications and will form the basis of future attempts to develop novel therapies aimed at inhibiting cranial suture fusion. (+info)
Twist2, a novel ADD1/SREBP1c interacting protein, represses the transcriptional activity of ADD1/SREBP1c.
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Adipocyte determination and differentiation dependent factor 1 (ADD1)/sterol regulatory element binding protein isoform (SREBP1c) is a key transcription factor in fatty acid metabolism and insulin- dependent gene expression. Although its transcriptional and post-translational regulation has been extensively studied, its regulation by interacting proteins is not well understood. To identify cellular proteins that associate with ADD1/SREBP1c, we employed the yeast two-hybrid system with an adipocyte cDNA library. Using the N-terminal domain of ADD1/SREBP1c as bait, we identified Twist2 (also known as Dermo-1), a basic helix-loop-helix (bHLH) protein, as a novel ADD1/SREBP1c interacting protein. Over-expression of Twist2 strongly repressed the transcriptional activity of ADD1/SREBP1c, primarily by reducing its binding to target sequences. Inhibition of histone deacetylase (HDAC) activity with HDAC inhibitors relieved this repression. Our data suggest that physical interaction between Twist2 and ADD1/SREBP1c attenuates transcriptional activation by ADD1/SREBP1c by inhibiting its binding to DNA, and that this inhibition is at least partly dependent on chromatin modification by HDACs. (+info)
Twist genes regulate Runx2 and bone formation.
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Runx2, a master regulator of osteoblast differentiation, is expressed several days before osteoblast genes in bone anlage. In this issue of Developmental Cell, show that Twist-1 and Twist-2 suppress the activity of Runx2 and thereby regulate bone formation. (+info)
A twist code determines the onset of osteoblast differentiation.
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Runx2 is necessary and sufficient for osteoblast differentiation, yet its expression precedes the appearance of osteoblasts by 4 days. Here we show that Twist proteins transiently inhibit Runx2 function during skeletogenesis. Twist-1 and -2 are expressed in Runx2-expressing cells throughout the skeleton early during development, and osteoblast-specific gene expression occurs only after their expression decreases. Double heterozygotes for Twist-1 and Runx2 deletion have none of the skull abnormalities observed in Runx2(+/-) mice, a Twist-2 null background rescues the clavicle phenotype of Runx2(+/-) mice, and Twist-1 or -2 deficiency leads to premature osteoblast differentiation. Furthermore, Twist-1 overexpression inhibits osteoblast differentiation without affecting Runx2 expression. Twist proteins' antiosteogenic function is mediated by a novel domain, the Twist box, which interacts with the Runx2 DNA binding domain to inhibit its function. In vivo mutagenesis confirms the antiosteogenic function of the Twist box. Thus, relief of inhibition by Twist proteins is a mandatory event precluding osteoblast differentiation. (+info)
The RhoGEF Pebble is required for cell shape changes during cell migration triggered by the Drosophila FGF receptor Heartless.
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The FGF receptor Heartless (HTL) is required for mesodermal cell migration in the Drosophila gastrula. We show that mesoderm cells undergo different phases of specific cell shape changes during mesoderm migration. During the migratory phase, the cells adhere to the basal surface of the ectoderm and exhibit extensive protrusive activity. HTL is required for the protrusive activity of the mesoderm cells. Moreover, the early phenotype of htl mutants suggests that HTL is required for the adhesion of mesoderm cells to the ectoderm. In a genetic screen we identified pebble (pbl) as a novel gene required for mesoderm migration. pbl encodes a guanyl nucleotide exchange factor (GEF) for RHO1 and is known as an essential regulator of cytokinesis. We show that the function of PBL in cell migration is independent of the function of PBL in cytokinesis. Although RHO1 acts as a substrate for PBL in cytokinesis, compromising RHO1 function in the mesoderm does not block cell migration. These data suggest that the function of PBL in cell migration might be mediated through a pathway distinct from RHO1. This idea is supported by allele-specific differences in the expressivity of the cytokinesis and cell migration phenotypes of different pbl mutants. We show that PBL is autonomously required in the mesoderm for cell migration. Like HTL, PBL is required for early cell shape changes during mesoderm migration. Expression of a constitutively active form of HTL is unable to rescue the early cellular defects in pbl mutants, suggesting that PBL is required for the ability of HTL to trigger these cell shape changes. These results provide evidence for a novel function of the Rho-GEF PBL in HTL-dependent mesodermal cell migration. (+info)
Notch signaling patterns Drosophila mesodermal segments by regulating the bHLH transcription factor twist.
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One of the first steps in embryonic mesodermal differentiation is allocation of cells to particular tissue fates. In Drosophila, this process of mesodermal subdivision requires regulation of the bHLH transcription factor Twist. During subdivision, Twist expression is modulated into stripes of low and high levels within each mesodermal segment. High Twist levels direct cells to the body wall muscle fate, whereas low levels are permissive for gut muscle and fat body fate. We show that Su(H)-mediated Notch signaling represses Twist expression during subdivision and thus plays a critical role in patterning mesodermal segments. Our work demonstrates that Notch acts as a transcriptional switch on mesodermal target genes, and it suggests that Notch/Su(H) directly regulates twist, as well as indirectly regulating twist by activating proteins that repress Twist. We propose that Notch signaling targets two distinct 'Repressors of twist' - the proteins encoded by the Enhancer of split complex [E(spl)C] and the HLH gene extra machrochaetae (emc). Hence, the patterning of Drosophila mesodermal segments relies on Notch signaling changing the activities of a network of bHLH transcriptional regulators, which, in turn, control mesodermal cell fate. Since this same cassette of Notch, Su(H) and bHLH regulators is active during vertebrate mesodermal segmentation and/or subdivision, our work suggests a conserved mechanism for Notch in early mesodermal patterning. (+info)