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B Z Shilo

Publications and source records attributed to B Z Shilo.

At least 37 records · Page 2Linked to original sources
PubMed1997

Interactions between the EGF receptor and DPP pathways establish distinct cell fates in the tracheal placodes.

The formation of the tracheal network in Drosophila is driven by stereotyped migration of cells from the tracheal pits. No cell divisions take place during tracheal migration and the number of cells in each branch is fixed. This work examines the basis for the determination of tracheal branch fates, prior to the onset of migration. We show that the EGF receptor pathway is activated by localized processing of the ligand SPITZ in the tracheal placodes and is responsible for the capacity to form the dorsal trunk and visceral branch. The DPP pathway, on the contrary, is induced in the tracheal pit by local presentation of DPP from the adjacent dorsal and ventral ectodermal cells. This pathway patterns the dorsal and lateral branches. Elimination of both pathways blocks migration of all tracheal branches. Antagonistic interactions between the two pathways are demonstrated. The opposing activities of two pathways may refine the final determination of tracheal branch fates.

Animals↗
PubMed1997

Spitz and Wingless, emanating from distinct borders, cooperate to establish cell fate across the Engrailed domain in the Drosophila epidermis.

A key step in development is the establishment of cell type diversity across a cellular field. Segmental patterning within the Drosophila embryonic epidermis is one paradigm for this process. At each parasegment boundary, cells expressing the Wnt family member Wingless confront cells expressing the homeoprotein Engrailed. The Engrailed-expressing cells normally differentiate as one of two alternative cell types. In investigating the generation of this cell type diversity among the 2-cell-wide Engrailed stripe, we previously showed that Wingless, expressed just anterior to the Engrailed cells, is essential for the specification of anterior Engrailed cell fate. In a screen for additional mutations affecting Engrailed cell fate, we identified anterior open/yan, a gene encoding an inhibitory ETS-domain transcription factor that is negatively regulated by the Rasl-MAP kinase signaling cascade. We find that Anterior Open must be inactivated for posterior Engrailed cells to adopt their correct fate. This is achieved by the EGF receptor (DER), which is required autonomously in the Engrailed cells to trigger the Ras1-MAP kinase pathway. Localized activation of DER is accomplished by restricted processing of the activating ligand, Spitz. Processing is confined to the cell row posterior to the Engrailed domain by the restricted expression of Rhomboid. These cells also express the inhibitory ligand Argos, which attenuates the activation of DER in cell rows more distant from the ligand source. Thus, distinct signals flank each border of the Engrailed domain, as Wingless is produced anteriorly and Spitz posteriorly. Since we also show that En cells have the capacity to respond to either Wingless or Spitz, these cells must choose their fate depending on the relative level of activation of the two pathways.

Animals↗
PubMed1996

Heartless, a Drosophila FGF receptor homolog, is essential for cell migration and establishment of several mesodermal lineages.

A Drosophila FGF receptor homolog (DFGF-R2/DFRI) termed Heartless (Htl) is expressed in the embryonic mesoderm. The phenotypes of null mutant embryos demonstrated that Htl is a central player that is required for the development of several mesodermal lineages. No abnormalities in the primary specification of the mesoderm were observed. The first defects were seen as irregular migration and spreading of the mesoderm over the ectoderm. Subsequently, cell fates were not induced in several lineages including the visceral mesoderm, heart, and the dorsal somatic muscles. The defects in the induction of cell fates are likely to result from failure of the mesoderm to spread over the ectoderm and receive patterning signals. The defective spreading could be circumvented in htl mutant embryos by providing an ectopic Dpp patterning signal, leading to the formation of heart and dorsal muscle cells. Htl appears to be required also subsequently during the migration and morphogenesis of the different lineages. Expression of a dominant-negative htl construct after the initial induction of cell fates gave rise to aberrant migration and organization of the visceral mesoderm, heart, and somatic muscles. Thus, a common role for Htl in cell migration and tissue organization may account for the pleiotropic defects of the htl mutation.

Animals↗
PubMed1996

trachealess encodes a bHLH-PAS protein that is an inducer of tracheal cell fates in Drosophila.

The embryonic tracheal system in Drosophila develops from placodes of precursor cells on the ectoderm. A transcription factor of the basic helix-loop-helix (bHLH)-PAS family, which is expressed in the nuclei of the tracheal cells throughout development, was identified. The protein shows the highest degree of homology to the Single-minded (Sim) protein. The transcript represents the previously identified trachealess (trh) locus, essential for tracheal development. Ectopic expression of trh leads to generation of extra tracheal pits and branches and to the expression of tracheal markers by patches of ectodermal cells. The expression of trh is consistent with a biphasic mode of transcriptional regulation. Expression is first induced by exogenous cues and is subsequently autoregulated. trh is also expressed and required in the posterior spiracles and the salivary gland ducts. The role of Trachealess in the formation of several tubular tissues in the embryo suggests that it may induce a general fate of branched tubular structures of epithelial origin.

Amino Acid Sequence↗
PubMed1996

Argos transcription is induced by the Drosophila EGF receptor pathway to form an inhibitory feedback loop.

Argos is a secreted molecule with an atypical EGF motif. It was recently shown to function as an inhibitor of the signaling triggered by the Drosophila EGF receptor (DER). In this work, we determine the contribution of Argos to the establishment of cell fates in the embryonic ventral ectoderm. Graded activation of DER is essential for patterning the ventral ectoderm. argos mutant embryos show expansion of ventral cell fates suggesting hyperactivation of the DER pathway. In the embryonic ventral ectoderm, argos is expressed in the ventralmost row of cells. We show that argos expression in the ventral ectoderm is induced by the DER pathway: argos is not expressed in DER mutant embryos, while it is ectopically expressed in the entire ventral ectoderm following ubiquitous activation of the DER pathway. argos expression appears to be triggered directly by the DER pathway, since induction can also be observed in cell culture, following activation of DER by its ligand, Spitz. Argos therefore functions in a sequential manner, to restrict the duration and level of DER signaling. This type of inhibitory feedback loop may represent a general paradigm for signaling pathways inducing diverse cell fates within a population of non-committed cells.

Animals↗
PubMed1996

EGF receptor signaling induces pointed P1 transcription and inactivates Yan protein in the Drosophila embryonic ventral ectoderm.

The induction of different cell fates along the dorsoventral axis of the Drosophila embryo requires a graded activity of the EGF receptor tyrosine kinase (DER). Here we have identified primary and secondary target genes of DER, which mediate the determination of discrete ventral cell fates. High levels of DER activation in the ventralmost cells trigger expression of the transcription factors encoded by ventral nervous system defective (vnd) and pointed P1 (pntPl). Concomitant with the induction of pntP1, high levels of DER activity lead to inactivation of the Yan protein, a transcriptional repressor of Pointed-target genes. These two antagonizing transcription factors subsequently control the expression of secondary target genes such as otd, argos and tartan. The simultaneous effects of the DER pathway on pntP1 induction and Yan inactivation may contribute to the definition of the border of the ventralmost cell fates.

Animals↗
PubMed1996

The Drosophila embryonic midline is the site of Spitz processing, and induces activation of the EGF receptor in the ventral ectoderm.

The Drosophila EGF receptor (DER) is activated by secreted Spitz to induce different cell fates in the ventral ectoderm. Processing of the precursor transmembrane Spitz to generate the secreted form was shown to be the limiting event, but the cells in which processing takes place and the mechanism that may generate a gradient of secreted Spitz in the ectoderm were not known. The ectodermal defects in single minded (sim) mutant embryos, in which the midline fails to develop, suggested that the midline cells contribute to patterning of the ventral ectoderm. This work shows that the midline provides the site for Spitz expression and processing. The Rhomboid and Star proteins are also expressed and required in the midline. The ectodermal defects of spitz, rho or Star mutant embryos could be rescued by inducing the expression of the respective normal genes only in the midline cells. Rho and Star thus function non-autonomously, and may be required for the production or processing of the Spitz precursor. Secreted Spitz is the only sim-dependent contribution of the midline to patterning the ectoderm, since the ventral defects observed in sim mutant embryos can be overcome by expression of secreted Spitz in the ectoderm. While ectopic expression of secreted Spitz in the ectoderm or mesoderm gave rise to ventralization of the embryo, increased expression of secreted Spitz in the midline did not lead to alterations in ectoderm patterning. A mechanism for adjustment to variable levels of secreted Spitz emanating from the midline may be provided by Argos, which forms an inhibitory feedback loop for DER activation. The production of secreted Spitz in the midline, may provide a stable source for graded DER activation in the ventral ectoderm.

Animals↗
PubMed1995

Inhibition of Drosophila EGF receptor activation by the secreted protein Argos.

The Drosophila homologue of the mammalian epidermal growth factor (EGF) receptor (DER) is a receptor tyrosine kinase involved in many stages of fly development, including photoreceptor determination, and wing-vein formation. Its primary activating ligand is the Spitz protein, which is similar to mammalian TGF-alpha. Argos is a secreted protein that, like Spitz, contains a single EGF motif. It is a repressor of cell determination in the eye, and acts in other tissues, including the wing. Because Argos has the opposite effects to DER in the eye (the former blocks photoreceptor determination, the latter promotes it) we have tested whether it acts by blocking the DER pathway. We show that Argos does indeed repress this pathway in vivo and find that, in vitro, Argos protein can inhibit the activation of DER by Spitz. Thus the determination of cells by the DER pathway is regulated by a balance between extracellular activating and inhibiting signals. This is the first in vivo example of an extracellular inhibitor of a receptor tyrosine kinase.

Animals↗
PubMed1995

Secreted Spitz triggers the DER signaling pathway and is a limiting component in embryonic ventral ectoderm determination.

The spitz gene encoding a TGF-alpha homolog, has been shown to affect a subset of developmental processes that are similar to those regulated by DER, the Drosophila EGF receptor homolog. This work demonstrates that Spitz triggers the DER signaling cascade. Addition of a secreted, but not the membrane-associated form of Spitz to S2 Drosophila cells expressing DER gives rise to a rapid tyrosine autophosphorylation of DER. Following autophosphorylation, DER associates with the Drk adapter protein. Consequently, activation of MAP kinase is observed. The profile of MAP kinase activation provides a quantitative assay for DER activation. A dose response between the levels of Spitz and MAP kinase activity was observed. The secreted Spitz protein was expressed in embryos to assess its biological activity. An alteration in cell fates was observed in the ventral ectoderm, such that lateral cells acquired the ventral-most fates. The result indicates that graded activation of the DER pathway may normally give rise to a repertoire of discrete cell fates in the ventral ectoderm. Spatially restricted processing of Spitz may be responsible for this graded activation. The Rhomboid (Rho) and Star proteins were suggested, on the basis of genetic interactions, to act as modulators of DER signaling. No alteration in DER autophosphorylation or the pattern of MAP kinase activation by secreted Spitz was observed when the Rho and Star proteins were coexpressed with DER in S2 cells. In embryos mutant for rho or Star the ventralizing effect of secreted Spitz is epistatic, suggesting that Rho and Star may normally facilitate processing of the Spitz precursor.

Animals↗
PubMed1995

Breathless, a Drosophila FGF receptor homolog, is required for the onset of tracheal cell migration and tracheole formation.

Breathless, a Drosophila FGF receptor homolog (DFGF-R1), was shown to be essential for the migration of the tracheal cells and the posterior midline glia cells. The temporal requirement for the activity of this receptor was dissected by a dominant-negative construct lacking a functional cytoplasmic tyrosine-kinase domain. Induction of the construct prior to the onset of tracheal or glial cell migration produced phenotypes that were similar to those observed in the corresponding tissues of breathless null mutant embryos. However, this effect is not detected if the dominant-negative receptor is induced after the initiation of tracheal cell migration, indicating that Breathless is required primarily at the onset of the migration process. Induction of the construct after the tracheal branches are completed, blocked the formation of tracheoles, i.e. extension of cellular processes by the terminal tracheal cells, demonstrating that Breathless plays an essential role in this process as well. The requirement for Breathless at the onset of migration and the diversity of processes in which it participates, suggest that the receptor is involved in triggering transcription factors, which may be distinct for each context.

Animals↗
PubMed1995

The role of yan in mediating the choice between cell division and differentiation.

An allele of the yan locus was isolated as an enhancer of the Ellipse mutation of the Drosophila epidermal growth factor receptor (Egfr) gene. This yan allele is an embryonic lethal and also fails to complement the lethality of anterior open (aop) mutations. Phenotypic and complementation analysis revealed that aop is allelic to yan and genetically the lethal alleles act as null mutations for the yan gene. Analysis of the lethal alleles in the embryo and in mitotic clones showed that loss of yan function causes cells to overproliferate in the dorsal neuroectoderm of the embryo and in the developing eye disc. Our studies suggest that the role of yan is defined by the developmental context of the cells in which it functions. An important role of this gene is in allowing a cell to choose between cell division and differentiation. The relationship of the Egfr and Notch pathways to this developmental role of yan is discussed.

Alleles↗
PubMed1995

The breathless FGF receptor homolog, a downstream target of Drosophila C/EBP in the developmental control of cell migration.

To investigate the molecular mechanisms responsible for the temporal and spatial control of cell movements during development, we have been studying the migration of a small group of follicle cells, called the border cells, in the Drosophila ovary. Timely initiation of border cell migration requires the product of the slow border cells (slbo) locus, which encodes the Drosophila homolog of the transcription factor C/EBP. Here we report evidence that one target of C/EBP in the control of border cell migration is the FGF receptor homolog encoded by the breathless (btl) locus. btl expression in the ovary was border cell-specific, beginning just prior to the migration, and this expression was reduced in slbo mutants. btl mutations dominantly enhanced the border cell migration defects found in weak slbo alleles. Furthermore, C/EBP-independent btl expression was able to rescue the migration defects of hypomorphic slbo alleles. Purified Drosophila C/EBP bound eight sites in the btl 5' flanking region by DNAse I footprinting. Taken together these results suggest that btl is a key, direct target for C/EBP in the regulation of border cell migration.

Animals↗
PubMed1994

Elucidation of the role of breathless, a Drosophila FGF receptor homolog, in tracheal cell migration.

DFGF-R1 (breathless), a Drosophila FGF receptor homolog, is required for the migration of tracheal cells and the posterior midline glial cells during embryonic development. To define the role of this receptor in cell migration, we have monitored the biological effects of a deregulated receptor containing the extracellular and transmembrane regions of the torso dominant allele and the cytoplasmic domain of DFGF-R1. Ubiquitous expression of the chimeric receptor at the time of tracheal cell migration did not disrupt migration in wild-type embryos. However, induction of the chimeric receptor corrected the tracheal defects of breathless (btl) mutant embryos, allowing the tracheal cells to migrate along their normal tracts. This result indicates that the normal activity of DFGF-R1 in promoting cell migration does not require spatially restricted cues. Late inductions of the chimeric construct, after the normal initiation of tracheal migration, allowed the definition of a broad time window during which the external signals guiding migration persist and the tracheal cells retain the capacity to respond to these cues. Rescue of tracheal migration in btl mutant embryos by the chimeric construct provides a sensitive biological assay for the activity of other Drosophila receptor tyrosine kinases (RTKs). Deregulated receptors containing the cytoplasmic domains of DFGF-R2, DER, torso, and sevenless were all able to partially rescue the migration defects. Consistent with the notion that these RTKs share a common signaling pathway, constructs containing the activated downstream elements Dras1 and Draf were also able to rescue tracheal migration, demonstrating that these two proteins are key players in the DFGF-R1 signaling pathway.

Animals↗
PubMed1993

Activation and repression of Drosophila alcohol dehydrogenase distal transcription by two steroid hormone receptor superfamily members binding to a common response element.

Developmental activation of the Drosophila alcohol dehydrogenase (Adh) distal promoter is controlled by the Adh adult enhancer (AAE). Within this 150 bp, complex enhancer is a small (12 bp) positive cis-acting element that is required for high levels of distal transcription in adult flies and ADH-expressing tissue culture cells. We previously reported that the steroid receptor superfamily member FTZ-F1 binds to this site. We have identified a second steroid receptor superfamily member, DHR39, which also binds to this site. DHR39 is expressed throughout development in transcripts of several sizes. In situ hybridization to embryos has shown that DHR39 RNA is found primarily in the central nervous system, and not in embryonic tissues that express ADH. FTZ-F1 RNA, however, shows temporal-specific patterns similar to those of the distal promoter. FTZ-F1 and DHR39 have identical amino acids in the 'P-box' of the DNA binding domain, suggesting that they have identical DNA recognition characteristics. By electrophoretic mobility shift analysis we show that a DHR39 fusion protein binds specifically to two FTZ-F1 binding sites. By over expressing the full length DHR39 protein in a transient co-transfection assay we have shown that it represses distal Adh expression in a dosage- and binding site-dependent manner. Over expression of an alternative DHR39 open reading frame that lacks part of the putative ligand binding domain does not alter Adh expression. In contrast, over expression of FTZ-F1 specifically activates distal Adh expression.

Alcohol Dehydrogenase↗
PubMed1993

Establishment of ventral cell fates in the Drosophila embryonic ectoderm requires DER, the EGF receptor homolog.

The embryonic ectoderm in Drosophila displays a highly organized arrangement of specific structures along the dorsal-ventral axis. To establish this characteristic design, cells must receive instructive cues regarding their position. We present evidence that during stages 8-9 of embryonic development, the Drosophila EGF receptor homolog (DER) is essential for determining the identity of cells within the ventral ectoderm. In the absence of DER activity at this phase, alterations in cell fate are observed: Ventral cells acquire more dorsal fates, as visualized by the expression profile of specific markers. The ventralizing effect of DER appears to function later than that of the dorsalizing dpp pathway, and the spatial overlap between them is minimal. A model for the determination of cell fates along the dorsal-ventral axis involving the two pathways is presented. Some aspects of the mutant ectodermal and CNS phenotypes of the DER locus (faint little ball, flb) resemble the phenotype of mutations from the spitz group. Synergistic interactions between flb and spitz or Star mutations suggest that these genes participate in a common signaling pathway.

Animals↗
PubMed1992

Localization of DER and the pattern of cell divisions in wild-type and Ellipse eye imaginal discs.

The compound eye of Drosophila develops from a uniform layer of epithelial cells in the eye imaginal disc. One intriguing aspect of eye development is the establishment of the correct number and spacing of the photoreceptor clusters which give rise to the mature ommatidia. Ellipse (Elp) has been implicated as playing a role in this process because the Elp dominant gain of function mutation dramatically reduces the number of photoreceptor clusters in the compound eye without affecting the morphology of individual clusters that are formed (Baker and Rubin, 1989). Since Elp represents an allele of the Drosophila EGF receptor (DER) locus, it encodes a protein which is structurally capable of mediating inductive cell-cell interactions. In an effort to better understand the role of the DER locus in ommatidial patterning, we compared the localization of DER protein in eye imaginal discs of wild-type and Elp larvae. The distribution of this receptor is consistent with the notion of its mediating interactions between cells at the initial stages of photoreceptor precluster positioning and differentiation. However, the basis of the Elp gain of function mutation is not ectopic or increased expression of the DER protein. Rather, expression of the Elp form of the EGF receptor homolog in the normal localization leads to changes in the proliferative pattern of cells dividing posterior to the morphogenetic furrow.

Animals↗