bluuebunny/crossref_metadata_2025 huggingface dataset:
Notch signalling regulates veinlet expression and establishes boundaries between veins and interveins in the Drosophila wing
Authors: Celis Jose F. de , Bray Sarah , Garcia-Bellido Antonio
Published: April 2021

The veins in the Drosophila wing have a characteristic width, which is regulated by the activity of the Notch pathway. The expression of the Notch-ligand Delta is restricted to the developing veins, and coincides with places where Notch transcription is lower. We find that this asymmetrical distribution of ligand and receptor leads to activation of Notch on both sides of each vein within a territory of Delta-expressing cells, and to the establishment of boundary cells that separate the vein from adjacent interveins. In these cells, the expression of the Enhancer of split gene m β is activated and the transcription of the veinpromoting gene veinlet is repressed, thus restricting vein differentiation. We propose that the establishment of vein thickness utilises a combination of mechanisms that include: (1) independent regulation of Notch and Delta expression in intervein and vein territories, (2) Notch activation by Delta in cells where Notch and Delta expression overlaps, (3) positive feedback on Notch transcription in cells where Notch has been activated and (4) repression of veinlet transcription by E(spl)m β and maintenance of Delta expression by veinlet/torpedo activity.
Competition and cooperation among receptor tyrosine phosphatases control motoneuron growth cone guidance in Drosophila
Authors: Desai Chand J. , Krueger Neil X. , Saito Haruo , Zinn Kai
Published: April 2021

The neural receptor tyrosine phosphatases DPTP69D, DPTP99A and DLAR are involved in motor axon guidance in the Drosophila embryo. Here we analyze the requirements for these three phosphatases in growth cone guidance decisions along the ISN and SNb motor pathways. Any one of the three suffices for the progression of ISN pioneer growth cones beyond their first intermediate target in the dorsal muscle field. DLAR or DPTP69D can facilitate outgrowth beyond a second intermediate target, and DLAR is uniquely required for formation of a normal terminal arbor. A different pattern of partial redundancy among the three phosphatases is observed for the SNb pathway. Any one of the three suffices to allow SNb axons to leave the common ISN pathway at the exit junction. When DLAR is not expressed, however, SNb axons sometimes bypass their ventrolateral muscle targets after leaving the common pathway, instead growing out as a separate bundle adjacent to the ISN. This abnormal guidance decision can be completely suppressed by also removing DPTP99A, suggesting that DLAR turns off or counteracts a DPTP99A signal that favors the bypass axon trajectory. Our results show that the relationships among the tyrosine phosphatases are complex and dependent on cellular context. At growth cone choice points along one nerve, two phosphatases cooperate, while along another nerve these same phosphatases can act in opposition to one another.
Transcriptional regulation of Notch and Delta: requirement for neuroblast segregation in Drosophila
Authors: Seugnet Laurent , Simpson Pat , Haenlin Marc
Published: April 2021

Segregation of a single neural precursor from each proneural cluster in Drosophila relies on Notch-mediated lateral signalling. Studies concerning the spacing of precursors for the microchaetes of the peripheral nervous system suggested the existence of a regulatory loop between Notch and its ligand Delta within each cell that is under transcriptional control. Activation of Notch leads to repression of the achaete-scute genes which themselves regulate transcription of Delta, perhaps directly. Here we have tested a requirement for transcriptional regulation of Notch and/or Delta during neuroblast segregation in embryos, by providing Notch and Delta ubiquitously at uniform levels. Neuroblast segregation occurs normally under conditions of uniform Notch expression. Under conditions of uniform Delta expression, a single neuroblast segregates from each proneural group in 80% of the cases, more than one in the remaining 20%. Thus transcriptional regulation of Delta is largely dispensable. We discuss the possibility that segregation of single precursors in the central nervous system may rely on a heterogeneous distribution of neural potential between different cells of the proneural group. Notch signalling would enable all cells to mutually repress each other and only a cell with an elevated neural potential could overcome this repression.
Flk-1 expression defines a population of early embryonic hematopoietic precursors
Authors: Kabrun Neil , Bühring Hans-Jörg , Choi Kyunghee , Ullrich Axel , Risau Werner , Keller Gordon
Published: April 2021

We have investigated the expression pattern of the Flk-1 receptor tyrosine kinase in mouse embryonic and fetal hematopoietic tissues as well as on hematopoietic precursor cells derived from these tissues. RNA analysis indicated that flk-1 was expressed in the yolk sac at day 10 of gestation, in the whole embryo at day 10 and 12 of gestation, in the liver throughout fetal life and in embryoid bodies (EBs) generated from ES cells differentiated in culture. Flk-1 message was also detected in erythroid and macrophage colonies generated from precursors of yolk sac, fetal liver, adult marrow and eB origin. Using an antibody directed against the extracellular portion of the molecule we have found that up to 50% of cells from EBs differentiated for 4 days express Flk-1. Following the development of this early Flk-1+ population the number of receptor-positive cells declines progressively to represent less than 5% of the EBs by day 12 of differentiation. Kinetic analysis revealed that the establishment of the EB Flk-1+ population precedes the development of cells which express CD34, Ly6A (Sca-1) and AA4.1. Cell sorting experiments demonstrated that all day-4 EB-derived hematopoietic precursors are Flk-1+ whereas greater than 95% of those found within the day-12 EBs are Flk-1−, suggesting that the precursor population which expresses this receptor represents an early but transient wave of hematopoietic development. Analysis of yolk sac and whole embryos at day 8.5 of gestation revealed a small but distinct Flk-1+ population that contained hematopoietic precursors. Day-12.5 fetal liver contained few Flk-1+ cells that showed little hematopoietic potential. Together these findings indicate that Flk-1 is expressed on an early population of hematopoietic precursors that may represent the onset of embryonic hematopoiesis.
Cell movements, neuronal organisation and gene expression in hindbrains lacking morphological boundaries
Authors: Nittenberg Ronald , Patel Ketan , Joshi Yogish , Krumlauf Robb , Wilkinson David G. , Brickell Paul M. , Tickle Cheryll , Clarke Jonathan D. W.
Published: April 2021

Rhombomeres are segmental units of the hindbrain that are separated from each other by a specialised zone of boundary cells. Retinoic acid application to a recently segmented hindbrain leads to disappearance of posterior rhombomere boundaries. Boundary loss is preceded by changes in segmental expression of Krox-20 and Cek-8 and followed by alterations in Hox gene expression. The characteristic morphology of boundary cells, their expression of follistatin and the periodic accumulation of axons normally associated with boundaries are all lost. In the absence of boundaries, we detect no change in anteroposterior dispersal of precursor cells and, in most cases, no substantial cell mixing between former rhombomeric units. This is consistent with the idea that lineage restriction can be maintained by processes other than a mechanical barrier composed of boundary cells. Much of the early organisation of the motor nuclei appears normal despite the loss of boundaries and altered Hox expression.
Bmp-4 acts as a morphogen in dorsoventral mesoderm patterning in Xenopus
Authors: Dosch Roland , Gawantka Volker , Delius Hajo , Blumenstock Claudia , Niehrs Christof
Published: April 2021

The marginal zone is a ring of tissue that gives rise to a characteristic dorsoventral pattern of mesoderm in amphibian embryos. Bmp-4 is thought to play an important role in specifying ventral mesodermal fate. Here we show (1)that different doses of Bmp-4 are sufficient to pattern four distinct mesodermal cell types and to pattern gene expression in the early gastrula marginal zone into three domains, (2) that there is a graded requirement for a Bmp signal in mesodermal patterning, and (3) that Bmp-4 has long-range activity which can become graded in the marginal zone by the antagonizing action of noggin. The results argue that Bmp-4 acts as a morphogen in dorsoventral patterning of mesoderm.
The Pax protein Noi is required for commissural axon pathway formation in the rostral forebrain
Authors: Macdonald Rachel , Scholes John , Strähle Uwe , Brennan Caroline , Holder Nigel , Brand Michael , Wilson Stephen W.
Published: April 2021

No-isthmus (Noi) is a member of the zebrafish Pax family of transcriptional regulators that is expressed in restricted domains of the developing CNS. In the developing eye and optic nerve, the Noi + cells are primitive glial cells that line the choroid fissure and optic stalk/nerve to its junction with the optic tract. This pattern of Noi expression is retained in the adult, defining the optic nerve astroglia, which wrap the left and right nerves separately at the midline, thus forming the bodily crossed optic chiasm found in fish. In embryos carrying mutations in the noi gene, the choroid fissure fails to close, glial cells of the optic nerve fail to dif-ferentiate and optic axons exhibit abnormal trajectories exiting the eye and at the midline of the diencephalon. Optic axons select inappropriate pathways into the con-tralateral optic nerve, rostrally towards the anterior com-missure and along the ipsilateral optic tract. Noi+ cells also border the pathway of axons in the postoptic commissure, which is located adjacent to the optic chiasm. These postoptic commissural axons are defasciculated and also exhibit pathfinding defects in noi− embryos. These results indicate that Noi is required in cells that line the pathways taken by optic and non-optic commissural axons for guidance across the midline of the diencephalon. We find that expression of two members of the Netrin family of axon guidance molecules and the signalling protein Sonic hedgehog is disturbed in noi− embryos, whereas several members of the Eph family of receptors and ligands show no obvious alterations in expression at the diencephalic midline.
Analysis of growth factor and receptor mRNA levels during development of the rat seminal vesicle and prostate
Authors: Thomson Axel A. , Foster Barbara A. , Cunha Gerald R.
Published: April 2021

Development of the mammalian male accessory sexual organs requires both androgens and mesenchymal/epithelial interactions. Paracrine acting factors whose expression is mesenchymal and androgen dependent have been proposed to regulate development of these organs, although the identity of these paracrine mediators is unknown. Keratinocyte growth factor (Kgf) has been shown to play an important role in the development of the mouse seminal vesicle and rat ventral prostate. Also, Kgf is expressed in mesenchymal cells and has been shown to be regulated by androgens in prostatic cells grown in vitro. Thus Kgf has been proposed as a mediator of androgen action. We have investigated the expression of Kgf mRNA during development of the rat seminal vesicle and prostate, both in vitro and in vivo. Additionally we have examined mRNAs for Kgf receptor (KgfR), transforming growth factor alpha (Tgf α), epidermal growth factor receptor (EgfR) and cytokeratin 19 (CK19). The levels of growth factor and receptor mRNAs fluctuated during androgen-regulated development; however, these changes reflected variations in the mesenchymal/epithelial ratio rather than regulation by testosterone. Expression of Kgf is mesenchymal, while KgfR is epithelial and Tgf α is predominantly epithelial. The changes in the levels of mRNAs for these factors correlated well with changes in the level of an epithelial marker, CK19, suggesting they were due to alterations in the relative abundance of tissue compartments in which they were expressed. Kgf has been shown to mimic androgen action in explant cultures of seminal vesicle and prostate. We demonstrate here that anti-androgens are able to block Kgf stimulated development, suggesting that Kgf and androgen receptor signalling pathways may interact. Taken together our data suggest that, in vivo, Kgf may interact with androgen receptor signalling but it is not a direct target of androgen action.
A novel group of pumilio mutations affects the asymmetric division of germline stem cells in the Drosophila ovary
Authors: Lin Haifan , Spradling Allan C.
Published: April 2021

Germline stem cells play a pivotal role in gametogenesis; yet little is known about how they are formed, how they divide to self-renew, and how these processes are genetically controlled. Here we describe the self-renewing asymmetric division of germline stem cells in the Drosophila ovarian germline, as marked by the spectrosome, a cytoplasmic structure rich in membrane skeletal proteins. The ontogeny of the spectrosome marks the lineage of germline stem cells. We identified two new groups of mutations in which the divisional asymmetry is disrupted. The first, which we refer to as ovarette (ovt) mutations, was shown to correspond to a novel class of mutations in the pumilio locus. Since pumilio is known to posttranscriptionally repress the expression of target genes at earlier stages of germ cell development, our results suggest that a similar activity is needed to maintain germ line stem cells. We have also identified a second and novel gene, piwi, whose mutations abolish germline stem cell division.
Mechanisms of dorsal-ventral patterning in noggin-induced neural tissue
Authors: Knecht Anne K. , Harland Richard M.
Published: April 2021

We have investigated mechanisms of dorsal-ventral patterning of neural tissue, using Xenopus ectoderm neuralized by noggin protein. This tissue appears to be patterned dorsoventrally; cpl-1, a gene expressed in the dorsal brain, and etr-1, a gene largely excluded from the dorsal brain, are expressed in separate territories in noggin-treated explants (Knecht, A. K., Good, P. J., Dawid, I. B. and Harland, R. M. (1995) Development 121, 1927-1936). Here we show further evidence that this pattern represents a partial dorsal-ventral organization. Additionally, we test two mechanisms that could account for this pattern: a dosedependent response to a gradient of noggin protein within the explant, and regulative cell-cell interactions. We show that noggin exhibits concentration-dependent effects, inducing cpl-1 at low doses but repressing it at high doses. Since noggin acts by antagonizing Bone Morphogenetic Protein (BMP) signaling, this result suggests that BMPs also may act in a dose-dependent manner in vivo. However, in the absence of a noggin gradient, regulative cell-cell interactions can also pattern the tissue. Such regulation is facilitated by increased motility of noggin-treated cells. Finally, the response of cells to both of these patterning mechanisms is ultimately controlled by a third process, the changing competence of the responding tissue.
Myogenin can substitute for Myf5 in promoting myogenesis but less efficiently
Authors: Wang Yukang , Jaenisch Rudolf
Published: April 2021

The myogenic basic Helix-Loop-Helix transcription factors, including Myf5, MyoD, myogenin (myg) and MRF4, play important roles in skeletal muscle development. The phenotypes of mutant mice deficient in either gene are different, suggesting that each gene may have a unique function in vivo. We previously showed that targeting myogenin into the Myf5locus (Myf5myg-ki) rescued the rib cage truncation in the Myf5-null mutant, hence demonstrating functional redundancy between Myf5 and myogenin in skeletal morphogenesis. Here we present the results of crossing myogenin knock-in (myg-ki) mice with either MyoD-null or myogenin-null mutants. The Myf5myg-ki allele rescued early myogenesis, but Myf5(myg-ki/myg-ki);MyoD−(/−) mutant mice died immediately after birth owing to reduced muscle formation. Therefore, myogenin, expressed from the Myf5 locus, is not able to completely replace the function of Myf5 in muscle devel-opment although it is capable of determining and/or maintaining myogenic lineage. Myf5(myg-ki/myg-ki);myg−(/−) mutant mice displayed the same phenotype as myg(−/−) mutants. This indicates that the earlier expression of myogenin cannot promote myogenic terminal differentiation, which is normally initiated by the endogenous myogenin. Thus, our results are consistent with the notion that Myf5 and myogenin are functionally interchangeable in determining myogenic lineage and assuring normal rib formation. Our experiment revealed, however, that some aspects of myogenesis may be unique to a given myogenic factor and are due to either different regulatory sequences that control their temporal and spatial expression or different functional protein domains.
β3-tubulin is directly repressed by the Engrailed protein in Drosophila
Authors: Serrano Nuria , Brock Hugh W. , Maschat Florence
Published: April 2021

In Drosophila, Engrailed is a nuclear regulatory protein with essential roles during embryonic development. Although Engrailed is a transcription factor, little progress has been achieved in identifying its target genes. We report here the identification of an effector gene, the β3-tubulin gene, as a direct target of Engrailed. The cytological location of β3-tubulin, 60C, is a strong site of Engrailed binding on polytene chromosomes. Immunostaining analysis of a transgenic line containing a P[β3-tubulin-lacZ] construct shows an additional site of Engrailed binding at the location of the transgene. Molecular analysis allowed identification of several Engrailed binding sites, both in vitro and in vivo, within the first intron of the β3-tubulin locus. Engrailed binding sites identified in vitro are active in larvae. Furthermore, expression of β3-tubulin is de-repressed in the ectoderm of engrailed mutant embryos. Repression of β3-tubulin by Engrailed is also obtained when Engrailed is ectopically expressed in embryonic mesoderm. Finally, two different sets of Engrailed binding sites are shown to be involved in the early and late regulation of β3-tubulin by Engrailed during embryogenesis.
The Drosophila G-protein-coupled receptor kinase homologue Gprk2 is required for egg morphogenesis
Authors: Schneider Lynne E. , Spradling Allan C.
Published: April 2021

G protein signaling is a widely utilized form of extracellular communication that is mediated by a family of serpentine receptors containing seven transmembrane domains. In sensory neurons, cardiac muscle and other tissues, G protein-coupled receptors are desensitized through phosphorylation by a family of kinases, the G protein-coupled receptor kinases (GRKs). Desensitization allows a cell to decrease its response to a given signal, in the continued presence of that signal. We have identified a Drosophila mutant, gprk26936 that disrupts expression of a putative member of the GRK family, the G protein-coupled receptor kinase 2 gene (Gprk2). This mutation affects Gprk2 gene expression in the ovaries and renders mutant females sterile. The mutant eggs contain defects in several anterior eggshell structures that are produced by specific subsets of migratory follicle cells. In addition, rare eggs that become fertilized display gross defects in embryogenesis. These observations suggest that developmental signals transduced by G protein-coupled receptors are regulated by receptor phosphorylation. Based on the known functions of G protein-coupled receptor kinases, we speculate that receptor desensitization assists cells that are migrating or undergoing shape changes to respond rapidly to changing external signals.
Genetic evidence that heparin-like glycosaminoglycans are involved in wingless signaling
Authors: Binari Richard C. , Staveley Brian E. , Johnson Wayne A. , Godavarti Ranga , Sasisekharan Ram , Manoukian Armen S.
Published: April 2021

We have identified the Drosophila UDP-glucose dehydro-genase gene as being involved in wingless signaling. Mutations in this gene, called kiwi, generate a phenotype identical to that of wingless.UDP-glucose dehydrogenase is required for the biosynthesis of UDP-glucuronate, which in turn is utilized in the biosynthesis of glycosaminoglycans. By rescuing the kiwiphenotype with both UDP-glucuronate and the glycosaminoglycan heparan sulfate, we show that kiwi function in the embryo is crucial for the production of heparan sulfate in the extracellular matrix. Further, injection of heparin degrading enzyme, heparinase (and not chondroitin, dermatan or hyaluronic acid degrading enzyme) into wild-type embryos leads to the degradation of heparin-like glycosaminoglycans and a ‘wingless-like’ cuticular phenotype. Our study thus provides the first genetic evidence for the involvement of heparin-like gly-cosaminoglycans in signal transduction.
Development of branchiomotor neurons in zebrafish
Authors: Chandrasekhar Anand , Moens Cecilia B. , Warren James T. , Kimmel Charles B. , Kuwada John Y.
Published: April 2021

The mechanisms underlying neuronal specification and axonogenesis in the vertebrate hindbrain are poorly under-stood. To address these questions, we have employed anatomical methods and mutational analysis to character-ize the branchiomotor neurons in the zebrafish embryo. The zebrafish branchiomotor system is similar to those in the chick and mouse, except for the location of the nVII and nIX branchiomotor neurons. Developmental analyses of genes expressed by branchiomotor neurons suggest that the different location of the nVII neurons in the zebrafish may result from cell migration.

To gain insight into the mechanisms underlying the organization and axonogenesis of these neurons, we examined the development of the branchiomotor pathways in neuronal mutants. The valentinob337 mutation blocks the formation of rhombomeres 5 and 6, and severely affects the development of the nVII and nIX motor nuclei. The cyclopsb16 mutation deletes ventral midline cells in the neural tube, and leads to a severe disruption of most bran-chiomotor nuclei and axon pathways. These results demon-strate that rhombomere-specific cues and ventral midline cells play important roles in the development of the bran-chiomotor pathways.
TETRASPORE is required for male meiotic cytokinesis in Arabidopsis thaliana
Authors: Spielman Melissa , Preuss Daphne , Li Feng-Lan , Browne William E. , Scott Rod J. , Dickinson Hugh G.
Published: April 2021

In flowering plants, male meiosis occurs in the microsporocyte to produce four microspores, each of which develops into a pollen grain. Here we describe four mutant alleles of TETRASPORE (TES), a gene essential for microsporocyte cytokinesis in Arabidopsis thaliana. Following failure of male meiotic cytokinesis intes mutants, all four microspore nuclei remain within the same cytoplasm, with some completing their developmental programmes to form functional pollen nuclei. Both of the mitotic divisions seen in normal pollen development take place in tes mutants, including the asymmetric division required for the differentiation of gametes; some tes grains perform multiple asymmetric divisions in the same cytoplasm. tes pollen shows a variety of abnormalities subsequent to the cytokinetic defect, including fusion of nuclei, formation of ectopic internal walls, and disruptions to external wall patterning. In addition, ovules fertilized by tes pollen often abort, possibly because of excess paternal genomes in the endosperm. Thus tes mutants not only reveal a gene specific to male meiosis, but aid investigation of a wide range of processes in pollen development and function.
TGFβ2 knockout mice have multiple developmental defects that are non-overlapping with other TGFβ knockout phenotypes
Authors: Sanford L. Philip , Ormsby Ilona , Groot Adriana C. Gittenberger-de , Sariola Hannu , Friedman Rick , Boivin Gregory P. , Cardell Emma Lou , Doetschman Thomas
Published: April 2021

The growth and differentiation factor transforming growth factor-β2 (TGFβ2) is thought to play important roles in multiple developmental processes. Targeted disruption of the TGF β2 gene was undertaken to determine its essential role in vivo. TGFβ2-null mice exhibit perinatal mortality and a wide range of developmental defects for a single gene disruption. These include cardiac, lung, craniofacial, limb, spinal column, eye, inner ear and urogenital defects. The developmental processes most commonly involved in the affected tissues include epithelial-mesenchymal interac-tions, cell growth, extracellular matrix production and tissue remodeling. In addition, many affected tissues have neural crest-derived components and simulate neural crest deficiencies. There is no phenotypic overlap with TGFβ1- and TGF β3-null mice indicating numerous non-compen-sated functions between the TGFβ isoforms.
Induction of identified mesodermal cells by CNS midline progenitors in Drosophila
Authors: Lüer Karin , Urban Joachim , Klämbt Christian , Technau Gerhard M.
Published: April 2021

The Drosophila ventral midline cells generate a discrete set of CNS lineages, required for proper patterning of the ventral ectoderm. Here we provide the first evidence that the CNS midline cells also exert inductive effects on the mesoderm. Mesodermal progenitors adjacent to the midline progenitor cells give rise to ventral somatic mucles and a pair of unique cells that come to lie dorsomedially on top of the ventral nerve cord, the so-called DM cells. Cell ablation as well as cell transplantation experiments indicate that formation of the DM cells is induced by midline progenitors in the early embryo. These results are corroborated by genetic analyses. Mutant single minded embryos lack the CNS midline as well as the DM cells. Embryos mutant for any of the spitz group genes, which primarily express defects in the midline glial cell lineages, show reduced formation of the DM cells. Conversely, directed overexpression of secreted SPITZ by some or all CNS midline cells leads to the formation of additional DM cells. Furthermore we show that DM cell development does not depend on the absolute concentration of a local inductor but appears to require a graded source of an inducing signal. Thus, the Drosophila CNS midline cells play a central inductive role in patterning the mesoderm as well as the underlying ectoderm.
DPP controls tracheal cell migration along the dorsoventral body axis of the Drosophila embryo
Authors: Vincent Stéphane , Ruberte Esther , Grieder Nicole C. , Chen Chao-Kung , Haerry Theo , Schuh Reinhard , Affolter Markus
Published: April 2021

We report that DPP signaling is required for directed tracheal cell migration during Drosophila embryogenesis. The failure of tracheal cells to receive the DPP signal from adjacent dorsal and ventral cells results in the absence of dorsal and ventral migrations. Ectopic DPP signaling can reprogram cells in the center of the placode to adopt a dorsoventral migration behavior. The effects observed in response to ectopic DPP signaling are also observed upon the tracheal-specific expression of a constitutive active DPP type I receptor (TKVQ253D), indicating that the DPP signal is received and transmitted in tracheal cells to control their migration behavior. DPP signaling determines localized gene expression patterns in the developing tracheal placode, and is also required for the dorsal expression of the recently identified BRANCHLESS (BNL) guidance molecule, the ligand of the BREATHLESS (BTL) receptor. Thus, DPP plays a dual role during tracheal cell migration. It is required to control the dorsal expression of the BNL ligand; in addition, the DPP signal recruits groups of dorsal and ventral tracheal cells and programs them to migrate in dorsal and ventral directions.
Cellularization in locust embryos occurs before blastoderm formation
Authors: Ho Karen , Dunin-Borkowski Olga M. , Akam Michael
Published: April 2021

In Drosophila intracellular gradients establish the pattern of segmentation by controlling gene expression during a critical syncytial stage, prior to cellularization. To investigate whether a similar mechanism may be exploited by other insects, we examined the timing of cellularization with respect to blastoderm formation in an insect with extreme short-germ development, the African desert locust, Schistocerca gregaria. Using light and electron microscopic techniques, we show that the islands of cytoplasm surrounding cleavage nuclei are largely isolated from their neighbours, allowing cleavage to proceed asynchronously. Within a short time of their arrival at the surface and prior to blastoderm formation, nuclei become surrounded by complete cell membranes that block the free uptake of dye (10,000 kDa) from the yolk. Our results imply that the formation of the blastoderm disc involves the aggregation of cells at the posterior pole of the egg and not the migration of nuclei within a syncytial cytoplasm. These findings suggest that the primary cleavage syncytium does not play the same role in patterning the locust embryo as it does in Drosophila. However, we do identify a syncytial nuclear layer that underlies the forming blastoderm and remains in continuity with the yolk.
Positional apoptosis during vertebrate CNS development in the absence of endogenous retinoids
Authors: Maden Malcolm , Graham Anthony , Gale Emily , Rollinson Chris , Zile Maija
Published: April 2021

We have previously shown that quail embryos that develop in the absence of vitamin A have severe defects in their central nervous system. One defect is a completely missing posterior hindbrain. Here we have studied how this comes about by examining cell death using a wholemount technique. In these A− embryos we observe two narrow bands of ectopic apoptosis. One is in the mesenchyme in the region of the first somite and occurs at the 4–6 somite stage, before neural tube closure. The second band follows immediately afterwards and occurs in the neuroepithelium of the presumptive posterior hindbrain at the 6-8 somite stage. Electron microscopy shows that the dying neuroepithelial cells exhibit the characteristics of apoptosis. Rescuing the embryos by injecting retinol before gastrulation completely prevents these apoptotic events. In an effort to identify some of the genes that may be involved in the apoptotic pathway we show that Msx-2 is upregulated in the apoptotic neuroepithelium and thus may be involved, whereas Bmp-4 is not altered and thus presumably not involved. Since these apoptotic event take place at the time of specification of axial identity and segmentation in the mesenchyme and neuroepithelium we conclude that these cells die because they are wrongly specified in terms of their rostrocaudal position, a novel phenomenon which we refer to as positional apoptosis.
Chimeric analysis of fibroblast growth factor receptor-1 (Fgfr1) function: a role for FGFR1 in morphogenetic movement through the primitive streak
Authors: Ciruna Brian G. , Schwartz Lois , Harpal Kendraprasad , Yamaguchi Terry P. , Rossant Janet
Published: April 2021

Fibroblast growth factor (FGF) signaling has been implicated in the patterning of mesoderm and neural lineages during early vertebrate development. In the mouse, FGF receptor-1 (FGFR1) is expressed in an appropriate spatial and temporal manner to be orchestrating these functions. Mouse embryos homozygous for a mutated Fgfr1 allele (fgfr1Δtmk) die early in development, show abnormal growth and aberrant mesodermal patterning. We have performed a chimeric analysis to further study FGFR1 function in the morphogenesis and patterning of the mesodermal germ layer at gastrulation. At E9.5, fgfr1Δtmk/fgfr1Δtmk cells showed a marked deficiency in their ability to contribute to the extra-embryonic, cephalic, heart, axial and paraxial mesoderm, and to the endoderm of chimeric embryos. Analysis at earlier stages of development revealed that fgfr1Δtmk/fgfr1Δtmk cells accumulated within the primitive streak of chimeric embryos, and consequently failed to populate the anterior mesoderm and endodermal lineages at their inception. We suggest that the primary defect associated with the fgfr1Δtmk mutation is a deficiency in the ability of epiblast cells to traverse the primitive streak. fgfr1Δtmk/fgfr1Δtmk cells that accumulated within the primitive streak of chimeric embryos tended to form secondary neural tubes. These secondary neural tubes were entirely fgfr1Δtmk/fgfr1Δtmk cell derived. The adoption of ectopic neural fate suggests that normal morphogenetic movement through the streak is essential not only for proper mesodermal patterning but also for correct determination of mesodermal/neurectodermal cell fates.
goosecoid and HNF-3β genetically interact to regulate neural tube patterning during mouse embryogenesis
Authors: Filosa Stefania , Rivera-Pérez Jaime A. , Gómez Aitana Perea , Gansmuller Anne , Sasaki Hitoshi , Behringer Richard R. , Ang Siew-Lan
Published: April 2021

The homeobox gene goosecoid (gsc) and the winged-helix gene Hepatic Nuclear Factor-3β (HNF-3β) are co-expressed in all three germ layers in the anterior primitive streak and at the rostral end of mouse embryos during gastrulation. In this paper, we have tested the possibility of functional synergism or redundancy between these two genes during embryogenesis by generating double-mutant mice for gsc and HNF-3β. Double-mutant embryos of genotype gsc−/−;HNF-3β+/− show a new phenotype as early as embryonic days 8.75. Loss of Sonic hedgehog (Shh) and HNF-3β expression was observed in the notochord and ventral neural tube of these embryos. These results indicate that gsc and HNF-3β interact to regulate Shh expression and consequently dorsal-ventral patterning in the neural tube. In the forebrain of the mutant embryos, severe growth defects and absence of optic vesicles could involve loss of expression of fibroblast growth factor-8, in addition to Shh. Our results also suggest that interaction between gsc and HNF-3β regulates other signalling molecules required for proper development of the foregut, branchial arches and heart.
Genetic separation of the neural and cuticular patterning functions of gooseberry
Authors: Duman-Scheel Molly , Li Xuelin , Orlov Irena , Noll Markus , Patel Nipam H.
Published: April 2021

In addition to their role in the specification of the epidermal pattern in each segment, several segment polarity genes, including gooseberry (gsb), specify cell fate in the Drosophila central nervous system (CNS). Analyses of the gsb CNS phenotype have been complicated by the fact that the previously available gsb mutants, all caused by cyto-logically visible deficiencies, have severe segmentation defects and also lack a number of additional genes. We have characterized two novel gsb mutants which, due to their hypomorphic nature, have CNS defects, but have only weak or no segmentation defects. These gsb alleles, as well as gsb rescue experiments, have allowed us to determine which aspects of the deficiency mutant phenotypes can be attrib-uted to loss of gsb. gsb mutants lack U and CQ neurons, have duplicated RP2 neurons, and display posterior com-missure defects. gsb neural defects, as well as the gsb cuticle defect, are differentially sensitive to the level of functional Gsb. We have used one of the novel gsb alleles in order to understand the genetic interactions between gsb, wingless (wg), and patched (ptc) during the patterning of the ventral neuroectoderm. In contrast to epidermal patterning, where Gsb is required to maintain wg transcription, we find that Gsb antagonizes the Wg signal that confers neuroblast (NB) 4–2 fate.
Hepatocyte growth factor (HGF/SF) is a muscle-derived survival factor for a subpopulation of embryonic motoneurons
Authors: Yamamoto Yoichi , Livet Jean , Pollock Richard A. , Garces Alain , Arce Vilma , deLapeyrière Odile , Henderson Christopher E.
Published: April 2021

Muscle-derived factors are known to be important for the survival of developing spinal motoneurons, but the molecules involved have not been characterized. Hepatocyte growth factor/scatter factor (HGF/SF) plays an important role in muscle development and motoneuron axon outgrowth. We show that HGF/SF has potent neurotrophic activity (EC50=2 pM) for a subpopulation (40%) of purified embryonic rat motoneurons. Moreover, HGF/SF is an essential component of muscle-derived support for motoneurons, since blocking antibodies to HGF/SF specifically inhibited 65% of the trophic activity of media conditioned by C2/C7 skeletal myotubes, but did not inhibit the trophic activity secreted by Schwann cell lines. High levels of expression of the HGF/SF receptor c-Met in the spinal cord are restricted to subsets of motoneurons, mainly in limb-innervating segments. Consistent with this distribution, cultured motoneurons from limb-innervating brachial and lumbar segments showed a more potent response to HGF/SF than did thoracic motoneurons. By the end of the period of motoneuron cell death, levels of c-Met mRNA in motoneurons were markedly reduced, suggesting that the effects of HGF/SF may be limited to the period of motoneuron cell death. HGF/SF may play an important role during motoneuron development as a muscle-derived survival factor for a subpopulation of limb-innervating motoneurons.
Lhx2, a LIM homeobox gene, is required for eye, forebrain, and definitive erythrocyte development
Authors: Porter Forbes D. , Drago John , Xu Yang , Cheema Surindar S. , Wassif Chris , Huang Sing-Ping , Lee Eric , Grinberg Alexander , Massalas Jim S. , Bodine David , Alt Frederick , Westphal Heiner
Published: April 2021

We investigated the function of Lhx2, a LIM homeobox gene expressed in developing B-cells, forebrain and neural retina, by analyzing embryos deficient in functional Lhx2 protein. Lhx2 mutant embryos are anophthalmic, have malformations of the cerebral cortex, and die in utero due to severe anemia. In Lhx2−/− embryos specification of the optic vesicle occurs; however, development of the eye arrests prior to formation of an optic cup. Deficient cellular proliferation in the forebrain results in hypoplasia of the neocortex and aplasia of the hippocampal anlagen. In addition to the central nervous system malformations, a cell non-autonomous defect of definitive erythropoiesis causes severe anemia in Lhx2−/− embryos. Thus Lhx2 is necessary for normal development of the eye, cerebral cortex, and efficient definitive erythropoiesis.
Nanos and pumilio establish embryonic polarity in Drosophila by promoting posterior deadenylation of hunchback mRNA
Authors: Wreden Christopher , Verrotti Arturo C. , Schisa Jennifer A. , Lieberfarb Marshal E. , Strickland Sidney
Published: April 2021

Nanos protein promotes abdominal structures in Drosophila embryos by repressing the translation of maternal hunchback mRNA in the posterior. To study the mechanism of nanos-mediated translational repression, we first examined the mechanism by which maternal hunchback mRNA is translationally activated. In the absence of nanos activity, the poly(A) tail of hunchback mRNA is elongated concomitant with its translation, suggesting that cyto-plasmic polyadenylation directs activation. However, in the presence of nanos the length of the hunchback mRNA poly(A) tail is reduced. To determine if nanos activity represses translation by altering the polyadenylation state of hunchback mRNA, we injected various in vitro transcribed RNAs into Drosophila embryos and determined changes in polyadenylation. Nanos activity reduced the polyadenylation status of injected hunchback RNAs by accelerating their deadenylation. Pumilio activity, which is necessary to repress the translation of hunchback, is also needed to alter polyadenylation. An examination of translation indicates a strong correlation between poly(A) shortening and suppression of translation. These data indicate that nanos and pumilio determine posterior morphology by promoting the deadenylation of maternal hunchback mRNA, thereby repressing its translation.
Mouse-chick chimera: a developmental model of murine neurogenic cells
Authors: Fontaine-Pérus Josiane , Halgand Philippe , Chéraud Yvonnick , Rouaud Thierry , Velasco Maria Elena , Diaz Carmen Cifuentes , Rieger François
Published: April 2021

Chimeras were prepared by transplanting fragments of neural primordium from 8- to 8.5- and 9-day postcoital mouse embryos into 1.5- and 2-day-old chick embryos at different axial levels. Mouse neuroepithelial cells differentiated in ovo and organized to form the different cellular compartments normally constituting the central nervous system. The graft also entered into the development of the peripheral nervous system through migration of neural crest cells associated with mouse neuroepithelium. Depending on the graft level, mouse crest cells participated in the formation of various derivatives such as head components, sensory ganglia, orthosympathetic ganglionic chain, nerves and neuroendocrine glands. Tenascin knockout mice, which express lacZ instead of tenascin and show no tenascin production (Saga, Y., Yagi, J., Ikawa, Y., Sakakura, T. and Aizawa, S. (1992) Genes and Development 6, 1821-1838), were specifically used to label Schwann cells lining nerves derived from the implant. Although our experiments do not consider how mouse neural tube can participate in the mechanism required to maintain myogenesis in the host somites, they show that the grafted neural tube behaves in the same manner as the chick host neural tube. Together with our previous results on somite development (Fontaine-Pérus, J., Jarno, V., Fournier Le Ray, C., Li, Z. and Paulin, D. (1995) Development 121, 1705-1718), this study shows that chick embryo constitutes a privileged environment, facilitating access to the developmental potentials of normal or defective mammalian cells. It allows the study of the histogenesis and precise timing of a known structure, as well as the implication of a given gene at all equivalent mammalian embryonic stages.
Loss-of-function mutations in the maize homeobox gene, knotted1, are defective in shoot meristem maintenance
Authors: Kerstetter Randall A. , Laudencia-Chingcuanco Debbie , Smith Laurie G. , Hake Sarah
Published: April 2021

The product of the maize homeobox gene, knotted1 (kn1), localizes to the nuclei of cells in shoot meristems, but is absent from portions of the meristem where leaf primordia or floral organs initiate. Recessive mutant alleles of kn1 were obtained by screening for loss of the dominant leaf phenotype in maize. Mutant kn1 alleles carrying nonsense, splicing and frame shift mutations cause severe inflorescence and floral defects. Mutant tassels produce fewer branches and spikelets. Ears are often absent, and when present, are small with few spikelets. In addition, extra carpels form in female florets and ovule tissue proliferates abnormally. Less frequently, extra leaves form in the axils of vegetative leaves. These mutations reveal a role for kn1 in meristem maintenance, particularly as it affects branching and lateral organ formation.
Defects in glucuronate biosynthesis disrupt Wingless signaling in Drosophila
Authors: Haerry Theodor E. , Heslip Tim R. , Marsh J. Lawrence , O’Connor Michael B.
Published: April 2021

In vitro experiments suggest that glycosaminoglycans (GAGs) and the proteins to which they are attached (proteoglycans) are important for modulating growth factor signaling. However, in vivo evidence to support this view has been lacking, in part because mutations that disrupt the production of GAG polymers and the core proteins have not been available. Here we describe the identification and characterization of Drosophila mutants in the suppenkasper (ska) gene. The ska gene encodes UDP-glucose dehydrogenase which produces glucuronic acid, an essential component for the synthesis of heparan and chondroitin sulfate. ska mutants fail to put heparan side chains on proteoglycans such as Syndecan. Surprisingly, mutant embryos produced by germ-line clones of this general metabolic gene exhibit embryonic cuticle phenotypes strikingly similar to those that result from loss-of-function mutations in genes of the Wingless (Wg) signaling pathway. Zygotic loss of ska leads to reduced growth of imaginal discs and pattern defects similar to wg mutants. In addition, genetic interactions of ska with wg and dishevelled mutants are observed. These data demonstrate the importance of proteoglycans and GAGs in Wg signaling in vivo and suggest that Wnt-like growth factors may be particularly sensitive to perturbations of GAG biosynthesis.
Early- and late-migrating cranial neural crest cell populations have equivalent developmental potential in vivo
Authors: Baker Clare V. H. , Bronner-Fraser Marianne , Douarin Nicole M. Le , Teillet Marie-Aimée
Published: April 2021

We present the first in vivo study of the long-term fate and potential of early-migrating and late-migrating mesencephalic neural crest cell populations, by performing isochronic and heterochronic quail-to-chick grafts. Both early- and late-migrating populations form melanocytes, neurons, glia, cartilage and bone in isochronic, isotopic chimeras, showing that neither population is lineagerestricted. The early-migrating population distributes both dorsally and ventrally during normal development, while the late-migrating population is confined dorsally and forms much less cartilage and bone. When the latemigrating population is substituted heterochronically for the early-migrating population, it contributes extensively to ventral derivatives such as jaw cartilage and bone. Conversely, when the early-migrating population is substituted heterochronically for the late-migrating population, it no longer contributes to the jaw skeleton and only forms dorsal derivatives. When the late-migrating population is grafted into a late-stage host whose neural crest had previously been ablated, it migrates ventrally into the jaws. Thus, the dorsal fate restriction of the late-migrating mesencephalic neural crest cell population in normal development is due to the presence of earlier-migrating neural crest cells, rather than to any change in the environment or to any intrinsic difference in migratory ability or potential between early- and late-migrating cell populations. These results highlight the plasticity of the neural crest and show that its fate is determined primarily by the environment.