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A F Schier

Publications and source records attributed to A F Schier.

At least 19 recordsLinked to original sources
PubMed2001

Nodal signaling and the zebrafish organizer.

Systematic genetic screens in zebrafish have led to the discovery of mutations that affect organizer function and development. The molecular isolation and phenotypic analysis of the affected genes have revealed that TGF-beta signals of the Nodal family play a key role in organizer formation. The activity of the Nodal signals Cyclops and Squint is regulated extracellularly by the EGF-CFC cofactor One-eyed Pinhead and by antagonists belonging to the Lefty family of TGF-beta molecules. In the absence of Nodal signaling, the fate of cells in the organizer is transformed from dorsal mesoderm to neural ectoderm. Differential Nodal signaling also patterns the organizer along the anterior-posterior axis, with high levels required for anterior cell fates and lower levels for posterior fates. In addition, Nodal signaling cooperates with the homeodomain transcription factor Bozozok in organizer formation and neural patterning. The combination of genetic, molecular and embryological approaches in zebrafish has thus provided a framework to understand the mechanisms underlying organizer development.

Animals↗
PubMed2000

Fast1 is required for the development of dorsal axial structures in zebrafish.

Nodal-related signals comprise a subclass of the transforming growth factor-beta (TGF-beta) superfamily and regulate key events in vertebrate embryogenesis, including mesoderm formation, establishment of left-right asymmetry and neural patterning [1-8]. Nodal ligands are thought to act with EGF-CFC protein co-factors to activate activin type I and II or related receptors, which phosphorylate Smad2 and trigger nuclear translocation of a Smad2/4 complex [8-12]. The winged-helix transcription factor forkhead activin signal transducer-1 (Fast-1) acts as a co-factor for Smad2 [12-20]. Xenopus Fast-1 is thought to function as a transcriptional effector of Nodal signals during mesoderm formation [17], but no mutations in the Fast-1 gene have been identified. We report the identification of the zebrafish fast1 gene and show that it is disrupted in schmalspur (sur) mutants, which have defects in the development of dorsal midline cell types and establishment of left-right asymmetry [21-25]. We find that prechordal plate and notochord are strongly reduced in maternal-zygotic sur mutants, whereas other mesendodermal structures are present - a less severe phenotype than that caused by complete loss of Nodal signaling. These results show that fast1 is required for development of dorsal axial structures and left-right asymmetry, and suggest that Nodal signals act through Fast1-dependent and independent pathways.

Amino Acid Sequence↗
PubMed2000

Nodal-related signals establish mesendodermal fate and trunk neural identity in zebrafish.

The vertebrate body plan arises during gastrulation, when morphogenetic movements form the ectoderm, mesoderm, and endoderm. In zebrafish, mesoderm and endoderm derive from the marginal region of the late blastula, and cells located nearer the animal pole form the ectoderm [1]. Analysis in mouse, Xenopus, and zebrafish has demonstrated that Nodal-related proteins, a subclass of the TGF-beta superfamily, are essential for mesendoderm development [2], but previous mutational studies have not established whether Nodal-related signals control fate specification, morphogenetic movements, or survival of mesendodermal precursors. Here, we report that Nodal-related signals are required to allocate marginal cells to mesendodermal fates in the zebrafish embryo. In double mutants for the zebrafish nodal-related genes squint (sqt) and cyclops (cyc) [3] [4] [5], dorsal marginal cells adopt neural fates, whereas in wild-type embryos, cells at this position form endoderm and axial mesoderm. Involution movements characteristic of developing mesendoderm are also blocked in the absence of Nodal signaling. Because it has been proposed [6] that inhibition of Nodal-related signals promotes the development of anterior neural fates, we also examined anteroposterior organization of the neural tube in sqt;cyc mutants. Anterior trunk spinal cord is absent in sqt;cyc mutants, despite the presence of more anterior and posterior neural fates. These results demonstrate that nodal-related genes are required for the allocation of dorsal marginal cells to mesendodermal fates and for anteroposterior patterning of the neural tube.

Animals↗
PubMed2000

Nodal signalling in vertebrate development.

Communication between cells during early embryogenesis establishes the basic organization of the vertebrate body plan. Recent work suggests that a signalling pathway centering on Nodal, a transforming growth factor beta-related signal, is responsible for many of the events that configure the vertebrate embryo. The activity of Nodal signals is regulated extracellularly by EGF-CFC cofactors and antagonists of the Lefty and Cerberus families of proteins, allowing precise control of mesoderm and endoderm formation, the positioning of the anterior-posterior axis, neural patterning and left-right axis specification.

Animals↗
PubMed2000

The EGF-CFC gene family in vertebrate development.

EGF-CFC genes encode extracellular proteins that play key roles in intercellular signaling pathways during vertebrate embryogenesis. Mutations in zebrafish and mouse EGF-CFC genes lead to defects in germ-layer formation, anterior-posterior axis orientation and left-right axis specification. In addition, members of the EGF-CFC family have been implicated in carcinogenesis. Although formerly regarded as signaling molecules that are distant relatives of epidermal growth factor (EGF), recent findings indicate that EGF-CFC proteins act as essential cofactors for Nodal, a member of the transforming growth factor beta (TGF-beta) family. Here, we review molecular genetic evidence from mouse and zebrafish on biological and biochemical roles of the EGF-CFC family, and discuss differing models for EGF-CFC protein function.

Amino Acid Sequence↗
PubMed2000

A nodal signaling pathway regulates the laterality of neuroanatomical asymmetries in the zebrafish forebrain.

Animals show behavioral asymmetries that are mediated by differences between the left and right sides of the brain. We report that the laterality of asymmetric development of the diencephalic habenular nuclei and the photoreceptive pineal complex is regulated by the Nodal signaling pathway and by midline tissue. Analysis of zebrafish embryos with compromised Nodal signaling reveals an early role for this pathway in the repression of asymmetrically expressed genes in the diencephalon. Later signaling mediated by the EGF-CFC protein One-eyed pinhead and the forkhead transcription factor Schmalspur is required to overcome this repression. When expression of Nodal pathway genes is either absent or symmetrical, neuroanatomical asymmetries are still established but are randomized. This indicates that Nodal signaling is not required for asymmetric development per se but is essential to determine the laterality of the asymmetry.

Animals↗
PubMed2000

Loss-of-function mutations in the EGF-CFC gene CFC1 are associated with human left-right laterality defects.

All vertebrates display a characteristic asymmetry of internal organs with the cardiac apex, stomach and spleen towards the left, and the liver and gall bladder on the right. Left-right (L-R) axis abnormalities or laterality defects are common in humans (1 in 8,500 live births). Several genes (such as Nodal, Ebaf and Pitx2) have been implicated in L-R organ positioning in model organisms. In humans, relatively few genes have been associated with a small percentage of human situs defects. These include ZIC3 (ref. 5), LEFTB (formerly LEFTY2; ref. 6) and ACVR2B (encoding activin receptor IIB; ref. 7). The EGF-CFC genes, mouse Cfc1 (encoding the Cryptic protein; ref. 9) and zebrafish one-eyed pinhead (oep; refs 10, 11) are essential for the establishment of the L-R axis. EGF-CFC proteins act as co-factors for Nodal-related signals, which have also been implicated in L-R axis development. Here we identify loss-of-function mutations in human CFC1 (encoding the CRYPTIC protein) in patients with heterotaxic phenotypes (randomized organ positioning). The mutant proteins have aberrant cellular localization in transfected cells and are functionally defective in a zebrafish oep-mutant rescue assay. Our findings indicate that the essential role of EGF-CFC genes and Nodal signalling in left-right axis formation is conserved from fish to humans. Moreover, our results support a role for environmental and/or genetic modifiers in determining the ultimate phenotype in humans.

Abnormalities, Multiple↗
PubMed2000

Analysis of chromosomal rearrangements induced by postmeiotic mutagenesis with ethylnitrosourea in zebrafish.

Mutations identified in zebrafish genetic screens allow the dissection of a wide array of problems in vertebrate biology. Most screens have examined mutations induced by treatment of spermatogonial (premeiotic) cells with the chemical mutagen N-ethyl-N-nitrosourea (ENU). Treatment of postmeiotic gametes with ENU induces specific-locus mutations at a higher rate than premeiotic regimens, suggesting that postmeiotic mutagenesis protocols could be useful in some screening strategies. Whereas there is extensive evidence that ENU induces point mutations in premeiotic cells, the range of mutations induced in postmeiotic zebrafish germ cells has been less thoroughly characterized. Here we report the identification and analysis of five mutations induced by postmeiotic ENU treatment. One mutation, snh(st1), is a translocation involving linkage group (LG) 11 and LG 14. The other four mutations, oep(st2), kny(st3), Df(LG 13)(st4), and cyc(st5), are deletions, ranging in size from less than 3 cM to greater than 20 cM. These results show that germ cell stage is an important determinant of the type of mutations induced. The induction of chromosomal rearrangements may account for the elevated frequency of specific-locus mutations observed after treatment of postmeiotic gametes with ENU.

Animals↗
PubMed2000

Genetic linkage mapping of zebrafish genes and ESTs.

Genetic screens in zebrafish (Danio rerio) have isolated mutations in hundreds of genes essential for vertebrate development, physiology, and behavior. We have constructed a genetic linkage map that will facilitate the identification of candidate genes for these mutations and allow comparisons among the genomes of zebrafish and other vertebrates. On this map, we have localized 771 zebrafish genes and expressed sequence tags (ESTs) by scoring single-stranded conformational polymorphisms (SSCPs) in a meiotic mapping panel. Of these sequences, 642 represent previously unmapped genes and ESTs. The mapping panel was comprised of 42 homozygous diploid individuals produced by heat shock treatment of haploid embryos at the one-cell stage (HS diploids). This "doubled haploid" strategy combines the advantages of mapping in haploid and standard diploid systems, because heat shock diploid individuals have only one allele at each locus and can survive to adulthood, enabling a relatively large quantity of genomic DNA to be prepared from each individual in the mapping panel. To integrate this map with others, we also scored 593 previously mapped simple-sequence length polymorphisms (SSLPs) in the mapping panel. This map will accelerate the molecular analysis of zebrafish mutations and facilitate comparative analysis of vertebrate genomes.

Animals↗
PubMed2000

bozozok and squint act in parallel to specify dorsal mesoderm and anterior neuroectoderm in zebrafish.

In vertebrate embryos, maternal (beta)-catenin protein activates the expression of zygotic genes that establish the dorsal axial structures. Among the zygotically acting genes with key roles in the specification of dorsal axial structures are the homeobox gene bozozok (boz) and the nodal-related (TGF-(beta) family) gene squint (sqt). Both genes are expressed in the dorsal yolk syncytial layer, a source of dorsal mesoderm inducing signals, and mutational analysis has indicated that boz and sqt are required for dorsal mesoderm development. Here we examine the regulatory interactions among boz, sqt and a second nodal-related gene, cyclops (cyc). Three lines of evidence indicate that boz and sqt act in parallel to specify dorsal mesoderm and anterior neuroectoderm. First, boz requires sqt function to induce high levels of ectopic dorsal mesoderm, consistent with sqt acting either downstream or in parallel to boz. Second, sqt mRNA is expressed in blastula stage boz mutants, indicating that boz is not essential for activation of sqt transcription, and conversely, boz mRNA is expressed in blastula stage sqt mutants. Third, boz;sqt double mutants have a much more severe phenotype than boz and sqt single mutants. Double mutants consistently lack the anterior neural tube and axial mesoderm, and ventral fates are markedly expanded. Expression of chordin and noggin1 is greatly reduced in boz;sqt mutants, indicating that the boz and sqt pathways have overlapping roles in activating secreted BMP antagonists. In striking contrast to boz;sqt double mutants, anterior neural fates are specified in boz;sqt;cyc triple mutants. This indicates that cyc represses anterior neural development, and that boz and sqt counteract this repressive function. Our results support a model in which boz and sqt act in parallel to induce dorsalizing BMP-antagonists and to counteract the repressive function of cyc in neural patterning.

Animals↗
PubMed2000

Essential role of Bmp7 (snailhouse) and its prodomain in dorsoventral patterning of the zebrafish embryo.

Bone morphogenetic proteins (Bmps) are signaling molecules that have been implicated in a variety of inductive processes. We report here that zebrafish Bmp7 is disrupted in snailhouse (snh) mutants. The allele snh(st1) is a translocation deleting the bmp7 gene, while snh(ty68) displays a Val->Gly exhange in a conserved motif of the Bmp7 prodomain. The snh(ty68) mutation is temperature-sensitive, leading to severalfold reduced activity of mutant Bmp7 at 28 degrees C and non-detectable activity at 33 degrees C. This prodomain lesion affects secretion and/or stability of secreted mature Bmp7 after processing has occurred. Both snh(st1) and snh(ty68) mutant zebrafish embryos are strongly dorsalized, indicating that bmp7 is required for the specification of ventral cell fates during early dorsoventral patterning. At higher temperature, the phenotype of snh(ty68) mutant embryos is identical to that caused by the amorphic bmp2b mutation swirl swr(ta72) and similar to that caused by the smad5 mutation somitabun sbn(dtc24). mRNA injection studies and double mutant analyses indicate that Bmp2b and Bmp7 closely cooperate and that Bmp2b/Bmp7 signaling is transduced by Smad5 and antagonized by Chordino.

Amino Acid Sequence↗
PubMed2000

Nodal signaling patterns the organizer.

Spemann's organizer plays an essential role in patterning the vertebrate embryo. During gastrulation, organizer cells involute and form the prechordal plate anteriorly and the notochord more posteriorly. The fate mapping and gene expression analyses in zebrafish presented in this study reveal that this anteroposterior polarity is already initiated in the organizer before gastrulation. Prechordal plate progenitors reside close to the blastoderm margin and express the homeobox gene goosecoid, whereas notochord precursors are located further from the margin and express the homeobox gene floating head. The nodal-related genes cyclops and squint are expressed at the blastoderm margin and are required for prechordal plate and notochord formation. We show that differential activation of the Nodal signaling pathway is essential in establishing anteroposterior pattern in the organizer. First, overexpression of cyclops and squint at different doses leads to the induction of floating head at low doses and the induction of both goosecoid and floating head at higher doses. Second, decreasing Nodal signaling using different concentrations of the antagonist Antivin inhibits goosecoid expression at low doses and blocks expression of both goosecoid and floating head at higher doses. Third, attenuation of Nodal signaling in zygotic mutants for the EGF-CFC gene one-eyed pinhead, an essential cofactor for Nodal signaling, leads to the loss of goosecoid expression and expansion of floating head expression in the organizer. Concomitantly, cells normally fated to become prechordal plate are transformed into notochord progenitors. Finally, activation of Nodal signaling at different times suggests that prechordal plate specification requires sustained Nodal signaling, whereas transient signaling is sufficient for notochord development. Together, these results indicate that differential Nodal signaling patterns the organizer before gastrulation, with the highest level of activity required for anterior fates and lower activity essential for posterior fates.

Animals↗
PubMed1999

Conserved requirement for EGF-CFC genes in vertebrate left-right axis formation.

Specification of the left-right (L-R) axis in the vertebrate embryo requires transfer of positional information from the node to the periphery, resulting in asymmetric gene expression in the lateral plate mesoderm. We show that this activation of L-R lateral asymmetry requires the evolutionarily conserved activity of members of the EGF-CFC family of extracellular factors. Targeted disruption of murine Cryptic results in L-R laterality defects including randomization of abdominal situs, hyposplenia, and pulmonary right isomerism, as well as randomized embryo turning and cardiac looping. Similarly, zebrafish one-eyed pinhead (oep) mutants that have been rescued partially by mRNA injection display heterotaxia, including randomization of heart looping and pancreas location. In both Cryptic and oep mutant embryos, L-R asymmetric expression of Nodal/cyclops, Lefty2/antivin, and Pitx2 does not occur in the lateral plate mesoderm, while in Cryptic mutants Lefty1 expression is absent from the prospective floor plate. Notably, L-R asymmetric expression of Nodal at the lateral edges of the node is still observed in Cryptic mutants, indicating that L-R specification has occurred in the node but not the lateral plate. Combined with the previous finding that oep is required for nodal signaling in zebrafish, we propose that a signaling pathway mediated by Nodal and EGF-CFC activities is essential for transfer of L-R positional information from the node.

Animals↗
PubMed1999

The EGF-CFC protein one-eyed pinhead is essential for nodal signaling.

The zebrafish EGF-CFC gene one-eyed pinhead (oep) is required zygotically for the formation of the ventral neuroectoderm, endoderm, and prechordal plate. Here we report that embryos lacking both maternal and zygotic Oep activity are defective in germ layer formation, organizer development, and the positioning of the anterior-posterior axis. An identical phenotype is displayed by double mutants for the nodal-related genes squint and cyclops. Mutations in oep eliminate the response to Squint and Cyclops overexpression but are suppressed by expression of Activin and activated forms of the type I receptor ActRIB and Smad2. Expression of the murine EGF-CFC gene cripto rescues oep mutants. These results suggest a conserved role for EGF-CFC proteins as essential extracellular cofactors for Nodal signaling during vertebrate development.

Activins↗
PubMed1999

Comparative synteny cloning of zebrafish you-too: mutations in the Hedgehog target gli2 affect ventral forebrain patterning.

Zebrafish you-too (yot) mutations interfere with Hedgehog (Hh) signaling during embryogenesis. Using a comparative synteny approach, we isolated yot as a zinc finger transcription factor homologous to the Hh target gli2. Two alleles of yot contain nonsense mutations resulting in carboxy-terminally truncated proteins. In addition to causing defects in midline development, muscle differentiation, and retinal axon guidance, yot mutations disrupt anterior pituitary and ventral forebrain differentiation. yot mutations also cause ectopic lens formation in the ventral diencephalon. These findings reveal that truncated zebrafish Gli2 proteins interfere with Hh signaling necessary for differentiation and axon guidance in the ventral forebrain.

Amino Acid Sequence↗
PubMed1999

Mouse Lefty2 and zebrafish antivin are feedback inhibitors of nodal signaling during vertebrate gastrulation.

Mammalian lefty and zebrafish antivin form a subgroup of the TGF beta superfamily. We report that mouse mutants for lefty2 have an expanded primitive streak and form excess mesoderm, a phenotype opposite to that of mutants for the TGF beta gene nodal. Analogously, overexpression of Antivin or Lefty2 in zebrafish embryos blocks head and trunk mesoderm formation, a phenotype identical to that of mutants caused by loss of Nodal signaling. The lefty2 mutant phenotype is partially suppressed by heterozygosity for nodal. Similarly, the effects of Antivin and Lefty2 can be suppressed by overexpression of the nodal-related genes cyclops and squint or the extracellular domain of ActRIIB. Expression of antivin is dependent on Nodal signaling, revealing a feedback loop wherein Nodal signals induce their antagonists Lefty2 and Antivin to restrict Nodal signaling during gastrulation.

Activin Receptors, Type II↗