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U Strähle

Publications and source records attributed to U Strähle.

At least 37 records · Page 2Linked to original sources
PubMed1998

Zebrafish Clock rhythmic expression reveals independent peripheral circadian oscillators.

The only vertebrate clock gene identified by mutagenesis is mouse Clock, which encodes a bHLH-PAS transcription factor. We have cloned Clock in zebrafish and show that, in contrast to its mouse homologue, it is expressed with a pronounced circadian rhythm in the brain and in two defined pacemaker structures, the eye and the pineal gland. Clock oscillation was also found in other tissues, including kidney and heart. In these tissues, expression of Clock continues to oscillate in vitro. This demonstrates that self-sustaining circadian oscillators exist in several vertebrate organs, as was previously reported for invertebrates.

Amino Acid Sequence↗
PubMed1998

Sonic hedgehog is not required for the induction of medial floor plate cells in the zebrafish.

Sonic hedgehog (Shh) is a secreted protein that is involved in the organization and patterning of several tissues in vertebrates. We show that the zebrafish sonic-you (syu) gene, a member of a group of five genes required for somite patterning, is encoding Shh. Embryos mutant for a deletion of syu display defects in patterning of the somites, the lateral floor plate cells, the pectoral fins, the axons of motorneurons and the retinal ganglion cells. In contrast to mouse embryos lacking Shh activity, syu mutant embryos do form medial floor plate cells and motorneurons. Since ectopic overexpression of shh in zebrafish embryos does not induce ectopic medial floor plate cells, we conclude that shh is neither required nor sufficient to induce this cell type in the zebrafish.

Animals↗
PubMed1997

Cleavage of the BMP-4 antagonist chordin by zebrafish tolloid.

Dorsoventral patterning of vertebrate and Drosophila embryos requires bone morphogenetic proteins (BMPs) and antagonists of BMP activity. The Drosophila gene tolloid encodes a metalloprotease similar to BMP-1 that interacts genetically with decapentaplegic, the Drosophila homolog of vertebrate BMP-2/4. Zebrafish embryos overexpressing a zebrafish homolog of tolloid were shown to resemble loss-of-function mutations in chordino, the zebrafish homolog of the Xenopus BMP-4 antagonist Chordin. Furthermore, Chordin was degraded by COS cells expressing Tolloid. These data suggest that Tolloid antagonizes Chordin activity by proteolytically cleaving Chordin. A conserved function for zebrafish and Drosophila Tolloid during embryogenesis is proposed.

Animals↗
PubMed1997

Axial (HNF3beta) and retinoic acid receptors are regulators of the zebrafish sonic hedgehog promoter.

The signalling molecule Sonic hedgehog is involved in a multitude of distinct patterning processes during vertebrate embryogenesis. In the nascent body axis of the zebrafish embryo, sonic hedgehog is co-expressed with axial (HNF3beta in mammals), a transcription regulator of the winged helix family. We show here that misexpression of axial leads to ectopic activation of sonic hedgehog expression in the zebrafish, suggesting that axial is a regulator of sonic hedgehog transcription. The sonic hedgehog gene was cloned from zebrafish and its promoter was characterized with respect to activation by axial. Expression of axial or rat HNF3beta in HeLa cells results in activation of co-transfected sonic hedgehog promoter-CAT fusion genes. This effect is mediated by two Axial (HNF3beta) recognition sequences. We furthermore identified a retinoic acid response element (RARE) in the sonic hedgehog upstream region which can be bound by retinoic acid receptor (RAR) and retinoid X receptor (RXR) heterodimers in vitro and confers retinoic acid inducibility to the sonic hedgehog promoter in the HeLa cell system. Our results suggest that both Axial (HNF3beta) and retinoic acid receptors are direct regulators of the sonic hedgehog gene.

Animals↗
PubMed1997

Expression and regulation of a netrin homologue in the zebrafish embryo.

Proteins of the Netrin family have been implicated in axon guidance in both C. elegans and vertebrates. Here, we report the cloning and expression analysis of a zebrafish netrin homologue (net1). net1 is expressed in the floor plate and the anterior ventral neural tube. Its expression is ectopically induced by misexpression of sonic hedgehog (shh) and a dominant negative mutant of the regulatory subunit of protein kinase A (dnReg). Ectopic activation of net1, however, is restricted to distinct regions in the brain. Upon overexpression of shh or dnReg in cyclops mutants, which have strongly impaired net1 expression in the ventral neural tube, rescue of net1 expression was observed in the brain but not in the spinal cord. Ectopic expression of dnReg and Shh protein can be detected at high levels throughout injected embryos from pre-gastrula stages onwards suggesting that the competence of the neural plate to respond to Shh signalling activity differs regionally. Similar to net1, axial, the zebrafish homologue of mammalian HNF3beta, which is also expressed along the ventral neural tube, is ectopically induced in the brain of embryos injected with dnReg mRNA. Neurons differentiate normally within domains of ectopic net1 and axial expression. Thus, dorsal neuronal differentiation appears to be unaffected despite co-expression of a gene program specific for the ventral neural tube. This also suggests that these ectopically expressing regions have not differentiated into floor plate.

Amino Acid Sequence↗
PubMed1997

one-eyed pinhead is required for development of the ventral midline of the zebrafish (Danio rerio) neural tube.

Axial midline structures play a central role as signalling centres during the development of the vertebrate embryo. We have isolated mutant alleles of a new locus, one-eyed pinhead (oep), in the zebrafish that are characterized by cyclopia and impaired development of the floor plate. oep mutants fail to establish expression of axial (zebrafish HNF3beta) and sonic hedgehog in the midline of the neural plate but form a notochord that expresses both genes. In the spinal cord of the 1-day-old embryo, mutation of oep impairs floor-plate but not motor-neuron development. Floor-plate development is absolutely dependent on oep only at early stages, since partial recovery of the floor plate can be detected at 48 h in the spinal cord, suggesting compensatory pathways. Ectopic expression of sonic hedgehog and a dominant-negative protein kinase A regulatory subunit induces expression of floor-plate marker genes in the oep mutant neural tube in a manner indistinguishable from wild-type embryos. Our data suggest that the oep mutation does not impair Sonic hedgehog signalling and thus implicate a second process that acts synergistically with Sonic hedgehog signalling in the specification of the midline of the neuroectoderm and that can partially be compensated for during later development.

Alleles↗
PubMed1997

The Pax protein Noi is required for commissural axon pathway formation in the rostral forebrain.

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 differentiate and optic axons exhibit abnormal trajectories exiting the eye and at the midline of the diencephalon. Optic axons select inappropriate pathways into the contralateral optic nerve, rostrally towards the anterior commissure 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.

Animals↗
PubMed1997

The activity of neurogenin1 is controlled by local cues in the zebrafish embryo.

Zebrafish neurogenin1 encodes a basic helix-loop-helix protein which shares structural and functional characteristics with proneural genes of Drosophila melanogaster. neurogenin1 is expressed in the early neural plate in domains comprising more cells than the primary neurons known to develop from these regions and its expression is modulated by Delta/Notch signalling, suggesting that it is a target of lateral inhibition. Misexpression of neurogenin1 in the embryo results in development of ectopic neurons. Markers for different neuronal subtypes are not ectopically expressed in the same patterns in neurogenin1-injected embryos suggesting that the final identity of the ectopically induced neurons is modulated by local cues. Induction of ectopic motor neurons by neurogeninl requires coexpression of a dominant negative regulatory subunit of protein kinase A, an intracellular transducer of hedgehog signals. Moreover, the pattern of endogenous neurogenin1 expression in the neural plate is expanded in response to elevated levels of Hedgehog (Hh) signalling or abolished as a result of inhibition of Hh signalling. Together these data suggest that Hh signals regulate neurogenin1 expression and subsequently modulate the type of neurons produced by Neurogenin1 activity.

Amino Acid Sequence↗
PubMed1997

Wnt5 is required for tail formation in the zebrafish embryo.

Intercellular signaling molecules, such as those encoded by the Wnt gene family, have a fundamental role in various aspects of pattern formation in the developing embryo. The zebrafish wnt5 gene encodes a member of a subfamily of Wnt molecules thought to be involved in modulating cell behavior during vertebrate development. Here, we show that the zebrafish pipetail gene is identical to wnt5. The pipetail mutant phenotype is characterized by defects in tail formation and impaired maturation of the cells that contribute to cartilaginous elements of the head skeleton. This suggests a major role for wnt5 in morphogenetic processes underlying tail outgrowth and cartilage differentiation in the head. To investigate the function of maternally derived wnt5 mRNA, we generated females that were homozygous for pipetail. The lack of a maternal effect phenotype in the progeny of these females suggests that no obvious function for the maternal wnt5 expression can be deduced.

Amino Acid Sequence↗
PubMed1996

The zebrafish Fgf-3 gene: cDNA sequence, transcript structure and genomic organization.

We report the isolation and characterization of genomic and cDNA clones encoding zebrafish fibroblast growth factor 3 (FGF3). An initial cDNA clone was generated by PCR amplification using degenerate oligo primers corresponding to a conserved region of protein found in the mouse and human homologues. Screening a cDNA library made from 30-33-h-old zebrafish embryos with this PCR product led to the isolation of two cDNA clones. Sequence analysis of the longest cDNA insert (1810 bp) revealed a 256-amino-acid (aa) orf. The central region, composed of approx. 155 aa, shares 78% identity with the analogous region of Xenopus laevis FGF3 and 72% identity with the product of the more distantly related human gene. However, the N-and C-terminal domains of zebrafish FGF3 are very different from those of other known homologues. The cDNA was used as a probe on genomic DNA to create a physical map of the locus and to isolate a genomic clone encompassing the entire coding region and 5' sequences. DNA sequencing and RNase protection analyses indicate that zebrafish Fgf-3 (ZFgf-3) is structurally analogous to the mouse gene and regulated through two different promoters. The transcription start point of the proximal promoter aligns to that of mouse promoter P3 and lies within a conserved region of sequence.

Amino Acid Sequence↗
PubMed1996

Active complex formation of type I and type II activin and TGF beta receptors in vivo as studied by overexpression in zebrafish embryos.

We have investigated the involvement of activin receptors and TGF beta type I receptor in zebrafish development. Overexpression of either full-length or a truncated form of mouse ActR-IIA interferes with the development. Different splice variants of mouse ActR-IIB have distinct effects; ActR-IIB4 induces abnormal embryos, whereas ActR-IIB2 does not. Activin and TGF beta type I receptors can induce axis duplications. Co-expression of ActR-IA or ActR-IB with the type II activin receptors results in a synergistic increase of the frequency of axis duplication. Moreover, ActR-IIB2 is synergistic with ActR-IA and ActR-IB, demonstrating that ActR-IIB2 can interact with the zebrafish ligand. Overexpression of TGF beta R-I with ActR-IIA or ActR IIB4 results in a synergistic increase in frequency of abnormal embryos, whereas in combination with ActR-IIB2 no such increase occurs.

Activins↗
PubMed1996

Expression of axial and sonic hedgehog in wildtype and midline defective zebrafish embryos.

We present a description of the expression of the HNF-3 beta homolog axial (axl) in the developing zebrafish up to larva stages and compare it with that of sonic hedgehog (shh). Both genes are expressed in derivatives of all three germ layers in dynamic patterns that show substantial overlap, consistent with mutual regulatory interactions between the two genes. However, we also describe unique sites of expression of both axl and shh indicating that some aspects of their regulation are independent of one another. In no tail (ntl, zebrafish Brachyury) and floating head (flh, zebrafish Xnot1) mutants, both of which affect notochord development, early expression of axl in the organizer is unaffected, excluding a function for these genes in establishment of axl expression. At later stages, ntl and flh mutants show different effects on the expression of both axl and shh in the neuroectoderm of the trunk and tail reflecting their distinct contributions to the development of the midline mesoderm; in contrast to flh and ntl mutations whose effects are restricted to the trunk and tail, mutation of cyclops (cyc) affects axl and shh expression along the entire midline of the neuroectoderm. Endodermal expression of axl and shh is not affected by the mutations showing that development of the endoderm is under distinct control.

Animals↗
PubMed1994

Early neurogenesis in the zebrafish embryo.

Zebrafish embryos represent an attractive system for the study of early vertebrate neurogenesis. The embryos develop outside of the mother and are transparent allowing analysis at the cellular level in the living embryo during all phases of early neurogenesis. The teleostean neural tube is generated by a mechanism different from that of other vertebrates. A massive keel is formed first and the central canal appears by subsequent cavitation. Despite this, however, the organization of the neural plate and the neural keel resembles that of other vertebrates in many aspects. Oriented cell divisions coupled with oriented cell intercalations appear to be involved in the morphogenesis of the neural keel. Embryos mutant in the cyclops gene show deficiencies in the ventral neural tube. They lack the floor plate and the ventral parts of the diencephalon. Two recently cloned genes, axial and sonic hedgehog, have been implicated in the development of the ventral midline of the neural tube. Expression of axial and sonic hedgehog is impaired by the cyclops mutation in the midline of the neural plate. This, together with the effects of ectopic expression of the two cloned genes, suggests that axial, sonic hedgehog, and cyclops are part of the regulatory cascade leading to floor plate formation.

Animals↗
PubMed1993

Expression of the mouse glucocorticoid receptor and its role during development.

Genes encoding enzymes involved in gluconeogenesis are activated in liver shortly after birth by the synergistic effect of glucagon and glucocorticoids. This induction is achieved by the synergistic action of hormone responsive and liver-specific enhancer elements. In the case of glucocorticoids, this enhancer is composed of a glucocorticoid-response element (GRE) and a number of cell-specific hepatocyte nuclear factor 3 (HNF-3) binding sites. The GRE binds the ligand-activated glucocorticoid receptor (GR) which is ubiquitously expressed and the HNF-3 element binds a cell-specific protein factor. To further understand the role of cell-specific glucocorticoid signalling in the perinatal period and earlier during development we have studied the expression of the mouse GR gene. The gene has been cloned and fully characterized. Expression of the gene is controlled by at least three promoters, one of which is only active in T-lymphocytes. Expression of GR mRNA has been detected back to day 9.5 of mouse development. The role of GR during mouse development has been further analysed by disruption of the GR gene in vivo by homologous recombination in mouse embryonic stem cells.

Animals↗
PubMed1993

Axial, a zebrafish gene expressed along the developing body axis, shows altered expression in cyclops mutant embryos.

Here, we report the cloning of a cDNA from zebrafish encoding a member of the fork head/HNF3 gene family. The gene, which we have called Axial, begins to be expressed just before gastrulation in a narrow region on the dorsal side of the embryo, the fish equivalent of the amphibian organizer. Expression can be detected in the involuted cells comprising the mesendoderm of the developing axis. At the end of gastrulation expression is turned on in the ventral neural plate in cells adjacent to the Axial-expressing mesodermal cells. Thus, Axial appears to be a target of both mesoderm induction and neural induction, leading to expression in cells of all three germ layers along the developing axis. Like the Brachyury gene. Axial is strongly induced by activin A, suggesting a role for endogenous activins in specifying the overlapping domains of expression of these two genes along the axis. Axial-expressing cells in the neuroectoderm include those of the future floor plate and cells of the ventral forebrain. In embryos homozygous for the cyclops mutation, expression is normal in mesendodermal cells but is absent from the ventral neural tube. The primary defects of cyclops mutants (lack of floor plate, deficiencies in the brain and cyclopia) correlate well with the expression domain of the Axial gene in wild-type neuroectoderm. The lack of Axial expression in cyclops neuroectoderm suggests that activation of Axial may be an immediate response of cyclops gene activity. Taken together, our data suggest that Axial plays a crucial role in specification of both the axial mesendoderm and the ventral central nervous system.

Activins↗
PubMed1993

Ultraviolet irradiation impairs epiboly in zebrafish embryos: evidence for a microtubule-dependent mechanism of epiboly.

Early morphogenesis of the teleost embryo is characterized by three orchestrated cell movements. Epiboly leads to spreading of the blastoderm over an uncleaved yolk cell while involution around the blastoderm margin and convergence movements towards the dorsal side generate the mes-endodermal inner cell sheet and the axis rudiment, respectively. Irradiation of zebrafish zygotes with ultraviolet light selectively impairs epiboly resulting in embryos with open blastopores but well-formed anterior axes. Gastrulation movements are only marginally affected by ultraviolet irradiation. Involution of marginal cells in epiboly-retarded embryos takes place prior to 50% epiboly and thus appears independent of epiboly. Expression of dorsal and anterior marker genes is unaffected by ultraviolet irradiation. The ultraviolet light effect is not restricted to the zygote stage as irradiation of later embryonic stages also impairs epiboly. The ultraviolet-sensitive targets may thus be maternally encoded components of the machinery driving epiboly. These targets appear to be microtubules: firstly, irradiated embryos show disorganized and less microtubules in the cytoplasmic layer of the yolk sphere; secondly, the ultraviolet light effect can be mimicked by the microtubule-depolymerizing agent nocodazole. We suggest that epiboly is driven, at least partially, by motors that use microtubules radiating from the yolk syncytial layer into the yolk cytoplasmic layer. Together with an observed constrictive behaviour of the blastoderm margin, we propose a two-force model of epiboly: epiboly is initiated and driven by a pulling force dependent on microtubules in the yolk cytoplasmic layer; contraction at the margin operates in addition to aid closure of the blastopore.

Animals↗
PubMed1992

At least three promoters direct expression of the mouse glucocorticoid receptor gene.

We have characterized the gene for the mouse glucocorticoid receptor. The gene spans approximately 110 kilobases, and glucocorticoid receptor transcripts are assembled from nine exons. Expression of the gene is controlled by at least three promoters, resulting in glucocorticoid receptor transcripts with different 5' nontranslated exons. One promoter is cell-specific, found to be active only in T lymphocytes. The other two promoters are active to various degrees in all cell lines and tissues so far analyzed and are located in a CpG island. The promoter activities are accompanied by DNase I hypersensitivity sites in chromatin. In contrast to a conservation of exon-intron structure, differences in promoter organization suggest a divergence between the evolution of regulatory and coding regions among members of the steroid receptor super-family.

Animals↗