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PubMed1999

Signals from the yolk cell induce mesoderm, neuroectoderm, the trunk organizer, and the notochord in zebrafish.

We have analyzed the role of the zebrafish yolk cell in the processes of mesoderm induction and establishment of the organizer. By recombining blastomere-free yolk cells and animal cap tissue we have shown that the yolk cell itself can induce mesoderm in neighboring blastomeres. We further demonstrate the competence of all blastomeres to form mesoderm, suggesting the endogenous mesoderm inducing signal to be locally restricted. Ablation of the vegetal third of the yolk cell during the first 20 min of development does not interfere with mesoderm formation in general, but results in completely ventralized embryos. These embryos lack the notochord, neuroectoderm, and the anterior-most 14-15 somites, demonstrating that the ablation affects the formation of the trunk-, but not the tail region of the embryo. This suggests the presence of a trunk organizer in fish. The dorsalized mutant swirl (zbmp-2b) shows expanded dorsal structures and missing ventral structures. In contrast to the phenotypes obtained upon the ablation treatment in wild-type embryos, removal of the vegetal-most yolk in swirl mutants results in embryos which do form neuroectoderm and anterior trunk somites. However, both wild-type and swirl mutants lack a notochord upon vegetal yolk removal. These ablation experiments in wild-type and swirl mutant embryos demonstrate that in zebrafish dorsal determining factors originate from the vegetal part of the yolk cell. These factors set up two independent activities: one induces the notochord and the other is involved in the formation of the neuroectoderm and the trunk region by counteracting the function of swirl. In addition, these experiments show that the establishment of the anteroposterior axis is independent of the dorsoventral axis.

Animals↗
PubMed2003

Shadoo, a new protein highly conserved from fish to mammals and with similarity to prion protein.

We report evidence from cDNA isolation and expression analysis as well as analyses of genome, expressed sequence tag (EST), cDNA and expression databases for a new gene named SPRN (shadow of prion protein). SPRN comprises two exons, with the open reading frame (ORF) contained within exon 2, and codes for a protein of 130-150 amino acids named Shadoo (Japanese shadow), predicted to be extracellular and GPI-anchored. The SPRN gene was found in fish (zebrafish, Fugu) and mammals (mouse, rat, human). Conservation of order and transcription orientation of two proximal genes between fishes and mammals strongly indicates gene orthology. Sequence comparison shows: a highly conserved N-terminal signal sequence; Arg-rich basic region containing up to six tetrarepeats of consensus XXRG (where X is G, A or S); a hydrophobic region of 20 residues with strong homology to PrP; a less conserved C-terminal domain containing a conserved glycosylation motif; and a C-terminal peptide predicted to be a signal sequence for glycophosphotidylinositol (GPI)-anchor attachment. Fish Shadoos (Sho) show well conserved sequences (identity 54%) over 106 amino acids (zebrafish length), and conservation among the mammalian sequences is very high (identity 81-96%). The fish and mammalian sequences are also well conserved, particularly for zebrafish, to beyond the end of the hydrophobic sequence (identity 41-53%, 78 amino acids, zebrafish length). The overall structure appears closely related to prion proteins (PrPs), although the C-terminal domains of Shos are quite different from those of PrPs, for which conformational changes in mammals are implicated in disease. The structural similarity is particularly interesting given recent reports of three new genes with similarities to PrPs found in Fugu (PrP-like, PrP-461/stPrP-1 and stPrP-2) and other fish, but for which direct evolution to higher vertebrate PrPs is unlikely and for which no other mammalian homologues have been found. Database information indicates expression of SPRN in embryo, brain and retina of mouse and rat, hippocampus of human, and in embryo and retina of zebrafish, and we directly confirmed a strikingly specific expression of the mammalian (human, mouse, rat) transcripts in whole brain. This result together with some common structural features led to the suggestive hypothesis of a possible functional link between mammalian PrP and Sho proteins.

Amino Acid Sequence↗
PubMed1998

Overexpression of the forebrain-specific homeobox gene six3 induces rostral forebrain enlargement in zebrafish.

The Drosophila homeobox gene sine oculis is expressed in the rostral region of the embryo in early development and is essential for eye and brain formation. Its murine homolog, Six3, is expressed in the anterior neural plate and eye anlage, and may have crucial functions in eye and brain development. In this study, we describe the cloning and expression of zebrafish six3, the apparent ortholog of the mouse Six3 gene. Zebrafish six3 transcripts are first seen in hypoblast cells in early gastrula embryos and are found in the anterior axial mesendoderm through gastrulation. six3 expression in the head ectoderm begins at late gastrula. Throughout the segmentation period, six3 is expressed in the rostral region of the prospective forebrain. Overexpression of six3 in zebrafish embryos induced enlargement of the rostral forebrain, enhanced expression of pax2 in the optic stalk and led to a general disorganization of the brain. Disruption of either the Six domain or the homeodomain abolish these effects, implying that these domains are essential for six3 gene function. Our results suggest that the vertebrate Six3 genes are involved in the formation of the rostral forebrain.

Amino Acid Sequence↗
PubMed2000

Phenotypic effects in Xenopus and zebrafish suggest that one-eyed pinhead functions as antagonist of BMP signalling.

Zebrafish one-eyed pinhead (oep) is essential for embryonic axis and dorsal midline formation by promoting Nodal signalling and is thought to act as a permissive factor. Here we describe that oep elicits profound phenotypic effects when overexpressed in Xenopus and zebrafish. In Xenopus, wild-type oep inhibits mesoderm induction, disrupts axis formation and neuralizes animal caps. A secreted Oep dorsoanteriorizes and neuralizes Xenopus embryos indicative of BMP inhibition. In zebrafish, misexpression of smad1 in oep mutant embryos also reveals an interaction of oep with BMP signalling. Furthermore, the phenotypic effect of nodal overexpression can be rescued by coexpression of oep both in Xenopus and zebrafish. Taken together, our results support an interaction between oep and nodal but they suggest also (1) that the role of oep in Nodal signalling may include negative as well as positive regulation, (2) that oep is able to function in an active fashion and (3) that oep exerts a regulatory effect on the BMP signalling pathway.

Animals↗
PubMed2002

Expression and splice variant analysis of the zebrafish tcf4 transcription factor.

Wnt signalling has been implicated in antero-posterior patterning of the vertebrate embryonic body axis and in a number of other developmental processes. One of the downstream effectors of Wnt signalling is the beta-catenin protein which complexes with members of the Lef/tcf transcription factor family. In the zebrafish, specification of the head has been shown to be dependent on the Tcf3 protein which acts as a repressor of the posteriorizing activity of Wnt (Nature 407 (2000) 913). Here, we report the cloning and expression pattern of the zebrafish tcf4 gene. In embryos, we find that the tcf4 gene is highly regulated at the level of RNA splicing such that the variant proteins that are produced contain or lack domains proposed to be essential in repression or activation of transcription. Expression of tcf4 mRNA is first detected in a graded fashion in the anterior brain and subsequently becomes restricted to the dorsal diencephalon and anterior midbrain. There is also transient expression in the anterior rhombomeres of the hindbrain and in the developing gut.

Alternative Splicing↗
PubMed2006

Zebrafish BarH-like genes define discrete neural domains in the early embryo.

BarH (Barhl) genes encode for highly conserved homeodomain-containing transcription factors involved in critical functions during development, including cell fate specification, migration and survival. Here, we report the dynamic and restricted expression of three zebrafish barhl within the developing central nervous system. barhl2 becomes expressed in the late gastrula as a transverse diencephalic domain located immediately caudal to the prospective eyes. At early somitogenesis, barhl1.1 and barhl1.2 are expressed in the diencephalon in domains that partially overlap with the ventral and dorsal aspects of barhl2 expression, respectively. At later stages, expression of all zebrafish barhl shows large extent of overlap in the pretectum, tectum and dorsal hindbrain. The presence of a unique territory of barhl2 expression in the dorsal telencephalon and the high levels of expression in the retina are both consistent with expression reports of other Barhl2 orthologues, and support the subdivision of vertebrate Barhl into two paralogue groups based on the phylogenetic analysis of nucleotide and amino acid sequences.

Amino Acid Sequence↗
PubMed2002

Identification of a second presenilin gene in zebrafish with similarity to the human Alzheimer's disease gene presenilin2.

Presenilins play prominent roles in the molecular pathogenesis of Alzheimer's disease and during embryo development. We have isolated a zebrafish presenilin orthologue (pre2), which shows a high degree of sequence identity to the human PS2 protein. Zebrafish pre2 is maternally and ubiquitously expressed during early embryo development, whereas Pre2 protein expression is initiated between 6 and 12 hours post fertilisation (hpf), suggesting strict regulation of pre2 translation. pre2 expression is especially high in neural-crest-derived melanocytes.

Alzheimer Disease↗
PubMed2007

Effects of prolonged exposure to perchlorate on thyroid and reproductive function in zebrafish.

The objectives of this study were to determine the effects of prolonged exposure to perchlorate on (1) thyroid status and reproductive performance of adult zebrafish (Danio rerio) and (2) F1 embryo survival and early larval development. Using a static-renewal procedure, mixed sex populations of adult zebrafish were exposed to 0, 10, and 100 mg/l nominal concentrations of waterborne perchlorate for 10 weeks. Thyroid histology was qualitatively assessed, and females and males were separated and further exposed to their respective treatments for six additional weeks. Eight females in each tank replicate (n = 3) were paired weekly with four males from the same respective treatment, and packed-egg (spawn) volume (PEV) was measured each of the last five weeks. At least once during weeks 14-16 of exposure, other end points measured included fertilization rate, fertilized egg diameter, hatching rate, standard length, and craniofacial development of 4-day-postfertilization larvae and thyroid hormone content of 3.5-h embryos and of exposed mothers. At 10 weeks of exposure, perchlorate at both concentrations caused thyroidal hypertrophy and colloid depletion. A marked reduction in PEV was observed toward the end of the 6-week spawning period, but fertilization and embryo hatching rates were unaffected. Fertilized egg diameter and larval length were increased by parental exposure to perchlorate. Larval head depth was unaffected but the forward protrusion of the lower jaw-associated cartilage complexes, Meckel's and ceratohyal, was decreased. Exposure to both concentrations of perchlorate inhibited whole-body thyroxine content in mothers and embryos, but triiodothyronine content was unchanged. In conclusion, prolonged exposure of adult zebrafish to perchlorate not only disrupts their thyroid endocrine system but also impairs reproduction and influences early F1 development.

Analysis of Variance↗
PubMed1997

The molecular nature of zebrafish swirl: BMP2 function is essential during early dorsoventral patterning.

Early dorsoventral pattern formation in vertebrate embryos is regulated by opposing activities of ventralizing bone morphogenetic proteins (BMPs) and dorsal-specific BMP antagonists such as Chordin, Noggin and Follistatin. Specific defects in early dorsoventral patterning have been recently found in a number of zebrafish mutants, which exhibit either a ventralized or dorsalized phenotype. One of these, the ventralized mutant chordino (originally called dino) is caused by a mutation in the zebrafish chordin homologue and interacts genetically with the dorsalized mutant swirl. In swirl mutant embryos, dorsal structures such as notochord and somites are expanded while ventral structures such as blood and nephros are missing. Here we demonstrate that the swirl phenotype is caused by mutations in the zebrafish bmp2 gene (zbmp2). While injection of mRNAs encoded by the mutant alleles has no ventralizing effect, injection of wild-type zbmp2 mRNA leads to a complete rescue of the swirl mutant phenotype. Fertile adult mutant fish were obtained, showing that development after gastrulation is not dependent on zbmp2 function. In addition zBMP2 has no maternal role in mesoderm induction. Our analysis shows that swirl/BMP2, unlike mouse BMP2 but like mouse BMP4, is required for early dorsoventral patterning of the zebrafish embryo.

Alleles↗
PubMed1995

Cell-autonomous shift from axial to paraxial mesodermal development in zebrafish floating head mutants.

Zebrafish floating head mutant embryos lack notochord and develop somitic muscle in its place. This may result from incorrect specification of the notochord domain at gastrulation, or from respecification of notochord progenitors to form muscle. In genetic mosaics, floating head acts cell autonomously. Transplanted wild-type cells differentiate into notochord in mutant hosts; however, cells from floating head mutant donors produce muscle rather than notochord in wild-type hosts. Consistent with respecification, markers of axial mesoderm are initially expressed in floating head mutant gastrulas, but expression does not persist. Axial cells also inappropriately express markers of paraxial mesoderm. Thus, single cells in the mutant midline transiently co-express genes that are normally specific to either axial or paraxial mesoderm. Since floating head mutants produce some floor plate in the ventral neural tube, midline mesoderm may also retain early signaling capabilities. Our results suggest that wild-type floating head provides an essential step in maintaining, rather than initiating, development of notochord-forming axial mesoderm.

Animals↗
PubMed2003

Direct binding of Lef1 to sites in the boz promoter may mediate pre-midblastula-transition activation of boz expression.

The Nieuwkoop center provides signals essential for the establishment of the dorsal gastrula organizer in vertebrates. Activation of beta-catenin is one of the events in the Nieuwkoop center that lead to activation of dorsal-specific genes during blastula and early gastrula stages. Zebrafish bozozok (boz) mutant embryos have severe defects in axial mesoderm and anterior neuroectoderm. The boz gene is activated in the organizer in response to beta-catenin signaling, and Boz protein has been demonstrated to contribute to organizer formation by repression of ventralizing genes, including bmp2b, vega1, and vega2. Here, we investigate the timing and molecular mechanism by which boz expression is activated in the organizer. We demonstrate that boz is already expressed before midblastula transition (MBT). We further identify high-affinity binding sites for Tcf/Lef1 within the boz promoter region. These sites, together with the finding that beta-catenin induces boz expression, indicate that transcription of boz may be activated directly by beta-catenin/Lef1. We hypothesize that pre-MBT activation of boz may be important to build up a sufficiently strong antagonizing activity against zygotic ventralizing genes activated immediately post-MBT. Thus, the early onset of boz expression may be crucial for organizer establishment in the presence of ubiquitous maternal activators of ventralizing genes.

Animals↗
PubMed1996

The identification of genes with unique and essential functions in the development of the zebrafish, Danio rerio.

In a large-scale screen, we isolated mutants displaying a specific visible phenotype in embryos or early larvae of the zebrafish, Danio rerio. Males were mutagenized with ethylnitrosourea (ENU) and F2 families of single pair matings between sibling F1 fish, heterozygous for a mutagenized genome, were raised. Egg lays were obtained from several crosses between F2 siblings, resulting in scoring of 3857 mutagenized genomes. F3 progeny were scored at the second, third and sixth day of development, using a stereomicroscope. In a subsequent screen, fixed embryos were analyzed for correct retinotectal projection. A total of 4264 mutants were identified. Two thirds of the mutants displaying rather general abnormalities were eventually discarded. We kept and characterized 1163 mutants. In complementation crosses performed between mutants with similar phenotypes, 894 mutants have been assigned to 372 genes. The average allele frequency is 2.4. We identified genes involved in early development, notochord, brain, spinal cord, somites, muscles, heart, circulation, blood, skin, fin, eye, otic vesicle, jaw and branchial arches, pigment pattern, pigment formation, gut, liver, motility and touch response. Our collection contains alleles of almost all previously described zebrafish mutants. From the allele frequencies and other considerations we estimate that the 372 genes defined by the mutants probably represent more than half of all genes that could have been discovered using the criteria of our screen. Here we give an overview of the spectrum of mutant phenotypes obtained, and discuss the limits and the potentials of a genetic saturation screen in the zebrafish.

Animals↗
PubMed2000

The bHLH transcription factor hand2 plays parallel roles in zebrafish heart and pectoral fin development.

The precursors of several organs reside within the lateral plate mesoderm of vertebrate embryos. Here, we demonstrate that the zebrafish hands off locus is essential for the development of two structures derived from the lateral plate mesoderm - the heart and the pectoral fin. hands off mutant embryos have defects in myocardial development from an early stage: they produce a reduced number of myocardial precursors, and the myocardial tissue that does form is improperly patterned and fails to maintain tbx5 expression. A similar array of defects is observed in the differentiation of the pectoral fin mesenchyme: small fin buds form in a delayed fashion, anteroposterior patterning of the fin mesenchyme is absent and tbx5 expression is poorly maintained. Defects in these mesodermal structures are preceded by the aberrant morphogenesis of both the cardiogenic and forelimb-forming regions of the lateral plate mesoderm. Molecular analysis of two hands off alleles indicates that the hands off locus encodes the bHLH transcription factor Hand2, which is expressed in the lateral plate mesoderm starting at the completion of gastrulation. Thus, these studies reveal early functions for Hand2 in several cellular processes and highlight a genetic parallel between heart and forelimb development.

Animals↗
PubMed2004

Turning heads: development of vertebrate branchiomotor neurons.

The cranial motor neurons innervate muscles that control eye, jaw, and facial movements of the vertebrate head and parasympathetic neurons that innervate certain glands and organs. These efferent neurons develop at characteristic locations in the brainstem, and their axons exit the neural tube in well-defined trajectories to innervate target tissues. This review is focused on a subset of cranial motor neurons called the branchiomotor neurons, which innervate muscles derived from the branchial (pharyngeal) arches. First, the organization of the branchiomotor pathways in zebrafish, chick, and mouse embryos will be compared, and the underlying axon guidance mechanisms will be addressed. Next, the molecular mechanisms that generate branchiomotor neurons and specify their identities will be discussed. Finally, the caudally directed or tangential migration of facial branchiomotor neurons will be examined. Given the advances in the characterization and analysis of vertebrate genomes, we can expect rapid progress in elucidating the cellular and molecular mechanisms underlying the development of these vital neuronal networks. Developmental Dynamics 229:143-161, 2004.

Animals↗
PubMed2004

Cloning and embryonic expression of zebrafish neuropilin genes.

Neuropilin (Nrp), a cell surface receptor for class 3 semaphorins and for certain heparin forms of vascular endothelial growth factors, functions in many biological processes including axon guidance, neural cell migration and angiogenesis in the development of the nervous system and the cardiovascular system. To understand the role of neuropilins in zebrafish embryogenesis, we have cloned three zebrafish neuropilin homologues, nrp1b, nrp2a and nrp2b. Based on synteny, zebrafish nrp1b and the previously cloned nrp1a are orthologous to human nrp1, and zebrafish nrp2a and 2b orthologous to human nrp2. We have characterized the expression patterns of these four zebrafish neuropilin genes in wild type embryos from the beginning of somitogenesis to 48 h post-fertilization. Zebrafish nrp1a is expressed in the neural tube including telencephalon, epithalamus, cells along the axonal trajectory of the posterior commissure and the medial longitudinal fascicle, hindbrain neurons, vagus motor neurons and spinal motoneurons. Zebrafish nrp1b is expressed in the nose, the cranial neural crest cell (NCC) derived tissue underlying the hypothalamus, endothelial precursors and the trunk and tail vasculature. Zebrafish nrp2a is expressed in telencephalon, anterior pituitary, oculomotor and trochlear motor neurons, cells along the supra-optic and posterior commissures, hindbrain rhombomere 1, hindbrain neurons, cranial NCCs and sclerotome. Zebrafish nrp2b is expressed in telencephalon, thalamus, hypothalamus, epiphysis, cells along the anterior and posterior commissures, post-optic and supra-optic commissures and the olfactory axonal trajectory, hindbrain neurons, cranial NCCs, somites and spinal cord neurons.

Animals↗
PubMed2003

A novel interleukin-17 receptor-like protein identified in human umbilical vein endothelial cells antagonizes basic fibroblast growth factor-induced signaling.

We have previously utilized a combination of high throughput sequencing and genome-wide microarray profiling analyses to identify novel cell-surface proteins expressed in human umbilical vein endothelial cells. One gene identified by this approach encodes a type I transmembrane receptor that shares sequence homology with the intracellular domain of members of the interleukin-17 (IL-17) receptor family. Real-time quantitative PCR and Northern analyses revealed that this gene is highly expressed in human umbilical vein endothelial cells and in several highly vascularized tissues such as kidney, colon, skeletal muscle, heart, and small intestine. In addition, we also found that it is also highly expressed in the ductal epithelial cells of human salivary glands, seminal vesicles, and the collecting tubules of the kidney by in situ hybridization. This putative receptor, which we have termed human SEF (hSEF), is also expressed in a variety of breast cancer tissues. In co-immunoprecipitation assays, this receptor is capable of forming homomeric complexes and can interact with fibroblast growth factor (FGF) receptor 1. Overexpression of this receptor inhibits FGF induction of an FGF-responsive reporter gene in human 293T cells. This appears to occur as a result of specific inhibition of p42/p44 ERK in the absence of upstream MEK inhibition. This inhibitory effect is dependent upon a functional intracellular domain since deletion mutants missing the IL-17 receptor-like domain lack this inhibitory effect. These findings are consistent with the recent discovery of the zebrafish homologue, Sef (similar expression to fgf genes), which specifically antagonizes FGF signaling when ectopically expressed in zebrafish or Xenopus laevis embryos. Based on sequence and functional similarities, this novel IL-17 receptor homologue represents a potential human SEF and is likely to play critical roles in endothelial or epithelial functions such as proliferation, migration, and angiogenesis.

Amino Acid Sequence↗
PubMed2005

Interactions between muscle fibers and segment boundaries in zebrafish.

The most obvious segmental structures in the vertebrate embryo are somites: transient structures that give rise to vertebrae and much of the musculature. In zebrafish, most somitic cells give rise to long muscle fibers that are anchored to intersegmental boundaries. Therefore, this boundary is analogous to the mammalian tendon in that it transduces muscle-generated force to the skeletal system. We have investigated interactions between somite boundaries and muscle fibers. We define three stages of segment boundary formation. The first stage is the formation of the initial epithelial somite boundary. The second "transition" stage involves both the elongation of initially round muscle precursor cells and somite boundary maturation. The third stage is myotome boundary formation, where the boundary becomes rich in extracellular matrix and all muscle precursor cells have elongated to form long muscle fibers. It is known that formation of the initial epithelial somite boundary requires Notch signaling; vertebrate Notch pathway mutants show severe defects in somitogenesis. However, many zebrafish Notch pathway mutants are homozygous viable suggesting that segmentation of their larval and adult body plans at least partially recovers. We show that epithelial somite boundary formation and slow-twitch muscle morphogenesis are initially disrupted in after eight (aei) mutant embryos (which lack function of the Notch ligand, DeltaD); however, myotome boundaries form later ("recover") in a Hedgehog-dependent fashion. Inhibition of Hedgehog-induced slow muscle induction in aei/deltaD and deadly seven (des)/notch1a mutant embryos suggests that slow muscle is necessary for myotome boundary recovery in the absence of initial epithelial somite boundary formation. Because we have previously demonstrated that slow muscle migration triggers fast muscle cell elongation in zebrafish, we hypothesize that migrating slow muscle facilitates myotome boundary formation in aei/deltaD mutant embryos by patterning coordinated fast muscle cell elongation. In addition, we utilized genetic mosaic analysis to show that somite boundaries also function to limit the extent to which fast muscle cells can elongate. Combined, our results indicate that multiple interactions between somite boundaries and muscle fibers mediate zebrafish segmentation.

Animals↗