Stages of embryonic development of the zebrafish

Distinct subdomains of the EphA3 receptor mediate ligand binding and receptor dimerization

Eph receptor tyrosine kinases and their ligands (ephrins) are highly conserved protein families implicated in patterning events during development, particularly in the nervous system. In a number of functional studies, strict conservation of structure and function across distantly related vertebrate species has been confirmed. In this study we make use of the observation that soluble human EphA3 (HEK) exerts a dominant negative effect on somite formation and axial organization during zebrafish embryogenesis to probe receptor function. Based on exon structure we have dissected the extracellular region of EphA3 receptor into evolutionarily conserved subdomains and used kinetic BIAcore analysis, mRNA injection into zebrafish embryos, and receptor transphosphorylation analysis to study their function. We show that ligand binding is restricted to the N-terminal region encoded by exon III, and we identify an independent, C-terminal receptor-dimerization domain. Recombinant proteins encoding either region in isolation can function as receptor antagonists in zebrafish. We propose a two-step mechanism of Eph receptor activation with distinct ligand binding and ligand-independent receptor-receptor oligomerization events.

Role of sonic hedgehog in branchiomotor neuron induction in zebrafish

The role of zebrafish hedgehog genes in branchiomotor neuron development was analyzed by examining mutations that affect the expression of the hedgehog genes and by overexpressing these genes in embryos. In cyclops mutants, reduction in sonic hedgehog (shh) expression, and elimination of tiggy-winkle hedgehog (twhh) expression, correlated with reductions in branchiomotor neuron populations. Furthermore, branchiomotor neurons were restored in cyclops mutants when shh or twhh was overexpressed. These results suggest that Shh and/or Twhh play an important role in the induction of branchiomotor neurons in vivo. In sonic-you (syu) mutants, where Shh activity was reduced or eliminated due to mutations in shh, branchiomotor neurons were reduced in number in a rhombomere-specific fashion, but never eliminated. Similarly, spinal motor neurons were reduced, but not eliminated, in syu mutants. These results demonstrate that Shh is not solely responsible for inducing branchiomotor and spinal motor neurons, and suggest that Shh and Twhh may function as partially redundant signals for motor neuron induction in zebrafish.

The zebrafish organizer

Signals from the organizer play a crucial role in patterning the vertebrate embryo. Recent molecular analysis of zebrafish mutations has established an essential role for BMP2 and chordin in organizer function and has identified one-eyed pinhead as a novel EGF-like gene involved in prechordal plate and endoderm formation. In addition, embryological studies have suggested that the zebrafish organizer is induced by extraembryonic cues and have defined two novel organizing centers that pattern the nervous system along the anterior-posterior axes.

Zebrafish nodal-related genes are implicated in axial patterning and establishing left-right asymmetry

Nodal-related 1 (ndr1) and nodal-related 2 (ndr2) genes in zebrafish encode members of the nodal subgroup of the transforming growth factor-beta superfamily. We report the expression patterns and functional characteristics of these factors, implicating them in the establishment of dorsal-ventral polarity and left-right asymmetry. Ndr1 is expressed maternally, and ndr1 and ndr2 are expressed during blastula stage in the blastoderm margin. During gastrulation, ndr expression subdivides the shield into two domains: a small group of noninvoluting cells, the dorsal forerunner cells, express ndr1, while ndr2 RNA is found in the hypoblast layer of the shield and later in notochord, prechordal plate, and overlying anterior neurectoderm. During somitogenesis, ndr2 is expressed asymmetrically in the lateral plate as are nodal-related genes of other organisms, and in a small domain in the left diencephalon, providing the first observation of asymmetric gene expression in the embryonic forebrain. RNA injections into Xenopus animal caps showed that Ndr1 acts as a mesoderm inducer, whereas Ndr2 is an efficient neural but very inefficient mesoderm inducer. We suggest that Ndr1 has a role in mesoderm induction, while Ndr2 is involved in subsequent specification and patterning of the nervous system and establishment of laterality.

Fgf8 is mutated in zebrafish acerebellar (ace) mutants and is required for maintenance of midbrain-hindbrain boundary development and somitogenesis

We describe the isolation of zebrafish Fgf8 and its expression during gastrulation, somitogenesis, fin bud and early brain development. By demonstrating genetic linkage and by analysing the structure of the Fgf8 gene, we show that acerebellar is a zebrafish Fgf8 mutation that may inactivate Fgf8 function. Homozygous acerebellar embryos lack a cerebellum and the midbrain-hindbrain boundary organizer. Fgf8 function is required to maintain, but not initiate, expression of Pax2.1 and other marker genes in this area. We show that Fgf8 and Pax2.1 are activated in adjacent domains that only later become overlapping, and activation of Fgf8 occurs normally in no isthmus embryos that are mutant for Pax2.1. These findings suggest that multiple signaling pathways are independently activated in the midbrain-hindbrain boundary primordium during gastrulation, and that Fgf8 functions later during somitogenesis to polarize the midbrain. Fgf8 is also expressed in a dorsoventral gradient during gastrulation and ectopically expressed Fgf8 can dorsalize embryos. Nevertheless, acerebellar mutants show only mild dorsoventral patterning defects. Also, in spite of the prominent role suggested for Fgf8 in limb development, the pectoral fins are largely unaffected in the mutants. Fgf8 is therefore required in development of several important signaling centers in the zebrafish embryo, but may be redundant or dispensable for others.

Regulation of netrin-1a expression by hedgehog proteins

Netrins, a family of growth cone guidance molecules, are expressed both in the ventral neural tube and in subsets of mesodermal cells. In an effort to better understand the regulation of netrins, we examined the expression of netrin-1a in mutant cyclops, no tail, and floating head zebrafish embryos, in which axial midline structures are perturbed. Netrin-1a expression requires signals present in notochord and floor plate cells. In the myotome, but not the neural tube, netrin-1a expression requires sonic hedgehog. In embryos lacking sonic hedgehog, the sonic-you locus, netrin-1a expression is reduced or absent in the myotomes but present in the neural tube. Embryos lacking sonic hedgehog express tiggy-winkle hedgehog in the floor plate, suggesting that, in the neural tube, tiggy-winkle hedgehog can compensate for the lack of sonic hedgehog in inducing netrin-1a expression. Ectopic expression of sonic hedgehog, tiggy-winkle hedgehog, or echidna hedgehog induces ectopic netrin-1a expression in the neural tube, and ectopic expression of sonic hedgehog or tiggy-winkle hedgehog, but not echidna hedgehog, induces ectopic netrin-1a expression in somites. These data demonstrate that in vertebrates netrin expression is regulated by Hedgehog signaling.

Conversion of zebrafish blastomeres to an endodermal fate by TGF-beta-related signaling

The endoderm contributes cells to the gut, and participates in the induction and patterning of the vertebrate head and heart. The mechanisms controlling the formation of endoderm are poorly understood. Commitment of endoderm cells occurs at the onset of gastrulation and requires cell interactions; studies in vitro have implicated transforming growth factor Beta (TGF-beta)-related molecules in this process. TARAM-A is a zebrafish receptor kinase that is related to the type I subunit of the TGF-beta receptor, and is expressed in presumptive endomesodermal cells at gastrulation. We provide here evidence for its involvement in endoderm formation in vivo. Activation of TARAM-A was found to drive blastomeres towards an endodermal fate. The induced endoderm behaved ad endogenous endoderm during gastrulation: it migrated in contact with the yolk and expressed endoderm-specific markers. Loss-of-function mutations in the zebrafish one-eyed-pinhead (OEP) gene lead to defects in heart formation, defects of the ventral central nervous system (CNS) and cyclopia. Mutant embryos also lack endoderm and anterior mesoderm. Endoderm formation in oep mutant embryos was found to be restored by the activation of the TARAM-A signaling pathway. Cardiac and ocular defects, but not midline CNS structures, were rescued non-autonomously, demonstrating that endoderm may provide signals that can pattern the eye anlage, and which are distinct form those specifying the ventral midline of the CNS.

Patterns of free calcium in zebrafish embryos

Direct knowledge of Ca2+ patterns in vertebrate development is largely restricted to early stages, in which they control fertilization, ooplasmic segregation and cleavage. To explore new roles of Ca2+ in vertebrate development, we injected the Ca2+ indicator aequorin into zebrafish eggs and imaged Ca2+ throughout the first day of development. During early cleavages, a high Ca2+ zone is seen in the cleavage furrows. The high Ca2+ zone during first cleavage spreads as a slow wave (0.5 microm/second) and is preceded by three Ca2+ pulses within the animal pole region of the egg. When Ca2+ concentrations are clamped at the resting level by BAPTA buffer injection into the zygote, all signs of development are blocked. In later development, Ca2+ patterns are associated with cell movements during gastrulation, with neural induction, with brain regionalization, with formation of the somites and neural keel, with otic placode formation, with muscle movements and with formation of the heart. Particularly remarkable is a sharp boundary between high Ca2+ in the presumptive forebrain and midbrain versus low Ca2+ in the presumptive hindbrain starting at 10 hours of development. When Ca2+ changes are damped by injection of low concentrations of BAPTA, fish form with greatly reduced eyes and hearts. The present study provides a first overview of Ca2+ patterns during prolonged periods of vertebrate development and points to new roles of Ca2+ in cellular differentiation and pattern formation.

Comparison of sequence and function of the Oct-6 genes in zebrafish, chicken and mouse

To examine the role of the Oct-6 gene in Schwann cell differentiation we have cloned and characterized the chicken and zebrafish homologues of the mouse Oct-6 gene. While highly homologous in the Pit1-Oct1/2-Unc86 (POU) domain, sequence similarities are limited outside this domain. Both genes are intronless and both proteins lack the amino acid repeats that are a characteristic feature of the mammalian Oct-6 proteins. However as in mammals, the aminoterminal parts of the chicken and zebrafish Oct-6 proteins are essential for transactivation of octamer containing promoters. By immunohistochemistry we have found that the chicken Oct-6 protein is expressed in late embryonic ensheathing Schwann cells of the sciatic nerve and is rapidly downregulated when myelination proceeds. This expression profile in glial cells is identical to that in the mouse and rat. Furthermore the zebrafish Oct-6 homolog is expressed in the posterior lateral nerve at a time when it contains actively myelinating Schwann cells. Thus despite extensive primary sequence divergence among the vertebrate Oct-6 proteins, the expression of the chicken and zebrafish Oct-6 proteins is consistent with the notion that Oct-6 functions as a 'competence factor' in promyelin cells to execute the myelination program.

Specification of the hindbrain fate in the zebrafish

We determine the timing of neural commitment by hindbrain tissue in the zebrafish using microsurgical transplantation. When transplanted at shield stage to the ventral side of the embryo, presumptive hindbrain cells are not committed, as they can adapt to their environment and give rise to epidermis. In contrast, when transplanted at 80% epiboly, hindbrain cells retain their neural fate and express neural-specific antigens. Moreover, they are able to maintain regional fate, as is evident by the expression of the hindbrain-specific marker, Krox20. In addition, we observe that committed hindbrain tissues are able to induce presumptive ventral epidermis to form neural crest derivatives, otic vesicles, and neural tissues. We propose that hindbrain progenitors have acquired regional identity as a group at 80% epiboly even before making vertical contact with axial mesoderm. These results suggest that planar induction may constitute a significant component in the zebrafish neural patterning pathway.

Initial anteroposterior pattern of the zebrafish central nervous system is determined by differential competence of the epiblast

Analyses using amphibian embryos proposed that induction and anteroposterior patterning of the central nervous system is initiated by signals that are produced by the organizer and organizer-derived axial mesoderm. However, we show here that the initial anteroposterior pattern of the zebrafish central nervous system depends on the differential competence of the epiblast and is not imposed by organizer-derived signals. This anteroposterior information is present throughout the epiblast in ectodermal cells that normally give rise both to neural and non-neural derivatives. Because of this information, organizer tissues transplanted to the ventral side of the embryo induce neural tissue but the anteroposterior identity of the induced neural tissue is dependent upon the position of the induced tissue within the epiblast. Thus, otx2, an anterior neural marker, was only ever induced in anterior regions of the embryo, irrespective of the position of the grafts. Similarly, hoxa-1, a posterior neural marker was induced only in the posterior regions. Furthermore, the boundary of each ectopic expression domain on the ventral side was always at an equivalent latitude to that of the endogenous expression of the dorsal side of the embryo. The anteroposterior specification of the epiblast is independent of the dorsoventral specification of the embryo because neural tissues induced in the ventralized embryos also showed anteroposterior polarity. Cell transplantation and RNA injection experiments showed that non-axial marginal mesoderm and FGF signalling is required for anteroposterior specification of the epiblast. However, the requirement for FGF signalling is indirect in that cells with compromised ability to respond to FGF can still respond to anteroposterior positional information.

The cloche and spadetail genes differentially affect hematopoiesis and vasculogenesis

In vertebrates, hematopoietic and vascular progenitors develop from ventral mesoderm. The first primitive wave of hematopoiesis yields embryonic red blood cells, whereas progenitor cells of subsequent definitive waves form all hematopoietic cell lineages. In this report we examine the development of hematopoietic and vasculogenic cells in normal zebrafish and characterize defects in cloche and spadetail mutant embryos. The zebrafish homologs of lmo2, c-myb, fli1, flk1, and flt4 have been cloned and characterized in this study. Expression of these genes identifies embryonic regions that contain hematopoietic and vascular progenitor cells. The expression of c-myb also identifies definitive hematopoietic cells in the ventral wall of the dorsal aorta. Analysis of b316 mutant embryos that carry a deletion of the c-myb gene demonstrates that c-myb is not required for primitive erythropoiesis in zebrafish even though it is expressed in these cells. Both cloche and spadetail mutant embryos have defects in primitive hematopoiesis and definitive hematopoiesis. The cloche mutants also have significant decreases in vascular gene expression, whereas spadetail mutants expressed normal levels of these genes. These studies demonstrate that the molecular mechanisms that regulate hematopoiesis and vasculogenesis have been conserved throughout vertebrate evolution and the clo and spt genes are key regulators of these programs.

Isolation of the zebrafish homologues for the tie-1 and tie-2 endothelium-specific receptor tyrosine kinases

Several characteristics of the zebrafish embryo make it an attractive model in which to study the development of the cardiovascular system. The utility of the zebrafish as a model of mammalian vascular development will depend on the conservation of molecular and morphogenetic mechanisms of vessel growth. Here, we report the cloning of the zebrafish homologues of the endothelium-specific receptor tyrosine kinases tie-1 and tie-2. The Z tie-2 clone represents the first report of a full-length zebrafish endothelium-specific gene. The zebrafish tie family members have significant structural homology with their murine and human counterparts. In addition, like the murine tie-1 and tie-2 genes, expression was found predominantly in endothelial cells. At 24-hr postfertilization (HPF), Z tie-1 was expressed in all observed populations of endothelial cells. Interestingly, Z tie-2 exhibited a similar, although slightly more restricted, expression pattern. Taken together, these data strongly suggest that mechanisms of vascular development are highly conserved across species and that zebrafish will continue to be a useful model for the investigation of vertebrate embryonic vascular development.

Promoting notochord fate and repressing muscle development in zebrafish axial mesoderm

Cell fate decisions in early embryonic cells are controlled by interactions among developmental regulatory genes. Zebrafish floating head mutants lack a notochord; instead, muscle forms under the neural tube. As shown previously, axial mesoderm in floating head mutant gastrulae fails to maintain expression of notochord genes and instead expresses muscle genes. Zebrafish spadetail mutant gastrulae have a nearly opposite phenotype; notochord markers are expressed in a wider domain than in wild-type embryos and muscle marker expression is absent. We examined whether these two phenotypes revealed an antagonistic genetic interaction by constructing the double mutant. Muscle does not form in the spadetail;floating head double mutant midline, indicating that spadetail function is required for floating head mutant axial mesoderm to transfate to muscle. Instead, the midline of spadetail;floating head double mutants is greatly restored compared to that of floating head mutants; the floor plate is almost complete and an anterior notochord develops. In addition, we find that floating head mutant cells can make both anterior and posterior notochord when transplanted into a wild-type host, showing that enviromental signals can override the predisposition of floating head mutant midline cells to make muscle. Taken together, these results suggest that repression of spadetail function by floating head is critical to promote notochord fate and prevent midline muscle development, and that cells can be recruited to the notochord by environmental signals.

Zebrafish semaphorin Z1a collapses specific growth cones and alters their pathway in vivo

The semaphorin/collapsin gene family encodes secreted and transmembrane proteins several of which can repulse growth cones. Although the in vitro activity of Semaphorin III/D/Collapsin 1 is clear, recent analyses of two different strains of semaphorin III/D/collapsin 1 knockout mice have generated conflicting findings. In order to clarify the in vivo action of this molecule, we analyzed sema Z1a, a zebrafish homolog of semaphorin III/D/collapsin 1. The expression pattern of sema Z1a suggested that it delimited the pathway of the growth cones of a specific set of sensory neurons, the posterior ganglion of the lateral line, in zebrafish. To examine the in vivo action of this molecule, we analyzed (1) the pathways followed by lateral line growth cones in mutants in which the expression of sema Z1a is altered in an interesting way, (2) response of lateral line growth cones to exogenous Sema Z1a in living embryos, and (3) the pathway followed by lateral line growth cones when Sema Z1a is misexpressed by cells along their normal route. The results suggest that a repulsive action of Sema Z1a helps guide the growth cones of the lateral line along their normal pathway.

zALK-8, a novel type I serine/threonine kinase receptor, is expressed throughout early zebrafish development

Here, we report the isolation and characterization of zebrafish activin receptor-like kinase-8 (zALK-8), a novel type I serine/threonine (ser/thr) kinase receptor of the transforming growth factor beta (TGF-beta) family. zALK-8 is novel, in that it contains an extracellular domain that is quite distinct from that of previously identified ALK receptors 1 through 7. Analysis of the predicted amino acid sequence of the 506 amino acid zALK-8 receptor reveals an ser/thr kinase domain characteristic of type I TGF-beta family member receptors. zALK-8, therefore, is a traditional type I ser/thr kinase receptor of the TGF-beta family, but it may exhibit novel ligand-binding activities. The developmental expression of zALK-8 mRNA was examined by wholemount in situ hybridization analysis using a probe from the 3'-untranslated sequence of zALK-8, which does not cross react with other members of the highly conserved TGF-beta receptor family. zALK-8 mRNA is present as a maternal message that is expressed ubiquitously before the start of zygotic transcription. By 16 hr postfertilization (hpf), zALK-8 mRNA is still expressed fairly evenly throughout the embryo. In 24-hpf embryos, zALK-8 mRNA is expressed predominantly in the developing eye and neural structures. By 48 hpf, zALK-8 mRNA is faintly detectable as a diffuse signal throughout the head. zALK-8 mRNA is not detectable by this method in 72-hpf or 96-hpf embryos. Northern analysis of zALK-8 mRNA in poly(A+) mRNA isolated from 6-9 hpf embryos detects a major transcript of 3.6 kb and a minor transcript of 4.3 kb. zALK-8 mRNA expression correlates well with known functions of TGF-beta family members as early axial patterning and mesoderm-inducing growth factors and as potent growth and differentiation factors in craniofacial development.

Regulation in the heart field of zebrafish

In many vertebrates, removal of early embryonic heart precursors can be repaired, leaving the heart and embryo without visible deficit. One possibility is that this ‘regulation’ involves a cell fate switch whereby cells, perhaps in regions surrounding normal progenitors, are redirected to the heart cell fate. However, the lineage and spatial relationships between cells that are normal heart progenitors and those that can assume that role after injury are not known, nor are their molecular distinctions. We have adapted a laser-activated technique to label single or small patches of cells in the lateral plate mesoderm of the zebrafish and to track their subsequent lineage. We find that the heart precursor cells are clustered in a region adjacent to the prechordal plate, just anterior to the notochord tip. Complete unilateral ablation of all heart precursors with a laser does not disrupt heart development, if performed before the 18-somite stage. By combining extirpation of the heart precursors with cell labeling, we find that cells anterior to the normal cardiogenic compartments constitute the source of regulatory cells that compensate for the loss of the progenitors. One of the earliest embryonic markers of the premyocardial cells is the divergent homeodomain gene, Nkx2.5. Interestingly, normal cardiogenic progenitors derive from only the anterior half of the Nkx2.5-expressing region in the lateral plate mesoderm. The posterior half, adjacent to the notochord, does not include cardiac progenitors and the posterior Nkx2.5-expressing cells do not contribute to the heart, even after ablation of the normal cardiogenic region. The cells that can acquire a cardiac cell fate after injury to the normal progenitors also reside near the prechordal plate, but anterior to the Nkx2.5-expressing domain. Normally they give rise to head mesenchyme. They share with cardiac progenitors early expression of GATA 4. The location of the different elements of the cardiac field, and their response to injury, suggests that the prechordal plate supports and/or the notochord suppresses the cardiac fate.

Oriented cell divisions and cellular morphogenesis in the zebrafish gastrula and neurula: a time-lapse analysis

We have taken advantage of the optical transparency of zebrafish embryos to investigate the patterns of cell division, movement and shape during early stages of development of the central nervous system. The surface-most epiblast cells of gastrula and neurula stage embryos were imaged and analysed using a computer-based, time-lapse acquisition system attached to a differential interference contrast (DIC) microscope. We find that the onset of gastrulation is accompanied by major changes in cell behaviour. Cells collect into a cohesive sheet, apparently losing independent motility and integrating their behaviour to move coherently over the yolk in a direction that is the result of two influences: towards the vegetal pole in the movements of epiboly and towards the dorsal midline in convergent movements that strengthen throughout gastrulation. Coincidentally, the plane of cell division becomes aligned to the surface plane of the embryo and oriented in the anterior-posterior (AP) direction. These behaviours begin at the blastoderm margin and propagate in a gradient towards the animal pole. Later in gastrulation, cells undergo increasingly mediolateral-directed elongation and autonomous convergence movements towards the dorsal midline leading to an enormous extension of the neural axis. Around the equator and along the dorsal midline of the gastrula, persistent AP orientation of divisions suggests that a common mechanism may be involved but that neither oriented cell movements nor shape can account for this alignment. When the neural plate begins to differentiate, there is a gradual transition in the direction of cell division from AP to the mediolateral circumference (ML). ML divisions occur in both the ventral epidermis and dorsal neural plate. In the neural plate, ML becomes the predominant orientation of division during neural keel and nerve rod stages and, from late neural keel stage, divisions are concentrated at the dorsal midline and generate bilateral progeny (C. Papan and J. A. Campos-Ortega (1994) Roux's Arch. Dev. Biol. 203, 178–186). Coincidentally, cells on the ventral surface also orient their divisions in the ML direction, cleaving perpendicular to the direction in which they are elongated. The ML alignment of epidermal divisions is well correlated with cell shape but ML divisions within the neuroepithelium appear to be better correlated with changes in tissue morphology associated with neurulation.

SCL/Tal-1 transcription factor acts downstream of cloche to specify hematopoietic and vascular progenitors in zebrafish

SCL/Tal-1 is a transcription factor necessary for hematopoietic stem cell differentiation. Although SCL is also expressed in endothelial and neural progenitors, SCL function in these cells remains unknown. In the zebrafish mutant cloche (clo), SCL expression is nearly abolished in hematopoietic and vascular tissues. Correspondingly, it was shown previously that clo fails to differentiate blood and angioblasts. Genetic analysis demonstrates that the clo mutation is not linked to the SCL locus. Forced expression of SCL in clo embryos rescues the blood and vascular defects, suggesting that SCL acts downstream of clo to specify hematopoietic and vascular differentiation.

Regulation of neuronal specification in the zebrafish spinal cord by Delta function

The vertebrate spinal cord consists of a large number of different cell types in close proximity to one another. The identities of these cells appear to be specified largely by information acquired from their local environments. We report here that local cell-cell interactions, mediated by zebrafish homologues of the Drosophila melanogaster neurogenic gene, Delta, regulate specification of diverse neuronal types in the ventral spinal cord. We describe identification of a novel zebrafish Delta gene expressed specifically in the nervous system and show, by expressing a dominant negative form of Delta protein in embryos, that Delta proteins mediate lateral inhibition in the zebrafish spinal cord. Furthermore, we find that Delta function is important for specification of a variety of spinal cord neurons, suggesting that lateral inhibition serves to diversify neuronal fate during development of the vertebrate spinal cord.

Equivalence in the genetic control of hindbrain segmentation in fish and mouse

The vertebrate hindbrain is subdivided into a series of rhombomeres whose segmental organization serves to pattern the architecture and innervation of the developing head. The zebrafish gene valentino is required cell-autonomously in the development of rhombomeres 5 and 6, and valentino mutants lack visible hindbrain segmentation caudal to the r3/4 boundary (Moens, C. B., Yan, Y.-L., Appel, B., Force, A. G., and Kimmel, C. B. (1996) Development 122, 3981–3990). Here we show that valentino is the zebrafish homologue of the mouse segmentation gene kreisler, which encodes a bZip transcription factor. The valentino gene is expressed in a manner consistent with its proposed role in subdividing rhombomeres 5 and 6 from their common precursor ‘proto-segment’ in the presumptive hindbrain, a process that we also demonstrate is reflected in the normal order of appearance of rhombomere boundaries. As well as having similar phenotypes with respect to visible hindbrain segmentation and patterns of marker gene expression, valentino and kreisler mutants have similar pharyngeal arch and inner ear defects, consistent with a conserved role for this gene in hindbrain segmentation and in patterning of the head periphery.

Zebrafish wnt11: pattern and regulation of the expression by the yolk cell and No tail activity

This study analyzed the spatial and temporal expression pattern of zebrafish wnt11 and the regulation of the expression during zebrafish early development, focusing on the interaction with the no tail (ntl) gene, a zebrafish orthologue of mouse Brachyury (T). Zygotic expression of wnt11 was first detected at the late blastula stage in the blastoderm margin, a presumptive mesoderm region. wnt11 expression coincided with mesoderm induction, and the expression was induced by mesoderm inducers such as the yolk cell (Mizuno, T., Yamaha, E., Wakahara, M., Kuroiwa, A., Takeda, H., 1996. Mesoderm induction in zebrafish. Nature 383, 131-132) or FGFs, indicating that, like ntl, wnt11 is one of the immediate-early genes in mesoderm induction. Initial expression domains of wnt11 and ntl overlapped, and these genes showed a similar response to mesoderm inducers. However, analysis of the ntl mutant embryos suggested that wnt11 and ntl are placed in distinct genetic pathways; the ntl mutation had no effect on wnt11 expression in the blastoderm margin. This was further supported by the result of RNA injection experiments showing that overexpression of Wnt11 did not affect ntl expression in the margin. Thus, wnt11 and ntl expression are induced and maintained independently in their initial phase of expression. In later stages, wnt11 was expressed in various organs, such as the somites, particularly in the developing notochord. Since no wnt gene has been reported to be expressed in the axial mesoderm, which is known to act as a signaling source that patterns the neural tube and somites, zebrafish wnt11 is the first wnt gene expressed in the notochord. Furthermore, in contrast to early expression, wnt11 expression in the notochord depended on Ntl activity. In the ntl mutant in which somite patterning is severely affected, wnt11 expression was completely lost, while another signaling molecule, sonic hedgehog is expressed in the mutant notochord precursor cells (Krauss, S., Concordet, J.-P., Ingham, P.W., 1993. A functionally conserved homolog of the Drosophila segment polarity gene hh is expressed in tissues with polarizing activity in zebrafish embryos. Cell 75, 1431-1444). wnt11 expression in the somite also shows a characteristic pattern, correlated with the migration and differentiation of slow muscle precursors. These observations suggest a role for wnt11 in patterning the somites.

Zebrafish hox genes: expression in the hindbrain region of wild-type and mutants of the segmentation gene, valentino

The developing hindbrain is organized into a series of segments termed rhombomeres which represent lineage restricted compartments correlating with domains of gene expression and neuronal differentiation. In this study, we investigate the processes of hindbrain segmentation and the acquisition of segmental identity by analyzing the expression of zebrafish hox genes in the hindbrains of normal fish and fish with a loss-of-function mutation in the segmentation gene valentino (val, the homologue of mouse kreisler; Moens, C. B., Cordes, S. P. Giorgianni, M. W., Barsh, G. S. and Kimmel, C. B. (1998). Development 125, 381–391). We find that zebrafish hox genes generally have similar expression profiles to their murine and avian counterparts, although there are several differences in timing and spatial extent of expression which may underlie some of the functional changes that have occurred along the separate evolutionary lineages of teleosts and tetrapods. Our analysis of hox gene expression in val- embryos confirms that the val gene product is important for subdivision of the presumptive rhombomere 5 and 6 territory into definitive rhombomeres, suggests that the val gene product plays a critical role in regulating hox gene transcription, and indicates that some neural crest cells are inappropriately specified in val- embryos. Our analysis of gene expression at several developmental stages has allowed us to infer differences between primary and secondary defects in the val mutant: we find that extended domains of expression for some hox genes are secondary, late phenomena potentially resulting from inappropriate cell mixing or lack of normal inter-rhombomeric interactions in the caudal hindbrain.

Zebrafish hox genes: genomic organization and modified colinear expression patterns in the trunk

The Hox genes are implicated in conferring regional identity to the anteroposterior axis of the developing embryo. We have characterized the organization and expression of hox genes in the teleost zebrafish (Danio rerio), and compared our findings with those made for the tetrapod vertebrates. We have isolated 32 zebrafish hox genes, primarily via 3′RACE-PCR, and analyzed their linkage relationships using somatic cell hybrids. We find that in comparison to the tetrapods, zebrafish has several additional hox genes, both within and beyond the expected 4 hox clusters (A-D). For example, we have isolated a member of hox paralogue group 8 lying on the hoxa cluster, and a member of hox paralogue group 10 lying on the b cluster, no equivalent genes have been reported for mouse or human. Beyond the 4 clusters (A-D) we have isolated a further 3 hox genes (the hoxx and y genes), which according to their sequence homologies lie in paralogue groups 4, 6, and 9. The hoxx4 and hoxx9 genes occur on the same set of hybrid chromosomes, hinting at the possibility of an additional hox cluster for the zebrafish. Similar to their tetrapod counterparts, zebrafish hox genes (including those with no direct tetrapod equivalent) demonstrate colinear expression along the anteroposterior (AP) axis of the embryo. However, in comparison to the tetrapods, anterior hox expression limits are compacted over a short AP region; some members of adjacent paralogue groups have equivalent limits. It has been proposed that during vertebrate evolution, the anterior limits of Hox gene expression have become dispersed along the AP axis allowing the genes to take on novel patterning roles and thus leading to increased axial complexity. In the teleost zebrafish, axial organization is relatively simple in comparison to that of the tetrapod vertebrates; this may be reflected by the less dispersed expression domains of the zebrafish hox genes.

Multiple delta genes and lateral inhibition in zebrafish primary neurogenesis

In Drosophila, cells are thought to be singled out for a neural fate through a competitive mechanism based on lateral inhibition mediated by Delta-Notch signalling. In tetrapod vertebrates, nascent neurons express the Delta1 gene and thereby deliver lateral inhibition to their neighbours, but it is not clear how these cells are singled out within the neurectoderm in the first place. We have found four Delta homologues in the zebrafish--twice as many as reported in any tetrapod vertebrate. Three of these--deltaA, deltaB and deltaD--are involved in primary neurogenesis, while two--deltaC and deltaD--appear to be involved in somite development. In the neural plate, deltaA and deltaD, unlike Delta1 in tetrapods, are expressed in large patches of contiguous cells, within which scattered individuals expressing deltaB become singled out as primary neurons. By gene misexpression experiments, we show: (1) that the singling-out of primary neurons, including the unique Mauthner cell on each side of the hindbrain, depends on Delta-Notch-mediated lateral inhibition, (2) that deltaA, deltaB and deltaD all have products that can deliver lateral inhibition and (3) that all three of these genes are themselves subject to negative regulation by lateral inhibition. These properties imply that competitive lateral inhibition, mediated by coordinated activities of deltaA, deltaB and deltaD, is sufficient to explain how primary neurons emerge from proneural clusters of neuroepithelial cells in the zebrafish.

Positional cloning identifies zebrafish one-eyed pinhead as a permissive EGF-related ligand required during gastrulation

The zebrafish one-eyed pinhead (oep) mutation disrupts embryonic development, resulting in cyclopia and defects in endoderm, prechordal plate, and ventral neuroectoderm formation. We report the molecular isolation of oep using a positional cloning approach. The oep gene encodes a novel EGF-related protein with similarity to the EGF-CFC proteins cripto, cryptic, and FRL-1. Wild-type oep protein contains a functional signal sequence and is membrane-associated. Following ubiquitous maternal and zygotic expression, highest levels of oep mRNA are found in the gastrula margin and in axial structures and forebrain. Widespread misexpression of both membrane-attached and secreted forms of oep rescues prechordal plate and forebrain development in mutant embryos but does not lead to the ectopic induction of these cell types in wild-type fish. These results establish an essential but permissive role for an EGF-related ligand during vertebrate gastrulation.

Mindin/F-spondin family: novel ECM proteins expressed in the zebrafish embryonic axis

F-spondin is a secreted protein expressed at high levels by the floor plate cells. The C-terminal half of the protein contains six thrombospondin type 1 repeats, while the N-terminal half exhibited virtually no similarity to any other protein until recently, when a Drosophila gene termed M-spondin was cloned; its product was found to share two conserved domains with the N-terminal half of F-spondin. We report the molecular cloning of four zebrafish genes encoding secreted proteins with these conserved domains. Two are zebrafish homologs of F-spondin, while the other two, termed mindin1 and mindin2, encode mutually related novel proteins, which are more related to the Drosophila M-spondin than to F-spondin. During embryonic development, all four genes are expressed in the floor plate cells. In addition to the floor plate, mindin1 is expressed in the hypochord cells, while mindin2 is expressed in the sclerotome cells. When ectopically expressed, Mindin proteins selectively accumulate in the basal lamina, suggesting that Mindins are extracellular matrix (ECM) proteins with high affinity to the basal lamina. We also report the spatial distribution of one of the F-spondin proteins, F-spondin2. F-spondin2 is localized to the thread-like structure in the central canal of the spinal cord, which is likely to correspond to Reissner's fiber known to be present in the vertebrate phylum. In summary, our study has defined a novel gene family of ECM molecules in the vertebrate, all of which may potentially be involved in development of the midline structure.

Differential regulation of chordin expression domains in mutant zebrafish

Patterning along the dorsal-ventral (D-V) axis of Xenopus and Drosophila embryos is believed to occur through a conserved molecular mechanism, with homologous proteins Chordin and Short gastrulation (Sog) antagonizing signaling by bone morphogenetic protein 4 (BMP-4) and Decapentaplegic (Dpp), respectively. We have isolated a zebrafish gene that is highly homologous to chordin and sog within cysteine-rich domains and exhibits conserved aspects of expression and function. As in Xenopus embryos, zebrafish chordin is expressed in the organizer region and transiently in axial mesoderm. Injection of zebrafish chordin mRNA to the ventral side of Xenopus embryos induced secondary axes. Ectopic overexpression in zebrafish resulted in an expansion of paraxial mesoderm and neurectoderm at the expense of more lateral and ventral derivatives, producing a range of defects similar to those of dorsalized zebrafish mutants (Mullins et al., 1996). In accordance with the proposed function of chordin in D-V patterning, dorsalized zebrafish mutants showed expanded domains of chordin expression by midgastrulation, while some ventralized mutants had reduced expression; however, in all mutants examined, early organizer expression was unaltered. In contrast to Xenopus, zebrafish chordin is also expressed in paraxial mesoderm and ectoderm and in localized regions of the developing brain, suggesting that there are additional roles for chordin in zebrafish embryonic development. Surprisingly, paraxial mesodermal expression of chordin appeared unaltered in spadetail mutants that later lack trunk muscle (Kimmel et al., 1989), while axial mesodermal expression was affected. This finding reveals an unexpected function for spadetail in midline mesoderm and in differential regulation of chordin expression during gastrulation.

Neuronal and neuroendocrine expression of lim3, a LIM class homeobox gene, is altered in mutant zebrafish with axial signaling defects

LIM class homeobox genes code for a family of transcriptional regulators that encode important determinants of cell lineage and cell type specificity. The lim3 gene from the zebrafish, Danio rerio, is highly conserved in sequence and expression pattern compared to its homologs in other vertebrates. In this paper we report immunocytochemical analysis of Lim3 protein expression in the pituitary, pineal, hindbrain, and spinal cord of the embryo, revealing an asymmetrical, lateral and late program of pituitary development in zebrafish, distinct from the pattern in other vertebrates. We studied Lim3 expression in no tail, floating head, and cyclops mutant embryos, all of which have midline defects, with special reference to spinal cord differentiation where Lim3 marks mostly motoneurons. cyclops embryos showed essentially normal Lim3 expression in the hindbrain and spinal cord despite the absence of the floor plate, while no tail mutant embryos, which lack a differentiated notochord, displayed an excess of Lim3-expressing cells in a generally normal pattern. In contrast, Lim3-positive cells largely disappeared from the posterior spinal cord in floating head mutants, except in patches that correlated with remnants of apparent floor plate cells. These results support the view that either notochord or floor plate signaling can specify Lim3-positive motoneurons in the spinal cord.

Cloning and expression of the quaking gene in the zebrafish embryo

The responsible gene for hypomyelinating quaking deficiency, qkI, encoding a KH RNA binding protein, is expressed abundantly in the developing mouse nervous system, whereas who/how/struthio, a homologue of the qkI in Drosophila, is expressed predominantly in the mesoderm. Here we describe the isolation and early developmental expression of a zebrafish homologue of qkI. The zebrafish quaking cDNA, zqk, exhibits striking conservation with qkI across the coding region, accompanied by a unique 123 nucleotide insertion sequence. Maternal and zygotic zqk transcripts are ubiquitously distributed during cleavage and blastula periods, and then accumulate in the dorsal midline of the body trunk during gastrulation. During segmentation and pharyngula periods zqk transcripts are expressed in the neural tissue of the head region, and in the paraxial mesoderm of the body trunk. Subsequently they diminish until the hatching period, when they are expressed only in the cardiac sac and pectoral finbuds. We also found that the zqk transcript is alternatively spliced with the transcript containing a 123 nucleotide additional segment localized in neural tissue in the head region, but not in the paraxial mesoderm in the body trunk. The data suggest that the quaking gene family originated in the mesoderm and evolved to become expressed in the nervous system in lower vertebrates. The insertion of the 123 nucleotide sequence could be related to the acquisition of a neural function for the gene.

Effects of hirudin (thrombin specific inhibitor) in zebrafish embryos: a developmental role for thrombin

To address the role of thrombin in early embryogenesis, hirudin, a thrombin specific inhibitor was microinjected into developing zebrafish embryos to inhibit the temporal activity of thrombin during early embryonic development. We found that the fibrin forming activity is inhibited by the presence of hirudin. We also found that hirudin affects development in zebrafish embryos suggesting thrombin's role in early embryogenesis. This ability to inhibit thrombin activity in developing embryos should facilitate studies on identifying signal transduction pathways affected by thrombin during embryogenesis.

The role of thyroid hormone in zebrafish and axolotl development

Exogenous thyroid hormone (TH) induces premature differentiation of the zebrafish pectoral fins, which are analogous to the forelimbs of tetrapods. It accelerates the growth of the pelvic fins but not precociously. Goitrogens, which are chemical inhibitors of TH synthesis by the thyroid gland, inhibit the transition from larva to juvenile fish including the formation of scales, and pigment pattern; they stunt the growth of both pectoral and pelvic paired fins. Inhibition by goitrogens is rescued by the simultaneous addition of thyroxine. The effect of adding TH to the rearing water of the postembryonic Mexican axolotl was reinvestigated under conditions that permit continued growth and development. In addition to morphological changes that have been described, TH greatly stimulates axolotl limb growth causing the resulting larva to be proportioned as an adult in about two months. This study extends the known evolutionary relatedness of tetrapod limbs and fish fins to include the TH stimulation of salamander limb and zebrafish fin growth, and suggests that TH is required to complete the life cycle of a typical bony fish and a salamander at the same developmental stage that it controls anuran and flounder metamorphosis.

Vsx-1 and Vsx-2: differential expression of two paired-like homeobox genes during zebrafish and goldfish retinogenesis

Vsx-1 and Vsx-2 are two homeobox genes that were cloned originally from an adult goldfish retinal library. They are members of the paired-like:CVC gene family, which is characterized by the presence of a paired homeodomain and an additional conserved region, termed the CVC domain. To analyze the possible roles for Vsx-1 and Vsx-2 in eye development, we used in situ hybridization to examine their expression patterns in zebrafish and goldfish embryos. Vsx-2 is initially expressed by proliferating neuroepithelial cells of the presumptive neural retina, then it is down-regulated as differentiation begins, and it is finally reexpressed at later stages of differentiation in a subset of cells, presumed to be bipolar cells, in the inner nuclear layer. In contrast, Vsx-1 is expressed only weakly in undifferentiated, presumptive neural retina and is then up-regulated selectively in presumptive bipolar cells at early stages of differentiation (when Vsx-2 is turned off), before decreasing to an intermediate level, which is maintained in the differentiated (adult) retina. The restricted expression patterns of Vsx-2 correspond to the observed phenotypes in mice with the ocular retardation mutation (orJ), further supporting the notion that Vsx-2 and Chx10 are homologues. The sequential complimentary and then corresponding expression patterns of Vsx-1 and Vsx-2 suggest that these similar transcription factors may be recruited for partially overlapping, but distinct, functions during the development of the retina.

A critical period of ear development controlled by distinct populations of ciliated cells in the zebrafish

The zebrafish (Danio rerio) is a useful model system for analyzing development of the inner ear. A number of mutations affecting the inner ear have been identified. Here we investigate the initial stages of otolith morphogenesis in wild-type embryos as well as in monolith (mnl) mutant embryos, which fail to form anterior otoliths but otherwise appear normal. Otolith growth is initiated at 18-18.5 h by localized accretion of free-moving precursor particles. This process, referred to as otolith seeding, is regulated by two classes of cilia: First, kinocilia of precociously forming hair cells (tether cells) bind seeding particles, thereby localizing otolith formation. Tether cells usually occur in pairs at the anterior and posterior ends of the ear. Despite the presence of functional kinocilia, tether cells initially appear immature and do not acquire the characteristics of mature hair cells until approximately 21.5 h. Second, beating cilia distributed throughout the ear agitate seeding particles, thereby inhibiting premature agglutination. Constraining particles with laser tweezers caused them to fuse into large untethered masses. Bringing such masses into contact with tethered otoliths caused them to fuse, greatly enhancing otolith growth. Selectively enhancing one otolith greatly inhibited growth of the second, creating an imbalance that persisted for many days. Seeding particles and beating cilia disappear soon after 24 h, and the rate of otolith growth decreases by nearly 90%. In mnl mutant embryos, tethers and beating cilia are distributed normally, but anterior otoliths fail to form in 80-85% of mutant ears. The binding properties of seeding particles appear normal, as shown by their ability to fuse when entrapped by laser tweezers and their binding to posterior tethers. We infer that anterior tethers have a weakened ability to bind seeding particles in mnl embryos. Immobilizing mnl embryos with the anterior end of the ear oriented downward effectively concentrated the dense seeding particles near the anterior tethers and permitted all to form anterior otoliths. However, immobilizing mnl embryos after 24 h when seeding particles were depleted did not facilitate anterior otolith formation. Together, these data demonstrate that the ability to initiate otolith formation is limited to a critical period, from 18.5 to 24 h, and that interfering with the functions of tether cell kinocilia or beating cilia impairs otolith seeding and subsequent otolith morphogenesis.

Rapid identification and isolation of zebrafish cDNA clones

A fast and economical approach, referred to as cDNA clone tagging, was adapted to identify and isolate zebrafish cDNA clones. The basic approach was to partially sequence the coding region of size selected cDNA clones and the partial sequences were then used as tags for identifying the clones through homology search. To benefit maximally from the tagging approach, two cDNA libraries, derived from embryonic and adult fish poly(A)+ RNAs, respectively, were constructed by unidirectional cloning; conceptually, they have the potential to represent all expressed zebrafish genes. A total of 1084 clones were sequenced from the two libraries, and 511 clones were identified, based on sequence homology. These identified clones were derived from at least 261 genes, encoding 48 translational machinery proteins, 47 cytosolic proteins, 43 cytoskeletal proteins, 41 nuclear proteins, 32 membrane proteins, 22 secreted proteins, 20 mitochondrial proteins and 8 proteins with an unknown location. Of the 261 distinct cDNA clones identified, 254 were isolated for the first time in the zebrafish. These tagged cDNA clones, identified and unidentified, provide rich resources for developmental analysis as well as mapping of zebrafish genome. The long-term objective of this study is to establish a tagged zebrafish gene library that can be accessed both by hybridization screening against the plasmid DNAs and by electronic screening using the sequence information.

Activation of the metaphase checkpoint and an apoptosis programme in the early zebrafish embryo, by treatment with the spindle-destabilising agent nocodazole

We have studied the developmental activation of the metaphase checkpoint, and the consequences of activating this checkpoint, in the zebrafish embryo. (1) Treatment with nocodazole (a microtubule destabiliser) before mid-blastula transition (MBT) produces complete destruction of all nuclei in the deep cell layer of the embryo. In contrast, nocodazole treatment after MBT efficiently produces metaphase arrest in this cell layer. Thus, the metaphase checkpoint becomes activated at MBT. (2) Although a metaphase arrest is induced by nocodazole, it is not induced by paclitaxel (a microtubule stabiliser). Thus the metaphase checkpoint appears to sense a destabilisation, but not a stabilisation, of spindle microtubules. (3) Metaphase-arrested cells (in nocodazole) can be driven into the next interphase by adding the Ca2+-specific ionophore A23187. Thus, a Ca2+-signalling pathway lies downstream of, or parallel to, the metaphase checkpoint. (4) After mid-gastrula stage, treatment with nocodazole produces DNA fragmentation in all three cell layers. In the enveloping epithelial monolayer (EVL), this is associated with a classical apoptotic phenotype. In the deep layer, it is associated with an unusual, highly condensed nuclear state that is entered directly from metaphase arrest. Thus, after the mid-gastrula stage, the embryo responds to nocodazle by undergoing apoptosis. (5) Nocodazole-induced apoptosis in the deep cell layer can be blocked by the caspase-1,4,5 inhibitors Ac-YVAD-CHO and Ac-YVAD-CMK. This suggests that a homologue of the C. elegans ced-9-ced-4-ced-3 pathway is involved in control over apoptosis in the early zebrafish embryo.

Anf: a novel class of vertebrate homeobox genes expressed at the anterior end of the main embryonic axis

Five novel genes homologous to the homeobox-containing genes Xanf-1 and Xanf-2 of Xenopus and Hesx-1/Rpx of mouse have been identified as a result of a PCR survey of cDNA in sturgeon, zebrafish, newt, chicken and human. Comparative analysis of the homeodomain primary structure of these genes revealed that they belong to a novel class of homeobox genes, which we name Anf. All genes of this class investigated so far have similar patterns of expression during early embryogenesis, characterized by maximal transcript levels being present at the anterior extremity of the main embryonic body axis. The data obtained also suggest that, despite considerable high structural divergence between their homeodomains, all known Anf genes may be orthologues, and thus represent one of the most quickly evolving classes of vertebrate homeobox genes.

Pathfinding by identified zebrafish motoneurons in the absence of muscle pioneers

To identify the cellular cues that guide zebrafish neuronal growth cones to their targets, we examined interactions between identified motor growth cones and identified muscle fibers and tested whether these fibers were required for growth cone navigation. Caudal primary motoneurons (CaPs) and middle primary motoneurons (MiPs) are identified motoneurons that innervate cell-specific regions of the myotome. Growth cones of both cells initially extend along a common pathway and then pause at a set of identified muscle fibers, called muscle pioneers, before diverging along cell-specific pathways. Muscle pioneers are intermediate targets of both CaP and MiP (Westerfield et al., 1986; Liu and Westerfield, 1990); both motoneurons extend their growth cones directly to the muscle pioneers on which the first functional neuromuscular contacts form, suggesting that muscle pioneers may provide guidance information to these growth cones. We tested this idea by ablating muscle pioneers and observing the resulting motor axonal trajectories. Both CaP and MiP ultimately formed normal axonal arbors after muscle pioneer ablation, showing that muscle pioneers are unnecessary for formation of correct axonal trajectories; however, although final cellular morphology was correct in the absence of muscle pioneers, MiP growth cones branched abnormally or extended ventrally beyond the common pathway. Ablation of CaP and the muscle pioneers together increased the aberrant behavior of the MiP growth cone. Our results provide evidence that an intermediate target, the muscle pioneers, affects motor axonal extension without altering target choice, suggesting that other cues also contribute to proper pathway navigation.

Positive and negative regulation of muscle cell identity by members of the hedgehog and TGF-beta gene families

We have examined whether the development of embryonic muscle fiber type is regulated by competing influences between Hedgehog and TGF-beta signals, as previously shown for development of neuronal cell identity in the neural tube. We found that ectopic expression of Hedgehogs or inhibition of protein kinase A in zebrafish embryos induces slow muscle precursors throughout the somite but muscle pioneer cells only in the middle of the somite. Ectopic expression in the notochord of Dorsalin-1, a member of the TGF-beta superfamily, inhibits the formation of muscle pioneer cells, demonstrating that TGF-beta signals can antagonize the induction of muscle pioneer cells by Hedgehog. We propose that a Hedgehog signal first induces the formation of slow muscle precursor cells, and subsequent Hedgehog and TGF-beta signals exert competing positive and negative influences on the development of muscle pioneer cells.

High-resolution imaging at the cellular and subcellular levels in flattened whole mounts of early zebrafish embryos

We describe a rapid and sensitive method for high-resolution imaging at the cellular and subcellular levels in the whole-mount zebrafish embryo. The procedure involves fixing and staining the embryo, followed by deyolking and flattening it under a cover slip, to produce a planar mount that is 20 to 100 µm thick. Such a flattened whole mount allows imaging with a spatial resolution of ~500 nm in the x-y plane and does not require the use of embedding, sectioning, confocal microscopy, or computational deblurring procedures. We can resolve all individual nuclei and chromosome sets in the embryo, up to the late gastrula stage (10 000 cell stage). In addition, older embryos (through the segmentation stage) can also be examined, with the preservation of significant morphological detail. Because of its ability to resolve subcellular detail, the flattened whole-mount method can provide significant biological information beyond what can be obtained from conventional (three-dimensional) whole mounts. We have used the flattened whole-mount method to study subcellular events related to progression through the cell cycle or to apoptosis, in cells of the early zebrafish embryo. A specific DNA-binding dye (Hoechst 33258) or an antibody against a chromosomal protein (histone H1) was used to stain the nuclei of individual cells in the embryo. This allowed us to determine the spatial positions of all the individual cells, and also their stages in the cell cycle. A terminal transferase (TUNEL) assay was used to detect apoptotic cells. This combination of specific stains allowed us to study the behaviors of groups of cells in situ, within the developing zebrafish embryo.

Spatially distinct domains of cell behavior in the zebrafish organizer region

To determine the sequence of cell behaviors that is involved in the morphogenesis of the zebrafish organizer region, we have examined the dorsal marginal zone of vitally stained zebrafish embryos using time-lapse confocal microscopy. During the late-blastula stage, the zebrafish dorsal marginal zone segregates into several cellular domains, including a group of noninvoluting, highly endocytic marginal (NEM) cells. The NEM cell cluster, which lies in a superficial location of the dorsal marginal zone, is composed of both enveloping layer cells and one or two layers of underlying deep cells. The longitudinal position of this cellular domain accurately predicts the site of embryonic shield formation and occupies a homologous location to the organizer epithelium in Xenopus laevis. At the onset of gastrulation, deep cells underneath the superficial NEM cell domain undergo involution to form the nascent hypoblast of the embryonic shield. Deep cells within the NEM cell cluster, however, do not involute during early shield formation, but instead move in front of the blastoderm margin to form a loose mass of cells called forerunner cells. Forerunner cells coalesce into a wedge-shaped mass during late gastrulation and eventually become overlapped by the converging lateral lips of the germ ring. During early zebrafish tail elongation, most forerunner cells are incorporated into the epithelial lining of Kupffer's vesicle, a transient teleostean organ rudiment long thought to be an evolutionary vestige of the neurenteric canal. Owing to the location of NEM cells at the dorsal margin of blastula-stage embryos, as well as their early segregation from other deep cells, we hypothesized that NEM cells are specified by an early-acting dorsalizing signal. To test this possibility, we briefly treated early-blastula stage embryos with LiCl, an agent known to produce hyperdorsalized zebrafish embryos with varying degrees of expanded organizer tissue. In Li + -treated embryos, NEM cells appear either within expanded spatial domains or in ectopic locations, primarily within the marginal zone of the blastoderm. These results suggest that NEM cells represent a specific cell type that is specified by an early dorsal patterning pathway.

Heat shock genes and the heat shock response in zebrafish embryos

Heat shock genes exhibit complex patterns of spatial and temporal regulation during embryonic development in a wide range of organisms. Our laboratory has initiated an analysis of heat shock protein gene expression in the zebrafish, a model system that is now utilized extensively for the examination of early embryonic development of vertebrates. We have cloned members of the zebrafish hsp47, hsp70,\i and hsp90 gene families and shown them to be closely related to their counterparts in higher vertebrates. Whole mount in situ hybridization and Northern blot analyses have revealed that these genes are regulated in distinct spatial, temporal, and stress-specific manners. Furthermore, the tissue-specific expression patterns of the hsp47 and hsp90 alpha genes correlate closely with the expression of genes encoding known chaperone targets of Hsp47 and Hsp90 in other systems. The data raise a number of interesting questions regarding the function and regulation of these heat shock genes in zebrafish embryos during normal development and following exposure to environmental stress.

Mutations resulting in transient and localized degeneration in the developing zebrafish brain

In a large-scale mutagenesis screen in the zebrafish, Danio rerio, we have identified a heterogeneous group of 30 recessive, embryonic lethal mutations characterized by degeneration in the developing central nervous system that is either transient or initially localized to one area of the brain. Transient degeneration is defined as abnormal cell death occurring during a restricted period of development. Following degeneration, the affected structures do not appear to regenerate. In each case degeneration is identified after somitogenesis is complete and is not associated with visually identified patterning defects. These 30 mutations, forming 21 complementation groups, have been classified into four phenotypic groups: group 1, transient degeneration (13 mutations); group 2, spreading degeneration, early onset, in which degeneration is initially confined to the optic tectum but subsequently spreads to other areas of the central nervous system (7 mutations); group 3, late-onset degeneration, initially identified after 4 days (6 mutations); and group 4, degeneration with abnormal pigmentation (4 mutations). Although apoptotic cells are seen in the retina and tectum of all mutants, the distribution, temporal progression, and severity of degeneration vary between mutations. Several mutations also show pleiotropic effects, with degeneration involving extraneural structures including the pharyngeal arches and pectoral fins. We discuss some of the pathways important for cell survival in the nervous system and suggest that these mutations will provide entry points for identifying genes that affect the survival of restricted neural populations.

The zebrafish's swim to fame as an experimental model in biology

The zebrafish has long been the favorite organism in many scientific disciplines. Although its attributes as a model were expounded for many years and thus were no secret, the zebrafish sat in the wings while other more popular vertebrates such as chick, amphibians, and mouse were examined at length. We cannot say there was a resurgence in popularity, but more an explosion of research utilizing the zebrafish beginning in the late 1970s when investigators at the University of Oregon began using it as their model in neuroscience. Prior to this reawakening, the zebrafish was one of the significant organisms in the study of teratology and toxicology, development, and, to some extent, behavior. Recently, however, the field of zebrafish genetics has gained immense popularity and success, in part owing to the fact that zebrafish are diploid and are amenable to genetic manipulations. Here we present an overview of the multidisciplinary research that has laid some of the foundation of our present understanding of the biochemical, cell biological, and molecular genetic events accompanying zebrafish development.

Zebrafish vasa homologue RNA is localized to the cleavage planes of 2- and 4-cell-stage embryos and is expressed in the primordial germ cells

Identification and manipulation of the germ line are important to the study of model organisms. Although zebrafish has recently emerged as a model for vertebrate development, the primordial germ cells (PGCs) in this organism have not been previously described. To identify a molecular marker for the zebrafish PGCs, we cloned the zebrafish homologue of the Drosophila vasa gene, which, in the fly, encodes a germ-cell-specific protein. Northern blotting revealed that zebrafish vasa homologue (vas) transcript is present in embryos just after fertilization, and hence it is probably maternally supplied. Using whole-mount in situ hybridization, we investigated the expression pattern of vas RNA in zebrafish embryos from the 1-cell stage to 10 days of development. Here we present evidence that vas RNA is a germ-cell-specific marker, allowing a description of the zebrafish PGCs for the first time. Furthermore, vas transcript was detected in a novel pattern, localized to the cleavage planes in 2- and 4-cell-stage embryos. During subsequent cleavages, the RNA is segregated as subcellular clumps to a small number of cells that may be the future germ cells. These results suggest new ways in which one might develop techniques for the genetic manipulation of zebrafish. Furthermore, they provide the basis for further studies on this novel RNA localization pattern and on germ-line development in general.

Both apolipoprotein E and A-I genes are present in a nonmammalian vertebrate and are highly expressed during embryonic development

Apolipoprotein E (apoE) is associated with several classes of plasma lipoproteins and mediates uptake of lipoproteins through its ability to interact with specific cell surface receptors. Besides its role in cardiovascular diseases, accumulating evidence has suggested that apoE could play a role in neurodegenerative diseases, such as Alzheimer disease. In vertebrates, apoA-I is the major protein of high-density lipoprotein. ApoA-I may play an important role in regulating the cholesterol content of peripheral tissues through the reverse cholesterol transport pathway. We have isolated cDNA clones that code for apoE and apoA-I from a zebrafish embryo library. Analysis of the deduced amino acid sequences showed the presence of a region enriched in basic amino acids in zebrafish apoE similar to the lipoprotein receptor-binding region of human apoE. We demonstrated by whole-mount in situ hybridization that apoE and apoA-I genes are highly expressed in the yolk syncytial layer, an extraembryonic structure implicated in embryonic and larval nutrition. ApoE transcripts were also observed in the deep cell layer during blastula stage, in numerous ectodermal derivatives after gastrulation, and after 3 days of development in a limited number of cells both in brain and in the eyes. Our data indicate that apoE can be found in a nonmammalian vertebrate and that the duplication events, from which apoE and apoA-I genes arose, occurred before the divergence of the tetrapod and teleost ancestors. Zebrafish can be used as a simple and useful model for studying the role of apolipoproteins in embryonic and larval nutrition and of apoE in brain morphogenesis and regeneration.

Activator effect of coinjected enhancers on the muscle-specific expression of promoters in zebrafish embryos

The transient expression of reporter gene constructs in embryos provides a powerful tool to characterise cis-acting transcriptional elements of the genes involved in development. In the present study, we have analysed the expression pattern of several muscle-specific and ubiquitous regulatory sequences in microinjected zebrafish embryos. By using a fast and reproducible coinjection strategy, the mosaic expression of lacZ reporter gene was monitored in wholemount embryos injected with sequences containing putative enhancer elements and a carp myosin heavy chain promoter/lacZ reporter construct. We have found that a 0.9-kb myosin heavy chain (MyHC) proximal promoter containing several putative myogenic regulatory factors (MRF) binding sites is sufficient to restrict lacZ expression to the skeletal muscle fibres of prim-6 stage zebrafish embryos. Expression of a rat-derived foetal myosin light chain enhancer (MyLC) and different fragments of a carp beta-actin regulatory region together with the MyHC promoter were compared by accumulating the type, number and spatial distribution of beta-galactosidase-expressing cells on an expression map. beta-galactosidase activity increased similarly whether the MyLC enhancer was ligated to the promoter/ reporter construct directly or when coinjected as a separate fragment whilst skeletal muscle specificity was retained. The coinjection of two different forms of the beta-actin regulatory elements also showed a marked effect on the MyHC promoter activity. The coinjection of putative enhancers with minimal promoter constructs and subsequent analysis of the transient expression pattern in the developing embryos provides a rapid and simple technique to identify cis acting activator elements of genes expressed in the vertebrate embryo.