Wnt1 and wnt10b function redundantly at the zebrafish midbrain-hindbrain boundary

Wnt signals have been shown to be involved in multiple steps of vertebrate neural patterning, yet the relative contributions of individual Wnts to the process of brain regionalization is poorly understood. Wnt1 has been shown in the mouse to be required for the formation of the midbrain and the anterior hindbrain, but this function of wnt1 has not been explored in other model systems. Further, wnt1 is part of a Wnt cluster conserved in all vertebrates comprising wnt1 and wnt10b, yet the function of wnt10b during embryogenesis has not been explored. Here, we report that in zebrafish wnt10b is expressed in a pattern overlapping extensively with that of wnt1. We have generated a deficiency allele for these closely linked loci and performed morpholino antisense oligo knockdown to show that wnt1 and wnt10b provide partially redundant functions in the formation of the midbrain-hindbrain boundary (MHB). When both loci are deleted, the expression of pax2.1, en2, and her5 is lost in the ventral portion of the MHB beginning at the 8-somite stage. However, wnt1 and wnt10b are not required for the maintenance of fgf8, en3, wnt8b, or wnt3a expression. Embryos homozygous for the wnt1-wnt10b deficiency display a mild MHB phenotype, but are sensitized to reductions in either Pax2.1 or Fgf8; that is, in combination with mutant alleles of either of these loci, the morphological MHB is lost. Thus, wnt1 and wnt10b are required to maintain threshold levels of Pax2.1 and Fgf8 at the MHB.     

Wnt1 regulates neurogenesis and mediates lateral inhibition of boundary cell specification in the zebrafish hindbrain

The formation of localised signalling centres is essential for patterning of a number of tissues during development. Previous work has revealed that a distinct population of boundary cells forms at the interface of segments in the vertebrate hindbrain, but the role of these cells is not known. We have investigated the function of the Wnt1 signalling molecule that is expressed by boundary and roof plate cells in the zebrafish hindbrain. Knockdown of wnt1 or of tcf3b, a mediator of Wnt signalling, leads to ectopic expression of boundary cell markers, rfng and foxb1.2, in non-boundary regions of the hindbrain. Ectopic boundary marker expression also occurs following knockdown of rfng, a modulator of Notch signalling required for wnt1 expression at hindbrain boundaries. We show that the boundary and roof plate expression of wnt1 each contribute to upregulation of proneural and delta gene expression and neurogenesis in non-boundary regions, which in turn blocks ectopic boundary marker expression. Boundary cells therefore play a key role in the regulation of cell differentiation in the zebrafish hindbrain. The network of genes underlying the regulation of neurogenesis and lateral inhibition of boundary cell formation by Wnt1 has a striking similarity to mechanisms at the dorsoventral boundary in the Drosophila wing imaginal disc.     

Combinatorial Wnt control of zebrafish midbrain-hindbrain boundary formation

Wnt signaling is known to be required for the normal development of the vertebrate midbrain and hindbrain, but genetic loss of function analyses in the mouse and zebrafish yield differing results regarding the relative importance of specific Wnt loci. In the zebrafish, Wnt1 and Wnt10b functionally overlap in their control of gene expression in the ventral midbrain-hindbrain boundary (MHB), but they are not required for the formation of the MHB constriction. Whether other wnt loci are involved in zebrafish MHB development is unclear, although the expression of at least two wnts, wnt3a and wnt8b, is maintained in wnt1/wnt10b mutants. In order to address the role of wnt3a in zebrafish, we have isolated a full length cDNA and examined its expression and function via knockdown by morpholino antisense oligonucleotide (MO)-mediated knockdown. The expression pattern of wnt3a appears to be evolutionarily conserved between zebrafish and mouse, and MO knockdown shows that Wnt3a, while not uniquely required for MHB development, is required in the absence of Wnt1 and Wnt10b for the formation of the MHB constriction. In zebrafish embryos lacking Wnt3a, Wnt1 and Wnt10b, the expression of engrailed orthologs, pax2a and fgf8 is not maintained after mid-somitogenesis. In contrast to acerebellar and no isthmus mutants, in which midbrain and hindbrain cells acquire new fates but cell number is not significantly affected until late in embryogenesis, zebrafish embryos lacking Wnt3a, Wnt1 and Wnt10b undergo extensive apoptosis in the midbrain and cerebellum anlagen beginning in mid-somitogenesis, which results in the absence of a significant portion of the midbrain and cerebellum. Thus, the requirement for Wnt signaling in forming the MHB constriction is evolutionarily conserved in vertebrates and it is possible in zebrafish to dissect the relative impact of multiple Wnt loci in midbrain and hindbrain development.     

Regulatory gene expression patterns reveal transverse and longitudinal subdivisions of the embryonic zebrafish forebrain

To shed light on the organization of the rostral embryonic brain of a lower vertebrate, we have directly compared the expression patterns of dlx, fgf, hh, hlx, otx, pax, POU, winged helix and wnt gene family members in the fore- and midbrain of the zebrafish. We show that the analyzed genes are expressed in distinct transverse and longitudinal domains and share expression boundaries at stereotypic positions within the fore- and midbrain. Some of these shared expression boundaries coincide with morphological landmarks like the pathways of primary axon tracts. We identified a series of eight transverse diencephalic domains suggestive of neuromeric subdivisions within the rostral brain. In addition, we identified four molecularly distinct longitudinal subdivisions and provide evidence for a strong bending of the longitudinal rostral brain axis at the cephalic flexure. Our data suggest a strong conservation of early forebrain organization between lower and higher vertebrates.     

Zebrafish wnt4b expression in the floor plate is altered in sonic hedgehog and gli-2 mutants

The floor plate of the neural tube serves an important function as a source of signals that pattern cell fates in the nervous system as well as directing proper axon pathfinding. We have cloned a novel zebrafish wnt family member, wnt4b, which is expressed exclusively in the floor plate. To place wnt4b in the context of known regulators of midline development, its expression was analyzed in the zebrafish mutants cyclops (cyc), floating head (flh), you-too (yot), and sonic you (syu). wnt4b expression in the medial and lateral floor plate are shown to be regulated independently: medial floor plate expression occurs in the absence of a notochord, while lateral floor plate expression requires a functional notochord, sonic hedgehog and gli-2.     

Zebrafish no isthmus reveals a role for pax2.1 in tubule differentiation and patterning events in the pronephric primordia

Pax genes are important developmental regulators and function at multiple stages of vertebrate kidney organogenesis. In this report, we have used the zebrafish pax2.1 mutant no isthmus to investigate the role for pax2.1 in development of the pronephros. We demonstrate a requirement for pax2.1 in multiple aspects of pronephric development including tubule and duct epithelial differentiation and cloaca morphogenesis. Morphological analysis demonstrates that noi(- )larvae specifically lack pronephric tubules while glomerular cell differentiation is unaffected. In addition, pax2.1 expression in the lateral cells of the pronephric primordium is required to restrict the domains of Wilms' tumor suppressor (wt1) and vascular endothelial growth factor (VEGF) gene expression to medial podocyte progenitors. Ectopic podocyte-specific marker expression in pronephric duct cells correlates with loss of expression of the pronephric tubule and duct-specific markers mAb 3G8 and a Na(+)/K(+) ATPase (&agr;)1 subunit. The results suggest that the failure in pronephric tubule differentiation in noi arises from a patterning defect during differentiation of the pronephric primordium and that mutually inhibitory regulatory interactions play an important role in defining the boundary between glomerular and tubule progenitors in the forming nephron.     

Pax2.1 is required for the development of thyroid follicles in zebrafish

The thyroid gland is an organ primarily composed of endoderm-derived follicular cells. Although disturbed embryonic development of the thyroid gland leads to congenital hypothyroidism in humans and mammals, the underlying principles of thyroid organogenesis are largely unknown. In this study, we introduce zebrafish as a model to investigate the molecular and genetic mechanisms that control thyroid development. Marker gene expression suggests that the molecular pathways of early thyroid development are essentially conserved between fish and mammals. However during larval stages, we find both conserved and divergent features of development compared with mammals. A major difference is that in fish, we find evidence for hormone production not only in thyroid follicular cells, but also in an anterior non-follicular group of cells. We show that pax2.1 and pax8, members of the zebrafish pax2/5/8 paralogue group, are expressed in the thyroid primordium. Whereas in mice, only Pax8 has a function during thyroid development, analysis of the zebrafish pax2.1 mutant no isthmus (noi(-/-)) demonstrates that pax2.1 has a role comparable with mouse Pax8 in differentiation of the thyroid follicular cells. Early steps of thyroid development are normal in noi(-/-), but later expression of molecular markers is lost and the formation of follicles fails. Interestingly, the anterior non-follicular site of thyroid hormone production is not affected in noi(-/-). Thus, in zebrafish, some remaining thyroid hormone synthesis takes place independent of the pathway leading to thyroid follicle formation. We suggest that the noi(-/-) mutant serves as a new zebrafish model for hypothyroidism.     

Identification and expression analysis of mical family genes in zebrafish

Mical(molecule interacting with CasL)represent a conserved family of cytosolic multidomain proteins that has been shown to be associated with a variety of cellular processes,including axon guidance,cell movement,cell-cell junction formation,vesicle trafficking and cancer cell metastasis.However,the expression and function of these genes during embryonic development have not been comprehensively characterized,especially in vertebrate species,although some limited in vivo studies have been carried out in neural and musculature systems of Drosophila and in neural systems of vertebrates.So far,no mica/family homologs have been reported in zebrafish,an ideal vertebrate model for the study of developmental processes.Here we report eight homologs of m/ca/family genes in zebrafish and their expression profiles during embryonic development.Consistent with the findings in Drosophila and mammals,most zebrafish mical family genes display expression in neural and musculature systems.In addition,five mica/homologs are detected in heart,and one,micall2a,in blood vessels.Our data established an important basis for further functional studies of mica/family genes in zebrafish,and suggest a possible role for mica/genes in cardiovascular development.