Zebrafish smoothened functions in ventral neural tube specification and axon tract formation

Sonic hedgehog (Shh) signaling patterns many vertebrate tissues. shh mutations dramatically affect mouse ventral forebrain and floor plate but produce minor defects in zebrafish. Zebrafish have two mammalian Shh orthologs, sonic hedgehog and tiggy-winkle hedgehog, and another gene, echidna hedgehog, that could have overlapping functions. To examine the role of Hedgehog signaling in zebrafish, we have characterized slow muscle omitted (smu) mutants. We show that smu encodes a zebrafish ortholog of Smoothened that transduces Hedgehog signals. Zebrafish smoothened is expressed maternally and zygotically and supports specification of motoneurons, pituitary cells and ventral forebrain. We propose that smoothened is required for induction of lateral floor plate and a subpopulation of hypothalamic cells and for maintenance of medial floor plate and hypothalamic cells.     

A temperature-sensitive mutation in the nodal-related gene cyclops reveals that the floor plate is induced during gastrulation in zebrafish

The floor plate, a specialized group of cells in the ventral midline of the neural tube of vertebrates, plays crucial roles in patterning the central nervous system. Recent work from zebrafish, chick, chick-quail chimeras and mice to investigate the development of the floor plate have led to several models of floor-plate induction. One model suggests that the floor plate is formed by inductive signalling from the notochord to the overlying neural tube. The induction is thought to be mediated by notochord-derived Sonic hedgehog (Shh), a secreted protein, and requires direct cellular contact between the notochord and the neural tube. Another model proposes a role for the organizer in generating midline precursor cells that produce floor plate cells independent of notochord specification, and proposes that floor plate specification occurs early, during gastrulation. We describe a temperature-sensitive mutation that affects the zebrafish Nodal-related secreted signalling factor, Cyclops, and use it to address the issue of when the floor plate is induced in zebrafish. Zebrafish cyclops regulates the expression of shh in the ventral neural tube. Although null mutations in cyclops result in the lack of the medial floor plate, embryos homozygous for the temperature-sensitive mutation have floor plate cells at the permissive temperature and lack floor plate cells at the restrictive temperature. We use this mutant allele in temperature shift-up and shift-down experiments to answer a central question pertaining to the timing of vertebrate floor plate induction. Abrogation of Cyc/Nodal signalling in the temperature-sensitive mutant embryos at various stages indicates that the floor plate in zebrafish is induced early in development, during gastrulation. In addition, continuous Cyclops signalling is required through gastrulation for a complete ventral neural tube throughout the length of the neuraxis. Finally, by modulation of Nodal signalling levels in mutants and in ectopic overexpression experiments, we show that, similar to the requirements for prechordal plate mesendoderm fates, uninterrupted and high levels of Cyclops signalling are required for induction and specification of a complete ventral neural tube.     

A revised model of Xenopus dorsal midline development: differential and separable requirements for Notch and Shh signaling

The development of the vertebrate dorsal midline (floor plate, notochord, and hypochord) has been an area of classical research and debate. Previous studies in vertebrates have led to contrasting models for the roles of Shh and Notch signaling in specification of the floor plate, by late inductive or early allocation mechanisms, respectively. Here, we show that Notch signaling plays an integral role in cell fate decisions in the dorsal midline of Xenopus laevis, similar to that observed in zebrafish and chick. Notch signaling promotes floor plate and hypochord fates over notochord, but has variable effects on Shh expression in the midline. In contrast to previous reports in frog, we find that Shh signaling is not required for floor plate vs. notochord decisions and plays a minor role in floor plate specification, where it acts in parallel to Notch signaling. As in zebrafish, Shh signaling is required for specification of the lateral floor plate in the frog. We also find that the medial floor plate in Xenopus comprises two distinct populations of cells, each dependent upon different signals for its specification. Using expression analysis of several midline markers, and dissection of functional relationships, we propose a revised allocation mechanism of dorsal midline specification in Xenopus. Our model is distinct from those proposed to date, and may serve as a guide for future studies in frog and other vertebrate organisms.     

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.     

Notch signaling regulates neural precursor allocation and binary neuronal fate decisions in zebrafish

Notch signaling plays a well-described role in regulating the formation of neurons from proliferative neural precursors in vertebrates but whether, as in flies, it also specifies sibling cells for different neuronal fates is not known. Ventral spinal cord precursors called pMN cells produce mostly motoneurons and oligodendrocytes, but recent lineage-marking experiments reveal that they also make astrocytes, ependymal cells and interneurons. Our own clonal analysis of pMN cells in zebrafish showed that some produce a primary motoneuron and KA' interneuron at their final division. We investigated the possibility that Notch signaling regulates a motoneuron-interneuron fate decision using a combination of mutant, transgenic and pharmacological manipulations of Notch activity. We show that continuous absence of Notch activity produces excess primary motoneurons and a deficit of KA' interneurons, whereas transient inactivation preceding neurogenesis results in an excess of both cell types. By contrast, activation of Notch signaling at the neural plate stage produces excess KA' interneurons and a deficit of primary motoneurons. Furthermore, individual pMN cells produce similar kinds of neurons at their final division in mib mutant embryos, which lack Notch signaling. These data provide evidence that, among some postmitotic daughters of pMN cells, Notch promotes KA' interneuron identity and inhibits primary motoneuron fate, raising the possibility that Notch signaling diversifies vertebrate neuron type by mediating similar binary fate decisions.     

Two distinct cell populations in the floor plate of the zebrafish are induced by different pathways

The floor plate is a morphologically distinct structure of epithelial cells situated along the midline of the ventral spinal cord in vertebrates. It is a source of guidance molecules directing the growth of axons along and across the midline of the neural tube. In the zebrafish, the floor plate is about three cells wide and composed of cuboidal cells. Two cell populations can be distinguished by the expression patterns of several marker genes, including sonic hedgehog (shh) and the fork head-domain gene fkd4: a single row of medial floor plate (MFP) cells, expressing both shh and fkd4, is flanked by rows of lateral floor plate (LFP) cells that express fkd4 but not shh. Systematic mutant searches in zebrafish embryos have identified a number of genes, mutations in which visibly reduce the floor plate. In these mutants either the MFP or the LFP cells are absent, as revealed by the analysis of the shh and fkd4 expression patterns. MFP cells are absent, but LFP cells are present, in mutants of cyclops, one-eyed pinhead, and schmalspur, whose development of midline structures is affected. LFP cells are absent, but MFP cells are present, in mutants of four genes, sonic you, you, you-too, and chameleon, collectively called the you-type genes. This group of mutants also shows defects in patterning of the paraxial mesoderm, causing U- instead of V-shaped somites. One of the you-type genes, sonic you, was recently shown to encode the zebrafish Shh protein, suggesting that the you-type genes encode components of the Shh signaling pathway. It has been shown previously that in the zebrafish shh is required for the induction of LFP cells, but not for the development of MFP cells. This conclusion is supported by the finding that injection of shh RNA causes an increase in the number of LFP, but not MFP cells. Embryos mutant for iguana, detour, and umleitung share the lack of LFP cells with you-type mutants while somite patterning is not severely affected. In mutants that fail to develop a notochord, MFP cells may be present, but are always surrounded by LFP cells. These data indicate that shh, expressed in the notochord and/or the MFP cells, induces the formation of LFP cells. In embryos doubly mutant for cyclops (cyc) and sonic you (syu) both LFP and MFP cells are deleted. The number of primary motor neurons is strongly reduced in cyc;syu double mutants, while almost normal in single mutants, suggesting that the two different pathways have overlapping functions in the induction of primary motor neurons.     

Paraxial mesoderm specifies zebrafish primary motoneuron subtype identity

We provide the first analysis of how a segmentally reiterated pattern of neurons is specified along the anteroposterior axis of the vertebrate spinal cord by investigating how zebrafish primary motoneurons are patterned. Two identified primary motoneuron subtypes, MiP and CaP, occupy distinct locations within the ventral neural tube relative to overlying somites, express different genes and innervate different muscle territories. In all vertebrates examined so far, paraxial mesoderm-derived signals specify distinct motoneuron subpopulations in specific anteroposterior regions of the spinal cord. We show that signals from paraxial mesoderm also control the much finer-grained segmental patterning of zebrafish primary motoneurons. We examined primary motoneuron specification in several zebrafish mutants that have distinct effects on paraxial mesoderm development. Our findings suggest that in the absence of signals from paraxial mesoderm, primary motoneurons have a hybrid identity with respect to gene expression, and that under these conditions the CaP axon trajectory may be dominant.     

The Cdc14B phosphatase contributes to ciliogenesis in zebrafish

Progression through the cell cycle relies on oscillation of cyclin-dependent kinase (Cdk) activity. One mechanism for downregulating Cdk signaling is to activate opposing phosphatases. The Cdc14 family of phosphatases counteracts Cdk1 phosphorylation in diverse organisms to allow proper exit from mitosis and cytokinesis. However, the role of the vertebrate CDC14 phosphatases, CDC14A and CDC14B, in re-setting the cell for interphase remains unclear. To understand Cdc14 function in vertebrates, we cloned the zebrafish cdc14b gene and used antisense morpholino oligonucleotides and an insertional mutation to inhibit its function during early development. Loss of Cdc14B function led to an array of phenotypes, including hydrocephaly, curved body, kidney cysts and left-right asymmetry defects, reminiscent of zebrafish mutants with defective cilia. Indeed, we report that motile and primary cilia were shorter in cdc14b-deficient embryos. We also demonstrate that Cdc14B function in ciliogenesis requires its phosphatase activity and can be dissociated from its function in cell cycle control. Finally, we propose that Cdc14B plays a role in the regulation of cilia length in a pathway independent of fibroblast growth factor (FGF). This first study of a loss of function of a Cdc14 family member in a vertebrate organism reveals a new role for Cdc14B in ciliogenesis and consequently in a number of developmental processes.     

Hedgehog signaling is required for differentiation of endocardial progenitors in zebrafish

Endocardial cells form the inner endothelial layer of the heart tube, surrounded by the myocardium. Signaling pathways that regulate endocardial cell specification and differentiation are largely unknown and the origin of endocardial progenitors is still being debated. To study pathways that regulate endocardial differentiation in a zebrafish model system, we isolated zebrafish NFATc1 homolog which is expressed in endocardial but not vascular endothelial cells. We further demonstrate that Hedgehog (Hh) but not VegfA or Notch signaling is required for early endocardial morphogenesis. Pharmacological inhibition of Hh signaling with cyclopamine treatment resulted in nearly complete loss of the endocardial marker expression. Simultaneous knockdown of the two zebrafish sonic hedgehog homologs, shh and twhh or Hh co-receptor smoothened (smo) resulted in similar defects in endocardial morphogenesis. Inhibition of Hh signaling resulted in the loss of fibronectin (fn1) expression in the presumptive endocardial progenitors as early as the 10-somite stage which suggests that Hh signaling is required for the earliest stages of endocardial specification. We further show that the endoderm plays a critical role in migration but not specification or differentiation of the endocardial progenitors while notochord-derived Hh is a likely source for the specification and differentiation signal. Mosaic analysis using cell transplantation shows that Smo function is required cell-autonomously within endocardial progenitor cells. Our results argue that Hh provides a critical signal to induce the specification and differentiation of endocardial progenitors.     

The evolution of the hedgehog gene family in chordates: insights from amphioxus hedgehog

 The hedgehog family of intercellular signalling molecules have essential functions in patterning both Drosophila and vertebrate embryos. Drosophila has a single hedgehog gene, while vertebrates have evolved at least three types of hedgehog genes (the Sonic, Desert and Indian types) by duplication and divergence of a single ancestral gene. Vertebrate Sonic-type genes typically show conserved expression in the notochord and floor plate, while Desert- and Indian-type genes have different patterns of expression in vertebrates from different classes. To determine the ancestral role of hedgehog in vertebrates, I have characterised the hedgehog gene family in amphioxus. Amphioxus is the closest living relative of the vertebrates and develops a similar body plan, including a dorsal neural tube and notochord. A single amphioxus hedgehog gene, AmphiHh, was identified and is probably the only hedgehog family member in amphioxus, showing the duplication of hedgehog genes to be specific to the vertebrate lineage. AmphiHh expression was detected in the notochord and ventral neural tube, tissues that express Sonic-type genes in vertebrates. This shows that amphioxus probably patterns its ventral neural tube using a molecular pathway conserved with vertebrates. AmphiHh was also expressed on the left side of the pharyngeal endoderm, reminiscent of the left-sided expression of Sonic hedgehog in chick embryos which forms part of a pathway controlling left/right asymmetric development. These data show that notochord, floor plate and possibly left/right asymmetric expression are ancestral sites of hedgehog expression in vertebrates and amphioxus. In vertebrates, all these features have been retained by Sonic-type genes. This may have freed Desert-type and Indian-type hedgehog genes from selective constraint, allowing them to diverge and take on new roles in different vertebrate taxa. Received: 20 July 1998 / Accepted: 23 September 1998     

Methoprene Photolytic Compounds Disrupt Zebrafish Development,Producing Phenocopies of Mutants in the Sonic Hedgehog Signaling Pathway

Environmental chemicals have been proposed to impact endocrine or retinoid pathways, causing developmental abnormalities in humans and other vertebrates. Presented evidence shows that exposure of zebrafish embryos to sunlight-induced photolytic products of the pesticide methoprene results in developmental defects in the head, heart, pectoral fins, and somites, and in spinal motor and optic nerve axons. Exposed embryos are phenocopies of zebrafish you-type mutants and, as in the mutant sonic-you, show underexpression of the signaling protein sonic hedgehog. Reduced expression of sonic hedgehog is also displayed in embryos treated with the retinoic acid synthesis inhibitor citral. This study identifies citral-related compounds as embryonic signaling disruptors of potential environmental concern.     

Function for Hedgehog genes in zebrafish retinal development

The hedgehog (hh) genes encode secreted signaling proteins that have important developmental functions in vertebrates and invertebrates. In Drosophila, expression of hh coordinates retinal development by propagating a wave of photoreceptor differentiation across the eye primordium. Here we report that two vertebrate hh genes, sonic hedgehog (shh) and tiggy-winkle hedgehog (twhh), may perform similar functions in the developing zebrafish. Both shh and twhh are expressed in the embryonic zebrafish retinal pigmented epithelium (RPE), initially in a discrete ventral patch which then expands outward in advance of an expanding wave of photoreceptor recruitment in the subjacent neural retina. A gene encoding a receptor for the hedgehog protein, ptc-2, is expressed by retinal neuroepithelial cells. Injection of a cocktail of antisense (alphashh/alphatwhh) oligonucleotides reduces expression of both hh genes in the RPE and slows or arrests the progression of rod and cone photoreceptor differentiation. Zebrafish strains known to have mutations in Hh signaling pathway genes similarly exhibit retardation of photoreceptor differentiation. We propose that hedgehog genes may play a role in propagating photoreceptor differentiation across the developing eye of the zebrafish.     

Hedgehog signaling is required for primary motoneuron induction in zebrafish

Sonic hedgehog (Shh) is crucial for motoneuron development in chick and mouse. However, zebrafish embryos homozygous for a deletion of the shh locus have normal numbers of motoneurons, raising the possibility that zebrafish motoneurons may be specified differently. Unlike other vertebrates, zebrafish express three hh genes in the embryonic midline: shh, echidna hedgehog (ehh) and tiggywinkle hedgehog (twhh). Therefore, it is possible that Twhh and Ehh are sufficient for motoneuron formation in the absence of Shh. To test this hypothesis we have eliminated, or severely reduced, all three Hh signals using mutations that directly or indirectly reduce Hh signaling and antisense morpholinos. Our analysis shows that Hh signals are required for zebrafish motoneuron induction. However, each of the three zebrafish Hhs is individually dispensable for motoneuron development because the other two can compensate for its loss. Our results also suggest that Twhh and Shh are more important for motoneuron development than Ehh.     

Discontinuous organization and specification of the lateral floor plate in zebrafish

The floor plate is a signaling center in the ventral neural tube of vertebrates with important functions during neural patterning and axon guidance. It is composed of a centrally located medial floor plate (MFP) and a bilaterally positioned lateral floor plate (LFP). While the role of the MFP as source of signaling molecules like, e.g., Sonic Hedgehog (Shh) is well understood, the exact organization and function of the LFP are currently unclear. Based on expression analyses, the one cell wide LFP in zebrafish has been postulated to be a homogenous structure. We instead show that the zebrafish trunk LFP is discontinuously arranged. Single LFP cells alternate with p3 neuronal precursor cells, which develop V3 interneurons along the anteroposterior (AP) axis. Our mutant analyses indicate that both, formation of LFP and p3 cells require Delta-Notch signaling. Importantly, however, the two cell types are differentially regulated by Hedgehog (HH) and Nkx2.2 activities. This implicates a novel mechanism of neural tube patterning, in which distinct cell populations within one domain of the ventral neural tube are differently specified along the AP axis. We conclude that different levels of HH and Nkx2.2 activities are responsible for the alternating appearance of LFP and p3 neuronal progenitor cells in the zebrafish ventral neural tube.