Expression of zebrafish nk2.2 is influenced by sonic hedgehog/vertebrate hedgehog-1 and demarcates a zone of neuronal differentiation in the embryonic forebrain

A small population of anterior cells patterns the forebrain during zebrafish gastrulation

During gastrulation in vertebrates, dorsal ectoderm is induced to form neural tissue that later gives rise to the brain and spinal cord. This induction depends on signals arising from a group of cells on the dorsal side of the gastrula. This group of cells constitutes the organizer. It is thought that the organizer initially induces neural tissue with anterior, or forebrain, character, and that other signals subsequently posteriorize neural tissue in the trunk. Here we show that development of the anterior central nervous system of the zebrafish embryo also depends on a small group of ectodermal cells located in the prospective head region. Removal of these ectodermal cells during gastrulation perturbs subsequent neural patterning and results in widespread cell death. Transplantation of these cells shows that they can induce forebrain-specific gene expression in more posterior regions of the neural plate. Our results indicate that an early step in neural patterning is the establishment of a small population of signalling cells within the most anterior region of the embryo. These cells are required for patterning and survival of the anterior brain.

Patterning of the embryonic forebrain

The vertebrate forebrain is derived from the anterior neural plate, where anteroposterior, dorsoventral and local patterning mechanisms specify regional identify. The recent identification of genetic regulators of these processes has opened the way to elucidating how the major forebrain regions (i.e. cerebral cortex, basal ganglia, thalamus, and hypothalamus) are formed, and how molecular lesions in these processes cause human birth defects.

Mediolateral patterning of somites: multiple axial signals, including Sonic hedgehog, regulate Nkx-3.1 expression

The axial structures, the notochord and the neural tube, play an essential role in the dorsoventral patterning of somites and in the differentiation of their many cell lineages. Here, we investigated the role of the axial structures in the mediolateral patterning of the somite by using a newly identified murine homeobox gene, Nkx-3.1, as a medial somitic marker in explant in vitro assays. Nkx-3.1 is dynamically expressed during somitogenesis only in the youngest, most newly-formed somites at the caudal end of the embryo. We found that the expression of Nkx-3.1 in pre-somitic tissue explants is induced by the notochord and maintained in newly-differentiated somites by the notochord and both ventral and dorsal parts of the neural tube. We showed that Sonic hedgehog (Shh) is one of the signaling molecules that can reproduce the effect of the axial structures by exposing explants to either COS cells transfected with a Shh expression construct or to recombinant SHH. Shh could induce and maintain Nkx-3.1 expression in pre-somitic mesoderm and young somites but not in more mature, differentiated ones. The effects of Shh on Nkr-3.1 expression were antagonized by a forskolin-induced increase in the activity of cyclic AMP-dependent protein kinase A. Additionally, we confirmed that the expression of the earliest expressed murine myogenic marker, myf 5, is also regulated by the axial structures but that Shh by itself is not capable of inducing or maintaining it. We suggest that the establishment of somitic medial and lateral compartments and the early events in myogenesis are governed by a combination of positive and inhibitory signals derived from the neighboring structures, as has previously been proposed for the dorsoventral patterning of somites.

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.

Notochord to endoderm signaling is required for pancreas development

The role of the notochord in inducing and patterning adjacent neural and mesodermal tissues is well established. We provide evidence that the notochord is also required for one of the earliest known steps in the development of the pancreas, an endodermally derived organ. At a developmental stage in chick embryos when the notochord touches the endoderm, removal of notochord eliminates subsequent expression of several markers of dorsal pancreas bud development, including insulin, glucagon and carboxypeptidase A. Pancreatic gene expression can be initiated and maintained in prepancreatic chick endoderm grown in vitro with notochord. Non-pancreatic endoderm, however, does not express pancreatic genes when recombined with the same notochord. The results suggest that the notochord provides a permissive signal to endoderm to specify pancreatic fate in a stepwise manner.

Patterning of the chick forebrain anlage by the prechordal plate

We analysed the role of the prechordal plate in forebrain development of chick embryos in vivo. After transplantation to uncommitted ectoderm a prechordal plate induces an ectopic, dorsoventrally patterned, forebrain-like vesicle. Grafting laterally under the anterior neural plate causes ventralization of the lateral side of the forebrain, as indicated by a second expression domain of the homeobox gene NKX2.1. Such a lateral ventralization cannot be induced by the secreted factor Sonic Hedgehog alone, as this is only able to distort the ventral forebrain medially. Removal of the prechordal plate does not reduce the rostrocaudal extent of the anterior neural tube, but leads to significant narrowing and cyclopia. Excision of the head process results in the caudal expansion of the NKX2.1 expression in the ventral part of the anterior neural tube, while PAX6 expression in the dorsal part remains unchanged. We suggest that there are three essential steps in early forebrain patterning, which culminate in the ventralization of the forebrain. First, anterior neuralization occurs at the primitive streak stage, when BMP-4-antagonizing factors emanate from the node and spread in a planar fashion to induce anterior neural ectoderm. Second, the anterior translocation of organizer-derived cells shifts the source of neuralizing factors anteriorly, where the relative concentration of BMP-4-antagonists is thus elevated, and the medial part of the prospective forebrain becomes competent to respond to ventralizing factors. Third, the forebrain anlage is ventralized by signals including Sonic Hedgehog, thereby creating a new identity, the prospective hypothalamus, which splits the eye anlage into two lateral domains.

Inhibitory effects of ventral signals on the development of Brn-3.0-expressing neurons in the dorsal spinal cord

Brn-3.0, a POU-domain transcription factor, is expressed in specific postmitotic neurons in the dorsal part of the neural tube which are among the first spinal cord neurons to appear in development. In the mature spinal cord, the Brn-3.0 cells form a numerous population of scattered neurons in the intermediate spinal gray. Ablation of the notochord in chick embryos extends the domain of Brn-3.0 expression into the ventral neural tube, while ectopic grafts of notochord tissue suppress Brn-3.0 expression. The notochord effects on Brn-3.0 expression are reproduced in vivo by the implantation of a local source of recombinant Shh protein. The down-regulation of Brn-3.0 expression in the dorsal spinal cord by the notochord and Shh contrasts with the known inductive effects of these ventral signals on the approximately simultaneous development of the spinal motor neurons. In cultured explants of neural plate from the region of the presumptive spinal cord, Brn-3.0 neurons develop in the absence of surface ectoderm and ventral midline tissue, suggesting that the Brn-3.0 phenotype may represent a "default" developmental pathway for early spinal cord neurons. Together these results advance the understanding of the mechanism of the generation of neuronal diversity in the developing vertebrate CNS.

Expression patterns of Brx1 (Rieg gene), Sonic hedgehog, Nkx2.2, Dlx1 and Arx during zona limitans intrathalamica and embryonic ventral lateral geniculate nuclear formation

The Brx1 homeobox gene has been isolated and shown to be expressed in the zona limitans intrathalamica (ZLI) of the mouse embryo. Brx1 is a member of the Brx gene family and comprises the genes for Brx1a and Brx1b, which differ in the sequence in the region located on the 5'-terminal side of the homeobox. The complete amino acid sequences of the open reading frame of Brx1a and Brx1b were determined and each was found to be similar to that of Rgs, the mouse homologue of the Rieger syndrome associated human RIEG gene (RGS), to the extent that the sequence of Rgs has been clarified. Brx1 was strongly expressed in the mammillary area as well as in the ZLI of the mouse embryonic brain. Homologues of Brx1a and Brx1b were isolated in chick in which the expression of Brx1 in the ventral diencephalon was well conserved. The expression of Brx1 along with that of Sonic hedgehog (Shh), Nkx2.2, Dlx1 and Arx was examined at the time of the formation of ZLI in mouse embryos. The expression of Shh was initially noted in the ventricular zone of the presumptive ZLI and was then replaced by that of Brx1 at the time of radial migration of the neuroepithelial cells. Nkx2.2 was widely expressed in the ventricular zone of presumptive ZLI and also as a narrow band in the mantle zone. The expression of Dlx1 and Arx in the presumptive ventral thalamus extended as far as ZLI and overlapped with that of Brx1. The Dlx1- and Arx-expressing cells in ZLI, which extended towards the lateral (pial) surface of the diencephalic wall, differed from those expressing Nkx2.2 and Brx1. The embryonic ventral lateral geniculate nucleus present in the visual pathway was eventually formed from these cells. Each homeobox gene was also expressed regionally in the nucleus, suggesting that the nucleus is comprised of subdivisions.

Medaka spalt acts as a target gene of hedgehog signaling

In vertebrates, pattern formation in the eye, central nervous system, somites, and limb depends on hedgehog activity, but a general target gene controlled by hedgehog in all these signaling centers has remained largely elusive. The medaka fish gene spalt encodes a zinc-finger transcription factor, which is expressed in all known hedgehog signaling centers of the embryo and in the organizer region at the midbrain-hindbrain boundary. We show that the spalt expression domains expand in response to ectopic hedgehog activity and narrow in the presence of protein kinase A activity, an antagonist of hedgehog signaling, indicating that spalt is a hedgehog target gene. Our results also suggest a signaling mechanism for anterior-posterior patterning of the vertebrate brain that controls spalt expression at the midbrain-hindbrain boundary in a protein kinase A dependent manner likely to involve an unknown member of the hedgehog family.

Chicken Nkx-2.8: a novel homeobox gene expressed in early heart progenitor cells and pharyngeal pouch-2 and -3 endoderm

Members of the NK family of homeobox transcription factors regulate critical steps of organogenesis during vertebrate development. In the studies described in this report, we have isolated and functionally characterized the chicken Nkx-2.8 (cNkx-2.8) cDNA and protein and defined the temporal and spatial pattern of cNkx-2.8 gene expression during chicken development. cNkx-2.8 transcripts are first detectable at HH stage 7 in the splanchnopleura. At stage 10(+), the cNkx-2.8 gene is expressed in the linear heart tube and the dorsal half of the vitelline vein. However, after looping, HH stage 13, cNkx-2.8 is no longer expressed in the looped heart tube, but is expressed in the ventral pharyngeal endoderm. At stage 15, in addition to the pharyngeal expression pattern, cNkx-2.8 is expressed in the ectoderm of the pharyngeal arches and the aortic sac. By HH Stage 17, cNkx-2.8 expression is detectable in lateral endoderm of the second and third pharyngeal pouches, the posterior portion of the aortic sac, and the sinus venosus. cNkx-2.8 binds to previously characterized Nkx2-1 and Nkx2-5 DNA-binding sites and overexpression of cNkx-2.8 transactivates a minimal promoter which contains multimerized Nkx-2 DNA-binding sites. In addition, cNkx-2.8 and serum response factor can coactivate a minimal cardiac alpha-actin promoter. These data are consistent with a model in which cNkx-2.8 performs a unique temporally and spatially restricted function in the developing embryonic heart and pharyngeal region. Moreover, the coexpression of cNkx-2.5 and -2.8 raises the possibility that cNkx-2. 8 may have a redundant role with cNkx-2.5 in the coalescing heart tube and may play an important role in the transcriptional program(s) that underlies thymus formation. The existence of multiple NK-2 family members and their partially overlapping patterns of expression are discussed within the framework of a "Nkx code."

Cooperation of BMP7 and SHH in the induction of forebrain ventral midline cells by prechordal mesoderm

Ventral midline cells at different rostrocaudal levels of the central nervous system exhibit distinct properties but share the ability to pattern the dorsoventral axis of the neural tube. We show here that ventral midline cells acquire distinct identities in response to the different signaling activities of underlying mesoderm. Signals from prechordal mesoderm control the differentiation of rostral diencephalic ventral midline cells, whereas notochord induces floor plate cells caudally. Sonic hedgehog (SHH) is expressed throughout axial mesoderm and is required for the induction of both rostral diencephalic ventral midline cells and floor plate. However, prechordal mesoderm also expresses BMP7 whose function is required coordinately with SHH to induce rostral diencephalic ventral midline cells. BMP7 acts directly on neural cells, modifying their response to SHH so that they differentiate into rostral diencephalic ventral midline cells rather than floor plate cells. Our results suggest a model whereby axial mesoderm both induces the differentiation of overlying neural cells and controls the rostrocaudal character of the ventral midline of the neural tube.

Genetic interactions in zebrafish midline development

Mutational analyses have shown that the genes no tail (ntl, Brachyury homolog), floating head (flh, a Not homeobox gene), and cyclops (cyc) play direct and essential roles in the development of midline structures in the zebrafish. In both ntl and flh mutants a notochord does not develop, and in cyc mutants the floor plate is nearly entirely missing. We made double mutants to learn how these genes might interact. Midline development is disrupted to a greater extent in cyc;flh double mutants than in either cyc or flh single mutants; their effects appear additive. Both the notochord and floor plate are completely lacking, and other phenotypic disturbances suggest that midline signaling functions are severely reduced. On the other hand, trunk midline defects in flh;ntl double mutants are not additive, but are most often similar to those in ntl single mutants. This finding reveals that loss of ntl function can suppress phenotypic defects due to mutation at flh, and we interpret it to mean that the wild-type allele of ntl (ntl+) functions upstream to flh in a regulatory hierarchy. Loss of function of ntl also strongly suppresses the floor plate deficiency in cyc mutants, for we found trunk floor plate to be present in cyc;ntl double mutants. From these findings we propose that ntl+ plays an early role in cell fate choice at the dorsal midline, mediated by the Ntl protein acting to antagonize floor plate development as well as to promote notochord development.

Pax6 controls progenitor cell identity and neuronal fate in response to graded Shh signaling

Distinct classes of motor neurons and ventral interneurons are generated by the graded signaling activity of Sonic hedgehog (Shh). Shh controls neuronal fate by establishing different progenitor cell populations in the ventral neural tube that are defined by the expression of Pax6 and Nkx2.2. Pax6 establishes distinct ventral progenitor cell populations and controls the identity of motor neurons and ventral interneurons, mediating graded Shh signaling in the ventral spinal cord and hindbrain.

Expression of a novel aristaless related homeobox gene 'Arx' in the vertebrate telencephalon, diencephalon and floor plate

We have isolated a novel homeobox gene that is expressed in the vertebrate central nervous system and which shows striking similarity to the Drosophila al gene in the homeodomain (85% identity) and in a 17 amino acid-sequence near the carboxyl-terminus. This gene was designated Arx (aristaless related homeobox gene) in consideration of its structural similarity to the al gene. Arx was highly conserved between mouse and zebrafish. Neuromeric expression in the forebrain and longitudinal expression in the floor plate were observed in mouse and zebrafish. The expression of Arx in the ganglionic eminence and ventral thalamus overlapped regionally with that of Dlx1, but the cell layer where Arx is expressed differed from that of the Dlx1. This gene was also found to be expressed in the dorsal telencephalon (presumptive cerebral cortex) of mouse embryos. The structure and expression pattern of Arx with respect to any possible relationship to al and Dlx1, as well as the function of Arx in the floor plate are discussed.

Gli1 is a target of Sonic hedgehog that induces ventral neural tube development

The vertebrate zinc finger genes of the Gli family are homologs of the Drosophila gene cubitus interruptus. In frog embryos, Gli1 is expressed transiently in the prospective floor plate during gastrulation and in cells lateral to the midline during late gastrula and neurula stages. In contrast, Gli2 and Gli3 are absent from the neural plate midline with Gli2 expressed widely and Gli3 in a graded fashion with highest levels in lateral regions. In mouse embryos, the three Gli genes show a similar pattern of expression in the neural tube but are coexpressed throughout the early neural plate. Because Gli1 is the only Gli gene expressed in prospective floor plate cells of frog embryos, we have investigated a possible involvement of this gene in ventral neural tube development. Here we show that Shh signaling activates Gli1 transcription and that widespread expression of endogenous frog or human glioma Gli1, but not Gli3, in developing frog embryos results in the ectopic differentiation of floor plate cells and ventral neurons within the neural tube. Floor-plate-inducing ability is retained when cytoplasmic Gli1 proteins are forced into the nucleus or are fused to the VP16 transactivating domain. Thus, our results identify Gli1 as a midline target of Shh and suggest that it mediates the induction of floor plate cells and ventral neurons by Shh acting as a transcriptional regulator.

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

No-isthmus (Noi) is a member of the zebrafish Pax family of transcriptional regulators that is expressed in restricted domains of the developing CNS. In the developing eye and optic nerve, the Noi+ cells are primitive glial cells that line the choroid fissure and optic stalk/nerve to its junction with the optic tract. This pattern of Noi expression is retained in the adult, defining the optic nerve astroglia, which wrap the left and right nerves separately at the midline, thus forming the bodily crossed optic chiasm found in fish. In embryos carrying mutations in the noi gene, the choroid fissure fails to close, glial cells of the optic nerve fail to differentiate and optic axons exhibit abnormal trajectories exiting the eye and at the midline of the diencephalon. Optic axons select inappropriate pathways into the contralateral optic nerve, rostrally towards the anterior commissure and along the ipsilateral optic tract. Noi+ cells also border the pathway of axons in the postoptic commissure, which is located adjacent to the optic chiasm. These postoptic commissural axons are defasciculated and also exhibit pathfinding defects in noi- embryos. These results indicate that Noi is required in cells that line the pathways taken by optic and non-optic commissural axons for guidance across the midline of the diencephalon. We find that expression of two members of the Netrin family of axon guidance molecules and the signalling protein Sonic hedgehog is disturbed in noi- embryos, whereas several members of the Eph family of receptors and ligands show no obvious alterations in expression at the diencephalic midline.

Expression and regulation of a netrin homologue in the zebrafish embryo

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

Regionalization in the mammalian telencephalon

Regionalization in the telencephalon results in the formation of functionally and anatomically distinct territories. Cell fate analysis and gene expression studies suggest these subdivisions arise relatively late in development compared with the spinal cord or hindbrain. The mechanisms underlying the commitment of telencephalic cells to specific regional identities have been examined through recent transplantation experiments.

Genetics of neural development in zebrafish

Large-scale mutant screens in zebrafish have led to the identification of more than 50 genes affecting various aspects of neural development and function, including neural induction, anteroposterior and dorsoventral regionalization, axon pathfinding, neuronal differentiation and survival, and behavior. Phenotypic analysis of mutants for some of these genes has already uncovered important genetic and cellular interactions during development. Ongoing molecular analyses promise to further elucidate the mechanisms underlying neural development in vertebrates.

The one-eyed pinhead gene functions in mesoderm and endoderm formation in zebrafish and interacts with no tail

The zebrafish locus one-eyed pinhead (oep) is essential for the formation of anterior axial mesoderm, endoderm and ventral neuroectoderm. At the beginning of gastrulation anterior axial mesoderm cells form the prechordal plate and express goosecoid (gsc) in wild-type embryos. In oep mutants the prechordal plate does not form and gsc expression is not maintained. Exposure to lithium, a dorsalizing agent, leads to the ectopic induction and maintenance of gsc expression in wild-type embryos. Lithium treatment of oep mutants still leads to ectopic gsc induction but not maintenance, suggesting that oep acts downstream of inducers of dorsal mesoderm. In genetic mosaics, wild-type cells are capable of forming anterior axial mesoderm in oep embryos, suggesting that oep is required in prospective anterior axial mesoderm cells before gastrulation. The oep gene is also essential for endoderm formation and the early development of ventral neuroectoderm, including the floor plate. The loss of endoderm is already manifest during gastrulation by the absence of axial-expressing cells in the hypoblast of oep mutants. These findings suggest that oep is also required in lateral and ventral regions of the gastrula margin. The sonic hedgehog (shh).gene is expressed in the notochord of oep animals. Therefore, the impaired floor plate development in oep mutants is not caused by the absence of the floor plate inducer shh. This suggests that oep is required downstream or in parallel to shh signaling. The ventral region of the forebrain is also absent in oep mutants, leading to severe cyclopia. In contrast, anterior-posterior brain patterning appears largely unaffected, suggesting that underlying prechordal plate is not required for anterior-posterior pattern formation but might be involved in dorsoventral brain patterning. To test if oep has a wider, partially redundant role, we constructed double mutants with two other zebrafish loci essential for patterning during gastrulation. Double mutants with floating head, the zebrafish Xnot homologue, display enhanced floor plate and adaxial muscle phenotypes. Double mutants with no tail (ntl), the zebrafish homologue of the mouse Brachyury locus, display severe defects in midline and mesoderm formation including absence of most of the somitic mesoderm. These results reveal a redundant function of oep and ntl in mesoderm formation. Our data suggest that both oep and ntl act in the blastoderm margin to specify mesendodermal cell fates.

Dynamo, a new zebrafish DVR member of the TGF-beta superfamily is expressed in the posterior neural tube and is up-regulated by Sonic hedgehog

Dynamo, a new zebrafish DVR detected from late gastrula on in the posterior neural plate, becomes restricted to the ventral region of the trunk neural tube, with the exclusion of floor plate and adjacent cells. Analysis of dynamo expression in zebrafish axial mutants indicated that dynamo expression in the ventral region of the central nervous system (CNS) is induced by axial mesoderm and maintained by notochord, but is independent of a differentiated floor plate. Ectopic Sonic hedgehog (shh) expression can up-regulate dynamo expression in the posterior neural tube providing evidence that cells of the posterior neural tube are competent to respond to shh signalling and that the close relationship between DVR members and hedgehog-related genes might also apply to vertebrate CNS development.

Frontal eye circuitry, rostral sensory pathways and brain organization in amphioxus larvae: evidence from 3D reconstructions

The cells comprising the frontal eye of a 12.5 day amphioxus larva are described based on 3D reconstructions from serial electron micrographs, along with the fibre tracts and more caudal groupings of cells in the nerve cord to which the frontal eye appears to be linked. The frontal eye consists of a pigment cup, two transverse rows of receptor cells, and clusters of neurons whose close association with the medial receptor cells suggests they may function as an integral part of the eye complex. Neurites from both the receptor cells and neurons supply the ventrolateral nerve tracts, which consist mainly of axons arising from sensory cells located at the rostral tip of the larva. A core group of 3—4 rostral fibres on each side innervate two ventral giant cells located just behind the cerebral vesicle in the primary motor centre (PMC). The circuitry suggests these cells may be responsible for triggering the larval startle response. The ventrolateral tracts also include two types of axial dendrite-like fibres: (i) a single unpaired fibre, a forward continuation of the principal dendrite of the left giant cell, which is the main target for synapses from neurons in the frontal eye; and (ii) sets of paired fibres from cells in the tectum, a dorsal cortex-like structure located at the back of the cerebral vesicle through which the dorsal sensory nerves pass in transit to the PMC. Recent behavioural studies show that larvae feed in a hovering posture that maximally shades the frontal eye. They also orient to light in this position. The shape and orientation of the frontal eye suggests it could be responsible for this response. The existence of separate pathways from lateral and medial receptor cells, both directly and indirectly to the PMC, suggests the frontal eye may also be involved in modulating locomotory behaviour during hovering. The visual ‘system’ described here for amphioxus larvae is more like that of vertebrates than has previously been recognized. Specifically: (i)the medial nerve cells of the frontal eye appear to form local circuits with relay and integrative functions similar to those of the retina, involving cell types that resemble specific retinal interneurons; and (ii) output is directed to a region at the back of the posterior c.v. that resembles the vertebrate midbrain, and which may be its homologue. This region has a dorsal tectum and, like the midbrain, includes the anterior part of a ventral zone of motoneurons and reticulospinal interneurons. The morphological evidence supports the idea that the ‘brain’ of amphioxus is sufficiently like that of vertebrates to provide important clues concerning the basic organization and subdivision of the vertebrate brain.

Conservation of BMP signaling in zebrafish mesoderm patterning

To investigate the conservation of mechanisms for mesodermal patterning between zebrafish and Xenopus, we isolated two cDNA clones encoding bone morphogenetic protein (BMP)-related proteins from a zebrafish cDNA library. Based on their predicted amino acid sequences, these two clones were designated as zbmp-2 and zbmp-4. Whole-mount in situ hybridization analysis revealed that in gastrula embryo, both genes were localized in the ventral part of the embryo, consistent with the proposed function of Xenopus BMP-4 in ventral mesoderm specification. zbmp-4 expression, however, was also seen in the embryonic shield, the most dorsal mesodermal structure. To examine the ability of zbmp-2 to ventralize mesoderm, we injected synthetic mRNA into zebrafish embryos and found that overexpression of this gene eliminated dorsal structures including notochord at both morphological and molecular level. In contrast, expression of ventral marker gene eve1 was expanded to the dorsal side. These effects are analogous to the ventralization of embryos caused by ectopic xBMP-4 expression. Taken together, one may conclude that the developmental mechanisms for mesodermal patterning regulated by BMPs are evolutionarily conserved between amphibians and teleosts.

Patterning and specification of the cerebral cortex

Regionalization of the cerebral cortex occurs during development by the formation of anatomically and functionally discrete areas of the brain. Descriptive evidence based on expression of molecules and structural features suggests that an early parcelation of the cerebral wall may occur during fetal development. Experimental strategies using tissue transplants and cell culture models have explored the nature of the timing of areal specification. New signaling systems displaying the sensitivity of precursor cells to environmental cues that define the fate of neurons destined for specific areas of the cortex have been discovered. Studies in the field now suggest mechanisms of regulating cell phenotype in the cortex that are common to all parts of the neuraxis.

floating head and masterblind regulate neuronal patterning in the roof of the forebrain

The epiphysial region of the dorsal diencephalon is the first site at which neurogenesis occurs in the roof of the zebrafish forebrain. We show that the homeobox containing gene floating head (flh) is required for neurogenesis to proceed in the epiphysis. In flh- embryos, the first few epiphysial neurons are generated, but beyond the 18 somite stage, neuronal production ceases. In contrast, in masterblind- (mbl-) embryos, epiphysial neurons are generated throughout the dorsal forebrain. We show that mbl is required to prevent the expression of flh in dorsal forebrain cells rostral to the epiphysis. Furthermore, epiphysial neurons are not ectopically induced in mbl-/flh- embryos, demonstrating that the epiphysial phenotype of mbl- embryos is mediated by ectopic Flh activity. We propose a role for Flh in linking the signaling pathways that regulate regional patterning to the signaling pathways that regulate neurogenesis.

Spatial regulation of a zebrafish patched homologue reflects the roles of sonic hedgehog and protein kinase A in neural tube and somite patterning

Signalling by members of the Hedgehog family of secreted proteins plays a central role in the development of vertebrate and invertebrate embryos. In Drosophila, transduction of the Hedgehog signal is intimately associated with the activity of protein kinase A and the product of the segment polarity gene patched. We have cloned a homologue of patched from the zebrafish Danio rerio and analysed the spatiotemporal regulation of its transcription during embryonic development in both wild-type and mutant animals. We find a striking correlation between the accumulation of patched1 transcripts and cells responding to sonic hedgehog activity both in the neurectoderm and mesoderm, suggesting that like its Drosophila counterpart, patched1 is regulated by sonic hedgehog activity. Consistent with this interpretation, mis-expression of sonic hedgehog results in ectopic activation of patched1 transcription. Using dominant negative and constitutively active forms of the protein kinase A subunits, we also show that expression of patched1 as well as of other sonic hedgehog targets, is regulated by protein kinase A activity. Taken together, our findings suggest that the mechanism of signalling by Hedgehog family proteins has been highly conserved during evolution.

Roles of cell-autonomous mechanisms for differential expression of region-specific transcription factors in neuroepithelial cells

Although a number of genes have been found to have restricted expression domains in the embryonic forebrain and midbrain, it remains largely unknown how the expression of these genes is regulated at the cellular level. In this study, we explored the mechanisms for the differential expression of region-specific transcription factors in neuroepithelial cells by using both primary and immortalized neuroepithelial cells from the rat brain at embryonic day 11.5. We found that differential expression patterns of Pax-3, Pax-5, Pax-6, Dlx-1, Dlx-2, Emx2, Otx1 and Dbx observed in vivo were maintained even when the cells were isolated and cultured in vitro, free from environmental influences. Furthermore, in response to Sonic hedgehog, which is a major inductive signal from the environment for regional specification, neuroepithelial cells that maintain distinct regional identities expressed different sets of ventral-specific genes including Islet-1, Nkx-2.1 and Nkx-2.2. These results suggest that certain cell-autonomous mechanisms play important roles in regulating both environmental signal-dependent and -independent expression of region-specific genes. Thus, we propose that use of the in vitro culture systems we describe in this study facilitates the understanding of regulatory mechanisms of region-specific genes in neuroepithelial cells.

msh may play a conserved role in dorsoventral patterning of the neuroectoderm and mesoderm

Many of the mechanisms that govern the patterning of the Drosophila neuroectoderm and mesoderm are still unknown. Here we report the sequence, expression, and regulation of the homeobox gene msh, which is likely to play an important role in the early patterning events of these two tissue primordia. msh expression is first observed in late blastoderm embryos and occurs in longitudinal bands of cells that are fated to become lateral neuroectoderm. This expression is under the control of dorsoventral axis-determination genes and depends on dpp-mediated repression in the dorsal half of the embryo and on fib-(EGF-) mediated repression ventrally. The bands of msh expression define the cells that will form the lateral columns of proneural gene expression and give rise to the lateral row of SI neuroblasts. This suggests that msh may be one of the upstream regulators of the achaete-scute (AS-C) genes and may play a role that is analogous to that of the homeobox gene vnd/NK2 in the medial sector of the neuroectoderm. During neuroblast segregation, msh expression is maintained in a subset of neuroblasts, indicating that msh, like vnd/NK2, could function in both dorsoventral patterning of the neuroectoderm and neuroblast specification. The later phase of msh expression that occurs after the first wave of neuroblast segregation in defined ectodermal and mesodermal clusters of cells points to similar roles of msh in patterning and cell fate specification of the peripheral nervous system, dorsal musculature, and the fat body. A comparison of the expression patterns of the vertebrate homologs of msh, vnd/NK2, and AS-C genes reveals striking similarities in dorsoventral patterning of the Drosophila and vertebrate neuroectoderm and indicates that genetic circuitries in neural patterning are evolutionarily conserved.

Complex expression of the zp-50 pou gene in the embryonic zebrafish brain is altered by overexpression of sonic hedgehog

We report the characterization of the zebrafish zp-50 class III POU domain gene. This gene is first activated in the prospective diencephalon after the end of the gastrula period. During somitogenesis, zp-50 is expressed in a very dynamic and complex fashion in all major subdivisions of the central nervous system. After one day of development, zp-50 transcripts are present in the fore- and midbrain in several distinct cell clusters. In the hindbrain, zp-50 expression is found in two types of domains. Correct zp-50 expression in the ventral fore- and midbrain requires genes known to be involved in dorsoventral patterning of the zebrafish CNS. Transcripts of the sonic hedgehog (shh) gene encoding an intercellular signaling molecule are detected in the forming diencephalon shortly prior to the appearance of zp-50 mRNA. Correct expression in this region of both shh, and zp-50, requires a functional cyclops (cyc) locus: shh and zp-50 transcripts are likewise absent from the ventral rostral brain of mutant cyc−/− embryos. Injection of synthetic shh mRNA into fertilized eggs causes ectopic zp-50 expression at more dorsal positions of the embryonic brain. The close spatial and temporal coincidence of expression in the rostral brain, the similar response to the cyc- mutation, and the ectopic zp-50 expression in the injection experiments all suggest that zp-50 may directly respond to the reception of the Shh signal.

Patterning cascades in the neural tube. Neural development

The vertebrate central nervous system comprises an intricate array of neurons generated in a highly organized way. Examination of the genes expressed and required at early stages of neural differentiation reveals that a coordinated signalling cascade transforms progenitor cells into discrete neuronal subsets.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

The cDNA sequence of murine Nkx-2.2

We isolated and sequenced a 2026-bp murine Nkx-2.2 cDNA clone that contains an open reading frame encoding 273 amino acids (aa). The 273-aa protein includes a homeobox, an NK-2 box and a N-terminal decapeptide found in other Nk family members.

Pax proteins and eye development

Homologous members of the Pax gene family are required for eye development in Drosophila and vertebrates. Despite superficial similarities in the phenotypes of vertebrates with mutations in pax-6 and Drosophila eyeless mutants, it remains uncertain whether the two proteins encoded by these genes have comparable functions. The genetic cascade triggered by eyeless leads to eye formation, whereas pax-6 is not necessary for optic vesicle formation, but is required at other stages of eye development. A second vertebrate Pax gene, pax-2, is also required during eye development and appears to play a role during closure of the choroid fissure.

Tripartite signaling of pattern: interactions between Hedgehogs, BMPs and Wnts in the control of vertebrate development

A central issue in embryonic development is the resolution of how groups of equivalent cells are transformed into orderly and patterned arrays of distinct cell types. Recent studies suggest the involvement of the Hedgehog, Wnt and bone morphogenetic protein families in the patterning of different tissue types in vertebrate embryos. The integrated actions of members of these three families of signaling proteins appear to have been recruited in the patterning of neural tissue in addition to several different tissues. Over the past year, a clearer picture of the diverse roles of these signaling proteins in embryonic development has begun to emerge.

Requirement for LIM homeobox gene Isl1 in motor neuron generation reveals a motor neuron-dependent step in interneuron differentiation

Motor neuron differentiation is accompanied by the expression of a LIM homeodomain transcription factor, Islet1 (ISL1). To assess the involvement of ISL1 in the generation of motor neurons, we analyzed cell differentiation in the neural tube of embryos in which ISL1 expression has been eliminated by gene targeting. Motor neurons are not generated without ISL1, although many other aspects of cell differentiation in the neural tube occur normally. A population of interneurons that express Engrailed1 (EN1), however, also fails to differentiate in Isl1 mutant embryos. The differentiation of EN1+ interneurons can be induced in both wild-type and mutant neural tissue by regions of the neural tube that contain motor neurons. These results show that ISL1 is required for the generation of motor neurons and suggest that motor neuron generation is required for the subsequent differentiation of certain interneurons.

Engrailed, retinotectal targeting, and axonal patterning in the midbrain during Xenopus development: an antisense study

Axonal tracts in the vertebrate brain seem to respect domains of homeobox gene expression. To test the role of engrailed in tract formation in the midbrain, we inhibited its expression using antisense (AS) oligonucleotides. Phosphorothioate-modified AS oligos caused navigational errors in both the optic projection (OP) and the intertectal commissure (ITC). These oligos, however, also inhibited bFGF binding to the brain. To determine whether these tract phenotypes were due to inhibition of bFGF function or engrailed expression, we used partially phosphorothioate-modified (pp) oligos, which inhibit engrailed expression but do not affect bFGF binding. These ppAS oligos caused the ITC phenotype but had no effect on the OP. Thus, interference with bFGF function correlates with the OP phenotype, while the ITC phenotype is directly related to engrailed expression.

Midline signalling is required for Pax gene regulation and patterning of the eyes

Pax6 and Pax2 are members of the Pax family of transcription factors that are both expressed in the developing visual system of zebrafish embryos. Pax6 protein is present in all cells that form the neural retina and pigment epithelium, whereas Pax2 is located primarily in cells that will give rise to the optic stalk. In this study, we have addressed the role of midline signalling in the regulation of Pax2 and Pax6 distributions and in the subsequent morphogenesis of the eyes. Midline signalling is severely perturbed in cyclops mutant embryos resulting in an absence of ventral midline CNS tissue and fusion of the eyes. Mutant embryos ectopically express Pax6 in a bridge of tissue around the anterior pole of the neural keel in the position normally occupied by cells that form the optic stalks. In contrast, Pax2 protein is almost completely absent from this region in mutant embryos. Concommitant with the changes in Pax protein distribution, cells in the position of the optic stalks differentiate as retina. These results suggest that a signal emanating from the midline, which is absent in cyclops mutant embryos, may be required to promote Pax2 and inhibit Pax6 expression in cells destined to form the optic stalks. Sonic hedgehog (Shh also known as Vhh-1 and Hhg-1) is a midline signalling molecule that is absent from the neuroepithelium of cyclops mutant embryos at early developmental stages. To test the possibility that Shh might be able to regulate the spatial expression of Pax6 and Pax2 in the optic primordia, it was overexpressed in the developing CNS. The number of cells containing Pax2 was increased following shh overexpression and embryos developed hypertrophied optic stalk-like structures. Complimentary to the changes in Pax2 distribution, there were fewer Pax6-containing cells and pigment epithelium and neural retina were reduced. Our results suggest that Shh or a closely related signalling molecule emanating from midline tissue in the ventral forebrain either directly or indirectly induces the expression of Pax2 and inhibits the expression of Pax6 and thus may regulate the partitioning of the optic primordia into optic stalks and retinal tissue.

Sonic hedgehog induces the differentiation of ventral forebrain neurons: a common signal for ventral patterning within the neural tube

The vertebrate hedgehog-related gene Sonic hedgehog (Shh) is expressed in ventral domains along the entire rostrocaudal length of the neural tube, including the forebrain. We show here that SHH induces the differentiation of ventral neuronal cell types in explants derived from prospective forebrain regions of the neural plate. Neurons induced in explants derived from both diencephalic and telencephalic levels of the neural plate express the LIM homeodomain protein Isl-1, and these neurons possess distinct identities that match those of the ventral neurons generated in these two subdivisions of the forebrain in vivo. A single inducing molecule, SHH, therefore appears to mediate the induction of distinct ventral neuronal cell types along the entire rostrocaudal extent of the embryonic central nervous system.