D4 Dopamine receptor genes of zebrafish and effects of the antipsychotic clozapine on larval swimming behaviour

Zebrafish, a model developmental genetic organism, is being increasingly used in behavioural studies. We have initiated studies designed to evaluate the response of zebrafish to antipsychotic drugs. This study focuses on characterization of zebrafish D4 dopamine receptors (D4Rs) and the response of larval zebrafish to the atypical antipsychotic clozapine. The D4R is of interest because of its high affinity for clozapine, while interest in clozapine stems from its effectiveness in reducing symptoms in acutely psychotic, treatment-resistant schizophrenic patients. By mining the zebrafish genomic database, we identified three distinct D4R genes, drd4a, drd4b and drd4c, and generated full-length open reading frames encoding each of the three D4Rs by reverse transcription-polymerase chain reaction. Gene mapping studies showed that each D4R gene mapped to a distinct chromosomal location in the zebrafish genome, and each gene exhibited a unique expression profile during embryogenesis. When administered to larval zebrafish, clozapine produced a rapid and profound effect on locomotor activity. The effect of clozapine was dose-dependent, resulted in hypoactivity and was prevented by the D4-selective agonist ABT-724. Our data suggest that the inhibitory effect of clozapine on the locomotor activity of larval zebrafish may be mediated through D4Rs.

Calsenilin is required for endocrine pancreas development in zebrafish

Calsenilin/DREAM/Kchip3 is a neuronal calcium-binding protein. It is a multifunctional protein, mainly expressed in neural tissues and implicated in regulation of presenilin processing, repression of transcription, and modulation of A-type potassium channels. Here, we performed a search for new genes expressed during pancreatic development and have studied the spatiotemporal expression pattern and possible role of calsenilin in pancreatic development in zebrafish. We detected calsenilin transcripts in the pancreas from 21 somites to 39 hours postfertilization stages. Using double in situ hybridization, we found that the calsenilin gene was expressed in pancreatic endocrine cells. Loss-of-function experiments with anti-calsenilin morpholinos demonstrated that injected morphants have a significant decrease in the number of pancreatic endocrine cells. Furthermore, the knockdown of calsenilin leads to perturbation in islet morphogenesis, suggesting that calsenilin is required for early islet cell migration. Taken together, our results show that zebrafish calsenilin is involved in endocrine cell differentiation and morphogenesis within the pancreas.

Sequence and expression pattern of ziro7, a novel, divergent zebrafish iroquois homeobox gene

We have isolated the zebrafish ziro7 gene, a novel, divergent member of the Iroquois family. ziro7 is expressed at early epiboly stages in the dorsal half of the zebrafish embryo, with a higher level in the dorso-lateral margin. From mid-gastrulation stages onward, ziro7 is expressed in a large transversal stripe in the future neural plate, which subsequently divides into thinner stripes located in the diencephalon, midbrain and hindbrain.

A zebrafish nanos-related gene is essential for the development of primordial germ cells

Asymmetrically distributed cytoplasmic determinants collectively termed germ plasm have been shown to play an essential role in the development of primordial germ cells (PGCs). Here, we report the identification of a nanos-like (nanos1) gene, which is expressed in the germ plasm and in the PGCs of the zebrafish. We find that several mechanisms act in concert to restrict the activity of Nanos1 to the germ cells including RNA localization and control over the stability and translatability of the RNA. Reducing the level of Nanos1 in zebrafish embryos revealed an essential role for the protein in ensuring proper migration and survival of PGCs in this vertebrate model organism.

Zebrafish early macrophages colonize cephalic mesenchyme and developing brain, retina, and epidermis through a M-CSF receptor-dependent invasive process

The origin of resident (noninflammatory) macrophages in vertebrate tissues is still poorly understood. In the zebrafish embryo, we recently described a specific lineage of early macrophages that differentiate in the yolk sac before the onset of blood circulation. We now show that these early macrophages spread in the whole cephalic mesenchyme, and from there invade epithelial tissues: epidermis, retina, and brain--especially the optic tectum. In the panther mutant, which lacks a functional fms (M-CSF receptor) gene, early macrophages differentiate and behave apparently normally in the yolk sac, but then fail to invade embryonic tissues. Our video recordings then document for the first time the behavior of macrophages in the invaded tissues, revealing the striking propensity of early macrophages in epidermis and brain to wander restlessly among epithelial cells. This unexpected behavior suggests that tissue macrophages may be constantly "patrolling" for immune and possibly also developmental and trophic surveillance. At 60 h post-fertilization, all macrophages in the brain and retina undergo a specific phenotypic transformation, into "early (amoeboid) microglia": they become more highly endocytic, they down-regulate the L-plastin gene, and abruptly start expressing high levels of apolipoprotein E, a well-known neurotrophic lipid carrier.

Pattern formation in the lateral line of zebrafish

The lateral line of fish and amphibians is a sensory system that comprises a number of individual sense organs, the neuromasts, arranged in a defined pattern on the surface of the body. A conspicuous part of the system is a line of organs that extends along each flank (and which gave the system its name). At the end of zebrafish embryogenesis, this line comprises 7-8 neuromasts regularly spaced between the ear and the tip of the tail. The neuromasts are deposited by a migrating primordium that originates from the otic region. Here, we follow the development of this pattern and show that heterogeneities within the migrating primordium prefigure neuromast formation.

sprouty4 acts in vivo as a feedback-induced antagonist of FGF signaling in zebrafish

In looking for novel factors involved in the regulation of the fibroblast growth factor (FGF) signaling pathway, we have isolated a zebrafish sprouty4 gene, based on its extensive similarities with the expression patterns of both fgf8 and fgf3. Through gain- and loss-of-function experiments, we demonstrate that Fgf8 and Fgf3 act in vivo to induce the expression of Spry4, which in turn can inhibit activity of these growth factors. When overexpressed at low doses, Spry4 induces loss of cerebellum and reduction in size of the otic vesicle, thereby mimicking the fgf8/acerebellar mutant phenotype. Injections of high doses of Spry4 cause ventralization of the embryo, an opposite phenotype to the dorsalisation induced by overexpression of Fgf8 or Fgf3. Conversely we have shown that inhibition of Spry4 function through injection of antisense morpholino oligonucleotide leads to a weak dorsalization of the embryo, the phenotype expected for an upregulation of Fgf8 or Fgf3 signaling pathway. Finally, we show that Spry4 interferes with FGF signaling downstream of the FGF receptor 1 (FGFR1). In addition, our analysis reveals that signaling through FGFR1/Ras/mitogen-activated protein kinase pathway is involved, not in mesoderm induction, but in the control of the dorsoventral patterning via the regulation of bone morphogenetic protein (BMP) expression.

casanova encodes a novel Sox-related protein necessary and sufficient for early endoderm formation in zebrafish

Early endoderm formation in zebrafish requires at least three loci that function downstream of Nodal signaling but upstream of the early endodermal marker sox17: bonnie and clyde (bon), faust (fau), and casanova (cas). cas mutants show the most severe phenotype as they do not form any gut tissue and lack all sox17 expression. Activation of the Nodal signaling pathway or overexpression of Bon or Fau/Gata5 fails to restore any sox17 expression in cas mutants, demonstrating that cas plays a central role in endoderm formation. Here we show that cas encodes a novel member of the Sox family of transcription factors. Initial cas expression appears in the dorsal yolk syncytial layer (YSL) in the early blastula, and is independent of Nodal signaling. In contrast, endodermal expression of cas, which begins in the late blastula, is regulated by Nodal signaling. Cas is a potent inducer of sox17 expression in wild-type embryos as well as in bon and fau/gata5 mutants. Cas is also a potent inducer of sox17 expression in MZoep mutants, which cannot respond to Nodal signaling. In addition, ectopic expression of cas in presumptive mesodermal cells leads to their transfating into endoderm. Altogether, these data indicate that Cas is the principal transcriptional effector of Nodal signaling during zebrafish endoderm formation.

Zebrafish evx1 is dynamically expressed during embryogenesis in subsets of interneurones, posterior gut and urogenital system

The even-skipped-related homeobox genes (evx) are widely distributed through animal kingdom and are thought to play key role in posterior body patterning and neurogenesis. We have cloned and analyzed the expression of evx1 in zebrafish (see also Borday et al. (Dev. Dyn. 220 (2001) in press) which displays a dynamic and restricted expression pattern during neurogenesis. In spinal cord, rhombencephalon, and epiphysis, evx1 is expressed in several subsets of emerging interneurones prior to their axonal outgrowth, identified as primary interneurones and a subset of Pax2.1(+) commissural interneurones. In the hindbrain, evx1 is expressed in reticulospinal interneurones of rhombomeres 5 and 6 as well as in rhombomere 7 interneurones. The latest emerging evx1(+) interneurones in the hindbrain correspond to commissural interneurones. evx1 is also dynamically transcribed during the formation of the posterior gut and the uro-genital system in mesenchymal cells that border the pronephric ducts, the wall of the pronephric duct, and later in the posterior gut and the wall of the uro-genital opening. In larvae, the ano-rectal epithelium and the muscular layer that surrounds the analia-genitalia region remain stained up to 27 days. In contrast other vertebrates, evx1displays no early nor caudal expression in zebrafish.

Evolutionary origins of vertebrate appendicular muscle

The evolution of terrestrial tetrapod species heralded a transition in locomotor strategies. While most fish species use the undulating contractions of the axial musculature to generate propulsive force, tetrapods also rely on the appendicular muscles of the limbs to generate movement. Despite the fossil record generating an understanding of the way in which the appendicular skeleton has evolved to provide the scaffold for tetrapod limb musculature, there is, by contrast, almost no information as to how this musculature arose. Here we examine fin muscle formation within two extant classes of fish. We find that in the teleost, zebrafish, fin muscles arise from migratory mesenchymal precursor cells that possess molecular and morphogenetic identity with the limb muscle precursors of tetrapod species. Chondrichthyan dogfish embryos, however, use the primitive mechanism of direct epithelial somitic extensions to derive the muscles of the fin. We conclude that the genetic mechanism controlling formation of tetrapod limb muscles evolved before the Sarcopterygian radiation.

The Mdm2 gene of zebrafish (Danio rerio): preferential expression during development of neural and muscular tissues, and absence of tumor formation after overexpression of its cDNA during early embryogenesis

The Mdm2 protein is most probably the main negative cellular regulator of the p53 tumor-suppressor protein. It was found to be overexpressed in a great number of human tumors and is considered as a potential target for anti-tumor therapies. Mdm2 is an essential gene in mice, yet its role in normal development and tissue differentiation is unknown. In order to study the role of this important protein in an evolutionary perspective, we cloned an Mdm2 cDNA from the fish Danio rerio and analyzed its expression pattern as well as the phenotypic consequences of its overexpression. The main functional domains as well as the interaction between Mdm2 and p53 are conserved in zebrafish. Moreover, we show here that the gene is expressed specifically during early development in neural and muscular tissues. Surprisingly, microinjection of Mdm2 mRNA in two-cell-stage embryos led to inhibition of cellular convergence during gastrulation. The clones derived from Mdm2 microinjected blastomeres were significantly smaller than those derived from control microinjections, and, in contrast to what was observed in Xenopus, did not develop tumors. Our results suggest that Mdm2 expression may be important during the differentiation of neural and muscular tissues of zebrafish. They also point to important differences between phyla in the susceptibility to tumor formation.

An inherited functional circadian clock in zebrafish embryos

Circadian clocks are time-keeping systems found in most organisms. In zebrafish, expression of the clock gene Period3 (Per3) oscillates throughout embryogenesis in the central nervous system and the retina. Per3 rhythmic expression was free-running and was reset by light but not by the developmental delays caused by low temperature. The time of fertilization had no effect on Per3 expression. Per3 messenger RNA accumulates rhythmically in oocytes and persists in embryos. Our results establish that the circadian clock functions during early embryogenesis in zebrafish. Inheritance of maternal clock gene products suggests a mechanism of phase inheritance through ovogenesis.

Roles for Fgf signaling during zebrafish fin regeneration

Following amputation of a urodele limb or teleost fin, the formation of a blastema is a crucial step in facilitating subsequent regeneration. Using the zebrafish caudal fin regeneration model, we have examined the hypothesis that fibroblast growth factors (Fgfs) initiate blastema formation from fin mesenchyme. We find that fibroblast growth factor receptor 1 (fgfr1) is expressed in mesenchymal cells underlying the wound epidermis during blastema formation and in distal blastemal tissue during regenerative outgrowth. fgfr1 transcripts colocalize with those of msxb and msxc, putative markers for undifferentiated, proliferating cells. A zebrafish Fgf member, designated wfgf, is expressed in the regeneration epidermis during outgrowth. Furthermore, we show that a specific inhibitor of Fgfr1 applied immediately following fin amputation blocks blastema formation, without obvious effects on wound healing. This inhibitor blocks the proliferation of blastemal cells and the onset of msx gene transcription. Inhibition of Fgf signaling during ongoing fin regeneration prevents further outgrowth while downregulating the established expression of blastemal msx genes and epidermal sonic hedgehog. Our findings indicate that zebrafish fin blastema formation and regenerative outgrowth require Fgf signaling.

Anteroposterior patterning is required within segments for somite boundary formation in developing zebrafish

Somite formation involves the establishment of a segmental prepattern in the presomitic mesoderm, anteroposterior patterning of each segmental primordium and formation of boundaries between adjacent segments. How these events are co-ordinated remains uncertain. In this study, analysis of expression of zebrafish mesp-a reveals that each segment acquires anteroposterior regionalisation when located in the anterior presomitic mesoderm. Thus anteroposterior patterning is occurring after the establishment of a segmental prepattern in the paraxial mesoderm and prior to somite boundary formation. Zebrafish fss(−), bea(−), des(−) and aei(−) embryos all fail to form somites, yet we demonstrate that a segmental prepattern is established in the presomitic mesoderm of all these mutants and hox gene expression shows that overall anteroposterior patterning of the mesoderm is also normal. However, analysis of various molecular markers reveals that anteroposterior regionalisation within each segment is disturbed in the mutants. In fss(−), there is a loss of anterior segment markers, such that all segments appear posteriorized, whereas in bea(−), des(−) and aei(−), anterior and posterior markers are expressed throughout each segment. Since somite formation is disrupted in these mutants, correct anteroposterior patterning within segments may be a prerequisite for somite boundary formation. In support of this hypothesis, we show that it is possible to rescue boundary formation in fss(−) through the ectopic expression of EphA4, an anterior segment marker, in the paraxial mesoderm. These observations indicate that a key consequence of the anteroposterior regionalisation of segments may be the induction of Eph and ephrin expression at segment interfaces and that Eph/ephrin signalling subsequently contributes to the formation of somite boundaries.

Equivalent genetic roles for bmp7/snailhouse and bmp2b/swirl in dorsoventral pattern formation

A bone morphogenetic protein (BMP) signaling pathway acts in the establishment of the dorsoventral axis of the vertebrate embryo. Here we demonstrate the genetic requirement for two different Bmp ligand subclass genes for dorsoventral pattern formation of the zebrafish embryo. From the relative efficiencies observed in Bmp ligand rescue experiments, conserved chromosomal synteny, and isolation of the zebrafish bmp7 gene, we determined that the strongly dorsalized snailhouse mutant phenotype is caused by a mutation in the bmp7 gene. We show that the original snailhouse allele is a hypomorphic mutation and we identify a snailhouse/bmp7 null mutant. We demonstrate that the snailhouse/bmp7 null mutant phenotype is identical to the presumptive null mutant phenotype of the strongest dorsalized zebrafish mutant swirl/bmp2b, revealing equivalent genetic roles for these two Bmp ligands. Double mutant snailhouse/bmp7; swirl/bmp2b embryos do not exhibit additional or stronger dorsalized phenotypes, indicating that these Bmp ligands do not function redundantly in early embryonic development. Furthermore, overexpression experiments reveal that Bmp2b and Bmp7 synergize in the ventralization of wild-type embryos through a cell-autonomous mechanism, suggesting that Bmp2b/Bmp7 heterodimers may act in vivo to specify ventral cell fates in the zebrafish embryo.

The lefty-related factor Xatv acts as a feedback inhibitor of nodal signaling in mesoderm induction and L-R axis development in xenopus

In mouse, lefty genes play critical roles in the left-right (L-R) axis determination pathway. Here, we characterize the Xenopus lefty-related factor antivin (Xatv). Xatv expression is first observed in the marginal zone early during gastrulation, later becoming restricted to axial tissues. During tailbud stages, axial expression resolves to the neural tube floorplate, hypochord, and (transiently) the notochord anlage, and is joined by dynamic expression in the left lateral plate mesoderm (LPM) and left dorsal endoderm. An emerging paradigm in embryonic patterning is that secreted antagonists regulate the activity of intercellular signaling factors, thereby modulating cell fate specification. Xatv expression is rapidly induced by dorsoanterior-type mesoderm inducers such as activin or Xnr2. Xatv is not an inducer itself, but antagonizes both Xnr2 and activin. Together with its expression pattern, this suggests that Xatv functions during gastrulation in a negative feedback loop with Xnrs to affect the amount and/or character of mesoderm induced. Our data also provide insights into the way that lefty/nodal signals interact in the initiation of differential L-R morphogenesis. Right-sided misexpression of Xnr1 (endogenously expressed in the left LPM) induces bilateral Xatv expression. Left-sided Xatv overexpression suppresses Xnr1/XPitx2 expression in the left LPM, and leads to severely disturbed visceral asymmetry, suggesting that active ‘left’ signals are critical for L-R axis determination in frog embryos. We propose that the induction of lefty/Xatv in the left LPM by nodal/Xnr1 provides an efficient self-regulating mechanism to downregulate nodal/Xnr1 expression and ensure a transient ‘left’ signal within the embryo.

Activin- and Nodal-related factors control antero-posterior patterning of the zebrafish embryo

Definition of cell fates along the dorso-ventral axis depends on an antagonistic relationship between ventralizing transforming growth factor-beta superfamily members, the bone morphogenetic proteins and factors secreted from the dorsal organizer, such as Noggin and Chordin. The extracellular binding of the last group to the bone morphogenetic proteins prevents them from activating their receptors, and the relative ventralizer:antagonist ratio is thought to specify different dorso-ventral cell fates. Here, by taking advantage of a non-genetic interference method using a specific competitive inhibitor, the Lefty-related gene product Antivin, we provide evidence that cell fate along the antero-posterior axis of the zebrafish embryo is controlled by the morphogenetic activity of another transforming growth factor-beta superfamily subgroup--the Activin and Nodal-related factors. Increasing antivin doses progressively deleted posterior fates within the ectoderm, eventually resulting in the removal of all fates except forebrain and eyes. In contrast, overexpression of activin or nodal-related factors converted ectoderm that was fated to be forebrain into more posterior ectodermal or mesendodermal fates. We propose that modulation of intercellular signalling by Antivin/Activin and Nodal-related factors provides a mechanism for the graded establishment of cell fates along the antero-posterior axis of the zebrafish embryo.

Asymmetric expression of antivin/lefty1 in the early chick embryo

Mammalian lefty and zebrafish antivin, highly related to lefty, are shown to be expressed asymmetrically and involved in the specification of the left body side of early embryos. We isolated a chick homologue of the antivin/lefty1 cDNA and studied its expression pattern during early chick development. We found that antivin/lefty1 is expressed asymmetrically on the left side of the prospective floorplate, notochord and lateral plate mesoderm of the chick embryo.

Three different noggin genes antagonize the activity of bone morphogenetic proteins in the zebrafish embryo

The dorsoventral polarity of the vertebrate embryo is established through interactions between ventrally expressed bone morphogenetic proteins and their organizer-borne antagonists Noggin, Chordin, and Follistatin. While the opposing interactions between Short Gastrulation/Chordin and Decapentaplegic/BMP4 have been evolutionarily conserved in arthropods and vertebrates, there has been up to now no functional evidence of an implication of Noggin in the early patterning of organisms other than Xenopus. We have studied the contribution of Noggin to the embryonic development of the zebrafish. While single-copy noggin genes have been characterized in several vertebrate species, we report that the zebrafish genome harbors three noggin homologues. Overexpression experiments show that Noggin1, Noggin2, and Noggin3 can antagonize ventralizing BMPs. While all three factors have similar biological activities, their embryonic expression is different. The combined expression of the three genes recapitulates the different aspects of the expression of the single-copy noggin genes of other organisms. This suggests that the three zebrafish noggin genes and the single noggin genes of other vertebrates have evolved from a common ancestor and that subsequent differential loss of tissue-specific elements in the promoters of the different zebrafish genes accounts for their more restricted spatiotemporal expression. Finally we show that noggin1 is expressed in the fish organizer and able to dorsalize the embryo, suggesting its implication in the dorsoventral patterning of the zebrafish.

Maternal and zygotic activity of the zebrafish ogon locus antagonizes BMP signaling

The dorsal-ventral axis of vertebrate embryos is thought to be specified by a gradient of bone morphogenetic protein (BMP) activity, which, in part, arises through the interaction of dorsally expressed antagonists Chordin and Noggin with the ventralizing BMPs. The zebrafish mercedes(tm305), ogon(m60), and short tail(b180) mutations produce ventralized phenotypes, including expanded bmp2b/4 expression domains. We find that the three mutations are allelic and that the locus they define, renamed ogon (ogo), maps to linkage group 25. The ogo(m60) and ogo(b180) mutations are deficiencies and thus represent null alleles, whereas the ENU-induced allele ogo(tm305) retains partial function. Aspects of the ogo(m60) and ogo(tm305) mutant phenotypes are fully suppressed by overexpression of BMP antagonists. Moreover, swirl(tc300), a null mutation in bmp2b, is epistatic to ogo(m60) mutation, providing further evidence that ogo normally functions in a BMP-dependent manner. Embryonic patterning is highly sensitive to maternal and zygotic ogo gene dosage, especially when the level of zygotic chordin activity is also reduced. However, elimination of the zygotic activity of both genes does not result in a completely ventralized embryo. Thus, while ogo and chordin are required to limit activity of BMPs, additional mechanisms must exist to block these ventralizing signals. We have ruled out zebrafish noggin homologues as candidates for the ogo gene, including a newly identified gene, nog1, which is specifically expressed in the gastrula organizer. The results suggest that ogo encodes an as yet unidentified dorsalizing factor that mediates dorsoventral patterning by directly or indirectly antagonizing BMP activity.

Ontogeny and behaviour of early macrophages in the zebrafish embryo

In the zebrafish embryo, the only known site of hemopoieisis is an intra-embryonic blood island at the junction between trunk and tail that gives rise to erythroid cells. Using video-enhanced differential interference contrast microscopy, as well as in-situ hybridization for the expression of two new hemopoietic marker genes, draculin and leucocyte-specific plastin, we show that macrophages appear in the embryo at least as early as erythroid cells, but originate from ventro-lateral mesoderm situated at the other end of the embryo, just anterior to the cardiac field. These macrophage precursors migrate to the yolksac, and differentiate. From the yolksac, many invade the mesenchyme of the head, while others join the blood circulation. Apart from phagocytosing apoptotic corpses, these macrophages were observed to engulf and destroy large amounts of bacteria injected intravenously; the macrophages also sensed the presence of bacteria injected into body cavities that are isolated from the blood, migrated into these cavities and eradicated the microorganisms. Moreover, we observed that although only a fraction of the macrophage population goes to the site of infection, the entire population acquires an activated behaviour, similar to that of activated macrophages in mammals. Our results support the notion that in vertebrate embryos, macrophages endowed with proliferative capacity arise early from the hemopoietic lineage through a non-classical, rapid differentiation pathway, which bypasses the monocytic series that is well-documented in adult hemopoietic organs.

Antivin, a novel and divergent member of the TGFbeta superfamily, negatively regulates mesoderm induction

Mesoderm induction and patterning are mediated by members of the TGFbeta superfamily. We have isolated a novel zebrafish member, antivin, that structurally is highly related to mouse lefty. Overexpression of antivin completely abolishes mesoderm induction at blastula stage, yet resultant embryos develop well-patterned epidermal and neural derivatives. The mesoderm-inhibiting activity of antivin can be mimicked by lefty and is suppressed by increasing levels of the mesodermal inducer Activin or its receptors. On the basis of its expression and activity, we propose that Antivin normally functions as a competitive inhibitor of Activin to limit mesoderm induction in the early embryo.

A role for FGF-8 in the dorsoventral patterning of the zebrafish gastrula

Signals released from Spemann's organizer, together with ventralizing factors such as BMPs, are necessary to pattern the dorsoventral axis of the vertebrate embryo. We report that a member of the FGF family, fgf-8, not secreted by the axial mesoderm but expressed in a dorsoventral gradient at the margin of the zebrafish gastrula, also contributes to the establishment of the dorsoventral axis of the embryo. Ectopic expression of FGF-8 leads to the expansion of dorsolateral derivatives at the expense of ventral and posterior domains. Moreover, FGF-8 displays some organizer properties as it induces the formation of a partial secondary axis in the absence of factors released from Spemann's organizer territory. Analysis of its interaction with the ventralizing factors, BMPs, reveals that overexpression of FGF-8 inhibits the expression of these factors in the ventral part of the embryo as early as blastula stage, suggesting that FGF-8 acts upstream of BMP2 and BMP4. We conclude that FGF-8 is involved in defining dorsoventral identity and is an important organizing factor responsible for specification of mesodermal and ectodermal dorsolateral territories of the zebrafish gastrula.

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.

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.

The expression pattern of the mouse receptor tyrosine kinase gene MDK1 is conserved through evolution and requires Hoxa-2 for rhombomere-specific expression in mouse embryos

Segmentation of the hindbrain has been conserved throughout the vertebrate species and results in the transient formation of rhombomeres, which are lineage-restricted compartments. Studies on the molecular mechanisms underlying the segmentation process have revealed that rhombomeric boundaries coincide with the expression limits of several evolutionary conserved genes such as the zinc-finger transcription factor Krox-20 and homeobox genes which are expressed in a specific spatial and temporal order and have been shown to be important regulators of segmental identity. In addition to Krox-20 and Hox genes, several members of the Eph subfamily of receptor protein tyrosine kinase (RTK) genes are also expressed in a segment-restricted manner in the hindbrain, suggesting that these receptors may act in concert with Hox genes to establish regional identity. In the cascade of regulatory interactions leading to segmental identity, Krox-20 appears to act "upstream" of Hox genes, but the identity of the "downstream" effectors has not yet been identified. We report here the isolation of the zebrafish orthologue of the mouse RTK gene MDK1 which belongs to the Eph receptor subfamily and show that the major expression domains of the mouse and the zebrafish genes have been conserved through evolution. Since the coincident spatial and temporal expression of Hoxa-2 and MDK1 in the mouse hindbrain suggested a possible regulatory link between them, we analyzed the expression of the MDK1 in Hoxa-2 null mutant embryos. A selective lack of MDK1 expression in rhombomere 3 of Hoxa-2 mutant hindbrains together with an overall altered expression pattern in the other rhombomeres was observed, thus demonstrating that MDK1 lies downstream of Hoxa-2 in the morphogenetic signaling cascade.

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

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

Expression of snail2, a second member of the zebrafish snail family, in cephalic mesendoderm and presumptive neural crest of wild-type and spadetail mutant embryos

Transcripts of a newly discovered gene called snail2, encoding a zinc finger protein of the Snail family, first appear in rows of cephalic mesendodermal cells in gastrulating zebrafish embryos. At the end of gastrulation, snail2 RNA accumulates in a domain of ectodermal cells that mark the border between the epidermal epithelium and the neural plate and includes precursors of the neural crest. During somitogenesis, snail2 expression becomes restricted to neural crest. snail2 is thus one of the earliest genes yet known to be specifically expressed in neural crest in zebrafish embryos. Since snail2 is expressed in mesendoderm, a tissue layer whose convergence in the trunk is known to be altered in embryos homozygous for the spadetail mutation, we examined snail2 expression in spadetail embryos. In these mutants, the number of cephalic mesendodermal cells expressing snail2 is strongly reduced and the distribution of cells containing snail2 and no tail transcripts in the axial mesoderm is much broader than normal Moreover, the embryos are shorter than normal at the end of gastrulation. This shows that, in addition to the failure of paraxial mesoderm to converge normally in the trunk during gastrulation, spadetail also affects the elongation of the embryo and the convergence of axial and lateral mesendoderm in both trunk and head.

Novel FGF receptor (Z-FGFR4) is dynamically expressed in mesoderm and neurectoderm during early zebrafish embryogenesis

We have identified a novel FGF receptor, Z-FGFR4, in zebrafish embryos. Z-FGFR4 is closely related to both chicken FREK (Marcelle et al. [1994] Development 120:683-694) and the Pleurodeles cDNA clone Pw-FGFR4 (also named PFR4). The Z-FGFR4 cDNA clones contain consensus sequences for two groups of two Ig-like domains, separated by eight acidic residues referred to as the "acid box." Z-FGFR4, therefore, is the first FGFR molecule yet described in vertebrates that contains four Ig domains in its amino-terminal region. Whole-mount in situ hybridization of staged zebrafish embryos, using probes prepared from a variety of domains of the Z-FGFR4 cDNA, reveal complex temporal and spatial expression patterns. Expression of Z-FGFR4 mRNA is first detected in embryos prior to gastrulation and then appears in prechordal plate mesendoderm. At this time, Z-FGFR mRNA is expressed in the epiblast in two distinct stripes which ultimately contribute to the brain. Eventually Z-FGFR4 transcripts are observed in forebrain, anterior hindbrain (rhombomeres 1, 3), and caudal hindbrain (rhombomere 7), as well as in the dorsal-most portion of the rostral spinal cord. Expression in axial mesendoderm appears transiently in notochord and segmental plate mesoderm. Eventually, Z-FGFR4 mRNA becomes restricted to the posterior somites and is absent in differentiated notochord. These detailed expression studies provide the basis for understanding FGFR function through an analysis, currently in progress, of the developmental consequences of Z-FGFR4 misexpression.

Goosecoid expression in neurectoderm and mesendoderm is disrupted in zebrafish cyclops gastrulas

RNA from goosecoid, a homeobox-containing gene expressed during gastrulation in the anterior mesoderm of vertebrate embryos, can generate organizer activity when injected into ventral mesoderm, resulting in a secondary body axis; it is not yet understood, however, how goosecoid performs its organizer function. We report here that in the zebrafish gastrula, a domain of goosecoid expression arises in presumptive anterior neurectoderm which lies directly above goosecoid-expressing mesendodermal cells. From this position, goosecoid expression then spreads gradually across the ectodermal layer. In cyclops mutant embryos, which lack a ventral anterior brain, expression of goosecoid is abnormal in the mesendoderm and completely absent in the overlying neurectoderm. These results indicate that cyclops is required for correct specification of the mesendoderm and suggest that goosecoid expression in the ectoderm may result from vertical induction from the mesoderm. We propose that in the gastrula head, goosecoid may be important in organizing the ventral neurectoderm.

Structure of the zebrafish snail1 gene and its expression in wild-type, spadetail and no tail mutant embryos

Mesoderm formation is critical for the establishment of the animal body plan and in Drosophila requires the snail gene. This report concerns the cloning and expression pattern of the structurally similar gene snail1 from zebrafish. In situ hybridization shows that the quantity of snail1 RNA increases at the margin of the blastoderm in cells that involute during gastrulation. As gastrulation begins, snail1 RNA disappears from the dorsal axial mesoderm and becomes restricted to the paraxial mesoderm and the tail bud. snail1 RNA increases in cells that define the posterior border of each somite and then disappears when somitic cells differentiate. Later in development, expression appears in cephalic neural crest derivatives. Many snail1-expressing cells were missing from mutant spadetail embryos and the quantity of snail1 RNA was greatly reduced in mutant no tail embryos. The work presented here suggests that snail1 is involved in morphogenetic events during gastrulation, somitogenesis and development of the cephalic neural crest, and that no tail may act as a positive regulator of snail1.

Sequence-specific transactivation of the Drosophila twist gene by the dorsal gene product

The maternal gene dorsal encodes a nuclear protein acting as a morphogen that determines the size and fate of regions along the dorsal-ventral axis of the Drosophila embryo. From previous genetic and biochemical studies it was hypothesized that dorsal might be responsible for the activation of the zygotic gene twist. In this report, regulatory sequences required for correct spatial and quantitative expression of twist are defined, by using phenotypic rescue and studying twist-beta-galactosidase expression. In addition, by transient cotransfection assays, we show that the dorsal protein specifically activates expression from the twist promoter. We demonstrate that dorsal is a sequence-specific DNA-binding protein that recognizes a motif similar to that recognized by the mammalian transcriptional activator NF-kappa B.

The M-twist gene of Mus is expressed in subsets of mesodermal cells and is closely related to the Xenopus X-twi and the Drosophila twist genes

The twist gene was characterized in Drosophila as being necessary at gastrulation for the establishment of the mesodermal germ layer. It codes for a nuclear DNA-binding protein that is probably a transcription factor. We have cloned and sequenced the M-twist gene of Mus musculus. The deduced proteins encoded by the Mus, Xenopus, and Drosophila twist cDNAs, respectively, show a high degree of similarity. Northern blot analyses and in situ hybridizations reveal that the 1.7-kb murine M-twist m-RNA is present at early stages, starting at 8 days post coitum, and is expressed the most at 9.5 days in the cephalic and branchial mesectoderm, in some derivatives of the mesodermal layer (sclerotoma and somatopleura), and in the limb buds.

Sequence of the twist gene and nuclear localization of its protein in endomesodermal cells of early Drosophila embryos

The twist gene is involved in the establishment of germ layers in Drosophila embryos: twist homozygous mutant embryos fail to form the ventral furrow at gastrulation and lack mesoderm and all internal organs. We have determined the sequence of the twist gene, that contains 'CAX' repeats in its 5' moiety, and codes for a protein of 490 amino acids. We have raised anti-twist antibodies that were used to study the distribution of the twist protein in whole mounts and tissue sections of wild-type embryos. Twist protein appears to be a nuclear protein at all developmental stages. It is present over both poles and in the midventral region (endoderm and mesoderm anlagen) at cellular blastoderm stage; later in development, it is detected within the mesodermal layer until its differentiation into somatopleura and splanchnopleura in which some cells are still labelled by anti-twist antibodies.

The twist gene: isolation of a Drosophila zygotic gene necessary for the establishment of dorsoventral pattern

The twist zygotic gene appears to be involved in the establishment of the dorso-ventral pattern in Drosophila embryos. Homozygous twist embryos are partially dorsalized, their gastrulation is abnormal, and they fail to differentiate mesoderm. We determined the temperature-sensitive period of twist around the gastrulation time, and we isolated the gene. A 300 kb chromosomic walk allowed the detection of the 70 kb deletion that delimits the twist region in Df(2R)twiS60. Southern blot analyses of 21 EMS induced twist allele DNAs and systematic Northern blot analyses all over this 70 kb region lead to the localization of the twist gene: within about 10 kb at the left border of the deletion, 2 twist alleles show each a small deletion that uncover a transcription unit whose expression occurs about at the time of gastrulation.