Ras-mediated FGF signaling is required for the formation of posterior but not anterior neural tissue in Xenopus laevis

Fibroblast growth factor (FGF) has been proposed to be involved in the specification and patterning of the developing vertebrate nervous system. There is conflicting evidence, however, concerning the requirement for FGF signaling in these processes. To provide insight into the signaling mechanisms that are important for neural induction and anterior-posterior neural patterning, we have employed the dominant negative Ras mutant, N17Ras, in addition to a truncated FGF receptor (XFD). Both N17Ras and XFD, when expressed in Xenopus laevis animal cap ectoderm, inhibit the ability of FGF to generate neural pattern. They also block induction of posterior neural tissue by XBF2 and XMeis3. However, neither XFD nor N17Ras inhibits noggin, neurogenin, or XBF2 induction of anterior neural markers. MAP kinase activation has been proposed to be necessary for neural induction, yet N17Ras inhibits the phosphorylation of MAP kinase that usually follows explantation of explants. In whole embryos, Ras-mediated FGF signaling is critical for the formation of posterior neural tissues but is dispensable for neural induction.     

Xenopus Meis3 protein forms a hindbrain-inducing center by activating FGF/MAP kinase and PCP pathways

Knockdown studies in Xenopus demonstrated that the XMeis3 gene is required for proper hindbrain formation. An explant assay was developed to distinguish between autonomous and inductive activities of XMeis3 protein. Animal cap explants caudalized by XMeis3 were recombined with explants neuralized by the BMP dominant-negative receptor protein. XMeis3-expressing cells induced convergent extension cell elongations in juxtaposed neuralized explants. Elongated explants expressed hindbrain and primary neuron markers, and anterior neural marker expression was extinguished. Cell elongation was dependent on FGF/MAP-kinase and Wnt-PCP activities. XMeis3 activates FGF/MAP-kinase signaling, which then modulates the PCP pathway. In this manner, XMeis3 protein establishes a hindbrain-inducing center that determines anteroposterior patterning in the brain.     

Xenopus Polycomblike 2 (XPcl2) controls anterior to posterior patterning of the neural tissue

A novel gene, Xenopus Polycomblike 2 (XPcl2), which encodes a protein similar to Drosophila Polycomblike was cloned and characterized. Polycomblike belongs to the Polycomb group proteins, which maintain stable expression patterns for the clustered homeotic genes in the Drosophila embryo by forming multimeric complexes on chromatin. XPcl2 shows greater amino acid sequence homology to human and mouse M96 (hPcl2, mPcl2) than Xenopus Pcl1 (XPcl1), mouse Tctex3 (mPcl1) and human PHF1 (hPcl1), indicating that at least two types of Polycomblike genes are conserved between amphibians and mammals. XPcl2 mRNA is present both maternally and zygotically, and the temporal expression profile is distinct from XPcl1, another member of the Polycomblike family in Xenopus. XPcl2 is highly expressed in the anterior-dorsal region of Xenopus following the neurula stage in a manner similar to XPcl1. Overexpression of XPcl2 disturbs the development of the anterior central nervous system, eye and cement gland. In the XPcl2-overexpressing embryo, a hindbrain marker, Krox20, and a spinal cord marker, HoxB9, are expressed more posteriorly, suggesting an alteration in the anterior-posterior patterning of the neural tissue. In addition, XPcl2 represses Zic3- and noggin-induced anterior neural markers, but not neural crest markers in animal cap explants. These results indicate that XPcl2 regulates anterior neural tissue development and the anterior-posterior patterning of the neural tissue.     

Neural expression of the Xenopus homeobox gene Xhox3: evidence for a patterning neural signal that spreads through the ectoderm

The Xenopus laevis homeobox gene Xhox3 is expressed in the axial mesoderm of gastrula and neurula stage embryos. By the late neurula-early tailbud stage, mesodermal expression is no longer detectable and expression appears in the growing tailbud and in neural tissue. In situ hybridization analysis of the expression of Xhox3 in neural tissue shows that it is restricted within the neural tube and the cranial neural crest during the tailbud-early tadpole stages. In late tadpole stages, Xhox3 is only expressed in the mid/hindbrain area and can therefore be considered a marker of anterior neural development. To investigate the mechanism responsible for the anterior-posterior (A-P) regionalization of the neural tissue, the expression of Xhox3 has been analysed in total exogastrula. In situ hybridization analyses of exogastrulated embryos show that Xhox3 is expressed in the apical ectoderm of total exogastrulae, a region that develops in the absence of anterior axial mesoderm. The results provide further support for the existence of a neuralizing signal, which originates from the organizer region and spreads through the ectoderm. Moreover, the data suggest that this neural signal also has a role in A-P patterning the neural ectoderm.     

Expression of Sox1 during Xenopus early embryogenesis

Sox B1 group genes, Sox1, Sox2, and Sox3 (Sox1-3), are involved in neurogenesis in various species. Here, we identified the Xenopus homolog of Sox1, and investigated its expression patterns and neural inducing activity. Sox1 was initially expressed in the anterior neural plate of Xenopus embryos, with expression restricted to the brain and optic vesicle by the tailbud stage. Expression subsequently decreased in the eye region by the tadpole stage. Sox1 expression in animal cap explants was induced by inhibition of BMP signaling in the same manner as Sox2, Sox3, and SoxD. In addition, overexpression of Sox1 induced neural markers in ventral ectoderm and in animal caps. These results implicate Xenopus Sox1 in neurogenesis, especially brain and eye development.     

Induction of proopiomelanocortin mRNA expression in animal caps of Xenopus laevis embryos

To convert animal pole cells of a frog embryo from an ectodermal fate into a neural one, inductive signals are necessary. The alkalizing agent NH4Cl induces the expression of several anterior brain markers and the early pituitary marker XANF-2 in Xenopus animal caps. Here it is demonstrated that NH4Cl also induced proopiomelanocortin (POMC)-expressing cells (the first fully differentiated pituitary cell type) in stage 9 and 10 Xenopus animal caps, and that all-trans retinoic acid, a posteriorizing agent, was able to block this induction when it was administered within 2 h after the start of NH4Cl incubation. Thus, after 2 h, the fate of Xenopus animal cap cells was determined. Microinjection of ribonucleic acid (RNA) encoding noggin, an endogenous neural inducer, led to the induction of POMC gene expression in animal caps of stage 10 embryos, suggesting that noggin represents a candidate mesodermal signal leading to the POMC messenger (m) RNA producing cell type in uncommitted ectoderm. Hence, an alkalizing agent and a neural inducer can generate a fully differentiated POMC cell lineage from Xenopus animal caps.     

Xenopus laevis POU91 protein, an Oct3/4 homologue, regulates competence transitions from mesoderm to neural cell fates

Cellular competence is defined as a cell's ability to respond to signaling cues as a function of time. In Xenopus laevis, cellular responsiveness to fibroblast growth factor (FGF) changes during development. At blastula stages, FGF induces mesoderm, but at gastrula stages FGF regulates neuroectoderm formation. A Xenopus Oct3/4 homologue gene, XLPOU91, regulates mesoderm to neuroectoderm transitions. Ectopic XLPOU91 expression in Xenopus embryos inhibits FGF induction of Brachyury (Xbra), eliminating mesoderm, whereas neural induction is unaffected. XLPOU91 knockdown induces high levels of Xbra expression, with blastopore closure being delayed to later neurula stages. In morphant ectoderm explants, mesoderm responsiveness to FGF is extended from blastula to gastrula stages. The initial expression of mesoderm and endoderm markers is normal, but neural induction is abolished. Churchill (chch) and Sip1, two genes regulating neural competence, are not expressed in XLPOU91 morphant embryos. Ectopic Sip1 or chch expression rescues the morphant phenotype. Thus, XLPOU91 epistatically lies upstream of chch/Sip1 gene expression, regulating the competence transition that is critical for neural induction. In the absence of XLPOU91 activity, the cues driving proper embryonic cell fates are lost.     

A novel member of the Xenopus Zic family, Zic5, mediates neural crest development

We characterized Xenopus Zic5 which belongs to a novel class of the Zic family. Zic5 is more specifically expressed in the prospective neural crest than other Zic genes. Overexpression of Zic5 in embryos led to ectopic expression of the early neural crest markers, Xsna and Xslu, with the loss of epidermal marker expression. In Zic5-overexpressing animal cap explants, there was marked induction of neural crest markers, without mesodermal and anterior neural markers. This was in contrast to other Xenopus Zic genes, which induce both anterior and the neural crest markers in the same assay. Injection of a dominant-negative form of Zic5 can block neural crest formation in vivo. These results indicate that Zic5 expression converts cells from an epidermal fate to a neural crest cell fate. This is the first evidence for neural crest tissue inductive activity separate from anterior neural tissue inductive activity in a Zic family gene.     

Neural crest determination by co-activation of Pax3 and Zic1 genes in Xenopus ectoderm

A number of regulatory genes have been implicated in neural crest development. However, the molecular mechanism of how neural crest determination is initiated in the exact ectodermal location still remains elusive. Here, we show that the cooperative function of Pax3 and Zic1 determines the neural crest fate in the amphibian ectoderm. Pax3 and Zic1 are expressed in an overlapping manner in the presumptive neural crest area of the Xenopus gastrula, even prior to the onset of the expression of the early bona fide neural crest marker genes Foxd3 and Slug. Misexpression of both Pax3 and Zic1 together efficiently induces ectopic neural crest differentiation in the ventral ectoderm, whereas overexpression of either one of them only expands the expression of neural crest markers within the dorsolateral ectoderm. The induction of neural crest differentiation by Pax3 and Zic1 requires Wnt signaling. Loss-of-function studies in vivo and in the animal cap show that co-presence of Pax3 and Zic1 is essential for the initiation of neural crest differentiation. Thus, co-activation of Pax3 and Zic1, in concert with Wnt, plays a decisive role for early neural crest determination in the correct place of the Xenopus ectoderm.     

XMAN1, an inner nuclear membrane protein,antagonizes BMP signaling by interacting with Smad1 in Xenopus embryos

A family of inner nuclear membrane proteins is implicated in gene regulation by interacting with chromatin, nuclear lamina and intranuclear proteins; however, the physiological functions of these proteins are largely unknown. Using a Xenopus expression screening approach with an anterior neuroectoderm cDNA library, we have identified an inner nuclear membrane protein, XMAN1, as a novel neuralizing factor that is encoded by the Xenopus ortholog of human MAN1. XMAN1 mRNA is expressed maternally, and appears to be restricted to the entire ectoderm at the early gastrula stage, then to the anterior neuroectoderm at the neurula stage. XMAN1 induces anterior neural markers without mesoderm induction in ectodermal explants, and a partial secondary axis when expressed ventrally by dorsalizing the ventral mesoderm. Importantly, XMAN1 antagonizes bone morphogenetic protein (BMP) signaling downstream of its receptor Alk3, as judged by animal cap assays, in which XMAN1 blocks expression of downstream targets of BMP signaling (Xhox3 and Msx1), and by luciferase reporter assays, in which XMAN1 suppresses BMP-dependent activation of the Xvent2 promoter. Deletion mutant analyses reveal that the neuralizing and BMP-antagonizing activities of XMAN1 reside in the C-terminal region, and that the C-terminal region binds to Smad1, Smad5 and Smad8, which are intracellular mediators of the BMP pathway. Interference with endogenous XMAN1 functions with antisense morpholino oligos leads to the reduction of anterior neuroectoderm. These results provide the first evidence that the nuclear envelope protein XMAN1 acts as a Smad-interacting protein to antagonize BMP signaling during Xenopus embryogenesis.     

Temporal and spatial expression patterns of Cdc25 phosphatase isoforms during early Xenopus development

In early animal development, cell proliferation and differentiation are tightly linked and coordinated. It is important, therefore, to know how the cell cycle is controlled during early development. Cdc25 phosphatases activate cyclin-dependent kinases (Cdks) and thereby promote cell-cycle progression. In Xenopus laevis, three isoforms of cdc25 have been identified, viz. cdc25A, cdc25B and cdc25C. In this study, we isolated a cDNA encoding a novel Xenopus Cdc25 phosphatase (named cdc25D). We investigated the temporal and spatial expression patterns of the four cdc25 isoforms during early Xenopus development, using RT-PCR and whole-mount in situ hybridization. cdc25A and cdc25C were expressed both maternally and zygotically, whereas cdc25B and cdc25D were expressed zygotically. Both cdc25A and cdc25C were expressed mainly in prospective neural regions, whereas cdc25B was expressed preferentially in the central nervous system (CNS), such as the spinal cord and the brain. Interestingly, cdc25D was expressed in the epidermal ectoderm of the late-neurula embryo, and in the liver diverticulum endoderm of the mid-tailbud embryo. In agreement with the spatial expression patterns in whole embryos, inhibition of bone morphoge- netic protein (BMP), a crucial step for neural induction, induced an upregulation of cdc25B, but a downregulation of cdc25D in animal cap assays.These results indicate that different cdc25 isoforms are differently expressed and play different roles during early Xenopus development.     

FGF signaling and the anterior neural induction in Xenopus

We previously showed that FGF was capable of inducing Xenopus gastrula ectoderm cells in culture to express position-specific neural markers along the anteroposterior axis in a dose-dependent manner. However, conflicting results have been obtained concerning involvement of FGF signaling in the anterior neural induction in vivo using the same dominant-negative construct of Xenopus FGF receptor type-1 (delta XFGFR-1 or XFD). We explored this issue by employing a similar construct of receptor type-4a (XFGFR-4a) in addition, since expression of XFGFR-4a was seen to peak between gastrula and neurula stages, when the neural induction and patterning take place, whereas expression of XFGFR-1 had not a distinct peak during that period. Further, these two FGFRs are most distantly related in amino acid sequence in the Xenopus FGFR family. When we injected mRNA of a dominant-negative version of XFGFR-4a (delta XFGFR-4a) into eight animal pole blastomeres at 32-cell stage, anterior defects including loss of normal structure in telencephalon and eye regions became prominent as examined morphologically or by in situ hybridization. Overexpression of delta XFGFR-1 appeared far less effective than that of delta XFGFR-4a. Requirement of FGF signaling in ectoderm for anterior neural development was further confirmed in culture: when ectoderm cells that were overexpressing delta XFGFR-4a were cocultured with intact organizer cells from either early or late gastrula embryos, expression of anterior and posterior neural markers was inhibited, respectively. We also showed that autonomous neuralization of the anterior-type observed in ectoderm cells that were subjected to prolonged dissociation was strongly suppressed by delta XFGFR-4a, but not as much by delta XFGFR-1. It is thus indicated that FGF signaling in ectoderm, mainly through XFGFR-4, is required for the anterior neural induction by organizer. We may reconcile our data to the current "neural default model," which features the central roles of BMP4 signaling in ectoderm and BMP4 antagonists from organizer, simply postulating that the neural default pathway in ectoderm includes constitutive FGF signaling step.     

FRL-1, a member of the EGF-CFC family,is essential for neural differentiation in Xenopus early development

Recent studies indicate an essential role for the EGF-CFC family in vertebrate development, particularly in the regulation of nodal signaling. Biochemical evidence suggests that EGF-CFC genes can also activate certain cellular responses independently of nodal signaling. Here, we show that FRL-1, a Xenopus EGF-CFC gene, suppresses BMP signaling to regulate an early step in neural induction. Overexpression of FRL-1 in animal caps induced the early neural markers zic3, soxD and Xngnr-1, but not the pan-mesodermal marker Xbra or the dorsal mesodermal marker chordin. Furthermore, overexpression of FRL-1 suppressed the expression of the BMP-responsive genes, Xvent-1 and Xmsx-1, which are expressed in animal caps and induced by overexpressed BMP-4. Conversely, loss of function analysis using morpholino-antisense oligonucleotides against FRL-1 (FRL-1MO) showed that FRL-1 is required for neural development. FRL-1MO-injected embryos lacked neural structures but contained mesodermal tissue. It was suggested previously that expression of early neural genes that mark the start of neuralization is activated in the presumptive neuroectoderm of gastrulae. FRL-1MO also inhibited the expression of these genes in dorsal ectoderm, but did not affect the expression of chordin, which acts as a neural inducer from dorsal mesoderm. FRL-1MO also inhibited the expression of neural markers that were induced by chordin in animal caps, suggesting that FRL-1 enables the response to neural inducing signals in ectoderm. Furthermore, we showed that the activation of mitogen-activated protein kinase by FRL-1 is required for neural induction and BMP inhibition. Together, these results suggest that FRL-1 is essential in the establishment of the neural induction response.     

Conversion of ectoderm into a neural fate by ATH-3, a vertebrate basic helix-loop-helix gene homologous to Drosophila proneural gene atonal.

We have isolated a novel basic helix-loop-helix (bHLH) gene homologous to the Drosophila proneural gene atonal, termed ATH-3, from Xenopus and mouse. ATH-3 is expressed in the developing nervous system, with high levels of expression in the brain, retina and cranial ganglions. Injection of ATH-3 RNA into Xenopus embryos dramatically expands the neural tube and induces ectopic neural tissues in the epidermis but inhibits non-neural development. This ATH-3-induced neural hyperplasia does not require cell division, indicating that surrounding cells which are normally non-neural types adopt a neural fate. In a Xenopus animal cap assay, ATH-3 is able to convert ectodermal cells into neurons expressing anterior markers without inducing mesoderm. Interestingly, a single amino acid change from Ser to Asp in the basic region, which mimics phosphorylation of Ser, severely impairs the anterior marker-inducing ability without affecting general neurogenic activities. These results provide evidence that ATH-3 can directly convert non-neural or undetermined cells into a neural fate, and suggest that the Ser residue in the basic region may be critical for the regulation of ATH-3 activity by phosphorylation.     

Transgenic Xenopus embryos reveal that anterior neural development requires continued suppression of BMP signaling after gastrulation.

In vertebrates, BMP signaling before gastrulation suppresses neural development. Later in development, BMP signaling specifies a dorsal and ventral fate in the forebrain and dorsal fate in the spinal cord. It is therefore possible that a change in the competence of the ectoderm to respond to BMP signaling occurs at some point in development. We report that exposure of the anterior neural plate to BMP4 before gastrulation causes suppression of all neural markers tested. To determine the effects of BMP4 after gastrulation, we misexpressed BMP4 using a Pax-6 promoter fragment in transgenic frog embryos and implanted beads soaked in BMP4 in the anterior neural plate. Suppression of most anterior neural markers was observed. We conclude that most neural genes continue to require suppression of BMP signaling into the neurula stages. Additionally, we report that BMP4 and BMP7 are abundantly expressed in the prechordal mesoderm of the neurula stage embryo. This poses the paradox of how the expression of most neural genes is maintained if they can be inhibited by BMP signaling. We show that at least one gene in the anterior neural plate suppresses the response of the ectoderm to BMP signaling. We propose that the suppressive effect of BMP signaling on the expression of neural genes coupled with localized suppressors of BMP signaling result in the fine-tuning of gene expression in the anterior neural plate.