Development of the ectoderm in Xenopus: tissue specification and the role of cell association and division

Bidirectional multiciliated cell extrusion is controlled by Notch-driven basal extrusion and Piezo1-driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late-stage development, there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional, whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, response to the Notch signal is age dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo1, such that premature activation of Piezo1 leads to early extrusion while blocking Piezo1 leads to MCC maintenance. Distinct mechanisms for MCC loss underlie the importance of their removal during epithelial remodeling.

Bidirectional multiciliated cell extrusion is controlled by Notch driven basal extrusion and Piezo 1 driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late stage development there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, receptivity to the Notch signal is age-dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo 1 such that premature activation of Piezo 1 leads to early extrusion while blocking Piezo 1 leads to MCC maintenance. Distinct mechansms for MCC loss underlie the importance of their removal during epithelial remodeling.

Summay Statement

Cell extrusion typically occurs unidirectionally. We have identified a single population of multiciliated cells that extrudes bidirectionally: Notch-driven basal extrusion and Piezo 1-mediated apical extrusion.

Kinematic self-replication in reconfigurable organisms

All living systems perpetuate themselves via growth in or on the body, followed by splitting, budding, or birth. We find that synthetic multicellular assemblies can also replicate kinematically by moving and compressing dissociated cells in their environment into functional self-copies. This form of perpetuation, previously unseen in any organism, arises spontaneously over days rather than evolving over millennia. We also show how artificial intelligence methods can design assemblies that postpone loss of replicative ability and perform useful work as a side effect of replication. This suggests other unique and useful phenotypes can be rapidly reached from wild-type organisms without selection or genetic engineering, thereby broadening our understanding of the conditions under which replication arises, phenotypic plasticity, and how useful replicative machines may be realized.

SPIN90 associates with mDia1 and the Arp2/3 complex to regulate cortical actin organization

Cell shape is controlled by the submembranous cortex, an actomyosin network mainly generated by two actin nucleators: the Arp2/3 complex and the formin mDia1. Changes in relative nucleator activity may alter cortical organization, mechanics and cell shape. Here we investigate how nucleation-promoting factors mediate interactions between nucleators. In vitro, the nucleation-promoting factor SPIN90 promotes formation of unbranched filaments by Arp2/3, a process thought to provide the initial filament for generation of dendritic networks. Paradoxically, in cells, SPIN90 appears to favour a formin-dominated cortex. Our in vitro experiments reveal that this feature stems mainly from two mechanisms: efficient recruitment of mDia1 to SPIN90-Arp2/3 nucleated filaments and formation of a ternary SPIN90-Arp2/3-mDia1 complex that greatly enhances filament nucleation. Both mechanisms yield rapidly elongating filaments with mDia1 at their barbed ends and SPIN90-Arp2/3 at their pointed ends. Thus, in networks, SPIN90 lowers branching densities and increases the proportion of long filaments elongated by mDia1.

The Xenopus animal cap transcriptome: building a mucociliary epithelium

With the availability of deep RNA sequencing, model organisms such as Xenopus offer an outstanding opportunity to investigate the genetic basis of vertebrate organ formation from its embryonic beginnings. Here we investigate dynamics of the RNA landscape during formation of the Xenopus tropicalis larval epidermis. Differentiation of non-neural ectoderm starts at gastrulation and takes about one day to produce a functional mucociliary epithelium, highly related to the one in human airways. To obtain RNA expression data, uncontaminated by non-epidermal tissues of the embryo, we use prospective ectodermal explants called Animal Caps (ACs), which differentiate autonomously into a ciliated epidermis. Their global transcriptome is investigated at three key timepoints, with a cumulative sequencing depth of ∼10⁸ reads per developmental stage. This database is provided as online Web Tool to the scientific community. In this paper, we report on global changes in gene expression, an unanticipated diversity of mRNA splicing isoforms, expression patterns of repetitive DNA Elements, and the complexity of circular RNAs during this process. Computationally we derive transcription factor hubs from this data set, which may help in the future to define novel genetic drivers of epidermal differentiation in vertebrates.

Foxi2 is an animally localized maternal mRNA in Xenopus, and an activator of the zygotic ectoderm activator Foxi1e

Foxi1e is a zygotic transcription factor that is essential for the expression of early ectodermal genes. It is expressed in a highly specific pattern, only in the deep cell layers of the animal hemisphere, and in a mosaic pattern in which expressing cells are interspersed with non-expressing cells. Previous work has shown that several signals in the blastula control this expression pattern, including nodals, the TGFβ family member Vg1, and Notch. However, these are all inhibitory, which raises the question of what activates Foxi1e. In this work, we show that a related Forkhead family protein, Foxi2, is a maternal activator of Foxi1e. Foxi2 mRNA is maternally encoded, and highly enriched in animal hemisphere cells of the blastula. ChIP assays show that it acts directly on upstream regulatory elements of Foxi1e. Its effect is specific, since animal cells depleted of Foxi2 are able to respond normally to mesoderm inducing signals from vegetal cells. Foxi2 thus acts as a link between the oocyte and the early pathway to ectoderm, in a similar fashion to the vegetally localized VegT acts to initiate endoderm and mesoderm formation.

Frog skin epithelium: electrolyte transport and chytridiomycosis

One unique physiological characteristic of frogs is that their main route for intake of water is across the skin. In these animals, the skin acts in concert with the kidney and urinary bladder to maintain electrolyte homeostasis. Water absorption across the skin is driven by the osmotic gradient that develops as a consequence of solute transport. Our recent study demonstrated that chytridiomycosis, an infection of amphibian skin by the fungal pathogen, Batrachochytrium dendrobatidis, inhibits epithelial Na(+) channels, attenuating Na(+) absorption through the skin. In frogs that become severely affected by this fungus, systemic depletion of Na(+), K(+) and Cl(-) is thought to cause deterioration of cardiac electrical function, leading to cardiac arrest. Here we review the ion transport mechanisms of frog skin, and discuss the effect of chytridiomycosis on these mechanisms.

An essential role for transcription before the MBT in Xenopus laevis

Most zygotic genes remain transcriptionally silent in Drosophila, Xenopus, and zebrafish embryos through multiple mitotic divisions until the midblastula transition (MBT). Several genes have been identified in each of these organisms that are transcribed before the MBT, but whether precocious expression of specific mRNAs is important for later development has not been examined in detail. Here, we identify a class of protein coding transcripts activated before the MBT by the maternal T-box factor VegT that are components of an established transcriptional regulatory network required for mesendoderm induction in Xenopus laevis, including the Nodal related ligands xnr5, xnr6, and derrière and the transcription factors bix4, and sox17α. Accumulation of phospho-Smad2, a hallmark of active Nodal signaling, at the onset of the MBT requires preMBT transcription and activity of xnr5 and xnr6. Furthermore, preMBT activation of the Nodal pathway is essential for mesendodermal gene expression and patterning of the embryo. Finally, xnr5 and xnr6 can also activate their own expression during cleavage stages, indicating that preMBT transcription contributes to a feed-forward system that allows robust activation of Nodal signaling at the MBT.

Normal levels of p27 are necessary for somite segmentation and determining pronephric organ size

The Xenopus laevis cyclin dependent kinase inhibitor p27(Xic1) has been shown to be involved in exit from the cell cycle and differentiation of cells into a quiescent state in the nervous system, muscle tissue, heart and retina. We show that p27(Xic1) is expressed in the developing kidney in the nephrostomal regions. Using overexpression and morpholino oligonucleotide (MO) knock-down approaches we show normal levels of p27(Xic1) regulate pronephros organ size by regulating cell cycle exit. Knock-down of p27(Xic1) expression using a MO prevented myogenesis, as previously reported; an effect that subsequently inhibits pronephrogenesis. Furthermore, we show that normal levels of p27(Xic1) are required for somite segmentation also through its cell cycle control function. Finally, we provide evidence to suggest correct paraxial mesoderm segmentation is not necessary for pronephric induction in the intermediate mesoderm. These results indicate novel developmental roles for p27(Xic1), and reveal its differentiation function is not universally utilised in all developing tissues.

The apicobasal polarity kinase aPKC functions as a nuclear determinant and regulates cell proliferation and fate during Xenopus primary neurogenesis

During neurogenesis in Xenopus, apicobasally polarised superficial and non-polar deep cells take up different fates: deep cells become primary neurons while superficial cells stay as progenitors. It is not known whether the proteins that affect cell polarity also affect cell fate and how membrane polarity information may be transmitted to the nucleus. Here, we examine the role of the polarity components, apically enriched aPKC and basolateral Lgl2, in primary neurogenesis. We report that a membrane-tethered form of aPKC (aPKC-CAAX) suppresses primary neurogenesis and promotes cell proliferation. Unexpectedly, both endogenous aPKC and aPKC-CAAX show some nuclear localisation. A constitutively active aPKC fused to a nuclear localisation signal has the same phenotypic effect as aPKC-CAAX in that it suppresses neurogenesis and enhances proliferation. Conversely, inhibiting endogenous aPKC with a dominant-negative form that is restricted to the nucleus enhances primary neurogenesis. These observations suggest that aPKC has a function in the nucleus that is important for cell fate specification during primary neurogenesis. In a complementary experiment, overexpressing basolateral Lgl2 causes depolarisation and internalisation of superficial cells, which form ectopic neurons when supplemented with a proneural factor. These findings suggest that both aPKC and Lgl2 affect cell fate, but that aPKC is a nuclear determinant itself that might shuttle from the membrane to the nucleus to control cell proliferation and fate; loss of epithelial cell polarity by Lgl2 overexpression changes the position of the cells and is permissive for a change in cell fate.

Notch activates Wnt-4 signalling to control medio-lateral patterning of the pronephros

Previous studies have highlighted a role for the Notch signalling pathway during pronephrogenesis in the amphibian Xenopus laevis, and in nephron development in the mammalian metanephros, yet a mechanism for this function remains elusive. Here, we further the understanding of how Notch signalling patterns the early X. laevis pronephros anlagen, a function that might be conserved in mammalian nephron segmentation. Our results indicate that early phase pronephric Notch signalling patterns the medio-lateral axis of the dorso-anterior pronephros anlagen, permitting the glomus and tubules to develop in isolation. We show that this novel function acts through the Notch effector gene hrt1 by upregulating expression of wnt4. Wnt-4 then patterns the proximal pronephric anlagen to establish the specific compartments that span the medio-lateral axis. We also identified pronephric expression of lunatic fringe and radical fringe that is temporally and spatially appropriate for a role in regulating Notch signalling in the dorso-anterior region of the pronephros anlagen. On the basis of these results, along with data from previous publications, we propose a mechanism by which the Notch signalling pathway regulates a Wnt-4 function that patterns the proximal pronephric anlagen.

Maternal Interferon Regulatory Factor 6 is required for the differentiation of primary superficial epithelia in Danio and Xenopus embryos

Early in the development of animal embryos, superficial cells of the blastula form a distinct lineage and adopt an epithelial morphology. In different animals, the fate of these primary superficial epithelial (PSE) cells varies, and it is unclear whether pathways governing segregation of blastomeres into the PSE lineage are conserved. Mutations in the gene encoding Interferon Regulatory Factor 6 (IRF6) are associated with syndromic and non-syndromic forms of cleft lip and palate, consistent with a role for Irf6 in development of oral epithelia, and mouse Irf6 targeted null mutant embryos display abnormal differentiation of oral epithelia and skin. In Danio rerio (zebrafish) and Xenopus laevis (African clawed frog) embryos, zygotic irf6 transcripts are present in many epithelial tissues including the presumptive PSE cells and maternal irf6 transcripts are present throughout all cells at the blastula stage. Injection of antisense oligonucleotides with ability to disrupt translation of irf6 transcripts caused little or no effect on development. By contrast, injection of RNA encoding a putative dominant negative Irf6 caused epiboly arrest, loss of gene expression characteristic of the EVL, and rupture of the embryo at late gastrula stage. The dominant negative Irf6 disrupted EVL gene expression in a cell autonomous fashion. These results suggest that Irf6 translated in the oocyte or unfertilized egg suffices for early development. Supporting the importance of maternal Irf6, we show that depletion of maternal irf6 transcripts in X. laevis embryos leads to gastrulation defects and rupture of the superficial epithelium. These experiments reveal a conserved role for maternally-encoded Irf6 in differentiation of a simple epithelium in X. laevis and D. rerio. This epithelium constitutes a novel model tissue in which to explore the Irf6 regulatory pathway.

Grainyhead-like 3, a transcription factor identified in a microarray screen, promotes the specification of the superficial layer of the embryonic epidermis

The Xenopus ectoderm consists of two populations of cells, superficial polarised epithelial cells and deep, non-epithelial cells. These two cell types differ in their developmental fate. In the neural ectoderm, primary neurons are derived only from the deep cells. In the epidermal ectoderm, superficial cells express high levels of differentiation markers, while most of the deep cells do not differentiate until later when they produce the stratified adult epidermis. However, few molecular differences are known between the deep and superficial cells. Here, we have undertaken a systematic approach to identify genes that show layer-restricted expression by microarray analysis of deep and superficial cells at the gastrula stage, followed by wholemount in situ hybridisation. We have identified 32 differentially expressed genes, of which 26 show higher expression in the superficial layer and 6 in the deep layer and describe their expression at the gastrula and neurula stage. One of the identified genes is the transcription factor Grhl3, which we found to be expressed in the superficial layer of the gastrula ectoderm and the neurula epidermis. By using markers identified in this work, we show that Grlh3 promotes superficial gene expression in the deep layer of the epidermis. Concomitantly, deep layer specific genes are switched off, showing that Grlh3 can promote deep cells to take on a superficial cell identity in the embryonic epidermis.

XIC is required for Siamois activity and dorsoanterior development

Siamois is the transcriptional mediator of the dorsal Wnt signaling pathway and is necessary for formation of the Spemann organizer and dorsoanterior development in Xenopus. We have determined that XIC, a Xenopus I-mfa domain protein that regulates Tcf3 binding, is required for dorsoaxial development and specifically for Siamois activity in establishing the dorsal organizer. In loss-of-function studies, we found that embryos injected with a morpholino to XIC mRNA (XIC morphpolino) are missing head structures, neural tube, notochord, and paraxial mesoderm as well as NCAM and XMyoD expression. Although Siamois, Twin, and Xnr3 expression is normal in morpholino-injected embryos, levels of downstream organizer factors, including goosecoid, Xnot, Cerberus, and chordin, are severely reduced. Ectopic axis formation induced by Siamois is repressed by injection of the XIC morpholino and further repressed by coinjection of beta-catenin or a constitutively active Tcf3/HMG/G4A fusion. Activation of reporters driven by the Siamois-responsive proximal element of the goosecoid promoter is inhibited in the presence of the morpholino and can be rescued by murine I-mfa and by a dominant-negative Tcf3. The data indicate a role for XIC in limiting Tcf3-dependent repression of Siamois activities that are required for goosecoid transcription and for dorsal organizer formation.

Specification of the enveloping layer and lack of autoneuralization in zebrafish embryonic explants

We have analyzed the roles of cell contact during determination of the outermost enveloping layer (EVL) and deeper neurectoderm in zebrafish embryos. Outer cells, but not deeper cells, are specified to express the EVL-specific marker, cyt1 by late blastula. EVL specification requires cell contact or close cell proximity, because cyt1 is not expressed after explant dissociation. The EVL may be homologous to the Xenopus epithelial layer, including the ventral larval epidermis. While Xenopus epidermal cytokeratin gene expression is activated by bone morphogenetic protein (BMP) signaling, zebrafish cyt1 is not responsive to BMPs. Zebrafish early gastrula ectodermal explants are specified to express the neural markers opl (zic1) and otx2, and this expression is prevented by BMP4. Dissociation of zebrafish explants prevents otx2 and opl expression, suggesting that neural specification in zebrafish requires cell contact or close cell proximity. This finding is in contrast to the case in Xenopus, where ectodermal dissociation leads to activation of neural gene expression, or autoneuralization. Our data suggest that distinct mechanisms direct development of homologous lineages in different vertebrates.

G1/S phase cyclin-dependent kinase overexpression perturbs early development and delays tissue-specific differentiation in Xenopus

Cell division and differentiation are largely incompatible but the molecular links between the two processes are poorly understood. Here, we overexpress G1/S phase cyclins and cyclin-dependent kinases in Xenopus embryos to determine their effect on early development and differentiation. Overexpression of cyclin E prior to the midblastula transition (MBT), with or without cdk2, results in a loss of nuclear DNA and subsequent apoptosis at early gastrula stages. By contrast, overexpressed cyclin A2 protein does not affect early development and, when stabilised by binding to cdk2, persists to tailbud stages. Overexpression of cyclin A2/cdk2 in post-MBT embryos results in increased proliferation specifically in the epidermis with concomitant disruption of skin architecture and delay in differentiation. Moreover, ectopic cyclin A2/cdk2 also inhibits differentiation of primary neurons but does not affect muscle. Thus, overexpression of a single G1/S phase cyclin/cdk pair disrupts the balance between division and differentiation in the early vertebrate embryo in a tissue-specific manner.

Annexin IV (Xanx-4) has a functional role in the formation of pronephric tubules

Vertebrate kidney organogenesis is characterised by the successive formation of the pronephros, the mesonephros and the metanephros. The pronephros is the first to form and is the functional embryonic kidney of lower vertebrates; although it is vestigial in higher vertebrates, it is a necessary precursor for the other kidney types. The Xenopus pronephros is a simple paired organ; each nephron consists of a single large glomus, one set of tubules and a single duct. The simple organisation of the pronephros and the amenability of Xenopus laevis embryos to manipulation make the Xenopus pronephros an attractive system in which to study organogenesis. It has been shown that pronephric tubules can be induced to form in presumptive ectodermal tissue by treatment with RA and activin. We have used this system in a subtractive hybridisation screen that resulted in the cloning of Xenopus laevis annexin IV (Xanx-4). Xanx-4 transcripts are specifically located to the developing pronephric tubules, and the protein to the luminal surface of these tubules. Temporal expression shows zygotic transcription is upregulated at the time of pronephric tubule specification and persists throughout pronephric development. The temporal and spatial expression pattern of Xanx-4 suggests it may have a role in pronephric tubule development. Overexpression of Xanx-4 yields no apparent phenotype, but Xanx-4 depletion, using morpholinos, produces a shortened, enlarged tubule phenotype. The phenotype observed can be rescued by co-injection of Xanx-4 mRNA. Although the function of annexins is not yet clear, studies have suggested a role for annexins in a number of cellular processes. Annexin IV has been shown to have an inhibitory role in the regulation of epithelial calcium-activated chloride ion conductance. The enlarged pronephric tubule phenotype observed may be attributed to incorrect modulation of exocytosis, membrane plasticity or ion channels and/or water homeostasis. In this study, we demonstrate an in vivo role for annexin IV in the development of the pronephric tubules in Xenopus laevis.

Early anteroposterior division of the presumptive neurectoderm in Xenopus

We analyze the timing of neural patterning in Xenopus and the mechanism by which the early pattern is generated. With regard to timing, we show that by early gastrula, two domains of the anteroposterior (A/P) pattern exist in the presumptive neurectoderm, since the opl gene is expressed throughout the future neural plate, while the fkh5 gene is expressed only in more posterior ectoderm. By mid-gastrula, this pattern has become more elaborate, with an anterior domain defined by expression of opl and otx2, a middle domain defined by expression of opl and fkh5, and a posterior domain defined by expression of opl, fkh5 and HoxD1. Explant assays indicate that the late blastula dorsal ectoderm is specified as the anterior domain, but is not yet specified as middle or posterior domains. With regard to the mechanism by which the A/P pattern is generated, gain and loss of function assays indicate that quantitatively and qualitatively different factors may be involved in inducing the early A/P neural pattern. These data show that neural patterning occurs early in Xenopus and suggest a molecular basis for initiating this pattern.

Xenopus Enhancer of Zeste (XEZ); an anteriorly restricted polycomb gene with a role in neural patterning

We have identified the Xenopus homologue of Drosophila Enhancer of Zeste using a differential display strategy designed to identify genes involved in early anterior neural differentiation. XEZ codes for a protein of 748 amino acids that is very highly conserved in evolution and is 96% identical to both human and mouse EZ(H)2. In common with most other Xenopus Pc-G genes and unlike mammalian Pc-G genes, XEZ is anteriorly restricted. Zygotic expression of XEZ commences during gastrulation, much earlier than other anteriorly localized Pc-G genes; expression is restricted to the anterior neural plate and is confined later to the forebrain, eyes and branchial arches. XEZ is induced in animal caps overexpressing noggin; up-regulation of XEZ therefore represents a response to inhibition of BMP signalling in ectodermal cells. We show that the midbrain/hindbrain junction marker En-2,and hindbrain marker Krox-20, are target genes of XEZ and that XEZ functions to repress these anteroposterior marker genes. Conversely, XEZ does not repress the forebrain marker Otx-2. XEZ overexpression results in a greatly thickened floor of the forebrain. These results implicate an important role for XEZ in the patterning of the nervous system.

Human truncated Smad 6 (Smad 6s) inhibits the BMP pathway in Xenopus laevis

A previously identified truncated form of the human Smad 6 gene containing a unique 12 amino acid motif at its N-terminus was studied. We have named this truncated form of the gene Smad 6s, for 'short-form', to distinguish it from the full-length form (Smad 6fl). Reverse transcription-polymerase chain reaction and immunohistochemistry revealed that Smad 6s has a unique pattern of expression in human coronary tissue and is upregulated in diseased heart tissue. We used the expression of human Smad 6s in Xenopus laevis as a model system to assess Smad 6s function. Injection of Smad 6fl RNA (4-cell embryos, 2 x ventral) produced tadpoles with partial secondary axes. In contrast, Smad 6s RNA injected in a similar manner produced tadpoles with a severe 'head-only' phenotype with no morphological appearance of a secondary axis. Mutant Smad 6s RNA lacking the unique 12 amino acids at the N-terminus of the Smad 6s isoform produced no embryonic phenotype, suggesting that this region is important in conferring biological activity. Ectodermal explant assays show that Smad 6s has activity consistent with being a BMP antagonist and can synergize with and enhance the activities of the activin and fibroblast growth factor pathways, all of which are novel findings in this study.

Xbra3 induces mesoderm and neural tissue in Xenopus laevis

Homologues of the murine Brachyury gene have been shown to be involved in mesoderm formation in several vertebrate species. In frogs, the Xenopus Brachyury homologue, Xbra, is required for normal formation of posterior mesoderm. We report the characterisation of a second Brachyury homologue from Xenopus, Xbra3, which has levels of identity with mouse Brachyury similar to those of Xbra. Xbra3 encodes a nuclear protein expressed in mesoderm in a temporal and spatial manner distinct from that observed for Xbra. Xbra3 expression is induced by mesoderm-inducing factors and overexpression of Xbra3 can induce mesoderm formation in animal caps. In contrast to Xbra, Xbra3 is also able to cause the formation of neural tissue in animal caps. Xbra3 overexpression induces both geminin and Xngnr-1, suggesting that Xbra3 can play a role in the earliest stages of neural induction. Xbra3 induces posterior nervous tissue by an FGF-dependent pathway; a complete switch to anterior neural tissue can be effected by the inhibition of FGF signalling. Neither noggin, chordin, follistatin, nor Xnr3 is induced by Xbra3 to an extent different from their induction by Xbra nor is BMP4 expression differentially affected.

Xenopus Zic family and its role in neural and neural crest development

We characterized two new members of the Zic family, Xenopus Zic1 and Zic2. They are very similar to mouse Zic1 and Zic2 in the protein coding region including the zinc finger domain. In early gastrula, Zic1 expression was restricted to the prospective neural plate region whereas Zic2 was expressed widely in the ectoderm. We observed enhanced neural and neural crest-derived tissue formation in the Zic1 or Zic2 overexpressed embryos and neural and neural crest marker induction in the Zic1 or Zic2 overexpressed animal cap explants. Our findings suggest that Zic1 and Zic2 have essentially the same properties as Zic3 and that the Xenopus Zic family may act cooperatively in the initial phase of neural and neural crest development.

Neural induction and patterning by fibroblast growth factor, notochord and somite tissue in Xenopus

Two natural neural inducing sources have been used, the notochord and the somites together with the growth factor bFGF, to investigate the anterior/posterior patterning of neural tissue in an animal cap explant model in Xenopus laevis. Notochord and somite tissue from stages 12.5/13 and 16, respectively, were manually isolated, and combined heterochronically with responding animal cap ectoderm aged to gastrula stages. Somite recombinants were also constructed with animal caps injected with noggin mRNA. The responses of the ectoderm were analyzed by reverse transcription polymerase chain reaction (RT-PCR) detection of marker gene expression, and in some cases by in situ hybridization. The requirement for FGF receptor function was analyzed using the dominant negative FGF receptor (XFD). The experiments showed that bFGF is capable of direct neural induction in caps aged to stage 10.5. It was also shown that notochords are capable of inducing anterior neural tissue in gastrula stage animal cap ectoderm, and this induction is sensitive to XFD in the responding tissue. Injection of noggin mRNA results in the induction of anterior neural differentiation, and it was demonstrated that this induction was insensitive to the expression of XFD in the responding tissue. It was also shown that somite tissue recombined with gastrula stage animal cap ectoderm, can induce both anterior and posterior nervous tissue and can also posteriorize noggin-induced anterior neural tissue when combined with noggin-injected animal cap ectoderm. This response is partially sensitive to XFD expression. The results shed light on the role of competence of animal cap ectoderm and the signals from postgastrulation axial and paraxial mesoderm in the patterning of the amphibian nervous system.

Autonomous neural axis formation by ectopic expression of the protooncogene c-ski

The ski oncogene was originally isolated as an avian retroviral gene with the ability to induce quail embryonic cells to differentiate into muscle. Mice containing a chicken c-ski transgene exhibit postnatal hypertrophy of skeletal muscle. Xenopus ski (Xski) protein is maternal and present throughout early development. We show that overexpression of Xski RNA in Xenopus embryos results in the cell-autonomous induction of secondary neural axis formation. Injection of Xski RNA into prospective endodermal cells resulted in the formation of an ectopic neural tube-like structure and cells derived from the injected blastomeres populated the spinal cord. Injected Xski RNA was able to induce neural-specific gene expression directly in ectodermal explants in the absence of the expression of mesodermal markers. The widespread distribution of ski protein in the early gastrula embryo including the dorsal animal region supports a role for ski in neural axis formation in vivo.

Xenopus Zic3, a primary regulator both in neural and neural crest development

Xenopus Zic3 is a Xenopus homologue of mouse Zic and Drosophila pair-rule gene, odd-paired. We show here that Zic3 has significant roles both in neural and neural crest development in Xenopus embryo. Expression of Zic3 is first detected in prospective neural plate region at gastrulation. Onset of the expression was earlier than most proneural genes and followed chordin expression. The expression was induced by blockade of BMP4 signal. Overexpression of Zic3 resulted in hyperplastic neural and neural crest derived tissue. In animal cap explant, the overexpression of Zic3 induced expression of all the proneural genes and neural crest marker genes. These findings suggest that Zic3 can determine the ectodermal cell fate and promote the earliest step of neural and neural crest development.

Vertebrate neural induction

During early vertebrate development, the cells of the ectoderm choose between two possible fates: neural and epidermal. The process of neural induction was discovered nearly 70 years ago in vertebrates, and molecular analyses in recent years using Xenopus laevis embryos have identified several secreted factors with direct neural-inducing ability. There is considerable evidence that the mechanism of neuralization by these inducing factors is under inhibitory control and involves derepression. This review focuses on factors involved in the specification of neural fate within the frame of the default model of neural induction.

Positive and negative signals modulate formation of the Xenopus cement gland

The cement gland is a simple secretory organ that marks the anterior-most dorsal ectoderm in Xenopus embryos. In this study, we examine the timing of cement gland induction and the cell interactions that contribute to cement gland formation. Firstly, we show that the outer ectodermal layer, from which the cement gland arises, becomes specified as cement gland by mid-gastrula. Curiously, at early gastrula, the inner layer of the dorsal ectoderm, which does not contribute to the mature cement gland, is strongly and transiently specified as cement gland. Secondly, we show that the mid-gastrula dorsoanterior yolky endoderm, which comes to underlie the cement gland primordium, is a potent inducer of cement gland formation and patterning. The cement gland itself has an anteroposterior pattern, with the gene XA expressed only posteriorly. Dorsoanterior yolky endoderm greatly enhances formation of large, patterned cement glands in partially induced anterodorsal ectoderm, but is unable to induce cement gland in naive animal caps. Neural tissue is induced less frequently than cement gland by the dorsoanterior yolky endoderm, suggesting that the endoderm induces cement gland directly. Thirdly, we demonstrate that the ventral ectoderm adjacent to the cement gland attenuates cement gland differentiation late during gastrulation. The more distant ventral mesendoderm is also a potent inhibitor of cement gland formation. These are the first data showing that normal ventral tissues can inhibit cement gland differentiation and suggest that cement gland size and position may be partly regulated by negative signals. Previous work has shown that cement gland can be induced by neural plate and by dorsal mesoderm. Together, these data suggest that cement gland induction is a complex process regulated by multiple positive and negative cell interactions.

Developmental effects of over-expression of normal and mutated forms of a Xenopus NF-kappa B homologue

High level over-expression of XrelA1, a homologue of the p65 sub-unit of NF-kappa B and of Drosophila dorsal, arrests Xenopus development at the gastrula stage, producing a reduction in the levels of expression of various genes of developmental interest without general reduction in transcription or cessation of cell division. There is little Goosecoid expression, even though a dorsal lip forms. At lower levels XrelA1 mRNA primarily produces disruption of the mid-dorsal axis. A dominant interference gene product, delta 222, produces mainly posterior, but also anterior abnormalities. On the basis of these results we postulate that the role of XrelA1 in the vertebrate embryo is unlikely to be in dorsoventral development, but more likely in the formation of the termini.

Conversion of Xenopus ectoderm into neurons by NeuroD, a basic helix-loop-helix protein

Basic helix-loop-helix (bHLH) proteins are instrumental in determining cell type during development. A bHLH protein, termed NeuroD, for neurogenic differentiation, has now been identified as a differentiation factor for neurogenesis because (i) it is expressed transiently in a subset of neurons in the central and peripheral nervous systems at the time of their terminal differentiation into mature neurons and (ii) ectopic expression of neuroD in Xenopus embryos causes premature differentiation of neuronal precursors. Furthermore, neuroD can convert presumptive epidermal cells into neurons and also act as a neuronal determination gene. However, unlike another previously identified proneural gene (XASH-3), neuroD seems competent to bypass the normal inhibitory influences that usually prevent neurogenesis in ventral and lateral ectoderm and is capable of converting most of the embryonic ectoderm into neurons. The data suggest that neuroD may participate in the terminal differentiation step during vertebrate neuronal development.

Molecular mechanisms of tissue determination and pattern formation in amphibian embryos

Factors of the TGF-beta superfamily (activin, vegetalizing factor) and the FGF family determine endoderm and mesoderm. The dorsoventral polarity of the mesoderm depends on additional factors (BMP-4, Wnt-8, noggin). Activin can directly activate gene transcription by signal transduction. Mesoderm is determined by factors prelocalized in the marginal zone. Its differentiation depends also on the animal ectoderm. Neural inducing factors have been partially purified. A masked neuralizing factor in the ectoderm is activated by induction of the ectoderm to the nervous system. Phorbolester can evoke neuralization signaling.

Isolation of a novel RXR from Xenopus that most closely resembles mammalian RXR beta and is expressed throughout early development

In a search for nuclear receptors that may mediate the teratogenic effects of the potential morphogen, retinoic acid, on the early development of Xenopus we have isolated a novel Xenopus RXR that most closely resembles the mammalian beta-type RXR. Xenopus RXR beta mRNA is expressed throughout early embryogenesis, and functions as an accessory protein to enhance the DNA-binding of other members of the nuclear receptor superfamily.

Expression of achaete-scute homolog 3 in Xenopus embryos converts ectodermal cells to a neural fate

In Drosophila, the proneural genes of the achaete-scute complex encode transcriptional activators that can commit cells to a neural fate. We have isolated cDNAs for two Xenopus achaete-scute homologs, ASH3a and ASH3b, which are expressed in a subset of central nervous system (CNS) neuroblasts during early neurogenesis. After expressing either ASH3 protein in developing Xenopus embryos, we find enlargement of the CNS at the expense of adjacent non-neural ectoderm. Analysis of molecular markers for neural, epidermal, and neural crest cells indicates that CNS expansion occurs as early as neural plate formation. ASH3-dependent CNS enlargement appears to require neural induction, as it does not occur in animal cap explants. Inhibition of DNA synthesis shows that additional CNS tissue does not depend on cell division--rather it reflects conversion of prospective neural crest and epidermal cells to a neural fate. The differentiation of the early forming primary neurons also seems to be prevented by ASH3 expression. This may be secondary to the observed activation of Xotch transcription by ASH3.

Expression of Xenopus snail in mesoderm and prospective neural fold ectoderm

Expression of the Xsna gene during Xenopus laevis embryogenesis has been analysed by in situ hybridisation. Like its homologue snail in Drosophila, Xsna is expressed zygotically in all early mesoderm. Expression starts during stage 9 in the dorsal marginal zone and spreads to the ventral side by stage 10. During gastrulation, each cell begins to express as it involutes so that cells newly expressing Xsna are added to the forming mesoderm mantle in an anterior-to-posterior progression. Xsna expression is then down-regulated in a tissue-specific fashion that reveals the subdivision of the mesoderm before its derivatives are overtly differentiated; e.g., the appearance of the notochord, myotomes, and pronephroi are preceded by the disappearance of Xsna mRNA, while undifferentiated mesoderm remains labelled, even into tadpole stages. Xsna is expressed in the suprablastoporal endoderm during gastrulation and in its derivatives, the prechordal and sub-notochordal endoderm, during neurulation. Relationships between Xbra, Xtwi, and Xsna expression are examined. Xsna is also expressed in the prospective neural fold ectoderm from stage 11 in a low arc above the dorsal marginal zone, precisely identifying a distinct band of cells that surrounds the prospective neural plate that we designate the neural plate border. The anterior transverse neural fold, which becomes forebrain, ceases Xsna expression during neurulation. In the longitudinal neural folds, the deep and superficial ectoderm compartments labelled by Xsna expression are the prospective neural crest and prospective roof of the neural tube, respectively. Xsna expression persists in the neural crest during migration and in some derivatives at least until metamorphosis but ceases in the roof of the neural tube soon after neurulation.

Expression of an extracellular deletion of Xotch diverts cell fate in Xenopus embryos

Xotch is a Xenopus homolog of Notch, a receptor involved in cell fate decisions in Drosophila. Using an extracellular deletion construct, Xotch delta E, we show that Xotch has a similar role in Xenopus embryos. Broad expression causes the loss of dorsal structures and the expansion and disorganization of the brain. Single blastomere injections of Xotch delta E induce autonomous neural and mesodermal hypertrophy, even in the absence of cell division. Xotch delta E inhibits the early expression of epidermal and neural crest markers yet enhances and extends the response of animal caps to mesodermal and neural induction. Our data suggest a mechanism for the function of Notch homologs in which they delay differentiation and leave undetermined cells competent to respond to later inductive signals.

Xenopus blastulae show regional differences in competence for mesoderm induction: correlation with endogenous basic fibroblast growth factor levels

Single cells from the animal cap and marginal zone (MZ) of mid-blastula stage embryos can undergo mesodermal or ectodermal differentiation as small clones under defined conditions in culture. Here we report that cells treated with Xenopus basic fibroblast growth factor (XbFGF), a mesoderm-inducing factor, usually differentiated into muscle. MZ cells, which normally give rise to most of the mesoderm, responded to lower concentrations of XbFGF than animal pole (AP) presumptive ectoderm cells. This difference in sensitivity correlated with immunocytochemical staining patterns that showed much greater levels of endogenous bFGF within MZ than AP cells in early embryos. At the mid-late blastula stage, nuclei of MZ cells were strongly immunoreactive. Nuclear staining persisted during gastrula and neurula stages, and extracellular bFGF also became apparent. Subsequently in somites, immunoreactivity of nuclei declined while that of the extracellular matrix was retained during tailbud stages. Nuclear localization of bFGF appeared to be temporally correlated with new transcription of muscle-specific genes, and extracellular bFGF was present during morphological differentiation. The results suggest that a cell's competence for mesoderm induction is related to its position in the embryo.

The cell cycle dependence of the secretory pathway in developing Xenopus laevis

The cell cycle during the cleavage period of the amphibian Xenopus laevis is about 30 min long and oscillates between equal periods of mitosis and interphase. At the midblastula transition (MBT) the length of interphase begins to elongate and brings about corresponding changes in the activities of cell cycle-dependent processes. In this study protein secretion and Golgi processing during embryonic Xenopus development were examined. The elongation of interphase, either during normal development or experimentally induced, resulted in an increase in the secretion of both endogenous and exogenous proteins. Secretion was found to increase linearly with the increase in interphase length, indicating that the rate of secretion was constant and was regulated by the length of interphase. M-phase arrest in embryos and oocytes produced an inhibition of protein secretion that was reversible if the cell cycle was returned to interphase. This M-phase block of the secretory pathway was found to take place between the trans Golgi compartment and the plasma membrane. The developmental increase in the function of this pathway after the MBT may affect the expression of surface and secreted proteins important for the cell-cell interactions necessary for subsequent development through gastrulation.

Occurrence of nonlymphoid leukocytes that are not derived from blood islands in Xenopus laevis larvae

Previous immunohistochemical observations using the monoclonal antibody (XL-1) which recognizes all types of leukocytes in Xenopus laevis revealed the occurrence of XL-1+ cells in the mesenchyme throughout the early larval body, before the appearance of any lymphocytes. The present experiments were performed to determine whether these leukocytes originate, like lymphocytes and red blood cells (RBCs), in the ventral blood islands (VBI) or the dorsolateral plate (DLP). For tracing the derivation of cells, a specific staining by quinacrine to nuclei of X. laevis and Xenopus borealis hybrid (LB) cells was used to distinguish them from X. laevis (LL) cells. Orthotopic graftings of VBI tissue from st.22-23 LB embryos to the stage-matched LL embryos and examinations at st.44-45 before differentiation of the lymphocytes showed that the proportion of XL-1+ LB cells was always significantly lower than that of RBCs with the same marker in all experimental larvae. The head (LB)-body (LL) chimeras from st.22-23 embryos and culture of the head-portions as VBI- and DLP-free explants from st.14-23 embryos both demonstrated that a significant number of XL-1+ cells which had originated in the head portions had begun to differentiate by st.42-43. These results indicate that there is a significant population of larval nonlymphoid leukocytes (mostly macrophages) that do not originate from either the VBI or DLP region, and are distributed in the mesenchyme throughout the body.

Modulation of neural commitment by changes in target cell contacts in Pleurodeles waltl

In amphibian development, neural structures arise from the presumptive ectoderm at the gastrula stage by an inductive interaction with the chordamesoderm. It has been previously reported that early gastrula presumptive ectoderm can be neuralized when it is dissociated into single cells. A similar result is reported here with regard to Pleurodeles waltl presumptive ectoderm. Using this experimental model system we demonstrate: first, that neuronal and glial lineages can be specified from the presumptive ectoderm without any intervention of the natural inducing tissue; and second, that whereas rupture of cell-cell contacts evoked neural induction, dissociation immediately followed by reaggregation reduces the neuralizing response, pointing toward an active role played by cell-cell contacts of presumptive ectodermal cells in the modulation of neural commitment.

Identification of a retinoic acid-sensitive period during primary axis formation in Xenopus laevis

Retinoic acid (RA) is able to profoundly alter patterning of the primary body axis in embryos of the frog Xenopus laevis. The response to RA is dose-dependent, and leads to progressive truncation of the anteroposterior axis, with anterior structures most sensitive. Both mesodermal and ectodermal tissues are affected, and in vitro assays demonstrate that induced dorsal ectoderm is one direct target of RA. RA represses expression of anterior-specific genes and concomitantly induces expression of at least one posterior-specific gene. Resistance to RA is acquired gradually, during gastrula and early neurula stages, with posterior structures becoming resistant before anterior structures. These data demarcate in the embryo an anterior "domain," which may define the head rudiment and which transcends germ layers. RA can alter the axial pattern after its initial induction; thus, RA sensitivity defines a labile intermediate that occurs during axial patterning. These data suggest a possible role for RA in normal axis formation.

Appearance and distribution of laminin during development of Xenopus laevis

A cloned cDNA fragment, homologous to domain II of the mouse laminin B1 chain, was obtained from a Xenopus neurula cDNA library. Using this probe Northern-blot analysis over the course of embryogenesis revealed a first signal of a laminin transcript at midgastrula (stage 11). Affinity-purified polyclonal antibodies directed against Xenopus laminin were obtained via a lacZ fusion protein. Immunohistology demonstrated the appearance of the antigen at about one developmental stage after the detection of the transcript. At stage 12 1/2 a faint immunofluorescence staining surrounded the developing notochord and somitogenic mesodermal cells. Thereafter laminin appeared in distinct locations outlining notochord, somites and neural tube. The epidermal basement membrane seemed to be endowed with laminin only at the relatively late postneurula stage. The observation that laminin could not be detected before stage 12 1/2 is discussed with respect to a proposed role of laminin in the gastrulation process.

Differential keratin gene expression during the differentiation of the cement gland of Xenopus laevis

The expression of both epidermal and nonepidermal keratins has been detected in the cement gland of Xenopus laevis by antibody staining. Northern blot and in situ hybridizations with gene-specific probes indicated the expression of the nonepidermal keratin, XK endo B, and the embryonic epidermal keratin, XK70, in the cement gland. Furthermore, since explanted animal pole cells can be induced to differentiate into cement gland cells in vitro by incubation in NH4Cl, we have demonstrated the in vitro induction of XK endo B, maintenance of XK70, and repression of another embryonic epidermal keratin, XK81. This is the first report of keratin gene expression in the cement gland.

Developmental expression of regionally specific cell surface antigens in the Xenopus gastrula

Molecular markers for specific cell lineages would be useful in studies of cellular differentiation. To isolate such markers monoclonal antibodies (MoABs) were raised against plasma membranes isolated from gastrulating Xenopus embryos. Those antibodies that recognized subsets of cells within the embryo were selected by indirect immunofluorescence. The analysis of eight such MoAbs is presented. Western blot analysis showed that all but one MoAb recognized a complex pattern of glycoconjugates associated with glycoproteins. All the antigens recognized by the MoAbs were maternal in origin and displayed similar spatial patterns of pregastrular expression. This pattern of immunoreactivity at the apical surface was inherited passively during cleavage by the resulting superficial blastomeres suggesting that ectodermal specific markers of maternal origin are pre-localized to the cortical ooplasm in mature oocytes. We suggest that these maternal components may be specific glycosyl transferases. Three different patterns of expression were observed during gastrulation as exemplified by MoAbs 1F10C1, 3A4D1, and 6F10B6. MoAb 6F10B6 was specific for both neural and non-neural epithelium. MoAb 3A4D1 was specific for non-neural epidermis. MoAb 1F10C1 appeared to recognize a protein epitope on an extracellular component expressed by the superficial and involuting epithelial cells. The pattern of expression for the 1F10C1 antigen suggests that it may play a role in facilitating the movement of the involuting cells during gastrulation.