Enzymatic removal of vitelline membrane and other protocol modifications for whole mount in situ hybridization of Xenopus embryos

Whole-Mount In Situ Hybridization of Xenopus Embryos

Historically, techniques to analyze the localized distribution of mRNAs during development were performed on sectioned embryos using radioactively labeled riboprobes. The processing of the tissues and the use of emulsion autoradiography were laborious and time-consuming, leading to the development of more direct approaches. The nonradioactive whole-mount in situ hybridization method was first introduced in Drosophila embryos, and later adapted to Xenopus embryos for abundant transcripts such as muscle actin. Since then, the technique has been improved and is now broadly used for the spatial detection of even less abundant transcripts in Xenopus The technique has been especially powerful in the analysis of changes in gene expression in embryos manipulated by mRNA or antisense oligonucleotides microinjection, and in animal cap explants exposed to exogenous factors. The protocol described here provides an excellent signal-to-noise ratio for most labeled probes. It also is relatively high-throughput: With a little practice, approximately 50 samples can easily be processed simultaneously.

Agonistic and antagonistic roles for TNIK and MINK in non-canonical and canonical Wnt signalling

Wnt signalling is a key regulatory factor in animal development and homeostasis and plays an important role in the establishment and progression of cancer. Wnt signals are predominantly transduced via the Frizzled family of serpentine receptors to two distinct pathways, the canonical ß-catenin pathway and a non-canonical pathway controlling planar cell polarity and convergent extension. Interference between these pathways is an important determinant of cellular and phenotypic responses, but is poorly understood. Here we show that TNIK (Traf2 and Nck-interacting kinase) and MINK (Misshapen/NIKs-related kinase) MAP4K signalling kinases are integral components of both canonical and non-canonical pathways in Xenopus. xTNIK and xMINK interact and are proteolytically cleaved in vivo to generate Kinase domain fragments that are active in signal transduction, and Citron-NIK-Homology (CNH) Domain fragments that are suppressive. The catalytic activity of the Kinase domain fragments of both xTNIK and xMINK mediate non-canonical signalling. However, while the Kinase domain fragments of xTNIK also mediate canonical signalling, the analogous fragments derived from xMINK strongly antagonize this signalling. Our data suggest that the proteolytic cleavage of xTNIK and xMINK determines their respective activities and is an important factor in controlling the balance between canonical and non-canonical Wnt signalling in vivo.

Developmental expression patterns of Tbx1, Tbx2, Tbx5, and Tbx20 in Xenopus tropicalis

T-box genes have diverse functions during embryogenesis and are implicated in several human congenital disorders. Here, we report the identification, sequence analysis, and developmental expression patterns of four members of the T-box gene family in the diploid frog Xenopus tropicalis. These four genes-Tbx1, Tbx2, Tbx5, and Tbx20-have been shown to influence cardiac development in a variety of organisms, in addition to their individual roles in regulating other aspects of embryonic development. Our results highlight the high degree of evolutionary conservation between orthologs of these genes in X. tropicalis and other vertebrates, both at the molecular level and in their developmental expression patterns, and also identify novel features of their expression. Thus, X. tropicalis represents a potentially valuable vertebrate model in which to further investigate the functions of these genes through genetic approaches.

Cdc42 Effector Protein 2 (XCEP2) is required for normal gastrulation and contributes to cellular adhesion in Xenopus laevis

Background

Rho GTPases and their downstream effector proteins regulate a diverse array of cellular processes during embryonic development, including reorganization of cytoskeletal architecture, cell adhesion, and transcription. Changes in the activation state of Rho GTPases are converted into changes in cellular behavior by a diversity of effector proteins, which are activated in response to changes in the GTP binding state of Rho GTPases. In this study we characterize the expression and function of one such effector, XCEP2, that is present during gastrulation stages in Xenopus laevis.

Results

In a search for genes whose expression is regulated during early stages of embryonic development in Xenopus laevis, a gene encoding a Rho GTPase effector protein (Xenopus Cdc42 effector protein 2, or XCEP2) was isolated, and found to be highly homologous, but not identical, to a Xenopus sequence previously submitted to the Genbank database. These two gene sequences are likely pseudoalleles. XCEP2 mRNA is expressed at constant levels until mid- to late- gastrula stages, and then strongly down-regulated at late gastrula/early neurula stages. Injection of antisense morpholino oligonucleotides directed at one or both pseudoalleles resulted in a significant delay in blastopore closure and interfered with normal embryonic elongation, suggesting a role for XCEP2 in regulating gastrulation movements. The morpholino antisense effect could be rescued by co-injection with a morpholino-insensitive version of the XCEP2 mRNA. Antisense morpholino oligonucleotides were found to have no effect on mesodermal induction, suggesting that the observed effects were due to changes in the behavior of involuting cells, rather than alterations in their identity. XCEP2 antisense morpholino oligonucleotides were also observed to cause complete disaggregation of cells composing animal cap explants, suggesting a specific role of XCEP2 in maintenance or regulation of cell-cell adhesion in early embryos. This loss of cell adhesion could be rescued by co-injection with a morpholino-insensitive version of the XCEP2 mRNA.

Conclusions

XCEP2 appears to be an essential component in the early developmental program in Xenopus laevis. XCEP2 is involved in maintenance of cell-cell adhesion, and as such may constitute a regulatory component that could help to balance the need for tissue integrity and plasticity during the dynamic cellular rearrangements of gastrulation.

A trial for induction of supernumerary primordial germ cells in Xenopus tadpoles by injecting RNA of Xenopus vasa homologue into germline cells of 32-cell embryos

Whether overexpression of Xenopus vasa homologue or Xenopus vasa-like gene 1 (XVLG1) in germline cells of Xenopus embryos can induce supernumerary primordial germ cells (PGC) at tadpole stage was investigated. XVLG1 RNA (0.1-2.0 ng) and beta-gal RNA (0.5 ng) were injected into one of, usually, four germ plasm-bearing cells (GPBC) of 32-cell embryos, with the beta-gal RNA (2.0 ng) serving as both lineage tracer and control for XVLG1 RNA. The total number of PGC, including X-gal-stained and unstained PGC of injected and uninjected GPBC origins respectively, was examined in the experimental tadpoles developed from the injected embryos. The injected RNA, XVLG1 and beta-gal RNA, were translated, resulting in a large amount of corresponding proteins in presumptive PGC (pPGC) as well as in somatic cells derived from the injected GPBC. Nevertheless, the average number of total PGC per tadpole found in the experimental tadpoles from the XVLG1 RNA-injected embryos was not significantly different from that of beta-gal RNA-injected ones, irrespective of the injected dose of XVLG1 RNA. This indicates that the extra XVLG1 protein in pPGC is not sufficient to increase the number of PGC in the tadpoles.

Vertebrate HoxB gene expression requires DNA replication

To study the relationship between DNA replication and transcription in vivo, we investigated Hox gene activation in two vertebrate systems: the embryogenesis of Xenopus and the retinoic acid-induced differentiation of pluripotent mouse P19 cells. We show that the first cell cycles following the mid- blastula transition in Xenopus are necessary and sufficient for HoxB activation, whereas later cell cycles are necessary for the correct expression pattern. In P19 cells, HoxB expression requires proliferation, and the entire locus is activated within one cell cycle. Using synchronous cultures, we found that activation of HoxB genes is colinear within a single cell cycle, occurs during S phase and requires S phase. The HoxB locus replicates early, whereas replication is still required for maximal expression later in S phase. Thus, induction of HoxB genes occurs in a DNA replication-dependent manner and requires only one cell cycle. We propose that S-phase remodelling licenses the locus for transcriptional regulation.

A tissue restricted role for the Xenopus Jun N-terminal kinase kinase kinase MLK2 in cement gland and pronephric tubule differentiation

The MLK family of mitogen activated protein kinase kinase kinases (MAPKKK) has been shown to activate Jun N-terminal kinase/stress-activated protein kinase 1 (JNK/SAPK1). However, little is known of the in vivo functions of the MLKs. We have identified a Xenopus laevis MLK that shows highest homology with mammalian MLK2 (62%) and, like MLK2, interacts preferentially with the Rho-family GTPase Rac. xMLK2 was expressed zygotically from late gastrula/early neurula. Surprisingly, this expression was restricted to the cement gland, the brain, and the pronephros. In the differentiating cement gland, xMLK2 expression correlated with cell elongation and the onset of a previously unobserved apoptotic phase, while in the pronephros, expression corresponded with the differentiation and opening of the nephric tubules. Overexpression of xMLK2 in COS7 cells led to a SEK1/MKK4 (MAPKK)-dependent hyperactivation of JNK in response to UV irradiation. xMLK2 was shown to be required for normal cement gland development and pronephric tubule formation using antisense inactivation and a dominant negative xMLK2. The data suggest a novel role for the MLKs as tissue-restricted mediators of signal transduction. They also suggest that tissue-specific responses to common extracellular signals may in part result from the programmed expression of MAPKKKs with differing specificities.

Multiple isoforms of the high molecular weight microtubule associated protein XMAP215 are expressed during development in Xenopus

We have cloned and sequenced cDNAs encoding two isoforms of XMAP215, a high molecular weight microtubule-associated protein identified in Xenopus eggs. XMAP215 is approximately 80% identical in amino acid sequence to the product of ch-TOG, a cDNA that is over expressed in certain human tumors [Charrasse et al., 1995: Eur J Biochem 234:406-413]. Northern and Western blots demonstrated that XMAP215 is expressed throughout development, from oogenesis to tadpole. We identified two XMAP215 transcripts differing only in the presence of a 108-bp sequence encoding a 36 amino acid insert. RT-PCR revealed that the transcripts encoding these two isoforms are expressed at distinct times during development: a transcript containing the insert (encoding XMAP215(M)) is expressed during oogenesis and is present through gastrulation. The second transcript (encoding XMAP215(Z)) lacks the 108-bp insert and is expressed from gastrulation onward. In situ hybridization demonstrated that XMAP215 transcripts are localized to the ectoderm of early embryos and in the developing nervous system during later development. These results suggest that XMAP215 plays important roles in at least two phases of development: (1) regulating the assembly of MTs during the rapid cell divisions after fertilization, and (2) regulating MT assembly during the development of the nervous system.

Spatio-temporal expression of Xenopus vasa homolog, XVLG1, in oocytes and embryos: the presence of XVLG1 RNA in somatic cells as well as germline cells

The expression of Xenopus vasa homolog or XVLG1 was examined in oocytes and embryos by whole-mount in situ hybridization and reverse transcription polymerase chain reaction (RT-PCR). To confirm the results in embryos, both methods were also applied to explants of germ plasm-bearing cells (GPBC) from 32-cell embryos and to those of partial embryos deprived of GPBC. By hybridization, XVLG1 ribonucleic acid (RNA) was shown to be present throughout the cytoplasm in oocytes at stages I-III, except for the mitochondrial cloud. It was barely recognizable in a portion of germline cells of embryos at specific stages, notwithstanding that XVLG1 protein was present in those cells almost throughout their life-span. A weak signal for the RNA was detectable in some of the presumptive primordial germ cells (pPGC, descendants of GPBC from the gastrula stage onward) from the late gastrula (stage 12) to the hatching tadpole stage (stage 33/34), and in some of the PGC at stages 49-50. The results for pPGC were confirmed by the hybridization of explants of GPBC at equivalent stages in control embryos. In contrast, XVLG1 RNA was detected in certain somatic cells of embryos until stage 46. These observations were supported in part by the results of RT-PCR for embryos and explants. The possible role of the product of XVLG1 was reconsidered given its presence in both germline and somatic cells.

Xenopus axin interacts with glycogen synthase kinase-3 beta and is expressed in the anterior midbrain

Axin is encoded by the fused locus in mice and is required for normal vertebrate axis formation. It has recently been shown that axin associates with APC, beta-catenin and glycogen synthase kinase-3 (GSK-3) in a complex that appears to regulate the level of cytoplasmic beta-catenin. We have identified the Xenopus homologue of axin through its interaction with GSK-3b. Xenopus axin (Xaxin) is expressed maternally and throughout early development with a low level of ubiquitous expression. Xaxin also shows remarkably high expression in the anterior mesencephalon adjacent to the forebrain-midbrain boundary.

Frizzled-8 is expressed in the Spemann organizer and plays a role in early morphogenesis

Wnts are secreted signaling molecules implicated in a large number of developmental processes. Frizzled proteins have been identified as likely receptors for Wnt ligands in vertebrates and invertebrates, but a functional role for vertebrate frizzleds has not yet been defined. To assess the endogenous role of frizzled proteins during vertebrate development, we have identified and characterized a Xenopus frizzled gene (xfz8). It is highly expressed in the deep cells of the Spemann organizer prior to dorsal lip formation and in the early involuting marginal zone. Ectopic expression of xfz8 in ventral cells leads to complete secondary axis formation and can synergize with Xwnt-8 while an inhibitory form of xfz8 (Nxfz8) blocks axis duplication by Xwnt-8, consistent with a role for xfz8 in Wnt signal transduction. Expression of Nxfz8 in dorsal cells has profound effects on morphogenesis during gastrulation and neurulation that result in dramatic shortening of the anterior-posterior axis. Our results suggest a role for xfz8 in morphogenesis during the gastrula stage of embryogenesis.

Xpat, a gene expressed specifically in germ plasm and primordial germ cells of Xenopus laevis

cDNAs specific to vegetal poles of Xenopus gastrula embryos were used as a probe to screen a gastrula vegetal pole cDNA library. One of the novel clones isolated had an RNA expression pattern consistent with it being a component of germ plasm and it was thus named Xpat (Xenopus primordial germ cell associated transcript). The open reading frame encodes a 35 kDa protein with no clear homologies. The RNA is localised to the vegetal pole throughout oogenesis and early cleavage. During gastrulation cells containing this message move internally and at tailbud stages they migrate in an antero-dorsal direction. Xpat mRNA is not detectable once the dorsal mesentery forms. We show that the 3'-UTR is required and is sufficient for localisation of exogenous RNA to the vegetal pole of oocytes. We propose that Xpat UTR-containing transcripts can be localised by the Vg1 or late pathway of mRNA localisation during stage III of oogenesis, but endogenous Xpat appears to be localised earlier by a mitochondrial cloud mechanism similar to that proposed for Xcat-2.

XCoe2, a transcription factor of the Col/Olf-1/EBF family involved in the specification of primary neurons in Xenopus

BACKGROUND

Primary neurogenesis in Xenopus is a model for studying the control of neural cell fate decisions. The specification of primary neurons appears to be driven by transcription factors containing a basic region and a helix-loop-helix (HLH) motif: expression of Xenopus neurogenin-related-1 (X-ngnr-1) defines the three prospective domains of primary neurogenesis, and expression of XNeuroD coincides with neuronal differentiation. The transition between neuronal competence and stable commitment to a neuronal fate remains poorly characterised, however.

RESULTS

Drosophila Collier and rodent early B-cell factor/olfactory-1 define a family of HLH transcription factors containing a previously unknown type of DNA-binding domain. We isolated an orthologous gene from Xenopus, Xcoe2, which is expressed in precursors of primary neurons. Xcoe2 is transcribed after X-ngnr-1 and before XNeuroD. Overexpression of a dominant-negative mutant of XCoe2 prevented neuronal differentiation. Conversely, overexpressed wild-type Xcoe2 could promote ectopic differentiation of neurons, in both the neural plate and the epidermis. In contrast to studies with X-ngnr-1 or XNeuroD, the supernumerary neurons induced by Xcoe2 appeared in a 'salt-and-pepper' pattern, resulting from the activation of X-Delta1 expression and feedback regulation by lateral inhibition.

CONCLUSIONS

XCoe2 may play a pivotal role in the transcriptional cascade that specifies primary neurons in Xenopus embryos: by maintaining Delta-Notch signalling, XCoe2 stabilises the higher neural potential of selected progenitor cells that express X-ngnr-1, ensuring the transition between neural competence and irreversible commitment to a neural fate; and it promotes neuronal differentiation by activating XNeuroD expression, directly or indirectly.

Xsox17alpha and -beta mediate endoderm formation in Xenopus

We have isolated two Xenopus relatives of murine Sox17 expressed in gastrula presumptive endoderm. Xsox17alpha and -beta expression can be induced in animal caps by activin, but not by FGF. Ectopic expression of these genes in animal caps induces the expression of endoderm markers; this induction is blocked by overexpression of a fusion of the Xsox17beta HMG domain to the Drosophila Engrailed repressor domain, as is induction of endoderm markers by activin and the expression of endodermal markers in whole embryos and isolated vegetal poles. These experiments, as well as the effects of the mRNAs on embryo phenotypes, suggest that the Xsox17 genes mediate an activin-induced endoderm differentiation pathway in animal caps and are involved in normal endoderm differentiation in embryos.

Identification of a chick homologue of Fringe and C-Fringe 1: involvement in the neurogenesis and the somitogenesis

In Drosophila, Serrate plays an important role to the wing margin formation. A putative secretory protein, Fringe, is indispensable for the wing margin formation inducing Serrate and other genes. Recently, Xenopus homologues of Fringe were identified and one of them, lunatic Fringe (X-lFng), was demonstrated to be involved in mesoderm induction. We have identified two chick Fringe homologous genes by reverse transcription-polymerase chain reaction and cDNA library screening. One of them, C-Fringe 1, showed sequence similarity to X-lFng. In situ hybridization study of C-Fringe 1 has demonstrated its expression in the developing nervous system and in the presomitic mesoderm. The hindbrain and spinal cord showed the distinct stripe pattern expression which was complementary to that of C-Serrate, indicating the correlation between them in vertebrate.