The POU-er of gene nomenclature.

The pluripotency factor POU5F1 (OCT4) is well known as a key regulator of stem cell fate. Homologues of POU5F1 exist throughout vertebrates, but the evolutionary and functional relationships between the various family members have been unclear. The level to which function has been conserved within this family provides insight into the evolution of early embryonic potency. Here, we seek to clarify the relationship between POU5F1 homologues in the vertebrate lineage, both phylogenetically and functionally. We resolve the confusion over the identity of the zebrafish gene, which was originally named pou2, then changed to pou5f1 and again, more recently, to pou5f3. We argue that the use of correct nomenclature is crucial when discussing the degree to which the networks regulating early embryonic differentiation are conserved.

Long- and short-range signals control the dynamic expression of an animal hemisphere-specific gene in Xenopus.

Little is known of the control of gene expression in the animal hemisphere of the Xenopus embryo. Here we show that expression of FoxI1e, a gene essential for normal ectoderm formation, is expressed regionally within the animal hemisphere, in a highly dynamic fashion. In situ hybridization shows that FoxI1e is expressed in a wave-like fashion that is initiated on the dorsal side of the animal hemisphere, extends across to the ventral side by the mid-gastrula stage, and is then turned off in the dorsal ectoderm, the neural plate, at the neurula stage. It is confined to the inner layers of cells in the animal cap, and is expressed in a mosaic fashion throughout. We show that this dynamic pattern of expression is controlled by both short- and long-range signals. Notch signaling controls both the mosaic, and dorsal/ventral changes in expression, and is controlled, in turn, by Vg1 signaling from the vegetal mass. FoxI1e expression is also regulated by nodal signaling downstream of VegT. Canonical Wnt signaling contributes only to late changes in the FoxI1e expression pattern. These results provide new insights into the roles of vegetally localized mRNAs in controlling zygotic genes expressed in the animal hemisphere by long-range signaling. They also provide novel insights into the role of Notch signaling at the earliest stages of vertebrate development.

Microarray-based identification of VegT targets in Xenopus.

The Xenopus T box family member VegT is expressed maternally in the vegetal hemisphere of the embryo. Mis-expression of VegT in prospective ectodermal tissue causes ectopic activation of mesodermal and endodermal markers, and ablation of VegT transcripts prevents proper formation of the mesendoderm, with the entire embryo developing as epidermis. These observations define VegT as a key initiator of mesendodermal development in the Xenopus embryo, and in an effort to understand how it exerts its effects we have used microarray analysis to compare gene expression in control animal caps with that in ectodermal tissue expressing an activated form of VegT. This procedure allowed the identification of 99 potential VegT targets, and we went on to study the expression patterns of these genes and then to ask, for those that are expressed in mesoderm or endoderm, which are direct targets of VegT. The putative regulatory regions of the resulting 14 genes were examined for T domain binding sites, and we also asked whether their expression is down-regulated in embryos in which VegT RNA is ablated. Finally, the functions of these genes were assayed by both over-expression and by use of antisense morpholino oligonucleotides. Our results provide new insights into the function of VegT during early Xenopus development.

Cloning and expression of Xenopus Lrp5 and Lrp6 genes.

LRP5 and LRP6 comprise a subfamily of lipoprotein-receptor related proteins that function as co-receptors for Wnt proteins. Mutation of human LRP5 is responsible for osteoporosis-pseudoglioma syndrome and disruption of Lrp6 in mice causes similar effects to mutation of several different Wnt genes. We have cloned Xenopus homologues of Lrp5 and Lrp6 (Xlrp5, Xlrp6) and examined their expression during embryogenesis. Both genes are expressed maternally and ubiquitously through early development. At later stages, Xlrp5 is found in the eye, forebrain, hindbrain, branchial arches and the tip of the tail bud. Xlrp6 is expressed throughout the central nervous system, branchial arches, in the eye and otic vesicle. Both genes are also expressed at the intersomitic boundary. These results suggest roles for Wnt signaling via LRP proteins in these tissues.

Ablating hedgehog signaling in tenocytes during development impairs biomechanics and matrix organization of the adult murine patellar tendon enthesis.

Restoring the native structure of the tendon enthesis, where collagen fibers of the midsubstance are integrated within a fibrocartilaginous structure, is problematic following injury. As current surgical methods fail to restore this region adequately, engineers, biologists, and clinicians are working to understand how this structure forms as a prerequisite to improving repair outcomes. We recently reported on the role of Indian hedgehog (Ihh), a novel enthesis marker, in regulating early postnatal enthesis formation. Here, we investigate how inactivating the Hh pathway in tendon cells affects adult (12-week) murine patellar tendon (PT) enthesis mechanics, fibrocartilage morphology, and collagen fiber organization. We show that ablating Hh signaling resulted in greater than 100% increased failure insertion strain (0.10 v. 0.05 mm/mm, p<0.01) as well as sub-failure biomechanical deficiencies. Although collagen fiber orientation appears overtly normal in the midsubstance, ablating Hh signaling reduces mineralized fibrocartilage by 32%, leading to less collagen embedded within mineralized tissue. Ablating Hh signaling also caused collagen fibers to coalesce at the insertion, which may explain in part the increased strains. These results indicate that Ihh signaling plays a critical role in the mineralization process of fibrocartilaginous entheses and may be a novel therapeutic to promote tendon-to-bone healing.

Transcriptional profiling identifies genes differentially expressed during and after migration in murine primordial germ cells.

Mouse primordial germ cells (PGCs) are migratory until they colonize the genital ridges, assemble with the somatic tissue, and start to differentiate into oocytes or spermatogonia. Using cell transplantation experiments, we show here that germ cells isolated during migration (at E10.5) will migrate actively to the genital ridges, whereas post-migratory PGCs isolated from E12.5 embryos are non-motile even when transferred into a permissive environment (e.g. E10.5 host tissue). Major transcriptional changes must take place between E10.5 and E12.5 that convert germ cells from a migratory to a non-migratory state. To identify the genes involved, we have performed transcriptional profiling of motile and non-motile populations of PGCs. We have identified 55 transcripts that are expressed in E10.5 PGCs at levels at least 3 x their expression at E12.5, and 48 transcripts with the reciprocal expression levels. Additionally, 309 transcripts were found to be expressed in both populations. Many of the E10.5 transcripts encode proteins involved in controlling cytoskeletal and adhesive interactions implicated in cell motility. Many of the E12.5 transcripts encode proteins implicated in germ cell differentiation.

Steel factor controls primordial germ cell survival and motility from the time of their specification in the allantois, and provides a continuous niche throughout their migration.

Steel factor is an essential survival and proliferation factor for primordial germ cells (PGCs) during their migration in the early mouse embryo. PGCs arise during gastrulation, and migrate into the posterior endoderm that becomes the hindgut. Previous reports have suggested that PGCs become dependent on Steel factor when they colonize the hindgut. However, in the absence of a good marker for living PGCs, their behavior before hindgut colonization has not been previously studied. We report here the normal behavior of PGCs in live embryos before hindgut colonization, and the roles of Steel factor, using a reporter line in which GFP is driven by the promoter of the Stella gene, whose activation accompanies the initial specification of PGCs. We show first that PGCs are surrounded by Steel factor-expressing cells from their first appearance in the allantois to the time they enter the genital ridges. Second, fewer PGCs are found in the allantois in Steel-null embryos, but this is not due to a failure of PGC specification. Third, the analysis of cultured Steel-null early embryos shows that Steel factor is required for normal PGC motility, both in the allantois and in the hindgut. Germ cells migrate actively in the allantois, and move directionally from the allantois into the proximal epiblast. In the absence of Steel factor, caused by either null mutation or antibody blockade, PGC motility is dramatically decreased, but directionality is maintained, demonstrating a primary role for Steel factor in PGC motility. This was found both before and after colonization of the hindgut. These data, together with previously published data, show that PGCs are Steel factor dependent from their initial specification until they colonize the genital ridges, and suggest the existence of a ;spatio-temporal niche' that travels with this important pluripotential cell population in the embryo.

Vg 1 is an essential signaling molecule in Xenopus development.

Xenopus Vg 1, a transforming growth factor beta (Tgfbeta) family member, was one of the first maternally localized mRNAs identified in vertebrates. Its restriction to the vegetal pole of the egg made it the ideal candidate to be the mesoderm-inducing signal released by vegetal cells, but its function in vivo has never been resolved. We show that Vg 1 is essential for Xenopus embryonic development, and is required for mesoderm induction and for the expression of several key Bmp antagonists. Although the original Vg 1 transcript does not rescue Vg 1-depleted embryos, we report that a second allele is effective. This work resolves the mystery of Vg 1 function, and shows it to be an essential maternal regulator of embryonic patterning.

Maternal XTcf1 and XTcf4 have distinct roles in regulating Wnt target genes.

Wnt signaling pathways have essential roles in developing embryos and adult tissue, and alterations in their function are implicated in many disease processes including cancers. The major nuclear transducers of Wnt signals are the Tcf/LEF family of transcription factors, which have binding sites for both the transcriptional co-repressor groucho, and the co-activator beta-catenin. The early Xenopus embryo expresses three maternally inherited Tcf/LEF mRNAs, and their relative roles in regulating the expression of Wnt target genes are not understood. We have addressed this by using antisense oligonucleotides to deplete maternal XTcf1 and XTcf4 mRNAs in oocytes. We find that XTcf1 represses expression of Wnt target genes ventrally and laterally, and activates their expression dorsally. Double depletions of XTcf1 and XTcf3 suggest that they act cooperatively to repress Wnt target genes ventrally. In contrast, XTcf4 has no repressive role but is required to activate expression of Xnr3 and chordin in organizer cells at the gastrula stage. This work provides evidence for distinct roles for XTcfs in regulating Wnt target gene expression.

Jun NH2-terminal kinase (JNK) prevents nuclear beta-catenin accumulation and regulates axis formation in Xenopus embryos.

Jun NH(2)-terminal kinases (JNKs) regulate convergent extension movements in Xenopus embryos through the noncanonical Wnt/planar cell polarity pathway. In addition, there is a high level of maternal JNK activity spanning from oocyte maturation until the onset of gastrulation that has no defined functions. Here, we show that maternal JNK activation requires Dishevelled and JNK is enriched in the nucleus of Xenopus embryos. Although JNK activity is not required for the glycogen synthase kinase-3-mediated degradation of beta-catenin, inhibition of the maternal JNK signaling by morpholino-antisense oligos causes hyperdorsalization of Xenopus embryos and ectopic expression of the Wnt/beta-catenin target genes. These effects are associated with an increased level of nuclear and nonmembrane-bound beta-catenin. Moreover, ventral injection of the constitutive-active Jnk mRNA blocks beta-catenin-induced axis duplication, and dorsal injection of active Jnk mRNA into Xenopus embryos decreases the dorsal marker gene expression. In mammalian cells, activation of JNK signaling reduces Wnt3A-induced and beta-catenin-mediated gene expression. Furthermore, activation of JNK signaling rapidly induces the nuclear export of beta-catenin. Taken together, these results suggest that JNK antagonizes the canonical Wnt pathway by regulating the nucleocytoplasmic transport of beta-catenin rather than its cytoplasmic stability. Thus, the high level of sustained maternal JNK activity in early Xenopus embryos may provide a timing mechanism for controlling the dorsal axis formation.

Lysophosphatidic acid signaling controls cortical actin assembly and cytoarchitecture in Xenopus embryos.

The mechanisms that control shape and rigidity of early embryos are not well understood, and yet are required for all embryonic processes to take place. In the Xenopus blastula, the cortical actin network in each blastomere is required for the maintenance of overall embryonic shape and rigidity. However, the mechanism whereby each cell assembles the appropriate pattern and number of actin filament bundles is not known. The existence of a similar network in each blastomere suggests two possibilities: cell-autonomous inheritance of instructions from the egg; or mutual intercellular signaling mediated by cell contact or diffusible signals. We show that intercellular signaling is required for the correct pattern of cortical actin assembly in Xenopus embryos, and that lysophosphatidic acid (LPA) and its receptors, corresponding to LPA1 and LPA2 in mammals, are both necessary and sufficient for this function.

A novel G protein-coupled receptor, related to GPR4, is required for assembly of the cortical actin skeleton in early Xenopus embryos.

As the fertilized Xenopus egg undergoes sequential cell divisions to form a blastula, each cell develops a network of cortical actin that provides shape and skeletal support for the whole embryo. Disruption of this network causes loss of shape and rigidity of the embryo, and disrupts gastrulation movements. We previously showed that lysophosphatidic acid (LPA) signaling controls the change in cortical actin density that occurs at different stages of the cell cycle. Here, we use a gain-of-function screen, using an egg cDNA expression library, to identify an orphan G protein-coupled cell-surface receptor (XFlop) that controls the overall amount of cortical F-actin. Overexpression of XFlop increases the amount of cortical actin, as well as embryo rigidity and wound healing, whereas depletion of maternal XFlop mRNA does the reverse. Both overexpression and depletion of XFlop perturb gastrulation movements. Reciprocal rescue experiments, and comparison of the effects of their depletion in early embryos, show that the XLPA and XFlop signaling pathways play independent roles in cortical actin assembly, and thus that multiple signaling pathways control the actin skeleton in the blastula.

aPKC, Crumbs3 and Lgl2 control apicobasal polarity in early vertebrate development.

In early vertebrate development, apicobasally polarised blastomeres divide to produce inner non-polarised cells and outer polarised cells that follow different fates. How the polarity of these early blastomeres is established is not known. We have examined the role of Crumbs3, Lgl2 and the apical aPKC in the polarisation of frog blastomeres. Lgl2 localises to the basolateral membrane of blastomeres, while Crumbs3 localises to the apical and basolateral membranes. Overexpression aPKC and Crumbs3 expands the apical domain at the expense of the basolateral and repositions tight junctions in the new apical-basolateral interface. Loss of aPKC function causes loss of apical markers and redirects basolateral markers ectopically to the apical membrane. Cell polarity and tight junctions, but not cell adhesion, are lost and outer polarised cells become inner-like apolar cells. Overexpression of Xenopus Lgl2 phenocopies the aPKC knockout, suggesting that Lgl2 and aPKC act antagonistically. This was confirmed by showing that aPKC and Lgl2 can inhibit the localisation of each other and that Lgl2 rescues the apicalisation caused by aPKC. We conclude that an instrumental antagonistic interaction between aPKC and Lgl2 defines apicobasal polarity in early vertebrate development.

The roles of FGF signaling in germ cell migration in the mouse.

Fibroblast growth factor (FGF) signaling is thought to play a role in germ cell behavior. FGF2 has been reported to be a mitogen for primordial germ cells in vitro, whilst combinations of FGF2, steel factor and LIF cause cultured germ cells to transform into permanent lines of pluripotent cells resembling ES cells. However, the actual function of FGF signaling on the migrating germ cells in vivo is unknown. We show, by RT-PCR analysis of cDNA from purified E10.5 germ cells, that germ cells express two FGF receptors: Fgfr1-IIIc and Fgfr2-IIIb. Second, we show that FGF-mediated activation of the MAP kinase pathway occurs in germ cells during their migration, and thus they are potentially direct targets of FGF signaling. Third, we use cultured embryo slices in simple gain-of-function experiments, using FGF ligands, to show that FGF2, a ligand for FGFR1-IIIc, affects motility, whereas FGF7, a ligand for FGFR2-IIIb, affects germ cell numbers. Loss of function, using a specific inhibitor of FGF signaling, causes increased apoptosis and inhibition of cell shape change in the migrating germ cells. Lastly, we confirm in vivo the effects seen in slice cultures in vitro, by examining germ cell positions and numbers in embryos carrying a loss-of-function allele of FGFR2-IIIb. In FGFR2-IIIb(-/-) embryos, germ cell migration is unaffected, but the numbers of germ cells are significantly reduced. These data show that a major role of FGF signaling through FGFR2-IIIb is to control germ cell numbers. The data do not discriminate between direct and indirect effects of FGF signaling on germ cells, and both may be involved.

The role of Mixer in patterning the early Xenopus embryo.

The transcription factor VegT, is required in early Xenopus embryos for the formation of both the mesoderm and endoderm germ layers. Inherited as a maternal mRNA localized only in vegetal cells, VegT activates the transcription of a large number of transcription factors, as well as signaling ligands that induce cells in the vegetal mass to form endoderm, and the marginal zone to form mesoderm. It is important now to understand the extent to which transcription factors downstream of VegT play individual, or overlapping, roles in the specification and patterning of the endoderm and mesoderm. In addition, it is important to understand the mechanism that specifies the boundary between endoderm and mesoderm. One of the downstream targets of VegT, the homeodomain protein Mixer, is expressed at high levels at the mesoderm/endoderm boundary at the late blastula stage. We therefore examined its functions by blocking its translation using morpholino oligos. In Mixer-depleted embryos, the expression of many signaling ligands and transcription factors was affected. In particular, we found that the expression of several genes, including several normally expressed in mesoderm, was upregulated. Functional assays of Mixer-depleted vegetal cells showed that they have increased mesoderm-inducing activity. This demonstrates that Mixer plays an essential role in controlling the amount of mesoderm induction by the vegetal cells.

The role of maternal CREB in early embryogenesis of Xenopus laevis.

In Xenopus embryos, body patterning and cell specification are initiated by transcription factors, which are themselves transcribed during oogenesis, and their mRNAs are stored for use after fertilization. We have previously shown that the T-box transcription factor VegT is both necessary and sufficient to initiate transcription of all endoderm, and most mesoderm genes. In the absence of maternal VegT, no mesodermal organs (including the heart) or endodermal organs form. A second maternal transcription factor XTcf3 acts as a global repressor of transcription of dorsal genes, whose repression is inactivated on the dorsal side by a maternally encoded Wnt signaling pathway. In the absence of beta-catenin, no mesodermal or endodermal organs form. We show here that the maternally encoded transcription factor CREB is also essential for development. It is required for the initiation of expression of several mesodermal genes, including Xbra, Xcad2, and -3 and also regulates the cardiogenic gene Nkx 2-5. We show that maternal CREB-depleted embryos develop gastrulation defects that are rescued by the reintroduction of activated CREB mRNA. We conclude that maternal CREB must be added to the list of essential maternal transcription factors regulating cell specification in the early embryo.

New roles for FoxH1 in patterning the early embryo.

FoxH1 (Fast1) was first characterized as the transcriptional partner for Smad proteins. Together with Smad2/4, it forms the activin response factor (ARF) that binds to the Mix.2 promoter in Xenopus embryos. Foxh1 is expressed maternally in Xenopus. Depletion of maternal Foxh1 mRNA results in abnormalities of head and dorsal axis formation. We show that FoxH1 is required, together with XTcf3/beta catenin, to activate the zygotic expression of the nodal gene, Xnr3 in a Smad2-independent manner. In contrast, maternal FoxH1 acts as an inhibitor of Xnr5 and 6 transcription, preventing their upregulation on the ventral side of the embryo, by the maternal T-box transcription factor VegT. We conclude that maternal FoxH1 has essential, context-dependent roles in regulating the pattern of zygotic gene expression in the early embryo.

A novel role for a nodal-related protein; Xnr3 regulates convergent extension movements via the FGF receptor.

Convergent extension behaviour is critical for the formation of the vertebrate body axis. In Xenopus, components of the Wnt signaling pathway have been shown to be required for convergent extension movements but the relationship between cell fate and morphogenesis is little understood. We show by loss of function analysis that Xnr3 activates Xbra expression through FGFR1. We show that eFGF activity is not essential in the pathway, and that dishevelled acts downstream of Xnr3 and not in a parallel pathway. We provide evidence for the involvement of the EGF-CFC protein FRL1, and suggest that the pro-domain of Xnr3 may be required for its activity. Since Xnr3 is a direct target of the maternal betacatenin/XTcf3 signaling pathway, it provides the link between the initial, maternally controlled, allocation of cell fate, and the morphogenetic movements of cells derived from the organizer.

Primordial germ cell migration in the chick and mouse embryo: the role of the chemokine SDF-1/CXCL12.

As in many other animals, the primordial germ cells (PGCs) in avian and reptile embryos are specified in positions distinct from the positions where they differentiate into sperm and egg. Unlike in other organism however, in these embryos, the PGCs use the vascular system as a vehicle to transport them to the region of the gonad where they exit the blood vessels and reach their target. To determine the molecular mechanisms governing PGC migration in these species, we have investigated the role of the chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) in guiding the cells towards their target in the chick embryo. We show that sdf-1 mRNA is expressed in locations where PGCs are found and towards which they migrate at the time they leave the blood vessels. Ectopically expressed chicken SDF-1alpha led to accumulation of PGCs at those positions. This analysis, as well as analysis of gene expression and PGC behavior in the mouse embryo, suggest that in both organisms, SDF-1 functions during the second phase of PGC migration, and not at earlier phases. These findings suggest that SDF-1 is required for the PGCs to execute the final migration steps as they transmigrate through the blood vessel endothelium of the chick or the gut epithelium of the mouse.

The chemokine SDF1/CXCL12 and its receptor CXCR4 regulate mouse germ cell migration and survival.

In mouse embryos, germ cells arise during gastrulation and migrate to the early gonad. First, they emerge from the primitive streak into the region of the endoderm that forms the hindgut. Later in development, a second phase of migration takes place in which they migrate out of the gut to the genital ridges. There, they co-assemble with somatic cells to form the gonad. In vitro studies in the mouse, and genetic studies in other organisms, suggest that at least part of this process is in response to secreted signals from other tissues. Recent genetic evidence in zebrafish has shown that the interaction between stromal cell-derived factor 1 (SDF1) and its G-protein-coupled receptor CXCR4, already known to control many types of normal and pathological cell migrations, is also required for the normal migration of primordial germ cells. We show that in the mouse, germ cell migration and survival requires the SDF1/CXCR4 interaction. First, migrating germ cells express CXCR4, whilst the body wall mesenchyme and genital ridges express the ligand SDF1. Second, the addition of exogenous SDF1 to living embryo cultures causes aberrant germ cell migration from the gut. Third, germ cells in embryos carrying targeted mutations in CXCR4 do not colonize the gonad normally. However, at earlier stages in the hindgut, germ cells are unaffected in CXCR4(-/-) embryos. Germ cell counts at different stages suggest that SDF1/CXCR4 interaction also mediates germ cell survival. These results show that the SDF1/CXCR4 interaction is specifically required for the colonization of the gonads by primordial germ cells, but not for earlier stages in germ cell migration. This demonstrates a high degree of evolutionary conservation of part of the mechanism, but also an area of evolutionary divergence.