Secretion and mesoderm-inducing activity of the TGF-beta-related domain of Xenopus Vg1

Spemann-Mangold organizer and mesoderm induction

The discovery of the Spemann-Mangold organizer strongly influenced subsequent research on embryonic induction, with research aiming to elucidate the molecular characteristics of organizer activity being currently underway. Herein, we review the history of research on embryonic induction, and describe how the mechanisms of induction phenomena and developmental processes have been investigated. Classical experiments investigating the differentiation capacity and inductive activity of various embryonic regions were conducted by many researchers, and important theories of region-specific induction and the concept for chain of induction were proposed. The transition from experimental embryology to developmental biology has enabled us to understand the mechanisms of embryonic induction at the molecular level. Consequently, many inducing substances and molecules such as transcriptional factors and peptide growth factors involved in the organizer formation were identified. One of peptide growth factors, activin, acts as a mesoderm- and endoderm-inducing substance. Activin induces several tissues and organs from the undifferentiated cell mass of amphibian embryos in a concentration-dependent manner. We review the extent to which we can control in vitro organogenesis from undifferentiated cells, and discuss the application to stem cell-based regenerative medicine based on insights gained from animal experiments, such as in amphibians.

Evolution of the gene regulatory network of body axis by enhancer hijacking in amphioxus

A central goal of evolutionary developmental biology is to decipher the evolutionary pattern of gene regulatory networks (GRNs) that control embryonic development, and the mechanism underlying GRNs evolution. The Nodal signaling that governs the body axes of deuterostomes exhibits a conserved GRN orchestrated principally by Nodal, Gdf1/3, and Lefty. Here we show that this GRN has been rewired in cephalochordate amphioxus. We found that while the amphioxus Gdf1/3 ortholog exhibited nearly no embryonic expression, its duplicate Gdf1/3-like, linked to Lefty, was zygotically expressed in a similar pattern as Lefty. Consistent with this, while Gdf1/3-like mutants showed defects in axial development, Gdf1/3 mutants did not. Further transgenic analyses showed that the intergenic region between Gdf1/3-like and Lefty could drive reporter gene expression as that of the two genes. These results indicated that Gdf1/3-like has taken over the axial development role of Gdf1/3 in amphioxus, possibly through hijacking Lefty enhancers. We finally demonstrated that, to compensate for the loss of maternal Gdf1/3 expression, Nodal has become an indispensable maternal factor in amphioxus and its maternal mutants caused axial defects as Gdf1/3-like mutants. We therefore demonstrated a case that the evolution of GRNs could be triggered by enhancer hijacking events. This pivotal event has allowed the emergence of a new GRN in extant amphioxus, presumably through a stepwise process. In addition, the co-expression of Gdf1/3-like and Lefty achieved by a shared regulatory region may have provided robustness during body axis formation, which provides a selection-based hypothesis for the phenomena called developmental system drift.

The Role of Aplnr Signaling in the Developmental Regulation of Mesenchymal Stem Cell Differentiation from Human Pluripotent Stem Cells

Stem cells are invaluable resources for personalized medicine. Mesenchymal stem cells (MSCs) have received great attention as therapeutic tools due to being a safe, ethical, and accessible option with immunomodulatory and controlled differentiation properties. Apelin receptor (Aplnr) signaling is reported to be involved in biological events, including gastrulation, mesoderm migration, proliferation of MSCs. However, the knowledge about the exact role and mechanism of Aplnr signaling during mesoderm and MSCs differentiation is still primitive. The current study aims to unveil the role of Aplnr signaling during mesoderm and MSC differentiation from pluripotent stem cells (PSCs) through peptide/small molecule activation, overexpression, knock down or CRISPR/Cas9 mediated knock out of the pathway components. Morphological changes, gene and protein expression analysis, including antibody array, LC/MS, mRNA/miRNA sequencing, reveal that Aplnr signaling promotes mesoderm commitment possibly via EGFR and TGF-beta signaling pathways and enhances migration of cells during mesoderm differentiation. Moreover, Aplnr signaling positively regulates MSCs differentiation from hPSCs and increases MSC characteristics and differentiation capacity by regulating pathways, such as EGFR, TGFβ, Wnt, PDGF, and FGF. Osteogenic, chondrogenic, adipogenic, and myogenic differentiations are significantly enhanced with Aplnr signaling activity. This study generates an important foundation to generate high potential MSCs from PSCs to be used in personalized cell therapy.

Molecular mechanisms controlling the biogenesis of the TGF-β signal Vg1

The TGF-beta signals Vg1 (Dvr1/Gdf3) and Nodal form heterodimers to induce vertebrate mesendoderm. The Vg1 proprotein is a monomer retained in the endoplasmic reticulum (ER) and is processed and secreted upon heterodimerization with Nodal, but the mechanisms underlying Vg1 biogenesis are largely elusive. Here, we clarify the mechanisms underlying Vg1 retention, processing, secretion, and signaling and introduce a Synthetic Processing (SynPro) system that enables the programmed cleavage of ER-resident and extracellular proteins. First, we find that Vg1 can be processed by intra- or extracellular proteases. Second, Vg1 can be processed without Nodal but requires Nodal for secretion and signaling. Third, Vg1-Nodal signaling activity requires Vg1 processing, whereas Nodal can remain unprocessed. Fourth, Vg1 employs exposed cysteines, glycosylated asparagines, and BiP chaperone-binding motifs for monomer retention in the ER. These observations suggest two mechanisms for rapid mesendoderm induction: Chaperone-binding motifs help store Vg1 as an inactive but ready-to-heterodimerize monomer in the ER, and the flexibility of Vg1 processing location allows efficient generation of active heterodimers both intra- and extracellularly. These results establish SynPro as an in vivo processing system and define molecular mechanisms and motifs that facilitate the generation of active TGF-beta heterodimers.

Quantitative analysis of transcriptome dynamics provides novel insights into developmental state transitions

Background

During embryogenesis, the developmental potential of initially pluripotent cells becomes progressively restricted as they transit to lineage restricted states. The pluripotent cells of Xenopus blastula-stage embryos are an ideal system in which to study cell state transitions during developmental decision-making, as gene expression dynamics can be followed at high temporal resolution.

Results

Here we use transcriptomics to interrogate the process by which pluripotent cells transit to four different lineage-restricted states: neural progenitors, epidermis, endoderm and ventral mesoderm, providing quantitative insights into the dynamics of Waddington’s landscape. Our findings provide novel insights into why the neural progenitor state is the default lineage state for pluripotent cells and uncover novel components of lineage-specific gene regulation. These data reveal an unexpected overlap in the transcriptional responses to BMP4/7 and Activin signaling and provide mechanistic insight into how the timing of signaling inputs such as BMP are temporally controlled to ensure correct lineage decisions.

Conclusions

Together these analyses provide quantitative insights into the logic and dynamics of developmental decision making in early embryos. They also provide valuable lineage-specific time series data following the acquisition of specific lineage states during development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08953-3.

Multivalent interactions with RNA drive recruitment and dynamics in biomolecular condensates in Xenopus oocytes

RNA localization and biomolecular condensate formation are key biological strategies for organizing the cytoplasm and generating cellular polarity. In Xenopus oocytes, RNAs required for germ layer patterning localize in biomolecular condensates, termed Localization bodies (L-bodies). Here, we have used an L-body RNA-binding protein, PTBP3, to test the role of RNA–protein interactions in regulating the biophysical characteristics of L-bodies in vivo and PTBP3–RNA condensates in vitro. Our results reveal that RNA–protein interactions drive recruitment of PTBP3 and localized RNA to L-bodies and that multivalent interactions tune the dynamics of the PTBP3 after localization. In a concentration-dependent manner, RNA becomes non-dynamic and interactions with the RNA determine PTBP3 dynamics within these biomolecular condensates in vivo and in vitro. Importantly, RNA, and not protein, is required for maintenance of the PTBP3–RNA condensates in vitro, pointing to a model where RNA serves as a non-dynamic substructure in these condensates.

•

RNA–protein interactions drive recruitment of both RNA and protein to L-bodies

•

RNA is non-dynamic in both L-bodies and in vitro condensates

•

Multivalent interactions with RNA tune protein dynamics both in vivo and in vitro

•

RNA, but not protein, is required for maintenance of the in vitro condensates

Phylogenetic evidence for independent origins of GDF1 and GDF3 genes in anurans and mammals

Growth differentiation factors 1 (GDF1) and 3 (GDF3) are members of the transforming growth factor superfamily (TGF-β) that is involved in fundamental early-developmental processes that are conserved across vertebrates. The evolutionary history of these genes is still under debate due to ambiguous definitions of homologous relationships among vertebrates. Thus, the goal of this study was to unravel the evolution of the GDF1 and GDF3 genes of vertebrates, emphasizing the understanding of homologous relationships and their evolutionary origin. Our results revealed that the GDF1 and GDF3 genes found in anurans and mammals are the products of independent duplication events of an ancestral gene in the ancestor of each of these lineages. The main implication of this result is that the GDF1 and GDF3 genes of anurans and mammals are not 1:1 orthologs. In other words, genes that participate in fundamental processes during early development have been reinvented two independent times during the evolutionary history of tetrapods.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Molecular regulation of Nodal signaling during mesendoderm formation

One of the most important events during vertebrate embryogenesis is the formation or specification of the three germ layers, endoderm, mesoderm, and ectoderm. After a series of rapid cleavages, embryos form the mesendoderm and ectoderm during late blastulation and early gastrulation. The mesendoderm then further differentiates into the mesoderm and endoderm. Nodal, a member of the transforming growth factor β (TGF-β) superfamily, plays a pivotal role in mesendoderm formation by regulating the expression of a number of critical transcription factors, including Mix-like, GATA, Sox, and Fox. Because the Nodal signal transduction pathway is well-characterized, increasing effort has been made to delineate the spatiotemporal modulation of Nodal signaling during embryonic development. In this review, we summarize the recent progress delineating molecular regulation of Nodal signal intensity and duration during mesendoderm formation.

Heterodimers reign in the embryo

Experiments by three independent groups on zebrafish have clarified the role of two signaling factors, Nodal and Gdf3, during the early stages of development

Vg1-Nodal heterodimers are the endogenous inducers of mesendoderm

Nodal is considered the key inducer of mesendoderm in vertebrate embryos and embryonic stem cells. Other TGF-beta-related signals, such as Vg1/Dvr1/Gdf3, have also been implicated in this process but their roles have been unclear or controversial. Here we report that zebrafish embryos without maternally provided vg1 fail to form endoderm and head and trunk mesoderm, and closely resemble nodal loss-of-function mutants. Although Nodal is processed and secreted without Vg1, it requires Vg1 for its endogenous activity. Conversely, Vg1 is unprocessed and resides in the endoplasmic reticulum without Nodal, and is only secreted, processed and active in the presence of Nodal. Co-expression of Nodal and Vg1 results in heterodimer formation and mesendoderm induction. Thus, mesendoderm induction relies on the combination of two TGF-beta-related signals: maternal and ubiquitous Vg1, and zygotic and localized Nodal. Modeling reveals that the pool of maternal Vg1 enables rapid signaling at low concentrations of zygotic Nodal.

All animals begin life as just one cell – a fertilized egg. In order to make a recognizable adult, each embryo needs to make the three types of tissue that will eventually form all of the organs: endoderm, which will form the internal organs; mesoderm, which will form the muscle and bones; and ectoderm, which will generate the skin and nervous system.

All vertebrates – animals with backbones like fish and humans – use the so-called Nodal signaling pathway to make the endoderm and mesoderm. Nodal is a signaling molecule that binds to receptors on the surface of cells. If Nodal binds to a receptor on a cell, it instructs that cell to become endoderm or mesoderm. As such, Nodal is critical for vertebrate life. However, there has been a 30-year debate in the field of developmental biology about whether a protein called Vg1, which has a similar molecular structure as Nodal, plays a role in the early development of vertebrates.

Zebrafish are often used to study animal development, and Montague and Schier decided to test whether these fish need the gene for Vg1 (also known as Gdf3) by deleting it using a genome editing technique called CRISPR/Cas9. It turns out that female zebrafish can survive without this gene. Yet, when the offspring of these females do not inherit the instructions to make Vg1 from their mothers, they fail to form the endoderm and mesoderm. This means that the embryos do not have hearts, blood or other internal organs, and they die within three days. Two other groups of researchers have independently reported similar results.

The findings reveal that Vg1 is critical for the Nodal signaling pathway to work in zebrafish. Montague and Schier then showed that, in this pathway, Nodal does not activate its receptors on its own. Instead, Nodal must interact with Vg1, and it is this Nodal-Vg1 complex that activates receptors, and instructs cells to become endoderm and mesoderm.

Scientists currently use the Nodal signaling pathway to induce human embryonic stem cells growing in the laboratory to become mesoderm and endoderm. As such, these new findings could ultimately help researchers to grow tissues and organs for human patients.

Genome organization of the vg1 and nodal3 gene clusters in the allotetraploid frog Xenopus laevis

Extracellular factors belonging to the TGF-β family play pivotal roles in the formation and patterning of germ layers during early Xenopus embryogenesis. Here, we show that the vg1 and nodal3 genes of Xenopus laevis are present in gene clusters on chromosomes XLA1L and XLA3L, respectively, and that both gene clusters have been completely lost from the syntenic S chromosome regions. The presence of gene clusters and chromosome-specific gene loss were confirmed by cDNA FISH analyses. Sequence and expression analyses revealed that paralogous genes in the vg1 and nodal3 clusters on the L chromosomes were also altered compared to their Xenopus tropicalis orthologs. X. laevis vg1 and nodal3 paralogs have potentially become pseudogenes or sub-functionalized genes and are expressed at different levels. As X. tropicalis has a single vg1 gene on chromosome XTR1, the ancestral vg1 gene in X. laevis appears to have been expanded on XLA1L. Of note, two reported vg1 genes, vg1(S20) and vg1(P20), reside in the cluster on XLA1L. The nodal3 gene cluster is also present on X. tropicalis chromosome XTR3, but phylogenetic analysis indicates that nodal3 genes in X. laevis and X. tropicalis were independently expanded and/or evolved in concert within each cluster by gene conversion. These findings provide insights into the function and molecular evolution of TGF-β family genes in response to allotetraploidization.

Analysis of Active Transport by Fluorescence Recovery after Photobleaching

Fluorescence recovery after photobleaching (FRAP) is a well-established experimental technique to study binding and diffusion of molecules in cells. Although a large number of analytical and numerical models have been developed to extract binding and diffusion rates from FRAP recovery curves, active transport of molecules is typically not included in the existing models that are used to estimate these rates. Here we present a validated numerical method for estimating diffusion, binding/unbinding rates, and active transport velocities using FRAP data that captures intracellular dynamics through partial differential equation models. We apply these methods to transport and localization of mRNA molecules in Xenopus laevis oocytes, where active transport processes are essential to generate developmental polarity. By providing estimates of the effective velocities and diffusion, as well as expected run times and lengths, this approach can help quantify dynamical properties of localizing and nonlocalizing RNA. Our results confirm the distinct transport dynamics in different regions of the cytoplasm, and suggest that RNA movement in both the animal and vegetal directions may influence the timescale of RNA localization in Xenopus oocytes. We also show that model initial conditions extracted from FRAP postbleach intensities prevent underestimation of diffusion, which can arise from the instantaneous bleaching assumption. The numerical and modeling approach presented here to estimate parameters using FRAP recovery data is a broadly applicable tool for systems where intracellular transport is a key molecular mechanism.

RNA Whole-Mount In situ Hybridisation Proximity Ligation Assay (rISH-PLA), an Assay for Detecting RNA-Protein Complexes in Intact Cells

Techniques for studying RNA-protein interactions have lagged behind those for DNA-protein complexes as a consequence of the complexities associated with working with RNA. Here we present a method for the modification of the existing In Situ Hybridisation-Proximity Ligation Assay (ISH-PLA) protocol to adapt it to the study of RNA regulation (rISH-PLA). As proof of principle we used the well-characterised interaction of the Xenopus laevis Staufen RNA binding protein with Vg1 mRNA, the complex of which co-localises to the vegetal pole of Xenopus oocytes. The applicability of both the Stau1 antibody and the Locked Nucleic Acid probe (LNA) recognising Vg1 mRNA were independently validated by whole-mount Immunohistochemistry and whole-mount in situ hybridisation assays respectively prior to combining them in the rISH-PLA assay. The rISH-PLA assay allows the identification of a given RNA-protein complex at subcellular and single cell resolution, thus avoiding the lack of spatial resolution and sensitivity associated with assaying heterogenous cell populations from which conventional RNA-protein interaction detection techniques suffer. This technique will be particularly usefully for studying the activity of RNA binding proteins (RBPs) in complex mixtures of cells, for example tissue sections or whole embryos.

Molecular specification of germ layers in vertebrate embryos

In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. Classic experiments in amphibian embryos revealed the tissue interactions involved in germ layer formation and provided the groundwork for the identification of secreted and intracellular factors involved in this process. We will begin this review by summarising the key findings of those studies. We will then evaluate them in the light of more recent genetic studies that helped clarify which of the previously identified factors are required for germ layer formation in vivo, and to what extent the mechanisms identified in amphibians are conserved across other vertebrate species. Collectively, these studies have started to reveal the gene regulatory network (GRN) underlying vertebrate germ layer specification and we will conclude our review by providing examples how our understanding of this GRN can be employed to differentiate stem cells in a targeted fashion for therapeutic purposes.

Identification of 3'UTR sequence elements and a teloplasm localization motif sufficient for the localization of Hro-twist mRNA to the zygotic animal and vegetal poles

The early localization of mRNA transcripts is critical in sorting cell fate determinants in the developing embryo. In the glossiphoniid leech, Helobdella robusta, maternal mRNAs, such as Hro-twist, localize to the zygotic teloplasm. Ten seven nucleotide repeat elements (AAUAAUA) called ARE2 and a predicted secondary structural motif, called teloplasm localization motif (TLM), are present in the 3'UTR of Hro-twist mRNA. We used site-directed mutagenesis, deletions, and microinjection of labeled, exogenous transcripts to determine if ARE2 elements, and the TLM, play a role in Hro-twist mRNA localization. Deleting the poly-A tail and the cytoplasmic polyadenylation element (CPE) had no effect on Hro-twist mRNA localization. Site-directed mutagenesis of nucleotides that altered ARE2 element sequences or the TLM suggest that the ARE2 elements and the TLM are important for Hro-twist mRNA localization to the teloplasm of pre-cleavage zygotes. Hro-Twist protein expression data suggest that the localization of Hro-twist transcripts in zygotes and stage two embryos is not involved in ensuring mesoderm specification, as Hro-Twist protein is expressed uniformly in most cells before gastrulation. Our data may support a shared molecular mechanism for leech transcripts that localize to the teloplasm.

Putting RNAs in the right place at the right time: RNA localization in the frog oocyte

Localization of maternal mRNAs in many developing organisms provides the basis for both initial polarity during oogenesis and patterning during embryogenesis. Prominent examples of this phenomenon are found in Xenopus laevis, where localized maternal mRNAs generate developmental polarity along the animal/vegetal axis. Targeting of mRNA molecules to specific subcellular regions is a fundamental mechanism for spatial regulation of gene expression, and considerable progress has been made in defining the underlying molecular pathways.

Polarizing animal cells via mRNA localization in oogenesis and early development

The localization of mRNAs in developing animal cells is essential for establishing cellular polarity and setting up the body plan for subsequent development. Cellular and molecular mechanisms by which maternal mRNAs are localized during oogenesis have been extensively studied in Drosophila and Xenopus. In contrast, evidence for mechanisms used in the localization of mRNAs encoded by developmentally important genes has also been accumulating in several other organisms. This offers the opportunity to unravel the fundamental mechanisms of mRNA localization shared among many species, as well as unique mechanisms specifically acquired or retained by animals based on their developmental needs. In addition to maternal mRNAs, the localization of zygotically expressed mRNAs in the cells of cleaving embryos is also important for early development. In this review, mRNA localization dynamics in the oocytes/eggs of Drosophila and Xenopus are first summarized, and evidence for localized mRNAs in the oocytes/eggs and cleaving embryos of other organisms is then presented.

Cloning and characterization of a new gene from the PAT protein family, in a marsupial, the stripe-faced dunnart (Sminthopsis macroura)

Recent studies of PAT proteins in Drosophila and Xenopus have revealed significant roles for this family of proteins in the polarized transport of lipid droplets and maternal determinants during early embryogenesis. In mammals, PAT proteins are known to function mainly in lipid metabolism, yet research has yet to establish a role for PAT proteins in mammalian embryogenesis. Oocytes and early cleavage stages in Sminthopsis macroura show obvious polarized cytoplasmic distribution of organelles, somewhat similar to Drosophila and Xenopus, suggesting that a PAT protein may also be involved in S. macroura embryonic development. In the present study, we identified a new marsupial gene for PAT family proteins, DPAT, from S. macroura. Expression analyses by RT-PCR and whole mount fluorescent in situ hybridization revealed that DPAT expression was specific to oocytes and cleavage stage conceptuses. Analysis of the localization of lipid droplets during S. macroura early embryonic development found a polarized distribution of lipid droplets at the two- and four-cell stage, and an asymmetric enrichment in blastomeres on one side of conceptuses from two- to eight-cell stage. Lipid droplets largely segregate to pluriblast cells at the 16-cell stage, suggesting a role in pluriblast lineage allocation.

Untranslated regions of a mobile transcript mediate RNA metabolism

BEL1-like transcription factors are ubiquitous in plants and interact with KNOTTED1 types to regulate numerous developmental processes. In potato (Solanum tuberosum subsp. andigena), the BEL1-like transcription factor StBEL5 and its Knox protein partner regulate tuber formation by targeting genes that control growth. RNA detection methods and heterografting experiments demonstrated that StBEL5 transcripts are present in phloem cells and move across a graft union to localize in stolon tips, the site of tuber induction. This movement of RNA originates in leaf veins and petioles and is induced by a short-day photoperiod, regulated by the untranslated regions, and correlated with enhanced tuber production. Assays for RNA mobility suggest that both 5' and 3' untranslated regions contribute to the preferential accumulation of the StBEL5 RNA but that the 3' untranslated region may contribute more to transport from the leaf to the stem and into the stolons. Addition of the StBEL5 untranslated regions to another BEL1-like mRNA resulted in its preferential transport to stolon tips and enhanced tuber production. Transcript stability assays showed that the untranslated regions and a long-day photoperiod enhanced StBEL5 RNA stability in shoot tips. Upon fusion of the untranslated regions of StBEL5 to a beta-glucuronidase marker, translation in tobacco (Nicotiana tabacum) protoplasts was repressed by those constructs containing the 3' untranslated sequence. These results demonstrate that the untranslated regions of the mRNA of StBEL5 are involved in mediating its long-distance transport, in maintaining transcript stability, and in controlling translation.

Translational control during early development

Translational control of specific messenger RNAs, which themselves are often asymmetrically localized within the cytoplasm of a cell, underlies many events in germline development, and in embryonic axis specification. This comprehensive, but by no means exhaustive, review attempts to present a picture of the present state of knowledge about mechanisms underlying mRNA localization and translational control of specific mRNAs that are mediated by trans-acting protein factors. While RNA localization and translational control are widespread in evolution and have been studied in many experimental systems, this article will focus mainly on three particularly well-characterized systems: Drosophila, Caenorhabditis elegans, and Xenopus. In keeping with the overall theme of this volume, instances in which translational control factors have been linked to human disease states will also be discussed.

Germ layer induction in ESC--following the vertebrate roadmap

Controlled differentiation of pluripotential cells takes place routinely and with great success in developing vertebrate embryos. It therefore makes sense to take note of how this is achieved and use this knowledge to control the differentiation of embryonic stem cells (ESCs). An added advantage is that the differentiated cells resulting from this process in embryos have proven functionality and longevity. This unit reviews what is known about the embryonic signals that drive differentiation in one of the most informative of the vertebrate animal models of development, the amphibian Xenopus laevis. It summarizes their identities and the extent to which their activities are dose-dependent. The unit details what is known about the transcription factor responses to these signals, describing the networks of interactions that they generate. It then discusses the target genes of these transcription factors, the effectors of the differentiated state. Finally, how these same developmental programs operate during germ layer formation in the context of ESC differentiation is summarized.

Multiple kinesin motors coordinate cytoplasmic RNA transport on a subpopulation of microtubules in Xenopus oocytes

RNA localization is a widely conserved mechanism for generating cellular asymmetry. In Xenopus oocytes, microtubule-dependent transport of RNAs to the vegetal cortex underlies germ layer patterning. Although kinesin motors have been implicated in this process, the apparent polarity of the microtubule cytoskeleton has pointed instead to roles for minus-end-directed motors. To resolve this issue, we have analyzed participation of kinesin motors in vegetal RNA transport and identified a direct role for Xenopus kinesin-1. Moreover, in vivo interference and biochemical experiments reveal a key function for multiple motors, specifically kinesin-1 and kinesin-2, and suggest that these motors may interact during transport. Critically, we have discovered a subpopulation of microtubules with plus ends at the vegetal cortex, supporting roles for these kinesin motors in vegetal RNA transport. These results provide a new mechanistic basis for understanding directed RNA transport within the cytoplasm.

Vg1 has specific processing requirements that restrict its action to body axis patterning centers

Unlike most transforming growth factor-beta (TGF-beta) superfamily members, Vg1 has been shown not to produce gross phenotypic alterations in Xenopus embryos when overexpressed by mRNA injection. Experiments with artificial chimeric constructs and a recently identified second allele of Vg1 suggest that this may be due to unusually stringent requirements for proteolytic processing. We provide biological and biochemical evidence that cleavage by two distinct proteolytic enzymes is required for effective activation of Vg1. We demonstrate a tightly restricted overlap in expression patterns of Vg1 with the proteases required to release the mature peptide. The data presented may account for the long-standing observation that the vast majority of Vg1 protein, in vivo, is present in its unprocessed form. Taken together, these observations provide a plausible mechanism for local action of Vg1 consistent with requirements imposed by current models of pattern formation in the developing body axis.

Xenopus hairy2b specifies anterior prechordal mesoderm identity within Spemann's organizer

Spemann's organizer is a region of the gastrula stage embryo that contains future anterior endodermal and dorsal mesodermal tissues. During gastrulation, the dorsal mesoderm is divided into the prechordal mesoderm and the chordamesoderm. However, little is known regarding how this division is established. We analyzed the role of the anterior prechordal mesoderm-specific gene Xhairy2b in the regionalization of the organizer. We found that mesoderm-inducing transforming growth factor-beta signaling induced Xhairy2b expression. On the other hand, the ectopic expression of Xhairy2b induced the expression of organizer-specific genes and resulted in the formation of a secondary dorsal axis lacking head and notochord structures. We also showed that Xhairy2b down-regulated the expression of ventral mesodermal, anterior endodermal, and chordamesodermal genes. In Xhairy2b-depleted embryos, defects in the specification of anterior prechordal mesoderm identity were observed as the border between the prechordal mesoderm and the chordamesoderm was anteriorly shifted. These results suggest that Xhairy2b establishes the identity of the anterior prechordal mesoderm within Spemann's organizer by inhibiting the formation of neighboring tissues.

Ribonucleoprotein remodeling during RNA localization

Cytoplasmic RNA localization is a means to create polarity by restricting protein expression to a discrete subcellular location. RNA localization is a multistep process that begins with the recognition of cis-acting sequences within the RNA by specific trans-factors, and RNAs are localized in ribonucleoprotein (RNP) complexes that contain both the RNA and numerous protein components. Components of the localization machinery transport the RNP complex, usually in a translationally repressed state, to a distinct subcellular region, resulting in spatially restricted gene expression. Recent efforts to identify both the cis- and trans-factors required for RNA localization have elucidated RNA-protein interactions that are remodeled during localization.

Maternal control of pattern formation inXenopus laevis

We review the essential role of maternal factors in pattern formation for Xenopus laevis, focusing on VegT, Vg1, and Wnt11. Results from loss of function experiments demonstrate a clear requirement for these genes in germ layer specification, dorsal-ventral axis formation, and convergence extension. We also discuss these genes in the broader context of metazoan development, exploring whether and how their functions in the X. laevis model organism may or may not be conserved in other species. Wnt11 signaling in particular provides a classic example where understanding context in development is crucial to understanding function. Genomic sequencing, gene expression, and functional screening data that are becoming available in more species are providing invaluable aid to decoding and modeling signaling pathways. More work is needed to develop a comprehensive catalog of the Wnt signaling, T-box, and TGF-beta genes in metazoans both near and far in evolutionary distance. We finally discuss some specific experimental and modeling efforts that will be needed to understand the behavior of these signaling networks in vivo so that we can interpret these critical pathways in an evolutionary framework.

Molecular Basis of Vertebrate Endoderm Development

The embryonic endoderm gives rise to the epithelial lining of the digestive and respiratory systems and organs such as the thyroid, lungs, liver, gallbladder, and pancreas. Studies in Xenopus, zebrafish, and mice have revealed a conserved molecular pathway controlling vertebrate endoderm development. The TGFbeta/Nodal signaling pathway is at the top of this molecular hierarchy and controls the expression of a number of key transcription factors including Mix-like homeodomain proteins, Gata zinc finger factors, Sox HMG domain proteins, and Fox forkhead factors. Here we review the function of these molecules comparing and contrasting their roles in each model organism. Finally, we will describe how our understanding of the molecular pathway governing endoderm development in embryos is being used to differentiate embryonic stem cells in vitro along endodermal lineages, with the ultimate goal of making therapeutically useful tissue.

Emergence of Organizer function: A lot of “stuff” involved

Spemann's Organizer is a critical signaling center for patterning the embryo. It arises during blastula stages through the combined influences of dorsal modifying signals and general mesendoderm inducers. Dorsal modifying signals require the nuclear accumulation of beta-catenin, but how this is initiated remained a mystery until recently. New findings now demonstrate that maternal Wnt11 activates the canonical Wnt signaling pathway and is essential for organizer formation. Furthermore, two of the earliest identified mesendoderm inducers, activin and Vg-1, have now been shown to be required for induction of a fully functional organizer. Finally, while it has been clear for a number of years that the Organizer secretes a cocktail of growth factor antagonists, their necessity for organizer function has been in question. Their requirement has now been demonstrated through a multiple "knockdown" approach in frog embryos. Here, we discuss the impact these recent findings have on our understanding of formation and function of the Organizer.

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.

Expression patterns of the DAZ-associated protein DAZAP1 in rat and human ovaries

OBJECTIVE

To evaluate the expression of DAZAP1 (deleted in azoospermia-associated protein 1) in rat and human ovaries.

DESIGN

Experimental study.

SETTING

University hospital.

PATIENT(S)

Twelve corpus luteum (CL) specimens were collected during operation, either by laparoscopic surgery for CL rupture or by laparotomy for benign gynecologic conditions.

INTERVENTION(S)

Surgical excision of 12 human CL.

MAIN OUTCOME MEASURE(S)

Proteins analyzed by immunohistochemical staining, Western blotting, and co-immunoprecipitation experiments.

RESULT(S)

DAZAP1 is expressed in rat and human luteal cells. Expression of DAZAP1 decreases with advancing stages of CL. Co-immunoprecipitation experiments show in vivo interaction of DAZ-like (DAZL) protein with DAZAP1 in the ovarian tissues.

CONCLUSION(S)

The expression patterns of DAZAP1 and DAZL are identical within rat and human ovaries. In mammalian species, DAZAP1 may be involved in diverse reproductive functions, ranging from cell cycle regulation and maturation of oocytes to differentiation of luteal cells.

Overexpression of Nodal promotes differentiation of mouse embryonic stem cells into mesoderm and endoderm at the expense of neuroectoderm formation

Understanding how to direct the fate of embryonic stem (ES) cells upon differentiation is critical to their eventual use in therapeutic applications. Clues for controlling ES cell differentiation may be found in the early embryo because mouse ES cells form derivatives of all three embryonic germ layers upon injection into blastocysts. One promising candidate for influencing the differentiation of ES cells into the embryonic germ layers is the transforming growth factor-beta (TGF-beta) growth factor, Nodal. Nodal null mouse mutants lack mesoderm, and injection of Nodal mRNA into nonmammalian embryos induces mesodermal and endodermal tissues. We find that overexpression of Nodal in mouse ES cells leads not only to up-regulation of mesodermal and endodermal cell markers but also to downregulation of neuroectodermal markers. These findings demonstrate the importance of Nodal's influence on the differentiation of pluripotent cells to all three of the primary germ layers. Accordingly, altering expression of factors responsible for cell differentiation in the intact embryo provides an approach for directing ES cell fates in vitro toward therapeutically useful cell types.

Nodal inhibits differentiation of human embryonic stem cells along the neuroectodermal default pathway

Genetic studies in fish, amphibia, and mice have shown that deficiency of Nodal signaling blocks differentiation into mesoderm and endoderm. Thus, Nodal is considered as a major inducer of mesendoderm during gastrulation. On this basis, Nodal is a candidate for controlling differentiation of pluripotent human embryonic stem cells (hESCs) into tissue lineages with potential clinical value. We have investigated the effect of Nodal, both as a recombinant protein and as a constitutively expressed transgene, on differentiation of hESCs. When control hESCs were grown in chemically defined medium, their expression of markers of pluripotency progressively decreased, while expression of neuroectoderm markers was strongly upregulated, thus revealing a neuroectodermal default mechanism for differentiation in this system. hESCs cultured in recombinant Nodal, by contrast, showed prolonged expression of pluripotency marker genes and reduced induction of neuroectoderm markers. These Nodal effects were accentuated in hESCs expressing a Nodal transgene, with striking morphogenetic consequences. Nodal-expressing hESCs developing as embryoid bodies contained an outer layer of visceral endoderm-like cells surrounding an inner layer of epiblast-like cells, each layer having distinct gene expression patterns. Markers of neuroectoderm were not upregulated during development of Nodal-expressing embryoid bodies, nor was there induction of markers for definitive mesoderm or endoderm differentiation. Moreover, the inner layer expressed markers of pluripotency, characteristic of undifferentiated hESCs and of epiblast in mouse embryos. These results could be accounted for by an inhibitory effect of Nodal-induced visceral endoderm on pluripotent cell differentiation into mesoderm and endoderm, with a concomitant inhibition of neuroectoderm differentiation by Nodal itself. There could also be a direct effect of Nodal in the maintenance of pluripotency. In summary, analysis of the Nodal-expressing phenotype suggests a function for the transforming growth factor-beta (TGF-beta) growth factor superfamily in pluripotency and in early cell fate decisions leading to primary tissue layers during in vitro development of pluripotent human stem cells. The effects of Nodal on early differentiation illustrate how hESCs can augment mouse embryos as a model for analyzing mechanisms of early mammalian development.

XPACE4 is a localized pro-protein convertase required for mesoderm induction and the cleavage of specific TGFbeta proteins in Xenopus development

XPACE4 is a member of the subtilisin/kexin family of pro-protein convertases. It cleaves many pro-proteins to release their active proteins, including members of the TGFbeta family of signaling molecules. Studies in mouse suggest it may have important roles in regulating embryonic tissue specification. Here, we examine the role of XPACE4 in Xenopus development and make three novel observations: first, XPACE4 is stored as maternal mRNA localized to the mitochondrial cloud and vegetal hemisphere of the oocyte; second, it is required for the endogenous mesoderm inducing activity of vegetal cells before gastrulation; and third, it has substrate-specific activity, cleaving Xnr1, Xnr2, Xnr3 and Vg1, but not Xnr5, Derriere or ActivinB pro-proteins. We conclude that maternal XPACE4 plays an important role in embryonic patterning by regulating the production of a subset of active mature TGFbeta proteins in specific sites.

Activin redux: specification of mesodermal pattern in Xenopus by graded concentrations of endogenous activin B

Mesoderm formation in the amphibian embryo occurs through an inductive interaction in which cells of the vegetal hemisphere of the embryo act on overlying equatorial cells. The first candidate mesoderm-inducing factor to be identified was activin, a member of the transforming growth factor type beta family, and it is now clear that members of this family are indeed involved in mesoderm and endoderm formation. In particular, Derrière and five nodal-related genes are all considered to be strong candidates for endogenous mesoderm-inducing agents. Here, we show that activin, the function of which in mesoderm induction has hitherto been unclear, also plays a role in mesoderm formation. Inhibition of activin function using antisense morpholino oligonucleotides interferes with mesoderm formation in a concentration-dependent manner and also changes the expression levels of other inducing agents such as Xnr2 and Derrière. This work reinstates activin as a key player in mesodermal patterning. It also emphasises the importance of checking for polymorphisms in the 5' untranslated region of the gene of interest when carrying out antisense morpholino experiments in Xenopus laevis.

Conserved and clustered RNA recognition sequences are a critical feature of signals directing RNA localization in Xenopus oocytes

Although it is widely regarded that the targeting of RNA molecules to subcellular destinations depends upon the recognition of cis-elements found within their 3' untranslated regions (UTR), relatively little is known about the specific features of these cis-sequences that underlie their function. Interaction between specific repeated motifs within the 3' UTR and RNA-binding proteins has been proposed as a critical step in the localization of Vg1 RNA to the vegetal pole of Xenopus oocytes. To understand the relative contributions of repeated localization element (LE) sequences, we used comparative functional analysis of Vg1 LEs from two frog species, Xenopus laevis and Xenopus borealis. We show that clusters of repeated VM1 and E2 motifs are required for efficient localization. However, groups of either site alone are not sufficient for localization. In addition, we present evidence that the X. borealis Vg1 LE is recognized by the same set of RNA-binding proteins as the X. laevis Vg1 LE and is capable of productive interactions with the X. laevis transport machinery as it is sufficient to direct vegetal localization in X. laevis oocytes. These results suggest that clustered sets of cis-acting sites within the LE direct vegetal transport through specific interactions with the localization machinery.

Xenopus Staufen is a component of a ribonucleoprotein complex containing Vg1 RNA and kinesin

RNA localization is a key mechanism for generating cell and developmental polarity in a wide variety of organisms. We have performed studies to investigate a role for the Xenopus homolog of the double-stranded RNA-binding protein, Staufen, in RNA localization during oogenesis. We have found that Xenopus Staufen (XStau) is present in a ribonucleoprotein complex, and associates with both a kinesin motor protein and vegetally localized RNAs Vg1 and VegT. A functional role for XStau was revealed through expression of a dominant-negative version that blocks localization of Vg1 RNA in vivo. Our results suggest a central role for XStau in RNA localization in Xenopus oocytes, and provide evidence that Staufen is a conserved link between specific mRNAs and the RNA localization machinery.

Nuclear RNP complex assembly initiates cytoplasmic RNA localization

Cytoplasmic localization of mRNAs is a widespread mechanism for generating cell polarity and can provide the basis for patterning during embryonic development. A prominent example of this is localization of maternal mRNAs in Xenopus oocytes, a process requiring recognition of essential RNA sequences by protein components of the localization machinery. However, it is not yet clear how and when such protein factors associate with localized RNAs to carry out RNA transport. To trace the RNA-protein interactions that mediate RNA localization, we analyzed RNP complexes from the nucleus and cytoplasm. We find that an early step in the localization pathway is recognition of localized RNAs by specific RNA-binding proteins in the nucleus. After transport into the cytoplasm, the RNP complex is remodeled and additional transport factors are recruited. These results suggest that cytoplasmic RNA localization initiates in the nucleus and that binding of specific RNA-binding proteins in the nucleus may act to target RNAs to their appropriate destinations in the cytoplasm.

ALK4 functions as a receptor for multiple TGF beta-related ligands to regulate left-right axis determination and mesoderm induction in Xenopus

In Xenopus, several TGF betas, including nodal-related 1 (Xnr1), derriere, and chimeric forms of Vg1, elicit cardiac and visceral organ left-right (LR) defects when ectopically targeted to right mesendoderm cell lineages, suggesting that LR axis determination may require activity of one or more TGF betas. However, it is not known which, if any, of these ligands is required for LR axis determination, nor is it known which type I TGF beta receptor(s) are involved in mediating left-side TGF beta signaling. We report here that similar to effects of ectopic TGF betas, right-side expression of constitutively active activin-like kinase (ALK) 4 results in LR organ reversals as well as altered Pitx2 expression in the lateral plate mesoderm. Moreover, left-side expression of a kinase-deficient, dominant-negative ALK4 (DN-ALK4) or an ALK4 antisense morpholino also results in abnormal embryonic body situs, demonstrating a left-side requirement for ALK4 signaling. To determine which TGF beta(s) utilize the ALK4 pathway to mediate LR development, biochemical and functional assays were performed using an Activin-Vg1 chimera (AVg), Xnr1, and derriere. Whereas ALK4 can co-immunoprecipitate all of these TGF betas, including endogenous Vg1 protein from embryo homogenates, functional assays demonstrate that not all of these ligands require an intact ALK4 signaling pathway to modulate LR asymmetry. When AVg and DN-ALK4 are co-expressed, LR defects otherwise induced by AVg alone are attenuated by DN-ALK4; however, when functional assays are performed with Xnr1 or derriere, LR defects otherwise elicited by these ligands alone still occur in the presence of DN-ALK4. Intriguingly, when any of these TGF betas is expressed at a higher concentration to elicit primary axis defects, DN-ALK4 blocks gastrulation and dorsoanterior/ventroposterior defects that otherwise occur following ligand-only expression. Together, these results suggest not only that ALK4 interacts with multiple TGF betas to generate embryonic pattern, but also that ALK4 ligands differentially utilize the ALK4 pathway to regulate distinct aspects of axial pattern, with Vg1 as a modulator of ALK4 function in LR axis determination and Vg1, Xnr1, and derriere as modulators of ALK4 function in mesoderm induction during primary axis formation.

VgRBP71 stimulates cleavage at a polyadenylation signal in Vg1 mRNA, resulting in the removal of a cis-acting element that represses translation

Translation of Vg1 mRNA is repressed in Xenopus oocytes until it is localized to the vegetal cortex. Localization and translational repression are mediated by separate elements in the 3'UTR of the mRNA. VgRBP71 binds to the 3' end of the localization element and stimulates cleavage at an adjacent polyadenylation signal. The protein has an RNA strand-separation activity that likely underlies this event. Polyadenylation occurs at this site in Vg1 mRNA with the consequence of removing the downstream translational repressor element. Ectopic expression of VgRBP71 in stage II oocytes results in cleavage of the mRNA and premature expression of Vg1 protein. These results support a model in which VgRBP71 activates translation of Vg1 mRNA by promoting the removal of a cis-acting repressor element.

A homolog of FBP2/KSRP binds to localized mRNAs in Xenopus oocytes

A Xenopus oocyte expression library was screened for proteins that bind to the 340-nucleotide localization element of Vg1 mRNA. Four different isolates encoded a Xenopus homolog of the human transcription factor, FUSE-binding protein 2 (FBP2). This protein has been independently identified as the splicing regulatory factor KSRP. The only significant difference between the Xenopus protein, designated VgRBP71, and KSRP is the absence of a 58 amino acid segment near the N-terminal of the former. In vivo binding assays show that VgRBP71 is associated with mRNAs localized to either the vegetal or animal hemispheres, but was not found with control mRNAs. Unlike other factors that bind to the localization element of Vg1 mRNA, VgRBP71 does not accumulate at the vegetal cortex with the mRNA; rather, it is present in the nucleus and throughout the cytoplasm at all stages of oogenesis. Cytoplasmic VgRBP71 appears to be most concentrated at the cell cortex. VgRBP71 interacts with Prrp, another protein that binds to the Vg1 localization element; this association does not require the presence of Vg1 mRNA.

Transcriptional regulation of Xbr-1a/Xvent-2 homeobox gene: analysis of its promoter region

Xvent homeobox proteins are induced by BMP-4 signaling and have been known to mediate many BMP-4 activities as key downstream transcriptional factors. In order to investigate the regulatory mode of Xvent transcription, we isolated genomic DNA of the Xbr-1a/Xvent-2 containing the promoter region responsive to BMP-4 signaling. The cis-acting elements located within the Xbr-1a/Xvent-2 promoter and the regulation modes by BMP-4 signaling were analyzed by serial deletion and site-directed mutagenesis experiments. The upstream -235bp of the promoter retained the full transcriptional activity and BMP-4-response when compared with the longest promoter construct. Further analysis indicated that two separated 15bp regions contained a strong positive element and BMP-4-response element. Site-directed mutagenesis of those regions suggests that those two regions cooperate for the promoter activity and BMP-4-response. Moreover, we found that the transcription factors, Oaz and PEBP2alphaA, were able to elicit additive effects with BMP-4 signaling on Xbr-1a/Xvent-2 reporter activities. These results indicate that transcriptional regulation of the Xbr-1a/Xvent-2 gene occurs in a complex mode through the cooperation of various transcription factors.

A consensus RNA signal that directs germ layer determinants to the vegetal cortex of Xenopus oocytes

RNA localization is an important mechanism for generating cellular diversity and polarity in the early embryo. In Xenopus, the correct localization of the RNA encoding the T-box transcription factor VegT is essential for the correct spatial organization and identity of endoderm and mesoderm. Although localization signals in the 3' UTR have been identified for many localized RNAs, insight into what constitutes an RNA localization signal remains elusive. To investigate possible common features between signals that direct different RNAs to the same subcellular region, we carried out a detailed analysis of the uncharacterized VegT RNA localization signal and compared it with the well-studied Vg1 localization signal. Both RNAs localize to the vegetal cortex during the same period of oogenesis. Our results suggest a common RNA localization signal at the level of clustered redundant protein-binding motifs and trans-acting factors. We propose that what characterizes RNA localization signals in general is not the nucleotide sequence or secondary structure per se, but the critical clustering of specific redundant protein-binding motifs.

Ectodermal syndecan-2 mediates left-right axis formation in migrating mesoderm as a cell-nonautonomous Vg1 cofactor

Heparan sulfate proteoglycans expressed on the Xenopus animal cap ectoderm have been implicated in transmitting left-right information to heart and gut primordia. We report here that syndecan-2 functions in the ectoderm to mediate cardiac and visceral situs, upstream of known asymmetrically expressed genes but independently of its ability to mediate fibronectin fibrillogenesis. Left-right development is dependent on a distinct subset of glycosaminoglycan attachment sites on syndecan-2. A novel in vivo approach with enterokinase demonstrates that syndecan-2 functions in left-right patterning during early gastrulation. We describe a cell-nonautonomous role for ectodermal syndecan-2 in transmitting left-right information to migrating mesoderm. The results further suggest that this function may be related to the transduction of Vg1-related signals.

Development of an MFG-based retroviral vector system for secretion of high levels of functionally active human BMP4

We sought to develop a retroviral vector system that would produce secretion of high levels of bone morphogenetic protein (BMP)-4 by optimizing the expression construct and developing an improved retroviral vector. Replacement of the propeptide domain of BMP4 with that of BMP2 increased the secretion level of mature BMP4 protein in transduced cells. The intact BMP2 pro-peptide sequence was essential, as deletion of a small part of the propeptide sequence of BMP2 from the BMP2/4 hybrid construct diminished BMP4 expression and secretion. Addition of a hemaglutinin tag to the carboxy terminus of BMP4 abolished the bioactivity of secreted BMP4. Transduction of rat marrow stromal cells (and fibroblasts) with an MFG-based retroviral vector pseudotyped with VSV-G envelope containing this BMP2/4 hybrid expression construct led to secretion of very high levels of mature BMP4 in conditioned medium (up to 1 microg/10(6) cells/24 hours). The secreted BMP4 was biologically active, as it induced alkaline phosphatase expression in C2C12 cells. The transduced rat marrow stromal cells expressing mature BMP4 induced de novo ectopic bone formation in syngenic immune-competent rats. We have developed an MFG-based retroviral vector system that causes secretion of high levels of functionally active human BMP4 protein.

Patterning and lineage specification in the amphibian embryo

Xenopus has been widely used to study early embryogenesis because the embryos allow for efficient functional assays of gene products by the overexpression of RNA. The first asymmetry of the embryo is initiated during oogenesis and is manifested by the darkly pigmented animal hemisphere and lightly pigmented vegetal hemisphere. Upon fertilization a second asymmetry, the dorsal-ventral asymmetry, is established, with the sperm entry site defining the prospective ventral region. During the cleavage stage, a vegetal cortical cytoplasm (VCC)/beta-catenin signaling pathway is differentially activated on the prospective dorsal side of the embryo. The overlapping of the VCC/beta-catenin and transforming growth factor beta (TGF-beta) pathways in the dorsal vegetal quadrant specifies dorsal-vental axis formation by regulating formation of the Spemann organizer, including the anterior endomesoderm. The organizer initiates gastrulation to form a triploblastic embryo in which the mesoderm layer is located between the ectoderm layer and the endoderm layer. The interplay between maternal and zygotic TGF-beta s and the T-box transcription factors in the vegetal hemisphere initiates the specification of germ-layer lineages. TGF-beta signaling originating from the vegetal region induces mesoderm in the equatorial region, and initiates endoderm differentiation directly in the vegetal region. The ectoderm develops from the animal region, which does not come into contact with the vegetal TGF-beta signals. A large number of the downstream components and transcriptional targets of early developmental pathways have been identified and characterized. This review gives an overview of recent advances in the understanding of the functional roles and interactions of the molecular players important for axis determination and germ-layer specification during early Xenopus embryogenesis.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

In synergy with noggin and follistatin, Xenopus nodal-related gene induces sonic hedgehog on notochord and floor plate

In early development of vertebrates, sonic hedgehog functions in dorsal-ventral patterning of dorsal tissue (nervous system and somites). In Xenopus, sonic hedgehog (Xshh) is first expressed in the Spemann organizer/notochord and floor plate. We report here the mechanism governing Xshh mRNA induction in these regions. In animal cap assays, the antagonizing BMPs signal was not sufficient to induce Xshh mRNA expression; however, it could induce Xshh mRNA expression in the presence of Xnr-1. In whole embryos, when secondary axes were induced by coexpressing noggin and Xnr-1 or follistatin and Xnr-1, Xshh mRNA expression was observed in the notochord and floor plate within the induced axes. It seems apparent that spatially restricted Xshh mRNA expression is determined as intersection of the two signals.