Role of activin and other peptide growth factors in body patterning in the early amphibian embryo

The functions of maternal Dishevelled 2 and 3 in the early Xenopus embryo

Of the three Dishevelled (Dvl) genes, only Dvl2 and Dvl3 are maternally encoded in the frog, Xenopus laevis. We show here by loss of function analysis that single depletion of either Dvl2 or Dvl3 from the oocyte causes the same embryonic phenotype. We find that the effects of loss of function of Dvl2 and 3 together are additive, and that the proteins physically interact, suggesting that both are required in the same complex. We show that maternal Dvl2 and 3 are required for convergence extension movements downstream of the dorsally localized signaling pathway activated by Xnr3, but not downstream of the pathway activated by activin. Also, depletion of maternal Dvl2 and 3 mRNAs causes the up-regulation of a subset of zygotic ectodermal genes, including Foxi1e, with surprisingly no significant effect on the canonical Wnt direct target genes Siamois and Xnr3. We suggest that the likely reason for continued expression of the Wnt target genes in Dvl2/3-depleted embryos is that maternal Dvl mRNA depletion is insufficient to deplete stored punctae of Dvl protein in the oocyte cortex, which may transduce dorsal signaling after fertilization.

The nephrogenic potential of the transcription factors osr1, osr2, hnf1b, lhx1 and pax8 assessed in Xenopus animal caps

BACKGROUND

The three distinct types of kidneys, pronephros, mesonephros and metanephros, develop consecutively in vertebrates. The earliest form of embryonic kidney, the pronephros, is derived from intermediate mesoderm and the first expressed genes localized in the pronephros anlage are the transcription factors osr1, osr2, hnf1b, lhx1 and pax8, here referred to as the early nephrogenic transcription factors. However, the pathway inducing nephrogenesis and the network of theses factors are poorly understood. Treatment of the undifferentiated animal pole explant (animal cap) of Xenopus with activin A and retinoic acid induces pronephros formation providing a powerful tool to analyze key molecular events in nephrogenesis.

RESULTS

We have investigated the expression kinetics of the early nephrogenic transcription factors in activin A and retinoic acid treated animal caps and their potential to induce pronephric differentiation. In treated animal caps, expression of osr1, osr2, hnf1b and lhx1 are induced early, whereas pax8 expression occurs later implying an indirect activation. Activin A alone is able to induce osr2 and lhx1 after three hours treatment in animal caps while retinoic acid fails to induce any of these nephrogenic transcription factors. The early expression of the five transcription factors and their interference with pronephros development when overexpressed in embryos suggest that these factors potentially induce nephrogenesis upon expression in animal caps. But no pronephros development is achieved by either overexpression of OSR1, by HNF1B injection with activin A treatment, or the combined application of LHX1 and PAX8, although they influenced the expression of several early nephrogenic transcription factors in some cases. In an additional approach we could show that HNF1B induces several genes important in nephrogenesis and regulates lhx1 expression by an HNF1 binding site in the lhx1 promoter.

CONCLUSIONS

The early nephrogenic transcription factors play an important role in nephrogenesis, but have no pronephros induction potential upon overexpression in animal caps. They activate transcriptional cascades that partially reflect the gene activation initiated by activin A and retinoic acid. Significantly, HNF1B activates the lhx1 promoter directly, thus extending the known activin A regulation of the lhx1 gene via an activin A responsive element.

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.

Transcriptional frequency and cell determination

The relative base composition of DNA regulatory sequences of certain genes of undetermined multipotent progenitor cells may account for the frequency of transcription of these genes in cell determination. The sequences of these regulatory regions of cell determination genes that are more AT-rich would create the potential for transcription at a higher frequency due to their lower melting temperature, as well as propensity to bend. An increase of one or more of the high mobility group (HMG) chromatin proteins would preferentially bind the more AT-rich regulatory sequences, thereby increasing the rate of transcription. The amount of unphosphorylated H1 histone reacting with these same regulatory sites may decrease transcription frequency. The level of cell growth, i.e. total protein synthesis of a cell, is correlated positively with the synthesis of HMG proteins. H1 histone synthesis is linked to DNA replication. Unbalanced growth would alter the amounts of HMG proteins and H1 histone, thus changing transcriptional frequency. The greater the enrichment of AT sequences in the regulatory regions of the cell determination genes, the greater may be the extent of evolutionary conservation. Higher frequency of transcription of the cell determination genes with the more AT-rich regulatory sequences could account for the earlier expression of the more conserved cell determination genes during embryonic development. Preferential binding of H1 histone to the more AT-rich regulatory sequences would subsequently restrict their transcription before that of less conserved cell determination genes.

MODULATION OF ACTIVIN A–INDUCED DIFFERENTIATION IN VITRO BY VASCULAR ENDOTHELIAL GROWTH FACTOR IN XENOPUS PRESUMPTIVE ECTODERMAL CELLS

We have previously demonstrated that activin A at low concentrations induced ventral mesoderm including blood-like cells from Xenopus animal caps and that beating heart could be also induced from animal caps treated with 100 ng/ml activin A, suggesting that activin A might be involved in cardiac vasculogenesis. A vascular endothelial growth factor (VEGF) is a powerful mitogen for endothelial cells and is an inducer and regulator of angiogenesis. However, VEGF function in Xenopus development is not clearly identified. In this study, we determined the effect of VEGF on activin A-induced differentiation of animal cap. The VEGF induced duct-like structure composed of Flk-1-positive cells together with the induction of nonvascular tissues, such as neural tissues. This histological result was coincident with our reverse transcriptase-polymerase chain reaction analysis that VEGF together with activin A promoted the expression of Xenopus N-CAM and Xenopus brachyury. This study suggests that VEGF has additional biological activities besides angiogenesis, and arises a different function that VEGF induces stroma cell migration or recruitment that are required for blood vessel formation. This differentiation system will aid in the understanding of angiogenesis during early development.

A molecular recognition paradigm: promiscuity associated with the ligand-receptor interactions of the activin members of the TGF-β superfamily

The structure-function properties of the pleiotropic activins and their relationship to other members of the transforming growth factor-beta superfamily of proteins are described. In order to highlight the molecular promiscuity of these growth factors, emphasis has been placed on molecular features associated with the recognition by activin A and the bone morphogenic proteins of the corresponding extracellular domains of the ActRI and ActRII receptors. The available evidence suggests that the homodimeric activin A in its various functional roles has the propensity to fulfill key tasks in the regulation of mammalian cell behaviour, through coordination of numerous transcriptional and translational processes. Because of these profound effects, under physiologically normal conditions, activin A levels are closely controlled by a variety of binding partners, such as follistatin-288 and follistatin-315, alpha(2)-macroglobulin and other proteins. Moreover, the subunits of other members of the activin subfamily, such as activin B or activin C, are able to form heterodimers with the activin A subunit, thus providing a further avenue to positively or negatively control the physiological concentrations of activin A that are available for interaction with specific receptors and induction of cell signaling events. Based on data from X-ray crystallographic studies and homology modeling experiments, the molecular architecture of the ternary receptor-activin ligand complexes has been dissected, permitting rationalization in structural terms of the pattern of interactions that are the hallmark of this protein family.

Assessment of the function of the betaC-subunit of activin in cultured hepatocytes

We assessed the function of the beta(C)-subunit of activin in hepatocytes. We studied the effect of conditioned medium of Chinese hamster ovary (CHO) cell line stably expressing the beta(C) gene (CHO-beta(C)) on growth of AML12 hepatocytes. We also examined the effect of recombinant activin C and transfection of the beta(C) gene by using adenovirus vector. CHO-beta(C) secreted activin C, a homodimer of the beta(C), as well as precursors of the beta(C). The conditioned medium of CHO-beta(C) increased both [(3)H]thymidine incorporation and the cell number in AML12 cells. It also supported survival of AML12 cells in a serum-free condition. Recombinant human activin C also increased both [(3)H]thymidine incorporation and the number of AML12 cells. Transfection of AML12 cells with the beta(C)-subunit led to the stimulation of [(3)H]thymidine incorporation. Analysis of the conditioned medium revealed that the beta(C)-subunit formed a heterodimer with the endogenous beta(A), the formation of which was dependent on the amount of beta(C) expressed. Recombinant activin C did not affect the binding of (125)I-activin A to its receptor or follistatin. These results indicate that activin C stimulates growth of AML12 cells. The beta(C)-subunit modifies the function of the beta(A)-subunit by multiple mechanisms.

Current state of and outlook for organogenesis from undifferentiated cells

PURPOSE

The aims of regenerative medicine and artificial internal organ development are, respectively, to regenerate and provide for transplant tissue or organs.

METHODS

We summarize recent research on tissue differentiation and organogenesis using stem cells and report our laboratory's research using amphibian undifferentiated cells.

RESULTS

We have successfully induced differentiation in cells from Xenopus laevis and generated structures in vitro that function in a similar way to organs when transplanted to Xenopus and newt embryos. We are attempting to establish a system to induce sensory organs, including eyes.

CONCLUSION

Our experimental systems in amphibians are useful for organogenetic research, and it is hoped that our techniques can, in the future, be applied to mammals.

Long-term culture of Xenopus presumptive ectoderm in a nutrient-supplemented culture medium

Animal cap assay is a useful experimental model for investigating the activity of inducers in amphibian development. This assay has revealed that activin A is a potent mesoderm-inducing factor. However, it has been very difficult to induce highly differentiated tissues such as cartilage in a 3-4 day culture period. It was recently reported that jaw cartilage was induced in vitro in an animal cap that had been cultured for 14 days in Steinberg's solution using the sandwich culture method and activin A. Under these conditions, necrosis was occasionally observed in the explants. In this study, we have achieved long-term animal cap cultures in a nutrient-supplemented culture medium designated RDX. This medium was made by modifying the saline concentration of the RD medium previously developed as a basal medium for the serum-free culture of various kinds of mammalian cells. The explants cultured in RDX grew more vigorously compared with those in Steinberg's solution. RDX medium promoted a wider variety of tissue induction and gene expression in the animal caps than Steinberg's solution, and also increased the frequency of cartilage induction. Therefore, the supplemental nutrients may support and promote the differentiation of cartilage. This long-term culture method using RDX medium is useful for studying the differentiation of tissues or organs such as cartilage in vitro.

Normal reproductive function in InhBP/p120-deficient mice

The inhibins are gonadal transforming growth factor beta superfamily protein hormones that suppress pituitary follicle-stimulating hormone (FSH) synthesis. Recently, betaglycan and inhibin binding protein (InhBP/p120, also known as the product of immunoglobulin superfamily gene 1 [IGSF1]) were identified as candidate inhibin coreceptors, shedding light on the molecular basis of how inhibins may affect target cells. Activins, which are structurally related to the inhibins, act within the pituitary to stimulate FSH production. Betaglycan increases the affinity of inhibins for the activin type IIA (ACVR2) receptor, thereby blocking activin binding and signaling through this receptor. InhBP/p120 may not directly bind inhibins but may interact with the activin type IB receptor, ALK4, and participate in inhibin B's antagonism of activin signaling. To better understand the in vivo functions of InhBP/p120, we characterized the InhBP/p120 mRNAs and gene in mice and generated InhBP/p120 mutant mice by gene targeting in embryonic stem cells. InhBP/p120 mutant male and female mice were viable and fertile. Moreover, they showed no alterations in FSH synthesis or secretion or in ovarian or testicular function. These data contribute to a growing body of evidence indicating that InhBP/p120 does not play an essential role in inhibin biology.

Immunocytochemical study of activin type IB receptor (XALK4) in Xenopus oocytes

Studies have shown that the activin type IB receptor is specific for activin/nodal signaling. Activin is produced by follicle cells in the ovary, and is incorporated into the oocytes. Antisera against three peptides were prepared, encompassing the extracellular, intracellular and serine/threonine kinase domains of the Xenopus type IB activin receptor (XALK4). Immunocytochemistry was done using these antisera to investigate the distribution of XALK4 in the Xenopus ovary. All three antisera stained the mitochondrial cloud of Xenopus previtellogenic oocytes. Purified antibody against the intracellular domain also recognized the mitochondrial cloud. Immunoelectron microscopy localized XALK4 on the endoplasmic reticulum of the mitochondrial cloud, although not on mitochondria.

Activin A induces craniofacial cartilage from undifferentiated Xenopus ectoderm in vitro

Activin A has potent mesoderm-inducing activity in amphibian embryos and induces various mesodermal tissues in vitro from the isolated presumptive ectoderm. By using a sandwich culture method established to examine activin A activity, we previously demonstrated that activin-treated ectoderm can function as both a head and trunk-tail organizer, depending on the concentration of activin A. By using activin A and undifferentiated presumptive ectoderm, it is theoretically possible to reproduce embryonic induction. Here, we test this hypothesis by studying the induction of cartilage tissue by using the sandwich-culture method. In the sandwiched explants, the mesenchymal cell condensation expressed type II collagen and cartilage homeoprotein-1 mRNA, and subsequently, cartilage were induced as they are in vivo. goosecoid (gsc) mRNA was prominently expressed in the cartilage in the explants. Xenopus distal-less 4 (X-dll4) mRNA was expressed throughout the explants. In Xenopus embryos, gsc expression is restricted to the cartilage of the lower jaw, and X-dll4 is widely expressed in the ventral head region, including craniofacial cartilage. These finding suggest that the craniofacial cartilage, especially lower jaw cartilage, was induced in the activin-treated sandwiched explants. In addition, a normal developmental pattern was recapitulated at the histological and genetic level. This work also suggests that the craniofacial cartilage-induction pathway is downstream of activin A. This study presents a model system suitable for the in vitro analysis of craniofacial cartilage induction in vertebrates.

Axial protocadherin is a mediator of prenotochord cell sorting in Xenopus

Prenotochord cell sorting is regarded as one of the first cell sorting events in early chordate development. We recently demonstrated that this sorting event occurs in vitro, although the mediator of this activity remains unidentified. Herein, we report the isolation of a full-length cDNA clone of Axial protocadherin (AXPC), the homologue of human protocadherin-1 (PCD1). AXPC encodes a transmembrane protein (AXPC) that is expressed exclusively in the notochord at the neurula stage and in the pronephros, somites, heart, optic vesicle, otic vesicle, and distinct parts of the brain at the tailbud stage. Cell dissociation and reaggregation assays and in vivo microinjection experiments demonstrated that cells overexpressing a membrane-tethered form of AXPC (MT-AXPC) acquired the same adhesive properties as prenotochord cells. Moreover, microinjection of either mRNA encoding the dominant negative form of AXPC (DN-AXPC) or morpholino oligonucleotides interferes with the sorting activity of prenotochord cells and normal axis formation. This study suggests that AXPC is necessary and sufficient for prenotochord cell sorting in the gastrulating embryo, and may also mediate sorting events later in development.

Overexpression of the secreted factor Mig30 expressed in the Spemann organizer impairs morphogenetic movements during Xenopus gastrulation

The Spemann organizer secretes several antagonists of growth factors during gastrulation. We describe a novel secreted protein, Mig30, which is expressed in the anterior endomesoderm of the Spemann organizer. Mixer-inducible gene 30 (Mig30) was isolated as a target of Mixer, a homeobox gene required for endoderm development. The Mig30 gene encodes a secreted protein containing a cysteine-rich domain and an immunoglobulin-like domain that belongs to the insulin-like growth factor-binding protein family. Overexpression of Mig30 in the dorsal region results in the retardation of morphogenetic movements during gastrulation and leads to microcephalic embryos. Overexpression of Mig30 also inhibits activin-induced elongation of ectodermal explants without affecting gene expression patterns in mesoderm and endoderm. These results suggest that Mig30 is involved in the regulation of morphogenetic movements during gastrulation in the extracellular space of the Spemann organizer.

Total chemical synthesis of human activin beta(A)[12-116] and related large-loop polypeptides

We report here the synthesis, purification, and characterization of several large polypeptides related to the human activin beta(A) subunit and their cyclic counterparts. In particular, we describe for the first time the total chemical synthesis of a 105-mer polypeptide, des[1-11] activin beta(A), and related large-loop polypeptide, by an optimized solid phase synthetic protocol based on 9-flouroenylmethyoxycarbonyl (Fmoc) chemistry. These studies show that automated chemical synthesis utilizing Fmoc-based solid phase synthetic strategies provides a practical alternative to recombinant DNA technology for the production of activin-related subunits, with the opportunity to rapidly provide different analogues and structural variants for subsequent structure-function and associated biophysical investigations.

Biology of kidney cells: ontogeny-recapitulating phylogeny

Biology of kidney cells can be used as a model for further understanding of ontogeny-recapitulating phylogeny. The common and species-specific structural and functional relationship between blood capillaries and the environment via a filtration barrier of nephrons is a biological phenomenon resulting from renal cell memory acquired through evolution. Genetically programmed development, a subsequent series of gene expression, and inductive interactions played a key role in differentiation and maintenance of specific activities of kidneys in birds and mammals. Various environmental factors may alter kidney development and specific activities at the levels of gene expression, repression, or derepression, and defensive mechanisms involved in reaction to risk factors are developed. Autoimmunity and cancerogenesis are closely dependent on a variety of environmental agents, such as antigens originating from infections with some viruses and toxins, or irradiation, advanced industrialization, and progress of civilization. As a result of gene mutation, delation, rearrangement, and/or susceptibility to different agents, renal cell memory is altered. Instead of cell-specific activities, the abilities for regeneration, and other genetically programmed activities, the genesis of kidney diseases are common. Balkan endemic nephropathy, as regional disease, is an important example of the role, of environmental agents, at the level of genes. Research programs on molecular genetics will contribute to our efforts both to prevent infections and to elucidate the genesis, diagnosis, prognosis, prevention, and therapy of kidney diseases.

Role of BMP-4 in the inducing ability of the head organizer in Xenopus laevis

BMP-4 has been implicated in the patterning of the Dorsal-Ventral axis of mesoderm and ectoderm. In this study, we describe the posteriorizing effect of BMP-4 on the neural inducing ability of dorsal mesoderm (dorsal lip region) in Xenopus gastrulae. Dorsal lip explants dissected from stage 10.25 embryos retained anterior inducing ability when precultured for 6 hrs until sibling embryos reach stage 12. When the dorsal lips from stage 10.25 embryos were treated with a range of BMP-4 concentrations, posterior tissues were induced in adjacent ectoderm in a dose-dependent manner. Thus activin-treated explants able to act as head inducers can also induce posterior structures in the presence of BMP-4. To investigate whether BMP-4 directly affects the inducing ability of dorsal mesoderm, we blocked the BMP-4 signaling pathway by injection of mRNA encoding a truncated form of the BMP-4 receptor (tBR) mRNA. Under these conditions, activin-treated explants induced anterior tissues following BMP-4 treatment. Taken together, these results indicate that BMP-4 may affect the head inducing ability of dorsal mesoderm and confer trunk-tail inducing ability during Xenopus gastrulation.

Antagonism of activin by inhibin and inhibin receptors: a functional role for betaglycan-glycan

Activin and inhibin research has provided important insight into reproductive physiology as well as many areas involving regulation of cell growth, differentiation and function. Progress in understanding the roles of these hormones in various cell and tissue types has been complimented by novel discoveries at the molecular level that have shed light on ligand/receptor interactions, signaling mechanisms and regulation. While the receptors and signaling pathway for activin are now well characterized, the molecular basis for inhibin action has remained relatively unclear. Here we summarize recent advances in understanding inhibin's mode of action focusing on our recent identification of betaglycan-glycan as an inhibin co-receptor capable of mediating inhibin action.

Activin A induces expression of rat Sel-1l mRNA, a negative regulator of notch signaling, in rat salivary gland-derived epithelial cells

We previously established a rat submandibular gland (SMG)-derived epithelial cell line (RSMG-1) to study the mechanism of morphogenesis in salivary gland development and regeneration. We found that activin A regulated the branching morphogenesis of RSMG-1 cells, suggesting that it is involved in SMG morphogenesis. We used a subtraction cloning procedure with activin-A-treated and untreated RSMG-1 cells to identify activin-A-induced genes. One of the genes detected encoded a rat homologue of Sel-1l (rSel-1l). rSel-1l is a mammalian homologue of C. elegans sel-1, which is a negative regulator of Notch signaling. In this study, we confirmed that activin A induces rSel-1l mRNA expression in RSMG-1 cells, and that rSel-1l is expressed in SMG acinar cells. These results suggest that activin A regulates the differentiation of RSMG-1 cells to acinar cells.

In vitro pancreas formation from Xenopus ectoderm treated with activin and retinoic acid

In the present study, isolated presumptive ectoderm from Xenopus blastula was treated with activin and retinoic acid to induce differentiation into pancreas. The presumptive ectoderm region of the blastula consists of undifferentiated cells and is fated to become epidermis and neural tissue in normal development. When the region is isolated and cultured in vitro, it develops into atypical epidermis. Isolated presumptive ectoderm was treated with activin and retinoic acid. The ectoderm frequently differentiated into pancreas-like structures accompanied by an intestinal epithelium-like structure. Sections of the explants viewed using light and electron microscopy showed some cells clustered and forming an acinus-like structure, including secretory granules. The pancreas-specific molecular markers insulin and XIHbox8 were also expressed in the treated explants. The pancreatic hormones, insulin and glucagon, were detected in the explants using immunohistochemistry. Therefore, sequential treatment with activin and retinoic acid can induce presumptive ectoderm to differentiate into a morphological and functional pancreas in vitro. When ectoderm was immediately treated with retinoic acid after treatment with activin, well-differentiated pronephric tubules were seen in a few of the differentiated pancreases. Treatment with retinoic acid 3-5 h after activin treatment induced frequent pancreatic differentiation. When the time lag was longer than 15h, the explants developed into axial mesoderm and pharynx. The present study provides an effective system for analyzing pancreas differentiation in vertebrate development.

Bottle cell formation in relation to mesodermal patterning in the Xenopus embryo

The appearance of bottle cells at the dorsal vegetal/marginal boundary of Xenopus embryos marks the onset of blastopore formation. The conditions leading to this epithelial activity were investigated by inducing bottle cells ectopically in the animal region with VegT or different members of the transforming growth factor (TGF)-beta family. Morphological studies on the ectopic bottle cells indicate their close similarity to the endogenous bottle cells at the dorsal blastopore lip. The subepithelial cells of the induced animal region express mesodermal genes in a pattern reminiscent to that observed on the dorsal lip. Relating this expression pattern to the position of the ectopic bottle cells leads to the conclusion that bottle cells form in regions of high TGF-beta signalling. The specific inhibitory effects of cerberus on ectopically induced bottle cells revealed that nodal related growth factors are the intrinsic signals that elicit bottle cell formation in the normal embryo. In addition, fibroblast growth factor signalling is an essential precondition for this epithelial response as it is for mesoderm formation. We conclude that bottle cell formation in the epithelial layer of the gastrula is closely linked to mesodermal patterning in the subepithelial tissues.

Future opportunities for life science programs in space

Most space-related life science programs are expensive and time-consuming, requiring international cooperation and resources with trans-disciplinary expertise. A comprehensive future program in "life sciences in space" needs, therefore, well-defined research goals and strategies as well as a sound ground-based program. The first half of this review will describe four key aspects such as the environment in space, previous accomplishments in space (primarily focusing on amphibian embryogenesis), available resources, and recent advances in bioinformatics and biotechnology, whose clear understanding is imperative for defining future directions. The second half of this review will focus on a broad range of interdisciplinary research opportunities currently supported by the National Aeronautics and Space Administration (NASA), National Institute of Health (NIH), and National Science Foundation (NSF). By listing numerous research topics such as alterations in a diffusion-limited metabolic process, bone loss and skeletal muscle weakness of astronauts, behavioral and cognitive ability in space, life in extreme environment, etc., we will attempt to suggest future opportunities.

In vitro control of organogenesis and body patterning by activin during early amphibian development

In the process of amphibian development, an embryonic body plan is established through cell division, sequential gene expression, morphogenesis and cell differentiation. The mechanism of body patterning is complex and includes multiple induction events. Activin, a TGF-beta family protein, can induce several kinds of mesodermal and endodermal tissues in animal cap explants in a dose-dependent manner. In a recent study of the role of activin in organogenesis, we succeeded in raising a beating heart by treating animal caps with a high concentration of activin. Activin also participates in kidney organogenesis in combination with retinoic acid. An embryonic kidney induced by activin and retinoic acid in vitro can function in vivo when it is transplanted into a larva in which pronephros rudiments have already been removed. Further, the activin-treated animal caps clearly show organizer actions that are closely related to body patterning along the anteroposterior axis. These experiments will help to serve as a model system for understanding organogenesis and body patterning at the cellular and molecular levels.