Using stem cells to study and possibly treat type 1 diabetes

Stem cells with the potential to form many different cell types are actively studied for their possible use in cell replacement therapies for several diseases. In addition, the differentiated derivatives of stem cells are being used as reagents to test for drugs that slow or correct disease phenotypes found in several degenerative diseases. This paper explores these approaches in the context of type 1 or juvenile diabetes, pointing to recent successes as well as the technical and theoretical challenges that lie ahead in the path to new treatments and cures.

Pathways to new beta cells

Diabetes is a leading health problem of the world and its prevalence continues to rise. With Type I diabetes, and in some patients with Type II, the lack of insulin can be counterbalanced by providing new beta (insulin-producing) cells. For Type I diabetes, treating the autoimmune attack remains a serious challenge. Several strategies to produce new beta cells have been proposed. These include differentiation from embryonic stem cells, proliferation of existing adult beta cells, derivation from putative adult progenitors/stem cells, and reprogramming of non-beta cells to beta cells. Each of these strategies has distinct merits and risks, and they are at different stages of understanding and development. In particular, the approach based on differentiation from embryonic stem cells has had strong support and in recent years has made notable progress. Nevertheless, significant hurdles remain to transform the current research into future therapies. To expedite this transformation, we believe particular emphasis should be placed on overcoming key knowledge gaps in beta-cell biology, developing strategies that produce patient-specific beta cells, and carefully addressing potential treatment-related complications or limitations.

Nuclear reprogramming in cells

Nuclear reprogramming describes a switch in gene expression of one kind of cell to that of another unrelated cell type. Early studies in frog cloning provided some of the first experimental evidence for reprogramming. Subsequent procedures included mammalian somatic cell nuclear transfer, cell fusion, induction of pluripotency by ectopic gene expression, and direct reprogramming. Through these methods it becomes possible to derive one kind of specialized cell (such as a brain cell) from another, more accessible, tissue (such as skin) in the same individual. This has potential applications for cell replacement without the immunosuppression treatments that are required when cells are transferred between genetically different individuals. This article provides some background to this field, a discussion of mechanisms and efficiency, and comments on prospects for future nuclear reprogramming research.

Localization of mRNA and axis formation during Xenopus embryogenesis

This paper summarizes our recent work concerned with the developmental polarity of the frog egg and the patterning of the embryonic body plan. In two separate projects, we are studying genes involved in setting up basic embryonic axes. One of these genes, Vg1, codes for a maternal mRNA that is localized in the frog egg. The Vg1 gene is used in studies on the induction of mesoderm and as a probe to understand how the polarity of an egg is established. A second gene, Xhox3, contains a homeodomain and is differentially expressed in the axial mesoderm. Our studies suggest that this homeobox gene is critically involved in setting up different positional values along the anteroposterior axis.

Analysis of pancreatic development in living transgenic zebrafish embryos

Using DNA constructs containing regulatory sequences of the zebrafish Pdx-1 and insulin genes, germline transgenic zebrafish expressing the green fluorescent protein (GFP) reporter gene in the pancreas were generated. For both constructs, the GFP expression patterns in transgenic embryos were consistent with the mRNA expression patterns detected by RNA in situ hybridization. A deletion promoter analysis revealed that positive and negative cis-acting elements were involved in regulation of insulin gene expression. Three-dimensional reconstructions imaged from living embryos using two-photon laser-scanning microscopy (TPLSM) demonstrated that the zebrafish pancreas is formed from a single dorsal pancreatic cell mass. This is in contrast to mammals where the pancreas derives from both dorsal and ventral anlage. These transgenic fish should be useful for in vivo studies of factors involved in specifying and regulating pancreatic development and function.

Key events of pancreas formation are triggered in gut endoderm by ectopic expression of pancreatic regulatory genes

The mechanisms by which the epithelium of the digestive tract and its associated glands are specified are largely unknown. One clue is that several transcription factors are expressed in specific regions of the endoderm prior to and during organogenesis. Pdx-1, for example, is expressed in the duodenum and pancreas and Pdx-1 inactivation results in an arrest of pancreatic development after buds formation. Similarly, ngn3 is transiently expressed in the developing pancreas and a knockout results in the absence of endocrine cells. This paper focuses on the question of whether these and other transcription factors, known to be necessary for pancreatic development, are also sufficient to drive a program of pancreatic organogenesis. Using in ovo electroporation of chick embryos, we show that ectopic expression of Pdx-1 or ngn3 causes cells to bud out of the epithelium like pancreatic progenitors. The Pdx-1-expressing cells extinguish markers for other nonpancreatic regions of the endoderm and initiate, but do not complete, pancreatic cytodifferentiation. Ectopic expression of ngn3 is sufficient to turn endodermal cells of any region into endocrine cells that form islets expressing glucagon and somatostatin in the mesenchyme. The results suggest that simple gene combinations could be used in stem cells to achieve specific endodermal tissue differentiation.

Regulation of pancreas development by hedgehog signaling

Pancreas organogenesis is regulated by the interaction of distinct signaling pathways that promote or restrict morphogenesis and cell differentiation. Previous work has shown that activin, a TGF(beta+) signaling molecule, permits pancreas development by repressing expression of Sonic hedgehog (Shh), a member of the hedgehog family of signaling molecules that antagonize pancreas development. Here we show that Indian hedgehog (Ihh), another hedgehog family member, and Patched 1 (Ptc1), a receptor and negative regulator of hedgehog activity, are expressed in pancreatic tissue. Targeted inactivation of Ihh in mice allows ectopic branching of ventral pancreatic tissue resulting in an annulus that encircles the duodenum, a phenotype frequently observed in humans suffering from a rare disorder known as annular pancreas. Shh(−)(/)(−) and Shh(−)(/)(−) Ihh(+/)(−) mutants have a threefold increase in pancreas mass, and a fourfold increase in pancreatic endocrine cell numbers. In contrast, mutations in Ptc1 reduce pancreas gene expression and impair glucose homeostasis. Thus, islet cell, pancreatic mass and pancreatic morphogenesis are regulated by hedgehog signaling molecules expressed within and adjacent to the embryonic pancreas. Defects in hedgehog signaling may lead to congenital pancreatic malformations and glucose intolerance.

Effects of eight growth factors on the differentiation of cells derived from human embryonic stem cells

Human embryonic stem (ES) cells are pluripotent cells derived from the inner cell mass of in vitro fertilized human blastocysts. We examined the potential of eight growth factors [basic fibroblast growth factor (bFGF), transforming growth factor beta1 (TGF-beta1), activin-A, bone morphogenic protein 4 (BMP-4), hepatocyte growth factor (HGF), epidermal growth factor (EGF), beta nerve growth factor (betaNGF), and retinoic acid] to direct the differentiation of human ES-derived cells in vitro. We show that human ES cells that have initiated development as aggregates (embryoid bodies) express a receptor for each of these factors, and that their effects are evident by differentiation into cells with different epithelial or mesenchymal morphologies. Differentiation of the cells was assayed by expression of 24 cell-specific molecular markers that cover all embryonic germ layers and 11 different tissues. Each growth factor has a unique effect that may result from directed differentiation and/or cell selection, and we can divide the overall effects of the factors into three categories: growth factors (Activin-A and TGFbeta1) that mainly induce mesodermal cells; factors (retinoic acid, EGF, BMP-4, and bFGF) that activate ectodermal and mesodermal markers; and factors (NGF and HGF) that allow differentiation into the three embryonic germ layers, including endoderm. None of the growth factors directs differentiation exclusively to one cell type. This analysis sets the stage for directing differentiation of human ES cells in culture and indicates that multiple human cell types may be enriched in vitro by specific factors.

Activin receptor patterning of foregut organogenesis

Foregut development produces a characteristic sequence of gastrointestinal and respiratory organs, but the signaling pathways that ensure this developmental order remain largely unknown. Here, mutations of activin receptors ActRIIA and ActRIIB are shown to disrupt the development of posterior foregut-derived organs, including the stomach, pancreas, and spleen. Foregut expression of genes including Shh and Isl1 is shifted in mutant mice. The endocrine pancreas is particularly sensitive to the type and extent of receptor inactivation. ActRIIA(+/-)B(+/-) animals lack axial defects, but have hypoplastic pancreatic islets, hypoinsulinemia, and impaired glucose tolerance. Thus, activin receptor-mediated signaling regulates axial patterning, cell differentiation, and function of foregut-derived organs.

Development of the pancreas in Xenopus laevis

Xenopus embryos have several experimental advantages for studying development. Although these advantages have traditionally been used to elucidate mechanisms of early development, they can also be exploited to investigate issues later in development such as organogenesis. We have begun to study pancreatic organogenesis in Xenopus. Using histological and molecular marker analysis, we characterized the anatomy of the developing pancreas in Xenopus embryos from the time of initial pancreatic rudiment formation to the time when the tadpole starts to feed. We examined the expression of various endocrine hormones, exocrine gene products, and pancreatic transcription factors. Interestingly, the endocrine hormone insulin has restricted expression in the dorsal pancreas. Investigation of pancreatic specification during gastrulation demonstrates that insulin expression is regionalized along the dorsoventral axis early in development.

Hedgehog signals regulate multiple aspects of gastrointestinal development

The gastrointestinal tract develops from the embryonic gut, which is composed of an endodermally derived epithelium surrounded by cells of mesodermal origin. Cell signaling between these two tissue layers appears to play a critical role in coordinating patterning and organogenesis of the gut and its derivatives. We have assessed the function of Sonic hedgehog and Indian hedgehog genes, which encode members of the Hedgehog family of cell signals. Both are expressed in gut endoderm, whereas target genes are expressed in discrete layers in the mesenchyme. It was unclear whether functional redundancy between the two genes would preclude a genetic analysis of the roles of Hedgehog signaling in the mouse gut. We show here that the mouse gut has both common and separate requirements for Sonic hedgehog and Indian hedgehog. Both Sonic hedgehog and Indian hedgehog mutant mice show reduced smooth muscle, gut malrotation and annular pancreas. Sonic hedgehog mutants display intestinal transformation of the stomach, duodenal stenosis (obstruction), abnormal innervation of the gut and imperforate anus. Indian hedgehog mutants show reduced epithelial stem cell proliferation and differentiation, together with features typical of Hirschsprung's disease (aganglionic colon). These results show that Hedgehog signals are essential for organogenesis of the mammalian gastrointestinal tract and suggest that mutations in members of this signaling pathway may be involved in human gastrointestinal malformations.

Notch gene expression during pancreatic organogenesis

Notch receptors are involved in regulating the balance between cell differentiation and stem cell proliferation during the development of numerous tissues (Artavanis-Tsakonas, S., Matsuno, K., Fortini, M. E., 1995. Notch signaling. Science 268, 225-232). Here the expression of all four vertebrate Notch genes, their ligands, and some down-stream targets is analyzed during mouse pancreatic organogenesis. Notch 1 is the first Notch gene expressed in the pancreatic epithelium, and coexpression with HES 1 suggests that the Notch 1 pathway is activated. Notch 2 expression follows later when pancreatic buds branch and is restricted to embryonic ducts, believed to be the source for endocrine and exocrine stem cells. Notch 3 and Notch 4 are expressed in pancreatic mesenchyme and later in endothelial cells. Together these descriptive data comprise a framework for understanding the cellular basis for Notch function during pancreatic development.

Early mouse endoderm is patterned by soluble factors from adjacent germ layers

Endoderm that forms the respiratory and digestive tracts is a sheet of approximately 500–1000 cells around the distal cup of an E7.5 mouse embryo. Within 2 days, endoderm folds into a primitive gut tube from which numerous organs will bud. To characterize the signals involved in the developmental specification of this early endoderm, we have employed an in vitro assay using germ layer explants and show that adjacent germ layers provide soluble, temporally specific signals that induce organ-specific gene expression in endoderm. Furthermore, we show that FGF4 expressed in primitive streak-mesoderm can induce the differentiation of endoderm in a concentration-dependent manner. We conclude that the differentiation of gastrulation-stage endoderm is directed by adjacent mesoderm and ectoderm, one of the earliest reported patterning events in formation of the vertebrate gut tube.

Endoderm development: from patterning to organogenesis

Although the ectoderm and mesoderm have been the focus of intensive work in the recent era of studies on the molecular control of vertebrate development, the endoderm has received less attention. Because signaling must occur between germ layers in order to achieve a properly organized body, our understanding of the coordinated development of all organs requires a more thorough consideration of the endoderm and its derivatives. This review focuses on present knowledge and perspectives concerning endoderm patterning and organogenesis. Some of the classical embryology of the endoderm is discussed and the progress and deficiencies in cellular and molecular studies are noted.

Vertebrate endoderm development

Endoderm, one of the three principal germ layers, contributes to all organs of the alimentary tract. For simplicity, this review divides formation of endodermal organs into four fundamental steps: (a) formation of endoderm during gastrulation, (b) morphogenesis of a gut tube from a sheet of cells, (c) budding of organ domains from the tube, and (d) differentiation of organ-specific cell types within the growing buds. We discuss possible mechanisms that regulate how undifferentiated endoderm becomes specified into a myriad of cell types that populate the respiratory and gastrointestinal tracts.

Screening for novel pancreatic genes expressed during embryogenesis

We have combined suppressive subtractive hybridization with in situ hybridization to identify genes expressed at early stages of pancreas development. By using polymerase chain reaction amplification and subtractive hybridization, this protocol for screening can be applied when the amount of RNA is limited. Seven genes expressed in or adjacent to the pancreas anlage were isolated, three of which show similarity to known genes. The expression pattern and sequence information indicate that some of the genes could govern pancreas development.

Pancreas development is promoted by cyclopamine, a hedgehog signaling inhibitor

Exposure to cyclopamine, a steroid alkaloid that blocks Sonic hedgehog (Shh) signaling, promotes pancreatic expansion in embryonic chicks. Heterotopic development of pancreatic endocrine and exocrine structures occurs in regions adjacent to the pancreas including stomach and duodenum, and insulin-producing islets in the pancreas are enlarged. The homeodomain transcription factor PDX1, required for pancreas development, is expressed broadly in the posterior foregut but pancreas development normally initiates only in a restricted region of PDX1-expressing posterior foregut where endodermal Shh expression is repressed. The results suggests that cyclopamine expands the endodermal region where Shh signaling does not occur, resulting in pancreatic differentiation in a larger region of PDX1-expressing foregut endoderm. Cyclopamine reveals the capacity of a broad region of the posterior embryonic foregut to form pancreatic cells and provides a means for expanding embryonic pancreas development.

Mutant Vg1 ligands disrupt endoderm and mesoderm formation in Xenopus embryos

The Xenopus Vg1 gene, a TGFbeta superfamily member, is expressed as a maternal mRNA localized to prospective endoderm, and mature Vg1 protein can induce both endodermal and mesodermal markers in embryonic cells. Most previous work on embryonic inducers, including activin, BMPs and Vg1, has relied on ectopic expression to assay for gene function. Here we employ a mutant ligand approach to block Vg1 signaling in developing embryos. The results indicate that Vg1 expression is essential for normal endodermal development and the induction of dorsal mesoderm in vivo.

Mixer, a homeobox gene required for endoderm development

An expression cloning strategy in Xenopus laevis was used to isolate a homeobox-containing gene, Mixer, that can cause embryonic cells to form endoderm. Mixer transcripts are found specifically in the prospective endoderm of gastrula, which coincides with the time and place that endodermal cells become histologically distinct and irreversibly determined. Loss-of-function studies with a dominant inhibitory mutant demonstrate that Mixer activity is required for endoderm development. In particular, the expression of Sox17alpha and Sox17beta, two previously identified endodermal determinants, require Mixer function. Together, these data suggest that Mixer is an embryonic transcription factor involved in specifying the endodermal germ layer.

Notochord repression of endodermal Sonic hedgehog permits pancreas development

Notochord signals to the endoderm are required for development of the chick dorsal pancreas. Sonic hedgehog (SHH) is normally absent from pancreatic endoderm, and we provide evidence that notochord, in contrast to its effects on adjacent neuroectoderm where SHH expression is induced, represses SHH expression in adjacent nascent pancreatic endoderm. We identify activin-betaB and FGF2 as notochord factors that can repress endodermal SHH and thereby permit expression of pancreas genes including Pdx1 and insulin. Endoderm treatment with antibodies that block hedgehog activity also results in pancreatic gene expression. Prevention of SHH expression in prepancreatic dorsal endoderm by intercellular signals, like activin and FGF, may be critical for permitting early steps of chick pancreatic development.

Notochord to endoderm signaling is required for pancreas development

The role of the notochord in inducing and patterning adjacent neural and mesodermal tissues is well established. We provide evidence that the notochord is also required for one of the earliest known steps in the development of the pancreas, an endodermally derived organ. At a developmental stage in chick embryos when the notochord touches the endoderm, removal of notochord eliminates subsequent expression of several markers of dorsal pancreas bud development, including insulin, glucagon and carboxypeptidase A. Pancreatic gene expression can be initiated and maintained in prepancreatic chick endoderm grown in vitro with notochord. Non-pancreatic endoderm, however, does not express pancreatic genes when recombined with the same notochord. The results suggest that the notochord provides a permissive signal to endoderm to specify pancreatic fate in a stepwise manner.

Xnr4: a Xenopus nodal-related gene expressed in the Spemann organizer

We have isolated a novel TGF beta-like gene from Xenopus which is highly related to mouse nodal The gene, Xnr4, is expressed at the gastrula stage in the Spemann organizer and at later stages in the notochord and neural tube. Ectopic expression of Xnr4 can induce and dorsalize mesoderm. These studies suggest that Xnr4, along with other nodal-related genes (Xnr1-3), may participate in mesodermal patterning and possibly neural development.

Short-range signaling by candidate morphogens of the TGF beta family and evidence for a relay mechanism of induction

The specification and patterning of cell fates by a morphogen gradient is a unifying theme of developmental biology, yet little evidence exists for the presence of gradients in vivo or to show how such putative gradients form. Vg1 and activin are candidate morphogens involved in Xenopus mesoderm induction. This study suggests that these TGF beta family members act on adjacent cells but do not travel through the intact extracellular space to induce distant cells directly. Moreover, we present evidence for the presence of secondary inducing signals that could be involved in relaying signals to distant cells. These results suggest that if a localized cellular source of an inducer acts to pattern mesodermal cells at a distance in Xenopus embryos, it does so by a relay mechanism.

A molecular mechanism for the effect of lithium on development

Lithium, one of the most effective drugs for the treatment of bipolar (manic-depressive) disorder, also has dramatic effects on morphogenesis in the early development of numerous organisms. How lithium exerts these diverse effects is unclear, but the favored hypothesis is that lithium acts through inhibition of inositol monophosphatase (IMPase). We show here that complete inhibition of IMPase has no effect on the morphogenesis of Xenopus embryos and present a different hypothesis to explain the broad action of lithium. Our results suggest that lithium acts through inhibition of glycogen synthase kinase-3 beta (GSK-3 beta), which regulates cell fate determination in diverse organisms including Dictyostelium, Drosophila, and Xenopus. Lithium potently inhibits GSK-3 beta activity (Ki = 2 mM), but is not a general inhibitor of other protein kinases. In support of this hypothesis, lithium treatment phenocopies loss of GSK-3 beta function in Xenopus and Dictyostelium. These observations help explain the effect of lithium on cell-fate determination and could provide insights into the pathogenesis and treatment of bipolar disorder.

Mothers against dpp encodes a conserved cytoplasmic protein required in DPP/TGF-beta responsive cells

The proteins necessary for signal transduction in cells responding to ligands of the TGF-beta family are largely unknown. We have previously identified Mad (Mothers against dpp), a gene that interacts with the TGF-beta family member encoded by decapentaplegic (dpp) in Drosophila. Assay of Mad's role in the DPP-dependent events of embryonic midgut development demonstrates that Mad is required for any response of the visceral mesoderm or endoderm to DPP signals from the visceral mesoderm. Replacement of the normal DPP promoter with a heterologous (hsp70) promoter fails to restore DPP-dependent responses in Mad mutant midguts. Experiments utilizing Mad transgenes regulated by tissue-specific promoters show that MAD is required specifically in cells responding to DPP. Immunohistochemical studies localize MAD to the cytoplasm in all tissues examined. Experiments in Xenopus embryos demonstrate that Drosophila MAD can function in the signaling pathway of BMP-4, a vertebrate homolog of dpp. Based on these results, we propose that Mad is a highly conserved and essential element of the DPP signal transduction pathway.

Xenopus Mad proteins transduce distinct subsets of signals for the TGF beta superfamily

Xenopus cDNAs homologous to the Drosophila Mad gene and C. elegans CEM genes have been cloned and functionally analyzed by microinjection into frog embryos. The results show that these genes (Xmad) encode intracellular proteins that act downstream of TGF beta superfamily ligands. Most interesting is the fact that different Xmad proteins produce distinct biological responses. Xmad1 produces ventral mesoderm, apparently transducing a signal for BMP2 and BMP4, whereas Xmad2 induces dorsal mesoderm like Vg1, activin, and nodal. These results suggest that an individual Xmad protein waits poised in the cytoplasm for instruction from a distinct subset of TGF beta ligands and then conveys specific information to the nucleus.

Induction of axial mesoderm by zDVR-1, the zebrafish orthologue of Xenopus Vg1

The zebrafish DVR-1 (zDVR-1) gene, like Xenopus Vg1, is present maternally as an unprocessed precursor protein which is distributed ubiquitously along the future dorsoventral axis. Also, like Vg1, overexpression of zDVR-1 in zebrafish directs synthesis of more precursor, but no processed protein. However, the native zDVR-1 precursor is processed to mature protein when expressed in Xenopus. Like processed Vg1, mature zDVR-1 is a potent inducer of axial mesoderm. The parallels in expression pattern, apparent regulation of protein processing, and mesoderm-inducing activity support the hypothesis that localized protein processing controls production of a dorsal mesoderm inducer in these two species. Furthermore, using mutant mRNAs, we show that cleavage site sequences of the precursor protein are important in regulating protein processing.

TGF-beta signals and a pattern in Xenopus laevis endodermal development

We have analyzed two gene products expressed in the early endoderm of Xenopus laevis: Xlhbox-8, a pancreas-specific transcription factor and intestinal fatty acid binding protein (IFABP), a marker of small intestinal epithelium. Expression of the pancreas marker relies on cell signaling mediated by both the TGF-beta and FGF classes of secreted peptide growth factors, whereas, expression of the more posterior small intestinal marker does not. Endodermal explants devoid of mesoderm express both markers in a regionalized manner. Cortical rotation is required for the expression of the more anterior marker, Xlhbox-8, but not for the small intestinal marker, IFABP. These findings suggest that endodermal patterning is dependent, in part, on the same events and signals known to play important roles in mesodermal development. Furthermore, inhibition of TGF-beta signaling in the endoderm leads to ectopic expression of both mesodermal and ectodermal markers, suggesting the TGF-beta signaling may play a general role in the segregation of the three embryonic germ layers.

Induction and patterning of the vertebrate nervous system

The formation of the vertebrate central nervous system depends on a series of inductive interactions. Ventral ectoderm gives rise to epidermis (skin), whereas dorsal ectoderm forms neurectoderm, which is regionalized along the anteroposterior axis in response to inductive signals from dorsal mesoderm. Specific neural cell types are established along the dorsoventral axis of the neural tube by inductive signals from the dorsal mesoderm and prospective epidermis. We review the induction and patterning of the nervous system, with an emphasis on molecules that might play a role in these interactions.

Induction of dorsal mesoderm by soluble, mature Vg1 protein

Mesoderm induction during Xenopus development has been extensively studied, and two members of the transforming growth factor-beta family, activin beta B and Vg1, have emerged as candidates for a natural inducer of dorsal mesoderm. Heretofore, analysis of Vg1 activity has relied on injection of hybrid Vg1 mRNAs, which have not been shown to direct efficient secretion of ligand and, therefore, the mechanism of mesoderm induction by processed Vg1 protein is unclear. This report describes injection of Xenopus oocytes with a chimeric activin-Vg1 mRNA, encoding the pro-region of activin beta B fused to the mature region of Vg1, resulting in the processing and secretion of mature Vg1. Treatment of animal pole explants with mature Vg1 protein resulted in differentiation of dorsal, but not ventral, mesodermal tissues and dose-dependent activation of both dorsal and ventrolateral mesodermal markers. At high doses, mature Vg1 induced formation of ‘embryoids’ with a rudimentary axial pattern, head structures including eyes and a functional neuromuscular system. Furthermore, truncated forms of the activin and FGF receptors, which block mesoderm induction in the intact embryo, fully inhibited mature Vg1 activity. To examine the mechanism of inhibition, we have performed receptor-binding assays with radiolabeled Vg1. Finally, follistatin, a specific inhibitor of activin beta B which is shown not to block endogenous dorsal mesoderm induction, failed to inhibit Vg1. The results support a role for endogenous Vg1 in dorsal mesoderm induction during Xenopus development.

Translational control of activin in Xenopus laevis embryos

Activin is a potent mesoderm inducing factor present in embryos of Xenopus laevis. Recent evidence has implicated activin in the inhibition of neural development in addition to the well-established induction of mesoderm in ectodermal explants. These diverse effects are critically dependent on the concentration of activin yet little is known about the mechanisms regulating the level of activin in the embryo. We report that the 3' untranslated region (3' UTR) of activin beta B mRNA inhibits the translation of activin in embryos. Micro-injection of activin mRNA from which the 3' UTR has been deleted is 8-10-fold more potent in inducing mesoderm than mRNA containing the 3' UTR. Truncation of the 3' UTR also leads to a marked enhancement of activin protein levels in embryos but has no effect when the truncated mRNA is translated in vitro. The 3' UTR also confers translational inhibition on a heterologous mRNA. These data show that a maternal factor(s) present in X. laevis regulates the translation of injected activin beta B mRNA. This factor(s) could be responsible for regulating the levels of endogenous activin beta B protein during mesoderm induction and the specification of ectodermal derivatives such as neural and epidermal tissues.

Vertebrate embryonic induction: mesodermal and neural patterning

Within the fertilized egg lies the information necessary to generate a diversity of cell types in the precise pattern of tissues and organs that comprises the vertebrate body. Seminal embryological experiments established the importance of induction, or cell interactions, in the formation of embryonic tissues and provided a foundation for molecular studies. In recent years, secreted gene products capable of inducing or patterning embryonic tissues have been identified. Despite these advances, embryologists remain challenged by fundamental questions: What are the endogenous inducing molecules? How is the action of an inducer spatially and temporally restricted? How does a limited group of inducers give rise to diversity of tissues? In this review, the focus is on the induction and patterning of mesodermal and neural tissues in the frog Xenopus laevis, with an emphasis on families of secreted molecules that appear to underlie inductive events throughout vertebrate embryogenesis.

Studies with a Xenopus BMP receptor suggest that ventral mesoderm-inducing signals override dorsal signals in vivo

We report the isolation of a Xenopus BMP receptor that is expressed maternally in the appropriate location to play a role in mesoderm induction. This receptor binds both BMP-2 and BMP-4 with high affinity. A truncated form of this BMP receptor specifically blocks BMP-4 signaling. Expression of this truncated BMP receptor during embryogenesis converts ventral mesoderm to dorsal mesoderm. Contrary to the popularly held view that ventral is the ground state for all mesoderm, our results suggest that formation of ventral mesoderm requires an active signal and that, in the absence of this ventral signal, dorsal mesoderm is formed.

Induction of mesoderm in Xenopus laevis embryos by translation initiation factor 4E

The microinjection of messenger RNA encoding the eukaryotic translation initiation factor 4E (eIF-4E) into early embryos of Xenopus laevis leads to the induction of mesoderm in ectodermal explants. This induction occurs without a stimulation of overall protein synthesis and is blocked by the co-expression of a dominant negative mutant of the proto-oncogene ras or a truncated activin type II receptor. Although other translation factors have been studied in vertebrate and invertebrate embryos, none have been shown to play a direct role in development. The results here suggest a mechanism for relaying and amplifying signals for mesoderm induction.

Mesodermal patterning by an inducer gradient depends on secondary cell-cell communication

BACKGROUND

Gradients of inducing molecules, or morphogens, could impose pattern on early embryos. Although there are candidates for morphogens in several systems, it is not well understood how cells might translate differences in extracellular inducer concentration into an orderly arrangement of cell types. With this question in mind, we have re-examined mesodermal patterning in Xenopus in response to the secreted growth factor activin. Previous work has shown that activin can initiate the formation of a variety of mesodermal tissues in a concentration-dependent fashion. We have sought to disentangle the roles played by individual cell responses to activin and subsequent interactions among induced cells in producing this outcome.

RESULTS

We find that the initial response of dispersed cells to activin concentration is unexpectedly simple, showing neither the thresholds of activin concentration nor the distinct domains of gene expression that characterize the later response. The eventual emergence of an ordered series of coherent differentiation steps requires the reaggregation of the induced cells, implying that secondary interactions occur. Furthermore, when cells induced at different doses of activin are mixed, the final response apparently represents a consensus, rather than a mosaic, of the mixed populations.

CONCLUSIONS

We conclude that communication among responding cells underlies much of the remarkable patterning influence of activin. Moreover, we suggest that these findings can inform thinking about how inducer gradients might act in other systems, shifting emphasis from the initial response of cells to inducer concentration toward the elaboration of complex pattern by secondary interactions.

Inhibition of activin receptor signaling promotes neuralization in Xenopus

Expression of a truncated activin type II receptor, which blocks signaling by activin, neuralizes explants of embryonic cells that would otherwise become epidermal cells. This neuralization is direct and does not require the presence of mesoderm. The induced neural tissue expresses general molecular markers of the central nervous system as well as an array of neural markers along the anteroposterior axis. In the context of the whole embryo, expression of this truncated activin receptor diverts prospective ectoderm and endoderm to a neural fate. We propose that inhibition of the activin type II receptor signaling causes the cells of Xenopus embryos to adopt a neural fate. These results, along with previous experiments performed in Drosophila, suggest that the formation of the nervous system in vertebrates and invertebrates occurs by a common strategy.

Follistatin, an antagonist of activin, is expressed in the Spemann organizer and displays direct neuralizing activity

In the accompanying paper, we show that the expression of a dominant negative activin receptor can convert prospective ectoderm into neural tissue, which suggests that activin is an inhibitor of neuralization. Here we report the isolation and characterization of an activin antagonist, follistatin, that can induce neural tissue directly in vivo. Follistatin RNA is localized in the Spemann organizer and notochord, tissues known to be potent neural inducers. We demonstrate that follistatin RNA and protein are able to block the activity of activin in embryonic explants. Furthermore, we show that follistatin RNA directly neuralizes ectodermal explants in the absence of detectable mesoderm. Thus, follistatin is present at the correct time and location to play a role in neural induction in vivo.

Xwnt-11: a maternally expressed Xenopus wnt gene

We have isolated and characterized a novel Xenopus wnt gene, Xwnt-11, whose expression pattern and overexpression phenotype suggest that it may be important for dorsal-ventral axis formation. Xwnt-11 mRNA is present during oogenesis and embryonic development through swimming tadpole stages. Xwnt-11 mRNA is ubiquitous in early oocytes and is localized during mid-oogenesis. By late oocyte stages, Xwnt-11 mRNA is localized to the vegetal cortex, with some mRNA in the vegetal cytoplasm. After egg maturation, Xwnt-11 mRNA is released from the vegetal cortex and is found in the vegetal cytoplasm. This early pattern of Xwnt-11 mRNA localization is similar to another vegetally localized maternal mRNA, Vg1 (D. A. Melton (1987) Nature 328, 80–82). In the late blastula, Xwnt-11 mRNA is found at high levels in the dorsal marginal zone. As gastrulation proceeds, Xwnt-11 mRNA appears in the lateral and ventral marginal zone and, during tadpole stages, it is found in the somites and first branchial arch. Injection of Xwnt-11 mRNA into UV-ventralized embryos can substantially rescue the UV defect by inducing the formation of dorsal tissues. The rescued embryos develop somitic muscle and neural tube; however, they lack notochord and anterior head structures.

Processed Vg1 protein is an axial mesoderm inducer in Xenopus

Vg1 is a TGF beta-related growth factor encoded by a maternal mRNA localized to vegetal blastomeres in Xenopus embryos. Vg1 precursor protein is abundant in vegetal cells, but the processed mature form has not been readily detected and no activity has been demonstrated for the putative Vg1 mature protein. We have engineered a BMP2-Vg1 fusion (BVg1) that promotes formation of mature Vg1 protein in vivo. Injection of BVg1 mRNA induces dorsal mesoderm in animal cap cells, and BVg1 expression in ultraviolet-ventralized embryos fully restores a normal dorsal axis. Blastomeres expressing BVg1 act as a Nieuwkoop center, the region that induces the Spemann organizer. our results lead us to suggest that localized posttranslational processing of Vg1 precursor protein on the future dorsal side of the embryo is a key step in generating dorsal mesoderm and the body axis in Xenopus.

Xenopus axis formation: induction of goosecoid by injected Xwnt-8 and activin mRNAs

In this study, we compare the effects of three mRNAs-goosecoid, activin and Xwnt-8- that are able to induce partial or complete secondary axes when injected into Xenopus embryos. Xwnt-8 injection produces complete secondary axes including head structures whereas activin and goosecoid injection produce partial secondary axes at high frequency that lack head structures anterior to the auditory vesicle and often lack notochord. Xwnt-8 can activate goosecoid only in the deep marginal zone, i.e., in the region in which this organizer-specific homeobox gene is normally expressed on the dorsal side. Activin B mRNA, however, can turn on goosecoid in all regions of the embryo. We also tested the capacity of these gene products to restore axis formation in embryos in which the cortical rotation was blocked by UV irradiation. Whereas Xwnt-8 gives complete rescue of anterior structures, both goosecoid and activin give partial rescue. Rescued axes including hindbrain structures up to level of the auditory vesicle can be obtained at high frequency even in the absence of notochord structures. The possible functions of Wnt-like and activin-like signals and of the goosecoid homeobox gene, and their order of action in the formation of Spemann's organizer are discussed.

Expression of activin mRNA during early development in Xenopus laevis

Activins are members of the transforming growth factor-beta superfamily, a class of peptide growth factors that can regulate the growth and differentiation of a variety of cell types. In mesoderm induction assays, activins A and B were shown to be very potent inducers and it was only recently demonstrated that they are crucial for initial mesoderm induction in Xenopus embryos. To determine the source of activin protein for initial mesoderm induction and to investigate whether activins may play further roles in embryonic development we have examined the localization of the mRNAs encoding the activin beta A and beta B subunits in Xenopus embryos. Activin beta A and beta B mRNAs are found in the follicle cells surrounding oocytes but not in oocytes themselves or fertilized eggs. During embryogenesis activin mRNA is first detected after the midblastula transition and expression increases as development proceeds. Activin beta B mRNA is homogeneously distributed during blastula and early gastrula stages but restricted to the dorso-anterior region in neurula stage embryos. At the early tailbud stage activin expression becomes confined to the brain, eye analgen, visceral pouches, otic vesicles, and the anterior notochord.

Interaction of Wnt and activin in dorsal mesoderm induction in Xenopus

Both the activin and Wnt families of peptide growth factors are capable of inducing dorsal mesoderm in Xenopus embryos. Presumptive ventral ectoderm cells isolated from embryos injected with Xwnt8 mRNA were cultured in the presence of activin A to study the possible interactions between these two classes of signaling proteins. We find that overexpression of Xwnt8 RNA alters the response of ventral ectoderm to activin such that ventral explants differentiate dorsoanterior structures including notochord and eyes. This response is similar to the response of dorsal ectoderm to activin alone. When embryos are irradiated with uv light to inhibit dorsal axis formation, ectodermal explants differentiate notochord when they are induced by a combination of both signaling factors, but not when cells receive only one inducing signal (activin or Xwnt8). This result is further supported by the observation that goosecoid (gsc) mRNA, an early marker for dorsal mesoderm, is expressed in these explants only when they are injected with Xwnt8 mRNA followed by exposure to activin. Early morphogenetic movements of the induced cells and activation of muscle-specific actin and Brachyury (Xbra) genes also reveal a cooperation of activin A and Xwnt8 in mesoderm induction.

A truncated activin receptor inhibits mesoderm induction and formation of axial structures in Xenopus embryos

Activins can induce mesoderm in embryonic explants and have been proposed as the natural inducer in Xenopus. A mutant activin receptor that inhibits activin signalling is used to show that activin is required for the induction of mesoderm in vivo and the patterning of the embryonic body plan. Blocking the activin signal transduction pathway also reveals autonomous induction of a neural marker and unmasks a relationship between activin and fibroblast growth factor.

Specificity of antisense oligonucleotides in vivo

Antisense oligonucleotides are widely used as inhibitors of gene expression in cultured cells and have been proposed as potential therapeutic agents, but it is not known to what extent they are specific for their intended target RNAs. Statistical considerations indicate that if oligonucleotides can form hybrids with mRNA molecules in vivo by means of short or imperfect regions of complementarity, then the specificity of oligonucleotides as antisense reagents will be greatly compromised. We have used Xenopus oocytes as a model system in which to investigate the potential specificity of antisense oligonucleotides in vivo. We injected perfect and partially matched antisense oligonucleotides into oocytes and measured the resulting degradation of the target RNA in each case. On the basis of the extent to which antisense oligonucleotides can cause cleavage of RNAs at imperfectly matched target sites, we conclude that in this system it is probably not possible to obtain specific cleavage of an intended target RNA without also causing at least the partial destruction of many nontargeted RNAs.

Involvement of p21ras in Xenopus mesoderm induction

During early vertebrate embryogenesis, mesoderm is specified by a signal emanating from prospective endoderm. This signal can respecify Xenopus prospective ectoderm as mesoderm, and can be mimicked by members of the fibroblast growth factor and transforming growth factor-beta families. In other systems, the p21c-ras proto-oncogene product has been implicated in signal transduction for various polypeptide growth factors. We report here that a dominant inhibitory ras mutant blocks the mesoderm-inducing activity of fibroblast growth factor and activin, as well as the endogenous inducing activity of prospective endoderm. A constitutively active ras mutant partially mimics these activities. These results indicate that p21ras may have a central role in the transduction of the mesoderm inductive signal. Basic fibroblast growth factor and activin have emerged as candidates for endogenous mesoderm-inducing molecules. The character of the mesoderm induced by these two factors is overlapping but distinct when assessed both by histological and molecular criteria. The signal transduction pathways used during induction by these factors are unknown. We used messenger RNA microinjection of Xenopus eggs to express a dominant inhibitory mutant ras, p21(Asn 17)Ha-ras, in cells competent to respond to inducing factors to examine the role of p21ras in this response. This mutant, which has a reduced affinity for GTP relative to GDP, blocks a variety of mitogenic signals in 3T3 fibroblasts as well as the differentiation of pheochromocytoma cells in response to nerve growth factor.

Embryonic expression and functional analysis of a Xenopus activin receptor

We report the isolation and characterization of a Xenopus activin receptor (XAR1). The amino acid sequence of this protein shows extensive homology with a murine activin receptor. The mRNA is expressed maternally and is ubiquitously distributed during the early stages of embryogenesis. Consistent with a possible role in mesoderm induction and patterning, interference with the normal expression of the receptor by overexpression in the early embryo results in the formation of ectopic dorsal axial structures. During neurulation the XAR1 mRNA is expressed predominantly in the presumptive brain and spinal cord, suggesting an additional function for XAR1 in neurogenesis.

Vegetal messenger RNA localization directed by a 340-nt RNA sequence element in Xenopus oocytes

Contained within a single cell, the fertilized egg, is information that will ultimately specify the entire organism. During early embryonic cleavages, cells acquire distinct fates and their differences in developmental potential might be explained by localization of informational molecules in the egg. The mechanisms by which Vg1 RNA, a maternal mRNA, is translocated to the vegetal pole of Xenopus oocytes may indicate how developmental signals are localized. Data presented here show that a 340-nucleotide localization signal present in the 3' untranslated region of Vg1 RNA is sufficient to direct RNA localization to the vegetal pole.