Suramin changes the fate of Spemann's organizer and prevents neural induction in Xenopus laevis

Proteins and small molecules for cellular regenerative medicine

Regenerative medicine seeks to understand tissue development and homeostasis and build on that knowledge to enhance regeneration of injured tissues. By replenishing lost functional tissues and cells, regenerative medicine could change the treatment paradigm for a broad range of degenerative and ischemic diseases. Multipotent cells hold promise as potential building blocks for regenerating lost tissues, but successful tissue regeneration will depend on comprehensive control of multipotent cells-differentiation into a target cell type, delivery to a desired tissue, and integration into a durable functional structure. At each step of this process, proteins and small molecules provide essential signals and, in some cases, may themselves act as effective therapies. Identifying these signals is thus a fundamental goal of regenerative medicine. In this review we discuss current progress using proteins and small molecules to regulate tissue regeneration, both in combination with cellular therapies and as monotherapy.

Xenopus: An emerging model for studying congenital heart disease

Congenital heart defects affect nearly 1% of all newborns and are a significant cause of infant death. Clinical studies have identified a number of congenital heart syndromes associated with mutations in genes that are involved in the complex process of cardiogenesis. The African clawed frog, Xenopus, has been instrumental in studies of vertebrate heart development and provides a valuable tool to investigate the molecular mechanisms underlying human congenital heart diseases. In this review, we discuss the methodologies that make Xenopus an ideal model system to investigate heart development and disease. We also outline congenital heart conditions linked to cardiac genes that have been well studied in Xenopus and describe some emerging technologies that will further aid in the study of these complex syndromes.

Programming pluripotent precursor cells derived from Xenopus embryos to generate specific tissues and organs

Xenopus embryos provide a rich source of pluripotent cells that can be differentiated into functional organs. Since the molecular principles of vertebrate organogenesis appear to be conserved between Xenopus and mammals, this system can provide useful guidelines for the directional manipulation of human embryonic stem cells. Pluripotent Xenopus cells can be easily isolated from the animal pole of blastula stage Xenopus embryos. These so called "animal cap" cells represent prospective ectodermal cells, but give rise to endodermal, mesodermal and neuro-ectodermal derivatives if treated with the appropriate factors. These factors include evolutionary conserved modulators of the key developmental signal transduction pathways that can be supplied either by mRNA microinjection or direct application of recombinant proteins. This relatively simple system has added to our understanding of pancreas, liver, kidney, eye and heart development. In particular, recent studies have used animal cap cells to generate ectopic eyes and hearts, setting the stage for future work aimed at programming pluripotent cells for regenerative medicine.

Xenopus explants as an experimental model system for studying heart development

Many developmental processes are highly conserved in all vertebrate organisms. This conservation has allowed developmental biologists to use numerous animal models to further our understanding of the molecular mechanisms that govern heart development and congenital heart disease. Amphibian embryos represent a useful model for such studies because their relatively large embryos are available in large numbers and survive simple microsurgery. In addition, until swimming tadpole stages, an amphibian embryo develops using nutrients stored in each of its many cells. This feature has the advantage that explants isolated from embryonic tissue will continue to survive in isolation and differentiate in culture. Furthermore, cells from the ectodermal layer of the blastula or gastrula embryos are stem cell like in that they are pluripotent and can be induced to form various tissues in vitro. Here, we will review work from recent studies in which explants from the amphibian embryos were used to further our understanding of vertebrate heart development. We will bring together the key facts needed for using Xenopus explants as experimental approaches for studying molecular pathways and gene regulatory networks in vertebrate cardiogenesis. The knowledge generated with these approaches supports the usefulness of amphibian explants, and the relevance of the findings strongly validates the conservation of molecular pathways that underlie heart development in all vertebrates.

Differentiation induction of mouse embryonic stem cells into sinus node-like cells by suramin

BACKGROUND

Embryonic stem (ES) cells differentiate into cardiac phenotypes representing early pacemaker-, atrial-, ventricular-, and sinus node-like cells, however, ES-derived specification into sinus nodal cells is not yet known. By using the naphthylamine derivative of urea, suramin, we were able to follow the process of cardiac specialization into sinus node-like cells.

METHODS

Differentiating mouse ES cells were treated with suramin (500 µM) from day 5 to 7 of embryoid body formation, and cells were analysed for their differentiation potential via morphological analysis, flow cytometry, RT-PCR, immunohistochemistry and patch clamp analysis.

RESULTS

Application of suramin resulted in an increased number of cardiac cells, but inhibition of neuronal, skeletal muscle and definitive endoderm differentiation. Immediately after suramin treatment, a decreased mesendoderm differentiation was found. Brachyury, FGF10, Wnt8 and Wnt3a transcript levels were significantly down-regulated, followed by a decrease in mesoderm- and cardiac progenitor-specific markers BMP2, GATA4/5, Wnt11, Isl1, Nkx2.5 and Tbx5 immediately after removal of the substance. With continued differentiation, a significant up-regulation of Brachyury, FGF10 and GATA5 transcript levels was observed, whereas Nkx2.5, Isl1, Tbx5, BMP2 and Wnt11 levels were normalized to control levels. At advanced differentiation stages, sinus node-specific HCN4, Tbx2 and Tbx3 transcript levels were significantly up-regulated. Immunofluorescence and patch-clamp analysis confirmed the increased number of sinus node-like cells, and electrophysiological analysis revealed a lower number of atrial- and ventricular-like cardiomyocytes following suramin treatment.

CONCLUSION

We conclude that the interference of suramin with the cardiac differentiation process modified mesoderm- and cardiac-specific gene expression resulting in enhanced formation of sinus node-like cells.

Global gene expression profiling and cluster analysis in Xenopus laevis

We have undertaken a large-scale microarray gene expression analysis using cDNAs corresponding to 21,000 Xenopus laevis ESTs. mRNAs from 37 samples, including embryos and adult organs, were profiled. Cluster analysis of embryos of different stages was carried out and revealed expected affinities between gastrulae and neurulae, as well as between advanced neurulae and tadpoles, while egg and feeding larvae were clearly separated. Cluster analysis of adult organs showed some unexpected tissue-relatedness, e.g. kidney is more related to endodermal than to mesodermal tissues and the brain is separated from other neuroectodermal derivatives. Cluster analysis of genes revealed major phases of co-ordinate gene expression between egg and adult stages. During the maternal-early embryonic phase, genes maintaining a rapidly dividing cell state are predominantly expressed (cell cycle regulators, chromatin proteins). Genes involved in protein biosynthesis are progressively induced from mid-embryogenesis onwards. The larval-adult phase is characterised by expression of genes involved in metabolism and terminal differentiation. Thirteen potential synexpression groups were identified, which encompass components of diverse molecular processes or supra-molecular structures, including chromatin, RNA processing and nucleolar function, cell cycle, respiratory chain/Krebs cycle, protein biosynthesis, endoplasmic reticulum, vesicle transport, synaptic vesicle, microtubule, intermediate filament, epithelial proteins and collagen. Data filtering identified genes with potential stage-, region- and organ-specific expression. The dataset was assembled in the iChip microarray database, , which allows user-defined queries. The study provides insights into the higher order of vertebrate gene expression, identifies synexpression groups and marker genes, and makes predictions for the biological role of numerous uncharacterized genes.

Teratogenic effects of suramin on the chick embryo

Suramin, a polysulfonated naphthylamine, has been used for the chemotherapy of trypanosomiasis and onchocerciasis since about the 1920s. Currently, it is also being tested as an anticancer agent. It is hoped that suramin might stop the progression of some kinds of cancer since it has been found to inhibit the proliferation and migration of cells and the formation of new blood vessels. These processes are not only essential for the development and progression of cancer, but also for normal embryonic development. Suramin might, therefore, be a potent teratogen. In the literature, however, we have found only scant information on this subject. In the present study, we demonstrate the teratogenic effects of suramin on chick embryos. Suramin was injected into the coelomic cavity of chick embryos on incubation day (ID) 3. Following reincubation until ID 8, suramin-treated embryos ( n=50) were examined for congenital malformations and compared with a control group ( n=30). The survival rate of suramin-treated embryos was markedly reduced compared with controls (50% vs 90%). Among the 25 survivors the following malformations were recorded: caudal dysgenesia (100%), median facial clefts with hypertelorism (92%), malformations of the aortic arch arteries (88%), hypo-/aplasia of the allantoic vesicle (84%), microphthalmia (52%), abnormalities of the great arterial trunks (44%), unilateral or bilateral cleft lips (40%), heart defects with juxtaposition of the right atrial appendage (36%), persistence of the lens vesicle (32%), median clefts of the lower beak (8%), omphalocele (4%), and cloacal exstrophy (4%). These results show that suramin is a potent teratogen. The possible implications of our findings for human beings and the possible teratogenic mechanisms of suramin are discussed. Use of suramin in experimental teratology might help to clarify the morphogenesis of median facial clefts and of some congenital heart defects.

Regional specification of the head and trunk-tail organizers of a urodele (Cynops pyrrhogaster) embryo is patterned during gastrulation

The dorsal marginal zone (DMZ) of an amphibian early gastrula is thought to consist of at least two distinct domains: the future head and trunk-tail organizers. We studied the mechanism by which the organizing activities of the lower half of the DMZ (LDMZ) of the urodelean (Cynops pyrrhogaster) embryo are changed. The uninvoluted LDMZ induces the notochord and then organizes the trunk-tail structures, whereas after cultivation in vitro or suramin treatment, the same LDMZ loses the notochord-inducing ability and organizes the head structures. A cell-lineage experiment indicated that the change in the organizing activity of the LDMZ was reflected in the transformation of the inductive ability: from notochord-inducing to neural-inducing activity. Using RT-PCR, we showed that the LDMZ expressed gsc, lim-1, chordin, and noggin, but not the mesoderm marker bra. In the sandwich assay, the LDMZ induced bra expression in the animal cap ectoderm, but the inductive activity was inhibited by cultivation or suramin treatment. The present study indicates that the change in the organizing activity of the LDMZ from trunk-tail to head is coupled with the loss of notochord-inducing activity. Based on these results, we suggest that this change is essential for the specification of the head and trunk-tail organizers during gastrulation.

Molecular cloning of tyrosine kinases in the early Xenopus embryo: identification of Eck-related genes expressed in cranial neural crest cells of the second (hyoid) arch

Growth factors and their receptors play an important role in controlling cellular proliferation, migration, and differentiation during vertebrate embryogenesis. We have used the reverse transcription-polymerase chain reaction to survey the repertoire of receptor tyrosine kinases (TK) expressed during early embryogenesis of Xenopus laevis. Twelve distinct Xenopus TK cDNA classes were identified among a total of 352 cDNAs screened. A single TK cDNA class has been described previously and encodes the fibroblast growth factor receptor FGFR-A1. The remaining 11 TK cDNA classes appear to encode novel genes of the FGFR, platelet-derived growth factor receptor (PDGFR), Eph, Csk, Tyk2, and Klg subfamilies. By RNase protection assays, Xenopus TK mRNAs are rare transcripts (< 10(7) mRNA molecules/embryo), and are usually found to be expressed also maternally in the embryo. Most Xenopus TK genes examined by whole-mount in situ hybridization were expressed widely in tissues derived from multiple germ layers. Two Eck-related genes, however, were found to be restricted in their expression to neural crest of the second (hyoid) arch. Our findings are consistent with the proposed function of TKs in the regulation of specification and differentiation of embryonic tissues.

Nucleo-cytoplasmic translocation and secretion of fibroblast growth factor-2 during avian gastrulation

The expression and distribution of the fibroblast growth factor-2 (FGF-2 or bFGF) proteins during early avian embryogenesis has been analysed in detail. Three FGF-2 protein isoforms of 18.5, 20.0 and 21.5 kDa are expressed during gastrulation of chicken embryos. Using whole mount immunohistochemistry, these proteins were found to be predominantly nuclear in prestreak blastodiscs during mesoderm induction. Distribution of positive cells in the epiblast was mosaic, whereas all cells of the forming hypoblast expressed the FGF-2 proteins. During primitive streak formation, the proteins started to translocate to the cytoplasm in epiblast cells but remained nuclear in the hypoblast. The FGF-2 proteins became predominantly cytoplasmic in all cells during the subsequent developmental stages. Their highest levels were detected in endodermal cells underlying Hensen's node and the newly formed notochord, the dorsal apex of all epiblast cells and, most interestingly, in the extra-cellular basal lamina separating the epiblast from newly formed mesoderm. Heparin and suramin treatment of these advanced embryos (stage 4) revealed a dose-dependent inhibition on the regression of Hensen's node and formation of mesodermal derivatives such as somites. The results are discussed with respect to current models on FGF-mediated functions during vertebrate mesoderm induction and regionalization.

Heparan sulfate proteoglycans are required for mesoderm formation in Xenopus embryos

Mesoderm forms in the vertebrate embryo as a result of inductive interactions involving secreted growth factors and cell surface molecules. Proteoglycans have recently been implicated in the control of cell adhesion, migration and growth factor responsiveness. We have found that removal of glycosaminoglycan chains of proteoglycans from Xenopus ectodermal explants by heparinase, but not by chondroitinase, results in inhibition of elongation and mesodermal differentiation in response to signaling factors: activin, FGF and Wnt. Heparinase treatment differentially affected expression of early general and region-specific mesodermal markers, suggesting that mesodermal cell fates become specified in the early embryo via at least two signaling pathways which differ in their requirements for heparan sulfate proteoglycans. Addition of soluble heparan sulfate restored activin-mediated induction of muscle-specific actin gene in heparinase-treated explants. Finally, heparinase inhibited autonomous morphogenetic movements and mesodermal, but not neural, differentiation in dorsal marginal zone explants, which normally give rise to mesoderm in the embryo. These results directly demonstrate that heparan sulfate proteoglycans participate in gastrulation and mesoderm formation in the early embryo.

Inducing factors in Xenopus early embryos

Recent results make it possible to postulate credible candidates for each of the known inducing signals that act to determine cell fate during Xenopus early development. Experiments on biological activity, expression patterns and inhibition of function suggest that Vg-1 and Wnt-11 may act as the primary mesoderm-inducing signals, FGF and activin may serve to relay their effects, and noggin may be a major component of the dorsalizing and neural-inducing signals from the organizer.

Suramin and heparin: aspecific inhibitors of mesoderm induction in the Xenopus laevis embryo

Xenopus embryos in solutions containing suramin show a dose-dependent decrease in the formation of dorsoanterior structures. Continuous treatment with 1 mM suramin produces embryos without mesodermal derivatives but with mesenchymal cells. Brief immersions of 20 min were used to determine the most sensitive stages and to establish dose-effect curves: a 20 min treatment with 3 mM suramin at stages 7-8.5 produces blastula-like embryos, never classified before, with atypical epidermis, cells full of yolk and mesenchyme in between. The lack of dorsal mesoderm was confirmed by an RNase protection assay with alpha-cardiac actin probe. Heparin also causes a reduction in dorsal structures, but its action is weaker and and there are also strong toxic effects such as superficial cell dissociation. The effect of heparin is dose-dependent and brief immersions show a very sensitive period around stage 6.5. The lowest DAI obtained is 1.5, an extremely microcephalic embryo with forked tail codes, a stocky notochord, and abnormally shaped, abundant neural tissue. Immunofluorescence shows that the distribution of fibronectin-containing fibrils is normal in heparin-treated embryos, whereas there are no such fibrils in suramin-treated embryos at control stage 12.

Suramin prevents transcription of dorsal marker genes in Xenopus laevis embryos, isolated dorsal blastopore lips and activin A induced animal caps

Suramin, a polyanionic compound which is known to interact with the receptors of growth factors inhibits the expression of dorsal marker genes in whole embryos and isolated dorsal blastopore lips. Suramin also prevents activin A induced dorsalization of animal cap explants from blastula stage embryos, but it simultaneously evokes a shift of the differentiation pattern from dorsal mesodermal structures (notochord, somites) to ventral mesodermal derivatives (mesothelium and erythroid precursor cells). The results are consistent with the assumption that the dorsal vegetal zone (Nieuwkoop center) primarily releases more general/ventral mesodermalization signals. They further suggest a dual role of activin A in early embryogenesis. While the maternal component may contribute to a more general/ventral type of induction, increasing concentrations of the zygotic component along with the activation of primary response genes may contribute to the dorsalization of the organizer.