Mesoderm formation in Xenopus

Dissecting hematopoiesis and disease using the zebrafish

The study of blood has often defined paradigms that are relevant to the biology of other vertebrate organ systems. As examples, stem cell physiology and the structure of the membrane cytoskeleton were first described in hematopoietic cells. Much of the reason for these successes resides in the ease with which blood cells can be isolated and manipulated in vitro. The cell biology of hematopoiesis can also be illuminated by the study of human disease states such as anemia, immunodeficiency, and leukemia. The sequential development of the blood system in vertebrates is characterized by ventral mesoderm induction, hematopoietic stem cell specification, and subsequent cell lineage differentiation. Some of the key regulatory steps in this process have been uncovered by studies in mouse, chicken, and Xenopus. More recently, the genetics of the zebrafish (Danio rerio) have been employed to define novel points of regulation of the hematopoietic program. In this review, we describe the advantages of the zebrafish system for the study of blood cell development and the initial success of the system in this pursuit. The striking similarity of zebrafish mutant phenotypes and human diseases emphasizes the utility of this model system for elucidating pathophysiologic mechanisms. New screens for lineage-specific mutations are beginning, and the availability of transgenics promises a better understanding of lineage-specific gene expression. The infrastructure of the zebrafish system is growing with an NIH-directed genome initiative, providing a detailed map of the zebrafish genome and an increasing number of candidate genes for the mutations. The zebrafish is poised to contribute greatly to our understanding of normal and disease-related hematopoiesis.

An analysis of Xenopus tyrosine kinase genes and their expression in early development

Xenopus laevis and X. borealis were screened for tyrosine kinase genes using the polymerase chain reaction (PCR) and reverse transcriptase (RT)-PCR and 34 X. laevis and 23 X. borealis tyrosine genes were identified. Eighteen of the genes represented novel tyrosine kinase family members. The rest could be classified into known tyrosine kinase subfamilies, of which however only three have been previously identified in Xenopus. Eight clones, including bFGFR (xFGFR1) and potential Trk, Ins R., Fak, Fyn, and Abl homologs, were used to probe temporal and spatial gene expression in early development. Quantitative RT-PCR and whole-mount and in situ hybridization showed that most of these mRNAs were present throughout development and were broadly distributed, mainly in ectodermal and mesodermal derived tissues. At the blastula stage, bFGFR mRNA was detected within the ectoderm and a gradient of expression was noted within the invaginating mesoderm. The unexpected promiscuous expression of many tyrosine kinase genes in early development is discussed.

Amniotic band sequence: Streeter's hypothesis reexamined

Recently published reports of 54 subjects with the amniotic band syndrome (ABS) were reviewed, paying particular attention to internal anomalies. Evidence from the internal anomalies suggests that in most cases reviewed, damage occurred in a definable time period, probably prior to 26 days postconception and before the establishment of effective embryonic circulation. Most defects are explicable in terms of interference with neuropore closure, malmigration of cephalic neural crest tissue, and damage to the mesonephros consistent with local interference of the graded expression of organizational genes resulting in a local defect in the organization of the embryo.

Ventrolateral regionalization of Xenopus laevis mesoderm is characterized by the expression of alpha-smooth muscle actin

Mesodermal patterning in the amphibian embryo has been extensively studied in its dorsal aspects, whereas little is known regarding its ventrolateral regionalization due to a lack of specific molecular markers for derivatives of this type of mesoderm. Since smooth muscles (SM) are thought to arise from lateral plate mesoderm, we have analyzed the expression of an alpha-actin isoform specific for SM with regard to mesoderm patterning. Using an antibody directed against alpha-SM actin that recognized specifically this actin isoform in Xenopus, we have found that the expression of alpha-SM actin is restricted to visceral and vascular SM with a transient expression in the heart. The overall expression of the alpha-SM actin appears restricted to the ventral aspects of the differentiating embryo. alpha-SM actin expression appears to be activated following mesoderm induction in animal cap derivatives. Moreover, at the gastrula stage, SM precursor cells are regionalized since they will only differentiate from ventrolateral marginal zone explants. Using the animal cap assay, we have found that alpha-SM actin expression is specifically induced in treated animal cap with bFGF or a low concentration of XTC-MIF, which induce ventral structures, but not with a high concentration of XTC-MIF, which induces dorsal structures. Altogether, these results establish that alpha-SM actin is a reliable marker for ventrolateral mesoderm. We discuss the importance of this novel marker in studying mesoderm regionalization.

Poland anomaly with a limb body wall disruption defect: case report and review

We describe a female infant with apparent Poland anomaly (PA) and limb body wall defect. Analysis of the defects suggest that a disruption of the lateral embryonic plate mesoderm may have been responsible for the observed lesions. Because of the overlap of this case with PA, we re-examined previous reports of this syndrome. We think that the lesions could be equally well explained as a mesodermal disruption, and point out a previously unrecognised discrepancy between sex and affected side in sporadic PA and inherited PA which supports this view.

Expression of a dominant negative mutant of the FGF receptor disrupts mesoderm formation in Xenopus embryos

Peptide growth factors may play a role in patterning of the early embryo, particularly in the induction of mesoderm. We have explored the role of fibroblast growth factor (FGF) in early Xenopus development by expressing a dominant negative mutant form of the FGF receptor. Using a functional assay in frog oocytes, we found that a truncated form of the receptor effectively abolished wild-type receptor function. Explants from embryos expressing this dominant negative mutant failed to induce mesoderm in response to FGF. In whole embryos the mutant receptor caused specific defects in gastrulation and in posterior development, and overexpression of a wild-type receptor could rescue these developmental defects. These results demonstrate that the FGF signaling pathway plays an important role in early embryogenesis, particularly in the formation of the posterior and lateral mesoderm.

Carbon metabolism in early amphibian embryos

Xenopus embryos undergoing cleavage utilize amino acids as their main carbon source for metabolism. Glycolysis (from stored glycogen) begins near the onset of gastrulation. Thus, a major transition in the metabolism of the early embryo occurs before morphological differentiation. The enzymology that supports the carbon metabolism of the cleaving amphibian embryo resembles that of many mammalian tumor cells.

Pattern formation during animal development

At the beginning of this century, embryologists defined the central problems of developmental biology that remain today. These questions include how differentiated cells arise and form tissues and organs and how pattern is generated. In short, how does an egg give rise to an adult? In recent years, the application of molecular biology to embryological problems has led to significant advances and recast old problems in molecular and cellular terms. Although not necessarily comprehensive, this idiosyncratic review is intended to highlight selected findings and indicate where there are important gaps in our knowledge for those less than familiar with developmental biology.

Bone morphogenetic protein 4 (BMP-4), a member of the TGF-beta family, in early embryos of Xenopus laevis: analysis of mesoderm inducing activity

We have screened a Xenopus ovary cDNA library using a synthetic oligonucleotide derived from that part of the inhibin beta A sequence, which is highly conserved within the TGF-beta family. Out of several clones yielding autoradiographic signals four turned out to represent Xenopus counterparts to the human bone morphogenetic protein 4 (BMP-4). Each two of the four sequences are nearly identical and probably account for different alleles whereas the two pairs showing 5% divergence may have arisen by genome duplication in this tetraploid species. The amino acid sequence of the Xenopus protein is 80% homologous to the human sequence showing no single exchange within the last 100 amino acids at the C-terminus. This region, which constitutes the main part of the mature, biologically active protein, also exhibits substantial homologies to other representatives of the TGF-beta family, especially to the Drosophila DPPC protein. Transfection of COS-1 cells with the Xenopus BMP-4 sequence under control of the CMV-promoter leads to the secretion of a protein which exhibits mesoderm inducing activity when tested with animal cap explants from Xenopus blastula stage embryos.

Embryonic angiogenesis factors

The vascular system develops during embryonic development by at least two distinct processes; vasculogenesis is the development of blood vessels from in situ differentiating angioblasts and angiogenesis is the sprouting of capillaries from pre-existing vessels. The molecular mechanisms involved in the regulation of these processes are poorly understood. Endoderm-mesoderm interactions seem to play an important role in angioblast differentiation and vasculogenesis. Soluble angiogenic factors may be involved in the vascularization of some embryonic organs, e.g. kidney and brain. Angiogenic growth factors have been isolated and purified from embryonic brain and identified as acidic and basic fibroblast growth factors. More specific endothelial cell growth factors such as platelet-derived endothelial cell growth factor and vascular endothelial growth factor may also play a role in embryonic angiogenesis.

Cellular and genetic responses to mesoderm induction in Xenopus

Mesodermal cell differentiation begins in response to an inductive interaction early in frog development. In parallel with the recent finding that certain peptide growth factors can induce mesoderm, early cellular and genetic responses to the induction have been discovered. I review here recent work on these responses, work that aims to understand how cells respond to inducers to form the complex pattern of the vertebrate mesoderm.

Differential regulation of skeletal alpha-actin transcription in cardiac muscle by two fibroblast growth factors

In cardiac muscle, acidic and basic fibroblast growth factors (aFGF and bFGF) regulate at least five genes in common (including alpha and beta myosin heavy chains, atrial natriuretic factor, and the sarcoplasmic reticulum calcium ATPase), provoking a generalized "fetal" phenotype similar to events in pressure-overload hypertrophy; however, aFGF and bFGF differentially control the striated alpha-actins. bFGF stimulates and aFGF inhibits skeletal alpha-actin transcripts associated with the embryonic heart, whereas cardiac alpha-actin mRNA is inhibited by aFGF but not bFGF. To elucidate mechanisms for these selective and discordant actions of aFGF and bFGF on cardiac muscle, chicken skeletal and cardiac alpha-actin promoter-driven reporter genes were introduced into neonatal rat cardiac myocytes by electroporation. Skeletal alpha-actin transcription was selectively stimulated by bFGF, whereas the cardiac alpha-actin promoter was unaffected. In contrast, aFGF suppressed both transfected alpha-actin genes. The differential regulation of skeletal alpha-actin transcription was equivalent with either purified or recombinant FGFs and was observed with 5' flanking sequences from either nucleotide -202 or -2000 to nucleotide -11. Positive and negative modulation of alpha-actin transcription by growth factors corresponded accurately to the endogenous genes in all permutations studied. These investigations provide a model for reciprocal control of gene transcription by aFGF vs. bFGF.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

Angiogenic growth factors

Angiogenesis, the formation of new blood vessels, plays a central role in a variety of physiological and pathological processes such as embryonic development, wound healing and tumor growth. It is a complex, multi-step process that involves the migration and proliferation of capillary endothelial cells. Several factors that stimulate the proliferation of endothelial cells in vitro have been shown to induce angiogenesis in vivo. Among these angiogenic growth factors are wide-spectrum multifunctional mitogens (e.g. the fibroblast growth factors) and the recently identified factors with distinct specificity for vascular endothelial cells (e.g. the platelet-derived endothelial cell growth factor). Another group of factors apparently induce angiogenesis indirectly (e.g. transforming growth factor-beta) by stimulating target cells to release angiogenic factors or by other mechanisms. The differential expression, release and activation of these factors might regulate angiogenesis under various physiological and pathological conditions.