Transforming growth factor-beta5 expression during early development of Xenopus laevis

Conserved and novel functions of programmed cellular senescence during vertebrate development

Cellular senescence, a form of stable cell cycle arrest that is traditionally associated with tumour suppression, has been recently found to occur during mammalian development. Here, we show that cell senescence is an intrinsic part of the developmental programme in amphibians. Programmed senescence occurs in specific structures during defined time windows during amphibian development. It contributes to the physiological degeneration of the amphibian pronephros and to the development of the cement gland and oral cavity. In both contexts, senescence depends on TGFβ but is independent of ERK/MAPK activation. Furthermore, elimination of senescent cells through temporary TGFβ inhibition leads to developmental defects. Our findings uncover conserved and new roles of senescence in vertebrate organogenesis and support the view that cellular senescence may have arisen in evolution as a developmental mechanism.

Anterior-posterior regionalized gene expression in the Ciona notochord

BACKGROUND

In the simple ascidian chordate Ciona, the signaling pathways and gene regulatory networks giving rise to initial notochord induction are largely understood and the mechanisms of notochord morphogenesis are being systematically elucidated. The notochord has generally been thought of as a non-compartmentalized or regionalized organ that is not finely patterned at the level of gene expression. Quantitative imaging methods have recently shown, however, that notochord cell size, shape, and behavior vary consistently along the anterior-posterior (AP) axis.

RESULTS

Here we screen candidate genes by whole mount in situ hybridization for potential AP asymmetry. We identify 4 genes that show non-uniform expression in the notochord. Ezrin/radixin/moesin (ERM) is expressed more strongly in the secondary notochord lineage than the primary. CTGF is expressed stochastically in a subset of notochord cells. A novel calmodulin-like gene (BCamL) is expressed more strongly at both the anterior and posterior tips of the notochord. A TGF-β ortholog is expressed in a gradient from posterior to anterior. The asymmetries in ERM, BCamL, and TGF-β expression are evident even before the notochord cells have intercalated into a single-file column.

CONCLUSIONS

We conclude that the Ciona notochord is not a homogeneous tissue but instead shows distinct patterns of regionalized gene expression.

The role and regulation of GDF11 in Smad2 activation during tailbud formation in the Xenopus embryo

A key role for phosphorylation of Smad2 by TGFβ superfamily ligands in the axial patterning of early embryos is well established. The regulation and role of Smad2 signaling in post-neurula embryonic patterning, however, is less well understood. While a variety of TGFβ superfamily ligands are implicated in various stages of anterior-posterior patterning, the ligand GDF11 has been shown to have a particular role in post-gastrula patterning in the mouse. Mouse GDF11 is specifically localized to the developing tail and is essential for normal posterior axial patterning. Mature GDF11 ligand is inhibited by its own prodomain, and extracellular proteolysis of this prodomain is thought to be necessary for GDF11 activity. The contribution of this proteolytic regulatory mechanism to Smad activation during embryogenesis in vivo, and to the development of posterior pattern, has not been characterized. We investigate here the role of Xenopus GDF11 in the activation of Smad2 during the development of tailbud-stage embryos, and the role of this activation in larval development. We also demonstrate that the activity of BMP-1/Tolloid-like proteases is necessary for the normal GDF11-dependent activation of Smad2 phosphorylation during post-gastrula development. These data demonstrate that GDF11 has a central role in the activation of Smad2 phosphorylation in tailbud stage Xenopus embryos, and provide the first evidence that BMP-1/Tolloid-mediated prodomain cleavage is important for activation of GDF11 in vivo.

TGF-beta signaling is required for multiple processes during Xenopus tail regeneration

Xenopus tadpoles can fully regenerate all major tissue types following tail amputation. TGF-beta signaling plays essential roles in growth, repair, specification, and differentiation of tissues throughout development and adulthood. We examined the localization of key components of the TGF-beta signaling pathway during regeneration and characterized the effects of loss of TGF-beta signaling on multiple regenerative events. Phosphorylated Smad2 (p-Smad2) is initially restricted to the p63+ basal layer of the regenerative epithelium shortly after amputation, and is later found in multiple tissue types in the regeneration bud. TGF-beta ligands are also upregulated throughout regeneration. Treatment of amputated tails with SB-431542, a specific and reversible inhibitor of TGF-beta signaling, blocks tail regeneration at multiple points. Inhibition of TGF-beta signaling immediately following tail amputation reversibly prevents formation of a wound epithelium over the future regeneration bud. Even brief inhibition immediately following amputation is sufficient, however, to irreversibly block the establishment of structures and cell types that characterize regenerating tissue and to prevent the proper activation of BMP and ERK signaling pathways. Inhibition of TGF-beta signaling after regeneration has already commenced blocks cell proliferation in the regeneration bud. These data reveal several spatially and temporally distinct roles for TGF-beta signaling during regeneration: (1) wound epithelium formation, (2) establishment of regeneration bud structures and signaling cascades, and (3) regulation of cell proliferation.

Transforming growth factor beta inhibits proliferation of somatic cells without influencing germ cell number in the chicken embryonic ovary

The gonadal development of chicken embryo is regulated by hormones and growth factors. Transforming growth factor beta (TGF-beta) isoforms may play a critical role in the regulation of growth in chicken gonads. We have investigated the effect of the TGF-beta isoforms on the number of germ and somatic cells in the ovary of the chicken embryo. Ovaries were obtained from chicken embryos at 9 days of incubation. They were organ-cultured for 72 h in groups treated with TGF-beta1, TGF-beta2, soluble betaglycan, TGF-beta1 plus soluble betaglycan, or TGF-beta2 plus soluble betaglycan, and untreated (control). TGF-beta1 and TGF-beta2 diminished the somatic cell number in the ovary of the chicken embryo at this age by inhibiting the proliferation of the somatic cells without increasing apoptosis. On the other hand, TGF-beta1 and TGF-beta2 did not affect the number of germ cells in the cultured ovary. The capacity of TGF-beta1 and TGF-beta2 to diminish the number of somatic cells in the ovary was blocked with soluble betaglycan, a natural TGF-beta antagonist. However, changes in the location of germ cells within the ovary suggested that TGF-beta promoted the migration of the germ cells from the ovarian cortex to the medulla. Thus, TGF-beta affects germ and somatic cells in the ovary of the 9-day-old chicken embryo and inhibits the proliferation of somatic cells.

Developmental analysis of activin-like kinase receptor-4 (ALK4) expression in Xenopus laevis

The type I transforming growth factor-beta (TGFbeta) receptor, activin-like kinase-4 (ALK4), is an important regulator of vertebrate development, with roles in mesoderm induction, primitive streak formation, gastrulation, dorsoanterior patterning, and left-right axis determination. To complement previous ALK4 functional studies, we have analyzed ALK4 expression in embryos of the frog, Xenopus laevis. Results obtained with reverse transcriptase-polymerase chain reaction indicate that ALK4 is present in both the animal and vegetal poles of blastula stage embryos and that expression levels are relatively constant amongst embryos examined at blastula, gastrula, neurula, and early tail bud stages. However, the tissue distribution of ALK4 mRNA, as assessed by whole-mount in situ hybridization, was found to change over this range of developmental stages. In the blastula stage embryo, ALK4 is detected in cells of the animal pole and the marginal zone. During gastrulation, ALK4 is detected in the outer ectoderm, involuting mesoderm, blastocoele roof, dorsal lip, and to a lesser extent, in the endoderm. At the onset of neurulation, ALK4 expression is prominent in the dorsoanterior region of the developing head, the paraxial mesoderm, and midline structures, including the prechordal plate and neural folds. Expression in older neurula stage embryos resolves to the developing brain, somites, notochord, and neural crest; thereafter, additional sites of ALK4 expression in tail bud stage embryos include the spinal cord, otic placode, developing eye, lateral plate mesoderm, branchial arches, and the bilateral heart fields. Together, these results not only reflect the multiple developmental roles that have been proposed for this TGFbeta receptor but also define spatiotemporal windows in which ALK4 may function to modulate fundamental embryological events.

Transforming growth factor beta and prostate cancer

The TGF-beta superfamily is the most versatile considering the ability of its members to regulate proliferation, growth arrest, differentiation, and apoptosis of prostatic stromal and epithelial cells as well as the formation of osteoblastic metastases. TGF-beta mediated action in prostate cells follows a complex signaling pathway from binding and phosphorylation of receptor type II to the TbetaRI kinase to Smad activation, resulting in ligand-induced transcription. TGF-beta as an indirect tumor suppressor, its role of regulating tumor induction, as well as tumor suppression depending on the tissue microenvironment merits further exploration. The rationale for targeting growth factors and their receptors for therapeutic intervention is based upon the fact that these proteins represent the most proximate component of the signal transduction cascade. The alternate targeting of intracellular effectors in the signal transduction may be thwarted by cross talk between signaling pathways (such as the Smads in a dynamic interplay with the androgen receptor). TGF-beta within the context of its well-documented apoptosis regulatory actions in the prostate and the significance its key receptor TbetaRII as a potential tumor suppressor, provides a highly attractive candidate for such targeting with high clinical significance for the treatment and diagnosis of prostate cancer.

Notochord Patterning of the Endoderm

Endodermally derived organs of the gastrointestinal and respiratory system form at distinct anterioposterior and dorsoventral locations along the vertebrate body axis. This stereotyped program of organ formation depends on the correct patterning of the endodermal epithelium so that organ differentiation and morphogenesis occur at appropriate positions along the gut tube. Whereas some initial patterning of the endoderm is known to occur early, during germ-layer formation and gastrulation, later signaling events, originating from a number of adjacent tissue layers, are essential for the development of endodermal organs. Previous studies have shown that signals arising from the notochord are important for patterning of the ectodermally derived floor plate of the neural tube and the mesodermally derived somites. This review will discuss recent evidence indicating that signals arising from the notochord also play a role in regulating endoderm development.

Isolation and characterization of a transforming growth factor-beta Type II receptor cDNA from Xenopus laevis

Transforming Growth Factor-beta (TGF-beta) and their receptors have been characterized from many organisms. Two TGF-beta signaling receptors called Type I and II have been described for various ligands of the superfamily from organisms ranging from Drosophila to humans. In Xenopus laevis, TGF-beta2 and 5 have been reported and presumably, play important roles during early development. Several Type I and type II receptors for many ligands of the TGF-beta superfamily except TGF-beta type II receptor (TbetaIIR), have been characterized in Xenopus laevis. A chemical cross linking experiment using iodinated TGF-beta1 and -beta5, revealed four specific binding proteins on XTC cells. In order to understand the TGF-beta involvement during Xenopus development, a TGF-beta type II receptor (XTbetaIIR) has been isolated from a XTC cDNA library. XTbetaIIR was a partial cDNA lacking a portion of the signal peptide. The sequence analysis and homology comparison with the human TbetaIIR revealed 67% amino acid similarity in the extra cellular domain, 60% similarity in the transmembrane domain and 87% similarity in the cytoplasmic kinase domain, suggesting that XTbetaIIR is a putative TGF-beta type II receptor. In addition, the consensus amino acid motif for serine threonine receptor kinases was also present. Further, a dominant negative expression construct lacking the cytoplasmic kinase domain (engineered with the signal peptide from human TGF-beta type II receptor), was able to abolish TGF-beta mediated induction of a luciferase reporter plasmid, in a transient cell transfection assay. This substantiates the notion that XTbetaIIR cDNA can act as a receptor for TGF-beta. RT-PCR analysis using RNA isolated from various developmental stages of Xenopus laevis revealed expression of this gene in all the early stages of development and in the adult organs, except in stages 46/48.