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

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

The Caenorhabditis elegans presenilin sel-12 is required for mesodermal patterning and muscle function

Mutations in presenilin genes impair Notch signalling and, in humans, have been implicated in the development of familial Alzheimer's disease. We show here that a reduction of the activity of the Caenorhabditis elegans presenilin sel-12 results in a late defect during sex muscle development. The morphological abnormalities and functional deficits in the sex muscles contribute to the egg-laying defects seen in sel-12 hermaphrodites and to the severely reduced mating efficiency of sel-12 males. Both defects can be rescued by expressing sel-12 from the hlh-8 promoter that is active during the development of the sex muscle-specific M lineage, but not by expressing sel-12 from late muscle-specific promoters. Both weak and strong sel-12 mutations cause defects in the sex muscles that resemble the defects we found in lin-12 hypomorphic alleles, suggesting a previously uncharacterised LIN-12 signalling event late in postembryonic mesoderm development. Together with a previous study indicating a role of lin-12 and sel-12 during the specification of the pi cell lineage required for proper vulva-uterine connection, our data suggest that the failure of sel-12 animals to lay eggs properly is caused by defects in at least two independent signalling events in different tissues during development.     

Well-defined growth factors promote cardiac development in axolotl mesodermal explants

The effect of growth factors on the formation of cardiac mesoderm in the urodele, Ambystoma mexicanum (axolotl), has been examined using an in vitro explant system. It has previously been shown that cardiac mesoderm is induced by pharyngeal endoderm during neurula stages in urodeles. In this study, explants of prospective cardiac mesoderm from early neurula stage embryos rarely formed beating cardiac tissue in culture. When transforming growth factor beta-1 (TGF-beta 1) or platelet-derived growth factor BB (PDGF) was added to such explants, the frequency of heart tissue formation increased markedly. The addition of other growth factors to these explants did not enhance cardiac mesoderm formation. The addition of basic fibroblast growth factor (bFGF) to prospective heart mesoderm derived from later stage embryos resulted in a decreased tendency to form cardiac tissue. These results suggest that growth factors analogous to TGF-beta 1, PDGF, and bFGF may regulate the initial stages of vertebrate cardiac development in vivo.     

Induction of mesodermal tissues by acidic and basic heparin binding growth factors

The inducing activity of two heparin binding growth factors HBGF-1 (prostate epithelial cell growth factor; acidic pI) and HBGF-2 (fibroblast growth factor; basic pI) from bovine brain has been tested on totipotent ectoderm from early amphibian (Xenopus laevis, Ambystoma mexicanum) embryos. Both factors induced, at high concentrations, mostly compact spheres surrounded by a non-epidermal epithelium. When the concentration or time of incubation was reduced, large muscle inductions frequently organized as somites were formed besides endothelial vesicles, mesenchyme and smaller areas of intestine-like epithelium. Further reduction of the concentrations or the time of incubation led to an increase in size and number of endothelium-lined vesicles and of mesenchyme, whereas the induction of muscle decreased. At still lower concentrations the overall rate of inductions decreased. The relationship of the growth factors to the vegetalizing factor from chicken embryos, dilution of which shows a similar shift in induced organs, is discussed. The present and previous experiments suggest that different mesodermal and endodermal tissues are induced by secondary interactions in which additional factors are involved. The induced organs derive from dorsal as well as from ventral mesoderm.     

An antagonistic role for the C. elegans Schnurri homolog SMA-9 in modulating TGFbeta signaling during mesodermal patterning

In C. elegans, the Sma/Mab TGFbeta signaling pathway regulates body size and male tail patterning. SMA-9, the C. elegans homolog of Schnurri, has been shown to function as a downstream component to mediate the Sma/Mab TGFbeta signaling pathway in these processes. We have discovered a new role for SMA-9 in dorsoventral patterning of the C. elegans post-embryonic mesoderm, the M lineage. In addition to a small body size, sma-9 mutant animals exhibit a dorsal-to-ventral fate transformation within the M lineage. This M lineage defect of sma-9 mutants is unique in that animals carrying mutations in all other known components of the TGFbeta pathway exhibit no M lineage defects. Surprisingly, mutations in the core components of the Sma/Mab TGFbeta signaling pathway suppressed the M lineage defects of sma-9 mutants without suppressing their body size defects. We show that this suppression specifically happens within the M lineage. Our studies have uncovered an unexpected role of SMA-9 in antagonizing the TGFbeta signaling pathway during mesodermal patterning, suggesting a novel mode of function for the SMA-9/Schnurri family of proteins.     

Analysis of the expression pattern of Mysidium columbiae wingless provides evidence for conserved mesodermal and retinal patterning processes among insects and crustaceans

The Wnt family includes a number of genes, such as wingless ( wg), which encode secreted glycoproteins that function in numerous developmental patterning processes. In order to gain a better understanding of crustacean pattern formation, a wg orthologue was cloned from the malacostracan crustacean Mysidium columbiae(mysid), and the expression pattern of this gene was compared with that of Drosophila wg. Although Drosophila wg is expressed in many developing tissues, such as the ventral neuroectoderm, M. columbiae wg (mcowg)expression is detected within only a subset of these tissues. mcowg is expressed in the dorsal part of each developing segment and within the developing eye, but not within the ventral neuroectoderm. Dorsal wg expression in Drosophila is required for heart and muscle development, and conservation of this dorsal wgexpression pattern suggests that mcowgmay function to pattern these tissues in mysids. Consistent with this, expression of Even-skipped (Eve) protein in heart precursor and muscle cells, which is dependent on Wg signaling in Drosophila, is also conserved in mysids. Within the developing mysid eye, mcowg is expressed in a pattern that is similar to the expression pattern of Drosophila wg in the fly eye disc. In Drosophila,Wg inhibits neural differentiation at the anterior margin of the eye disc and patterns the dorsal/ventral axis of the eye. These data indicate that mcowg may function similarly during mysid eye development. Analysis of mcowgexpression provides molecular evidence suggesting that the processes of heart, muscle, and eye patterning are likely to be conserved among insects and crustaceans.     

Mox-1 and Mox-2 define a novel homeobox gene subfamily and are differentially expressed during early mesodermal patterning in mouse embryos.

We have isolated two mouse genes, Mox-1 and Mox-2 that, by sequence, genomic structure and expression pattern, define a novel homeobox gene family probably involved in mesodermal regionalization and somitic differentiation. Mox-1 is genetically linked to the keratin and Hox-2 genes of chromosome 11, while Mox-2 maps to chromosome 12. At primitive streak stages (approximately 7.0 days post coitum), Mox-1 is expressed in mesoderm lying posterior of the future primordial head and heart. It is not expressed in neural tissue, ectoderm, or endoderm. Mox-1 expression may therefore define an extensive 'posterior' domain of embryonic mesoderm before, or at the earliest stages of, patterning of the mesoderm and neuroectoderm by the Hox cluster genes. Between 7.5 and 9.5 days post coitum, Mox-1 is expressed in presomitic mesoderm, epithelial and differentiating somites (dermatome, myotome and sclerotome) and in lateral plate mesoderm. In the body of midgestation embryos, Mox-1 signal is restricted to loose undifferentiated mesenchyme. Mox-1 signal is also prominent over the mesenchyme of the heart cushions and truncus arteriosus, which arises from epithelial-mesenchymal transformation and over a limited number of craniofacial foci of neural crest-derived mesenchyme that are associated with muscle attachment sites. The expression profile of Mox-2 is similar to, but different from, that of Mox-1. For example, Mox-2 is apparently not expressed before somites form, is then expressed over the entire epithelial somite, but during somitic differentiation, Mox-2 signal rapidly becomes restricted to sclerotomal derivatives. The expression patterns of these genes suggest regulatory roles for Mox-1 and Mox-2 in the initial anterior-posterior regionalization of vertebrate embryonic mesoderm and, in addition, in somite specification and differentiation.     

The possible role of mesodermal growth factors in the formation of endoderm inXenopus laevis

We have raised a monoclonal antibody, 4G6, against gut manually isolated from stage 42Xenopus laevis embryos. It is specific for endoderm and recognises an epitope that is first expressed at stage 19 and which persists throughout subsequent development. The antibody maintains gut specificity through metamorphosis and into adulthood. The epitope is conserved in the mouse, where it is also found in the gut. Isolated vegetal poles fromXenopus blastula stage embryos express the epitope autonomously after culturing to the appropriate stage. This shows that certain aspects of endoderm differentiation do not require germ layer interactions. Animal cap cells from stage 9 blastulae cultured in the presence of the mesodermal growth factors FGF, XTC-MIF and PIF form both endodermal and mesodermal tissues, assessed by the binding of tissue-specific monoclonal antibodies. Endoderm is typically found in those caps which form intermediate and ventral forms of mesoderm, that is muscle and lateral plate. Correspondence to: E.A. Jones     

Structural and functional evidences for a type 1 TGF-beta sensu stricto receptor in the lophotrochozoan Crassostrea gigas suggest conserved molecular mechanisms controlling mesodermal patterning across bilateria

The transforming growth factor beta (TGFbeta) superfamily includes bone morphogenetic proteins, activins and TGF-betasensu stricto (s.s.). These ligands have been shown to play a key role in numerous biological processes including early embryonic development and immune regulation. They transduce their signal through a hetromeric complex of type I and type II receptors. Such receptors have been identified in ecdysozoans but none have been found as yet in the other major protostomal clade, the lophotrochozoans. Here, we report the identification of the first lophotrochozoan TGFbetas.s. type I receptor (Cg-TGFbetaRI) from the mollusk Crassostrea gigas. The phylogenetic and structural analyses as well as the expression pattern during early development suggest Cg-TGFbetaRI to belong to the TGFbetas.s./activin type I receptor clade and functional studies corroborate these deductions. The use of the zebrafish embryo as a reporter organism reveals that either Cg-TGFbetaRI or its dominant negative acting truncated form, when overexpressed during gastrulation, resulted in a range of phenotypes displaying severe disturbance of anterioposterior patterning due to a strong modulation of ventrolateral mesoderm patterning. Finally, a Cg-TGFbetaRI cytokine activity during immune regulation in C. gigas has been investigated by real-time PCR in haemocytes and mantle edge during an in vivo bacterial LPS challenge. One piece of evidence from this study suggests that the molecular mechanisms controlling mesodermal patterning and some immune regulations across all bilateria could be conserved through a functional TGF-beta s.s. pathway in lophotrochozoans.