The role of BMP signaling in outgrowth and patterning of the Xenopus tail bud

Early fin primordia of zebrafish larvae regenerate by a similar growth control mechanism with adult regeneration

Some vertebrate species, including urodele amphibians and teleost fish, have the remarkable ability of regenerating lost body parts. Regeneration studies have been focused on adult tissues, because it is unclear whether or not the repairs of injured tissues during early developmental stages have the same molecular base as that of adult regeneration. Here, we present evidence that a similar cellular and molecular mechanism to adult regeneration operates in the repair process of early zebrafish fin primordia, which are composed of epithelial and mesenchymal cells. We show that larval fin repair occurs through the formation of wound epithelium and blastema-like proliferating cells. Cell proliferation is first induced in the distal-most region and propagates to more proximal regions, as in adult regeneration. We also show that fibroblast growth factor signaling helps induce cell division. Our results suggest that the regeneration machinery directing cell proliferation in response to injury may exist from the early developmental stages.

Morphological Features of Tail Bud Development in Truncate Mouse Mutants

A key malformation in the homozygous truncate mouse mutants is a partial lack of the notochord in the embryo tail. In order to analyze if tail bud development was affected by the truncate (tc) mutation, serial semithin sections of tails of the homozygous mutant embryos were compared to the wild-type controls. In the wild-type embryos morphologically uniform mesenchyme of the tail bud was continuous via the medullary cord to the secondary neural tube, and via the tail cord to the notochord and the gut. In truncate embryos the tail cord was not continuous to the notochord, but to the additional lumen of the tail gut resulting in tail gut duplication. Toward the base of the tail two tail guts subsequently fused together or the additional one disappeared. If present in the tip of the tail, the notochord in truncate embryos ended near the ventral border of the secondary neural tube. The rest of the tail notochord was fragmented and the posterior ends of the fragments were frequently adjacent or even continuous to the neural tube. We suggest that the improper regionalization of the tail bud, where notochord is associated with the neural tube rather than with the tail gut, is related to the subsequent segmental lack of the notochord in truncate mutants.

Conservation and divergence of BMP2/4 genes in the lamprey: expression and phylogenetic analysis suggest a single ancestral vertebrate gene

Bone morphogenetic protein (BMP) molecules are members of a large family of signaling molecules important in numerous developmental pathways throughout the metazoa. Single members of the BMP2/4 class have been found in invertebrates such as cnidarians, arthropods, nematodes, echinoderms, ascidians, and cephalochordates. In all vertebrates studied, there are at least two copies, BMP2 and BMP4, that play important roles in axial patterning, tissue specification, and organogenesis. The basal vertebrate, lamprey, diverged near the time of vertebrate origins and is useful for understanding the gene duplication events that led to the increased complexity of the vertebrate genome. We characterized the sequence and expression pattern of BMP2/4 class genes in the sea lamprey, Petromyzon marinus. We uncovered three genes that we named PmBMP2/4A, PmBMP2/4B, and PmBMP2/4C. Phylogenetic analysis indicates that PmBMP2/4A is closer than PmBMP2/4B or PmBMP2/4C in sequence identity to both BMP2 and BMP4 of gnathostomes. The developmental expression pattern of PmBMP2/4A also more closely resembles the combined early expression patterns of gnathostome BMP2 and BMP4, whereas PmBMP2/4B and PmBMP2/4C appear to play roles only later in development. Cell labeling showed that the BMP-expressing cells in the branchial arches of lampreys are of neural crest origin. Taken together, our sequence and expression data support the duplication of BMP2/4 genes in the lamprey from a single ancestral vertebrate BMP2/4 gene.

Cellular and molecular mechanisms of regeneration in Xenopus

We have employed transgenic methods combined with embryonic grafting to analyse the mechanisms of regeneration in Xenopus tadpoles. The Xenopus tadpole tail contains a spinal cord, notochord and segmented muscles, and all tissues are replaced when the tail regenerates after amputation. We show that there is a refractory period of very low regenerative ability in the early tadpole stage. Tracing of cell lineage with the use of single tissue transgenic grafts labelled with green fluorescent protein (GFP) shows that there is no de-differentiation and no metaplasia during regeneration. The spinal cord, notochord and muscle all regenerate from the corresponding tissue in the stump; in the case of the muscle the satellite cells provide the material for regeneration. By using constitutive or dominant negative gene products, induced under the control of a heat shock promoter, we show that the bone morphogenetic protein (BMP) and Notch signalling pathways are both essential for regeneration. BMP is upstream of Notch and has an independent effect on regeneration of muscle. The Xenopus limb bud will regenerate completely at the early stages but regenerative ability falls during digit differentiation. We have developed a procedure for making tadpoles in which one hindlimb is transgenic and the remainder wild-type. This has been used to introduce various gene products expected to prolong the period of regenerative capacity, but none has so far been successful.

Ventral tail bud mesenchyme is a signaling center for tail paraxial mesoderm induction

A large body of evidence from several systems indicates that formation of the vertebrate tail is morphogenetically continuous with gastrulation, including neural inducing activity in descendants of the gastrula organizer. However, the signaling centers and molecular events regulating tail mesoderm induction and its organized elongation remain poorly defined. In mammals, the ventral ectoderm ridge (VER) is essential to maintain ongoing formation of paraxial mesoderm and somitogenesis in cultures of intact tail. Avian tail buds contain a similar VER structure. Here, we report that the chick ventral tail bud operates as a signaling center for paraxial mesoderm induction. By using "organizer" style grafting assays to early host embryos, we found that ventral tail bud was able to induce elongated paraxial mesodermal extensions and that the ventral tail bud mesenchyme underlying the VER is both necessary and sufficient for the induction in this assay system. Our observations combined with those of others suggest that interplay between several different signaling centers in the amniote tail bud regulates the coordinate induction and elongation of axial and paraxial structures in the developing tail.

A Notch feeling of somite segmentation and beyond

Vertebrate segmentation is manifested during embryonic development as serially repeated units termed somites that give rise to vertebrae, ribs, skeletal muscle and dermis. Many theoretical models including the "clock and wavefront" model have been proposed. There is compelling genetic evidence showing that Notch-Delta signaling is indispensable for somitogenesis. Notch receptor and its target genes, Hairy/E(spl) homologues, are known to be crucial for the ticking of the segmentation clock. Through the work done in mouse, chick, Xenopus and zebrafish, an oscillator operated by cyclical transcriptional activation and delayed negative feedback regulation is emerging as the fundamental mechanism underlying the segmentation clock. Ubiquitin-dependent protein degradation and probably other posttranslational regulations are also required. Fgf8 and Wnt3a gradients are important in positioning somite boundaries and, probably, in coordinating tail growth and segmentation. The circadian clock is another biochemical oscillator, which, similar to the segmentation clock, is operated with a negative transcription-regulated feedback mechanism. While the circadian clock uses a more complicated network of pathways to achieve homeostasis, it appears that the segmentation clock exploits the Notch pathway to achieve both signal generation and synchronization. We also discuss mathematical modeling and future directions in the end.

Independent induction and formation of the dorsal and ventral fins inXenopus laevis

It has been known since the 1930s that the dorsal fin is induced by the underlying neural crest. The inducer of the ventral fin, however, has remained elusive. We have investigated the source of the inducer of the ventral fin in Xenopus and show that it is the ventral mesoderm and not the neural crest. This induction takes place during mid-neurula stages and is completed by late neurulation. In terms of cell composition, the dorsal fin mesenchyme core arises from neural crest cells, while the mesenchyme of the ventral fin has a dual origin. The ventral fin contains neural crest cells that migrate in from the dorsal side of the embryo, but a contribution is also made by cells from the ventral mesoderm.

No tail co-operates with non-canonical Wnt signaling to regulate posterior body morphogenesis in zebrafish

The vertebrate posterior body is formed by a combination of the gastrulation movements that shape the head and anterior trunk and posterior specific cell behaviors. Here, we investigated whether genes that regulate cell movements during gastrulation [no tail (ntl)/brachyury, knypek (kny) and pipetail (ppt)/wnt5] interact to regulate posterior body morphogenesis. Both kny;ntl and ppt;ntl double mutant embryos exhibit synergistic trunk and tail shortening by early segmentation. Gene expression analysis in the compound mutants indicates that anteroposterior germ-layer patterning is largely normal and that the tail elongation defects are not due to failure to specify or maintain posterior tissues. Moreover, ntl interacts with ppt and kny to synergistically regulate the posterior expression of the gene encoding bone morphogenetic protein 4 (bmp4) but not of other known T-box genes, fibroblast growth factor genes or caudal genes. Examination of mitotic and apoptotic cells indicates that impaired tail elongation is not simply due to decreased cell proliferation or increased cell death. Cell tracing in ppt;ntl and kny;ntl mutants demonstrates that the ventral derived posterior tailbud progenitors move into the tailbud. However, gastrulation-like convergence and extension movements and cell movements within the posterior tailbud are impaired. Furthermore, subduction movements of cells into the mesendoderm are reduced in kny;ntl and ppt;ntl mutants. We propose that Ntl and the non-canonical Wnt pathway components Ppt and Kny function in parallel, partially redundant pathways to regulate posterior body development. Our work initiates the genetic dissection of posterior body morphogenesis and links genes to specific tail-forming movements. Moreover, we provide genetic evidence for the notion that tail development entails a continuation of mechanisms regulating gastrulation together with mechanisms unique to the posterior body.

The pro-BMP activity of Twisted gastrulation is independent of BMP binding

The determination of the vertebrate dorsoventral body axis is regulated in the extracellular space by a system of interacting secreted molecules consisting of BMP, Chordin, Tolloid and Twisted Gastrulation (Tsg). Tsg is a BMP-binding protein that forms ternary complexes with BMP and Chordin. We investigated the function of Tsg in embryonic patterning by generating point mutations in its two conserved cysteine-rich domains. Surprisingly, Tsg proteins with mutations in the N-terminal domain were unable to bind BMP, yet ventralized the embryo very effectively, indicating strong pro-BMP activity. This hyperventralizing Tsg activity required an intact C-terminal domain and could block the anti-BMP activity of isolated BMP-binding modules of Chordin (CRs) in embryonic assays. This activity was specific for CR-containing proteins as it did not affect the dorsalizing effects of Noggin or dominant-negative BMP receptor. The ventralizing effects of the xTsg mutants were stronger than the effect of Chordin loss-of-function in Xenopus or zebrafish. The results suggest that xTsg interacts with additional CR-containing proteins that regulate dorsoventral development in embryos.

Molecular pathways needed for regeneration of spinal cord and muscle in a vertebrate

The tail of the frog tadpole, comprising spinal cord, muscle, and notochord, regenerates following partial amputation. We show that, in Xenopus, this occurs throughout development, except for a "refractory period" between stages 45 and 47, when tails heal over without regeneration. Regeneration can be enabled during this refractory period by activation of either the BMP or Notch signaling pathways. Conversely, regeneration can be prevented during the later, regenerative, stages by inhibition of either pathway. BMP signaling will cause regeneration of all tissues, whereas Notch signaling activates regeneration of spinal cord and notochord, but not muscle. An activated form of Msx1 can promote regeneration in the same way as BMP signaling. Epistasis experiments suggest that BMP signaling is upstream of Notch signaling but exerts an independent effect on muscle regeneration. The results demonstrate that regenerative capability can be enabled by genetic modifications that reactivate specific components of the developmental program.

What's your position? the Xenopus cement gland as a paradigm of regional specification

The correct positioning of organs during embryonic development requires multiple cues. The Xenopus cement gland is a mucus-secreting epithelium that is a simple model for organogenesis, allowing detailed analysis of this complex process. The cement gland forms at a conserved anterior position, where embryonic ectoderm and endoderm touch. In all deuterostomes, this region will form the stomodeum (primitive mouth) and, in some aquatic larva, will also form a cement gland. In recent years, a model has been put forward suggesting that an intermediate level of BMP signaling in the ectoderm leads to cement gland formation. We propose an alternative model whereby, during gastrulation, the cement gland (CG) is positioned by the overlap of three domains, corresponding to anterodorsal identity (AD), ventrolateral identity (VL), and ectodermal outer layer identity (EO), defining the equation (AD + VL + EO = CG). Anterodorsal identity requires a contribution by the transcription factor Otx2 while ventrolateral identity requires the BMP4 signaling pathway. These postional cues are integrated to activate cement gland differentiation. This integration appears to require intermediate steps, including expression of pitx genes, and members of the ATF/CREB and Ets transcription factor families.

Chordin is required for the Spemann organizer transplantation phenomenon in Xenopus embryos

We analyzed the Chordin requirement in Xenopus development. Targeting of both chordin Xenopus laevis pseudoalleles with morpholino antisense oligomers (Chd-MO) markedly decreased Chordin production. Embryos developed with moderately reduced dorsoanterior structures and expanded ventroposterior tissues, phenocopying the zebrafish chordino mutant. A strong requirement for Chordin in dorsal development was revealed by experimental manipulations. First, dorsalization by lithium chloride treatment was completely blocked by Chd-MO. Second, Chd-MO inhibited elongation and muscle differentiation in Activin-treated animal caps. Third, Chd-MO completely blocked the induction of the central nervous system (CNS), somites, and notochord by organizer tissue transplanted to the ventral side of host embryos. Unexpectedly, transplantations into the dorsal side revealed a cell-autonomous requirement of Chordin for neural plate differentiation.

Xolloid-related: a novel BMP1/Tolloid-related metalloprotease is expressed during early Xenopus development

We have identified a novel Tolloid-like metalloprotease, called Xolloid-related (Xlr), that is expressed during early Xenopus development. Transcripts for xlr are localized to the marginal zone of mid-gastrulae and are most abundant in ventral and lateral sectors. At neurula stages xlr is strongly expressed around the blastopore and in the pharyngeal endoderm, and more weakly expressed throughout the ventral half of the embryo. Transcripts are detected in the nervous system, particularly the hindbrain and spinal cord, and tailbud of tailbud stage embryos, with weaker expression in the anterior nervous system, otic vesicle, heart, and pronephric duct. Transcription of xlr is increased by BMP4 and decreased by Noggin and tBR, indicating that xlr is regulated by BMP signalling. Injection of xlr mRNA inhibits dorsoanterior development and the dorsal axis inducing ability of coinjected chordin, but not noggin or tBR, mRNA. Xlr conditioned media cleaves Chordin in vitro, indicating that this protease may regulate the availability of Chordin in vivo.