Syndecan-1 regulates BMP signaling and dorso-ventral patterning of the ectoderm during early Xenopus development

Extracellular regulation of growth factor signaling is a key event for embryonic patterning. Heparan sulfate proteoglycans (HSPG) are among the molecules that regulate this signaling during embryonic development. Here we study the function of syndecan1 (Syn1), a cell-surface HSPG expressed in the non-neural ectoderm during early development of Xenopus embryos. Overexpression of Xenopus Syn1 (xSyn1) mRNA is sufficient to reduce BMP signaling, induce chordin expression and rescue dorso-ventral patterning in ventralized embryos. Experiments using chordin morpholinos established that xSyn1 mRNA can inhibit BMP signaling in the absence of chordin. Knockdown of xSyn1 resulted in a reduction of BMP signaling and expansion of the neural plate with the concomitant reduction of the non-neural ectoderm. Overexpression of xSyn1 mRNA in xSyn1 morphant embryos resulted in a biphasic effect, with BMP being inhibited at high concentrations and activated at low concentrations of xSyn1. Interestingly, the function of xSyn1 on dorso-ventral patterning and BMP signaling is specific for this HSPG. In summary, we report that xSyn1 regulates dorso-ventral patterning of the ectoderm through modulation of BMP signaling.     

HyBMP5-8b, a BMP5-8 orthologue, acts during axial patterning and tentacle formation in hydra

Developmental gradients play a central role in axial patterning in hydra. As part of the effort towards elucidating the molecular basis of these gradients as well as investigating the evolution of the mechanisms underlying axial patterning, genes encoding signaling molecules are under investigation. We report the isolation and characterization of HyBMP5-8b, a BMP5-8 orthologue, from hydra. Processes governing axial patterning are continuously active in adult hydra. Expression patterns of HyBMP5-8b in normal animals and during bud formation, hydra's asexual form of reproduction, were examined. These patterns, coupled with changes in patterns of expression in manipulated tissues during head regeneration, foot regeneration as well as under conditions that alter the positional value gradient indicate that the gene is active in two different processes. The gene plays a role in tentacle formation and in patterning the lower end of the body axis.     

Identification and expression analysis of BMP signaling inhibitors genes of the DAN family in amphioxus

Bone morphogenetic proteins (BMPs) are members of the Transforming Growth Factor-β (TGF-β) family implicated in many developmental processes in metazoans such as embryo axes specification. Their wide variety of actions is in part controlled by inhibitors that impede the interaction of BMPs with their specific receptors. Here, we focused our attention on the Differential screening-selected gene Aberrative in Neuroblastoma (DAN) family of inhibitors. Although they are well-characterized in vertebrates, few data are available for this family in other metazoan species. In order to understand the evolution of potential developmental roles of these inhibitors in chordates, we identified the members of this family in the cephalochordate amphioxus, and characterized their expression patterns during embryonic development. Our data suggest that the function of Cerberus/Dand5 subfamily genes is conserved among chordates, whereas Gremlin1/2 and NBL1 subfamily genes seem to have acquired divergent expression patterns in each chordate lineage. On the other hand, the expression of Gremlin in the amphioxus neural plate border during early neurulation strengthens the hypothesis of a conserved neural plate border gene network in chordates.     

斑马鱼胚胎背腹轴建立的分子机制

脊椎动物胚胎发育起始于体轴的建立,是胚胎早期发育过程中最重要的事件之一。Wnt、BMP、Nodal和FGF等多个信号通路协同调控细胞分化和细胞运动,促进胚胎胚层的形成和空间上的分离,调控胚胎背腹轴、前后轴和左右轴线的分化,为胚胎进一步发育勾勒出蓝图。本文主要综述斑马鱼胚胎背腹轴建立的分子机制,包括背部组织中心简介;母源Wnt/β-catenin信号调控背部组织中心形成的分子机制;BMP信号调控背腹轴建立的分子机制。     

Vg1 RNA localization in oocytes in the absence of xVICKZ3 RNA-binding activity

Vg 1 RNA becomes localized at the vegetal cortex of Xenopus oocytes in a process requiring both intact microtubules (MT) and microfilaments. This localization occurs during a narrow window of oogenesis, when a number of RNA-binding proteins associate with the RNA. xVICKZ3 (Vg1 RBP/Vera), the first Vg1 RNA-binding protein identified, helps mediate the association of Vg1 RNA with MT and is co-localized with the RNA at the vegetal cortex. Given the complexity of the Vg1 RNA ribonucleoprotein (RNP) complex, it has remained unclear how xVICKZ3 functions in Vg1 RNA localization. Here, we have taken a closer look at the process of xVICKZ3 localization in oocytes. We have made use of deletion constructs to perform a structure-function analysis of xVICKZ3. The ability of xVICKZ3-GFP constructs to vegetally localize correlates with their association to MT but not with Vg1 RNA-binding ability. We find that when the ability of xVICKZ3 to bind Vg1 RNA is inhibited by the injection of a construct that dominantly inhibits RNA binding, both the construct and Vg1 RNA still localize, apparently through their continued association with a Vg1 RNA-containing RNP complex. These results emphasize the importance of protein-protein interactions in both xVICKZ3 and Vg1 RNA localization.     

Differential regulation of ParaHox genes by retinoic acid in the invertebrate chordate amphioxus (Branchiostoma floridae)

The ParaHox cluster is the evolutionary sister to the Hox cluster. Like the Hox cluster, the ParaHox cluster displays spatial and temporal regulation of the component genes along the anterior/posterior axis in a manner that correlates with the gene positions within the cluster (a feature called collinearity). The ParaHox cluster is however a simpler system to study because it is composed of only three genes. We provide a detailed analysis of the amphioxus ParaHox cluster and, for the first time in a single species, examine the regulation of the cluster in response to a single developmental signalling molecule, retinoic acid (RA). Embryos treated with either RA or RA antagonist display altered ParaHox gene expression: AmphiGsx expression shifts in the neural tube, and the endodermal boundary between AmphiXlox and AmphiCdx shifts its anterior/posterior position. We identified several putative retinoic acid response elements and in vitro assays suggest some may participate in RA regulation of the ParaHox genes. By comparison to vertebrate ParaHox gene regulation we explore the evolutionary implications. This work highlights how insights into the regulation and evolution of more complex vertebrate arrangements can be obtained through studies of a simpler, unduplicated amphioxus gene cluster.     

Computational analysis of BMP gradients in dorsal-ventral patterning of the zebrafish embryo

The genetic network controlling early dorsal-ventral (DV) patterning has been extensively studied and modeled in the fruit fly Drosophila. This patterning is driven by signals coming from bone morphogenetic proteins (BMPs), and regulated by interactions of BMPs with secreted factors such as the antagonist short gastrulation (Sog). Experimental studies suggest that the DV patterning of vertebrates is controlled by a similar network of BMPs and antagonists (such as Chordin, a homologue of Sog), but differences exist in how the two systems are organized, and a quantitative comparison of pattern formation in them has not been made. Here, we develop a computational model in three dimensions of the zebrafish embryo and use it to study molecular interactions in the formation of BMP morphogen gradients in early DV patterning. Simulation results are presented on the dynamics BMP gradient formation, the cooperative action of two feedback loops from BMP signaling to BMP and Chordin synthesis, and pattern sensitivity with respect to BMP and Chordin dosage. Computational analysis shows that, unlike the case in Drosophila, synergy of the two feedback loops in the zygotic control of BMP and Chordin expression, along with early initiation of localized Chordin expression, is critical for establishment and maintenance of a stable and appropriate BMP gradient in the zebrafish embryo.     

Essential role of Dkk3 for head formation by inhibiting Wnt/β‐catenin and Nodal/Vg1 signaling pathways in the basal chordate amphioxus

To dissect the molecular mechanism of head specification in the basal chordate amphioxus, we investigated the function of Dkk3, a secreted protein in the Dickkopf family, which is expressed anteriorly in early embryos. Amphioxus Dkk3 has three domains characteristic of Dkk3 proteins—an N‐terminal serine rich domain and two C‐terminal cysteine‐rich domains (CRDs). In addition, amphioxus Dkk3 has a TGFβ‐receptor 2 domain, which is not present in Dkk3 proteins of other species. As vertebrate Dkk3 proteins have been reported to regulate either Nodal signaling or Wnt/β‐catenin signaling but not both in the same species, we tested the effects of Dkk3 on signaling by these two pathways in amphioxus embryos. Loss of function experiments with an anti‐sense morpholino oligonucleotide (MO) against amphioxus Dkk3 resulted in larvae with truncated heads and concomitant loss of expression of anterior gene markers. The resemblance of the headless phenotype to that from upregulation of Wnt/β‐catenin signaling with BIO, a GSK3β inhibitor, suggested that Dkk3 might inhibit Wnt/β‐catenin signaling. In addition, the Dkk3 MO rescued dorsal structures in amphioxus embryos treated with SB505124, an inhibitor of Nodal signaling, indicating that amphioxus Dkk3 can also inhibit Nodal signaling. In vitro assays in Xenopus animal caps showed that Nodal inhibition is largely due to domains other than the TGFβ domain. We conclude that amphioxus Dkk3 regulates head formation by modulating both Wnt/β‐catenin and Nodal signaling, and that these functions may have been partitioned among various vertebrate lineages during evolution of Dkk3 proteins.     

An amphioxus snail gene: Expression in paraxial mesoderm and neural plate suggests a conserved role in patterning the chordate embryo

 Homologs of the Drosophila snail gene have been characterized in several vertebrates. In addition to being expressed in mesoderm during gastrulation, vertebrate snail genes are also expressed in presumptive neural crest and/or its derivatives. Given that neural crest is unique to vertebrates and is considered to be of fundamental importance in their evolution, we have cloned and characterized the expression of a snail gene from amphioxus, a cephalochordate widely accepted as the sister group of the vertebrates. We show that, at the amino acid sequence level, the amphioxus snail gene is a clear phylogenetic outgroup to all the characterized vertebrate snail genes. During embryogenesis snail expression initially becomes restricted to the paraxial or presomitic mesoderm of amphioxus. Later, snail is expressed at high levels in the lateral neural plate, where it persists during neurulation. Our results indicate that an ancestral function of snail genes in the lineage leading to vertebrates is to define the paraxial mesoderm. Furthermore, our results indicate that a cell population homologous to the vertebrate neural crest may be present in amphioxus, thus providing an important link in the evolution of this key vertebrate tissue. Received: 11 May 1998 / Accepted: 2 August 1998     

zen and the art of phenotypic maintenance: Canalization of embryonic dorsal-ventral patterning in Drosophila

We recently uncovered a novel genetic mechanism that generates the phenotypic uniformity, or canalization, of BMP signaling and cell fate specification during patterning of the dorsal-ventral (D/V) axis in D. melanogaster embryos. We went on to show that other wild-type Drosophila species lack this canalizing genetic circuitry and, consequently, have non-robust D/V patterning. In this review, we propose molecular mechanisms that may give rise to stereotyped BMP signaling, and we identify an additional species that could have decanalized D/V patterning. Extension of these analyses could in turn help explain why canalization is not a universal necessity for species survival.     

Dpp and Gbb exhibit different effective ranges in the establishment of the BMP activity gradient critical for Drosophila wing patterning

Morphogen gradients ensure the specification of different cell fates by dividing initially unpatterned cellular fields into distinct domains of gene expression. It is becoming clear that such gradients are not always simple concentration gradients of a single morphogen; however, the underlying mechanism of generating an activity gradient is poorly understood. Our data indicate that the relative contributions of two BMP ligands, Gbb and Dpp, to patterning the wing imaginal disc along its A/P axis, change as a function of distance from the ligand source. Gbb acts over a long distance to establish BMP target gene boundaries and a variety of cell fates throughout the wing disc, while Dpp functions at a shorter range. On its own, Dpp is not sufficient to mediate the low-threshold responses at the end points of the activity gradient, a function that Gbb fulfills. Given that both ligands signal through the Tkv type I receptor to activate the same downstream effector, Mad, the difference in their effective ranges must reflect an inherent difference in the ligands themselves, influencing how they interact with other molecules. The existence of related ligands with different functional ranges may represent a conserved mechanism used in different species to generate robust long range activity gradients.     

Induction and axial patterning of the neural plate: Planar and vertical signals

In this review I summarize recent findings on the contributions of different cell groups to the formation of the basic plan of the nervous system of vertebrate embryos. Midline cells of the mesoderm—the organizer, notochord, and prechordal plate—and midline cells of the neural ectoderm—the notoplate and floor plate—appear to have a fundamental role in the induction and patterning of the neural plate. Vertical signals acting across tissue layers and planar signals acting through the neural epithelium have distinct roles and cooperate in induction and pattern formation. Whereas the prechordal plate and notochord have distinct vertical signaling properties, the initial anteroposterior (A-P) pattern of the neural plate may be induced by planar signals originating from the organizer region. Planar signals from the notoplate may also contribute to the mediolateral (M-L) patterning of the neural plate. These and other findings suggest a general view of neural induction and axial patterning. © 1993 John Wiley & Sons, Inc.     

Zebrafish chordin-like and chordin are functionally redundant in regulating patterning of the dorsoventral axis

Chordin is the prototype of a group of cysteine-rich domain-containing proteins that bind and modulate signaling of various TGFβ-like ligands. Chordin-like 1 and 2 (CHL1 and 2) are two members of this group that have been described in human, mouse, and chick. However, in vivo roles for CHL1 and 2 in early development are unknown due to lack of loss-of-function analysis. Here we identify and characterize zebrafish, Danio rerio, CHL (Chl). The chl gene is on a region of chromosome 21 syntenic with the area of murine chromosome 7 bearing the CHL2 gene. Inability to identify a separate zebrafish gene corresponding to the mammalian CHL1 gene suggests that Chl may serve roles in zebrafish distributed between CHL1 and CHL2 in other species. Chl is a maternal factor that is also zygotically expressed later in development and has spatiotemporal expression patterns that differ from but overlap those of zebrafish chordin (Chd), suggesting differences but also possible overlap in developmental roles of the two proteins. Chl, like Chd, dorsalizes embryos upon overexpression and is cleaved by BMP1, which antagonizes this activity. Loss-of-function experiments demonstrate that Chl serves as a BMP antagonist with functions that overlap and are redundant with those of Chd in forming the dorsoventral axis.