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PubMed2003

Induction of neuron-specific glycosylation by Tollo/Toll-8, a Drosophila Toll-like receptor expressed in non-neural cells.

Specific glycan expression is an essential characteristic of developing tissues. Our molecular characterization of a mutation that abolishes neural-specific glycosylation in the Drosophila embryo demonstrates that cellular interactions influence glycan expression. The HRP epitope is an N-linked oligosaccharide expressed on a subset of neuronal glycoproteins. Embryos homozygous for the TM3 balancer chromosome lack neural HRP-epitope expression. Genetic and molecular mapping of the relevant locus reveals that Tollo/Toll-8, a member of the Toll-like receptor family, is altered on the TM3 chromosome. In wild-type embryos, Tollo/Toll-8 is expressed by ectodermal cells that surround differentiating neurons and precedes HRP-epitope appearance. Re-introduction of Tollo/Toll-8 into null embryos rescues neural-specific glycan expression. Thus, loss of an ectodermal cell surface protein alters glycosylation in juxtaposed differentiating neurons. The portfolio of expressed oligosaccharides in a cell reflects its identity and also influences its interactions with other cells and with pathogens. Therefore, the ability to induce specific glycan expression complements the previously identified developmental and innate immune functions of Toll-like receptors.

Animals↗

PubMed1995

Expression of L-type Ca2+ channel during early embryogenesis in Xenopus laevis.

The mechanisms involved in the first step of neurogenesis, i.e. neural induction, are poorly understood, particularly in terms of the signalling pathway. In a recent work it has been shown that in urodeles the activation of L-type calcium channels is sufficient to trigger neural induction. In order to substantiate a possible role of this channel in early development in anurans, we have detailed the kinetics of the expression and the localization of the alpha 1 subunit of L-type calcium channel in the early stages of Xenopus laevis embryogenesis using immunological techniques. We observed that the expression of the alpha 1 subunit started during blastulation, where a cytoplasmic labeling was observed. At the onset of gastrulation alpha 1 was targeted to the plasma membrane of the dorsal and the ventral ectoderm. Some labeling was found in the mesoderm but never in the endoderm. This expression seems to be general, since similar results have been obtained in anurans (Xenopus) and in urodeles (Pleurodeles). In addition, we found that the alpha 0 subunit of the G(o) protein is expressed simultaneously and strictly colocalized with the alpha 1 subunit of the L-type calcium channel. The role of this channel and its regulation by G(o) protein during early neurogenesis is discussed.

Animals↗

PubMed1999

Anterior endomesoderm specification in Xenopus by Wnt/beta-catenin and TGF-beta signalling pathways.

In Xenopus, XHex and cerberus are early marker genes of the anterior endomesoderm (AE), a subset of endoderm cells fated to form the liver and foregut and implicated in head induction. Using XHex and cerberus as markers we have examined the signals underlying AE induction. We show that the AE is specified by the early blastula in the absence of mesodermal signals but that cell-cell contact between presumptive AE cells is required. In overexpression experiments maternal Wnt/beta-catenin and TGF-beta signals (Vg1, Xnr1-2) can induce ectopic XHex and cerberus. Inhibiting these pathways with dominant interfering signalling components blocks endogenous XHex and cerberus expression. We assess the role of signals from the organiser and show that the BMP antagonists noggin and chordin are important for maintaining XHex and cerberus expression. Finally, ventral injection of XHex mRNA can induce ectopic cerberus. Our results indicate that endodermal and mesodermal patterning are closely coordinated and that the AE is likely to be specified by the combined action of dorsal Wnt/beta-catenin signals and endoderm-specific factors mediated by TGF-beta signalling. These results provide a starting point for understanding the molecular events underlying the progressive determination of endodermally derived organs, such as the liver and foregut.

Animals↗

PubMed2000

Two novel nodal-related genes initiate early inductive events in Xenopus Nieuwkoop center.

In vertebrates, Nodal-related protein plays crucial roles in mesoderm and endoderm induction. Here we describe two novel Xenopus nodal-related genes, Xnr5 and Xnr6, which are first zygotically expressed at the mid-blastula transition, in the dorsal-vegetal region including the Nieuwkoop center. Xnr5 and Xnr6 were isolated by expression screening of a library enriched with immediate-early-type transcripts, and are strong inducers of both mesoderm and endoderm. They also induce the other nodal-related genes in the animal cap. In embryos, cerberus-short (nodal-specific inhibitor) can inhibit Xnr1 and Xnr2 express to the same extent goosecoid, but not Xnr5 and Xnr6 transcription. Xnr5 and Xnr6 are regulated completely cell autonomously, differently from other Xnrs in the cell-dissociated embryos. The expression of Xnr5 and Xnr6 is regulated by maternal VegT and (beta)-catenin, but does not require TGF-(beta) signaling. Therefore, expression of Xnr5 and Xnr6 is controlled by different mechanisms from other Xnr family genes.

Amino Acid Sequence↗

PubMed1981

Shift in collagen type as an early response to induction of the metanephric mesenchyme.

Conversion of the nephrogenic mesenchyme into epithelial tubules requires an inductive stimulus from the ureter bud. Here we show with immunofluorescence techniques that the undifferentiated mesenchyme before induction expresses uniformly type I and type III collagens. Induction both in vivo and in vitro leads to a loss of these proteins and to the appearance of basement membrane components including type IV collagen. This change correlates both spatially and temporally with the determination of the mesenchyme and precedes and morphological events. During morphogenesis, type IV collagen concentrates at the borders of the developing tubular structures where, by electron microscopy, a thin, often discontinuous basal lamina was seen to cover the first pretubular cell aggregates. Subsequently, the differentiating tubules were surrounded by a well-developed basal lamina. No loss of the interstitial collagens was seen in the metanephric mesenchyme when brought into contact with noninducing tissues or when cultured alone. Similar observations were made with nonnephrogenic mesenchyme (salivary, lung) when exposed to various heterotypic tissues known to induce tubules in the nephrogenic mesenchyme. The sequential shift in the composition of the extracellular matrix from an interstitial, mesenchymal type to a differentiated, epithelial type is so far the first detectable response of the nephrogenic mesenchyme to the tubule-inducing signal.

Animals↗

PubMed2005

A novel signal induces a segmentation fissure by acting in a ventral-to-dorsal direction in the presomitic mesoderm.

We describe here a novel inductive action that operates during somitic segmentation in chicken embryos. We previously reported that the posterior border cells located at a next-forming boundary in the anterior end of the presomitic mesoderm (PSM) exhibit an inductive activity that acts on the anterior cells to cause the formation of a somitic fissure (Sato, Y., Yasuda, K., Takahashi, Y., 2002. Morphological boundary forms by a novel inductive event mediated by Lunatic fringe and Notch during somitic segmentation. Development 129, 3633-3644). In this study, we have found a second inductive action along the dorso-ventral (D-V) axis during fissure formation. When relocated into a non-segmenting region of PSM, the ventral-most cells taken from the presumptive boundary are sufficient to induce an ectopic fissure in host cells. The ventrally derived signal acts in a ventral-to-dorsal direction but not ventrally, regardless of where the ventral cells are placed. This directional signaling is governed, at least in part, by the signal-receiving cells of the PSM, which we found to be polarized along the D-V axis, and also by intimate cell-cell interactions. Finally, we have observed that morphological segmentation is able to rearrange the anterior and posterior regionalization of individual somites. These findings suggest that discrete unidirectional signals along both the antero-posterior and the D-V axes act coordinately to achieve the formation of the intersomitic fissure, and also that fissure formation is important for the fine-tuning of A-P regionalization in individual somites.

Animals↗

PubMed1996

Conditioned medium from a rat ureteric bud cell line in combination with bFGF induces complete differentiation of isolated metanephric mesenchyme.

Differentiation of metanephric mesenchyme is triggered by an inductive signal(s) from the epithelial ureteric bud. As a result of this induction, most of the metanephric mesenchyme converts into epithelium of a nephron. We have developed and characterized an explant culture system, in which metanephric mesenchyme can grow and completely differentiate in vitro in the absence of an inductive tissue. When separated 13 dpc rat metanephric mesenchymes were cultured in serum-free conditioned medium from a rat ureteric bud cell line (RUB1) in the presence of bFGF and TGFalpha, they were induced to differentiate into nephron epithelia and glomeruli-like structures. The nephric type of differentiation was confirmed by both morphological and molecular criteria and paralleled the developmental changes of nephron differentiation in vivo. Expression patterns of brush-border antigen as well as molecular markers of kidney differentiation Wt1, Lim1, Hgf and c-met, c-ret, Shh, Wnt4, Wnt7b, and Wnt11 were analyzed in explants by whole mount and tissue section in situ hybridization following 1-9 days in culture. The expression of secreted patterning molecules Bmp7 and Wnt7b, but not Shh or Wnt11, were demonstrated by RT-PCR and northern blot hybridization with RNA from the RUB1 cells. Our culture system lends itself to examining the relevance of these and other signaling molecules required for nephron differentiation.

Animals↗

PubMed2004

Activin-like signaling activates Notch signaling during mesodermal induction.

Both activin-like signaling and Notch signaling play fundamental roles during early development. Activin-like signaling is involved in mesodermal induction and can induce a broad range of mesodermal genes and tissues from prospective ectodermal cells (animal caps). On the other hand, Notch signaling plays important roles when multipotent precursor cells achieve a specific cell fate. However, the relationship between these two signal pathways is not well understood. Here, we show that activin A induces Delta-1, Delta-2 and Notch expression and then activates Notch signaling in animal caps. Also, in vivo, ectopic activin-like signaling induced the ectopic expression of Delta-1 and Delta-2, whereas inhibition of activin-like signaling abolished the expression of Delta-1 and Delta-2. Furthermore, we show that MyoD, which is myogenic gene induced by activin A, can induce Delta-1 expression. However, MyoD had no effect on Notch expression, and inhibited Delta-2 expression. These results indicated that activin A induces Delta-1, Delta-2 and Notch by different cascades. We conclude that Notch signaling is activated when activin-like signaling induces various tissues from homogenous undifferentiated cells.

Activins↗

PubMed1999

Regulative development in a nematode embryo: a hierarchy of cell fate transformations.

Cell specification during embryogenesis of the model system Caenorhabditis elegans involves a combination of inductive and autonomous mechanisms. We have begun to study the development of other nematodes to investigate how well cell-specification mechanisms are preserved among closely related species. Here we report that the embryo of the soil nematode Acrobeloides nanus expresses a so far undescribed regulative potential. When, for instance, the first somatic founder cell AB is eliminated it is replaced by its posterior neighbor EMS, which in turn is replaced by the C cell. This allows-different from C. elegans-the development of partial embryos up to hatching and sometimes to fertile adults. Thus, early somatic blastomeres in A. nanus are multipotent, each being capable of giving rise to more than one somatic founder cell. Lost germ-line cells, however, are not replaced. A model is presented, according to which in A. nanus cellular identities are assigned by specific reciprocal inhibitory cell-cell interactions absent in C. elegans. Differences and similarities in cell specification between the two species are discussed and related to different developmental strategies.

Animals↗

PubMed1994

XASH genes promote neurogenesis in Xenopus embryos.

Neural development in Drosophila is promoted by a family of basic helix-loop-helix (bHLH) transcription factors encoded within the Achaete Scute-Complex (AS-C). XASH-3, a Xenopus homolog of the Drosophila AS-C genes, is expressed during neural induction within a portion of the dorsal ectoderm that gives rise to the neural plate and tube. Here, we show that XASH-3, when expressed with the promiscuous binding partner XE12, specifically activates the expression of neural genes in naive ectoderm, suggesting that XASH-3 promotes neural development. Moreover, XASH-3/XE12 RNA injections into embryos lead to hypertrophy of the neural tube. Interestingly, XASH-3 misexpression does not lead to the formation of ectopic neural tissue in ventral regions, suggesting that the domain of XASH proneural function is restricted in the embryo. In contrast to the neural inducer noggin, which permanently activates the NCAM gene, the activation of neural genes by XASH-3/XE12 is not stable in naive ectoderm, yet XASH-3/XE12 powerfully and stably activates NCAM, Neurofilament and type III beta-tubulin gene expression in noggin-treated ectoderm. These results show that the XASH-3 promotes neural development, and suggest that its activity depends on additional factors which are induced in ectoderm by factors such as noggin.

Animals↗

PubMed1995

Induction of avian cardiac myogenesis by anterior endoderm.

An experimental system was devised to study the mechanisms by which cells become committed to the cardiac myocyte lineage during avian development. Chick tissues from outside the fate map of the heart (in the posterior primitive streak (PPS) of a Hamburger & Hamilton stage 4 embryo) were combined with potential inducing tissues from quail embryos and cultured in vitro. Species-specific RT-PCR was employed to detect the appearance of the cardiac muscle markers chick Nkx-2.5 (cNkx-2.5), cardiac troponin C and ventricular myosin heavy chain in the chick responder tissues. Using this procedure, we found that stage 4-5 anterior lateral (AL) endoderm and anterior central (AC) mesendoderm, but not AL mesoderm or posterior lateral mesendoderm, induced cells of the PPS to differentiate as cardiac myocytes. Induction of cardiogenesis was accompanied by a marked decrease in the expression of rho-globin, implying that PPS cells were being induced by anterior endoderm to become cardiac myocytes instead of blood-forming tissue. These results suggest that anterior endoderm contains signaling molecules that can induce cardiac myocyte specification of early primitive streak cells. One of the cardiac muscle markers induced by anterior endoderm, cNkx-2.5, is here described for the first time. cNkx-2.5 is a chick homeobox-containing gene that shares extensive sequence similarity with the Drosophila gene tinman, which is required for Drosophila heart formation. The mesodermal component of cNkx-2.5 expression from stage 5 onward, as determined by in situ hybridization, is strikingly in accord with the fate map of the avian heart. By the time the myocardium and endocardium form distinct layers, cNkx-2.5 is found only in the myocardium. cNkx-2.5 thus appears to be the earliest described marker of avian mesoderm fated to give rise to cardiac muscle.

Amino Acid Sequence↗

PubMed1995

Sequential signalling during Caenorhabditis elegans vulval induction.

During the induction of the Caenorhabditis elegans vulva, cell signalling causes initially equipotent cells to express a reproducible pattern of cell fates. The position of the anchor cell determines the pattern of vulval precursor cell fates, such that the closest precursor cell (P6.p) expresses the primary cell fate, the next closest cells (P5.p and P7.p) both express the secondary cell fate, and each of the precursor cells located at a distance (P3.p, P4.p and P8.p) express the tertiary cell fate (Fig. 1a). We present data indicating that this stereotypical pattern of cell fates can be generated by sequential signals. We identified genetic mosaic animals in which P5.p and P7.p were defective in the anchor-cell signal-transduction pathway and observed that these cells adopted the secondary cell fate, indicating that anchor-cell signal transduction is not required for the expression of the secondary cell fate. These results suggest that the anchor cell induces P6.p to express the primary cell fate, and that P6.p subsequently induces P5.p and P7.p to express the secondary cell fate.

Animals↗

PubMed1994

Computations of post-inductive dynamics in axolotl heart formation.

This paper reports modelling of heart localization in the axolotl (Ambystoma mexicanum). The region of heart specification in the mesoderm defined by classical induction from the endoderm is larger than the area of final myocardial differentiation. For localizing the area of differentiation within the area of specification, we postulate a reaction-diffusion system that arises within the mesoderm in response to induction from the endoderm. This mechanism generates a spatial pattern for two chemicals, an activator and an inhibitor, corresponding to the area of myocardial differentiation. We postulate a diffusible chemical rescuer, which is absent in the cardiac lethal mutant, and which is a precursor to the reaction-diffusion mechanism. The activator, inhibitor, rescuer, and product of endodermal induction are presented in an enzyme mechanism with rate equations similar to the Gierer-Meinhardt equations. These equations were solved numerically in both one and two spatial dimensions. We have attained quantitative agreement with the experimental data for sizes of tissue regions and for times to heartbeat. Experiments modelled include wild-type heart localization as well as both in vitro and in vivo rescue of cardiac lethal mesoderm with wild-type mesoderm. Based upon the parameters necessary to model heart localization, we make a series of predictions. We predict: a specific profile for the endodermal inducer gradient; the possibility of producing multiple hearts in vivo; and a greater contribution to the heart from the wild-type mesoderm for in vivo transplants with cardiac lethal mesoderm. We make some suggestions as to the possible chemical nature of the substances in the model. We indicate that the inhibitory field and mechanochemical theories are probably not as promising as reaction-diffusion for the mechanism of heart localization.

Ambystoma mexicanum↗

PubMed1995

Dorsal-ventral patterning and differentiation of noggin-induced neural tissue in the absence of mesoderm.

In Xenopus development, dorsal mesoderm is thought to play a key role in both induction and patterning of the nervous system. Previously, we identified a secreted factor, noggin, which is expressed in dorsal mesoderm and which can mimic that tissue's neural-inducing activity, without inducing mesoderm. Here the neural tissue induced in ectodermal explants by noggin is further characterized using four neural-specific genes: two putative RNA-binding proteins, nrp-1 and etr-1; the synaptobrevin sybII; and the lipocalin cpl-1. First we determine the expression domain of each gene during embryogenesis. Then we analyze expression of these genes in noggin-treated explants. All markers, including the differentiated marker sybII, are expressed in noggin-induced neural tissue. Furthermore, cpl-1, a marker of dorsal brain, and etr-1, a marker absent in much of the dorsal forebrain, are expressed in non-overlapping territories within these explants. We conclude that the despite the absence of mesoderm, noggin-induced neural tissue shows considerable differentiation and organization, which may represent dorsal-ventral patterning of the forebrain.

Amino Acid Sequence↗

PubMed1999

Essential role for Sonic hedgehog during hair follicle morphogenesis.

The hair follicle is a source of epithelial stem cells and site of origin for several types of skin tumors. Although it is clear that follicles arise by way of a series of inductive tissue interactions, identification of the signaling molecules driving this process remains a major challenge in skin biology. In this study we report an obligatory role for the secreted morphogen Sonic hedgehog (Shh) during hair follicle development. Hair germs comprising epidermal placodes and associated dermal condensates were detected in both control and Shh -/- embryos, but progression through subsequent stages of follicle development was blocked in mutant skin. The expression of Gli1 and Ptc1 was reduced in Shh -/- dermal condensates and they failed to evolve into hair follicle papillae, suggesting that the adjacent mesenchyme is a critical target for placode-derived Shh. Despite the profound inhibition of hair follicle morphogenesis, late-stage follicle differentiation markers were detected in Shh -/- skin grafts, as well as cultured vibrissa explants treated with cyclopamine to block Shh signaling. Our findings reveal an essential role for Shh during hair follicle morphogenesis, where it is required for normal advancement beyond the hair germ stage of development.

Adipose Tissue↗

PubMed2001

Cellular and molecular responses of the uterus to embryo implantation can be elicited by locally applied growth factors.

The implantation of a blastocyst into a receptive uterus is associated with a series of events, namely the attachment reaction followed by decidualization of the stroma. Previous studies established that the gene encoding heparin-binding EGF-like growth factor (HB-EGF) is expressed in the luminal epithelium solely at the site of blastocyst apposition preceding the attachment reaction. We report here the expression during implantation of 21 genes encoding other signaling proteins, including those belonging to the Bone morphogenetic protein (BMP), fibroblast growth factor (FGF), WNT, and Hedgehog (HH) pathways. We find that the attachment reaction is associated with a localized stromal induction of genes encoding BMP-2, FGF-2, and WNT-4. Despite efforts by many investigators, a simple in vitro model of implantation is not yet available to study either the hierarchy of the events triggered in the uterus by the embryo or the function of individual signaling proteins. We have therefore approached these questions by introducing beads loaded with purified factors into the receptive uterus. We show that beads soaked in HB-EGF or insulin-like growth factor-1 (IGF-1), but not other proteins, induce many of the same discrete local responses elicited by the blastocyst, including increased localized vascular permeability, decidualization, and expression of Bmp2 at the sites of the beads. By contrast, the expression domains of Indian hedgehog (Ihh), patched, and noggin become restricted as decidualization proceeds. Significantly, beads containing BMP-2 do not themselves elicit an implantation response but affect the spacing of implantation sites induced by blastocysts cotransferred with the beads.

Animals↗

PubMed2001

The role of Otx2 in organizing the anterior patterning in mouse.

Understanding the molecular mechanism controlling induction and maintenance of signals required for specifying anterior territory (forebrain and midbrain) of the central nervous system is a major task of molecular embryology. The current view indicates that in mouse, early specification of the anterior patterning is established at the beginning of gastrulation by the anterior visceral endoderm, while maintenance and refinement of the early specification is under the control of epiblast-derived tissues corresponding to the axial mesendoderm and rostral neuroectoderm. In vertebrates a remarkable amount of data has been collected on the role of genes contributing to brain morphogenesis. Among these genes,the orthodenticle group is defined bythe Drosophila orthodenticle and the vertebrate Otx1 and Otx2 genes, which contain a bicoid-like homeodomain. Mouse models and chimera experiments have provided strong evidence that Otx2 plays an important role in the specification and maintenance of the rostral neuroectoderm destined to become forebrain and midbrain. In evolutionary terms, some of these findings lead us to hypothesize a fascinating and crucial contribution of the Otx genes to the genetic program underlying the establishment of the mammalian brain.

Animals↗

PubMed1993

Xenopus axis formation: induction of goosecoid by injected Xwnt-8 and activin mRNAs.

In this study, we compare the effects of three mRNAs-goosecoid, activin and Xwnt-8- that are able to induce partial or complete secondary axes when injected into Xenopus embryos. Xwnt-8 injection produces complete secondary axes including head structures whereas activin and goosecoid injection produce partial secondary axes at high frequency that lack head structures anterior to the auditory vesicle and often lack notochord. Xwnt-8 can activate goosecoid only in the deep marginal zone, i.e., in the region in which this organizer-specific homeobox gene is normally expressed on the dorsal side. Activin B mRNA, however, can turn on goosecoid in all regions of the embryo. We also tested the capacity of these gene products to restore axis formation in embryos in which the cortical rotation was blocked by UV irradiation. Whereas Xwnt-8 gives complete rescue of anterior structures, both goosecoid and activin give partial rescue. Rescued axes including hindbrain structures up to level of the auditory vesicle can be obtained at high frequency even in the absence of notochord structures. The possible functions of Wnt-like and activin-like signals and of the goosecoid homeobox gene, and their order of action in the formation of Spemann's organizer are discussed.

Activins↗