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PubMed2002

Patterning the marginal zone of early ascidian embryos: localized maternal mRNA and inductive interactions.

Early animal embryos are patterned by localized egg cytoplasmic factors and cell interactions. In invertebrate chordate ascidians, larval tail muscle originates from the posterior marginal zone of the early embryo. It has recently been demonstrated that maternal macho-1 mRNA encoding transcription factor acts as a localized muscle determinant. Other mesodermal tissues such as notochord and mesenchyme are also derived from the vegetal marginal zone. In contrast, formation of these tissues requires induction from endoderm precursors at the 32-cell stage. FGF-Ras-MAPK signaling is involved in the induction of both tissues. The responsiveness for induction to notochord or mesenchyme depends on the inheritance of localized egg cytoplasmic factors. Previous studies also point to critical roles of directed signaling in polarization of induced cells and in subsequent asymmetric divisions resulting in the formation of two daughter cells with distinct fates. One cell adopts an induced fate, while the other assumes a default fate. A simple model of mesoderm patterning in ascidian embryos is proposed in comparison with that of vertebrates.

Animals↗
PubMed1990

Regional neural induction in Xenopus laevis.

During development of the Xenopus embryo, the formation of the nervous system depends on an inductive interaction between mesoderm and ectoderm. The result is a neural tube that is regionally differentiated along the anterior-posterior axis from forebrain to spinal cord. The discovery of genes whose transcripts can be used as molecular markers for different regions of the nervous system has permitted reassessment of the existing theories of neural tissue formation. Although the neural inducing molecules remain elusive, the mechanism by which cells interact to form a regionally differentiated nervous system is becoming clearer.

Animals↗
PubMed1994

Xenopus chordin: a novel dorsalizing factor activated by organizer-specific homeobox genes.

A Xenopus gene whose expression can be activated by the organizer-specific homeobox genes goosecoid and Xnot2 was isolated by differential screening. The chordin gene encodes a novel protein of 941 amino acids that has a signal sequence and four Cys-rich domains. The expression of chordin starts in Spemann's organizer subsequent to that of goosecoid, and its induction by activin requires de novo protein synthesis. Microinjection of chordin mRNA induces twinned axes and can completely rescue axial development in ventralized embryos. This molecule is a potent dorsalizing factor that is expressed at the right time and in the right place to regulate cell-cell interactions in the organizing centers of head, trunk, and tail development.

Activins↗
PubMed1987

Expression of Xenopus N-CAM RNA in ectoderm is an early response to neural induction.

We have isolated Xenopus laevis N-CAM cDNA clones and used these to study the expression of N-CAM RNA during neural induction. The results show that the first marked increase in N-CAM RNA levels occurs during gastrulation when mesoderm comes in contact with ectoderm and induces neural development. In situ hybridization results show that the early expression of N-CAM RNA is localized to the neural plate and its later expression is confined to the neural tube. Induction experiments with explanted germ layers show that N-CAM RNA is not expressed in ectoderm unless there is contact with inducing tissue. Together these results suggest an approach to studying how ectoderm is committed to form neural rather than epidermal tissue. Specifically, the data suggest that neural commitment is marked and perhaps mediated by the transcriptional activation of genes, like N-CAM, in the neural ectoderm.

Animals↗
PubMed1978

[Experimental and biochemical aspects of crystalline lens induction during embryogenesis].

The lens induction is a two-step process and involves morphogenetic influences from the archencepalic endoderm and optic vesicle. One can suggest that the lens induction is primed by specific proteins which are synthesized and secreted by the optic vesicle cells. The proteins-inductors appear to penetrate in the cells and, while interacting (directly or via the cytoplasm) with the nuclei, "programme" the ectodermal cells towards the lens differentiation. The contact interactions and extracellular matrix are of substantial, but not crucial value for the lens induction. The synthesis of specific proteins (crystallins) is to be considered as the most objective criterion of lens differentiation. In vertebrates, there is a lag-period between the moment of lens induction and synthesis of crystallins which is the most long-term in amphibians. The chick embryos constitute an exception and the synthesis of crystallin mRNA occurs in them a few hours after the lens induction. The developing retina loses its capacity to induce lens but stimulates the processes of fiber formation and synthesis of crystallins. A factor was found in the definitive lens epithelium which may be considered as a possible regulator of lens differentiation. On the basis of experiments with heterogenous and native lens inductors, a suggestion is put forward to the effect that the activity of inducing substances is determined by a definite determinant group of the molecule, rather than by the whole molecule.

Animals↗
PubMed2004

Reevaluation of in vitro differentiation protocols for bone marrow stromal cells: disruption of actin cytoskeleton induces rapid morphological changes and mimics neuronal phenotype.

Bone marrow stromal cells (MSC), which represent a population of multipotential mesenchymal stem cells, have been reported to undergo rapid and robust transformation into neuron-like phenotypes in vitro following treatment with chemical induction medium including dimethyl sulfoxide (DMSO; Woodbury et al. [2002] J. Neurosci. Res. 96:908). In this study, we confirmed the ability of cultured rat MSC to undergo in vitro osteogenesis, chondrogenesis, and adipogenesis, demonstrating differentiation of these cells to three mesenchymal cell fates. We then evaluated the potential for in vitro neuronal differentiation of these MSC, finding that changes in morphology upon addition of the chemical induction medium were caused by rapid disruption of the actin cytoskeleton. Retraction of the cytoplasm left behind long processes, which, although strikingly resembling neurites, showed essentially no motility and no further elaboration during time-lapse studies. Similar neurite-like processes were induced by treating MSC with DMSO only or with actin filament-depolymerizing agents. Although process formation was accompanied by rapid expression of some neuronal and glial markers, the absence of other essential neuronal proteins pointed toward aberrantly induced gene expression rather than toward a sequence of gene expression as is required for neurogenesis. Moreover, rat dermal fibroblasts responded to neuronal induction by forming similar processes and expressing similar markers. These studies do not rule out the possibility that MSC can differentiate into neurons; however, we do want to caution that in vitro differentiation protocols may have unexpected, misleading effects. A dissection of molecular signaling and commitment events may be necessary to verify the ability of MSC transdifferentiation to neuronal lineages.

Actin Cytoskeleton↗
PubMed1997

Hox9 genes and vertebrate limb specification.

Development of paired appendages at appropriate levels along the primary body axis is a hallmark of the body plan of jawed vertebrates. Hox genes are good candidates for encoding position in lateral plate mesoderm along the body axis and thus for determining where limbs are formed. Local application of fibroblast growth factors (FGFs) to the anterior prospective flank of a chick embryo induces development of an ectopic wing, and FGF applied to posterior flank induces an ectopic leg. If particular combinations of Hox gene expression determine where wings and legs develop, then formation of additional limbs from flank should involve changes in Hox gene expression that reflect the type of limb induced. Here we show that the same population of flank cells can be induced to form either a wing or a leg, and that induction of these ectopic limbs is accompanied by specific changes in expression of three Hox genes in lateral plate mesoderm. This then reproduces, in the flank, expression patterns found at normal limb levels. Hox gene expression is reprogrammed in lateral plate mesoderm, but is unaffected in paraxial mesoderm. Independent regulation of Hox gene expression in lateral plate mesoderm may have been a key step in the evolution of paired appendages.

Animals↗
PubMed1993

Establishment of gut fate in the E lineage of C. elegans: the roles of lineage-dependent mechanisms and cell interactions.

The gut of C. elegans derives from all the progeny of the E blastomere, a cell of the eight cell stage. Previous work has shown that gut specification requires an induction during the four cell stage (Goldstein, B. (1992) Nature 357, 255-257). Blastomere isolation and recombination experiments were done to determine which parts of the embryo can respond to gut induction. Normally only the posterior side of the EMS blastomere contacts the inducing cell, P2. When P2 was instead placed in a random position on an isolated EMS, gut consistently differentiated from the daughter of EMS contacting P2, indicating that any side of EMS can respond to gut induction. Additionally, moving P2 around to the opposite side of EMS in an otherwise intact embryo caused EMS's two daughter cells to switch lineage timings, and gut to differentiate from the descendents of what normally would be the MS blastomere. The other cells of the four cell stage, ABa, ABp, and P2, did not form gut when placed in contact with the inducer. To determine whether any other inductions are involved in gut specification, timed blastomere isolations were done at the two and eight cell stages. In the absence of cell contact at the two cell stage, segregation of gut fate proceeded normally at both the two and four cell stages. Gut fate also segregated properly in the absence of cell contact at the eight cell stage. A model is presented for the roles of lineage-dependent mechanisms and cell interactions in establishing gut fate in the E lineage.

Animals↗
PubMed2001

spalt-dependent switching between two cell fates that are induced by the Drosophila EGF receptor.

Signaling from the EGF receptor (EGFR) can trigger the differentiation of a wide variety of cell types in many animal species. We have explored the mechanisms that generate this diversity using the Drosophila peripheral nervous system. In this context, Spitz (SPI) ligand can induce two alternative cell fates from the dorsolateral ectoderm: chordotonal sensory organs and non-neural oenocytes. We show that the overall number of both cell types that are induced is controlled by the degree of EGFR signaling. In addition, the spalt (sal) gene is identified as a critical component of the oenocyte/chordotonal fate switch. Genetic and expression analyses indicate that the SAL zinc-finger protein promotes oenocyte formation and supresses chordotonal organ induction by acting both downstream and in parallel to the EGFR. To explain these findings, we propose a prime-and-respond model. Here, sal functions prior to signaling as a necessary but not sufficient component of the oenocyte prepattern that also serves to raise the apparent threshold for induction by SPI. Subsequently, sal-dependent SAL upregulation is triggered as part of the oenocyte-specific EGFR response. Thus, a combination of SAL in the responding nucleus and increased SPI ligand production sets the binary cell-fate switch in favour of oenocytes. Together, these studies help to explain how one generic signaling pathway can trigger the differentiation of two distinct cell types.

Animals↗
PubMed1993

The pattern of retinoic acid receptor gamma (RAR gamma) expression in normal development of Xenopus laevis and after manipulation of the main body axis.

Nuclear receptors are a prerequisite for the transduction of retinoid signals in vertebrate embryogenesis. We have raised an antiserum against the X. laevis retinoic acid receptor gamma (xRAR gamma), which specifically detects a polypeptide with an M(r) of 54 x 10(3) in embryos at gastrula and neurula stages. The antiserum reveals xRAR gamma as a nuclear protein in the head endomesoderm, the neuroectoderm of the hindbrain region, and the entire posterior region during neurula stages. Explanted animal caps express low amounts of xRAR gamma in the absence of mesoderm induction. Treatment of animal caps with activin leads to an increase of the amount and to a regionalization of the xRAR gamma expression. In exogastrulae and in Keller sandwiches, the pattern of expression in the endomesoderm is as expected by analogy to the normal embryo and therefore independent of the vertical contact between the germ layers. A planar signal coming from the dorsal mesoderm is sufficient to impose region-specific expression of xRAR gamma in the neuroectoderm of Keller sandwiches.

Activins↗
PubMed1995

Regulation of Spemann organizer formation by the intracellular kinase Xgsk-3.

Dorsal axis formation in the Xenopus embryo can be induced by the ectopic expression of several Wnt family members. In Drosophila, the protein encoded by the Wnt family gene, wingless, signals through a pathway that antagonizes the effects of the serine/threonine kinase zeste-white 3/shaggy. We describe the isolation and characterization of a Xenopus homolog of zeste-white 3/shaggy, Xgsk-3. A kinase-dead mutant of Xgsk-3, Xgsk-3K-->R, has a dominant negative effect and mimics the ability of Wnt to induce a secondary axis by induction of an ectopic Spemann organizer. Xgsk-3K-->R, like Wnt, induces dorsal axis formation when expressed in the deep vegetal cells, which do not contribute to the axis. These results indicate that the dorsal fate is actively repressed by Xgsk-3, which must be inactivated for dorsal axis formation to occur. Furthermore, our work suggests that the effects of Xgsk-3K-->R are mediated by an additional intercellular signal.

Amino Acid Sequence↗
PubMed1995

Dorsalization of the neural tube by the non-neural ectoderm.

The patterning of cell types along the dorsoventral axis of the spinal cord requires a complex set of inductive signals. While the chordamesoderm is a well-known source of ventralizing signals, relatively little is known about the cues that induce dorsal cell types, including neural crest. Here, we demonstrate that juxtaposition of the non-neural and neural ectoderm is sufficient to induce the expression of dorsal markers, Wnt-1, Wnt-3a and Slug, as well as the formation of neural crest cells. In addition, the competence of neural plate to express Wnt-1 and Wnt-3a appears to be stage dependent, occurring only when neural tissue is taken from stage 8-10 embryos but not from stage 4 embryos, regardless of the age of the non-neural ectoderm. In contrast to the induction of Wnt gene expression, neural crest cell formation and Slug expression can be induced when either stage 4 or stage 8-10 neural plates are placed in contact with the non-neural ectoderm. These data suggest that the non-neural ectoderm provides a signal (or signals) that specifies dorsal cell types within the neural tube, and that the response is dependent on the competence of the neural tissue.

Animals↗
PubMed2003

Tracking mesoderm induction and its specification to the hemangioblast during embryonic stem cell differentiation.

The hematopoietic and endothelial lineages derive from mesoderm and are thought to develop through the maturation of a common progenitor, the hemangioblast. To investigate the developmental processes that regulate mesoderm induction and specification to the hemangioblast, we generated an embryonic stem cell line with the green fluorescent protein (GFP) targeted to the mesodermal gene, brachyury. After the in vitro differentiation of these embryonic stem cells to embryoid bodies, developing mesodermal progenitors could be separated from those with neuroectoderm potential based on GFP expression. Co-expression of GFP with the receptor tyrosine kinase Flk1 revealed the emergence of three distinct cell populations, GFP(-)Flk1(-), GFP(+)Flk1(-) and GFP(+)Flk1(+) cells, which represent a developmental progression ranging from pre-mesoderm to prehemangioblast mesoderm to the hemangioblast.

Animals↗
PubMed2001

Noggin is required for induction of the hair follicle growth phase in postnatal skin.

During postnatal development, the hair follicle (HF) shows cyclic activity with periods of relative resting, active growth (anagen), and regression. We demonstrate that similar to the HF induction in embryonic skin, initiation of a new hair growth phase in postnatal skin requires neutralization of the inhibitory activity of bone morphogenetic protein 4 (BMP4) by the BMP antagonist noggin. In the resting HF, BMP4 mRNA predominates over noggin in the epithelium and mesenchyme, and the BMP receptor IA is prominently expressed in the follicular germ. Anagen development is accompanied by down-regulation of the BMP4 and increased noggin mRNA in the HF. Furthermore, administration of noggin protein induces new hair growth phase in postnatal telogen skin in vivo. In contrast, BMP4 induces selective arrest of anagen development in the non-tylotrich (secondary) HF. As a hair growth inducer, noggin increases Shh mRNA in the HF whereas BMP4 down-regulates Shh. This suggests that modulation of BMP4 signaling by noggin is essential for hair growth phase induction in postnatal skin and that the hair growth-inducing effect of noggin is mediated, at least in part, by Shh.

Animals↗
PubMed2002

Ectodermal Wnt function as a neural crest inducer.

Neural crest cells, which generate peripheral nervous system and facial skeleton, arise at the neural plate/ectodermal border via an inductive interaction between these tissues. Wnts and bone morphogenetic proteins (BMPs) play roles in neural crest induction in amphibians and zebrafish. Here, we show that, in avians, Wnt6 is localized in ectoderm and in vivo inhibition of Wnt signaling perturbs neural crest formation. Furthermore, Wnts induce neural crest from naive neural plates in vitro in a defined medium without added factors, whereas BMPs require additives. Our data suggest that Wnt molecules are necessary and sufficient to induce neural crest cells in avian embryos.

Animals↗
PubMed1989

Differential gene expression in the anterior neural plate during gastrulation of Xenopus laevis.

We have isolated three cDNA clones that are preferentially expressed in the cement gland of early Xenopus laevis embryos. These clones were used to study processes involved in the induction of this secretory organ. Results obtained show that the induction of this gland coincides with the process of neural induction. Genes specific for the cement gland are expressed very early in the anterior neural plate of stage-12 embryos. This suggests that the anteroposterior polarity of the neural plate is already established during gastrulation. At later stages of development, two of the three genes have secondary sites of expression. The expression of these genes can be induced in isolated animal caps by incubation in 10 mM-NH4Cl, a treatment that is known to induce cement glands.

Animals↗
PubMed2001

Kermit, a frizzled interacting protein, regulates frizzled 3 signaling in neural crest development.

Wnts are a family of secreted glycoproteins that are important for multiple steps in early development. Accumulating evidence suggests that frizzled genes encode receptors for Wnts. However, the mechanism through which frizzleds transduce a signal and the immediate downstream components that convey that signal are unclear. We have identified a new protein, Kermit, that interacts specifically with the C-terminus of Xenopus frizzled-3 (Xfz3). Kermit is a 331 amino acid protein with a central PDZ domain. Kermit mRNA is expressed throughout Xenopus development and is localized to neural tissue in a pattern that overlaps Xfz3 expression temporally and spatially. Co-expression of Xfz3 and Kermit results in a dramatic translocation of Kermit to the plasma membrane. Inhibition of Kermit function with morpholino antisense oligonucleotides directed against the 5' untranslated region of Kermit mRNA blocks neural crest induction by Xfz3, and this is rescued by co-injection of mRNA encoding the Kermit open reading frame. These observations suggest that Kermit is required for Wnt/frizzled signaling in neural crest development. To the best of our knowledge, Kermit is the first protein identified that interacts directly with the cytoplasmic portion of frizzleds to modulate their signaling activity.

Adaptor Proteins, Signal Transducing↗
PubMed1998

The molecular ZPA.

Signals that instruct the developing limb bud mesenchyme to form the appropriate elements along the anterior posterior axis emanate from the zone of polarizing activity (ZPA). Studies of the last five years have illustrated that the Sonic hedgehog (Shh) protein mediates the activities of the ZPA. This review will focus on three aspects of the ZPA biology. We will address induction of the ZPA in terms of the factors required for activation and maintenance of Shh expression in the posterior limb bud mesenchyme and the evidence for negative regulatory mechanisms that prevent inappropriate Shh expression. Second, we will discuss the current understanding of Shh processing and transduction of the Shh signal, inferring much from the elegant studies on Drosophila Hedgehog (Hh) protein. Finally, we will address the interpretation of the secreted Shh signal by the limb bud mesenchyme that leads to a predictable morphological response.

Animals↗