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PubMed2004

Sonic hedgehog patterning in chick neural plate is antagonized by a Wnt3-like signal.

Sonic hedgehog (Shh) patterns the dorsal-ventral axis of the neural tube by promoting the differentiation of ventral neural cell types while suppressing dorsal neural fates. Other signals impinge upon the Shh response, biasing the differentiation of a cell. Three dorsally expressed transforming Wnts, of which the most broadly expressed is Wnt3, may be among the signals that influence the Shh response. We demonstrate that activation of Wnt signaling results in an inhibition of the Shh response in neural tissue. Additionally, we show that the expression pattern of chick Wnt3 is consistent with a role in neural patterning. These results indicate that differentiating neural tube cells, besides integrating signals from Hedgehogs and BMPs, may also incorporate a Wnt response to make cell fate decisions.

Amino Acid Sequence↗
PubMed1992

The cellular basis of the convergence and extension of the Xenopus neural plate.

There is great interest in the patterning and morphogenesis of the vertebrate nervous system, but the morphogenetic movements involved in early neural development and their underlying cellular mechanisms are poorly understood. This paper describes the cellular basis of the early neural morphogenesis of Xenopus laevis. The results have important implications for neural induction. Mapping the fate map of the midneurula (Eagleson and Harris: J. Neurobiol. 21:427-440, 1990) back to the early gastrula with time-lapse video recording demonstrates that the prospective hindbrain and spinal cord are initially very wide and very short, and thus at the beginning of gastrulation all their precursor cells lie within a few cell diameters of the inducing mesoderm. In the midgastrula, the prospective hindbrain and spinal cord undergo very strong convergence and extension movements in two phases: In the first phase they primarily undergo thinning in the radial direction and lengthening (extension) in the animal-vegetal direction, and the second phase is characterized primarily by mediolateral narrowing (convergence) and anterior-posterior lengthening (extension). These movements also occur in sandwich explants of the gastrula, thus demonstrating the local autonomy of the forces producing them. Tracing cell movements with fluorescein dextran-labeled cells in embryos or explants shows that the initial thinning and extension occurs by radial intercalation of deep cells to form fewer layers of greater area, all of which is expressed as increased length. The subsequent convergence and extension occurs by mediolateral intercalation of deep cells to form a longer, narrower array. These results establish that a similar if not identical sequence of radial and mediolateral cell intercalations underlie convergence and extension of the neural and the mesoderm tissues (Wilson and Keller: Development, 112:289-300, 1991). Moreover, these results establish that radial and mediolateral intercalation are the principal neural cell behaviors induced by the planar signals emanating from the dorsal involuting marginal zone (the Spemann organizer) in the early gastrula (Keller et al: Develop. Dynamics, 193: 218-234, 1992). Radial and mediolateral intercalation are induced among the 5 to 7 rows of cells comprising the prospective hindbrain and spinal cord, thus producing the massive convergence and extension movements that narrow and elongate these regions of the nervous system in the late gastrula. A more general significance of these results is that neural induction is best analyzed and understood in terms of the dynamics of the morphogenetic processes involved.

Animals↗
PubMed2004

Disturbing endoderm signaling to anterior neural plate of vertebrates by the teratogen cadmium.

Cadmium accumulation in the mouse gut endoderm occurs until the closure of the vitelline duct (day 9 post-coitus; p.c.), producing anterior neural tube defects (NTD). The anterior part of the primitive endoderm, designated as the primary signaling center for anterior patterning, expresses several transcription factors of importance for head formation. Here, we studied the expression levels of some of these transcription factors (Hesx1, HNF3beta, Cerl, Otx2 and Sox2), and cell death induced after single cadmium administration to dams on days 7, 8 and 9 p.c. Stage specific down-regulation of Hesx1, Cerl, and Sox2, and an up-regulation of HNF3beta were observed. No effect was seen in Otx2 expression levels. Cell death was increased in the neuroepithelium of the cranial neural folds, and in areas where neural crest cells migrate, but not in the gut endoderm. It is proposed that cadmium-induced NTD is due to interference with head-inductive signals from the endoderm to the adjacent layers.

Animals↗
PubMed1997

Chick noggin is expressed in the organizer and neural plate during axial development, but offers no evidence of involvement in primary axis formation.

We have cloned and examined the early developmental expression of the chick homolog of noggin, a gene originally isolated in Xenopus that can dorsalize gastrular mesoderm and induce anterior neural tissue from gastrular ectoderm when expressed experimentally. Chick noggin is expressed at relatively low levels, but at sites equivalent to those seen in amphibian development, namely Hensen's node and the endo- and mesodermal head process. There is also diffuse expression in the early CNS, centered on the ventral midline, and later hindbrain-associated expression. Since the earlier of these expression sites are consistent with endogenous organizer functions suggested by the properties of the protein in Xenopus experiments, we have used recombinant mammalian Noggin protein secreted by CHO cells in tests for developmental disturbance on the early gastrula-staged chick blastoderm. Comparable tests sensitively detect effects, on chick, of various other secreted proteins that simulate or replicate early developmental signals in Xenopus. We have been unable to observe such effects with a range of Noggin concentrations including those that dramatically dorsalize Xenopus ventral marginal zones. To illustrate effects observed in such tests with secreted proteins active on early stages, we show results with the known Xenopus ventralizer Bone Morphogenetic Protein 4 (BMP-4).

Amino Acid Sequence↗
PubMed2003

Xenopus Xlmo4 is a GATA cofactor during ventral mesoderm formation and regulates Ldb1 availability at the dorsal mesoderm and the neural plate.

We have identified and functionally characterized the Xenopus Xlmo4 gene, which encodes a member of the LIM-domain-only protein family. Xlmo4 is activated at gastrula stages in the mesodermal marginal zone probably in response to BMP4 signaling. Soon after, Xlmo4 is downregulated in the dorsal region of the mesoderm. This repression seems to be mediated by organizer-expressed repressors, such as Gsc. Xlmo4 downregulation is necessary for the proper formation of this territory. Increasing Xlmo4 function in this region downregulates Spemman Organizer genes and suppresses dorsal-anterior structures. By binding to Ldb1, Xlmo4 may restrict the availability of this cofactor for transcription factors expressed at the Spemman Organizer. In the ventral mesoderm, Xlmo4 is required to establish the identity of this territory by acting as a positive cofactor of GATA factors. In the neural ectoderm, Xlmo4 expression depends on Xiro homeoprotein activity. In this region, Xlmo4 suppresses differentiation of primary neurons and interferes with gene expression at the Isthmic Organizer, most likely by displacing Ldb1 from active transcription factor complexes required for these processes. Together, our data suggest that Xlmo4 uses distinct mechanisms to participate in different processes during development.

Adaptor Proteins, Signal Transducing↗
PubMed1999

Successful survival of grafted transgenic neural plate cells in adult central nervous system environment.

1. Accumulating evidence indicates that damaged brain functions can be ameliorated in a variety of animal models by the grafting of fetal neuronal cell or tissue into damaged brain. Clinical trials are under way to determine whether human fetal mesencephalic tissue can ameliorate motor functions in patients with Parkinson's disease. 2. Autopsy findings of parkinsonian patient implanted with human fetal mesencephalic tissue clearly revealed that the fetal neuronal graft can survive for an extended period of time in the human brain and densely reinnervate the surrounding host striatal tissue. 3. It is, however, still important to obtain more practical, effective, and ethically justifiable donor material for the future clinical application of the procedures. Desirable properties for the donor cells include long-term survival in the brain, neuronal cell type for the reconstruction of damaged neural circuits, and susceptibility to genetic manipulation for the practical use. 4. With the development of molecular biology techniques, genetic modification and transplantation of the donor neuronal cells might be a feasible way to cure many kinds of central nervous system diseases toward a "graft-gene therapy."

Animals↗
PubMed1999

Constitutive and stress-inducible expression of the endoplasmic reticulum heat shock protein 70 gene family member, immunoglobulin-binding protein (BiP), during Xenopus laevis early development.

We have characterized the constitutive and stress-inducible pattern of immunoglobulin-binding protein (BiP) gene expression during Xenopus early development. Whole mount in situ hybridization analysis revealed that BiP mRNA was detected in unfertilized eggs, cleavage and blastula stage embryos. In gastrulae, BiP mRNA was present across the surface of the embryo, while in neurulae BiP mRNA was enriched in the neural plate, neural fold, and around the blastopore. In early and late tailbud embryos, BiP mRNA was found primarily in the dorsal region. Tunicamycin and A23187, the calcium ionophore, enhanced BiP mRNA accumulation first at the neurula stage, while heat shock induced BiP mRNA accumulation first at the gastrula stage. Compared to control, A23187- and heat shock-treated neurulae displayed relatively high levels of BiP mRNA in selected tissues, including the neural plate, neural folds, around the blastopore, and ectoderm. At the early tailbud stage, A23187 and heat shock enhanced BiP mRNA accumulation primarily in the head, somites, tail, and along the spinal cord. A similar situation was found with A23187- and heat shock-treated late tailbud embryos, except that heat-shocked embryos also displayed enhanced BiP mRNA accumulation in the epidermis. These studies demonstrate a preferential accumulation of BiP mRNA in selected tissues during development and in response to stress.

Animals↗
PubMed1995

Pax-2 expression in the murine neural plate precedes and encompasses the expression domains of Wnt-1 and En-1.

In the Drosophila embryo, activation of wingless and engrailed in the parasegment requires paired, a member of the Pax family of transcription factors. We have explored the possible conservation of this regulatory hierarchy in the developing mouse brain. We find that Pax-2 is expressed prior to somite formation in the presumptive mid/hindbrain region. Shortly thereafter, Wnt-1 (the wingless orthologue) and Engrailed-1 are expressed in overlapping regions within the Pax-2 domain. Pax-5 expression commences later, at the 3-somite stage. Thus, the spatial and temporal expression of Pax-2 is consistent with a possible regulatory role in the activation of Wnt-1 and En-1.

Animals↗
PubMed1996

Anterior primitive endoderm may be responsible for patterning the anterior neural plate in the mouse embryo.

BACKGROUND: After implantation, the basic body plan of the mammalian embryo is established during gastrulation when the epithelial founder tissue of the fetus, the epiblast, gives rise to new tissues by ingression through the primitive streak. Formation of the primitive streak defines the caudal aspect of the embryo and thus the anteroposterior axis. Further patterning of this axis has been attributed to signals produced by tissues arising from the primitive streak, and in particular the mesendoderm located along the midline of the embryo is thought to be responsible for the correct anteroposterior subdivision of the neurectoderm as it begins to form the central nervous system (CNS). RESULTS: In situ hybridization studies show that the onset of expression of the homeobox-containing gene Hesx1 coincides with the formation of the primitive streak, but occurs on the opposite side of the embryo, in a small domain of anterior endoderm. Lineage tracing using a lipophilic fluorescent label shows that the first endoderm cells to express Hesx1 are not destined to contribute to the future embryo, but instead belong to the primitive endoderm lineage and will be displaced by definitive endoderm arising from the primitive streak during gastrulation. Approximately 24 hours after Hesx1 transcripts are first detected in the endoderm, they start to appear in adjacent ectoderm that gives rise to the most anterior component of the developing CNS, the prosencephalon, which continues to express Hesx1. Eventually, Hesx1 transcripts are detectable only in Rathke's pouch as the pituitary starts to develop. Removal of endoderm cells expressing Hesx1 during the earlier stages of gastrulation either prevents or severely curtails the later expression of Hesx1 in ectoderm and neurectoderm, but does not affect gene expression in more caudal regions of the developing CNS. CONCLUSIONS: As overt anterior pattern is present in the visceral embryonic endoderm prior to formation of any axial mesendoderm, a mechanism for bestowing anterior pattern must exist which is independent of primitive streak descendants. Furthermore, correct molecular patterning of the most rostral neurectoderm appears to depend on the presence of this anterior visceral embryonic endoderm during the early stages of gastrulation. We propose that primitive endoderm is responsible for the initial induction of rostral identity in the embryo, and in particular for the correct definition of the future prosencephalic neurectoderm. Subsequently, this identity will be reinforced and maintained by axial mesendoderm when it displaces the visceral embryonic endoderm during the course of gastrulation.

Animals↗
PubMed2004

Six1 promotes a placodal fate within the lateral neurogenic ectoderm by functioning as both a transcriptional activator and repressor.

Cranial placodes, which give rise to sensory organs in the vertebrate head, are important embryonic structures whose development has not been well studied because of their transient nature and paucity of molecular markers. We have used markers of pre-placodal ectoderm (PPE) (six1, eya1) to determine that gradients of both neural inducers and anteroposterior signals are necessary to induce and appropriately position the PPE. Overexpression of six1 expands the PPE at the expense of neural crest and epidermis, whereas knock-down of Six1 results in reduction of the PPE domain and expansion of the neural plate, neural crest and epidermis. Using expression of activator and repressor constructs of six1 or co-expression of wild-type six1 with activating or repressing co-factors (eya1 and groucho, respectively), we demonstrate that Six1 inhibits neural crest and epidermal genes via transcriptional repression and enhances PPE genes via transcriptional activation. Ectopic expression of neural plate, neural crest and epidermal genes in the PPE demonstrates that these factors mutually influence each other to establish the appropriate boundaries between these ectodermal domains.

Animals↗