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PubMed2006

Mechanisms of ventral patterning in the vertebrate nervous system.

Dorsoventral patterning of the neural tube has a crucial role in shaping the functional organization of the CNS. It is well established that hedgehog signalling plays a key role in specifying ventral cell types throughout the neuroectoderm, and major progress has been made in elucidating how hedgehog signalling works in this ventral specification. In addition, other molecular pathways, including nodal, retinoic acid and fibroblast growth factor signalling, have been identified as important molecular cues for ventral patterning of the spinal cord, telencephalon and eye. Here, we discuss recent advances in this field, highlighting the emerging interplay of these signalling pathways in the molecular specification of ventral patterning at different rostrocaudal levels of the CNS.

Animals↗
PubMed2006

Spot pattern of leopard Danio is caused by mutation in the zebrafish connexin41.8 gene.

Leopard, a well-known zebrafish mutant that has a spotted skin pattern instead of stripes, is a model for the study of pigment patterning. To understand the mechanisms underlying stripe formation, as well as the spot variation observed in leopard, we sought to identify the gene responsible for this phenotype. Using positional cloning, we identified the leopard gene as an orthologue of the mammalian connexin 40 gene. A variety of different leopard alleles, such as leo(t1), leo(tq270) and leo(tw28), show different skin-pattern phenotypes. In this manuscript we show that the mutation in allele leo(t1) is a nonsense mutation, whereas alleles leo(tq270) and leo(tw28) contain the missense mutations I202F and I31F, respectively. Patch-clamp experiments of connexin hemichannels demonstrated that the I202F substitution in allele leo(tq270) disrupted the channel function of connexin41.8. These results demonstrate that mutations in this gene lead to a variety of leopard spot patterns.

Alleles↗
PubMed2007

FGF4 regulates blood and muscle specification in Xenopus laevis.

BACKGROUND INFORMATION: FGF (fibroblast growth factor) signalling is known to be required for many aspects of mesoderm formation and patterning during Xenopus development and has been implicated in regulating genes required for the specification of both blood and skeletal muscle lineages. RESULTS: In the present study, we have specifically knocked down the expression of FGF4 using AMO (antisense morpholino oligonucleotide)-mediated inhibition and demonstrate that FGF4 acts in the dorsal marginal zone to restrict blood development and promote the development of skeletal muscle. In addition, we used a drug inhibitor of FGF signalling and an inducible form of FGFR1 (FGF receptor 1) to identify a period of competence during late blastula and gastrula stages when FGF signalling acts to regulate blood versus muscle specification. Notably, we found that it is the dorsal activity of FGF that is required to restrict the expression of SCL (stem cell leukaemia) to the ventral blood island. CONCLUSIONS: Our data indicate that FGF4 is a key organizer-derived signal involved in the process of dorsoventral patterning of the mesoderm.

Animals↗
PubMed1999

Overexpression of thyroid hormone receptor alpha 1 during zebrafish embryogenesis disrupts hindbrain patterning and implicates retinoic acid receptors in the control of hox gene expression.

Nuclear receptors play key roles in anterior/posterior (A/P) axis formation during vertebrate embryogenesis. Within this gene family, retinoic acid receptors and retinoic acid itself have profound influences on the establishment of the A/P axis. Thyroid hormone receptors are expressed during early periods of development, long before the establishment of the thyroid gland, and are able to interact with retinoic acid receptors. Here we examined the ability of the thyroid hormone receptor alpha 1 to affect early embryonic development by mRNA injection of either repressor or activator forms of the thyroid hormone receptor. Overexpression of either the thyroid hormone receptor alpha 1 or a constitutive repressor form, v-erbA, caused a swelling in the rostral hindbrain. These defects were associated with disorganization and loss of rhombomere borders as well as an increase in the number of acetylcholine esterase positive cells. This phenotype correlated with a reduction in hoxa1 expression during gastrulation. Furthermore, injection of either thyroid hormone receptor alpha 1 or v-erbA mRNA repressed a reporter gene that contained a retinoic acid response element, demonstrating the ability of the thyroid hormone receptor alpha 1 to repress retinoic acid signaling during gastrulation. In contrast, embryos treated with retinoic acid alone or embryos injected with thyroid hormone receptor alpha 1 and treated with the thyroid hormone analog TRIAC displayed a similar set of defects, including loss of the midbrain-hindbrain border and severe disruption of the rostral hindbrain. These studies support the involvement of retinoic acid and its receptors in the direct control of Hox gene expression and the early patterning of the zebrafish central nervous system.

Animals↗
PubMed2002

Essential role of the transcription factor Ets-2 in Xenopus early development.

The fibroblast growth factor (FGF)/MAPK pathway plays an important role in early Xenopus developmental processes, including mesoderm patterning. The activation of the MAPK pathway leads to induction of Xenopus Brachyury (Xbra), which regulates the transcription of downstream mesoderm-specific genes in mesoderm patterning. However, the link between the FGF/MAPK pathway and the induction of Xbra has not been fully understood. Here we present evidence suggesting that Ets-2 is involved in the induction of Xbra and thus in the development of posterior mesoderm during early embryonic development. Overexpression of Ets-2 caused posteriorized embryos and led to the induction of mesoderm in ectodermal explants. Expression of a dominant-negative form of Ets-2 or injection of antisense morpholino oligonucleotides against Ets-2 inhibited the formation of the trunk and tail structures. Overexpression of Ets-2 resulted in the induction of Xbra, and expression of the dominant-negative Ets-2 inhibited FGF- or constitutively active MEK-induced Xbra expression. Moreover, overexpression of Ets-2 up-regulated the transcription from Xbra promoter reporter gene constructs. Ets-2 bound to the Xbra promoter region in vitro. These results taken together indicate that Xenopus Ets-2 plays an essential role in mesoderm patterning, lying between the FGF/MAPK pathway and the Xbra transcription.

Animals↗
PubMed1981

Studies on the effect of nutritional deprivation during childhood on body composition of adolescent boys: creatinine excretion.

Forty-eight hour urinary creatinine excretion was measured in 50 boys aged 14 yr. who had had varying degrees of growth retardation during their preschool years. Subjects with growth retardation continued to be shorter and lighter and also had lower urinary creatinine excretion values per cm height or per kg weight. Creatinine height index values showed a deficit of about 40%. It is not clear whether this reduction in creatinine index represents a reduction in muscle mass or an altered creatine turnover. There is a need to study the body composition pattern among undernourished communities using direct methods.

Adolescent↗
PubMed2004

Essential role of BCL9-2 in the switch between beta-catenin's adhesive and transcriptional functions.

beta-Catenin controls both cadherin-mediated cell adhesion and activation of Wnt target genes. We demonstrate here that the beta-catenin-binding protein BCL9-2, a homolog of the human proto-oncogene product BCL9, induces epithelial-mesenchymal transitions of nontransformed cells and increases beta-catenin-dependent transcription. RNA interference of BCL9-2 in carcinoma cells induces an epithelial phenotype and translocates beta-catenin from the nucleus to the cell membrane. The switch between beta-catenin's adhesive and transcriptional functions is modulated by phosphorylation of Tyr 142 of beta-catenin, which favors BCL9-2 binding and precludes interaction with alpha-catenin. During zebrafish embryogenesis, BCL9-2 acts in the Wnt8-signaling pathway and regulates mesoderm patterning.

Animals↗
PubMed2006

Pattern formation in a gene network model with boundary shape dependence.

A fundamental task in developmental biology is to identify the mechanisms which drive morphogenesis. Traditionally pattern formation have been modeled mainly using Turing-type mechanisms, where complex patterns arise by symmetry breaking. However, there is a growing experimental evidence that the influence of signals derived from surrounding tissues can contribute to the patterning processes. In this paper, we show that the interplay between the shape of surrounding tissues and a hierarchically organized gene regulatory network can be able to induce stable complex patterns. The rise of these patterns depends strongly on the shape of the surrounding tissues.

Animals↗
PubMed2005

Role of radial glia in cytogenesis, patterning and boundary formation in the developing spinal cord.

Radial glial fibres provide a transient scaffold and impose constraints in the developing central nervous system (CNS) that facilitate cell migration and axon growth. Recent reports have raised doubts about the distinction between radial glia and precursor cells by demonstrating that radial glia are themselves neuronal progenitor cells in the developing cortex, indicating a dual role for radial glia in both neurogenesis and migration guidance. Radial glia shift toward exclusive generation of astrocytes after neurogenesis has ceased. Radial progenitor cell differentiation and lineage relationships in CNS development are complex processes depending on genetic programming, cell-cell interaction and microenvironmental factors. In the spinal cord, radial cells that arise directly from the neuroepithelium have been identified. At least in the spinal cord, these radial cells appear to be the precursors to radial glia. It remains unknown whether radial glial cells or their precursors, the radial cells, or both can give rise to neurons in the spinal cord. Radial glial cells are also important in regulating the axon out-growth and pathfinding processes that occur during white matter patterning of the developing spinal cord.

Animals↗
PubMed1996

Developmental control of cell cycle regulators: a fly's perspective.

During early development in many species, maternally supplied gene products permit the cell cycle to run at maximum velocity, subdividing the fertilized egg into smaller and smaller cells. As development proceeds, zygotic controls are activated that first limit divisions to defined spatial and temporal domains, coordinating them with morphogenesis, and then halt proliferation altogether, to allow cell differentiation. Analysis of the regulation of cyclin-dependent kinases (Cdks) in Drosophila has provided insights into how this embryonic program of cell proliferation is controlled at the molecular level and how it is linked to developmental cues. Recent studies have also begun to reveal how cell proliferation is controlled during the second phase of Drosophila development, which occurs in imaginal tissues. In contrast to their embryonic progenitors, imaginal cells proliferate with a cycle that requires cell growth and is linked to patterning processes controlled by secreted cell signaling molecules. The functions of these signaling molecules appear to be nearly as conserved between vertebrates and invertebrates as the cell cycle control apparatus itself, suggesting that the mechanisms that coordinate growth, patterning, and cell proliferation in developing tissues have ancient origins.

Animals↗
PubMed2004

The Wnt co-receptors Lrp5 and Lrp6 are essential for gastrulation in mice.

Recent work has identified LDL receptor-related family members, Lrp5 and Lrp6, as co-receptors for the transduction of Wnt signals. Our analysis of mice carrying mutations in both Lrp5 and Lrp6 demonstrates that the functions of these genes are redundant and are essential for gastrulation. Lrp5;Lrp6 double homozygous mutants fail to establish a primitive streak, although the anterior visceral endoderm and anterior epiblast fates are specified. Thus, Lrp5 and Lrp6 are required for posterior patterning of the epiblast, consistent with a role in transducing Wnt signals in the early embryo. Interestingly, Lrp5(+/-);Lrp6(-/-) embryos die shortly after gastrulation and exhibit an accumulation of cells at the primitive streak and a selective loss of paraxial mesoderm. A similar phenotype is observed in Fgf8 and Fgfr1 mutant embryos and provides genetic evidence in support of a molecular link between the Fgf and Wnt signaling pathways in patterning nascent mesoderm. Lrp5(+/-);Lrp6(-/-) embryos also display an expansion of anterior primitive streak derivatives and anterior neurectoderm that correlates with increased Nodal expression in these embryos. The effect of reducing, but not eliminating, Wnt signaling in Lrp5(+/-);Lrp6(-/-) mutant embryos provides important insight into the interplay between Wnt, Fgf and Nodal signals in patterning the early mouse embryo.

Animals↗
PubMed2005

Hedgehog signaling controls dorsoventral patterning, blastema cell proliferation and cartilage induction during axolotl tail regeneration.

Tail regeneration in urodeles requires the coordinated growth and patterning of the regenerating tissues types, including the spinal cord, cartilage and muscle. The dorsoventral (DV) orientation of the spinal cord at the amputation plane determines the DV patterning of the regenerating spinal cord as well as the patterning of surrounding tissues such as cartilage. We investigated this phenomenon on a molecular level. Both the mature and regenerating axolotl spinal cord express molecular markers of DV progenitor cell domains found during embryonic neural tube development, including Pax6, Pax7 and Msx1. Furthermore, the expression of Sonic hedgehog (Shh) is localized to the ventral floor plate domain in both mature and regenerating spinal cord. Patched1 receptor expression indicated that hedgehog signaling occurs not only within the spinal cord but is also transmitted to the surrounding blastema. Cyclopamine treatment revealed that hedgehog signaling is not only required for DV patterning of the regenerating spinal cord but also had profound effects on the regeneration of surrounding, mesodermal tissues. Proliferation of tail blastema cells was severely impaired, resulting in an overall cessation of tail regeneration, and blastema cells no longer expressed the early cartilage marker Sox9. Spinal cord removal experiments revealed that hedgehog signaling, while required for blastema growth is not sufficient for tail regeneration in the absence of the spinal cord. By contrast to the cyclopamine effect on tail regeneration, cyclopamine-treated regenerating limbs achieve a normal length and contain cartilage. This study represents the first molecular localization of DV patterning information in mature tissue that controls regeneration. Interestingly, although tail regeneration does not occur through the formation of somites, the Shh-dependent pathways that control embryonic somite patterning and proliferation may be utilized within the blastema, albeit with a different topography to mediate growth and patterning of tail tissues during regeneration.

Ambystoma↗
PubMed2005

Vg 1 is an essential signaling molecule in Xenopus development.

Xenopus Vg 1, a transforming growth factor beta (Tgfbeta) family member, was one of the first maternally localized mRNAs identified in vertebrates. Its restriction to the vegetal pole of the egg made it the ideal candidate to be the mesoderm-inducing signal released by vegetal cells, but its function in vivo has never been resolved. We show that Vg 1 is essential for Xenopus embryonic development, and is required for mesoderm induction and for the expression of several key Bmp antagonists. Although the original Vg 1 transcript does not rescue Vg 1-depleted embryos, we report that a second allele is effective. This work resolves the mystery of Vg 1 function, and shows it to be an essential maternal regulator of embryonic patterning.

Amino Acid Sequence↗
PubMed2006

Ci-FoxA-a is the earliest zygotic determinant of the ascidian anterior ectoderm and directly activates Ci-sFRP1/5.

This work focuses on the anteroposterior patterning of the ectoderm in the invertebrate chordate Ciona intestinalis. Previous work indicated that, by the eight-cell stage, the anterior and posterior animal blastomeres have acquired different properties, including a differential responsiveness to inducing signals from the underlying mesendoderm. Here, we investigated the molecular basis of this distinction. For this, we studied the regulation of the earliest marker specific for the anterior ectoderm, Ci-sFRP1/5, which is activated at the 64-cell stage. We first found that the activation of this marker in the anterior ectoderm does not involve communication with other lineages. We then identified, by phylogenetic footprinting and deletion analysis, a short conserved minimal enhancer driving the onset of expression of Ci-sFRP1/5. We showed that this enhancer was a direct target of the Ci-FoxA-a gene, a FoxA/HNF3 orthologue expressed in anterior ectodermal and mesendodermal lineages from the eight-cell stage. Gain- and loss-of-function experiments revealed that Ci-FoxA-a is necessary and sufficient within the ectoderm to impose an ectodermal anterior identity, and to repress the posterior programme. Thus, Ci-FoxA-a constitutes a major early zygotic anterior determinant for the ascidian ectoderm, acting autonomously in this territory, prior to the onset of vegetal inductions. Interestingly, while vertebrate FoxA2 are also involved in the regionalization of the ectoderm, they are thought to act during gastrulation to control, in the mesendoderm, the expression of organizer signals. We discuss the evolution of chordate ectodermal patterning in light of our findings.

Animals↗
PubMed2006

Shisa2 promotes the maturation of somitic precursors and transition to the segmental fate in Xenopus embryos.

In vertebrate somitogenesis, FGF and Wnt signals constitute a morphogenetic gradient that controls the maturation of the presomitic mesoderm (PSM) as well as the transition to segmental units. It remains unclear, however, whether there is a regulatory mechanism that promotes the transition by a direct regulation of FGF and Wnt signaling in the PSM. Here we show that Shisa2, a member of a novel Shisa gene family, plays an essential role in segmental patterning during Xenopus somitogenesis. Shisa2 encodes an endoplasmic reticulum (ER) protein that cell-autonomously inhibits FGF and Wnt signaling by preventing the maturation and the cell-surface expression of their receptors. Shisa2 is expressed in the PSM and its knockdown caused a reduction in somite number by the delayed maturation of PSM and anterior shift of the transition; however, the phase of the segmental clock remained intact. These phenotypes were abolished by the inhibition of both FGF and Wnt signals, but by neither alone. We therefore propose that the individual inhibition of both types of signaling by the regulation of receptor maturation in the ER plays an essential role in the establishment of proper segmental patterning.

Amino Acid Sequence↗
PubMed1996

A posteriorising factor, retinoic acid, reveals that anteroposterior patterning controls the timing of neuronal differentiation in Xenopus neuroectoderm.

During early development of the Xenopus central nervous system (CNS), neuronal differentiation can be detected posteriorly at neural plate stages but is delayed anteriorly until after neural tube closure. A similar delay in neuronal differentiation also occurs in the anterior neural tissue that forms in vitro when isolated ectoderm is treated with the neural inducer noggin. Here we examine the factors that control the timing of neuronal differentiation both in embryos and in neural tissue induced by noggin (noggin caps). We show that the delay in neuronal differentiation that occurs in noggin caps cannot be overcome by inhibiting the activity of the neurogenic gene, X-Delta-1, which normally inhibits neuronal differentiation, suggesting that it represents a novel level of regulation. Conversely, we show that the timing of neuronal differentiation can be changed from late to early after treating noggin caps or embryos with retinoic acid (RA), a putative posteriorising agent. Concommittal with changes in the timing of neuronal differentiation, RA suppresses the expression of anterior neural genes and promotes the expression of posterior neural genes. The level of early neuronal differentiation induced by RA alone is greatly increased by the additional expression of the proneural gene, XASH3. These results indicate that early neuronal differentiation in neuralised ectoderm requires posteriorising signals, as well as signals that promote the activity of proneural genes such as XASH3. In addition, these result suggest that neuronal differentiation is controlled by anteroposterior (A-P) patterning, which exerts a temporal control on the onset of neuronal differentiation.

Animals↗
PubMed1997

Patterning of the embryo along the anterior-posterior axis: the role of the caudal genes.

Patterning along the anterior-posterior axis takes place during gastrulation and early neurulation. Homeobox genes like Otx-2 and members of the Hox family have been implicated in this process. The caudal genes in Drosophila and C. elegans have been shown to determine posterior fates. In vertebrates, the caudal genes begin their expression during gastrulation and they take up a posterior position. By injecting sense and antisense RNA of the Xenopus caudal gene Xcad-2, we have studied a number of regulatory interactions among homeobox genes along the anterior-posterior axis. Initially, the Xcad-2 and Otx-2 genes are mutually repressed and, by late gastrulation, they mark the posterior- or anterior-most domains of the embryo, respectively. During late gastrulation and neurulation, Xcad-2 plays an additional regulatory function in relation to the Hox genes. Hox genes normally expressed anteriorly are repressed by Xcad-2 overexpression while those normally expressed posteriorly exhibit more anterior expression. The results show that the caudal genes are part of a posterior determining network which during early gastrulation functions in the subdivision of the embryo into anterior head and trunk domains. Later in gastrulation and neurulation these genes play a role in the patterning of the trunk region.

Animals↗
PubMed1998

The Drosophila EGF receptor controls the formation and specification of neuroblasts along the dorsal-ventral axis of the Drosophila embryo.

The segmented portion of the Drosophila embryonic central nervous system develops from a bilaterally symmetrical, segmentally reiterated array of 30 unique neural stem cells, called neuroblasts. The first 15 neuroblasts form about 30-60 minutes after gastrulation in two sequential waves of neuroblast segregation and are arranged in three dorsoventral columns and four anteroposterior rows per hemisegment. Each neuroblast acquires a unique identity, based on gene expression and the unique and nearly invariant cell lineage it produces. Recent experiments indicate that the segmentation genes specify neuroblast identity along the AP axis. However, little is known as to the control of neuroblast identity along the DV axis. Here, I show that the Drosophila EGF receptor (encoded by the DER gene) promotes the formation, patterning and individual fate specification of early forming neuroblasts along the DV axis. Specifically, I use molecular markers that identify particular neuroectodermal domains, all neuroblasts or individual neuroblasts, to show that in DER mutant embryos (1) intermediate column neuroblasts do not form, (2) medial column neuroblasts often acquire identities inappropriate for their position, while (3) lateral neuroblasts develop normally. Furthermore, I show that active DER signaling occurs in the regions from which the medial and intermediate neuroblasts will later delaminate. In addition, I demonstrate that the concomitant loss of rhomboid and vein yield CNS phenotypes indistinguishable from DER mutant embryos, even though loss of either gene alone yields minor CNS phenotypes. These results demonstrate that DER plays a critical role during neuroblast formation, patterning and specification along the DV axis within the developing Drosophila embryonic CNS.

Animals↗