Embryonic expression of Lim-1, the mouse homolog of Xenopus Xlim-1, suggests a role in lateral mesoderm differentiation and neurogenesis

p38 and a p38-interacting protein are critical for downregulation of E-cadherin during mouse gastrulation

During vertebrate gastrulation, an epithelial to mesenchymal transition (EMT) is necessary for migration of mesoderm from the primitive streak. We demonstrate that p38 MAP kinase and a p38-interacting protein (p38IP) are critically required for downregulation of E-cadherin during gastrulation. In an ENU-mutagenesis screen we identified the droopy eye (drey) mutation, which affects splicing of p38IP. p38IP(drey) mutant embryos display incompletely penetrant defects in neural tube closure, eye development, and gastrulation. A stronger allele (p38IP(RRK)) exhibits gastrulation defects in which mesoderm migration is defective due to deficiency in E-cadherin protein downregulation in the primitive streak. We show that p38IP binds directly to p38 and is required for p38 activation in vivo. Moreover, both p38 and p38IP are required for E-cadherin downregulation during gastrulation. Finally, p38 regulates E-cadherin protein expression downstream from NCK-interacting kinase (NIK) and independently of the regulation of transcription by Fibroblast Growth Factor (Fgf) signaling and Snail.

Lhx5 promotes forebrain development and activates transcription of secreted Wnt antagonists

In vertebrate embryos, induction and patterning of the forebrain require the local inhibition of caudalizing signals, such as Wnts, emanating from the mesendoderm and caudal brain. Here, we report that Lhx5, expressed in the rostral neuroectoderm, regulates the local inhibition of Wnts. Activation of Lhx5 expands forebrain structures, whereas inhibition of Lhx5 function compromises forebrain development in zebrafish embryos. Lhx5 can rescue forebrain deficiencies caused by excess Wnt activity, and inhibition of Lhx5 function results in ectopic activation of Wnt signaling. Lhx5 regulates the expression of two secreted Frizzled-related Wnt antagonists, Sfrp1a and Sfrp5. These Sfrps can reduce the ectopic activation of Wnt signaling and rescue the forebrain deficiencies caused by inhibition of Lhx5 function. Our results demonstrate that Lhx5 is a required factor that promotes forebrain development and inhibits Wnt signaling by activating the transcription of secreted Wnt antagonists.

Retinoic acid signalling is required for specification of pronephric cell fate

The mechanisms by which a subset of mesodermal cells are committed to a nephrogenic fate are largely unknown. In this study, we have investigated the role of retinoic acid (RA) signalling in this process using Xenopus laevis as a model system and Raldh2 knockout mice. Pronephros formation in Xenopus embryo is severely impaired when RA signalling is inhibited either through expression of a dominant-negative RA receptor, or by expressing the RA-catabolizing enzyme XCyp26 or through treatment with chemical inhibitors. Conversely, ectopic RA signalling expands the size of the pronephros. Using a transplantation assay that inhibits RA signalling specifically in pronephric precursors, we demonstrate that this signalling is required within this cell population. Timed antagonist treatments show that RA signalling is required during gastrulation for expression of Xlim-1 and XPax-8 in pronephric precursors. Moreover, experiments conducted with a protein synthesis inhibitor indicate that RA may directly regulate Xlim-1. Raldh2 knockout mouse embryos fail to initiate the expression of early kidney-specific genes, suggesting that implication of RA signalling in the early steps of kidney formation is evolutionary conserved in vertebrates.

An amphioxus LIM-homeobox gene, AmphiLim1/5, expressed early in the invaginating organizer region and later in differentiating cells of the kidney and central nervous system

A LIM-homeobox gene, AmphiLim1/5, from the Florida amphioxus (Branchiostoma floridae) encodes a protein that phylogenetic analysis positions at the base of a clade comprising vertebrate Lim1 and Lim5. Amphioxus AmphiLim1/5 is expressed in domains that are a composite of those of vertebrate Lim1 and Lim5, which evidently underwent subfunctionalization after duplication of an ancestral protochordate Lim1/5. During amphioxus development, transcription is first detected in the ectoderm of the blastula. Then, in the gastrula, a second expression domain appears in the mesendoderm just within the dorsal lip of the blastopore, a region known to have organizer properties in amphioxus. This mesendodermal expression corresponds to Lim1 expression in the Spemann organizer of vertebrates. At least one of the functions of vertebrate Lim1 in the organizer is to control the transcription of genes involved in cell and tissue movements during gastrulation, and a comparable early function seems likely for AmphiLim1/5 during gastrular invagination of amphioxus. Later embryos and larvae of amphioxus express AmphiLim1/5 in clusters of cells, probably motoneurons, in the anterior part of the central nervous system, in the hindgut, in Hatschek's right diverticulum (a rudiment of the rostral coelom), and in the wall of the first somite on the left side (a precursor of Hatschek's nephridium). In the early larva, expression continues in neural cells, in Hatschek's nephridium, in the wall of the rostral coelom, in the epidermis of the upper lip, and in mesoderm cells near the opening of the second gill slit. The developmental expression in Hatschek's nephridium is especially interesting because it helps support the homology between this amphioxus organ and the vertebrate pronephros.

Cells differentiated from mouse embryonic stem cells via embryoid bodies express renal marker molecules

Differentiation of mouse embryonic stem (ES) cells via embryoid bodies (EB) is established as a suitable model to study cellular processes of development in vitro. ES cells are known to be pluripotent because of their capability to differentiate into cell types of all three germ layers including germ cells. Here, we show that ES cells differentiate into renal cell types in vitro. We found that genes were expressed during EB cultivation, which have been previously described to be involved in renal development. Marker molecules characteristic for terminally differentiated renal cell types were found to be expressed predominantly during late stages of EB cultivation, while marker molecules involved in the initiation of nephrogenesis were already expressed during early steps of EB development. On the cellular level--using immunostaining--we detected cells expressing podocin, nephrin and wt-1, characteristic for differentiated podocytes and other cells, which expressed Tamm-Horsfall protein, a marker for distal tubule epithelial cells of kidney tissue. Furthermore, the proximal tubule marker molecules renal-specific oxido reductase, kidney androgen-related protein and 25-hydroxyvitamin D3alpha-hydroxylase were found to be expressed in EBs. In particular, we could demonstrate that cells expressing podocyte marker molecules assemble to distinct ring-like structures within the EBs. Because the differentiation efficiency into these cell types is still relatively low, application of fibroblast growth factor (FGF)-2 in combination with leukaemia inhibitory factor was tested for induction, but did not enhance ES cell-derived renal differentiation in vitro.

LIM-homeodomain genes in mammalian development and human disease

The human and mouse genomes each contain at least 12 genes encoding LIM homeodomain (LIM-HD) transcription factors. These gene regulatory proteins feature two LIM domains in their amino termini and a characteristic DNA binding homeodomain. Studies of mouse models and human patients have established that the LIM-HD factors are critical for the development of specialized cells in multiple tissue types, including the nervous system, skeletal muscle, the heart, the kidneys, and endocrine organs such as the pituitary gland and the pancreas. In this article, we review the roles of the LIM-HD proteins in mammalian development and their involvement in human diseases.

Efficient differentiation of human embryonic stem cells to definitive endoderm

The potential of human embryonic stem (hES) cells to differentiate into cell types of a variety of organs has generated much excitement over the possible use of hES cells in therapeutic applications. Of great interest are organs derived from definitive endoderm, such as the pancreas. We have focused on directing hES cells to the definitive endoderm lineage as this step is a prerequisite for efficient differentiation to mature endoderm derivatives. Differentiation of hES cells in the presence of activin A and low serum produced cultures consisting of up to 80% definitive endoderm cells. This population was further enriched to near homogeneity using the cell-surface receptor CXCR4. The process of definitive endoderm formation in differentiating hES cell cultures includes an apparent epithelial-to-mesenchymal transition and a dynamic gene expression profile that are reminiscent of vertebrate gastrulation. These findings may facilitate the use of hES cells for therapeutic purposes and as in vitro models of development.

Mice deficient in Ext2 lack heparan sulfate and develop exostoses

Hereditary multiple exostoses (HME) is a genetically heterogeneous human disease characterized by the development of bony outgrowths near the ends of long bones. HME results from mutations in EXT1 and EXT2, genes that encode glycosyltransferases that synthesize heparan sulfate chains. To study the relationship of the disease to mutations in these genes, we generated Ext2-null mice by gene targeting. Homozygous mutant embryos developed normally until embryonic day 6.0, when they became growth arrested and failed to gastrulate, pointing to the early essential role for heparan sulfate in developing embryos. Heterozygotes had a normal lifespan and were fertile; however, analysis of their skeletons showed that about one-third of the animals formed one or more ectopic bone growths (exostoses). Significantly, all of the mice showed multiple abnormalities in cartilage differentiation, including disorganization of chondrocytes in long bones and premature hypertrophy in costochondral cartilage. These changes were not attributable to a defect in hedgehog signaling, suggesting that they arise from deficiencies in other heparan sulfate-dependent pathways. The finding that haploinsufficiency triggers abnormal cartilage differentiation gives insight into the complex molecular mechanisms underlying the development of exostoses.

Eya 1 acts as a critical regulator for specifying the metanephric mesenchyme

Although it is well established that the Gdnf-Ret signal transduction pathway initiates metanephric induction, no single regulator has yet been identified to specify the metanephric mesenchyme or blastema within the intermediate mesoderm, the earliest step of metanephric kidney development and the molecular mechanisms controlling Gdnf expression are essentially unknown. Previous studies have shown that a loss of Eya 1 function leads to renal agenesis that is a likely result of failure of metanephric induction. The studies presented here demonstrate that Eya 1 specifies the metanephric blastema within the intermediate mesoderm at the caudal end of the nephrogenic cord. In contrast to its specific roles in metanephric development, Eya 1 appears dispensable for the formation of nephric duct and mesonephric tubules. Using a combination of null and hypomorphic Eya 1 mutants, we now demonstrated that approximately 20% of normal Eya 1 protein level is sufficient for establishing the metanephric blastema and inducing the ureteric bud formation but not for its normal branching. Using Eya 1, Gdnf, Six 1 and Pax 2 mutant mice, we show that Eya 1 probably functions at the top of the genetic hierarchy controlling kidney organogenesis and it acts in combination with Six 1 and Pax 2 to regulate Gdnf expression during UB outgrowth and branching. These findings uncover an essential function for Eya 1 as a critical determination factor in acquiring metanephric fate within the intermediate mesoderm and as a key regulator of Gdnf expression during ureteric induction and branching morphogenesis.

Lim 1 is required for nephric duct extension and ureteric bud morphogenesis

The nephric duct plays a central role in orchestrating the development of the mammalian urogenital system. Lim 1 is a homeobox gene required for head and urogenital development in the mouse but most Lim 1-deficient embryos die by embryonic day 10. To determine the role of Lim 1 in the development of the nephric duct, we conditionally removed Lim 1 in the nephric epithelium just after the nephric duct begins to form using a floxed allele of Lim 1 and Pax2-cre transgenic mice. We report that Lim 1 conditional knockout mice have renal hypoplasia and hydronephrosis. Developmental studies revealed that the caudal portion of the nephric duct did not reach the urogenital sinus at embryonic day 10.5, formation of the ureteric bud was delayed, the ureteric bud was smaller and branching of the ureteric bud reduced. We also found that the nephric duct was generally not maintained and extension of the Müllerian duct inhibited. Molecular analysis indicated that Pax2 was expressed normally but the expression of Wnt9b and E-cadherin in the nephric duct was markedly altered. These results suggest that Lim 1 influences nephric duct extension and ureteric bud outgrowth by regulating and or maintaining the differentiation of the nephric epithelium.

Distinct and sequential tissue-specific activities of the LIM-class homeobox gene Lim1 for tubular morphogenesis during kidney development

Kidney organogenesis requires the morphogenesis of epithelial tubules. Inductive interactions between the branching ureteric buds and the metanephric mesenchyme lead to mesenchyme-to-epithelium transitions and tubular morphogenesis to form nephrons, the functional units of the kidney. The LIM-class homeobox gene Lim1 is expressed in the intermediate mesoderm, nephric duct, mesonephric tubules, ureteric bud, pretubular aggregates and their derivatives. Lim1-null mice lack kidneys because of a failure of nephric duct formation, precluding studies of the role of Lim1 at later stages of kidney development. Here, we show that Lim1 functions in distinct tissue compartments of the developing metanephros for both proper development of the ureteric buds and the patterning of renal vesicles for nephron formation. These observations suggest that Lim1 has essential roles in multiple steps of epithelial tubular morphogenesis during kidney organogenesis. We also demonstrate that the nephric duct is essential for the elongation and maintenance of the adjacent Mullerian duct, the anlage of the female reproductive tract.

LIM-homeodomain genes as territory markers in the brainstem of adult and developing Xenopus laevis

We investigated expression patterns of the LIM-homeodomain (LIM-hd) genes x-Lhx1, x-Lhx2, x-Lhx5, and x-Lhx9 in the brainstem of Xenopus laevis during larval development and in the adult. The two groups of paralogous genes, x-Lhx1/x-Lhx5 and x-Lhx2/x-Lhx9, showed fundamentally different expression patterns, being expressed in ventral versus dorsal territories of the midbrain and hindbrain, respectively. Indeed, prominent expression of x-Lhx1/5 was found in the mesencephalic tegmentum and the hindbrain reticular formation, whereas conspicuous x-Lhx2/9 expression was observed in the torus semicircularis and isthmic nucleus. A few shared expression domains for the two pairs of paralogs included the optic tectum and the anterodorsal and pedunculopontine nuclei. In each structure, expression of the two paralogs was almost identical, indicating that the regulation of their expression in this part of the brain has evolved slightly since gene duplication occurred. Notable exceptions included the expression of x-Lhx1 but not x-Lhx5 in the Purkinje cells and the expression of x-Lhx9 but not x-Lhx2 in the lateral line nucleus. The analysis of LIM-hd expression patterns throughout development allowed the origin of given structures in early embryos to be traced back. x-Lhx1 and x-Lhx5 were relevant to locate the cerebellar anlage and to follow morphogenesis of the cerebellar plate and cerebellar nuclei. They also highlighted the rhombomeric organization of the hindbrain. On the other hand, x-Lhx2 and x-Lhx9 showed a dynamic spatiotemporal pattern relative to tectal development and layering, and x-Lhx9 was useful to trace back the origin of the isthmus in early development.

Overexpression of connexin43 alters the mutant phenotype of midgestational wnt-1 null mice resulting in recovery of the midbrain and cerebellum

The midbrain-hindbrain (MHB) junction plays a key role in the patterning of the embryonic neural tube and the formation of brain structures such as the cerebellum. The mitogen wnt-1 is critical for cerebellar development, as evidenced by the lack of MHB region and cerebellar formation in the wnt-1 null embryo. We have generated wnt-1 null embryos overexpressing the gap junction gene connexin43 by crossing wnt-1 null heterozygotes into the CMV43 mouse line. We have confirmed that these mice show an increase in gap junctional communication by dye coupling analysis. Two-thirds of wnt-1 null CMV43(+) mouse embryos at E18.5 have a cerebellum. In addition, changes in the wnt-1 null phenotype in mouse embryos overexpressing connexin43 are observed as early as E9.5. At this stage, one-quarter of wnt-1 null CMV43(+) embryos display extra or expanded tissue present at the MHB boundary (a wnt-1 null enlarged phenotype). In situ hybridization studies conducted on these embryos have indicated no changes in the expression of embryonic brain positional markers in this region. We conclude from these studies that overexpression of the connexin43 gap junction restores cerebellar formation by compensating for the loss of wnt-1.

Developmental gene expression profiling of mammalian, fetal orofacial tissue

BACKGROUND

The embryonic orofacial region is an excellent developmental paradigm that has revealed the centrality of numerous genes encoding proteins with diverse and important biological functions in embryonic growth and morphogenesis. DNA microarray technology presents an efficient means of acquiring novel and valuable information regarding the expression, regulation, and function of a panoply of genes involved in mammalian orofacial development.

METHODS

To identify differentially expressed genes during mammalian orofacial ontogenesis, the transcript profiles of GD-12, GD-13, and GD-14 murine orofacial tissue were compared utilizing GeneChip arrays from Affymetrix. Changes in gene expression were verified by TaqMan quantitative real-time PCR. Cluster analysis of the microarray data was done with the GeneCluster 2.0 Data Mining Tool and the GeneSpring software.

RESULTS

Expression of >50% of the approximately 12,000 genes and expressed sequence tags examined in this study was detected in GD-12, GD-13, and GD-14 murine orofacial tissues and the expression of several hundred genes was up- and downregulated in the developing orofacial tissue from GD-12 to GD-13, as well as from GD-13 to GD-14. Such differential gene expression represents changes in the expression of genes encoding growth factors and signaling molecules; transcription factors; and proteins involved in epithelial-mesenchymal interactions, extracellular matrix synthesis, cell adhesion, proliferation, differentiation, and apoptosis. Following cluster analysis of the microarray data, eight distinct patterns of gene expression during murine orofacial ontogenesis were selected for graphic presentation of gene expression patterns.

CONCLUSIONS

This gene expression profiling study identifies a number of potentially unique developmental participants and serves as a valuable aid in deciphering the complex molecular mechanisms crucial for mammalian orofacial development.

Ssdp1 regulates head morphogenesis of mouse embryos by activating the Lim1-Ldb1 complex

The transcriptional activity of LIM-homeodomain (LIM-HD) proteins is regulated by their interactions with various factors that bind to the LIM domain. We show that reduced expression of single-stranded DNA-binding protein 1 (Ssdp1), which encodes a co-factor of LIM domain interacting protein 1 (Ldb1), in the mouse mutant headshrinker (hsk) disrupts anterior head development by partially mimicking Lim1 mutants. Although the anterior visceral endoderm and the anterior definitive endoderm, which together comprise the head organizer, were able to form normally in Ssdp1(hsk/hsk) mutants, development of the prechordal plate was compromised. Head development is partially initiated in Ssdp1(hsk/hsk) mutants, but neuroectoderm tissue anterior to the midbrain-hindbrain boundary is lost, without a concomitant increase in apoptosis. Cell proliferation is globally reduced in Ssdp1(hsk/hsk) mutants, and approximately half also exhibit smaller body size, similar to the phenotype observed in Lim1 and Ldb1 mutants. We also show that Ssdp1 contains an activation domain and is able to enhance transcriptional activation through a Lim1-Ldb1 complex in transfected cells, and that Ssdp1 interacts genetically with Lim1 and Ldb1 in both head development and body growth. These results suggest that Ssdp1 regulates the development of late head organizer tissues and body growth by functioning as an essential activator component of a Lim1 complex through interaction with Ldb1.

Transcript profiling of Wilms tumors reveals connections to kidney morphogenesis and expression patterns associated with anaplasia

Anaplasia (unfavorable histology) is associated with therapy resistance and poor prognosis of Wilms tumor, but the molecular basis for this phenotype is unclear. Here, we used a cDNA array with 9240 clones relevant to cancer biology and/or kidney development to examine the expression profiles of 54 Wilms tumors, five normal kidneys and fetal kidney. By linking genes differentially expressed between fetal kidney and Wilms tumors to kidney morphogenesis, we found that genes expressed at a higher level in Wilms tumors tend to be expressed more in uninduced metanephrogenic mesenchyme or blastema than in their differentiated structures. Conversely, genes expressed at a lower level in Wilms tumors tend to be expressed less in uninduced metanephrogenic mesenchyme or blastema. We also identified 97 clones representing 76 Unigenes or unclustered ESTs that clearly separate anaplastic Wilms tumors from tumors with favorable histology. Genes in this set provide insight into the nature of the abnormal nuclear morphology of anaplastic tumors and may facilitate identification of molecular targets to improve their responsiveness to treatment.

Hypoblast controls mesoderm generation and axial patterning in the gastrulating rabbit embryo

Gastrulation in higher vertebrate species classically commences with the generation of mesoderm cells in the primitive streak by epithelio-mesenchymal transformation of epiblast cells. However, the primitive streak also marks, with its longitudinal orientation in the posterior part of the conceptus, the anterior-posterior (or head-tail) axis of the embryo. Results obtained in chick and mouse suggest that signals secreted by the hypoblast (or visceral endoderm), the extraembryonic tissue covering the epiblast ventrally, antagonise the mesoderm induction cascade in the anterior part of the epiblast and thereby restrict streak development to the posterior pole (and possibly initiate head development anteriorly). In this paper we took advantage of the disc-shape morphology of the rabbit gastrula for defining the expression compartments of the signalling molecules Cerberus and Dickkopf at pre-gastrulation and early gastrulation stages in a mammal other than the mouse. The two molecules are expressed in novel expression compartments in a complementary fashion both in the hypoblast and in the emerging primitive streak. In loss-of-function experiments, carried out in a New-type culturing system, hypoblast was removed prior to culture at defined stages before and at the beginning of gastrulation. The epiblast shows a stage-dependent and topographically restricted susceptibility to express Brachyury, a T-box gene pivotal for mesoderm formation, and to transform into (histologically proven) mesoderm. These results confirm for the mammalian embryo that the anterior-posterior axis of the conceptus is formed first as a molecular prepattern in the hypoblast and then irrevocably fixed, under the control of signals secreted from the hypoblast, by epithelio-mesenchymal transformation (primitive streak formation) in the epiblast.

TGFβ superfamily signals are required for morphogenesis of the kidney mesenchyme progenitor population

The TGFβ superfamily plays diverse and essential roles in kidney development. Gdf11 and Bmp4 are essential for outgrowth and positioning of the ureteric bud, the inducer of metanephric mesenchyme. During nephrogenesis, Bmp7 is required for renewal of the mesenchyme progenitor population. Additionally, in vitro studies demonstrate inhibitory effects of BMPs and TGFβs on collecting duct branching and growth. Here,we explore the predicted models of TGFβ superfamily function by cell-specific inactivation of Smad4, a key mediator of TGFβsignaling. Using a HoxB7cre transgene expressed in ureteric bud and collecting duct, we find that development of the collecting duct is Smad4 independent. By contrast, removal of Smad4 in nephrogenic mesenchyme using the Bmp7cre/+ allele leads to disorganization of the nephrogenic mesenchyme and impairment of mesenchyme induction. Smad4-deficient metanephric mesenchyme does not display defects in inducibility in LiCl or spinal cord induction assays. However, in situ hybridization and lineage analysis of Smad4 null mesenchyme cells at E11.5 show that the nephrogenic mesenchyme does not aggregate tightly around the ureteric bud tips, but remains loosely associated, embedded within a population of cells expressing markers of both nephrogenic mesenchyme and peripheral stroma. We conclude that the failure of recruitment of nephrogenic mesenchyme leaves a primitive population of mesenchyme at the periphery of the kidney. This population is gradually depleted, and by E16.5 the periphery is composed of cells of stromal phenotype. This study uncovers a novel role for TGFβ superfamily signaling in the recruitment and/or organization of the nephrogenic mesenchyme at early time-points of kidney development. Additionally, we present conclusive genetic lineage mapping of the collecting duct and nephrogenic mesenchyme.

The forkhead genes, Foxc1 and Foxc2, regulate paraxial versus intermediate mesoderm cell fate

During vertebrate embryogenesis, the newly formed mesoderm is allocated to the paraxial, intermediate, and lateral domains, each giving rise to different cell and tissue types. Here, we provide evidence that the forkhead genes, Foxc1 and Foxc2, play a role in the specification of mesoderm to paraxial versus intermediate fates. Mouse embryos lacking both Foxc1 and Foxc2 show expansion of intermediate mesoderm markers into the paraxial domain, lateralization of somite patterning, and ectopic and disorganized mesonephric tubules. In gain of function studies in the chick embryo, Foxc1 and Foxc2 negatively regulate intermediate mesoderm formation. By contrast, their misexpression in the prospective intermediate mesoderm appears to drive cells to acquire paraxial fate, as revealed by expression of the somite markers Pax7 and Paraxis. Taken together, the data indicate that Foxc1 and Foxc2 regulate the establishment of paraxial versus intermediate mesoderm cell fates in the vertebrate embryo.

Differential requirements for Smad4 in TGFbeta-dependent patterning of the early mouse embryo

Genetic and biochemical data have identified Smad4 as a key intracellular effector of the transforming growth factor beta (TGFbeta superfamily of secreted ligands. In mouse, Smad4-null embryos do not gastrulate, a phenotype consistent with loss of other TGFbeta-related signaling components. Chimeric analysis reveals a primary requirement for Smad4 in the extra-embryonic lineages; however, within the embryo proper, characterization of the specific roles of Smad4 during gastrulation and lineage specification remains limited. We have employed a Smad4 conditional allele to specifically inactivate the Smad4 gene in the early mouse epiblast. Loss of Smad4 in this tissue results in a profound failure to pattern derivatives of the anterior primitive streak, such as prechordal plate, node, notochord and definitive endoderm. In contrast to these focal defects, many well-characterized TGFbeta- and Bmp-regulated processes involved in mesoderm formation and patterning are surprisingly unaffected. Mutant embryos form abundant extra-embryonic mesoderm, including allantois, a rudimentary heart and middle primitive streak derivatives such as somites and lateral plate mesoderm. Thus, loss of Smad4 in the epiblast results not in global developmental abnormalities but instead in restricted patterning defects. These results suggest that Smad4 potentiates a subset of TGFbeta-related signals during early embryonic development, but is dispensable for others.

Whole-embryo culture of E5.5 mouse embryos: development to the gastrulation stage

This study reports establishment of an in vitro culture system for E5.5 mouse embryos that supports development to the gastrulation stage and allows the use of experimental approaches to study gastrulation during mouse embryogenesis. Recent experiments suggest that the extraembryonic tissues may play a critical role for gastrulation from as early as E5.5. To apply whole embryo culture to E5.5 embryos and analyze gastrulation, it is essential to optimize the conditions so that most of the embryos develop to the gastrulation stage in culture. For this purpose, we established a protocol in which embryos were isolated using micromanipulator and cultured with 50-75% rat serum. Although cultured embryos tended to grow a larger extraembryonic portion, more than 80% of them developed the primitive streak and induce mesoderm, which corresponds to the mid-streak stage.

Comprehensive transcriptome analysis of differentiation of embryonic stem cells into midbrain and hindbrain neurons

Neurogenesis is one of the most complex events in embryonic development. However, little information is available regarding the molecular events that occur during neurogenesis. To identify regulatory genes and underlying mechanisms involved in the differentiation of embryonic stem (ES) cells to neurons, gene expression profiling was performed using cDNA microarrays. In mouse ES cells, we compared the gene expression of each differentiated cell stage using a five-stage lineage selection method. Of 10,368 genes, 1633 (16%) known regulatory genes were differentially expressed at least 2-fold or greater at one or more stages. At stage 3, during which ES cells differentiate into neural stem cells, modulation of nearly 1000 genes was observed. Most of transcription factors (Otx2, Ebf-3, Ptx3, Sox4, 13, 18, engrailed, Irx2, Pax8, and Lim3), signaling molecules (Wnt, TGF, and Shh family members), and extracellular matrix/adhesion molecules (collagens, MAPs, and NCAM) were up-regulated. However, some genes which may play important roles in maintaining the pluripotency of ES cells (Kruppel-like factor 2, 4, 5, 9, myeloblast oncogene like2, ZFP 57, and Esg-1) were down-regulated. The many genes identified with this approach that are modulated during neurogenesis will facilitate studies of the mechanisms underlying ES cell differentiation, neural induction, and neurogenesis.

Requirement of Lim1 for female reproductive tract development

Lim1 encodes a LIM-class homeodomain transcription factor that is essential for head and kidney development. In the developing urogenital system, Lim1 expression has been documented in the Wolffian (mesonephric) duct, the mesonephros, metanephros and fetal gonads. Using, a Lim1 lacZ knock-in allele in mice, we identified a previously unreported urogenital tissue for Lim1 expression, the epithelium of the developing Müllerian duct that gives rise to the oviduct, uterus and upper region of the vagina of the female reproductive tract. Lim1 expression in the Müllerian duct is dynamic, corresponding to its formation and differentiation in females and regression in males. Although female Lim1-null neonates had ovaries they lacked a uterus and oviducts. A novel female mouse chimera assay was developed and revealed that Lim1 is required cell autonomously for Müllerian duct epithelium formation. These studies demonstrate an essential role for Lim1 in female reproductive tract development.

The orphan steroid receptor Nur77 family member Nor-1 is essential for early mouse embryogenesis

Nur77 and its family members, Nor-1 and Nurr1, are orphan steroid receptors implicated in a wide variety of biological processes, including apoptosis and dopamine neuron agenesis. Expression of these family members can be detected at low levels in many tissues but they are expressed at very high levels when cells are stimulated by outside signals, including serum, nerve growth factor, and receptor engagement. Introduction of a dominant negative Nur77 protein that blocks the activities of all family members led to inhibition of apoptosis in T cells. Nur77-deficient mice, however, exhibit no phenotype, and a line of Nor-1 mutant mice was reported to exhibit a mild ear development phenotype but no other gross abnormalities. Here, we report the generation of Nor-1-deficient mice with a block in early embryonic development. Nor-1 is expressed early during embryogenesis, and its loss leads to embryonic lethality around embryonic day 8.5 of gestation. The mutant embryos fail to complete gastrulation and display distinct morphological abnormalities, including a decrease in overall size, developmental delay and an accumulation of mesoderm in the primitive streak during gastrulation. Abnormal expression of a number of early developmental markers and defects in growth or distribution of emerging mesoderm cells were also detected. These data suggest that Nor-1 plays a crucial role in gastrulation.

Specification of germ cell fate in mice

An early fundamental event during development is the segregation of germ cells from somatic cells. In many organisms, this is accomplished by the inheritance of preformed germ plasm, which apparently imposes transcriptional repression to prevent somatic cell fate. However, in mammals, pluripotent epiblast cells acquire germ cell fate in response to signalling molecules. We have used single cell analysis to study how epiblast cells acquire germ cell competence and undergo specification. Germ cell competent cells express Fragilis and initially progress towards a somatic mesodermal fate. However, a subset of these cells, the future primordial germ cells (PGCs), then shows rapid upregulation of Fragilis with concomitant transcriptional repression of a number of genes, including Hox and Smad genes. This repression may be a key event associated with germ cell specification. Furthermore, PGCs express Stella and other genes, such as Oct-4 that are associated with pluripotency. While these molecules are also detected in mature oocytes as maternally inherited factors, their early role is to regulate development and maintain pluripotency, and they do not serve the role of classical germline determinants.

Sexy transgenes: the impact of gene transfer and gene inactivation technologies on the understanding of mammalian sex determination

Amongst the various developmental pathways ending in a sound mammal, sex determination presents the peculiarity of a choice between two equally viable options: female or male. Therefore, destroying a 'male-determining gene' or a 'female-determining gene' should generally not be lethal. Genetic sex determination is divided into two consecutive steps: construction of the bipotential gonad, and then sex determination per se. The genes involved in the first step are in fact involved in the development of various body compartments, and their mutation is generally far from innocuous. From transgenic and inactivation studies carried out on the laboratory mouse, a complete picture of the two steps is beginning to emerge, where the gonad itself and the necessary ducts are shown to evolve in a very coordinate way, with well-defined sex-specificities. Compared with testis determination, the ovarian side of the picture is still relatively empty, but this situation can change rapidly as candidate ovarian genes for inactivation studies are beginning to be identified.

Sry expression level and protein isoform differences play a role in abnormal testis development in C57BL/6J mice carrying certain Sry alleles

Transfer of certain Mus domesticus-derived Y chromosomes (Sry(DOM) alleles, e.g., Sry(POS) and Sry(AKR)) onto the C57BL/6J (B6) mouse strain causes abnormal gonad development due to an aberrant interaction between the Sry(DOM) allele and the B6-derived autosomal (tda) genes. For example, B6 XY(POS) fetuses develop ovaries and ovotestes and B6 XY(AKR) fetuses have delayed testis cord development. To test whether abnormal testis development is caused by insufficient Sry(DOM) expression, two approaches were used. First, gonad development and relative Sry expression levels were examined in fetal gonads from two strains of B6 mice that contained a single M. domesticus-derived and a single M. musculus-derived Sry allele (B6-Y(POS,RIII) and B6-Y(AKR,RIII)). In both cases, presence of the M. musculus Sry(RIII) allele corrected abnormal testis development. On the B6 background, Sry(POS) was expressed at about half the level of Sry(RIII) whereas Sry(AKR) and Sry(RIII) were equally expressed. On an F(1) hybrid background, both Sry(POS) and Sry(RIII) expression increased, but Sry(POS) expression increased to a greater extent. Second, sexual development and Sry expression levels were determined in XX mice carrying a transgene expressing Sry(POS) controlled by POS-derived or MUS-derived regulatory regions. In both cases one B6 transgenic line was recovered in which XX transgenic mice developed only testicular tissue but cord development was delayed despite normal Sry transcriptional initiation and overexpression. For three transgenes where B6 XX transgenic mice developed as females, hermaphrodites, or males, the percentage of XX transgenic males increased on an F(1) background. For the one transgene examined, Sry expression increased on an F(1) background. These results support a model in which delayed testis development is caused by the presence of particular DOM SRY protein isoforms and this, combined with insufficient Sry expression, causes sex reversal. These results also indicate that at least one tda gene regulates Sry expression, possibly by directly binding to Sry regulatory regions.

Angiomotin regulates visceral endoderm movements during mouse embryogenesis

In pregastrula stage mouse embryos, visceral endoderm (VE) migrates from a distal to anterior position to initiate anterior identity in the adjacent epiblast. This anterior visceral endoderm (AVE) is then displaced away from the epiblast by the definitive endoderm to become associated with the extra-embryonic ectoderm and subsequently contributes to the yolk sac. Little is known about the molecules that regulate this proximal displacement. Here we describe a role for mouse angiomotin (amot) in VE movements. amot expression is initially detected in the AVE and subsequently in the VE associated with the extra-embryonic ectoderm. Most amot mutant mice die soon after gastrulation with distinct furrows of VE located at the junction of the embryonic and extra-embryonic regions. Mutant analysis suggests that VE accumulation in these furrows is caused by defects in cell migration into proximal extra-embryonic regions, although distal-to-anterior movements associated with the epiblast, definitive endoderm formation, and anterior specification of the epiblast appear to be normal. These results suggest that amot acts within subregions of the VE to regulate morphogenetic movements that are required for embryo viability.

Regulation of the Lim-1 gene is mediated through conserved FAST-1/FoxH1 sites in the first intron

The Lim-1 gene encodes a LIM-homeodomain transcription factor that is highly conserved among vertebrates and is required for successful gastrulation and head formation. The expression of this gene in the mesoderm of the gastrula is known to require an activin/nodal signal. Earlier studies have shown that the Xenopus Lim-1 (Xlim-1) gene contains an activin response element (ARE) in its first intron, which cooperates with an activin-unresponsive upstream promoter in the regulation of the gene. Here, we show that the Xlim-1 ARE contains a cluster of FAST-1/FoxH1 and Smad4 recognition sites; such sites have been shown to mediate activin/nodal responses in other genes. By using reporter constructs with mutated FAST-1/FoxH1 sites and FAST-1/FoxH1 protein chimeras, we show that the regulation of Xlim-1 by activin depends on FAST-1/FoxH1 function. Comparative studies on the zebrafish lim1 gene indicate the presence of FoxH1 sites in the first intron of this gene and provide evidence for the requirement for FoxH1 function in its regulation. These results illuminate the conserved nature of the transcriptional regulation of the Lim-1 gene in different vertebrate animals.

Nephric lineage specification by Pax2 and Pax8

The mammalian kidney develops in three successive steps from the initial pronephros via the mesonephros to the adult metanephros. Although the nephric lineage is specified during pronephros induction, no single regulator, including the transcription factor Pax2 or Pax8, has yet been identified to control this initial phase of kidney development. In this paper, we demonstrate that mouse embryos lacking both Pax2 and Pax8 are unable to form the pronephros or any later nephric structures. In these double-mutant embryos, the intermediate mesoderm does not undergo the mesenchymal-epithelial transitions required for nephric duct formation, fails to initiate the kidney-specific expression of Lim1 and c-Ret, and is lost by apoptosis 1 d after failed pronephric induction. Conversely, retroviral misexpression of Pax2 was sufficient to induce ectopic nephric structures in the intermediate mesoderm and genital ridge of chick embryos. Together, these data identify Pax2 and Pax8 as critical regulators that specify the nephric lineage.

Requirement for Foxd3 in maintaining pluripotent cells of the early mouse embryo

Critical to our understanding of the developmental potential of stem cells and subsequent control of their differentiation in vitro and in vivo is a thorough understanding of the genes that control stem cell fate. Here, we report that Foxd3, a member of the forkhead family of transcriptional regulators, is required for maintenance of embryonic cells of the early mouse embryo. Foxd3-/- embryos die after implantation at approximately 6.5 days postcoitum with a loss of epiblast cells, expansion of proximal extraembryonic tissues, and a distal, mislocalized anterior organizing center. Moreover, it has not been possible to establish Foxd3-/- ES cell lines or to generate Foxd3-/- teratocarcinomas. Chimera analysis reveals that Foxd3 function is required in the epiblast and that Foxd3-/- embryos can be rescued by a small number of wild-type cells. Foxd3-/- mutant blastocysts appear morphologically normal and express Oct4, Sox2, and Fgf4, but when placed in vitro the inner cell mass initially proliferates and then fails to expand even when Fgf4 is added. These results establish Foxd3 as a factor required for the maintenance of progenitor cells in the mammalian embryo.

Gonadal differentiation, sex determination and normalSryexpression in mice require direct interaction between transcription partners GATA4 and FOG2

In mammals, Sry expression in the bipotential, undifferentiated gonad directs the support cell precursors to differentiate as Sertoli cells, thus initiating the testis differentiation pathway. In the absence of Sry, or if Sry is expressed at insufficient levels, the support cell precursors differentiate as granulosa cells, thus initiating the ovarian pathway. The molecular mechanisms upstream and downstream of Sry are not well understood. We demonstrate that the transcription factor GATA4 and its co-factor FOG2 are required for gonadal differentiation. Mouse fetuses homozygous for a null allele of Fog2 or homozygous for a targeted mutation in Gata4 (Gata4ki) that abrogates the interaction of GATA4 with FOG co-factors exhibit abnormalities in gonadogenesis. We found that Sry transcript levels were significantly reduced in XY Fog2–/– gonads at E11.5, which is the time when Sry expression normally reaches its peak. In addition, three genes crucial for normal Sertoli cell function (Sox9, Mis and Dhh) and three Leydig cell steroid biosynthetic enzymes (p450scc, 3βHSD and p450c17) were not expressed in XY Fog2–/– and Gataki/ki gonads, whereas Wnt4, a gene required for normal ovarian development, was expressed ectopically. By contrast, Wt1 and Sf1, which are expressed prior to Sry and necessary for gonad development in both sexes, were expressed normally in both types of mutant XY gonads. These results indicate that GATA4 and FOG2 and their physical interaction are required for normal gonadal development.

A molecular programme for the specification of germ cell fate in mice

Germ cell fate in mice is induced in proximal epiblast cells by the extra-embryonic ectoderm, and is not acquired through the inheritance of any preformed germ plasm. To determine precisely how germ cells are specified, we performed a genetic screen between single nascent germ cells and their somatic neighbours that share common ancestry. Here we show that fragilis, an interferon-inducible transmembrane protein, marks the onset of germ cell competence, and we propose that through homotypic association, it demarcates germ cells from somatic neighbours. Using single-cell gene expression profiles, we also show that only those cells with the highest expression of fragilis subsequently express stella, a gene that we detected exclusively in lineage-restricted germ cells. The stella positive nascent germ cells exhibit repression of homeobox genes, which may explain their escape from a somatic cell fate and the retention of pluripotency.

The Foxh1-dependent autoregulatory enhancer controls the level of Nodal signals in the mouse embryo

The TGFβ-related growth factor Nodal governs anteroposterior (AP) and left-right (LR) axis formation in the vertebrate embryo. A conserved intronic enhancer (ASE), containing binding sites for the fork head transcription factor Foxh1, modulates dynamic patterns of Nodal expression during early mouse development. This enhancer is responsible for early activation of Nodal expression in the epiblast and visceral endoderm, and at later stages governs asymmetric expression during LR axis formation. We demonstrate ASE activity is strictly Foxh1 dependent. Loss of this autoregulatory enhancer eliminates transcription in the visceral endoderm and decreases Nodal expression in the epiblast, but causes surprisingly discrete developmental abnormalities. Thus lowering the level of Nodal signaling in the epiblast disrupts both orientation of the AP axis and specification of the definitive endoderm. Targeted removal of the ASE also dramatically reduces left-sided Nodal expression, but the early events controlling LR axis specification are correctly initiated. However loss of the ASE disrupts Lefty2 (Leftb) expression and causes delayed Pitx2 expression leading to late onset, relatively minor LR patterning defects. The feedback loop is thus essential for maintenance of Nodal signals that selectively regulate target gene expression in a temporally and spatially controlled fashion in the mouse embryo.

Activating transcription factor 1 and CREB are important for cell survival during early mouse development

Activating transcription factor 1 (ATF1), CREB, and the cyclic AMP (cAMP) response element modulatory protein (CREM), which constitute a subfamily of the basic leucine zipper transcription factors, activate gene expression by binding as homo- or heterodimers to the cAMP response element in regulatory regions of target genes. To investigate the function of ATF1 in vivo, we inactivated the corresponding gene by homologous recombination. In contrast to CREB-deficient mice, which suffer from perinatal lethality, mice lacking ATF1 do not exhibit any discernible phenotypic abnormalities. Since ATF1 and CREB but not CREM are strongly coexpressed during early mouse development, we generated mice deficient for both CREB and ATF1. ATF1(-/-) CREB(-/-) embryos die before implantation due to developmental arrest. ATF1(+/-) CREB(-/-) embryos display a phenotype of embryonic lethality around embryonic day 9.5 due to massive apoptosis. These results indicate that CREB and ATF1 act in concert to mediate signals essential for maintaining cell viability during early embryonic development.

Maintenance of the specification of the anterior definitive endoderm and forebrain depends on the axial mesendoderm: a study using HNF3beta/Foxa2 conditional mutants

In mouse embryo, the early induction of the head region depends on signals from the anterior visceral endoderm (AVE) and the anterior primitive streak. Subsequently, node derivatives, including anterior definitive endoderm and axial mesendoderm, are thought to play a role in the maintenance and elaboration of anterior neural character. Foxa2 encodes a winged-helix transcription factor expressed in signaling centers required for head development, including the AVE, anterior primitive streak, anterior definitive endoderm, and axial mesendoderm. To address Foxa2 function during formation of the head, we used conditional mutants in which Foxa2 function is preserved in extraembryonic tissues during early embryonic stages and inactivated in embryonic tissues after the onset of gastrulation. In Foxa2 conditional mutants, the anterior neural plate and anterior definitive endoderm were initially specified. In contrast, the axial mesendoderm failed to differentiate. At later stages, specification of the anterior neural plate and anterior definitive endoderm was shown to be labile. As a result, head truncations were observed in Foxa2 conditional mutants. Our results therefore indicate that anterior definitive endoderm alone is not sufficient to maintain anterior head specification and that an interaction between the axial mesendoderm and the anterior definitive endoderm is required for proper specification of the endoderm. Foxa2 therefore plays an integral role in the formation of axial mesendoderm, which is required to maintain the specification of the forebrain and the anterior definitive endoderm.

The genetics of male undermasculinization

A review of the genetics of male undermasculinization must encompass a description of the embryology of the genital system. The dimorphism of sex development consequent upon the formation of a testis and the subsequent secretion of hormones to impose a male phenotype is highlighted. Thus, an understanding of the causes of male undermasculinization (manifest as XY sex reversal, complete and partial) includes reviewing the genetic factors which control testis determination and the production and action of testicular hormones. The study of disorders of male sex development has contributed substantially to knowledge of normal male development before birth. This knowledge has been complimented in recent years by the use of targeted murine gene disruption experiments to study the sex phenotype, although murine and human phenotypes are not always concordant. The investigation of disorders associated with male undermasculinization of prenatal onset is described briefly to complete the review.

The mouse snail gene encodes a key regulator of the epithelial-mesenchymal transition

Snail family genes encode DNA binding zinc finger proteins that act as transcriptional repressors. Mouse embryos deficient for the Snail (Sna) gene exhibit defects in the formation of the mesoderm germ layer. In Sna(-/-) mutant embryos, a mesoderm layer forms and mesodermal marker genes are induced but the mutant mesoderm is morphologically abnormal. Lacunae form within the mesoderm layer of the mutant embryos, and cells lining these lacunae retain epithelial characteristics. These cells resemble a columnar epithelium and have apical-basal polarity, with microvilli along the apical surface and intercellular electron-dense adhesive junctions that resemble adherens junctions. E-cadherin expression is retained in the mesoderm of the Sna(-/-) embryos. These defects are strikingly similar to the gastrulation defects observed in snail-deficient Drosophila embryos, suggesting that the mechanism of repression of E-cadherin transcription by Snail family proteins may have been present in the metazoan ancestor of the arthropod and mammalian lineages.

Low levels of Sry transcripts cannot be the sole cause of B6-Y(TIR) sex reversal

Sry, a single-copy gene on the Y-chromosome, triggers the fetal gonad to begin testis differentiation in mammals. On the other hand, mutation or absence of Sry results in ovary differentiation and the female phenotype. However, cases of XY sex reversal in the presence of wild-type Sry exist in mice and man. One such example is the B6-Y(TIR) mouse, whose autosomes and X-chromosome are from the C57BL/6J mouse (an inbred strain of Mus musculus molossinus), whereas the Y-chromosome is from a Mus musculus domesticus mouse originating in Tirano, Italy. The B6-Y(TIR) mouse never develops normal testes and instead develops ovaries or ovotestes in fetal life. It has been suggested that low levels of Sry transcription may account for the aberrant testis differentiation in the B6-Y(TIR) mouse. In this study, however, we observed relatively low levels of Sry transcripts not only in B6-Y(TIR) but also in B6 mice, which develop normal testes. We conclude that low dosage of Sry transcripts cannot be the sole cause of sex reversal in the B6-Y(TIR) gonad.

C57BL/6J-T-associated sex reversal in mice is caused by reduced expression of a Mus domesticus Sry allele

C57BL/6J-T-associated sex reversal (B6-TAS) in XY mice results in ovarian development and involves (1) hemizygosity for Tas, a gene located in the region of Chromosome 17 deleted in T(hp) and T(Orl), (2) homozygosity for one or more B6-derived autosomal genes, and (3) the presence of the AKR Y chromosome. Here we report results from experiments designed to investigate the Y chromosome component of this sex reversal. Testis development was restored in B6 T(Orl)/+ XY(AKR) mice carrying a Mus musculus Sry transgene. In addition, two functionally different classes of M. domesticus Sry alleles were identified among eight standard and two wild-derived inbred strains. One class, which includes AKR, did not initiate normal testis development in B6 T(Orl)/+ XY mice, whereas the other did. DNA sequence analysis of the Sry ORF and a 5' 800-bp segment divided these inbred strains into the same groups. Finally, we found that Sry is transcribed in B6 T(Orl)/+ XY(AKR) fetal gonads but at a reduced level. These results pinpoint Sry as the Y-linked component of B6-TAS. We hypothesize that the inability of specific M. domesticus Sry alleles to initiate normal testis development in B6 T(Orl)/+ XY(AKR) mice results from a biologically insufficient level of Sry expression, allowing the ovarian development pathway to proceed.

Emx2directs the development of diencephalon in cooperation withOtx2

The vertebrate brain is among the most complex biological structures of which the organization remains unclear. Increasing numbers of studies have accumulated on the molecular basis of midbrain/hindbrain development, yet relatively little is known about forebrain organization. Nested expression among Otx and Emx genes has implicated their roles in rostral brain regionalization, but single mutant phenotypes of these genes have not provided sufficient information. In order to genetically determine the interaction between Emx and Otx genes in forebrain development, we have examined Emx2−/−Otx2+/− double mutants and Emx2 knock-in mutants into the Otx2 locus (Otx2+/Emx2). Emx2−/−Otx2+/− double mutants did not develop diencephalic structures such as ventral thalamus, dorsal thalamus/epithalamus and anterior pretectum. The defects were attributed to the loss of the Emx2-positive region at the three- to four-somite stage, when its expression occurs in the laterocaudal forebrain primordia. Ventral structures such as the hypothalamus, mammillary region and tegmentum developed normally. Moreover, dorsally the posterior pretectum and posterior commissure were also present in the double mutants. In contrast, Otx2+/Emx2 knock-in mutants displayed the majority of these diencephalic structures; however, the posterior pretectum and posterior commissure were specifically absent. Consequently, development of the dorsal and ventral thalamus and anterior pretectum requires cooperation between Emx2 and Otx2, whereas Emx2 expression is incompatible with development of the commissural region of the pretectum.

Mesodermal patterning defect in mice lacking the Ste20 NCK interacting kinase (NIK)

We have previously shown that the Drosophila Ste20 kinase encoded by misshapen (msn) is an essential gene in Drosophila development. msn function is required to activate the Drosophila c-Jun N-terminal kinase (JNK), basket (Bsk), to promote dorsal closure of the Drosophila embryo. Later in development, msn expression is required in photoreceptors in order for their axons to project normally. A mammalian homolog of msn, the NCK-interacting kinase (NIK) (recently renamed to mitogen-activated protein kinase kinase kinase kinase 4; Map4k4), has been shown to activate JNK and to bind the SH3 domains of the SH2/SH3 adapter NCK. To determine whether NIK also plays an essential role in mammalian development, we created mice deficient in NIK by homologous recombination at the Nik gene. Nik(−/−) mice die postgastrulation between embryonic day (E) 9.5 and E10.5. The most striking phenotype in Nik(−/−) embryos is the failure of mesodermal and endodermal cells that arise from the anterior end of the primitive streak (PS) to migrate to their correct location. As a result Nik(−/−)embryos fail to develop somites or a hindgut and are truncated posteriorly. Interestingly, chimeric analysis demonstrated that NIK has a cell nonautonomous function in stimulating migration of presomitic mesodermal cells away from the PS and a second cell autonomous function in stimulating the differentiation of presomitic mesoderm into dermomyotome. These findings indicate that despite the large number of Ste20 kinases in mammalian cells, members of this family play essential nonredundant function in regulating specific signaling pathways. In addition, these studies provide evidence that the signaling pathways regulated by these kinases are diverse and not limited to the activation of JNK because mesodermal and somite development are not perturbed in JNK1-, and JNK2-deficient mice.

The role of prechordal mesendoderm in neural patterning

Prechordal mesendoderm is formed in response to Nodal and maternal beta-Catenin signaling and is regulated by signals from anterior endoderm and chordamesoderm. Prechordal mesendodermal cells are involved in neural induction and in anteroposterior and dorsoventral neural patterning. Inhibitors of Wnt and BMP growth factors secreted by prechordal mesendoderm mediate neural induction and anteroposterior and dorsoventral patterning, whereas SHH and TGF betas mediate dorsoventral patterning.

Functional domains of the LIM homeodomain protein Xlim-1 involved in negative regulation, transactivation, and axis formation in Xenopus embryos

The Xenopus LIM homeodomain protein Xlim-1 is specifically expressed in the Spemann organizer region and assumed to play a role in the establishment of the body axis as a transcriptional activator. To further elucidate the mechanism underlying the regulation of its transcriptional activity, we focused on the region C-terminal to the homeodomain of Xlim-1 (CT239-403) and divided it into five regions, CCR1-5 (C-terminal conserved regions), based on similarity between Xlim-1 and its paralog, Xlim-5. The role of Xlim-1 CT239-403 in the Spemann organizer was analyzed by assaying the axis-forming ability of a series of CCR-mutated constructs in Xenopus embryos. We show that high doses of Xlim-1 constructs deleted of CCR1 or CCR2 initiate secondary axis formation in the absence of its coactivator Ldb1 (LIM-domain-binding protein 1), suggesting that CCR1 and CCR2 are involved in negative regulation of Xlim-1. In contrast, while Xlim-1 is capable of initiating secondary axis formation at low doses in the presence of Ldb1, deletion of CCR2 (aa 275-295) or substitution of five conserved tyrosines in CCR2 with alanines (CCR2-5YA) abolished the activity. In addition, UAS-GAL4 one-hybrid reporter assays in Xenopus showed that CCR2, but not CCR2-5YA, with its flanking regions (aa 261-315) functions as a transactivation domain when fused to the GAL4 DNA-binding domain. Finally, we show that none of the known transcriptional coactivators tested (CBP, SRC-1, and TIF2) interacts with the Xlim-1 transactivation domain (aa 261-315). Thus, Xlim-1 not only contains a unique tyrosine-rich activation domain but also contains a negative regulatory domain in CT239-403, suggesting a complex regulatory mechanism underlying the transcriptional activity of Xlim-1 in the organizer.

Genomic structure, chromosomal localization and expression of the human LIM-homeobox gene LHX5

The LIM-homeobox gene Lhx5 plays an essential role in the regulation of neuronal differentiation and migration during development of the central nervous system. Mice lacking Lhx5 function show severely disorganized brain morphology and are impaired in cognition and motor coordination. In this study, we characterized the cDNA and genomic organization of the human LHX5 gene and analyzed its expression and chromosomal location. The human gene was found to contain five exons encoding a protein composed of 402 amino acids that is 98.8% identical to mouse Lhx5. By reverse transcriptase polymerase chain reaction, LHX5 transcripts were detected in fetal brain and in various regions of the adult central nervous system including the spinal cord, the thalamus, and the cerebellum. Fluorescence in situ hybridization mapped the LHX5 gene to chromosome 12, position 12q24.31-24.32. These results provide a framework for future analysis of possible association of human hereditary disorders with mutations in LHX5.

Requirements of Lim1, a Drosophila LIM-homeobox gene, for normal leg and antennal development

During Drosophila leg development, the distal-most compartment (pretarsus) and its immediate neighbour (tarsal segment 5) are specified by a pretarsus-specific homeobox gene, aristaless, and tarsal-segment-specific Bar homeobox genes, respectively; the pretarsus/tarsal-segment boundary is formed by antagonistic interactions between Bar and pretarsus-specific genes that include aristaless (Kojima, T., Sato, M. and Saigo, K. (2000) Development 127, 769–778). Here, we show that Drosophila Lim1, a homologue of vertebrate Lim1 encoding a LIM-homeodomain protein, is involved in pretarsus specification and boundary formation through its activation of aristaless. Ectopic expression of Lim1 caused aristaless misexpression, while aristaless expression was significantly reduced in Lim1-null mutant clones. Pretarsus Lim1 expression was negatively regulated by Bar and abolished in leg discs lacking aristaless activity, which was associated with strong Bar misexpression in the presumptive pretarsus. No Lim1 misexpression occurred upon aristaless misexpression. The concerted function of Lim1 and aristaless was required to maintain Fasciclin 2 expression in border cells and form a smooth pretarsus/tarsal-segment boundary. Lim1 was also required for femur, coxa and antennal development.

Formation of the definitive endoderm in mouse is a Smad2-dependent process

TGFbeta growth factors specify cell fate and establish the body plan during early vertebrate development. Diverse cellular responses are elicited via interactions with specific cell surface receptor kinases that in turn activate Smad effector proteins. Smad2-dependent signals arising in the extraembryonic tissues of early mouse embryos serve to restrict the site of primitive streak formation and establish anteroposterior identity in the epiblast. Here we have generated chimeric embryos using lacZ-marked Smad2-deficient ES cells. Smad2 mutant cells extensively colonize ectodermal and mesodermal populations without disturbing normal development, but are not recruited into the definitive endoderm lineage during gastrulation. These experiments provide the first evidence that TGFbeta signaling pathways are required for specification of the definitive endoderm lineage in mammals and identify Smad2 as a key mediator that directs epiblast derivatives towards an endodermal as opposed to a mesodermal fate. In largely Smad2-deficient chimeras, asymmetric nodal gene expression is maintained and expression of pitx2, a nodal target, is also unaffected. These results strongly suggest that other Smad(s) act downstream of Nodal signals in mesodermal populations. We found Smad2 and Smad3 transcripts both broadly expressed in derivatives of the epiblast. However, Smad2 and not Smad3 mRNA is expressed in the visceral endoderm, potentially explaining why the primary defect in Smad2 mutant embryos originates in this cell population.

Lim1 activity is required for intermediate mesoderm differentiation in the mouse embryo

During gastrulation and early organogenesis, Lim1 is expressed in the visceral endoderm, the anterior mesendoderm, and the lateral mesoderm that comprises the lateral plate and intermediate mesoderm. A previous study has reported that kidneys and gonads are missing in the Lim1 null mutants (W. Shawlot and R. R. Behringer, 1995, Nature 374, 425-430). Results of the present study show that in the early organogenesis stage mutant embryo, the intermediate mesoderm that contains the urogenital precursor tissues is disorganized and displays diminished expression of PAX2 and the Hoxb6-lacZ transgene. When posterior epiblast cells of the Lim1 null mutant embryo were transplanted to the primitive streak of wild-type host embryos, they were able to colonize the lateral plate and intermediate mesoderm of the host, suggesting that Lim1 activity is not essential for the allocation of epiblast cells to these mesodermal lineages. However, most of the mutant cells that colonized the lateral and intermediate mesoderm of the host embryo did not express the Hoxb6-lacZ transgene, except for some cells that were derived from the distal part of the posterior epiblast. Lim1 activity may therefore be required for the full expression of this transgene that normally marks the differentiation of the lateral plate and intermediate mesoderm.